Title: Failure analysis of DSS 2205 GTA welds and evaluation of the cooling rate effects on localized corrosion with mini-electrochemical cell

Abstract

Duplex stainless steel 2205 is widely used in various applications, especially in petrochemical industry, owing to its resistance to chloride stress corrosion cracking. However, during manufacturing processes, the 50:50 ratio of austenite and ferrite phase is often disturbed by welding, which often leads to the deterioration of corrosion resistance in the heat affected zone (HAZ). In this study, a real world localized corrosion failure of DSS 2205 welds is illustrated, and welding experiments were conducted in the attempt to prevent such failure. The results of failure analysis indicate that the localized corrosion took place in the HAZ immediately adjacent to the fusion boundary and was caused by the welding of two components with different thickness. The higher cooling rate of the thicker component leads to the formation of austenite-depleted-zone, thus lowering the corrosion resistance of the HAZ. Hence, a multipass GTA welding experiment of DSS 2205 was carried out with different cooling conditions to evaluate the effects of cooling rate on localized corrosion resistance and resultant mechanical properties. Slower and faster cooling rates were achieved by a 300°C preheating and water cooling of the base metal, respectively. A mini-electrochemical cell and potentiodynamic polarization were used to evaluate the corrosion resistance of the HAZ. Results show that the water-cooled HAZ demonstrates a more noticeable austenite-depleted-zone, which results in higher passive current and lower Ecorr value. On the other hand, preheating significantly enhances the formation of grain boundary austenite, Widmanstätten austenite, intragranular austenite and partially transformed austenite, thus increases the corrosion resistance. Although preheating is beneficial to the resistance to localized corrosion, DSS 2205 is also sensitive to 475 °C embrittlement. Considering that only the inner surface of the weld is exposed to corrosive environment in real world applications, preheating is recommended only for the first welding pass.