Environmental Engineering Reference
In-Depth Information
Figure 3.26
Micrograph showing difference in dezincification of inside and outside
surfaces of a plated 70-30 brass pipe for domestic water supply. (A) Plug-type attack of
the outside surface. (B) Layer-type attack of the inside surface.
over the metal surface or impinging on it provides the usual condition for erosion
corrosion. However, the mechanical deterioration may be aggravated by sheer
presence of a corrodent as in the case of fretting corrosion or corrosive wear.
Sometimes movement of environment decreases the rate of attack, e.g., pitting
of stainless steels in flowing seawater as compared to pitting in stagnant seawater,
but this is not erosion corrosion because the attack is not increased.
The attack takes the form of grooves, i.e., scooped-out, rounded areas, horse-
shoe-shaped depressions, gullies, waves, all of which often showing directional-
ity. Sometimes the attack appears as an assembly of pits. Ultimate perforation
due to thinning or progress of pits and rupture due to the failure of the thinned-
down wall to sustain the internal fluid pressure are also common. All sorts of
equipment exposed to flowing fluid are subject to erosion corrosion among which
piping systems and heat exchangers are most common.
3.8.1 Environmental Factors
The environmental factors that affect erosion corrosion are: velocity, turbulence,
impingement, presence of suspended solids, temperature, and prevailing cavita-
tion conditions. The acceleration of attack is due to destruction or removal of
the protective surface film by mechanical forces exposing fresh metal surfaces
that are anodic to the uneroded neighboring film. The nature of the surface film
is important. A hard, dense, adherent, and continuous film such as on stainless
