WO2002007902A2

WO2002007902A2 - Anticorrosive black coating on zinc alloy and method for making same

Info

Publication number: WO2002007902A2
Application number: PCT/FR2001/002376
Authority: WO
Inventors: Jean-Jacques Duprat; Lionel Thiery; Nicolas Pommier; Anne Charpentier
Original assignee: Coventya
Priority date: 2000-07-25
Filing date: 2001-07-20
Publication date: 2002-01-31
Also published as: EP1409157A2; EP1409157B1; FR2812307B1; WO2002007902A3; DE60127870D1; DE60127870T2; ES2282278T3; FR2812307A1

Abstract

The invention concerns a method for depositing an anticorrosive black coating on a zinc alloy which consists in treating the zinc alloy in a first step with an aqueous acid solution of Cr3+ containing one or several complexing agents and in treating the resulting passivated zinc alloy in a second step with an aqueous suspension containing organic polymers and anticorrosive metal oxides and black pigments. The invention also concerns a black anticorrosive coating with two layers on a zinc alloy, the first layer of the anticorrosive coating having a weight (in Cr) of 1 to 4 mg/dm2.

Description

Black anticorrosive layer on a zinc alloy and its preparation process.

The invention relates to a black anticorrosive layer formed from two layers on a zinc alloy and to a method for preparing this anticorrosive layer.

To protect them from corrosion, metallic surfaces, for example steel surfaces, are coated with a zinc alloy (for example Zn-Fe, Zn-Ni, Zn-Co, Zn-Mn). The deposit of alloyed zinc can then be protected by a chromating layer. This can be obtained by treating the deposit of alloyed zinc with a solution containing Cr ⁶⁺ . However, the use of solutions containing Cr ⁶⁺ is no longer desired for reasons of toxicity and environmental protection. US Pat. No. 5,415,702 proposes instead of treating a zinc / nickel or zinc / iron alloy with an acid solution containing Cr ³⁺ ions and phosphate ions. The surface thus coated with chromate can be brought into contact with an alkali metal silicate solution. Finally in another operating stage, an organic coating (for example a paint) can be deposited on the silicate layer thus obtained.

This process has the following drawbacks: the protection against corrosion of a layer prepared by this process on a zinc / iron alloy is not sufficient compared to that of a layer prepared by this process on a zinc / iron alloy nickel. In addition, the protection against corrosion of a layer prepared by this process on a zinc / iron alloy is defective after heating for one hour beyond 150JC (thermal shock). Finally, it may occur due to the silicate coating after a certain time, very little desired white traces on the surface. In addition, the surface prepared by this process exhibits adhesion insufficient for subsequent painting.

The present invention aims to overcome the drawbacks of the prior art, to further improve the protection of zinc alloys against corrosion, and in particular to provide an anticorrosive black layer on zinc alloys which after one hour heating up to 120 ° C or up to 150 ° C or in some cases beyond (thermal shock) and a salt spray test according to DIN 50 021 corrosion resistance greater than 200 h. The invention further aims to give the anticorrosive layer a uniform black appearance which cannot be changed over time.

The method according to the invention is characterized in that it consists in treating the zinc alloy in a first step with an acidic aqueous solution of Cr ³⁺ , containing a complexing agent or several complexing agents, and in treating the alloy of zinc thus passive in a second stage of the process with an aqueous solution containing an organic polymer, an anticorrosive metal oxide and optionally a black pigment. The anticorrosion coating produced on a zinc alloy according to the invention consists of a superposition of two individual layers, prepared according to a method according to the invention, the first layer having a layer weight (expressed in Cr) from 1 to 4 mg / dm ² .

The process described according to the invention consists of two stages. In the first stage, the deposit of alloyed zinc is treated with an acidic aqueous solution of Cr ³⁺ free of Cr ⁶⁺ containing one or more complexing agents. The deposit is thus passive. The treatment time ranges from 20 to 400 seconds. A preferred treatment time is between 60 and 240 seconds. In a second step of the process, the passive zinc alloy, rinsed and drained, is treated with an aqueous suspension containing organic polymers, anticorrosive metal oxides and possibly black pigments. The treatment time ranges from 5 to 120 seconds. Finally, the zinc alloy thus treated can be dried in hot air (for example between 50 and 75 ° C, preferably at 70 ° C).

The pH of the Cr ³⁺ acid solution will be between 1 and 4, preferably between 1 and 3. The pH is preferably adjusted using mineral acids such as HNO ₃ , H ₂ SO ₄ , HCI or H ₃ PO ₄ . Treatment of the zinc alloy with the passivation solution and by the suspension can be carried out in different ways, for example by immersion, by projection, with the brush or with the roller. The immersion application will be used preferentially. The temperature of the Cr ³⁺ solution used in the first step of the process is between 20 and 80 ° C, preferably between 25 and 60 ° C; the temperature of the aqueous suspension used in the second step of the process is between 15 and 35 ° C, preferably between 20 and 30 ° C.

Trivalent chromium can be introduced into the solution in the form of trivalent chromium salts, for example CrCI ₃ , 6H ₂ O, Cr (NO ₃ ) ₃ , 9H ₂ O, chromium (III) acetate or in the form trivalent chromium sulphate ("pure" chromium sulphate or chromium sulphate and other cations such as for example KCr (SO ₄ ) ₂ -12H ₂ O). Alternatively, a Cr ³⁺ solution can be prepared from a Cr ⁶⁺ solution with the addition of a mineral acid in the presence of a reducing agent such as formaldehyde or sodium hydrogen sulfite. The lower limit of the chromium content of the solution (expressed as Cr ³⁺ ) is 1 g / l (which corresponds to 0.02 mol / l), preferably 5 g per liter (which corresponds to 0 , 1 mol / l); the upper limit of the chromium (III) content of the solution is 30 g / l (which corresponds to 0.58 mol / l), but preferably 20 g / l (which corresponds to 0.38 mol / l).

To complex the Cr ³⁺ ions and thus regulate the precipitation of chromium hydroxide III on the surface of the part during the conversion operation, the solution contains one or more complexing agents such as phosphates, hypophosphites, acid salts organic (e.g. citric acid, oxalic acid, tartaric acid, malonic acid, maleic acid, formic acid, acetic acid, lactic acid, aminoacetic acid, l 'iminodiacetic acid) or urea.

Phosphates will be used preferentially because they particularly favor the formation of a dark conversion layer. The phosphates act as a "proton reservoir", these protons allow to activate and maintain the passivation process of the zinc alloy deposit. The phosphates will be incorporated for example in the form of Na ₃ PO ₄ , Na ₂ HPO ₄ , NaH ₂ P0 ₄ or in the form of phosphoric acid. Phosphates can be present for a minimum concentration in the solution (expressed as PO ₄ ^{3 "} ) of 10 g / l (which corresponds to 0.01 mol / l), preferably 15 g / l (which corresponds to 0.15 mol / l). The upper limit of the phosphate concentration in the solution is preferably 44 g / l (which corresponds to 0.42 mol / l), in particular preferably 25 g / l (which corresponds to 0.26 mol / l l). Hypophosphite can be added (for example in the form of an alkali metal hypophosphite such as NaH ₂ PO ₂ or in the form of hypophosphorous acid), alone or preferably with phosphates. An advantageous effect of hypophosphite lies in the fact that it delays the precipitation of chromium hydroxide in the solution on the surface of the zinc alloy until a pH of 4.5. Without addition of hypophosphite, chromium hydroxide precipitates from a pH of the solution of 3.0. The minimum concentration of hypophosphite (expressed as H ₂ PO ₂ ^" ) is 0.08 mol / l, and preferably 0.23 mol / l. The upper limit of the hypophosphite concentration is 0.65 mol / l and preferably 0.5 mol / l and more preferably 0.45 mol / l.

The other complexing agents mentioned can be incorporated at a level of 0.025 mol / l to 0.6 mol / l with a preference in particular for the range between 0.06 and 0.2 mol / l. It is preferable to use them at the same time as phosphates and / or hipophosphite. Protection against corrosion can also be improved as well as the uniformity of the black color by the addition of ions of the elements iron, molybdenum, cobalt and / or nickel to the Cr ³⁺ solution. The amounts to be added can for example be between 0.001 mol / l and 0.1 mol / l.

The protection against corrosion and the adhesion of the topcoat can be improved by incorporating into the passivation solution silicon dioxide in the form of colloidal particles such as, for example, the Ludox AM30 dispersion sold by the company Du Bridge. The amount incorporated may be between 0.07 mol / l and 1.3 mol / l. The passivation layer or stratum ¹ has a thickness of between 0.5 and 5 μ and preferably between 0.5 μ and 1, 5 μ

The aqueous suspension used in the second step of the process contains organic polymers, anticorrosive metal oxides and possibly black pigments. The organic polymers used are of the copolymer type acrylic, methacrylic or vinyl resins. They are polymers of the ester of acrylic acid or of methacrylic acid and they can have, as alcoholic constituent, an alkyl group which is unsubstituted or substituted by functional groups, for example a methyl, ethyl, propyl, isopropyl group, n-butyl, isobutyl, tert-butyl, pentyl and hexyl and their isomers and higher homologs, 2-ethylhexyl, phenoxyethyl, hydroxyethyl, 2-hydroxypropyl, caprolactone-hydroxyethyl or dimethylaminoethyl. The commercially available acrylate copolymers are, for example, Lugalvan DC from the company BASF or Carboset 560 from the company BF Goodrich.

As organic polymers, polyethylene waxes may also be used (in the form of an emulsion, for example, Polygene PE from the company BASF or Luciwax EN 41 from the company Morton). The waxes advantageously increase the wear resistance of the surfaces and make it possible to obtain, depending on the wax used, advantageous sliding properties such as a low friction coefficient of between 0.08 and 0.18. The use of suitable waxes in the suspension makes it possible to obtain constant coefficient of friction values of ± 0.03.

The amount of organic polymers added (expressed respectively as dry matter) is in total from 5 to 150 g / l, preferably from 10 to 100 g / l. A combination of 5 to 90 g / l of acrylate or methacrylate copolymer and 1 to 60 g / l of polyethylene wax has been found to be very favorable.

As anticorrosive metal oxides, it is possible to use silicon dioxide, titanium dioxide, zirconium dioxide, and / or rare earth oxides such as cerium oxide or La ₂ O ₃ or Y ₂ O ₃ or Ces metal oxides are preferably used in the form of an aqueous suspension of nanoparticles. The concentration of metal oxides (based on solid matter) in the aqueous suspension used in the second step of the process is preferably from 20 to 60 g / l with in particular a preference for the interval between 30 and 45 g / l. The metal oxides are fixed by organic polymers. The introduction of mineral substances into the anticorrosive layer in this form has, compared to the known treatment with solutions of these substances (for example sodium silicate solutions) the advantage of improving the protection against corrosion and maintain the uniform black color of the anticorrosive layer over time.

The uniform black appearance of the anticorrosive layer can be further reinforced by adding carbon black pigments (for example Derussol P130 from Degussa) or other pigments but such as Sanodal Black sold by Clariant in the suspension. aqueous used during the second step of the process. Preferably, the concentration of carbon black pigments in the aqueous suspension is between 5 and 20 g / l. The friction layer or second layer deposited in the second stage has a thickness of 0.5 to 5 μ and preferably 0.5 to 2.5 μ. The anticorrosive layer thus has in all a thickness generally between 1 μ and 5 μ and preferably between 1 μ and 4 μ.

The anticorrosive layer produced in two stages according to the method according to the invention which has just been described on galvanized metal surfaces has a layer weight in the first layer (expressed in Cr) of 0.5 to 4 mg / dm ² . To determine this weight, the chromium layer formed (first layer) is dissolved on the galvanized metal surface after the first step of the process (passivation) with 10% hydrochloric acid and the chromium content is determined using a atomic absorption spectroscopy. The second layer of the anticorrosive layer consists of anticorrosive metal oxides, black pigments and, where appropriate, other additives incorporated in organic polymers. An advantage of this anticorrosive layer according to the invention lies in the absence of Cr ⁶⁺ and in resistance to corrosion after heating for one hour at 120 ° C or 150 ° C, and resistance to salt spray following the standard DIN 50 021 of 200 h and sometimes well beyond.

The invention is described more precisely using the following examples:

Example 1

19 g of CrO _{3 are} dissolved in approximately 250 ml of water. 11 g of 96% H ₂ SO ₄ are added very slowly. 60 g of H ₃ PO ₄ are then added thereto.

85% and complete with water to 1 I of solution in all. The temperature of the solution is approximately 65 ° C. The solution is kept stirring, formaldehyde is then carefully added until the color turns greenish yellow. Cool to about 25 ° C. To this solution of Cr ³⁺ is added per liter 30 g of sodium hypophosphite and then 20 g of citric acid. The pH is adjusted to 2.7 with a 50% NaOH solution and the mixture is maintained at a temperature of 25 ° C.

The aqueous suspension making it possible to produce the second layer consists of 90 g / l of a colloidal dispersion containing 44% of particles of silicon dioxide in water (Ludox AS 40 from the company Du Pont), of 50 g / l a 27% acrylic copolymer (Carboset 560 from the company BF Goodrich) and 70 g / l of an ethylene grafted acrylic copolymer (Lugalvan DC from the company BASF). To enhance the black appearance of the surface treated with the suspension, 50 g / l of a 20% carbon black dispersion (Derussol P 130 from Degussa) are introduced as black pigment into the aqueous solution. Standard steel plates of 100 mm x 70 mm are coated in a conventional manner with a zinc / iron alloy (Fe 0.5%), they are activated in a solution of nitric acid at 5 ml / l , then after rinsing they are immersed for about 180 seconds in the Cr ³⁺ solution (temperature of the solution 25 ° C), they are rinsed, they are drained in a stream of air (room temperature), they are soaked for About 30 seconds in the aqueous suspension and they are dried for about 15 minutes in a stream of hot air (70 ° C). The appearance of the plates is then uniform black. The chromium content of the anticorrosion layer is (before treatment with the aqueous suspension) 2 mg / dm ² . The steel plates thus treated are brought for 1 h at 150 ° C. (thermal shock) and they are then subjected to a salt spray test according to DIN 50 021. An appearance of white rust is observed after 300 h.

EXAMPLE 2 60 g of a solution of chromium nitrate (chromium content 11.5%) are diluted in 200 ml of water. 20 g of 85% phosphoric acid, 0.3 g of 69% nitric acid and 75 g of colloidal silica dispersion type Ludox AM 30 are added thereto and the volume is adjusted to 1 liter with water. The pH is adjusted between 1.5 and 1.6 using a 50% NaOH solution. Standard steel plates are coated in a conventional manner 100 mm x 70 mm of a zinc / nickel alloy (Ni 12 to 15%), they are soaked for approximately 90 seconds in the Cr ³⁺ solution (solution temperature 25 ° C), they are rinsed, they are drained in a stream of air (room temperature), they are soaked for about 30 seconds in the aqueous suspension (of Example 1) and they are dried for about 10 minutes in a stream of hot air (70 ° VS). The steel plates have a uniform black appearance. The chromium content of the anticorrosion layer is (before treatment with the aqueous suspension)> 2 mg / dm ² . The steel plates thus treated are brought for 1 h to 120 ° C (thermal shock) and then subjected to a salt spray test according to DIN 50 021. The parts do not show white rust after 240 hours and no red rust after 800 hours.

Example 3 The Cr ³⁺ solution consists of 55 g / l of Chromitan

(trademark) (chromium (III) sulfate-based salt), which corresponds to a Cr ³⁺ content in the solution of 9.35 g / l, 25.5 g / l of H ₃ PO ₄ , 30 g / l of sodium hypophosphite, 20 g / l of citric acid and 0.9 g / l of NHO ₃ , the pH of the solution is adjusted to 2.5 per using 20% NaOH solution.

The aqueous suspension making it possible to produce the second layer consists of 90 g / l of a colloidal dispersion containing 44% of particles of silicon dioxide in water (Ludox AS40 from the company Du Pont), of 50 g / l of a 27% acrylic copolymer (Carboset 560 from BF Gooirich) and 70 g / l of an ethylene grafted acrylic copolymer (Cugalvan DC from BASF)

Standard steel plates of 100 mm x 70 mm are coated with a zinc / iron alloy (Fe 0.5%), they are cleaned, they are immersed for about 240 seconds in the solution of Cr ³⁺ (temperature of the solution 25 ° C), they are rinsed, they are drained in a stream of air (room temperature), they are soaked for about 30 seconds in the aqueous suspension described above and they are dried for about 15 minutes in a stream of hot air (70 ° C). The appearance of the plates is then uniform black. The chromium content of the anticorrosion layer is (before treatment with the aqueous suspension) greater than 2 mg / dm ² . We wear the steel plates thus treated for 1 h at 120 ° C (thermal shock) and they are then subjected to a salt spray test according to DIN 50021. The parts do not show white rust after 320 hours.

Example 4

Dissolve 45 g (corresponding to a Cr ³⁺ content of 7.65 g) of Chromitan (trademark) (chromium (III) sulfate salt) in 500 ml of water, and bring the pH above 5 by addition of a 20% NaOH solution. To this solution are added 45 g of sodium hypophosphite and 54 g of 85% phosphoric acid and the solution is made up to 1 I with water.

The solution is filtered on a filter paper having a pore diameter of 100 μm and it is brought for 2 h at 60 ° C. The pH is adjusted between 2.3 and 2.5 with a 50% NaOH solution. To this solution is added 1 g of cobalt sulfate CoSO ₄ , 7H ₂ O.

Standard steel plates of 100 mm x 70 mm are coated with a zinc / iron alloy (Fe 0.5%), they are activated in a 5 ml / l nitric acid solution, soak them for approximately 60 seconds in the Cr ³⁺ solution (solution temperature 25 ° C), rinse them and drain them in a stream of air (room temperature). The appearance of the plates is then uniform black. The chromium content of the anticorrosion layer is greater than 2 mg / dm ² .

The steel plates thus treated are immersed for approximately 30 seconds in the aqueous suspension (example 1) and they are dried for approximately 15 minutes in a stream of hot air (70 ° C.), they are brought for 24 h to 120 ° C (thermal shock) and they are then subjected to a salt spray test according to DIN 50021. After 300 h, no white rust is observed.

Example 5

100 g of KCr (SO ₄ ) ₂ , 12H ₂ O are dissolved in 200 ml of water. 20 g of NaOH are dissolved in approximately 100 ml and added to them. The mixture is brought to 80 ° C. for 1 h. After cooling to approximately 25 ° C., 20 g of sodium hypophosphite, 20 g of citric acid, 15 g of 85% phosphoric acid, 0.6 g of 69% nitric acid and 100 g of sodium hydroxide are added. typical silica dispersion Ludox AM30, then complete the 1 I solution with water. The pH of the solution is 2.5.

Standard steel plates of 100 mm x 70 mm are coated with a zinc / iron alloy (Fe 0.5%), they are activated in a 5 ml / l nitric acid solution, soak them for 180 seconds in the Cr ³⁺ solution (solution temperature 25 ° C), rinse and drain in a stream of air (room temperature). The steel plates have a uniform black appearance. The chromium content of the anticorrosive layer is between 2 and 4 mg / dm ² . The steel plates are soaked for approximately 30 seconds in the aqueous suspension (of Example 1) and they are dried for approximately 15 minutes in a stream of hot air (70 ° C.), they are heated for 24 h at 120 ° C (thermal shock) and then subjected to a salt spray test according to DIN 50 021. After 200 h, no white rust is observed.

Example 6

35 g of a solution of chromium nitrate (chromium content 11.5%) in 100 ml of water are added. 6.5 g of NaOH, 15 g of oxalic acid and 2 g of malonic acid are added thereto. The temperature becomes above 60 ° C.

After cooling to 25 ° C., 2 g of Co (NO ₃ ) ₂ , 6H ₂ O are added, and the solution is made up to 1 I with water. The pH is adjusted to 1.5.

Standard steel plates of 100 mm x 70 mm are coated with a zinc / nickel alloy (Ni 15%) 12 μm thick, they are quenched for 75 seconds in the Cr ³⁺ solution (temperature of the solution 25 ° C.), they are rinsed and they are drained in a stream of air (room temperature). The steel plates have a uniform black appearance. The chromium content of the anticorrosive layer is 2 mg / dm ² .

The steel plates are soaked for approximately 30 seconds in the aqueous suspension (of Example 3) and they are dried for approximately 15 minutes in a stream of hot air (70 ° C.).

The plates are then brought for 1 h at 120 ° C. (thermal shock), and they are then subjected to a salt spray test according to DIN 50 021. After 200 h, no white rust is observed, and after 800 h there is no red rust observed.

Claims

1. A method of depositing an anti-corrosion black layer on a zinc alloy, characterized in that it consists in treating the zinc alloy in a first process step with an acidic aqueous solution of Cr ³⁺ , free of Cr ⁶⁺ containing a complexing agent or several complexing agents, and in treating the zinc alloy thus passive in a second process step with an aqueous solution containing an organic polymer, an anticorrosive metal oxide and optionally a black pigment.

2. Method according to claim 1, characterized in that, in the first step of the method, the pH of the Cr ³⁺ solution ranges from 1 to 4.

3. Method according to claim 2, characterized in that, in the first step of the method, the pH of the Cr ³⁺ solution ranges from 1.0 to 3.0.

4. Method according to one of claims 1 to 3, characterized in that it consists in using as complexing agent a phosphate, a hypophosphite, citric acid, oxalic acid, tartaric acid, malonic acid, maleic acid, formic acid, acetic acid, lactic acid, aminoacetic acid, iminodiacetic acid or urea.

5. Method according to one of claims 1 to 4, characterized in that it consists in adding in the first step of the method to the solution of Cr3 + ions Fe, Co, Mo and / or Ni in an amount of 0.001 mol / l to 0.1 mol / l.

6. Method according to one of claims 1 to 5, characterized in that it consists in adding in the first step of the method to the solution

Cr ³⁺ of colloidal particles of silicon dioxide in an amount which can

7. Method according to one of claims 1 to 6, characterized in that it consists in using as organic polymers copolymers of acrylate or methacrylate, polyethylene wax or vinyl resins-

8. Method according to one of claims 1 to 7, characterized in that it consists in using as anticorrosive metal oxides silicon dioxide, titanium dioxide or rare earth oxides.

9. Method according to one of claims 1 to 8, characterized in that the anti-corrosive metal oxides are in the form of a suspension of nanoparticles.

10. Anti-corrosion black layer on a zinc alloy, characterized in that it comprises two layers, that it is prepared by a process according to one of claims 1 to 8, and that the first layer has a weight (expressed in Cr) from 1 to 4 mg / dm ² .

11. Layer according to claim 10, characterized in that it has a thickness between 1 μ and 4 μ.