Stainless steel corrosion in Ultrahigh-Purity Water: Surface analyses and metal release

The present work focuses on corrosion behavior of 316L stainless steel in three typical pharmaceutical environments: tap water, distilled water and WFI (Water For Injection). Stainless steel is widely used for its corrosion resistance, however, in certain service conditions typical of pharmaceutical industry, the degradation of stainless steel may lead to the formation of a thin yellow surface film, sometimes evolving to a black and thick oxide layer. This phenomenon is known as rouge and is often observed in equipments of the pharmaceutical industries where stainless steel is in contact with high purity water, at about 70/80 °C. The purpose of this research was to examine the influence on rouge of parameters like environment (tap water, distilled water and WFI) and type of welding. To this aim, AISI 316L (GTAW-welded with or without filler material) coupons underwent “exposure tests” in the three types of water at 70°C for 3 weeks. The evolution of the surfaces was followed as a function of exposure time. In particular, the morphology and composition of corrosion products were studied by VP-SEM (Variable Pressure Scanning Electron Microscope) integrated with EDS microprobe and μ-Raman probe. In order to monitor the metal release in the water, the ageing solutions were collected at the end of the test and the amount of dissolved Fe was analyzed. Finally, polarization curves (in tap water, distilled water and WFI, at 70° C) were recorded, so as to assess the corrosion process. Preliminary results showed the occurrence of corrosion phenomena only on coupons tested in WFI, while in tap water only calcareous deposits were observed. In distilled water the highest Fe release was detected as well as the highest current density in the polarization curves. However, in all cases, the release was under the limit defined by USP XXI standard.