Analysis of pack chromizing of Nickel superalloys for gas turbines

The pack-cementation process is a widespread technique used to obtain protective coatings formed by diffusion mechanisms. The protection of the base component is achieved by the enrichment of the outer layer with scale formers, such as Al, Cr and Si which react with the substrate material to form new phases and intermetallics. During the working life these chemical species react with the pollutant elements to form stable oxides and compounds, increasing the oxidation and hot corrosion resistance of gas-turbine components. The component is embedded inside a mixture of powder and then exposed at high temperature. The pack-mix is based on a scale former source, an activator (usually halide salts) and an inert filler (Al2O3). The pack-cementation process consists of a heating step, an isothermal stage and a cooling step.During the thermal treatment the activator reacts with the metal source, forming a vapor phase that transports the metal source to the surface of the component, where chemical reactions lead to the inward diffusion of the selected element into the substrate, forming new phases and intermetallics.The present investigation deals with the study of formation mechanisms of diffusion coatings using chromium as scale former, in order to obtain hot corrosion resistant coating, for gas turbine applications. According to the pertinent literature aluminide coatings provide the protection of the component against oxidation, whereas chromium rich coating are more suitable against hot corrosion type II, that usually occurs at about 700 degrees C due to pollutant agentsThe main phases of these coatings depend on many parameters, such as temperature, duration of the isothermal stage and pack-mix composition. However a systematic investigation about the effect of the main process parameters on the formation mechanisms is not available in the pertinent literature.Therefore this work aims to realize an accurate investigation that leads to a better comprehension of the influence of time and pack-mix composition on the microstructure and thickness of these coatings.A Ni-base superalloy, Inconel 738, was used as substrate material in this work. The samples (approx. size 15x15x3mm) were polished with a 1000 SiC paper and the cleaned in acetone.In order to investigate the effect of the pack-mix composition on the final microstructure and on the coating thickness different formulations were selected, varying the amount of the chromium source, based on pure Cr powder (10, 25 wt.%) and the activator (NH4Cl, 1, 2 and 5 wt.%). Before each pack-chromizing cycle two samples were embedded inside a steel retort with the chosen pack-mix.Three different isothermal stage were used to evaluate the effect of the thermal cycle duration on the diffusion phenomena: 0, 12 and 24 hours at 1100 degrees C. A summary of the process parameters and the pack-mix formulations is listed in tab. 1.At the end of the pack-chromizing one sample for each deposition condition was heat treated in vacuum to evaluate the effect of the interdiffusion phenomena that occur during the vacuum cycle.The surface of the specimens were characterized by X-Ray diffraction (XRD), the cross-section of the coatings were embedded in an epoxy resin and analyzed by Scanning Electron Microscope (SEM) and Energy-Dispersion X-Ray Spectroscopy (EDS).The analysis on the coating C10A1t0 shows that the diffusion mechanism takes place even before the selected process temperature (1100 degrees C), leading to the formation of a thin Cr-rich layer (fig. 2). The dark region is based on Al2O3 rich particles, formed due to secondary reactions occurred between the salt activator and the inert filler that caused the oxidation of the aluminum originally present in the Inconel 738 composition.The comparison between the coating C10A1t0 and C10A1t12 reveals that during the isothermal stage the Cr atoms diffused inward and Ni outward. The coating is based on 3 different layer: a Cr-rich outer region, an alumina rich zone and an inner layer, mostly formed by Cr and Ti nitrides and carbides dispersed in a -Ni matrix.The XRD analysis (fig. 4) revealed that the external region in based on a Cr-rich -Ni matrix formed mainly due to the Cr inward diffusion. The Ti, Cr carbides and nitrides were formed due to the precipitation of these two elements caused by the depletion of Ni and their reaction with carbon and nitrogen. The former is an alloy element, the latter comes from the activator (NH4Cl).Increasing the amount of activator (from 1 wt.% to 2 wt.%) within the pack-mix made the transport of Cr to the sample surface easier: the concentration of this element in the outer layer is higher (fig. 3) whereas the microstructure and the coating thickness remain almost the same.The duration of the isothermal stage (from 12 to 24 h) affected the interdiffusion movements of Cr and Ni, bringing to a decrease of the Cr content in the outer layer, due to an improvement of the Ni outward diffusion (fig. 6).Increasing the Cr content from 10 to 25 wt.% made the Cr diffusion in the outer layer more relevant. Despite this it was found that the amount of activator (2 wt.%) was not enough to provide a good transport of Cr inside the coatings. Therefore, compared to the previous samples, no important changes in thickness were observed (fig. 8).To obtain thicker coatings a higher amount of activator is required (5 wt.%), in order to transport the Cr atoms not only to the substrate surface, but also inside the component (fig. 10).In order to realize a coating with a good resistance against both oxidation and hot corrosion a Cr,Al-rich diffusion coating was prepared, with a 2 step pack-chromizing and pack-aluminzing process. The coating C10A2t24 was embedded inside a pack-mix based on CoAl as aluminum source, AIF3 as activator and alumina as inert filler and then heat treated for 2.5 hours. Compared to the pack-chromized coating, the new resulting coating show a higher thickness, due to the inward diffusion of the new element. The microstructure is also different: SEM investigation shows the presence of an interdiffusion layer, that demonstrates also the outward diffusion of Ni (Fig. 12).