Effect of Secondary Phases Precipitation on Corrosion Resistance of Duplex Stainless Steels

Abstract

Duplex Stainless steels (DSS) are biphasic austeno-ferritic steels in which the best combination of mechanical and corrosion resistance properties is achieved for almost equal volume fraction of the phases. These steels are classified according to their pitting corrosion resistance, assessed by the PREN index (Pitting Resistance Equivalent Number) which, although qualitatively, is widely employed as comparison. The present work is aimed to study the pitting resistance of four DSS grades (SAF 2101, 2304, 2205 and 2507) in the as-received condition and after isothermal aging in the critical range 750°C-900°C, to highlight the effect of secondary phases precipitation on the corrosion behavior. The materials were potentiodynamically tested in artificial seawater (pH7) at room temperature and the corresponding Critical Pitting Temperatures (CPT) were determined according to ASTM G150. Secondary phase precipitation mainly affected the lean duplex grades whereas the high-alloyed DSS were more stable even if large precipitation occurred.

Resistance of Thermally Aged DSS 2304 against Localized Corrosion Attack

“…shows a micrograph acquired on the surface of DSS 2304 electrode aged for 10 min at 750 • C after CPT test in 0.1 M NaCl solution. In agreement with the results of other authors[23,30], the localized corrosion attack appeared to start at the γ phase boundary, most likely in correspondence of Cr and Mo depleted areas around precipitates, then propagated in the ferrite phase, where relatively large pits formed.Metals 2018, 8, x FOR PEER REVIEW 8 of 19…”

“…The images of samples aged at 650 and 750 • C for 60 min show that the attack mainly occurred around the precipitates at the α/γ interfaces, that is, on γ and likely on γ 2 phases, due to passivating element depletion. The line elemental analysis performed by EDS across the precipitates revealed peaks related to chromium, molybdenum and carbon, confirming the presence of chromium and molybdenum carbides (M 23 C 6 -type precipitates [5]), while the small peak related to nitrogen suggests the likely concomitant formation of some nitrides [23,39]. Table 4 collects the average εf% values obtained with SSRT carried out in air at 25 °C and in NACE TM-0177 in the presence of 10 −3 M S2O3 2− , before and after thermal aging.…”

“…Few studies in the literature deal with the corrosion resistance of DSS 2304 after microstructural modifications. Some concern the influence of welding conditions [20][21][22], while others the effect of thermal aging on DSS 2304 corrosion behaviour [23][24][25]. Thus, a wider characterization of pitting and intergranular corrosion behaviour of this grade after thermal aging within a temperature range in which deleterious secondary phases can form is evidently an interesting topic.…”