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 constant monitoring.
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.
How does it occur?
How does sulfur get into the engine in the first place? All hydrocarbon, or petroleum-based fuels contain trace amounts of sulfur after refining. The specification for Jet A, A-1 fuel (JP-8) allows a maximum of 0.3 percent by mass of sulfur, and this percentage goes up by 0.1 percent for other turbine fuels. No turbine engine is immune from this process, 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 constant monitoring. The blade root is obviously the worst place for corrosion to form, since weakening of the blade in this area can cause catastrophic turbine failure. Turbine blade airfoils are designed to decrease as much as possible the accumulation of the compounds that lead to sulfidation, and blade coating materials are used to keep the most corrosion prone areas of the turbine as corrosion free as possible.
The Rolls-Royce RB211-535E4 engine is a triple spool high bypass fan type with a low pressure wide chord fan as the main propulsion element (Figure 1). The three-spool design provides a high bypass ratio in a short nose to exhaust cone space, and in the case of the RB211, a greatly simplified compressor section. There are no movable compressor guide vanes to tinker with, since compressor airflow control is done through a series of bleed valves. This design uses airflow bleeds from various compressor stages to cool the hot section through the use of hollow turbine blades and controlled bleed film cooling of the turbine wheels. There is one high-pressure (HP) turbine wheel attached to a six-stage HP compressor, one intermediate pressure (IP) turbine wheel attached to a six-stage IP compressor and three low-pressure (LP) turbine wheels attached to a single-stage LP compressor (the fan). All turbine blades regardless of stage are attached to the turbine disks using the standard fir tree serration method. This arrangement retains the blades radially, and circumferential lock plates retain the blades axially. HP turbine blades are made from a directionally solidified nickel alloy casting, and there are 102 blades on HP turbine disk. Sulfidation corrosion is usually confined to the blade root shanks and shrouds in all stages of the RB211 turbine section, with light corrosion occurring on the airfoils.
Sulfidation exists in the blistering environment of a turbine engine hot section.
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