Uniform Corrosion

Uniform Corrosion.

If a metal is not immune to attack and corrosion cannot be completely eliminated, uniform corrosion is considered the form of corrosion that can be tolerated in marine structures and equipment. It is also relatively easy to control uniform corrosion to acceptable levels through judicious selection of materials, the application of corrosion control measures, and to allow for any corrosion which does occur.

Definition.

Uniform corrosion is the attack of a metal at essentially the same at all exposed areas of its surface. At no point is the penetration of the metal by corrosion twice as great as the average rate.

Mechanism.

Examples.

Rusting steel in the atmosphere and the corrosion of copper alloys in seawater are common examples where uniform corrosion is usually encountered. Steel submerged in seawater also suffer uniform corrosion but can also suffer from non-uniform attack under some circumstances.

Appearance.

Corrosion products commonly remain on uniformly corroding surfaces but these can be removed by velocity, by mechanical action or by other mechanisms.

Significant Measurements.

Weight loss is the most commonly used method of measuring the corrosion rate of metals when uniform corrosion occurs. In this method, a test sample is cleaned, weighed, and its surface area is measured. It is then exposed for a specific period of time, re-cleaned and re-weighed.

The amount of metal loss as measured by the weight loss is used to calculate the loss in thickness of the metal assuming that the corrosion was absolutely uniform. In some cases this is further verified by thickness measurements.

These results are commonly expressed in “Mils perYear” or “Microns per Year.” It must be remembered that these rates are usually calculated from weight loss rather than thickness loss and are only valid if the attack was uniform. The maximum error of this measurement is theoretically a factor of two if the rule that attack can be no greater than twice the average rate at any given point is properly applied.