Resistance of Thermally Aged DSS 2304 against Localized Corrosion Attack

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.

Investigation on the Corrosion Behavior of Lean Duplex Stainless Steel 2404 after Aging within the 650–850 °C Temperature Range

“…Aging at 650 and 750 °C created a reduction of pitting corrosion resistance, in comparison to the as-received sample (CPT value of 49.7 °C), particularly after the longest aging time (60 min). On the contrary, in agreement with previous results on LDSS 2101 and DSS 2304 [7,10], at 850 °C a recovery of pitting resistance in comparison to the corresponding aging at 750 °C was obtained. In Figure 5, the micrographs of the as-received LDSS 2404 electrode after CPT test, before and after etching with Beraha's reagent, evidenced that pits developed in the α phase and selectively grew within this phase.…”

“…Mechanical and corrosion properties in duplex stainless steels (DSSs) closely depend on microstructural characteristics including α/γ phase ratio and secondary phase formation during production or welding operations. In particular, in previous works [6][7][8][9][10][11], we found that short-term thermal aging between 650 and 850 • C negatively affected both localized and stress corrosion cracking (SCC) resistance of LDSS 2101 and DSS 2304. In 2101 grade, because of its high N alloyed-content (0.22 wt.%), thermal aging from 5 to 30 min in the 650-850 • C range mainly induced chromium nitride formation at grain boundaries.…”

“…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]. Precipitation of chromium compounds in the studied alloys led to formation of chromium-depleted areas adjacent to these secondary phases, which stimulated pitting corrosion initiation, induced sensitization to intergranular corrosion (IGC), and also susceptibility to SCC in chloride environments containing thiosulphate ions (S 2 O 3 2− ) [6,8,10]. In particular, 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.…”

“…Starting from the results of previous research studies carried out in our laboratory on LDSS 2101 and DSS 2304 [6][7][8][9][10][11][21][22][23], in this work the effects of brief (5, 10, and 60 min) thermal aging between 650 and 850 • C on the pitting and IGC corrosion behaviors of LDSS 2404 were investigated. With this aim, both electrochemical tests and scanning electron microscope (SEM) observations, coupled to energy-dispersive X-ray spectroscopy analyses (SEM-EDS), were performed also with the help of the focused ion beam coupled to SEM imaging (FIB-SEM) technique.…”

Corrosion and Protection of Metals

“…The study by Zanotto et al found that microstructural modifications imparted by heat treatment produced sensitization and influenced the localized corrosion and stress corrosion cracking of 2304 duplex stainless steel [5]. Pitting and intergranular corrosion mainly initiated in Cr-and Mo-depleted regions (ferrite/austenite interphases), near to the Cr 23 C 6 precipitates within the γ 2 and γ phases, then propagated in the ferrite matrix.…”