Abstract A process is applied for the production of colored, protective coatings on articles of aluminum or aluminum alloys by passing an alternating current between a previously anodically oxidized aluminum article and a counter-electrode immersed in an acid aqueous bath containing salts of metals capable of coloring the aluminum oxide layer, the alternating voltage supplied being modulated as to its amplitude and/or frequency so as to make it asymmetrical thereby to control the color tone on the aluminum article. The asymmetrical voltage may be produced, for example, by supplying two or more alternating voltages of different amplitude, frequency and/or phase, or by using a direct voltage superimposed on the alternating voltage.
A process claimed in claim 1, wherein initially a symmetrical alternating voltage is passed followed by the asymmetrical alternating voltage.
Claim 2
Dependent Claim
26 words
A process as claimed in claim 1, wherein the asymmetrical voltage is produced by supplying at least two alternating voltages of different amplitude, frequency and/or phase.
Claim 3
Dependent Claim
23 words
A process as claimed in claim 1, wherein the asymmetrical alternating voltage is produced by superimposing a direct voltage on the alternating voltage.
Claim 4
Dependent Claim
24 words
A process as claimed in claim 1, wherein the alternating voltage is regulated relatively slowly from zero to the voltage used for the coloring.
Claim 5
Dependent Claim
38 words
A process as claimed in claim 1, wherein the alternating current is passed through two oppositely directed rectifiers connected in parallel each connected in series to a rheostat, and regulating the current by means of the two rheostats.
Claim 6
Dependent Claim
48 words
A process as claimed in claim 1, wherein the alternating current is passed through two oppositely directed controlled rectifiers connected in parallel, and controlling the rectifiers so as to disrupt the current during a part of each of the positive and negative half waves of the alternating current.