4. Forms of Corrosion

4.4 Pitting Corrosion [1/3]

Pitting Corrosion

It is a localized attack on a metal surface which forms holes or pits. It can be very deep and penetrate through a sheet without much warning or prior indication.

Shapes:

Pitting corrosion occurs in various shapes. They can be small, large, circular, hexagonal or irregular shaped. Puts usually grow downward in the direction of gravity and most pits form on top of a horizontal surface.

Following examples show different pit morphologies.

1. Pits on a 314 Stainless Steel coupon exposed to H2SO4 solution containing Ferric Chloride.

2. Pits along a grain boundary of an Inconel autoclave casing.

3. Pits in an 8-inch aluminum pipe exposed to moist saline alkaline soil.

4. Various morphologies of pits in aluminum.

(a) pitting on surface

(b) pitting on edge

(c) underpinning

(d) cubic pitting

Section of an oil pipeline showing pitting and general corrosion.