EP1801262A1 - Treatment by carboxylation of metal surfaces, use of this process as temporary protection against corrosion and manufacturing process of a shaped sheet coated with a carboxylated conversion coating - Google Patents

Abstract

Conversion process comprises carboxylation of a metallic surface e.g. zinc, iron, aluminum, copper, lead and their alloys, galvanized steels, electrozincated, aluminized, coppered, under oxidizing conditions with an aqueous/organic water bath containing a mixture of organic acids comprising 10-18 carbon atoms. Conversion process comprises carboxylation of a metallic surface e.g. zinc, iron, aluminum, copper, lead and their alloys or galvanized, electrozincated, aluminized or coppered steels under oxidizing with an aqueous/organic water bath containing a mixture of organic acids comprising 10-18 carbon atoms in a binary or ternary form, where respective proportions of for the binary acid mixture is x+-5:y+-5 molar ratio, where x and y are two acids in a mixture in the eutectic composition and for a ternary mixture x+-3:y+-3:Z +-3 molar ratio, x, y and z are three acids in a mixture in the eutectic composition in which the concentration of the mixture is >=20 g/l.

Description

The invention relates to a method for the formation of conversion layers on a metal surface selected from zinc, iron, aluminum, copper, lead and their alloys, as well as galvanized, electrozinced, aluminized, coppered steels. , making it possible to produce at high speed conversion layers formed of very small crystals, from 1 to 20 μm.

• l'amélioration des propriétés de frottement sous lubrification en mécanique, par exemple pour l'emboutissage de tôles, sans avoir recours à des huiles minérales polluantes ;
• la protection temporaire contre la corrosion, la couche de conversion pouvant être éliminée facilement lorsqu'elle n'est plus utile.

When applied prior to forming the sheet metal, these metal surface conversion treatments generally have at least one of the following effects:

• the improvement of friction properties under mechanical lubrication, for example for stamping sheets, without the use of polluting mineral oils;
• temporary protection against corrosion, the conversion layer can be easily removed when it is no longer needed.

For this first type of application, it is possible to use treatments that are identical to the treatments that are usually called pre-phosphatation and result in the deposition of a layer of metal phosphate whose weight (weight of layer) is of the order from 1 to 1.5 g / m ² .

These different conversion treatments generally consist of anodic dissolution of the metal elements of the surface, followed by precipitation on this surface of the compounds formed by the reaction of the dissolved metal elements with the species present in the conversion bath. Dissolution requires the creation of oxidizing conditions vis-à-vis the metal surface and usually takes place in an acidic environment. The precipitation of the metal compounds to form the conversion layer requires a sufficiently high concentration and is favored by a medium that is locally less acidic under the effect of the dissolution of the metal. It is the nature and structure of the precipitated compounds on the treated surface that determine the degree of protection against corrosion, improvement of tribological properties and / or adhesion, as well as the other properties of the layer.

To ensure the superficial oxidation of the metal of the surface to be treated and to promote its dissolution, it is possible to proceed chemically or electrochemically, using a chemical agent for oxidation of the metal that is introduced into the treatment solution, and / or by electric polarization of the surface while subjecting it to the action of the treatment solution.

In addition to a possible oxidizing agent, the conversion baths essentially contain anions and cations capable of forming insoluble compounds with the dissolved metal of the surface. The main conversion treatments applied to the steels are thus chromating treatments on galvanized steel (by dip galvanizing or electrogalvanizing) or aluminized, phosphating on unalloyed bare steels or coated steels, or even oxalation on alloy steels such as as stainless steels, for example.

After being brought into contact with a conversion bath, the treated surface is generally rinsed to remove unreacted surface and / or treatment solution components, and this surface is dried, particularly to harden the coating layer. conversion and / or to improve its properties.

The conditions of application, the nature and the concentration of the additives have an important influence on the structure, the morphology and the compactness of the conversion layer obtained, therefore on its properties.

The conversion treatment may itself be preceded by a pretreatment, generally consisting of a preliminary degreasing and rinsing of the surface followed by a so-called refining operation using a pretreatment solution adapted to create and / or promote germination sites on the surface to be treated.

For this purpose, commonly used, as a refining solution on galvanized surfaces, sols or colloidal suspensions of titanium salts which allow the subsequent obtaining of a conversion layer having smaller crystals in a denser layer.

At the end of the conversion process, it is also possible to carry out a post-treatment to improve the properties of the conversion layer. Thus, a chromatin post-treatment can be carried out on a conversion layer obtained by phosphatation.

The various treatments of the prior art, such as chromating, phosphating and oxalation treatments, have a major drawback which is the toxicity of these products vis-à-vis people and the environment in general. In addition, when spot welding plates carrying such conversion layers, it creates fumes toxic fumes.

It was proposed in the document WO-A-02/677324 to use a carboxylation treatment to achieve the conversion of surfaces WO 02 077 324 metal. For this purpose, conversion layers are formed by bringing the surface into contact with an aqueous, organic or hydro-organic bath comprising one or more carboxylic acids in solution or in emulsion at a concentration of at least 0.1 mol / liter, and this in oxidizing conditions vis-à-vis the metal surface. This or these acids are saturated or unsaturated aliphatic monocarboxylic or dicarboxylic acids.

The precise treatments that have been used so far and which used the latter technique have yielded satisfactory results from many points of view, but need to be further improved in some respects.

The best results have heretofore been obtained with the use of a hydro-organic bath, thus containing, in addition to water, an organic co-solvent which it would be desirable to optimally be able to do without, in particular to simplify the manufacture of the treatment solution and improve hygiene and safety in the workshops. Only a mixture comprising water, the organic acid or acids, the optional oxidant and a surfactant would then be retained, this mixture constituting an emulsion.

On the other hand, it has been found on the treatment lines using known carboxylate solutions and emulsions, a phenomenon known as "dusting" which is attributed to the brittleness of the coating soap crystals during the treatment. winding of the sheet metal coils or during contact with the forming tools. This phenomenon results from the significant friction exerted on the metal surface during these operations. Thus, during the shaping of a galvanized sheet, it is covered with a powder consisting of zinc-based particles, generated by the degradation of the coating. The accumulation of these particles in or on the forming tools can cause damage to the formed parts, by formation of pins or strictions. There is also a risk of rupture of the sheet if this degradation of the coating results in insufficient slippage of the sheet in the grip of the shaping tool, even if beforehand a lubricating film is applied to the surface. sheet metal.

Finally, there is still a demand from the users for obtaining a still improved corrosion resistance.

The object of the invention is to propose treatments by carboxylation of metal surfaces, in particular zinc and zinc alloy layers coating the galvanized and electrogalvanized steel sheets, solving better than the existing treatments the problems that we have just described. to quote.

• lesdits acides organiques sont des acides carboxyliques linéaires saturés comportant de 10 à 18 atomes de carbone ;
• ledit mélange est un mélange binaire ou ternaire de tels acides ;
• les proportions respectives de ces acides sont telles que :
  • * pour un mélange binaire x ± 5% - y ± 5%, x et y étant, en pourcentages molaires, les proportions respectives des deux acides dans un mélange à la composition de l'eutectique ;
  • * pour un mélange ternaire x ± 3% - y ± 3% - z ± 3%, x, y et z étant, en pourcentage molaire, les propositions respectives des trois acides dans un mélange à la composition de l'eutectique ;
• la concentration dudit mélange dans ledit bain est supérieure ou égale à 20 g/l.

For this purpose, the subject of the invention is a conversion process by carboxylation of a metal surface chosen from zinc, iron, aluminum, copper, lead and their alloys, galvanized or electrozinced, aluminized steels. , coppered, under oxidizing conditions with respect to the metal, by contact with an aqueous or hydro-organic bath containing a mixture of organic acids, characterized in that:

• said organic acids are saturated linear carboxylic acids having from 10 to 18 carbon atoms;
• said mixture is a binary or ternary mixture of such acids;
• the respective proportions of these acids are such that:
  • * for a binary mixture x ± 5% - y ± 5%, x and y being, in molar percentages, the respective proportions of the two acids in a mixture to the composition of the eutectic;
  • * for a ternary mixture x ± 3% - y ± 3% - z ± 3%, x, y and z being, in molar percentage, the respective proposals of the three acids in a mixture to the composition of the eutectic;
• the concentration of said mixture in said bath is greater than or equal to 20 g / l.

Preferably, for a binary mixture, the respective proportions of the acids are x ± 3% - y ± 3%.

Said oxidizing conditions can be created by the presence in the bath of an oxidizing compound for the metal surface.

Said oxidizing compound may be hydrogen peroxide.

Said oxidizing compound may be sodium perborate.

Said oxidizing conditions can be created by the bath application of an electric current.

The bath may be a hydro-organic bath and contain a co-solvent.

This co-solvent may be chosen from 3-methoxy-3-methylbutan-1-ol, ethanol, n-propanol, dimethylsulfoxide, N-methyl-2-pyrrolidone and 4-hydroxy-4-methyl -2-pentanone, diacetone alcohol.

Said bath may be an aqueous bath and include a surfactant and / or a dispersant.

Said surfactant may be chosen from alkylpolyglycosides, ethoxylated fatty alcohols, ethoxylated fatty acids, ethoxylated oils, ethoxylated nonylphenols and ethoxylated sorbitan esters.

Said dispersant may be chosen from high molecular weight polyols, carboxylic acid salts such as (meth) acrylic copolymers, polyamide derivatives such as polyamide waxes.

Said saturated carboxylic acids may each have an even number of carbon atoms.

Said saturated carboxylic acids may be lauric acid and palmitic acid.

Said metal surface may be a galvanized steel sheet, and the bath may contain an Al ³⁺ complexing ^agent .

The invention also relates to a use of the above method for the temporary protection against corrosion of said metal surface.

The subject of the invention is also a process for producing a shaped sheet having a metal surface chosen from zinc, iron, aluminum, copper, lead, and their alloys as well as galvanized, aluminized, copper-plated steels, in which a carboxylation treatment of said sheet is carried out and is shaped, characterized in that said carboxylation treatment is performed by the above method.

Said sheet may be steel coated with zinc or a zinc alloy and is shaped by stamping.

As will be understood, the invention is based on the use, for composing the solution or the carboxylation emulsion, of a binary or ternary eutectic of saturated C ₁₀ -C ₁₈ linear fatty acids. Preferably, the acids used are all even-numbered acids of carbon atoms. The binary eutectic of C ₁₂ - C ₁₆ acids is particularly preferred. The concentration of the eutectic in the carboxylate bath is greater than or equal to 20 g / l.

It should be understood that in this specification the term "eutectic" refers to a mixture in the composition of the eutectic containing two or three C ₁₀ -C ₁₈ saturated linear fatty acids.

Under these conditions, it becomes possible, although not mandatory, to dispense with organic co-solvent, and the treatment bath may contain only the eutectic, a surfactant and water, if the necessary oxidizing conditions are obtained by electrochemical means. This is very beneficial from an ecological point of view. These oxidizing conditions can also be obtained by chemical means, namely by adding an oxidizing compound, such as hydrogen peroxide. It may also be desirable to add one or more compounds which lower the pH of the medium, but in most cases the pH of 3 to 5 obtained naturally by the mixture of the compounds mentioned above will be sufficiently acidic, especially in the context of the carboxylation of galvanized steel sheets.

The minimum concentration of 20 g / l of the eutectic is chosen because, below this limit, the rate of formation of the carboxylated layer is no longer sufficient to obtain an effective conversion layer with a duration of treatment compatible with industrial requirements.

• la figure 1 qui schématise le diagramme d'équilibre d'un mélange de deux acides gras A et B en fonction de la température ;
• la figure 2 qui représente les diagrammes binaires de mélanges d'acides gras linéaires saturés HC₁₀/HC₁₂ (fig.2a), HC₁₂/HC₁₆ (fig.2b), HC₁₆/HC₁₈ (fig.2c) et HC₁₂/HC₁₈ (fig.2d),
• la figure 3 qui montre l'évolution de la résistance de polarisation au cours du temps pour différents eutectiques et une tôle électrozinguée de référence, la carboxylation étant réalisée en milieu hydro-organique ;
• la figure 4 qui montre l'évolution du potentiel de corrosion au cours du temps, dans les mêmes conditions que les essais de la figure 3 ;
• la figure 5 qui montre les résultats d'essais tribologiques effectués sur un échantillon de tôle électrozinguée carboxylatée par un eutectique HC₁₂/HC₁₆ et sur un échantillon de référence ;
• la figure 6 qui montre les résultats d'essais analogues à ceux de la figure 3, réalisés en milieu eau + tensioactif ;
• la figure 7 qui montre les résultats d'essais analogues à ceux de la figure 4, réalisés en milieu eau + tensioactif.

The invention will be better understood with the aid of the description which follows, given with reference to the appended figures:

• Figure 1 which shows schematically the equilibrium diagram of a mixture of two fatty acids A and B as a function of temperature;
• FIG. 2 which represents the binary diagrams of mixtures of linear saturated fatty acids HC ₁₀ / HC ₁₂ (FIG. 2a), HC ₁₂ / HC ₁₆ (FIG. 2b), HC ₁₆ / HC ₁₈ (FIG. ₁₂ / HC ₁₈ (fig.2d),
• FIG. 3 which shows the evolution of the polarization resistance over time for different eutectics and a reference electrogalvanized sheet, the carboxylation being carried out in a hydro-organic medium;
• Figure 4 which shows the evolution of the corrosion potential over time, under the same conditions as the tests of Figure 3;
• FIG. 5, which shows the results of tribological tests carried out on a sample of electrozinc plated sheet carboxylated by an HC ₁₂ / HC ₁₆ eutectic and on a reference sample;
• FIG. 6 which shows the results of tests analogous to those of FIG. 3, carried out in a water + surfactant medium;
• Figure 7 which shows the results of tests similar to those of Figure 4, carried out in water + surfactant medium.

We will first briefly recall the principle of carboxylation of metal surfaces.

The ability of saturated linear aliphatic monocarboxylates to inhibit the aqueous corrosion of metals (Cu, Fe, Pb, Zn and Mg) in neutral and aerated solution has been widely demonstrated. The protection provided is due to the presence of a thin film consisting of crystals of metal soap and hydroxide of the treated metal. The protective layer is formed under oxidizing conditions and has a corrosion resistance closely dependent on the length of the carbon chain and the concentration of the carboxylate.

The carboxylation process, known in itself, has been applied primarily to zinc and zinc coatings. A carboxylation bath contains a linear C _n saturated carboxylic acid of general formula (CH ₃ (CH ₂ ) _n-2 COOH), with n ≥ 7, denoted HC _n , dissolved in water or in a generally equivolumic water-non-aqueous solvent mixture (ethanol, etc.). An oxidant, such as hydrogen peroxide or sodium perborate, is added to the bath to produce a sufficient amount of Zn ⁺⁺ cations at the zinc / solution interface. The pH of the bath is close to 5. Alternatively, the oxidizing conditions producing the Zn ⁺⁺ cations are obtained by circulating an electric current between the surface to be protected and a counter electrode immersed in the bath.

If the carboxylic acid HC _n is noted, the essential formation reaction of the carboxylated layer on the zinc surface is:

Zn ²⁺ + 2 C _n ^- → Zn (C _n ) ₂ ↓

The compounds that can be used in the context of the invention, both the acids and the surfactants, may be derived from products of the green chain, that is to say from agricultural production for non-food use (sunflower oils, flax, colza ...). They advantageously replace the polluting mineral oils used for the lubrication of metal surfaces and the phosphating and chromating solutions used for the protection of these same surfaces against corrosion.

The effectiveness of the carboxylation treatment has been verified essentially in the case of saturated linear carboxylic acid baths containing 7 to 18 carbon atoms, and stearic acid HC ₁₈ has hitherto appeared as a particularly advantageous compound for optimize the resistance to aqueous corrosion and atmospheric corrosion of zinc soap coatings.

However, the inventors have found that still improved results, both in terms of protection against corrosion and behavior of the carboxylate coating in use (reduction of dusting) could be obtained in the case where a mixture is used. the composition of the eutectic of two or three C ₁₀ to C ₁₈ saturated linear carboxylic acids, called "saturated C ₁₀ -C ₁₈ fatty acids". Such a mixture provides a significant improvement in corrosion protection compared to coatings obtained with a single acid or mixture of acids of composition not close to a eutectic. Also, the lubricating properties of these coatings according to the invention are excellent. They make it possible to do without an oiling of the coated product during its shaping.

Of these saturated fatty acids, those containing an even number of carbon atoms are preferred.

• l'acide caprique HC₁₀ ;
• l'acide laurique HC₁₂ ;
• l'acide myristique HC₁₄ ;
• l'acide palmitique HC₁₆ ;
• l'acide stéarique HC₁₈.

The saturated fatty acids with an even number of carbon atoms that can be used in the context of the invention are:

• capric acid HC ₁₀ ;
• lauric acid HC ₁₂ ;
• myristic acid HC ₁₄ ;
• palmitic acid HC ₁₆ ;
• stearic acid HC ₁₈ .

The study of their binary mixtures makes it possible to highlight the existence of two particular proportions for which appear respectively an inflection and a minimum in the curve of the melting point. Figure 1 shows the equilibrium diagram of mixtures of fatty acids A and B as a function of temperature. The minimum e indicates the formation of a eutectic and the change of slope at point u is due, in general, to the existence of a defined molecular compound c of formula A _m B _n (m and n denote the molar fractions of A and B respectively).

Studies have been carried out on binary mixtures of saturated fatty acids, one of which has two carbon atoms more than the other, that is to say of the HC _n + HC _{n + 2 type} . In these cases, the eutectic is always formed for the composition corresponding to one molecule of the longest chain acid for three molecules of the other. Similarly, the break (Figure 1, point u) corresponding to the complex always appears for a proportion 1/1 molar approximately.

Figures 2b and 2d show the HC ₁₂ / HC ₁₆ and HC ₁₂ / HC ₁₈ binary diagrams. It is found that the eutexte point e, as well as the point of inflection u corresponding to the complex, do not appear respectively at 25 and 50%, as is the case with acid mixtures whose lengths of chains differ only by two carbon atoms (Fig. 2a for HC ₁₀ / HC ₁₂ and Fig. 2c for HC ₁₆ / HC ₁₈ ). Eutectic is shifted to higher molar concentrations of the shortest fatty acid. The shape of the binary diagram and the positions of the points u and e are a function of the more or less limited stability of the complex. The form depends on the difference between the chain lengths of the constituents, and more exactly, the difference between the melting points of these two fatty acids. Table 1 shows the compositions of the eutectics e of various binary mixtures and their melting points T _{f (e)} .

The eutectic compositions given in Table 1 are approximate. According to the publications, they can vary from a few percent. These differences are due to the purity of the fatty acids used. Table 1 - Properties of the studied fatty acid mixtures HC blends _n Composition e (% _mol ) T _{f (e)} (° C) HC ₁₀ / HC ₁₂ 65/35 18 HC ₁₂ / HC ₁₄ 69/31 34.2 HC ₁₂ / HC ₁₆ 81/19 32.7 HC ₁₂ / HC ₁₈ 81.5 / 18.5 37.0 HC ₁₄ HC ₁₆ 58/42 42.6 HC ₁₄ / HC ₁₈ 61/39 44.1 HC ₁₆ / HC ₁₈ 72.5 / 27.5 51.1

Carboxylation treatments of electrogalvanized steel sheets on both sides making use of these eutectics have been carried out.

The sheets were degreased in an alkaline degreasing bath, similar to those used in industrial alkaline phosphating. They were then rinsed. Then the carboxylation treatment took place chemically (presence of an oxidant in the bath, such as hydrogen peroxide or a sodium perborate tetrahydrate) or electrochemical.

The oxidizing conditions allow a rapid reaction between Zn ²⁺ and C _n ^- , providing fine crystals of Zn carboxylate.

In the case of the use of an oxidant, experience shows that hydrogen peroxide and sodium perborate tetrahydrate provide comparable results. The advantages of using an oxidant are explained by increasing the amount of Zn solution at the substrate / solution interface, and / or by the local increase in pH due to the reduction of the oxidant next :

BO ₃ ^- + 2H ⁺ + 2e ^- → BO ₂ ^- + H ₂ O

H ₂ O ₂ + 2H ⁺ + 2e ^- → 2H ₂ O

Regarding the amount of oxygenated water, it should not be too important to obtain a good coverage of the surface by the carboxylate crystals. Excess hydrogen peroxide results in faster dissolution of the carboxylate into peracid. The concentration of H ₂ O ₂ in the solution is, for example, from 2 to 15 g / l. Below 2 g / l the medium is generally not enough oxidizing to form enough Zn ²⁺ in solution. The duration of the reaction may not be compatible with industrial requirements. Above 15 g / l, the medium is generally too oxidizing and the crystals are poorly formed. The optimum concentration is about 8 to 12 g / l H ₂ O ₂ in the solution.

Compared to hydrogen peroxide, sodium perborate has the disadvantage of less solubility in water. The use of hydrogen peroxide thus provides greater flexibility in the choice of oxidant concentrations.

The preferred co-solvent is 3-methoxy-3-methylbutan-1-ol (MMB). It is a green and biodegradable solvent. In addition, its flash point, which is the temperature from which it becomes flammable, is 71 ° C, compared for example with that of ethanol which is 12 ° C. MMB therefore provides better safety conditions than ethanol. It is also possible to use, in particular, ethanol, n-propanol, dimethylsulfoxide, N-methyl-2-pyrrolidone, 4-hydroxy-4-methyl-2-pentanone or diacetone alcohol.

Regarding the use of a mixture in the composition of the fatty acid eutectic, a first advantage is the lowering of the melting temperature compared to the use of a single fatty acid, as is apparent from the figure 2. This keeps the carboxylate bath at a relatively low temperature of about 45 ° C in many cases.

Eutectic is prepared by melting for several hours the mixture of fatty acids component. The mixture is then slowly cooled to room temperature.

In the examples which will be described, electrogalvanized steel sheets (thickness of the Zn layer: 7.5 μm) were treated to obtain a weight of carboxylated layer of between 1 and 2 g / m ² , of which experience shows that it provides a maximum coverage rate of the sheet.

The weight of the carboxylate layer is evaluated by measuring the difference in mass between the carboxylate substrate and the substrate etched with dichloroethane under ultrasound, which treatment causes the dissolution of the carboxylate layer.

The aqueous corrosion resistance of the test samples was tested in a conventional three-electrode electrochemical cell, by monitoring the corrosion potential and measuring the polarization resistance. The electrolyte used is water according to ASTM D1384-87 (148 mg / l of Na ₂ SO ₄ , 138 mg / l of NaHCO ₃ , 165 mg / l of NaCl, pH 7.8). This corrosive solution is usually used to evaluate the effectiveness of corrosion inhibitors in the laboratory.

The resistance to atmospheric corrosion of 50 cm ² samples was studied according to DIN 50017 using a climatic chamber where the samples were placed vertically and subjected to cycles of 24 h, each with successive exposure. from 8 h to 100% humidity (twice water at 40 ° C) and then to ambient air for 16 h. The degradation of the coating was estimated by visual observation and X-ray diffraction.

Dusting of the samples was evaluated by measuring the mass difference of the substrate before and after successive passes between two squeezing rollers. The loss of mass thus measured can be related to the dusting tendency of the coating.

Tribological tests were carried out to evaluate the lubricating properties of the coating during stamping. They were carried out on a plane / plane tribometer with tightening force control, by scrolling the sample of pressed sheet at a speed of 1 to 100 mm / s, and by measuring the evolution of the distance between the tools. planes ensuring the tightening of the sample. It is thus possible to determine the coefficient of friction as a function of the clamping pressure.

• HC₁₀/HC₁₂;
• HC₁₂/HC₁₆;
• HC₁₂/HC₁₈.

The binary eutectics of fatty acids with an even number of carbon atoms have been particularly studied:

• HC ₁₀ / HC ₁₂ ;
• HC ₁₂ / HC ₁₆ ;
• HC ₁₂ / HC ₁₈ .

The coatings obtained with these three mixtures were first studied in the composition of the eutectic in a hydro-organic medium in the presence of hydrogen peroxide. The compositions of the baths were as follows:
medium 50% by volume of water and 50% by volume of 3-methoxy-3-methylbutan-1-ol (MMB);
concentration of H ₂ O ₂ 5 g / l;
temperature 45 ° C .;
compositions and concentrations of eutectics and duration of carboxylation according to Table 2: Table 2: compositions and concentrations of eutectics tested and duration of carboxylation Mixed % _mol eutectic Concentration (g / l) Duration of carboxylation (s) HC ₁₀ / HC ₁₂ 64/35 85 4 HC ₁₂ / H ₁₆ ; 81/19 55 4 HC ₁₂ / HC ₁₈ 81.5 / 18.5 45 2

The residence times of the sheet samples in the bath were determined to obtain a carboxylate layer weight of between 1 and 1.5 g / m ² .

Visual observation with a scanning electron microscope shows that each of these deposits provides a satisfactory coverage of the sample surface. Small parallelepipedic crystals with a size of between 5 and 10 μm are observed for the HC ₁₂ / HC ₁₆ and HC ₁₂ / HC ₁₈ eutectics. For eutectic HC ₁₀ / HC ₁₂ the crystals are rather spherical or cylindrical.

X-ray diffraction analysis shows that these deposits are poorly crystallized. This is not a defect in itself for the desired properties, but it complicates the characterization of the deposits. However, it has been possible to determine, by synthesizing the carboxylates of Zn in powder form, that the compounds formed have a structure close to ZnC _n1 , C _n2 , C _n1 and C _n2 denoting the carboxylate ions corresponding to the two acids of the mixture in the composition. of the eutectic at n ₁ and n ₂ carbon atoms.

Figure 3 shows the evolution over time of the polarization resistance Rp of the coatings, and Figure 4 shows this same trend for the corrosion potential E _corr in corrosive water, for the three coatings tested previously defined and, in reference title, for a non-carboxylated electrogalvanized EG coating.

It can be seen that the coatings according to the invention have much better performances than coatings resulting from simple electrogalging. For these, the polarization resistance is of the order of 2 kΩ.cm ² , and the carboxylation coatings usually made with water-solvent solutions based on a single fatty acid provide only one relatively weak improvement of this value (up to 15 kΩ.cm ² ). In contrast, the coatings according to the invention provide values of the order of 5 to 15 times higher than those observed for coatings electrozinced alone. Coatings obtained with HC ₁₂ / HC ₁₆ first, and with HC ₁₂ / HC ₁₈ second, provide the best results in absolute value and stability in time. As for the corrosion potentials, those of the coatings according to the invention are 80 to 140 mV higher than the values obtained for the electrogalvanized coating. The HC ₁₂ / HC _{16 again} gives the best result. The coatings obtained with a single fatty acid in a water-solvent medium usually provide corrosion potentials of the order of -1020 to -1080 mV, and therefore less favorable than those of the coatings according to the invention.

The resistance to atmospheric corrosion was also estimated by observing the percentage of the surface of the corroded sample after 20 cycles of exposure, as defined above.

While 100% of the surface of the electrogalvanized sample is corroded after 10 cycles, no degradation is observed after 20 cycles for the mixture HC ₁₂ / HC ₁₆ which has the best performance. For the other mixtures, the surface corroded after 20 cycles represents approximately 7% (for HC ₁₀ HC ₁₂ ) and 10% (for HC ₁₂ / HC ₁₈ ) of the total surface. These performances are comparable to or better than those obtained using single fatty acids in a water-organic solvent medium.

In addition, no recrystallized corrosion product was observed in X-ray diffraction.

Tribology tests were carried out on the coating formed with HC ₁₂ / HC ₁₆ as compared to electrogalvanized coating. The result is shown in FIG. 5 which shows the coefficient of friction of the coating as a function of the contact pressure for the two coatings. The tribological behavior of uncoated electrogalvanized steel degrades substantially with the increase of the contact pressure, which is not the case with the coating according to the invention which constantly has a low coefficient of friction, of the same order of magnitude. larger than coatings formed with single fatty acids. This coating is well suited to be used as a lubricant during stamping of a steel sheet coated with zinc or zinc alloy.

It has also been verified that this coating is not very subject to dusting. After 20 passes on spinning rolls, a layer weight loss of 0.2 g / m ² is measured against 0.4 g / m ² for a steel coated with a Zn (C ₇ ) ₂ conversion layer.

In general, the carboxylation coatings obtained using binary mixtures of fatty acids with the composition of the eutectic have at least equal performances, and often superior in all points of view, to those of the coatings obtained at using unique fatty acids in water-solvent medium. Overall, the HC ₁₂ / HC ₁₆ mixture is the most satisfactory of those tested.

Further tests have shown that in the sample preparation process, a refining step to activate the metal surface to be treated did not provide a significant improvement in the quality of the carboxylation coating formed in the next step. It can therefore usually be omitted without major drawbacks, which is very advantageous from an economic and ecological point of view.

Other tests have also shown that the invention is also applicable with advantage to galvanized coatings. In this case, however, it is necessary to remove the alumina layer Al ₂ O ₃ usually present on the surface of the coating, because it reduces the reactivity of the surface and inhibits the dissolution of zinc. This can be done by adding complexing agents of Al ³⁺ to the conversion bath, such as NaF, diethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid NTA, citrates, oxalates, certain amino acids, an oxalic acid mixture and aluminum phosphate.

• par un dégraissage alcalin (NaOH , tensioactifs, complexants) pour dissoudre l'Al₂O₃, suivi d'une oxydation alcaline (NaOH, sels de fer et cobalt, complexants) qui parachève l'élimination de Al₂O₃ et précipite une fine couche contenant Fe et Co qui améliore la dissolution du zinc lors de la conversion ;
• ou par une attaque acide (H₂SO₄) en présence d'ions Ni ; le Ni précipite sur le substrat à l'état métallique et accélère la dissolution du zinc lors de la conversion.

Another method is to prepare the surface before carboxylation by removing the Al ₂ O ₃ layer:

• by an alkaline degreasing (NaOH, surfactants, complexing agents) to dissolve the Al ₂ O ₃ , followed by an alkaline oxidation (NaOH, iron and cobalt salts, complexing agents) which completes the removal of Al ₂ O ₃ and precipitates a thin layer containing Fe and Co which improves the dissolution of zinc during the conversion;
• or by an acid attack (H ₂ SO ₄ ) in the presence of Ni ions; Ni precipitates on the substrate in the metallic state and accelerates the dissolution of zinc during the conversion.

In addition, tests were carried out on the HC ₁₂ / HC ₁₆ mixture with compositions deviating from the 81-19% eutectic. It turns out that the mixtures 77/23% and 85/15% already have degraded properties compared to the eutectic 81/19%, concerning in particular the polarization resistance. However, these performances remain better than those obtained with solutions containing HC ₁₂ or HC ₁₆ alone.

In general, it is considered that the composition difference (in mol%) with respect to the eutectic x% - y% should not exceed x ± 5% - y ± 5% and preferably x ± 3% - y ± 3%, for binary eutectics or x ± 3%, y ± 3% - z ± 3% for ternary eutectics.

On the other hand, there is a need for a process where the fatty acids would not need the presence of an organic solvent in the carboxylation medium. For this purpose, it was verified in particular on the mixture with the composition of the eutectic HC ₁₂ / HC ₁₆ 81/19% that it was possible to obtain good results by removing the organic solvent and adding a surfactant and / or or a dispersant in the carboxylate bath.

It is then necessary to provide a rinsing step to remove the surfactant, which is hydrophilic, in order to recover the hydrophobic character of the Zn carboxylate layer, and thus prevent corrosion of the sheet.

• des alkylpolyglycosides (APG) tels que l'Agrimul PG 215 CS VP et le Glucopon 225 DK/HH de la société COGNIS ; ces tensioactifs sont à base de sucre, sont non toxiques et ont une résistance exceptionnelle aux agents alcalins et aux sels ;
• des alcools gras éthoxylés tels que le Brij 58 de la société ACROS ;
• des acides gras éthoxylés saturés ou non ;
• des huiles éthoxylées ;
• des nonylphénols éthoxylés ;
• des esters de sorbitan éthoxylés.

As surfactants, a wide variety of compounds have been used, generally chosen from nonionic surfactants and especially:

• alkylpolyglycosides (APG) such as Agrimul PG 215 CS VP and Glucopon 225 DK / HH from the company COGNIS; these surfactants are sugar-based, non-toxic and have exceptional resistance to alkalis and salts;
• ethoxylated fatty alcohols such as Brij 58 from ACROS;
• saturated or unsaturated ethoxylated fatty acids;
• ethoxylated oils;
• ethoxylated nonylphenols;
• ethoxylated sorbitan esters.

As dispersants, it is possible to use especially high molecular weight polyols, carboxylic acid salts such as (meth) acrylic copolymers, polyamide derivatives such as polyamide waxes.

Under these conditions, the optimum for the oxygenated water concentration is between 2 and 8 g / l.

With single fatty acids, carboxylation without an organic solvent by means of a simple aqueous emulsion does not provide optimal coatings for corrosion protection because the weight of the carboxylate layer is relatively low. It has therefore been verified whether the use of eutectic fatty acids under these conditions could be more satisfactory.

Thus, carboxylation emulsions containing water, the surfactant APG 215 mentioned above and the mixture with the composition of the eutectic HC ₁₂ / HC ₁₆ at 81/19% were prepared.

It has been established that at 45 ° C a stable emulsion can be obtained for at least 1 hour containing up to at least 6% APG 215 and up to 4% eutectic. The percentages for surfactant and eutectic are percentages by weight.

The following experiments were carried out with an emulsion containing 3% of eutectic and 0.1 to 3% of APG 215, in the presence of 5 or 10 g / l of hydrogen peroxide.

• A : eau - HC₁₂/HC₁₆ 3% - APG 215 0,1 % - H₂O₂ 5 g/l
• B : eau - HC₁₂/HC₁₆ 3% - APG 215 1 % - H₂O₂ 5 g/l
• C : eau - HC₁₂/HC₁₆ 3% - APG 215 3% - H₂O₂ 5 g/l
• D : eau - HC₁₂/HC₁₆ 3% - APG 215 3% - H₂O₂ 10 g/l

The tested emulsions had the following compositions:

• A: water - HC ₁₂ / HC ₁₆ 3% - APG 215 0.1% - H ₂ O ₂ 5 g / l
• B: water - HC ₁₂ / HC ₁₆ 3% - APG 215 1% - H ₂ O ₂ 5 g / l
• C: water - HC ₁₂ / HC ₁₆ 3% - APG 215 3% - H ₂ O ₂ 5 g / l
• D: water - HC ₁₂ / HC ₁₆ 3% - APG 215 3% - H ₂ O ₂ 10 g / l

It has been found that emulsion A with a low concentration of APG 215 makes it possible to release the fatty acids more rapidly. A coat weight of 1.2 g / m ² is reached in 5 s, while 10 s are needed to achieve a layer weight comparable with other emulsions. For contents of APG 215 of 1 to 3%, no marked effect of the surfactant concentration is observed. The Oxidant concentration also has no very appreciable effect in the explored range.

The size of the crystals does not seem to be related to the composition of the emulsion. Again, the product of the carboxylation is not well crystallized, and its composition is close to ZnC ₁₂ C ₁₆ .

Polarization resistance and corrosion potential measurements were made under the same conditions as above, and compared to those obtained on an electrogalvanized coating EG. The results are illustrated in Figures 6 and 7 respectively.

It appears that in aqueous corrosion, all the coatings provide a higher polarization resistance than the electrogalvanized coating alone during the first minutes of immersion, then stabilize at values equal to or slightly greater than that of the electrogalvanized coating. Emulsions that are less rich in surfactant provide the best results. For the corrosion potential, the different coatings have comparable behavior and provide a more favorable corrosion potential than that of electrogalvanized sheet.

In atmospheric corrosion, it is the emulsions C and D, the richest in surfactant, which have the best results, with respectively 10 and 20% corroded surface after 20 cycles. The results in tribology are also favorable.

In summary, the performance of carboxylation coatings formed from eutectics in a water / organic solvent medium is generally greater than that of similar coatings formed by emulsions in a water / surfactant medium. However, when the performance of the coatings formed without organic solvent are considered sufficient, for example because the coated products are not intended to remain long in a corrosive atmosphere, it is advantageous to use them because the toxicological risks are less for the manipulators and for the environment. Moreover their implementation does not require or little control and post-treatment effluents.

In the experiments which have been described, the oxidizing conditions have been obtained using hydrogen peroxide. But, as is known, they could have been obtained with other oxidants, or by the application to the carboxylation bath of an electrical current of the order of intensity, for example, from 10 to 25 mA / cm. ² .

The invention is not limited to the examples that have been described. In particular the eutectics of the other couples of saturated C ₁₀ -C ₁₈ linear fatty acids would be usable, whether these acids each have an even or odd number of carbon atoms. Eutectics of ternary mixtures of such fatty acids can also be used.

It is, however, the use of even-numbered fatty acids which is the preferred embodiment of the invention. These even fatty acids are of plant origin and are generally derived from the green products sector, from renewable sources. Odd fatty acids do not exist in nature and must be synthesized. In addition, odd fatty acid eutectics require chemical treatments for their preparation.

• des agents de régulation du pH ou agents tampons pour réguler les conditions de formation de la couche de conversion sur la surface ;
• des additifs facilitant la mise en oeuvre du traitement et la répartition du bain sur la surface à traiter, comme des agents tensioactifs (étant entendu que la présence d'un tensioactif est obligatoire lorsque le bain est une émulsion aqueuse) ;
• des additifs permettant d'augmenter la durée de vie du bain comme, par exemple, des agents chélatants pour retarder la précipitation d'autres composés que ceux que l'on souhaite obtenir dans la couche de conversion, ou des agents bactéricides ;
• des agents accélérateurs de traitement ; et
• des additifs permettant la dispersion des acides gras en milieu aqueux.

The conversion baths may optionally contain:

• pH regulating agents or buffering agents for controlling the conditions of formation of the conversion layer on the surface;
• additives facilitating the implementation of the treatment and the distribution of the bath on the surface to be treated, such as surfactants (it being understood that the presence of a surfactant is mandatory when the bath is an aqueous emulsion);
• additives to increase the life of the bath such as, for example, chelating agents to delay the precipitation of other compounds than those desired in the conversion layer, or bactericidal agents;
• accelerating agents for treatment; and
• additives for the dispersion of fatty acids in an aqueous medium.

The conversion treatments according to the invention are applicable to other metal surfaces than galvanized steels. They may concern any metal surface susceptible to carboxylation, namely zinc, iron, aluminum, copper, lead and their alloys, aluminized or copper-coated steels.

Claims (17)

Process for conversion by carboxylation of a metal surface selected from zinc, iron, aluminum, copper, lead and their alloys, galvanized or electrozinc, aluminized, copper-plated steels, under oxidizing conditions vis-à-vis screwing the metal, by contact with an aqueous or hydro-organic bath containing a mixture of organic acids, characterized in that : said organic acids are saturated linear carboxylic acids containing from 10 to 18 carbon atoms; said mixture is a binary or ternary mixture of such acids; the respective proportions of these acids are such that: * for a binary mixture x ± 5% - y ± 5%, x and y being, in molar percentages, the respective proportions of the two acids in a mixture to the composition of the eutectic; * for a ternary mixture x ± 3% - y ± 3% - z ± 3%, x, y and z being, in molar percentage, the respective proposals of the three acids in a mixture to the composition of the eutectic; the concentration of said mixture in said bath is greater than or equal to 20 g / l. Process according to claim 1, characterized in that the mixture is binary and in that the respective proportions of the acids are x ± 3% - y ± 3%. Process according to claim 1 or 2, characterized in that said oxidative conditions are created by the presence in the bath of an oxidizing compound for the metal surface. Process according to Claim 3, characterized in that the said oxidizing compound is hydrogen peroxide. Process according to claim 3, characterized in that said oxidizing compound is sodium perborate. Process according to Claim 1 or 2, characterized in that the said oxidizing conditions are created by the bath application of an electric current. Process according to one of Claims 1 to 6, characterized in that the bath is a hydro-organic bath and contains a co-solvent. Process according to Claim 7, characterized in that the cosolvent is chosen from 3-methoxy-3-methylbutan-1-ol, ethanol, n-propanol, dimethylsulfoxide and N-methyl-2-pyrrolidone. , 4-hydroxy-4-methyl-2-pentanone, diacetone alcohol. Process according to one of Claims 1 to 8, characterized in that the said bath is an aqueous bath and contains a surfactant and / or a dispersant. Process according to Claim 9, characterized in that the said surfactant is chosen from alkylpolyglycosides, ethoxylated fatty alcohols, ethoxylated fatty acids, ethoxylated oils, ethoxylated nonylphenols and ethoxylated sorbitan esters. Process according to one of claims 9 or 10, characterized in that the dispersant is selected from high molecular weight polyols, carboxylic acid salts such as (meth) acrylic copolymers, polyamide derivatives such as waxes polyamides. Process according to one of Claims 1 to 11, characterized in that the said saturated carboxylic acids each have an even number of carbon atoms. Process according to Claim 12, characterized in that the said saturated carboxylic acids are lauric acid and palmitic acid. Method according to one of claims 1 to 13, characterized in that said metal surface is a galvanized steel sheet, and in that the bath contains an Al ³⁺ complexing ^agent . Use of the method according to any one of claims 1 to 14 for the temporary protection against corrosion of said metal surface. A method of manufacturing a shaped sheet having a metal surface selected from zinc, iron, aluminum, copper, lead, and their alloys as well as galvanized, aluminized, copper-plated steels, wherein carboxylate treatment of said sheet and is shaped, characterized in that said carboxylation treatment is carried out according to one of claims 1 to 14. Process according to claim 16, characterized in that said sheet is made of steel coated with zinc or a zinc alloy and in that it is shaped by stamping.

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