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ഀ Uniform Corrosion. ഀ ഀഀ ഀഀ
If a metal is not immune to attackഀ
and corrosion cannot be completely eliminated, uniform ഀ
corrosion is considered theഀ
form of corrosion that can be tolerated in marine structures and equipment. It isഀ
also relatively easy to control uniform corrosion to acceptable levels through judiciousഀ
selection of materials, the application of corrosion control measures, and to allowഀ
for any corrosion which does occur. ഀ ഀഀ ഀഀ Definition. ഀ ഀഀ ഀഀ
Uniform corrosion is the attackഀ
of a metal at essentially the same at all exposed areas of its surface. At no pointഀ
is the penetration of the metal by corrosion twice as great as the average rate. ഀ ഀഀ ഀഀ Mechanism. ഀ ഀഀ ഀഀ
Uniform corrosionഀ
occurs whenഀ
there are local anodic and cathodic sites on the surface of the metal. Due to polarizationഀ
effects, these locations shift from time to time and a given area on a metal willഀ
be act as both an anode and as a cathode over any extended period of time. The averagingഀ
effect of these shifting local action cells results in a rather uniform attack andഀ
general loss of material and roughening of the surface. ഀ ഀഀ ഀഀ Examples. ഀഀ ഀഀ
ഀ
Rusting steel in the atmosphereഀ
and the corrosion of copper alloys in seawater are common examples where uniformഀ
corrosion is usually encountered. Steel submerged in seawater can also corrode uniformlyഀ
but can also suffer from non-uniform attack under some circumstances. ഀ ഀഀ ഀഀ Appearance. ഀഀ ഀഀ
In uniform corrosion,ഀ
the metalഀ
loss occurs at essentially the same rate over the entire metal surface. Smooth surfacesഀ
are usually roughened during uniform corrosion. This ഀ
form of corrosion is characterizedഀ
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 corrodingഀ
surfaces but these can be removed by velocity, by mechanical action or by otherഀ
mechanisms. ഀ ഀഀ ഀഀ Significant Measurements. ഀ ഀഀ ഀഀ
Weight loss is the most commonlyഀ
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. Theഀ
amount of metal loss as measured by the weight loss is used to calculate the lossഀ
in thickness of the metal assuming that the corrosion was absolutely uniform. Inഀ
some cases this is further verified by thickness measurements. These results areഀ
commonly expressed in “Mils perYear” or “Microns per Year.” It must be remembered thatഀ
these rates are usually calculated from weight loss rather than thickness loss andഀ
are only valid if the attack was uniform. The maximum error of this measurementഀ
is theoretically a factor of two if the rule that attack can be no greater thanഀ
twice the average rate at any given point is properly applied. ഀ ഀഀ ഀ ഀ ഀ Source : "Corrosion Control" NAVFAC MO-307ഀ September 1992 ഀ | ഀ
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