Treatment of aluminium or aluminium alloys

Details Treatment of aluminium or aluminium alloys Treatment of aluminium or aluminium alloys

1997-05-22

1999-09-21

Sheehan, John

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

148247, 148256, 148273, 148275, 427216, 428658, C23C 2282

Patent

active

059548932

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to the protection of surfaces and more particularly relates to the protection of surfaces with corrosion inhibitors.
Airframes and weapons systems need to be protected against corrosion. A conventional technique is to anodise the surface of aluminium or aluminium alloy. This provides some protection as a barrier layer also promotes good paint adhesion. To get adequate levels of corrosion resistance, chromic acid anodising is frequently used, which imparts a degree of corrosion resistance to the base metal, partly due to the presence of inhibiting chromate species in the anodised film. A paint scheme often used is an epoxy primer pigmented with a chromate salt corrosion inhibitor followed by a polyurethane top coat. When the paint scheme becomes damaged the chromate salt leaches out from the primer and inhibits the corrosion of the exposed metal. The main drawback of the chromic acid anodising process is that the chemicals used are toxic and the process is potentially harmful to the environment. Thus the process is effective but has environmental disadvantages and alternative non-environmentally harmful techniques are desirable.
It has been previously suggested to use other acids in the anodising process. such as sulphuric acid, as replacements for chromic acid. Such a technique could offer a lower toxicity and generally less expensive alternative to chromic acid anodising, but sulphuric acid films do not contain an inherent, corrosion inhibiting component and the treatment can have a harmful effect on fatigue performance of the metals. The present invention relates to improved corrosion inhibiting systems which overcome or alleviate one or more of the drawbacks of previous systems.
Thus according to the present invention there is provided a method for treating the surface or surfaces of an aluminium or aluminium alloy containing substrate comprising the steps of (a) creating a porous layer on the surface or surfaces of the aluminium or aluminium alloy, (b) treating the surface or surfaces with a solution or gel comprising a metavanadate ion, (c) preferably washing the surface or surfaces to remove excess metavanadate ion and (d) treating the surface or surfaces with a solution comprising a metal ion selected to coprecipitate with the metavanadate ion to form a sparingly soluble compound within the pores of the oxide layer.
The metal ion is preferably selected from cerium, nickel, zinc, strontium, barium, lanthanum and calcium; more preferably from cerium (III), nickel (II) and zinc(II). These offer corrosion inhibition from non-carcinogenic species, so that the protective treatment provides an effective and lower toxicity alternative to chromate anodising. The solution comprising a metal ion is conveniently the sulphate and the metavanadate solution or gel conveniently comprises sodium metavanadate. These two solutions effect ready precipitation, by a simple double decomposition reaction, of the desired sparingly soluble metavanadate species into the pores of the anodic film.
In practice, the porous layer will usually be an oxide layer, although it will be understood that the precise chemistry of the layer is not of importance to the working of the invention. The exact process by which the porous oxide layer is produced is not critical to the invention, and various methods will suggest themselves to those skilled in the art. However, a convenient technique will utilise a porous film anodising process step, suitably the step of anodising the aluminium or aluminium alloy by treating the surface or surfaces with a solution comprising a suitable acid.
Particularly preferred acids are for example sulphuric, phosphoric, or oxalic acid, which produce a porous film oxide layer without the toxicity associated with chromic acid anodising, although any acid which produced a suitably porous film (including chromic) could be used at this stage. These acid anodising treatments will be known to those skilled in the art of protection of aluriniurn, and it will be understood that it will invol

REFERENCES:
patent: 2018388 (1935-10-01), Tosterud
patent: 4264378 (1981-04-01), Oppen et al.
patent: 4828615 (1989-05-01), Cape
patent: 4992115 (1991-02-01), Ikeda
patent: 5362335 (1994-11-01), Rungta

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