KCl-induced high temperature corrosion of selected commercial alloys Part I: chromia-formers S. Kiamehr*, K. V. Dahl, M. Montgomery and M. A. J. Somers Laboratory testing of selected chromia-forming alloys was performed to rank the materials and gain further knowledge on the mechanism of KCl-induced high temperature corrosion. The investigated alloys were stainless steels EN1.4021, EN1.4057, EN1.4521, TP347H (coarse-grained), TP347HFG (fine-grained), Sanicro 28 and the nickel-based alloys 625, 263 and C276. Exposure was performed at 600 8C for 168 h in flowing N 2 (g)þ5%O 2 (g)þ15% H 2 O(g) (vol.%). Samples were covered with KCl powder prior to exposure. A salt-free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). It was observed that in the salt-free exposure, stainless steels TP347H (coarse-grained) and EN1.4521 failed to form a thin protective oxide layer compared to the oxide formed on the other alloys. In the presence of solid KCl, all the alloys showed significant corrosion. Measurement of corrosion extent indicated that alloys EN1.4057, Sanicro 28 and 625 show a better performance compared to the industrial state of the art material TP347HFG under laboratory conditions. An additional test was performed with KCl vapor in static air for the same duration and at the same temperature.This was undertaken to investigate the role of the vapor phase and revealed that KCl vapor at 600 8C can initiate attack. 1 Introduction During the past decades, increasing concern about the global warming effects has been a large driving force for utilization of CO 2 -neutral fuels. Combustion of biomass instead of fossil fuels is an attractive option, especially in countries with large forest and agricultural resources. However, employing biomass is not free from challenges. Compared to the fossil fuels, such as coal and oil, biomass contains large amounts of alkali and chlorine. Consequently combustion of biomass leads to deposits rich in alkali chlorides, especially potassium chloride KCl, on the heat exchangers. Such deposits are so corrosive that obtaining sufficient life time of components in biomass fired power plants necessitates a reduction of the outlet steam temperature to a value below that for fossil fuel fired power plants. Currently, the maximum outlet steam temperature for biomass firing is 540 8C for state of the art power plants using stainless steels such as TP347HFG as superheater tube material [1]. The relatively low steam temperature makes the biomass based power plants less efficient compared to plants firing fossil fuels. Therefore reducing the extent of KCl-induced corrosion will directly affect the feasibility of substituting fossil fuels with biomass. High temperature alloys often rely on the formation of a chromium-rich oxide layer for protection against corrosion. Therefore any interaction with the environment that damages this oxide layer accelerates corrosion of the underlying alloy. It has been shown [2] that in alkali-containing environments, a reaction between the alkali elements and chromia in the oxide layer can lead to the formation of alkali chromate. The reaction for the case of potassium chloride is: 4KClðs; gÞþ Cr 2 O 3 þð5=2ÞO 2 ðgÞ¼ 2K 2 CrO 4 þ 2Cl 2 ðgÞ ð1Þ Chromate formation depletes the original oxide of chromium and hence the oxide is no longer protective [2,3]. Consequently it is anticipated that with an increase in the alloy’s chromium content, the overall damage could be mitigated as a protective S. Kiamehr, K. V. Dahl, M. Montgomery, M. A. J. Somers Technical University of Denmark (DTU), Department of Mechanical Engineering, Produktionstorvet, Building 425, 2800 Kgs., Lyngby, (Denmark) E-mail: sabag@mek.dtu.dk M. Montgomery COWI A/S Parallelvej 2, 2800 Kgs., Lyngby, (Denmark) 1414 DOI: 10.1002/maco.201408213 Materials and Corrosion 2015, 66, No. 12 www.matcorr.com wileyonlinelibrary.com © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim