The effect of Mo on the corrosion behavior of Ni20Cr–xMo alloys in an oxidizing chlorine‐containing atmosphere using air mixed with the salt‐vapor mixture of NaCl–KCl–CaCl2 at 570°C was investigated. The results revealed that the corrosion performance of the Ni20Cr alloys in the oxidizing chlorine atmosphere was improved by Mo addition of up to 3 wt%. The Mo‐free alloy formed a potassium chromate during corrosion as a result of the reaction between the Cr2O3 scale and KCl vapor. The chromate formation increased the chlorine potential at the scale surface and induced the breakdown of the protective Cr2O3 scale, resulting in internal chromium chloride precipitates and a Cr‐depleted zone. In contrast, the presence of Mo resulted in the formation of a NiO scale, which did not react with the salt vapors and, therefore, prevented the formation of chromates. The beneficial effect of Mo on the high‐temperature chlorination of Ni–Cr alloys in salt‐vapor‐containing atmospheres was ascribed to the suppression of chlorine generation due to NiO scale formation.
The transition behavior of an Al-rich amorphous oxide layer to an external Al 2 O 3 layer on Fe-(4, 24)Cr-(6, 10)Al (at.%) alloys was investigated during heating to 1000°C at a heating rate of 50°C/min, by means of in situ hightemperature X-ray diffraction measurement and TEM observation. In the alloy containing 6Al, internal amorphous Al 2 O 3 was initially developed below the Al-rich amorphous surface layer. The amorphous internal precipitates transformed to be crystalline and grew laterally with time. The internal precipitates subsequently connected with each other to form a continuous a-Al 2 O 3 scale. In the case of 10Al alloy, an Al-rich amorphous layer transitioned to a crystalline a-Al 2 O 3 layer from the interface between transient/amorphous layers during heating. The Al 2 O 3 scale developed on high Al alloys contained Fe and Cr with relatively higher contents, but that formed on low Al alloy contained low Fe and Cr. The effect of Cr on promoting an external Al 2 O 3 scale formation was found to be weaker for alloys with higher Al content compared to the alloys with lower Al content, if Al 2 O 3 scale was directly transitioned from the amorphous layer.
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