Chemistry: electrical and wave energy – Processes and products
Patent
1983-10-07
1985-02-26
Tufariello, Thomas
Chemistry: electrical and wave energy
Processes and products
204 28, 204 321, C25D 534
Patent
active
045016470
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of electroplating metal, primarily chromium, onto a workpiece connected as cathode in a current circuit, said workpiece being fed through the electrolyte at a predetermined speed past the anode and any auxiliary anodes in the current circuit.
Electroplating metal on a cathode from an electrolyte entails relatively difficult and sensitive processes in which small variations in the current density between anode and cathode in the electrolyte may give rise to completely different properties in the coating and adhesion to the coated surface.
The present invention relates both to a method of achieving better adhesion to the coated surface and to a method of improving the density of the coating itself.
Over the years a considerable number of patents have been granted describing various methods of electroplating metal objects.
German patent No. 484.206, dealing with chromium plating, proposes that initially the workpiece to be chromium plated is permitted to act as anode in order to etch the original surface to give better adhesion at subsequent electroplating with the workpiece as cathode. Nowadays this method is used generally.
Furthermore, the German patent No. 923.405 maintains that a more easily polished chromium surface is obtained if electroplating is performed in periods broken by short periods when the current is cut but the workpiece is allowed to remain in the electrolyte.
Swiss patent No. 498 941 describes a method of chromium plating elongate objects by gradually moving them through an anode.
Swedish published specification No. 310 970 also reveals that when electroplating with chrome, for instance, the current density must be controlled over the entire area to be plated since differences in area, geometry or accessibility may cause the current density at some parts of the cathode to be so low that no plating at all occurs there. On the contrary, a warning is given that particularly unfavourable surfaces may be etched instead. From the second paragraph on page 3 of the published specification it is evident that cast-iron and steel cathodes are considered especially liable to such undesired etching in chromium-plating baths.
To avoid the above problems the published specification proposes placing an auxiliary electrode close to the area where the current density is either too low to give the desired plating or gives plating which is not desired on a particular part of the surface, because the current density is too high. The auxiliary electrode shall in this case be connected to a current source which is independent of the current circuit connected between anode and cathode.
The problem of etching in chromium baths with too low current density has also been discussed in U.S. Pat. No. 4,062,741 where it is suggested to connect a protective voltage of a few volts across those objects which must remain in the chromium-plating bath even after the current has been cut.
The method most frequently used in practice has otherwise been to first etch the object in question with inverse polarity and then plate it in the same bath.
The present invention relates to a new method resulting in a considerable improvement in the adhesion of the plated surface coating as well as its quality, by performing the etching and plating closer together in time and by enabling the pole-changing method to be avoided.
The method according to the invention is based on experience of electroplating gathered over the years, also verified in the patents discussed above. At the same time, however, the inventive concept offers a completely independent solution to previously unsolved problems. As already mentioned in the introduction, the method according to the invention relates to electroplating a metal, primarily chromium, onto a workpiece acting as cathode, said workpiece being fed through an electrolyte at a predetermined speed past an anode where depositing of the metals is effected.
The method according to the invention is based on the cathode being continuously etched immediately before it
REFERENCES:
patent: 3582479 (1971-06-01), Urban
patent: 3650935 (1972-03-01), Andersson
patent: 3720596 (1973-03-01), Draghicescu
patent: 3871982 (1975-03-01), Idstein
patent: 3926767 (1975-12-01), Brendlinger
patent: 4183799 (1980-01-01), Sellitto
Korpi Jouko K.
Korpi Teuvo T.
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