WO1997020641A1

WO1997020641A1 - Method for protecting a metal part from corrosion and/or preparing it for shaping

Info

Publication number: WO1997020641A1
Application number: PCT/FR1996/001864
Authority: WO
Inventors: Philippe Antoine; Frédéric CALDERARA; Gérard Riess
Original assignee: Sollac S.A.
Priority date: 1995-12-07
Filing date: 1996-11-26
Publication date: 1997-06-12
Also published as: FR2742159A1; FR2742159B1

Abstract

A method comprising the use of a non-cross-linked block or graft copolymer film having a thickness of 0.5-5 νm and including at least one 'polar' block (ii) and at least one 'apolar' block (i). The solubility parameters of the monomeric units of the apolar block (i) are all no higher than 19.1 J?1/2.cm-3/2¿, and at least one of the monomeric units of the polar block (ii) is non-ionic, has a solubility parameter higher than 19.1 J?1/2.cm-3/2¿, and constitutes at least 5 wt.% of said copolymer.

Description

Method for protecting against corrosion and / or preparing a metal part for shaping

The invention relates to a treatment of the metal surface of parts, in particular of steel sheets, which consists in applying, to these parts, organic thin films, for temporary protection against corrosion and / or their preparation for operations of shaping, especially stamping.

By temporary protection against corrosion, it means that this treatment provides protection limited in time, compatible with the duration of a period - one month or even several months - of parts storage, before their transformation into finished or semi-finished product. finished.

The transformation of said parts may include shaping operations, in particular stamping, and the treatment (coating) must be able to contribute to the reduction of the coefficient of friction during these operations.

The transformation of said parts may include other operations, such as painting operations, and the coating must be such that it is not necessarily necessary to remove it, in particular by degreasing, before these operations. A painting operation in fact conventionally begins with a phosphating step which, to be effective, often requires prior degreasing.

If certain transformations nevertheless require the prior elimination of the coating, it is important that this coating be easily eliminated.

For this purpose, to produce a coating polymer film, it is possible to use, according to document FR 2 677 375, a block copolymer, where a first block consists of siloxane monomer units, where a second block comprises acrylic or vinyl monomer units. Thus, by using a copolymer according to this document, a film with a thickness of less than 3 micrometers is sufficient to protect a steel sheet against corrosion for more than a month, or even three months, and also suffices to significantly reduce its coefficient. friction.

This copolymer thus provides greater corrosion protection than that provided by conventional oiling treatments.

This same copolymer also contributes to reducing the coefficient of friction more than a treatment with an alkaline phosphate solution known elsewhere for this specific application would do. As indicated in the example described in this same document, the acrylic monomers that are used have a higher solubility parameter than that of the siloxane monomer.

For example, by retaining the definition of the solubility parameter of the work "Polymer hand book" by J. BANDRUP (Wiley interscience edition of 1976):

- The solubility parameter of the siloxane monomer such as dimethylsiloxane is 7.5 (cal / cm ³ ) ⁰ - ⁵ or 15.4 (J / cm ³ ) ⁰ - ⁵ .

- the solubility parameters of acrylic monomers such as methyl methacrylate, hydroxyethyl methacrylate and butyl methacrylate are respectively 10, 11 and 10 (cal / cm ³ ) ⁰ _' ⁵ , ie 20.5, 22.5 and 20.5 (J / cm ³ ) 0-5.

These solubility parameters relate here to the monomers themselves and not to the monomer units in the co-polymer, the difference in chemical structure between the monomer and the monomer unit bearing on a carbon-carbon double bond.

In order to improve the effectiveness of the protection and / or of the preparation for shaping, and to reduce the cost of the treatment, inherent in particular to the nature of the copolymers and to the coating method, other methods have been sought. copolymers which may be more suitable for this application for protecting metals, in particular steel.

The prior art describes other copolymers which can be used for protecting metals against corrosion:

- in the summary CA (Chemical Abstracts) n ° 079 (10) 054966 - corresponding to the article "Protective action of coatings made from copolymers of styrene and vinylpyridine." of authors V.l. Zavrazhina, Ju.N. Mikhailovskii, P.l. Zubov, T.N. Pavlinova in the journal Lakokrasoch. Mater. Ikh Primen., 26 (1973) - describes the use of random copolymers based on styrene and vinylpyridine to increase the corrosion resistance of iron films; the thickness necessary to obtain effective protection is of the order of 100 μ.

- patent applications JP 74 101412 "Corrosion-resistant temporary coating materials for steel." and JP 74 97835 "Protective coating composition for steel." describe the temporary protection of steel sheets with copolymers based on methacrylic esters, acrylic acid and styrene.

- The article by M. Orbay, R. Laible and L. Dulog entitled "Preparation of amide and amine groups containing copolymers of methyl methacrylate and their performance in solid polymer composites." in Die Makromolare Chemie, 183, 47 (1982) mentions the use of random copolymers based on methyl methacrylate and nitrogenous monomers to improve the corrosion resistance of steel parts; the thickness of the protective layer reaches 25 μm. - Polish patent PL 152076 "Agent for temporary corrosion protection of iron and steel." mentions the use of copolymers based on butadiene, styrene, methacrylic esters, butadiene carboxylated, the use of polymers of the poly (vinyl acetate) type, poly (methacrylic salts) and the use of resins of the type phenolic, melamine and coumarone for the preparation of coatings for the temporary protection of iron and steel. However, nothing indicates that effective protection can be obtained with thicknesses of only a few micrometers.

- US patent 4,942,193 "Temporary protective coating compositions." describes coatings based on waxes - and not on copolymers - for protection against corrosion, with relatively low surface densities (between 1.6 g / m ² and 3.2 g / m ² ), which correspond to lower thicknesses at 5 μ; in the coating composition, wetting agents are added in small proportions; as wetting agent, random copolymers based on (meth) acrylic acid, styrenic monomers, (meth) acrylic monomers (methyl methacrylate, butyl acrylate, ethylhexyl acrylate) and amino alkyl (meth) monomers are used ) acrylic.

Thus, apart from the document FR 2 677 375 mentioned above, the other copolymers, cited in the other documents of the prior art for the protection against corrosion of metal parts, are random copolymers; these documents describe very thick coatings, in particular greater than 10 μm, necessary to obtain effective protection.

Furthermore, it is not possible to deduce from these documents any teaching concerning the tribological properties and the aptitude for shaping of the metals coated with these copolymers.

The object of the invention is to improve, economically and in terms of efficiency, the temporary protection against corrosion of metal parts, in particular steel sheets.

Another object of the invention is to provide copolymers for coating metal parts which, in addition to their anti-corrosion properties, simultaneously offer an improvement in the friction properties facilitating the shaping of said parts. The object of the invention is also to protect against corrosion and / or to prepare the shaping of metal parts with a coating which is easily removable - ie "degreasable" - if necessary before further processing. said parts, on the other hand sufficiently thin to precisely avoid having to eliminate it before most processing operations (for example, before painting).

To this end, the subject of the invention is a method of treating the metal surface of a part, in particular of a steel sheet, intended to protect it against corrosion and / or to prepare its lubrication for an operation of shaping, characterized in that it consists in applying to said part a non-crosslinked film of block or graft copolymer having a thickness of between 0.5 and 5 μm, said copolymer comprising at least one so-called "apolar" block (i ) and at least one so-called "polar" sequence (ii), said apolar sequence (i) consisting of one or more monomeric units whose solubility parameters are all less than or equal to 19.1 J ^1/2 .cnτ ^{3 / 2} , and said polar sequence (ii) comprising at least one nonionic monomer unit having a solubility parameter greater than 19.1 J ^1/2 .cπr ^3/2 in mass proportion of at least 5% in said copolymer, excluding block copolymers combining patterns s siloxane monomers (as units of apolar sequence (i)) with acrylic or vinyl monomer units (as units of polar sequence (ii)).

The term “block copolymer” is intended to mean a copolymer which, if it were composed solely of a block represented by A and of a block represented by B (each block which may itself be composed of several monomer units), would have a chemical formula of type -AB-

A-B-A-B-, where the different sequences are distributed periodically (and not statistically).

The term “graft copolymer” is intended to mean a copolymer which, if it were composed solely of a main block represented by A (which may itself be composed of several monomer units) and of a grafted block represented by B, would be described by formula :

B B B B B

I I I I I

- A - A - A - A - A - The invention may also have one or more of the following characteristics:

- The mass proportion of said polar block (ii) in the copolymer is between 5% and 95%, preferably between 20% and 80%. - The mass proportion of said at least one nonionic monomer unit of the polar block (ii) in the copolymer is between 5% and 95%, preferably between 5% and 50%.

- Said monomer units of the apolar sequence (i) are preferably of diene type such as butadiene, isoprene or pentadiene; olefinic type such as ethylene, propylene or butylene; of chlorofluoroethylene type such as tetrafluoroethylene or chlorotrifluoroethylene; vinyl type such as vinyl laurylate or vinyl stearate; of styrenic type such as styrene, α-methylstyrene or styrene substituted by one or more alkyl groups; acrylic ester type such as n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, n-hexyl acrylate or lauryl acrylate; methacrylic ester type such as methyl methacrylate (in the polar sequence of ex. 7) ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, sec-butyl methacrylate, tertiobutyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, ethylhexyl methacrylate or lauryl methacrylate; or of siloxane type such as dimethylsiloxane or diphenylsiloxane. - Said at least one monomer unit of the polar sequence (ii) preferably has an amine function, such as 2-vinylpyridine, 4-vinylpyhdin, acrylate or dimethylaminoethyl methacrylate, or diisobutylarπinoethyl acrylate or methacrylate ; and / or an amide function such as acrylamides such as N-methylacrylamide, N, N-dimethylacrylamide, hydroxymethylacrylamide or N-vinylpyrrolidone; and / or a nitrile function such as acrylonitrile or methacrylonitrile; and / or a hydroxyl function such as hydroxyethyl methacrylate or hydroxypropyl methacrylate; or an epoxy function such as glycidyl methacrylate.

The invention will be better understood on reading the description which follows, given by way of example, and with reference to the copolymer described above concerning the subject of the invention.

The metal part to be protected against corrosion can for example be a sheet or wire, a piece of bare or already coated steel, in particular zinc-plated or aluminized. By extension, it may be a non-metallic part having a metallic surface.

Before depositing the film of said copolymer on the metal part, the metal part is prepared as necessary to have a surface sufficiently clean: degreasing and / or stripping can therefore be carried out.

Conversely, for certain metallic parts already treated, in particular by phosphating and / or chromating, it is not necessarily necessary to carry out a preliminary treatment of the surface.

In order to protect against corrosion and / or prepare the metal part for shaping, according to the invention, a film of the copolymer is deposited on the surface of the metal part: a process known per se is used. even for depositing a film, for example a process in which an application solution containing said copolymer is applied to said surface, then the applied solution is dried.

The operating conditions of said deposition process are determined in a manner known per se, as well as, if necessary, the composition of the solution, so that the film obtained has a thickness of less than 5 μm and covers d '' homogeneously the surface of the metal part to be protected.

Preferably, the thickness of the coating copolymer film remains greater than 0.5 μm, and, better still, is between 1 and 2.5 μm.

In the case of the use of a solution of the copolymer in the film deposition process, this solution is preferably prepared from a solvent for the copolymer; this solvent is for example from the ketone family, such as acetone, methyl ethyl ketone, butanone or cyclohexanone; or from the ester family, such as ethyl acetate or butyl acetate; or from the family of chlorinated alkanes such as dichloromethane, 1, 1-dichloroethane, 1, 2-dichloroethane or 1, 1, 1 -trichloroethane; or from the family of alcohols such as methanol, ethanol, propanol, isopropanol, butanol, ethylene glycol or glycerol; or an alcohol / water mixture such as a methanol / water, ethanol / water or isopropanol / water mixture; or alternatively an aromatic solvent such as toluene or xylene. For this application solution, the solvent which has the lowest surface tension is preferably retained in order to obtain a film of very uniform thickness on the part to be protected.

Advantageously, the mass proportion of copolymer in the solution is between 0.2% and 10%, preferably between 1% to 6%. Still in the case of the use of a copolymer solution in the film deposition process, the copolymer solution is applied to the metal part in a manner known per se, for example by spraying spray, dip, rotate, brush, brush or roll, or strip lacquer.

After applying the copolymer solution to the part, the metal part is dried, preferably at a temperature between 50 ° C. and 150 ° C., chosen in particular according to the nature of the application solvent used (because the drying has to remove the application solvent).

This temperature is for example of the order of 80 ° C. with toluene, ethanol and 1, 2-dichloroethane.

A metal part is obtained coated with a copolymer film according to the invention, which has a "dry" appearance, that is to say in particular neither greasy nor wet, and which is not crosslinked.

This copolymer film is not crosslinked because no crosslinking means are used at any stage of the process: the application solution does not contain any crosslinking initiator and, after application of the application solution and drying, the treated surface is not subjected to any crosslinking radiation or any heat treatment.

The uncrosslinked nature of the copolymer film is controlled by conventional methods, for example:

- by a destructive method: the glass transition temperature (T _g ,) of the film deposited is measured, then suitable crosslinking means are applied to the film (for example, heat treatment) and the glass transition temperature is measured a second time (T _g2 ) of the crosslinked film; if T _g2 ≠ T _g] , the deposited film was not crosslinked.

- by a method based on the measurement of the swelling index in a suitable solvent: a high swelling index indicates the absence of crosslinking.

Advantageously, the copolymer film provides insensitivity for marking the surface with fingerprints.

Thanks to the invention, in particular to the block or grafted nature of the copolymer used, to the different polarities of certain monomer units of the blocks which compose it, and to the proportion of said blocks and / or of said units, the copolymer film exhibits strong adhesion to the metal part, and, despite its small thickness, offers high resistance to corrosion and significantly facilitates possible subsequent shaping operations.

It is in particular the polar nature of said at least one monomer unit of the polar sequence (ii), which gives strong adhesion of the copolymer film to the metal part. The copolymer film creates a barrier limiting the diffusion of water and ionic species which can cause corrosion of the metal.

The shaping operations are facilitated by reducing the coefficient of friction on the tools used for this shaping, thanks in part to the adhesion of the film to the metal part and on the other hand to the compatibility of the copolymer with the lubricating oils normally used for these operations.

It is in particular the apolar nature of said apolar sequence (i) which gives good compatibility of the copolymer with lubricating oils.

Thus, the copolymer prepares the lubrication of the metal part by giving its surface advantageous tribological properties in the field of forming metal parts by cold plastic deformation and more particularly in the field of stamping. These advantageous tribological properties make it possible to increase the service life of the shaping tools, to obtain stamped parts of better quality, in particular of better surface condition, to produce metal parts of complex geometry or even to increase the stamped depth. This advantage is particularly advantageous when encountering operating drawing limits linked to the sheet / tool interface, such as the drawing speed, the shape of the sheet blank and the pressure of the blank holder.

Finally, the advantage of a thin coating according to the invention is that, for certain subsequent operations to be carried out on the metal part, for example painting or enamelling, it is not even necessary to remove the copolymer film , which avoids a degreasing operation.

Conversely, when it is nevertheless desired to remove the copolymer film before an operation to be carried out on the metal part, it is found that the copolymer film is easily removable, in particular because it is not crosslinked.

For degreasing (elimination of the film), it is possible in particular to use the same solvent as that which served for its application.

Thanks to the invention, there are criteria which make it possible to choose the copolymers which are at the same time the most economical, the easiest to apply in coating of metal part and the most effective in thin coating thickness, while remaining compatible with any subsequent treatment products for the metal part, such as lubricating oils for shaping or surface preparation products for painting or enamelling.

According to the invention, these selection criteria relate in particular to the block or grafted nature of the copolymer, as opposed to the statistical character, and the solubility parameters of the monomer units of the so-called "apolar" block (i) as well as that of said monomer unit of the block (ii) called "polar".

The coating of a metal part with a copolymer film according to the invention can be carried out continuously, in particular in the case of steel sheets, for example at the end of rolling.

The following examples illustrate the invention.

Standard tests are described in the preamble which make it possible to evaluate the corrosion resistance (test 1 and test 2) and the coefficient of friction (test 3) of a treated metal surface. Corrosion resistance test 1, by electrochemical impedance measurements.

Electrochemical impedance measurements are carried out in a conventional manner on metal test pieces of dimensions 100 × 100 mm ² coated with copolymer according to the invention. The copolymer, applied in the form of a film with a thickness of between 1 and 2.5 mμ, is obtained by dipping metal test pieces in an organic solution containing between 1% and 6% of copolymer, followed by evaporation of the solvent.

The electrochemical equipment used includes a potentiostat / galvanostat of type EG&G Princeton Applied Research model 273, a frequency analyzer of type Schiumberger Solartron SI 1255 and an electrochemical cell made up of 3 electrodes: a reference electrode of saturated mercuric sulfate (Hg / HgSO ₄ ), a platinum counter electrode and a working electrode constituted by the metal test piece to be tested, the exposed surface of which is 7 cm ² . A COMPAQ 386SX microcomputer equipped with M388 (Electrochemical Impedance Software System version 2.90) and M398 (Electrochemical Impedance Software 1.01) software marketed by EG&G Princeton Applied Research allows you to control, acquire and use measurement impedance.

The electrolyte used is an aqueous solution of sodium sulphate Na ₂ SO ₄ at a concentration of 0.05 mol per liter, not deaerated and not agitated.

Impedance measurements are performed at the corrosion-free potential, in galvanostatic mode (zero impressed current), after 30 minutes immersion in the electrolyte. The amplitude of the sinusoidal disturbance imposed on the system is 10 mV. The frequency sweep extends from 10 ⁵ Hz to 10 ^-2 Hz at the rate of 5 points per decade.

In order to compare the corrosion resistance of the coating according to the invention, with that of the temporary protection oils commonly used, identical metal specimens are prepared, but coated on the one hand with PREVOX 6767 oil marketed by HENKEL and on the other hand share of AQUA 2 oil marketed by CASTROL. These oils are applied by spraying and the amount of oil deposited on the metal substrate is 0.5 g / m ² .

An uncoated metal specimen serves as a witness. Corrosion resistance test 2, or atmospheric exposure test.

The atmospheric exposure tests are carried out on metal test pieces of dimensions 100 × 100 mm ² coated with copolymer according to the invention. The conditions of application of the copolymer are identical to those described in test 1.

For the same purpose of comparison as in test 1, oiled test pieces are prepared in the same way as in test 1 and an uncoated control metal test piece. According to the procedure of test 2, the metal test pieces are exposed to the atmosphere of a storage hall, which offers particularly severe climatic conditions, namely a temperature of the order of 30 ° C and a relative humidity close to 70 %.

The evaluation of the degree of corrosion as a function of the duration of exposure is carried out visually by applying a rating scale ranging from 1 to

5, the rating 1 corresponding to the appearance of the first punctures and the rating 5 corresponding to generalized corrosion of the metal specimen.

Friction measurement test 3. This test makes it possible to highlight the influence of the coating based on copolymer according to the invention on the coefficient of friction of the metallic substrate, in the case of a plane-plane friction.

The friction measurements are carried out on metal test pieces of dimensions 400 × 50 mm ² coated with copolymer according to the invention. The conditions of application of the copolymer are identical to those described in tests 1 and 2.

In order to compare the effectiveness of the copolymer-based coating with that of other known treatments, in terms of reducing the coefficient of friction, identical metallic test pieces are prepared which are treated with a conventional friction additive. A 1% by mass aqueous solution of potassium phosphate K ₃ P0 _{4 is} thus applied by spraying to these test pieces. An uncoated and untreated control metal specimen is also prepared.

To carry out the friction test, all the test specimens are oiled in the same way, with an oil sold by QUAKER under the reference 8021, the lubricating performance of which is deemed to be average among all the oils used in friction.

The determination of the coefficient of friction as a function of the normal clamping force F _s is carried out using a conventional plane-plane tribometer. The normal clamping force varies from 200 daN to 2000 daN and the traction speed is fixed at 10 mm / s. We will now describe examples of implementation of the invention with reference to the following figures:

- Figures 1, 3, 5 and 7 each give, in a so-called Niquyst representation, three diagrams for measuring impedances according to the procedure of test 1; the 3 diagrams A, B, C, correspond successively to a control specimen (not coated) and to treated or coated specimens; according to the representation of Niquyst, the impedances are plotted on the abscissa for the real part (marked with "Zre") and on the ordinate for the imaginary part (marked with "Zim"); the units are kΩ. cm ² .

- Figures 2, 4 and 6 show in three separate diagrams the coefficient of friction (indicated by the symbol μ) of test pieces A, B, C as a function of the normal tightening force, according to the procedure of test 3; test pieces A, B, C successively designate a control test piece (uncoated) and treated or coated test pieces; the normal tightening force, F _s , on the abscissa, is expressed in decaNewton (daN). Example 1.

Low carbon steel sheet steel samples leaving hot rolling are used, the chemical composition of which is given in Table I below.

To prepare the coating according to the invention, metal test specimens are treated in an acid medium, hydrochloric for example, in order to remove the chemical surface impurities which are detrimental to the subsequent application of the coating; then rinsed several times with water to remove traces of acid. Table I: chemical composition of steel.

Element Mass content

Al 0.034%

C 0.054%

Cr 0.015%

Cu 0.006%

Mn 0.291%

N 21 ppm

Ni 0.019%

P 0.009%

S 0.011%

If 0.013%

A bisequenced copolymer consisting of a so-called apolar block (i) with butadiene monomer units and a so-called polar block (ii) with 4-vinylpyridine monomer units is synthesized; for the synthesis, the procedure is carried out, for example, anionically according to the procedure described by R.l. Stankovic, R.W. Lenz and F.E. Karasz, in the European Polymer Journal, 26, 359 (1990). This copolymer is characterized by gel permeation chromatography and by NMR (nuclear magnetic resonance) of the proton.

To calculate or evaluate the solubility parameter of the monomer units, the Hoftyzer-Van Krevelen method described in

"Properties of polymers. Their correlation with chemical structure; their numerical estimation and prediction from additive group contributions." by D.W.

Van Krevelen (Third Edition, Elsevier 1990).

A copolymer is obtained, hereinafter called X, which, according to the invention, is block and has the following characteristics: 1 / Apolar block (i): - average molecular mass: 11000 g / mol.

- mass proportion in the copolymer: 73%.

- Solubility parameter of the butadiene monomer unit: 17.5 J ^1/2 .cnr ^3/2 , therefore less than 19.1 according to the invention.

2 / Polar sequence (ii): - average molecular mass: 4000 g / mol.

- mass proportion of the sequence (ii): 27%. - Solubility parameter of the 4-vinylpyridine monomer unit: 19.4 J ^1/2 .cnr ^3/2 , therefore greater than 19.1 according to the invention.

A 5% solution by mass of the copolymer is then prepared in toluene, as solvent. The application of the copolymer is carried out by dipping metal test pieces in the solution.

The copolymer film formed on the surface of the metal test pieces after evaporation of the solvent is dried with hot air at a temperature of the order of 80 ° C., in order to remove all traces of solvent. The conditions for applying the solution are adapted in a manner known per se to obtain, after drying, test specimens coated with approximately 1.5 g / m2 of copolymer, corresponding to a film approximately 1.5 μ thick. , or less than 5 μ according to the invention.

The corrosion resistance of the test pieces thus coated with product X is then measured, with reference to the procedure of test 1 which provides for a comparison with oiled test pieces and an untreated and uncoated control test piece.

FIG. 1 shows the impedance diagrams of a control specimen (curve A), of a specimen treated with AQUA 2 oil (curve B) and of a specimen coated according to the invention with product X (curve C ).

Table II combines the values of the electrical resistances determined from these electrochemical impedance diagrams, by adding the result obtained on a test specimen treated with PREVOX 6767 oil. Table II: results of the electrochemical impedance measurements.

Product Total electrical resistance (Ω.cm ² )

Witness 7.0.10 ²

PREVOX 6767 5.0.10 ³

AQUA 2 7.0.10 ³

X 1.3.10 ⁴

Electrochemical impedance measurements therefore illustrate the superiority of copolymer X, even in film with a thickness of less than 5 μ, compared to conventional oils for temporary protection against corrosion of steel parts.

Table III groups the results of the atmospheric exposure tests, referring to the procedure of test 2. Table III: results of atmospheric exposure tests.

Product Corrosion resistance in days

Witness 7

PREVOX 6767 30

AQUA 2 45

X 80

The metal specimens coated with product X have a corrosion resistance of at least 80 days, while in the case of conventional temporary protection oils, the corrosion resistance does not exceed 45 days.

The friction measurements are then carried out according to the procedure of test 3. FIG. 2 illustrates the variation of the plane-plane friction coefficient as a function of the normal clamping force for a control specimen (FIG. 2- A), for a treated specimen with a potassium phosphate solution (figure 2-B), and for a test tube coated with product X according to the invention (figure 2-C). Table IV summarizes the values of the coefficients of average friction

(μ _medium ), maximum (μ _max ) and minimum (μ _m j _n ) corresponding to Figures 2-A to 2- C.

Table IV: plane-plane friction coefficients, u

It is therefore found that the combined effect of the coating of product X and of a conventional stamping oil (cf. procedure of test 3) results in a significant reduction in the coefficient of friction, at a level much lower than that which obtained with the same oil alone and without prior coating, at a level even lower than that obtained with the same oil on a test tube treated with a phosphate solution.

As an anti-friction product, product X thus exhibits superior performance to a conventional product based on alkaline phosphate. In conclusion, the copolymer film according to the invention therefore HERE provides both an improvement in corrosion resistance and a reduction in the coefficient of friction, compared to other known treatments, such as oiling for resistance to corrosion and treatment with phosphates to reduce the coefficient of friction

These advantages result in part from the monomeric units of 4-vinylpyridine of the polar block (il) of the copolymer which confer a strong adhesion of the copolymer film to the metal part. Example 2 This example illustrates the influence of the chemical composition of the copolymer described in example 1 on the resistance to corrosion and on the reduction of the coefficient of friction

Three block copolymers of different compositions are synthesized comprising an apolar block (i) with butadiene monomer unit and a polar block (M) with 4-vιnylpyπdιne monomer unit, these copolymers are applied as before to identical steel test pieces, and we test then these coated test pieces according to the same procedures (tests 1 to 3) as in Example 1

The characteristics of the copolymers obtained, called successively X1, X2, X3, namely the average molecular weights (M _n ) of the sequences (i) and (n) and the mass percentage of the sequence (n) in the copolymer, are given in table V, the copolymer X1 corresponds to the copolymer X of example 1

Table V molecular characteristics of the copolymers X1 to X3

Copolymer M _n sequence (i) M _n sequence (ii)% by mass (g / mol) (g / mol) sequence (n)

X1 11,000 4,000 27 X2 10,000 7,500 43 X3 4,500 11,000 71

Table VI below groups together the values of the electrical resistances determined as in Example 1 from the electrochemical impedance diagrams of test 1, in the case of a control test piece (uncoated), and test pieces coated with products. X1, X2 and X3 Table VI: comparative results of the electrochemical impedance measurements.

Product Total electrical resistance (Ω.cm ² )

Witness 7.0.10 ² X1 1, 3.10 ⁴ X2 9.0.10 ³ X3 1.6.10 ³

The results of Table VI show that the corrosion resistance provided by the copolymer on the metal specimen, evaluated using the total electrical resistance of the surface, decreases when the mass proportion in sequence (ii) increases in the copolymer .

Table VII groups together the results of the atmospheric exposure tests (test 2) for the same test pieces.

Table VII: comparative results of atmospheric exposure tests.

Product Corrosion resistance in days

Witness 7 X1 80 X2 65 X3 25

The results of the atmospheric exposure tests confirm the results of the electrochemical impedance measurements, namely that the effectiveness of the copolymer, in terms of resistance to corrosion imparted to the metal substrate, decreases when the mass content in sequence (ii) increases in the copolymer.

The results of the average coefficient of friction measurements (μ _m0 yen) measured in plane-plane friction (test 3) are collated in Table VIII. Table VIII: comparative results of the plane-plane friction measurements.

Μmriyfm product

Control 0.16 X1 0.05 X2 0.07 X3 0.10 The results of the plane-plane friction measurements show that the efficiency of the copolymer, in terms of reducing the coefficient of friction, decreases when the content of block (ii) increases.

In conclusion, among the various chemical compositions tested, the copolymer having the lowest mass proportion of polar block (ii) (here 27%), and / or having the lowest mass proportion (here 27%) of monomer units having a solubility parameter greater than 19 J ^1/2 .crτr ^3/2 , appears to be the most effective for improving both the corrosion resistance and the tribological properties of a metal part. We note however that when the proportion of polar sequence

(ii) exceeds 43% in the copolymer, the tribological and anti-corrosion properties no longer substantially exceed those obtained with conventional treatments, of the potassium phosphate or protective oil type; however, the advantage of a bi-functional product (both anti-friction and anti-corrosion) and a very low coating thickness is retained. Example 3.

The same steel sheet test pieces are used and they are prepared in the same way as in Example 1.

A bisequenced copolymer is synthesized consisting of a so-called apolar block (i) with styrene monomer units and a so-called polar block (ii) with 4-vinylρyridine monomer units; for the synthesis, the procedure is carried out, for example, anionically according to the procedure described in the article by K. Ishizu, Y. Kashi, T. Fukutomi and T. Kakurai of the journal Die Makromolekuiàre Chemie, 183, 3099 (1982). This copolymer is characterized as in Example 1.

A copolymer is obtained, hereinafter called Y, which, according to the invention, is block and has the following characteristics: 1 / Apolar block (i):

- average molecular mass: 16000 g / mol. - mass proportion in the copolymer: 60%.

- Solubility parameter of the styrene monomer unit: 18.7 J ^1/2 .cnr ^3/2 , therefore less than 19.1 according to the invention.

2 / Polar sequence (ii):

- average molecular mass: 10500 g / mol. - mass proportion of the sequence (ii): 40%.

- Solubility parameter of the 4-vinylpyridine monomer unit: 19.4 J ^1/2 .cnr ^3/2 , therefore greater than 19.1 according to the invention. A 3% by mass solution of the copolymer in toluene, as solvent, is then prepared and this solution is applied to the test pieces as in Example 1.

The conditions for applying the solution are adapted in a manner known per se to obtain, after drying, test specimens coated with approximately 1 g / m2 of copolymer Y, corresponding to a film approximately 1 μ thick, ie less at 5 μ according to the invention.

The corrosion resistance of the test pieces thus coated with product Y is then measured, with reference to the procedure of test 1.

FIG. 3 shows the impedance diagrams of a control specimen (curve A), of a specimen treated with AQUA 2 oil (curve B) and of a specimen coated according to the invention with product Y (curve C ).

Table IX groups the values of the electrical resistances determined from these electrochemical impedance diagrams, by adding the result obtained on a test specimen treated with PREVOX 6767 oil.

Table IX: results of the electrochemical impedance measurements.

Product Total electrical resistance (Ω.cm ² )

Witness 7.0.10 ²

PREVOX 6767 5.0.10 ³

AQUA 2 7.0.10 ³

Y 1.3.10 ⁴

Electrochemical impedance measurements therefore illustrate the superiority of copolymer Y, even in films with a thickness of less than 5 μ, compared to conventional oils for temporary protection against corrosion of steel parts.

Table X shows the results of test 2.

Table X: results of atmospheric exposure tests.

Product Corrosion resistance in days

Witness 7

PREVOX 6767 30

AQUA 2 45

Y 90 The metal specimens coated with product Y have a corrosion resistance of at least 90 days, while in the case of temporary protection oils, the corrosion resistance does not exceed 45 days.

The friction measurements are then carried out according to the procedure of test 3.

FIG. 4 illustrates the variation of the plane-plane coefficient of strength as a function of the normal clamping force for a control specimen (FIG. 4A), for a specimen treated with a potassium phosphate solution (FIG. 4-B), and for a test tube coated with product Y according to the invention (FIG. 4-C).

Table XI summarizes the values of the mean (μrηoyeπ), maximum (μ _max ) and minimum (μ _m j _n ) friction coefficients corresponding to Figures 4-A to 4- C.

Table XI: results of the plane-plane friction tests.

From the copolymer Y, one arrives at conclusions identical to those of Example 1.

Example 4 By analogy with Example 2, this example illustrates the influence of the chemical composition of the copolymer described in Example 3 on the resistance to corrosion and on the reduction of the friction coefficient.

Three block copolymers of different compositions are synthesized comprising an apolar block (i) with styrene monomer unit and a polar block (ii) with 4-vinylpyridine monomer unit), these copolymers are applied as in Example 3 on steel test pieces identical, and then these coated test pieces are tested according to the same procedures (tests 1 to

3) as in Example 3.

The characteristics of the copolymers obtained, successively called Y1, Y2, Y3, namely the average molecular weights (M _n ) of the blocks (i) and (ii) and the mass percentage of the block (ii) in the copolymer, are given in Table XII; the copolymer Y1 corresponds to the copolymer Y of Example 3. Table XII: molecular characteristics of the copolymers Y1 to Y3.

Copolymer M _n sequence (i) M _n sequence (ii)% by mass.

(g / mol) (g / mol) sequence (ii)

Y1 16000 10500 40

Y2 16,000 18,000 53

Y3 11000 19500 64

Table XIII groups together the values of the electrical resistances determined as in Example 3 from the electrochemical impedance diagrams of test 1, in the case of a control test piece (uncoated), and test pieces coated with products Y1, Y2 and Y3. Table XIII: comparative results of the electrochemical impedance measurements.

Product Total electrical resistance (Ω.cm ² )

Control 7.0.10 ² Y1 1.3.10 ⁴ Y2 1, 1.10 * Y3 9.0.10 ³

As in Example 2, the results in Table XIII show that the corrosion resistance provided by the copolymer film on the metal specimen, evaluated using the total electrical resistance of the surface, decreases when the mass proportion in sequence (ii) increases in the copolymer.

Table XIV groups together the results of the atmospheric exposure tests (test 2) for the same test pieces.

Table XIV: comparative results of atmospheric exposure tests.

Product Corrosion resistance in days

Witness 7 Y1 90 Y2 80 Y3 60 The results of the atmospheric exposure tests therefore also confirm the results of the electrochemical impedance measurements, namely that the efficiency of the copolymer, in terms of resistance to corrosion imparted to the metallic substrate, decreases when the mass content in sequence (ii ) increases in the copolymer.

The results of the measurements of average coefficients of friction ( ^m average) measured in plane-plane friction (test 3) are grouped in Table XV.

Table XV: comparative results of the plane-plane friction measurements.

Medium product

Witness 0.16

Y1 0.10

Y2 0.11

Y3 0.13

The results of the plane-plane friction measurements show that the efficiency of the copolymer, in terms of reducing the coefficient of friction, decreases when the content of block (ii) increases. The conclusion of Example 2 is therefore confirmed: the copolymer having the lowest mass proportion of polar block (ii) (here 40%), and / or having the lowest mass proportion (here 40%) of monomer units having a solubility parameter greater than 19.1 J ^1/2 .cπr ^3/2 , appears to be the most effective for improving the corrosion resistance and the tribological properties of a metal part.

However, it can be seen that when the proportion of polar block (ii) exceeds 40% in the copolymer, the tribological properties no longer substantially exceed those obtained with potassium phosphate treatments; an advantage is nevertheless retained for the anti-corrosion properties, even when this proportion of polar block (ii) reaches 64%. Example 5.

The same steel sheet test pieces are used and they are prepared in the same way as in Example 1. A bisequenced copolymer is synthesized consisting of a so-called apolar block (i) with tertiary butyl acrylate monomer units and a so-called polar block (ii) with 2-vinylpyridine monomer units; for the synthesis, we proceed for example by anionic route in accordance with the procedure described in the article by JW Klein, JP Lamps, Y. Gnanou and P. Rempp of the review Polymer, 32 (12), 2278 (1991).

This copolymer is characterized as in Example 1. A copolymer is obtained, hereinafter called Z, which, according to the invention, is block and has the following characteristics: 1 / Apolar block (i):

- average molecular mass: 58,500 g / mol.

- mass proportion in the copolymer: 74%. - Solubility parameter of the tertiary butyl acrylate monomer unit: 18.8 J ^1/2 .crτr ^3/2 , therefore less than 19.1 according to the invention. 2 / Polar sequence (ii):

- average molecular mass: 20,500 g / mol.

- mass proportion of the sequence (ii): 26%. - Solubility parameter of the 2-vinylpyridine monomer unit: 19.4 J ^1/2 .crτr ^3/2 , therefore greater than 19.1 according to the invention.

A solution at 3% by mass of the copolymer is then prepared in ethanol, as solvent, which is applied as in Example 1.

The conditions of application of the solution provide test pieces coated with approximately 1 g / m2 of copolymer Z, corresponding to a film of approximately 1 μ in thickness, ie less than 5 μ according to the invention.

The corrosion resistance of the test pieces thus coated with product Z is then measured, with reference to the procedure of test 1; FIG. 5 shows the impedance diagrams of a control specimen (curve A), of a specimen treated with AQUA 2 oil (curve B) and of a specimen coated according to the invention with product Z (curve C ).

Table XVI groups together the values of the electrical resistances determined from these electrochemical impedance diagrams, by adding the result obtained on a test specimen treated with PREVOX 6767 oil. Table XVI: results of the electrochemical impedance measurements.

Product Total electrical resistance (Ω.cm ² )

Witness 7.0.10 ²

PREVOX 6767 5.0.10 ³

AQUA 2 7.0.10 ³

Z 1, 0.10 ⁴ Electrochemical impedance measurements also illustrate the superiority of copolymer Z, even in films with a thickness of less than 5 μ, compared to conventional oils for temporary protection against corrosion.

Table XVII groups the results of the atmospheric exposure tests (test 2).

Table XVII: results of atmospheric exposure tests.

Product Corrosion resistance in days

Witness 7

PREVOX 6767 30

AQUA 2 45

Z 60

Metal specimens coated with product Z have a corrosion resistance of at least 60 days, while in the case of temporary protection oils, the corrosion resistance does not exceed 45 days.

Figure 6 illustrates the variation of the plane-plane strength coefficient as a function of the normal clamping force for a control specimen (Figure 6-

A), for a test tube treated with a potassium phosphate solution

(Figure 6-B), and for a test tube coated with product Z according to the invention

(figure 6-C).

Table XVIII summarizes the values of the average friction coefficients (μ _moye n), maximum (μ χ _my) ^and minimum (μ _m j _n) corresponding to Figures 6 A to 6-C.

Table XVIII: results of the plane-plane friction tests.

As in Example 1, it can be seen that the combined effect of the coating of product Z and of a conventional stamping oil (cf. procedure of test 3) results in a significant reduction in the coefficient of friction, compared to that obtained with an oil alone or combined with a phosphate treatment.

Example 6.

By analogy with Examples 2 and 4, this example illustrates the influence of the chemical composition of the copolymer described in Example 5 on the resistance to corrosion and on the reduction of the friction coefficient.

Two block copolymers of different compositions are synthesized comprising an apolar block (i) with a tert-butyl acrylate monomer unit and a polar block (ii) with a 2-vinylpyridine monomer unit, these copolymers are applied as in Example 5 to identical steel, and these coated test pieces are then tested according to the same procedures (tests 1 to 3) as in Example 5.

The characteristics of the copolymers obtained, called Z1 and Z2, namely the average molecular weights (M _n ) of the blocks (i) and (ii) and the mass percentage of the block (ii) in the copolymer, are given in Table XIX below; the copolymer Z1 corresponds to the copolymer Z of example 5.

Table XX groups together the values of the electrical resistances determined as in Example 5 from the electrochemical impedance diagrams of test 1, in the case of a control specimen (uncoated), and of the specimens coated with products Z1 and Z2 .

Table XIX: molecular characteristics of the Z1 and Z2 copolymers

Table XX: comparative results of electrochemical impedance measurements.

Product Total electrical resistance (Ω.cm ² )

Control 7.0.10 ² Z1 1.0.10 ⁴ Z2 2.0.10 ³ As in Examples 2 and 4, the results in Table XX show that the corrosion resistance provided by the copolymer film on the metal specimen, evaluated using the total electrical resistance of the surface, decreases when the proportion mass in sequence (ii) increases in the copolymer.

Table XXI combines the results of the atmospheric exposure tests (test 2) which confirm this conclusion.

Table XXI: comparative results of atmospheric exposure tests.

Product Corrosion resistance in days

Witness 7 Z1 60 Z2 30

Table XXII: comparative results of the plane-plane friction measurements.

Medium Product

Witness 0.16

Z1 0.10

Z2 0.12

The results of the measurements of mean friction coefficients (μ _mean ) measured in plane-plane friction (test 3) are collated in table XXII above.

The results of the plane-plane friction measurements show that the efficiency of the copolymer, in terms of reducing the coefficient of friction, decreases slightly when the proportion of the block (ii) is increased.

It can be seen here that, when the proportion of the polar block (ii) exceeds 26% in the copolymer, the tribological and anti-corrosion properties no longer substantially exceed those obtained with conventional treatments, of the potassium phosphate or oil type. protection ; nevertheless, the advantage of a bi-functional product (both anti-friction and anti-corrosion) and the advantages linked to a very low coating thickness are retained.

The conclusions of Examples 2 and 4 are confirmed: the most efficient copolymer is that which has a low mass proportion in polar sequence (ii) (26% to 40% according to Examples 2, 4 and 6), and / or a low mass proportion of nonionic monomer units having a solubility parameter greater than 19.1 J ^1/2 .cπr ^3/2 (here the same values 26% to 40% for the same examples). Example 7. The purpose of this example is to illustrate the invention in the case of a grafted and not block copolymer.

This example relates to grafted (and not block) copolymers associating siloxane monomer units (as units of apolar sequence (i)) with acrylic or vinyl monomer units (as units of polar sequence (ii)).

A graft copolymer consists of a trunk and grafts, and although the term block is reserved for block polymers, it has been used here by extension for graft copolymers and here denotes a trunk or grafts. A graft copolymer is synthesized comprising several apolar sequences (i) with monomer units of the dimethylsiloxane type and a polar block (ii) consisting of monomer units of the methyl methacrylate and dimethylaminoethyl methacrylate type; for the synthesis, one proceeds by radical route according to a procedure analogous to that described by Y. Kawakami, R.A.N. Murthy and Y. Yamashita in the article Die Makromolekulâre Chemie, 185, 9 (1984).

A copolymer is obtained, hereinafter called G, which, according to the invention, is grafted and has the following characteristics: 1 / Apolar sequences (i): - average molecular mass: 30,000 g / mol.

- mass proportion in the copolymer: 40%.

- solubility parameter of the dimethylsiloxane monomer unit: between 14.9 and 15.6 J ^1/2 .crτr ^3/2 , therefore less than 19, 1 according to the invention, these values of the solubility parameter are here evaluated experimentally and appear in the work already cited in Example 1 by DW Van Krevelen (Third Edition, Elsevier 1990). 2 / Polar sequence (ii):

- average molecular mass: 105,000 g / mol.

- mass proportion of the sequence (ii): 60% .... distributed in: - 83% by mass of methyl methacrylate units;

- And 17% by mass of dimethylaminoethyl methacrylate units.

- solubility parameter of the dimethylaminoethyl methacrylate unit: 20.9

J1 / 2.cm-3/2 According to the invention, there is therefore 60% × 17%, or 10.2% of monomer units in the copolymer having a solubility parameter greater than 19.1, ie here 20.9; this proportion is much higher than 5% according to the invention. A 3% solution by mass of the copolymer is then prepared in

1, 2-dichloroethane, as solvent and this solution is applied to test tubes as in Example 1.

According to the conditions of application of the solution, after drying, test pieces coated with approximately 1 g / m2 of copolymer G are obtained, corresponding to a film approximately 1 μ thick, ie less than 5 μ according to the invention .

The resistance to corrosion of the test specimens thus coated with product G is then measured, with reference to the procedure of test 1.

FIG. 7 shows the impedance diagrams of a control specimen (curve A), of a specimen treated with AQUA 2 oil (curve B) and of a specimen coated according to the invention with product G.

Table XXIII groups together the values of the electrical resistances determined from these electrochemical impedance diagrams, by adding the result obtained on a test specimen treated with PREVOX 6767 oil. Table XXIII: results of the electrochemical impedance measurements.

Product Total electrical resistance (Ω.cm ² )

Witness 7.0.10 ²

PREVOX 6767 5.0.10 ³

AQUA 2 7.0.10 ³

G 1, 1.10 ⁴

Table XXIV then groups the results of the atmospheric exposure tests. Table XXIV: results of atmospheric exposure tests.

Product Corrosion resistance in days

Witness 7

PREVOX 6767 30

AQUA 2 45

G 130 Metal substrates coated with product G have a corrosion resistance of at least 130 days, while in the case of temporary protection oils, the corrosion resistance does not exceed 45 days.

The organic coating described above gives the metal substrate a corrosion resistance superior to that of common temporary protection oils. The metal substrates can therefore be stored in the open air without undergoing alteration.

FIG. 8 illustrates the variation of the plane-plane coefficient of strength as a function of the normal clamping force for a control specimen (FIG. 8-A), for a specimen treated with a potassium phosphate solution (FIG. 8-B), and for a test tube coated with product G according to the invention (FIG. 8-C).

Table XXV summarizes the values of the average friction coefficients (μ _m0 yen). maximum (μ _ma χ) ^and minimum (μ _m in) corresponding to Figures 8- A to 8-C.

Table XXV: results of the plane-plane friction tests.

As in the previous examples of block copolymers, it can be seen that the combined effect of the coating of product G and of a conventional stamping oil (cf. procedure of test 3) results in a significant reduction in the coefficient of friction, compared to that obtained with an oil alone or combined with a potassium phosphate treatment.

Claims

1 - Method for treating the metal surface of a part, in particular a steel sheet, intended to protect it against corrosion and / or to prepare its lubrication for a shaping operation, characterized in that it consists in applying to said part a non-crosslinked film of block or graft copolymer with a thickness of between 0.5 and 5 μm, said copolymer comprising at least one block called "apolar" (i) and at least one block called "polar""(ιι), said apolar sequence (i) consisting of one or more monomer units whose solubility parameters are all less than or equal to 19.1 J ^1/2 crrr ^3/2 , and said polar sequence (n) comprising at least one nonionic monomer unit having a solubility parameter greater than 19.1 J ^1/2 .crτr ^3/2 in mass proportion of at least 5% in said copolymer, excluding block copolymers associating siloxane monomer units (as motive s of apolar sequence (i)) with acrylic or vinyl monomer units (as units of polar sequence (n))

2 - Process according to claim 1, characterized in that the mass proportion of said polar block (ii) in said copolymer is between 5% and 95%, preferably between 20% and 80%

3 - Method according to any one of the preceding claims, characterized in that the mass proportion of said at least one nonionic monomer unit of the polar block in the copolymer is between 5% and 95%, preferably between 5% and 50 %

4 - Process according to any one of the preceding claims, characterized in that said monomer units of the apolar sequence (i) are chosen from monomers of diene type such as butadiene, isoprene or pentadiene, of olefinic type such as 'ethylene, propylene or butylene, of chlorofluoroethylene type such as tetrafluoroethylene or chlorotπfluoroethylene; of vinyl type such as vinyl laurylate or vinyl stearate, of styrene type such as styrene, α-methylstyrene or styrene substituted by one or more alkyl groups, of acrylic ester type such as n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, sec-butyl acrylate, tert-butyl acrylate, n-hexyl acrylate or lauryl acrylate, of methacrylic ester type such as methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, sec-butyl methacrylate, tert-butyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, methacrylate ethylhexyl or lauryl methacrylate; or of siloxane type such as dimethylsiloxane or diphenylsiloxane.

5.- Method according to any one of the preceding claims, characterized in that said at least one monomer unit of the polar sequence (ii) has a function chosen from an amine function, such as 2-vinylpyridine, 4-vinylpyhdine, dimethylaminoethyl acrylate or methacrylate, or diisobutylaminoethyl acrylate or methacrylate; and / or an amide function such as acrylamides such as N-methylacrylamide, N, N-dimethylacrylamide, hydroxymethylacrylamide or N-vinylpyrrolidone; and / or a nitrile function such as acrylonitrile or methacrylonitrile; and / or a hydroxyl function such as hydroxyethyl methacrylate or hydroxypropyl methacrylate; or an epoxy function such as glycidyl methacrylate.

6. Part, in particular sheet steel, having a metal surface treated by a process according to any one of the preceding claims.

7.- A method of stamping a part with a metal surface, in particular a steel sheet, characterized in that it comprises the following steps: treatment of the metal surface according to any one of claims 1 to 5, then oiling, then stamping proper for shaping.