Corrosion resistance of heat-treated alumina-forming austenitic steels in high-temperature molten NaCl–MgCl2–KCl

• The chloride salt corrosion behaviors of the as-forged and heat-treated AFA steels were compared. • Heat treatment compromises the chloride salt corrosion resistance of AFA steel. • A discontinuous MgCr 2 O 4 spinel structure was formed on the surface. • The corrosion depth of AFA steel is only 15–33% that of 316 L stainless steel. The molten chloride salt corrosion behavior of two heat-treated aluminum-containing austenitic (AFA) alloys, designated AFA1 (18Ni–14Cr–3Al) and AFA2 (25Ni–14Cr–3Al), was investigated at 700 °C in a NaCl–MgCl 2 –KCl (30.2–47.1–22.7 mol%) environment. A solution treatment of 1130 °C/1h followed by 700 °C/24 h aging was employed to precipitate secondary phases, including B2-NiAl, Laves, and NbC, which were characterized along with corrosion depth using scanning electron microscopy and energy-dispersive spectroscopy. The heat treatment significantly influenced the protective nature of the surface oxide scale. A less compact composite MgCr 2 O 4 spinel was formed. Despite this, both heat-treated AFA alloys demonstrated superior long-term resistance, with corrosion depths of less than 100 μm after 400 h. This performance markedly exceeds that of commercial 316 L stainless steel, which exhibited a corrosion depth of 260.1 μm under the identical condition. The underlying corrosion mechanisms are discussed.