Sulfidation: Turbine Blade Corrosion

A large number of Pratt & Whitney Canada (P&WC) PT6 Service Bulletins and maintenance manuals have been used in the preparation of this article. For this reason the specific recommendations are most closely applicable to P&WC PT6 engines, but the...


Deep inside every turbine engine there’s a conflict occurring: Sulfur versus contaminants in engine intake air such as sodium (salt), air pollution, soot, volcanic ash, agricultural chemicals, and other airborne particles.

When the turbine’s scorching heat joins the mix, it results in a corrosive weapon that eats away at the clean, efficient surfaces of compressor turbine blades. There are ways to minimize the collateral damage, (e.g. delay expensive part replacement or avoid massive engine failures). The name of this conflict is sulfidation.

Causes
Every technician knows the old saying “rust never sleeps” and has witnessed first hand the results of the oxidation that takes place when ferrous-based metals (e.g. iron)come in contact with moisture-laden air. Protective coatings and air drying techniques minimize and impede the corrosion process but cannot stop it completely. So it is with sulfidation (e.g. sulfur corrosion).

Every turbine engine uses fuel contaminated with sulfur, that when burned at high temperatures, emits varying amounts of sodium sulfate gas. Sulfidation is accelerated by the presense of catalysts. Just as there is no way to dry the air completely to eliminate rust-producing moisture, no one has yet built an effective filter to purify turbine engine intake air completely. If the aircraft operates near oceans, industrial complexes, cities, or volcanic regions, the engine’s intake system provides even stronger allies to link up with the sulfur waiting in the combustion chamber. The hot section becomes the battlefield where sulfidation begins. Once the sulfidation attack successfully penetrates a CT blade’s protective layer and reaches the base metal, there is no reversing the process.

For decades, turbine designers and engineers have come up with effective defenses for their turbine blades. These include using strong materials capable of withstanding high temperatures such as nickel and cobalt base metal alloys. In addition, several very effective defensive coatings have been added as “armor” for the blades. These coatings have included materials such as diffused aluminide and silicone aluminide (Sermaloy J). Each coating has its own particular advantages and disadvantages. The research and testing continue to search for that perfect alliance of base metal and protective coating that can effectively fight off sulfidation while not sacrificing protection against the even higher temperature oxidation forces. The goal is to try to extend operating intervals and to keep costs down — a daunting task. Different blade airfoil designs have also improved the defenses against both Type I sulfidation (1,500 F – 1,750 F; 825 C to 950 C) and Type II (1,300 F to 1,500 F; 700 C to 800 C).

Effects
Currently, there is no “magic weapon” in winning this war. There are only delaying actions that postpone what is considered “nonacceptable” damage before a turbine blade or allied part must be removed from service. It falls to the service technician and inspector to collect and analyze the intelligence data and physical evidence to determine the true condition and strategic plan of defense. There are four main levels of sulfidation attack that occur and must be evaluated at inspection time. It’s important for the technician/inspector to correctly determine which stage of attack is evident on each part.

STAGE 1: Light (or mild) sulfidation (initial coating deterioration) — The surface of the blade is slightly roughened with localized areas of breakdown evident on the protective coating and slight depletion of the base metal layer. None of this has, so far, reduced the mechanical integrity of the blade, but without treatment, the condition will eventually worsen. With this degree of damage, stripping and recoating the blade is effective.

STAGE 2: Failure of the protective layer (initial substrate corrosion) — Scale breakdown and surface roughness is more advanced than in Stage 1. Obvious depletion of the underlying alloy has begun. While mechanical integrity is maintained, there is no way to salvage this component back to original condition.

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