Page 215 - Corrosion Engineering Principles and Practice

190 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 191

industry owe their corrosion resistance to the formation and retention
of a protective film. Protective films fall into two categories [28]:
• Relatively thick and porous diffusion barriers such as formed
on carbon steel as red rust and copper oxide on copper
• Thin invisible passive films such as formed on stainless steels,
nickel alloys, and other passive metals such as titanium.
However, if the flow of liquid becomes turbulent, the random
liquid motion impinges on the surface to remove this protective film.
Additional oxidation then occurs by reaction with the liquid. This
alternate oxidation and removal of the film will accelerate the rate of
corrosion. The resulting erosive attack may be uniform, but quite
often produces pitted areas over the surface that can result in full
perforation (Fig. 6.40).
Obviously, the presence of solid particles or gaseous bubbles in
the liquid can accentuate the attack. Also, if the fluid dynamics are
such that impingement or cavitation attack is developed, even more
severe corrosion can occur.
Chromium has proven to be most beneficial toward improving
the properties of the passive film of ferrous and nickel-based alloys
while molybdenum, when added to these alloys, improves their
pitting resistance. Oxide passive films that contain insufficient
molybdenum, such as in many nickel-based alloys and stainless
steels, are susceptible to pitting in stagnant and low-flowing seawater,
but perform well on boldly exposed surfaces at intermediate and
high flow velocities. In oilfield conditions, fluid velocity acts in

FIGURE 6.40 Erosion–corrosion of a brass tube carrying out seawater.
(Courtesy of Defence R&D Canada-Atlantic)