Platinum Metals Rev., 1961, 5, (1), 12
Cathodic Protection of Water Heaters
Use Of Platinum-Protected Titanium Anodes
The short life, due to corrosion, of galvanised iron domestic water tanks has led to the development of glass-lined water heaters in the United States. Less serious tank corrosion caused by small holes in the glass linings may be controlled by the use of a cathodic protection system requiring a small impressed current. The design of such a system is discussed in a recent paper by H. C. Fischer of the Thermo-Craft Corporation, New York (Corrosion, 1960, 16, (9), 9-17).
It has been found that the bare area of a single-coated, glass-lined tank may be protected adequately by a current of 5 milli-amperes even in waters of high resistance. The presence in the system of copper ions derived from copper plumbing gives rise to local cell corrosion which renders magnesium and zinc anodes unsuitable. However, at the low current densities required, non-sacrificial anodes of bare titanium or titanium wound with platinum-clad tantalum ribbon have proved both effective and economic. One anode described consists of a 30-inch titanium wire, 0.051 inch in diameter, around which is wrapped a 0.002 by 0.008 by 36-inch platinum-clad tantalum ribbon. Field trials are being conducted using platinum-plated titanium wire anodes which have proved satisfactory in preliminary tests.
Cathodic protection has been found effective for glass-lined domestic water tanks heated either electrically or by gas. Power for the impressed current system for gas water heaters is supplied by a thermoelectric generator. In this case, in order to keep the anode voltage as low as possible, only platinum-plated titanium or the platinum-tantalum-titanium anodes may be used.