Environmental Engineering Reference
In-Depth Information
Figure 3.30
Schematic representation of sequence in cavitation damage.
mium alloys, and nickel-chromium-molybdenum alloys are useful under ex-
tremely severe conditions. Nickel-copper, copper-nickel, and other copper alloys
in general perform better than carbon and low-alloy steels. Hard facing of the
surface with a resistant alloy is beneficial. In an opposite approach, resilient coat-
ings of rubber and other elastomers have been successfully used to combat cavita-
tion damage. They essentially reflect the shock waves and reduce the damage on
metals. Aeration of water or hydrogen produced on cathodic protection serves
to cushion the shock waves. As smooth surfaces do not provide sites for bubble
nucleation, smooth finishes on pump impeller or ship propellers are advocated.
Damage can be reduced by operating rotary pumps at the highest possible head
of pressure to avoid bubble nucleation. In closed systems like the cooling water
in diesel engine cylinder liners, use of appropriate inhibitors like the addition of
2000 ppm sodium chromate has proved effective.
3.8.5 Corrosive Wear and Fretting Corrosion
Wear is a surface phenomenon that occurs by displacement and detachment of
materials. Corrosive wear refers to the aggravation by corrosion of the wear pro-
duced by the hard projection of a mating surface or with hard particles moving
relative to the wearing surface. The chemical reaction may take place first fol-
lowed by the removal of corrosion products by mechanical abrasion. Mechanical
action may also precede chemical action in which the small particles dislodged
by abrasion subsequently react with the environment. In both cases, the wear rate
enhances.
