High temperature corrosion of some selected stainless steels and Ni-base alloys – an advanced microscopy study

Licentiatavhandling, 2015

The thesis deals with high temperature corrosion behavior of some selective stainless steels and Ni-base alloys with applications in power generation technologies, e.g. boilers fired by biomass and waste. The initial stages of KCl-induced oxidation behavior of two alumina formers (alloys Kanthal APMT and TH1) and one chromia former (alloy Sanicro 25) were analyzed in an O2/H2O environment using in-situ ESEM method. Besides, the effects of thermal cycling on the oxidation behaviour of a Ni-base alloy HR-214 was studied in air at 1200°C. The in-situ oxidation experiments provided an opportunity to view dynamic processes occurring during the oxidation process ′′live′′. The in-situ results were validated by ex-situ exposures, i.e. reference tube furnace. The alloys were corroded in the matter of minutes in the studied environment. Quite evidently, the severest oxidation attack occurred locally in the vicinity of KCl particles, where oxide crusts and oxide shells/rims (consisting of Fe-, Cr- and Al- oxides) were formed. STEM studies showed that all the three alloys formed a thin base oxide scale. Chlorine-induced oxidation caused chlorination of the alloys as evidenced by detection of chlorine below the protective scales. In the case of the Ni-base alloy HR-214, both isothermal and cyclic exposures led to the formation of a duplex oxide morphology, composed of a columnar alumina layer overlaid by a complex Ni(Al,Cr)2O4 spinel. It was evident that thermal cycling resulted in the formation of vertical cracks in the multi-layered scale. Additionally, STEM/EDX revealed outwards transport of Cr through the cracks/ alumina grain boundaries, which caused thickening of the outer spinel layer. Moreover, an attempt was made to develop the newly introduced TKD method to study nano-sized oxide scales. This was conducted by (a) designing a dedicated sample holder, (b) specimen preparation and (c) acquisition parameters. These efforts made it possible to achieve data-rich TKD orientation maps (with indexing rates > 85%). Thus, the technique was effectively employed to obtain useful information from the microstructure and microtexture of the fine-grained oxide scales. Besides, the technique provided information concerning the crystallographic orientation relationship at oxide/oxide and oxide/alloy interfaces. Keywords: high-temperature materials; oxidation; KCl; STEM; STEM/EDX; TKD.