Some Aspects of the Hot Corrosion of Cobalt-base Alloys

Abstract

In an effort to develop a laboratory method suitable for studying the salt-induced hot corrosion encountered in gas turbines, the corrosion of a number of cobalt-based alloys and two comparable nickel-based alloys has been examined at 900°c in molten sodium sulphate and in an apparatus in which small concentrations of salt can be introduced into a gas stream. The molten salt was clearly too aggressive, in that the presence of tungsten in the cobalt-based alloys produced an extremely rapid attack, whereas practical experience in gas turbines has shown that tungsten is certainly not harmful. The nickelbased alloys were corroded less than their cobalt-based equivalents. The gas-phase apparatus seemed much more promising as a practical test. In an oxidising environment as little as 5 ppm NaCI produced a very considerable increase in attack, even though theoretically there should have been no condensation of salt on the metal, and none was detected. With high concentrations of sulphur dioxide the corrosion of nickel-containing alloys was extremely rapid, with the formation of liquid nickel sulphides. It is suggested that tantalum might be preferable to tungsten as a solid solution strengthener for cobaltbase alloys, and that while it is clearly necessary to stabilise the fcc phase, since rapid penetration of corrosion along the phase boundaries in two-phase alloys leads to fragmentation of the metal surface, iron might be it better stabiliser than nickel because of the higher melting point of its sulphides.