Letter 8086

Chromic acid anodize 

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This is probably a little basic but can anyone tell me why it is important to paint aircraft parts within a set time of chromic acid anodising?

Ian Moore
- North Wales

Chromic acid anodising is at times used a bond preparation rather than corrosion prevention in the aircaraft industry. (Particularly in Europe). The formation of the oxide coating if exposed to the atmosphere particularly a warm and wet atmosphere can lead to hydrolysis of the oxide structure. This is detrimental to the bond strength and durability. The presence of water in the oxide layer can lead to the formation of an oxy-hydroxide which is more massive than the oxide leading to a build up of stresses at a crack tip and can promote crack propagation along the adhesive, metal interface. Ultimately leading to bond failure. Adhesion mode of failure.

If the anodise coating has primer applied to the surface it prevents this degradation of the oxide structure and extends the bond strength and in particular its resistance to a warm wet environment. The finger like structure of the oxide also ensures any crack tip is maintained in the adhesive layer rather than migrating to the metal surface. This structure also acts to diperses any forces at a crack tip effectively stop drilling any crack. Cohesive failure mode.

Ciaron Murphy
- South Wales

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