Process for surface treatment of aluminum or aluminum alloys by an alkaline chemical bath

1. A process for surface treatment of aluminum or aluminum alloy semi-finished products, comprising the steps of:

(a) preparing an aqueous solution of sodium hydroxide (NaOH) in a concentration between 100 g/l and 250 g/l and dissolved metallic aluminum in a concentration between 50 g/l and 70 g/l;

(b) adding to said aqueous solution an aluminum complexing agent, comprising gluconate and sorbitol, in a concentration between 5 g/l and 25 g/l, the ratio between the concentration of sorbitol in grams per liter of solution and the concentration of gluconate in grams per liter of solution being between 0.7 and 0.75;

(c) placing a semi-finished product in contact with the aqueous solution, for a time necessary to carry out a desired surface treatment; and

(d) during step (c), maintaining temperature of said aqueous solution in a range between 50° C. and 100° C., and maintaining the concentration of metallic aluminum dissolved in said aqueous solution in a range between 50 g/l and 70 g/l.

2. The process of claim 1 , wherein concentrations of sodium hydroxide and metallic aluminum are maintained in the ranges indicated in step (a) by titration of the aqueous solution.

3. The process of claim 1 , wherein step (c) is implemented by immersing the semi-finished product in a tank containing the aqueous solution.

4. The process of claim 1 , further comprising the step of masking the semi-finished product prior to contact of the semi-finished product with the aqueous solution.

5. The process of claim 1 , further comprising the step of periodically checking dimensions and/or finishing state of the semi-finished product in contact with said aqueous solution.

6. The process of claim 1 , further comprising the step of filtering the aqueous solution with a filter configured to catalyze the dissolved metallic aluminum and dissociate the dissolved metallic aluminum from said aqueous solution.