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Use of an assumption that electric current flows only in one direction in seawater-carrying equipment constructed of more than one metal enables rapid calculation of the corrosion rate distribution employing small-memory microcomputers. There is negligible error associated with this assumption for typical seawater systems of essentially cylindrical geometry. Analytical expressions can usually be obtained, depending upon the experimentally determined electrode kinetics. The scope of the modelling procedure is indicated by examples of various practical seawater systems that have been examined at the design stage.
The distribution of cathodic protection has been analyzed within copper-alloy pipework attached to a steel hull, or to a small sacrificial anode, and also in a mixed-metal marine propulsion unit protected by zinc anodes. The cathodic electrode kinetics involved various combinations of potential-independent current density and linearly potential-dependent current density.
The distribution of galvanic corrosion has been calculated within a heat-exchanger system consisting of a cathodic tube-bundle and tubeplate connected via a copper-alloy header to a 7-m long copper-alloy pipe. Assessment was made for linear cathodic electrode kinetics and anodic electrode kinetics of the Butler-Volmer type. Calculations were also made for when the header was electrically insulated at both of its flanges. Assessment of the heat-exchanger system has been extended to cater for the copper-alloy pipe being electrically connected to a steel hull, although physically separated by an intervening plastic pipe, and to cover the case where the header is made from a cathodic material.
galvanic corrosion, impressed current protection, cathodic protection, seawater corrosion, mathematical models, computers, BASIC (programming language), mathematical prediction, one-dimensional flow, electrode potential, cathodic polarization, anodic polarization, electric fields, differential equations, heat exchangers, condensers, steam condensers, cooling systems, tubes, metal pipes, plastic pipes, piping systems, marine propulsion, marine engineering, ship hulls, sacrificial anodes, copper alloys, nickel alloys, titanium alloys
Senior technical officer, Research and Development Department, IMI, Birmingham,