ALUMINUM DIFFUSION

Gas turbine hot section components (blades / vanes / seals / cases) of land based engines are routinely coated with Aluminum Diffusion Coatings as this process has been vastly accepted for improved oxidation and corrosion protection. Aluminum diffusion coatings have been engineered and developed for an improved and balanced oxidation life. Components treated with this process also enjoy a bonus - increased material mechanical performance, adding up to extended life cycles and lower operating costs! The coating microstructure compositions are modified by diffusion coatings in that a chemically bonded, resolute coating, which acts as a barrier and inhibits oxygen and other elements from entering into the base materials substrate. Active elements are incorporated onto the surfaces (external and internally) of the components. Then through controlled heat treatment become thermally activated and diffuse into the surfaces of the components. This environmentally friendly, safe and relatively inexpensive process has been developed to produce a highly reliable, improved oxidation resistant coating while enhancing mechanical properties of critical components. This coating continues to be widely used in: Power Generation components, Pump Impellers, Valve Gates, Diesel Applications. Basic materials that are enhanced by the Diffusion Coating Process are Cobalt, Nickel, and Iron based metals especially when operating in severe service conditions.

Pack Cementation

In the pack-cementation process, the parts to be coated are placed inside a vented or purged retort and embedded in a pack mixture consisting of an inert powder (e.g., aluminum oxide), a source (e.g., pure or pre-alloyed aluminum), and typically a halide activator salt to generate the transporting vapor species. The retort is placed in a furnace and brought to the coating temperature. A protective atmosphere (generally argon or hydrogen) contacts the pack powders to prevent their oxidation. Aluminizing is by far the more widely used process throughout the ground turbine industry but packs have been used to transfer chromium, silicon, and hafnium . Variations of pack cementation include slurry-fusion and electrophoretic plating where the pack is either sprayed or electroplated onto the part to be coated. The components are then heat treated in a protective atmosphere to form the coating.

Control and repeatable coating thicknesses and aluminum concentration are key issues of the process and many different production processes are used to meet customer requirements. The effective pack aluminum activity is controlled through a choice of the source-powder composition (commonly alloys of aluminum and chromium or cobalt), the amount of the source powder , inert oxide, respective powder size distributions and the halide activator used. The temperature used for forming the coating determines to a large extent the degree of outward nickel diffusion obtained during processing. The low-temperature high-activity coatings may require an additional thermal diffusion step out-of-pack to reduce the composition and property gradients. Out-of-Pack methods or above-the-pack processes, are achieved by fixturing the parts out of contact with the pack mixture. The coating vapors are transported to the parts by an inert carrier gas or purge gas and, through the use of two packs, both internal and external surfaces can be separately coated. Other coating variations are also in use, such as pulse aluminizing and SNECMA vapor phase aluminizing.

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