Abstract
This paper reports the effects of thermal aging between 650 and 850 °C on the localized corrosion behavior of lean duplex stainless steel (LDSS 2404). Critical pitting temperature (CPT) and double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were performed. The localization of pitting attack and intergranular corrosion (IGC) attack after DL-EPR was investigated by optical (OM) and scanning electron microscopy (SEM) and by focused ion beam (FIB) coupled to SEM. Thermal aging caused the precipitation of mainly chromium nitrides at grain boundaries. Aging at 650 °C or short aging times (5 min) at 750 °C caused nitride precipitation mainly at α/α grain boundaries as a result of fast diffusion of chromium in this phase. Aging at 850 °C or aging times from 10 to 60 min at 750 °C also allowed the precipitation at the α/γ interface. Nitrides at γ/γ grain boundaries were observed rarely and only after long aging times (60 min) at 850 °C. Electrochemical tests showed that in as-received samples, pitting attack only affected the α phase. Conversely, in aged samples, pitting and IGC attack were detected close to nitrides in correspondence of α/α and α/γ grain boundaries depending on aging temperatures and times.
Highlights
Lean duplex stainless steels (LDSSs) are ferritic/austenitic steels characterized by a low nickel content, which makes them a cost-effective alternative to ordinary stainless steels (SSs) like SAE 304 and 316 [1]
Growth of the sigma (σ) phase was detected, as a new brighter intermetallic phase was observed at the α/γ interface
In the FEG-scanning electron microscope (SEM) chamber, each sample was tilted at 52°, and a preliminary in situ platinum deposition was performed in order to protect the area of interest during ion milling
Summary
Lean duplex stainless steels (LDSSs) are ferritic/austenitic steels characterized by a low nickel content, which makes them a cost-effective alternative to ordinary stainless steels (SSs) like SAE 304 and 316 [1]. In DSS 2304, which is characterized by a lower N concentration (0.1 wt.%), essentially chromium (and molybdenum) carbides formed with aging up to 60 min in the same temperature range In both alloys, sigma (σ) phase formation was not detected after thermal aging, because of their low Mo concentration (0.3 wt.%), which slowed down the precipitation rate of this deleterious phase [12,13,14]. The longest thermal aging at 650 and 750 ◦ C resulted most deleterious for the resistance to localized corrosion on both LDSS 2101 and DSS 2304, while generally a recovery of the performances was observed after aging at 850 ◦ C This effect was due to a rediffusion of chromium from the grain bulk towards the depleted regions at the grain boundaries. Both electrochemical tests and scanning electron microscope (SEM) observations, coupled to energy-dispersive X-ray spectroscopy analyses (SEM-EDS), were performed with the help of the focused ion beam coupled to SEM imaging (FIB-SEM) technique
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