Page 73 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 51

failure can occur in a short time (17). The resistance of metals to erosion–corrosion
damage depends on the physical and chemical properties of the products and/or the
passive layers and their adhesion to the surface. Passivating metals such as stainless
steels and titanium are relatively immune to erosion–corrosion in many oxidizing
environments.
A stainless steel pump impeller with a projected life of 2 years failed in three
weeks in a reducing solution. Soft metals such as copper and lead are readily
damaged. Even noble metals such as silver, gold, and platinum are subject to
erosion–corrosion (9, 17).

1.7.14 Impingement
This involves a liquid in turbulent flow, containing air bubbles and suspended particles
that strike the metallic surface with a strong force and destroy the protective film on
the surface. The shock of water bubbles or air against a metal surface results in wear
of the surface. The wear takes the shape of the directional progress of the attack. The
bottom part becomes anodic with respect to the adjacent outer surfaces. This mode
of attack can occur even in the absence of air bubbles or suspended particles. Typical
impingement is illustrated in Figure 1.16. Erosion–corrosion is characterized by the
appearance of grooves, gullies, waves, rounded holes, and valleys and usually shows
a directional pattern.

Figure 1.16 Erosion corrosion of a pump propeller. (Reproduced by permission, Elsevier
Ltd., (2).)