The corrosion by the mechanism of oxygen absorption can occur if
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This type of corrosion occurs in following possibilities. (1) Where a conducting liquid is in contact with metal OR (2) When two dissimilar metals or alloys are either immersed or ‘dipped partially in a solution. The corrosion is due to the formation of ANODIC and CATHODIC area between which current flows through the conducting solution. Graphite block pipeline Cathodic protection by immersed current in burried pipeline The Anodic Reaction involves dissolution of metal as corresponding metallic ions with the liberation of free electrons. At Anode : M M→n+ + ne-(oxidation) On the contrary, the cathodic reaction consumes electrons with either by (i) Evolution of Hydrogen or (ii) Absorption of oxygen, depending upon the nature of corrosive environment. When two different metals are in contact with a common conducting liquid(medium), metal which has higher oxidation potential acts as an anode and It is this metal which undergoes corrosion. Second metal with lower oxidation potential acts as a cathode and it is this metal this remains protected. [i.e. Anode undergoes corrosion and cathode remains protected]. This is also called as Galvanic cell corrosion. This type of electrochemical corrosion takes place in two different ways. (i) With evolution of hydrogen (ii) With absorption of oxygen. Evolution of Hydrogen Electrochemical corrosion with evolution of H2 occurs in acidic Environment At anode Fe →Fe2+ + 2e-(oxidation /corrosion) At cathode 2H+ + + 2e-→H2 Overall reaction Fe +2H+ →Fe Explanation (1) Iron tank which acts as an anode undergoes corrosion as Fe atoms from the tank pass into the acidic solution as Fe++ ions as shown in reaction above. Since nothing but loss of Fe atom as Fe++ ions i.e. corrosion. (2) Free electrons accumulate at cathode Hydrogen ions present in acidic solution take up these electrons forming H2 gas as shown in the reaction above. H2 gas liberates in the form of bubbles near the cathode. Thus, hydrogen evolution type of corrosion is nothing but displacement of H+ from acidic solution by metal ions. All the metals above H2 in the electro chemical series get dissolved in acidic solution with simultaneous evolution of hydrogen. In hydrogen evolution type of corrosion anode h as a large area (like metallic tank) and cathode has a smaller area.