METHOD OF ELECTROPLATING

Abstract: Method of electroplating metal, primarily chromium, onto a workpiece connected as cathode in a current circuit, said workpiece being fed through an electrolyte (3) at a predetermined speed past the anode (2) in the current circuit. According to the invention, immediately before the anode (2) a member (4) is arranged which controls the current density between itself and the cathode so that etching of the cathode is obtained. This means that the electroplating is performed on a newly etched surface. The invention also covers various methods of achieving this etching.

Description: Method of electroplating The present invention relates to a method of electroplating metal, pri¬ marily chromium, onto a workpiece connected as cathode in a current cir¬ cuit, 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 des¬ cribing various methods of electroplating metal objects. German patent 484.206, dealing with chromium plating, proposes that ini¬ tially 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 workpiec.e, as cathode. Nowadays this method is used generally. Furthermore, German patent 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 498 941 describes a method of chromium plating elongate objects by gradually moving them through an anode. - ϋ!Ω_ OMPI Swedish published specification 310 970 also reveals that when electro¬ plating 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 US patent No.4,062,7 1 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 electro¬ plating gathered over the years, also verified in the patents discussed above. At the same time, however, the inventive concept offers a com- * pletely 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 5 acting as cathode, said workpiece being fed through an electrolyte at a predetermined speed past an anode where depositing of the metals is ef¬ fected. The method according to the invention is based on the cathode being con¬ tinuously etched immediately before it reaches the anode. Since this takes 10 place continuously the pole-changing method, which has a number of draw¬ backs as already intimated, cannot be used. According to the invention this continuous etching is achieved by arrang¬ ing a member immediately before the anode, said member controlling the current density between itself and the cathode so that the surface is 5 etched. This member may ' either be entirely electrically insulating or connected -in a current circuit with the cathode in such a way that the current density provides etching of .the cathode when it passes the member in question. The method according to the invention can also be performed by arranging-several pairs of etching members and anodes successively in 20 the same electrolyte. The quality of the plated coating can also be improved by varying the distance between cathode and etching member and between cathode and anode along a_distance along which the cathode is moved past these. In this way the current density, and thus the degree of etching, and the density of the electroplating can be varied to the desired 25 . value at each point along the surface of the cathode. The opportunity of giving the plated surface different hardness at different depths in this way may be of particular value. Certain other advantages can also be achieved and the entire etching-plating process can be carried out under partial vacuum. -The method according to the invention is defined in the 30 following claims and will now be further described in connection with a number of basic sketches of arrangements for performing the method. OMPI λ WIPO * In this connection it may be mentioned that the method according to the invention has been tested with good results at the State Institute of Technical Research in Helsingfors, test report MRG 1776. Figures 1 - 5 are basic sketches and such conventional elements as electro- plating baths, measuring means and complete electrical connecting systems have been omitted or merely intimated. Figure 1 shows the basic principle of the method according to the inven¬ tion. A workpiece K is connected as cathode In the current circuit 1 with current source U. The anode is designated 2 and the electrolyte 3- The cathode is fed continuously in the direction of the arrow V. Immediately before the workpiece K (cathode) reaches the anode 2, it passes under the member 4, characteristic for the invention, which -constitutes an elec¬ trically insulating shield in the basic form shown in this figure. The distance between the anode 2 and the cathode K and the voltage of the " current souce U are essential variables with respect to the plating, while the distance a between the .Insulating member 4 and the cathode K and the distance B .between the member 4 and the anode 2, together with the current strength over the anode, determine the etching. It is the current density which controls both etching and plating. All the variables discussed above are values which must be empirically determined. Etching takes place In the region 10 and plating in the region 11. In the embodiment shown in Figure 2 the insulating member 4 is replaced by an electrically conducting member 5 which will thus in practice function in the same current circuit as the anode 2 and cathode K. This means that the previously mentioned valiables must be adjusted depending on the condi¬ tions prevailing. In the embodiment shown in Figure 3 a member 6 to intensify the etching has been connected into its own current circuit 7 and has its own current source. The conditions discussed earlier apply here except that the pre- vlously mentioned variables must be given other values. OMPI In the embodiment shown in Figure 4 an insulating layer 8 has been arranged between the anode 2 and the member 6 intensifying the etching. It should be noted that the Insulating layer 8 extends some way between the member 6 and the cathode K. This is not always necessary but may sometimes be advis- able. A current circuit 7 may be connected to the member 6 as shown in Figure 3. Figure 5 illustrates a modification in which the distance between anode and cathode (A1-A2) and between the etching-intensifying member 5 (4) and cathode (a1-a2) varies along the path of the cathode past said member and the anode. The member 5(1) may consist of an electrically conducting member 5 as in Figure 2 or of an insulating member 4 as in Figure 1. According to this embodiment it is possible to influence the etching pro¬ cess along the member -5(4) in order to produce plating with gradually changing properties between the bottom and surface layers, for instance. The variants shown in the drawings can to a great extent be combined with each other to achieve desired properties in the plating layer. For instance an insulating member 4 as well as an electrically conducting member 5 may be used arranged one after the other in the direction of movement of the workpiece (cathode).- Practical experiments have proved that the quality of the coating can be highly Improved by having the ωαrkpiece passing an anode, that is divided up in several parts by an insulating and shielding protection or by using several successive anodes having insulating and shielding protection betueen each other. The anodes may have different sources of current supply and different voltages. The quality of the coating can also be improved by giving the anode at the end an insulating and shielding protection resulting in a gradually decreasing current density.

Claims: CLAIMS 1. Method of electroplating metal onto a uorkplece (k) con¬ nected as cathode in a current circuit, said ωαrkpiece (k) being fed through the electrolyte (3) at a predetermined speed past the anode (2) and any auxiliary anodes in the -current circuit, c h a r a c t e r i s e d in that, Im¬ mediately prior -to passing the anode (2), the uorkpiece (k) is caused to pass * a member (4) controlling the current density in the electrolyte (3) between itself (4) and the cathode (k) so that the latter (k) is etched (10) on its ωay past the member (4), after ωhich the metal (11) is deposited on the ωorkpiece (k) at the subssquent anode (2). 2. Method according to claim 1, c h a r a c t e r i s e d in that, in order to achieve the desired etching, the uork¬ piece (k) is caused to pass an electrically insulated shield (4) arranged Immediately before the anode (2). 3. Method according to claim 1, c ha r a c t e r i s e d in that, before reaching the anode (2), the ωorkpiece (k) Is caused to pass an electrically conducting member (6), the conductivity of ωhich is such that, together ωith the anode (2) and the cathode (k) , the member (6) forms a current circuit producing such current density betωeen the member (6) in question and the cathode (k) that the cathode (k) Is etched (10) ωheπ It passes the member (6) before It reaches the anode (2) ωhere the metal (11) is deposited. 4. Method according to claim 1, c h a r a c t e r i s e d In that-, said member (6) is connected in a current circuit (7) of its oωn, together uith the cathode (k) and that this current circuit (7) is such that it results in etching (10) of the cathode (k) uhen it passes the member (6). OMPI SUBSTITUTE SHE S 5. Method according to any of claims 3 or 4, c h a r a c t e r i s e d in that said electrically conducting member (6) is separated from the anode (2) by means of an electricall insulating layer (θ), also extending in under the member (5) in question. 6. Method according to any of claims 1-5, c h a ra c t e¬ r i s e d in that the anode (2) is so arranged in rela¬ tion to the direction of movement of the ωorkpiece (k) that the distance (Al, A2) betωeen the anode (2) and the ωorkpiece (k) varies in the direction of movement of the ωorkpiece, thus producing a varying current density bet- ueen the anode (2) and the uorkpiece (k). 7. Method according to claim δ, c h a r a c t e r i s e d in that the distans (A1, A2) betueen the anode (2) and the ωorkpiece (k) decreases in the direction of movement of the ωorkpiece (k). Method according to any of the preceding claims, c h a¬ r a c t e r i s e d in that the member (5) producing the etching is arranged so that the distance (al a2) betωeen this and the ωαrkpiece (k) varies in the direc¬ tion of movement of the ωαrkpiece (k).- Method according to any of the preceding claims, c h a¬ r a c t e r i s e d in that the ωorkpiece (k) is caused to pass under several pairs of etching members (4) and electroplating anodes (2) where the etching is controlled so that the layer (11) of electroplating on the uorkpiece (k) gradually increases. 10. Method according to any of claims 1-9, c h a r a c t e¬ r i s e d in that the entire process is performed under partial vacuum. - - "- - - - . - ** ■ — - n

Inventors: KORPI JOUKO KALEVI, KORPI TEUVO TAPIO, KORPI, JOUKO, KALEVI, KORPI, TEUVO, TAPIO
Applicants: KORPI JOUKO KALEVI, KORPI TEUVO TAPIO, KORPI, JOUKO, KALEVI, KORPI, TEUVO, TAPIO
Application year: 1983
Main classification: C25D5
IPC codes: C23F1, C25D3, C25D5, C25D7