Hot Corrosion / Protection Lecture Questions Flashcards
What is the hot corrosion effect and describe the process?
The formation of condensed phases on the surface of alloys under high temperatures, hinders the passivation layer on the alloy protecting it. DURING COMBUSTION in the gas turbine, sulphur 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.” Condensed
alkali metal salts (eg. Na2SO4) are thought to be primarily responsible. Sources could be; Ingestion of sea salt in a
marine environment; or combustion of fuels.
Explain diffusion protection coatings and how they operate? Provide an example?
A reservoir with a very high
concentration of the element that should be enriched is created at the surface either in the gas phase or in a solid/liquid
state. For example, with Nb, Mo or W, silicon is deposited onto the surface and diffused into the substrate to form a
silicide eg. MoSi2, this will then oxidize to form Si rich oxides that subsequently provide protection. Alternatively Al
could be deposited and diffused to form aluminides eg. NbAl3, which subsequently forms Al rich oxides. Diffusion
coatings involve, and rely on, the substrate for their formation and therefore the composition of the coatings is limited. If
the majority of the diffusion is from the coating to the substrate it is described as “net inward”. If the majority of the
diffusion is from the substrate to the coating it is described as “net outward”.
Explain Overlay protection coatings and how they operate? Provide an example?
A layer is applied to a material and diffusion is not the primary formation mechanism. Coatings are relatively thick (~ 50 – 150um) compared to diffusion coatings. Overlay coatings are produced by one of the following processes: Physical vapor deposition (PVD) process;
Thermal spraying process, or Overlay welding (also called cladding The protective life increases in proportion to the coating
thickness. Thicker coatings are more prone to cracking as the cyclically induced stresses are higher. One of the main types
used is based on “M”-Cr-Al-Y (where M = Ni or Co). The “M” (Ni or Co) is the matrix of the coating. The matrix is ductile to provide better resistance to thermal fatigue. The Cr content is high to provide corrosion resistance. The Al is included for Al2O3 formation. The Y (Yttrium) is included to enhance the oxide adherence.
