Laboratory-Scale Replication of Deposit-Induced Degradation of High-Temperature Turbine Components

This study investigated the linkages between deposit chemistry and degradation morphology of field-exposed aero-turbine parts and the associated development of effective laboratory-scale replication testing procedures. Inductively coupled plasma-optimal emission spectroscopy (ICP-OES) analysis indicated that the water-soluble deposit constituents were mainly Na-rich, Ca-rich or a mix of these, with the relative amounts having a dependence on geographic region in which a given part was mainly exposed. Degradation of the field-exposed parts was characterized using scanning electron microscopy (SEM) and revealed unconventional hot-corrosion attack. Microstructures included duplex oxide scales, Ca-containing oxide, internal oxidation, sulfidation, and nitridation. Degradation mechanisms elucidated by replication testing were deposit-induced selective depletions at high-temperature and low-temperature oxidation. Field degradation was replicated when considering the thermal dependences of both stages. Replication of a chromide-coated field part required calcium chromate liquid formation during high-temperature testing. Electron probe microanalysis (EPMA) of the coating subsurface revealed extensive Cr depletion after exposure to CaO above the calcium chromate eutectic temperature. Oxidation temperature also contributed to replicating field degradation microstructures. Specifically, matching degradation was observed after the depleted superalloy or chromide coating was oxidized in air at an intermediate temperature.