Fundamental studies on the high temperature corrosion mechanisms of a 22Cr austenitic stainless steel in environments related to oxy-fuel combustion

Doktorsavhandling, 2013

This thesis concern the high temperature corrosion mechanisms of stainless steels, with focus on a 22 wt.% Cr FeNiCr alloy (Sanicro 25), in environments related to oxy-fuel combustion, which is a promising technology that can provide enhanced CO2 recovery and sequestration from power plants by using post-combustion capture. The aim is to describe the oxidation and corrosion mechanisms by combining controlled laboratory exposures with detailed microstructural analysis. Isothermal exposures were performed at different temperatures in different environments in horizontal tube furnaces. The oxide scale and the subjacent metal were investigated by a number of analytical techniques, including, GI-XRD, GDOES, SEM, FIB, TEM, EDX and t-EBSD. The results show that the oxidation behaviour of the alloy in dry-O2 is relatively independent of temperature in the range 600-750°C. The material forms a slow-growing relatively smooth Cr-rich oxide in dry environment at all temperatures. In contrast, the addition of water vapour induces a depletion of chromium in the oxide scale due to chromium volatilization, which may result in breakaway oxidation depending on the temperature. The alloy exhibits a protective behaviour at 600°C in the presence of water vapour, local breakaway occurring at 700 and 750°C. Thus, at 600°C the chromium depletion in the oxide is not enough to trigger accelerated corrosion of the alloy. However, as the temperature increases (700-750°C) the chromium supply from the bulk is not able to counteract the increasing chromium evaporation and breakaway occurs. The addition of CO2 to the wet environment at 700°C resulted in an enhanced breakaway oxidation and a rapid increase of the mass gain due to the fast formation of large oxide nodules. It is suggested that the enhanced breakaway oxidation in O2+H2O+CO2 is attributed to the additional oxidizing capability provided by the presence of CO2. No enhanced internal carburization of the alloy was observed in presence of CO2, suggesting that the breakaway oxidation observed is not caused by metal carburization. In contrast, a reduced oxidation rate was observed in a wet environment containing SO2 and Na2SO4. This was attributed to the inhibition of chromium volatilization, due to the presence of the sulphates on the surface. This limits the Cr-depletion of the oxide scale, stabilizing its protectiveness and inhibiting breakaway oxidation. The influence of KCl was investigated by in-situ ESEM exposures at 450°C of FeNiCr, FeCrAl and FeNiCrAl alloys. It was found that the presence of KCl in a O2/H2O environment is locally very corrosive towards the three investigated stainless steels even at this relatively low temperature. Two types of oxide morphologies were obtained; far away from KCl particles a thin (~100 nm) base oxide formed, while large (~20 µm) porous oxide formations were created at the KCl particles.