CHEMICAL AND ELECTROCHEMICAL INVESTIGATIONS OF COFFEE HUSK ... · IJRRAS 23 (1) April 2015 28 CHEMICAL AND ELECTROCHEMICAL INVESTIGATIONS OF COFFEE HUSK AS GREEN CORROSION INHIBITOR

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CHEMICAL AND ELECTROCHEMICAL INVESTIGATIONS OF

COFFEE HUSK AS GREEN CORROSION INHIBITOR FOR ALUMINUM

IN HYDROCHLORIC ACID SOLUTIONS

A.S.Fouda1,H.S.Gadow1,* & K.Shalabi1 1Chemistry Department, Faculty of science, El-Mansoura, Egypt

Fax:+20502202271, Tel:+20502365730, [email protected] 2Higher Institute For Engineering and Technology, New Damietta, Egypt,

*Email:hsgado73 @gmail.com

ABSTRACT

The inhibition action of coffee husk extract towards the corrosion of aluminum in 0.5 M HCl solution was studied at

temperatures 25 and 450C by weight loss method, potentiodynamic polarization, electrochemical impedance

spectroscopy (EIS) and electrochemical frequency modulation (EFM) methods. Surface morphology was tested using

energy dispersive X-ray (EDX) and scanning electron microscopy (SEM). The results showed that the inhibition

efficiency increased with increasing concentration of extracts. Polarization data showed that this extract acts as mixed

type inhibitor. Adsorption of extract on aluminum surface was found to obey Langmuir and Temkin isotherms. The

thermodynamic parameters were calculated and discussed. The inhibition efficiencies obtained from all techniques

employed are in good agreement with each other.

Keywords: Corrosion inhibition, aluminum, coffee husk extract, HCl, SEM-EDX.

1. INTRODUCTION

Hydrochloric acid solutions are widely used for acid cleaning, pickling, oil well acidizing and acid decaling [1–4].

Aluminum has a remarkable economic, light weight, high thermal and electrical conductivity. The most important

feature in aluminum is its corrosion resistance due to the formation of a protective film on its surface upon its exposure

to atmosphere or aqueous solutions. Prevention of corrosion of aluminum has been a subject of numerous studies due

to their high technological value and wide range of industrial and house hold applications. Using organic compounds

as corrosion inhibitors may be the main choice to decrease the corrosion rate of metals and their alloys in acidic media.

The presence of hetero atoms in the structure of inhibitor molecules such as O, N, S, P; enhance the adsorption process

and inhibition efficiency [5-9].Despite the broad spectrum of organic compounds, the choice of appropriate inhibitor

for a particular application is restricted by several factors. These include increased environmental awareness and the

need to promote environmentally friendly process. Many previous studies showed that the naturally occurring

substances of plant are successfully used as inhibitors for corrosion [10-12]. The natural products of plant origin are

inexpensive, ecofriendly corrosion inhibitors. The extracts from their leave, barks, seeds, and roots compose of a

mixture of organic compounds and some have been reported as effective inhibitors for metal corrosion [13-20].

The present work, was aimed to investigate the inhibition of aluminum corrosion in 0.5 M HCl solution in presence

of different concentrations of the extract and to study the effect of temperature on the extract efficiency. Also, to study

the surface morphology of Al in presence and absence of extract.

2. EXPERIMENTAL

2.1. Materials and solutions

2.1.1 Aluminum electrode

Corrosion tests have been carried out on electrodes cut from sheets of aluminum with composition more than 99.9%.

tel:+20502365730,%[email protected]

IJRRAS 23 (1) ● April 2015 Fouda et al. ● Coffee Husk as Green Corrosion Inhibitor

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2.1.2 Hydrochloric acid The corrosive medium (0.5 M HCl) was prepared from a stock 5 M HCl solution by dilution with bi-distilled water

from the concentrated acid solution (37 %, Merck). The concentration was checked by standard solution of Na2CO3.

0.5 M HCl solution was prepared by dilution from 5 M acid with bi-distilled water. This solution was used as a blank.

2.1.3 Coffee husk solution extract

Coffee husk extract was obtained directly from the powder of dried coffee husk, then soaked in methanol and left

standing for 7 days. The solution was filtered and further bi-distilled at 40°C to remove the methanol from the coffee

husk solution and then concentrated to dryness.

2.1.4 Chemical composition of coffee husk extract

Coffee husk is rich in organic matter (cellulose, hemicelluloses, pectin and lignin), and chemical nutrients such as

nitrogen (N) and potassium (K). Additionally, coffee husk also contains secondary compounds such as caffeine, tannin

and polyphenol [21, 22].

Table 1: chemical structure of coffee husk

Name Structure

Cellulose

(C6H10O5)n

Hemicellulose

pectin

http://en.wikipedia.org/wiki/Carbon

http://en.wikipedia.org/wiki/Hydrogen

http://en.wikipedia.org/wiki/Oxygen


30

Lignin

structure

Caffeine

2.2. Weight Loss Measurements

The weight experiments were carried out using specimens of aluminum having dimensions (2x2x0.05cm). The test

pieces of aluminum samples were weight up to fourth decimal place using digital electronic balance. The test samples

were immersed in 50 mL of 0.5 M HCl in absence and presence of varying concentrations of coffee husk extract taken

in beaker at temperatures 25and 400C in water thermostat. Initial weight of samples were measured before immersion

and after specified period of exposed time, each piece was taken out of the test solution, rinsed with bi-distilled water,

dried between two filter papers and weighed again. The difference in weight for an exposed period of 30-180 minutes

was taken as the total weight loss. The experiments were carried out at various concentrations (100-500 ppm) of coffee

husk extract. Triplicate samples were used to check reproducibility of results. From the average weight loss results

(average of three replicate analysis), the corrosion rate, the percentage of inhibition efficiency (% IE) and the degree

of surface coverage() were calculated using equations[23]:

Corrosion rate = W/AT mg/cm2h (1)

Where “W” is the weight loss in mg, “A” is the area of the specimen in sq-cm and “T” is the exposure time in min.

Inhibition efficiency (%IE) = () x 100 = [(W1-W2)/W1] x100 (2)

where W2and W1are the weight losses (mg) for aluminum sample in the presence and absence of the inhibitor and

is the degree of surface coverage of the inhibitor. The percentage of inhibition efficiency (% IE) and the degree of

surface coverage () were tabulated.

http://en.wikipedia.org/wiki/File:Koffein_-_Caffeine.svg


31

2.3. Electrochemical measurements

All electrochemical measurements (potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and

electrochemical frequency modulation (EFM)) were carried out using Gamry Potentiostat/Galvanostat/ZRA (model

PCI4/300) with a Gamry framework system based on ESA400. Gamry applications include software DC105 for

potentiodynamic polarization, EIS300 for EIS measurements and EFM140 for EFM measurements; computer was

used for collecting data. Echem Analyst 5.5 Software was used for plotting, graphing and fitting data. All

electrochemical measurements carried out using a conventional cell with three electrodes were used. The aluminum

sheet (1 cm2) was used as working electrode; the counter electrode was Pt foil and saturated calomel electrode (SCE)

as reference electrode. The working electrode was polished and cleaned as mentioned before.

2.3.1Potentiodynamic polarization measurements

For potentiodynamic polarization measurements, the polarization cell was filled with 100 ml of test solution. The

potentiodynamic current – potential curves were recorded by changing the electrode potential automatically from -0.8

to 0.5 V at a scan rate of 1 mVs-1. The degree of surface coverage (θ) and inhibition efficiency (% IE) were calculated

using equation (3):

% IE = θ x 100 = [1-(i/ i°)] x 100 (3)

where i° and i are the current densities in the absence and presence of the extract, respectively.

2.3.2 Electrochemical impedance spectroscopy (EIS) measurements

Electrochemical impedance spectroscopy (EIS) is a powerful technique for the characterization of electrochemical

systems and mechanistic information. For this reason this technique is being applied to an increasing extent to

understand corrosion process in solution, to study rate determination, inhibitor performance, coating performance and

passive layer characteristics [24-29].The electrochemical impedance measurements were carried out over a frequency

range of 105 Hz to 0.1 Hz with a signal amplitude perturbation of 5 mV. Experiments were always repeated at least

three times. The impedance diagrams were given in Nyquist representations. In the represented electrical equivalent

circuit (Figure 1), Rs is the solution resistance, Rct is the charge transfer resistance and Cdl is the double layer

capacitance.

The inhibition efficiency was calculated from the charge transfer resistance (Rct) values using the following equation

(4):

%IE = [(Rct-R0ct)/Rct] ×100 (4)

Where Roct and Rct are the charge transfer resistance in the absence and presence of inhibitor, respectively.

Figure (1): Equivalent circuit proposed to fit the EIS experimental data


32

2.3.3 Electrochemical frequency modulation (EFM) measurements

The electrochemical frequency modulation (EFM) technique is a new tool for monitoring the electrochemical

corrosion. The theory of EFM technique is previously reported [30]. The electrochemical frequency modulation has

many features [31-35]. EFM is a nondestructive technique, rapid test, gives directly value of the corrosion current

without a prior of knowledge of Tafel constants and has a great strength due to casually factors, which serve an internal

check on the validity of the EFM measurement.

2.4 Scanning electron microscopy studies

A scanning electron microscope (SEM) model HITACHI S-3000H coupled to an analyzer EDAX –RONTEC, were

used to analyze the morphology of the aluminum surface without and with inhibitor added. Images of the samples

were recorded after 24h exposure time in 0.5M HCl without and with different concentration of coffee husk extract.

These samples underwent the same pre-treatment as those used in the experiments of weight loss and electrochemistry.

3. Results

3.1 weight loss measurements

The weight loss of aluminum both in 0.5 M HCl in the absence and presence of various concentrations (100-500 ppm)

of coffee husk extract were determined. In all cases the weight loss decreases with increasing of extract concentration.

The experimental data of weight loss (w), percentage of inhibition efficiency (%IE), corrosion rate (C.R.) and degree

of surface coverage () for aluminum in 0.5 M HCl and in the presence of various concentrations of coffee husk extract

at different temperatures are shown inTable.2. The values of corrosion rate were plotted against the concentration of

the extract in 0.5 M HCl (Figure 2).

blank 100ppm 200ppm 300ppm 400ppm 500ppm

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

CR(wt

.loss.m

g/cm2 .h)

concentration(ppm)

25

45

Figure (2): Corrosion rates of various concentrations of inhibitor on aluminum in 0.5 M HCl at 25 and 450C

The characterization of the corrosion of aluminum in the different corrosive solution will carried out by assessment

of the inhibition efficiency (%IE) using equation 2. A bar chart of inhibition efficiency to concentration of inhibitor

in 0.5 M HCl was plotted as shown in (Figure 3).


33

100ppm 200ppm 300ppm 400ppm 500ppm

0

10

20

30

40

50

60

70

80

Concentration(ppm)

25

45

%IE

Figure (3): The variation of inhibition efficiency with inhibitor concentration of aluminum in 0.5 M HCl solutions

Table 2: data from weight loss of Al in 0.5 M HCl for various concentration of coffee husk after 1.5 h at 25 and

450C

Conc.,

ppm

298 K 318K

W

mg cm-2

% IE C.R., mg cm-

2h-1 W

mg cm-2

%IE C.R., mg cm-2h-

1

Blank 0.84 0.558 1.00 0.667

100 0.43 0.488 48.9 0.286 0.55 0.450 45.0 0.364

200 0.26 0.690 69.0 0.175 0.40 0.600 60.0 0.267

300 0.25 0.702 70.2 0.167 0.36 0.64 64.0 0.242

400 0.17 0.798 80.0 0.111 0.24 0.760 76.0 0.159

500 0.14 0.833 83.3 0.095 0.20 0.800 80.0 0.133

3.2 Adsorption isotherms

To understand the mechanism of corrosion inhibition, the adsorption behavior of the extract adsorbents on the metal

surface must be known. The predominant adsorption mode will be dependent on factors such as the extract

composition, chemical changes to the extract and the nature of the surface charge on metal. There are a number of

mathematical expressions having thus developed to take into consideration of non-ideal effects. The most used

isotherms are, Frumkin, De Boer, Parsons, Temkin, Flory-Huggins and Bockris-Swinkless [36-40].The values of

surface coverage, θ, corresponding to different concentrations of extract at 25 and 45 0C have been used to explain the

best isotherm to determine adsorption isotherm process. Figure 4 confirms that the inhibition processes due to

adsorption of the coffee husk extract on the Al surface. This is because a straight line is obtained when log (C/) is

plotted against log C and the linear correlation coefficient of the fitted data is close to 1. This indicates that the

adsorption of coffee husk extract molecules obeys the Langmuir adsorption [41] expressed as: C

=

1

K+ C (5)

Where C is the inhibitor concentration and K is the equilibrium constant for the adsorption /desorption process of the

inhibitor molecules on the metal surface.


34

0.1 0.2 0.3 0.4 0.5

0.2

0.3

0.4

0.5

0.6

0.7

25

45

C/

C

Figure (4): Langmuir adsorption isotherm for coffee husk extract adsorption on aluminum in 0.5 M HCl at two

different temperature after 90 min immersion

The extract also obeys Temkin adsorption isotherm which is present in Figure 5 equation 6. Values of adsorption

parameters deduced from the plots are recorded on Table 3

a = ln K C (6)

'a' is a molecular interaction parameter depending upon molecular interactions in the adsorption layer and the degree

of heterogeneity of the surface.

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

25

45

logC

Figure (5): Temkin adsorption isotherm plot as Ɵ against Log C for coffee husk extract at different temperatures

Plot of log θ/ 1-θ against log C (Figure 6) at different concentrations of coffee husk extract, straight lines were obtained

indicating that adsorption follows kinetic thermodynamic model according Equation (7) [42]:

Log θ/1-θ = log (K’) + y log C (7)

The equilibrium constant of adsorption is Kads= K’(1/y) , where 1/y is the number of surface active sites occupied by

one inhibitor molecules and C is the bulk concentration of the extract. According to the data in this table, it is seen

that the values of 1/y are nearly 1 indicating that each inhibitor molecule occupies one active site.


35

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

25oC

45oC

log

logC

Figure (6): The kinetic thermodynamic for coffee husk extract determined on aluminum at 25 and 450C

It is well known that the equilibrium constant of adsorption (K) is related to the standard adsorption free energy

(ΔGoads) and can be calculated by the following equation [43]:

Kads = 1/55.5 exp [−ΔGoads/RT] (8)

Values of free energy of adsorption calculated from equation 8 using K values obtained from Langmuir adsorption

and Temkin adsorption isotherm are presented in Table 3. The values are negative and less than -40 kJ mol-1. This

implies that the adsorption of the extract on aluminum surface is spontaneous and confirms the physical adsorption

isotherm mechanism [44]

Table 3: Langmuir, Temkin and Kinetic model adsorption parameters for adsorption of coffee husk extract on

aluminum in 0.5 M HCl for 90 min immersion period at different temperatures

The corrosion data fit the first- order reaction rate law according to equation (10) [45]

log[Wi-W] = -kt /2.303+LogWi (10)

where Wi is the initial weight of aluminum specimen, W is the weight loss of aluminum specimen at time t, [Wi-

W] is the residual weight of aluminum coupon at time t and k is the first–order rate constant. The linear plots obtained

with correlation coefficients close to 1 confirm first -order kinetics for the corrosion of aluminum in 0.5 M HCl

solution in the presence and absence of coffee husk extract Figure (7).

Temp.,

ºC

Langmuir isotherm Temkin isotherm Kinetic model

K R2 -Gºads a log K R2 -Gºads 1/y log K R2 -Gºads

25 9.61 0.99 15.54 4.83 2.056 0.9

5

21.68 1.02 0.99 0.95 15.6

45 7.43 0.98 15.92 4.69 1.91 0.9

6

22.3 1.04 0.89 0.93 16.0


36

20 40 60 80 100 120 140 160 180 200

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

blank

100ppm

200ppm

300ppm

400ppm

500ppm

log(W

i-W

)

Time(min.)

Figure (7): Plot of log (Wi-W) versus time for Al in 0.5 M HCl solution without and with coffee husk extract

3.3Effect of temperature

Temperature is one of the main factors likely to modify the behavior of materials in a corrosive medium. The

adsorption of organic compounds on the corroding system by physical and chemical adsorption was described by

studying the effect of temperature. Chemisorbed molecules protect anodic areas and reduce the inherent reactivity of

the metal at the sites where they are attached. The data revealed that the protection efficiency decreases with an

increase in temperature whereas the physisorbed molecules are attached to the metal at the cathode and essentially

retard metal dissolution by stifling the cathodic reaction [46]. From our study the protection efficiency decreases with

an increase in temperature. This can be due to the decrease in the strength of adsorption process at higher temperature,

suggesting that physical adsorption of the inhibitor on the sample surface. The apparent activation energies (E*a) for

the corrosion process in absence and presence of inhibitor can be evaluated from Arrhenius equation (11):

Log (C.R.)2 / (C.R.)1= Ea/2.303R x(1

𝑇1 -1

𝑇2) (11)

Whereas estimates of the heats of adsorption (Qads) can be obtained from the trend of surface coverage with

temperature as follows [46]:

Qads = 2.303R [log (𝜃2

1−𝜃2) – log (

𝜃1

1−𝜃1)] x

𝑇1𝑇2

𝑇2−𝑇1 (13)

Table 4: Calculated values of apparent activation energy (E*a) and heat of adsorption (Qads) of coffee husk extract on

aluminum in 0.5 M HCl at different temperatures

3.4 Potentiodynamic polarization measurements

Polarization curves for aluminium in HCl solutions in the absence and presence of different concentrations of coffee

husk extract at are shown in Figure 8. These polarization curves demonstrate, as a first sight, that in presence of coffee

husk extract the cathodic and anodic branches of the polarization curves are shifted towards lower currents to similar

Concentration,

ppm

E*a, kJmol-1

-Qads, kJmol-1

0.5M HCl 7.03

100 9.50 6.09

200 16.65 15.54

300 14.62 11.09

400 14.16 9.20

500 13.26 8.70


37

extent, probably as a consequence of the blocking effect of the adsorbed inhibitor molecules. As it can be seen from

Figure 8, the anodic and cathodic reactions are affected by the coffee husk extract. Based on this result, coffee husk

extract inhibits corrosion by controlling both anodic and cathodic reactions i.e. mixed-type inhibitor. Meaning that the

addition of coffee husk extract reduces the anodic dissolution of aluminium and also retards the cathodic reaction.

Electrochemical parameters derived from polarization curves including corrosion potential (Ecorr), corrosion current

(icorr), anodic and cathodic Tafel slopes (βa and βc) have been measured and by Tafel extrapolation and presented in

Table 5.

-5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

blank

100ppm

200ppm

300ppm

400ppm

500ppm

E,(V

)vs(

SC

E)

logi,A/cm2

Figure (8): Anodic and cathodic Tafel polarization curves for aluminum in the absence and presence of various

concentrations of coffee husk extract

Table (5): Electrochemical kinetic parameters obtained by Tafel polarization technique for aluminum in absence and

presence of various concentration of coffee husk extract.

Conc., ppm -Ecorr, mV vs. SCE icorr, A cm-2 βa, mV dec-1 βc, mV dec.-1 θ %IE

Blank 729 2600 174.4 358.0

100 744 750 111.7 236.3 0.712 71.2

200 748 565 112.9 252.6 0.783 78.3

300 739 533 148.8 312.6 0.795 79.5

400 748 449 175.6 319.9 0.827 82.7

500 752 320 200.7 334.9 0.877 87.7

3.5 Electrochemical impedance spectroscopy (EIS) measurements

The corrosion behavior of aluminum in 0.5 M HCl solution in the presence of coffee husk extract was investigated by

EIS at 25°C. Figure 9 shows the results of EIS experiments in the Nyquist representation. After analyzing the shape

of the Nyquist plots, it is concluded that the curves approximated by a single capacitive semi-circles, showing that the

corrosion process was mainly charge transfer controlled [47]. The general shape of the curves is very similar for all

samples; the shape is maintained throughout the whole concentrations, indicating that almost no change in the

corrosion mechanism occurred due to the extract addition (48). The diameter of Nyquist plots (Rp) increases on

increasing the coffee husk extract concentration. These results suggest the inhibition behavior of coffee husk extract.

The Nyquist plots are analyzed in terms of the equivalent circuit composed with classic parallel capacitor and resistor

(Figure 1) [49]. The impedance of a CPE is described by the equation 14:


38

ZCPE = Y0-1 (jω)-n (14)

where Y0 is the magnitude of the constant phase element ( CPE), j is an imaginary number, ω is the angular frequency

at which the imaginary component of the impedance reaches its maximum values and n is the deviation parameter of

the CPE: -1 ≤ n ≤ 1.

The values of the interfacial capacitance Cdl can be calculated from CPE parameter values Y0 and n using equation

15(50):

Cdl= (2πfmaxRct)−1 (15)

Where fmax is the frequency value at which the imaginary component (Z”) of impedance is maximum. The degree of

surface coverage (θ) and the inhibition efficiency(% IE) was calculated from the EIS data using equation 15, listed in

Table (6). By increasing the inhibitor concentration, the Rct values increase and the calculated Cdl values decrease, as

it can be seen from Table (6), the Cdl values tend to decrease with the increase of the concentration of coffee husk in

0.5 M HCl. The decrease in the Cdl, which can result from a decrease in local dielectric constant and/or an increase in

the thickness of the electrical double layer, suggests that coffee husk extract molecules function by adsorption at the

metal/solution interface. Deviations from the ideal semi-circle are generally attributed to the frequency dispersion as

well as in homogeneities, roughness of metal surface and mass transport process [51-53]. The resistances between the

metal and outer Helmholtz plane (OHP) must be equal to the Rct. The adsorption of inhibitor molecules on the metal

surface decreases its electrical capacity because they displace the water molecules and other ions originally adsorbed

on the metal surface. This modification results in an increase of charge-transfer resistance. The Rct values increased

with inhibitors concentrations may suggest the formation of a protective layer on the aluminum surface. This layer

makes a barrier for mass and charge-transfer. The Bode plot, Figure (10) Shows resistive region at high frequencies

and capacitive region at intermediate frequencies but do not show a clear resistive region (horizontal line and a phase

angle = 0) at low frequencies. These plots show two overlapped phase maxima at intermediate and low frequencies.

According to act circuit theory, an impedance plot obtained for a given electrochemical system can be correlated to

one or more equivalent circuits.

Table 6: EIS data of aluminum in 0.5 M HCl and in the presence of different concentration of coffee husk extract

extract

Conc.,

ppm

Rct,

Ω cm-2

Y° x 106

μΩ-1sn

n Cdl,

μF cm-2 % IE

Blank 28.1 53.01 0.970 43.34

100 67.0 85.67 0.954 66.79 0.58 58.1

200 106.6 41.17 0.946 30.2 0.74 73.6

300 135.2 45.95 0.935 32.27 0.79 79.2

400 153.9 49.53 0.918 32.1 0.82 81.7

500 197.8 38.8 0.892 21.52 0.86 85.8


39

-20 0 20 40 60 80 100 120 140 160 180 200 220 240 260

-80

-60

-40

-20

0

20

40

60

80

100

blank

100ppm

200ppm

300ppm

400ppm

500ppm

-Z imag

e/ohmc

m2

Zreal

/ohmcm2

Figure (9): Nyquist plots for aluminum in 0.5 M HCl in absence and presence of various concentrations of coffee husk

extract

0.01 0.1 1 10 100 1000 10000 100000 1000000

1

10

100

Blank

100ppm

200ppm

300ppm

400ppm

500ppm

logZ mo

d,ohmc

m2

log Freq,Hz

-100

-50

0

50

Figure (10): Bode plots for aluminum in 0.5 M HCl at different concentrations of coffee husk extract.

3.6 Electrochemical frequency modulation (EMF) measurements

The electrochemical frequency modulation (EFM) technique is a new tool for monitoring the electrochemical

corrosion. The EFM is a nondestructive corrosion measurement technique that can directly give values of the corrosion

current without prior knowledge of Tafel constants. The theory of EFM technique is previously reported [54,55]. The

great strength of the EFM is the causality factors which serve as an internal check on the validity of EFM measurement.

The causality factors CF-2 and CF-3 are calculated from the frequency spectrum of the current responses. Figure 11

shows the intermodulation spectra obtained from EFM measurements of aluminum in 0.5 M HCl solution in the

absence and presence of different concentrations of the coffee husk extract. The causality factor is calculated from the

frequency spectrum of the current response[56-60]. If the causality factors differ significantly from the theoretical

values of 2.0 and 3.0, then it can be deduced that the measurements are influenced by noise. If the causality factors


40

are approximately equal to the predicted values of 2.0 and 3.0, there is a causal relationship between the perturbation

signal and the response signal. Then the data are assumed to be reliable [61]. The calculated corrosion kinetic

parameters at different concentrations of the coffee husk extract in 0.5 M HCl at 25oC (icorr, βa, βc, CF-2, CF-3 and %

IE) is given in Table7. From this Table, the corrosion current densities decrease and the inhibition efficiencies increase

by increasing the concentration of coffee husk extract. Inhibition efficiency (%IEEFM) depicted in Table 7 calculated

from the following equation:

IE% = [i0corr - icorr/i0

corr] x100 (16)

Where iocorr and icorr are corrosion current density in the absence and presence of the studied compounds, respectively.

Table 7: Electrochemical kinetic parameters obtained by EFM technique for aluminum in 0.5 M HCl in the absence

and presence of different concentrations of coffee husk extract

Conc.,

ppm

icorr ,

μA cm-2

βa,

mV dec-1

βc,

mV dec-

1

CF-2 CF-3 θ

% IE Rcorr.

mmy-1

Blank 984.8 42.4 75.36 1.58 2.45 593.2

100 291.3 91.49 142.2 1.134 2.054 0.704 70.4 175.4

200 187.0 105.9 205.8 1.19 1.505 0.810 81.0 112.7

300 142.3 91.76 142.5 1.279 3.26 0.856 85.6 85.72

400 112.7 105.5 153 1.258 2.14 0.886 88.6 67.89

500 71.79 108.2 154 1.15 5.75 0.927 92.7 43.24

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

log

I,A

cm

-2

Freq,Hz

blank

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

log

I,A

cm

-2

Freq,HZ

100PPM


41

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

log

I,A

cm

-2

Freq.,HZ

200ppm

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

log

I,A

cm

-2

Freq., HZ

300ppm

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-9

-8

-7

-6

-5

-4

log

I,A

cm

-2

Freq,Hz

400ppm

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-9

-8

-7

-6

-5

-4

log

I,A

cm

-2

Freq.,Hz

500ppm

Figure (11): Intermodulation spectra recorded for aluminum electrode in 0.5 M HCl solutions in the absence and

presence various concentrations of coffee husk extract

3.7 Scanning electron microscopy (SEM) studies

Figure (12) represents the micrograph obtained for aluminum samples in presence and in absence of coffee husk

extract after exposure for 24h immersion. It can be seen from Fig. 12(a) that the aluminum samples before immersion

seems smooth and appears some abrading scratches on the surface. It is clear that aluminum surfaces suffer from

severe corrosion attack in the blank sample as shown in Fig. 12(b). Furthermore, the corrosion products appear very

uneven and cube-shaped morphology. It is important to stress out that when the coffee husk extract is present in the

solution Fig12(c), the morphology of aluminum surfaces is quite different from the previous one, and the specimen

surfaces were smoother. We noted the formation of a film which is distributed in a random way on the whole surface

of the aluminum. This may be interpreted as due to the adsorption of the coffee husk extract on the aluminum surface

incorporating into the passive film in order to block the active site present on the aluminum surface. Or due to the

involvement of inhibitor molecules in the interaction with the reaction sites of aluminum surface, resulting in a

decrease in the contact between aluminum and the aggressive medium and sequentially exhibited excellent inhibition

effect [62, 63].


42

a) b) c)

c)

Figure (12): Scanning electron microgram of polished aluminum (1500x) (a) alone (b) after exposure to 0.5 M HCl

(c) after exposure to 0.5 M HCl containing 500 ppm of extract

The EDX spectra were used to determine the elements present on the surface of aluminum and after 24h of exposure

in the uninhibited and inhibited0.5 M HCl. Figure (13) shows the EDX analysis result on the composition of aluminum

only without the acid and inhibitor treatment. Figure 13 show the EDX spectrum in the absence and presence of

inhibitor figure show an additional line characteristic for the existence of O. in addition, the intensities of C signal are

enhanced. The appearance of O signal and this enhancement in the C signal is due to the C and O atoms constituting

the inhibitor, which indicate that the inhibitor molecules have adsorbed on the metal surface. Data obtained from

spectra are presented in Table 8.The spectra show also that Al peaks are considerably suppressed in the presence of

inhibitor which is due to the overlying inhibitor film. These results confirm those from electrochemical measurements

which suggest that a surface film inhibits the metal dissolution, and hence retard the hydrogen evolution reaction

[64,65].

Table 8: Surface composition (weight %) of aluminum before and after immersion in 0.5 M HCl without and with

500 ppm of coffee husk extract at 25 0C

(wt %) Al C O Si

Pure 73.24 26.4 -- 0.27

Blank 90.32 9.23 -- 0.45

inhibitor 71.54 25.73 2.53 0.19

a) Metal free

b) 0.5 M HCl


43

c) Presence of coffee husk extract

Figure (13): EDX spectra of

aluminum: (a) before of

immersion in 0.5 M M HCl, (b) after 24 h of immersion in 0.5 M HCl and (c) after 24h of immersion in 0.5 M HCl

+ 500 ppm coffee husk extract at 25°C

DISCUSSION

From Table 1 coffee husk contains many O atoms in functional groups (O-H, C=O, C-O, O-heterocyclic ring) as well

as conjugated double bonds or aromatic rings which are the major characteristic for typical corrosion inhibitors [66].

As discussed above the thermodynamic and kinetic parameters, the adsorption is mainly electrostatic. Physical

adsorption requires presence of both electrically charged surface of the metal and charged species in the bulk of the

solution. In acid solution, the molecules of coffee husk components could be protonated in the acid media due to the

interaction between O and H+. Aluminum surface is positively charged due to the accumulation of Al-OH2+ species in

the acidic solution [67]. The acid anions (Cl-) adsorb electrostatically on the positively charged , giving rise in for a

net negative charge on the metal surface ; and the organic cations are physically attracted to the anions layer which is

formed on the metal surface , forming electrostatic protective layer on aluminum.

CONCLUSION

1- Coffee husk acts as good inhibitor for aluminum corrosion in 0.5 M HCl solutions.

2- The adsorption of coffee husk on aluminum surface obey Langmuir and Temkin isotherm.

3- The negative values of Gºads shows the spontaneity of the adsorption.

4- Corrosion current densities (icorr) obtained using EFM technique was in good agreement with those obtained

from Tafel extrapolation technique. In addition, the Causality factors were good internal check for verifying

the validity of data obtained by this technique.

5- EIS measurement reveals that charge transfer resistance increases and the double layer capacitance decreases

with increase in concentration of the extract.

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CHEMICAL AND ELECTROCHEMICAL INVESTIGATIONS OF COFFEE HUSK ... · IJRRAS 23 (1) April 2015 28 CHEMICAL AND ELECTROCHEMICAL INVESTIGATIONS OF COFFEE HUSK AS GREEN CORROSION INHIBITOR