During the dismantling and removal of fire-water-pump (FWP) columns of superduplex stainless steel barely 1 year after plant startup in a redeveloped brownfield offshore Nigeria, the unexpected was observed: severe localized pitting corrosion affected the columns. This paper explores the finding that, even though seawater chlorination was present, the chlorination was not the dominant factor that led to the corrosion; instead, manufacturing imperfections were responsible.
Introduction
After failures of duplex stainless steels in extreme environments (such as in solutions containing chloride, bromide, sulfides, or hypochlorite ions) were noted, superduplex stainless steel was developed. Since the 1970s, superduplex stainless steel has been preferred for use in a variety of marine and seawater applications and so far has recorded proven high performance. Superduplex stainless steel typically has high resistance to pitting and crevice corrosion, which is the most likely type of corrosion seen in other stainless steels.
Material resistance to pitting corrosion is measured by a pitting resistance equivalent number (PREN) value, used as the value to evaluate the resistance to pitting in an overall corrosive environment.
