Attack on superalloys by chemical and electrolytic processes

In the production and overhaul of aero engines chemical and electrolytic surface treatment processes such as electrolytic degreasing, chemical and electrochemical etching, chemical descaling and chemical stripping of thermally sprayed coatings are used. In the course of some of these processes the heat resistant nickel and cobalt base alloys found in the “hot end” of the newer gas turbines are attacked. These superalloys have a common characteristic, namely, they are precipitation hardened. The alloying elements added for this purpose give rise to intermetallic compounds (carbides, nitrides, carbonitrides) which appear as inclusions of various shapes and sizes within the grain or at the grain boundaries. The conclusion from this work is that these intermetallics present as separate phases can be dissolved out by oxidative attack leaving pits. The separate processes and their objectives are detailed. It is shown in which process stages and in which solutions attack occurs. The inclusions have been analysed and the nature of the attack is illustrated. We conclude from this work, which has embraced seven nickel and cobalt base materials, that certain processes or process steps cannot be permitted on these alloys. For cleaning and etching alternative processes are given. For descaling a new compatible process was evaluated and will be discussed in a second report.