EP2770088B1 - Extremely corrosion-resistant steel parts and method for their production - Google Patents

Description

The present invention relates to a high corrosion resistant steel part provided with a multi-layered coating, which coating comprises an electrodeposited, ductile nickel layer, a zinc nickel layer and a Cr (VI) -free passivation layer. Furthermore, the present invention relates to a method for producing this highly corrosion-resistant steel part.

The galvanic coating of components made of iron or steel with zinc or zinc alloy layers is carried out on a large scale and has a high economic importance. The deposited layers protect the base material against corrosion and thus lead to a high increase in the value of the components. The corrosion protection of iron or steel by zinc and zinc alloys is based on the cathodic protection mechanism. Zinc or zinc alloys are less electrochemical than the base material. The corrosion attack therefore begins at the coating. The base material remains protected against corrosion as long as there is still a closed zinc or zinc alloy layer.

The suitability of corrosion protection systems can be assessed by various short-term tests or by outdoor weathering tests. For zinc or zinc alloy coatings, the salt spray test according to DIN EN ISO 9227 is a standard test. The specimens are exposed in a test chamber to a salt mist of 5% sodium chloride solution. The corrosion protection effect is evaluated by the time until the occurrence of coating corrosion (white rust), if on the zinc or zinc alloy layer still another protective layer was applied, or until the occurrence of base metal corrosion (red rust).

Zinc nickel layers with an alloy content of 12-16 wt.% Nickel show a significantly better corrosion resistance compared to pure zinc coatings. The requirement for the corrosion resistance of a zinc layer for rack goods at 8 μm Zinc coating followed by chromium (VI) -free passivation is, according to DIN 50979, at least 264 hours long in the salt spray test until the beginning of base metal corrosion. For zinc nickel layers with the same layer thickness, the value is 720 hours. This has been specified in the standard DIN 50979 but to limit the testing effort to this value. The resistance of the zinc nickel layers is usually much higher. The use of zinc-nickel-coated components has therefore established itself everywhere, where high demands are placed on the corrosion resistance. The high corrosion resistance of zinc nickel layers is based on the formation of the γ-phase Ni ₅ Zn ₂₁ . The layers have a high hardness, but at the same time only a small deformability. This is disadvantageous in applications in which a component still has to be deformed after the coating. In the bending area, this results in cracking in the layer, which can be associated with a reduced corrosion resistance. It is not possible to change the deformability of the zinc nickel layers, without simultaneously deteriorating the corrosion resistance of the coatings significantly. Some improvement in ductility is achieved by adjusting low nickel levels in the layer (about 10 wt%) and setting moderate layer thicknesses of 6-8 microns. Both measures already lead to a deterioration in corrosion resistance. The best corrosion protection is provided by a zinc nickel layer with a nickel content of 12-16 wt.%.

Corrosion damage, which was observed when using zinc nickel coatings in practice, raised doubts about the general significance of the salt spray test according to DIN EN ISO 9227. Various measures in the construction of automobiles have led to increased stress on the zinc-nickel-plated components. Improved measures for noise reduction lead to higher temperatures in the engine compartment. During breaks in operation, it can then, for seasonal and geographical reasons, come to a strong cooling, combined with condensation of the surfaces. The industry is therefore increasingly developing new test methods in order to be able to better map the actual load of components in real operation by means of short-term tests. A test specification of a German car manufacturer (hereafter referred to as OEM 1) combines the VDA replacement test 621-415 with an upstream climate change test. The entire exam consists of the following individual steps:

Part A of the OEM 1 exam:

1. 1.) 1 h Aufheizphase von 23 °C auf 120 °C (ohne Regelung der relativen Luftfeuchte)
2. 2.) 3 h Haltezeit bei 120 °C (ohne Regelung der relativen Luftfeuchte)
3. 3.) 1 h Abkühlphase von 120 °C auf 80 °C (Regelung der relativen Luftfeuchte so, dass bei 80°C 90% relative Luftfeuchte erreicht werden)
4. 4.) 6 h Haltezeit bei 80 °C und 90 % relative Luftfeuchte
5. 5.) 1 h Abkühlphase von 80 °C und 90°C relative Luftfeuchte auf 23 °C und 90 % relative Luftfeuchte
6. 6.) 3 h Haltezeit bei 23 °C und 90 % relative Luftfeuchte
7. 7.) 1 h Abkühlphase von 23 °C auf - 40 °C (ohne Regelung der relativen Luftfeuchte)
8. 8.) 4 h Haltezeit bei - 40 °C (ohne Regelung der relativen Luftfeuchte)
9. 9.) 1 h Aufheizphase von - 40 °C auf 23 °C (Regelung der relativen Luftfeuchte so, dass bei 23°C 90% relative Luftfeuchte erreicht werden)
10. 10.) 3 h Haltezeit bei 23 °C und 90 % relative Luftfeuchte

Die Prüfschritte 1 bis 10 werden an sieben Tagen nacheinander wiederholt.
Dann erfolgt eine siebentägige Auslagerung der Teile unter Wechseltestbelastung nach VDA 621-415 (siehe Teil B).

1. 1.) 1 h heating phase from 23 ° C to 120 ° C (without regulation of relative humidity)
2. 2.) 3 h hold time at 120 ° C (without regulation of relative humidity)
3. 3.) 1 h cooling phase from 120 ° C to 80 ° C (regulation of the relative humidity so that 80% relative humidity is reached at 80 ° C)
4. 4.) 6 h hold time at 80 ° C and 90% relative humidity
5. 5.) 1 h cooling phase of 80 ° C and 90 ° C relative humidity to 23 ° C and 90% relative humidity
6. 6.) 3 h hold time at 23 ° C and 90% relative humidity
7. 7.) 1 h cooling phase from 23 ° C to - 40 ° C (without control of relative humidity)
8. 8.) 4 h hold time at - 40 ° C (without control of relative humidity)
9. 9.) 1 h heating phase from - 40 ° C to 23 ° C (regulation of the relative humidity so that at 23 ° C 90% relative humidity are reached)
10. 10.) 3 h hold time at 23 ° C and 90% relative humidity

The test steps 1 to 10 are repeated on seven days in succession.
This is followed by a seven-day outsourcing of the parts under alternating load according to VDA 621-415 (see Part B).

Part B of the OEM 1 exam:

• 11.) 24 h Salzsprühtest nach DIN EN ISO 9227 NSS
• 12.) 96 h Kondenswasserwechseltest nach DIN EN ISO 6270 AHT
• 13.) 48 h Lagerung unter Raumklimabedingungen nach DIN 50014

Die Prüfschritte 11 bis 13 werden innerhalb von sieben Tagen durchgeführt.
Ein Prüfzyklus bestehend aus Teil A und Teil B dauert somit 2 Wochen.
Die Gesamtprüfdauer beträgt insgesamt 6 Zyklen (12 Wochen).
Ein weiterer deutscher Automobilhersteller schlägt eine Kombination mehrerer Einzelprüfungen vor (im folgenden OEM 2 genannt). An fünf Tagen erfolgt eine Wechseltestprüfung, gestartet wird mit einem Salzsprühtest nach DIN EN ISO 9227 mit modifizierter Prüflösung. Danach erfolgt eine Feucht-Wärme-Lagerung, Prüfklima CH nach DIN EN ISO 6270-2. Die gesamte Prüfung besteht aus den folgenden Einzelschritten:VDA change test according to VDA 621-415:

• 11.) 24 h salt spray test according to DIN EN ISO 9227 NSS
• 12.) 96 h condensed water change test according to DIN EN ISO 6270 AHT
• 13.) 48 h storage under room climate conditions according to DIN 50014

The test steps 11 to 13 are carried out within seven days.
A test cycle consisting of Part A and Part B thus takes 2 weeks.
The total test duration is 6 cycles in total (12 weeks).
Another German car manufacturer proposes a combination of several individual tests (referred to below as OEM 2). On five days, a change test is carried out, starting with a salt spray test according to DIN EN ISO 9227 with modified test solution. This is followed by a damp heat storage, test climate CH according to DIN EN ISO 6270-2. The entire exam consists of the following individual steps:

Part A of the OEM2 exam:

1. 1.) 4 h Salzsprühtest nach DIN EN ISO 9227 NSS mit modifizierter Prüflösung (40 g/l NaCl + 10 g/l CaCl₂)
2. 2.) 4 h Abkühlphase bei RT (18-28 °C) und 40-60 % relative Luftfeuchte
3. 3.) 16 h Feucht-Wärme-Lagerung, Prüfklima CH nach DIN EN ISO 6270-2

Die Prüfschritte 1 bis 3 werden an fünf Tagen nacheinander wiederholt.
Dann erfolgt eine zweitägige Auslagerung der Teile unter Klimawechseltestbelastung (siehe Teil B).

1. 1.) 4 h salt spray test according to DIN EN ISO 9227 NSS with modified test solution (40 g / l NaCl + 10 g / l CaCl ₂ )
2. 2.) 4 h cooling phase at RT (18-28 ° C) and 40-60% relative humidity
3. 3.) 16 h wet heat storage, test climate CH according to DIN EN ISO 6270-2

Test steps 1 to 3 are repeated on five consecutive days.
This is followed by a two-day outsourcing of the parts under climatic cycle test load (see Part B).

Part B of the OEM 2 exam:

• 4.) 1 h Aufheizphase von 23 °C und 30 % relative Luftfeuchte auf 80° C und 80 % relative Luftfeuchte
• 5.) 4 h Haltezeit bei 80 °C und 80 % relative Luftfeuchte
• 6.) 2 h Abkühlphase von 80 °C und 80 % relative Luftfeuchte auf - 40 °C (Regelung der relativen Luftfeuchte auf 30 % bei T = 0 °C, keine Regelung der relativen Luftfeuchte bei T < 0 °C)
• 7.) 4 h Haltezeit bei - 40 °C (ohne Regelung der relativen Luftfeuchte)
• 8.) 1 h Aufheizphase von - 40 °C auf 23 °C (ab T = 0 °C Regelung der relativen Luftfeuchte auf 30 %)

Die Prüfschritte 4 bis 8 werden an zwei Tagen nacheinander wiederholt.
Ein Prüfzyklus bestehend aus Teil A und Teil B dauert somit 7 Tage.
Die Gesamtprüfdauer beträgt insgesamt 4 Zyklen (4 Wochen).

• 4.) 1 h heating phase of 23 ° C and 30% relative humidity to 80 ° C and 80% relative humidity
• 5.) 4 h hold time at 80 ° C and 80% relative humidity
• 6.) 2 h cooling phase from 80 ° C and 80% relative humidity to - 40 ° C (control of relative humidity to 30% at T = 0 ° C, no control of relative humidity at T <0 ° C)
• 7.) 4 h hold time at - 40 ° C (without control of relative humidity)
• 8.) 1 h heating phase from - 40 ° C to 23 ° C (from T = 0 ° C regulation of the relative humidity to 30%)

Test steps 4 to 8 are repeated two days in succession.
A test cycle consisting of Part A and Part B thus takes 7 days.
The total test duration is 4 cycles in total (4 weeks).

The strictest test requirements include the climate chamber replacement test of the OEM 1. Here, in the case of bent pipes with zinc nickel coating, base material corrosion can already be detected after two test cycles in the bending area. Although it would be technically possible to carry out the forming of the tubes before coating and thus to prevent their damage during bending. This variant is very unfavorable for the coating process. Straight pieces of pipe can be arranged closely on a Galvanogestell. It can be a uniform current density distribution and thus a uniform coating thickness distribution and alloy composition can be achieved on all components. On the other hand, pre-bent tubes have a partly three-dimensional structure for fitting on a galvanic frame. As a result, a uniform current density distribution on the entire product is no longer given. The layer thickness distribution and alloy composition can be adversely affected. In addition, significantly more electrolyte is carried away by the curved tubes. On the one hand, this leads to higher costs due to carry-over losses and, at the same time, the risk of contamination of downstream process baths. The aim is therefore to make the coating of the pipe sections in the unbent state and then to bend. In the modified corrosion tests, however, it was also found that early base material corrosion also occurred on straight areas of the components, ie where there is no damage to the layer. Improving the bendability of the zinc nickel layers, therefore, can not solve the problem of occurrence of corrosion. A change in the alloy composition of about 12-16 wt.% Nickel is also not useful, because the layer would then no longer be completely in the γ-phase.

• eine Zinknickellegierungsschicht mit einem Nickelanteil von 2 - 20 %, die auf der äußeren Oberfläche des Basisstahlrohres aufgebracht ist,
• eine Polyvinylfluoridschicht, die über der Zink-Nickel-Legierungsüberzugsschicht gebildet ist und eine Dicke von 15 - 25 µm aufweist,
• eine als Zwischenlage zwischen der Zink-Nickel-Legierungsüberzugsschicht und der Polyvinylfluoridschicht liegende Epoxidharzschicht, die eine Dicke von 3 - 10 µm aufweist,
• und eine zwischen der Zink-Nickel-Legierungsüberzugsschicht und der äußeren Oberfläche des Basis-Stahlrohres liegende Nickelüberzugsschicht mit einer Dicke von 0,2 - 10 µm.

DE 39 31 046 C2 claims a heat- and corrosion-resistant steel pipe having a multilayer coating of a metal alloy layer, a conductive intermediate layer, an adhesive intermediate layer of a synthetic resin and an outer layer of highly resistant polyvinyl fluoride, characterized by

• a zinc nickel alloy layer having a nickel content of 2 - 20% applied to the outer surface of the base steel pipe,
• a polyvinyl fluoride layer formed over the zinc-nickel alloy coating layer and having a thickness of 15-25 μm,
• an epoxy resin layer having a thickness of 3 - 10 μm as an intermediate layer between the zinc-nickel alloy coating layer and the polyvinyl fluoride layer,
• and a nickel plating layer having a thickness of 0.2 - 10 μm between the zinc-nickel alloy plating layer and the outer surface of the base steel pipe.

The corrosion resistance is tested in a combination of heat load (150 ° C, 24 hours) and then after cooling to room temperature in a salt spray test according to the standard JIS Z 2371 (comparable to DIN EN ISO 9227). A temperature cycling test as described above was not performed. To test the impact resistance, a simulated stone impact test was conducted followed by a salt spray test. The corrosion resistance is based in this multilayer system mainly on the combination of an epoxy resin layer with a thickness of 3 - 10 microns and the polyvinyl fluoride layer with a layer thickness of 15 - 25 microns.

The DE 10 2005 054 944 A1 deals with a method for surface treatment of a valve stem of an electromagnetic valve used in a water-conducting household appliance, which is formed of a magnetizable and alloyed material. In this case, a first layer of nickel is applied to the surface of the valve tappet and, following this, at least one second layer which covers the layer of nickel is applied. This second layer may be formed of a zinc-nickel layer. After coating the valve stem with the zinc-nickel layer, the surface can be chromated and then sealed. This is to provide a valve stem that retains its corrosion resistance over the entire life of a water-conducting household appliance.

The DE 39 34 111 A1 discloses a heat and corrosion resistant steel tube having a multi-layered coating with a protective layer that is resistant to a variety of corrosive agents. According to one embodiment, a nickel plating layer having a thickness of 0.2-10 μm is interposed the outer surface of the steel tube and a zinc-nickel alloy coating layer over which in turn a chromate layer is formed. The nickel coating layer as the lowermost layer can be produced by electrodeposition from a watt bath. The nickel content in the zinc-nickel alloy coating layer is preferably 2-20%.

EP 2 233 611 relates to a layer system for coating a substrate surface. The layer system consists of at least two layers, one layer is a tin-nickel layer, the other a layer of a metal selected from the group consisting of nickel, copper, tin, molybdenum, niobium, cobalt, chromium, vanadium, manganese, titanium and magnesium or an alloy of at least one of these metals. A layer combination of a nickel layer and a tin-nickel layer deposited thereon has a more positive electrochemical potential than steel and can therefore only provide corrosion protection by a pure barrier effect. As soon as there is a defect in the layer, eg a pore or a scratch, ground metal corrosion occurs immediately at this point. That is, the layer combination of EP 2 233 611 can not provide cathodic corrosion protection, which is disadvantageous.

DE 694 04 730 T2 describes a method of electroplating a zinc alloy coating on a steel substrate and coated steel substrate. The deposition is carried out from an additive-free zinc nickel electrolyte containing only zinc chloride and nickel chloride without further additives. To improve the liability of Coating is proposed that first a thin nickel layer is deposited by electroless contact of the steel strip with the zinc-nickel electrolyte. In such an electroless charge-exchange reaction (cementation), amorphous, wipeable layers are deposited which can not improve the adhesion of subsequently deposited layers. In the examples deposits are described, where by applying certain potentials first zinc nickel layers with a higher nickel content (up to 24 wt.%) Are deposited. Subsequently, the potential of depositing the alloy with 12-16 wt.% Nickel is then polarized and the desired layer thickness is deposited. Such a strong dependence of the alloy composition on the current density can be obtained only from an additive-free electrolyte, as it is used in the steel coating in continuous flow systems. There is due to the system a very constant distance between the anode and cathode and thus a very uniform current density distribution on the cathode, whereby a variation of the alloy composition can be achieved by setting a certain potential. In contrast, zinc nickel electrolytes used for coating in conventional electroplating plants must be able to achieve a constant alloy composition, regardless of the local current density on the entire component. This is achieved by using special additives. At the same time, these additives improve the adhesion of the deposited zinc nickel layer. Components which are coated in such zinc-nickel electrolytes can be subsequently deformed without the layer being spalled. Cracking, however, may occur in the bending region due to the low ductility of zinc nickel layers deposited in the γ-phase.

JL Chen, JH Liu, SM Li and M. Yu describe in an article in Materials and Corrosion 2012, 63, no. 7 pp. 607-613 a modular layer system consisting of alternating layers nickel + zinc nickel. This should improve the resistance in the salt spray test. An indication of the corrosion behavior in a combination of temperature cycling test followed by salt spray test is not given. The best result is achieved with a layer combination of six layers (3 layers of nickel, 3 layers of zinc nickel) with a layer thickness of 0.8 μm nickel and 1.2 μm zinc nickel in one layer Total layer thickness of 6 microns specified. For the practical application of a zinc or zinc alloy coating as cathodic corrosion protection zinc or zinc alloy coated components are usually provided with a conversion layer of passivation of trivalent chromium compounds, optionally also with an additional polymer-containing or silicate-containing seal. These post-treatment steps delay the corrosion attack on the zinc or zinc alloy layer. The components are optically unchanged. In a passivation reaction, the zinc or zinc alloy layer is removed on the order of 1-2 μm. Chen's optimally described modular layer of six layers (3 layers of nickel, 3 layers of zinc nickel) with a layer thickness of 0.8 μm nickel and 1.2 μm zinc nickel with a total layer thickness of 6 μm has a zinc nickel layer on the outside surface of 1.2 μm thickness. In a passivation reaction, this zinc nickel layer would thus be completely dissolved. Thus, no passivation layer can be produced on the above-described Chen modular layer.

The deposition of a modular layer system of alternating layers of nickel and zinc nickel also carries the risk of reduced layer adhesion. The resting potential of the nickel layer is about -60 mV, whereas the zinc nickel layer is about -615 mV. Nickel is thus much more noble electrochemically. When the component having a base zinc-nickel surface is brought into contact with the nickel electrolyte to deposit the next nickel layer, nickel is precipitated from the nickel electrolyte in the charge exchange on the zinc-nickel surface. Such electroless deposited layers generally reduce the adhesion of subsequently deposited layers. In practice, therefore, measures are taken to avoid such deposits in the de-energized state.

The object of the present invention is therefore to provide a steel part coated with a layer system which overcomes the above drawbacks so that the steel part has a corrosion resistance which meets the stringent testing requirements of the above-described climatic chamber change tests OEM 1 and OEM 2.

Surprisingly, it has been found that a substantial improvement in the corrosion resistance in the described corrosion alternating climate test can be achieved by a layer combination of a ductile layer of nickel and a zinc nickel layer of the known composition 12-16% by weight nickel deposited thereon. This result was completely surprising in this form. The protective mechanism of a zinc-nickel layer on steel as base material is based on the cathodic corrosion protection. The quiescent potential of a zinc nickel layer in 5% sodium chloride solution is about -615 mV, against normal hydrogen electrode, steel has a value of about -292 mV. The zinc-nickel layer is less noble than steel and protects it cathodically, in which it itself goes anodically into solution. A z. B. deposited from an additional nickel sulphamate nickel layer has a rest potential of -60 mV and is thus much nobler than the base material steel. In a corrosion element of nickel and steel, steel is the less noble component. When corrosion occurs, steel therefore goes into solution. It was therefore surprising that no base material corrosion occurs in a layer combination Fe / Ni / ZnNi in the presence of cracks in the layer, as is the case in the layer combination Fe / Ni.

• eine direkt auf der Oberfläche des Stahlteils galvanisch abgeschiedene, duktile Nickelschicht, die im Test gemäß DIN EN ISO 8401, Abschnitt 3.4, eine Bruchdehnung von 10 - 25% zeigt,
• eine direkt auf der Nickelschicht galvanisch abgeschiedene Zinknickelschicht mit einem Nickelgehalt von 12 - 16 Gew.%,
• eine auf der Zinknickelschicht aufgebrachte Chrom(VI)-freie Passivierungsschicht.

The above object is thus achieved by a steel part provided with a multilayer coating, the coating comprising the following layers:

• a ductile nickel layer which is electrodeposited directly on the surface of the steel part and exhibits an elongation at break of 10 to 25% in the test according to DIN EN ISO 8401, section 3.4,
• a zinc nickel layer electrodeposited directly on the nickel layer, with a nickel content of 12-16% by weight,
• a chromium (VI) -free passivation layer applied to the zinc nickel layer.

The nickel layer preferably has a layer thickness of 2-6 .mu.m, more preferably 3-6 .mu.m. The zinc nickel layer preferably has a layer thickness of 3 to 9 μm, very particularly preferably 6 μm. The layer thicknesses are determined by means of metallographic cross sections (DIN EN ISO 1463).

Chromium (VI) -free passivation layers usually have a layer thickness of about 100 nanometers. These small layer thicknesses can not be evaluated by microscopic methods. A measurement would be z. Example by scanning electron microscopic examination at high magnification or by glow discharge spectroscopy (GDOS) possible. The measurement of the layer thicknesses of passivation layers is not carried out in practice, as it does not allow a direct statement about the corrosion resistance.

The above multi-layer coating can be deposited from commercial nickel electrolytes and zinc nickel electrolytes as well as a Cr (VI) -free passivation bath.

• 50 - 70 g/l Nickel; vorzugsweise 60 g/l Nickel;
• 3 - 6 g/l Chloridionen; vorzugsweise 3 g/l Chloridionen;
• 25 - 40 g/l Borsäure; vorzugsweise 35 g/l Borsäure.

For nickel deposition, it is necessary to use a process which deposits ductile and thus readily deformable layers. This is best suited for a nickel sulfamate bath, ie a bath in which nickel is present in the form of nickel sulfamate. Such a nickel sulfamate bath typically has the following composition:

• 50-70 g / l nickel; preferably 60 g / l nickel;
• 3 - 6 g / l chloride ions; preferably 3 g / l chloride ions;
• 25-40 g / l boric acid; preferably 35 g / l boric acid.

The pH of the nickel sulphamate bath is in the range of 3.5-4.5, preferably pH 4. The nickel deposition from the nickel sulphamate bath is preferably carried out at a temperature of 50-60.degree. C., more preferably at 60.degree.

A nickel sulfamate bath of the type described above can be used to deposit ductile nickel layers which, in the test according to DIN EN ISO 8401, achieve elongation at break values in the range from 10 to 25%, preferably from 20 to 25%. The elongation at break is measured according to Section 3.4 (Hydraulic buckling test) of DIN EN ISO 8401. In this case, a corresponding nickel layer under the deposition conditions of the nickel sulphamate bath, as used for the production of the steel part according to the invention, in a thickness of 25-40 microns on a passivated, polished stainless steel plate and then removed as a foil, and the elongation at break of this film is determined (Appendix C of DIN EN ISO 8401). As a measuring device is z. B. "Ductsiomat" the company Atotech Germany GmbH.

Decisive for maintaining the high elongation at break of the deposited nickel layers are the purity of the chemicals used for the approach of the nickel sulfamate bath (nickel sulfamate, boric acid and nickel chloride) as well as measures for continuous purification. In order to reliably maintain the upper elongation at break values, it is recommended that the nickel sulphamate bath be continuously cleaned, eg by filtration over activated charcoal candles and electrolysis at low current densities. Organic contaminants can be removed by continuous filtration of the bath over filter cartridges filled with activated charcoal granules. Metallic impurities that can be deposited in the layer can be removed by a so-called selective cleaning. For this purpose, in shunt on a connected as a cathode sheet at 0.5 A / dm ^{2 is} deposited. Copper is preferably deposited at this low current density. With these measures, it is possible to obtain the upper elongation at break values, in particular also in the preferred range. Hydrolysis of the sulfamate anion upon operation of the nickel sulfamate bath produces ammonium and sulfate anions. High ammonium concentrations worsen the elongation at break values of the deposited nickel layer. Therefore, it may be necessary to dispose of the bath and re-start when high concentrations of ammonium.

• 6 - 9 g/l Zink; vorzugsweise 9 g/l Zink;
• 0,5 - 1,5 g/l Nickel; vorzugsweise 1 g/l Nickel;
• 100 - 130 g/l NaOH; vorzugsweise 120 g/l NaOH;
• 8 - 12 g/l Diethylentriamin; vorzugsweise 10 g/l Diethylentriamin;
• 20 - 40 g/l Tetrakishydroxypropylethylendiamin, vorzugsweise 30 g/l.

For the deposition of a zinc nickel layer having a nickel content of 12-16% by weight, a bath having the following composition is typically used:

• 6 - 9 g / l zinc; preferably 9 g / l zinc;
• 0.5-1.5 g / l nickel; preferably 1 g / l nickel;
• 100-130 g / l NaOH; preferably 120 g / l NaOH;
• 8-12 g / l diethylenetriamine; preferably 10 g / l diethylenetriamine;
• 20-40 g / l of tetrakishydroxypropylethylenediamine, preferably 30 g / l.

The deposition of the zinc nickel layer is preferably carried out at a bath temperature of 33 - 37 ° C. Particularly preferred is a bath temperature of 35 ° C is set.

The nickel content of the deposited zinc nickel layer is determined by X-ray fluorescence analysis according to DIN EN ISO 3497. The measurement is carried out here on a corresponding zinc nickel layer, which was deposited directly on steel. If the analysis of the composition of the zinc nickel layer on the previously deposited nickel layer is carried out, the X-ray fluorescence measurement would not only excite the nickel in the zinc nickel layer, but also that from the pure nickel layer. The X-ray fluorescence analysis is therefore carried out on a zinc-nickel layer, which was deposited directly on steel in the deposition conditions of the zinc nickel bath, as used for the production of the steel part to be coated.

For the deposition of the passivation layer, a chromium (IV) -free passivation bath comprising trivalent chromium compounds, cobalt sulfate and sodium nitrate is usually used. This contains z. 1.5 g / l Cr (III), 1.5 g / l Co (II) and 5 g / l sodium nitrate. Such a bath is commercially available with SLOTOPAS ZNT 80 available. In the passivation reaction, the zinc nickel layer is easily dissolved by the oxidant NaNO ₃ . In this reaction, at the zinc nickel layer / electrolyte interface, the pH value of 3.4 to 3.6 is shifted to the neutral range. This results in a precipitation of a mixture of the oxides or hydroxides of zinc, nickel (from the alloy layer) and chromium and cobalt (from the passivation solution). These mixed oxides or hydroxides together form the passivation layer.

The steel part according to the present invention is preferably a steel pipe. This steel pipe can be deformed after the application of the multi-layered coating without significantly deteriorating the corrosion resistance.

As deformations such as bending, flanging, crimping and squeezing come into question.

Examples

Steel tubes, outer diameter 8 mm, inner diameter 6 mm, length 500 mm were used for the experiments.

pretreatment:

• Abkochentfettung: SLOTOCLEAN AK 160
  (30 g/l SLOTOCLEAN AK 161 + 10 ml/l Entfetterzusatz SLOTOCLEAN RV 111), 65°C, 12 min.
• Spülen
• Beizen: 500 ml/l Salzsäure, 32 Gew.%
  (40 ml/l Beizentfetterzusatz BEF 30),
  21°C, 6 min.
• Spülen
• Anodische Entfettung: SLOTOCLEAN EL DCG
  (120 g/l Entfettersalz SLOTOCLEAN EL DCG),
  35°C, 6 A/dm², 2 min.
• Spülen
• Dekapieren: 50 ml/l Salzsäure 32 Gew.%,
  21°C, 60 s
• Spülen

A practical pretreatment procedure was used, consisting of:

• Boiling degreasing: SLOTOCLEAN AK 160
  (30 g / l SLOTOCLEAN AK 161 + 10 ml / l degreaser additive SLOTOCLEAN RV 111), 65 ° C, 12 min.
• do the washing up
• Pickling: 500 ml / l hydrochloric acid, 32% by weight
  (40 ml / l pickle degreaser BEF 30),
  21 ° C, 6 min.
• do the washing up
• Anodic degreasing: SLOTOCLEAN EL DCG
  (120 g / l defatting salt SLOTOCLEAN EL DCG),
  35 ° C, 6 A / dm ² , 2 min.
• do the washing up
• Pickling: 50 ml / l hydrochloric acid 32% by weight,
  21 ° C, 60 s
• do the washing up

Rinsing is done by immersing in deionized water at room temperature. By multiple immersion and removal (lifting and lowering) of the steel pipes in the Spülwannen forced flooding of the pipe interior was achieved. The rinsing time was about 30 seconds per rinse step.

Nickel deposit:

For nickel deposition, the nickel sulfamate bath MS (method of Dr.-Ing. Max Schlötter) was used. Here, nickel sulfamate and nickel chloride according to the quality requirements of DIN 50970 and boric acid according to the quality requirements of DIN 50973 were used.

The concentration and working conditions were:

• 420 g / l (= 280 ml / l) nickel sulfamate solution 60% (equivalent to 45 g / l nickel)
• 20 g / l nickel chloride hexahydrate
• 35 g / l boric acid
• 0.5 g / l sodium lauryl sulfate
• pH 4.0
• Temperature: 55 ° C
• Current density: 3 A / dm ²
• Separation period: 9 minutes
• Achieved layer thickness: 3 μm

Approach of the electrolyte:

The mixture consisting of nickel sulfamate solution, nickel chloride, boric acid and water was mixed with 1 ml / l of hydrogen peroxide 30 wt.% And with 6 g / l of activated carbon granules, stirred for 1 hour and then filtered. Thereafter, 0.5 g / l of sodium lauryl sulfate was added and worked through at a current density of 0.5 A / dm ² with a charge amount of 2.5 Ah / l. This measure was carried out to remove organic impurities (hydrogen peroxide and activated carbon treatment) and metallic impurities, in particular copper (working through). This ensured that optimally ductile nickel layers could be deposited.

Zinc nickel deposit:

• 6,5 g/l Zink
• 0,5 g/l Nickel
• 120 g/l NaOH
• 10 g/l Diethylentriamin
• 8 g/l Triethanolamin
• 30 g/l Tetrakishydroxypropylethylendiamin
• Temperatur: 35°C
• Stromdichte: 2,5 A/dm²
• Abscheidedauer: 25 Minuten
• Erzielte Schichtdicke: 6 µm

For the zinc nickel deposition, a zinc nickel bath SLOTOLOY ZN 80 (process of Dr.-Ing. Max Schlötter) was used with the following values:

• 6.5 g / l zinc
• 0.5 g / l nickel
• 120 g / l NaOH
• 10 g / l diethylenetriamine
• 8 g / l triethanolamine
• 30 g / l tetrakishydroxypropylethylenediamine
• Temperature: 35 ° C
• Current density: 2.5 A / dm ²
• Separation period: 25 minutes
• Achieved layer thickness: 6 μm

passivation:

• Ansatz: 70 ml/l
• Temperatur: 40°C
• pH: 3,5
• Tauchzeit: 45 Sekunden

After the zinc nickel coating, passivation was carried out in the transparent passivation SLOTOPAS ZNT 80 (method of the Dr.-Ing. Max Schlötter).

• Batch: 70 ml / l
• Temperature: 40 ° C
• pH: 3.5
• Dive time: 45 seconds

The deposition of the layer combination was carried out on a welded precision steel tube according to DIN 10305-2, as used for the production of e.g. Transmission oil lines are used in the automotive industry.

• Vorbehandlung (Parameter siehe oben)
• Spülen
• Nickelabscheidung im Nickelsulfamatbad (Parameter siehe oben)
• Spülen
• Zinknickelabscheidung (Parameter siehe oben)
• Spülen
• Passivieren (Parameter siehe oben)
• Spülen
• Trocknen bei 80°C, 20 Minuten

The deposition involved the following steps:

• Pretreatment (parameters see above)
• do the washing up
• Nickel deposition in the nickel sulfamate bath (parameters see above)
• do the washing up
• Zinc nickel deposition (parameters see above)
• do the washing up
• Passivate (parameters see above)
• do the washing up
• Dry at 80 ° C, 20 minutes

The individual rinsing steps were carried out under the same conditions of rinsing as in the pretreatment described above.

The coating of the comparative examples was carried out by depositing a zinc nickel layer from the zinc nickel bath SLOTOLOY ZN 80 and transparent passivation with SLOTOPAS ZNT 80 as indicated above, but without deposition of a nickel layer.

In order to simulate the damage of the coating during mechanical deformation, bends of 110 ° were mounted with bending tools from Jutec (Hand Bender KBVS) with the bending tool 1230 from both pipe ends such that the apex of the bend is approx. 120 mm from the pipe end was removed.

Corrosion tests:

4 bent tubes each were subjected to a corrosion test according to the above-mentioned test methods of OEM 1 and OEM 2, respectively. Part A of the test OEM1 and Part B of the test OEM2 were carried out in a climate chamber, manufactured by ESPEC, type ARS-0680 from Thermotec. Furthermore, Part B of the test OEM1 and Part A of the test OEM2 was carried out in a test chamber for corrosion change test, Köhler Automobiltechnik, type HKT 1000 WTG. The corrosion attack was evaluated by estimating the area fraction of the pipes in the bending area covered by red rust.

The assessment of the corrosion attack was carried out after each cycle. The damage pictures were photographed for later evaluation. The test specimen was returned to the test for further testing.

Evaluation: Testing according to the test specification OEM 1:

Comparative example, tubes with 9 μm zinc nickel layer with 14% by weight Ni, without nickel layer 1 cycle 2 cycles 3 cycles 4 cycles 5 cycles No red rust 15-30% red rust 50-70% red rust 90-95% red rust 95-100% red rust

Example according to the invention, tubes with 3 μm nickel (elongation at break: 20%, measured in the test according to DIN EN ISO 8401, section 3.4.) + 6 μm zinc nickel (14% by weight nickel) 1 cycle 2 cycles 3 cycles 4 cycles 5 cycles No red rust No red rust 5-10% red rust 30-40% red rust 50-70% red rust

Testing according to the test specification OEM 2:

Comparative example, tubes with 9 μm zinc nickel layer with 14% by weight Ni, without nickel layer 2 cycles 4 cycles 6 cycles 8 cycles No red rust 1-2% red rust 5-10% red rust 50-70% red rust

Example according to the invention, tubes with 3 μm nickel (elongation at break: 20%, measured in the test according to DIN EN ISO 8401, section 3.4.) + 6 μm zinc nickel (14% by weight nickel) 2 cycles 4 cycles 6 cycles 8 cycles No red rust No red rust No red rust <1% red rust

The above tests indicate that the provision of a ductile nickel layer is essential for achieving effective corrosion protection for the steel part.

Claims (5)

1. Steel part having a multi-layered coating, wherein the coating comprises the following layers:

   - a ductile nickel layer deposited galvanically directly on the surface of the steel part and which in the test according to DIN EN ISO 8401, Section 3.4, exhibits an extension at break of 10 - 25 %,

   - a zinc-nickel layer deposited galvanically directly on the nickel layer and having a nickel content of 12-16 wt.%,

   - a chromium(VI)-free passivation layer applied to the zinc-nickel layer.
2. Steel part according to claim 1, wherein the extension at break of the ductile nickel layer is 20 - 25 %.
3. Steel part according to claim 1 or 2, wherein the nickel layer has a layer thickness of 2 - 6 µm and the zinc-nickel layer has a layer thickness of 3 - 9 µm.
4. Steel part according to claims 1 to 3, wherein the steel part is a steel pipe.
5. Method for producing a coated steel part comprising the steps:

   a) galvanic deposition of a ductile nickel layer which in the test according to DIN EN ISO 8401, Section 3.4, exhibits an extension at break of 10 - 25 %,

   b) galvanic deposition of a zinc-nickel layer on the nickel layer from step a), wherein the zinc-nickel layer has a nickel content of 12-16 wt.%, and

   c) application of a Cr(VI)-free passivation layer onto the zinc-nickel layer from step b).