CorrosionDOE-HDBK-1015/1-93CRUD AND GALVANIC CORROSIONRev. 0CH-02Page 25Crud and Galvanic Corrosion SummaryCrud is corrosion products in the form of finely divided, insoluble oxide particlessuspended in the reactor coolant or loosely adhered to metal surfaces or activatedcorrosion and wear products.Scale is the deposition on the surfaces of the piping from the formation of insolublecompounds from normally soluble salts. Most common are calcium or magnesiumcarbonates.Galvanic corrosion is the corrosion that results when two dissimilar metals withdifferent potentials are placed in electrical contact in an electrolyte.The problems of crud in reactor plants are:Fouling of coolant flow pathsFouling of heat transfer surfacesHigh general background (ambient) radiation levelsRadiation hot spotsRadioactive waste disposalGalvanic corrosion can also be limited by: 1) using only metals that are close on the activityseries (discussed in the chapter on Corrosion Theory), 2) electrical insulation of dissimilar metals,and 3) using poorly-conducting electrolytes (very pure water).The relative surface areas of the two metals are also important. A much larger surface area ofthe non-active metal, compared to the active metal, will accelerate the attack. It has beendetermined that the relative surface area is the determining factor in the corrosion rates. The required electrical current for galvanic corrosion will be stopped if the dissimilar metals are:separated by a non-conducting junction, separated from a conductive environment, andlocated in a poorly conducting electrolyte (pure water).SummaryThe important information of this chapter is summarized below.
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