Validation of Hot Corrosion and Fatigue Models in HOTPITS

HOTPITS is a set of physics-based modeling tools for treating Type II hot corrosion in Ni-based superalloys. The methodology includes modeling the nucleation, growth, and coalescence of pits and microcracks as a random process, as well as the transition of pits to micrcracks and the propagation of the resulting large crack to failure. In this investigation, critical experiments were performed on coupon and low-cycle fatigue (LCF) specimens in order to validate the hot corrosion and the fatigue models in HOTPITS. The pit nucleation, growth, and coalescence models in HOTPITS including the assumption of a random process are validated by the hot corrosion critical experiments performed at two salt contents. The LCF critical experiments, performed using a marker band protocol, validated the stress concentration factor-based models used to predict the pit-to-crack transition in the HOTPITS tool.

Hot Corrosion Resistance of Laser-Sealed Thermal-Sprayed Cermet Coatings

Hot corrosion affects the components of diesel engines and gas turbines working at high temperatures, in the presence of low-melting salts and oxides, such as sodium sulfate and vanadium oxide. Thermal-sprayed coatings of nickel–chromium-based alloys reinforced with ceramic phases, can improve the hot corrosion and erosion resistance of exposed metals, and a sealing thermal, post-treatment can prove effective in reducing the permeability of aggressive species. In this study, the effect of purposely-optimized high-power diode laser reprocessing on the microstructure and type II hot corrosion resistance of cermet coatings of various compositions was investigated. Three different coatings were produced by high velocity oxy-fuel and was tested in the presence of a mixture of Na2SO4 and V2O5 at 700 °C, for up to 200 h: (i) Cr3C2–25% NiCr, (ii) Cr3C2–25% CoNiCrAlY, and (iii) mullite nano–silica–60% NiCr. Results evidenced that laser sealing was not effective in modifying the mechanism, on the basis of the hot corrosion degradation but could provide a substantial increase of the surface hardness and a significant decrease of the overall coating material consumption rate (coating recession), induced by the high temperature corrosive attack.