High-temperature corrosion of recuperators used in steel mills

Abstract

High-temperature corrosion is a serious problem on the heat exchanger pipes of recuperators used in steel mills, because they encounter severe corrosive environments at high temperatures. These pipes are found to be corroded via oxidation, sulfidation, and molten salt corrosion. Molten salt corrosion occurs by the formation of alkali-iron sulfate as reaction products of sulfur oxide in fuel gas and sodium chloride from seawater. Many integrated steel mills are located near the sea for convenience of transportation. Salt from the seawater can affect the corrosion of the heat exchanger pipes by inducing hot corrosion. Through inspection of the used pipes of recuperators, a corrosion mechanism of the heat exchanger pipes is proposed. To evaluate the corrosion resistance of possible protective coatings for recuperators, nickel- and cobalt-based self-fluxing alloys, iron-based alloys, and chromium carbide cermet coatings were tested. The coatings with high chromium content, either as a form of carbide or as an alloying element, had excellent corrosion resistance. A Cr 3C 2–NiCr coating in carbide form, and Ni–45Cr–4Ti as an alloy coating can be recommended as promising coatings for the heat exchanger pipes, even in cases where they may encounter molten salt corrosion attack. The microstructure of a coating is as important as the chemical composition of the coating material. The inter-lamella pores may act as passages for molten salts. Therefore, the coatings should be carefully sprayed so that the molten salts do not penetrate into the coating layer.