Sulphuric acid is known as type II anodising under the MIL-A-8625 standard. It is widely used in the anodising of aluminium due to its effectiveness in producing a protective and functional oxide layer on aluminium surfaces. Producing a high-quality type II anodised finish requires a precisely managed series of chemical and electrochemical processes.
In the sulphuric acid anodising process, aluminium parts are immersed in a dilute sulphuric acid electrolyte, with an electric current causing the formation of a controlled aluminium oxide coating. This coating typically ranges from 2.5 to 25 microns thick, offering substantial corrosion resistance without significantly changing the part dimensions.
The benefits of porosity
The aluminium oxide layer formed has a unique porous microstructure with hexagonal cells, allowing it to absorb dyes for vibrant colouring and secondary treatments such as sealants. This porosity also creates an excellent mechanical anchor for paints, primers, and adhesives. When sealed, the coating provides excellent protection against atmospheric corrosion and mild chemical exposure.
The dominant anodising process
Type II anodising, the most popular form of industrial anodising, is widely used across the world by specialists such as https://www.poeton.co.uk/surface-treatments/anodising/sulphuric-acid-anodising/.
High performance, low cost
Sulphuric acid anodising is favoured because it balances performance, versatility, and cost. The chemicals involved are relatively inexpensive, and the process operates at ambient or slightly chilled temperatures, making it economical. Additionally, it avoids the environmental and health hazards associated with chromic acid anodising, providing a more environmentally friendly solution.
This versatility and cost-effectiveness explain why sulphuric acid is the primary electrolyte in anodising aluminium for many industrial and decorative applications.