High-temperature corrosion properties of chromia- and alumina-forming alloys

Sammanfattning: 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 therefore crucial for making these processes more energy-efficient, thereby reducing greenhouse gas emissions. For an alloy to resist high-temperature corrosion, it must form a protective, slow-growing and adherent oxide scale on the metal surface. The type of oxide scale formed and how it evolves depend on the composition of the alloy and the operating conditions. Two common oxide scales formed on high-temperature alloys are chromia (Cr2O3) and alumina (Al2O3) scales. In this thesis, the formation and behaviour of these scales were studied. Focus was on two corrosion mechanisms: (i) how different scale microstructures are able to withstand the formation of volatile chromium-oxy-hydroxide; and (ii) how permeable the scales are to nitrogen. The study involved exposures of a wide variety of high-temperature alloys in environments with either high oxygen and water concentrations or high nitrogen concentration and low oxygen activity. In environments with high concentrations of oxygen and water, chromia-forming Ni-base alloys suffered extensive volatilization of chromium-oxy-hydroxide. The resulting chromium depletion of the alloy triggered nickel oxidation which, in turn, caused a NiO layer to form on top of the chromia scale. The NiO ‘cap-layer’ reduced chromium evaporation rate, resulting in a secondary chromia scale being established at the oxide/metal interface. Cr-containing alloys forming alumina scales, showed a very limited evaporation rate of chromium-oxy-hydroxide. In the nitriding environment, the ability of the scale to prevent nitridation was studied. Gravimetric and GD-OES analyses showed that the presence of a chromia scale decreased the nitridation by 50-95%. A beneficial effect was observed for a two-layered scale that contained both chromia and silica, as this gave a nitridation reduction at the higher limit of the interval, i.e., 83-95%. Furthermore, the study showed that alumina scales without macro defects completely block the ingress of nitrogen.