[0001] The present invention relates to a process for the preparation of corrosion resistant
zinc and zinc nickel deposits on linear or complex shaped parts, usually made of steel.
The deposition according to the present invention consists of a two step deposition,
wherein a layer of zinc from an alkaline zinc bath and a layer of zinc nickel alloy
deposited from an acid bath is deposited.
[0002] Such coated substrates possess a similar corrosion protection and similar thickness
distribution as zinc nickel coated parts deposited from an alkaline zinc nickel electrolyte
with the advantage of not using high concentrations of amines or ammonia as strong
complexing agents and not forming cyanide while at the same time providing reasonable
post-forming capability.
[0003] A lot of components in a diversity of applications are plated for protection from
corrosion. In order to provide the required corrosion protection, zinc or zinc alloy
coatings are plated onto these components to the desired thickness. Often a passivating
conversion coating is applied on top of the zinc or zinc alloy coating, typically
by immersion of the substrate into an acidic solution containing trivalent chromium
salts. For even further improvement, such passivated parts are often immersed in sealer
solutions and dried.
[0004] Substrates to be plated either have a linear shape (like window frames) or complex
shapes (like lock housings). These kind of parts are preferably plated from an alkaline
electrolyte as the current efficiency in the high current density area (on the end
of linear parts or at the edges of complex shaped parts) is reduced by using special
polymeric inhibitors. Due to this fact the deposited coating exhibits a very homogeneous
thickness distribution over the entire component surface and also provides excellent
corrosion protection.
[0005] As corrosion is very aggressive when the component is used in a humid environment
the corrosion protection requirement for window frames and automotive components is
steadily increasing. For this reason zinc-nickel coatings become more and more interesting
to the industry and alkaline electrolytes were developed. Such bath usually contain
zinc ions, nickel ions and poly(alkyleneimines) obtained from ethyleneimine, 1,2-propyleneimine,
1,2-butyleneimine and 1,1-dimethylethyleneimine. The poly(alkyleneimines) may have
molecular weights of from about 100 to about 100,000.
[0006] US 6,652,728 discloses a zinc and zinc alloy bath which contains zinc ions, hydroxide ions, optionally
nickel ions and a polymer having the following formula:
Wherein m has a value of 2 or 3, n has a value of at least 2; R
1, R
2, R
3, which may be the same or different, each independently denote methyl, ethyl or hydroxyethyl;
p has a value in the range from 3 to 12; and X
- denotes chloride, bromide and/or iodide.
[0007] Such bath is suitable for depositing alkaline zinc deposits.
[0008] The best corrosion protection is realized when the zinc-nickel coating contains a
gamma-phase zinc-nickel alloy. This is realized with a nickel content of an average
of 15 %. However, this coating is significantly less ductile than pure zinc coatings.
[0009] There is a disadvantage of using alkaline zinc-nickel electrolytes, because all such
electrolytes used in the plating industry today contain require high amounts of strong
amine complexing agents for success plating. In the plating industry a lot of rinse
water has to be waste water treated and with complexing agents in the water it is
very difficult to realize the limit values for metals. For this reason high investments
have to be made by the plating industry to treat the waste water.
EP 1 369 505 A2 teaches to evaporate a part of the process bath until phase separation occurs and
separate the organic from the aqueous phase. However, this procedure is both energy
and time consuming. Another disadvantage when using alkaline baths is the formation
of cyanide by an anodic oxidation. This can only be avoided by other important investments
like membranes to prevent anodic oxidation of amine complexing agents to cyanide as
for example described in
WO 00/06807 and
WO 01/96631).
[0010] GB 2 294 949 A relates to anti-corrosive multilayer-plated steel materials, e.g. plates, pipes,
joints, clamps, bolts and nuts made of two zinc-nickel alloy layers, wherein the first
zinc-nickel layer is deposited from an acidic plating bath and the second zinc-nickel
layer is deposited from an alkaline plating bath. The disadvantage of using an alkaline
zinc-nickel bath as described above cannot be avoided by using such a layer sequence.
[0011] DE 198 37 431 A1 A describes a coating process for hardened steel or cast iron which comprises electrodeposition
of a zinc-cobalt or zinc-nickel alloy layer from an acidic electrolyte and then a
zinc-nickel alloy layer from an alkaline electrolyte. It is disadvantageous to apply
an alkaline zinc-nickel alloy plating bath.
[0012] DE 31 29 129 A1 relates to a method for the multi-stage electrodeposition of very bright, strongly
adhering zinc coatings employing alkaline cyanide-free baths for the top layer, characterised
in that a zinc, nickel, tin, copper and/or cadmium layer is first deposited, followed
by deposition on this layer of a zinc layer.
[0013] It is the object underlying the present invention to provide a plating process for
coating of linear and complex shaped parts, which satisfies the stringent requirements
for corrosion protection and post-forming capability and at the same time does not
apply high concentrations of amines or ammonia as complexing agent in the electrolytes
used. Furthermore, it is the aim to obtain a deposit with homogeneous surface distribution
also for workpieces having a complex shape.
[0014] This object is achieved by a process for the preparation of corrosion resistant two
layer system of zinc and zinc-nickel onto substrates comprising the steps of:
- (i) plating of a zinc coating from an alkaline electrolyte providing very homogeneous
thickness distribution and resistance against corrosion of the coating with good ductility,
- (ii) plating a layer of a zinc nickel alloy from an acidic zinc nickel plating bath
having a high corrosion protection, particularly against red rust,
- (iii) optionally, providing a Cr(VI)-free passivating conversion coating for the zinc
nickel surface.
- (iv) optionally, applying a sealer.
[0015] Thus, according to the present invention a process is provided wherein the homogeneous
coating thickness and ductility is provided by a pure zinc layer also providing corrosion
protection and the very high corrosion protection particularly regarding red rust
is provided by a subsequent zinc-nickel, which is deposited from an acid electrolyte
and which does not contain high concentrations of amines or ammonia.
[0016] The pure zinc layer is deposited on the metal substrate and the zinc nickel layer
is deposited on the zinc layer:
[0017] Such process for the preparation of corrosion resistant substrate comprises the steps
of
- (i) providing a metal substrate
- (ii) plating a layer of zinc from an alkaline electrolyte thereon,
- (iii) plating a layer of zinc-nickel from an acid electrolyte.
[0018] The thickness of the zinc layer typically is between 1 - 12 µm, preferably between
2 - 8 µm, and more preferably 4-6 µm.
[0019] The thickness of the zinc nickel layer typically is between 1 - 12 µm, preferably
between 2 - 8 µm, and more preferably 4-6 µm.
[0020] The alkaline zinc electrolyte preferably has the following composition:
5 - 20 g/l Zn ions from a souble zinc source, e.g. ZnO
80 - 250 g/l hydroxide like NaOH
0.1 - 5 g/l Polymer (as described in Example 2.1 of US 6,652,728).
[0021] Brightening additives are used depending on the demand of the optical aspect. Such
brightening agents are well known in the art and for example described in
US 6,652,728.
[0022] A suitable process for preparing such an alkaline zinc electrolyte is described,
for example in European patent application
EP 1 114 206.
[0023] Acid zinc nickel electrolytes are known in the art and for example described in
US 4,699,696. Such acid zinc-nickel electrolytes solutions contain zinc ions and nickel Ions,
and an additive agent of a class selected from the group consisting of (a) aromatic
sulfonic acids, (b) aromatic sulfonamides, sulfonimides and mixed carboxamides/sulfonamides,
(c) acetylene alcohols.
[0024] Solutions for passivating zinc and zinc alloy surfaces are known and for example
described in
EP 1 484 432. Such solutions typically contain a water soluble Cr(III)-salt in a concentration
of 0.5-80 g/l, preferably 1-40 g/l, more preferably 1-10 g/l. The pH of such solution
is 0.8 - 4.0 and the temperature 10-80°C. Furthermore, the passivate solution can
contain additional metals such as cobalt, nickel, zinc, iron, zircon, titanium, aluminium,
silver, copper, pigments. Optional additional components contain silicates, nitrates,
phosphates, fluorides and polymer resins. For treatment the zinc plated surface is
immersed in the passivation solution, rinsed and thereafter optionally treated at
elevated temperatures of 100 to 250°C for 10 to 300 minutes.
[0025] According to the following examples coatings are obtained by electroplating in zinc
and an acid zinc-nickel electrolytes. Thickness distribution was tested according
to the following test procedure:
Comparative Example 1 - Deposition of a pure zinc layer from an alkaline zinc plating
bath
[0026] 250 ml of an alkaline zinc electrolyte, which contains zinc ions in an amount of
10 g/l, 120 g/l NaOH, 0.5 g/l polymeric inhibitor according to Example 2.1 of
US 6,652,728 and a brightening additive, the composition of which is described in
US 6,652,728, is filled into a Hull cell.
[0027] Zinc is used as anode material. The cathode steel panel will be deposited for 15
minutes at 1 A. The temperature is 28°C. The panel is rinsed and dried. The thickness
distribution is measured at 2 positions on the panel: 3 cm of the lower edge and 2.5
cm of the right and left edge of the panel at high (app. 2.8 A/dm
2) and low current density (app 0.5 A/dm
2). The thickness of the coating is measured four times at the two positions to avoid
measuring mistakes. The thickness distribution is the ratio of the measured layer
thicknesses between the two positions (thickness distribution = thickness at high
current density : thickness at low current density).
Comparative Example 2 - Deposition of a zinc nickel layer from a commercially available
alkaline zinc nickel plating bath
[0028] The same test was performed with a zinc nickel layer obtained from an alkaline zinc
nickel electrolyte Reflectalloy® ZNA (commercially available from Atotech Deutschland
GmbH and described in
US 5,417,840) with 1 A for 15 minutes plating at a temperature of 28°C in the Hull cell.
Comparative Example 3 - Deposition of a zinc nickel layer from a commercially available
acidic zinc nickel plating bath
[0029] The same test was performed with a zinc nickel layer obtained from an acid zinc nickel
electrolyte The composition of the bath acidic zinc nickel plating bath was as follows:
ZnCl2 |
0.3 M/l |
NiCl2 |
0.5 M/l |
KCI |
2.7 M/l |
H3BO3 |
0.3 M/l |
aromatic sulfonamide |
1.1 g/l |
acetylenic alcohol |
0.02 g/l |
sodium acetate |
0.7 M/l |
alkylene oxide polymer |
1.1 g/l |
sulfosuccinate-alkyl diester |
1.1 g/l |
[0030] Plating is with 1 A for 15 minutes plating at a temperature of 35°C in the Hull cell.
Example according to the present invention:
[0031] The same test was performed with a surface containing two layers, a first layer of
pure zinc plated from the alkaline zinc plating bath described in Comparative Example
1 (time: 7.5 minutes, 1 A, temperature: 28°C). Thereafter a second layer of zinc nickel
from the acidic zinc nickel plating bath as described in Comparative Example 3 is
plated (time: 7.5 minutes, 1A, temperature: 35°C).
[0032] The coating thicknesses and the thickness distribution of all coatings is shown in
Table 1.
Table 1: Thickness distribution for different plated layers
|
hcd |
lcd |
hcd : lcd |
Comp. Example 1 |
4.1 micron |
2.8 micron |
1.5 |
Comp. Example 2 |
5.5 micron |
1.8 micron |
3 |
Comp. Example 3 |
8.1 micron |
2.2 micron |
3.7 |
Example 4 (invention) |
6.1 micron |
2.6 micron |
2.4 |
[0033] The plated thickness for the same plating time and current clearly shows that higher
thicknesses and better thickness distributions - represented by the high current density
(hcd) to low current density (lcd) thickness ration - can be achieved with the two
layer sequence of the present invention according to Example 4 compared to the alkaline
zinc nickel process Reflectalloy ZNA (Comparative Example 2). 20% higher thickness
is obtained in the low current density regime.
[0034] Ductility of deposits according to the Comparative Example 2 and Example 4 was evaluated
by hardness measurements.
Comparative Example 2: |
600 Vicker's Hardness at 50 mN |
Example 4: |
525 Vicker's Hardness at 50 mN |
[0035] Hardness is lower for deposits from acidic zinc nickel baths (Example 4) compared
to zinc nickel deposits from alkaline baths (Comparative Example 2). Consequently
the inelastic deformation work is higher for the acidic alloy process of the process
according to the present invention and such deposit has superior post-forming capability.
[0036] The grain size in the zinc nickel layer also influences the ductility of the coating.
With increasing grain size the ductility of the coating is increasing. A good compromise
between grain size and current density is observed at a value of 2 A/dm
2.
[0037] Corrosion protection of the coatings obtained according to Example 4 was evaluated
in DIN 50021 neutral salt spray exposure and found >240 h to first appearance of white
corrosion and >1000 h to substrate corrosion.
[0038] The formation of gamma-phase ZnNi from the acid zinc nickel obtained by Example 4
was confirmed by XRD. Such phase exhibits very good corrosion resistance. The deposit
shows a homogeneous alloy composition over a wide range of current density. The gamma
phase zinc-nickel layer is consistently obtained even on complex shaped parts, as
confirmed by X-ray diffraction.
1. A process for the preparation of a corrosion resistant substrate comprising the steps
of
(i) providing a metal substrate
(ii) plating a layer of zinc from an alkaline electrolyte thereon,
(iii) thereafter, plating a layer of zinc-nickel from an acid electrolyte.
2. The process according to claim 1 wherein the metal substrate is a steel substrate.
3. The process according to any of the foregoing claims wherein the zinc coating is deposited
from an alkaline zinc electrolyte containing a polymeric inhibitor in a concentration
of 0.2 - 2 g/l.
4. The process according to any of the foregoing claims wherein the plating is operated
for 5-50 minutes at 0.5 - 5 A/dm2 and a temperature of 18-35°C.
5. The process according to any of the foregoing claims wherein the zinc layer has a
thickness of 1-12 µm, preferably 2-8 µm, more preferably 4-6 µm.
6. The process according to any of the foregoing claims wherein the acid zinc-nickel
electrolyte comprises zinc in an amount of 10 - 60 g/l and nickel in an amount of
10 - 50 g/l.
7. The process according to any of the foregoing claims wherein the layer of zinc-nickel
is additional treated with a Cr(VI)-free passivating conversion coating.
8. The process according to claim 7 wherein the passivate comprises 1 to 5 g/l Cr(III)
ions.
9. The process according to any of the foregoing claims wherein the substrate is finally
treated with a sealer.
10. The process according to claim 9 wherein the sealer comprises oxygenated silicon compounds,
organo-silicon compounds, polymer dispersions made from acrylic resins, polyurethane
resins, or polyethylene resins or mixtures thereof.
1. Verfahren zur Herstellung eines korrosionsbeständigen Substrats, bei dem man
(i) ein Metallsubstrat bereitstellt,
(ii) darauf eine Schicht aus Zink aus einem alkalischen Elektrolyten abscheidet,
(iii) danach eine Schicht aus Zink-Nickel aus einem sauren Elektrolyten abscheidet.
2. Verfahren nach Anspruch 1, bei dem es sich bei dem Metallsubstrat um ein Stahlsubstrat
handelt.
3. Verfahren nach einem der vorhergehenden Ansprüche, bei dem man den Zinküberzug aus
einem alkalischen Zinkelektrolyten, der einen polymeren Inhibitor in einer Konzentration
von 0,2-2 g/l enthält, durchführt.
4. Verfahren nach einem der vorhergehenden Ansprüche, bei dem man die Abscheidung über
einen Zeitraum von 5-50 Minuten bei 0,5-5 A/dm2 und einer Temperatur von 18-35°C durchführt.
5. Verfahren nach einem der vorhergehenden Ansprüche, bei dem die Zinkschicht eine Dicke
von 1-12 µm, vorzugsweise 2-8 µm und weiter bevorzugt 4-6 µm aufweist.
6. Verfahren nach einem der vorhergehenden Ansprüche, bei dem der saure Zink-Nickel-Elektrolyt
Zink in einer Menge von 10-60 g/l und Nickel in einer Menge von 10-50 g/l enthält.
7. Verfahren nach einem der vorhergehenden Ansprüche, bei dem man die Schicht aus Zink-Nickel
zusätzlich mit einem Cr(VI)-freien passivierenden Konversionsüberzug behandelt.
8. Verfahren nach Anspruch 7, wobei das Passivat 1 bis 5 g/l Cr (III) -Ionen enthält.
9. Verfahren nach einem der vorhergehenden Ansprüche, bei dem man das Substrat abschließend
mit einem Versiegelungsmittel behandelt.
10. Verfahren nach Anspruch 9, bei dem das Versiegelungsmittel sauerstoffhaltige Siliciumverbindungen,
Organosiliciumverbindungen, aus Acrylharzen, Polyurethanharzen oder Polyethylenharzen
hergestellte Polymerdispersionen oder Mischungen davon enthält.
1. Procédé de préparation d'un substrat résistant à la corrosion comprenant les étapes
consistant à
(i) se procurer un substrat métallique ;
(ii) plaquer par-dessus une couche de zinc à partir d'un électrolyte alcalin ;
(iii) plaquer ensuite une couche de zinc-nickel à partir d'un électrolyte acide.
2. Procédé selon la revendication 1 dans lequel le substrat métallique est un substrat
d'acier.
3. Procédé selon l'une quelconque des revendications précédentes dans lequel le revêtement
de zinc est déposé à partir d'un électrolyte de zinc alcalin contenant un inhibiteur
polymère à une concentration de 0,2-2 g/l.
4. Procédé selon l'une quelconque des revendications précédentes dans lequel le placage
est réalisé pendant 5-50 minutes à 0,5-5 A/dm2 et une température de 18-35 °C.
5. Procédé selon l'une quelconque des revendications précédentes dans lequel la couche
de zinc a une épaisseur de 1-12 µm, de préférence 2-8 µm, mieux 4-6 µm.
6. Procédé selon l'une quelconque des revendications précédentes dans lequel l'électrolyte
de zinc-nickel acide comprend du zinc dans une quantité de 10-60 g/l et du nickel
dans une quantité de 10-50 g/l.
7. Procédé selon l'une quelconque des revendications précédentes dans lequel la couche
de zinc-nickel est de plus traitée avec un revêtement de conversion passivant dépourvu
de Cr(VI).
8. Procédé selon la revendication 7 dans lequel le passivant comprend 1 à 5 g/l d'ions
Cr(III).
9. Procédé selon l'une quelconque des revendications précédentes dans lequel le substrat
est finalement traité avec un produit d'étanchéité.
10. Procédé selon la revendication 9 dans lequel le produit d'étanchéité comprend des
composés de silicium oxygénés, des composés organosiliciés, des dispersions polymères
constituées de résines acryliques, de résines de polyuréthane ou de résines de polyéthylène,
ou des mélanges de ceux-ci.