Sulphidation is a form of high temperature hot corrosion, a combined reaction made possible by the presence of molten salts on Compressor Turbine Blade surfaces and is the most common form of hot corrosion found in aircraft engines.
Sulphidation/Type 1 corrosion depends on the presence of sodium sulphate (Na2SO4) on the blade surface. This is most commonly found in engines which have been operating in marine environments and have ingested marine aerosol with the combustion air. In addition to the appreciable quantity of Na2SO4 present in sea spray, the sodium chloride (NaCl) present, may also combine with trace amounts of sulphur in the fuel to create NaSO4. The use of Avgas can also increase the levels of sulphidation as it increases the temperature band in which the combined reaction occurs.
What is the impact of sulphidation?
It has been proven over various long term field tests and observations that there is a correlation between the rate of power degradation and the atmospheric conditions in which aircraft operate. This causes the turbine engine to be less efficient, and more expensive to operate. Without proper maintenance of the engine, the replacement of compressor rotor parts can be costly. During a routine 400-600 hour Borescope Inspection, an engineer will be looking to categorise the components into the 4 stages of sulphidation;
Stages of Sulphidation
Stage 1 is light or mild sulphidation. This is where the coating of the part has initial deterioration. Without treatment the corrosion will continue and the part will need to be replaced. At this point the part is not considered to be at risk of mechanical break.
Stage 2 is where the protective coating has failed from the effects of corrosion and there is visible surface roughness. While the mechanical integrity is still considered to be maintained there is no way to return the part to original condition.
Stage 3 is severe corrosion of the part. This is where there is evidence of liquid sodium under the protective layer in addition to a build-up of scale on the surface. Mechanical integrity is severely compromised and the corrosion will continue regardless of exposure.
Stage 4 is a catastrophic attack on the component. The corrosion has penetrated the part and large blisters appear on the surface. The loss of structural material will lead to component failure. At the point of inspection, the technician will identify which stage categorises the parts and the estimated time of progression to stage 3. At stage 3 the parts must be removed and replaced.
Care should be taken to distinguish between corrosion build-up and possible light brown or rust coloured deposits which are essentially harmless combustion byproducts. The latter are more widespread over hot section components and while possibly affecting performance, will not directly affect blade integrity.
So how can we prevent this from occurring?
Since the first PT6 engines were brought into service, there have been numerous advances in coating technology. The most recent engines contain blades using a Diffused Platinum Aluminide protective coating. These coatings delay the actions of the corrosion, prolonging the Hot Section parts of your engine. These coatings improve the operational length of the Hot Section parts and therefore reduce the collateral damage to the combustion of the engine.
Further measures we recommend are de-salinized cold washes at the end of running to clean out the particles of which there are 4 types; Compressor, Compressor Turbine Compressor performance recovery wash and engine external wash. It is important to note that it is important to perform a drying ground run after washes, if you miss this stage it can leave you with the same problems you are trying to negate.