This can happen only when the protective film is weak in tension or non-adherent, and the surface is rough. Convective mass transfer controlled corrosion can be reduced by adding high molecular weight polymers, which have the effect of decreasing mass transfer coefficient, by affecting an increase in boundary layer thickness across which the transport of corrosive species and corrosion product occurs to and from the metal surface, respectively. Surface roughness also plays an important role in convective mass transfer, both in the laminar and the turbulent flow region. In the former it increases the effective area and in the latter decreases the both the turbulence and the rate of flow required for its onset. Very small amounts of aqueous phase dispersed in an innocuous hydrocarbon phase are a common cause of corrosion problems in most petroleum and petrochemical process industries.
Corrosion occurs when the aqueous phase is transported over the metal surface.
The secondary factors that determine the nature of flow are geometry and surface roughness. 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.