The formation of Cr(VI) compound at the interface between metal and heat-insulating material and the approach to prevent the formation by sol–gel process

Most heating apparatuses include Cr-containing metals and heat-insulating materials that are often placed in contact with each other. It was observed that hexavalent chromium [Cr(VI)] oxide occurred at the contact interface between Cr-containing metals and heat-insulating materials at elevated temperatures. Yellowish reaction products were confirmed on the contact surfaces of the heat-insulating materials and were identified as Cr(VI) compounds. The effect of heat-treatment temperature, time and metal Cr content on the amount of Cr(VI) was investigated. The amount of Cr(VI) compound increased as the heat-treatment temperature, time and metal Cr content increased. The compounds that occurred as a result of the reaction were CaCrO4 and Na2CrO4 for heat-insulating materials containing Ca and Na, respectively. The sol–gel coating technique was used to prevent the formation of Cr(VI) compounds. Al2O3-based coating films were formed on the surfaces of SUS304 metals by a dipping process. The coated metals were placed on the heat-insulating materials and these specimens were heat-treated at 873 K for 5,000 h. As a result, the formation of Cr(VI) was almost completely prevented by the coating film. It was concluded that the Al2O3-based coating films effectively suppressed the diffusion of Cr.