Chromic acid anodising
This special process is usually used when particularly high demands are placed on corrosion protection – and technical anodising is not an option because of the fatigue endurance limit.
Chromic acid anodising
Perfect adhesive primer
Chromic acid anodising is a special process not only because of its electrolyte composition. Parameters such as current density, exposure time and temperature have a strong effect on the oxide layer to be formed. A non-sealed chromic acid layer also provides a perfect adhesive base for aluminium components that still need to be bonded or painted.
Perfect for aviation
The process has a decisive advantage: Electrolyte residues that may remain on the layer, in recesses or in narrow cavities do not lead to corrosion. This is why this process is used particularly in the aviation industry.
The wear resistance and hardness are lower than with the sulphuric acid process. Chromic acid anodised surfaces that are subject to wear must not be chemically oxidised in addition.
Resistance to acids and strong bases is poor. However, the layer tolerates solutions with pH values between 6 and 9 well. The corrosion resistance is improved by post-sealing of the oxide layer, but in any case remains lower than that of the sulphuric acid anodising layer.
The process offers good electrical insulating properties with low electrical conductivity.
The oxide layers obtained by means of chromic acid are somewhat thinner than those obtained by sulphuric acid. However, they have a higher elasticity. Common coating thicknesses are usually in the range of 1-5 μm, depending on the alloy. The oxide layer grows from the original surface about 1/3 outwards and 2/3 inwards.
The layers do not colour well. The inherent colouring is more or less grey to greenish depending on the composition of the base metal, becoming darker with increasing silicon content.
The anodic oxidation makes the grain boundaries and the fibre course visible. The liquation of chromic acid left behind by capillary action can reveal cracks and folds in the metal. This makes it possible to crack test parts.
With aluminium and aluminium alloys, most alloys can be used, but cast alloys and alloys with more than 5% copper can only be used to a limited extent. As a pre-treatment for varnish and bonding, the coating must not be sealed.
All process technologies at a glance!
Would you like to learn more about how we refine surfaces? We offer these process technologies: