Properties of MetalsDOE-HDBK-1017/1-93CORROSIONGalvanic corrosionoccurs when two dissimilar metals with different potentials are placed inelectrical contact in an electrolyte. It may also take place with one metal with heterogeneities(dissimilarities) (for example, impurity inclusions, grains of different sizes, difference incomposition of grains, or differences in mechanical stress). A difference in electrical potentialexists between the different metals and serves as the driving force for electrical current flowthrough the corrodant or electrolyte. This current results in corrosion of one of the metals. Thelarger the potential difference, the greater the probability of galvanic corrosion. Galvaniccorrosion only causes deterioration of one of the metals. The less resistant, more active onebecomes the anodic (negative) corrosion site. The stronger, more noble one is cathodic (positive)and protected. If there were no electrical contact, the two metals would be uniformly attackedby the corrosive medium. This would then be called general corrosion.For any particular medium, a list can be made arranging metals sequentially from most active,or least noble, to passive, or most noble. The galvanic series for sea water is discussed in theChemistry Fundamentals Handbook. Galvanic corrosion is of particular concern in design and material selection. Material selectionis important because different metals come into contact with each other and may form galvaniccells. Design is important to minimize differing flow conditions and resultant areas of corrosionbuildup. Loose corrosion products are important because they can be transported to the reactorcore and irradiated.In some instances, galvanic corrosion can be helpful in the plant. For example, if pieces of zincare attached to the bottom of a steel water tank, the zinc will become the anode, and it willcorrode. The steel in the tank becomes the cathode, and it will not be effected by the corrosion.This technique is known as cathodic protection. The metal to be protected is forced to becomea cathode, and it will corrode at a much slower rate than the other metal, which is used as asacrificial anode.Localized corrosionis defined as the selective removal of metal by corrosion at small areas orzones on a metal surface in contact with a corrosive environment, usually a liquid. It usuallytakes place when small local sites are attacked at a much higher rate than the rest of the originalsurface. Localized corrosion takes place when corrosion works with other destructive processessuch as stress, fatigue, erosion, and other forms of chemical attack. Localized corrosionmechanisms can cause more damage than any one of those destructive processes individually. There are many different types of localized corrosion. Pitting, stress corrosion cracking, chloridestress corrosion, caustic stress corrosion, primary side stress corrosion, heat exchanger tubedenting, wastage, and intergranular attack corrosion are discussed in detail in the ChemistryFundamentals Handbook.Rev. 0Page 33MS-02
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