Process for the preparation of corrosion resistant zinc and zinc-nickel plated linear or complex shaped parts

The present invention relates to a process for the preparation of corrosion resistant zinc and zinc nickel deposits on linear particularly complex shaped parts, usually made of steel. The deposition according to the present invention consists of a two step deposition, wherein first a layer of pure zinc from an alkaline zinc bath and second a layer of zinc nickel alloy deposited from an acid bath is deposited.

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.

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.

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.

As corrosion is very aggressive when the component is used in a humid environment the corrosion protection requirement for window frames and automo- tive 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(alkylene- imines) 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.

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 hy- droxyethyl; p has a value in the range from 3 to 12; and X ^" denotes chloride, bromide and/or iodide.

Such bath is suitable for depositing alkaline zinc deposits.

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 av-

erage of 15 %. However, this coating is significantly less ductile than pure zinc coatings.

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.

GB 2 294 949 A relates to anti-corrosive multilayer-plated steel materials, e.g. plates, pipes, joints, clamps, bolts 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 dis- advantage of using an alkaline zinc-nickel bath as described above cannot be avoided by using such a layer sequence.

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 electro- lyte. It is disadvantageous to apply an alkaline zinc-nickel alloy plating bath.

US 2004/0197594 A1 relates to a steel material that has on its surface a non- hexavalent-chromium type corrosion-resistant coating film formed by a composite of a metal layer and a resin layer. The steel material having or not having a copper layer with 1) a Zn and/or Zn-based alloy layer, 2) a Zn/Ni alloy layer

-A-

which is on this Zn and/or Zn-based alloy layer. According to the description preferred is a layer sequence of zinc layer deposited from an acid zinc plating bath and a zinc-nickel layer deposited from a alkaline zinc-nickel plating bath. Again, it is disadvantageous to apply an alkaline zinc-nickel alloy plating bath.

It is, therefore, 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.

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) first plating of a pure zinc coating from an alkaline electrolyte providing very homogeneous thickness distribution and resistance against corrosion of the coating with good ductility,

(ii) second 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.

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. The very high corrosion protection particularly regarding red rust is provided by a subsequently deposited zinc-nickel layer,

which is deposited from an acid electrolyte and which does not contain high concentrations of amines or ammonia or other undesired complexing agents

It was found by the inventors that despite the low ductility of the zinc-nickel layer deposited from acid plating baths the overall ductility of the layer sequence is very high. Thus by depositing a combination of first a pure zinc layer and thereafter a zinc-nickel layer one obtains a coating with both excellent mechanical as well as corrosion properties.

In one embodiment, the pure zinc layer is deposited on the metal substrate and the zinc nickel layer is deposited on the zinc layer:

Such process for the preparation of corrosion resistant substrate comprises the steps of

(i) providing a metal substrate

(ii) plating a layer of pure zinc from an alkaline electrolyte thereon,

(iii) plating a layer of zinc-nickel from an acid electrolyte.

The thickness of the zinc layer typically is between 1 - 12 μm, preferably between 2 - 8 μm, and more preferably 4-6 μm.

The thickness of the zinc nickel layer typically is between 1 - 12 μm, preferably between 2 - 8 μm, and more preferably 4-6 μm.

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).

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.

A suitable process for preparing such an alkaline zinc electrolyte is described, for example in European patent application EP 1 114 206 and DE 31 29 129 A1.

Pure zinc layers according to this invention shall mean that the layer does not contain any alloying elements added on purpose. Slight impurities of the plating bath may result in very small amounts of metals being incorporated into the pure zinc layer however.

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.

Typically, zinc nickel alloys deposited from such plating baths contain 5 - 25 wt. % of nickel, preferably 9 - 18 [please confirm / amend values] wt.% of nickel in the alloy.

Solutions for passivating zinc and zinc alloy surfaces are known and for exam- pie described in EP 1 484 432. Such solutions typically contain a water soluble Cr(III)-SaIt 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 and zinc nickel plated surface is immersed in the passivation solution, rinsed and thereafter optionally treated at elevated temperatures of 100 to 250 ⁰ C for 10 to 300 minutes.

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

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.

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 meas- ured 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 com- mercially available alkaline zinc nickel plating bath

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

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:

ZnCI ₂ 0.3 M/l

NiCI ₂ 0.5 M/l

KCI 2.7 M/l

H ₃ BO ₃ 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

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:

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).

The coating thicknesses and the thickness distribution of all coatings is shown in Table 1.

hcd led hcd : led

Comp. Example 1 4.1 micron 2.8 micron 1.5

Comp. Example 2 5.5 micron 1.8 micron

Comp. Example 3 8.1 micron 2.2 micron 3.7

Example 4 (invention) 6.1 micron 2.6 micron 2.4

Table 1 : Thickness distribution for different plated layers

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 (led) 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.

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

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.

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 .

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.

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 resis- tance. 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.