develop under deposits that settle in the bottoms of pipes and
tanks or under material that may be splashed onto a surface.
Pitting corrosion will also occur at holidays (holes) in
Metals that rely on oxide films for their corrosion
resistance (stainless steels and aluminum) are prone to
developing pitting corrosion. In WWTPs, stainless steel or
aluminum slide gates, handrails, or other components often
develop pits, which initiate under a deposit. If chlorides are
present in the wastewater (possible in WWTPs near sea-coast
installations), they will tend to accelerate breakdown of the
oxide film and lead to pitting corrosion. The use of Type 316
stainless steel instead of Type 304 will provide improved
resistance to chloride-initiated pitting corrosion.
Intergranular Corrosion. Intergranular corrosion
is not usually encountered in the environments typically found in
WWTPs. Under certain conditions, the corrosion experienced will
be concentrated at the grain boundaries of the metal rather than
uniformly across the surface. Grain boundaries most commonly
become more sensitive to corrosion because of welding during
fabrication. For this reason, intergranular corrosion may also
be called weld decay, weld attack, weld corrosion, or corrosion
of the heat-affect zone.
Intergranular corrosion often occurs with stainless
steels, which develop sensitized areas around the welds because
of depletion of one of the alloying elements in the stainless
steel. The use of the low-carbon grade of stainless steel
(i.e., Type 304L or Type 316L) will usually control this problem.
Other methods include heat-treatment after fabrication or with
the use of stabilized stainless steels. Intergranular corrosion
usually occurs in very aggressive environments that are not
commonly found in WWTPs.
Selective Leaching. Selective leaching is the
removal of one element from a solid alloy by the corrosion
process. The results are unusual because the corroded metal may
not show any obvious signs of corrosion. Brass alloys are
composed primarily of zinc and copper. Selective corrosion of
alloy to take on a red, coppery color in contrast to the yellow
color of the brass. The result is a weak, porous, and brittle
material. Brasses containing more than 85 percent copper are
resistant to this form of corrosion.