1. Engineering Chemistry (revised edition)
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2. Corrosion in Metals and Alloys
The destruction of metals or alloys by surrounding environment through chemical or electrochemical changes.
Causes 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. The most familiar example of corrosion is rusting of iron exposed to the atmospheric conditions.
Engineering Chemistry (revised edition)
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3. Reduction of value of goods
Effects of Corrosion
Loss of metal
Loss of time
Reduction of value of goods
Contamination of fluids due to chemical reaction
Changes in surface properties
Mechanical damage
Engineering Chemistry (revised edition)
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4. Corrosion Cell Engineering Chemistry (revised edition)
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5. Types of Corrosion Dry Corrosion (Direct Chemical Attack)
There are three main types of dry corrosion.
Oxidation corrosion (reaction with oxygen):
The overall reaction can be given by
Engineering Chemistry (revised edition)
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6. Mechanism for oxidation corrosion
The oxide layer formed can be
Stable
Unstable
Volatile
Porous
Engineering Chemistry (revised edition)
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7. Corrosion by other gases such as Cl2, SO2, H2S, NOx: In dry atmosphere, these gases react with metal and form corrosion products, which may be protective or non-protective. Dry Cl2 reacts with Ag and forms AgCl which is a protective layer, while SnCl4 is volatile. In petroleum industries at high temperatures, H2S attacks steel forming FeS scale which is porous and interferes with normal operations.
Liquid metal corrosion: In several industries, molten metal passes through metallic pipes and causes corrosion due to dissolution or internal penetration. For example, liquid metal mercury dissolves most metals by forming amalgams, thereby corroding them.
Engineering Chemistry (revised edition)
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8. Wet corrosion (Electrochemical theory of corrosion)
Metal comes in contact with a conducting liquid or when two dissimilar metals are immersed dipped partly in a solution.
Engineering Chemistry (revised edition)
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9. The reactions involved in the galvanic cell are as follows:
Anodic reaction: Metal undergoes oxidation (corrosion) with positive release of electrons:
Cathodic reactions: These reactions are dependent on the constituents of the corrosion medium.
If the solution is aerated and almost neutral, oxygen is reduced in presence of H2O to OH− ions.
If the solution is deaerated and almost neutral, H2 is liberated along with OH− ions.
If the solution is deaerated and acidic, H+ ions are reduced to hydrogen gas
Engineering Chemistry (revised edition)
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10. In the presence of limited oxygen, black rust is formed as follows:
The metal ions formed at the anode combine with hydroxyl ions and form the corresponding metallic hydroxide Fe(OH)2, which further gets oxidized to hydrated ferric oxide [rust].
In the presence of limited oxygen, black rust is formed as follows:
Engineering Chemistry (revised edition)
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11. Dry Corrosion Wet Corrosion
Involves direct attack of atmospheric gases on metal in the absence of moisture liquid phase
Less prevalent
The corrosive media include vapors, gases, etc.
It is of various types: Oxidation corrosion, corrosion by other gases such as Cl2, SO2, H2S, NOx and liquid metal corrosion
Wet Corrosion
Involves electrochemical attack on metals in aqueous environments
More prevalent
The corrosive media is conducting liquid (aqueous) phase
It is of three types: differential metal corrosion, differential aeration corrosion and crevice corrosion
Engineering Chemistry (revised edition)
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12. Types of Electrochemical Corrosion
Differential Metal Corrosion (Galvanic Corrosion)
When two dissimilar metals are in contact in corrosive environment, metal with lesser standard reduction potential will act as anode and will undergo oxidation (corrosion). Metal with higher standard reduction potential will act as cathode and it will provide surface for reduction of oxygen. .
Engineering Chemistry (revised edition)
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13. Buried iron pipeline connected to zinc bar.
Other examples of differential metal corrosion (galvanic corrosion) include the following:
Buried iron pipeline connected to zinc bar.
Steel pipe connected to copper plumbing.
Steel propeller shaft in bronze bearing.
Zinc coating on mild steel.
Lead–tin solder around copper wires.
Preventive Measures
Placing a thin layer of an insulator between two metals or materials.
Selecting materials having very less potential difference.
Engineering Chemistry (revised edition)
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14. Differential Aeration Corrosion (Concentration Cell Corrosion)
When metal is exposed to differential air or oxygen concentration.
Engineering Chemistry (revised edition)
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15. The examples of differential aeration corrosion include:
Half-immersed iron plate in aqueous solution.
Steel pipe carrying any liquid exposed to atmosphere.
Ocean going ships.
Steel storage tanks.
Preventive Measures
Using metallic coating, electroless plating or chemical conversion methods.
Maintaining the material’s own protective fi lm.
Controlling the chemistry of fluids and using inhibitors.
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16. Waterline corrosion: (a) Water storage tank and (b) ocean going ship.
Engineering Chemistry (revised edition)
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17. Pitting corrosion Localized attack on iron surface in chloride medium
Preventive Measures
Preparing surfaces with best possible finish. Mirror-finish resists pitting best.
Removing all contaminants, especially free-iron by passivation.
Designing and fabricating to avoid trapped and pooled liquids.
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19. Other Types of Corrosion Intergranular corrosion:
Engineering Chemistry (revised edition)
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20. 2. Soil corrosion: Underground pipes, cables, tank bottoms, etc
2. Soil corrosion: Underground pipes, cables, tank bottoms, etc., get corroded due to moisture, pH of soil, ionic species like chlorides and micro- organisms like bacteria. It is further enhanced by differential aeration of various parts of the soil.
Engineering Chemistry (revised edition)
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21. Factors Influencing Rate of Corrosion
Primary Factors
Nature of the metal
Electrode potential difference
Surface state of the metal
Hydrogen overvoltage
Formation of protective films by metals
Secondary Factors pH
Temperature
Anodic and cathodic area effect
Conductance of the medium
Humidity
Engineering Chemistry (revised edition)
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22. Small ratio of anodic to cathodic region leading to high rate of corrosion
Large ratio of anodic to cathodic region leading to low rate of corrosion.
Engineering Chemistry (revised edition)
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23. Corrosion Control Methods
Design and material selection: number of angles, corners, edges etc. should be minimized
Protective coatings: prevents surface corrosion
Engineering Chemistry (revised edition)
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24. Cathodic protection: metal at cathodic region is unprotected
Impressed current method.
Anodic protection: impressed anodic corrosion provides anodic protection
Corrosion inhibitors: Examples of anodic inhibitors include chromate, tungstate, molybdate, etc.
Engineering Chemistry (revised edition)
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25. Corrosion Corrosion: Deterioration of metal by electrochemical process
e.g. Rust on iron, due to formation of hydrated ferric oxide i.e. Fe2O3.xH2O
Tarnish on silver
Green Patina on Cu
Corrosion causes enormous damage to building, bridges, ships and cars.

26. Fundamental Process in Corrosion
Conditions for rusting:
Oxygen gas and water
Electrochemical reaction:
Anode: Metal surface
Fe(s) → Fe2+ (aq) + 2e- E0red = V
Cathode: Another region of same metal surface
O2( g) + 4 H+ + 4e- → 2H2O (E0red = 1.23V
Overall reaction:
2Fe(s) + O2 (g) + 4H+ (aq) → 2Fe2+ (aq) + 2H2O
E0cell = Ec – EA = 1.23 – (-0.44) = 1.67V
4Fe2+ (aq) +O2 (g) + (4+2x)H2O(l) → 2Fe2O3.xH2O(s) +8H+
Rust

27. Corrosion Prevention Fe → Fe2+ (aq) + 2e- E0red = -0.44
Barrier Protection: Coating with paint
Alloy: Stainless steel i.e. Alloy of Fe-Cr
Sacrificial Protection: By covering Fe(s) with more active metal. Such as Zn and Sn
e.g. Fe surface covered with Zn metal
Zn → -Zn 2+(aq) + 2e E0red = -0.76
Fe → Fe2+ (aq) + 2e E0red = -0.44
The Process of covering Fe with Zn is known as Galvanization.
Sn → Sn e E0red =
Fe → Fe2+ (aq) + 2e E0red = -0.44

28. Corrosion Prevention
Electrical/Cathodic Protection: Fe is connected with more active metal Like Mg and Zn.
e.g. Under ground water pipes
Fe(s) → Fe2+ (aq) + 2e- E0red = V
Zn (s) → -Zn2+(aq) + 2e- E0red = V
Mg (s) → Mg 2+(aq) + 2e- E0cell = V

29. Corrosion on other metals
Aluminum: Al2O3 layer will formed
Copper: CuCO3
Silver: Ag2S