Abstract
The previous study has shown that stainless steel surfaces on piping could greatly be subject to rouging during the operation of high-purity water systems (Suzuki O et al.: Pharm Tech 22: 66-82, 1998). The follow-up study has hereupon been carried on to fed out what will potentially develop into such rouging on stainless steel, because the fouling-like rouging has always been an anxious matter to pharmaceutical engineers. The name of rouging is now given to a corrosive discoloration developed with various contaminants in the high-purity water treatment process, which will also occur in WFI (Water for Injection) systems or pure steam generators. Stainless steel elbow-piping sections, which surfaces were finished at such a high mill finish as mechanical polish and electropolish, were exposed to pure steam generated at 2 kg/cm2-g. The steam was also fed into the near-horizontal piping section not stagnant with distillate and the intentionally arranged section stagnant with distillate for 30 days. The inner surface after pure steam fed was visually inspected and analyzed by a physical technique of Auger electron spectroscopy so that rouging discoloration was detected to be mostly due to its surface oxidization of stainless steel. The oxidization is divided into two groups: namely, (1) reddish discoloration results from the vapor attack, and (2) blackish discoloration comes from long contact with condensate on stainless steel surfaces. The latter discoloration is further divided into two types in the chemical respect; namely, with carbon and without carbon. Although these kinds of discoloration may provide no deterioration for water quality because of very low solubility of metal oxides, a further study of this rouging mechanism should be made to acquire clear grounds for the final conclusion.
