Galvanic corrosion can be defined simply as that corrosion that occurs as a result of one metal being in electrical contact with another in a conducting corrosive environment. The corrosion is stimulated by the potential difference that exists between the two metals, the more noble material acting as a cathode where some oxidizing species is reduced, the more active metal, which corrodes, acting as the anode. To fully understand this process it is first necessary to understand the basic thermodynamics and kinetics of electrochemical reactions. These are considered with particular reference to exchange current densities and the factors that control them, linear and Tafel kinetics, concentration and mass transfer effects, and mixed potential theory. The practical side of galvanic corrosion and its relationship to electrochemical parameters is considered with particular reference to the most important cathodic processes, that is, oxygen reduction and hydrogen evolution, as they occur on engineering materials, for example, steel, stainless steel, copper base alloys, and so forth and to the general form of the anodic processes that occur. Finally the use of galvanic series as a predictive tool is discussed. In addition methods used to model galvanic situations with the aim of predicting corrosion are briefly mentioned and their advantages and limitations outlined.
In order to achieve an optimum compromise of cost and performance, it is necessary to construct large multistage flash (MSF) desalination plants from a variety of materials, notably carbon steel, copper-base alloys, stainless steels, and titanium. This produces a number of situations where two or more materials are in contact, and special care is needed to ensure that serious galvanic corrosion is avoided. Examples of such bimetallic and multimetallic junctions in plants operated by the Saline Water Conversion Corporation of Saudi Arabia are titanium tube/aluminum bronze tubeplate and waterbox; carbon steel/stainless steel flash chamber linings; and copper alloy/carbon steel waterboxes and piping. Problems associated with some of these junctions in older plants and the techniques used to avoid problems in modern plants are described in the paper. It is concluded that by application of good corrosion engineering practice, serious galvanic corrosion at mixed-metal joints can be avoided.
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