Hot corrosion of wrought and weld overlay alloy 625 in molten salts environments

KIVCET smelter is a modern direct smelting process which has been used to replace conventional sinter/blast furnace technology in pyrometallurgical process of lead and zinc. Although heavy metals including Cd, Pb, Zn, Fe as well as S, O and Cl, are the main elements playing a leading role in the formation of molten phase on the waterwall tubes of the KIVCET waste heat boiler, hot corrosion behavior and electrochemical properties of weld overlay and wrought alloy 625 have not been studied yet in the molten salt environments containing the above-mentioned elements. The present study was carried out to study hot corrosion behavior of the weld overlay and wrought alloy 625 as well as failure mechanism of the weld overlay alloy 625 under the corrosive conditions of the radiant boiler in the KIVCET smelter. It was found that the deposited salt mixtures on the waterwall tubes of the radiant boiler had a strong tendency to form a molten phase at the operating temperature range of the radiant boiler. Presence of the deposited salt mixtures and the formation of the molten phase led to the occurrence of the hot corrosion attack in the waterwall and the ultimate failure of the weld overlay. The dilution and, consequently, the presence of a significant amount of Fe in the weldment composition were the key issues in the alloy 625 weld overlay. High concentrations of sulfur and oxygen in the grain boundaries of the wrought alloy 625 are to blame for the occurrence of the intergranular corrosion together with the internal attack. An electrochemical model (a porous and non-protective barrier layer model) was used to explain the corrosion/electrochemical behavior of the wrought alloy which fit into the obtained EIS data well.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat