EP1373597B1 - Method for treating metal surfaces by carboxylation - Google Patents

Abstract

The invention relates to a method for treating a metal surface by carboxylation before forming, said metal being selected from zinc, iron, aluminium, copper, lead and the alloys thereof as well as copper-coated, aluminium-coated, galvanised steels. Said method is carried out in oxidising conditions in relation to the metal by bringing said metal into contact with an aqueous, organic or hydro-organic bath comprising at least one organic acid in free form or in salt form. The inventive method is characterised in that: the organic acid is a saturated or unsaturated aliphatic monocarboxylic or dicarboxylic acid; the organic acid is in solution and/or emulsion in the bath at a concentration greater than 0.1 mole/litre; the pH of the bath is acid.

Description

The invention relates to a method for the deposition of conversion layers on a metal surface selected from zinc, iron, aluminum, copper, lead, and their alloys as well as galvanized, aluminized, copper-coated steels to produce at high speed conversion layers formed of very small crystals.

• l'amélioration des propriétés de frottement sous lubrification en mécanique, par exemple pour l'emboutissage de tôles,
• la protection temporaire contre la corrosion.

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

• the improvement of friction properties under mechanical lubrication, for example for sheet metal stamping,
• temporary protection against corrosion.

For this first type of application, it is more particularly used so-called pre-phosphating treatments which result in the deposition of a metal phosphate layer whose weight is of the order of 1 to 1.5 g / m ² .

But, these conversion treatments can also be performed after shaping the sheet, to improve the adhesion of subsequently deposited organic coatings, such as paints. By way of example, mention may be made of the phosphating treatment which results in the deposition of a metal phosphate layer whose weight is greater than that of a pre-phosphatation treatment.

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 with respect to the metal of the surface and generally takes place in acidic medium. 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's nature and the structure of the precipitated compounds on the treated surface which determine the degree of protection against corrosion, improvement of the tribological properties and / or adhesion, as well as the other properties of the layer.

In order 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: by means of a chemical agent for oxidizing the metal to be 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 are thus chromating treatments on galvanized or aluminized steel, phosphating unalloyed bare steels or coated steels, or even oxalation on alloy steels such as stainless steels, for example.

After being brought into contact with a conversion bath, the treated surface is generally rinsed to remove the components of the surface and / or the treatment solution that have not reacted, then this surface is dried in particular to harden the conversion layer. and / or to improve the properties thereof.

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 on surfaces zinc plated colloidal sols or 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.

• un acide polyhydroxyaryl-carboxylique, par exemple l'acide gallique ou l'acide protocatéchuique, ou un depside de cet acide pouvant résulter de sa réaction avec du glucose, comme l'acide tannique, et
• un promoteur d'adhérence à base de silane.

International patent application WO95 / 21277 discloses an aqueous composition for treating zinc-coated surfaces comprising:

• a polyhydroxyaryl-carboxylic acid, for example gallic acid or protocatechuic acid, or a depside of this acid which may result from its reaction with glucose, such as tannic acid, and
• a silane adhesion promoter.

With this treatment composition, conventional accelerating agents are used for galvanized surfaces, such as phosphates, nitrates, fluorides or organic acids.

The treatment carried out using this composition provides both good protection against corrosion and good adhesion for paints.

The patent application FR 2 465 008 discloses a process for the deposition of conversion coats on galvanized surfaces, using a treatment solution which may contain a soluble oxalate, combined with activating agents which are carboxylic acids, especially short diacids.

The oxalic acid concentration of the treatment solutions described is less than 7.5 g / l, ie 0.08 mol / liter.

The document EP 494,431 (KAWASAKI ) teaches that the addition of a carboxylic acid to chromate treatment solutions containing colloidal silica makes it possible to improve the properties of the conversion layers obtained on galvanized surfaces and / or substantially reduces the chromium content of these solutions.

The weight of layer, or weight, resulting from these different treatments is very variable depending on the main objective pursued and can range from less than 1 g / m ² up to 8 g / m ² .

The different treatments of the prior art, such as chromate treatment, phosphatation and oxalation, have a major disadvantage 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.

The object of the invention is therefore to provide a treatment for converting metal surfaces before they are shaped, using a bath that is free from compounds that are harmful to the environment but also makes it possible to deposit on these surfaces layers of conversion more effective to protect against corrosion and / or prelubrication than those obtained by the processes of the prior art.

Another objective pursued by the invention is also to be able to apply an organic coating with good adhesion, in particular by cataphoresis, to a treated surface according to the invention, after it has been shaped and degreased, which supposes that the applied conversion layer is easily removed at the time of this degreasing.

• d'un agent chimique d'oxydation dudit métal adapté à sa nature,
• d'agents de régulation de pH,
• d'additifs facilitant la mise en oeuvre du traitement et de la répartition du bain sur la surface à traiter,
• d'additifs permettant d'augmenter la durée de vie du bain,
• d'agents accélérateurs de traitement,
• d'un co-solvant,
• d'ions de terres rares, et
• de cations métalliques Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, Mn²⁺, et Al³⁺,

en ce que ledit acide organique est un acide monocarboxylique ou dicarboxylique aliphatique saturé ou insaturé,
en ce que ledit acide organique est en solution et/ou en émulsion dans le bain à une concentration supérieure à 0,1 mol/litre, et
en ce que le pH du bain est acide.For this purpose, the subject of the invention is a method of treatment by carboxylation, before shaping, of a metal surface chosen from zinc, iron, aluminum, copper, lead, and their alloys, as well as galvanized, aluminized, copper-coated steels, under oxidizing conditions with respect to the metal, by contact with an aqueous, organic or hydro-organic bath consisting of one or more organic acids in free form or in the form of salt, and optionally:

• a chemical oxidation agent of said metal adapted to its nature,
• pH regulating agents,
• additives facilitating the implementation of the treatment and the distribution of the bath on the surface to be treated,
• additives to increase the life of the bath,
• accelerating agents,
• a co-solvent,
• rare earth ions, and
• Cu ²⁺ , Co ²⁺ , Ni ²⁺ , Fe ²⁺ , Mn ²⁺ , and Al ³⁺ metal cations,

in that said organic acid is a saturated or unsaturated aliphatic monocarboxylic or dicarboxylic acid,
in that said organic acid is in solution and / or emulsified in the bath at a concentration greater than 0.1 mol / liter, and
in that the pH of the bath is acidic.

The invention also relates to the use of said surface treatment method by carboxylation for the temporary protection against corrosion of said metal surface.

The invention further relates to a method of manufacturing a shaped sheet having a metal surface selected from zinc, iron, aluminum, copper, lead, and their alloys and galvanized steels, aluminized, coppered, in which a surface treatment of said sheet according to the invention is carried out, said treated sheet is oiled, and shaped. It is more particularly preferred to apply this method according to the invention to a steel sheet coated with zinc or zinc alloy which is then shaped by stamping.

The invention will be better understood on reading the description which follows, given by way of non-limiting example.

According to the invention, conversion layers are deposited by carboxylation of metal surfaces by bringing the surface into contact with an organic or hydro-organic aqueous bath consisting less of an organic acid in solution or in emulsion, under oxidizing conditions with respect to metal.

The present invention uses aliphatic monocarboxylic or dicarboxylic acids, optionally having one or more unsaturations.

Since the acid constants of these acids are of the order of 4.8, the natural pH of the baths according to the invention will generally be lower than this value. If it is desired to lower the pH, it will be possible to acidify the treatment bath, for example using nitric acid. The pH of the bath can also be increased by adding sodium hydroxide, for example. The acid bath used in the treatment according to the invention must in all cases have a pH value of between 1 and 7, the upper limit making it possible to avoid the presence of a metal hydroxide in the conversion layer. As for the minimum value of the pH, it will be adapted according to the metal of the surface to ensure a satisfactory deposit of the conversion layer.

The at least one organic acid used according to the invention is dissolved or emulsified in the bath at a concentration greater than 0.1 mol / liter.

If the concentration of organic acid in the treatment bath is less than 0.1 mole / liter, the formation rate of the metal carboxylate conversion layer is no longer sufficient to obtain a conversion layer effective over time assigned to the treatment.

In a preferred embodiment, the aliphatic organic acids according to the present invention, are selected from saturated monocarboxylic acids having from 5 to 16 carbon atoms. Hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid and decanoic acid are more particularly preferred.

In another preferred embodiment, the aliphatic organic acids according to the present invention are chosen from unsaturated monocarboxylic acids containing from 10 to 18 carbon atoms. Particularly preferred is undecenoic acid, oleic acid or linoleic acid.

In another preferred embodiment, the aliphatic organic acids according to the present invention are chosen from saturated dicarboxylic acids containing from 4 to 12 carbon atoms. Sebacic acid or azelaic acid is more particularly preferred.

In another preferred embodiment, the at least one aliphatic monocarboxylic acid retained is heptanoic acid.

More preferably, the bath comprises, in addition to heptanoic acid, decanoic acid or undecenoic acid.

Through the use of a mixture of organic acids, such as heptanoic acid and decanoic acid or undecenoic acid, a much denser and more efficient conversion layer is obtained to protect the metal surface against corrosion.

• soit par une addition dans le bain d'un agent chimique d'oxydation du métal adapté à sa nature, tel que le perborate tétrahydraté ou le peroxyde d'hydrogène,
• soit en faisant circuler un courant électrique entre la surface métallique préalablement immergée dans le bain et au moins une contre-électrode également immergée.

The oxidizing conditions of the carboxylation bath are obtained by the following means:

• either by adding to the bath a metal oxidation chemical agent adapted to its nature, such as perborate tetrahydrate or hydrogen peroxide,
• or by circulating an electric current between the metal surface previously immersed in the bath and at least one counter-electrode also immersed.

In the case where the oxidizing conditions with respect to zinc are obtained by addition, in the bath, of a chemical agent for oxidation of zinc, this oxidizing agent and / or accelerator is preferably chosen from the group comprising sodium perborate, nitrites, hydrogen peroxide, hydroxylamine sulfate and nitro-guanidine.

• des agents de régulation de 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 tensio-actifs,
• des additifs permettant d'augmenter la durée de vie du bain comme, par exemple, des agents chélatants pour retarder la précipitations d'autres composés que ceux que l'on souhaite obtenir dans la couche de conversion, ou des agents bactéricides, et
• des agents accélérateurs de traitement.

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,
• 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, and
• accelerators for treatment.

The conversion treatment bath according to the invention may also comprise a co-solvent such as ethanol, n-propanol, butanol, dimethylsulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), 4-hydroxy-4-methyl-2-pentanone, diacetone alcohol, glycol ethers, etc., or other organic or mineral acids. Diacetone alcohol is more particularly preferred.

A surprising result has been obtained when the carboxylation treatment bath comprises in solution rare earth ions such as gadolinium in the +3 oxidation state at a concentration greater than or equal to 1.10 ^-3 mol / liter.

The conversion layer obtained is then composed of two types of crystals of different composition and possibly of different shape and it proves an even greater efficiency for corrosion resistance.

Preferably, the concentration of organic acids in the bath, the conditions of use of this bath and the oxidizing conditions with respect to the metal are adapted to obtain on the metal surface a carboxylation layer of grammage between 1 and 6 g / m ² .

Other advantages of the method of the invention will become apparent on reading the examples presented below without limitation of the present invention.

MATERIALS to obtain the conversion layers: 1) Characteristics of the substrates tested

Sheet steel of ES quality (stamping steel) or HES (deep drawing steel), micro-alloyed or not, thickness between 0.7 and 1.2 mm, bare or coated with a layer of zinc approximately 10 μm thick, applied by electroplating in a bath based on chlorides or sulphates, or by hot-dip galvanizing.

2) Components of the conversion treatment baths tested 2.a - Solvent and co-solvent (s)

The treatment solutions tested are generally based on water as the main solvent and ethanol as co-solvent; propanol, DMSO, NMP or 4-hydroxy-4-methyl-2-pentanone (diacetone alcohol) or other polar solvents at least partially miscible with water may also be used as co-solvents.

2.b - Organic acid

For the examples illustrating the invention, it is heptanoic acid (abbreviation: HC ₇ , Aldrich product, reference 25,873-3) and / or decanoic acid (abbreviation: HC ₁₀ , Rectapur product, reference 243.298, sulfur ash 0.1% maximum).

2.c - Chemical agent of oxidation of the substrate

Unless otherwise indicated, zinc perborate tetrahydrate (NaBO ₃ , 4H ₂ O) at 2 g / l is used as the chemical oxidation agent for zinc. Hydrogen peroxide (or hydrogen peroxide H ₂ O ₂ ) can also be used.

2.d - pH

Unless otherwise indicated, the solutions are used at their natural acidic pH; the possible increases in pH are obtained by adding sodium hydroxide.

2.e - Other components and additives

Metallic cations can be added: Cu ²⁺ , Co ²⁺ , Ni ²⁺ , Fe ²⁺ , Zn ²⁺ , Mn ²⁺ , Al ³⁺ , Tr ^{n +} .

3) Standard procedure of treatment by conversion baths

Table I Ref. Step Composition of the bath Temperature Residence time 1 dry cleaning Ridoline ^™ 1.8% + Ridosol ^™ 0.18% 55 ° C 240s 2 dry cleaning Ridoline ^™ 0.4% + Ridosol ^™ 0.04% 55 ° C 240s 3 Rinsing Running water 25 ° C 90s 4 Pretreatment refining Fixodine ^™ 50CF: 0.5 g / l in deionized water 25 ° C 60s 5 Conversion processing variable 25 ° C variable 6 Rinsing Running water 25 ° C 60s 7 Drying Pulsed air 80 ° C 300s

Ridoline ^™ is an aqueous degreasing solution containing alkali metal salts, surfactants and potassium hydroxide.

Ridosol ^™ is a wetting base for liquid degreaser comprising a mixture of surfactants.

Fixodine ^™ 50CF is an activator (also referred to as a germinator) comprising compounds based on alkali metal salts such as titanium salts.

METHODS of evaluating conversion layers obtained: 1) Weight of deposited layer

In order to determine the grammage, the conversion layer formed is dissolved on a given surface with a complexing solution, the soaking time in this solution being adapted to obtain complete dissolution of the layer (of the order of one minute for one minute). layer made from HC ₇ , 0.3 mg / cm ² ).

The grammage, expressed in mg per cm ² , is evaluated from the difference in weight between the coated sample and the same sample having undergone the dissolution treatment.

• sel disodique de l'acide Éthylène Di-amine TétrAcétique : 0,1 M
• MBT (mercaptobenzothiazole) ou BZT (benzotriazole) : 0,1 g/l. Ces composés limitent la dissolution du substrat métallique

The composition of this solution of dissolution is as follows:

• Disodium salt of Ethylene Di-amine TetraAcetic acid: 0.1 M
• MBT (mercaptobenzothiazole) or BZT (benzotriazole): 0.1 g / l. These compounds limit the dissolution of the metal substrate

The pH of this solution is adjusted to a value close to 8 by addition of sodium hydroxide, in order to obtain an optimal complexation of the metal cation.

2) Resistance to aqueous corrosion

This test consists of a periodic potentiometric measurement of the polarization resistor R _p by a voltage sweep with an amplitude of +/- 10 mV around the corrosion potential E _cor. .

• une cuve en polyméthacrylate de méthyle, dont le fond est percé d'une fenêtre, et qui contient l'électrolyte aqueux standard ASTM D1384-87. Cet électrolyte comprend 148 mg/l de sulfate de sodium (Na₂SO₄), 138 mg/l de bicarbonate de sodium (NaHCO₃), et 165 mg/l de chlorure de sodium (NaCl) et présente un pH voisin de 8 ;
• trois électrodes immergées dans l'électrolyte : une électrode de travail constituée par la surface métallique revêtue de la couche de conversion à évaluer obstruant la fenêtre du fond de la cellule, une électrode de référence au calomel saturé (« ESC »), et une électrode de platine comme contre-électrode auxiliaire ;
• un potientiostat de type Princeton^™ 273 ou 263 connecté aux trois électrodes et piloté par un logiciel référencé m352 de la Société EG&G Princeton^™.

The device used for this evaluation comprises, in a conventional manner:

• a polymethyl methacrylate tank, the bottom of which is pierced by a window, and which contains the standard aqueous electrolyte ASTM D1384-87. This electrolyte comprises 148 mg / l of sodium sulphate (Na ₂ SO ₄ ), 138 mg / l of sodium bicarbonate (NaHCO ₃ ), and 165 mg / l of sodium chloride (NaCl) and has a pH of about 8 ;
• three electrodes immersed in the electrolyte: a working electrode constituted by the metal surface coated with the conversion layer to be evaluated obstructing the window of the bottom of the cell, a saturated calomel reference electrode ("ESC"), and an electrode platinum as an auxiliary counter-electrode;
• a Princeton ^™ type 273 or 263 potientiostat connected to the three electrodes and controlled by software referenced m352 from EG & G Princeton ^™ .

3) Resistance to atmospheric corrosion according to DIN 51160

• première partie : durée : 8 heures - 40°C ; 100% d'humidité relative ;
• deuxième partie : durée : 16 heures - température et humidité ambiante ;

The metal surface samples are stored first in a humid atmosphere in a humidotherm chamber regulated in successive cycles, each cycle having the following characteristics:

• first part: duration: 8 hours - 40 ° C; 100% relative humidity;
• second part: duration: 16 hours - ambient temperature and humidity;

Unless otherwise indicated, the quotation then consists of counting the number of cycles at the end of which it is found that at least 10% of the treated surface is covered with white rust.

4) Friction behavior

This test consists of measuring the plane-plane friction coefficient by increasing the clamping pressure to 80 MPa.

Before the test, the treated samples are covered with an oil referenced 6130 QUAKER Company, the amount of oil deposited is of the order of 1 to 2 g / m ² for example.

5) Behavior in stamping

This test consists of making complete stampings from sheet metal blank samples having undergone the conversion treatment. Thus, with an initial blank diameter of 64 mm, a bucket with a diameter of 32 mm and a depth of 25 mm is produced.

On the same type of sheet blank whose surface treatment is desired to be tested, the maximum stamping force beyond which there is a rupture of the blank during stamping is evaluated for a clamping force of the clamps. predetermined blanks. This maximum value is identified for a series of clamping force values of the blanks, so as to plot curves representing the variation of the maximum drawing force Fmax as a function of the blanking force.

6) Phosphatability after degreasing of the layer

• 2% en volume de Granodine 9580 M/CF (mélange phosphatant contenant de 10 à 25% d'acide phosphorique ainsi qu'une petite quantité d'acide fluorhydrique),
• 0,7% en volume de Starter 958/CF (solution aqueuse contenant des sels de zinc en présence d'un léger excès d'acide nitrique),
• 0,04% en volume d'Intensifier N°1 (solution aqueuse de sels métalliques en présence d'une petite quantité d'acide phosphorique),
• 0,5% en volume de Primaire M (solution aqueuse contenant de l'hydroxyde sodium > 2%) pour réguler le pH,
• 0,05% en volume de Compensateur M (solution aqueuse à base de nitrite de soude, accélératrice pour bain de phosphatation cristalline),
  • Conditions opératoires :
    • Eau déminéralisée
    • Traitement au trempé 2 min.
    • Température = 55°C +/- 2°C
  • Contrôle du bain par analyse chimique :
    Les analyses doivent se situer dans les fourchettes suivantes :
    • [Zn²⁺] = 0,9 à 1,1 g/l
    • [Mn²⁺] = 0,7 à 0,9 g/l
    • [Ni²⁺] = 0,18 à 0,25 g/l
    • [F^-] = 0,25 à 0,35 g/l

In a first step, the surface carrying a conversion layer is degreased according to the procedure specified in Table 1 - steps 1 and 2 of 3) "standard procedure for treatment with conversion baths" / Part "MATERIALS for obtaining the conversion layers" - in order to eliminate the conversion layer, then a conventional phosphating treatment is carried out on this degreased surface by applying to it an aqueous composition comprising:

• 2% by volume Granodine 9580 M / CF (phosphate mixture containing from 10 to 25% phosphoric acid and a small amount of hydrofluoric acid),
• 0.7% by volume of Starter 958 / CF (aqueous solution containing zinc salts in the presence of a slight excess of nitric acid),
• 0.04% by volume of Intensifier No. 1 (aqueous solution of metal salts in the presence of a small amount of phosphoric acid),
• 0.5% by volume of Primer M (aqueous solution containing sodium hydroxide> 2%) to regulate the pH,
• 0.05% by volume of Compensator M (aqueous solution based on sodium nitrite, accelerator for crystalline phosphating bath),
  • Operating conditions:
    • Demineralized Water
    • Dip treatment 2 min.
    • Temperature = 55 ° C +/- 2 ° C
  • Bath control by chemical analysis:
    The analyzes should be in the following ranges:
    • [Zn ²⁺ ] = 0.9 to 1.1 g / l
    • [Mn ²⁺ ] = 0.7 to 0.9 g / l
    • [Ni ²⁺ ] = 0.18 to 0.25 g / l
    • [F ^- ] = 0.25 to 0.35 g / l

In a second step, the quality of the phosphatation layer obtained is examined using an electron microscope.

7) Ability to organic coating by cataphoresis after degreasing and phosphating

• Formule du bain de contrôle :
  • 43,16 % en poids d'eau désionisée (10µS maxi),
  • 48,18 % en poids de liant dérivé des polyamine-uréthanne,
  • 8,66 % en poids de pâte (dispersion de pigments) pour peinture par électrodéposition cationique,
  Avec :
  • Extrait sec = 20 - 24%
  • pH = 5,7 à 6,1 (à 25°C)
  • Conductivité du mélange = 1500 - 2000 mS (à 25°C)
• Conditions d'application :
  • Température du bain = 28 à 35°C
  • Temps d'application = 120 à 180 s
  • Rapport anode / cathode = 1 / 4
  • Tension de travail = 200 - 320 V
  • Etuvage nominal (T° palier) = 15 min à 175°C
  • Epaisseur visée = 18 à 24 µm

At first, the treated surface is degreased according to the mode described in Table I - steps 1 and 2 of 3) "Standard conversion bath processing procedure" / Part "MATERIALS to obtain the conversion layers" above to remove the conversion layer, then performs, on this degreased surface, a phosphating treatment according to the procedure specified in 6) above. Finally, an organic coating is conventionally deposited by cataphoresis on the phosphated surface according to the following conditions (example of the formulation PPG 752 - 965 1):

• Control bath formula:
  • 43.16% by weight of deionized water (10μS max),
  • 48.18% by weight of binder derived from polyamine-urethanes,
  • 8.66% by weight of paste (pigment dispersion) for cationic electrodeposition painting,
  With:
  • Dry extract = 20 - 24%
  • pH = 5.7 to 6.1 (at 25 ° C)
  • Conductivity of the mixture = 1500 - 2000 mS (at 25 ° C)
• Conditions of application:
  • Bath temperature = 28 to 35 ° C
  • Application time = 120 to 180 s
  • Anode / cathode ratio = 1/4
  • Working voltage = 200 - 320 V
  • Nominal drying (T ° bearing) = 15 min at 175 ° C
  • Target thickness = 18 to 24 μm

Example 1

This example is intended to determine carboxylation treatment conditions according to the invention for obtaining, on a galvanized sheet surface, a conversion layer both compact and finely crystallized.

• préparation de la surface à traiter,
• trempage de la surface préparée dans le bain de conversion,
• séchage et observation de la couche de traitement obtenue.

The galvanized surfaces of steel sheets are treated according to the following steps:

• preparation of the surface to be treated,
• soaking the prepared surface in the conversion bath,
• drying and observation of the treatment layer obtained.

The initial preparation treatment is that mentioned in Table I. in the successive references 1 to 4. The solvent of the conversion bath is a water-ethanol mixture whose ratio is adjusted in order to completely solubilize the added organic acid.

In order to determine the optimal conditions of treatment, different parameters are varied.

1.1. pH

The pH is varied and it is found that the higher it is, the more the conversion layer appears poorly crystallized. In addition, too high a pH may result in parasitic precipitation of metal hydroxide. Therefore, in practice, it is limited to an acidic pH of between 1 and 7. The minimum pH value will be adapted according to the metal of the surface in order to ensure a satisfactory deposition of the conversion layer. Thus, in the case of zinc, the treatment will preferably be carried out at a pH of between 4 and 6.

1.2. Bath temperature

The bath temperature is varied between 20 and 80 ° C and it is observed that increasing the temperature accelerates the deposition rate.

1.3. Concentration of the corresponding organic acid or salt

The concentration of the organic acid is varied and it is found that the concentration range must be between 0.1 and 1.5 M. As mentioned above, if the concentration is less than 0.1M, the formation rate of the metal carboxylate conversion layer is no longer sufficient to obtain a conversion layer effective in the time given to the treatment. A concentration higher than 1.5 M is not possible because of the solubility limit of the organic acid close to 1.4 - 1.5M.

When the organic acid or acids are used in the form of salts, chlorides, nitrates or sulphates in a concentration of less than or equal to 3 g / l are preferably employed.

1.4. Length n of the carbon chain of organic acid

We vary the length of the carbon chain and we see that the longer the carbon chain of aliphatic acid is long (n high), plus the crystals of the layer are fine and more, under equivalent treatment conditions, the weight of the conversion layer is high: about 1.5 times higher for example, when n goes from value 7 to value 10.

In addition, the compactness of the conversion layer also increases with the length of the aliphatic chain of the carboxylic acid used, as well as the resistance to corrosion provided by this layer.

1.5. Nature and concentration of oxidizing agents

Various oxidants are used, such as dissolved oxygen (O ₂ ), nitrites (NO ₂ ^- ), hydrogen peroxide (H ₂ O ₂ ) or perborates (BO ₃ ^- ). It is found that the presence of oxidizing agent or accelerating agent has a favorable effect on the compactness of the conversion layer.

1.6. Duration of immersion in the bath or duration of treatment

• ratio eau/alcool : 1 ;
• n = 7 (acide heptanoïque), [HC₇] = 50 g/l,
• agent oxydant : perborate de sodium NaBO₃, 4 H₂O : 2 g/l ;
• pH ≈ 4,7 .

The galvanized surface to be treated is immersed for a few minutes in a bath having the following characteristics:

• water / alcohol ratio: 1;
• n = 7 (heptanoic acid), [HC ₇ ] = 50 g / l,
• oxidizing agent: sodium perborate NaBO ₃ , 4 H ₂ O: 2 g / l;
• pH ≈ 4.7.

The duration of the treatment is varied and the grammage of the carboxylation layer obtained is observed: <u> Table II </ u> Immersion time (min.) 0.5 3 5 10 Weight (g / m ² ) 1 2.4 3.1 3.2

It is found that the increase in immersion time in the bath is favorable to the compactness of the layer.

Example 2 - Comparison with the Prior Art

• 1. Tôle phosphatée portant une couche dont le grammage est de l'ordre de quelques g / m²
  On procède au traitement de phosphatation classique sur surface dégraissée selon les conditions de bain précisées au 6) (Aptitude à la phosphatation après dégraissage de la couche),
• 2. Tôle pré-phosphatée portant une couche dont le grammage est de l'ordre de 1 g / m²
  On procède au traitement de pré-phosphatation sur surface dégraissée selon les conditions suivantes :
  • bain de conversion composé d'acide phosphorique, de Mn et de Ni (formulation Chemetall VP 10118, par exemple)
  • séchage à 200°C,
  • grammage = 1,1 à 1,5 g/m².
• 3. Tôle oxalatée selon FR 2 465 008 : on procède selon le mode opératoire décrit dans l'exemple 4 de ce document à l'aide d'une solution de traitement contenant un fluorure complexe et un oxalate.
• 4. Tôle carboxylatée selon WO 95/21177 : on procède selon le mode opératoire décrit dans l'exemple 1 de ce document à l'aide d'une solution de traitement contenant de l'acide gallique et du 3-glycidoxypropyl-triméthoxy-silane.

In parallel, the following treatments of blanks of the same galvanized sheet are carried out under the conditions indicated below.

• 1. Phosphate sheet with a layer whose weight is of the order of a few g / m ²
  The conventional phosphatization treatment is carried out on a degreased surface according to the bath conditions specified in 6) (Phosphatability after degreasing of the layer),
• 2. Pre-phosphated sheet bearing a layer whose weight is of the order of 1 g / m ²
  The pre-phosphatation treatment is carried out on a degreased surface according to the following conditions:
  • conversion bath composed of phosphoric acid, Mn and Ni (Chemetall formulation VP 10118, for example)
  • drying at 200 ° C,
  • grammage = 1.1 to 1.5 g / m ² .
• 3. Oxalized sheet according to FR 2 465 008 The procedure described in Example 4 of this document is carried out using a treatment solution containing a complex fluoride and an oxalate.
• 4. Carboxylated sheet according to WO 95/21177 The procedure described in Example 1 of this document is carried out using a treatment solution containing gallic acid and 3-glycidoxypropyltrimethoxysilane.

By carrying out the evaluations of the different types of conversion layers as defined above in points 3, 4, 5, 6 and 7 of the Methods paragraph, the following results are obtained:

2.1. Resistance to atmospheric corrosion

Sample tested Number of cycles Galvanized sheet without conversion treatment - oiled QUAKER 2130 10 Pre-prephosphated sheet - not oiled 20 Oxalated sheet according to FR 2 465 008 4 Carboxylated sheet according to WO95 / 21277 7 Sheet treated according to the invention (with HC ₇ ): 1 min - not oiled 23 Treated sheet according to the invention (with HC ₇ ): 2 min - not oiled 24 Treated sheet according to the invention (with HC ₇ ): 2 min + rinsing - not oiled 27 Sheet treated according to the invention (with HC ₇ ): 5 min - not oiled 25

It can be seen that the treatment according to the invention provides better resistance to atmospheric corrosion than the treatments of the prior art.

2.2. Friction behavior

Type of treatment before oiling Weight (g / m ² ) Coefficient of friction comments max. min. way No 0 0.13 Strong growing chatter with clamping pressure Préphosphatation 1.5 0.19 0.17 0.11 No grazing FR 2 465 008 0.11 Little chatter WO95 / 21277 0.12 Average browsing Invention: HC ₇ 1.2 0.09 0.05 0.07 Very little chatter Invention: HC ₇ 5 0.09 0.04 0.05 No grazing Invention: HC ₁₀ 5 0.09 0.04 0.05 No grazing

The conversion treatments according to the invention carried out before oiling thus make it possible to eliminate grazing and a continuous decrease in the coefficient of friction is observed when the clamping pressure increases.

It is also noted that the treatment according to the invention makes it possible to obtain substantially improved coefficients of friction with respect to the pre-phosphatation treatment.

2.3. Behavior during stamping

• d'une part sur les tôles carboxylatées (durée : 5 min.) selon l'invention sans huilage complémentaire et avec huilage complémentaire à l'aide de l'huile référencée 6130 de la Société QUAKER ;
• d'autre part sur les mêmes tôles non traitées : exemple comparatif sont évalués par le tracé de courbes représentant la variation de la force maximale d'emboutissage en fonction de la force de serre-Flan.

The obtained results :

• on the one hand on the carboxylated plates (duration: 5 min.) according to the invention without additional oiling and with complementary oiling using the oil referenced 6130 of the QUAKER Company;
• on the other hand on the same untreated sheets: comparative example are evaluated by the plot of curves representing the variation of the maximum stamping force as a function of the greenhouse-blank force.

In general, the more the curves obtained are low and little inclined, the better the stamping behavior is good.

The invention very substantially improves the stamping behavior.

It can be deduced from these three series of tests that the conversion treatment according to the invention can advantageously replace the conventional pre-phosphatation treatment, all the more so as it avoids the pollution problems posed by the use of phosphates.

Example 3 - Ability to paint

The metal surfaces subjected to the conversion treatment according to the invention find their main use in the automotive field. It is therefore important to verify that these surfaces, once coated, are compatible with the methods usually used in industry for painting metal sheets, in particular galvanized sheets.

3.1. Suitability for degreasability

Embossing shaping generally requires that the metal surface be oiled, which requires, when it is desired to subsequently proceed with other operations such as painting or enameling, to remove any trace of oil from the surface. this surface and, at the same time, eliminate any conversion layer formed before shaping. This is done by a degreasing treatment, for which success it is important that the conversion layer is easily removed. It is therefore important to verify that the conversion layer according to the invention is easily degreasable.

In a first step, an alkaline degreasing of the metal surface carrying a conversion layer is carried out under the conditions specified in Table I - steps 1 and 2 of the 3) "standard operating procedure for treatment with conversion baths "/" MATERIALS part to obtain the conversion layers ".

In a second step, we observe the surface of the samples treated then degreased to see if the conversion layer has disappeared.

The complete disappearance of the carboxylation layer according to the invention is observed.

3.2. Phosphatability after degreasing

The following procedures are carried out according to the procedure defined in point 6) by treating the two faces of the sample according to the invention, namely the bare steel face and the galvanized face, and it is observed that the conversion layer obtained a covering aspect.

3.3. Ability to organic coating by cataphoresis after degreasing and phosphating

The observation of the samples treated according to the invention, degreased, phosphated and then coated, reveals no particular defect.

Example 4 - Incidence of co-solvent and immersion time on the weight of the conversion layer

The procedure is as described in the Materials and Methods section, using heptanoic acid at a concentration of 0.38 mol / liter.

Before the application of the conversion treatment, a refining pretreatment is applied.

• Cas 1 : 62% eau, 38% éthanol
• Cas 2 : 61% eau, 39% 1-propanol
• Cas 3 : 43% eau, 57% DMSO
• Cas 4 : 56% eau, 44% NMP
• Cas 5 : 60% eau, 40% diacétone alcool .

The grammage of the conversion layer obtained is evaluated as a function of the immersion time of the sheet blank to be treated in the solution according to the invention, for different solvent compositions (% by volume):

• Case 1: 62% water, 38% ethanol
• Case 2: 61% water, 39% 1-propanol
• Case 3: 43% water, 57% DMSO
• Case 4: 56% water, 44% NMP
• Case 5: 60% water, 40% diacetone alcohol.

The above mixtures are optimized to solubilize as much as 95% heptanoic acid.

• les profils de croissance de la couche de conversion sont similaires dans les cas 1, 4 et 5,
• pour des temps d'immersion inférieurs à 180 s, la vitesse de croissance de la couche est plus faible dans les cas 2 et 3 que dans les cas 1, 4 et 5.

It can be deduced from the curves obtained that:

• the growth profiles of the conversion layer are similar in cases 1, 4 and 5,
• for immersion times less than 180 s, the growth rate of the layer is lower in cases 2 and 3 than in cases 1, 4 and 5.

• après 30 s de traitement dans les cas 1, 4 et 5,
• après 300 s de traitement dans les cas 1, 3 et 4.

From observations using a scanning electron microscope, it can be seen that the morphologies of the crystals of the layers are identical:

• after 30 s of treatment in cases 1, 4 and 5,
• after 300 s of treatment in cases 1, 3 and 4.

Other co-solvents could be envisaged to implement the invention, in particular amide solvents such as N-methylformamide, N-N-dimethylformamide or sulfonated solvents such as tetramethylsulfone.

Example 5 - Emulsion in the conversion treatment solution

To avoid the use of solvents which may pose safety problems, it is possible to obtain the highly concentrated organic acid treatment solutions according to the invention using suitable surfactants.

With 1% by volume of FORAFAC 1033 D product of ATOFINA which is a 30% perfluorinated anionic surfactant in water, of the perfluoroalkylsulfonic acid family, an emulsion containing 50 g / l of heptanoic acid is prepared and 2 g / l of sodium perborate tetrahydrate. The emulsion contains no other solvent than water and, thanks to the surfactant, all of the acid is in emulsion and / or solution in water.

Two treatment solutions differing in pH are prepared: one at a natural pH = 4, the other at a pH adjusted to about 4.7 by adding sodium hydroxide.

The procedure is carried out according to the procedure of the Methods section by dipping for five minutes samples of sheets identical to those of Example 3 in one or the other treatment solution, then rinsing and Dry the treated surface.

• à pH = 4, on obtient un dépôt de carboxylatation de 0,3 mg/cm² comparable à ceux obtenus dans l'exemple 4 pour la même durée d'immersion ; le dépôt semble contenir des traces de l'agent tensio-actif, ce qui montre que cet agent n'est pas passif vis à vis de la réaction de précipitation ;
• à pH = 4,7, le grammage obtenu est un peu plus faible (- 30% environ) mais la morphologie du dépôt est comparable à celles obtenues dans l'exemple 4 ; le dépôt ne contient plus de traces d'agent tensio-actif.

From the observations made on the treated surfaces:

• at pH = 4, a carboxylation deposit of 0.3 mg / cm ² comparable to those obtained in Example 4 is obtained for the same immersion time; the deposit seems to contain traces of the surfactant, which shows that this agent is not passive with respect to the precipitation reaction;
• at pH = 4.7, the grammage obtained is a little lower (-30% approximately) but the morphology of the deposit is comparable to those obtained in Example 4; the deposit no longer contains traces of surfactant.

Example 6 Carboxylation Under Electrochemical Oxidation of the Substrate

This is to produce the metal cations from the metal surface electrochemically. The substrate which is here a galvanized sheet is subjected to a potential and an anode current by immersion of the galvanized sheet in the treatment solution between two titanium plates electrically connected to the same potential.

As a treatment solution, a 50% water / 50% ethanol solution containing 0.38 mol / liter of heptanoic acid and whose pH is between 3.2 and 4.7, depending on the amount of sodium hydroxide added, is used.

The procedure described in point 3) of the Equipment paragraph is applied; the conversion treatment according to the invention is applied by soaking the sheet sample in the treatment solution and passing an electric current between the submerged sheet and the titanium plates. We proceed under current densities of 10 and 25 mA / cm ² for treatment times of 1, 3, 5 and 10 seconds.

According to the results of these tests and from the observations made on the treated surfaces, it is found that the conversion layers obtained after 1 to 10 seconds have surface densities, morphologies and crystallizations comparable to those obtained by chemical oxidation after 60 to 300 seconds. This mode of application of the treatment according to the invention makes it possible to improve the formation rate of the conversion layer by at least a factor of 10 or even 100 and is therefore particularly advantageous.

These findings are confirmed by the results of atmospheric corrosion tests of samples treated at a current density of 25 mA / cm ² and summarized in Table III: Duration of treatment in s. 1 3 5 10 Number of cycles 2 5 30 30

Example 7 - Mixture of heptanoic acid and decanoic acid

The procedure is as in Example 4, replacing the heptanoic acid at 0.38 mol / liter with a mixture 80/20 (case A) or 50/50 (case B) of heptanoic acid (abbreviation: HC ₇ ) and decanoic acid (abbreviation: HC ₁₀ ), the immersion time being 5 minutes.

The atmospheric corrosion resistance of the samples obtained according to the test of the paragraph Methods is evaluated. <u> Table IV </ u> Composition of the mixture of acids in% Weight (mg / cm ² ) Number of cycles HC ₇ HC ₁₀ 100 0 0.30 25 80 20 0.30 50 50 50 0.32 50 0 100 0.45 * 50 * to note the presence after 5 cycles of spots not attributable to the white rust.

• la surface zinguée non traitée .
• la surface zinguée traitée selon l'invention sans post-traitement avec :
  • l'acide heptanoïque ,
  • l'acide décanoïque ,
  • un mélange d'acides heptanoïque et décanoïque dans un rapport HC7/HC10 égal à 80/20 en mole .

The resistance to aqueous corrosion is then evaluated by polarization resistance measurements of:

• the untreated galvanized surface.
• the galvanized surface treated according to the invention without post-treatment with:
  • heptanoic acid,
  • decanoic acid,
  • a mixture of heptanoic and decanoic acids in an HC7 / HC10 ratio of 80/20 mol.

By comparing the three curves, the mixture of heptanoic and decanoic acids has improved resistance to aqueous corrosion compared to heptanoic or decanoic acids used alone.

Observations made on the deposits obtained make it possible to see that the deposit corresponding to the solution of case B (50/50) has a morphology very different from that obtained with only one of the heptanoic or decanoic acids. The conversion layer obtained with this acid mixture seems much denser, which would explain the improvement of the resistance to aqueous corrosion. This effect is even more marked in case A (80/20). RX diffractograms of the layer obtained in these two cases demonstrate not only the presence of zinc decanoate but also that of a mixed species.

Example 8 - Post-treatments

It is common to perform a post-treatment to improve the properties of a conversion layer and the present inventors have therefore sought to determine the type of post-treatment most suitable for substrates carrying a conversion layer according to the invention. .

A galvanized sheet is treated by soaking for five minutes in a 50/50 water / ethanol solution containing 0.38 mole / liter of heptanoic acid and 2 g / l of sodium perborate hydrate.

The post-treatment is then carried out by soaking in a post-treatment solution, the characteristics of which are summarized in Table V for 60 seconds, before the final rinsing of the procedure described in point 3 of the Materials section. <u> Table V - Post-Processing Solutions </ u> Trial Ref. Society Composition Concentration pH If (1) 44.016-7 ALDRICH 3-glycodoxypropyl trimethoxysilane 5 g / l 4.7 Ti 30838.2 FLUKA K ₂ TiF ₆ 3.7 g / l 3.4 Gd ^III 19914-0250 Acros Gd (NO ₃ ) ₃ , 5 H ₂ O 0.01M 4.7 Lu ^III Lu (NO ₃ ) ₃ 0.01M 4.7 (1) solution comprising 10% methanol by weight

Once the post-treatment has been carried out, the treated surface of the samples is observed and / or analyzed and a potentiometric evaluation of the aqueous corrosion resistance according to item 2 of the Materials section is carried out.

It is found that the post-treatment Ti has no impact but that the post-treatment Si slightly improves the corrosion resistance, apparently without morphological modification of the conversion layer.

In the case of post-treatment solutions containing rare earth oxidation state 3 (Gd ^III , Lu ^III ), tested at the same concentration and at the same pH, a rare earth deposit is observed on the conversion layer, but the effect of this deposit on the polarization resistance R _p is different according to the nature of the lanthanide, as illustrated in FIG. 4: significant effect in the case of Lu ^III , a very important effect in the case of Gd ^III .

In conclusion, among the evaluated post-treatment solutions, it is the Gd ^III solution that seems to have the greatest impact on the morphology of the carboxylate layer and on the resistance to aqueous corrosion that it brings. It also makes it possible to increase the resistance to atmospheric corrosion because an improvement of approximately 10 cycles is observed during the tests carried out in the humidotherm.

Example 9 - Addition of gadolinium ions in the treatment solution

In point 2 e) of the paragraph Materials, it is indicated that other components or additives can be introduced into the carboxylate solution according to the invention. In the previous example, the effect of certain compounds on a post-treatment operation was evaluated and the effect of these same compounds on the treatment itself, using these compounds as the additive in the conversion solution.

The treatment according to the invention is carried out by dipping for five minutes in a 50/50 water / ethanol solution containing 0.38 mol / liter of heptanoic acid, 2 g / l of sodium perborate hydrate, and one of additives listed in Table VI, the pH can be adjusted by addition of nitric acid.

The procedure described in point 3 of the section Materials is applied without performing post-treatment. <u> Table VI - Treatment Additives </ u> Trial Ref. Society Composition Concentration pH If (1) 44.016-7 ALDRICH 3-glycodoxypropyl trimethoxysilane 5 g / l 4.8 Ti 30838.2 FLUKA K ₂ TiF ₆ 3.7 g / l 4.5 Gd ^III 19914-0250 Acros Gd (NO ₃ ) ₃ .5 H ₂ O 0.01M 3.5 Gd ^III Same Same Same 0.01M 4.7 (1) solution comprising 10% methanol by weight;

After observation and analysis of the treated surface of the samples, it is found that the case Ti has a negative effect on the carboxylation reaction, whereas the cases Si and Gd ^III at pH = 4.7 do not allow to observe any difference by compared to carboxylation without additives.

In contrast, the Gd ^III case at pH = 3.5 gives a conversion layer composed of two types of composition crystals and probably of different shapes. The appearance of this structure is observed for a pH greater than 4. By carrying out polarization measurements according to point 2 of the Methods paragraph, Rp values are obtained approximately 5 times higher than those obtained on the standard sample. obtained by carboxylation treatment without additive.

Claims (20)

1. Method for depositing conversion coatings on a metal surface, before shaping, chosen from zinc, iron, aluminium, copper, lead and their alloys, and galvanized, aluminized, coppered steels, said coating being formed based on metal carboxylate in oxidizing conditions with respect to said metal, by placing in contact with a bath,
   characterized in that said bath is aqueous, organic or hydro-organic, and comprises one or more organic acids in free form or in salt form, and optionally:

   - a chemical agent for oxidizing said metal suited to its nature,

   - pH regulating agents,

   - additives facilitating the implementation of the treatment and the distribution of the bath over the surface to be treated,

   - additives for increasing the life of the bath,

   - treatment accelerating agents,

   - a co-solvent,

   - rare earth ions, and

   - metal cations Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, Mn²⁺ and Al³⁺,

   in that said organic acid is a saturated or unsaturated, aliphatic monocarboxylic or dicarboxylic acid,
   in that said organic acid is in solution and/or in emulsion in the bath with a concentration greater than 0.1 mol/litre, and
   in that the pH of the bath is acid.
2. Method according to Claim 1, in which said organic acid is chosen from the saturated monocarboxylic acids with 5 to 16 carbon atoms.
3. Method according to Claim 1, in which said organic acid is chosen from the unsaturated monocarboxylic acids with 10 to 18 carbon atoms.
4. Method according to Claim 1, in which said organic acid is chosen from the saturated dicarboxylic acids with 4 to 12 carbon atoms.
5. Method according to Claim 2, in which said organic acid is chosen from hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid and decanoic acid.
6. Method according to Claim 3, in which said unsaturated monocarboxylic organic acid is undecenoic acid, oleic acid or linoleic acid.
7. Method according to Claim 4, in which said saturated dicarboxylic organic acid is sebacic acid or azelaic acid.
8. Method according to Claim 5, characterized in that said organic acid is heptanoic acid.
9. Method according to Claim 8, characterized in that the bath comprises, in addition to heptanoic acid, decanoic acid or undecenoic acid.
10. Method according to any one of Claims 1 to 9, characterized in that the organic or hydro-organic aqueous bath comprises a co-solvent chosen from ethanol, n-propanol, dimethyl sulphoxide, N-methyl-2-pyrrolidone, 4-hydroxy-4-methyl-2-pentanone or diacetone alcohol.
11. Method according to Claim 10, characterized in that the co-solvent is diacetone alcohol.
12. Method according to any one of Claims 1 to 11, characterized in that said bath also comprises multivalent cations in the + 3 oxidation state, chosen from the rare earths with a concentration greater than or equal to 1.10^-3 mol/litre, the pH of the bath being greater than 4.
13. Method according to Claim 12, characterized in that said multivalent cation is gadolinium.
14. Method according to any one of Claims 1 to 13, characterized in that said oxidizing conditions are obtained by the addition, in the bath, of a chemical agent suited to the metal to be treated.
15. Method according to any one of Claims 1 to 13, characterized in that said oxidizing conditions are obtained by making an electric current flow between said surface previously immersed in the bath and at least one counter-electrode that is also immersed.
16. Method according to any one of Claims 1 to 15, characterized in that the concentration of organic acids in the bath, the conditions of use of said bath and the oxidizing conditions with respect to the metal to be treated are adapted to obtain on said metal surface a carboxylation coating with a spread of between 1 and 6 g/m².
17. Method according to any one of Claims 1 to 16, characterized in that, on completion of the treatment of said surface, a post-treatment is applied using a bath containing multivalent cations in the + 3 oxidation state, chosen from the rare earths with a concentration greater than or equal to 1.10^-3 mol/litre.
18. Use of the method according to any one of Claims 1 to 17 for temporary protection against the corrosion of said metal surface.
19. Method according to any one of Claims 1 to 17, characterized in that said treated plate is oiled and shaped.
20. Method according to Claim 19, characterized in that said plate is made of steel coated with zinc or a zinc alloy, and in that it is shaped by stamping.