Material-Environment Interactions . This determines the nature of surface film. A continuous, hard, dense, impervious, and adherent film on a metal surface provides better protection, is not easily swept away by a flowing stream, and offers resistance to the impact of slurry. These condition occurs in corrosion-resistant materials, such as stainless steel, and more so in super stainless steels, Hastelloys, etc. Flow Characteristics .
Velocity exerts an overriding influence on the behavior pattern of the protective film.
Its influence has already been dealt with to some extent. Each system has a particular value of critical velocity below which there is no attack and above which corrosion is rapid as the protective film is swept away. Where suspensions are present, moderately high velocity may reduce corrosion by preventing deposits of slit or dirt, which can cause crevice corrosion and other fouling problems. On the other hand, a very rapid flow of hard solid suspensions has a scouring effect that destroys the protective film. The greater agitation of the processing fluid produced by this type of flow is such as to cause more intimate contact between it and the metal surface.