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J. Suresh Kumar Engineering Chemistry www.sureshchem.weebly.com

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SCIENCE OF CORROSION

Definition, Causes and Effects of Corrosion - Theories of Corrosion (Chemical and Electro

Chemical Corrosion) - Mechanism of Electro Chemical Corrosion (Oxygen Absorption

Type and Hydrogen Evolution Type) - Types of Corrosion (Galvanic Corrosion, Differential

Aeration Corrosion, Water Line Corrosion, Pitting Corrosion and Stress corrosion) -

Galvanic Series - Factors Effecting Rate of Corrosion (Nature of Metal and Nature of

Environment). Proper Designing - Modifying the Environment - Cathodic Protection

(Sacrificial Anodic and Impressed Current)


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UNIT –III: SCIENCE OF CORROSION

Definition: “Any process of deterioration or destruction and consequent loss of a solid metallic material,

through in chemical or electro chemical attack by its environment, starting at its surface is called

corrosion. Thus, corrosion is reverse of extraction of metals or metallurgy.

Corrosion Metal + oxide Metal oxide

(Higher Energy) Metallurgy (Lower Energy) Examples:

1. The most familiar example of corrosion is rusting of iron (Fe2O3 .3H2O)

2. Formation of green film of basic carbonate on the surface of copper [CuCO3 + Cu(OH)2]

Causes of Corrosion:

Metal exit nature in the form of oxides, sulphides, sulphates and carbonates. These chemically

combined states of metals known as mineral or ore has low energy and thermodynamically stable state

for the metal. A considerable amount of energy is required for extraction of metal from it ore. The

extracted metal has high energy and thermodynamically unstable state. Thus it is the naturally tendency

of a metal to back to the thermodynamically stable state. Metal do this by interacting chemically or

electrochemically with their environment is known as corrosion.

Effects of Corrosion: Effects of corrosion briefly given below.

1. Loss of useful properties of metals and thus loss of efficiency.

2. Decreasing production rate

3. Efficiency of machines decreases by corrosion

4. Increase in maintenance and production cost.

5. Contamination products

Types of Mechanism of Corrosion: The mechanism of corrosion classify as two types

1. Dry (or) Chemical Corrosion: This type of corrosion occurs mainly through the direct

chemical action of environment/atmospheric gases such as-oxygen, halogen, hydrogen sulphide, sulphur

dioxide, nitrogen or anhydrous inorganic liquid with metal surfaces immediate proximity.

Oxidation Corrosion: It is brought about by the direct action of oxygen on metals usually in the absence

of moisture. Alkali metals (Li, Na, K, Rb, etc.) and alkaline-earths (Be, Ca, Sr, etc.) are even rapidly

oxidized at low temperatures. At high temperatures, almost all (except Ag, Au, and Pt) are oxidized.

The reactions in the oxidation corrosion are:

M M2+ + 2e- (Loss of electrons) ½ O2 + 2e- O2- (Gain of electrons)

M + ½O2 M2+ + O2- (OR) MO (Formation of metal oxide)


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Mechanism: Oxidation occurs first at the surface of the metal and the resulting metal scale forms a

barrier that tends to restrict further oxidation. For oxidation to continue either metal must diffuse

outwards through the scale to the surface or the oxygen must diffuse inwards through the scale to the

underlying metal. Both transfers occur, but the outward diffusion metal is generally much more rapid

than the inward diffusion of oxygen, since the metal ion appreciably smaller than the oxygen ion and

consequently of much higher mobility. Nature of the oxide formed plays an important part in oxidation

corrosion process. A layer is called film, when its thickness is less than about 300 Ǻ and it s called scale,

when its thickness exceeds this value. The following types of films are there:

1. Stable:

A stable layer is fine grained in structure and can get adhered tightly to the parent metal surface.

Hence, such a layer can be of impermeable nature. Such a film behaves as protective coating in

nature thereby protecting the surface.

The oxide films on Al, Sn, Pb and Cu etc are stable film.

2. Unstable:

The oxide layer formed decomposes back into the metal and oxygen.

Metal oxide Metal + Oxygen.

Ag, Au and Pt metals form unstable film.

The film is unstable therefore do not undergo oxidation corrosion in these metals.

3. Volatile:

The oxide layer volatilizes as soon as it is formed.

Thereby leaving the original metal surface exposed for further attack.

This cause rapid and continuous corrosion, leading to excessive corrosion.

Molybdenum oxide (MoO3) is volatile.

2Mo + 3O2 2MoO3 (Volatile)


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4. Porous:

Having pores or cracks in the film or scale.

In such a case, the atmospheric oxygen has entrée to the original surface of metal through the

pores or cracks of the film.

Thereby the corrosion continues till the entire metal is completely converted into its oxide.

Alkali, alkaline earth metals and iron form porous films.

PILLING – BEDWORTH RULE: It states that an oxide layer is compact, non-porous as well as protective

preventing corrosion if the volume of metallic oxide is equal to or greater in volume to the metal

surface. The alkali metals like Li, Na, K and alkaline earth metals like Mg and metals like Fe produce

oxide film whose volume is less than the volume of the metal and as a result oxygen can diffuse through

the pores of the film producing oxide films continuously. This principle also explained according to

specific volume ratio.

1. Smaller the specific volume ratio greater the corrosion.

2. Volume of oxide ≥ volume of metal, thus non-porous film and protective film.

3. Volume of oxide < volume of metal, thus porous film and non protective film.

CORROSION BY OTHER GASES LIKE SO2, CO2, Cl2, H2, F2 etc:

1. AgCl film resulting from the attack of Cl2 on Ag. This film protects the metal from further attack.

2Ag + Cl2 2AgCl (Stable Film)

2. Cl2 gas attacks on tin (Sn) forming volatile SnCl4. This film doesn’t protect metal from further attack.

Sn + 2Cl2 SnCl4 (Volatile Film)

3. H2S at high temperature attacks steel forming a FeS scale, which is porous scale and don’t protect

metal from further attack.

LIQUID METAL CORROSION:

Liquid metal corrosion occurs when liquid metal is allowed to flow over solid metal at high

temperature. It leads to weakening of solid metal due to (i) its dissolution in liquid metal or (ii)

penetration of liquid metal into solid metal. For example, coolant (sodium metal) leads to corrosion of

cadmium in nuclear reactor.


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WET OR ELECTROCHEMICAL CORROSION:

This type of corrosion occurs, where a conducting liquid is in contact with metal or when two

dissimilar metals or alloys are either immersed or dipped partially in a solution.

The formation of anodic and cathodic areas or parts in contact with each other.

Presence a conducting medium.

Corrosion takes place at anodic areas only.

Formation of corrosion product somewhere between anodic and cathodic areas.

Mechanism of wet or electrochemical corrosion:

Electrochemical corrosion involves flow of electron-current between the anodic and cathodic.

The anodic reaction involves in dissolution of metal as corresponding metallic ions the liberation of free

electrons. A cathode reaction consumes electrons with either by two ways:

1. Evolution of Hydrogen Type Corrosion (In acidic environments): Considering metal like Fe, the anodic

reaction is dissolution of iron as ferrous ions with the liberation of electrons.

At Anode: Fe Fe2+ + 2e- ----------------------------- (Oxidation) At cathode: 2H+ + 2e- H2 --------------------------------- (Reduction)

½ O2 + 2e- + H2O 2 OH- Fe2+ + 2 OH- Fe (OH) 2

4 Fe (OH) 2 + O2 + 2H2O 4 Fe (OH) 3

2 Fe (OH) 3 Fe2O3.xH2O (Yellow rust) + (3-x) H2O (Or)

3 Fe (OH) 2+½ O2 Fe3O4 (Black rust) + 3H2O

Thus, this type of corrosion causes “displacement of

hydrogen ions from the acidic solution by metal ions.”

Consequently, all metals above hydrogen in the

electrochemical series have a tendency to get dissolved in

acidic solution with simultaneous

2. Absorption of Oxygen Type Corrosion (In neutral aqueous

solution): The surface of iron is, usually coated with at thin

film of iron oxide. However, if this iron oxide film develops

some cracks, anodic areas are created on the surface; while

the well-metal parts act as cathodes. It follows that the anodic

areas are small surface parts; while nearly the rest of the

surface of the metal forms large cathodes.


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At Anode: Fe Fe2+ + 2e- ------- (Oxidation) At cathode: ½ O2 + 2e- + H2O 2 OH-

Fe2+ + 2 OH- Fe (OH) 2

4 Fe (OH) 2 + O2 + 2H2O 4 Fe (OH) 3

2 Fe (OH) 3 Fe2O3.xH2O (Yellow rust) + (3-x) H2O (Or) 3 Fe (OH) 2+½ O2 Fe3O4 (Black rust) + 3H2O

Differences between Chemical and Electro Chemical Corrosion

S.NO Chemical Corrosion Electro Chemical Corrosion

1 It occurs in dry condition It occurs in wet condition (presence of moisture or electrolyte)

2 It involves direct chemical attack of the metal by environment

It involves setting up of a large number of galvanic cells

3 It is explained by absorption mechanism It is explained by mechanism of electrochemical reactions

4 It occurs in both homogeneous and heterogeneous surfaces

It occurs only on heterogeneous surface

5 Corrosion is uniform Corrosion is not uniform

6 It is a slow process It is a fast process

7 Corrosion product accumulate at the same place where corrosion occurs

Corrosion occurs at the anode but products accumulate near cathode

Types of Electro Chemical Corrosion:

1. Galvanic (or bimetallic) corrosion: When two dissimilar

metals (e.g., zinc and copper) are electrically connected and

exposed to an electrolyte, the metal higher in electrochemical

series undergoes corrosion. This type of corrosion is called

“galvanic corrosion”. In the above example, zinc (higher in

electrochemical series) forms the anode and is attacked and

gets dissolved; whereas copper (lower in electrochemical series

or nobler) acts as cathode.

Mechanism of galvanic corrosion: In acidic solution, the corrosion by hydrogen evolution type; while in

neutral or alkaline solution, the corrosion by oxygen absorption type occurs.

At anode: Zn Zn2+ + 2e- ----------------------------- (Oxidation) At cathode: 2H+ + 2e- H2 --------------------------------- (Reduction)

½ O2 + 2e- + H2O 2 OH-


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2. Concentration Cell Corrosion: The following types of

corrosions come under this heading:

a. Differential aeration corrosion:

The most common type of concentration cell corrosion

occurs when one part of metal is exposed to a different air

con cent ration from the other part. This causes a

difference in potential between differently aerated areas.

It has been found experimentally that “poor-oxygenated parts are anodic”. Consequently, a differential

aeration of metal causes a flow of current, called the differential current.

b. Waterline Corrosion: When water is stored in a

steel tank, it is generally found that the maximum amount

of corrosion takes place along a line just beneath the level

of the water meniscus. The area above the waterline

(highly-oxygenated) acts as the cathodic and is completely

unaffected by corrosion. The problem of waterline

corrosion is also that concerns marine engineers. In the case of ships, this kind of corrosion is often

accelerated by marine plants attaching themselves to the sides of ships. The uses of special anti fouling

paints restrict this to some extent.

3. Stress Cell Corrosion : Stress cells are formed between the stressed areas and the non-stressed areas

of metals. The stressed areas serve as anode due to higher energy and non-stressed areas serve as

cathode due to lower energy. Thus, corrosion occurs at the stressed areas of the metal. This type of

corrosion is observed in fabricated articles of certain alloys due to the presence of stresses caused by

rolling, drawing, welding and thermal treatment etc.,

In a nail, the head and the point are stressed areas (anode) which undergo corrosion.

In a bend pipe, the bend point is stressed area (anode) which undergoes corrosion.


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c. Pitting Corrosion: Pitting corrosion is a localized accelerated attack, resulting in the formation of

cavities around which the metal is relatively

unattached. Thus, pitting corrosion results in the

formation of pinholes, pits and cavities in the

metal. Pitting is, usually, the result of the

breakdown or cracking of the protective film on a

metal at specific points. This gives rise to the

formation of small anodic and large cathodic areas.

GALVANIC SERIES:

Electrochemical series give very useful information regarding chemical activity of metals. It did

not provide sufficient information regarding corrosion behavior of metals and alloys in environmental

conditions. Hence, oxidation potentials of various metals and alloys measured by using standard

electrodes immersing the metals and alloys in sea water. Those electrode potentials are arranged in the

decreasing order. This series is called galvanic series which give more practical information about the

corrosion tendency of various metals and alloys. In this series from top to bottom corrosion tendency

decreased i.e. top metals are anodic nature and bottom metals are cathodic nature. Thus it is clear that

corrosion occurs at anode part and cathode part to be protected.

FACTORS INFLUENCING CORROSION: The rate and extent of corrosion, depends on the following

factors:

1. Nature of the metal:

(i) Position in galvanic series: When two metals or alloys are in electrical contact, in presence of an

electrolyte, the more active metal (or higher up in the series) undergo corrosion. The rate of corrosion


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depends upon the difference in their positions, and greater is the difference the faster is the corrosion

of the anodic metal.

(ii) Relative areas of the anodic and cathodic parts: When two dissimilar metals or alloys are in

contact, the corrosion of the anodic part is directly proportional to the ratio of areas of the cathodic part

and the anodic part.

(iii) Purity of metal: Impurities in a metal, generally, and form Minute or tiny electrochemical cells and

the anodic parts get corroded. For example, zinc metal containing impurity (such as Pb or Fe)

undergoes corrosion of zinc, due to the formation of local electrochemical cells.

(iv) Physical state of metal: The rate of corrosion is influenced by physical state of the metal .The

smaller the grain-size of the metal or alloy, the greater will be its solubility and hence, greater will be its

corrosion.

(v) Nature of surface film: The ratio of the volumes of the metal oxide to the metal is known as a

“specific volume ratio.” Greater the specific volume ratio, lesser is the oxidation corrosion rate.

According to Pilling- bedworth rule the volume of oxide film is greater than metal from which metal

oxide formed, then the film is protective.

(vi) Solubility of corrosion products: In electrochemical corrosion, if the corrosion product is soluble in

the corroding medium, then corrosion proceeds at a faster rate. On the contrary, if the corrosion

product is insoluble in the medium or thereby suppressing further corrosion.

(vii) Volatility of corrosion products: If the corrosion product is volatile (MoO3), it volatilizes as soon

as it is formed, thereby leaving the underlying metal surface ex posed for flirt her attack. This causes

rapid and continuous corrosion.

(viii) Passivity of metal: Metals like Ti, Al, Cr and Ni are passive they exhibit much higher corrosion

resistance, due to the formation of highly protective on the metal. Moreover the film is self healing

nature. Thus corrosion resistance of stainless steel is due to passive character of Cr.

2. Nature of the corroding environment:

(i) Temperature: With increase of temperature of environment, the reaction as well as diffusion rate

increase, thereby corrosion rate is generally enhanced.

(ii) Humidity of air: “Critical humidity” is defined as the relative humidity above which the atmospheric

corrosion rate of metal increases sharply. The reason why corrosion of a metal becomes foster in humid


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atmosphere is that gases (CO2, O2, etc.) and vapors, present in atmosphere furnish water to the

electrolyte, essential for setting up an electrochemical corrosion cell.

(iii) Presence of impurities in atmosphere: Atmosphere, in the vicinity of industrial areas, contains

corrosive gases like CO2, H2SO4 and fumes of HCI, H2 etc. in presence of these gases, the acidity of the

liquid, adjacent to the metal surfaces, increases and its electrical conductivity also increases. This

consequently, results in an increase of corrosion.

(iv) Influence of pH: Generally, acidic media (i.e., pH <7) are more corrosive than alkaline and neutral

media.

(v) Nature of ions present: Presence of anions like silicate in the medium leads to the formation of

insoluble reaction products (e.g., silica gel), which inhibit further corrosion. On the other hand,

chloride ions, if present in the medium, destroy the protective and passive surface film.

Corrosion Control or Protection against Corrosion: Some of the corrosion control methods are

described as follows:

1. Proper designing: The design of metal should be such that corrosion even if it occurs is uniform and

does not result in intense and localized corrosion. Important design principles are:

Avoid the contact of dissimilar metals in corroding environment.

When contact two dissimilar metals, anode should be large are.

They should be as close as possible in electrochemical series.

Insulating fitting of two dissimilar metals.

Anode should not be painted or coated.

Welding rather than bolting.

Avoid Sharpe corners i.e., Smooth corner rather than sharp corners.

2. Using high purity of metal.

3. Using metal alloys.

4. Modifying the Environment:

The corrosive nature of the environment can be reduced either: (i) by the removal of harmful

constituents, or (ii) by the addition of specific substances, which neutralize the effect of corrosive

constituents of the environment.

Deaeration: In oxygen concentration type of corrosion, elimination of oxygen from aqueous

environment reduces metal corrosion. The method also reduces the CO2 content of water,

thereby decreasing the corrosion rate.

Deactivation: It involves the addition of chemicals, capable of combining rapidly with the oxygen


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in aqueous solution. For example, sodium sulphite (Na2S03).

2 Na2S03 + 02 -------------> 2 Na2S04

Hydrazine is useful over the sodium sulphite, because the reaction products are

N2 (g) and water.

N2H4+ 02 ------------->N2 + 2H20

Dehumidification: It reduces the moisture content of air to such an extent that the amount of

water reduced on metal is too small to cause corrosion. Alumina or silica gels, which adsorbs

moisture preferentially on their surfaces.

Alkaline neutralization: It is prevention of corrosion by neutralizing the acidic character of

corrosive environment due to the presence of acids like H2SO4, HCl, CO2, S02, etc. by alkali like NH3

NaOH,, lime,etc.) are, generally, injected either in vapour or liquid form to the corroding to its

parts.

5. Use of Inhibitors: A corrosion inhibitor is "a substance which when added in small quantities to

the aqueous corrosive environment effectively decreases the corrosion of a metal”. Inhibitors are

two types:

Anodic inhibitors: chromates, phosphates, tungstates or other ions of transition elements with

high oxygen content are those that suppress the corrosion reaction. They are adsorbed on the

metal surface, forming a protective film, thereby reducing the corrosion rate.

Cathodic inhibitors: Corrosion may be reduced by slowing down the diffusion of hydrated H+ ions

to cathode. The diffusion of H+ ions is considerably decreased by organic inhibitors like amines,

heterocyclic nitrogen compounds, substituted ureas and thioureas, heavy metal soaps, Antimony

and arsenic oxides.

6. Cathodic protection: The principle involved in this method is to force the metal to be protected to

behave like a cathode, thereby corrosion does not occur. There are two types of cathodic

protections.

(i) Sacrificial anodic protection method: In this protection method, the metallic structure (to be

protected) is connected by a wire to a more anodic

metal, so that all the corrosion is concentrated at

this more active metal. The more active metal itself

gets corroded slowly; while the parent structure

(cathodic) is protected. The more active metal so-

employed is called “sacrificial anode”. The corroded


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sacrificial anode block is replaced by a fresh one, when consumed completely. Metals commonly

employed as sacrificial anodes are magnesium, zinc, aluminum and their alloys. Important applications

of sacrificial anodic method include protection of buried pipelines, underground cables; marine

structures, ship-hulls, water-tanks, piers, etc.

(ii) Impressed current cathodic protection: In this method, an impressed current is applied in

opposite direction to nullify the corrosion current, and convert

the corroding metal from anode to cathode. Usually, the

impressed current is derived from a direct current source (like

battery or rectifier on a.c. line) with an insoluble anode (like

graphite, high silica iron, scrap iron, stainless steel or

platinum). This type of cathodic protection has been applied

to open water-box coolers, water-tanks, buried oil or water

pipes, condensers, transmission line towers, marine pier, laid-

up ships, etc.

Important Questions:

1. Describe dry corrosion. Explain the mechanism of dry corrosion with neat diagram and relevant

equations.

2. Describe Filling – Bedworth rule. Write its significance.

3. Distinguish dry and wet corrosion

4. Explain the mechanism of oxygen absorption corrosion with relevant chemical equations and

diagram.

5. Explain the mechanism of rusting of iron in acidic environment with relevant chemical equations.

6. Write the principle and process of corrosion control by sacrificial anodic protection with neat

diagram.

7. Write the principle and process of corrosion control by impressed current cathode protection with

neat diagram.

8. Explain how galvanic series is useful in assessment and control of galvanic corrosion.

9. Explain how the nature of metal and environmental factors influencing corrosion.

10. Write a brief notes on i) Galvanic corrosion ii) Concentration Cell Corrosion.

11. Write a brief notes on i) Stress corrosion ii) Pitting corrosion.

12. Discuss the role of inhibitors in corrosion control.

13. Illustrate with the aid of label diagrams that show how a (i) Magnesium Bar and (ii) D.C Electrical

power supply could be used to prevent or at least decrease the extent of corrosion of a steel

underground pipeline used for carrying gases.

SCIENCE OF CORROSION - SuReSh ChEmsureshchem.weebly.com/uploads/1/4/2/7/14275226/ec-unit-iii_2017-18.… · SCIENCE OF CORROSION ... preventing corrosion if the volume of metallic

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