Aluminizing Processes
1. Hot-Dip Aluminizing
Process Flow
Hot-dip aluminizing involves immersing clean steel parts into molten aluminum or an aluminum alloy at 680-780°C. The process steps are as follows:
- Workpiece → Degreasing → Rust removal → Pretreatment → Hot-dip aluminum.
Formation of Hot-Dip Aluminum Layer
The formation of the hot-dip aluminum layer occurs in three steps:
- The clean steel is immersed in molten aluminum, allowing the liquid aluminum to infiltrate the steel surface.
- A diffusion layer of aluminum-iron intermetallic compounds, including FeAl3 (θ phase) and Fe2Al5 (η phase), forms.
- After the workpiece is lifted from the molten aluminum, a coating of aluminum is attached to the surface.
The hot-dip aluminum layer consists of the diffusion layer from step two and the aluminum coating from step three.
2. Powder Aluminizing
Powder aluminizing is a diffusion-type aluminizing process. In this method, steel or high-temperature alloy is sealed with an aluminizing agent and heated to 800-950°C for several hours. After cooling, a diffusion-type aluminizing layer is formed.
The aluminizing agent primarily consists of the AI (or Al/Fe)-NH4Cl-Al2O3 type. The following reactions take place during the process:
- NH4Cl → NH3↑ + HCl↑
- 6HCl + 2Al → 2AlCl3 + 3H2↑
- Fe + AlCl3 → FeCl3 + [Al]
In these reactions, active aluminum atoms ([Al]) are deposited on the steel surface, penetrating into the material to form an aluminizing layer made entirely of aluminum-iron compounds.
3. Other Aluminizing Methods
Hot-Dip Diffusion Method
After applying a hot-dip aluminum coating, the steel workpiece is diffused at 800-950°C. This process transforms the aluminum coating into an aluminum-iron compound layer, creating a diffusion-type aluminizing layer.
Slurry Aluminizing Method
A solid aluminizing agent is mixed with a binder and water to form a slurry, which is applied to the workpiece surface. The workpiece is then heated to diffuse the aluminizing agent.
Electrophoresis-Diffusion Aluminizing
Aluminum powder is evenly applied to the workpiece using electrophoresis. The workpiece is then heated to diffuse the aluminum. If the heating temperature is below 500°C, only an aluminum sintering coating forms. At temperatures above 600°C, a diffusion aluminizing layer is created.
Thermal Spraying-Diffusion Aluminizing
Aluminum is coated onto the workpiece surface using thermal or electrostatic spraying. The workpiece is then subjected to thermal diffusion aluminizing.
