Electrochemical process for producing platings of chromium and similar metals, by means of pulsating current with periodic reversing polarity, and relevant equipment

Abstract

 Electrochemical process for producing platings of such metals as chromium, nickel and the like, consisting in depositing on the support to be plated at least a first layer of plating metal such as chromium, nickel and the like, by using a current rectifier equipped with an electronic device for the periodic and adjustable reversal of polarity, in such a way as to obtain said first layers with no cracks, then in subsequently depositing on said first layer at least a second layer by using the only positive polarity obtainable from said rectifier, so as to endow said second layer with the required values of hardness and elasticity.
Equipment for carrying out such a process, which consists in a generator of double-level pulsating electrical waves, comprising means for independently adjusting the time intervals between two consecutive negative pulses and the duration time and amplitude of the negative waves.

Description

[0001] The present invention relates to an electrochemical process for producing platings (deposits) of such metals as chromium, nickel, and the like, by using double-level pulsating electrical current, with said two levels being of reverse polarity relatively to each other, and adjustable, with said pulsating electrical current alternating with rectified current.

[0002] An object of the present invention is also a suitable equipment for carrying out said process, and consisting of a generator of electrical waves pulsating between two levels relatively to a zero line, i.e., of waves of periodically reversing polarity, and adjustable according to as required.

[0003] It is known that in the field of the electrodeposition of such metals as chromium, nickel, and still others, the use of square-wave currents pulsating between a plurality of levels relatively to a base level of direct current, made it possible layers of deposited metals to be attained in practice, which have considerably better characteristics of hardness, thickness uniformity and corrosion resistance than as obtainable by means of the traditional treatments with direct (rectified) current. In particular, in order to obtain the deposit of such a metal as chromium, nickel, and the like, free from cracks, and hence characterized by a very high corrosion resistance, a type of rectifier equipment showed to be particularly efficacious, which is equipped with an electronic device for the periodic reversal of polarity, provided with means for adjusting the parameters which define the wave form. By adjusting from time to time the parameters which define the form of the voltage, and hence current, wave, i.e., the duration times and the amplitudes, it resulted possible to obtain completely crack-free deposits, hence deposits endowed with a much higher corrosion resistance than as obtainable by the traditional routes. But in practice, the use of this type of pulsating current with the periodic reversal of polarity, involves the disadvantage that layers are obtained, whose hardness and elasticity values are considerably lower. This drawback, also on considering the fact that the outer appearance of the plating does not result shiny but after a mechanical polishing, causes a considerable limitation in the practical application of the process.

[0004] Therefore, the purpose of the present invention is to provide a plating process by electrodeposition of such metals as chromium, nickel and the like, which is capable of overcoming the drawbacks and disadvantages shown by the electrodeposition technique known from the prior art, including that technique which only uses pulsating currents with periodical reversing polarity.

[0005] Another purpose of the present invention is to provide a process for producing plating layers of chromium, and the like, which can be easily practiced, is adjustable relatively to the types and materials of the support to be plated, and to the characteristics of hardness, elasticity and corrosion resistance desired for the layers to be obtained, by using a current rectifier generating waves pulsating between a plurality of levels, of reverse polarity relatively to one another.

[0006] These and still further purposes, which will be evidenced in greater detail by the following disclosure, are practically achieved by means of a plating process by electrodeposition of such metals as chromium, nickel, and the like, which consists, according to the present invention, in depositing on the support to be plated at least a first layer of such a plating metal as chromium, nickel and the like, by using a current rectifier equipped with an electronic device for periodically reversing and adjusting the polarity, in such a way as to obtain said first layer with no cracks, then in depositing on said first layer at least a second layer by using the only positive polarity which is obtainable from said rectifier, in such a way as to endow said second layer with the required values of hardness and elasticity.

[0007] The passage from the periodically reversing polarity for obtaining said first, crack-free layer to the only positive polarity for obtaining said second layer having the required values of hardness and elasticity, can be advantageously carried out both in a sudden, or instantaneous, way, and in a progressive way, by adjusting said current rectifier with periodic reversal of polarity, so as to create a progressive increase in the time interval (duration) between two successive negative pulses, and obtain a gradual passage from the lower, crack-free layer, to the upper layer.

[0008] Still according to the present invention, said plating process can be carried out by creating a plurality of alternating layers, i.e., layers obtained by means of the use of current with reversing polarity, alternating with current with only positive polarity.

[0009] The invention is disclosed in greater detail in the following, according to a preferred form of practical embodiment, by referring to the hereto attached drawing tables, supplied for purely indicative purposes, wherein:

Figure 1 shows a schematic view of a pulsating wave form with the periodic reversal of polarity; and

Figure 2 shows a portion of a metal support to be plated with superimposed layers respectively obtained by means of a current with periodic polarity reversal, and by means of a direct current.

[0010] With reference to said figures, for practicing the process of the present invention, as already said an electronic equipment is used, which generates waves pulsating between two or more levels of reversed polarity relatively to one another, and adjustable, which equipment is substantially constituted by a main rectifier generating a unidirectional positive current of the controlled-diode type, and by a similar rectifier connected in antiparallel to the main rectifier, so as to obtain an output which is constituted by a current wave form showing periodic and extremely short polarity reversals, i.e., an at least double-level wave form, with at least one positive level alternating with a negative level, as shown in Figure 1, in which by the reference numeral 1 the positive square waves are indicated, which alternate with negative square waves 2, relatively to a zero level direct current; the values of amplitude B and of duration C of the negative waves and the time interval - or pause time - D between said negative waves can be independently adjusted by means of a suitable device or circuit controlling the negative or reverse polarity associated with said rectifiers.

[0011] The equipment furthermore comprises a static interblock circuit capable of excluding, alternatively and for preset time periods, one of the rectifiers, while the other rectifier is operating. More particularly, said means for adjusting the electrical quantities of the output waves are constituted by individual potentiometers, and said static interblock circuit interposed between said two rectifiers is of a traditional type. According to the present invention, in order to obtain a deposit of chromium, nickel, or of another metal, free from cracks, and therefore endowed with a high corrosion resistance combined with high hardness and elasticity values, said deposit is produced by depositing on the metal support 3 (Figure 2) a first chromium layer, indicated by the reference numeral 51, by using current with periodically reversing polarity and adjusting, relatively to the characteristics of the support 3 and to the desired thickness of the layer 51, the wave form parameters, i.e., the pulse durations and amplitudes, in such a way as to obtain a deposit which is completely free from cracks; then disabling the rectifier generating the waves with periodic polarity reversal, and letting only operate the rectifier which generates only positive waves, on the first layer 51 a second layer 52 is deposited. In practice, the present Applicant observed that by operating according to this procedure, the values of hardness and elasticity of the plating result comparable to those shown by deposits applied according to the traditionale route, whilst the corrosion resistance is considerably increased. Still according to the present invention, it was also observed that by applying a progressive increase of the time interval "D" between two successive negative pulses 2, a gradual passage can be attained from the underlying layer 51 to the external layer 52, i.e., a gradual passage from the crack-free layer, to the external layer affected by cracks.

[0012] By accomplishing the above-said two layers, the deposit is given a high corrosion resistance (by the layer 51) and high values of hardness and elasticity (by the layer 52).

[0013] Furthermore, according to another advantageous form of practical embodiment, the plating process can be accomplished by alternating with one another a plurality or layers obtained with current with periodic polarity reversal polarity, alternating with only positive current. According to this form of practical embodiment, values of hardness and corrosion resistance can be obtained, which are higher than those which can be obtained with two layers only. Furthermore, by accomplishing the double-layer, or the multi-­alternating-layer deposit, such a sufficiently shiny appearance of the outermost layer can be obtained, as to render useless the polishing operations.

[0014] In Figure 3, a diagram is shown, which represents an equipment capable of generating double-level pulsating waves characterized by periodic and extremely short reversals of polarity according to the present invention; said equipment is constituted by a main rectifier 3, preferably with an hexa-phase circuit structure and of the controlled-diode type, such as to output a unidirectional pulsating current; with said rectifier, a similar rectifier 5 is connected in antiparallel by means of the leads 4-4a, which generates a unidirectional pulsating current, with reversed polarity, i.e., capable of emitting, during adjustable times, and for adjustable durations, waves with a reverse polarity relatively to the waves generated by the main rectifier.

[0015] With said rectifiers a control circuit is connected, which is generally indicated by the reference numeral 6, for the independent adjustment of the times B and C and of the amplitude A of the reverse-polarity waves 2. Such adjustment means comprise three potentiometers A1, B1 and C1, and precisely: the potentiometer A1, for the adjustment of the amplitude - or intensity - A of the reverse-­polarity waves 2; the potentiometer B1, for the adjustment of the pause times B between two successive negative waves; and the third potentiometer C1, for adjusting the duration times C of said negative waves. The reversals of polarity are adjusted in such a way as to take place periodically and for extremely short times, of the order of milliseconds, so as to make it possible the main pulsating current to be interrupted and followed by negative pulsating current for very short times, and with adjustable intensities, according to the selected treatment type.

[0016] Then, in order to enable the output of the positive pulsating waves to be periodically interrupted, a circuit of static interblock (not shown, in that it is of a type known and suitable for the intended purpose) with the main rectifier is provided, which is capable of making operate, for the preset times, the only reverse-polarity-wave rectifier.

[0017] Obviously, when it is carried out in practice, the process can be used, by suitably adjusting the parameters of the wave form of the current with periodical polarity reversal, in order to obtain electrodepositions of any metals, with layers having desired thicknesses, and with a number of alternating layers which can be adjusted to meet the various requirements.

Claims

1. Electrochemical process for depositing platings of chromium, nickel, and other similar metals, characterized in that it consists in depositing on the support to be plated at least a first layer of such a plating metal as chromium, nickel and the like, by using a current rectifier equipped with an electronic device for periodically reversing and adjusting the polarity, in such a way as to obtain said first layer with no cracks, then in depositing on said first layer at least a second layer by using the only positive polarity which is obtainable from said rectifier, in such a way as to endow said second layer with the required values of hardness and elasticity.

2. Process according to claim 1, characterized in that the passage from the periodically reversing polarity for obtaining said first layer without cracks to the only positive polarity for obtaining said second layer having the required values of hardness and elasticity is carried out in an instantaneous way.

3. Process according to claim 1, characterized in that the passage from the periodically reversing polarity for obtaining said first layer without cracks to the only positive polarity for obtaining said second layer is carried out in a progressive way, by adjusting said current rectifier in such a way as to accomplish a progressive increase in the time interval between two successive negative pulses, in order to obtain a gradual passage from the lower, crack-free layer to the top layer.

4. Process according to claim 1, characterized in that the plating is carried out by depositing a plurality of alternating layers obtained by using reversing-polarity current alternating to current with only positive polarity.

5. Equipment generating electrical pulsating waves for facilities for the electrodeposition of plating metals, such as chromium, nickel, and the like, characterized in that it is constituted by a main current rectifier of the controlled-diode type, and by a similar rectifier connected in antiparallel with the main rectifier, so as to obtain an output constituted by a current wave form showing periodic and extremely short polarity reversals, i.e., an at least double-­level wave form, with at least one positive level alternating with a negative level relatively to the zero level direct current, with said rectifiers a control circuit for the negative or reverse polarity comprising means for independently adjusting the pause times, and the duration times of the negative waves, and the amplitude of said negative waves, as well as a circuit of static interblock with said main rectifier, capable of excluding, alternatively and for preset times, the main rectifier, while the other rectifier is emitting reverse-polarity waves, being associated.

6. Equipment according to claim 5, characterized in that said means for adjusting the electical quantities of the output waves are constituted by independent potentiometers, one of which is for adjusting the amplitude of the reverse-polarity waves, one is for adjusting the pause times between two successive negative waves, and another one is for adjusting the duration times of said negative waves.

7. Equipment according to clam 5, characterized in that said periodic reversals of polarity are adjusted by means of a circuit of interblock with the main rectifier, capable of causing said polarity reversals for duration times of the order of milliseconds.

Drawing