EP1565597A1

EP1565597A1 - Plain or zinc-plated steel plate coated with a zinc or zinc alloy layer comprising a polymer, and method for making same by electroplating

Info

Publication number: EP1565597A1
Application number: EP03782562A
Authority: EP
Inventors: Jacques Petitjean; Eric Jacqueson; Claude Arnoux; Guy Durand; Joseph Sliviack
Original assignee: USINOR SA
Current assignee: ArcelorMittal France SA
Priority date: 2002-11-19
Filing date: 2003-11-14
Publication date: 2005-08-24
Also published as: CN1714176A; JP2006506531A; KR20050075441A; CA2506355A1; FR2847275B1; MXPA05005285A; BR0316302A; US20060166031A1; AU2003290200A1; FR2847275A1; RU2005119188A; WO2004048645A1

Abstract

The invention concerns a plain or zinc-plated steel plate, additionally coated on at least one of its surfaces with a single zinc or zinc alloy layer containing 0.15 to 1 wt. % of a polymer consisting of 6 to 150 identical or different units of general formula: (CH2C(R)(CONH2))-, with R = H or CH3 and optionally comprising polyallyl units, and a method for making said plates by electroplating in sulphate baths.

Description

Bare steel or zinc plated steel sheet coated with a layer of zinc or zinc alloy comprising a polymer, and method of manufacturing by electrodeposition

The present invention relates to a sheet of bare steel or galvanized steel coated with a layer of zinc or zinc alloy comprising a polymer, as well as a method of manufacturing such a sheet by electrodeposition. These sheets are more particularly intended for the manufacture of automobiles.

In confined areas or crimped areas of the body of an automobile, accelerated corrosion phenomena can occur. Several strategies can be implemented, separately or jointly, by car manufacturers to avoid these phenomena. One of them consists in using steel sheets coated with a layer of zinc, deposited by electrodeposition or by galvanization, itself covered with a layer of thin organic coating, deposited by coating.

Conventionally, when it is desired to coat a metallic surface with an organic coating (be it a layer of thin organic coating or a layer of paint), a prior surface treatment is carried out, the main role of which is to ensure the adhesion of the organic coating on the metal surface. Among the most common surface treatments, mention may be made of chromating treatments, phosphating treatments, silane-based treatments or titanium-based treatments. The choice of surface treatment depends on the chemical nature of the subsequent organic coatings.

These treatments are essential if the subsequent coatings are to adhere properly, but their use poses a certain number of problems. Thus, their deposit requires having a specific installation dedicated to the implementation of the treatment, the treatment of the effluents generated can be costly as is the case for the phosphating treatments, and some of the products used are eco-friendly. -toxic, especially in the case of chromating. In the particular case of titanium-based treatments, it is also necessary to ensure perfect control of the layer weight deposited, since the range of layer weight leading to satisfactory characteristics is narrow. Silane-based treatments are relatively fragile and are in particular liable to be degraded by the degreasing and phosphating solutions used by automobile manufacturers, before the first layer of paint is deposited by cataphoresis.

The invention therefore aims to remedy the drawbacks of the prior art by providing a coated sheet allowing direct subsequent adhesion of organic coatings in thin layer, without harm to the environment, with improved productivity.

To this end, a first object of the invention is constituted by a sheet of bare steel or galvanized steel, characterized in that it is further coated on at least one of its faces by a single layer of zinc or zinc alloy containing 0.15 to 1% by weight of a polymer consisting of 6 to 150 identical or different units, preferably at most 80 identical or different units, and advantageously 20 to 30 identical or different units, of general formula:

- (CH ₂ -C (R) (CONH ₂ )) - with R = H or CH ₃ , and optionally comprising polyallyl units.

In a first preferred embodiment, the sheet according to the invention successively comprises:

- a layer of steel, then - a single layer of zinc or zinc alloy containing said polymer, then

- A layer based on epoxy resin, which can optionally be supplemented with polyurethane resin and optionally comprising electroconductive particles, such as, for example, zinc particles and / or iron phosphides. In this embodiment, the sheet can also further comprise a layer of zinc interposed between the steel layer and the single layer of zinc or zinc alloy containing the polymer.

In another preferred embodiment, the sheet according to the invention successively comprises:

- a layer of steel, then

- a single layer of zinc or zinc alloy containing said polymer, then

a layer based on polyurethane optionally comprising electroconductive particles, such as, for example, zinc particles and / or iron phosphides.

In this embodiment, it can also further comprise a layer of zinc interposed between the steel layer and the single layer of zinc or zinc alloy containing the polymer.

The sheet according to the invention as it has just been defined in its various embodiments can, in addition, be such that the single layer of zinc or zinc alloy containing the polymer is in turn covered with an organic coating layer, chosen from the group formed by polyurethanes, epoxy resins, polyesters, and their mixtures, said organic coating possibly also comprising electrically conductive particles.

The polymer content of its coating layer is between 0.15 and 1% by weight, preferably between 0.15 and 0.60% by weight.

A second object of the invention is constituted by a method of manufacturing a sheet according to the invention, in which a sheet of bare steel or galvanized steel is passed through in an electroplating bath comprising zinc sulphate , at least one support salt, 0.8 to 1.2 g / l of a polymer consisting of 6 to 150 identical or different units, and preferably at most 80 identical or different units, of general formula - (CH ₂ -C (R) (CONH ₂ )) - with R = H or CH ₃ , and optionally comprising polyallyl units, said bath having a pH between 0 and 3, and an electric plating current is passed between said sheet and at least one anode disposed in said bath, at an average current density between 60 and 160 A / dm ² and substantially constant.

The method according to the invention can also have the following characteristics, alone or in combination:

the polymer concentration in the bath is between 0.9 and 1.1 g / l,

- the sheet of bare steel or zinc-plated steel is passed through the electrodeposition bath at a speed of between 50 and 150 m / min, - the Zn ⁺⁺ ion concentration of the electrodeposition bath is between 40 and 100 g / l,

the temperature of the electrodeposition bath is between 30 and 70 ° C,

- the average current density is less than 120 A / dm ² .

The invention consists in depositing a zinc coating on a bare or galvanized steel sheet, said zinc coating incorporating a particular organic molecule, on the surface and / or in the mass of the zinc coating.

The sheet thus coated has excellent adhesion characteristics for a subsequent organic coating, without the need to carry out a pretreatment toxic to the environment.

From a practical point of view, the composite coating (zinc / organic polymer) can be deposited by any suitable process. In particular, the organic molecule can be added to an electrodeposition bath before the actual electrodeposition operation. The coating is then deposited under the conventional conditions of electrodeposition of a metal coating. The organic molecule present in the bath is incorporated into the mass and / or on the surface of the metal coating, which leads to the production of a composite coating (zinc + organic molecule). However, the inventors have shown that when the polymer concentration in the electroplating bath is insufficient, that is to say less than 0.8 g / l, the incorporation of the polymer into the composite coating is insufficient. Above 1.2 g / l of polymer, the surface appearance of the composite coating (zinc / polymer) is degraded. The polymer concentration in the bath is preferably included between 0.9 and 1.1 g / l. The coating can be deposited on one side, or on both sides of the steel sheet.

The electrodeposition bath that can be used to implement the method according to the invention is based on sulfates and comprises, in particular, zinc sulfate, and at least one support salt, as well as the organic molecule according to the invention.

Its pH can be adjusted, if necessary, by adding appropriate acids. We have seen that the pH must be between 0 and 3. Indeed, beyond a pH 3, the deposition of the composite coating (zinc / polymer) is impossible, because the conductivity of the electrolytic bath is insufficient. The inventors have demonstrated that the deposition of the composite coating (zinc / polymer) on the substrate only begins from an average current density greater than 60 A / dm ² . However, to prevent the adhesion of the composite coating with the subsequent organic coating to be degraded, the current density must remain less than 160 A / dm ² , since from 160 A / dm ² the burn-out limits of the coating are reached composite (zinc / polymer), which results in a dark and dendritic appearance of the coating. It is preferably less than 120 A / dm ² , because the inventors have demonstrated that from this value, the adhesion of the organic coating begins to deteriorate, and to avoid this degradation, it is also necessary to simultaneously increase the speed of the steel sheet in the electroplating bath.

When the running speed of the steel sheet is greater than 150 m / min, the deposition of the composite coating (zinc / polymer) becomes insufficient for the adhesion of the subsequent organic coating to be sufficient. If the running speed is less than 50 m / min, the adhesion of the organic coating is insufficient, because the threshold current density from which the burns of the composite coating begin to occur is reduced.

The concentration of Zn ²⁺ ion in the electrodeposition bath must be less than 100 g / l, since the inventors have demonstrated that beyond 100 g / l the adhesion of the organic coating is insufficient. It must be greater than 40 g / l to avoid the burn problems of the composite coating.

The bath temperature is between 30 and 70 ° C, in fact it is not conceivable, in industry, to work at temperatures above 70 ° C when it is not necessary. In addition, below 30 ° C, the conductivity of the bath becomes insufficient for the deposition of the composite coating on the sheet to be done correctly.

In another embodiment, it is also possible to deposit a bilayer coating (zinc) / (zinc + organic molecule), still by electrodeposition. In this case, it is necessary to have two different electroplating baths: a standard electrogalvanizing bath and an electrogalvanizing bath in which the organic molecule is added. The bath of pure zinc is then used on the first cells of the line in order to deposit a first layer of zinc, while the bath enriched in organic molecule is used on the cells at the end of the line in order to deposit a second layer of zinc. organic molecule.

Without wishing to be bound by a theory, the present inventors believe that the organic functions present on the surface of the zinc coating could be used as a base for attaching the organic coating, thus ensuring its adhesion to the metallic coating in the absence of any treatment. surface area.

The molecules used in the context of the present invention are polymers consisting of 6 to 150 identical or different units, preferably at most 80 identical or different units, and advantageously from 20 to 30 identical or different units, of general formula:

- (CH ₂ -C (R) (CONH ₂ )) - with R = H or CH ₃ , and optionally comprising polyallyl units. More particularly preferred are polymers such as polyacrylamides or polymethacrylamides, but also polyacrylamide / polyallyl copolymers.

The exemplary embodiments which will be described illustrate the invention without however limiting it.

Example 1 An electroplating bath having the following composition is prepared:

- ZnSO ₄ , 7 H ₂ O: 287.5 g / l, i.e. 65 g / l of Zn ²⁺

- H ₂ SO ₄ : 85 g / l - polyacrylamide, denoted M4, in aqueous solution at 50% by mass: 1 g / l of the aqueous solution (M4: polyacrylamide of which the group R is H, of molecular mass 1500 g, comprising about 25 patterns).

The pH of the bath is close to 0 and its bath temperature is maintained between 40 and 60 ° C.

A steel plate is placed on a cathode. The cathode is placed facing an insoluble anode. The support salt, previously prepared, is circulated in the interval between the cathode and the anode at speeds approaching 100 m / min (the width of the interval between the cathode and the anode is 10 mm). An electric current of about 100 A / dm ² is then passed until a coating with a thickness of 7.5 μm is obtained. The Zn-4 composite deposit thus obtained has a perfectly homogeneous appearance.

Using a coating tool of the roll-coat type, an organic layer based on polyurethane resin comprising iron phosphides, of the Granocoat LC type from the company Henkel, is applied to the surface coated with Zn-M4. The layer has a thickness of between 6 and 8 μm. The sheet A thus coated is in accordance with the invention.

For comparison, two coated steel sheets are also prepared according to the prior art:

- a steel sheet B covered with a layer of 7.5 μm of pure zinc, then directly with a layer of Granocoat LC,

a steel sheet C covered with a layer of 7.5 μm of pure zinc, then a conversion surface treatment obtained from a solution of Granodine 1456 marketed by Henkel (based on titanium), then a layer of Granocoat LC. The coatings of pure zinc were developed under the conditions of the prior art (without M4 in the bath). Granodin 1456 is applied using a coating tool of the roll-coat type and with a deposited layer weight included in the range recommended by the supplier (ie 8-12 mg / m ² of deposited titanium).

An adhesion test of the organic coating Granocoat LC is then carried out on the three metal substrates, respecting the following procedure: ^a two Erichsen stampings 8 mm deep are made by deforming from the uncoated face,

^B we apply a 3M standardized scotch tape to one of the two stampings, on the coated side, ^a we tear off the scotch tape and we score the tearing of the organic coating according to the following notation: - 0 no tearing

- 5 total removal, "the sheet thus deformed is then immersed in the conventional degreasing and phosphating baths used by automobile manufacturers, ¹⁸ a standardized tape 3 is applied to the second stamped, always on the coated side,

^H the scotch tape is torn off and the tearing of the organic coating is rated according to the same notation.

Two notes are therefore obtained for the adhesion of the organic coating: one before passing through the degreasing and phosphating baths and the other after passing through these baths.

The results of these tests are collated in the following table:

In view of these results, it appears that the adhesion of the organic coating applied directly to a Zn / M4 coating is excellent and is at the same level of performance as that of the zinc + pre-treatment + organic coating mode.

However, the direct application of the organic coating to a pure zinc substrate according to the prior art leads to unacceptable results in terms of adhesion.

Example 2

A steel sheet is made coated with a layer of Zn / lv.4 obtained under the same conditions as those described in the test carried out in Example 1.

Using a coating tool of the roll-coat type, a layer based on epoxy resin containing zinc micro-balls, of the Bonazinc 3005 type (sold by PPG) is applied to the surface coated with Zn-M4. ). The layer has a thickness of between 5 and 6 μm. The sheet D thus coated is in accordance with the invention.

- a steel sheet E covered with a layer of 7.5 μm of pure zinc, then directly with a layer of Bonazinc 3005,

- a steel sheet F covered with a layer of 7.5 μm of pure zinc, then with a silane-based pretreatment layer, of the Nupal type (marketed by PPG), then with a layer of Bonazinc 3005 .

The coatings of pure zinc were developed under the conditions of the prior art (without M4 in the bath). Nupal is applied using a tool coating of the roll-coat type and with a deposited layer weight included in the range recommended by the supplier (ie 80-120 mg / m ² of dry extract).

The results of these tests are collated in the following table:

In view of these results, it appears that the adhesion of the organic coating of epoxy resin type, applied directly to a Zn / M4 coating is excellent and is at the same level of performance as that of the zinc + pretreatment modality. silane base + epoxy resin.

However, the direct application of the organic coating on a pure zinc substrate according to the prior art also leads to unacceptable results in terms of adhesion.

Example 3

We manufacture different steel sheets (G to U) coated with a layer of zinc / polyacrylamide obtained by electrodeposition in an electrodeposition bath comprising zinc sulfate (ZnSO ₄ , 7 H ₂ O), sulfuric acid (H ₂ SO ₄ ), and a polyacrylamide (PAC) whose group R is H, of variable molecular mass MM, in aqueous solution at 50% by mass, under the conditions of concentration (g / l), pH, temperature T, of speed V of the support salt, and of current density DC which are described in the following table:

When the respective concentrations of ZnSO ₄ , 7H ₂ O are 373.6 g / l and 527.5 g / l, the Zn ²⁺ concentrations are respectively 85 g / l and 119 g / l. A polyacrylamide with a molecular weight of 10,000 g comprises approximately 166 units.

For this purpose, a steel plate is deposited on a cathode. The cathode is placed facing an insoluble anode. The support salt, previously prepared, is circulated in the interval between the cathode and the anode at speed V (the width of the interval between the cathode and the anode is 10 mm). An electric current is then passed with a DC current density until a coating of 7.5 μm in thickness is obtained. The zinc-polyacrylamide deposit thus obtained has a perfectly homogeneous appearance. The values of the parameters outside the invention have been underlined.

Using a coating tool of the roll-coat type, a layer comprising a resin based on epoxy and polyurethane containing zinc beads, of the Granocoat ZE type, is applied to the surface coated with Zn-polyacrylamide. (marketed by Henkel). For comparison, two coated steel sheets are also prepared according to the prior art: a steel sheet V covered with a layer of 7.5 μm of pure zinc, then directly one with a layer of Granocoat ZE , - a steel sheet W covered with a layer of 7.5 μm of pure zinc, then a conversion treatment obtained from a Granodine 1457 solution marketed by Henkel (based on titanium), then d '' a layer of Granocoat ZE with a thickness of 4 μm. The pure zinc coatings were developed under the conditions of the prior art (without polyacrylamide in the bath). Granodine 1457 is applied using a roll-coat type tool and with a deposited layer weight included in the range recommended by the supplier (ie 8 mg / m ² of titanium).

An adhesion test of the organic coating Granocoat ZE is then carried out on the seventeen steel sheets while respecting the procedure described in the test carried out in Example 1.

The removal of the organic coating is rated according to the following notation:

0 no tearing

1 very little tearing

2 little pullout 3 average pullout: barely acceptable limit

4 significant pullout

5 total removal.

The results of the adhesion tests are collated in the following table:

In view of these results, it appears that the adhesion of the organic coating of resin type based on epoxy and polyurethane, applied directly to a composite coating (zinc / polymer) is at the same level of performance as that of zinc mode + titanium-based pretreatment + epoxy and polyurethane-based resin, provided that:

The polyacrylamide concentration is between 0.8 and 1.2 g / l, and preferably between 0.9 and 1.1 g / l. Indeed, when the polyacrylamide concentration is 0.2 or 0.5 g / l (sheets G and H), this is insufficient to obtain good adhesion of the organic coating.

According to the invention, the number of units included in the polyacrylamide does not exceed 150. In fact, when a polyacrylamide of general formula is used in accordance with the invention but comprising approximately 166 units (sheet U), the adhesion of the organic coating is insufficient. The concentration of PAC of molecular weight of 10,000 g has been greatly reduced, since the inventors have found that when its concentration is 1 g / l, then the surface appearance of the composite coating is greatly degraded, which is unacceptable. Thus when the length of the polyacrylamide chains increases, and in particular from 150 units, either the surface appearance of the coating (zinc / polyacrylamide) is disadvantaged by maintaining the concentration of the polymer in the electrolytic bath at approximately 1 g / l in accordance with the invention, or an insufficient adhesion of the organic coating is obtained by reducing the concentration of the polymer in the electrolytic bath. When the current density is between 120 and 160 A / dm ² , the speed is adjusted by increasing it, so as to avoid the burning phenomena of the composite coating (zinc + polymer). In the same way, when the speed of the sheet decreases (or even the speed of the electrolyte according to the tests in accordance with the invention), care must also be taken to reduce the current density to avoid the burning phenomena of the composite coating.

(Zinc / polymer).

Claims

1. Sheet of bare steel or galvanized steel, characterized in that it is further coated on at least one of its faces by a single layer of zinc or of zinc alloy containing 0.15 to 1% by weight of a polymer consisting of 6 to 150 identical or different units, of general formula:

2. Sheet according to claim 1, characterized in that said layer contains from 0.15 to 0.60% by weight of said polymer.

3. Sheet according to claim 1 or 2, characterized in that said single layer of zinc or zinc alloy containing said polymer is in turn covered with an organic coating layer selected from the group formed by polyurethanes, epoxy resins, polyesters and their mixtures, said organic coating possibly also comprising electrically conductive particles.

4. Sheet according to any one of claims 1 to 3, characterized in that it successively comprises:

- a layer based on epoxy resin, which can optionally be added with polyurethane resin and optionally comprising electrically conductive particles.

5. Sheet according to claim 4, characterized in that said sheet further comprises a layer of zinc interposed between said layer of steel and said single layer of zinc or zinc alloy containing said polymer.

6. Sheet according to any one of claims 1 to 3, characterized in that it successively comprises:

- a layer of steel, then

- a single layer of zinc or zinc alloy containing said polymer, then - a polyurethane-based layer optionally comprising electrically conductive particles.

7. Sheet according to claim 6, characterized in that said sheet further comprises a layer of zinc interposed between said layer of steel and said single layer of zinc or zinc alloy containing said polymer.

8. Sheet according to any one of claims 1 to 7, characterized in that said polymer consists of at most 80 identical or different units.

9. Sheet according to claim 8, characterized in that said polymer consists of 20 to 30 identical or different units.

10. A method of manufacturing a sheet according to any one of claims 1 to 9, characterized in that a sheet of bare steel or galvanized steel is passed through in an electroplating bath comprising sulphate of zinc, at least one support salt, 0.8 to 1.2 g / l of a polymer consisting of 6 to 150 identical or different units, of general formula - (CH ₂ -C (R) (CONH ₂ )) - with R = H or CH ₃ , and optionally comprising polyallyl units, said bath having a pH between 0 and 3, and an electric current of electroplating is passed between said sheet and at least one anode disposed in said bath, an average current density between 60 and 160 A / dm ² and substantially constant.

11. Method according to claim 10, characterized in that the polymer concentration in the electroplating bath is between 0.9 and 1.1 g / l.

12. Method according to claims 10 or 11, characterized in that said sheet of bare steel or galvanized steel is passed through the electroplating bath at a speed between 50 and 150 m / min.

13. Method according to any one of claims 10 to 12, characterized in that the concentration of Zn ⁺⁺ ion is between 40 and 100 g / l.

14. Method according to any one of claims 10 to 13, characterized in that the temperature of the electrodeposition bath is between 30 and 70 ° C.

15. Method according to any one of claims 10 to 14, characterized in that the average current density is less than 120 A / dm ² .