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Adhesion solution

We are try to bond a composite to a aluminium spec 7075 which has been anodised and chromate sealed to MIL-STD 171 section 7.2.1, we are using a epoxy adhesion, and we are finding that it will not hold. We made some test pieces to the same spec and tensile pulled them and got a result of 1/2 a ton per sq. inch which we found acceptable. Why does it fail on are component, the component is 72" long and so is the composite, I believe it is the coating process when they apply the chromate, being such a large component it is flash drying on the surface before being put into the wash solutions. what do think.

Also by removing the anodise and chromate seal and bonding to the aluminium surfaces, do you no if the epoxy adhesion would be a preventive against corrosion.

warren vye
- United Kingdom
2001

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Anodising for improved bond strength relies on the porosity of the anodise coating to increase the bond strength. The usual sequence is to anodise then cold water rinse, dry and then apply a corrosion inhibiting primer to controlled thickness. Usually pretty thin (typically 0.00015"). The primer should give about 30 days salt spray corrosion resistance. However if you are getting good test figures (depending on adhesive type we would expect from about 1000 to 7000 lb./square inch) check if you are getting pressure across the whole component. It may help to know exactly which type of adhesive and how you are applying pressure. Other common faults are oil and silicon contamination, improper cleaning of the composite part and failing to get pressure on fast enough when using a two part epoxy.

Ciaron Murphy
- South Wales
2001