FR2906542A1 - Bath for electrodeposition of zinc or zinc nickel alloy on a conducting substrate having acid pH and containing zinc chloride, useful to protect parts of iron, cast iron or steel, comprises polyvinylimidazole and zinc in aqueous solution - Google Patents

Abstract

Bath for electrodeposition of zinc or zinc nickel alloy on a conducting substrate having an acid pH and containing zinc chloride and optionally nickel chloride hexahydrate, comprises polyvinylimidazole (0.1-30 g/l, preferably 1-10 g/l), zinc (>= 10 g/l, preferably 10-60 g/l) and optionally nickel (>= 5 g/l, preferably 10-50 g/l), in aqueous solution.

Description

The present invention relates to a zinc electrodeposition bath

  or zinc-nickel alloys on a conductive substrate, having an acidic pH and containing zinc chloride and optionally nickel chloride, preferably in hexahydrate form, in aqueous solution. It is well known that, in all areas of the industry such as automotive, household appliances, conservation ..., it is necessary to protect iron, cast iron or iron parts. steel against corrosion.

  Among the most commonly used methods for obtaining such protection, mention may be made of galvanization which has been known for many decades. This operation consisting in covering the parts to be protected with a protective layer based on zinc or on zinc alloys, in particular zinc-nickel alloys, can be carried out either by dipping in a zinc bath or of molten zinc alloys (or roll coating) or by electrodeposition. This ability of zinc to protect metal surfaces and more particularly steel against atmospheric corrosion phenomena is due to the fact that this metal has an anodic dissolution potential greater than that of iron. As a result, as long as there is zinc near a damaged area, it dissolves in favor of iron. This is the so-called sacrificial protection of steel by zinc. The present invention relates exclusively to electroplating (or electrogalvanizing) galvanizing process. This process consists diagrammatically in immersing the parts to be treated which act as cathodes in an electrolytic bath containing zinc salts and, where appropriate, other metals to be deposited, in aqueous solution, and for passing a current between these parts and metal anodes, in particular zinc anodes which also dive into the electrolytic bath. Under the action of the electric current, the metal ions present in the solution which are positively charged migrate to the cathode, so the workpieces are then coated with a galvanizing layer. Electrolytic galvanizing baths can have various compositions because many zinc salts are soluble in aqueous solution. Under specific conditions, the zinc can thus be deposited electrochemically from acidic solutions (sulphate, fluoborate, chloride, sulphamate, etc.) or from alkaline solutions (with cyanide, pyrophosphate, ethanolamine, etc.). ). Acidic acid plating baths have the advantage of allowing very high current densities and thereby achieving particularly high electrodeposition rates. A disadvantage of these acid baths, however, is that they have insufficient penetrating power. However, this disadvantage can be remedied by the addition of conductive salts (ammonium chloride, sodium chloride, sodium sulphate, etc.) which, in parallel, lead to an increase in the conductivity of the bath, and as a corollary a reduction of the consumption of electrical energy. Chloride electrodeposition baths are particularly advantageous in that - apart from the fact that they have a high conductivity, they can be used at high current densities and thus allow to operate very rapidly - they lead to obtaining galvanizing deposits having very fine grains. Many investigations have been made to try to improve the properties of galvanizing deposits obtained from zinc electrodeposition baths or zinc alloys, in particular from zinc-nickel alloys to chloride, in particular gloss or lime. the absence of pitting of these deposits, or the uniformity of their thickness over a wide range of current density, their resistance to corrosion, or the stability of their composition in the case of alloyed coatings.

  However, zinc electroplating baths or zinc-nickel, acidic, aqueous, chloride-based baths which have already been proposed by the specialists all have the disadvantage of forming deposits having insufficient ductility. In the case of zinc-nickel alloys, this disadvantage leads in particular to micro-cracking which significantly reduces the corrosion protection effect of the galvanizing deposit on the workpiece.

Another disadvantage of known acid-based zinc or nickel-zinc electroless plating baths is that they do not provide passivation films, in particular chromium III films. satisfactory.

It is in fact known to those skilled in the art that parts which have undergone galvanizing treatment generally remain too sensitive to corrosion, in particular following the formation of soluble zinc oxide: this is the so-called white rust phenomenon. zinc. For this reason, it is proposed to coat the parts already re-dressed with a layer of zinc or zinc alloyed with a complementary inhibiting film essentially based on metal oxides, which is called a passivation film. The conventional passivation process for zinc or zinc alloy parts is to immerse them in a chromic acid solution to cause a so-called zinc conversion reaction. This reaction uses two essential mechanisms, namely on the one hand an oxidation of zinc from the immersion of the workpiece, and on the other hand a series of electrochemical reduction reactions leading to a local alkalization leading to the formation of a gel by precipitation. This chemical reaction is, however, very difficult to control. The major drawback of this process is related to the fact that the passivation solutions used are very polluting because of their very high content of chromic anhydride which can be up to 100 g / l. It is also well known that chromium VI is a toxic compound. For this reason, hexavalent chromium is now tending to be replaced by trivalent chromium in the passivation processes of zinc-coated or alloy zinc-coated parts. Chromium III passivation processes, however, do not allow to achieve corrosion resistance equivalent to those obtained during the implementation of the conventional chromium VI passivation process.

As a result, it is often proposed to apply by dipping onto the chromium III passivated zinc a water-resistant and hydrophobic mineral or organominer topcoat.

It is an object of the invention to provide a zinc or zinc-nickel alloy plating bath on a conductive substrate having an acidic pH and containing zinc chloride and, where appropriate, nickel chloride. preferably in hexahydrate form in aqueous solution forming chromium III-passable galvanizing deposits so as to lead to passivation films having improved salt spray and corrosion behavior. Another object of the invention is to provide a zinc plating bath or zinc-nickel alloys of the aforementioned type forming galvanizing deposits having a substantially improved ductility following the decrease of the internal stresses of these deposits. According to the invention, such an electrodeposition bath is characterized in that it contains: between 0.1 and 30 g / l, preferably between 1 and 10 g / l of polyvinylimidazole, at least 10 g / l and preferably between 10 and 60 g / l of zinc, and where appropriate, at least 5 g / l and preferably between 10 and 50 g / l of nickel. Such a plating bath can without departing from the scope of the invention contain a zinc content up to the maximum solubility of zinc in the bath and a nickel content can also go to solubility maximum nickel in the bath. During the electrolysis process, the amounts of zinc and nickel consumed can be replenished in the bath by the use of zinc and / or nickel anodes which dissolve therein. If the amount of zinc and / or nickel thus dissolved is in-sufficient, the zinc and nickel concentrations of the bath can be maintained by addition of zinc chloride or nickel.

The essential characteristic of the electrodeposition bath according to the invention is however linked to the presence within it of a particular polymer, polyvinylimidazole (PVI) which makes it possible to reduce the internal stresses of the galvanizing coating obtained and improve its resistance to corrosion.

This is a polycarboxylate having the general formula - (C 5 H 6 N 2) n - CH 2 CH n 10 N This polymer is of course added to the electroplating bath before the electrolysis operation, which is carried out. works in the classical conditions. As a result, the polymer (PVI) present in the bath is incorporated in the mass and / or surface of the metal galvanizing coating, which leads to the production of a composite coating having particularly advantageous properties. According to another characteristic of the invention, the electrodeposition bath contains at least 90 g / l, preferably between 100 and 300 g / l of chloride. Such a chloride content is indeed essential to allow the bath to have sufficient conductivity. If the desired total chloride concentration is not obtained by the only chlorides from zinc and nickel, then other soluble chlorides in the bath are added, preferably sodium chloride. The electrodeposition bath may have a pH of between 2 and 8 without departing from the scope of the invention. This pH is, however, preferably set between 4 and 6, in particular by addition of hydrochloric acid to lower it or by addition of potassium hydroxide to increase it. According to another characteristic of the invention, the electroplating bath contains between 2.5 and 60 g / l, preferably between 5 and 40 g / l of boric acid.

Boric acid is a buffering agent whose function is to avoid increasing the pH in the vicinity of the cathode, i.e. the workpiece, which results in the precipitation of zinc hydroxide and its formation. possible incorporation into the galvanizing deposit.

Boric acid also makes it possible to limit the risks of burned deposits. According to another characteristic of the invention, the electrodeposition bath contains at least one chelating agent. This chelating agent is preferably selected from the group consisting of salts of hydroxycarboxylic acids such as citrates, tartrates, gluconates and glycollates, amino alcohols such as triethanolamine, polyamines such as ethylenediamine and polyhydric alcohols such as sorbitol and thiourea. These different constituents can of course be used alone or in mixture. Their function is to keep the metal ions in solution. According to another characteristic of the invention, the electroplating bath contains between 0.01 and 10 g / l of at least one brightening agent. Among these brighteners, it is necessary to distinguish between primary brighteners which are generally present in an amount of about 0.01 to 10 g / l and secondary brighteners which are generally present in amount of 0.2 to 10 g / l. The primary brighteners allow if they are used alone to obtain semi-gloss deposits. These primary brighteners may be, for example, dextrin, glucose or glycerin.

The secondary brighteners give the deposits a true shine; the most common are polyethers, aromatic carboxylic acids and their salts, quaternary nicotinate compounds, and aliphatic or aromatic aldehydes or ketones. These secondary glossing agents are generally used in the form of a combination of several of these agents. Indeed, used alone, they act only in a narrow area of current density which leads to embrittlement of galvanizing deposits and an increase in their internal stresses. The electrodeposition bath according to the invention may advantageously contain other constituents such as wetting agents which make it possible to avoid pinholes and / or grain refiners which make it possible to refine the structure of the coating.

The electrodeposition bath according to the invention may, by way of example, additionally contain anthraquinone sulphonate. According to another characteristic of the invention, the electrodeposition bath has a temperature of between 15 and 70.degree. C., preferably between 15 and 40.degree. C. The process for electroplating a galvanizing coating from the bath according to the invention is preferably carried out at cathodic current densities of between 0.1 and 100 A / dm 2, more particularly between 0.5 and 10 A / dm 2. The anodes used are preferably metal anodes made of zinc and / or nickel. During electrolysis, the bath must be stirred mechanically and / or by circulation of the solution. The parts to be treated can also be themselves in motion. The galvanizing bath according to the invention thus makes it possible to obtain a part coated with a uniform fine-grained Zn-PVI or ZnNi composite galvanizing deposit, containing at least 8% nickel, which has a satisfactory light gray appearance, more The galvanizing coatings obtained from the conventional galvanizing baths do not contain any polymer. The essential characteristics of such a galvanizing coating, however, are related to the reduction of its internal stresses and its ability to undergo chromium III passivation treatment.

The particularly advantageous characteristics of the electrocoating bath according to the invention will be brought to light by means of the examples below: EXAMPLE 1 Electrodeposition baths are prepared according to the following composition: ZnCl 2: 75 g / l NiC12, 6H20: 120 g / l KCl: 220 g / l H3B03: 20 g / 1 Wetting agent: 25 ml / l Primary brightener: 12 ml / 1 Secondary brightener: 1.5 ml / l PVI: from 0 to 5 g / l.

The pH of the bath is between 5 and 5.4 and its temperature is maintained at 35 C. Standard steel plates are treated with these baths and coated with a ZnNi deposit with and without a polymer, during an electrolysis. 20 minutes lysis, at a current density of 2 A / dm 2. Their X-Fluorescence analysis gives the following table: Bath Concentration Thickness (m) Alloy (% Ni) PVI (g / 1) ZnNi - 10.3 14.8 0.1 10.4 14.2 ZnNi + 0.5 11.0 15.8 PVI 1 10.7 15.5 5 11.5 15.6 The polymer does not modify the bath performance or the nickel content of the alloy obtained, since the uncertainty of this application at the X fluorescence is 0.9 m. Some platelets are observed under a scanning electron microscope (SEM) at a magnification of 1000 x, as shown in Figure 1.

This figure shows that black dots are observed in the ZnNi-PVI composite coating. The qualitative analysis of carbon with the EDAX probe, coupled with the SEM, is carried out, the carbon composing the structure, the skeleton of the polymer chain.

The deposition of ZnNi alone already contains carbon, resulting from the formulation of the bath (organic additives, brighteners, etc.). This deposit gives a carbon peak of 185 strokes. At 1 g / 1 PVI, the black spot analysis gave a peak at 1720 shots, and those at 5 g / 1 PVI peaked at 6406 shots.

It is concluded that the black spots observed correspond to polymer fragments.

EXAMPLE 2 In order to test the internal stresses of the deposits obtained from the present invention, electrocoating baths are prepared under the same conditions as those described in Example 1.

The principle of the measurement of internal stresses consists in carrying out the electrolysis on a test-specimen of known surface area under precise operating conditions and in determining the deformation of this specimen covered with the deposit to be characterized. This deformation will give the values of internal tensions of this deposit by the following equation: Tensions lnternes = IdV in kg / mm2 2LT Designation: 15 I: Young's modulus of steel: 2.119 X 10 000 kg / mm 2 D: thickness of the test piece: 0.05 mm V: deviation (m) L: length of the test piece: 150 mm These measurements are carried out using the IS METER specific apparatus. The operating conditions are set at a current density of 2 A / dm 2, at a temperature of 35 ° C., a pH of 5.4-5.5, a fairly slow mechanical stirring and nickel anodes. The electroplating baths tested were ZnNi alone, followed by 0.1-1.5-1 and 5 g / l PVI. The results obtained are collated on the curves shown in FIG. 2. According to this figure, positive internal voltages indicate compressive stresses.

The ZnNi deposit without polymer gives stresses of more than 85 MPa during the first microns, then the voltages decrease to reach 60 MPa at a thickness of 10 m, which corresponds to the reference thickness used in the tests. The addition of PVI in the bath, and therefore in the deposit, makes it possible to reduce the internal stresses. As soon as 0.1 g / l of PVI was introduced into the bath, the voltages decreased by 20 MPa initially.

This decrease in tension increases with the increase in the polymer concentration of the solution, up to 26 MPa for the bath at 5 g / l PVI, for a thickness of 10 m deposition.

Example 3: Mild steel plates are coated with a composite ZnNi deposit from electrodeposition baths formulated as in Example 1 under identical operating conditions. These platelets are passivated in a conversion solution based on trivalent chromium, at 60 ° C., for 1 minute, at pH 2, then, after rinsing, they are covered with a mineral topcoat, at pH 9, for 30 seconds. . The test pieces are then drained and dried at 60 ° C. for 15 minutes.

Before testing them with salt spray, the plates are baked for 10 minutes, at 180 ° C., in order to improve their anticorrosion performance. The performance of the salt spray tests are summarized in the following table: 20 PVI content Zn Ni + PVI White salts> 5% White salts Red rust - 216 h. 480 h. > 720 h. 0.1 g / 1216 hrs. 648 hrs. > 720 h. 0.5 g / 1504 hrs. 696 hrs. > 720 h. 1 g / 1,528 h. > 720 h. > 720 h. 5 g / 1> 720 h. > 720 h. > 720 h. The plate with non-composite coating (PVI-free) is used as a reference. The white salts appear in 216 hours of BS. In view of the results, it appears that the resistance to corro-Sion improves with the concentration of PVI in the electroplating bath 2906542 II, until a corrosion-free deposit is obtained. white after 720 hours of salt spray test. 5

Claims (5)

  1) Electroplating bath of zinc or of nickel-zinc alloys on a conductive substrate having an acidic pH and containing zinc chloride and, where appropriate, nickel chloride, preferably in hexahydrate form, in aqueous solution characterized in that it contains: between 0.1 and 30 g / l, preferably between 1 and 10 g / l of polyvinylimidazole, at least 10 g / l and preferably between 10 and 60 g / l of zinc, and where appropriate at least 5 g / l and preferably between 10 and 50 g / l of nickel. 2) Electroplating bath according to Claim 1, characterized in that it contains at least 90 g / l, preferably between 100 and 300 g / l of chloride. 3) Electroplating bath according to any one of claims 1 and 2, characterized in that it has a pH of between 4 and 6. 4) Electroplating bath according to any one of claims 1 to 3, characterized in that it contains between 2.5 and 60 g / l, preferably between 5 and 40 g / l of boric acid. 5) Electroplating bath according to any one of claims 1 to 4, characterized in that it contains at least one chelating agent preferably selected from the group consisting of salts of hydroxycarboxylic acids such as citrates, tartrates glucanates and glycollates, amino alcohols such as triethanolamine, polyamines such as ethylenediamine and polyhydric alcohols such as sorbitol and thiourea. 6) Electroplating bath according to any one of claims 1 to 5, characterized in that it contains between 0.01 and 10 g / l of at least one brightener preferably selected from group formed by polyethers, aromatic carboxylic acids and their salts, quaternary compounds of nicotinates, and aliphatic or aromatic aldehydes and ketones. 7) Electroplating bath according to any one of claims 1 to 6, characterized in that it contains wetting agents and / or grain refiners. 8) Electroplating bath according to any one of claims 1 to 7, characterized in that it has a temperature between 15 and 70 C, preferably between 15 and 40 C. 20