Abstract
Research started by A. O. Smith Corporation more than twenty years ago demonstrated that the effectiveness of cathodic protection was greately enhanced when coupled with a glass-fused-to-steel coating. This work resulted in the patent No. 2267361. Subsequent research and field experience with glass-lined domestic water heaters indicated this principle could, perhaps, be applied to advantage in this product. The glass-lining, being an inert, non-conductive coating would impose no current drain on the cathodic protection device used.
Heretofore the glass-lining was the sole protector of water heater tanks against corrosion, and (as in any manufactured product) there was always the possibility the coating might not be perfect. It was thought a magnesium anode might serve as a pinch hitter to protect steel areas exposed if the glass coverage was imperfect.
To evaluate cathodic protection in glass-lined water heaters a rather elaborate field test program was begun, involving installation of four glass-lined and two galvanized water heaters in each of twelve very carefully selected cities in the Eastern half of the United States. Construction of tanks used and the method of installing the magnesium anode is shown. Coatings on all tanks were damaged deliberately to make possible evaluation of the effectiveness of cathodic protection under abnormally adverse conditions. Plans included determination of current flow, anode life, effect of cathodic protection on the coating itself and the possibility of incorporating several cost saving features in glass-lined water heaters when magnesium anodes were used. After installation, these heaters were inspected at regular intervals both visually and electrically. After one year, half the heaters were removed and dissected.
Results of this field test program showed how magnesium anodes behave both in galvanized and glass-lined water heaters under a variety of water and performance conditions. Anode life under these conditions was determined and anode life expectancy graphs are shown. Photographs of many of the bisected water heaters are also shown.
