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.
(ii) Absorption of Oxygen [ If electrolyte is neutral or alkaline aqueous solution, corrosion takes place by absorption of O2 rusting of iron in water containing dissolved oxygen occurs by oxygen absorption mechanism. At anodic area iron will dissolve by oxidation. The surface of iron is usually coated with the thin film of iron oxide. But if this iron oxide film develops some cracks, anodic areas are created on the surface ; while the metal (iron) act as cathodes. Here the anodic areas are small surface while rest of the surface of the metal forms large cathodes. At anode, Fe →Fe2+→2e-
The electrons flow to cathodic area through and will be accepted by O2. At Cathode Fe2+ +2OH→Fe (OH)2 If enough O2 is present, ferrous hydroxide easily oxidizes to ferric hydroxide 2Fe (OH) 2 + ½ O2 +H2O→2Fe(OH)3 Ferric hydroxide rust This product called yellow rust, which is nothing but Fe2O3.H2O If O2 is limited, the corrosion product will be black unhydrous magnetite, Fe3O4. If environment is aqueous solution of NaCl containing dissolved O2 NaCl →H2O→Na+ + Cl- At cathode Na+ + OH→NaOH At anode Fe2+ + 2Cl→FeCl2 Ferrous Chloride
Both the products NaOH, FeCl2 are soluble in water they react with each other and ferrous hydroxide precipitates which further oxidizes to ferric hydroxide Fe(OH)3. 4 Fe (OH)2 + 2H2O + O2 →4Fe(OH)3 -
Fe2O3. H2O yellow rust