Corrosion of Three Commercial Steels Under ZnCl2 –KCl Deposits in a Reducing Atmosphere Containing HCl and H2S at 400–500°C

Abstract

The corrosion of three commercial steels in a reducing atmosphere containing HCl plus H 2 S in the presence of ZnCl 2 -KCl deposits has been investigated at 400-500°C and compared with the corrosion of the same materials in a similar gas mixture free from H 2 S. The presence of H 2 S in the gas accelerated the corrosion of the three commercial steels beneath ZnCl 2 -KCl deposits. All materials suffered from severe corrosion with partial detachment and spalling of the external scales. Degradation of the steels resulted from the penetration of chlorine-containing species through the scale formed initially down to the metal matrix, because chorine-rich species were detected close to the alloy/ scale interface. Although the corrosion resistance generally increased with increasing Cr content, even the high-Cr stainless steel SS304 is still unable to provide good corrosion resistance against the ZnCl 2 -KCl deposits in the presence of H 2 S due to the bad adherence of the scales to the alloy. The mechanisms of attack are discussed on the basis of thermodynamic considerations and of the active-oxidation model.

Performance of Alloy 625 under Combustion Gas Environments: A Review

“…In this environment, none of the oxides scales formed by Cr, Ni and Mo were sufficient to protect the alloy. Under the operating conditions of waterwall tubes of a WTE incinerator, where the gas temperature is high (850ºC or higher), high concentrations of the chlorides adhered to the surface is promoted, and the corrosion damage is caused mainly by chlorination reactions (6,8). This was in fact observed in the recently reported study (5).…”

“…These eutectic salts allow accelerated attack of metallic materials and affect the physical properties of the deposits such as the amount of molten phase and its permeability. The corrosion resistance of materials can be seriously challenged by a mixed attack involving oxidation, sulfidation, chlorination and hot corrosion (8)(9)(10). Furthermore, sulfidation is almost always involved in the corrosion of incinerators that burn hospital, chemical, and other industrial wastes.…”

“…The metal could be corroded by general corrosion and also intergranular corrosion and localized corrosion when the molten adhering ash causes the break-down of the protective oxide layer and attacks even alloys having a high corrosion resistance. In sprayed coatings, their high porosity allows diffusion into the substrate of chlorine and oxygen, leading to internal chlorination and oxidation (8). Several attempts have been made to understand the corrosion mechanism under such conditions with the objective to make a suitable material selection in actual applications.…”

“…2NaCl(g) + Fe 2 O 3 + 0.5O 2 → Na 2 Fe 2 O 4 + Cl 2 [8] Penetration of chlorine occurs through the metal oxide scale to the oxide/metal interface. This results in a reaction with iron or other metal component of the tube to form metal chlorides:…”

Improving Heat Recovery In Biomass-Fired Boilers

“…Much of the research on corrosion by lead and zinc impurities has been done for municipal waste incinerators. Spiegel [88] showed that, if HCl is present in the flue gases, temperatures above about Pan, Zeng and Niu [92] later studied the corrosion of 9% and 11% Cr steels and UNS S30400 (Type 304) stainless steel under ZnCl 2 -KCl deposits at 400-500°C (752-932°C) in a reducing CO 2 -2%H 2 -0.5%HCl gas mixture. This work and a subsequent study [93] confirmed that alloys containing more chromium showed greater corrosion resistance in reducing environments.…”