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Definition - What does Sulfidation mean?

Sulfidation is a chemical reaction of a solid substance, such as a metal or an alloy, with sulfur in some form in its working environment. This produces compounds of sulfur that usually form on the solid surface or under the surface of the substance, such as a metal or alloy. Sulfidation most often causes serious deterioration of the solid surface and the vital functional properties of the affected substance.

Corrosionpedia explains Sulfidation

Sulfidation most often affects many critical pieces of equipment such as turbo engines used in aviation, marine and other sectors. Normally, no turbine engine is free from sulfidation, because the root cause, sulfur, is present in fuel. This corrosive reaction starts at the turbine blade root, spreads to shrouds, and also to the blade airfoil. Sulfidation is a critical parameter that requires regular measurement and monitoring in many cases.

Sulfidation exists in the extreme high temperature, high pressure environment of turbine engines. Galvanic corrosion in contrast, occurs at ambient temperatures due to natural electro-chemical reaction, and causes deterioration of structures.

Sulfidation is a chemical reaction, classified as type1 and type 2. Type 1 is caused at temperatures of 825°C to 950°C, and type 2 is caused at 700°C to 800°C.

Generally sulfidation reactions occur on metals and alloys. Sulfur from airborne salts in some environments, and fuels in most cases, chemically react at high temperatures, with the oxides on the turbine blades, causing deterioration of the base alloy metal of the blades. During combustion of the fuel, the combination of water vapor and sulfur oxides produce sulfuric acid and finally result in chemicals like sodium sulfates that endanger the blades' protective coating.

Since fuels are petroleum based, they contain very small amounts of sulfur during and after complete refining. The fuel specification of a jet engine typically allows no more than 0.3 percent sulfur, and may go up to 0.4 percent in other specifications. The root of the blade is most seriously affected by sulfidation, and the weakening of this structural area and the affects balancing, can result in a catastrophic failure and an aviation disaster.

Sulfidation is studied in order to:

  • Understand the nature and rate of sulfidation and the impact on the surface
  • Prevent catastrophic failures of critical systems such as jet engines due to sulfidation of turbine blades
  • Continuously monitor deterioration of the surface of turbine blades and other affected critical components and systems
  • Ensure trouble-free operation of critical systems, such as turbines, susceptible to failure due to sulfidation
  • Minimize economic loss due to repair and restoration or replacement
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