Inhibitor IntroductionTypes and compositionApplication

IntroductionAn inhibitor is a substance that when added in

small concentrations to an environment, decreases the corrosion rate

Corrosion inhibitors are additives to the fluids that surround the metal or related object.

The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime.

A common mechanism for inhibiting corrosion involves formation of a coating, often a passivation layer, which prevents access of the corrosive substance to the metal.

Introduction An inhibitor can be considered as a retarding

catalystThere are numerous inhibitor types and

compositionsMost inhibitors have been developed by

empirical experimentation Inhibition is not completely understood but it

is possible to classify inhibitors according to their mechanism and composition

Introduction Inhibitors slow corrosion process by: - Increasing the anodic or cathodic

polarization behavior (Tafel slopes) - Reducing the movement or diffusion of

ions to the metallic surface - Increasing the electrical resistance to the

metallic surface

Inhibitors: types and compositionAdsorption-type inhibitorsHydrogen-evolution poisonsScavengersOxidizersVapor-phase inhibitors

Adsorption-type inhibitorsUsually designated as film forming protect

the metal by forming a hydrophobic film on the metal surface.

The effectiveness of this inhibitor depends on the chemical composition, their molecule structure and their affinities for the metal surface

These are organic compounds which adsorb on the metal surface and suppress metal dissolution and reduction reactions

Adsorption-type inhibitorsEx: organic amines (pyridine,

diethylamine, methylamine, etc.)

Chemisorption of an organic compound is facilitated by the presence of polar groups in the molecular structure, which can readily attach to the metal surface

Hydrogen-evolution poisonsRetard the hydrogen-evolution

reactionEx: arsenic and antimony ionsAs a consequence, these substances

are very effective in acid solutions but are ineffective in environments

where other reduction processes such as oxygen reduction are the controlling cathodic reactions.

Scavengers

These substances act by removing corrosive reagents from solution

Ex: sodium sulfite(Na2SO3) and hydrazine (N2H4)

It remove dissolved oxygen from aqueous solutions

O2HNOHN

SO2NaOSO2Na

22242

42232

Scavengers Work very effectively in solutions

where oxygen reduction is the controlling corrosion cathodic reaction

But will not be effective in strong acid solutions

Oxidizers Used to inhibit the corrosion of

metals and alloys that demonstrate active-passive transitions

Metals:iron and its alloy and stainless steels

Substance: chromate, nitrate and ferric salts.

Vapor-phase inhibitors

These are very similar to the organic adsorption-type inhibitors

Possess a very high vapor pressure (usually about 10-2 to 10-7 mmHg)

These materials can be used to inhibit atmospheric corrosion of metals without being placed in direct contact with the metal surface

Continue…Are placed in the vicinity of the

metal to be protectedTransferred by sublimation and

condensation to the metal surface

Only effective in closed spaces such as inside packages or on the interior of machinery during shipment

Continue…Dicyclohexylamine nitrite (DAN)

has been found especially effective for steel surfaces

But increases attack on copper alloy

Nitrobenzoate ( a universal vapor-phase inhibitor): protect ferrous, copper and other alloy system

Application Inhibitors find greatest use in

recirculating systems.Ex: recirculating cooling water,

automotive coolantsOnce-through sytems usually

consume too much of the inhibiting chemical to be economically feasible

ApplicationThe effectiveness of inhibitors is diminished

by ◦ Increasing solution corrosivity, ◦ Increasing concentration and ◦ Increasing temperature

Many inhibiting compounds are toxic (chromates, arsenic) and recent environmental regulations have limited their use

Application Many inhibitors are effective for more than

one type of alloy but◦ pH◦ temperature◦ and other condition

are unique for eachAn inhibitor for one metal may be corrosive

to others

Table 5.1: Effectiveness of inhibitors in near neutral pH water

Metal Chromates

Nitrites

Benzoates

Borates

Phosphates

Silicates

Tannins

Mild steel

E E E E E RE RE

Cast iron E E IE V E RE RE

Zinc E IE IE E RE RE

Copper E PE PE E E RE RE

Aluminium

E PE PE V V RE RE

Pb-Sn solder

E A E RE REE-effective, IE-ineffective, RE-reasonably effective, PE-partially effective, V-variable, A-aggressive

Application Three types of environment find greatest

use for inhibitors1) Natural, supply and industrial cooling waters

by nearly neutral (pH 5 to 9) rangeEx:

◦ organic phosphonate, acrylic copolymer as the main active 

ingredient. ◦ It has a good corrosion and scale inhibition effe

ct on the 

high hardness cooling water ◦ applicable to carbon steel, copper, copper alloy

 and 

stainless steel, etc.◦ widely used in the airconditioning cooling water

 in hotels, ◦ restaurants, etc..

Application2)Primary and secondary production of crude

oil and subsequent refining processesThe main ingredient :is two-imidazoline, soluble in oil, gas, but also dispersed in

water. This unique feature makes it be the preferred

component of gas corrosion inhibitor.

Application3) Acid solutions for pickling to remove rust

and mill scale during the production and fabrication of metal parts or for postservice cleaning of such parts

Ex: The pickling operation makes use hydrochloric acid; that presents many benefits, inclusive of a better finishing and appearance, higher efficiency, and a reduced iron build-up.

Table 5.2: Inhibitors used for industrial application

Environment Sample inhibitor

Waters

- Potable water CaCO3 deposition, silicates, polyphosphates, zinc salts

-Recirculing cooling water

Chromate, nitrate at 300-500 ppmCalcium polyphosphates at 15-37 ppmSilicates at 20-40 ppm

-Automotive coolants

Nitrite, benzoate, borax, phosphate, sodium mercaptobenzothiazole, benzotriazole

-Steam condensates

Neutralizers: ammonia, morpholine, cyclohexamine, benzylamine, long chain aliphatic amines such as octdecylamine at 1-3 ppm

-Brines and seawater

Refrigeration brines: chromates, 2000-3300 ppmDiluted seawater: sodium nitrite, 3-10%Hot desalting brines: mixed chromate and phosphate, 50-100 ppm

Continue…

Environment Sample inhibitor

Acid pickling solution

-Sulfuric acid Phenylthiourea, di-ortho-tolyl-thiourea, mercaptans, sulfides, (0.003-0.01%)

-Hydrochloric acid Pyridine, quinoline, various amines, decylamine, phenylthiourea, dibenzylsulfoxides

Oil production and refining

-Primary and secondary recovery

Fatty imidazolines, various amines including primary amine, diamines, amido-amines, oxyethylated primary amines, alkyl pyidines, quaternized amines

-Refining Imidazoline and derivatives

Cathodic inhibitorOn addition of the corrosion inhibitor, Ecorr

decreases and icorr decreases.Displacement of the corrosion potential in the

negative direction indicates mainly retardation of the cathodic process (cathodic control)

Cathodic inhibitorCathodic inhibitors are generally less

effective than the anodic type. They often form a visible film along the

cathode surface, which polarizes the metal by restricting the access of dissolved oxygen to the metal substrate.

The film also acts to block hydrogen evolution sites and prevent the resultant depolarizing effect.

Cathodic inhibitorExample:

◦Zinc hydroxide◦Zinc phosphate◦Calcium carbonate◦Calcium phosphate

Anodic inhibitorOn addition of the corrosion inhibitor, Ecorr

increases and icorr decreases.Displacement of the corrosion potential in the

positive direction indicates mainly retardation of the anodic process (anodic control)

Anodic inhibitors build a thin protective film along the anode, increasing the potential at the anode and slowing the corrosion reaction.

The film is initiated at the anode although it may eventually cover the entire metal surface.

Because the film is not visible to the naked eye, the appearance of the metal will be left unchanged.

Anodic inhibitor

Example:◦ Chromate, molybdate, and nitrite -- catalyze the

reaction between the metal and oxygen to form a passivating film. Chromate and nitrite are the only anodic inhibitors that function in the absence of oxygen.

◦ Orthophosphate -- also catalyzes the reaction between steel and oxygen to form a passivating gamma iron oxide film. Oxygen must be present in water for orthophosphate to function as an anodic inhibitor.

◦ Polyphosphate -- exhibits some anodic properties but functions primarily as a cathodic inhibitor.

Anodic inhibitor

Mixed inhibitorOn addition of the corrosion inhibitor, icorr

decreases but Ecorr changes very little (either positive or negative).

Little changes in the corrosion potential suggests that both anodic and cathodic processes are retarded.

Mixed inhibitor They are typically film forming compounds that cause

the formation of precipitates on the surface blocking both anodic and cathodic sites indirectly.

Examples:◦ Sodium silicate, for example, is used in many

domestic water softeners to prevent the occurrence of rust water. In aerated hot water systems, sodium silicate protects steel, copper and brass. However, protection is not always reliable and depends heavily on pH.

◦ Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites, however, they are very useful in situations where non-toxic additives are required.

Polarisation resistance, Rp

The efficiency of an inhibitor can also be studied by recording polarisation curves or by polarisation resistance measurements.

corrca

cap iR

)(3.2

Inhibition efficiency, p

It is important to be able to evaluate inhibitors and determine the conditions under which they are effective.

The inhibition efficiency can be expressed by,

100

%

inhibitedR

duninhibiteRinhibitedR

p

ppp