Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Controlling current distribution within bath
Patent
1994-06-29
1996-09-24
Niebling, John
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Controlling current distribution within bath
204228, 205128, 205145, C25D 1700, C25D 2112
Patent
active
055587577
DESCRIPTION:
BRIEF SUMMARY
The invention relates first to a process for improving the electrolytic coating on work pieces that are arranged in a row at intervals on cathode rails or cathode frames in a bath, whereby the cathodic treatment current runs in the rail or the frame parallel to the direction of the above mentioned row of work pieces. In practice, the cathodic rails are mainly brackets for articles on which the work pieces are clamped (in particular the concept is plate-shaped work pieces) like circuit boards. In this process, the row of work pieces runs horizontally corresponding to the arrangement of the brackets for articles. Such work pieces can also be clamped on carrier rods of frames, which extend vertically into the bath. The electrolytic treatment current, in particular the concept is a galvanizing process, i.e. a galvanizing current, is conducted to the work pieces to be treated via the brackets for articles or via the carrier rods of the frame. In addition, in the bath, anodes that supply the anode current are mounted on carriers or rods. The metal of the anodes is precipitated on the articles in a known way by the electrolytic procedure. The cathode current creates a voltage drop in its flow direction on the cathode rail or the corresponding frame carrier rod. The magnitude of the voltage drop depends both on the size of the current as well as the specific ohmic resistance per length unit of the cathode rail or of the frame carrier rod. In this way, a corresponding voltage drop results between the fastening point of a work piece on the cathode rail or the frame carrier rod up to the fastening point of the next work piece in the series. Of the opposing edge areas of two adjacent work pieces separated by a space, one acts as a local anode and the other like a local cathode with a cell voltage, i.e. like a partial electrolytic cell. The result of this is that the edge area of the cathode function gets more metal coating than the anodic edge area. During measurements, 20% and more differences were determined between the respective metal coatings. In order to eliminate this disadvantage, there could be an attempt to decrease the voltage drops along the cathode rail or corresponding frame carrier rods. To do this, for example, a decrease in the galvanizing current density, i.e. the currents in the rails and/or increase of the rail material conductivity or frame carrier rod material and/or an increase in the cross sections of the cathode rails or the frame carrier rods have been attempted. Each of these variations has disadvantages. The decreased current density requires longer galvanizing times, i.e. the system is not as economical. To increase the conductivity, in fact, copper can be used as a material. Still copper has a tendency to severe corrosion in the galvanizing technology or electrolytic bath environment, which leads to additional, uncontrolled and high voltage drops both at the anode contacts as well as at the work piece contacts. The above mentioned corrosion is not present with stainless steel so this material is preferably used to prevent corrosion. However, the disadvantage of a specific resistance that is 40 times higher compared to copper has to be taken into consideration. Increases in the cross section are in general not possible, and especially with stainless steel because of the high specific weight. Relatively high material costs also occur because of this. Instead of this, a two-sided contact of the anode and cathode rails has been planned. In fact this reduces the voltage drops on the rails by half, but simultaneously causes new uncertainties in the overall system. The anode and cathode rails can be lifted out, i.e. the current reaches these rails by contact. In practice, these contacts get dirty and corrode until complete failure of one side. This is not recognized by the system because the remaining second contact takes over the entire current. The result in turn is non-permissible high voltage drops on the rail. Monitoring the contacts is also not possible for cost reasons. In addition, arran
REFERENCES:
patent: 2558090 (1951-06-01), Jernstedt
patent: 3880725 (1975-04-01), Ven Raalte et al.
patent: 4184927 (1980-01-01), Takahashi et al.
Patent Abstracts of Japan, vol. 12, No. 365, C-532, 29 Sep. 1988.
Atotech Deutschland GmbH
Leader William T.
Niebling John
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