Industrial Chemistry B_ Lecture 6

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Industrial Engineering Chemistry B Instructor: Tracey-Ann Warner [email protected] Stream 1 Lecture 6 February 13, 2012

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Industrial Engineering Chemistry BInstructor: Tracey-Ann Warner [email protected] Stream 1

Lecture 6 February 13, 2012

Corrosion Definition Causes of Corrosion Formation of Corrosive Films Electrochemical Mechanisms for Corrosion Corrosion Prevention Methods Application

Definition of Corrosion

Corrosion is the deterioration of materials by chemical

interaction with their environment. The term corrosion is sometimes also applied to the

degradation of plastics, concrete and wood, but generally refers to metals.

Corrosion Corrosion is also defined as the oxidation of a metal to

produce compounds of the metal through interaction with its environment. Corrosion may be defined as the destruction of a metal or

an alloy because of chemical or electrochemical reaction with its surrounding environment or medium It is an inconvenient, costly and sometimes unavoidable

property of metals, and is an electrochemical process.


Environments in Corrosion1L.L., R.A. Jarman, and G.T. Burstein, eds. Corrosion. 3rd ed. Vol. 1. 2000, Butterworth-Heinemann: Oxford.

Corrosion: Metallurgy in Reverse2


M.G., Corrosion Engineering. 3rd ed. 1986, New York: McGraw-Hill. 6

Corrosion Basics Occurs mostly in ionic, aqueous environments Primarily a concern for metals Oxidation Reduction Reaction:

M Mn+ + nenH+ + ne- nH Loss of metal Become ions in solutions Combine with other species to form compound (oxides, hydroxides)

Corrosion Basics Only Cu and precious metals such as gold, silver and

platinum are found in nature in their metallic state. All other metals are processed from minerals or ores into

metals and are inherently unstable in their environments. The material with the greatest economic importance that is

most affected by corrosion is iron. The corrosion of iron is called rusting.

Corrosion Basics Iron, a common construction metal often used in forming

steel alloys, corrodes by being oxidized to ions of iron by oxygen. This corrosion is even faster in the presence of salts and

acids, because these materials make electrically conductive solutions that make electron transfer easy

Causes of Corrosion Multifactorial (many factors) problem-depends on geometry, metallurgy, stresses,

solution chemistry Driven by two primary factors: thermodynamic driving forces (Oxidation/Reduction) and kinetic barriers An electro-chemical attack resulting in material degradation Exacerbated by mechanical and biological attack Compromises Material Properties Mechanical Integrity Biocompatibility Aesthetics

Causes of Corrosion Metals corrode because we use them in environments where they

are chemically unstable. The most common causes of corrosion are contact with

water and oxygen, though other substances in the earth and in the atmosphere can also cause corrosion.

Effects of Corrosion The consequences of corrosion are many and varied and theeffects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal.

Failures of various kinds and the need for expensivereplacements may occur even though the amount of metal destroyed is quite small. Losses are economic and safety: Reduced Strength Downtime of equipment Escape of fluids Lost surface properties Reduced value of goods

Basic Types of Corrosion Uniform or General Corrosion

General corrosion, also referred to as uniform corrosion, is one of the most common types of corrosion

It damages the entire surface of the material at about the same rate, causing the metal to thin.It is a chemical attack, easily detected by its appearance. Controlling general corrosion is relatively easy. However, if not controlled, the metal surface continues to thin until nothing is left.

Basic Types of Corrosion contd General corrosion (uniform attack) that is evenly

distributed over entire corrosion region

Rusting of iron, tarnishing of silverware

Most readily detectable (visual) and preventable (alloying)

Basic Types of Corrosion contd

General Corrosion

Basic Types of Corrosion contd Crevice Corrosion contd:

Crevice corrosion, also called concentration cell corrosion This type of corrosion forms when a liquid corrosive is trapped in narrow gaps of space between metals, or between nonmetals and metals. such as chlorides must be present in the

Aggressive ions,


the liquid has settled in the gap, a corrosion reaction begins to take place.

Basic Types of Corrosion contd Crevice Corrosion contd:

The reaction consumes the oxygen in the bottom of the gap, and an anodic area develops adjacent to the oxygen-depleted zone. The material on the exterior acts as the cathode. Crevice corrosion develops quite similar to pitting corrosion after the initiation stage with a gradual decrease of the pH and an increase of the chloride concentration within the crevice.

Basic Types of Corrosion contd Crevice corrosion: Found in crevices or deep, narrow flaws (mismatch of components at interface

Can arise from localized oxygen depletion and metal ion concentration gradients


O2 O2 O2 OHOH-

Basic Types of Corrosion contd Crevice Corrosion contd:Micro-motion

between components results in fretting corrosion that can lead to initiation of crevice corrosion

Metallic implants rely on passive oxide film for protection from corrosion.Repetitive motion leads to continuous breakdown and repassivation. Repeated breakdown consumes oxygen in crevice and results in drop in pH--crevice corrosion.

Basic Types of Corrosion contd The design of certain

materials, such as gaskets, fasteners, surface deposits, washers, threads, clamps, and any close-fitting surfaces often initiates crevice corrosion.Crevice Corrosion Crevice corrosion can be

extremely aggressive; its detection and monitoring is a major challenge.

Basic Types of Corrosion contd Mechanically Assisted Crevice Corrosion : In the head-neck taper, tolerances are such that narrow crevices exist with

fluid present film

At onset of loading, interfacial shear stresses are sufficient to fracture oxide

Unpassivated metal is exposed to initially oxygen rich fluid. Oxidation occurs--depleting oxygen in crevice fluid--increases free metal

ions--which attract Cl ions-->metal chlorides pH

Metal chlorides react with water to form metal hydroxide and HCl--lowers

Cr2O3 is unstable below pH of 3-- results in active attack of CoCr alloy--

etched appearance (intergranular attack)

Basic Types of Corrosion contd Galvanic Corrosion:

Galvanic corrosion occurs in the presence of an electrolyte such as seawater when dissimilar types of metals join together. Most metals have different electrical potentials. When connected electrically and placed in an electrolyte, the more active metal becomes the anode because it has more negative potential and corrodes faster than if it were alone in the environment.

Basic Types of Corrosion contd Galvanic Corrosion contd:

The more noble (less active) metal becomes the cathode because it has more positive potential and corrodes at a slower rate than if it were alone in the environment. Electrical current flows between the metals until their potentials are equal. Galvanic corrosion typically appears in joints where the two dissimilar metals meet.

Basic Types of Corrosion contd Galvanic corrosion contd: Two different metals/alloys that are in close proximity in an

electrolytic environment

Distinct tendencies toward oxidation



nM =

nM+ +ne-


N+ N+

M+ M+

nN+ + ne- = NN+ N+



Metal 1

Metal 2

Common in orthopaedics Modular implants

Titanium femoral stems coupled with CoCr heads

Basic Types of Corrosion contd

Galvanic Corrosion

Basic Types of Corrosion contd Pitting Corrosion

Pitting corrosion causes damage by randomly attacking a limited section of the metal's surface, leaving behind holes that are larger in depth than width.

The "pit" that forms functions as the anode and the metal that is left undamaged functions as the cathode.The corrosion process starts with a chemical breakdown in a small spot such as a scratch or a nick, usually occurring under a surface coating that has experienced wear or damage.

Basic Types of Corrosion contd Pitting Corrosion contd:As the pitting progresses, the metal becomes thinner, and fatigue and stress corrosion cracking begins at the base of the corrosion pits. The failure can be catastrophic. For example, a gas line that was positioned over a sewer line killed 215 people in Guadalajara, Mexico, because of a single pit that had formed in the gas line.

Basic Types of Corrosion contd Pitting Corrosion contd: Subset of Crevice Corrosion

Formation of pits: local thickness reduction Difficult to detectO2 O2 OHClH+ OHClH+ O2 OH-

M+M+ H+ Cl-

M+H+ Cl-


Basic Types of Corrosion contd

Pitting Corrosion

Basic Types of Corrosion contd Intergranular Corrosion Preferential attack along grain


Results from localized differences in chemistry

Common in SS, nickel some Al alloys

Sensitive Regions precipitates

Corrosive Films Corrosive film is a formation on the surface of a materialdue to acidic substances and environments such as, Seawater, carbon steel rust and Iron Chloride (FeCl3). Corrosive films will deteriorate the material on a molecular

level, spreading through the material homogenously at a fast or slow rate depending on the type of material. Eg. Stainless steel isnt easily corroded, even in a coastal

atmosphere. But when it is contaminated by alien pollutants such as carbon steel rust or Iron Chloride (FeCl3) in combination with seawater, corrosive films begin to develop quicker.

Formation Of Corrosive Films The rate of development ofcorrosive films depends on the acidity of the contaminants and the resistance of the material to corrosion. In the figure on the right,

surface film formation on stainless steel contaminated by alien pollutants (a) clean bare surface after 60 days exposure; (b) carbon steel rust + seawater; (c) FeCl3 + seawater

Electrochemical Corrosion Electrochemical corrosion is a process through which a

metal returns to its lowest energy oxidation state. Some metals are found in nature in a metallic form already,

such as gold, and therefore are already stable in metallic form, which is why gold doesnt corrode. Unstable metals become stable throughout the years by

electrochemical corrosion if left unprotected.

Electrochemical Corrosion In order for electrochemical corrosion to take place a

corrosion cell must be present. A corrosion cell is a combination of four elements; an

anode, a cathode, an electrolyte and a metallic pathway connecting the anode and cathode. Without any of these four elements corrosion will not take

place, all elements must be present

Electrochemical Corrosion The anode is where metal loss takes place. By ionizing into the electrolyte the ions release electrons

through an elaborate series of chemical reactions producing ferric oxide (Fe2O3) and oxygen. Ferric oxide is Rust. The cathode is an important element of the corrosion cell

as it determines the rate of corrosion.

Electrochemical Corrosion

Electrons from the anode travel to the cathode and are accommodated there.The more electrons a cathode can accommodate the faster the anode will corrode.

This is the primary relationship between the anode and the cathode in a corrosion cell.

The Electrolyte is also an important element of the corrosion cell as it is the solution that surrounds the anode and the cathode.

Electrochemical Corrosion The electrolyte also has an effect over corrosion rate

since the conductivity of the electrolyte will either allow electrons to freely move from the anode to the cathode or restrict their flow, thus reducing corrosion rate. The metallic pathway is referred to as the internal

circuit as it is provided by the metal where the anode and cathode reside. The metallic pathway closes the corrosion circuit.

Electrochemical Corrosion Oxygen is considered to be an important factor to

corrosion since without oxygen corrosion slows down and eventually stops. ions that are released at the cathode due to the electron accommodation.

Oxygen is responsible for reacting with the hydrogen

In the lack of oxygen, hydrogen ions accumulate on

the cathode and prevent electron accommodation, thus stopping the corrosion process. This phenomenon is called polarization

Preventing Corrosion Corrosion of metals can be prevented if the contact between the

metal and air is cut off. Typically, corrosion can be prevented if the metal is coated with

something which does not allow moisture and oxygen to react with it. Coating of metals with paint, oil, grease or varnish prevents the

corrosion of metals. The coating of corrosive metals with non-corrosive metals also

prevents corrosion.

Corrosion prevention

Preventing Corrosion Coating to keep out air and water.

Galvanizing - Putting on a zinc coat Has a lower reduction potential, so it is more easily

oxidized. Alloying with metals that form oxide coats. Cathodic Protection - Attaching large pieces of an active metal like magnesium that get oxidized instead.

Corrosion Prevention Methods Methods of Corrosion Prevention are varied, and are

applied in several industries. Galvanizing: This is the process of giving a thin coating

of zinc on iron or steel to protect them from corrosion.

Iron is galvanized by dipping it in molten zinc. It is then taken out and allowed to cool. Galvanizing is an effective methods of protecting steel because even if the surface is scratched, the zinc still protects the underlying layer.

Corrosion Prevention Methods Tinning: This is the process of giving a coating of tin, i.e.,

molten tin.

Cooking vessels, made of copper and brass get a greenish coating due to corrosion.

This greenish coating is poisonous. Therefore they are given acoating of tin to prevent corrosion.

Electroplating: In this method of a metal is covered with

another metal using electrolysis.

Silver-plated spoons, gold-plated jewellery, etc., are electroplated.

Corrosion Prevention Methods Anodizing: In this method metals like copper and aluminium are

electrically coated with a thin strong film of their oxides.

This film protects the metals from corrosion.

We have seen that the oxide layer on the surface of aluminium metal

restricts further reaction of the metal with other chemicals. The corrosion resistance and hardness of this layer can be further increased

by a process known as anodising. In this process, the oxide layer is made about 1000 times thicker by

connecting the metal to the anode in an electrolytic cell with sulfuric acid as the electrolyte. The thick oxide film that forms is easily dyed to give an attractive


Corrosion Prevention Methods Alloying: Corrosion can be also prevented by alloying some metals

with other metals.

The resultant metals, called alloys, do not corrode easily, e.g. stainless steel.

Cathodic Protection: Another way to protect metals from

corrosion is to force the metal to behave as cathode in an electrochemical cell.

In cathodic protection, a second, more reactive metal is placed in electrical contact with the metal object being protected from corrosion. The more reactive metal behaves as the anode in the electrochemical cell, thus forcing the other metal to function as the cathode

Corrosion Prevention MethodsIt is not practical to galvanize large structures such as ships,

bridges and pipelines, but instead rusting can be minimized by cathodic protection.A large block of an active metal, such as zinc or magnesium, is

connected to the structure to be protected.It becomes the anode in a cell and supplies the electrons to

reduce oxygen.The active metal therefore corrodes, but is cheaper to replace

than the structure it protects.

Corrosion Prevention Methods Anodic Protection: In anodic protection the metal

is intentionally oxidized under carefully controlled conditions to form a thin, adhering layer of oxide on the surface of the metal. Treating iron with aqueous sodium chromate forms

a layer of Fe(III) and Cr(III) oxides that protects the iron from contact with oxygen and water:2Fe(s) + 2Na2CrO4(aq) + 2H2O Fe2O3(s) + Cr2O3(s) + 4NaOH(aq)

Anodic & Cathodic Reactions

Underground corrosion

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs, by which time considerable damage may be done.

Electronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections. Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits. These resistors form part of a radar installation.

Corrosion influenced by flow-1

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped. The high velocities in the pump accentuated the corrosion damage.

Corrosion influenced by flow 2

This is a bend in a copper pipe-work cooling system. Water flowed around the bend and then became turbulent at a roughly cut edge. Downstream of this edge two dark corrosion pits may be seen, and one pit is revealed in section.

Safety of aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet. Any failure of a structural component of an aircraft can lead to the most serious results.

Influence of corrosion on value

A very slight amount of corrosion may not interfere with the usefulness of an article, but can affect its commercial value. At the points where these scissors were held into their plastic case some surface corrosion has occurred which would mean that the shop would have to sell them at a reduced price.

Motor vehicle corrosion and safety

The safety problems associated with corrosion of motor vehicles is illustrated by the holes around the filler pipe of this petrol tank. The danger of petrol leakage is obvious. Mud and dirt thrown up from the road can retain salt and water for prolonged periods, forming a corrosive poultice.

Corrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel. This ship has suffered severe damage in the areas which are most buffeted by waves, where the protective coating of paint has been largely removed by mechanical action.

Aluminium CorrosionThe current trend for aluminium vehicles is not without problems. This aluminium alloy chassis member shows very advanced corrosion due to contact with road salt from gritting operations or use in coastal / beach regions.

Damage due to pressure of expanding rustThe iron reinforcing rods in this garden fence post have been set too close to the surface of the concrete. A small amount of corrosion leads to bulky rust formation which exerts a pressure and causes the concrete to crack. For structural engineering applications all reinforcing metal should be covered by 50 to 75 mm of concrete.

Corrosion of plasticsNot only metals suffer corrosion effects. This dished end of a vessel is made of glass fibre reinforced PVC. Due to internal stresses and an aggressive environment it has suffered environmental stress cracking.

Rainwater Guttering

This rainwater guttering is made of aluminium and would normally resist corrosion well. Someone tied a copper aerial wire around it, and the localised bimetallic cell led to a knife-cut effect.

Aluminium RimThis polished Aluminium rim was left over Christmas with road salt and mud on the rim. Galvanic corrosion has started between the chromium plated brass spoke nipple and the aluminium rim.

BicycleGalvanic corrosion can be even worse underneath the tyre in bicycles used all winter. Here the corrosion is so advanced it has penetrated the rim thickness.

Aircrafts hydraulic system

The tubing, shown here was part of an aircrafts hydraulic system. The material is an aluminium alloy and to prevent bimetallic galvanic corrosion due to contact with the copper alloy retaining nut this was cadmium plated. The plating was not applied to an adequate thickness and pitting corrosion resulted.