Corrosion inhibitors from plant extract and terephthal

UNIVERSITY OF GHANA

COLLEGE OF BASIC AND APPLIED SCIENCES

CORROSION INHIBITORS FROM PLANT EXTRACT AND TEREPHTHAL AMIDE

DERIVED FROM WASTE PET BOTTLES FOR MILD STEEL IN ACIDIC MEDIUM

NII ARDAY ARDAY

DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING

DEC, 2018

University of Ghana http://ugspace.ug.edu.gh

i

UNIVERSITY OF GHANA

COLLEGE OF BASIC AND APPLIED SCIENCES

CORROSION INHIBITORS FROM PLANT EXTRACT AND TEREPHTHAL AMIDE

DERIVED FROM WASTE PET BOTTLES FOR MILD STEEL IN ACIDIC MEDIUM

BY

NII ARDAY ARDAY

(ID. NO. 10340508)

THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN

PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF MPHIL

MATERIALS SCIENCE AND ENGINEERING DEGREE

DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING

DEC, 2018


ii

DECLARATION

I, Nii Arday Arday hereby declare that this dissertation is a presentation of my own research work

and effort. The work was done under the supervision and co-supervision of Dr. David Dodoo Arhin

and Dr. Lucas Damoah respectively, both of the Department of Materials Science and Engineering,

University of Ghana School of Engineering Sciences.

Signature………………………………………………………………

(Author: Nii Arday Arday)

Signature……………………………………………………………….

(Supervisor: Dr. David Dodoo-Arhin)

Signature……………………………………………………………….

(Co-Supervisor: Dr. Lucas Damoah)


iii

ABSTRACT

Powdered Cassia Siamea leaves were investigated as a possible corrosion inhibitor for mild steel

in 2 M HCl. Terephthal amide obtained from depolymerizing waste PET (Polyethylene

terephthalate) bottles through aminolysis using mono ethanol amine (MEA), served as an additive

to improve the efficiency of the powdered leaves. The plant inhibitor was prepared by dissolving

1.5 g of the powdered leaves in 300 ml of 2 M HCl and boiled for 3 hours. Corrosion rates of the

steel samples were determined using both electrochemical analysis and weight loss measurements.

The powdered leaves and terephthal amide were characterized using Fourier Transform Infrared

Spectroscopy (FTIR). Phytochemical screening was conducted on the powdered leaves to verify

the presence of heterocyclic compounds such as tannins and flavonoids since they are known to

have inhibitive properties.

Results obtained from electrochemical readings show a reduction in corrosion rate (CR) with

increasing plant inhibitor volume concentrations. The CRs decreased even further with the addition

of the terephthal amide thereby increasing the inhibitor efficiency (IE %) of the plant inhibitor. It

was observed from the polarization curves that the cathodic reactions were inhibited more as

compared to the anodic reactions and therefore the inhibitor can be classified as a cathodic

inhibitor. The CRs obtained from weight loss measurements also show a reduction in rates with

increasing concentration, corresponding to the electrochemical readings. Less weight was also lost

with increasing inhibitor concentration. The phytochemical screening confirmed the presence of

tannins, saponins and flavonoids in the powdered leaves. The FTIR also confirmed the presence

of heterocyclic compounds and amides in the powdered leaves and terephthal amide respectively.


iv

DEDICATION

I dedicate this work to my parents Mr. Daniel Nii Adu Arday and Mrs. Vivian Arday. I have

reached this level of education because of their sacrifices, love, emotional and financial support

and most importantly their prayers. I pray for God’s everlasting grace and blessings upon their

lives. Thank you all abundantly.


v

ACKNOWLEDGEMENT

My utmost gratitude goes to the Almighty God for his abundant grace, blessings, strength,

guidance and inspiration to come this far.

My thanks also goes to Dr. David Dodoo-Arhin and Dr. Lucas Damoah, for their supervision,

guidance and constructive criticisms throughout this project. My appreciation also goes to Mr.

Patrick of the Botany Department for his extremely helpful recommendations. My final thanks

goes to all friends and colleagues who helped in diverse ways especially Mr. Bright Nsolebna,

Miss Elsie Bowen-Dodoo and Dr. Audrey Allotey for their selfless provision of assistance. God

bless you all.


vi

TABLE OF CONTENTS

DECLARATION ............................................................................................................................ ii

ABSTRACT ................................................................................................................................... iii

DEDICATION ............................................................................................................................... iv

ACKNOWLEDGEMENT .............................................................................................................. v

TABLE OF CONTENTS ............................................................................................................... vi

TABLE OF FIGURES .................................................................................................................. vii

LIST OF TABLES ......................................................................................................................... ix

ABBREVIATIONS ........................................................................................................................ x

CHAPTER ONE ............................................................................................................................. 1

INTRODUCTION ....................................................................................................................... 1

1.1 BACKGROUND INFORMATION .................................................................................. 1

1.2 PROBLEM STATEMENT ............................................................................................... 3

1.3 RATIONALE .................................................................................................................... 4

1.4 AIMS ................................................................................................................................. 4

1.5 OBJECTIVES .................................................................................................................... 4

CHAPTER TWO ............................................................................................................................ 5

LITERATURE REVIEW ............................................................................................................ 5

2.1 CORROSION IN METALS .............................................................................................. 5

2.2 TYPES OF CORROSION ................................................................................................. 7

2.3 CORROSION INHIBITORS .......................................................................................... 28

2.4 CASSIA SIAMEA ........................................................................................................... 32

2.5 POLYETHYLENE TEREPHTHALATE (PET) ............................................................. 33

CHAPTER THREE ...................................................................................................................... 37

METHODOLOGY .................................................................................................................... 37

3.1 PREPARATION OF SPECIMENS ................................................................................ 37

3.2 PREPARATION OF 2M HCl ......................................................................................... 37

3.3 PREPARATION OF PLANT INHIBITOR .................................................................... 37

3.4 AMINOLYSIS OF PET WASTE ................................................................................... 38

3.5 ELECTROCHEMICAL ANALYSIS .............................................................................. 38

3.6 WEIGHT LOSS MEASUREMENTS ............................................................................. 40


vii

3.7 PREPARATION OF TEST SOLUTIONS ...................................................................... 40

3.8 CHARACTERIZATIONS............................................................................................... 41

CHAPTER FOUR ......................................................................................................................... 43

RESULTS AND DISCUSSION ............................................................................................... 43

4.1 ELECTROCHEMICAL ANALYSIS .............................................................................. 43

4.2 WEIGHT LOSS MEASUREMENTS ............................................................................. 49

4.3 PERFORMANCE COMPARISON OF PLANT INHIBITOR TO PLANT/PLASTIC

INHIBITOR ........................................................................................................................... 57

4.4 CHARACTERIZATIONS............................................................................................... 61

4.5 INHIBITION MECHANISM AND IR SPECTRA OF PRODUCTS FORMED ON

METAL SURFACE. ............................................................................................................. 64

4.6 COMPARISON WITH ALREADY REPORTED RESEARCHES ............................... 67

CHAPTER FIVE .......................................................................................................................... 69

CONCLUSION AND RECOMMENDATIONS ...................................................................... 69

5.1 CONCLUSION ............................................................................................................... 69

5.2 RECOMMENDATIONS................................................................................................. 70

REFERENCES ............................................................................................................................. 71

APPENDIX ................................................................................................................................... 77

TABLE OF FIGURES

Figure 2.1: Electrochemical reaction between a steel pipe and HCl .............................................. 6

Figure 2.2: Metal undergoing uniform corrosion. .......................................................................... 8

Figure 2.3: Pitting of a steel plate by an acid-chloride solution. .................................................. 11

Figure 2.4: Galvanic corrosion occurring between a magnesium shell and a steel core. ............. 13

Figure 2.5: The four stages of crevice corrosion. ......................................................................... 17

Figure 2.6: Crevice corrosion occurring at regions that were covered with washers after the plate

was immersed in sea water. .......................................................................................................... 15

Figure 2.7: Intergranular attack on stainless steel. ........................................................................ 19

Figure 2.8: Selective leaching of silver from gold (dark areas) leaving empty channels (light areas).

....................................................................................................................................................... 21

Figure 2.9: Intergranular stress corrosion cracking in brass. ........................................................ 23


viii

Figure 2.10: Microbiologically Induced Corrosion attack on a steel pipe. ................................... 26

Figure 2.11: Repeat units of PET. ................................................................................................. 33

Figure 2.12: Resin Identification Code for PET. .......................................................................... 33

Figure 2.13: Chemical structure of bis (2-hydroxy ethylene) terephthal amide ......................... 366

Figure 3.1: Process for Plant Inhibitor Preparation ...................................................................... 37

Figure 3.2: Aminolysis of waste PET ........................................................................................... 38

Figure 3.3: Potentiostat and electrodes ......................................................................................... 39

Figure 4.1: Corrosion Rate vs. Time Graph (Plant) ...................................................................... 43

Figure 4.2: IE% vs. Inhibitor Concentration Graph (Plant) .......................................................... 44

Figure 4.3: Plant Inhibitor Tafel Plots .......................................................................................... 45

Figure 4.4: CR vs. Time Graph (Plant/Plastic) ............................................................................. 46

Figure 4.5: IE% vs. Inhibitor Concentration Graph (Plant/Plastic) .............................................. 47

Figure 4.6: CR vs. Inhibitor Concentration Graph (Plant/Plastic) .............................................. 477

Figure 4.7: Tafel plots for Plant/Plastic Inhibitor ......................................................................... 48

Figure 4.8: Weight Loss vs. Time Graph (Plant) ........................................................................ 511

Figure 4.9: CRWL vs. Inhibitor Concentration Graph (Plant) ..................................................... 522

Figure 4.10: IE% vs. Inhibitor Concentration Graph with respect to Weight Loss (Plant) ........ 533

Figure 4.11: Weight Loss vs. Time Graph (Plant/Plastic) .......................................................... 555

Figure 4.12: CRWL vs. Inhibitor Concentration Graph (Plant/Plastic) ........................................ 566

Figure 4.13: IE% vs. Inhibitor Concentration Graph with respect to Weight Loss (Plant/Plastic)

..................................................................................................................................................... 577

Figure 4.14: Graph comparing Plant and Plant/Plastic CRs ....................................................... 588

Figure 4.15: Graph comparing Plant and Plant/Plastic IE% ....................................................... 588

Figure 4.16: Graph comparing Plant and Plant/Plastic Weight Losses ...................................... 599

Figure 4.17: Graph comparing Plant and Plant/Plastic CRWL ...................................................... 60

Figure 4.18: Graph comparing Plant and Plant/Plastic IE% with respect to Weight Loss ........... 60

Figure 4.19: Comparison of steel samples after corrosion using plant inhibitor and plant/plastic

inhibitor. ...................................................................................................................................... 611

Figure 4.20: IR Spectrum for Cassia leaves................................................................................ 622

Figure 4.21: IR Spectrum of Terephthal Amide ......................................................................... 633

Figure 4.22: IR Spectrum of TA obtained from literature. ......................................................... 644


ix

Figure 4.23: IR Spectrum of product formed on the metal surface when the plant inhibitor was

used. ............................................................................................................................................ 655

Figure 4.24: IR Spectrum of product formed on the metal surface when the plant-plastic inhibitor

was used. ..................................................................................................................................... 666

Figure 4.25: IR Spectrum of product formed on the metal surface when no inhibitor was used.

..................................................................................................................................................... 677

LIST OF TABLES

Table 3.1: Concentration of plant inhibitor used. ......................................................................... 40

Table 3.2: Combination of plant and plastic inhibitors. ................................................................ 41

Table 4.1: Initial weight of steel samples for plant inhibitor weight loss test. ............................. 49

Table 4.2: Final weight of steel samples for plant inhibitor weight loss test. ............................... 49

Table 4.3: Weight loss of steel samples after plant inhibitor test. ................................................ 50

Table 4.4: Initial weight of steel samples for plant/plastic inhibitor test. ..................................... 53

Table 4.5: Final weight of steel samples after plant/plastic inhibitor test. ................................. 544

Table 4.6: Weight losses of steel samples after plant/plastic inhibitor test. ............................... 544

Table 4.7: Phytochemical screening results ................................................................................ 611


x

ABBREVIATIONS

ASTM – American Standard for Testing Materials

CI – Corrosion Inhibitor

CR – Corrosion Rate

CRblank – Corrosion Rate in blank solution

CRinh – Corrosion Rate in inhibited solution

EMF – Electromotive Force

HCl – Hydrochloric acid

IE% - Inhibitor Efficiency

IR – Infrared

MEA – Mono Ethanol Amine

MPa – Mega Pascal

ppm – Parts per million

TA – Terephthal amide

wt% - Weight percent

%EL – Percentage Elongation

%WL – Percentage Weight Loss


1

CHAPTER ONE

INTRODUCTION

1.1 BACKGROUND INFORMATION

Metals in general are used in most constructional applications due to their relatively high strength

compared to other material classes. They are usually alloyed to improve certain qualities such as

toughness, ductility and corrosion resistance. There are two main classes of metal alloys: Ferrous

Alloys and Non-ferrous Alloys [1]. Ferrous alloys are those whose main constituent is iron while

non-ferrous alloys are not iron-based. The production of ferrous alloys occur in larger quantities

due to iron-based compounds being abundant in the earth’s crust [1]. A vast range of physical and

mechanical attributes of the alloys can be achieved through economical fabrication techniques [1].

In addition, extracting, refining and alloying these metals are known to be relatively cheaper [1].

Examples of ferrous alloys include steel and cast iron.

Steel is the most common ferrous alloy. Though it is mainly an iron-carbon alloy, other elements,

which usually include nickel, chromium and molybdenum are present in substantial amounts

during the alloying process. The carbon content is normally not more than 1.0 wt% [1]. Depending

on their carbon concentration, they are generally termed as low, medium or high-carbon steels [1].

Stainless steels are also another type of steel with higher chromium content compared to normal

carbon steels [1]. Carbon concentrations in high-carbon steel is usually between 0.60-1.4 wt% [1].

Among carbon steels, they are the strongest and hardest but least ductile. They usually contain

other alloying elements such as chromium, tungsten and vanadium. Upon reaction with carbon,

these elements form high strength carbide compounds which have very good wear resistance [1].

Due to this, high-carbon steel is used in making cutting tools such as hacksaw blades and drills.

Medium-carbon steels have carbon contents between 0.25-0.60 wt% [1]. They are also alloyed

with vanadium, molybdenum and nickel to give varying combinations of strength and ductility.

Compared to low-carbon steels they are stronger, but weaker than high-carbon steels. Examples

of medium-carbon steel applications include machine parts, crankshafts and railway lines.

Low-carbon steels also known as mild steels are produced in the largest quantities of all the various

steel types due to their low production cost [1]. Their carbon content is generally below 0.25 wt%

and despite the fact that they are soft and weak, they have exceptional ductility and toughness [1].

Their ductility is typically around 25 % EL, yield strength of 275 MPa and tensile strengths


2

between 415-550 MPa [1]. Furthermore, mild steels have good machinability and weldability.

Applications include nails, pipes, bridges and buildings to mention a few.

In numerous industrial and constructional applications, mild steel remains the preferred choice.

This can mainly be attributed to its high strength and economical value. In spite of this, certain

industrial applications expose mild steel to very corrosive environments. For instance, steel

industries employ acid solutions in descaling and pickling while oil industries use high acid

concentrations for oil well acidizing to stimulate oil flow. Metal dissolution therefore occurs due

to the aggressiveness of the solutions used. Corrosion may cause leaks in pipelines, tanks and

casings. It occurs in all stages of oil production from downhole to surface machinery and also at

refinery level. Corrosion if not properly dealt with can lead to catastrophic equipment or structural

failures and most detrimentally, loss of life. The economic costs incurred due to corrosion in oil

refineries and natural gas sweetening (Carbon dioxide corrosion) is reported to be between 10-

30% of the maintenance budget [2]. In the United States of America for instance, metal corrosion

is estimated to cost approximately $276 billion per annum [3].

To help solve the corrosion problem many methods have been explored. Some of these include

using corrosion inhibitors (CIs), anodic protection, painting, galvanizing, and cathodic protection.

Corrosion inhibitors are mostly used in industries because of its low cost and easy usage [2, 4].

One mechanism by which they reduce the corrosion rate is via molecular adsorption onto the metal

surface to form a passive layer which protects the metal. The application of CIs makes it possible

to use lower-grade carbon steels thereby reducing capital cost compared to using high-grade alloys

[2]. The effectiveness of the CI depends largely on the adsorption bond strength. Factors affecting

the strength of this bond include the type of metal, inhibitor concentration and the environment

[5]. CIs are very sensitive to change, in the sense that, minor alterations to these factors can cause

significant changes in performance. For example, some CIs perform poorly in 28-30 % HCl

although they are very good in 15 % HCl [2]. CI selection therefore depends on the acid and metal

type, acid strength, expected temperature and the required protection time [2].

CIs can either be organic or inorganic. Inorganic CIs are those which have inorganic compounds

as their active compounds. Inorganic compounds such as chromates, nitrates and arsenates

incorporate themselves into the oxide layer to passivate the metal surface [6]. However, there are

concerns over compounds such as chromates as CIs due to their toxicity and inability to fulfil


3

certain environmental protection standards. Arsenates are also known to produce poisonous arsine

gas in acidic environments and numerous lives have been lost due to arsenic poisoning [2]. Green

organic CIs are therefore necessary in solving this problem.

Green organic CIs are biocompatible compounds mainly due to their biological origin and eco-

friendly nature [3]. They are usually obtained from plant extracts either from leaves, bark of trees,

stems, seeds, fruits or roots [3,5]. This makes them very economical compared to the synthetically

produced ones. These compounds are generally heterocyclic in nature and contain nitrogen,

oxygen, phosphorus and sulphur heteroatoms of an aromatic character [2, 5, 7]. The sites for

corrosion are blocked when these aromatic compounds are adsorbed onto the metal surface.

Several works have been done in this area and some include the use of black radish juice together

with natural honey as CIs for tin, which was carried out by Radojcic et al. [8]. Kalaiselvi et al [9]

reduced the corrosion of mild steel in HCl using Artemisia pallens while Okafor et al [7] used

Phyllanthus amarus extracts as CIs for mild steel in different acidic media. Despite these works

further research needs to be undertaken in this field to unearth the full potential of green CIs.

Another material with numerous applications is the polymer known as Polyethylene Terephthalate

(PET). PET which is a thermoplastic belonging to the family of polyester is commonly used in the

production of water and soft drink bottles. Just like plastics in general, PET is quite difficult to

dispose of. Bottles and other products made from this polymer can be seen littered about in most

communities amidst other waste materials, choking gutters and creating a lot of nuisance. This

disturbing situation is commonly found in developing countries where there is severe lack of

proper waste management. Studies in Wa which is the capital city of the Upper West Region in

Ghana have shown that solid waste of approximately 810 tonnes are produced daily with only 216

tonnes being collected [10]. This leaves a staggering 594 tonnes of waste unattended to. About

25.46% of this waste generated is made up of plastics [10]. A study in Madina in the Greater Accra

Region of Ghana also showed that households generate 64.3 % of plastic waste [11]. There is

therefore the need to device a proper means to dispose of these waste plastic materials.

1.2 PROBLEM STATEMENT

Corrosion of mild steel is a major problem for most industries and there exists the need to control

it in a more efficient, cost effective and eco-friendly manner. More research into green corrosion


4

inhibitors is therefore necessary. Plastic waste also continues to be a huge menace worldwide and

there is hence the need to find appropriate disposal methods.

1.3 RATIONALE

The importance or reasons for this study are as follows,

i. This research will aid in tackling corrosion in mild steel in a cost effective and eco-friendly

manner.

ii. This work goes a long way to aid in the recycling of plastic waste.

iii. This research adds to the existing knowledge on green inhibitors.

iv. This research seeks to provide an innovative corrosion inhibitor by combining plant extract

with terephthal amide which is a by-product of PET.

1.4 AIMS

This project seeks to investigate the corrosion inhibitive properties of plant extracts from Cassia

Siamea leaves and a possible increase in its efficiency by combining it with terephthal amide

obtained from the depolymerization of waste PET bottles. This investigation will be carried out on

mild steel in 2 M HCl.

1.5 OBJECTIVES

To meet the aim of this study, the following objectives were set:

1. Extract inhibitor materials from Cassia Siamea leaves and waste PET bottles respectively.

2. Investigate the effect of various concentrations of Cassia Siamea leaves extracts on

corrosion rate of mild steel in 2 M HCl.

3. Investigate the combined effect of terephthal amide particles and various concentrations of

Cassia Siamea on the corrosion behaviour of mild steel in 2 M HCl solution.

4. Investigate the mechanism of corrosion inhibition using Cassia Siamea leaves extract. This

involves the characterization of active compounds adsorbed onto metal surface via FTIR.

5. Determine the type of inhibitor based on Tafel plots.

6. Quantify the inhibitor efficiencies obtained.


5

CHAPTER TWO

LITERATURE REVIEW

2.1 CORROSION IN METALS

Corrosion in metals is a chemical or electrochemical process which involves the reaction of a metal

with its environment, resulting in the destruction of the metal and its mechanical properties [12,

13]. This process ordinarily begins at the surface. Corrosion in steel is known as rusting due to

iron being its major constituent. In industrialized nations, corrosion is known to cause significant

economic problems with approximately 5 % of the country’s budget going into maintenance,

repairs or replacement of infrastructure or products affected by corrosion reactions [12].

The presence of four key elements is essential for the corrosion reaction to occur. The elements

include a cathode, an anode, a conductive material and an electrolyte. In the case of steel corroding

as a result of acid attack, the anode and cathode are found in the steel, the steel itself serves as the

conductive material and the acid serves as the electrolyte. The corrosion process involves two main

reactions:

1. An anodic (oxidation) reaction which occurs at the anode.

2. A cathodic (reduction) reaction which occurs at the cathode.

2.1.1 ANODIC OR OXIDATION REACTION

Iron has a high tendency to return to its natural state in order to minimize energy. This tendency is

the main driving force for corrosion to occur. Elemental iron which is unstable oxidizes and reacts

easily with oxygen and other elements. Iron ores such as siderite (FeCO3), hematite (Fe2O3), pyrite

(FeS2) and magnetite (Fe3O4) are produced in nature due to these oxidation reactions [14]. In the

case of corrosion, rust (Fe(OH)2 or Fe(OH)3) results from this oxidation reaction when the

electrolyte is water [14]. Considering a situation where a steel pipe is being corroded by HCl

(Figure 2.1), the breakdown of elemental iron at the anode is given in Equation 2.1:

𝐹𝑒 → 𝐹𝑒2+ + 2𝑒− 2. 1

In Equation 2.1 elemental iron (Fe) can be seen breaking down to ferrous iron (Fe2+) and two

electrons (2e-). Fe leaves the metal forming pits on the pipe surface at the anode. The ferrous iron

reacts with the HCl to form FeCl2 (green rust) and hydrogen ions as shown in Equation 2.2:


6

𝐹𝑒2+ + 𝐻𝐶𝑙 → 𝐹𝑒𝐶𝑙2 + 𝐻+ 2. 2

Green rust is a type of rust which forms when chloride ions react with iron. The rust accumulates

and forms a coating on the anode surface, eventually becoming tubercles. These tubercles actually

form a protective layer over the anode and in turn low down the corrosion rate. The continuous

flow of acid causes the tubercles to be dislodged exposing the anode again, thereby causing an

increase in corrosion rate. The electrons produced in Equation 2.1 are conducted to the cathode

through the pipe wall.

2.1.2 CATHODIC OR REDUCTION REACTION

Upon reaching the cathode, the electrons leave the metal and go into the acid by reacting with

hydrogen ions to produce hydrogen gas as shown in Equation 2.3:

2𝐻+ + 2𝑒− → 𝐻2 2. 3

Similar to the corrosion process being stalled when the tubercles cover the anode surface, the

hydrogen gas also coats the cathode surface thereby reducing the corrosion rate [14]. This process

of hydrogen gas protecting the cathode by forming a passive layer is known as polarization [14].

Water is produced when dissolved oxygen in the acid reacts with the hydrogen gas protecting the

cathode as shown in Equation 2.4:

2𝐻2 + 𝑂2 → 2𝐻2𝑂 2. 4

Figure 2.1: Electrochemical reaction between a steel pipe and HCl

Once the protective layer of hydrogen gas has been removed by the reaction, the corrosion rate

increases rapidly [14]. This process is known as depolarization and it explains why acids high in

dissolved oxygen are more corrosive.

e-

Cathode Anode

Fe HCl H2


7

2.2 TYPES OF CORROSION

Classification of corrosion attacks can be done based on the manner in which they manifest

themselves. Generally, corrosion can be classified into (but not limited to) nine groups and they

are as follows:

1. Uniform Corrosion

2. Pitting

3. Galvanic Corrosion

4. Crevice Corrosion

5. Intergranular Corrosion

6. Dealloying or Selective Leaching

7. Environmental Cracking: Stress Corrosion and Corrosion Fatigue

8. Hydrogen Damage

9. Microbiologically Induced Corrosion

Other forms of corrosion are also possible, such as exfoliation, which is only peculiar to high-

strength aluminium [15]. Each corrosion type has its unique characteristics, causes and control

measures though some control mechanisms cut across more than one corrosion type.

2.2.1 UNIFORM CORROSION

Uniform corrosion is the type of corrosion which occurs with the same intensity or rate across the

entire metal surface area with a scale of rust being produced [12, 16]. In simple terms, no single

region or area is favoured over the other as shown in Figure 2.2, with oxidation and reduction

reactions occurring sporadically over the surface. It is possibly the most common corrosion type

observed in everyday life. Examples of uniform corrosion include the rusting of iron and the

tarnishing of silver. Other forms of uniform attack are the fogging of nickel and the oxidation of

metals at high temperatures [14].

In the analysis of uniform corrosion and any other type of corrosion, two important principles must

be considered:

1. The tendency for the corrosion reaction.

2. The rate at which the reaction will occur.


8

Figure 2.2: Metal undergoing uniform corrosion [17].

Failure to consider one principle with regard for the other can lead to improper conclusions. In

uniform corrosion the tendency for the reaction to occur depends on the magnitude of the EMF of

the half-cell reactions (oxidation and reduction). The tendency can therefore be determined by

inspection of the thermodynamic equilibrium potentials of the oxidation and reduction reactions

[16].

Assuming the corrosion reactions detailed in sections 2.1.1 and 2.1.2 are uniform, the standard

thermodynamic equilibrium potential for Equation 2.1, V1 measured against a normal hydrogen

electrode is -0.44V [16]. The thermodynamic equilibrium potential for Equation 2.4, V2 also

measured against a normal hydrogen electrode is 1.229 V [16]. The overall cell EMF, V is given

as

𝑉 = 𝑉2 − 𝑉1 2. 5

𝑉 = 1.229𝑉 − (−0.44𝑉) = 𝟏. 𝟔𝟔𝟗 𝑽

If V > 0, then the reaction has a tendency to proceed as written. Meaning in this case since

1.669V>0 the iron will go into solution and the oxygen will be reduced. Similar calculations can

be made for different metals in different electrolytes. Despite the fact that these calculations

determine the tendency of corrosion to occur, they do not indicate the corrosion rate [12, 16].

Higher EMFs do not necessarily mean the corrosion rate will be high. The corrosion rate is a

parameter of concern in most practical problems. Corrosion reactions are made up of numerous

complex steps and the final corrosion rate can be no faster than the slowest of these steps [16]. The

slowest step therefore ends up being the rate determining step [16]. Each aspect of the reaction

sequence imparts a resistance on the corrosion reaction thereby limiting the corrosion currents

obtained from the cathodic and anodic reactions [16].


9

The key rate-controlling steps are referred to as the concentration control (concentration

polarization) and activation control (activation polarization) [12, 16]. Concentration control or

concentration polarization implies that the rate determining step is the rate at which reactants or

products move to or from the corroding surface [16]. In other words, diffusion within the

electrolyte controls the rate of reaction. An example of concentration control is the ability of a

reactant such as dissolved oxygen, to diffuse to a steel surface in an electrolyte or the ability of a

reaction product, for instance, hydroxide, to diffuse from a steel surface. High supply rates of

reactants favor a higher corrosion rate; low diffusion rates of products from a surface hinder

corrosion rate.

Activation polarization or control refers to the rate of oxidation and reduction that occurs at the

interface between the metal surface and the electrolyte [12, 16]. The term activation is used due to

the activation energy that accompanies the dilatory, rate-determining step in the corrosion process

[12]. Activation control is also referred to as charge transfer control [16]. Considering iron in an

acid environment, one reaction product is normally hydrogen gas. Given that the solution is

sufficiently acidic, there will be an abundance of hydrogen ions in the solution. In spite of this,

mass transfer of hydrogen ions to the iron surface is not usually the rate-controlling step. The rate-

controlling step can be the transfer of electrons from the iron to the hydrogen ions on the surface

to form adsorbed atomic hydrogen, the combination of the hydrogen atoms to form a hydrogen

molecule or the coalescence of numerous hydrogen molecules to form a bubble of hydrogen gas

[12, 16]. The overall reaction rate is determined by the most retarded step. Charge transfer rates

can vary depending on material properties and different chemical environments. Other factors may

influence uniform corrosion rates, such as the development of oxide layers on a metallic surface

which could limit the corrosion rate by affecting diffusion and imparting concentration control.

Units for describing the rate of uniform corrosion are generally expressed in millimeter penetration

per year (mm/y) and grams per meter squared day (g/m2d). Inches penetration per year (ipy), mil

per year (mpy) and milligram per decimeter squared day (mdd) are other commonly used units

[18]. Barring any corrosion product adhering to the metal surface, these units represent metal

penetration or weight loss of the metal [14]. For instance, there is a relatively uniform CR of steel

in sea water with values around 0.13 mm/y, 2.5 g/m2d, 25 mdd or 0.005 ipy [14]. These represent

time-averaged values.


10

Under normal circumstances, a light metal which loses a given weight per unit area will experience

a greater loss of thickness compared to a heavy metal which loses the same amount of weight per

unit area. Hence, it is essential to know the metal density when converting from one unit to the

other. In accordance with its intended purpose and CR, metals can be divided into three groups

[14]:

1. < 0.15 mm/y – This group represents metals that have good corrosion resistance and are

hence used for critical machine parts. Examples include springs, valve seats and pump

shafts.

2. 0.15-1.5 mm/y – Metals present in this category do not have a high corrosion resistance

and therefore can only be utilized if a higher CR can be tolerated. Examples are tanks,

pipes, valve bodies and bolt heads.

3. >1.5 mm/y – Usually not satisfactory.

2.2.1.1 Prevention and Control

The single most important control or protective measure against uniform corrosion is proper

materials selection for a project. Several corrosion-resistant alloys have been developed, with

stainless steels amongst the most widely used [15]. However, stainless steels cannot solve all

corrosion-related problems. Other alloys have therefore been developed to fill in where stainless

steel fails. For instance, special weathering steels have been produced to help curb the atmospheric

wearing of steel. These steels contain relatively low amounts of copper together with other alloying

elements such as nickel, chromium or phosphorus [15]. The rust on these steels adhere to the

surface, creating a passive layer which tends to protect.

In some instances, a cheaper material can be used as a protective coating against aggressive media.

Provision of a corrosion allowance has also proved to be quite useful. Example, if a CR of 6 mpy

is observed for a tank that is required to have a 7-year lifespan, an added tank wall thickness of

about 60 mils (or 0.06 inch) can be provided to exceed the necessary requirements needed to

conform to the operating conditions such as temperature, stress and pressure [15]. In addition to

this, the use of inhibitors and other common practices such as anodic and cathodic protection can

be applied.


11

2.2.2 PITTING

Pitting is the type of corrosion which is localized and more intense at some discrete regions while

a vast majority of other areas remain unaffected. This process differs largely from uniform

corrosion where the entirety of the exposed surface deteriorates at approximately the same rate.

Pitting usually penetrates from the top of a horizontal surface downwards in almost a vertical

manner [12]. This as a result of gravity [12]. As the pit grows deeper, the solution at the bottom of

the pit increases in concentration [12]. Pitting attack on a steel plate is shown in Figure 2.3.

Figure 2.3: Pitting attack on a steel plate [17].

A pit is termed as deep if the corrosion process is limited to a relatively small area, with that part

of the metal acting as the anode [14]. If the pits are not so deep and the affected area is relatively

broader, then the pits can be described as shallow [14]. The depth of the pit increases at a much

faster rate compared to the width of the pit [19]. In some circumstances, the pits can become hidden

due to the corrosion product covering them and thereby go undetected during inspection. There is

also very little material loss or obvious reduction in overall wall thickness until failure occurs. Due

to its unpredictability and difficulty to design against, pitting is considered to be more hazardous

than uniform attack [17].

Pitting can be induced by localized defects which include but not limited to scratches and

compositional differences. The existence of a passive state of a metal in an environment of interest

is a basic requirement for pitting [20]. Upon the initiation of pitting, certain regions of the metal

surface begin to corrode rapidly as they become passive [20]. This loss of passivity usually takes


12

place at heterogeneities like impurities which could be physical or chemical. Therefore metals with

polished surfaces have been observed to have a greater resistance to pitting [12].

The depth of the pit is normally expressed in terms of the pitting factor, P.F, which is the ratio of

the deepest metal penetration, P, to the average metal penetration as determined by the weight loss

of the specimen, d. It is expressed mathematically as:

𝑃. 𝐹 = 𝑃/𝑑 2. 6

A P.F of 1 indicates uniform corrosion. A metal’s resistance to pitting is often determined using

the Critical Pitting temperature (CPT) and the Pitting Resistance Equivalent Number (PREN) [19].

As generally accepted, a high PREN represents a better pitting resistance. Chromium,

molybdenum and nitrogen contents are used to measure the PREN:

𝑃𝑅𝐸𝑁 = %𝐶𝑟 + 3.3 (%𝑀𝑜) + 30(%𝑁) 2. 7

The temperature of the solution when pitting is first detected is known as the CPT of the alloy

[19]. These temperatures are normally measured in ferric chloride (10% FeCl3.6H2O) and an acidic

mixture of chlorides and sulphates [19].

One of the effects of pitting is the perforation of water pipes, rendering them inoperational although

only a relatively small amount of the metal has been lost. Structural failure from localized

weakening may also occur even though a considerable percentage of metal remains. By also

providing sites where stresses are concentrated, pits can assist in brittle and fatigue failure,

environmental cracking and corrosion fatigue [19].


Pitting can be prevented by

1. Reducing the aggressiveness of solutions through chloride elimination or decrement.

2. Adding passivating inhibitors to aid in curbing pitting attack. However, to ensure complete

passivation, the appropriate inhibitor concentrations have to be used. This is vital because

incomplete passivation can actually increase the rate of pitting [15].

3. Avoiding stagnation of the solution.

4. Applying a sacrificial anode or cathode.


13

2.2.3 GALVANIC CORROSION

This type of corrosion damage occurs when two dissimilar metals are electrically coupled in a

corrosive electrolyte [12, 18]. When two metals are in contact or close to each other in an

electrolyte, one of them will corrode when there is a flow of galvanic current between them. The

information as to which metal will corrode is provided by the galvanic series. This series represents

the relative reactivities of several metals in seawater [12, 15]. The more negative metal (anode) in

the galvanic series will have a driving force to lose electrons to a metal in electrical contact that is

more positive (cathode) in the series [21]. This will cause the corrosion rate of the more negative

metal to increase while the more inert metal will be protected when the electrical connection is

made. For example, in instances where a copper pipe is connected to a carbon steel pipe, the carbon

steel pipe will start corroding in the vicinity of the junction since it is more negative or reactive.

Similarly, as shown in Figure 2.4, galvanic corrosion can be seen occurring at the interface a

magnesium shell that has been cast around a steel core.

Figure 2.4: Galvanic corrosion occurring between a magnesium shell and a steel core [12].

Galvanic Corrosion Steel Core

Magnesium Shell


14

The rate at which galvanic corrosion occurs is directly proportional to the cathode–anode area ratio

that is exposed to the corrodent [12, 15]. Thus, considering a fixed cathode area, a relatively

smaller anode will corrode faster compared to a much larger anode [12]. This, as stated in section

2.2.1, is as a result of the CR not simply depending on the current but rather the current density.

The current density can be defined as the current per unit area of the corroding surface. The smaller

anode therefore experiences a high current density. The CR of the anode can be 100 to 1000 times

greater if its area is smaller than the cathodic area [15]. It is much preferred if the anodic area is

larger than the cathodic area.

Non-metallic conductors such as impermeable graphite in heat exchangers and carbon bricks in

metallic vessels and can also serve as cathodic sites when galvanically coupled with other metals

[15]. Other examples include conductive films that serve as cathodic sites to their base metal such

as iron sulphide on steel [15]. The galvanic behavior of metals in different environments is usually

based on the galvanic series obtained from seawater. Ideally every environment should have its

own galvanic series but this will result in an infinite number of tests making it impractical. It is

also worth noting that metals on their own will corrode at different rates in seawater compared to

when they are galvanically coupled with other metals [12, 15, 18].


Prevention and control measures that can be employed include:

1. Electrically insulating two dissimilar metals when joining them together by for example

using plastic washers as a separator when bolting together two dissimilar metal flanges.

2. Cathodic protection by coupling a third anode to the other anode and cathode.

3. Metals with vastly different potentials in the galvanic series should not be coupled together.

4. Avoid unfavourable cathode-anode ratios. Anodic areas must be larger to provide a longer

lifespan.

5. Favourable cathode-anode area ratios can be achieved by coating the cathodic area and not

the anodic area.


15

2.2.4 CREVICE CORROSION

Crevice corrosion is a localized form of attack that occurs within or around narrow gaps created

by metal-to-metal or metal-to-nonmetal contacts where there is the presence of a stagnant solution.

Varying ion concentrations or the concentration of dissolved gases found in these regions bring

about this type of corrosion and it usually occurs in areas where the concentration is lower [12,

15]. For instance, parts of metals including but not limited to flanged and lap joints which are not

directly exposed to the environment experience localized depletion of dissolved oxygen causing

them to corrode [12]. Furthermore, the crevice must be wide enough to trap the electrolyte but

small enough to stagnate it. The width is normally several thousandths of an inch but not more

than 3.18mm [12, 15]. An example of crevice corrosion is shown in Figure 2.5.

Figure 2.5: Crevice corrosion occurring at regions that were covered with washers after the

plate was immersed in sea water [12].

After the depletion of oxygen within the crevice, the metal begins to oxidize at this site. Electrons

generated from the oxidation are transferred to the external areas where they are used up in a

reduction reaction. Extremely corrosive agents such as solutions with high hydrogen and chloride

ion concentrations are seen to form within the crevices [12]. This occurs especially for metals that

have been exposed to aqueous or saline environments [12]. Metals that passivate through surface

film formation such as stainless steel and aluminum are susceptible to crevice corrosion since

chloride ions are known to destroy such films [12, 15]. However, it is important to take note of the

fact that the tendency of crevice corrosion to occur is not the attribute of a particular metal but

rather it is a function of the metal’s response to a given environment [22]. For example, carbon

steels do not undergo crevice corrosion in acidic media because they do not form a passive film in


16

such environments [22]. On the other hand, they are attacked by crevice corrosion in alkaline

environments even though they form a passive film [22].

The process of crevice corrosion can be classified into four stages as illustrated in Figure 2.6:

1. Deoxygenation - Crevice corrosion is initiated when localized areas on a passive surface

are isolated as already stated. Convection occurs at a slow rate between the solution

trapped within the crevice on the passive surface and the bulk environment, causing the

dissolved oxygen in a crevice solution to deplete quickly [15, 22].

2. Increase of the salt and acid concentrations - As metallic dissolution continues within

the crevice, the metal cations such as Fe2+ and Cr3+ become concentrated in the stagnant

solution. They also react with water in a hydrolysis reaction to generate acidity [15, 22] as

shown in Equation 2.8:

𝐹𝑒2+ + 2𝐻2𝑂 ↔ 𝐹𝑒(𝑂𝐻)2 + 2𝐻+ 2. 8

Chloride ions are drawn to the crevice solution increasing the metallic chloride concentration even

if small quantities of chloride ions are found in the bulk solution [22]. When chloride

concentrations are adequately high to break down the passive film, metallic dissolution occurs.

Low crevice solution pH also aids in metallic dissolution [22].

3. Depassivation – When the crevice which acts as the anode, and the surrounding surface

which acts as the cathode, have varying concentrations of oxygen in their respective

solutions, a localized galvanic cell is created. As the oxygen in the crevice solution

depletes, the corrosion potential is shifted from a passive to an active sense [22]. Once

corrosion has been initiated, the reaction can proceed rapidly due to the large difference

in cathode-anode area ratio where the anode (crevice) is small and the cathode

(surrounding surface) is large.

4. Propagation – Pit nucleation and growth occurs within the crevice as the potential is

applied. At a critical potential the metal dissolution increases steadily as the pits stabilize

[22].


17

Figure 2.6: The four stages of crevice corrosion.


Crevice corrosion can be prevented by:

1. Regular inspection and frequent removal of accumulated deposits.

2. Decreasing the acidity where possible. Chloride content and temperature can also be

reduced.

3. Avoiding stagnation through proper designing of containment vessels to enhance drainage.

4. Using tar or bitumen to seal the periphery of tanks to avoid seepage of rainwater.

5. Using non-absorbing gaskets. Wet packing materials should be discarded during prolonged

shutdowns.

6. Instead of riveted or bolted joints, welding can be done. Applying the right caulking

compound can also be considered.

7. Applying crevice corrosion resistant alloys could be taken into account. The resistance of

stainless steel for instance can be enhanced by increasing the amount of some alloying

elements such as chromium, nickel, molybdenum, and nitrogen [15].

Base Metal

Passive Film

1. Oxygen is consumed

within the crevice.

2. Acidification and

increase in chloride

concentration.

3. Potential decrease from

passive to active regime.

4. Metastable pits

nucleate and grow.


18

2.2.5 INTERGRANULAR CORROSION

This is a specialized kind of corrosion that occurs preferentially at the grain boundaries of metals

and in specific environments. The grain itself experiences little or no attack [19]. The grain

boundary, which is a small area, serves as the anode while the grain itself which has a relatively

large area serves as the cathode. This results in high cathode-anode surface area ratios. Therefore,

electrons flow from a small anode to a relatively large cathode leading to rapid attacks penetrating

deeply into the metal which eventually results failure.

Metals and alloys generally consist of collections of randomly oriented single crystals or grains.

At the boundaries between these crystals, different lattice orientations meet and zones of imperfect

structure are formed. That is, there is a crystallographic mismatch between the relative orientations

of the adjacent grains. As a result of these structural imperfections, the chemical composition and,

consequently, the electrochemical properties and corrosion resistance of the grain boundaries may

differ appreciably from those of the grains [23]. Intergranular attack on these boundaries may range

from light etching, which merely outlines the granular structure, to rapid penetration, which may

lead to loss of mechanical strength or even a residue of completely separated grains which is known

as "sugaring" [23].

Whether the type of corrosion observed will be intergranular or a uniform attack depends on the

difference between the CR of the grain boundaries and that of the grain surfaces [23]. The

characteristics of the electrolyte together with the metallurgical structure and composition of the

grain boundary account for the variations in CR [23]. Factors that lead to grain boundaries being

reactive include:

1. Equilibrium segregation of specific elements or compounds at grain boundaries while

remaining in solid solution [15, 23].

2. One of the alloying elements being preferentially enriched at the grain boundary. This can

be observed in brass [15, 23].

3. The corrosion-resistant element in the alloy being depleted at the grain boundary. This is

particularly seen in stainless steels [15, 23].


19

All contributing factors that bring about intergranular corrosion are due to the thermal exposure of

the metals. Examples of heat treatments that cause intergranular corrosion include welding,

fabrication and stress relief [12, 15, 19, 23].

Precipitation of carbide during welding is a typical factor for intergranular attack and this can

mainly be seen in austenitic stainless steels. This usually occurs when these alloys are heated

between 500 and 800 ºC [12]. When the heating duration is long enough, the metal becomes

sensitized to intergranular corrosion [12]. Sensitization is the process whereby the chromium and

carbon in the steel combine to produce chromium carbide (Cr23C6) precipitates as a result of the

heat treatment [15]. The sensitization of stainless steel during welding is called weld decay [12,

15, 19, 23]. The formation of the chromium carbide precipitates at the boundary as a result of

chromium-carbon diffusion, creates an area adjacent to the boundary which is deficient in

chromium, making the grain boundary extremely prone to attack [12]. Exposure of nickel-based

alloys to temperatures below their annealing temperature makes them susceptible to carbide

precipitation or the precipitation of other intermetallic phases [15]. For instance, when nickel-

based alloys are exposed to sulphur-bearing gaseous environments, nickel sulphide is produced

and this can bring about huge failures [14]. Intergranular corrosion is shown in Figure 2.7.

Figure 2.7: Intergranular attack on stainless steel [23].


20


Intergranular corrosion may be prevented by:

1. Redissolving all chromium carbide precipitates by heat treating the sensitized metal at high

temperatures.

2. Using alloys containing less than 0.03 wt% carbon to reduce the risk of carbide

precipitation.

3. Adding alloying elements like niobium or titanium during stainless steel production. These

metals have a higher tendency than chromium to form carbides, causing the chromium to

remain in solution.

4. Using low carbon-content alloys and alloying with tantalum, titanium or niobium can also

be applied to nickel-based alloys.

2.2.6 DEALLOYING OR SELECTIVE LEACHING

Dealloying is the process whereby a particular element is preferentially removed from an alloy

due to corrosion. Dezincification, which is a type of dealloying in zinc-based alloys such as brass,

occurs when the zinc corrodes leaving behind a porous copper residue together with other

corrosion products [14]. In addition, the colour changes from yellow to red or to the colour of

copper. Most often than not, the selectively leached metal is the least noble element found in the

alloy [15]. Aside surface tarnishing, a dealloyed metal will usually keep its original shape and

seem undamaged though its tensile strength and ductility may be badly affected.

Dezincification of brass can occur in two ways:

1. Plug-type which occurs in localized areas on the metal or

2. Layer-type which occurs uniformly over the surface [15].

Low zinc alloys favour plug type while layer-type attack is favoured by high-zinc alloys [15]. The

type of environment is a major determining factor of the kind of dezincification that will occur.

Plug-type dezincification is found in neutral, alkaline or high saline water environments [15]. It

also occurs above room temperature. Crevice conditions under a deposit of scale or salt tend to

aggravate the condition. A plug of dezincified brass may fall out, leaving a hole; whereas water

pipes experiencing layer-type dezincification may split open [15]. Conditions that favour selective

leaching are:


21

1. Porous inorganic scale formation.

2. High temperatures.

3. Stagnant solutions especially if acidic.

Another type of dealloying is known as parting. Parting, which is similar to dezincification, also

occurs when the least noble component of the alloy is selectively leached. It is generally limited

to novel metal alloys such as gold-copper alloy which is used in refining gold [14]. Other systems

that undergo dealloying are as follows:

1. Removal of nickel from copper-nickel alloys [24].

2. Leaching of silver from gold-silver alloys (Figure 2.8) [24].

3. Exposure to hydrofluoric acid or chloride-containing acids causes aluminum to leach out

of aluminum-bronze [15].

4. Manganese from copper-manganese [24].

5. Tin from bronze in hot brine or steam [15].

6. At high temperatures chromium will dissolve preferentially from steels [24].

7. Cobalt from Co-Ni-Cr alloys [24].

8. Gold from gold-platinum alloys [24].

9. Silver from silver-palladium [24].

10. Copper-aluminum alloys which have an aluminum content of more than 80 % are very

prone to dealloying [15] etc.

Figure 2.8: Selective leaching of silver from gold (dark areas) leaving empty channels (light

areas) [24].


22


Modes of prevention include:

1. Cathodic protection.

2. Frequent removal of scales and deposits.

3. Avoid the stagnation of corrosives especially acidic ones.

4. Using alloys with a high dealloying resistance. Alloys such as red brass which contain less

than 15% zinc are almost insusceptible to selective leaching [15].

5. Dezincification can be curbed by including alloying elements such as phosphorus,

aluminum or arsenic [15].

2.2.7 ENVIRONMENTAL CRACKING

Environmental cracking occurs when a tensile stress is applied to a metal in a corrosive

environment. There are two type of environmental cracking. They are stress corrosion cracking

and corrosion fatigue.

2.2.7.1 Stress Corrosion Cracking (SCC)

If a metal, being subjected to a constant tensile stress and simultaneous exposure to a specific

corrosive environment, cracks immediately or after a given time, the failure is said to be stress

corrosion cracking [12, 14, 15]. Alternating between the stress application and the corrosive

environment will not result in SCC [15]. Same way corrosion alone without applying any stress

does not lead to SCC [15]. Metals that are unreactive in certain environments can change and

become susceptible to corrosion in those environments once a stress is applied. Once there is the

formation and propagation of cracks that are perpendicular to the stress, failure becomes imminent.

The failure process is similar to that of a brittle material, though the alloy may be ductile [12].

Furthermore, the formation of cracks can occur at stresses significantly below the tensile strength

of the material and this can be attributed to the presence of corrosion. The minimum stress below

which SCC will occur is called the threshold stress. In some systems, this may be as low as 10 %

of the actual yield stress [15].

Most alloys are prone to SCC when exposed to certain environments, especially when the stress

being applied is moderate. Most stainless steels for instance stress corrode in solutions containing

chloride ions. Exposing brass to ammonia also makes it very susceptible to SCC. The stress that


23

produces SCC need not be externally applied. It may be a residual stress that occurs as a result of

spontaneous temperature changes and uneven contraction or expansion due to different

coefficients of expansion of the individual metals in an alloy [12]. The conditions therefore needed

for SCC to occur include

1. A constant tensile stress.

2. A suitable environment, that is, one which is corrosive.

3. A sensitive metal

4. Conducive temperature and pH values.

Precipitation and segregation processes as stated in section 2.2.5 can also lead to SCC [26]. Most

metals that are safe from general corrosion through protective film formation are susceptible to

SCC [15, 26]. This due to the breakdown of the film thereby creating galvanic cells. An example

of SCC is shown in Figure 2.9.

Figure 2.9: Intergranular stress corrosion cracking in brass [12].

2.2.7.2 Corrosion Fatigue

A metal is said to have failed by corrosion fatigue if it cracks due to the application of alternating

tensile stresses in a corrosive environment [14, 15, 27]. In the absence of a corrosive environment

the metal stresses similarly but at values below the critical stress called the fatigue limit or


24

endurance limit. A metal fails after a prescribed number of stressed cycles no matter how low the

stress [14].

The greater the applied stress, the fewer the number of cycles required for the metal fail. Failure

also occurs at a faster rate. Below the endurance limit, no failure will occur irrespective of the

number of cycles [15]. This is mainly observed in steels and other ferrous metals. In a corrosive

environment on the other hand, failure will occur at any applied stress as long as an adequate

number of cycles is applied. However, a large number of environments which are not particularly

specific can cause corrosion fatigue, unlike SCC [15]. An increase in temperature, decrease in pH

and an increase in corrodent concentration all lead to aggravation of corrosion fatigue [15].


1. Minimizing the external load can help lower the magnitude of stress. This can also be

achieved by increasing the cross-sectional area perpendicular to the applied stress.

2. Heat treatment can be used to anneal out any residual thermal stresses.

3. Cathodic protection

4. Uniform application of shot peening.

5. Use of inhibitors that prevent SCC such as Η3ΡΟ4 and Na2O4 which prevent SCC of iron

in nitrate environments.

2.2.8 HYDROGEN DAMAGE

This is the deterioration of physical and mechanical properties such as ductility and tensile strength

resulting from the penetration of atomic hydrogen into the crystal lattice. Basic aspects of the

problem of hydrogen in steels involve its limited solubility in the lattice, its high propensity for

adsorption on internal and external surfaces, its diffusion into the lattice due to a concentration

gradient, the transport by the motion of dislocations and the localization of hydrogen at internal

sites in the bulk metal [28]. This localization behavior is called trapping. A trap is a void, interface,

or other physical site but may include a region of high stress [28]. The metal may originally contain

some amount of hydrogen or accumulated it through absorption. Mostly, the damage is attributed

to residual or applied stresses [15]. Hydrogen damage may occur as:

1. Internal flaking, fissuring, cracking or blistering.

2. Internal damage due to defect formation.


25

3. Ductility and tensile strength reduction.

4. Continuous propagation of defects at stresses far lower than those required for mechanical

failure.

The availability of hydrogen is high and in abundance in environments such as hydrogen sulphide,

water, moist air, acids, and various liquids and gases utilized in chemical process operations.

During the manufacturing of equipment, corrosion by hydrogen damage may occur at several

stages even before the equipment exposed to service conditions. Furthermore, hydrogen can be

introduced into the metal’s lattice during welding, heat treating in hydrogen-bearing furnace

atmospheres or acid pickling. In the pickling of steel, the amount of hydrogen absorbed is relative

to both the bath temperature and the nature of the acid [15]. Atomic hydrogen produced at the

cathode during a corrosion process can also diffuse into the bulk material. The diffusion rate

increases when the material is stressed [15].

The Hydrogen Damage type of corrosion can be accelerated by sulphur and arsenic compounds

which are termed as poisons [12, 15]. The time atomic hydrogen remains on the metal surface is

extended due to the slow rate at which molecular hydrogen forms [12]. This is caused by the

poisons. The reaction between hydrogen sulphide and steel to form atomic hydrogen is given as

follows:

𝐹𝑒 + 𝐻2𝑆 → 𝐹𝑒𝑆 + 2𝐻 2.9

The chemisorbed sulphur specifically poisons the hydrogen recombination reaction and enhances

hydrogen absorption [15]. Hydrogen molecules are relatively large and only the smaller atomic

form of hydrogen can diffuse effectively [28]. The high mobility of the hydrogen contributes to

the damaging effects experienced [28]. In most circumstances, hydrogen-induced cracks are

transgranular, although some alloys undergo intergranular fracture [12]. When the solution has a

pH which is above 8, an iron sulphide film forms and provides some form of protection to the

metal surface, thereby retarding the CR [15]. The protective film will be destroyed once there is

the presence of cyanides. The CR of the unprotected surface increases, resulting in hydrogen

damage. Hydrogen concentrations as low as several ppm can lead to cracking [12, 15].

There are similarities between hydrogen damage and SCC in that a normally ductile metal

experiences brittle fracture when exposed to both a tensile stress and a corrosive environment.


26

However, whereas cathodic protection reduces or prevents SCC, it may, on the other hand, initiate

or promote hydrogen damage due to the production of hydrogen atoms at the cathode [12].


1. The alloy can be baked at an elevated temperature to drive out dissolved hydrogen.

2. Reducing corrosion by applying corrosion inhibitors to limit hydrogen production.

3. Data from Nelson curves can be used to determine the operating limits of steel to aid in

efficient materials selection.

4. Using coatings such as rubber linings that are impervious to hydrogen.

2.2.9 MICROBIOLOGICALLY INDUCED CORROSION (MIC)

MIC refers to corrosion induced by microbes or biological organisms [18]. Contributing to this

type of attack are both micro- and macro-organisms in different environments. Some of these

environments include domestic and industrial fresh waters, soils, groundwater, seawater and

natural petroleum products. MIC does not represent a special form of corrosion but rather the

aggravation of corrosion under environmental conditions in which CRs are expected to be rather

low [15]. MIC is shown in Figure 2.10.

Figure 2.10: Microbiologically Induced Corrosion attack on a steel pipe [18].


27

Microbes are categorized according to common characteristics such as their by-products example

sludge producing, or the compounds they affect example sulphur oxidizing [18]. Depending on

their oxygen requirements, these microbes can be classified as either aerobic (i.e. requires oxygen)

such as sulphur oxidizing bacteria or anaerobic (i.e. requires little or no oxygen) like sulphate

reducing bacteria [18]. Microorganisms increase the CR through the following ways:

1. Destruction of protective layers: Microorganisms may attack and destroy organic

coatings. This leaves the base metal exposed and prone to corrosion.

2. Hydrogen embrittlement: Certain microorganisms serve as a source of hydrogen by

producing hydrogen sulphides. This provision of hydrogen by the microorganisms makes

the base metal susceptible to hydrogen embrittlement.

3. Modification of corrosion inhibitors: The structure and properties of some CIs may be

altered by certain types of bacteria rendering them ineffective. Nitrite-based corrosion

inhibitors for example are used to protect aluminium and aluminium alloys from nitrate

and ammonia and this CI can be made inefficient by some bacteria.

4. Production of metabolites: Sulphides, organic or inorganic acids and ammonia may be

produced by bacteria. All these substances are capable of enhancing the corrosion of

metals.


1. Environmental or process parameter modification.

2. Use of biocides.

3. Combining coatings with cathodic protection.

4. Chlorination and periodic cleaning of surfaces and the inside of pipelines.


28

2.3 CORROSION INHIBITORS

A corrosion inhibitor is a substance or combination of substances when added in relatively small

quantities to a corrosive medium decreases the corrosion rate of metals in that medium. The type

of inhibitor used depends on the type of metal and the corrosive environment under consideration

[29]. Inhibitors are known to either be physically adsorbed (physisorption) or chemically adsorbed

(chemisorption) [5, 7, 15]. Physisorption is as a result of electrostatic forces between the organic

ions of the inhibitor and the electrically charged metal surface [15]. For chemisorption, the charge

of the inhibitor molecules is shared or transferred to the metal surface, thereby establishing a

coordinate-type bond [15].

The effectiveness of an inhibitor may be as a result of mechanisms such as

1. Elimination of chemically active species in the corrosive solution such as dissolved

oxygen.

2. The inhibitor physically or chemically attaching itself to the metal surface thereby

retarding the oxidation or reduction reactions or both.

3. Formation of a thin protective coating.

CIs are termed anodic, cathodic or mixed depending on the partial reaction (anodic oxidation

and/or cathodic reaction) they retard [15, 30].

2.3.1 ANODIC INHIBITOR

Generally, this type of inhibitor produces a large anodic shift of the corrosion potential by forming

a protective oxide film on the metal surface. The surface is hence forced into the passivation region

by this shift [18]. For this reason, anodic inhibitors are referred to as passivators on some occasions

[18]. Examples of anodic inhibitors include nitrates, tungstates and chromates. These ions can act

as passivators because they can be both oxidized and reduced readily [31]. That is, they have a

shallow cathodic polarization curve [31].

The mechanism for anodic inhibition involves the blocking of anodic sites in the metal and this

depends on the concentration of the inhibitor [32]. Weak anodic sites are blocked first and the

protective layer is not completely formed when the inhibitor concentration is low [32]. On the

other hand, an increase in concentration causes the inhibitor to react with the strong anodes thereby

forming a passive layer which eventually leads to a decrease in CRs [32].


29

2.3.2 CATHODIC INHIBITOR

Cathodic inhibitors are compounds that provide a protective film by causing the formation of

precipitates on the metal surface [31]. Due to this reason, they are sometimes called precipitation

inhibitors. They act by either slowing the cathodic reaction itself or selectively precipitating on

cathodic sites thereby retarding the diffusion of reducing species to the surface [18]. This can be

achieved by using cathodic poisons such as sulphides, selenides and compounds of antimony and

arsenic [18]. These poisons interrupt the formation of hydrogen gas and by so doing retard the

overall cathodic reaction [31]. They perform particularly well in strong acids where hydrogen gas

formation is the controlling step in the corrosion reaction [31]. In spite of this, cathodic poisons

can cause hydrogen embrittlement and blistering since formation of molecular hydrogen is

retarded increasing the possibility of atomic hydrogen entering the metal [18, 31].

The CRs can also be reduced by employing oxygen scavengers that react with dissolved oxygen

[18]. Sulphite and bisulphite ions are examples of oxygen scavengers that form sulphates by

reacting with oxygen [18].

2.3.3 MIXED INHIBITOR

For mixed inhibitors, both the anodic and cathodic reactions are retarded. They block both the

anode and cathode through the formation of precipitates on the surface [18]. They are typically

film forming compounds [18]. The most common mixed inhibitors are silicates and phosphates

[18]. Protection however depends heavily on pH and is not always reliable. Silicates and

phosphates do not also provide the same level of protection as chromates and nitrites but are less

toxic [18].

Globally there is a huge demand for eco-friendly products due to the alarming rate of

environmental degradation. As a result, inhibitors such as chromates and arsenics have had

injunctions placed on them because of their inability to meet certain environmental regulations

[31, 32]. They are toxic and chromates for example can cause a rash when in prolonged contact

with the skin [31]. Green inhibitors have therefore been researched into and developed to help

solve this environmental problem.


30

2.3.4 GREEN INHIBITORS

Green inhibitors are basically eco-friendly substances such as plant extracts that have been

developed to aid in CR reduction. They are organic in nature and usually contain aromatic or

heterocyclic compounds such as tannins, flavonoids and alkanoids [3, 5, 7]. These compounds

form a protective layer by attaching themselves onto the metal surface through their electroactive

sites which are normally nitrogen, oxygen and sulphur [5, 7, 31].

Several research work has gone on it this area. Loto had reported the application of the extracts of

Mangifera indica (mango) leaves and bark for corrosion of mild steel in diluted sulphuric acid

medium [33, 34]. Weight loss measurements and electrochemical impedance spectroscopy were

used to determine the IE % of the inhibitors. Though individually the extract of leaves and bark

showed a significant effect on the CR by reducing it, the combination of both extracts exhibited a

synergistic effect with higher efficiencies. Molasses treated with alkali solution was found by

Cabrera et al. to inhibit the corrosion of steel in HCl [35].

The extract of henna (Lawsonia inermis) which is a thorny shrub was investigated by Ostovari et

al. as a CI for mild steel in 1mol/L HCl. Its main constituents were lawsone, gallic acid, tannic

acid and α-ᴅ-glucose [36]. They found out that the extract was effective in preventing uniform

corrosion as well as pitting. However, as temperature increases from 25 oC to 60 oC the IE % drops.

After testing the constituents, they drew the conclusion that all the compounds acted as mixed

inhibitors with some doubling as oxygen scavengers. They also reported that the IE % for the

compounds increase in the following order: Lawsone > henna extract > gallic acid > α-ᴅ-glucose

> tannic acid.

Ashassi-Sorkhabi et al. [37] investigated the CI effect of three amino acids (glycine, alanine and

leucine for steel in HCl. These amino acids were used because they are relatively cheap, eco-

friendly and easy to produce with over 99 % purity. These compounds were efficient only when

the CI concentration of alanine and leucine was 1 mmol/L or higher and 10 mmol/L or higher for

glycine. Below these concentrations the authors observed an increase in CR, probably due to

complexations with iron.

Justicia gendarussa plant extract as CI for mild steel in 1 mol/L HCl at 25-70 oC was studied by

Satapathy et al [38]. They reported the major compounds to be friedelin, lupenol, phenolic


31

dimmers, β-sitosterol, ο-substituted aromatic amines and flavonoids. The authors claim this CI to

be a mixed type. They also reported that IE % increases with increasing CI concentration and

decreases with increasing temperature. Furthermore, they reported little or no inhibition effect at

80 oC and attributed it to the decomposition of the compounds at that temperature.

Abdel-Gaber et al. [39] tested the anti-corrosion behavior of the extract of the leguminous plant

lupine (Lupinous albus L.) on the corrosion of steel in aqueous solution of H2SO4 and HCl. To

reveal the adsorption mechanism of inhibition, the corrosion data were examined with kinetic-

thermodynamic model. The inhibitor showed comparatively higher IE % in HCl than in H2SO4.

Pennyroyal oil was extracted from Mentha pulegium (Pennyroyal Mint) and studied for corrosion

inhibition of steel in HCl by Bouyanzer et al. [40]. The major constituent of Pennyroyal oil was

R-(+)-pulegone [41]. The IE % of the inhibitor increased with temperature, which clearly indicated

the chemisorption of the inhibitor on the steel surface. It was also classified as a cathodic type

inhibitor.

The inhibition effect of Zanthoxylum alatum plant extract was investigated by Gunasekaran and

Chauhan on the corrosion of mild steel in 20 %, 50 % and 88 % aqueous orthophosphoric acid

using weight loss and electrochemical impedance spectroscopy (EIS) [42]. The experiment was

conducted at a temperature range of 50–80 oC. The plant extract was found to reduce the corrosion

of steel more effectively in 88 % than in 20 % phosphoric acid. The surface analysis using X-ray

photoelectron spectroscopy (XPS) showed the protective layer formed by the plant extract on the

surface of the mild steel. The results were indicative of the possible formation of iron phosphate,

which was catalyzed by the formation of the iron-plant extract organo-metallic complex.

Sribharathy and Rajendran [43] studied the inhibitive effect of Jeera (Cuminum cyminum) plant

extracts on the corrosion of mild steel in an aqueous solution of seawater using potentiodynamic

polarization and electrochemical impedance spectroscopy (EIS) techniques. The stability of the

IE% of Jeera extracts was examined using the weight-loss method. Polarization curves obtained

showed that the Jeera extract behaves as an anodic inhibitor. They also reported that the CRs of

the steel and the IE % of the extract obtained from impedance and polarization measurements were

in good agreement. In addition, inhibition was found to increase with an increasing concentration

of the plant extract.


32

The inhibitive action of natural honey on the corrosion of carbon steel in high saline water was

evaluated by El-Etre and Abdallah [44]. The IE % was calculated using weight loss and

potentiostatic polarization measurements. They found that natural honey exhibited a very good

performance as an inhibitor for steel corrosion in high saline water. The IE % increased with an

increase in natural honey concentration. However, the IE % decreased after some time due to the

growth of fungi in the medium.

El-Etre [45] again investigated the extract of khilah seeds (Ammi visnaga) against the corrosion

of SX 316 stainless steel in HCl solution. He reported the major constituents of the khilah seed

extract to be khellin and visnagin, and the inhibition mechanism was studied by comparing the

complexing ability of these compounds. The conductometric titrations showed the possible

formation of Fe-khellin or Fe-visnagin complex, which he attributed to chemisorption or chemical

bonding between the iron and inhibitor molecules. In spite of that, he observed that the IE % of

the extract significantly decreased with increasing temperature, which indicated that the

mechanism of adsorption of the inhibitor molecules were predominantly physisorption. Though

the decrease in the IE % was observed at elevated temperatures, the seed extract exhibited mostly

an IE % of 71 % for 120 ppm at 80 oC.

2.4 CASSIA SIAMEA

Cassia siamea also known as Senna siamea or Siamese cassia is a medium-sized evergreen

leguminous tree of the subfamily Caesalpinioideae [46]. It can grow up to 18 m high with beautiful

yellow flowers and leaves that are alternate, pinnately compound with slender, green-reddish

tinged axis [46]. There are about 6 to 12 pairs of leaflets on short stalks, rounded at both ends [46].

It can be found in South and South-east Asia as well as Africa. In Ghana it is commonly used as

ornamental plants or shade trees in cocoa plantations. Due to the edibility of its leaves, pods and

seeds, it is used in some Thai and Burmese cuisine. It also contains a compound called Barakol

which is used in traditional herbal medicine [47]. Its leaves are also a good source of heterocyclic

compounds such as tannins, flavonoids, alkanoids and saponins which have shown to have

corrosion inhibition properties [5, 7]. As a result of this, extracts from the cassia leaves were used

in the preparation of the plant inhibitor.


33

2.5 POLYETHYLENE TEREPHTHALATE (PET)

PET is a thermoplastic polymer from the polyester group [48]. It has its molecular formula to be

(C10H8O4)n. The repeat units of PET are given in Figure 2.11:

Figure 2.11: Repeat units of PET [48].

PET can exist as an amorphous or a semi-crystalline polymer. In its amorphous form, it has a

density of 1.370 g/cm3 and1.455 g/cm3 in its crystalline form [49]. Its melting point ranges from

250-260 oC and at a temperature of 350 oC PET decomposes. It is clear in nature and has a

refractive index of 1.575. A low thermal conductivity of 0.15-0.24 Wm-1K-1 gives PET a very good

heat resistance [50]. PET displays very good strength with its tensile strength ranging from 55 to

75 MPa [50].

Before recycling, plastics are sorted out according to their polymer resin type. The use of Resin

Identification Codes (RIC) on plastic products makes it easier to identify the type of polymer resin

used [51]. The RIC for PET is given in Figure 2.12:

Figure 2.12: Resin Identification Code for PET [51] [52].


34

Some of PETs common uses include soft drink and water bottles, ovenable food trays and

waterproof packaging. They are usually recycled into polyester carpets, fibre filling for pillows,

audio and video tapes [53-55].

The popularity of PET for disposable bottle production and packaging in general began in the

1980s, and within seven years more than 700 million pounds of PET had been consumed [56, 57].

Due to the diversity of its applications in a large volume of consumer products, extremely high

amounts of PET waste is also generated, which includes polymer manufacturing waste as well as

the products at the end of their useful life. As a result of its high resistance to atmospheric and

biological agents, PET is not considered as biodegradable. With the increasing pressure of keeping

the environment clean, recycling PET waste in an ecofriendly manner is the only viable option.

PET waste can be recycled either physically or chemically [57]. In chemical recycling, PET is

reacted with various chemical substances to obtain products that are used in the chemical industry

[57, 58]. With respect to this process, PET waste can be depolymerized to its base monomers or

oligomers. Depolymerization can generally be carried out through aminolysis (chemical

breakdown using ammonia or an amine), solvolysis (nucleophilic substitution or elimination where

the nucleophile is a solvent molecule), methanolysis (chemical breakdown using an alcohol) and

glycolysis (breakdown of glucose with the release of energy) [57, 59, 60]. Hydrolysis of PET is

also possible by using water under pressure [57]. Through these reactions, PET waste can be

converted to useful products such as corrosion inhibitors and several works have been done in this

area of research.

2.5.1 CORROSION INHIBITORS FROM PET WASTE

Abdul-Raheim et al. [61] synthesized a new water soluble corrosion inhibitor by depolymerizing

PET waste into a glycolyzed product using triethanolamine, followed by esterification with

bromoacetic acid in the presence of manganese acetate as a catalyst. The obtained ester was reacted

with thiourea to give thiol derivative. The effectiveness of the synthesized compound as a

corrosion inhibitor for API XL65 carbon steel, in 2 M HCl was evaluated using open circuit

potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The

results of these investigations showed improved IE % with increasing inhibitor concentration. The

protective film formed on the carbon steel surface was analyzed using an energy dispersive X-ray

analysis (EDX) technique. Also, a scanning electron microscope (SEM) was used to study the


35

surface morphology of steel surface in the absence and presence of 400 ppm of the inhibitor. The

analysis showed less damage on the steel surface in the presence of the inhibitor.

Abd El-Hameed et al. [62] investigated the corrosion inhibition of two compounds derived from

PET waste for carbon steel in phosphoric acid. The two compounds were obtained via the reaction

of PET waste with Ethylene glycol and ethylene di amine in the presence of catalysts. The

experiment was conducted at different concentrations and temperature by chemical and

electrochemical methods. They reported that the results obtained indicate the IE % of the di amine

based inhibitor to be higher than the glycol based inhibitor.

The study of the corrosion inhibition effect of Poly(oxyethylene)terephthylamine on carbon steel

in 1 M formic acid was done by El-Hameed et al [63]. PET waste was depolymerized with

diethanolamine to give the corresponding amine. Etherification was then done to produce different

molecular weights of poly ethylene glycol (PEG1000 and PEG4000) to give the corresponding

Poly(oxyethylene)terphthalamine). The IE % were determined at different concentrations and

temperatures by weight loss and electrochemical techniques (potentiodynamic polarization and

open circuit potential). They claimed the protection efficiency increased with increasing

concentration and number of ethylene oxide units. The inhibition was assumed to occur via the

adsorption of the inhibitors on the metal surface. The polarization curves indicated that these

compounds acted as mixed-type inhibitors. Addition of inhibitors molecules to the corrosive

medium produced a negative shift in the open circuit potential. It was found that the obtained data

from different techniques were in good agreement, and the increase in the numbers of ethylene

oxide units in the molecular structure of the modified plastic waste led to the increase in IE %.

Abd El-Hameed et al. [64] once again investigated the corrosion of mild steel in NaCl solution and

the effect recycled PET inhibitors obtained from aminolysis has on it. In this work, PET was

depolymerized with both Diethanolamine (DEA) and Triethanolamine (TEA) (ratios of 1:2 wt %)

followed by esterification with stearic acid, and the two compounds obtained were labelled as D2S

and T2S. The corrosion inhibition effect of D2S and T2S was determined using weight loss,

potentiodynamic polarization techniques and open circuit potential measurements. The results

showed that inhibition occurred through adsorption of the inhibitor molecules on the metal surface

and the IE % was found to increase with increasing inhibitor concentrations and temperature. The


36

inhibitors also acted as mixed type and the obtained results from weight loss and potentiodynamic

polarization techniques were in a good agreement.

The aminolysis of PET to produce terephthal amide was done by Abd El-Hameed [65]. The

depolymerization was done using MEA with sodium acetate serving as a catalyst to produce bis

(2-hydroxyethylene) terephthal amide, the chemical structure of which can be found in Figure

2.13. The terephthal amide was evaluated as a corrosion inhibitor for carbon steel in 1 M HCl by

using weight loss, open circuit potential and potentiodynamic polarization measurements. The

polarization curves indicated that the compound acted as mixed type inhibitor. The inhibition

efficiency increased with increasing inhibitor concentrations and decreased with increasing

temperature. The values of activation energy and free energy of adsorption indicated that the type

of adsorption was chemical adsorption. The inhibition occurred through adsorption of the inhibitor

molecules on the metal surface. The adsorption of the inhibitor on the steel surface was found to

follow the Langmuir adsorption isotherm. Scanning electron microscope (SEM) was used to study

the surface morphology of the steel surface in the absence and presence of 200 ppm of the used

inhibitor. It was found that in the presence of the inhibitor, less damage occurred on the surface as

compared to when no inhibitor was present.

Figure 2.13: Chemical structure of bis (2-hydroxy ethylene) terephthal amide [65]


37

CHAPTER THREE

METHODOLOGY

3.1 PREPARATION OF SPECIMENS

Eleven mild steel coupons each of dimensions 20 mm x 60 mm x 1 mm were degreased and

polished successively with different grades of emery paper (P80, P100 and finally P120) to remove

any corrosion product that might have formed on the surface prior to the experiment. The coupons

were washed with distilled water and dried with acetone. All eleven coupons were used in different

experiments with varying inhibitor type and concentrations.

3.2 PREPARATION OF 2M HCl

1 L of 2 M HCl with a pH of 0.2 was obtained from 35.4 % concentrated HCl by diluting 172 ml

of the concentrated acid in 1 L of distilled water. The calculations can be found in section A1 of

the appendix.

3.3 PREPARATION OF PLANT INHIBITOR

Cassia leaves were dried in open sunlight for a week and later milled into powder. Approximately

1.5 g of the powdered leaves was dissolved in 300 ml of the prepared 2 M HCl, giving a mass per

volume concentration of: 1.5/300 = 0.005 g/ml or 5 g/L. The solution was then boiled for 3 hours

at 90 oC and allowed to cool for 24 hours after which it was filtered with the filtrate serving as the

inhibitor. The solution had a pH of 0.2. A summary of the preparation process is shown in Figure

3.1.

Figure 3.1: Process for Plant Inhibitor Preparation

Cassia Siamea leaves Dry Cassia leaves milled to powder Plant extract as inhibitor


38

3.4 AMINOLYSIS OF PET WASTE

199.35 g of shredded waste PET bottles which were thoroughly washed and dried, was dissolved

in 300 ml of mono ethanol amine (MEA) (1:1 wt %) with 2 g of sodium acetate as a catalyst (1%

of PET weight). The solution was heated in a nitrogen atmosphere under vigorous stirring at 170-

190 oC for 4 hours and 200-210 oC for 3 hours and finally 100 oC for 1 hour [65]. It was then left

overnight to cool to room temperature. Distilled water was added in excess with vigorous agitation

to precipitate out the white crystalline powder of bis (2-hydroxy ethylene) terephthal amide. The

terephthal amide (TA) which is the precipitate was filtered out and dried at room temperature for

3 days after which it was ground to powder. The TA served as the plastic inhibitor. The preparation

process for the TA is shown in Figures 3.2.

Figure 3.2: Aminolysis of waste PET

3.5 ELECTROCHEMICAL ANALYSIS

The electrochemical tests done to determine the CRs and IE % were conducted using the Metrohm

Autolab Galvanostat/Potentiostat which is controlled by the NOVA 1.11 software. The set-up is a

three-electrode cell consisting of the steel coupon which is the working electrode (WE), an

Ag/AgCl electrode which is the reference electrode (RE) and a glassy carbon electrode which is

the counter electrode (CE) as shown in Figure 3.3.

Shredded PET bottles PET in molten state after

dissolution and heating in

MEA.

White crystalline TA powder


39

Figure 3.3: Potentiostat and electrodes

The CR depends on the kinetics of both anodic and cathodic reactions. According to Faraday’s

Law, there is a linear relationship between CR and the corrosion current, icorr [18]. This is shown

in Equation 3.2:

𝐶𝑅 =𝑀

𝑛𝐹𝜌𝑖𝑐𝑜𝑟𝑟 3. 1

Where M is the atomic weight of the metal, n is the charge number which indicates the number of

electrons exchanged in the dissolution or corrosion reaction, ρ is the density and F is the Faraday

constant, (96.485 C/mol). With the aid of Tafel plots icorr can be estimated and then be used in

calculating the CR. The NOVA software provides a convenient interface for making Tafel plots,

calculating Tafel slopes and CRs. Additionally, the software fits experimental data into the plot

based on the Butler-Volmer equation given in Equation 3.3:

𝑖 = 𝑖𝑐𝑜𝑟𝑟 [𝑒2.303

𝐸−𝐸𝑐𝑜𝑟𝑟𝑏𝑎 − 𝑒

2.303𝐸−𝐸𝑐𝑜𝑟𝑟

𝑏𝑐 ] 3. 2

Where i is the exchange current density, E is the applied potential, Ecorr is the corrosion potential

while ba and bc are the anodic and cathodic Tafel constants respectively [66]. From the CRs

obtained from the software, the IE% was calculated using Equation 3.4:

𝐼𝐸 % =𝐶𝑅𝑏𝑙𝑎𝑛𝑘 − 𝐶𝑅𝑖𝑛ℎ

𝐶𝑅𝑏𝑙𝑎𝑛𝑘 𝑥 100 % 3. 3


40

3.6 WEIGHT LOSS MEASUREMENTS

The CR with respect to weight loss of the steel coupons, CRWL (mg/cm2/day) and the Weight Loss

(WL) were calculated using Equations 3.5 and 3.6:

𝐶𝑅𝑊𝐿 =𝑊𝐿

𝐴 ∗ 𝑡 3. 4

𝑊𝐿(𝑊𝑒𝑖𝑔ℎ𝑡 𝐿𝑜𝑠𝑠) = 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑊𝑒𝑖𝑔ℎ𝑡 − 𝐹𝑖𝑛𝑎𝑙 𝑊𝑒𝑖𝑔ℎ𝑡 3. 5

Where A is the surface area of the metal and t is the duration of immersion. The coupons were

weighed using the Adam Equipment® analytical balance.

3.7 PREPARATION OF TEST SOLUTIONS

Five different concentrations of the prepared plant inhibitor were dissolved in 50 ml of 2 M HCl.

Table 3.1 shows the inhibitor concentrations that were used.

Table 3.1: Concentration of plant inhibitor used.

Volume concentration in 50ml HCl

(v/v %)

Actual Volume of Plant Inhibitor used

(ml)

10 5

20 10

30 15

40 20

50 25

To investigate the possible increase in plant inhibitor efficiency, some amount of the plastic

inhibitor was added. Both inhibitors were combined at varying concentrations in 50 ml 2 M HCl.

The concentrations are given in Table 3.2.


41

Table 3.2: Combination of plant and plastic inhibitors.

One steel coupon was immersed in each test solution and both weight loss and electrochemical

measurements were carried out simultaneously every 72 hours for a total of 21 days. All tests were

done at 24.5 oC.

3.8 CHARACTERIZATIONS

3.8.1 FTIR

The FTIR machine operates by using infra-red radiation to determine functional groups. The atoms

within each functional group have vibrating bonds existing between them [67]. Each bond vibrates

or oscillates at a specific frequency because the atoms and charges involved in each functional

group are different [67]. Due to these oscillations an electromagnetic field is produced. When

infra-red light encounters an oscillating electromagnetic field of the same frequency, the two

waves couple and infra-red light is absorbed. Therefore on a typical FTIR graph where

transmittance is on the y-axis and the wave number is on the x-axis, it can be seen that functional

groups that are clearly present in a particular sample have very low transmittance (0-35 %) since

they are being absorbed [67].

FTIR was used to determine the functional groups present in the TA and cassia leaves. It was also

used to determine the active compounds adsorbed onto the metal surface forming the protective

film.

3.8.2 PHYTOCHEMICAL SCREENING

Phytochemical screening was done to determine the heterocyclic compounds present in the cassia

leaves. About 15 g of the dried and pulverized leaves of Cassia siamea was extracted by cold

Amount of Plant Inhibitor (ml) Amount of Plastic inhibitor (mg)

0 25

5 20

10 15

15 10

20 5


42

maceration with 100 ml of methanol with continuous agitation for 3 days. The extract was filtered,

and concentrated using a rotary vacuum evaporator to evaporate all of methanol in the extract to

obtain dark green crude of mass 5 g. The crude extract was then screened for the presence of

phytochemicals such as tannins, saponins, flavonoids and alkanoids.


43

CHAPTER FOUR

RESULTS AND DISCUSSION

4.1 ELECTROCHEMICAL ANALYSIS

4.1.1 PLANT INHIBITOR

The CRs obtained from the electrochemical readings after 21 days of the steel coupons in HCl

with the plant inhibitor present, are given in AT1 (Appendix Tablea) and Figure 4.1.

Figure 4.1: Corrosion Rate vs. Time Graph (Plant). CR is seen to reduce with increasing

inhibitor concentrations.

It can be observed from Figure 4.1 that there are no corrosion rates for the blank test sample after

12 days. This is due to the fact that the steel coupon was dissolved by the acid after a 12 day period

while the coupons in the inhibited solutions remained intact for the maximum 21 days. It can also

be seen that the corrosion rate of the blank sample is very much higher compared to the inhibited

a All CR and IE% tables can be found in the appendix.

2

4

8

16

32

64

128

3 6 9 12 15 18 21 24

CO

RR

OSI

ON

RA

TE (

MM

/YR

)

DURATION (DAYS)

CORROSION RATE vrs TIME GRAPH (PLANT)

BLANK

10v/v%(5ml)

20v/v%(10ml)

30v/v%(15ml)

40v/v%(20ml)

50v/v%(25ml)


44

samples. The increase in corrosion rate of the blank is also more rapid whereas the inhibited

samples show a more steady and gradual increase in corrosion rate. Comparing the inhibited

samples, the corrosion rate is seen to drop as the inhibitor concentration increases with 50 v/v%

giving the lowest corrosion rate.

The IE % is also seen to increase with increase in inhibitor concentration as shown in Figure 4.2.

Figure 4.2: IE% vs. Inhibitor Concentration Graph (Plant). IE% increases with increasing

inhibitor concentrations.

In Figure 4.3, analyzing the cathodic branches which are the branches on the left, it can clearly be

observed that the plant inhibitor affected the cathodic reaction more than the anodic reaction which

is represented by the branches to the right. The cathodic current of the blank specimen is seen to

be much higher compared to the inhibited specimens. 50 v/v% is also seen to exhibit the lowest

cathodic current. Since the inhibitor decreased the cathodic reactions more than the anodic it can

therefore be termed as a cathodic inhibitor. Another characteristic of cathodic inhibitors is the

shifting of the corrosion potential to more negative values which can also be observed on the graph

[4, 30].

94.40

94.9895.12

95.49

95.87

93.5

94

94.5

95

95.5

96

10 20 30 40 50

INH

IBIT

OR

EFF

ICIE

NC

Y (

I.E%

)

INHIBITOR CONCENTRATION (v/v%)

INHIBITOR EFFICIENCY VRS INHIBITOR CONCENTRATION (PLANT) AFTER 12 DAYS


45

-0.55 -0.50 -0.45 -0.40 -0.35

1E-6

1E-5

1E-4

1E-3

0.01

Cu

rre

nt (L

og

i)

Potential (E)

blank

10v/v%

20v/v%

30v/v%

40v/v%

50v/v%

Figure 4.3: Plant Inhibitor Tafel Plots. Cathodic currents reduced more compared to the anodic

currents.

4.1.2 PLANT-PLASTIC INHIBITOR

The CRs from electrochemical measurements of the steel coupons after the TA was added to the

plant inhibitor are shown in Figure 4.4. It can be observed from the graph that from day 9 through

to day 18, adding 5 mg of plastic inhibitor (TA) to 20 ml of plant inhibitor actually causes it to

perform slightly better than 25 ml of solely plant inhibitor. Also, using solely 25 mg of TA reduces

the CR and the steel coupon does not dissolve as in the case of the blank test although it does not

perform as well as the combination of plant and TA. 20 ml/5 mg gave the lowest CR after 18 days.


46

Figure 4.4: CR vs. Time Graph (Plant/Plastic). 20 ml/5 mg is seen to have the lowest CR from

day 9 to day 18.

In comparing the IE % of the inhibited solutions, 25 mg of TA gave the lowest efficiency while

adding 5 mg of TA to 20 ml of plant inhibitor gave the highest efficiency. This can be seen in

Figure 4.5.

2

4

8

16

32

64

128

3 6 9 12 15 18 21

CO

RR

OSI

ON

RA

TE (

MM

/YR

)

DURATION (DAYS)

CORROSION RATE vrs TIME GRAPH (PLANT/PLASTIC)

BLANK

25mg

5ml/20mg

10ml/15mg

15ml/10mg

20ml/5mg

25ml


47

Figure 4.5: IE % vs. Inhibitor Concentration Graph (Plant/Plastic). 20 ml/5 mg is seen to have a

better efficiency than 25 ml.

When moving from 100 % plastic inhibitor (25 mg of TA) to 100 % plant inhibitor (25 ml of plant

inhibitor), the CR after 18 days reduces greatly in the region where the two inhibitors were

combined with 20 ml/5 mg giving the lowest rate as shown in Figure 4.6.

Figure 4.6: CR vs. Inhibitor Concentration Graph (Plant/Plastic). The CR reduced significantly

when both inhibitors were combined.

29.23

94.83 95.29 95.60 96.08 95.87

0

10

20

30

40

50

60

70

80

90

100

25mg 5ml/20mg 10ml/15mg 15ml/10mg 20ml/5mg 25ml

INH

IBIT

OR

EFF

ICIE

NC

Y (

I.E%

)

INHIBITOR CONCENTRATION

INHIBITOR EFFICIENCY VRS INHIBITOR CONCENTRATION (PLANT/PLASTIC) AFTER 12 DAYS

91.91

6.45 5.51 5.07 4.53 4.63

0

10

20

30

40

50

60

70

80

90

100


CO

RR

OSI

ON

RA

TE (

MM

/YR

)


CORROSION RATE vrs INHIBITOR CONCENTRATION (PLANT/PLASTIC) AFTER 18 DAYS


48

The Tafel plots for the plant/plastic inhibitor are shown in Figure 4.7.

-0.55 -0.50 -0.45 -0.40 -0.35

1E-6

1E-5

1E-4

1E-3

0.01

Curr

ent

(Log

i)

Potential (E)

blank

25mg

5ml/20mg

10ml/15mg

15ml/10mg

20ml/5mg

25ml

Figure 4.7: Tafel plots for Plant/Plastic Inhibitor. Cathodic currents reduced more as compared

to anodic currents.

The Tafel plots of the plant/plastic inhibitor in Figure 4.7 similar to that of the plant inhibitor show

that the inhibitor acts as a cathodic type since it interferes with the cathodic current more. The

potentials also shift to become more negative.


49

4.2 WEIGHT LOSS MEASUREMENTS

4.2.1 PLANT INHIBITOR

The initial weights of the steel coupons, their weights after reaction and their weight losses are

given in Tables 4.1-4.3.

Table 4.1: Initial weight of steel samples for plant inhibitor weight loss test.

INITIAL WEIGHT OF STEEL SAMPLES BEFORE REACTION (mg)

INHIBITOR CONCENTRATION WEIGHT (mg)

Blank 8898

10 v/v% 8896

20 v/v% 8898

30 v/v% 8899

40 v/v% 8895

50 v/v% 8898

Table 4.2: Final weight of steel samples for plant inhibitor weight loss test.

FINAL WEIGHT OF STEEL SAMPLES AFTER REACTION (mg)

DAY BLANK 10 v/v%

(5 ml)

20 v/v%

(10 ml)

30 v/v%

(15 ml)

40 v/v%

(20 ml)

50 v/v%

(25 ml)

3 8378 8856 8866 8874 8872 8880

6 7997 8803 8817 8821 8830 8837

9 7261 8763 8770 8779 8782 8789

12 6828 8746 8755 8760 8767 8778

15 6628 8719 8725 8735 8755 8771

18 6491 8712 8720 8729 8746 8769

21 6316 8699 8713 8722 8740 8766


50

Table 4.3: Weight loss of steel samples after plant inhibitor test.

WEIGHT LOSS [mg] (%WL) OF STEEL SAMPLES AFTER REACTION

DAY BLANK 10 v/v%

(5 ml)

20 v/v%

(10 ml)

30 v/v%

(15 ml)

40 v/v%

(20 ml)

50 v/v%

(25 ml)

3 520

(5.84 %)

40

(0.45 %)

32

(0.36 %)

25

(0.28 %)

23

(0.26 %)

18

(0.20 %)

6 901

(10.13 %)

93

(1.05 %)

81

(0.91 %)

78

(0.88 %)

65

(0.73 %)

61

(0.69 %)

9 1637

(18.40 %)

133

(1.50 %)

128

(1.44 %)

120

(1.35 %)

113

(1.27 %)

109

(1.22 %)

12 2070

(23.26 %)

150

(1.69 %)

143

(1.61 %)

139

(1.56 %)

128

(1.44 %)

120

(1.35 %)

15 2270

(25.51 %)

177

(1.99 %)

173

(1.94 %)

164

(1.84 %)

140

(1.57 %)

127

(1.43 %)

18 2407

(27.05 %)

184

(2.07 %)

178

(2.00 %)

170

(1.91 %)

149

(1.68 %)

129

(1.45 %)

21 2582

(29.02 %)

197

(2.21 %)

185

(2.08 %)

177

(1.99 %)

155

(1.74 %)

132

(1.48 %)

The blank sample experienced higher weight losses whereas the inhibited samples had lower

weight losses with 50 v/v% showing the least amount of weight loss across the 21 day period as

shown in Table 4.3.

The weight loss against time graph is shown in Figure 4.8. It is worth noting that unlike the

electrochemical analysis, there were data points for the blank sample after 12 days because the

broken pieces could be weighed.


51

Figure 4.8: Weight Loss vs. Time Graph (Plant). Less weight was lost as the inhibitor

concentrations increased

Figure 4.8 shows a reduction in weight loss with increasing inhibitor concentration though all

samples experienced higher weight losses as the duration increased. The increase in weight loss

was also more rapid in the blank sample compared to the inhibited samples.

15

30

60

120

240

480

960

1920

3 6 9 12 15 18 21

WEI

GH

T LO

SS (

mg)

DURATION (DAYS)

WEIGHT LOSS vrs TIME GRAPH (PLANT)

BLANK

10v/v%(5ml)

20v/v%(10ml)

30v/v%(15ml)

40v/v%(20ml)

50v/v%(25ml)


52

The CR with respect to weight loss (Figure 4.9) also decreases with increasing inhibitor

concentration and therefore in agreement with the CRs obtained from the electrochemical

measurements. Using Equation 3.5 with A=12 cm2 and t = 21 days, the CRWL obtained were:

Figure 4.9: CRWL vs. Inhibitor Concentration Graph (Plant). The CRWL is seen to decrease with

increasing inhibitor concentrations.

From Figure 4.10, the IE % increases with increasing inhibitor concentration and again in

agreement with the IE % obtained from the electrochemical measurements.

10.25

0.78 0.730.70 0.62 0.52

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

1

CO

RR

OSI

ON

RA

TE (

mg

cm-2

day

-1)


CORROSION RATE vrs INHIBITOR CONCENTRATION (PLANT) AFTER 21 DAYS

Blank 10v/v% 20v/v% 30v/v% 40v/v% 50v/v%


53

Figure 4.10: IE % vs. Inhibitor Concentration Graph with respect to Weight Loss (Plant). The

efficiency increased with increasing inhibitor concentrations.

4.2.2 PLANT/PLASTIC INHIBITOR

The initial weights of steel coupons that were used in the plant/plastic inhibitor weight loss test,

their weights after reaction and their weight losses are given in Tables 4.4-4.6.

Table 4.4: Initial weight of steel samples for plant/plastic inhibitor test.

INITIAL WEIGHT OF STEEL SAMPLES (mg)

INHIBITOR CONCENTRATION WEIGHT (mg)

Blank 8898

25 mg 8897

5 ml/20 mg 8897

10 ml/15 mg 8896

15 ml/10 mg 8898

20 ml/5 mg 8894

25 ml 8898

92.37

92.84

93.14

94.00

94.89

91

91.5

92

92.5

93

93.5

94

94.5

95

95.5

10v/v% 20v/v% 30v/v% 40v/v% 50v/v%

I.E%


INHIBITOR EFFICIENCY vrs INHIBITOR CONCENTRATION (PLANT) AFTER 21 DAYS


54

Table 4.5: Final weight of steel samples after plant/plastic inhibitor test.

WEIGHT OF STEEL SAMPLES AFTER REACTION (mg)

DAY BLANK 25 mg 5 ml/20 mg 10 ml/15 mg 15 ml/10 mg 20 ml/5 mg 25 ml

3 8378 8617 8860 8868 8876 8874 8880

6 7997 8228 8810 8816 8828 8831 8837

9 7261 7587 8771 8772 8785 8787 8789

12 6828 7105 8752 8756 8768 8775 8778

15 6628 7028 8722 8732 8740 8766 8771

18 6491 6952 8717 8727 8732 8764 8769

21 6316 6753 8706 8717 8724 8755 8766

Table 4.6: Weight losses of steel samples after plant/plastic inhibitor test.

WEIGHT LOSS (%WL) OF STEEL SAMPLES AFTER REACTION (mg)


3 520

(5.84 %)

280

(3.15 %)

37

(0.42 %)

28

(0.31 %)

22

(0.25 %)

20

(0.22 %)

18

(0.20 %)

6 901

(10.13 %)

669

(7.52 %)

87

(0.98 %)

80

(0.90 %)

70

(0.79 %)

63

(0.71 %)

61

(0.69 %)

9 1637

(18.40 %)

1310

(14.72 %)

126

(1.42 %)

124

(1.40 %)

113

(1.27 %)

107

(1.20 %)

109

(1.22 %)

12 2070

(23.26 %)

1792

(20.14 %)

145

(1.63 %)

140

(1.57 %)

130

(1.46 %)

119

(1.34 %)

120

(1.35 %)

15 2270

(25.51 %)

1869

(21.01 %)

175

(1.97 %)

164

(1.84 %)

158

(1.78 %)

128

(1.44 %)

127

(1.43 %)

18 2407

(27.05 %)

1945

(21.86 %)

180

(2.02 %)

169

(1.90 %)

166

(1.87 %)

130

(1.46 %)

129

(1.45 %)

21 2582

(29.02 %)

2144

(24.10 %)

191

(2.15 %)

179

(2.01 %)

174

(1.96 %)

139

(1.56 %)

132

(1.48 %)

From Table 4.6 it can be seen that 20 ml/5 mg and 25 ml gave the least weight losses with 25 ml

performing marginally better (0.08 % less weight lost) than 20 ml/5 mg after 21 days. This is

shown in Figure 4.11.


55

Figure 4.11: Weight Loss vs. Time Graph (Plant/Plastic). Combining both inhibitors performed

better than using only the terephthal amide.

In terms of CR 25 ml gave the lowest rate overall whereas 20 ml/5 mg gave the lowest rate amongst

the plant/plastic inhibitors as shown in Figure 4.12.

15

30

60

120

240

480

960

1920

3 6 9 12 15 18 21

WEI

GH

T LO

SS (

mg)

DURATION (DAYS)

WEIGHT LOSS vrs TIME GRAPH (PLANT/PLASTIC)

BLANK

25mg

5ml/20mg

10ml/15mg

15ml/10mg

20ml/5mg

25ml


56

Figure 4.12: CRWL vs. Inhibitor Concentration Graph (Plant/Plastic). Combining both inhibitors

reduced the CRWL significantly.

25 ml gave the highest IE % overall with 20 ml/5 mg giving the highest IE % among the inhibitor

combinations. 25 mg gave the lowest IE % overall and 5 ml/20 mg gave the lowest IE % among

the inhibitor combinations as shown in Figure 4.13.

10.25

8.51

0.76 0.71 0.69 0.55 0.52

0

2

4

6

8

10

12

Blank 25mg 5ml/20mg 10ml/15mg 15ml/10mg 20ml/5mg 25ml

CO

RR

OSI

ON

RA

TE (

mg

cm-2

day

-1)


CORROSION RATE vrs INHIBITOR CONCENTRATION (PLANT/PLASTIC) AFTER 21 DAYS


57

Figure 4.13: IE% vs. Inhibitor Concentration Graph with respect to Weight Loss (Plant/Plastic).

The efficiency increased when both inhibitors were combined.

4.3 PERFORMANCE COMPARISON OF PLANT INHIBITOR TO PLANT/PLASTIC

INHIBITOR

4.3.1 ELECTROCHEMICAL PERFORMANCE

Adding varying amounts of the plastic inhibitor to the plant inhibitor showed an improved

performance in terms of electrochemical CRs and efficiencies compared to using only the plant

inhibitor. For instance adding 20 mg of plastic inhibitor to 5 ml plant inhibitor reduces the CR and

improves the IE% further compared to using just 5 ml plant inhibitor. This pattern is seen to cut

across all the different inhibitor concentrations. These trends are displayed in Figures 4.14 and

4.15.

16.96

92.60 93.07 93.26 94.62 94.89

-5

5

15

25

35

45

55

65

75

85

95


I.E%


INHIBITOR EFFICIENCY vrs INHIBITOR CONCENTRATION (PLANT/PLASTIC) AFTER 21 DAYS


58

Figure 4.14: Graph comparing Plant and Plant/Plastic CRs

Figure 4.15: Graph comparing Plant and Plant/Plastic IE%

6.94

6.015.67

4.88

6.45

5.515.07

4.53

0

1

2

3

4

5

6

7

8

CO

RR

OSI

ON

RA

TE (

MM

/YR

)


COMPARISON BETWEEN PLANT & PLANT/PLASTIC CORROSION RATES AFTER 18 DAYS

PLANT

PLANT/PLASTIC

5ml 5ml/20mg 10ml 10ml/15mg 15ml 15ml/10mg 20ml 20ml/5mg

94.40

94.9895.12

95.49

94.83

95.29

95.60

96.08

93.5

94

94.5

95

95.5

96

96.5

INH

IBIT

OR

EFF

ICIE

NC

Y (

I.E%

)


COMPARISON BETWEEN PLANT & PLANT/PLASTIC INHIBITOR EFFICIENCIES AFTER 12 DAYS

PLANT PLANT/PLASTIC



59

4.3.2 WEIGHT LOSS PERFORMANCE

The plant/plastic inhibitor resulted in less weight loss compared to using only the plant inhibitor

(Figure 4.16). Similar to the electrochemical performance, the CRWL and IE% of the plant inhibitor

were improved by adding varying amounts of the plastic inhibitor (Figures 4.17 and 4.18).

Figure 4.16: Graph comparing Plant and Plant/Plastic Weight Losses

197

185177

155

191

179174

139

0

50

100

150

200

250

WEI

GH

T LO

SS (

mg)


COMPARISON BETWEEN PLANT & PLANT/PLASTIC WEIGHT LOSSES

PLANT

PLANT/PLASTIC



60

Figure 4.17: Graph comparing Plant and Plant/Plastic CRWL

Figure 4.18: Graph comparing Plant and Plant/Plastic IE% with respect to Weight Loss

0.78

0.730.70

0.62

0.76

0.710.69

0.55

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CO

RR

OSI

ON

RA

TE (

mg

cm-2

day

-1)


COMPARISON BETWEEN PLANT & PLANT/PLASTIC CORROSION RATES

PLANT

PLANT/PLASTIC


92.37

92.84

93.14

94.00

92.60

93.0793.26

94.62

91

91.5

92

92.5

93

93.5

94

94.5

95

I.E

%


COMPARISON BETWEEN PLANT & PLANT/PLASTIC INHIBITOR EFFICIENCIES

PLANT

PLANT/PLASTIC



61

From Figure 4.19 it can be seen that the steel samples that were protected by the plant/plastic

inhibitor did not corrode as much as those protected by only the plant inhibitor.

Figure 4.19: Comparison of steel samples after corrosion using plant inhibitor and plant/plastic

inhibitor.

4.4 CHARACTERIZATIONS

4.4.1 PHYTOCHEMICAL SCREENING

Table 4.7 shows the presence of heterocyclic or aromatic compounds detected in the cassia leaves.

Table 4.7: Phytochemical screening results

Phytochemical Tests Methanol Extract of leaves

Dragendorff’s test for Alkaloids +

Test for Tannin +

Test for Saponins +

Test for Anthraquinone -

Test for Flavonoids +


62

The tests conducted showed the presence of tannins, saponins, flavonoids and alkanoids in the

leaves.

4.4.2 FTIR

The infrared (IR) spectrum for the Cassia leaves is shown in Figure 4.20.

Figure 4.20: IR Spectrum for Cassia leaves

From Figure 4.20, the aromatic overtone region which is a weak grouping of peaks found in

aromatic systems can be observed between 1667 cm-1 and 2000 cm-1 [67]. Tannins, flavonoids,

saponins and alkanoids are all aromatic compounds and therefore can be attributed to this region.

Flavonoids in particular are ketone containing compounds which can be seen as a C=O stretch

occurring at 1728 cm-1 [67]. The presence of a secondary amine can be characterized by the N-H

stretch observed at 3283 cm-1 [67]. Alkanoids are sometimes considered as amines. The presence

of alkanes can also be characterized by the sp3 C-H stretch observed at 2849 cm-1 and 2918 cm-1

[67].

The IR spectrum for the TA is given in Figure 4.21:

3283, 95.51

2918, 94.32

2849, 95.39

2000, 99.05

1728, 96.52

1667, 97.32

94

95

96

97

98

99

100

40

00

39

04

38

08

37

12

36

16

35

20

34

24

33

28

32

32

31

36

30

40

29

44

28

48

27

52

26

56

25

60

24

64

23

68

22

72

21

76

20

80

19

84

18

88

17

92

16

96

16

00

15

04

14

08

13

12

12

16

11

20

10

24

92

88

32

73

66

40

54

4

TRA

NSM

ITTA

NC

E (%

T)

WAVE NUMBER (cm-1)

IR SPECTRUM FOR CASSIA LEAVES


63

Figure 4.21: IR Spectrum of Terephthal Amide

In Figure 4.21, the presence of primary amide can be characterized by the N-H stretch at 3359

cm-1 and 3284 cm-1 occurring as a doublet and the C=O stretch at 1621 cm-1. Generally the C=O

stretch for amides occur from 1650-1700 cm-1 [67], but when they are conjugated with aromatic

C=C stretches which can be observed at 1553 cm-1, 1501 cm-1 and 1463 cm-1, the wave number of

the C=O stretch is reduced by 20-40 cm-1 [67], accounting for the C=O stretch occurring at 1621

cm-1.

The IR spectrum obtained for TA is comparable to that in literature which is shown in Figure 4.22:

3359, 87.18

3284, 81.28 1621, 81.06

1553, 83.02

1501, 87.41

1463, 91.56

81

83

85

87

89

91

93

95

97

99

40

00

38

95

37

90

36

85

35

80

34

75

33

70

32

65

31

60

30

55

29

50

28

45

27

40

26

35

25

30

24

25

23

20

22

15

21

10

20

05

19

00

17

95

16

90

15

85

14

80

13

75

12

70

11

65

10

60

95

58

50

74

56

40

53

5

TRA

NSM

ITTA

NC

E (%

T)

WAVE NUMBER (cm-1)

IR SPECTRUM OF TEREPHTHAL AMIDE


64

Figure 4.22: IR Spectrum of TA obtained from literature [65].

4.5 INHIBITION MECHANISM AND IR SPECTRA OF PRODUCTS FORMED ON

METAL SURFACE.

The components of the plant inhibitor as seen in Table 4.7 and Figure 4.20 include tannins,

saponins, alkanoids and flavonoids. The polyphenolic fraction of tannin molecules react with ferric

ions to form a highly cross-linked passivating layer of ferric tannate which ensures effective

protection of the metal surface [5, 68]. Furthermore, alkanoids, tannins, saponins and flavonoids

are all aromatic compounds which are adsorbed through their electroactive sites where high

electron density is available [69]. Nitrogen, oxygen and sulphur are usually these sites where

adsorption occurs. Due to their high electron density and electron lone pairs they easily get

protonated giving the inhibitor a net positive charge [55]. It is therefore attracted to the metal

surface which is negatively charged due to its interaction with the chloride ions in HCl.


65

The adsorption of the aromatic compounds onto the metal surface was verified using FTIR to

determine the active compounds in the corrosion products as shown in Figures 4.23 and 4.24.

Figure 4.23: IR Spectrum of product formed on the metal surface when the plant inhibitor was

used.

3361, 97.31

2000, 99.921667, 99.72

97.2

97.7

98.2

98.7

99.2

99.7

100.2

100.7

40

00

38

89

37

78

36

67

35

56

34

45

33

34

32

23

31

12

30

01

28

90

27

79

26

68

25

57

24

46

23

35

22

24

21

13

20

02

18

91

17

80

16

69

15

58

14

47

13

36

12

25

11

14

10

03

89

27

81

67

05

59

TRA

NSM

ITTA

NC

E (%

T)

WAVE NUMBER (cm-1)

IR SPECTRUM OF PRODUCT FORMED ON METAL SURFACE (PLANT INHIBITOR)


66

Figure 4.24: IR Spectrum of product formed on the metal surface when the plant-plastic

inhibitor was used.

In Figure 4.23, the aromatic overtone at 1667-2000 cm-1 shows that aromatic compounds were

adsorbed. The formation of an N-H bond on the metal surface can also be observed at 3361 cm-1.

In Figure 4.24, the adsorption of a secondary amide onto the metal surface is evident as shown by

the N-H stretch at 3342 cm-1 and a C=O peak at 1624 cm-1. This could be as a result of delocalized

pi-electrons in the aromatic ring filling the vacant d-orbital of the iron thereby creating a pi-bond

with the iron [55]. The carbonyl group (C=O) found in the plastic inhibitor is also capable of

forming pi-bonds with iron and also accepting electrons from the d-orbitals to form feedback bonds

thereby creating more than one site for chemisorption on the metal surface [55].

Figure 4.25 shows the IR spectrum for the products formed on the unprotected metal surface:

3342, 78.17

1624, 94.4

78

83

88

93

98

40

00

38

92

37

84

36

76

35

68

34

60

33

52

32

44

31

36

30

28

29

20

28

12

27

04

25

96

24

88

23

80

22

72

21

64

20

56

19

48

18

40

17

32

16

24

15

16

14

08

13

00

11

92

10

84

97

68

68

76

06

52

54

4

TRA

NSM

ITTA

NC

E (%

T)

WAVE NUMBER (cm-1)

IR SPECTRUM OF PRODUCT FORMED ON METAL SURFACE (PLANT-PLASTIC INHIBITOR)


67

Figure 4.25: IR Spectrum of product formed on the metal surface when no inhibitor was used.

For the product formed on the metal surface without any inhibitor protection, a broad O-H peak

can be observed at 3276 cm-1 in Figure 4.25. This characterizes the formation of an O-H group

which is most likely Fe(OH)3 (rust).

4.6 COMPARISON WITH ALREADY REPORTED RESEARCHES

Several researches have been done in the area of green inhibitors. Oguzie [5] researched on the

leaf extracts of Occimum Viridis as a CI for mild steel in 2 M HCl. He investigated the IE % using

a gasometric test. His highest inhibitor concentration, which was 50 v/v% gave the highest

efficiency of 71 % at a temperature of 30 oC after about an hour. That is considerably lower

compared to the efficiencies of 95.87 % (electrochemical test after 12 days) and 94.89 % (weight

loss test after 21 days) obtained for the plant inhibitor in this research. He also reported on adding

potassium iodide to 10 v/v%. This additive greatly increased the IE % from 66.89 % to 91.56 %.

In this research adding the TA to 10 v/v% plant inhibitor did not cause a huge increase in its

3276, 32.4

30

40

50

60

70

804

00

03

89

23

78

43

67

63

56

83

46

03

35

23

24

43

13

63

02

82

92

02

81

22

70

42

59

62

48

82

38

02

27

22

16

42

05

61

94

81

84

01

73

21

62

41

51

61

40

81

30

01

19

21

08

49

76

86

87

60

65

25

44

TRA

NSM

ITTA

NC

E (%

T)

WAVE NUMBER (cm-1)

IR SPECTRUM OF PRODUCT FORMED ON METAL SURFACE (BLANK)


68

efficiency. That is, it only increased from 94.40 % to 94.83 % for the electrochemical test and

92.37 % to 92.60 % for the weight loss test. In spite of this, the efficiency was still higher in this

work.

Okafor et al. [7] reported on the inhibitory effect of extracts from Phyllanthus Amarus leaves on

mild steel in 2 M HCl. From their weight loss test, the highest inhibitor concentration of 4 g/L (100

v/v%) gave the highest efficiency of 94.1 % after 5 days at a temperature of 30 oC. This is quite

similar to the highest efficiency of 94.89 % (50 v/v% after 21 days) that was obtained from the

weight loss test conducted in this work.

Kalaiselvi et al. [9] also used Artemisia Pallens as a CI for mild steel in 4 M HCl. They obtained

their highest IE % to be 93 % at an inhibitor concentration of 1.5 g/L after 24 hours using weight

loss measurements. This is also quite similar to the efficiency of 94.89 % obtained from this

research.

El-Hameed [57] depolymerized PET and used the terephthal amide obtained as by-product as a CI

for mild steel in 1 M HCl. From his electrochemical tests performed at 25 oC, he obtained his

highest IE % to be 94.7 % at an inhibitor concentration of 200 ppm after 7 days. He also obtained

an efficiency of 93.5 % from his weight loss test. Using the TA solely did not perform so well in

this work with efficiencies of 29.23 % and 16.96 % being obtained from electrochemical and

weight loss tests respectively.


69

CHAPTER FIVE

CONCLUSION AND RECOMMENDATIONS

5.1 CONCLUSION

Extracts from Cassia Siamea leaves and Terephthal amide from waste PET bottles proved to be

good green corrosion inhibitors for mild steel in HCl solution. The efficiency of the plant inhibitor

increased with increasing inhibitor concentrations and acted as a cathodic inhibitor per the Tafel

plots obtained through electrochemical readings. Addition of TA to the plant inhibitor showed an

improved performance compared to using only the plant inhibitor and this was seen in both

electrochemical and weight loss measurements. Not only did this improvement show in reduced

CRs and increased IE% but it also reflected in the amount of weight-loss being reduced. The

combination of the plant/plastic inhibitor also acted as a cathodic inhibitor.

The inhibition was most likely as a result of a protective layer being formed due to the adsorption

of aromatic compounds such as tannins, flavonoids, saponins and alkanoids onto the metal surface.

An IR spectra showed the presence of such compounds in the corrosion product. The carbonyl

groups and aromatic rings in the TA also provided more sites for adsorption.

Based on the research objectives, the following major findings were made:

1. Extracts from the Cassia Siamea leaves displayed inhibitive properties by reducing the

CR of mild steel in 2M HCl. Electrochemical analysis showed a reduction in CR with

increasing inhibitor concentration. 50 v/v% gave the lowest rate of 5.12 mm/yr after 21

days, with an IE % of 95.87 %, while 10 v/v% gave the highest rate of 7.42 mm/yr after

21 days, with an IE % of 94.40 %. The blank sample did not last the 21 days and was

dissolved in the acid after 12 days. It had a CR of 95.77 mm/yr on the 12th day.

2. In terms of weight loss measurements, the plant inhibitor caused a reduction in weight

loss with increasing concentration. 50 v/v% caused the least weight loss of 1.48 %WL

while 10 v/v % caused the highest weight loss of 2.21 %WL, both after 21 days. The

blank sample had a weight loss of 29.02 %WL after 21 days. The CRWL also reduced

with increasing concentration, with 50 v/v% giving the lowest rate of 0.52 mg/cm2/day

and 10 v/v% giving the highest rate of 0.78 mg/cm2/day. The blank sample gave a rate

of 10.25 mg/cm2/day.


70

3. Adding the TA to the plant inhibitor showed an improvement in performance. For

instance, using solely 20ml (40 v/v%) of plant inhibitor gave a CR and CRWL of 4.88

mm/yr and 0.62 mg/cm2/day respectively. Adding 5 mg of TA to the 20 ml plant

inhibitor caused the CR and CRWL to drop to 4.53 mm/yr and 0.55 mg/cm2/day

respectively.

4. The Tafel plots obtained showed that both plant and plant/plastic inhibitors reduced the

cathodic current more and are therefore cathodic-type inhibitors.

5. The inhibition mechanism was as a result of, but not limited to the adsorption of aromatic

compounds such as tannins, flavonoids and saponins on to the metal surface to form a

protective layer. This was verified using FTIR.

5.2 RECOMMENDATIONS

Further research can be done by trying out different inhibitor concentrations and combinations to

see if the efficiencies can be further improved. The inhibitors can also be applied on other metals

and different corrosive environments to test its versatility.

Further characterizations can be done to determine the possibility of other compounds aiding in

inhibition and also to determine how the inhibitor affects the surface structures. Tests can also be

carried out at different temperatures if possible.


71

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77

APPENDIX

A1. Calculation of the volume of concentrated HCl used:

Density of 35.4 % HCl = 1.2 kg/L

Molecular Weight of 35.4 % HCl = 36.46 g/mol or 0.03646 kg/mol

Conversion from % concentration to kg/L concentration: 35.4

100× 1.2 = 0.4248 𝑘𝑔/𝑙

Conversion from kg/L to molar concentration: 0.4248/0.03646 = 11.65 M

Using the dilution equation, 𝑀1𝑉1 = 𝑀2𝑉2

Where M1 – Molar concentration of HCl, M2 – Molar concentration of final solution, V1 – Volume

of concentrated HCl, V2 – Volume of final solution

11.65𝑉1 = 2 𝑀 𝑥 1000 𝑚𝑙

𝑉1 = 2000/11.65

𝑉1 = 171.67 𝑚𝑙 ~ 𝟏𝟕𝟐 𝒎𝒍

AT1: CRs obtained from electrochemical readings (plant inhibitor)

CORROSION RATE (MM/YR)


DAY BLANK 10 v/v %

(5 ml)

20 v/v %

(10 ml)

30 v/v %

(15 ml)

40 v/v %

(20 ml)

50 v/v %

(25 ml)

3 51.19 3.74 3.16 2.94 2.88 2.47

6 64.47 3.98 3.52 3.43 3.08 2.89

9 89.21 4.60 4.38 4.04 3.86 3.55

12 95.77 5.36 4.80 4.68 4.32 3.96

15 - 6.23 5.75 5.26 4.52 4.27

18 - 6.94 6.00 5.67 4.88 4.63

21 - 7.42 6.59 5.88 5.59 5.12


78

AT1: IE% after 12 days

AT2: CRs for Plant-Plastic Inhibitor.

CORROSION RATE (MM/YR)



3 51.19 20.61 3.50 2.86 2.79 2.55 2.47

6 64.47 45.24 3.69 3.47 3.23 2.96 2.89

9 89.21 58.80 4.37 4.11 3.53 3.31 3.55

12 95.77 67.77 4.95 4.51 4.21 3.75 3.96

15 - 80.28 5.85 5.22 4.75 4.25 4.27

18 - 91.91 6.45 5.51 5.07 4.53 4.63

AT3: Plant/Plastic inhibitor IE% after 12 days

INHIBITOR EFFICIENCY AFTER 12 DAYS

INHIBITOR CONCENTRATION INHIBITOR EFFICIENCY (IE %)

25 mg 29.23

5 ml/20 mg 94.83

10 ml/15 mg 95.29

15 ml/10 mg 95.60

20 ml/5 mg 96.08

25 ml 95.87


INHIBITOR CONCENTRATION (v/v %) INHIBITOR EFFICIENCY (IE %)

10 94.40

20 94.98

30 95.12

40 95.49

50 95.87


79

AT5: CR with respect to weight loss (plant inhibitor)

CORROSION RATE AFTER 21 DAYS

INHIBITOR CONCENTRATION CORROSION RATE (mg cm-2 day-1)

Blank 10.25

10 v/v% 0.78

20 v/v% 0.73

30 v/v% 0.70

40 v/v% 0.62

50 v/v% 0.52

AT6: IE% with respect to weight loss (Plant)


INHIBITOR CONCENTRATION IE%

10 v/v% 92.37

20 v/v% 92.84

30 v/v% 93.14

40 v/v% 94.00

50 v/v% 94.89

AT7: CR with respect to weight loss (plant/plastic inhibitor)

CORROSION RATE AFTER 21 DAYS

INHIBITOR CONCENTRATION CORROSION RATE (mg cm-2 day-1)

Blank 10.25

25 mg 8.51

5 ml/20 mg 0.76

10 ml/15 mg 0.71

15 ml/10 mg 0.69

20 ml/5 mg 0.55

25 ml 0.52


80

AT8: IE% with respect to weight loss (plant/plastic)

INHIBITOR EFFICIENCY (IE %) AFTER 21 DAYS

INHIBITOR CONCENTRATION IE %

25 mg 16.96

5 ml/20 mg 92.60

10 ml/15 mg 93.07

15 ml/10 mg 93.26

20 ml/5 mg 94.62

25 ml 94.89


Documents

Corrosion inhibitors from plant extract and terephthal