Why is an oxide layer needed when bonding aluminium alloys?

[brewnog]

Hi all,

When bonding aluminium alloys, an oxide layer is needed between (I think) the substrate and the adhesive.

Why?

And does this oxide layer have to be supplied by some kind of primer, or is a natural oxide film acceptable? Does anyone know what chemical purpose the primer provides, or is this still an industry secret?

Cheers

xxx

 

[Astronuc]

Aluminium has very high resistance against corrosion because it develops an adherent, relatively impervious oxide layer. When aluminium is produced it is spontaneously covered by a thin but effective coating of oxide which protects the aluminium from further oxidation. If the coating of oxide is damaged mechanically this coating will be renewed immediately.

This is similar to stainless steel in which a thin layer of oxides of Ni and Cr protect the underlying Fe-based steel metal.

The industrial process most often used for oxidation of Al is Anodising (US - Anodizing). Anodising is a surface treatment which is used to give the aluminium product additional protection against corrosion, decorative surfaces with resistant colour and gloss, electrical insulating surfaces or foundations for gluing or painting. Anodising is an electro-chemical process that strengthens the natural oxide shift. Similar process is use to Ti-based alloys.

The oxide layer is generally rougher, and I believe organic coatings adhere better to the oxide layer. The oxide layer also protects the Al-metal substrate if the coating is damaged.

With respect to inorganic coatings, the oxide layer may also prevent undesirable impurities from diffusing into the Al metal. (I am speculating here).

 

[Gokul43201]

I've used West System Epoxy for aluminium bonding. They recommend and specifically sell an Aluminium Etch Kit for preparing the aluminium surfaces before applying the epoxy. The main component of this is phosphoric acid, which is also the primary component of the PAN wet etch solution for aluminium. So, I suspect what this does is it provides a clean, fresh surface, which in turn helps the oxide adhere better to the metal. And if your adhesive is strong, then the weak link may be the adhesion of the oxide to the metal.

 

[brewnog]

Cheers you two.

Gokul, this kit you use, aside from the etching (I presume once you've polished and etched your substrate you wash off those nasty etchy chemicals) are you supposed to provide a primer before you apply the adhesive itself?

 

[Gokul43201]

Cheers you two.

Gokul, this kit you use, aside from the etching (I presume once you've polished and etched your substrate you wash off those nasty etchy chemicals) are you supposed to provide a primer before you apply the adhesive itself?

No, I don't believe there's a primer involved.

The procedure is :sand -> etch -> wash -> wait -> glue away !

 

[brewnog]

Interesting, I guess it's the "wait" stage which allows the oxide to form. Is the length of this "wait" critical?