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Abstract
The main purpose of this work is to investigate the pitting corrosion behavior of sigmatized duplex stainless steel (DSS) exposed to two different chloride environments: simulated seawater solution and produced water solution. Specimens taken from a commercial DSS (UNS 31803) have been subjected to aging treatments at 850°C for different holding times to achieve different amounts of sigma phase. Metallographic examinations combined with X-ray diffraction technique were employed to follow the microstructure evolution. The pitting potential of the aged samples were determined in simulated seawater solution and produced water solution. It was established that solution treated DSS shows a high pitting corrosion resistance in both test solutions, while serious deterioration of corrosion properties occurs in presence of sigma phase. It was concluded that both sigma phase amount and chloride concentration worsen the pitting potential, the higher sigma content, the lower pitting potential and the higher chloride concentration, the lower pitting potential. SEM observation showed that pitting nucleation occurs preferentially at sigma phase interfaces due to the development of Cr- and Mo-depleted regions around sigma phase. It was also confirmed that the hardness behavior is only affected for long term aged samples.
Highlights
Duplex stainless steels (DSS) are Fe-Cr-Ni alloys having an approximately volumetric fraction of 50% ferrite and 50% austenite in their microstructures [1]
The presence of these phases was confirmed by X-ray diffraction analysis as shown in Fig. 2, no other phases or precipitates have been detected in the investigated DSS at the as received state
The effect of sigma phase on pitting corrosion resistance and hardness behavior was studied. It constitutes a good background for understanding the quantitative correlation between sigma phase amount and corrosion resistance
Summary
Duplex stainless steels (DSS) are Fe-Cr-Ni alloys having an approximately volumetric fraction of 50% ferrite and 50% austenite in their microstructures [1]. DSS combine some characteristics of each of these phases They have been introduced into the market during 1930s; since that time there has been an accentuated development and rapid spreading of their use [2]. The UNS S31803 alloy ( known as steel SAF 2205) is the most widely employed duplex stainless steels nowadays [6,7]. This material grade found widespread use in oil and gas industry, especially for high chloride containing process fluids like oily produced water and injection process systems. The use of DSS at high temperature is a typical concern to users of duplex stainless steels owing to their susceptibility to the formation of dangerous intermetallic phases, such as σ-
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