Environmental Engineering Reference
In-Depth Information
Figure 6.35
Schematic diagram illustrating the three categories of protective scale
breakdown to less protective reaction product during the process.
presence of salt deposits falls into three categories [58] as depicted in Fig. 6.35.
In one of these categories, the salt is innocuous, and accordingly degradation
during the propagation stage proceeds by the mechanisms determined by the alloy
and environment composition. Such a situation is likely to occur with porous
solid deposits through which the gas can penetrate. No definite example of such
an effect is yet reported. In the second category, reactions are considered between
the elements in the alloy and constituents of the gas phase in the presence of a
liquid salt in the formation of nonprotective reaction products. The reactions that
take place under such conditions are similar to those that occur in cleaning of
metal surfaces by the use of salt baths for descaling. Accordingly, this mode of
propagation is often referred to as
salt fluxing
.
The third category involves propagation stages where a constituent of the salt
deposit either is introduced to the alloy or reacts with the alloy or its corrosion
products, so that nonprotective reaction product barriers are formed. Such propa-
