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

In the present study as in our previous studies (Arikan and Doruk, 2008 and Arikan et al., 2012), similar specimens taken from a hot rolled cylindrical duplex stainless steel (DSS) bar with 22% Cr grade were solution annealed at 1050°C and then aged at 800°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. The samples aged for 100？min were less sensitized whereas samples aged for 316？min and more time were sensitized. The degree of sensitization (DOS) can be attributed to higher contribution of chromium and molybdenum depleted areas that result from intermetallic phases. However, especially the samples aged from 3162 to 31622？min have revealed chromium replenishment. Consequently, the degree of sensitization was lowered in comparison to the results obtained in previous studies. 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 favorable corrosion resistance of austenitic stainless steels with good mechanical properties [1–4]. However, duplex stainless steels are susceptible to the precipitation of some phases that affect both the corrosion and the mechanical properties. These phases may be formed during the solidification of the alloy or in subsequent heat treatments or plastic deformation processes or even due to ageing processes during its use, causing a marked effect upon the workability and the useful life of the material [5]. One of the possible phases to be formed is the sigma phase ( ), a hard and brittle intermetallic compound, rich in Cr and Mo, which is formed from ferrite [6]. This phase has deleterious effect upon both mechanical and the corrosion properties [7]. In most of the duplex stainless steels sigma phase is formed between 600°C and 950°C and its precipitation becomes faster between 700°C and 900°C. The embrittlement of some alloys due to the sigma phase precipitation may occur in very short times, of the order of 3？min [7–9]. Brandi and Padilha [10] and Maehara et al. [11] found the sigma phase precipitation in duplex stainless steels that as the amount of sigma phase increases the amount of ferrite decreases, until its total consumption. According to this result it was concluded that the sigma phase is formed from the ferrite. The proposed mechanism for this formation [10, 11] is the