Environmental effects of a simulated AGR coolant on oxidation and carburization behavior of type 316H stainless steel

This paper aims to further understand the environmental effects of oxidation and carburization of type 316H stainless steel, which is used within the super-heater and re-heater sections of the UK Advanced Gas-cooled Reactors. The effect of gas pressure and water vapour content, and applied stress on oxidation and carburization were investigated. Experiments were conducted using a simulated CO₂ /1% CO, AGR gas mixture at a temperature of 550˚C, which contained 300 vppm CH₄, 100 vppm H₂ and either 300, 500 or 700 ppm of water vapour. The results show that polishing the specimens with colloidal silica oxide polishing suspension (OPS) induced the formation of a duplex oxide structure, consisting of an outer magnetite and an inner chromium rich spinel layer. The thickness of the oxide layer followed a parabolic law with time, and changes in applied stress, did not affect the overall thickness. However, increased surface hardness beneath the scale, which was associated with a carburization process, was dependent on the applied stress. It is proposed that stress enhanced carburization is associated with the dislocation movement, which enhances carbon mobility. Furthermore, increasing the gas pressure led to thicker duplex oxide scales and most importantly, enhanced surface hardness beneath the scale, probably due to the higher carbon activity and associated diffusion through grain boundaries and pores in the oxide layer. Finally, unlike what found in other studies, no significant changes in oxidation or carburization as a function of water vapour in the gas were identified. The mechanistic interpretation of the environmental, total pressure and applied stress on oxidation and carburization is discussed.