Sulfidation: High temperature turbine blade corrosion

High temperature turbine blade corrosion By Chris Grosenick No turbine engine is immune from it, and since sulfidation tends to form in the blade root, shrouds, and to a lesser extent on the blade airfoil, it is a serious matter that requires...


Detection and treatment
In contrast, on low bypass fan engines like the Pratt & Whitney JT8D Series, sulfidation is allowed on the turbine blades as long as blistering doesn't cover more than 25 percent of the airfoil surface. Light corrosion is allowed on the entire surface of the blades, but these situations require monitoring on a flight hour basis. Another example is the P&W PT6 turboprop, where any sulfidation blistering or heavy deposits on the compressor or power turbine blades are cause for rejection. In these engines, blade airfoil sulfidation can be seen using a standard borescope and it can be assessed and monitored on-wing.

Because of the RB211's design, sulfidation usually is not apparent on the blade airfoils unless it is heavily corroded, so detecting and removing it is left for engine teardown. The use of blade and turbine wheel cooling, blade design, and high efficiency airflow paths, fuel nozzles, and combustor designs lessens considerably the formation of sulfidation.

Figure 2 shows typical sulfidation areas on the HP turbine blade root shank. Because of the high temperatures in the HP turbine and the flow of cooling air through the blades, sulfidation has not been indicated in the blade internal passages. While general blade cleaning processes are similar, acid immersion times, abrasive blast cleaning, acidic descale processes, and masking requirements vary from one blade type to another. HP and IP blades require a nitric acid bath to remove sulfidation deposits, followed in special cases by a ferric chloride etch and abrasive blast to recondition the etched area. The blade airfoil and fir tree areas must be masked to prevent acid or etch solution contact in some instances, and abrasive blast cleaning is called out for fir tree serrations on some blades. If the corrosion extends into the fir tree attachment slots past acceptable limits, the blade must be scrapped. Blades that exhibit this much corrosion are most likely cracked and will require replacement anyway.

IP turbine blades have different inspection and cleaning requirements (Figure 3). In addition to root shank and shroud corrosion, internal passage sulfidation is possible on the IP blades. If sulfidation is suspected in the internal passage, a magnetoscope is required to check for areas of corrosion in the blade throughout its length, and any readings past maintenance manual limits require blade rejection. IP blades also carry an aluminized coating that through service bulletins has been extended to noncontact areas of the shroud for added corrosion resistance. There is also a service bulletin that provides a chrome coating in the blade shank prior to application of the aluminum coating.

LP blades are inspected in a similar way to IP blades, and there are additional checks made using fixtures and visual inspections (Figure 4). Cleaning LP blades requires an acidic descale process, and if the corrosion is more stubborn, acid immersion for up to three hours may be needed. Abrasive blast cleaning and fiber brush removal are approved for stubborn deposits. Since inspection and cleaning processes are different for each blade, and special inspections and cleaning processes may be required, it is best to consult the applicable maintenance task for a given blade. Several service bulletins have addressed potential service issues concerning IP and LP turbine disks and blades. The fir tree serrations in the turbine disks have been coated with sermetal, which is a sacrificial coating, and there are also chrome and aluminum coatings that are used on the turbine blades. These coating processes and the high efficiency nature of the RB211 Series engines keep sulfidation to a minimum in the turbine section.

Depending on the engine make and model, sulfidation corrosion may be easy to spot using normal inspection techniques (a borescope), and it is worth the time to investigate this corrosion process when looking for mechanical damage in the turbine section. Turbine corrosion can be a contributing factor to decreased engine performance, so always consult the maintenance manual and look for corrosion inspection requirements when inspecting the engine hot section. A little time spent here could save an unscheduled trip to the overhaul facility, or worse, a catastrophic turbine failure that could take someone's life.

Additional ReSource
Rolls-Royce RB.211 Maintenance Manual
Rolls-Royce RB.211-535E4 Line Maintenance Course Notes
Rolls-Royce RB.211 Service Bulletins: 72-9495, 72-8474, and 72-8861
Pratt & Whitney JT8D Maintenance Manual
P&W Canada PT6A-38/41/42/42A Maintenance Manual
http://turbolab.tamu.edu/Texas A&M University Turbolab Symposium Papers
http://www.corrosionsource.com

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