EP0436408A1 - Process and bath for applying protective chrome hexavalent coatings on metallic substrates and trade form of the bath components - Google Patents

Abstract

Appropriate process for imparting to a metal substrate a coating protecting against corrosion, based on hexavalent chromium, not requiring recourse to any additional protective layer, and which comprises:   - a stage of preparation of the metal substrate by degreasing, optionally followed by scouring or pickling,   - a stage of treatment by dipping, spraying or with a roller by means of an appropriate bath for imparting the required protective coating to the metal substrate and   - a stage of baking at high temperature, is characterised in that the abovementioned bath comprises at least:   - a metal in particulate form,   - chromic acid and/or one of its derivatives as oxidising agent,   - at least one water-soluble solvent of high boiling point, chosen from those of aprotic polar type and of ketonic type,   - water.

Description

The subject of the invention is a method suitable for conferring on a metal substrate a protective coating based on hexavalent chromium; it also relates, and this as new industrial products, the bath used in this process and the commercial form of the constituents of the bath.

The metallic substrates intended to be protected against corrosion by means of the aforesaid process consist in particular of small metal parts and the screwed devices, for example made of steel or cast iron.

It is known (see for example French patent N ° 2,008,770) to fight effectively and without subsequent surface protection, against corrosion of metal substrates, by making them include coatings obtained, in general, using baths comprising
- a powdery metal, in particular zinc,
- chromic acid in solution,
an organic solvent comprising, in general, one or more alcohols or polyols such as, for example, tert-butanol, dipropylene glycol or diethylene glycol monoethyl ether,
- a wetting agent and
- some water.

• 1. une étape de préparation, à savoir un dégraissage suivi éventuellement d'un décapage mécanique ou chimique,
• 2. une étape de mise en oeuvre par trempage ou aspersion d'un bain du genre décrit ci-dessus, suivi d'un égout­tage ou d'un essorage et
• 3. une étape de cuisson du substrat ainsi traité à tempé­rature généralement élevée, par exemple d'environ 300°C.

From a practical point of view, a treatment is applied to the substrate to be protected comprising:

• 1. a preparation step, namely a degreasing optionally followed by a mechanical or chemical pickling,
• 2. an implementation step by soaking or spraying a bath of the kind described above, followed by draining or spinning and
• 3. a step of cooking the substrate thus treated at a generally high temperature, for example around 300 ° C.

A single layer coating is thus obtained; to obtain a multilayer coating, the treatment is repeated as many times as necessary.

The above baths have a significant drawback residing in the fact that they are unstable.

Indeed, the strong oxidizing nature of chromic acid reduces the life of the bath, the solvents used are oxidized, even at ordinary temperature, after a few days.

The quality of the coating obtained therefore decreases rapidly with the age of the bath.

The object of the invention is, above all, to remedy this drawback and to provide a bath of the kind in question whose stability is improved and which, consequently, allows prolonged use.

However, the Applicant Company has found, after extensive research that, surprisingly and unexpectedly, it is possible not only to decisively increase the stability of baths of the genus in question without altering their effectiveness but, moreover , to make these baths more efficient than those of the prior art with regard to the corrosion resistance of the coatings obtained when recourse is had, as organic solvent, to a water-soluble solvent with high point of boiling of the polar aprotic type and / or of the ketone type.

Consequently, the process according to the invention capable of imparting to a metallic substrate a protective coating against corrosion, based on hexavalent chromium, requiring no additional protective layer, and which comprises:
- a step of preparing the metal substrate by degreasing, possibly followed by mechanical or chemical pickling,
a step of treatment by soaking, spraying or using a roller by means of a clean bath to give the metal substrate the desired protective coating, and
- a high temperature cooking step,
is characterized by the fact that the above bath comprises at least:
- a metal in the form of particles,
- chromic acid and / or one of its derivatives as an oxidizing agent,
at least one water-soluble solvent with a high boiling point chosen from those of the aprotic polar type and of the ketone type,
- some water.

The above bath, according to the invention, advantageously comprises one or more surface-active agents and one or more rheological additives.

This bath is established by mixing its constituents at the time of use.

In a commercial form, the constituents of the bath are grouped into two or possibly three sets, separated from each other, and advantageously placed on the market in the form of what is designated in the art by the expression "kit".

In the case where there are two sets, the first will include chromic acid and / or its derivatives in the form of powder or concentrated solution while the second will include, in the form of a dispersion or possibly concentrated slurry, the metal in the form of particles, the water-soluble organic solvent and optionally one or more surface-active agents and one or more rheological additives, the first and the second set being arranged respectively in a first and in a second housing of the kit.

In the case where there are three sets, the third consists of part or all of the surfactants and rheological additives and placed in a third housing of the kit.

When the above first set is in the form of a concentrated solution, it will advantageously contain from 1 to 85% by weight of hexavalent chromium ion expressed as chromate.

Likewise, when the second set is in the form of a concentrated slurry, this will advantageously contain from 2 to 80% by weight of metallic particles in dispersion in a part of the organic solvent, the third set containing the surface-active agents. active ingredients and rheological additives in an amount of 1 to 70% by weight in another part of the organic solvent.

The metal in the form of particles can be chosen from metals, alloys or mixtures of metals or alloys having a negative normal electrode potential, of absolute value at least equal to that of the metal to be protected.

In practice, said metal is chosen from the group comprising zinc, iron, aluminum, cadmium, magnesium, manganese and their alloys; zinc, aluminum and their mixtures or alloys are particularly preferred.

The size of the metal particles must be compatible with the thickness desired for the protective coating.

It is generally from 1 to 25 µm (this last dimension corresponding to the length of the lamellae when the metal is in lamellar form).

Preferably, the bath according to the invention is in the form of an aqueous dispersion of metallic particles comprising approximately 5 to 40% of metal.

Hexavalent chromium is provided by chromic acid, its alkali or alkaline earth metal salts, its transition metal salts, its ammonium salt, chromic anhydride and, more generally, any substance capable of releasing ions containing hexavalent chromium; it is present in the bath at a rate of about 0.5 to 15% by weight, expressed as chromate ions.

The water-soluble solvent with a high boiling point of the ketone type and / or of the polar aprotic type is present in the bath in an amount such that the solvent / chromate ion ratio is between 0.5 and 20.

dans laquelle les radicaux R₁ et R₂ représentent des groupes alcoyles linéaires ou ramifiés en C₁ à C₁₈, identiques ou différents, pouvant former un cycle entre eux, ainsi que certains dérivés polycétoniques ou du type lactone; les susdits dérivés cétoniques sont choisis parmi ceux répondant à la formule (I) et dont la solubilité dans l'eau est au moins égale à 5% (P/P), dont le point éclair est supérieur ou égal à 50°C et dont le point d'ébullition est au moins égal à 100°C.The solvents of the ketone type used in the constitution of the bath according to the invention include those corresponding to formula I

in which the radicals R₁ and R₂ represent linear or branched C₁ to C₁₈ alkyl groups, identical or different, which can form a ring therebetween, as well as certain polyketonic or lactone type derivatives; the aforesaid ketone derivatives are chosen from those corresponding to formula (I) and whose solubility in water is at least equal to 5% (W / W), whose flash point is greater than or equal to 50 ° C and whose the boiling point is at least 100 ° C.

dans laquelle les radicaux R₁, R₂ et R₃ représentent des groupes alcoyles linéaires ou ramifiés en C₁ à C₁₈, iden­tiques ou différents, et pouvant former un cycle entre eux.The solvents of the aprotic polar type entering into the constitution of the bath in accordance with the invention are preferably constituted by the disubstituted amides of formula II

in which the radicals R₁, R₂ and R₃ represent linear or branched alkyl groups in C₁ to C₁₈, identical or different, and which can form a cycle between them.

According to an advantageous embodiment of the bath according to the invention, the incoming water-soluble solvent in its constitution is chosen from the group comprising cyclohexanone, 2,5-hexane-dione, gammabutyrolactone, dimethyl formamide, dimethyl acetamide, N-methyl-pyrrolidone.

The surfactants optionally comprising the bath used in accordance with the invention can be chosen from nonionic surfactants such as polyethoxy ethers of alkylphenol, of alcohol or of optionally substituted amine.

The rheological additives optionally included in the bath used in accordance with the invention can be chosen from thickening agents such as etherified or esterified celluloses, xanthan derivatives, and thixotropic derivatives of silica, montmorillonite or l aluminum such as aluminum stearate.

The metallic substrates which can be treated in accordance with the invention are those based on steel, cast iron and sintered steel used in fasteners, bolts and small parts devices; they can also consist of steel strips.

The step of treating the substrate by means of the bath in accordance with the invention can be carried out by soaking followed by draining, by spraying, by soaking followed by centrifugation for small pieces of hardware, or else by using rollers when processing flat metal strips.

Once provided with the protective coating which comprises the various constituents of the bath, the substrate is subjected to a baking step between 100 and 350 ° C for a few minutes, in particular for 5 to 45 minutes in order to harden the coating by evaporation of volatile materials.

Depending on the use for which the metal substrate is intended and depending on the thickness desired for the protective coating, the latter may be mono- or multilayer. To obtain a multilayer coating, the above-mentioned treatment cycle is repeated as many times as necessary, which may include, in the case of a substrate constituted by an article of fasteners:
- soaking in the bath according to the invention,
- draining or spinning,
- cooking at a temperature between 120 and 200 ° C.

A final baking treatment (at a temperature between 200 and 350 ° C) will be used in place of or in addition to the last baking at a temperature between 120 and 200 ° C.

The thickness of the coatings obtained in accordance with the invention is generally between 1 and 15 μm.

The substrates subjected to the treatment in accordance with the invention must be clean and suitable for receiving the anticorrosion protective coating. Depending on their degree of soiling, they should be pretreated by alkaline and / or solvent-based degreasing, rinsing and, if necessary, mechanical or chemical pickling.

One of the important advantages conferred by the invention resides in the fact that it is unnecessary to provide an additional protective layer, for example paint, on the coatings obtained.

The invention can be better understood with the aid of the nonlimiting examples which follow and in which advantageous embodiments of the invention are indicated.

EXAMPLE 1

A bath according to the invention is prepared from a kit according to the invention comprising three sets, one of which, designated by A, is in the form of a slurry containing the metal, including another, designated by B, is in the form of a solution providing the chromium ion and the third of which, designated by C, comprises a rheological agent.

Set A consists of 150 g of lamellar zinc (quality referenced ECKA zinc MP 31 129 / G and sold by the company ECKARTWERKE), 15 g of nonylphenol to 10 moles of ethylene oxide (brand CEMULSOL NP 10 marketed by RHONE-POULENC), 150 g of dimethyl formamide and 2 g of hydroxymethyl cellulose.

Set B consists of 27.6 g of chromic acid, 17.4 g of magnesium chromate and 223 g of water.

Set C, which is used to adjust the viscosity of the final mixture, consists of 4 g of hydroxymethyl cellulose in 581 g of water.

These three sets provide by mixing a quantity of 1170 g of ready-to-use bath.

EXAMPLE 2 (comparative)

A conventional formulation is prepared by mixing solutions A1 and B1 (compositions indicated below), which gives 100 g of ready-to-use bath. Components of solution A₁ Components of solution B₁ Lamellar zinc ... 20g Water ... 51.9 g Dipropylene glycol ... 10g Chromic acid ... 3 g Ethyl glycol acetate 5 g Zinc oxide ... 1 g White spirit 5% aromat. 5 g Boric acid ... 1 g REMCOPAL 334 ... 1 g Hydroxy ethyl cellulose ... 0.5 g REMCOPAL 349 ... 1.6g

The aging of this bath was compared to that according to Example 1 over a period of 10 days by following the change in viscosity.

This viscosity, expressed in cps, was measured in the case of the two baths on the 1st, 2nd, 3rd, 4th, 7th and 10th days.

The measured values are plotted on the graph of FIG. 1 which represents the evolution of the viscosity in cps as a function of time in days.

A curve C _{m is} thus obtained for the bath according to the prior art and a curve C _n for the bath conforms to the invention.

The examination of FIG. 1 shows that the viscosity of the two baths evolves differently: while the bath according to example 2 degrades gradually as a result of gelling from two days, to reach a maximum of viscosity at the end of 'About ten days, the bath according to Example 1, according to the invention, retains its fluidity.

These differences in aging behavior lead to differences in performance on the quality of the coating obtained when the coatings are not produced immediately after the formation of the bath by mixing the starting solutions.

To highlight this difference in performance between the two baths, the procedure is as described.

10 sheets of cold-rolled steel with 0.02% carbon (10 cm x 20 cm) of automotive quality "ZES" are provided with a protective coating according to the procedure comprising the following steps:
degreasing with a conventional alkaline aqueous composition, in this case that obtained with the product sold under the brand name RIDOLINE 1550 CF / 2 by the Applicant Company and used at 15 g / l at 60 ° C for 10 minutes,
- running cold rinse with tap water and drying at 80 ° C,
- Application of the baths according to examples 1 and 2, respectively to half of the 10 aforementioned sheets by the method known as the calibrated bar (one can use calibrated bar No. 3, for example that marketed by the company RK CHEMICALS Co thanks to which gives a wet film with a thickness of 24 µm),
- drying 10 minutes at 120 ° C, then cooking for 25 to 30 minutes at 300 ° C.

The dry coating thus obtained has a thickness of approximately 3.5 to 4 μm.

The sheets thus treated are subjected to adhesion and corrosion resistance tests, as set out below.

a) Adherence test

An Erichsen-type stamping [on the PEUGEOT vehicle standard B 53 3240 (§3.4.3)] with a depth of 8 mm is made on each sheet.

Is carried out on the top of the stamped cut with a reference adhesive tape 250, sold by the company 3 M.

The adhesion is all the better as the detachment of the coating is superficial.

When the detachment is very superficial, the strip of adhesive tape has a light gray appearance denoted "5".

The notation 5 corresponds to a coating from which no trace has been torn off.

When the covering is completely torn off, the support being visible, the notation "0" is applied.

The intermediate ratings are difficult to translate and correspond to the appearance which results from the corresponding photographs which appear in FIG. 2 in B "0" to B "5".

When the separation of the stamp is significant, the test is carried out on the flat non-deformed surface.

The corresponding notations from 0 (coating completely torn off, visible support) to 5 (no trace torn off) result from the appearance of the coating as it appears in FIG. 2 at A "0" to A "5".

The adhesion tests were carried out with baths aged 1, 2, 3, 4, 7 and 10 days corresponding to those used for measuring the viscosity of the bath.

The ratings for each test are collated in Table I. TABLE I Bath age (days) Score recorded in the adhesion test Coating according to Example 1 Coating according to Example 2 1 4/5 3 2 4/5 2 3 4/5 2 4 4/5 2 7 4/5 2 10 4/5 1

It follows from this table that for the bath according to Example 1, the quality of the adhesion remains unchanged, even when the application is carried out 10 days after preparation; on the contrary, in the case of the bath according to Comparative Example 2, the quality of the adhesion deteriorates from the second day after preparation of the bath.

b) Corrosion resistance

In the corrosion resistance test, the quantity of red rust produced is measured as a function of the duration of exposure to salt spray according to standard NF T 41002.

The degree of corrosion was measured (expressed as a percentage of corroded surface relative to the total surface) after an exposure of 168 hours, then 400 hours to salt spray with the baths of the previous comparative tests aged 1, 2, 3, 4, 7 and 10 days.

The results are collated in Table II. TABLE II Bath age (days) Bath according to the ex. 1 Bath according to the ex. 2 Corrosion (in%) after 168 h after 400 h after 168 h after 400 h 1 0 0 to 5% 0 from 20 to 50% 2 0 0 to 5% 0 from 20 to 50% 3 0 0 to 5% 0 from 20 to 50% 4 0 from 5 to 20% 0 from 20 to 50% 7 0 from 5 to 20% 0 from 20 to 50% 10 <5% from 20 to 50% > 50% > 50%

It appears, on examining the figures gathered in table 11, that the corrosion resistance is markedly improved by virtue of the bath in accordance with the invention since the bath used for obtaining the coating has not been prepared just for testing.

In addition, the presence of white rust is observed for the bath according to Comparative Example 2 well before the appearance of red rust. This phenomenon is very clearly reduced, even nonexistent, with the bath according to Example 1.

EXAMPLE 3

A bath identical to that of Example 1 is prepared, using zinc dust as the zinc source, for example the Ultra Fine Special quality sold by OLD MOUNTAIN (particle diameter 2-3 μm) and replacing weight for weight dimethylformamide by dimethyl acetamide:

On the adhesion test, the result corresponds to note 3/4.

The resistance to salt spray is greater than 600 hours.

EXAMPLE 4

Preparing a bath identical to that of Example 1, using as zinc source zinc dust identical to that of Example 2 and replacing weight for weight dimethyl formamide with N-methyl pyrrolidone.

On the adhesion test, the result corresponds to note 3/4.

The resistance to salt spray is greater than 800 hours.

EXAMPLE 5

A solution A is prepared consisting of 150 g of zinc dust (Ultra Fine Special quality sold by the company VIEILLE MONTAGNE), 30 g of nonyl phenol with 10 moles of ethylene oxide (CEMULSOL NP 10 sold by RHONE-POULENC), 60 g of cyclohexanone and 2 g of hydroxymethyl cellulose.

Solution B consists of 27.6 g of chromic acid, 17.4 g of magnesium chromate and 298 g of water.

Solution C is identical to that described in Example 1. The three mixed solutions provide 1170 g of ready-to-use bath.

On the adhesion test, the result corresponds to note 4/5.

The resistance to salt spray is greater than 600 hours.

EXAMPLE 6

A bath identical to that of Example 1 is prepared in which part of the lamellar zinc (namely 30 g) is replaced by lamellar aluminum (30 g). The lamellar aluminum used is that sold under the brand Chromal II by the company ECKARTWERKE and whose particle size is comparable to that of zinc (approximately 18 μm at its greatest length).

A coating on screws is made in two applications as described above with intermediate firing at 180 ° C.

The adhesion cannot be measured by the method described in Example 1, a stamping being not feasible.

The resistance to salt spray is greater than 500 hours.

Claims (9)

1. Method suitable for imparting to a metallic substrate a protective coating against corrosion, based on hexavalent chromium, requiring no additional protective layer, and which comprises:
a step of preparing the metal substrate by degreasing, optionally followed by mechanical or chemical pickling,
a step of treatment by soaking, spraying or using a roller by means of a clean bath to give the metal substrate the desired protective coating, and
- a high temperature cooking step,
is characterized by the fact that the above bath comprises at least:
- a metal in the form of particles,
- chromic acid and / or one of its derivatives as an oxidizing agent,
at least one water-soluble solvent with a high boiling point chosen from those of the aprotic polar type and of the ketone type,
- some water. 2. Bath suitable for imparting a protective coating against corrosion, based on hexavalent chromium, to a metal substrate and intended to be used within the framework of the method according to claim 1, characterized in that it comprises at least :
- a metal in the form of particles,
- chromic acid and / or one of its derivatives as an oxidizing agent,
at least one water-soluble solvent with a high boiling point chosen from those of the aprotic polar type and of the ketone type,
- some water. 3. Commercial form for the constituents of the bath according to claim 2, characterized in that it comprises two assemblies, the first of which comprises chromic acid and / or its derivatives in the form of powder or of concentrated solution while the second comprises , in the form of a dispersion or optionally concentrated slurry, the metal in the form of particles, the water-soluble organic solvent and optionally one or more surface-active agents and one or more rheological additives, the first and the second set being arranged respectively in a first and in a second housing of a kit. 4. Commercial form according to claim 3, characterized in that it comprises a third set consisting of part or all of the surfactants and rheological additives and disposed in a third housing of the kit. 5. Bath according to claim 2, characterized in that the water-soluble solvent with high boiling point of the ketone type and / or of the aprotic polar type is present in an amount such that the solvent / chromate ion ratio is between 0, 5 and 20. 6. Bath according to one of claims 2 and 5, characterized in that the water-soluble solvent with a high boiling point of ketone type corresponds to formula 1 in which the radicals R₁ and R₂ represent linear or branched alkyl groups in C₁ to C₁₈, identical or different, which can form a ring therebetween, as well as certain polyketonic derivatives or of the lactone type, said solvents being chosen from those whose solubility in water is at least equal to 5% (P / P) , whose flash point is greater than or equal to 50 ° C and whose boiling point is at least equal to 100 ° C. 7. Bath according to one of claims 2 and 5, characterized in that the water-soluble solvent with high boiling point of aprotic polar type corresponds to formula II in which the radicals R₁, R₂ and R₃ represent linear alkyl groups or branched in C₁ to C₁₈, identical or different, and being able to form a cycle between them. 8. Bath according to one of claims 2 and 5, characterized in that the water-soluble solvent is chosen from the group comprising cyclohexanone, hexane-2,5-dione, gammabutyrolactone, dimethyl formamide, dimethyl acetamide , N-methyl-pyrrolidone. 9. Metal substrate characterized in that it comprises an anticorrosion coating obtained by implementing the method according to claim 1.

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