Experimental study of pitting corrosion mechanism in hot water supply systems

One of the urgent problems in the operation of domestic hot water supply systems involves irregular cases of abnormally rapid development of pitting corrosion that results in significant economic damage to households. No definite explanation for the causes of accelerated corrosion of galvanized steel pipes has been found to date. This paper presents the results of a laboratory study of the corrosion of galvanized steel pipes in a hot water supply system. The conditions under which accelerated corrosion occurs are simulated and various factors that affect the corrosion process are evaluated, including the presence of a corrosion product sediment in the pipe. Based on an analysis of experimental data and observations of the operation of actual hot water supply systems, conditions under which initiation and abnormally fast development of pitting corrosion occurs in galvanized pipes have been identified. These include a low water flow rate and local accumulation of oxygen along the pipeline. Two possible ways of corrosion development with pitting and fistula formation leading to water leakage are considered. Namely, a slow rate of pit formation is typical of surfaces covered with a dense layer of corrosion deposits, while a rapid development of pits with formation of fistulas occurs on a relatively clean, deposit-free and passivated inner surface of galvanized pipes.