RESULTS AND DISCUSSION

The results from the three-month immersion by Ruhe are shown in Figure 1. In Figure 1a, the pure zinc showed twice the weight loss while exposed to the UV light versus the dark. The steel sample experienced crevice corrosion and its data was not useful. In Figure 1b, the copper showed 75% greater weight loss under the UV light. In Figure 1c, the titanium and 304 stainless steel showed weight gain during the immersion, and they exhibited photo-inhibition with 50% less weight gain under the UV light versus the dark. The other metals (zirconium, nickel, Al 5052, and brass 70-30) showed no consistent trend.

The results from the five-month immersion by Forsyth are shown in Figure 2 (the copper data is the three-month immersion). In Figures 2a and 2b, the zinc, carbon steel, aluminum 6061, pure copper and pure silver all showed greater weight loss under UV exposure versus the dark. In this investigation, the brass exhibited a slight photo-inhibition of corrosion, with the samples in the light showing slightly less weight loss. The zinc, aluminum and steel corroded by pitting corrosion, while the copper and the silver corroded by uniform dissolution. The zirconium, titanium, 304 stainless, nickel and the dark silver all showed weight gain due to thickening of their passive films. The zirconium showed an increased weight gain in the UV suggesting that there was photo-corrosion in that the oxide film thickened instead of dissolving. Notably, the 304 stainless again showed less weight gain in the UV, confirming a photo-inhibition of corrosion, similar to that reported by previous investigators. [6-8, 12] The silver exposed to the UV light became completely black (Ag2S?), while in the dark the silver maintained a silver surface. The black layer was washed from the UV silver with the warm soapy water, resulting in a weight loss, while in the dark the washed silver had a weight gain. In this second investigation, the nickel and the titanium showed no consistent trend in weight gain with or without light.

LABORATORY RESULTS

The field results discussed above were supplemented with laboratory studies that are beyond the scope of this internet report. Complete results of the total investigation are under review and scheduled for publication. [14]

SUMMARY

Zinc, carbon steel, aluminum 6061, copper, and silver showed increased corrosion rates under the ultraviolet light (photo-corrosion), as compared to duplicate samples immersed upstream in the dark. Stainless steel 304 consistently showed a photo-inhibition, that is, less weight gain in the UV exposure. Photo-corrosion is a real effect and not simply a laboratory artifact.

ACKNOWLEDGEMENTS

The authors extend their hearty thanks to the Jim Brucker, Manager, Gene Greco, Plant Superintendent, and Kevin Kaplan, Laboratory Technician, all of Franklin Township Municipal Sanitary Authority in Murrysville, PA, for their assistance and the use of their plant UV disinfection facilities for these investigations. These investigations were part of the senior theses of Christopher Ruhe and Jarred Forsyth at the Materials Science and Engineering Dept. at the University of Pittsburgh.

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