JPH03207884A - Direct or indirect deposition of high corrosion resisting industrial hard chrome layer - Google Patents

Abstract

PURPOSE: To obtain a chromium layer having high corrosion resistance with high current efficiency by executing an electrolysis with prescribed pulsed DC by using a chromate and sulfate ions-contg. soln. mixed with prescribed satd. aliphat. sulfonic acid and/or its salt, etc.

CONSTITUTION: The soln. optimized in current efficiency is prepd. by adding the prescribed satd. aliphat. sulfonic acid and/or its salt or halide deriv. to the soln. contg. the chromate and sulfate ions. The chromium layer having a thickness; ≥2μm and hardness; ≥900HV0.1 (in accordance with DIN ISO 4516) is deposited from this soln. At this time, the work is carried out by the pulsed DC between pulse frequencies Fu and Fo. The deposition having gloss free from cracks is executed and the industrial hard chromium layer having the high corrosion resistance is obtd.

COPYRIGHT: (C)1991,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention is based on the invention, which consists of the following:
from a working solution containing chromic acid and sulfate ions, optimized by adding saturated aliphatic sulfonic acids and/or their salts or halogen derivatives, with a thickness of at least 2 μl and a minimum hardness of 90 according to DIN 130 45161.1 m. 0 HVO. A method for directly or indirectly depositing a corrosion-resistant industrial chromium layer having a chromium content of 1. The sulfonic acid may be formed by chemical or electrochemical reaction of suitable starting materials in an electrolyte. The workpiece may be made of steel, aluminum or copper.

[Prior Art 1] Regarding the optimization of current efficiency by the use of additives, reference is made to German Patent No. 3402554. Current efficiency represents the ratio of actual to theoretical metal deposition at the cathode. The current efficiency is optimized by the amount of additive in the method of the above-mentioned principle structure. The working electrolyte can be modified by means of an otherwise cheap fluoride compound and adapted operating conditions, in particular also by constitution with additional fluorine ions or fluorine complexes. should be understood. However, it should always be possible to optimize the current efficiency description.
In practice, the optimization cannot be expressed as strongly when using fluorine compounds than in the case of electrolytes without fluorine compounds. According to the invention, it is advantageous to work with a working electrolyte without fluorine compounds. Technical hard chromium layers serve for the chromification of industrial functionality of workpieces made of metal, in particular steel or aluminum alloys. The chromium is often applied directly to the material. However, the material can also be previously provided with an underlayer of copper, copper alloys, nickel, zinc, zinc alloys or galvanically deposited nickel/phosphorus or nickel/boron alloys. Also with regard to the usual layer thicknesses, technical hard chrome layers differ from decorative bright chrome, which is applied exclusively within the layer range of 0.2 to 2 μR. The hard chrome layer is thicker. In practice, industrial chromation begins with 2 μl. Often in the range 5-100μ1. Such a layer serves as wear and corrosion protection. In certain cases, technical hard chromium layers whose thickness reaches the millimeter range are also applied. Therefore, different workpieces can be processed in the same way with a hard chromium layer.

The known means from which the present invention is based (in the scope of DE 34 02 554) is that during the precipitation it is operated with a direct current. Shiny hard chrome coatings can be deposited from working electrolytes of modified composition. In this case care is often taken that the DC has a residual ripple of less than 5%, even if the residual is too high. In the case of ribbles, matte precipitates may form to a greater or lesser extent, the hardness of which is disadvantageously reduced.Hard chromium layers produced within the range of known methods are finely divided with a large crack density. The cracks are therefore microcracks.The corrosion resistance is satisfactory, but there is room for improvement.

OBJECT OF THE INVENTION The object of the invention was to develop the method described in the introduction in such a way that the hard chromium layer is further improved and still achieves the highest possible current efficiency. .

1 Means for Solving the Problem] According to the invention, the problem depends on the lower critical pulse frequency Fu depending on the cathode current density selected during deposition and on the current efficiency optimized at the same current density. Working with a pulsed direct current with a pulse frequency in the range between the upper critical pulse frequency Pa and the lower critical pulse frequency Fu for the working electrolyte, the pulse frequency/current density curve is measured technically. Determined by taking the curve 1, a precipitation with a hardness less than the minimum hardness occurs at the -d of the ljh line, and a shiny crack-free deposition greater than the minimum hardness occurs at the 12 of the curve, and the working electrolytic Technically, the upper critical pulse frequency point Fo for the liquid is measured in the current efficiency/pulse frequency curve at the same current density for optimized current efficiency, and increasing the pulse frequency produces a shiny crack-free deposit. is determined by taking up to a critical pulse frequency point Fo on the E side where the precipitation first transitions to cracked precipitation, and within the range between the lower critical pulse frequency Fu and the upper critical pulse frequency point Fo, the shiny , is solved by choosing such that a practically crack-free precipitation takes place. Under the pulse frequency/current density curve, a deposition that is almost unusable for hard chroming purposes occurs due to the too low hardness.

The deposit is generally matte and coarser than the shiny deposit.

The deposits are often also cracked and have microcracks. The impact coefficient is the sum of the pulse times over the total processing time of the workpiece in the electrolyte. This is expressed in percent. Actually crack-free means that the number of cracks is extremely small compared to the number of cracks in known shiny hard chromium deposits, and that the phenomenon of cracking or precipitation is not actually observed in corrosion technology. It means that there is no shadow. According to an advantageous embodiment of the invention, within the scope of the basic technical idea mentioned above, it is provided that IO to I 2
We work within a cathode current range of 0 O A/da', which according to the invention means the arithmetic mean of the pulsed direct current. Within the scope of the present invention, the cathode current efficiency is usually in the range of 10-25%. According to the invention, the hard chromium deposition is multi-layered, in particular, first a crack-free hard chromium layer, on top of which a shiny cracked hard chromium layer is deposited from the same working electrolyte without interruption. can be done. To create this second layer of microcracks, simply select the parameters such that the basic idea and the range resulting therefrom deviate from between the lower critical pulse frequency P u and the upper critical pulse frequency point Fo. Just do it. In order to make this selection, and also to take the pulse frequency/current density curve for a given working electrolyte, in the present invention the current density, pulse frequency and impulse coefficient are variable. We recommend that you do so.

Of course, various pulse shapes can be operated within the scope of the invention.

In principle, it is known to work with pulsed direct current in hard chromium deposition (T. Pearson,
J. D. Dennis “Effect of pu
lsed current on the electr
odeposiLed chromium, 1 in Paris
(See lecture at the 12th World Conference on Surface Finishing held October 4-7, 1988). In this case too, deposited layers were obtained which were more or less glossy and virtually crack-free.

However, within the range of known means, in order to obtain a deposited layer that is more or less glossy and virtually crack-free,
Working at very low pulse frequencies in the range of 6 to 500 11z, reproducibility is not guaranteed. Working according to the technical idea of the present invention, the pulse frequency is usually 5
It is within the range of 00 to 5000Hz.

The attack coefficient is usually in the range of 30-70%.

1' Effects of the invention The advantages achieved are that the hard chromium layer defined at the beginning is shiny and practically crack-free, and at the same time is deposited with a relatively high cathodic current efficiency. The object of the present invention is that it can be manufactured reproducibly. A particular advantage lies in the fact that it is possible to additionally cover the microcracks with a hard chromium layer, which is often desired on account of its good smoothing properties. In this case, the crack network acts similarly to oil pockets.

Modifying the anodic pulsed current component of the integrated pulsed direct current does not change the result at all unless the pulse frequency is very low and the current density is not too high.

The present invention provides three methods for depositing a shiny, practically crack-free, highly corrosion-resistant hard chromium layer from a working electrolyte of the basic composition defined at the beginning. Two prerequisites: g) I<Russ frequency should be crystalline the higher the cathodic current density chosen for the chromation step.

b) However, the pulse frequency should not be too high for optimized current efficiency.

C) The shock coefficient should be chosen sufficiently far from DC operation.

should be satisfied.

The invention provides clear criteria for a) to b).

The corresponding impact coefficients based on C) can be ascertained by experiment without difficulty.

I Example 1] The present invention will now be described in detail with reference to the graphs and examples shown in the drawings.

FIG. 1 quantitatively shows the pulse frequency current density curve for a special working electrolyte constructed according to the invention. On the ordinate axis, the Norculus frequency was taken, and on the abscissa axis, the ball flow density was taken as the arithmetic mean. Under the curve shown, which increases with the current density, a hard chromium deposit of matte, cracked or non-cracked form is obtained with a hardness of 1 less than the minimum hardness. , a crack-free, shiny hard chromium deposit layer with a hardness greater than the minimum hardness is obtained.The curve therefore shows a lower critical pulse frequency Fu depending on the current density.As a parameter, the curve An impact coefficient E appears at .Other impact coefficients result in a family of curves shown in dotted lines in Figure 1.1
If + represents the duration of an ideal rectangular current pulse and 1 represents the duration of the current pulse, F = 1/(1++11) in Hz applies for the pulse frequency. The impact coefficient is expressed as percent E=100・t +/(t
,4-t? ) is defined. Of course, the pulse frequency/
The current density curve is plotted until a clear dependence of critical pulse frequency on current density occurs.

Figure 2 shows the determination of the current density/pulse frequency curves for a specific deposition task for the same electrolyte, selected current density I, and the same impulse coefficient E of the illustrated curve taken in Figure 1. In this case, the current density, expressed as / or its salt or by addition of 7% rogene derivative. The ordinate axis shows the current density,
The abscissa axis shows the pulse frequency. In the optimized current density curve, a critical pulse frequency point Fo is recognized. Behind this point Fo, the curve runs in dash-dot I to show that beyond this point an increasingly cracked precipitate layer is obtained. Different selected current densities and different equilibrium coefficients will result in different curves.

As such, a family of curves results, as illustrated by the dashed lines in FIG.

The impact coefficient, always within the range of Fu and FO, can be selected such that a shiny, practically crack-free hard chromium layer of normal thickness with a hardness greater than the minimum hardness results. Experience has shown that the range of Fu and FO can be increased if the hard chromium coating is kept as small as possible. In other words, the possibility of crack-free deposition is always particularly good if working with layer thicknesses that are not too large. Very low cathode current density, e.g. lO~20 A/d1
A high-gloss, crack-free chromium coating is achieved, which is also highly suitable as a decorative bright chrome [Example 2j A hard chromium layer is deposited on one surface of a workpiece made of sea bream. The electrolyte for this purpose was composed as follows: Chromic acid as CrOa 300g/Q sulfuric acid
13㎢ (based on Crys content) Ammonium fluorooctane sulfonate 10xg/(
1 (as crosslinking agent) Anode When using PbSns or platinized titanium or platinized Pb alloyed titanium platinized anodes, 1 g/(2) of lead carbonate was additionally added to the electrolyte. With this composition, the electrolyte exhibited a cathodic current efficiency of 16% in the direct current operation of the deposition.The cathodic current efficiency was 3.2% for a saturated aliphatic sulfonic acid having one carbon atom and one sulfonic acid group. By adding gIQ it was possible to optimize it to 27%. In both cases the cathode current density was 50 A/da'' and the electrolyte temperature was 55°C.

Thereafter, a switch was made to the deposition with pulsed direct current, and constant pulse frequency/current density curves, described further below, were plotted for various impact coefficients. In detail, it was carried out as follows.

A piston rod with a diameter of 71 N made of material C 45 K (micropolished and brushed to a surface roughness Rz < 1.5 μl) was deposited with hard chromium, after having been previously cleaned and degreased according to the usual electrotechnical rules. I let it happen. that time,
Depending on the averaged cathode current density adjusted in each case at a given impulse factor E of the pulse, the deposition can be as low as 900 HV.
O. Matte appearance with hardness less than 190
01YO. Critical pulse frequency Fu, dependent on current density, leading to a shiny appearance with hardness greater than I
was measured. In all experiments, the electrolyte temperature was maintained at 55°C. The deposited layer thickness of the hard chromium coating was approximately 25 μl in each case.

In this case, it should be understood that the transition range from matte to glossy appearance is fluid, and even more so that the actual local cathodic current density inevitably varies depending on the geometry of the workpiece. The critical pulse frequency Fu to be determined is therefore the average value within this transition range. In this case, the inventive shiny areas of hard chromium deposits were simultaneously examined for cracking status (with or without cracks) using an optical microscope.

FIG. 1 schematically shows the pulse frequency/current density curve recorded in this way.

For the specially implemented hard chroming, a periodic average cathode current density of 50^/da was selected depending on the workpiece surface.Furthermore, from Fig. 1, for an impact coefficient of 50%, a lower It is clear that the critical frequency F' u = I 000.

For this chosen cathode current density, the current efficiency/pulse frequency curve shows that at least the hard chroming is actually brighter at 900 HV O. Precipitation occurs with a hardness of 1, but the data were recorded up to the upper critical pulse frequency point Fo at which cracks appeared again. By varying the current efficiency and/or the impulse coefficient, a family of curves with an upper critical pulse frequency point is obtained in each case.

This current efficiency/pulse frequency curve family is shown schematically in FIG. In detail, to investigate this family of curves, the following procedure was carried out: Deposition was carried out on the piston rod as in FIG. 1 with a layer thickness of approximately 25 μl. In this case, the investigation of the cathode current efficiency in % as a ratio of the actual metal deposition to the theoretical metal deposition at the cathode was carried out by measuring the amount of chromium deposited and the ampere fraction used for this purpose. Faraday-based precipitation equation calculations are well known to those skilled in the art.

Optical microscopic confirmation of crack conditions starting at the upper pulse frequency point FO can pose difficulties. A surefire proof is the corrosion resistance evaluation in the salt spray test (DIN50021-SS).
, ^STM B 117-73 or Is0 3768-
(1976). Thus, for those skilled in the art, it is clear that a hard chlorine coating approximately 25 μl thick, deposited with direct current, is shiny and cracked, but with micro-cracks, in the salt spray test. It is well known that stability of only less than However, under pulsed conditions a clearly higher service life occurs in the spray test, which is proof, for example, of the practically crack-free nature of such coatings.

Table 1 relates to the above example with an impact coefficient of 50% and an average cathode temperature of 50^/da' at an electrolyte temperature of 55°C. This represents a hard chromation that is shiny, virtually crack-free and therefore highly corrosion resistant.

Table 1 shows that for any pulse frequency within the range of the lower critical pulse frequency Fo = I00 Hz and the upper critical pulse frequency Fo = 4.000 Hz, at the stated current density within the stated pulse range, the impulse coefficient is the desired This shows that the rule holds that the hardness can be selected so low that a shiny, virtually crack-free and highly corrosive hard chromium deposit results.

On the other hand, Table 2 shows that if the impact coefficient is selected too high within the stated pulse frequency range, a cracked hard chromium layer with much poorer corrosion resistance results. Furthermore, it is clear from Table 1 that cracked coatings are also obtained when working with pulse frequencies that lie outside the stated pulse frequency range.

The experiment may be carried out at 40 A/da' or 60 A/da, for example.
” was repeated with a different current density. Said rule 81 was confirmed. In the said method, gaps were also confirmed when experimenting with an electrolyte of a different composition. As already mentioned, the brightness A too high impact coefficient and/or a pulse frequency setting adjustment that is too high will result in a shiny, cracked deposit on a shiny and virtually crack-free base layer. The cover layer can be deposited as little as possible without interruption of the current flow, so that double hard chromium layers with improved smoothness properties can thus be produced.

If an impact coefficient E=100% occurs, this means that the deposition of the cover layer takes place with direct current.

The addition of alkyl sulfonic acids, formulated to optimize cathode current efficiency, during pulsed direct current deposition,
As long as the working conditions remain above the lower critical pulse frequency Fu, the gloss improving effect will be to an even higher extent. Apart from too low cathode current efficiency, the lack of this addition results in
It can also be noticed by the reduced gloss of the hard chrome coating.

In the production of the bilayer described, depending on the current and time /t4
The pulse frequency and/or
or automated at controlled speeds with freely selectable impact coefficients, pulse frequencies up to 5000 Hz or higher and/or such that a cracked deposit instead of a crack-free deposit occurs in the subsequent deposition process. Alternatively, it is also within the scope of the present invention to shift to a higher impact coefficient of 100%. In this case, after the end of chroming, the initial values for pulse frequency and/or impact coefficient are automatically controlled again for adjustment of the next chroming cycle.

4

[Brief explanation of drawings]

FIG. 1 shows a curve of critical pulse frequency as a function of current density, and FIG. 2 shows a curve of optimized current efficiency as a function of pulse frequency.

Claims (1)

[Claims] 1. A saturated aliphatic sulfonic acid and/or its salt or halogen derivative having a current efficiency of up to 2 carbon atoms and up to 6 sulfonic acid groups is applied to the surface of a workpiece made of metal. From a working solution containing chromic acid and sulphate ions optimized by adding a thickness of at least 2 μm and a minimum hardness of 900 HV0.0 according to DINISO 4516.
1, with a lower critical pulse frequency Fu depending on the cathodic current density chosen during the deposition and an optimized current at the same current density. Working with a pulsed direct current with a pulse frequency in the range between the efficiency-dependent upper critical pulse frequency point Fo, where the lower critical pulse frequency Fu for the working electrolyte is pulse frequency/current density determined by measuring a curve, below which a precipitation has a hardness less than the minimum hardness, and above which a shiny, crack-free precipitation takes place, which is greater than the minimum hardness; The upper critical pulse frequency point Fo for the working electrolyte is technically determined by measuring the current efficiency/pulse frequency curve at the same current density for optimized current efficiency.
Determined by taking up to the upper critical pulse frequency point Fo, where increasing the pulse frequency leads to a transition from a shiny crack-free deposit to an increasingly cracked deposit, and by taking the lower critical pulse frequency Fu and the upper critical pulse frequency point Fo A method for directly or indirectly depositing industrial hard chromium layers of high corrosion resistance, characterized in that the method is chosen such that a shiny, virtually crack-free deposition takes place within the range between . 2. The method as claimed in claim 1, wherein the method operates within a cathodic current density range of 10 to 1200 A/dm^2. 3. The method according to claim 1 or 2, wherein the method operates within a cathodic current efficiency range of 10-25%. 5. The method as claimed in claim 1, wherein the method operates at a pulse frequency in the range from 500 to 5000 Hz. 5. The method as claimed in claim 1, wherein the pulsed direct current has an impulse coefficient of between 30 and 70%. 6. Depositing first a crack-free, shiny, hard chromium layer and then, without particular interruption, a shiny, cracked, hard chromium layer in the same electrolyte. The method described in section. 7. The method as claimed in claim 1, characterized in that the current density, pulse frequency and equilibrium coefficient are variable.