Type I and Type II Hot Corrosion of Ni-Based Alloys Without an Incubation Period

Abstract

A series of results obtained with different types of Ni-based alloys for turbine discs and blades and exposed to various (Type I and Type II) hot corrosion conditions (air, air + 150–400 ppm SO₂) is reported. Both the continuous mass change measurements and the characterizations of alloys after a brief exposure at high temperature with sulphate deposits, in air or in air + SO₂ (g) atmospheres, clearly demonstrate fast oxidation rates from the earliest time of exposure in the presence of sulphate deposits, i.e. accelerated oxidation. In situ SEM observations also support these findings. It clearly appears that hot corrosion often starts with a direct reaction involving the metallic substrate, the gaseous species and the sulphate deposits, i.e. it occurs in most cases without any incubation period. The results can therefore be of interest for the development of complementary selection procedures for alloys and coatings.