Abstract
In this paper, the effects of thermal aging in the 650–850 °C range on the localized corrosion behaviour of duplex stainless steel (DSS) 2304 was investigated. Pitting corrosion resistance was assessed by pitting potential (Epitt) and critical pitting temperature (CPT) determination, while the degree of sensitisation (DOS) to intergranular corrosion (IGC) was evaluated by double loop electrochemical potentiokinetic reactivation (DL-EPR). The susceptibility to stress corrosion cracking (SCC), evaluated in standard NACE TM-0177 solution at pH 2.7 and 25 °C, with the addition of S2O32− at 10−3 M, resulted in general good agreement with pitting and IGC behaviour. In fact, as-received DSS 2304 aged for 5 min at 650 °C or 750 °C presented a high resistance to localized corrosion. The alloy corrosion behaviour was severely impaired with an aging time of 60 min at 650 °C and of 10 or 60 min at 750 °C, due to the precipitation of finely distributed M23C6-type chromium carbides at ferrite/austenite interphases, which determined the formation of chromium and molybdenum depleted areas. The behaviour of samples aged at 850 °C also depended on the aging time, but, at 60 min, the rediffusion of passivating elements produced a recovery of the alloy resistance to pitting, IGC and SCC.
Highlights
IntroductionAre essentially related to their biphasic microstructure, characterized by an austenite/ferrite ratio close to 1 and by their chemical composition, which includes elements like chromium, molybdenum and nitrogen
The significant mechanical and corrosion resistance performances of duplex stainless steels (DSS)are essentially related to their biphasic microstructure, characterized by an austenite/ferrite ratio close to 1 and by their chemical composition, which includes elements like chromium, molybdenum and nitrogen
In the sample aged for 5 min at 650 ◦ C (Figure 1b), no precipitates were observed at the grain boundaries
Summary
Are essentially related to their biphasic microstructure, characterized by an austenite/ferrite ratio close to 1 and by their chemical composition, which includes elements like chromium, molybdenum and nitrogen. These elements can cause the formation of deleterious secondary phases, such as χ (chi) and σ (sigma) phases and chromium nitrides and/or carbides, if DSS are held, for brief times, within the 550–950 ◦ C temperature range [1,2]. Both the χ and/or σ phase and the chromium nitride and/or carbide precipitation can determine a Cr-depletion in adjacent areas [5,6,7,8,9], with consequent impairment of Metals 2018, 8, 1022; doi:10.3390/met8121022 www.mdpi.com/journal/metals
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