Effect of pH on External Surface Corrosion of Stainless Steel Pipes for Hot Water Supply Systems

This study investigates the effect of pH on the occurrence of external corrosion in stainless steel pipes used for hot-water supply systems. Leaching tests of coating materials revealed that the solution pH decreased to approximately 2.0 within four months, indicating significant acidification due to released components. Corrosion behavior was evaluated using corrosion potential measurements, surface observations, and anodic polarization measurements. The polarization potential increased with decreasing pH for both plate and pipe specimens. Scanning electron microscope (SEM) observations showed no significant surface changes for plate specimens before and after testing, whereas pipe specimens exhibited pronounced grain boundary development, particularly under higher pH conditions. Anodic polarization measurements demonstrated that plate specimens maintained stable passive behavior regardless of pH. By contrast, pipe specimens exhibited stable passivity at pH 1.5, while fluctuations in passive current density were observed at pH 2.0 and 3.0, suggesting instability of the passive film. These findings indicate that corrosion behavior is strongly dependent on pH, with pipe specimens being more susceptible than plate specimens. The enhanced susceptibility of the pipe specimens is attributed to tensile stress introduced during the forming process, which promotes grain boundary widening and facilitates localized corrosion. Overall, the results highlight the critical role of solution pH in determining the corrosion resistance of stainlesss-steel piping systems.