Current generation and control systems for electrolytic vat

Details Current generation and control systems for electrolytic vat Current generation and control systems for electrolytic vat

1993-02-11

1994-10-04

Valentine, Donald R.

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C25B 904, C25B 1502, C25D 2112

Patent

active

053523466

DESCRIPTION:

BRIEF SUMMARY
OBJECT OF THE INVENTION

The present invention relates to a number of improvements to current control systems used in electrolytic processes such as the conventional electrolytic coloration processes, opacification processes, processes for obtaining a range of greys, and aluminum optical interference coloration processes, though clearly such improvements can also be applied to any other field requiring like current control systems.


BACKGROUND OF THE INVENTION

For aluminium electrolytic coloration processes to be carried out to full satisfaction, a very thorough control on the current applied must exist.
Thus, for instance, Spanish patent of invention no. 498,578 and its U.S. Pat. No. 4,421,610, sets forth an electrolytic coloration process for an aluminium or aluminium alloy element, consisting of a first phase where, inter alia, an alternating current with a peak voltage lying between 25 and 85 volts and a current density below 0.3 amps. per square decimeter must be applied.
More specifically, and in order to obtain such alternating current, a polyphasic network or the secondaries in a polyphasic network transformer are used conducting the positive and negative half-cycles with the same conduction angle and both variables as required, which conduction angles are in turn controlled by reverse shunt thyristors or by triacs.
Said control of the thyristors' conduction angle obviously allows the average voltage to be controlled, but not so the peak voltage, and therefore the results attained, though acceptable, cannot be deemed to be the most favourable.
Manifold solutions have been put forward so far as electrolytic coloration processes are concerned, and the essential problem common to all is the difficulty of suitably controlling the currents applied to the vat.
Furthermore, from the theoretical viewpoint , opacification processes are known to attain, likewise by electrolytic processes, a transformation of the anodic film rendering the same opaque, but such processes require very low voltages in practice, less than three volts, and moreover very specific values, and no current control means exists presently that may allow the same to be maintained within the limits the process requires.
Optical interference aluminum coloration processes are also known, where the above-mentioned problem is even worse, for within a given range of voltages, minor variations in the value of the voltage lead to significant changes in the colour obtained, for which reason this system has not been developed industrially either, for the different load characteristics and the actual installation determine variations in the voltage drop and, hence, variations in the voltage applied to the load, originating undesirable colour changes.
There is hence no doubt whatsoever that the fact that there are presently no suitable means for controlling the current applied to electrolytic processes significantly constrains progress in this field.
In order to grasp the difficulties of the different aluminum electrolytic coloration systems it is worthwhile to note some of the phenomena that take place when applying an alternating current to the previously anodized aluminum:
During the positive half-cycle there is no deposition whatsoever at the anodic film pores. In the event of the voltage applied allowing passage of current, oxidation takes place, leading to an increase in film barrier thickness. The final film barrier thickness is proportional to the peak voltage applied.
During the negative half-cycle there is a double deposition. On the other hand, deposition of the metallic cation present in the form of a metallic particle. For instance:
Furthermore, deposition of protons present in the electrolyte, that become atomic hydrogen:
The speed of migration of the protons toward the bottom of the pores depends upon the voltage applied and the density of the circulating current. This latter in turn depends upon the total circuit impedance (see electric model of the U.S. Pat. No. 4,421,602, namely FIG. 1 thereof).
Because of the semiconducting natur

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patent: 4170739 (1979-10-01), Frusztajer et al.
patent: 4338176 (1982-07-01), Pelaez
patent: 4666567 (1987-05-01), Loch
patent: 4839002 (1989-06-01), Pernick et al.
patent: 5102513 (1992-04-01), Pelkus

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