Question

Explain four factors affecting rate of immersed corrosion. 

Answer 1

A) Nature of metal:

1) Position of metal in a galvanic series: A metal having higher position in a galvanic series has more chemical reactivity and therefore, it gets attacked by gaseous and corroding medium faster. In the series the noble metals are at the bottom whereas the alkali metals are at the top.

2) Purity of the metal: - Impurities present in a metal cause heterogeneity and forms a large no. of tiny galvanic cells when an aqueous medium comes in contact with such metal if the impurity metal is highly placed in a galvanic series then it acts as a anode and gets corroded to produce small depressions on the surface of the base metal. If the metal is pure it is corrosion resistant.

3) Physical state of the metal:-The physical state of metal means orientation of crystals, grain size, stress The larger grain size of the metal the smaller will be its solubility and hence lesser will be its corrosion.eg :- mild steel grains are smaller than cast iron grains therefore mild steel gets corrodes faster. Areas under stress tend to be anodic and corrosion takes place at these stressed areas.

4) Solubility of the corrosion products:-Insoluble corrosion products function as a physical barrier thereby suppresses further corrosion. But if the corrosion product is soluble in the corroding medium the corrosion of the metal proceeds faster.

B) Nature of Environment:- 

1) Effect of PH:-Acidic media are more corrosive than alkaline and neutral media. e.g. corrosion of Zn can be minimized by increasing the pH to 11

2) Differential aeration: Corrosion occurs where oxygen access is least. eg :- When pipeline passes through moist soil as well as dry soil the part passing through moist soil having restricted oxygen access becomes anodic while the part passing through dry soil having more access of air becomes cathodic. This causes corrosion of pipe embedded in moist soil. 

3) Presence of impurities in the atmosphere:- Corrosion of metals is more in industrial areas because corrosive gases like H2S, SO2 , CO2 and fumes of H2SO4 and HCl in industrial areas increases conductivity of the liquid layer in contact with the metal surface thereby increases the rate of corrosion.

4) Humidity:- The greater the humidity greater is the rate and extent of corrosion. Moisture dissolves the atmospheric gases or chemical vapours and the reaction between such dissolved gases with metallic surface become faster. Hence water can acts as a conducting medium and promotes corrosion. e.g. Rusting of Fe is promoted in humid atmosphere.