4.2.1 DEFINITION

It is the uniform thinning of a metal without any localized attack. Corrosion does not penetrate very deep inside. The most familiar example is the rusting of steel in air.

4.2.2 ENVIRONMENT

1. Dry atmosphere.

2. Damp atmosphere.

3. Wet atmosphere.

4. Acids (HCl, HClO ₄, H ₃PO ₄).

5. Atmospheric contaminants.

6. Process water containing hydrogen sulfide.

7. Brines.

8. Industrial atmosphere.

9. Hydrocarbon containing wet hydrogen sulfide.

4.2.3 EFFECT OF POLLUTANTS

Corrosion can proceed in a dry environment without any moisture if traces of sulfur compounds or H ₂S or other pollutants are present in the air. Tarnishing of silver in dry air in the presence of H ₂S traces is an example of dry corrosion. Industrial atmospheres contain SO ₂ as the major contaminant.

The rate of corrosion in the presence of SO ₂ increases in the presence of moisture. The sulfur dioxide released in the atmosphere reacts with the rust formed on the metal surface as shown in reaction(4.3). Much of the SO2 is converted to SO ₃ in the upper atmosphere. The reaction is:

The FeSO ₄ formed accelerates corrosion. Once rusting has started, corrosion cannot be stopped even after SO ₂ is removed from the air. Iron corrodes faster than any other engineering material in an industrial and marine atmosphere (Fig. 4.1). Other contaminants are nitrogen compounds, H ₂S and also dust particles. CO ₂ does not play a significant role in uniform corrosion. Sulfur compounds...