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Steel Designers' Manual - 6th Edition (2003)
1032 Corrosion and corrosion prevention
However, after a period of time, polarization effects such as the growth of corrosion
products on the surface cause the corrosion process to be stifled. New, reactive
anodic sites may then be formed, thereby allowing further corrosion. Over long
periods the loss of metal is reasonably uniform over the surface and so this case is
usually described as general corrosion.
35.1.3 Other forms of corrosion
Various types of localized corrosion can also occur:
(1) Pitting corrosion. In some circumstances the attack on the original anodic area
is not stifled and continues deep into the metal, forming a corrosion pit. Pitting
more often occurs with mild steels immersed in water or buried in soil rather
than those exposed in air.
(2) Crevice corrosion. Crevices can be formed by design-detailing, welding, surface
debris, etc. Available oxygen in the crevice is quickly used by the corrosion
process and, because of limited access, cannot be replaced. The entrance to the
crevice becomes catholic, since it can satisfy the oxygen-demanding cathode
reaction. The tip of the crevice becomes a localized anode, and high corrosion
rates occur at this point.
(3) Bimetallic corrosion. When two dissimilar metals are joined together in an
electrolyte an electrical current passes between them and corrosion occurs on
the anodic metal. Some metals (e.g. nickel and copper) cause steel to corrode
preferentially whereas other metals corrode preferentially themselves, thereby
protecting the steel. The tendency of dissimilar metals to bimetallic corrosion
is partly dependent upon their respective positions in the galvanic series
(Table 35.1): the further apart the two metals are in the series the greater the
tendency. Other aspects which influence bimetallic corrosion are the nature of
the electrolyte and the respective surface areas of the anodic and cathodic
metals. Bimetallic corrosion is most serious for immersed or buried structures
but in less aggressive environments, e.g. stainless steel brick support angles
attached to mild steel structural sections, the effect on the mild steel sections is
practically minimal and no special precautions are required.
Further guidance for the avoidance of bimetallic corrosion can be found in BS
PD 6484, Commentary on corrosion at bimetallic contacts and its alleviation.
35.1.4 Corrosion rates
The principal factors that determine the rate of corrosion of steel in air are: