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Chromic
Anodising

Highest corrosion protection & optimum adhesion for aluminium

Maximum corrosion protection & perfect adhesion thanks to chromic acid anodisation

Chromic acid anodising is a special anodic oxidation process that is primarily used in aviation and other high-security areas. It is used where conventional anodising is not suitable due to fatigue strength and where the highest corrosion protection requirements must be met at the same time.

A particular advantage of this method is the excellent adhesion primer formation for subsequent painting or bonding, making it ideal for complex, further processed aluminium components.

Optimum adhesion & reliable corrosion protection in one process

Chromic acid anodising differs from other anodising processes due to its special process parameters. Factors such as current density, exposure time and temperature significantly influence the formation of the oxide layer.

Thanks to these unique properties, chromic acid anodising is indispensable in the aviation industry and is used wherever aluminium components need to remain mechanically flexible.

Perfect primer

Uncompressed layers enable the best paint & adhesive adhesion

Excellent corrosion protection

Reliably protects aluminium from environmental influences

Ideal for cavities & recesses

Electrolyte residues do not lead to corrosion

Lower hardness & wear resistance

Provides elasticity, but no mechanical resilience

Why is chromic acid anodising used in aviation?

Chromic acid anodised components are an essential component in aerospace technology, as the process offers decisive advantages:

High fatigue strength

No influence on the vibration resistance of the component

No corrosion due to electrolyte residues

Perfect for complex structures & cavities

Crack testing possible

The anodised oxide layer makes fine material defects visible

Ideal pre-treatment for painting & bonding

Perfect primer without additional compaction

Hohe Dauerfestigkeit

Kein Einfluss auf die Schwingfestigkeit des Bauteils

Keine Korrosion durch Elektrolytreste

Perfekt für komplexe Strukturen & Hohlräume

Rissprüfung möglich

Die anodische Oxidschicht macht feine Materialfehler sichtbar

Ideale Vorbehandlung für Lackierungen & Verklebungen

Perfekter Haftgrund ohne zusätzliche Verdichtung

    These properties mean that chromic acid anodising is used for aircraft structures, load-bearing aluminium parts and safety-critical components, among other things.

    Layer structure, growth & optical characteristics

    As the process makes capillary effects in the surface visible, it is also used for crack testing of safety-critical components.

    The layer formation in chromic acid anodising differs from classic sulphuric acid anodising:

    Growth of the layer

    1/3 to the outside, 2/3 to the inside

    Thinner layers

    1-5 μm depending on the alloy

    Elasticated structure

    No additional brittleness of the components

    Self-colouring

    Grey to greenish, depending on the silicon content of the alloy

    Properties of the chromic acid oxide layer

    Chromic acid anodised aluminium surfaces offer a range of unique properties:

    Elastic & thin oxide layer
    Typical layer thicknesses between 1-5 μm
    Good electrical insulation
    Low electrical conductivity for EMC applications
    Chemical resistance
    Withstands pH values between 6 and 9, but not strong acids or bases
    No additional corrosion protection necessary
    layer protects the base material independently
    Limited colouring options
    Depending on the alloy, grey to greenish

    Suitable materials for chromic acid anodising

    In principle, many aluminium alloys can be chromic acid anodised. There are restrictions for:

    Cast aluminium alloys

    Suitability must be checked individually

    Alloys with over 5 % copper content

    Not ideal for the process

    Gegossenen Aluminiumlegierungen

    Eignung muss individuell geprüft werden

    Legierungen mit über 5 % Kupferanteil

    Nicht optimal für das Verfahren

      Für Klebe- & Lackanwendungen darf die Schicht nicht verdichtet werden, um die optimale Haftung zu gewährleisten.

      FAQ about chromic acid anodising

      What is the difference between chromic acid and sulphuric acid anodising?

      Chromic acid anodising produces thinner, more elastic layers with better adhesive properties, while sulphuric acid anodising forms harder & thicker layers.

      Can chromic acid anodised components be coloured?

      No, the layered structure does not allow for intensive colouring. The natural colour varies between grey and green.

      Why is chromic acid anodising preferably used in aviation?

      The process offers perfect corrosion protection and does not impair fatigue strength, which is crucial for aircraft components.

      How thick is a chromic acid oxide layer?

      Typical layer thicknesses are between 1-5 μm, depending on the alloy.

      Is chromic acid anodising environmentally friendly?

      The process is subject to strict environmental regulations, particularly when handling chromic acid. Modern waste water and recycling plants minimise the environmental impact.