DefinePK

Abstract

Super super duplex stainless steel has shown excellent performance against corrosion attack. However, in corrosive media, 25Cr super duplex is susceptible to localized corrosion attack, especially in seawater and high temperatures. Therefore, the application of this high price material becomes useless when corrosion is likely to attack. The corrosiveness of media increases as the anion content increases. This paper presents the corrosion mechanism of 25Cr super duplex exposed to a high concentration of sulphate in the salinity of seawater. The solution (media) was prepared according to the same composition as seawater including pH, salinity and dissolved oxygen. The corrosion mechanism was characterized as breakdown potential (Eb) (also known as the pitting potential) of 25Cr super duplex, which is, the potential once reaches a sufficiently positive value. The Eb values of 25Cr super duplex were identified at 4°C, 20°C, 50°C and 80°C, and the Eb values in high sulphate content and in seawater were compared for the same temperature range. The results showed that the Eb values decreased in all solutions at increased temperatures due to the increasing attack of localized corrosion. The corrosion attack become worse as the sulphate content in seawater increase cause the decreasing the Eb value, which indicated that the sulphate anions accelerated the corrosion attack on 25Cr super duplex. The conclusion is, although some researchers claim that sulphate ion inhibite corrosion attack on corrosion resistant alloys (CRAs), but for super duplex alloy, the sulphate ion accelerate localized corrosion attack.