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UCTION

DUPLEX stainless steels (DSS) offer a combination of excellent corrosion resistance and mechanical properties.[1–3] The corrosion resistance and mechanical properties of DSS are even better than that of the austenitic stainless steels, and are attributed to the typical two-phase microstructure.[1,2,4,5] Like the austenitic stainless steels, DSS is also solution annealed at 1323 K to 1423 K (1050 C to 1150 C) and for appropriate durations.[1,4] Though the solution annealing in DSS serves the purpose of dissolving unwanted phases[1,4,6]; it is often associated with grain coarsening, possible austenite-to-ferrite phase transformation and changes in the phase-specific chemistry.[1,6–11] Naturally, such changes in the microstructures are expected to reflect on the corrosion performance.

RIYA MONDAL, ADITYA PRAKASH, D. FULORIA, and I. SAMAJDAR are with the Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. Contact e-mail: [email protected] A. RAJAGOPAL, P.V. SIVAPRASAD, and G. CHAI are with Sandvik Materials Technology, 811 81 Sandviken, Sweden. SUNIL KUMAR BONAGANI and V. KAIN are with the Materials Processing and Corrosion Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. Manuscript submitted September 12, 2019.

METALLURGICAL AND MATERIALS TRANSACTIONS A

The excellent corrosion performance of a stainless steel is related to the overall passivation: the formation of chromium oxide film.[12,13] This chromium oxide passive film imparts uniform corrosion resistance. The local breakdown(s) of the passive films, on the other hand, accounts for degradation in the corrosion behavior.[12–14] The DSS grades, for example, are often categorized by their pitting resistance equivalent number (PREN).[1,2,4,6] The PREN is a measure of the local breakdown(s) in the passive film under pitting environment. The chemistry (more specifically the chromium (Cr), molybdenum (Mo) and nitrogen (N) content) of the alloy decides the PREN.[1,2,11,12,15] However, there are enough examples[8,10,11,16–20] in the literature showing that pitting corrosion resistance, especially the critical pitting temperature,[8,16–18] depends on the substrate microstructure as well. More specifically, the grain size refinement is expected to enhance corrosion performance[21–24]: a combined effect of the nature[21,24] and electronic properties (donor-acceptor densities)[17,24,25] of the oxide film. The alloying of the substrate is also important. The exact chemistry of the substrate may affect the stability or retention of the oxide film.[26–36] The protective ability of the oxide film on duplex[26,33–35] and austenitic[27–32,36] stainless steels is reported to depend on the chromium and molybdenum contents.

The solution annealing, and associated microstructural evolution, thus may affect the (uniform) corrosion behavior. And there are several excellent research publications [8,10,11,16–20] relating solution annealing with pitting and general corrosion. However,