What Does Diffusion Coating Mean?
Diffusion coating is a process in which metal components that will be subjected to high temperature conditions and highly corrosive environments are coated with a non-corrosive material. The process is normally done at elevated temperatures in a controlled chamber.
The most widely used coatings are chromium, aluminum or silicon material. Substrate materials usually coated include cobalt and nickel-based super alloys, steels (including carbon, alloy and stainless steels) and refractory metals, among other alloys. As a result, the base metal develops extreme resistance to corrosion, oxidation and erosion in its severe working conditions. This makes the process highly reliable, enhancing the manufacture of critical components. Diffusion coating is normally used to process gas turbine engine components (vanes, blades and cases), pump impellers, gate valves and power generation components.
Diffusion coating is also called surface alloying.
Corrosionpedia Explains Diffusion Coating
Diffusion coating can be done using three processes:
- Solid state diffusion
- Liquid state diffusion
- Chemical vapor diffusion
Solid state diffusion is used with nickel, titanium and iron, among other metals, and the vapor pressure of the coating metal must be lower than the base metal. The process is normally performed in a hermetically sealed container with the base metal covered with the powdered coating material. The container is then heated in a vacuum, at a temperature of 1000°C 1500°C (1800°F to 2700°F). The coating metal melts to cover the entire surface of the base metal. This process is also referred to as pack cementation.
Zinc, chrome and copper are normally coated through liquid diffusion. Liquid diffusion is performed in tank furnaces in which the diffusing metal interacts with the base metal's surface at 800°C to 1300°C (1400°F to 2300°F). Complex diffusion coating can be achieved through this process, such as chrome calorization as well as chrome-nickel plating.
In chemical gas diffusion or out-of-contact gas phase diffusion, the coating material is heated into a gaseous form at a distance from the surface being saturated. The gaseous chemical compounds of the coating element react with the basic metal, resulting in diffusion of the metal. This gaseous phase consists of halides to ensure sublimation of the diffusing metal on the base metal's surface. The process is usually performed in specially designed furnaces at a temperature of 700°C to 1000°C (1300°F to 1800°F).