Uniform Corrosion

Uniform corrosion is the regular, uniform removal of metal from a surface. In uniform corrosion, microscopic anodic areas (where metal dissolution occur), and cathodic areas (where hydrogen evolution or oxygen reduction occur), frequently alternate. If, however, impurities are present on the metal surface, such as carbide precipitates, then corrosion can be localized around the precipitate.

In the oilfield, uniform corrosion may be observed in tubing and sucker rods, possibly following an acidizing treatment.

The rate of uniform corrosion can be calculated as shown in the example below.  Uniform corrosion is usually measured in mpy (mils per year, 1 mil = 1/1000 inch).

Example: A steel coupon of 4 x 2 x 1/8 inches is placed in an acid solution for one week, and loses 90 mg. Calculate the rate of corrosion in mpy. Assume that steel is iron only.

Surface Area = 2(4 in x 2 in) + 2(4 in x 1/8 in) + 2(2 in x 1/8 in) = 17.5 in²

90 mg Fe             x  1 cm³          x (365 days)  x 1 in   x   1000 mil
(17.5 in²)(7 days)  7870 mg Fe       1 year       2.54 cm³   1 in

= 2 mpy

The following is an example of uniform corrosion caused by CO₂.