Define corrosion. Explain the effect of the following factors on the rate of corrosion:

Position of metals in galvanic series

a) Relative area of the anodic and cathodic part.

b) PH of the medium

c) Temperature

d) Hydrogen overvoltage

e) Nature of corrosion product

f) Nature of surface film

Definition: Decay, destruction or deterioration of material (metal or nonmetal) due to unwanted chemical reaction with gases which Present in the environment or due to electrochemical reactions, starting at its surface
a. Position of metals in electrochemical series: Metals which are chemically very active they are present above hydrogen behave like anode & undergo corrosion due to oxidation reaction. Metals which are present above hydrogen posses negative electrode potential. Metals which are chemically inactive behave like cathode & protected from corrosion as they show positive electrode potential. More the difference in electrode potential value, higher will be rate of corrosion & vice versa Electrochemical Series

b.Cathodic & anodic area: If catholic area is small & anodic area is large & requirement of electron is very less which can be easily satisfied by large anode & corrosion is not very serious. On the contrary if anodic area is small & cathodic area is large so requirement of electron is larger which can not satisfied by small anode then corrosion is very serious which is beyond control

c.pH: In acidic pH the rate of corrosion is higher because mechanism of electrochemical corrosion proceeds by evolution of hydrogen gas at cathode. Whereas in case of neutral & alkaline electrolytic solution the corrosion takes place by formation of film over the surface & rate of corrosion depends upon nature of the film. amphoteric metals like Al, Zn and Pb dissolves in alkaline solution and undergo corrosion

d. Temperature: Higher the temp higher is the rate of corrosion because diffusion of gases increases with temp & hydrogen evolution type corrosion increases with increases in temp & O2 absorption type corrosion decreases because rate of polarization & solubility of O2 decreases with temperature

e.Hydrogen over voltage: When metal occupies higher position in electrochemical series for example Zn if placed in 0.1 N H2SO4 undergo corrosion by forming a film & evolving hydrogen gas. Initial rate of corrosion is very slow, because of high overvoltage (=0.70 volts) of Zn metal, If a drop of copper sulphate are added the corrosion rate of Zn is accelerated, because some copper get deposited on the Zn metal forming minute cathodes, Where the hydrogen overvoltage is 0.33 volts similarly corrosion rate become still faster if few drops of platinic chloride are added. This is because overvoltage of platinized plated spot is only 0.2 volts. Thus higher the overvoltage of impurity lower will be the rate of corrosion & lower the hydrogen voltage higher will be the rate of corrosion.

f. Solubility of corrosion product: In electrochemical corrosion if electrode is soluble in its electrolytic solution the rate of corrosion is faster. But if it is insoluble in its electrolytic solution then metal is protected from corrosion. E.g. Pb with H2SO4. It results in the formation of PbSO4 which acts as barrier there by suppressing further attack

g. Nature of the film:.Monovalent metals like Li, Na, K react with Oxygen & results in the formation of Li2O, Na2O, and K2O. Here of Oxygen is less than volume of Metal. The film developed over the surface is stable porous. Corrosion is moderate, as volume continues thro of oxygen is insufficient to cover the surface. Corrosion continues through cracks, Pores & portion which is free from cracks & pores is protected. Bivalent metals like Ba, Sr, Ca results in the formation of BaO, SrO & CaO In these cases also corrosion is moderate as film developed is porous stable. Trivalent metals like Al, Ti, V & Cr react with O2 & results in the formation of Al2O3, Ti2O3, and V2O3 & Cr2O3. Here these metals develop stable nonporous film & corrosion is minimum or negligible. For trivalent metals volume of oxygen is more than volume of metals. Gold & platinum develop unstable film with O2 & Corrosion is either minimum or negligible Here gold reacts with metal & results in the formation of its oxide & immediately it is separated in to metal & .oxygen. In short this is reversible reaction.

Molybdenum reacts with oxygen which results in the formation molybdenum oxide film which is developed over the surface. The film developed over surface is volatile & corrosion is maximum Here oxygen is completely separated from metal during evaporation process & underneath metal surface is for entry of impurities.

Volatile film – maximum corrosion Unstable film- negligible or minimum corrosion Stable nonporous –minimum corrosion Stable porous- moderate corrosion