Turbine Blade Sulfidation
 

Airlines use flex thrust or de-rated power settings on take-off to reduce the erosion on turbine blades called Sulfidation. Sulfidation, as I understand it happens at close to redline ITT/EGT.

As i understand my turbine theory, as a turbine engine climbs to high altitude, ITT etc becomes the limiting factor in the power/thrust that can be produced assuming a non flat-rated engine.

Does Sulfidation erosion in that case happen in cruise where the ITT/EGT is close to limit (ie less than 10 deg margin), or is the cruise limit ITT/EGT outside the range for Sulfidation erosion to occur.

Max continuous thrust or the lower max cruise thrust is normally limited to lower temperatures. Max takeoff thrust is usually time limited to 5 or 10 minutes.

Sulfidation: High temperature turbine blade corrosion

Mechanical erosion like pitting, FOD, cracking, and burning is the main form of damage to engine compressor and turbine sections, with corrosion usually reserved for the parts of the airplane in closest contact to the atmosphere.
But there is one form of corrosion that exists in the blistering environment of a turbine engine hot section, and it is called sulfidation (or sulphidation in some texts). The galvanic corrosion that eats away aircraft structures exists at normal temperatures and is an electro-chemical process. Sulfidation is a chemical process that occurs in high temperature environments, and typically has two types: Type I sulfidation occurs between about 1,500 F to 1,750 F (825 C to 950 C), and Type II sulfidation occurs in the 1,300 F to 1,500 F range (700 C to 800 C).


One definition of sulfidation is a reaction of a metal or alloy with some form of sulfur to produce a sulfur compound that forms on or under the surface of a metal or alloy. Sulfur in the fuel and airborne salts like sodium and chlorine reacts with the oxide layer on the blades in the high temperature environment of the turbine to attack the base metal of the blades. As a normal by-product of combustion, sulfur oxides are formed that combine with the salts and other elements ingested into the engine. This reaction forms sodium sulfates that expose the blade's protective oxide layer to decay. Water is also produced as a by-product of hydrocarbon fuel combustion, and this water can combine with the sodium sulfur compounds to produce sulfuric acid.

I keep being reminded of that old saying about "nothing new in the world".

Zaphod's link leads us to the remark about "requiring constant monitoring",,etc,,etc.

How about daily inspections of the fan/compressor/inlet, LP turbine blade , EGV's, jet pipe areas..Don't all airlines do it ? The one that I work for certainly does.

I seem to remember an Airworthiness Notice [ AWN.12, Experience from Incidents ]which dealt with molybdenum di-sulphide contamination of hot section blades [i.e. splashing on too much Molykote during fan lubes] as being a likely cause.

I have to say though , an engine will normally be removed from the wing for replacement long before any of that type of degradation has time to manifest itself.

Most airline base/line guys will confirm this.

From Boeing:

Takeoff thrust may be used only for a maximum of five minutes, with one exception: the time limit is extended to ten minutes for airlines that have purchased a special Airplane Flight Manual appendix called the “ten-minute appendix”.
The Certificate Limitations section of that appendix states specifically:
“The time limit on the use of takeoff thrust is increased to 10 minutes provided this use is limited to situations where an engine failure actually occurs and there is an obstacle in the takeoff flight path.”

https://i.imgur.com/GyQvKr3.jpg

As very well summarised by a previous post, sulphidation occurs to high temperature alloys when operated at such temperatures in contact with the sulphur content in aviation fuels. Not just turbine blades but also combustion components.
On turbine blades, the attack tends to occur to the pressure surface at about 2/3 height where the combustor peak outlet temperature occurs.
If sulphidation is a problem the manufacturer would normally coat the aerofoil with diffused materials which resist the effects of high temperature sulphides. For example aluminium or more recently platinum.