Unit 2- Class 5- CORROSION (Factors related to corrosive environment)

Factors related to corrosive environment:

1.    pH of the medium:  Usually higher acidic nature (low pH) higher is the rate of corrosion. If the pH is greater than 10 corrosion of iron is very less due to the formation of protective coating of hydrous oxides of iron.
 If pH is between 10 and 3, then presence of oxygen is essential for corrosion   of iron. If the pH is 3 or lower than 3, severe corrosion occurs in the absence of air due to the continuous evolution of H2 at cathode. However metals like Al, Zn etc undergo fast corrosion in highly alkaline medium.

2.    Temperature: On increasing the temp. Rate of corrosion process also gets increases because on increase of temp. Conductance of the aqueous medium increases hence rate of diffusion also.
In some cases on rise in temp. Decrease the passivity, which again leads to increase in the corrosion rate.

3.    Polarisation at anodic and cathodic area: Polarisation of cathode or anode decreases the rate of corrosion. If anodic polarization takes place due to some reaction, then tendency of metal to undergo oxidation decreases hence dissolution of metals as metal ion decreases. This is usually due to increase in conc. Of ions of the dissolved metals in the vicinity of electrode or also due to the anodic passivity. Cathode polarization decreases the cathodic reaction hence hindering the combination of cathode reactant and electron. For the corrosion to continue both anodic and cathodic reaction should take place simultaneously if any one reaction is slower then the rate of corrosion is slower. Use of depolarizers reduces the polarization effect hence the rate of corrosion reaction increases.