GENERAL CORROSIONDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 16The hydrogen normally dissolved in reactor coolant does not have any detectable direct effectupon the corrosion rate of the iron and steels exposed to reactor coolant. It does, however, havean important indirect effect by preventing the accumulation of dissolved oxygen in reactorcoolant, which would accelerate corrosion. Dissolved oxygen reacts with the protectivehydrogen gas layer at the cathode to form water.The condition and composition of the metal surfaces affects the corrosion rate. Deposits, scale,or irregular surfaces create areas on the metal where local corrosion can initiate and proceed ata faster rate than normal. Certain alloys of metals have higher corrosion resistance than others,as discussed in the Material Science Handbook.When iron or steel is exposed to high temperature water, the rate of corrosion of the metal isobserved to decrease with exposure time during the early period of exposure. After a fewthousand hours, the corrosion rate becomes relatively constant at a low value. During the earlyperiod of exposure, while the corrosion rate is decreasing, the oxide film on the surface of themetal grows in thickness. However, the rate at which the film grows decreases with time. Thethickness of the oxide film soon reaches a relatively constant value, and thereafter film thicknessdoes not change appreciably with further exposure. As might be expected, a relatively constantcorrosion rate and oxide film thickness are attained at about the same time. Because a tightlyadhering corrosion film inhibits further corrosion, great care is taken during the initial fill ofreactor plants to promote formation of the best possible corrosion film. This process, referredto as pretreatment, or pickling, involves careful control of reactor coolant water chemistry andtemperature during the pretreatment period.PreventionChemistryControlPlant chemistry is used to control corrosion. The type of corrosion determines the method usedfor preventing or minimizing the corrosion rate.PassivatorsandInhibitorsPassivation is the condition where a naturally active metal corrodes at a very low rate,probably due to an oxide coating or an absorbed layer of oxygen. Some chemicalsubstances, called passivators or inhibitors, if added to water, can provide this type ofpassivation by undergoing reduction at the metal surface. A common inhibitor ispotassium chromate.CathodicProtectionThe use of cathodic protection, supplying an external electric current to the iron so thatit acts as a cathode and has no anodic areas, is another method of preventative chemicalcontrol. This can be accomplished by the use of an external voltage source or by the useof a sacrificial anode (e.g., zinc) which will corrode and provide the current.
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