27.14.7Chemical attack

Corrosive chemicals may attack the metal components of control valves if those components are not carefully selected for the proper service. A close-up photograph of a chemically-pitted valve plug shows pitting characteristic of chemical attack:

As mentioned previously in this chapter, the e ects of corrosion are multiplied when combined with the e ects of cavitation. Most metals develop what is known as a passivation layer in response to chemical attack. The outer layer of metal corrodes, but the byproduct of that corrosion is a relatively inert compound acting to shield the rest of the metal from further attack. Rust on steel, or aluminum oxide on aluminum, are both common examples of passivation layers in response to oxidation of the metal. When cavitation happens inside a valve, however, the extremely high pressures caused by the liquid microjets will blast away any protection a orded by the passivation layer, allowing chemical attack to begin anew. The result is rapid degradation of the valve components.