Hot Corrosion of Pure Nickel Induced by Na₂SO₄-NaCl Mixtures

Abstract

The effect of salt composition on the Na₂SO₄-NaCl induced hot corrosion of pure nickel was examined by modified coating and whole immersion tests in still air. The NaCl content of the mixed salt was varied from 0 to 100 mass percent. The corrosion loss and the depth of penetration were measured.
The result of coating test showed that corrosion losses were very large at salt compositions from 0 to 25%NaCl and decreased with increasing NaCl content. Corrosion losses by the mixed salt in the whole immersion test were much smaller than those in the coating test. Catastrophic corrosion of pure nickel occurred only in the condition where molten Na₂SO₄ existed and oxygen was supplied sufficiently. Hot corrosion by NaCl was promoted through the dissolution of nickel metal into molten salt.
Except for the corrosion by 100%NaCl, internal penetrations were observed. The depths of internal penetration were almost independent of both the salt composition and test method. The internal penetration was caused by diffusion of sulfur, and proceeded in grain boundaries at an early stage and then within grains.
Hot corrosion behavior of pure nickel was the process controlled mainly by the action of Na₂SO₄ in the salt mixtures.