Effect of Mo on corrosion behavior of Ni20Cr–xMo alloys in air with NaCl–KCl–CaCl2 vapor at 570°C

Abstract

AbstractThe 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.