Hot Corrosion in Gas Turbines

DURING COMBUSTION in the gas turbine, sulfur from the fuel reacts with sodium chloride from ingested air at elevated temperatures to form sodium sulfate. The sodium sulfate then deposits on the hot-section components, such as nozzle guide vanes and rotor blades, resulting in accelerated oxidation (or sulfidation) attack. This is commonly referred to as “hot corrosion.” Sulfur in the fuel is generally limited to 0.3% for commercial jet engines and to 1.0% for marine gas turbines (Ref 1). Sodium chloride comes from seawater (see Table 9.1) (Ref 2). Seawater is also a source of sulfur. For aircraft engines, Tschinkel (Ref 1) suggested that runway dust may be a source of salts. Gas turbines generally use large amounts of excess air for combustion (a large fraction of air, in fact, is also used to cool the combustor), with a typical air-to-fuel ratio from about 40 to 1 (during takeoff) to 100 to 1 (at cruising speed) for aircraft gas turbine engines (Ref 2). These air-to-fuel ratios correspond to about 0.12 to 0.18 mole fractions of oxygen in the combustion zone (Ref 2). Thus, the combustion gas atmosphere is highly oxidizing. The sulfur partial pressure in the atmosphere can be extremely low, varying from 10−40 to 10−26 atm over the range from 330 to 1230 °C (620 to 2240 °F) (Ref 2). These sulfur partial pressures are well below those necessary to form chromium sulfides, which are frequently observed in alloys suffering hot corrosion attack. High-temperature alloys that suffered hot corrosion attack were generally found to exhibit both oxidation and sulfidation. The hot corrosion morphology is typically characterized by a thick, porous layer of oxides with the underlying alloy matrix depleted in chromium, followed by internal chromium-rich sulfides. It is generally believed that the molten sodium sulfate deposit is required to initiate hot corrosion attack. The temperature range for hot corrosion attack, although dependent on alloy composition, is generally 800 to 950 °C (1470 to 1740 °F). The lower threshold temperature is believed to be the melting temperature of the salt deposit, and the upper temperature is the salt dew point (Ref 3). This type of corrosion process is sometimes referred to as Type I hot corrosion to differentiate it from Type II hot corrosion, which occurs at lower temperatures (typically 670 to 750 °C, or 1240 to 1380 °F) (Ref 4). Type II hot corrosion is characterized by pitting attack with little or no internal attack underneath the pit (Ref 4). Type II hot corrosion is rarely observed in aeroengines because the blades are generally operated at higher temperatures (Ref 5). However, marine and industrial gas turbines, which operate at lower temperatures, can experience low-temperature Type II hot corrosion. Type I hot corrosion generally proceeds in two stages: an incubation period exhibiting a low corrosion rate, followed by accelerated corrosion attack. The incubation period is related to the formation of a protective oxide scale. Initiation of accelerated corrosion attack is believed to be related to the breakdown of the protective oxide scale. Many mechanisms have been proposed to explain accelerated corrosion attack; the salt fluxing model is probably the most widely accepted. Oxides can dissolve in Na2SO4 as anionic species (basic fluxing) or cationic species (acid fluxing), depending on the salt composition (Ref 6). Salt is acidic when it is high in SO3, and basic when low in SO3. The hot corrosion