Babylon University College of Materials EngineeringMetals Eng. Department

Corrosion Controlby Inhibition

Prepared by

Eng.Ahmed Salih Abd AL-Hussien

Corrosion Control

by Inhibition

Mechanism

Introduction

Example of inhibitor

Classification

Inorganic and Organic

Scavengers

Inhibitor Application Techniques

Inhibitor Efficiency

Scope of Inhibitor

INTRODUCTION

A corrosion inhibitor is a chemical substance which, when added in small concentrations to an environment, minimizes or prevents corrosion .Corrosion inhibitors are used to protect metals from corrosion, including temporary protection during storage or transport as well as localized protection, required, for example, to prevent corrosion that may result from accumulation of small amounts of an aggressive phase. One example is brine, in a nonaggressive phase, such as oil. An efficient inhibitor is compatible with the environment, is economical for application, and produces the desired effect when present in small concentrations.

Mechanisms of actions of inhibitors

Inhibitors are substances or mixtures that in low concentration and in aggressive environment inhibit, prevent or minimize the corrosion.Generally the mechanism of the inhibitor is one or more of three that are cited below:•The inhibitor is chemically adsorbed on the surface of the metal and forms a protective thin film with inhibitor effect or by combination between inhibitor ions and metallic surface.• The inhibitor leads a formation of a film by oxide protection of the base metal.• The inhibitor reacts with a potential corrosive component present in aqueous media and the product is a complex

Scope of Inhibitor

Corrosion control by use of inhibitors is extremely useful in many environments, however,there are certain exceptions , such as:(a) equipment and components subjected to turbulent flow.(b) systems operating above the stability limits of inhibitor.(c) equipment subjected to high velocity, beyond 4 m/s.

Examples of Application of Inhibitors

•Petroleum Industry. Corrosion phenomena in the petroleum industry occur in a two-phase medium of water and hydrocarbon.It is the presence of a thin layer of water which leads to corrosion, and rigorous elimination of water reduces the corrosion rate to a negligible value. The inhibitors used in petroleum industry, both in production and refining are either oil soluble–water insoluble types or oil soluble–water dispersible compounds.

•Packaging Industry. For transportation of machinery,components and equipment by sea, vapor phase cyclohexylamine and hexa-methylamine are used.

•Sour Gas Systems. A major problem is encountered in steel pipelines in various sour gas environments. Chemical inhibition is one of the effective methods used to mitigate sulfide induced corrosion. Inhibitors containing alkyl ammonium ions are found to suppress corrosion effectively.

•Potable Water Systems. Corrosion is experienced in potable water transportation pipes of steels and cast iron. Inhibitors, such as Ca(HCO3)2 and polyphosphates are commonly used to combat corrosion.

•Engine Coolants. Inhibitors, such as NaCrO4 (sodium chromate), borates and nitrites (NaNO2) and mercaptabenzothia-zole are widely used for protection of auto-mobile engines. Chromates are a health hazard.

Classification of Inhibitors

Inhibitors may be classified as shown in Fig( 1.1).There are two major classes: inorganic an organic. The anodic type of inorganic inhibitor includes chromates, nitrites, molybdates and phosphates, and the cathodic type includes zinc and polyphosphate inhibitors. The film forming class is the major class of organic inhibitor as it includes amines, amine salt and imidazoilnes – sodium benzoate mercaptans, esters, amines and ammonia derivatives.

Fig (1.1 ) : Classification of inhibitors

Inorganic Inhibitors

The addition of inorganic inhibitors causes suppression of electrochemical reaction at anodic– cathodic areas. Most of the times, inhibitors are used in a blended form. These inhibitors only react at an adequate level of concentration.

(A) Chromate InhibitorsThey are most effective inhibitors, but they are toxic and, hence, their application is restricted and is not advised. In industrial water, the threshold concentration is 120 mg/L. Chromate inhibitors contain either Na2CrO4 or Na2Cr2O7. Fe → Fe+2 + 2e- (Oxidation of iron) CrO-4 +8H+3e → Cr+3 + 4H2O (Formation of Cr+3 )

Fig (1.2) : Formation of a mixed iron oxide and chromium oxide film

(B) Nitrites

They are effective inhibitors for iron and a number of metals in a wide variety of waters. Like chromates, nitrites are anodic inhibitors and they inhibit the system by forming a passive film with ferric oxide. These are environmentally-friendly inhibitors. Besides steel, nitrites also inhibit the corrosion of copper. Nitrites should not be used in open systems as they would oxidize to nitrates in the presence of oxygen. NO-

2+O2↔2NO-3

Nitrites are not effective if presence of chloride and sulfate .

(C) Phosphate Inhibitors

Phosphate retards corrosion by promoting the growth of protective iron oxide films and by healing the defects in protective films.

(D) Molybdates

Molybdenum is an alloying element which is known to increase passivation of stainless steels of type 316. Sodium molybdates forms a complex passivation film at the iron anode.The passive film can only be formed in the presence of oxygen.

(E) Silicates

Silicates are strong anodic inhibitors and passive films can be formed even on the corroded surface. The monomeric silica does not provide any protection. In waters below pH levels of 6.0 , the silicate used is Na2O.2SiO3 and with a pH greater than 6.0, it is Na2O3.3SiO3 .

Silicate treatment also prevents dezincification in brass and corrosion of copper.

Anodic inhibitor

Cathodic inhibitor

Organic Inhibitors

Organic inhibitors are used in the oil industry to control oil and gas well corrosion. Most common types are long chain (C18) . The most common types of organic inhibitors are shown below:(1) Monoamine:Primary amine, RNH2 (4) Polyethoxylated compoundsSecondary amine, R2NH (a) AminesTertiary amine, R-N(CH3)2

(2) Diamines (x and y vary between 2 and 50)R – NHCH2CH2CH2NH2

(3) Amides (b) DiaminesR – CONH2

(x + y+ z varies between 3–10)

Organic inhibitors react by adsorption on a metallic surface. Cationic inhibitors (+), like amines, or anionic inhibitors (−), like sulfonates, are preferentially adsorbed depending on the charge of the metal surface (+)or(−). At zero point of charge, there is no particular preference for an anodic or cathodic inhibitor.

Organic inhibitor

Scavengers

Oxygen, even in very small amounts, may cause serious corrosion in feedwater lines, stage heaters, economizers, boiler metal, steam operated equipment and condensable piping. It must, there-fore, be removed from the closed system. The solubility of oxygen varies with both pressure and temperature. Oxygen is the main cause of corrosion. It reacts by consuming electrons at the cathode causing cathodic depolarization and enhancing the rate of corrosion. Chemicals which eliminate oxygen from the closed systems are called scavengers. Ammonium sulfite (NH4)2SO3, and hydrazine (N2H4) have been successfully used over the years to eliminate oxygen. Oxygen scavengers remove oxygen as shown below:

) NH4)2SO3 + 1/2 O2→ (NH4)2SO4

1. Org . molecule(aq) + nH2O(ads → ( Org . molecule(ads) + nH2O(soln(

2. N2H4+ O2 → N2 ↑+2H2O3. Na2SO3 + 1/2 O2 → Na2SO4

4. NH4HSO3 + 1/2 O2 → NH4HSO4 (Ammonium hydrogen sulfate)

• In reaction in equation 1 best at temperature (10 ċ) • In reaction in equation 2 increase the total dissolved solid content .• In reaction in equation 3 the rate of rection is slow at temperature below (15ċ) 4Fe3O4 + O2 → 6Fe2O3

6Fe2O3 + N2H4 → 4Fe3O4 + 2H2O + N2 ↑

• Table (1.2) : Advantages and disadvantages of sodium sulfite and hydrazine

Disadvantage Advantage Chemical

■ Does not reduce ferric oxide tomagnetite■May decompose to form corrosive gases■ Reacts less rapidly compared to sodium sulfite

■ Rapid reaction■ Non-toxic■ Contributes no solids■Reduces ferric oxide to magnetite

Sodiumsulfite

■ More expensive than sodiumsulfite■ Toxic and flammable

■ Less dosage for scavenging compared to sodium sulfiterequired

Hydrazine

Inhibitor Application TechniquesBasically, there are three well-known inhibitor application techniques: (1) Continuous injection(2) Batch treatment(3) Squeeze treatment

1 . Continuous Injection As the name suggests, inhibitors are injected in the system to achieve inhibition objectives through the system. Normally the inhibitor is injected into the system by means of an electric or gas driven chemical pump. The inhibitor is added at the point of turbulence to achieve uniform mixing. This method is used for municipal water supplies, cooling towers and oil wells, to minimize scaling and corrosion problems. In the continuous injection method, a constant supply of chemicals is maintained at a controlled rate.

2. Batch Treatment

This is a periodic treatment in which a large quantity of chemicals is used for an extended period of time. It is commonly used to treat flowing oil wells. Batch treatment is also called slug treatment. For batch treating, the tube displacement method is employed. Several barrels of inhibitor are pumped into the tubing at the top. The inhibitor is displaced to the bottom of the tubing with the fluids in the oil well. The well is closed for a specific period before operation. The batch is used mainly to treat water with biocides.

3 Squeeze Treatment

Continuous treatment of oil wells by inhibitors is achieved by this method. The liquid inhibitor is pumped down through the tubing into the oil producing geological formation under low pressure which acts as a chemical reserve. In oil wells, 1–2 drums of inhibitor is mixed with 10–20 bbl of water (1bbl (British barrel) = 36 gallons), and is pumped into the well followed by pumping in over-flush fluid (50–75 bbl). The inhibitor is absorbed by the formation.

Inhibitor Efficienc y and Inhibitor Concentration(1) The efficiency of corrosion inhibition can be expressed as Einh = (CR o – CRI) / CR O

where Einh = efficiency of a corrosion inhibitor CR o = corrosion rate with zero inhibitor CRI = corrosion rate in the presence of an inhibitor

(2) The quantity of inhibitor required for a fluid to be inhibited can be obtained by the relationship: Qinh =( Vfluid / 1*106) *Cinh (ppm)where Qinh = quantity of inhibitor, kg Vfluid = volume of fluid to be inhibited , liters Cinh = inhibitor concentration, ppm

Example

Calculate the dosage of sodium chromate required to be added to 500 000 liters ofwater, if the concentration of sodium chromate is 5 ppm.

Solution:QNn2CrO4= ( VNa2CrO4/ 1*106) *CNa2CrO4 (ppm) 06 ) *5 (ppm) =(500000Kg/ 1*1 =2.5 Kg

The Table (1.3) in the next page state some corrosive systems and the inhibitors used to protect them.