Environmental Engineering Reference
In-Depth Information
Figure 3.25
Graphitic corrosion of gray cast iron showing the residual skeleton of
graphite flakes and phosphide eutectic.
but this should be avoided because the term refers to graphite formation through
breakdown of Fe
3
C in steels on prolonged heating.
3.7.4 Practical Examples
The micrograph of dezincified area experienced in an alloy 260 (70-30 brass)
pipe for domestic water supply is shown in Fig. 3.26 [2]. Area A shows plug-type
attack on the nickel-chromium-plated exterior surface where untreated municipal
water supply leaking from the faucet packing gland ran down the pipe and at-
tacked the brass through a break in the plating caused by mechanical damage.
The attack on the interior surface of the pipe was a fairly uniform layer-type
dezincification. It may be noted that in the area shown only about one-third of
the original wall has remained as sound metal.
Figure 3.27 shows the fragmented parts of an aluminum bronze seawater pump
impeller that were retrieved from the casing of the pump [9]. The impeller was
missing; apparently, it had disintegrated in service. Sectioning of the fragments
revealed the porous structure caused by dealuminification from both sides. Weak-
ening of the impeller through dealloying was established as the cause of failure.
Solution heat treatment minimizes dealuminification, but this particular impeller
was in as-cast condition.
3.8 EROSION CORROSION
The term
erosion
applies to deterioration due to mechanical factors. When the
factors contributing to erosion accelerate the rate of corrosion of a metal, the
attack is called erosion corrosion. A corrosive fluid, aqueous or gaseous, flowing
