Uniform Corrosion

Uniform corrosion is a generalized corrosive attack that occurs over a large area on the surface of a material. It is only dependent upon the material’s composition and the environment. The result is a thinning of the material until failure occurs. Uniform corrosion rates are fairly predictable, following an exponential relationship as follows.

where,
p –corrosion rate
t – exposure time
A, B – constants, dependent upon material and environment

The decrease in corrosion rate with time is a direct result of an oxide scale layer forming on the metal’s surface, which then deters further corrosion from occurring. There are extreme cases however, where the corrosivity of the environment is severe and prevents an oxide layer from forming. In this case, the corrosion rate will be constant with time. Figure 6 depicts this relationship for uniform corrosion. Equation 3 may be used to predict the long-term corrosion damage from short-term tests. There are some problems with this prediction however. Environments usually change over time so that corrosion rates will deviate from those predicted by the equation. Also, the development of additional forms of corrosion will likely accelerate attack in localized areas.

where
t – thickness loss in mils/yr
w – weight loss in mg.
ρ – density in g/cm³
A – area of exposure in inches²
T – exposure time in hrs.