High Temperature Corrosion Behaviour of Ni-base Alloys and FeCrAl Alloys – The Influence of Water Vapour
Electricity production, transportation, and manufacturing industry are some of the largest sources of greenhouse gas emissions. In many cases, these processes are carried out at high temperature and energy efficiency is limited by material degradation, so-called ‘high-temperature corrosion’. Understanding material degradation at high temperature is of the utmost importance in making these processes more energy efficient, thereby reducing
greenhouse gas emissions. One of the corrosive species at high temperature is water vapour, which is present in many industrial processes. Exposure of chromia-forming alloys in air that contains water vapour can result in the formation of volatile chromium-containing compounds from the chromia scale that protects the metal surfaces. This reduces the life times of the components and leads to the uncontrolled release of poisonous hexavalent
chromium species. Strategies to mitigate the formation of volatile chromium-oxy-hydroxide species include: coating the metal surfaces; and adding alloying elements that promote the formation of an oxide scale that is less susceptible to the formation of volatile species. This thesis explores how Ni-base alloys and FeCrAl alloys, form oxide scales that exhibit low degrees of evaporation. The studies were carried out with a denuder technique
to measure evaporation rates in the temperature range of 500-800 °C in an environment that consisted of air with 20-40 vol.% water. In-depth analyses of the formed oxide scales were performed using electron microscopy and X-ray techniques. The studied Ni-base alloy contained approximately 30 wt.% chromium, 60 wt.% nickel, and 10 wt.% iron, and formed a chromium-rich oxide scale in the studied environments. Under the most severe condition, i.e., 800 °C and a high gas velocity, the initially very high evaporation rate decreased rapidly with exposure time. Examination of the oxide scale after 200 hours showed that an essentially chromium free and nickel-rich oxide had formed as a result of extensive chromium depletion in the oxide and near-surface region of the alloy. It is concluded that the formation of a Ni-rich cap layer drastically reduces the evaporation rate, which leads to the recovery of
chromium levels in the near-surface region of the substrate, thereby allowing a new chromia layer to form at the metal/oxide interface. The final oxide scale was shown to be highly resistant to both evaporation and further oxidation. One of the two studied FeCrAl alloys contained rather low concentrations of chromium (~12 wt.%) and aluminium (~4 wt.%), such that it had good structure stability, weldability, and formability. This alloy also contained about 1.3 wt.% silicon, to increase oxidation resistance. Under all the studied conditions (600-800 °C, dry and wet air), protective alumina scales were formed, showing very low levels of evaporation of chromium-oxy-hydroxides. From the exposure in wet air at 800 °C, a significantly high level of silicon was found in the oxide scale. GIXRDmeasurements indicated the presence of mullite and tridymite in the scale. The results described in thesis increase our knowledge of oxide formation on Ni-base alloys and FeCrAl alloys in wet air and can be used for guidance when selecting alloys in environments that cause the evaporation of chromium-oxy-hydroxides in the temperature range of 500-800 °C.