Modes of Sulfate-Deposit-Induced Metallic Corrosion at Elevated Temperatures: Mechanisms and Mitigation Strategies

Hot-section components of aero- and marine-gas turbines can be at risk of hot corrosion, a form of accelerated attack resulting from the presence of sulfate-containing deposits. There are two documented forms of hot corrosion, referred to as Type II (below Tm of Na2SO4 ; 884C) and Type I (above Tm of Na2SO4), both of which have been inferred to involve liquid formation and subsequent oxide-scale fluxing. Previous literature has focused on relatively high SO2-content environments (~1000 ppm SO2 and above). However, in actual engine applications, the SO2 contents are much lower. This study assessed the effect of SO2 content on the corrosion behavior of relevant Ni-based alloys in environments of dry air and O2-(2.5, 10, and 100) ppm SO2 from 550°C-1000°C. It was found in the Type II regime, as SO2 content increased, corrosion rate increased. The opposite trend was found in the Type I regime, where the dry-air environment resulted in the most severe attack. The reasons for these trends were deduced. It was also found that hot corrosion can occur at temperatures as low as 550°C, where liquid formation is not thermodynamically possible. Additional experiments were conducted on pure nickel samples to more clearly elucidate the mechanism of this newly observed form of low-temperature hot corrosion. Based on the results, a solid-state corrosion mechanism was inferred. The mechanism relies on the formation of a previously unreported compound phase, identified using TEM analysis to be nanocrystalline with a stoichiometry of Na2Ni2SO5. This phase was found to be a rapid diffusor of Ni2+ and Na+. To mitigate against Type II hot corrosion, a chromized coating is often used, but the application of a coating can result in varying Cr content across a part. The Cr level needed for protection was analyzed via exposure of comparable NiCrAl model alloys. Protection against the sulfate deposit was linked to the ability of the alloy to establish and maintain a Cr2O3 scale. The presence of a sodium sulfate deposit leads to chromium-sulfide formation, depleting the alloy of Cr needed to form a protective scale. The effect of Co, TaC, and AlN was also studied