A method for calculating the extent of galvanic corrosion and cathodic protection in metal tubes assuming unidirectional current flow

Abstract

A simple procedure for calculating how galvanic corrosion and cathodic protection effects are distributed within metallic tubular systems carrying electrolytes is described. The procedure involves the use of equations which are analytical solutions and, in some instances, equations which require numerical solutions which can easily be effected using microcomputers. The equations arc based on an assumption of unidirectional flow of electric current which is shown to be valid for appreciably conducting electrolytes, such as seawater. Various combinations of linear polarisation kinetics, Tafel law polarisation kinetics and totally diffusion-controlled kinetics can be used. Experimental verification of the procedure has been obtained using synthetic seawater for titanium and Cu-30Ni tubes in galvanic contact and for cathodically protected Cu-30Ni tube. It is proposed that the procedure could find useful application at the design stage of plant involving two or three different metals, or of cathodically protected pipework, when any necessary preventative measures indicated by an analysis could be adopted. Plant containing two or three metals can be, for example, in the form of heat-exchangers, pipe to plate combinations or pipework systems connected by plastic sections. Useful design data relating to cathodically-protected pipework can be obtained for systems based on either sacrificial protection or impressed current. © 1983.