DE1496823B1 - Process for the galvanic deposition of corrosion-resistant, three-layer nickel or nickel-cobalt alloy coatings on metals - Google Patents

Description

1 2

The invention relates to a method for narrow limits can be changed. Furthermore, vanic deposition of corrosion-resistant when a lower nickel layer consists of a sulfur three-layer Nickel or nickel-cobalt alloy-containing electroplating bath is deposited, the coatings on metals. Amounts of sulfur in the middle and upper

In recent times a lot of effort has been expended to increase the nickel layer, from which the emergency products are produced, which result in a decorative chrome turn, and the concentration of the other coating over two or three lower layers of nickel additives in the bath to be adjusted accordingly ,
exhibit. The composition and the electro- It has now been found that these nickel layers consist of three layers of chemical properties, in which the middle layer is balanced against one another in such a way that it contains the basic arsenic, not only more durable than the metal surface coatings The sulfur in the intermediate layer provides protection against corrosion without affecting the attractive appearance. but that they can also be produced more easily and advantageously. In order to be effective, the different ones must be produced. They only need to be deposited from normal nickel layers from electroplating baths, which are those that are designed to contain the nickel, not just the desired electrochemical compound, the correct concentration of a water-soluble arsenic layer. In addition, the Arsenver properties, but also other physical and bonding properties, are more stable in the bath than the sulfur-chemical properties that are required for good connections that serve to bring sulfur into the electroplated coating. In addition, introduce galvanic coating. This resistance must be the gloss and other surface properties, particularly with regard to air oxidation, so that the upper layer should be such that the mixing of the bath to any desired chrome coating is smooth, shiny and practically free of any extent with the aid of air can. λ is color, stain, and the like. The invention relates to a method for "

If three layers of nickel are used, the electrodeposition of corrosion-resistant, Composition of the middle layer adjusted to 25 three-layer nickel or nickel-cobalt alloy they opposite the upper and lower layers approximately coatings on metals, their middle layer is anodic, and the composition of the top is anodic compared to the other two layers, Layer is set in such a way that it is characterized by the fact that it is in the middle Lower layer is anodic. The decorative chrome layer is a nickel-arsenic alloy with an arsenic layer is more noble than the other nickel layers. 30 content from 0.025 to 8 percent by weight and one Furthermore, the middle layer is advantageously deposited with a very thin thickness of 0.25 to 5 μ. To held. this method gives an improved three-

Originally, the three-layer coatings were made with a lower layer coating, per se with intermediate layers, which usually consisted of known, low-sulfur electroplated nickel coating, metals other than nickel, but the 35 of preferably 7.6 to 50 µm. is thick when alloy nickel were anodic to. For some of these constituents up to about 25 percent by weight of cobalt coatings, a pronounced corrosion resistance and preferably a sulfur content of protection was observed (cf. Knapp, Trans. Inst. Met. Having less than 0.01 percent by weight, a Finishing, 1958, 35, p . 139 to 165). In most of the above, known galvanic bright nickel cases, however, these coatings turned out to be worthless, 4 ° coating, which is preferably 3.8 to 25.4 μm thick, either because the metal corroded too quickly and an alloy component up to 50 percent by weight Cobalt contains blistering or flaking of the coating and preferably caused a sulfur content, or because the 'metal has the decorative layer about 0.03 to 0.3 percent by weight, and one stained or discolored. _ 0.25 to 5 μ thick nickel intermediate layer with up to {

In recent years three-layer coatings have been made 45 to 25 percent by weight cobalt, which is about 0.025 to 8 percent been made, in which each layer consists of a weight percent arsenic, based on the total weight nickel-containing deposit exists, the potential of this deposit containing.

tiale of the layers by the amount of together The three-layer coatings according to the invention sulfur deposited with the nickel can be controlled advantageously with an intermediate layer. 50, in which the arsenic concentration is in the range of

These known three-layer coatings bring about 0.25 to 4 percent by weight. Preferably

but certain difficulties with it because the sulfur will be in the range of about the arsenic concentration

as such, not only kept potential differences in the 0.50 to 2.5 percent by weight,

galvanic deposits of nickel generated, The inventive method is particularly suitable

but also others: physical properties, 55 for iron, steel, copper, brass and aluminum,

such as ductility, gloss and the relationship between zinc and magnesium with or without a copper base

of the coating. For setting the top layer.

The galvanic top layer obtained according to the invention must be kept under control, the sulfur contents of both composite coatings are three to four times more effective than layers. It has 60 those in which the intermediate layer is found to contain sulfur but the negative potential. The rust-free service life of the bond over sulphurous nickel (and thus its corrosion, according to the results of the »Corrodkote« - susceptibility to corrosion) does not change linearly with the sulfur and »CASS« corrosion test for 5 to 6 years, but that the curve at sulfur. Corresponding three-layer composite overconcentrations in the range of about 0.2 to 0.3 puffs, in which the intermediate layer contains sulfur, flattens as a percentage by weight. It follows that these corrosion tests only result in a rust-sulfur concentration in the nickel layer for a free service life of at best 2 to 4 years.
Purpose of setting the potential only to a very great extent. Both the »Corrodkote« and the »CASS« -

Tests are used in technology as accelerated corrosion tests. ^: A detailed description of these tests can be found in the journal "Plating", Vol. 44, 1957, p. 763.

In some cases neither the »Corrodkote« - The »CASS« test provides a completely reliable prediction of the service life of galvanized parts Motor vehicles. Galvanized panels may pass the Corrodkote test as good and rated poorly in the CASS test, and this can be the case with other galvanized plates The reverse may be the case. Therefore, the test results given below were found showing the advantages to demonstrate the invention, obtained by the test plates three or four times in a row the "Corrodkote" test procedure and then, instead of more often, guided through the "Corrodkote" test cycle have been subjected to the »CASS« test. In all cases control plates were used for comparison also checked. A typical test procedure consists of 3 to 4 »Corrodkotee test cycles followed by

> 48-hour »CASS« test.
The mechanism by which the coating obtained by the process according to the invention protects the substrate against corrosion is described below with reference to the drawing.

F i g. Fig. 1 shows schematically a triple layer with a corrosion hole in its initial stage;

F i g. 2 shows the same triple layer with the corrosion hole in the advanced stage in which the intermediate layer has already been attacked;

F i g. 3 shows the same triple layer, with the corrosion hole widening towards the side has, but the lower class is not significantly affected.

The advantage of using a three-layer composite coating to protect corrodible substrates is shown in FIG. 1 to 3 of the drawing. These figures show the progressive corrosion of a typical three-layer coating on a base metal 6. When a corrosion hole 1 forms in the upper nickel layer 3,. as shown in FIG. 1 (which is usually caused by ) a pore or some other defect in the decorative coating 2 made of chrome), this hole widens progressively in a hemispherical manner. This corrosion point continues to enlarge until it reaches the more anodic intermediate layer 4.

In Fig. 2, the corrosion hole has already attacked the intermediate layer 4, has moved away from its original hemispherical expanded to a substantially cylindrical shape and has the Intermediate layer 4 infiltrated somewhat below the upper nickel layer 3. Since the lower class S is nobler is than the two upper nickel layers 3 and 4, the lower layer 5 remains relatively free of Corrosion.

If the intermediate layer 4 corrodes under the top layer 3, the intermediate layer 4 becomes for the corrosive electrolyte becomes more inaccessible, and the overall polarization increases as a result of local changes in concentration too, which increase the resistance of the electrolyte and other ohmic effects. Therefore, the rate of corrosion decreases the intermediate layer 4, and the top layer 3 begins to corrode again and lays the intermediate layer 4 free, which in turn creates an anodic protective effect on the top layer 3. This interplay repeats itself over and over again, and the lower layer 5 becomes at the expense of the attack the intermediate layer 4 and the upper layer 3 are protected. F i g. 3 shows a galvanic coating, in which the corrosion hole has widened, but the lower nickel layer 5 is only slightly attacked is. The rate of corrosion of the lower layer 5 corresponds to that of the upper layer 3 and the intermediate layer 4 usually in a ratio of less than 1: 100.

A preliminary study of the electrochemical properties of arsenic-containing nickel deposits shows that the relative potentials obtainable with these deposits are generally higher than those obtained with sulfur-containing nickel deposits, usually at least 75 mV higher when measured in acetic acid a pH of 3 adjusted 5 ° / _o solution of sodium chloride. The intermediate layer of the three-layer composite coating according to the invention is therefore always anodic with respect to the upper and lower nickel layers. There are also other advantages.

All conventional nickel baths can be used for the galvanic deposition of the nickel.

As soon as the first or lower nickel layer has reached the desired thickness, the bath becomes a Arsenic compound added and a thin, arsenic-containing nickel layer directly on the surface the first or lower nickel layer deposited.

A third layer of nickel is then deposited immediately by electroplating. Preferably is this layer a glossy layer in order to provide the most favorable surface for the decorative chrome plating form. The chrome layer is advantageously about 0.13 to 5 μm thick. Any of the above nickel layers can be deposited in more than one process stage.

In its preferred form, the composite coating obtained by the process according to the invention possesses a first or lower layer of ductile, sulfur-free nickel. This preferred coating is separated from a typical Watts or Fluoboratbad, which a sulfur-free leveler, such as coumarin and the like. The preferably shiny top layer of nickel is made of a conventional bath using a first class brightener (sulfur-oxygen carrier) and one Second class brightener deposited. If the top layer has a high sulfur content (0.1 to 0.3 percent by weight), the underlayer can also be deposited from a bath, which apart from the first class brightener (the sulfur-oxygen compound) small amounts of a brightener second class contains. The baths disclosed in U.S. Patent 3,090,733 as being suitable for deposition of the upper nickel layers are mentioned for the composite coatings described there, can also used for the deposition of the upper nickel layer in the context of the present invention will.

If cobalt is desired as an alloy former in the nickel layers, the baths in known Way cobalt salts are added. Particularly suitable are, for. B. the cobalt halides, such as cobalt chloride, Cobalt bromide. Cobalt sulfate can also be used.

5 6

Electroplated nickel baths, which are particularly useful in such. B. arsenic trichloride. To this class

Combination with these brighteners are suitable, compounds generally include all compounds

Watts and fluoborate baths with increased nickel gene, which are hydrolyzed to arsenic acid,

salary. Such baths are for use with the esters of arsenous acid, both

generally intended for organic brighteners; they 5 fully and partially substituted esters,

can be operated at higher current densities, can also be used if they are water-

as they are soluble enough for the effective application of the organically or by hydrolysis into arsenic

see brighteners are useful. The preferred acid or other water-soluble compounds

Watts bath has a total nickel content of 70 to go. These esters correspond to the general one

115 g / l and contains 270 to 450 g nickel sulfate per liter, ίο formula
about 20 to 90 g of nickel chloride per liter and about
30 to 40 g boric acid per liter as a buffer, while the
preferred fluoborate bath has a total nickel content

from 75 to 110 g / l and about 440 g of nickel fluorate in which η has the value 0, 1 or 2, R is a hydrogen

as well as about 30 g boric acid per liter. 15 atom, an alkyl, aryl, alkaryl, hydroxyaryl radical

For the purposes of the invention, boric acid is used as the od. The like. And R _{1 is} a hydrogen atom, a monovalent one

Buffers preferred to keep the pH constant to a metal atom or an alkyl, aryl or hydroxy

keep. The baths can, however, also denote acetic acid, an alkyl radical, and where R ₁ , if ti, is 0

Borax (sodium tetraborate), formic acid that has fluo- or 1, also a divalent metal atom, one

borate and other compounds known as buffers 20 alkylene, alkenylene or arylene radical or one

included without adverse effects. substituted divalent organic radical mean

Most anionic surfactants are used as wetting agents. In this case, R _{1 is} a single divalent a

surface-active agents usable. In particular, remainder that the two directly to the arsenic atom can "

Sulphates can be used. bonded oxygen atoms bridged.

Excellent precipitates are also made from 25 examples of particular compounds of the above

Baths are obtained without wetting agents, provided they are only free formula the trialkyl esters of arsenous acid, such as

of water-insoluble and hydrophobic substances. the trimethyl, triethyl, triisopropyl derivatives, the

The production of the galvanic arsenic-nickel triaryl esters of arsenic acid, such as the triphenyl

Baths are advantageously carried out by the mere addition of ester, the mixed alkyl and aryl triesters

about 0.5 to 8.0, preferably 0.5 to about 5 g / l of a 3 ° arsenic acid, such as ethylphenylene arsenite, the organic

Arsenic-supplying compound to one of the above nisch substituted arsenic acids, such as bis- (phenyl) -

the baths specified for nickel deposition. The men-arsenic acid, hydroxyphenyl-phenyl-arsenic acid,

the respective arsenic compound, the required ethylphenyl arsenic acid and the like, and the diesters,

are to add the desired amount of arsenic to the nickel where a hydrocarbon residue is immediate

Deposition vary depending on the type of compound the arsenic atom is bound to, e.g. B. Dibutoxy-

bond and of course with the temperature of the phenylarsine.

Bades and the cathode current density. In general, these organic derivatives have arsenic compounds, which can be used to introduce arsenic in only a limited solubility in water and offer nickel deposition, therefore no advantage over the very water-soluble not very water-soluble, thus reducing the amount in which 40 borrowed arsenic Acid and the metal arsenites.
These compounds can be applied, but good corrosion results are also achieved with entanglement. obtained from the five-

Preferably the working temperature of a valuable arsenic is produced. Allylarsonic acid

typical bath (Watts bath) in the range of about delivers particularly good results, which in many cases \

54 to 66 ° C. However, this area is not critical; 45 are as good as those made with sodium arsenite

good galvanic deposits can also still be obtained at.

Temperatures of only 10 ° C or less or compounds that produce arsenic can be mixed with higher Temperatures of 82 ° C, for example, or sometimes phenylarsonic acid and benzylarsonic acid are also possible at the boiling point of the electrolyte obtained. Also organic arsenic compounds, will. 5 ° that contain a triple bond, such as propargylic

The working current density at the cathode also depends on arsonic acid and butinarsonic acid, can be used

of the working current density for the chosen one.

Arsenic-free bath. When using a Watts bath According to a further embodiment of the invention

works, current densities are in the range from 1.1 to becomes a sulfur-oxygen compound or a

5.4 A 'dm ² preferred. 55 First class brightener in small quantities

From acidic nickel baths, the arsenic-containing baths are added. Preferably that is

Nickel deposits are advantageous at lower pH arsenic in the bath than arsenic ion or in some form

Values generated. The pH value should generally be in the range that supplies ions that contain trivalent arsenic.

from 1.5 to 5 and preferably in the range from 2 to 4. The sulfur-oxygen bond is created

lie. 60 surprisingly depending on the amount of in that

As arsenic compounds are contained arsenic semi-glossy up for the most effective precipitation Formation of the arsenic-containing intermediate layers - the gray deposits, while those without the sulfur Compounds used that oxygenate the bath at similar arsenic concentrations Arsenic ions provide, such as arsenic trioxide, arsenic acid, dark to black deposits are obtained, the alkali arsenite, nickel arsenite, cobalt arsenite and 65 Die to avoid the blackening of the thinned, acid-soluble alkaline earth arsenites, such as calcium separating suitable concentrations of calcium sulphurous arsenite and strontium arsenite. Also, hydrolyzing oxygen carriers are generally the same Bare arsenic compounds can be used with advantage as in arsenic-free baths. Effective concentrations

ϊ 496 823

the sulfur-oxygen compound is about 0.5 to 2 g / l; However, one can also use concentrations outside apply this area.

It has been found that relatively small amounts of sulfur are incorporated into the deposit in a much smaller amount than when using a high-gloss nickel bath in the

Deposition is stored, which contains a brightener of the second class.

In Table A below are particular ones

Bath compositions specified, which can be used for

5 separation of the underlayer in the manufacture of the

three-layer galvanic coatings according to the invention are suitable.

Table A.

Nickel salts, g / l 300 Additive, g / l Coumarin 0.2 Other additives, g / l chloral 0.1 ml / 1 Wetting agents, g / l Boric acid _TT bath NiSO ₄ · 6 H ₂ O 37 ___ Sodium- 0.02 as a buffer
g / l pH
value la NiCl ₂ · _{6H 2} O formaldehyde lauryl 37 3.5 300 Nickel ormiat 4.5 (10%) 0.5 ml / 1 sulfate NiSO ₄ · 6 H ₂ O 37 formaldehyde Saccharin Sodium- 0.02 2a NiCl ₂ · _{6H 2} O (40%) chloral lauryl 37 4.0 300 3-bromocoumarin 0.2 sulfate NiSO ₄ · 6 H ₂ O 45 formaldehyde - 3a NiCl ₂ · _{6H 2} O 30th Butynediol 0.1 0.05 30th 2.5 CoSO ₄ · _{6H 2} O 150 NiSO ₄ · 6 H ₂ O 150 Coumarin 0.1 0.1 ml / 1 4a NiCl ₂ · _{6H 2} O 300 30th 4.0 NiSO ₄ · 6 H ₂ O 30th 1.0 5a NiCl ₂ · _{6H 2} O 300 Coumarin 0.2 0.05 30th 4.0 NiSO ₄ · 6 H ₂ O 30th Sodium- 0.1 6a NiCl ₂ · _{6H 2} O octyl 35 4.0 300 sulfate Ni (BF ₄ ), 30th Sodium- 0.1 7a NiCl ₂ · _{6H 2} O octyl 35 3.5 sulfate

In the following table B special bath compositions are given which are used for the deposition of the top layer can be used in the manufacture of the three-layer galvanic coatings according to the invention can.

Nickel salts, • 6 H ₂ O 6H ₂ O g / l 300 Table B. 4th Gloss agent, - g / l Wetting agents , SU Boric acid
as a buffer
g / l pH
value bath NiSO ₄ 6H ₂ O 37 Organic sulfur
Oxygen compound, g / l Reduced 0.007 Sodium- 0.03 37 3.5 Ib NiCl ₂ -6H ₂ O naphthalene Vixen lauryl -6H ₂ O 6H ₂ O 300 disulfonic acid 1 sulfate NiSO ₄ 6H ₂ O 37 0.5 Butynediol 0.2 Sodium- 0.03 37 4.0 2 B NiCl ₂ Saccharin Coumarin 0.1 lauryl -6H ₂ O 300 Allyl sulfonate 3 sulfate NiSO ₄ 6H ₂ O 37 Allyl pyridinium 0.05 Sodium- 0.03 37 4.0 3b NiCl ₂ Dibenzenesulfone bromide lauryl -6H ₂ O 300 imid 3 sulfate NiSO ₄ 6H ₂ O 37 β, β'-ΎΙήο- 0.003 Sodium- 0.1 37 4.0 4b NiCl ₂ p, p'-oxy-bis- dipropionitrile octyl -6H ₂ O 150 (dibenzenesulfone- 2 sulfate NiSO ₄ 6 H ₂ O 150 amide) 1 ß, ß'-thio- 0.003 Sodium- 0.1 37 3.5 5b NiCl ₂ Saccharin dipropionitrile octyl NiC (NH ₂ ) SO ₄ I ₂ 400 Allyl sulfonate 1 sulfate NiCl ₂ 15th 0.5 Butynediol 0.2 Sodium- 0.03 37 4.0 6b Saccharin Coumarin 0.1 lauryl NiSO ₄ 300 Allyl sulfonate 2 sulfate NiCl ₂ 40 Sodium- 0.03 37 3.5 7b Benzenesulfonamide lauryl sulfate 009548/363

example 1

Using the bath 2a of Table A, a 11.4 μ thick layer of nickel is deposited on steel plates. The coated plates are rinsed with water, whereupon a 3.3 μ thick nickel layer ^{is deposited from the following bath at a current density of 4.3 A / dm 2} , a temperature of 60 ° C and a pH value of 4.0:

Components

amounts

NiSO ₄ -OH ₂ O 300g / l

NiCl ₂ -6H ₂ O. 45 g / l

H ₃ BO ₄ ... Λ .....:: 41 g / l

NaAsO ₂ , lg / 1

Wetting agent 3 ml / 1

The amount of arsenic in the deposit is 2.0 percent by weight based on the amount by weight the intermediate layer. ao

The plate is rinsed again, whereupon a 14 μ thick top layer of shiny nickel from the Bath 4b of Table B is deposited.

After rinsing again, a chrome layer 0.25 to 0.3 μ thick is applied by electroplating.

Examples 2 to 8

Under the conditions and using the baths according to Example 1, a series of steel plates coated with three layers of nickel and one layer of chrome. The thickness of the individual layers is shown in Table I. The amount of arsenic in the intermediate layer is approximately in all cases 2.0 percent by weight based on the weight of the intermediate layer.

Table I.

35

example Lower class Layer thickness, μ
Intermediate layer Upper class 2 11.9 3.3 13.5 3 11.4 3.8 11.4 4th 11.4 3.05 11.7 5 11.4 3.05 11.7 6th 12.7 2.54 13.2

With the same baths and under the same conditions, a different set of steel plates is made coated ,, but with the sodium arsenite in the bath used to create the intermediate layer is replaced by 1.27 g of allylarsonic acid per liter. The thickness of the individual layers is shown in Table II. The amount of arsenic in the intermediate layer is 2 percent by weight, based on that in both cases Weight of the intermediate layer.

Test and exposed to the conditions of the »CASS« test for 48 hours.

The plates according to Examples 1 to 8 remain practically unaffected and show no rust stains.

For comparison purposes, the same bath under the same conditions is used to coat one Another series of steel plates used, the the bath used to produce the intermediate layer contains varying amounts of sodium benzene sulfinate. The thickness of the nickel layers for each plate and the amounts of sodium benzene sulfinate in the Bad result from the following table.

Table ΠΙ

Layer thickness, Under Between μ Upper Amount of xiei-
gniel layer layer layer added sodium 11.4 1.78 9.4 benzenesulfinate, g / l 9 11.9 1.52 11.4 4.0 10 12.7 1.78 12.7 4.0 11th 12.7 2.54 14.5 4.0 12th 12.2 2.3 14.7 4.0 13th 12.7 2.54 14.7 0.5 14th 11.4 1.78 11.7 0.5 15th 11.9 2.54 13.5 0.5. 16 11.7 2.54 11.9 0.5 17th 11.4 2.54 12.2 0.25 18th 11.7 2.3 12.2 0.25 19th 11.4 . 3.05 12.2 0.25 20th 0.25

Table IV shows the results of the corrosion tests carried out on the plates according to Table III.

Table IV

Number of rust spots after the specified number
of »Corrodkote« test cycles *)

⁴⁰ example 1 2 3 4th CASS **) _: 9 13th 31 51 62 62 10 0 2 2 5 18th 11th 0 C-- 21 55 69 ⁴⁵ 12 0 2 6th 14th 20th 13th 0 0 1 22nd 45 14th 0 3 18th 34 58 15th 1 12th 45 53 83 16 0 10 21 40 58 ⁵⁰ 17 2 3 20th 39 72 18th 0 25th 43 39 66 19th 0 17th 27 44 54 20th 3 21 34 36 51

Table II Lower class Layer thickness, μ
Intermediate layer Upper class example 11.9
13.7 3.8
3.05 12.2
11.7 7th
8th

In each case three plates coated according to Examples 1 to 8 are subjected to the "Corrodkote" conditions for 80 hours - *) Each test cycle lasts 20 hours.
**) 48 hours.

Example 21

Using the bath described in Example 1 with an addition of 1 g of saccharine per liter, plates are coated according to Example 1. The concentration of sodium arsenite (NaAsO ₂ ) in the bath is adjusted to 0.5 to 3.0 g / l, the concentration in each subsequent bath being 0.5 g / l higher than in the previous one. The nickel deposits produced from these baths are semi-glossy to metallic gray. The corrosion resistance of all composite coatings is that of

Coatings produced according to Example 1 to 8 are comparable. The semi-glossy and metallic gray deposits are in marked contrast to the black deposits obtained from baths containing no saccharin if the amount of sodium arsenite in the bath exceeds 2.5 g / l.

Examples of multilayer coatings according to the invention are, besides those already described, those the (1) a sulfur-free undercoat and a sulfur-free topcoat, (2) a glossy one Undercoat and a glossy overcoat; and (3) a sulfur-containing undercoat and a sulfur-containing one Have upper class. Multi-layer coatings can therefore be produced according to the invention in which the sub-layer up to about 0.15 percent by weight or 0.15 percent by weight on average Sulfur and the top layer contains up to about 0.4 weight percent sulfur. It can also be larger Achieve sulfur levels in the nickel deposits. However, there is no further benefit from the; Use of larger amounts of sulfur (> 1.0 percent by weight).

Claims (4)

Patent claims: 1. Process for the galvanic deposition of corrosion-resistant, three-layer nickel or Nickel-cobalt alloy coatings on metals, the middle layer of which is anodic to the the other two layers, characterized in that the middle layer is a nickel-arsenic alloy with an arsenic content of 0.025 to 8 percent by weight and a thickness of 0.25 to 5 μ is deposited. 2. The method according to claim 1, characterized in that a middle layer with a Arsenic content of 0.25 to 4 percent by weight is deposited. 3. The method according to claim 1 or 2, characterized in that the nickel-arsenic alloy is deposited from a nickel bath containing 0.5 to 8 g / l of an arsenic compound. 4. The method according to claim 3, characterized in that a 0.5 to 5.0 g / l of an arsenic compound containing nickel bath is used. 1 sheet of drawings