Hydrothermal Sealing

The anodizing process results in a porous oxide layer on the surface of aluminum alloys. These pores need to be sealed in post-treatment processes such as hydrothermal sealing to enhance the corrosion and wear resistance of the alloy.

In the hydrothermal sealing process, the anodized aluminum parts are immersed in hot deionized water. This causes the aluminum oxide to react with the water, resulting in the formation of aluminum hydroxide. The latter occupies more space than aluminum oxide and effectively seals the tiny microscopic pores of the anodic aluminum oxide layer.

Since the pores are sealed, the pathway for corrosive species to reach the metal substrates is now blocked, effectively improving the corrosion resistance of the alloys. The sealing of the pores also improves the wear resistance of the metal. Additionally, the long-term aesthetic appearance of the alloy may be improved as contaminants can no longer enter the pores of the anodic aluminum oxide layer to stain the surface.

The immersion time in hot water for efficient hydrothermal sealing depends on the thickness of the anodic aluminum oxide layer. A proper hydrothermal sealing requires about 2 to 3 minutes of immersion per micrometer of coating thickness. Therefore, the efficient sealing time for a 10-micrometer thick oxide layer is about 30 minutes.

To determine the proper sealing time, it is important to measure the thickness of the anodic aluminum oxide layer. Furthermore, the water used for hydrothermal sealing should be deionized or distilled and have reproducible pH, conductivity, and temperature.

Here is an example of the complete procedure. First, a thickness gauge is used to measure the thickness of the anodic aluminum oxide layer by placing it onto the surface of the alloy as shown in the video. Next, the conductivity and pH of the sealing solution are measured with a multifunctional tester. After heating the sealing solution to the boiling point, its temperature is measured with a stainless-steel thermometer. If the parameters are suitable, the anodized aluminum part is immersed in the sealing solution for the appropriate calculated time.