Metallic construction materials in biomass-and waste-fired boilers are exposed to corrosive environments due to the considerable amounts of alkali chlorides and HCl(g) released in renewable fuels combustion. Alkali chlorides corrosivity toward stainless steels exposed at high temperature has been extensively studied. Nevertheless, the corrosion attack propagation is still not fully understood and it is expected that chlorine diffusivity through oxide layers plays a major role in accelerating the corrosion. In order to investigate the role of chlorine on the propagation step of the corrosion attack, tailor-made oxides were produced. The samples were subsequently exposed to chlorine-containing environments for short period of time. The reaction atmospheres were O 2 ? H 2 O ? KCl(s) and O 2 -? H 2 O ? HCl(g) at 600°C. Since in this study chlorine diffusivity through the corrosion product layer is of great interest, samples were analyzed with XRD and SEM/EDX. High-quality BIB cross sections were performed. Summarizing, for the preformed oxide layers on the stainless steel in the presence of HCl (g)
123Oxid Met (2017) 87:801-811 DOI 10.1007 seemed to penetrate to the oxide/metal interfaces in the material. However, in the presence of KCl(s) there seems to be no effect of the salt on the corrosion rate.
The high-temperature corrosion of low-alloyed steels and stainless steels in the presence of KCl(s) has been studied extensively in the last decades by several authors. The effect of KCl(s) on the initial corrosion attack has retained extra focus. However, the mechanisms behind the long-term behavior, e.g., when an oxide scale has already formed, in the presence of KCl(s) are still unclear. The aim of this study was to investigate the effect of the microstructure of a pre-formed oxide scale on low-alloyed steel (Fe-2.25Cr-1Mo) when exposed to small amounts of KCl(s). The pre-oxidation exposures were performed at different temperatures and durations in order to create oxide scales with different microstructures but with similar thicknesses. After detailed characterization, the pre-oxidized samples were exposed to 5%O 2 + 20%H 2 O + 75%N 2 (+KCl(s)) at 400 °C for 24, 48, and 168 h and analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and focused ion beam. The microstructural investigation indicated that Clinduced corrosion is a combination of oxide thickness and microstructure, and the breakaway mechanism in the presence of KCl(s) is diffusion-controlled as porosity changes prior to breakaway oxidation were observed.
The present study investigates the initial corrosion behaviour of HVAF-sprayed NiCr, NiAl and NiCrAlY coatings in two different environments, O 2 + H 2 O and O 2 + H 2 O + KCl at 600 °C for up to 168 h in order to evaluate the possibility of utilizing such coatings in biomass-and waste-fired boilers. SEM/EDX analysis showed that all coatings displayed a protective behaviour in O 2 + H 2 O. Upon addition of KCl (O 2 + H 2 O + KCl), the corrosion behaviour of the NiCr coating drastically changed as it formed a thick oxide layer and displayed major chlorine diffusion down to the substrate. The NiCrAlY coating displayed a significantly better corrosion resistance with only minor oxide formation. The NiAl coating exhibited a protective behaviour similar to when exposed in the absence of KCl indicating that a thin protective oxide has formed on the coating surface. The performance of the NiAl and NiCrAlY coatings is promising for future studies with long-term exposures in more corrosive environments such as in a biomass-and waste-fired boiler.
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