It has been reported that routine washing of a gas turbine engine with ZOK 27® gave a value of 0.85ppm cadmium in the effluent water whereas when another cleaner was used the figure increased to 3.8ppm. Although reportable figures vary from site to site, at this site the reportable figure is 1ppm, and the operator asked us for our comments on the difference between these figures.
Mr R W Haskellâ€™s ASME paper 89-GT-42 describes the mechanism where NiCd compressor blade coatings can be attacked in naturally occurring acidic conditions, particularly where pH drops to around pH4.0. We assume that this is the source of cadmium in washing effluent. Although Mr Haskellâ€™s paper refers to severe mechanical attack on the material it would seem that a much earlier indication that this attack is taking place is now apparent because of much stricter environmental monitoring requirements showing cadmium removal at ppm levels in the wash effluent.
In our laboratory, we took diluted samples of two cleaners and added 2.8% Hydrochloric Acid to them until the pH came down to pH4.0. It took eleven times as much acid to reduce the pH of ZOK 27® as it did with the other cleaner. We attribute the much greater acid resistance of ZOK 27® to the highly buffered corrosion inhibitor package it contains, and we believe that this is the reason why ZOK 27® reduces the amount of heavy metals leached from the compressor.
It seems to us likely that future specifications for compressor cleaning fluids may include a requirement for corrosion inhibition, particularly in acidic conditions, in order to minimize this removal of blade coating material.