High Temperature Corrosion of Stainless Steels in Low Oxygen Activity Environments -The effect of H2 and H2O

Doctoral thesis, 2016

Gasification of biomass and waste is promising for reducing the use of fossil fuels. While the operation conditions of superheaters and gas coolers in the gasifier are restricted by gas-side corrosion, scientific investigations are very scarce in this field. The corrosion of stainless steel (304L) and low-alloy steel (T22) in the 2 MW Chalmers gasifier was investigated. To understand corrosion in the real gasifier, simplified laboratory experiments were performed, focussing on the effect of H2 and H2O on different high temperature alloys. While 304L stainless steel showed protective behaviour in dry O2 at 600˚C due to formation of a slow-growing (FexCr1-x)2O3 oxide, it suffered rapid oxidation in H2 + H2O + Ar environment at the same temperature. It is argued that “breakaway” oxidation was triggered by a deep chromium depletion of the alloy substrate. The chromium depletion was deeper in H2-H2O environment than it is in, e.g. dry O2 because the pure chromia scale formed in H2-H2O environment grew faster that the (Cr1-xFex)2O3 scale formed in air and dry O2. The ability of stainless steel to resist corrosion at high temperature relies on selective oxidation of chromium to form a slow-growing (FexCr1-x)2O3 layer, the protective behaviour being the result of slow transport of ions through the oxide and the availability of chromium in the alloy. It is argued that if the alloy substrate becomes exhausted in chromium, another, faster mode of oxidation starts which is characterized by oxidation of iron to Fe2+. The deep chromium depletion of the metal substrate is also accompanied by dissolution of oxygen into the metal and internal oxidation. The latter corresponds to a mixture of nm size FeCr spinel oxide particles and Fe,Ni metal. Compared to 304L, the highly alloyed stainless steel Sanicro28 and the FeCrAl alloy Kanthal APMT showed superior performance in H2 + H2O + Ar environment. The corrosion resistance of Sanicro 28 is explained by its high Cr content which precluded a very deep Cr depletion of the alloy. The oxidation behaviour of Kanthal APMT was apparently almost the same in H2 + H2O + Ar environment and in O2.