Dry corrosion Mechanism: When a metal is exposed to air it gets oxidized by loosing its valence electrons & reduction of oxygen take place M+n/4 (o^2) => M^n+ + O^2- At point of contact of m^n+ & o^2- metallic oxide will form & that metallic oxide scale forms a barrier to restrict further oxidation of inside metal . Since size of cation (M^n+) is smaller than anion . Hence cation will diffuse much faster than anion through the scale for continuation of oxidation , it can be possible if the metallic oxide barrier is sufficiently porous . The nature of oxide film plays very important role in oxidation corrosion. a. When oxide film is stable and tightly adhering , it will act as protective coating and corrosion is prevented. b. When oxide film is unstable and has tendency to decompose back to metal and oxygen , it does not go into oxidation corrosion. When film is volatile then metal surface again come into contact with air and oxidation take place d. If film is sufficiently porous then continuous oxidation take place. 2. Liquid metal corrosion - When a liquid metal is allowed to flow over solid metal at high temperature is called liquid metal corrosion. Due to this solid metal gets weak. Example- In nuclear reactor sodium metal is used as coolant & it leads to corrosion of cadmium.
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Mechanism of wet corrosion: wet corrosion is a two step process. One is anodic or oxidation reaction and the other is cathodic or reduction process.
1) anodic reaction involves dissolution of metal
[M→Mn+ + ne- ] the anode are absorbed at the cathode.
2) There are different cathodic reactions in which the electrons are consumed depending upon the nature (acidic / basic / neutral) of the corrosion environment.
i) Hydrogen evolution type wet corrosion: it occurs in the acidic environment containing no oxygen or very less oxygen.
ii) Oxygen absorption type wet corrosion.: it occurs when the environment is alkaline / basic or neutral, and contains more oxygen, OH- ions will be given out.