HAO: Hard anodic oxidation
Hard anodizing is a specific application of sulphuric anodizing. This process is usually performed in a bath of sulphuric acid from 10 to 20% (by volume), at low temperature, generally from -5 to 5°C.
Description of the process
The use of low temperature allows the application of higher current densities (typically 3 A/dm2) and minimizes the dissolution of the oxide, which allows us to reach significant thicknesses (25 to 100 μm), and especially in dense structures. For coatings formed of hard anodizing we note an increase in the hardness, abrasion resistance and electrical and thermal insulation. Consequently, this treatment is particularly recommended when resistance to abrasive or erosive wear is paramount. Colouring and sealing of the coating are possible but they decrease hardness and wear resistance.
Applications aerospace, automobile, defence, household appliances, etc.
Choice criteria - Limitations
Considering the significant thicknesses of the oxides formed in hard anodizing, it may be necessary to consider variations in the dimensions of parts. It is sometimes necessary to grind the coating after anodizing to restore the surface condition
The process used has no critical influence on the environment, it contains no heavy metals or chlorinated solvents. Consequently, so far there is no threat to the continuation of this process.