Abstract
The corrosion of the Ni-base alloy 690 (60Ni, 30Cr and 10Fe) in humidified air was studied at 500–800 °C, and the rate of CrO2(OH)2 volatilization was measured quantitatively as a function of exposure time using a denuder technique. Different gas velocities were employed in exposures with a maximum duration of 200 h. Corrosion morphology was investigated by SEM/EDX using BIB-milled cross sections. The rate of chromium volatilization increased with increasing temperature and gas velocity. The rate of volatilization decreased with exposure time. Two oxide scale morphologies were observed, depending on temperature and gas velocity. In the 500–700 °C range, the scale consisted of chromia-rich corundum-type oxide, while exposures with high gas velocities at 800 °C produced an entirely different type of scale that included a Ni-rich and Cr-poor cap layer. The latter scale morphology is suggested to result from extensive chromium depletion of the alloy substrate which triggers a new mode of oxidation involving formation of NiCr spinel oxide. Continued volatilization of CrO2(OH)2 causes the NiCr spinel to decompose into a Ni-rich oxide that forms a cap layer on the scale surface. This cap layer is very efficient in decreasing the rate of chromium volatilization, allowing the chromium levels in the substrate to recuperate. We show that volatilization of chromium (VI) from the alloy can be mitigated by an oxidation pre-treatment that allows the Ni-rich cap layer to form.
Highlights
It is well known that exposure of metals and alloys to high-temperature environments containing a combination of O 2 and H 2O can result in evaporation of oxyhydroxide compounds [1]
The results show that the rate of chromium volatilization depends strongly on gas velocity in the beginning of the exposures, but that this dependence becomes weaker with time, the four curves tending to converge towards the end of the exposure
The results show a dramatic decrease in chromium volatilization for the as-exposed compared to the as-polished sample
Summary
It is well known that exposure of metals and alloys to high-temperature environments containing a combination of O 2 and H 2O can result in evaporation of oxyhydroxide compounds [1]. In humid air, CrO2(OH) is the dominant volatile chromium species up to very high temperatures [2, 3]. Volatilization of CrO2(OH) can cause problems for alloys that rely on forming a protective chromia scale, i.e. for stainless steels and Ni-base alloys. In the case of stainless steels, this may cause the protective chromia scale to be replaced by less protective and faster growing oxides such as spinel and iron oxides, resulting in so-called breakaway oxidation [1, 4]. Ni-base alloys with low amounts of iron are not likely to form iron oxide scales [5]. Volatilization of C rO2(OH) can have other detrimental effects depending on the application, e.g. cathode poisoning in solid oxide fuel cells (SOFCs) [7]
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