The Aluminizing Process
Surface treatments are an extremely important factor in preventing a wide variety of material failures including fatigue, wear, corrosion and oxidation.
Aluminizing and thermo-chemical diffusion process is carried out at high temperatures, typically in the range of 800–1000°C with a prolonged soaking time to aid the diffusion potential.
The surface of materials is the only part that has to co-exist with the external environment, however, majority of engineering failures originate from the surface which then results in the components failures such as fatigue, wear, corrosion and oxidation. To further improve these surface properties various surface modifications can be done through different techniques through which a suitable coating material is deposited onto the surface.
Aluminizing is a thermo-chemical diffusion treatment where in the surface layer of the material is impregnated with aluminum. It is primarily used on steels, but also on nickel and cobalt based alloys to obtain greater creep resistance, hardness and corrosion resistance.
The aluminizing process is usually operating at a relatively high temperature of 800–1000°C with prolonged soaking time taking into account the diffusion concept. To decrease the grain growth, carbide precipitation and distortion, there are many investigations relating a low-temperature aluminizing process, which is operating at a temperature below 700°C. However, the diffusion rate of aluminum atoms decreases with decreasing temperature, although many concerns are relieved by a low-temperature aluminizing process. Consequently, it cannot be refuted that the aluminizing time will be extended for the low-temperature aluminizing process. Bulk plastic deformation is a way to enhance the diffusion rate as reported in. However, the aluminizing process emphasizes only at the surface and in near-surface regions. Thus, plastic deformation only at the surface and in near-surface regions should be considered to improve the diffusion rate for the thermochemical surface treatments.
There are several techniques that have been used to obtain a layer of aluminium over a steel surface on a commercial scale. These include; electrolytic coating, cladding, pack, gas, spray (metalizing) and hot-dip aluminizing.
Coating Processes
Pack Coating:
Coatings can be applied by pack coating processes, where the compounds containing Al or Cr are placed inside a powder mixture of aluminum oxide and an activator to produce a metal halide vapor. These vapors will deposit on the surface of the work piece.
Vapor Phase Coating:
A more advanced method is to process parts in a vapor phase coating system, where the workpiece is placed above the donor materials, so the surface of the parts will not be contaminated by contact with the powder. Vapor Phase Aluminizing (VPA) and Vapor Phase Chromizing (VPC) are typical coating processes.
Chemical Vapor deposition (CVD):
The most advanced method of coating is to produce the elements to be applied either outside or inside the coating chamber by reacting donor material with halide forming gasses, controlling the rate of deposition by adjusting the temperature and gas flow rates over or through the work piece.
Characteristics:
- Oxidation resistance
- Carburization resistance
- Hydrogen permeation
- Sulfidation resistance
Applications:
- Oil and gas tubes and pipes
- Industrial furnace components
- Catalyst trays
Figure 1: Aluminizing on Inconel 718