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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.
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.
Uniform corrosion occurs when there are local anodic and cathodic sites on the surface of the metal (see Galvanic Corrosion). Due to polarization effects, these locations shift from time to time and a given area on a metal will be act as both an anode and as a cathode over any extended period of time. The averaging effect of these shifting local action cells results in a rather uniform attack and general loss of material and roughening of the surface.
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.
In uniform corrosion, the metal loss occurs at essentially the same rate over the entire metal surface. Smooth surfaces are usually roughened during uniform corrosion. This form of corrosion is characterized by the lack of any significant non-uniform attack such as pitting or crevice corrosion.
Corrosion products commonly remain on uniformly corroding surfaces but these can be removed by velocity, by mechanical action or by other mechanisms.
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.
Source : "Corrosion Control" NAVFAC MO-307 September 1992
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