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: The present invention relates to an electrochemical process for producing platings (deposits) of chromium, 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. 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 chromium, 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. It is a known fact (Anodizing and hardchromium plating with pulsed current; C.Colombini - Symposium held at Washington DC on October 28-29, 1986- Published by American Electroplaters and Surface Finishers Society - ORLANDO, FL) that the presence of cracks in chrome deposits causes a lower resistence to corrosion. In the past many researchers have, for this reason, made various efforts to solve this problem, working both from the chemical (concerning the chemical composition of the bath) and the electrical point of view, that is to say appropriately modifying the output voltage wave form. As far as this second method is concerned, some researchers have in the past obtained a strong (even though not complete) decrease of cracks adopting a voltage wave form that is characterized by periodical interruptions (fig.4). The method of periodical current interruptions does not completely eliminate cracks and what's more it has the disadvantage of decreasing the deposition rate due to the periods when the current is null. To have a strong decrease of cracks the interruption period must be 10-15% of the period of time in which current is flowing: consequently even the deposition rate decreases of the same value. To avoid this inconvenient and eliminate cracks completely a type of wave form in which periods of reverse current substitute periods of null current can be used: their duration is much lower than the periods of direct current (fig.5). In fact the periods of time used are the following: direct current periods: adjustable between 15-45 sec. inverted current periods: 20 msec. It is evident that this way there is hardly any diminution of the current average value and consequently of the deposition rate. Adopting this type of wave form a completely crack-free chrome deposit is obtained, with a considerable increase in resistence to corrosion. It must however be observed that the appearance of the chromed surface when it is taken out of the bath is not shiny, but grey. However a normal mechanical polishing is sufficient to obtain a normal shine. As regards hardness of the chrome deposit obtained with the periodical interruption of current, it must be observed that there is a diminution of hardness in comparison with deposits obtained with continuous current without interruptions. Tests have been carried out on cylindrical samples with a diameter of 30 mm (1.18 inch) with a chrome depth of 40/45 um (1.6/1.8 mils). A chrome bath with a classical composition, 250 gr/l CrO₃-2.5 gr/l H₂SO₄ was used. The tests were carried out even at different values of current density (fig.6). The results of the tests illustrated in the graph show that the chrome hardness of the chrome deposit increases as the time interval between two subsequent negative impulses increases. If this time interval is increased too much the first cracks appear. Examining the graph it must be concluded that this time interval must not be more than 30 sec., to be certain of not having any cracks even with a high current density (50 A/sqdm, about 500 A/sqft). It is however important to notice that the value of the hardness that can be reached is always lower (at a ratio of about 9/10) than the one that can be obtained with continuous current without reversing: however this has the disadvantage of causing various cracks as was mentioned above. Certain chemical compositions of the bath other the classical one can increase even more the hardness value. The level of the reverse current impulse is adjusted at about 1/3 of that of the direct current. Varying the level of the negative impulse, the limit values of cracks and hardness are not substantially modified. Crack-free chromium deposits have also been produced by Mortier er al. (Cetim Report 12 F 390 - 1984) and Sutter (Oberflache Surface 25, (1): 16-19 (1984)) by pulsed reverse current plating from a conventional chromium bath, operated at 50°C, and using direct current periods of 5-15 sec. and inverted current periods of 30-50 mseconds, and current density of 5-50 A/dm². The deposits produced were grey, mat in appearance, but readly polished. Hardness was reduced as compared with conventional chromium plate (850HV, cf 1000 HV), but nevertheless retained a useful level. 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. US 4092226, relates to a method of electroplating a crack-free hard chromium deposit applying a base voltage and periodically superimposing, on the base voltage, high voltage pulses 3 to 7 times greater than the base voltage. US 4666567 relates to a method and apparatus for electroplating the surface of a substrate providing alternatively forward and reverse polarity current pulses, the time ratio between the forward and reverse current pulses being varied to maintain the process voltage below the burn voltage. Two separate cycles are provided, a first conditioning cycle comprising short and constant duration pulses of both forward and reverse polarity which improves the deposit adhesion to the substrate and a second plating cycle divided into two phases, each of these phases including a reduction in the time ratio between the forward and the reverse polarity. US 4496436 relates to a method of elctrodepositing a metal using an electric current which is applied in the form of pulses having a pulse duration not greater than 100 microseconds (preferably 1-50 microseconds) while the off time between adjacent pulses is more than twice as long as the on time pulses, also interposing a reverse-polarity pulse between the successive pulses of the electrodepositing polarity. US 3975254 relates to a power supply apparatus for anodizing and electroplating wherein a series of discrete positive current pulses which are a portion of a sinusoidal wave are supplied to the load followed by a plurality of discrete negative partial sinusoidal pulses. The period of the ramp is variable from fractions of a second to several minutes or longer. Therefore, the purpose of the present invention is to provide a plating process by electrodeposition of chromium, 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. 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. These and still further purposes, which will be evidenced in greater detail by the following disclosure, are practically achieved by means of a process as claimed in the only appended claim. The passage from the periodically reversing polarity for obtaining said first, crack-free layer to the direct current for obtaining said second layer having the required values of hardness and elasticity, is carried out 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. 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. 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. 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. 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, 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 S.1, 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 S.1, 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 direct current, on the first layer S.1 a second layer S.2 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 S.1 to the external layer S.2, i.e., a gradual passage from the crack-free layer, to the external layer endowed with high hardness and elasticity. By accomplishing the above-said two layers, the deposit is given a high corrosion resistance (by the layer S.1) and high values of hardness and elasticity (by the layer S.2). 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 direct 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. 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. With said rectifiers a control circuit is connected, which is generally indicated by the reference numeral 6, for the independent adjustment of the times D and C and of the amplitude B 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 - B of the reverse-polarity waves 2; the potentiometer B1, for the adjustment of the pause times D 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. 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. 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: Process for depositing on a metal support a plating of chromium from a conventional chromium bath using the technique of pulsating currents with periodical reversing polarity which comprises the following steps: a) depositing on said metal support a first metal layer by using currents with periodically reversing polarity adjusted to take place for extremely short times, of the order of milliseconds, so obtaining a first metal layer which is completely free of cracks, by means of a current rectifier equipped with an electronic device for periodically reversing and adjusting polarity; b) depositing on said metal layer a second metal layer by using direct current so obtaining said second metal layer which is endowed with hardness and elasticity, by means of the only positive polarity from said rectifier, whereby the passage from the periodically reversing polarity step to the direct current step 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, said plating being free from cracks and endowed with a high corrosion resistance, high hardness and elasticity.

Inventors: BLESIO, LUIGI, LEONI, MARIO
Applicants: ELCA S.R.L.
Application year: 1988
Main classification: C25D5
IPC codes: C25D5