The Water-Cooled Steel-Tank Rectifier Corrosion Problem

The first mercury-are rectifiers utilized glass tubes and consequently the physical size and electrical capacity was limited. The efforts to increase the available output led to the development of water-cooled steel-tank vacuum chambers after many years of research and the solution of many problems. This paper is primarily concerned with the solution of the problem presented by corrosion. Corrosion is essentially the returning of a metal to its original state and the problem consists in finding a means to retard the return sufficiently so that corrosion will not be a limiting factor in the life of rectifiers. Analysis of the problem indicates that pitting corrosion causes the greater damage and is largely a function of the character and condition of the metal, the quantity of electricity passing, and also that it is self- propagative. The rate of corrosion is affected by the temperature of the cooling water, the chemical constituency of the water, and potential differences between points on the surface of the rectifier tank. The first efforts in the solution of the corrosion problems were directed toward finding a protective coating to isolate the steel tank from the cooling water. Various paints, varnishes and lacquers, and enamels were investigated, as well as sprayed metal, but were not found satisfactory. Chemical treatment of the water was then investigated and sodium chromate found to give excellent results.