Process for chemically treating aluminum work pieces

Surfaces of structural components made of aluminum or aluminum alloys are chemically treated to produce an aluminum oxide coating for improved adhesive bonding to other components or to other surface layers. The oxidizing is accomplished by exposing the surface to be oxidized to a combination bath of an inorganic mineral acid and an oxidation medium for a specific time duration, so that an oxide coating is produced on the surface of the structural components, as a prerequisite for a good adhesion stability of adhesive bonds to be formed after oxidizing.

FIELD OF THE INVENTION 
The invention relates to a process for chemically treating work pieces made 
of aluminum or aluminum alloys to produce surfaces having an oxide coating 
needed for an adhesive bond with surface layers or other structural 
components. 
BACKGROUND INFORMATION 
In the production of structural components by adhesive bonding of 
individual components of aluminum or aluminum alloy, it is necessary that 
the surfaces of these components are pretreated to assure the required 
bonding strength. In this pretreatment of the aluminum components, the 
component surfaces are covered with an oxide coating, which is a necessary 
prerequisite for the desired adhesion stability of the adhesive bond. 
Until now, it was customary to produce the oxide film on the aluminum 
components by means of a so-called pickling-process, which works on the 
basis of chromic acid which provides good pretreatment coatings with 
strong adhesive properties. 
The pickling-process, however, is negatively criticized due to the dangers 
that it can cause for the environment. Specifically, because the chromate 
necessary for the pickling should not be discharged into the environment 
after it has been used. In addition, energy is needed in the pickling 
process to heat the bath. 
OBJECTS OF THE INVENTION 
In view of the above it is the aim of the invention to achieve the 
following objects singly or in combination: 
to provide a process for chemically treating structural components made of 
aluminum or aluminum alloy to form an aluminum oxide coating on the 
surface of such components, whereby the process takes into account the 
demands for lower environmental burdens; 
to provide an oxidizing process that does not require additional energy for 
producing an adequate oxide layer on aluminum structural components; and 
to assure a tenacious bond between the oxide coating and the aluminum on 
the one hand and the oxide coating and the adhesive on the other hand. 
SUMMARY OF THE INVENTION 
According to the invention the surface of aluminum and aluminum alloy 
components is oxidized to form an aluminum oxide coating by exposing the 
respective surface, e.g. by immersion for a specific time duration in a 
combination bath of an inorganic mineral acid and an oxidation medium, 
said time duration being sufficient to form the oxide coating. 
The process of the invention has the advantage of working with inexpensive 
chemicals, and also does not need any additional energy, since the 
combination bath works best at room temperature.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE 
OF THE INVENTION 
Sulfuric or phosphoric acids are preferably used as the inorganic mineral 
acid. Peroxide-disulfide, such as the ammonium salt, or perborate, such as 
sodium salt are preferably used as the oxidation medium. 
A time duration for the exposure of about thirty minutes has been found to 
be practical and sufficient to obtain a satisfactory oxide coating. 
The combination bath should have an acid concentration of at least 3 mol 
per liter and an oxidation medium concentration of at least 0.3 mol per 
liter. 
The treatment of aluminum or aluminum alloy structural components according 
to the invention, leads to optimal surface conditions for the subsequent 
adhesive bonding, since the oxide coating on the aluminum components 
ensures an excellent adhesion stability for the adhesive bonding. The 
present process, as compared to known processes, can be carried out with 
greater reliability, and it does not produce any disposal problems, the 
present bath is chromate-free. 
As a further example, the process of the invention may be carried out with 
hydrogen peroxide (H.sub.2 O.sub.2) as the oxidation medium. However, 
H.sub.2 O.sub.2 has a lower redox potential than the above mentioned 
oxidation media. More specifically, a solution of hydrogen peroxide in an 
acidic carrier has a rather high decomposition rate. As a result, the 
effort and expense for properly controlled operating conditions may not be 
preferable. 
The pretreatment according to the invention may also be achieved by the 
application of a combination oxidizing paste or by spraying a combination 
oxidizing solution onto the surfaces of the structural components to be 
oxidized. 
Any of the treatments disclosed herein are followed by a proper rinsing 
with water. 
Although the invention has been described with reference to specific 
example embodiments, it will be appreciated that it is intended to cover 
all modifications and equivalents within the scope of the appended claims.