The duration of treatment is 20 to 60 min, the thickness of the oxide film obtained depends on the chemical composition of the alloy and varies according to the application sought and the industrial sector concerned. It is usually 5 to 25 µm. The treatment leads to a swelling equivalent to 33% of the thickness of the film and a slight increase in the surface roughness. The oxide layer is mainly composed of a porous structure. Sealing treatment by hydration of the porous layer is necessary to achieve the maximum corrosion resistance, but paint adhesion decreases significantly. It is possible to colour the coating, a bright metallic colour is obtained by chemical colouration: black, red, blue or green.
Electrolytic colouring, by the deposit of a mixture of oxides and metals, produces shades of gold, bronze and black that are very solid under light. The colouring operation is located before the sealing.
Appearance: metallic grey
Thickness: 5 to 25 µm
Microhardness: 200 / 250 Hv 0.050
Surface condition: slight degradation of the roughness
Electrical insulation: 500 to 1500 volts
Coefficient of friction: good under medium load
Reduction in fatigue: 5 to 40 % in alternating traction, depending on the alloy, may be offset by prior shot peening
- without sealing: > 100 h (BS ISO 9227 NSS)
- sealing with nickel salt: > 300 h
- sealing with chromium VI salt > 500 h