Pitting corrosion of the stainless steel ventilation duct in a roofed swimming pool

On the surface of an uptake ventilation duct made of austenitic stainless steel, installed in a roofed swimming pool with chlorinated water, corrosion pits appeared after two years service. Chemical analysis of the duct sheet has shown, that it was made of X10CrNi18-8 austenitic steel. Microanalysis results of the corrosion deposits on the sheet surface revealed a high concentration of chlorine, which was also found in deposits located inside the corrosion pits. Discussion of the examination results and the survey of literature has led to the conclusion, that the corrosion resistance of the applied classical austenitic steel is not adequate for the ventilation installation in roofed swimming pools with chlorinated water, which belong to the highest group of corrosion hazards. The X10CrNi18-8 steel should be replaced in the new duct installation by a higher alloyed and Mo containing austenitic steel with a lower carbon content.