Comparative laboratory testing of molten salt attack of superheater tube materials

In some practical superheater corrosion failure cases one finds that the corrosion environment has actually been a low-melting salt film spreading on the tube surface. In this study a modified crucible test method is introduced that simulates this corrosion condition. Metallic specimens exposed in air atmosphere as half-immersed into a molten alkali chloride - alkali sulphate salt mixture of eutectic composition in 520-530°C. Materials evaluation is based on the post-experiment examination of the specimens and their cross-sections. Low-alloy steels and various austenitic steels and nickel alloys were tested using this method. For low-alloy steels and austenitic stainless steels the characteristic corrosion attack mode was intergranular attack and growth of oxide scales. More corrosion resistant steels and alloys suffered from pitting attack. Corrosion morphologies observed had a close resemblance to practical corrosion morphologies in cases involving alkali sulphate - alkali chloride melt attack.