We religiously washed T53 (205), PT6 (212,412), C20 (76A) and Turbomeca (76C). Difference with the 76C was the rinse is done at 70% N1. We had three catastrophic engine failures with the 76C one of which will be found at
www.atsb.gov.au search the aviation site for VH-EXX engine failure. Is accompanied by an extensive technical analysis of the engine strip part of which says,
"The internal and external surfaces of the diffuser assembly showed significant levels of general and localised corrosion. The inside surfaces of the diffuser housing showed extensive pitting corrosion in many locations. The coincidence of several longitudinal cracks with large corrosion pits suggested the contribution of pitting to the initiation of fatigue cracking. Localised corrosion is known to increase the risk of fatigue damage and can appreciably lower the limiting stress levels required to initiate cracking.
The bulk of the scale-type deposit found over the inside surfaces of the diffuser housing was shown by analysis to be compounds of calcium, sulphur and oxygen, with iron, silicon, sodium and chromium also present in significant quantities. The iron and chromium most likely originated from the diffuser parent material, whereas the calcium, sulphur, sodium, silicon and oxygen are elements most likely entrained from external sources. The washing procedure adopted for the periodic maintenance of gas-turbine units commonly uses water introduced into the airflow path. In the presence of dissolved solids within the water (known as 'hardness'), resilient scales can form, particularly if the water is introduced into an operating or hot engine. Soluble elements can be readily precipitated from hard water when being exposed to hot surfaces.
Scale accumulations can also exacerbate corrosion problems by trapping corrosive compounds against metallic surfaces. The corrosion damage to the diffuser housing is evidence of this process, with chloride compounds (known promoters of pitting corrosion attack) being detected on the scaled surfaces."
We were using tap water at the time but after this went to demineralised/deionised water.