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.

  • Appearance: grey or brown (black colouration is possible)
  • Thickness: 5 to 60 µm
  • Microhardness: 300 to 500HV for 50 µm, depending on the alloy
  • Surface condition: slight deterioration of the roughness,
  • Electrical insulation: 500 to 1000 volts~
  • Coefficient of friction: good under high load
  • Reduction in fatigue: 30 to 50% depending on the alloy
  • Corrosion resistance:
    - without sealing: ~ 50 h
    - sealing with nickel salt ~ 300 h
    - sealing with chromium VI salt ~ 500 h


  • Aluminium alloys with a maximum of 4 to 5% of copper. However, the alloy used greatly influences the level of performance obtained and the appearance.
  • Thickness limited to 20/25 μm for alloys produced by melting under pressure


  • Use of baths comprising mixtures of sulphuric acid and organic acids (Sanford), application of alternating current (Hardas) or use of pulsing or periodic current (Durkaluminium).
  • Possibility of black colouring
  • PTFE impregnation: decrease in the coefficient of friction


  • Resistance to abrasive or erosion wear, slip properties.
  • Electrical insulation
  • Thermal insulation

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

Environmental impact

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.

Reference systems

  • EN 2536: aeronautical sector
  • ISO 10 074: building sector
  • EN 12 373-2 / 17: methods of measurement of anodic coating
  • ISO 9227-NSS: neutral salt spray test
  • EN 12 373-18: pitting corrosion rating system
  • ISO 4516: method of measuring Vickers and Knoop microhardness
  • FED.TEST METHOD STD.141C Method 6192-1: resistance to abrasive wear, Taber Method