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Nature of the metal
1)Corrosion directly proportional to smaller size:smaller the grain size of a metal/alloy➡greater solubility. Hence, corrosion rate will be higher
2)Grain orientation at metal/alloy surface: if G-O is linear, regular and uniform, corrosion rate is high
3)Porosity: If porosity is high then corrosion rate is also high
thus, corrosion directly proportional to porosity
4) Stress: areas of metals/alloys under stress become anodic and have higher corrosion rate. thus,....
5)Position in Galvanic Series: if 2 different metals are in corrosive environment, metal having higher position in the galvanic series acts as an anode and undergoes more corrosion. Thus, corrosion rate directly higher position.

Nature of the Corrosion product
1) Solubility: if corrosion product is soluble in the corroding medium, corrosion rate higher
2)Nature of Oxide film: same as role of oxide...

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Stress corrosion is characterized by highly localized attack occuring when overall corrosion is negligible.
Stress corrosion is the combined effect of static tensile stress and corrosive environment.
Pure metals are relatively immune to stress corrosion
It can be minimized by: i) suitable treatment
ii) removing critical environment.
iii) selecting better resistant material.

Consider an example of steam boiler
Boilers need soft water for which Na2CO3 is added to ordinary hard water
Na2CO3 undergoes hydrolysis at high temperature of the boiler to produce NaOH and CO2
reaction: Na2CO3 ➡(in presence of H2O) 2NaOH+CO2

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This type of corrosion occurs along grain boundaries, where the material is sensitive to corrosion attack. Further, the material is more anodic as compared to that at the center.
The grain boundary is always attacked leaving the interior or it is slightly attacked.
The intergranular corrosion is generally found in alloys. Eg: During welding of stainless steel.
It occurs microscopically and internally, hence, it is dangerous. Sudden failure of the material occurs without any previous symptom due to loss of cohesion between the grains.
The remedy is proper heat treatment before rapid cooling.

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It is localized attack, non uniform and takes place by electrochemical reaction resulting in formation of a pit.
Pitting corrosion is caused due to roughness of the surface, scratches, impurities, chemical attack, sliding under the load, etc.
When small particles of dust, dirt or scales is deposited on the metal surface, the area under them becomes less oxygenated(anode) and the balance, uncovered and more oxygenated area becomes cathode. Thus, Concentration cells are set up
In presence of moisture, containing dissolved O2, Corrosion underneath the impurity forms a small shallow depression in the metal surface called pits
Reaction:
Anode: Fe ➡ Fe²⁺ + 2e⁻
Cathode: 1/2O₂ + H₂0 + 2e⁻ ➡ 2OH⁻
Overall reaction: Fe + 1/2O₂+H₂O ➡ Fe²⁺ +2OH⁻

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1) For soluble corrosion product, rate is faster, since the product is an electrolyte.
2) For insoluble corrosion product, rate is slightly slower since the product may get deposited over the cathodic area.
3)When cathodic area>anodic area, cathode requires more electrons. Therefore, anode loses electrons faster, increasing the rate of corrosion
4) When anodic area> cathodic area, electrons released by anode are accepted slowly. Therefore, the rate of corrosion is decreased.

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Concentration cell corrosion occurs when a metal is subjected to different concentrations of air at its different areas. This causes potential difference between different areas
Consider a zinc rod partially dipped in ZnSO4 solution
Immersed part of Zn is less oxygenated, hence act as anode and undergoes corrosion.
The part of rod above the solution is more oxygenated, hence acts as cathode and remains unaffected.
Reaction:
Anode: Zn ➡ Zn²+ + 2e⁻
Cathode: 1/2O₂+ H₂O +2e- ➡ 2OH⁻
Overall reaction: Zn + 1/2O₂ + H2O ➡ Zn²⁺ + 2OH⁻

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When two different metals are electrically connected and placed in an electrolyte, the metal which occupies higher position in the electrochemical series undergoes corrosion known as galvanic cell corrosion.

Consider Zn and Cu plates are electrically connected and immersed in an electrolytic solution.
Zn being higher in the electrochemical series, acts as an anode and is attacked while Cu being lower in the electrochemical series, acts as Cathode and is protected
The electron current flows from anode to cathode as shown in fig
Anode: Zn ➡ Zn^2+ + 2e^-
Cathode is protected
Eg: Use of steel pipes in Cu storage tanks
Control of moisture

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In case of electrochemical corrosion, absorption of oxygen occurs metal is in contact with aqueous acidic/ alkaline/ neutral/ electrolytic solution, and this causes corrosion.
Electrochemical corrosion with absorption of oxygen also occurs when two different metals or alloys are immersed or dipped partially in aqueous solutions.

Consider an iron tank whose inner surface is coated with a thin film of Fe2O3.
If this film develops cracks, anode areas are created on the surface, and the metal acts as cathode
Reactions:
Anode: Fe ➡ Fe^2+ + 2e^-
Cathode: 1/2O2 +H2O + 2e^- ➡ 2OH^-
Fe^2+ + 2OH^- ➡Fe(OH)2

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this type of corrosion occurs in acidic medium/ environment.
consider an iron tank containing some acid.

Reactions:
At anode: Fe ➡ Fe^2+ +2e^-
At cathode: 2H + 2e^- ➡ H2
Overall Reaction: Fe + 2H^+ ➡  Fe^2+ + H2

Iron tank acts as an anode and undergoes corrosion as Fe atoms from the tank pass into the acidic solution as Fe^2+.
Free electrons accumulate at cathode, Hydrogen ions present in the acidic solution take up these electrons forming hydrogen gas.
Example: Iron tank or vessel containing acid
Iron pipelines carrying acidic industrial waste.

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This type of corrosion occurs under following 2 conditions
i) when the metal surface is in immediate contact with aqueous acidic/alkaline/neutral/electrolytic solution forming the short circuited galvanic cells
ii) when 2 different metals or alloys are immersed or dipped partially in aqueous solution

The corrosion is due to the formation of anodic and cathodic areas, between which the current flows through the conducting solution

The anodic reaction involves dissolution of metals as corresponding metal ions liberate free electrons.

M ➡ Mn+ + ne⁻

Conversely, Cathodic reaction consumes electrons by either evolution of H2 or by absorption of O2 depending upon the nature of the corrosive environment. This is the mechanism of wet/electrochemical corrosion

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1)Cl2: In case of silver, due to action of Cl2, silver chloride film is formed which is unstable and protects Ag from further corrosion
2Ag + Cl2 ➡ 2AgCl
But when, cl2 attacks tin, stanic chloride film is formed which is volatile, and hence volatizes as soon as it is formed, causing more metal to corrode due to Cl2 attack
Sn + 2Cl2 ➡SnCl4

2)SO2: When fossil fuel is burned, SO2 is formed which gets deposited on metallic surface. In presence of O2 and moisture, SO2 is oxidized too sulphuric and sulphurous acids which are highly corrosive to metallic parts
S+O2 ➡SO2
SO2 + H2O + 1/2 O2➡H2SO4

3)H2S: In an industrial environment, all types of contaminants by sulphur in the form of SO2 and H2S are corrosive. For Eg: In petroleum industry, H2S. at high temperature, corrodes iron
Fe + H2S ➡ FeS +H2

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Role of Oxide formed by oxidation corrosion

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Role of Oxide formed by oxidation corrosion

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During oxidation, a thin layer of oxide film is formed on the metal surface which can be divided into 3 parts
1)Stable: If the oxide film formed is stable, it acts as a protective coating to the metal surface. There are two types of stable coating:
i)Porous-If the oxide film formed on metal surface is porous, the rate of further corrosion is not reduced much. Eg. Iron oxide
ii)Non-Porous: if the oxide film formed on the metaal surface is non porous, the rate of reduction reduces considerably and in most cases, corrosion stops. For example, Al2O3,Cr2O3
2)Unstable: If the oxide film formed is unstable, it decomposes on the metal surface back to forming metal and oxygen. For eg: Oxides of noble metals.
Metal +O2 ➡ Metal Oxide ➡ Metal + O2
3)Volatile: If the oxide film formed is volatile, the oxide film volatizes as soon as it formed, thus exposing underlying metal surface for further attack

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This type of corrosion occurs due to ATTACK of atmospheric oxygen on metals at either low or high temperatures, usually in absence of moisture forming metal oxides as follows:
Metal + O2 ➡Metal oxide
Normally, more active metals get corroded faster than less active metals. For example, Alkali and Alkaline earth metals get oxidized even at low temperatures as compared to metals which are less active.
At high temperatures, almost all metals except Pt, Au, Ag are oxidized

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The corrosion occuring through direct chemical action of atmospheric gases like oxygen, sulphur dioxide, hydrogen sulphide, halogens etc. with metal or alloy surface.

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It is the process of deterioration and consequent loss of a solid metallic material through an unwanted chemical or an electrochemical attack by its environment, starting at its surface.