Sulfidation of Zinc by Traces of Hydrogen Sulfide in Air

The effect of H2S in the sub-parts per million range on the atmospher ic corrosion of zinc has been investigated in the laboratory. Each sample was exposed individually to air with careful control of pollutant concentration, relative humidity, and f lew conditions. Monitor ing of H2S concentration at the outlet of the corrosion chamber was used to study the reaction of H2S with zinc metal during the first 24 h. Four-week exposures were also per formed to study the corrosion rate and the corrosion products. The deposition of H2S on zinc strongly depended on the relative humidity. In dry air H2S deposition was limited to an uptake on the order of a monolayer. At 70 to 99 % RH the initial deposition rate of H2S was faster, and the deposition reached a steady state after 15 to 20 h of exposure. Analysis of the H2S evolved f rom the samples when treated with acid showed 95-+ 5 % of the H2S deposited on zinc to be present in sulfide form. X- ray photoelectron spectroscopy and ion chromatograph ic analysis supported this finding, small amounts of sulfate being detected besides sulfide. After a 4 week exposure to 410 ppb H2S at 95 % RH, zinc blende (sphalerite) (ZnS) was identified by grazing angle x-ray diffraction. It was conc luded that the zinc sulfide film grows by bulk diffusion of ions. The rate of zinc sulfidation was independent of H2S concentration between 25 and 780 ppb