Uniform Corrosion

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਍ഀ Uniform Corrosion. ਍ഀ

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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.

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਍ഀ Definition. ਍ഀ

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਍ഀ 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.

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਍ഀ Mechanism. ਍ഀ

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਍ഀ Uniform corrosion਍ഀ occurs when਍ഀ there are local anodic and cathodic sites on the surface of the metal. 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.

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਍ഀ Examples.

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਍ഀ ਍ഀ 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 can also corrode uniformly਍ഀ but can also suffer from non-uniform attack under some circumstances.

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਍ഀ Appearance.

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਍ഀ 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,਍ഀ which will be described later.

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

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਍ഀ Significant Measurements. ਍ഀ

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਍ഀ 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.

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