Basic copper sulfates and copper chlorides are known to form in addition to cuprite during outdoor atmospheric corrosion of copper. Among the basic copper sulfates, posnjakite is reported to form in the early stage and then transform into brochantite when during the course of exposure. In contrast, copper chlorides, such as atacamite and nantokite, are reported not to form in the early stage. Using thermodynamic data at 298 K, we have calculated the conditions under which basic copper sulfates and chlorides are deposited on the surface of exposed copper. For posnjakite, the minimum sulfate ion concentration needed for deposition was calculated to be 6.3×10⁻⁴ M given a cupric ion concentration of 1×10⁻⁶ M and a pH of 7. The posnjakite formation found on copper exposed in summer in urban areas is reasonably explained by the observation that the sulfate ion concentration in the surface electrolyte exceeded the minimum concentration. Although the sulfate ion concentration in the surface electrolyte exceeded the minimum concentration during winter, there was no evident posnjakite formation. This is explained by the low relative humidity and the short ‘time of wetness’. For atacamite, the minimum chloride ion concentration needed for deposition was calculated to be 2.5×10⁻² M given a cupric ion concentration of 1×10⁻⁶ M and a pH of 7. Although the chloride ion concentration in the surface electrolyte on copper exposed in urban areas exceeded the minimum concentration needed to deposit atacamite, peaks originating from this phase were not observed. A possible explanation for this is the slow formation rate of atacamite. Posnjakite depostition was found to be predominant, particularly in winter. This is reasonable give that posnjakite is more stable than atacamite. This also explains why there was a lack of atacamite formation in the early stage of the exposure.

View full abstract

In the aqueous natural gas plants, the injection well was corroded after a few years because of sulfuric acid addition to adjust pH for iodine separation. To identify the corrosion factor, microbial communities and corrosion in the plants were compared using sulfuric acid or hydrochloric acid as a pH conditioner. Genus Pseudomonas and methanogenic archaea were dominant in 16S rRNA gene library constructed from production water. The dominant SRB species were different depending on the pH conditioner. According to DAPI staining and DGGE analysis, the cell concentration increased and bacterial community changed. It was assumed that this phenomenon occurred due to contamination by microorganisms from the surrounding environment. The three month corrosion study showed that the general corrosion rate of carbon steel coupons in the injection tank using sulfuric acid as a pH conditioner was four times higher and the biofilm weight was one and a half times bigger than the injection tank using hydrochloric acid. One possible reason is that hydrogen sulfide produced by SRB corroded carbon steel. However, the pitting corrosion rate was the highest in the receiving tank which used no acid. This result indicated that metal surface corrosion is caused a mechanism which includes an oxygen concentration cell formed by the heterogeneously-attached biofilm.

View full abstract