Contribution to the problem of vapour-side corrosion of copper-nickel tubes in MSF distillers

Abstract

The paper describes the vapour-side corrosion (VSC) of 70 Cu30Ni condenser tubes in some MSF distillers. Corrosion is first recognized by above-normal Cu 2+ content of the distillate and, at advanced stages, by the increase in electrical conductivity of the product water. Attack takes place at the inlets of tubes in the first high temperature cells and is located mainly at the lower part of the tube bundle. Corrosion starts after a long time of operating the distiller (the induction period). A “corrosion factor” is proposed to characterize tube failure in distillers. It is the number of tubes failing per 1000 operation hours, following the induction period. Corrosion is not due to assault on tube material by CO 2 resulting from the thermal decomposition of HCO 3 − and CO 3 2− ions of seawater. Corrosion involves the primary oxidation of the metal by air leaking through flanges, glands and/or gaskets, followed by dissolution in CO 2-loaded vapours condensing on the colder ends of the tubes. Analysis of the distillate reveals the presence of large quantities of O 2 in the vapour. A simple experimental set-up to simulate VSC in the laboratory is described. The results obtained therefrom clearly show that: (a) pure water vapour does not produce VSC; (b) CO 2-loaded vapours have no measurable effect on VSC; (c) the presence of air together with CO 2 is necessary to cause measurable VSC; (d) bombardment of the tubes with water droplets condensing at an upper level greatly accelerates material loss; and (e) dissolved Cu 2+ ions catalyze the dissolution of copper-nickel alloy. Mitigation of VSC can be either preventive or curative. Preventive measures involve the monitoring of the O 2 content of both brine and distillate, the performance of pressure tests on the distiller, and the change of old gaskets at reasonable intervals. Curative measures involve the plugging and extraction of failed tubes, the application of protective sleeves, and the eventual retubing of the condenser. A change of tube material might be an appealing alternative.