Behaviour of NiAl APS‐coatings in chlorine‐containing atmospheres

Due to their thermal expansion coefficient being close to those of ferritic steels, NiAl atmospheric plasma spraying (APS)‐coatings have been chosen to be tested in chloridizing atmosphere. A new type of quasi‐stability diagram which couples thermodynamic and kinetic approaches was developed to define the stability domains of the pure metals aluminium and nickel in dynamic chlorine‐ and oxygen‐containing atmospheres. NiAl APS‐coatings (300 µm) with low porosity on steels were applied and tested in an atmosphere containing 0.2 v/o chlorine and 3 ppm oxygen. After 280 h of exposure one quarter of the outer coating was transformed into a protective alumina layer. Much nickel depletion, due to the evaporation of nickel chlorides, was observed.