Environmentally assisted cracking of T91 ferritic-martensitic steel in heavy liquid metals

Abstract

In order to advance material development for future nuclear systems, an insight into the cracking conditions of T91 ferritic-martensitic steel in heavy liquid metals (HLM) is provided. The paper critically reviews previous experimental data and summarizes them with new results. The new testing of T91 steel was performed in contact with slow flow and static HLM to study crack initiation, especially in liquid PbBi eutectic at 300°–350°C and Pb at 400°C with about 1 × 10−6 wt.% of oxygen. Pre-stressed coupons were exposed to the liquid metals for up to 2000 h. Constant extension rate tests (CERTs) were performed in the liquid metals to accelerate cracking development. Under static conditions, the testing resulted in oxidation without any crack observation. Under the CERT ones, the T91 steel showed a tendency to crack initiation in PbBi, while in Pb, cracks were not initiated even when the oxide layer was broken. Moreover, the environmentally assisted crack initiated at the maximum load and continued to grow under further loading without unstable failure. Both previous and new data have confirmed that high stress and plastic strain are pre-conditions for the environmentally assisted cracking of T91 in static HLM. It indicates that in the systems utilizing continuous oxygen control of HLM, the LME/EAC of the T91 could develop only in the beyond design load conditions. Further testing is necessary to address the HLM flow speed effect.

Liquid metal embrittlement of a dual-phase Al0.7CoCrFeNi high-entropy alloy exposed to oxygen-saturated lead-bismuth eutectic

“…Nevertheless, the deployment of this technology is technically impeded by incompatibility issues of steels when exposed to LBE, including the so-called liquid metal embrittlement (LME) and liquid metal corrosion (LMC) [1,2] . LME in presence of LBE occurs mainly in ferritic/martensitic steels (F/M, e.g., T91, HT9, and EP823) at around 350 °C, leading to severe deterioration of ductility and toughness of these steels when stressed in contact with this liquid metal [3][4][5][6][7][8] . LMC can occur in the F/M steels and austenitic stainless steels (AuSS, e.g., 316L and 15-15Ti), manifested by either surface oxidation when LBE is rich in oxygen or by selective leaching of the steel elements (e.g., Ni, Mn, Cr, and Fe) when oxygen in LBE is poor [9] .…”

Influence of surface treatment on corrosion resistance of steel in liquid Pb

“…Inhibitory se dělí do dvou kategorií -kovové a nekovové, přičemž nejčastěji používaným inhibitorem je kyslík, který ve vhodné koncentraci zajišťuje optimální podmínky pro vytváření ochranné vrstvy na povrchu a snižuje korozní poškození materiálu. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Nicméně při teplotách nad 500 °C se zvyšuje rychlost růstu oxidických vrstev, klesá jejich tepelná vodivost a efektivita působení kyslíku se tak snižuje. Je experimentálně prokázáno, že stabilní vrstva oxidu může vzniknout při teplotách nižších než 550 °C a při koncentraci kyslíku 10 -7 -10 -8 hm.%.…”

A Review of the Surface Modifications for Corrosion Mitigation of Steels in Lead and LBE

“…Austenitic stainless steels are much less sensitive to oxidation [26] and sometimes claimed as excellent corrosion resistance materials [27]. Nevertheless, it is important to note that the external magnetic layer and the oxide layer easily crack under stress, as shown in the scanning electron microscopy (SEM) image of Figure 2 [28]. In saturated oxygen lead or LBE, the mechanism of corrosion is oxidation for both families of steels.…”

“…The mechanical response of martensitic steel is much more sensitive to the liquid metal environment. T91 steel may exhibit liquid metal embrittlement (LME) e.g., [28,33,37,38], have fatigue lives e.g., [35,[39][40][41], and creep rupture times [42][43][44] are shorter in liquid metal than in air. It is important to note that LME and fatigue cracking initiate in liquid metals at the surface of the materials, as does corrosion.…”

“…Figure 2. Double-layer oxide scale developed and braked in T91 steel under constant extension rate test in Pb at 400 °C, showing many superficial cracks in the oxide; SEM-secondary electrons image from[28].…”