D U RI NG the past year, research on protective coatings in water distribution systems has been in progress at the sanitary engineering laboratories of Michigan State University. This work has been directed toward develing procedures that are available in water utility operation for anticorrosion protection for metal water pipes. Studies have been restricted to calcium carbonate-ferric oxide-ferric carbonate deposits, mostly because of the wide use of these "eggshell" coatings in modern water supply practice.All of the work reported here has been conducted with cast-iron and stainless-steel specimens exposed to demineralized waters containing only calcium hydroxide, carbon dioxide and dissolved gases of the atmosphere, oxygen and nitrogen.