During a recent high-temperature work, it was discovered that a high-temperature coating on a single crystal casting had completely failed after 1000 hours of exposure in a hot corrosion environment. Though previous tests showed this coating was somewhat resistant to hot corrosion at 1650 °F (899 °C), our results revealed catastrophic corrosion penetration of both the coating and the alloy substrate.
Differences in chemical compositions between a coating and a substrate alloy can lead to interdiffusion between these materials that can modify the oxidation and corrosion resistance of the coating and the mechanical properties of the coating-substrate system. The stress state may also significantly influence and increase the magnitude of the interdiffusion that may lead to deleterious precipitation reactions. The crystal orientation or alloy phase of the substrate may also contribute to interdiffusion rates.
This paper will look at the chemistry of a high-temperature coating and a substrate alloy before and after exposure to a hot corrosion environment to evaluate the degree of interdiffusion and discern what mechanistic pathways may cause precipitation reactions deleterious to alloy/coating performance.
High temperature coatings provide a barrier from corrosive or highly oxidative environments that may degrade the substrate alloy. To provide optimum performance coatings need to have good adherence to the substrate, ductility to resist thermal cycling, avoid defects that may provide easy pathways to deleterious gases, and maintain corrosion resistance over the design life of the high temperature component.
Aluminide coatings on transition metals and alloys such as iron, nickel and cobalt are [3-FeA1, [3- NiA1, and [3-CoAl, respectively. Diffusion aluminide coatings are typically processed via pack cementation, slurry-diffusion or chemical vapor deposition [1]. Subsequent heat treatments help develop the proper mechanical properties and trigger further diffusion within the coating. The principal protective oxide generated by the high-temperature oxidation of these aluminide intermetallics is A1203 which causes subsequent improved oxidation and corrosion resistance of aluminide coatings, although less protective oxides can form if other alloying elements are present in the substrate, either in solution or as precipitated phases [2].
Two diffusional growth mechanisms are: (a) low activity aluminide and (b) high activity aluminide that characterize the pack aluminizing process on nickel-base superalloys depending on the relative aluminum activity [3].
Low activity coatings grow predominately by outward diffusion of nickel from the substrate to form a two-zone structure. The outer zone is a single-phase 13-NiA1 layer saturated with other substrate alloying elements such as chromium, cobalt, titanium, and molybdenum that diffuse outwardly with the nickel from the substrate [3]. The content of any foreign element in the outer zone is limited by its solubility in NiA1. [3-NiA1 stoichiometry varies with aluminum content decreasing from the outer surface towards the inner coating zone. The inner zone is 13-NiA1 containing a variety of second phases such as carbides and cr phase [3]. The effects of nickel outward diffusion can lead to the enrichment of elements such as chromium, cobalt, and molybdenum, in certain cases, in the substrate underlying the coating proper [3].
