Atmospheric corrosion of copper in an indoor environment with a high H ₂S concentration

Atmospheric corrosion of copper was investigated in an indoor environment with a remarkably high concentration of hydrogen sulfide (H ₂S). The exposure site was a spa in the Noboribetsu hot springs area of Hokkaido, Japan. A copper plate was exposed for about 30 days inside a machine room to estimate the corrosivity. For reference, a copper plate was simultaneously exposed outdoors in the same area. After exposure, the corrosion product on the plates was characterized using X-ray fluorescence analysis, X-ray diffraction (XRD), scanning electron microscopy, and glow discharge optical emission spectroscopy (GDOES). The XRD pattern of the copper exposed indoors showed the presence of cuprite (Cu ₂O) and Cu ₂S, and the latter was preferentially formed. On the copper exposed outdoors, Cu ₂O, Cu ₂S, and two kinds of basic copper sulfates (posnjakite and brochantite) were found. GDOES depth profiles revealed a difference in the distributions of sulfur and oxygen in the corrosion product between the copper exposed indoors and outdoors. The thickness of the product on the copper exposed indoors estimated by GDOES depth profiling was more than 1 μm, which corresponds to the GX level (severe) prescribed by ISA Standard S71.04. There was also a difference in the surface morphology. These differences in the corrosion product can be explained by the difference in relative humidity, which affected the copper corrosion mechanism.