very small in area compared with the steel, they become badly corroded in comparison with the action
on the steel.
e. Intergranular. This corrosion is the result of the metallic grain boundaries and the grain
particles creating a cell in an electrolyte, such as a corrosive solution or atmosphere. This type of
corrosion is an attack on the metal's basic grain structure. A highly magnified metal cross section shows
its composition is made up of a number of tiny crystals or grains. Each of these tiny grains has a clearly
defined boundary, and each grain differs chemically from the one in the center of the metal. The
adjacent grains of different elements react with each other as anodes and cathodes when they are in
contact with an electrolyte. The early stages of this corrosion cannot be detected by normal visual
inspections. Some of the stainless steels are prone to intergranular corrosion if they are heated. This is
because corrosion may begin when heat from welding causes chromium carbides to collect at the grain
boundaries.
f. Exfoliation. This corrosion is the visible evidence of intergranular corrosion. It shows itself
by metal surface grains lifting up and lifting is caused by the force of expanding corrosion occurring at
the grain boundaries just below the metal's surface. This corrosion is most often seen on rough-finished
metal surfaces. The rougher, more strained, and less uniform a metal's surface, the sooner corrosion
starts and the more localized the corrosion develops.
g. Concentration Cell (Differential Environmental). This corrosion occurs when several areas
of a metal's surface are in contact with different concentrations of the same electrolyte. Corrosion
results from a difference in the composition of the electrolyte and from the difference in the
concentrations. Both conditions cause metal corrosion. An example of differential-environmental
corrosion is shown in Figure 5-7.
EN0562
5-8
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