Question

Explain how the following factors affect the rate pf corrosion 

i) pH

ii) Ratio of anode to cathode areas. 

iii) Position of metal in galvanic series.

Answer 1

pH
In acidic pH the rate of corrosion is higher, because the mechanism of electrochemical corrosion proceeds by evolution of hydrogen gas at cathode. On the contrary, in alkaline or neutral electrolytic medium the electrochemical corrosion occurs by following the mechanism of absorption of oxygen, thereby forming as oxide film as a cathodic product. Such a film gets adhered to the surface of the metal and further rate of corrosion is governed by nature of corrosion product which is discussed as above 

Ratio of anode to cathode areas.

1. If two dissimilar metals are in contact one forming anode while another cathode, then the corrosion of the anodic metal is directly proportional to the ratio of the areas occupied by cathode and anode.
Thus, corrosion at anode
α = area of cathodic part / area of anodic part
2. Hence, if cathode is large and anode is small, then corrosion at anode is higher and vice – versa.

Position of metal in galvanic series.

This is the major factor for corrosion of metals are in corroding environment, the metal having higher electrode potential in the galvanic series undergoes corrosion, i.e. it act as a anode.