Effects of 304 Stainless Steel Microstructure on Short-Time Corrosion in High Temperature Water

Austenitic stainless steels of the 300 series are commonly used for low to moderate temperature air and high temperature, high pressure water environments that are found in the primary coolant loop of pressurized water reactors due to their corrosion resistance, low cost, wide availability, weldability, and formability. Understanding the material condition and environmental combinations that may be present in a pressurized water reactor can help make the reactors safer with longer lifespans through material selection insight and predictive modelling. The austenite phase present in some grades of the 300 series, including 304, is metastable and can be transformed to martensite through cooling and deformation. In this work, surface condition and its oxidation effects are studied in air at 280, 400, and 700 ^◦C for 50 hours and plastic strain and surface finish oxidation effects are studied in pressurized water at 280 ◦C for 10, 50, and 100 hours. Deformation microstructures accompanying each condition vary: surface preparation can result in a deformation zone localized to the near surface while plastic strain results in uniform deformation throughout the thickness with potentially deformation-induced martensite. Martensite results in a higher density of grain boundaries and also has a different crystal structure than austenite leading to differences in diffusion kinetics between the two phases. Differences between the deformation microstructures can alter the oxidation behavior; various surface conditions and material plastic strain are present as a material starting condition in nuclear reactors and can affect material degradation. 

Uniaxial tension and plastic strain are characterized for 304 at 21, 250, and 338 ^◦C on cold worked and mill annealed conditions. Prior material condition leads to differences in mechanical behavior and martensite transformation upon deformation: the cold worked samples result in lower strain at failure and deformation induced martensite is found to occur in all samples at low temperature and in the cold worked samples at high temperatures. Local misorientation provides a proxy for plastic strain for specific testing conditions. A relationship between axial and lateral strain is derived and tested for isotropic and anisotropic materials up to necking. 

High temperature deformation induced martensite is shown to form in specimens that contain cold work prior to high temperature tension. Deformation induced martensite in the 300 series is reported to be limited to occur exclusively at low temperatures (less than 100 ^◦C), although it is shown in this work to occur as high as 347 ^◦C. High temperature deformation induced martensite can increase or decrease oxidation kinetics when formation occurs inside an environmentally assisted crack. 

Surface preparation and martensite content are studied in low temperature air at 280, 400, and 700 ^◦C in a thermogravimetric analyzer. Surface preparation can leave a deformation layer that extends a few microns in depth into the material; a fine grained layer forms on the outer most layer with deformation bands at a deeper level. The finer grain structure increases the grain boundary surface area which can act as fast diffusion paths for alloying elements, changing the oxidation behavior of the material. The mass of oxide formed on the surface is too small to be resolved in the thermogravimetric analyzer. Scanning electron microscopy did confirm that an oxide layer(s) formed as a result of the low temperature air exposure. 

Macroscale corrosion properties of austenitic stainless steel in high temperature water conditions have been studied extensively at long exposure time, although effects of microstructural and phase differences within these steels at short exposure times are largely unknown. Tensile specimens with a range of plastic strain and machined surface finish are exposed to high temperature water for 10, 50, and 100 hours. Polished surface finish with plastic strain and a range of surface finishes with and without cold work are oxidized for 100 hours in high temperature water. The resulting oxide thickness for all exposures is determined by plasma focused ion beam milling and scanning electron microscopy imaging of the cross sections. Inner oxide thickness is modeled as a parabolic rate law. The effect of plastic strain and surface finish are concomitant. The effect of plastic strain is linked with deformation induced martensite in the near surface structure. Surface finishing exhibits a positive correlation between oxide thickness and finishing particle size. A metallic nickel rich phase is seen in place of an outer oxide and the inner oxide contained metallic phase embedded within a variable density inner oxide. Faster oxidation kinetics are determined to be present in current experiments as compared to oxidation kinetics reported in literature. The presence of nickel was confirmed to be a result of the high flow in the loop system, while several mechanisms are proposed to account for the faster oxidation kinetics.