Initial corrosion attack of 304L and T22 in 2 MW biomass gasifier: a microstructural investigation H. Hooshyar* 1 , J. Liske 1 , L.-G. Johansson 1 , M. Seemann 2 and T. Jonsson 1 The work investigates the initial corrosion attack on a low alloyed steel and a stainless steel in a 2 MW test gasifier. The gasifier environment generates homogenous deposits that consist mainly of carbon containing species, potassium sulphate, potassium chloride and zinc sulphide. The stainless steel exhibits better corrosion resistance compared to the low alloyed steel and the analysis indicates a protective thin scale covering parts of the surface after 4 h exposure. However, in some areas the oxide scale has lost its protective properties and thicker oxide scales are seen. The thick oxide islands consist of an inward growing Fe,Cr,Ni oxide and an outward growing iron oxide. The low alloyed steel shows a more homogenous and faster initial corrosion attack. The thick scales exhibit a sharp straight line in the middle of the scale that separates the bottom spinel oxide from the outer iron rich parts of the scale. It is considered that this flat interface corresponds to the original sample surface. Keywords: Biomass gasification, Syngas coolers, Corrosion, Stainless steel This article is part of a special issue on Microscopy of Oxidation 9 Introduction Concerns about global warming and increasing CO 2 emissions have motivated an increased use of green energy, e.g. wind power, solar power and the use of renewables instead of fossil fuels. In Sweden, the transformation to a sustainable society started already in the 1990s and today the use of fossil fuels for heat and power production is minor. However, in the transporta- tion section (i.e. fuels for vehicles), fossil fuels are still dominant. In order to increase the share of green fuels in the transportation section, the utilisation of biogas from gasification of biomass and waste is a key issue. However, compared to biomass fired combustion plants, the experiences of corrosion issues in gasification plants are limited, especially when renewable fuels are used. Hence, it is of great importance to increase our knowledge about the complex corrosion processes occurring of the steels used in these, rather unexplored, environments. In general, the term gasification refers to a chemical process in which any carbonaceous fuel is converted to a gaseous fuel. 1 Compared to combustion, the available oxygen in the environment is not sufficient for complete oxidation of the fuel and the resulting gaseous products are dominated by H 2 (g) and CO(g). 2 A variety of other gas species increase the complexity of the environment, e.g. CO 2 , water vapour, alkali chlorides, sulphides as well as fly ash and aerosols. The chemical composition of the product gas dictates the boundaries in which components such as superheaters and product gas coolers will operate. It is of great importance to increase our knowledge about the complex corrosion processes occurring of the superheaters and product gas coolers in these environments. This study investigates the initial corrosion attack of a low alloyed steel (T22) and an austenitic stainless steel (304L) exposed in the 2 MW gasifier at Chalmers using biomass (wood chips) as fuel. Hence, this study uniquely combines an investigation of the environment produced in a biomass gasifier with detailed analysis of raw gas, deposit buildup and corrosion initiation. The character- isation includes not only a microstructural investigation but also analytical techniques aiming to characterise the complex deposit and corrosion products (e.g. X-ray diffraction (XRD)). In order to be able to perform a detailed microstructural investigation of the corrosion products below the thick deposit the novel broad ion beam (BIB) technique was employed. The corrosion attack was characterised in detail with scanning electron microscopy (SEM)/energy dispersive x-ray analysis (EDX), both as plan view and as cross-sectional analysis. Experimental methods Sample preparation and exposure conditions Two different alloys were exposed in the product gas stream in the Chalmers gasifier (steam blown indirect circulating fluidised bed gasifier), 3 for alloy compositions 1 Environmental Inorganic Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, S-412 96, Go ¨ teborg, Sweden 2 Energy Technology, Department of Energy and Environmnet, Chalmers University of Technology, S-412 96, Go ¨ teborg, Sweden *Corresponding author, email hamed.hooshyar@chalmers.se ß W. S. Maney & Son Ltd. 2015 Received 16 April 2014; accepted 2 July 2014 DOI 10.1179/0960340914Z.000000000101 Materials at High Temperatures 2015 VOL 32 NO 1–2 197