Determination of Susceptibility to Intergranular Corrosion of UNS 31803 Type Duplex Stainless Steel by Electrochemical Reactivation Method

Specimens taken from a hot-rolled cylindrical duplex stainless steel (DSS) bar with 22% Cr grade were solution annealed at C and then aged at C from 100 to 31622？min for sensitization treatment. Double loop electrochemical potentiodynamic reactivation and standard weight loss immersion acid tests were conducted. The solution-annealed samples were found unsensitized. Those samples aged for 100 and 316？min were less sensitized whereas samples aged for 1000？min and especially those aged for 3162, 10000, and 31622？min were heavily sensitized. The degree of sensitization (DOS) can be attributed to higher contribution of chromium- and molybdenum-depleted areas resulting from intermetallic phases. 1. Introduction Generally, duplex stainless steels (DSS) are Fe-Cr-Ni alloys having an approximately volumetric fraction of 50% ferrite and 50% austenite in their microstructures. Their main feature is that they compromise the favorable corrosion resistance of austenitic stainless steels with good mechanical properties [1–4]. In duplex stainless steels undesirable phases such as intermetallic phases (sigma and chi), carbides, and nitrides may exist if the manufacturing processes are not carefully controlled. High levels of elements stabilizing ferrite, such as chromium, molybdenum, and silicon, can promote the formation of sigma phase . Sigma phase is a hard and brittle intermetallic phase, which is generally formed between 600 and 950°C with rapid formation kinetics [5–7]. Additional phases found in duplex stainless steels can include chi , laves , and [8]. The nucleation sites for and phases are grain boundaries, incoherent twin boundaries, and dislocations. Duplex stainless steels are susceptible to sensitization due to the precipitation of additional phases when heated in a temperature range of 600–950°C. These phases have a reverse effect on the corrosion and mechanical properties [6, 7]. A substantial depletion of Cr and Mo due to a copious precipitation of and phases results in a decrease of the corrosion properties [9]. There are several test methods for determining the sensitization to intergranular corrosion. Weight loss acid test was first standardized, and the test procedure was presented in ASTM A262-91 [10]. Corrosion rate is determined by measuring the weight loss of the sample. Another test method of measuring the degree of sensitization to intergranular corrosion involves electrochemical reactivation of the steel samples as defined in ASTM G108-94 [11]. This reactivation process is named as electrochemical potentiodynamic reactivation (EPR) and has