High Temperature Corrosion and Descaling

Sammanfattning

High temperature corrosion presents a challenge both in predicting material loss over time and in cleaning and reusing the material after oxidation cycles. This thesis investigates the oxidation behavior and descaling of different austenitic stainless steels exposed to a range of temperatures in an oxygen-rich environment. The first part of the project focused on developing a descaling method. Two solutions were tested: one containing rock salt and nitric acid, and the other composed of nitric acid and hydrofluoric acid. The second solution was found to be more effective. The resulting samples were examined using both stereoscopic and optical microscopy in order to characterize the surface and cross-sections. The images highlighted a tendency for the descaled samples to exhibit intergranular corrosion or surface erosion, especially after treatment with the rock salt solution. Additionally, it was observed that 950◦C was the most critical temperature, producing the worst results in term of descaling. In the second part of the project, the previously developed descaling method was applied to a selected group of austenitic stainless steels: 304, 310S, 316L and 347. These materials were oxidized for almost 2000 hours at two different service temperatures: 600◦C and 800◦C. This allowed the development of thickness loss models over time for the tested materials, based on oxidation kinetics. Among the materials studied, 310S displayed the best performance over time at both temperatures, exhibiting a logarithmic thickness loss behavior. According to the oxidation kinetics, this is the optimal output, as it results in less material loss over time compared to parabolic and linear rates. 347 was found to be the second-best material, also displaying a logarithmic behavior at 800◦C. 304 performed well at 600◦C but poorly at 800◦C, in terms of thickness loss. 316L performed the worst overall, exhibiting a parabolic rate in both cases.