SUMMARY OF THE SESSIONS ON AUSTENITIC ALLOYS: STAINLESS STEELS – Chaired by Anders Jenssen, Toshio Yonezawa and Thierry Couvant

The first part of the stainless steel session on Monday before lunch comprised four papers.

Grace Burke (University of Manchester, UK) started the session with a presentation about recent results regarding microstructural aspects of EAC of austenitic stainless steels. The presentation was divided in two parts, of which the first aimed at providing mechanistic understanding regarding the corrosion fatigue (CF) crack growth retardation observed in type 304 stainless steel with high sulfur content. In previous testing, type 304 with low S exhibited enhanced crack growth rates (CGRs), while retarded CGRs were observed for type 304 with high S, as well as in low S type 304 with sulphide addition to the crack enclave. Examination of the cracks showed that crack retardation was associated with broad oxide filled cracks. Measurable enrichments of S and Ni was observed in the vicinity of the metal oxide interface in the high S 304. Addition of sulphide to the crack enclave of low S 304 was characterized by enrichment of S and Ni in the oxide filled crack-tip region, and NiS precipitation. The second part of the presentation focused on the effects of hydrogen on the microstructure of type 316L. The aim of the study was to find the effect of hydrogen-induced martensite phases on the oxidation behavior of austenitic stainless steels in PWR primary coolant environment. Examination of hydrogen charged samples revealed that martensite formation is dependent on the grain orientation, and that martensite oxidized rapidly and in preference to austenite.

Colin Judge (CNL, Canada) gave a presentation, on behalf of Suraj Persaud (Queen’s University, Canada), entitled “Precursors to Pb-caustic SCC in Alloy 800”. High-resolution microscopy techniques were applied to elucidate the mechanism for Pb-caustic SCC and to identify precursors to cracking. In the approach employed, different stages of the cracking process were characterized, and surface precursors for SCC could be hypothesized by studying the crack-tip region first. In the crack-tip region, evidence Pb deposition at oxide-metal interfaces enriched in Ni, and de-alloying ahead of the crack-tip were observed. These observations suggested a film rupture/de-alloying mechanism for Pb-caustic SCC in Alloy 800, where oxide passivity is impaired by Pb deposition at the metal/oxide interfaces, and easier rupture and slow film repair kinetics lead to selective dissolution of underlying Fe. Surface examination revealed metallic Ni “channels” in the Cr and Fe mixed oxide, where Pb tended to be deposited on the Ni channels. 3D FIB serial sectioning showed that the Ni channels formed an interconnected structure in three dimensions. These features were considered as precursors to Pb-caustic SCC. Incipient cracks extending down from the surface oxide also exhibited evidence of de-alloying and Pb deposition.

Dave Morton (Naval Nuclear Laboratory, USA) gave a presentation on “Stainless steel high-temperature SCC growth rate retardation (HTR) in hydrogen deaerated water”. The background to the study is the departure from Arrhenius temperature dependency in PWR primary water at elevated temperature that has been observed in medium cold worked (CW) type 316 (10-15 %), but not in 316 with higher degree of CW (20 %). In agreement with previous testing, the results of the study presented showed that heavily CW stainless steel has an Arrhenius temperature dependency with an activation energy of ~75 kJ/mol, while HTR occurred in low to moderate CW material. HTR was more pronounced in materials that were sensitized prior to CW. A phenomenological model has been developed to account for HTR in stainless steel.

Junjie Chen (KAIST, Korea) ended the session by presenting a paper entitled “Oxide film properties and SCC of austenitic alloys in simulated PWR primary water with the effect of water chemistry: ex/in-situ electrochemical and microstructural analysis”. The oxide films formed on crack initiation specimens and coupons of Alloy 182 after exposure to PWR environments with different dissolved hydrogen contents were examined by SEM, TEM/EDS, XRD and electrochemical impedance spectroscopy (EIS). Consistent with other observations, a peak in the SCC susceptibility was observed in the environments clearly in the NiO and Ni stable regimes. The oxide thickness increased with increasing hydrogen content. In another part of the study, compact tension (CT) specimens of 20 % CW type 316L were exposed to PWR primary water at 310 °C with 2.6 ppm hydrogen, no hydrogen and oxygen (deaerated), and 8 ppm oxygen (no hydrogen). The hydrogenated and deaerated environments produced similar oxide films with a Fe-enriched outer oxide layer and a Cr-rich inner oxide, whereas the inner oxide film formed in oxygenated water was rich in Ni. The CGR was higher in the latter environment, while it was almost identical in the other two environments.

After the lunch break the stainless steel session continued with four talks. New information on SCC growth and initiation in stainless steels (as well as some Ni-base alloys) in simulated PWR primary or BWR water environment were discussed.

The first presentation was done by Johan Stjärnsäter (Studsvik Nuclear AB, Sweden). He presented about the effect of C(T) specimen size on the SCC growth rate of stainless steel type 316L in simulated BWR environment. This study was conducted with eight C(T) specimens of three different sizes using 10% tensile pre-strained stainless steel. The tests are completed but post-test examinations are still on-going. From the current test results, the following preliminary conclusions can be drawn: The CGR is higher in the small specimens; it is difficult to obtain crack growth under constant K for the largest specimens; the K-dependency in the smallest specimens was lower than what have been presented in literature; the validity criterion in ASTM E399 appears to be over-conservative and may be exceeded without generating invalid data per se.

The second presentation was given by Kiyoko Takeda (Nippon Steel & Sumitomo Metal Co., Japan). She presented about the effect of Ni and Cr content in different austenitic alloys on SCC initiation using reverse U-bend specimens in 8 ppm DO, <5 ppb DO and <5 ppb DO and 30 cc/kg DH containing high-temperature water (320 and 360 °C). The oxide films were analyses by XPS after the tests. In the 316 stainless steel IGSCC could be seen in the 8 ppm DO high-temperature water, but the Ni-based alloys didn’t show any signs of SCC in the 8 ppm DO or <5 ppb DO water. The Alloy 600TT revealed SCC in the <5 ppb DO and 30 cc/kg DH containing high-temperature water at 360 °C. The oxide film which showed higher Cr-O contents seemd to have a better SCC resistance. In <5 ppb DO with 30 cc/kg DH high-temperature water, the Ni-O contents increased in the top layer of the oxide film and the Cr-O contained metallic Ni. The analysis of the oxide film suggests an effect of the inward oxidation on the SCC resistance. One of the questions raised was whether the Ni-oxide was bivalent or trivalent, the answer being that only XPS was conducted in this study, so it is impossible to distinguish between bivalent and trivalent state for the Ni-oxide. 

Kimihisa Sakima (MHI, Japan) talked about the surface finishing process on SCC initiation for austenitic stainless steel piping in simulated PWR primary environment. In this study, cold-bending, expansion, grinding, mechanical thinning, and one-directional rolling were applied as surface finishing for 316 and 304 stainless steels. Constant load SCC initiation tests were conducted at 290, 320 and 345 °C in simulated PWR primary water. One specimen from cold-bending pipe mock up ruptured after 23’353 h, but all other specimens showed no cracking up to about 57’000 h. Existence of both “hardened surface” and “multi-axially strained layer” seems to be an important factor for SCC initiation. Not only crack initiation on the surface layer, but also crack growth rate evaluation considering the hardness profile is important to prioritize CW stainless steel components that require further maintenance.

The fourth presentation was given by Yue-Tai Chen (National Tsing Hua University, Taiwan) and was dealing with the evaluation of SCC initiation for Pt-treated 304L stainless steel in simulated BWR water during start-up conditions. He conducted slow strain rate tensile tests at a strain rate of 3·10^-7 s-¹ and at temperatures of 200, 250 and 280 °C in 300 ppb DO containing high-temperature water. He used untreated, 1 ppm Pt-treated and 3 ppm Pt-treated 304L stainless steel round-bar tensile specimens. The following conclusions could be drawn: Fractographical investigations revealed that a primary crack appeared on the fracture surfaces at 288 °C with or without Pt-treatment and multiple cracks occurred at 250 °C; no SCC occurred at 200 °C; the mechanical performance of the Pt-treated sample tested at 250 °C was similar to that of the untreated one, however, the Pt-treated sample exhibited more IGSCC on the fracture surface and revealed a worse mechanical performance than the untreated sample for the test at 288 °C.

Two more presentations concluded the stainless steels session after the tea break.

Yu-Hsuan Li (National Tsing-Hua University, Taiwan) presented her work on the corrosion behavior of candidate materials used in dry cask storage systems in a simulated marine atmospheric environment. The susceptibility to chloride-induced SCC of candidate canister materials (304, 304L, 316L stainless steels and GGG-40 ductile cast iron) was evaluated testing U-bend and CBB specimens in a simulated marine atmospheric environment (50-80 °C, 40 % relative humidity, 27-43 g/m² NaCl). After testing (500-1000 h), SEM examinations were performed and cracking was quantified. Due to its significant susceptibility to pitting, ductile cast iron is not convenient for the manufacturing of canisters. Tests on sensitized 304 stainless steels showed pits and cracks growing between these pits. Large round cavities were noticed on sensitized 316L due to inclusions (Mo, Mn, S). Less cavities and cracks were reported on annealed stainless steels.

Reuben Holmes and Stacy Moore (NNL, UK) introduced their development of fundamental approaches for dynamic study of nanoscale corrosion initiation events in irradiated systems. In particular, high speed atomic force microscopy (HS-AFM) allows to map microstructures (typically 1 mm² with 4 nm pixel size in a day). HS-AFM can be coupled with mechanical and environmental devices or electrochemistry. Different mappings were presented: precipitation, slip bands, carburization, and pitting.

Please note that four more stainless steel papers were presented in one of the summary/poster presentation sessions:

Jiamei Wang (SJTU, China): “Effect of residual δ-ferrite on SCC behavior of 321 stainless steel”

Joshiyuki Kaji (JAEA, Japan): “SCC initiation susceptibility in L-grade austenitic stainless steels after long-term thermal aging treatment”

Yung-Hsiang Chang (National Tsing Hua University, Taiwan): “Electrochemical characteristics of platinum-treated type 304 stainless steels of different surface oxide structures in high-temperature water”

Ryan Matthews (ESKOM, South Africa): “Oxidation of 316L stainless steel in primary water environment”