Investigating the Impact of Surface Finishing and Residual Stress on the Oxidation Behavior of Alloy 800

Alloy 800 is extensively used in nuclear power plants (NPPs) as steam generator tubing material and is essentially the pressure barrier between radioactive and non-radioactive components of the heat transport system in a NPP. Alloy 800 tubes are subjected to various surface finishing operations such as shot peening, pickling and passivation, and electropolishing before installing it in nuclear plants. The finishing operations carried out could cause a significant change in surface conditions that may alter the high-temperature oxidation behavior of Alloy 800. Surface finishing operations such as electropolishing, pickling and passivation and shot peening were carried out on Alloy 800. Surface roughness was measured for different surface-treated samples by using laser profilometer. The residual stress on the treated surface was quantified using grazing incidence x-ray diffraction (Sin 2 Ψ method). The microhardness measurements were carried out to evaluate the effect of strain hardening. All of the surface-treated specimens along with the as-received specimen were exposed in steam environment in an autoclave at 400 °C, 10 MPa for a period of 72 h. The difference in color of oxide film correlated with weight change of specimen having different surface treatments. It was observed that surface roughness, residual stress and hardness of the surface influence the oxidation behavior. The oxidation was more in the presence of tensile residual stress compared to compressive residual stress on Alloy 800. The morphology, orientation and composition of oxide layer thus formed during exposure to high-temperature steam were characterized using scanning electron microscopy (SEM), microlaser Raman spectroscopy (MLRS). The oxide film formation rate varied on Alloy 800 due to different surface finishing operations. The oxide film morphology and composition plays an important role in localized corrosion resistance of Alloy 800. The oxide film instability would result in local breakage which acts as a precursor for stress corrosion cracking (SCC) initiation on Alloy 800. This study establishes the influence of surface treatments and the residual stress generated on high-temperature steam oxidation of Alloy 800.