Environmental Engineering Reference
In-Depth Information
Figure 3.11
Pitting attack on a type 304L stainless steel pump component.
far superior to type 304 in pitting resistance in seawater. Nickel alloys like
Hastelloy or Chlorimet are even better. Titanium and its alloys show excel-
lent pitting resistance in the most aggressive media, like those containing
FeCl
3
or CuCl
2
, and are often recommended for such applications.
3.4.5 Practical Examples
Figure 3.11 illustrates pitting attack on a pump component made of type 304L
stainless steel experienced in a medium comprising hydrochloric and nitic acid
mixtures [6]. Temperature and concentration are unknown. The damage marks
on the surface inflicted during material handling apparently provided the nucle-
ation sites for pitting. The lateral broadening of pits with time is also indicated.
Figure 3.12 shows the subsurface enlargement of one of the pits in a type 321
stainless steel aircraft freshwater storage tank that failed in service because of leak
formation as a result of pitting. The tank was sterilized with sodium hypochlorite
solution, but the rinsing was delayed for 68 h after the draining out of the solution.
Apparently, a small amount of the solution remained in the bottom of the tank
causing the damage. The tank eventually failed after only 10 h of service.
3.5 CREVICE CORROSION
A crevice is a small gap created by contact of a material with another material.
The crevice area of a metal or alloy in contact with another material, metal or
