EP0102284A1 - Composition and process for the chemical conversion of metallic substrates - Google Patents

Abstract

Acid solution for the chemical conversion of metallic substrates, comprising: a polyphosphate, soluble in water and of the formula (X PO3)n in which n>/=3 and in which X is an alkali, alkaline earth metal or ammonium, an organic chelating agent and the zinc iron, the pH being brought to the desired value by means of an inorganic acid selected from the group comprising sulphuric hydrochloric and nitric acid.

Description

The invention relates to an acidic aqueous solution for the chemical conversion of metallic substrates, in particular based on iron or its alloys.

It also relates to a chemical conversion process using said solution.

Finally, it relates to the metal parts obtained by implementing the conversion process according to the invention.

The expression “chemical conversion” denotes the surface chemical transformations of metals, in particular in an acid medium, making it possible to modify their intrinsic properties and to confer on them new physical or physico-chemical characteristics, in particular with a view to increasing their resistance. corrosion and / or to promote the adhesion of film-forming coatings applied subsequently.

Traditionally, this "chemical conversion" of metal substrates is carried out by conventional phosphating treatments and leads to the deposition on the surface of the metal of a thin layer of insoluble phosphate.

This phosphating layer can have a so-called amorphous or crystalline structure.

The amorphous structure is obtained by a conversion based on iron phosphate; the phosphating layer is then essentially composed of an iron phosphate, vivianite Fe ₃ (PO ₄ ) ₂ , 8 H ₂ O and iron oxide of the magnetite type Fe ₃ 0 ₄ . This type of layer allows excellent adhesion of paints and a significant increase in corrosion resistance.

The crystal structure is obtained when the treatment solution contains for example phosphate zinc; the essential constituents of the layer are then the homeopathy Zn ₃ (PO ₄ ) ₂ , 4 H ₂ O and the phosphophyllite Zn ₂ Fe (PO ₄ ) ₂ , 4 H ₂ O which are in the form of crystals oriented with respect to the support. This type of layer has a certain porosity due to the existence of intercrystalline gaps, which give it good wettability with respect to products such as paints and varnishes. The essential property of the crystalline layers is however to slow corrosion; this property is related to the dielectric strength of the phosphate coating which opposes the passage of local currents generated by the formation of galvanic microcouples on the surface of the metal.

• - acide phosphorique H₃PO₄
• - un phosphate métallique primaire (H₂P0₄)₂ Me, Me représentant souvent le zinc ou le fer, mais pouvant aussi représenter le manganèse, le calcium, le nickel, le cuivre et autres,
• - un accélérateur constitué par un élément oxydant généralement d'origine minérale, choisi parmi les chlorates, les nitrates et/ou les nitrites et associé à l'un des métaux précédemment cités, ou encore à du sodium ou à de l'ammonium.

In general, conventional phosphating treatments use acid solutions which, before use, contain the following constituents:

• - phosphoric acid H ₃ PO ₄
• - a primary metallic phosphate (H ₂ P0 ₄ ) ₂ Me, Me often representing zinc or iron, but which can also represent manganese, calcium, nickel, copper and others,
• - An accelerator consisting of an oxidizing element generally of mineral origin, chosen from chlorates, nitrates and / or nitrites and associated with one of the metals mentioned above, or with sodium or ammonium.

• - une étape de dégraissage,
• - une étape de rinçage (à froid, puis à chaud),
• - une étape de décapage pour la préparation de la surface à traiter,
• - une étape de rinçage,
• - l'étape de conversion chimique proprement dite,
• - une étape de lavage pour les produits acides non combinés,
• - une étape de passivation en milieu chromique destinée à augmenter la résistance de la couche de phosphate à la corrosion,
• - une étape de rinçage,
• - une étape de séchage et d'étuvage,
• - une étape de "graissage" (protection temporaire) de-la couche obtenue dans la mesure où le vernis'ou la peinture ne sont pas immédiatement appliqués.

These treatments can be carried out by spraying these solutions onto the objects to be treated or, most often, by immersing the objects to be treated in these solutions, generally at temperatures above 40 ° C .; the spraying or immersion treatment is inserted as follows in a sequence of operational steps which may include:

• - a degreasing step,
• - a rinsing step (cold, then hot),
• - a pickling step for the preparation of the surface to be treated,
• - a rinsing step,
• - the chemical conversion stage proper,
• - a washing step for non-combined acid products,
• - a passivation step in a chromic medium intended to increase the resistance of the phosphate layer to corrosion,
• - a rinsing step,
• - a drying and steaming stage,
• - a step of "lubrication" (temporary protection) of the layer obtained insofar as the varnish or the paint are not immediately applied.

It has already been proposed to improve in various ways the performance or the conduct of conventional phosphate baths.

In particular, it has been proposed to add to the above solutions a small amount, relative to the primary phosphate present, of polyphosphates such as, for example, pyrophosphate and metaphosphates with long chains, of the Graham salt type.

• - les faibles poids de couches obtenus qui améliorent l'adhérence des peintures et vernis,
• - la consommation moindre à l'enrichissement,
• - la diminution du volume de boues.

This addition to a conventional type phosphating bath, that is to say where a primary phosphate always appears as an essential component, has met with limited interest despite the advantages then highlighted, namely:

• - the low weight of layers obtained which improve the adhesion of paints and varnishes,
• - less consumption due to enrichment,
• - reduction in the volume of sludge.

• - aux difficultés de contrôle de l'évolution de la concentration en polyphosphates, dont la présence n'est pas facile à déceler et surtout
• - aux inconvénients entraînés par un excès de polyphosphates, par exemple en pyrophosphate, excès qui peut rapidement empêcher toute germination et, par conséquent, interdire toute formation de couche cristalline.

This lack of interest in polyphosphates is due to:

• - difficulties in controlling the evolution of the polyphosphates concentration, the presence of which is not easy to detect and above all
• - the drawbacks caused by an excess of polyphosphates, for example pyrophosphate, an excess which can quickly prevent any germination and, consequently, prohibit any formation of a crystalline layer.

It has also been proposed to add "chelating" agents to conventional phosphating baths having as essential component primary phosphates; these chelating agents were chosen from the group comprising EDTA (ethylene-diamine-tetracetic acid), monohydroxycarboxylic acids (in particular gluconic acid) and polycarboxylic acids such as citric acid, oxalic acid, l tartaric acid and others.

• - moindre formation de boues,
• - dépôts de phosphates plus épais, c'est-à-dire que, contrairement à ce que l'on observe en présence de polyphosphates, il se produit un accroissement de la proportion de dépôt cristallin.

This addition of chelating or complexing agents had resulted in the following advantages:

• - less sludge formation,
• - thicker phosphate deposits, that is to say that, contrary to what is observed in the presence of polyphosphates, there is an increase in the proportion of crystalline deposit.

However, despite all these improvements, the solutions or phosphating baths of the prior art still do not meet all the requirements of the technique.

In particular, one of the major drawbacks of conventional phosphating processes lies in the fact that, even after chromating passivation, the behavior of the chemical conversion layers obtained and the resistance to corrosion of the treated substrates are only very limited in time. .

• - de par la présence de quantités de boues de phosphate qui, bien que diminuées dans certains cas, restent toujours importantes et qui se forment lors de l'utilisation du bain et,
• - de par la présence d'ions chrome VI, amenés par l'étape de passivation et qui constituent des agents polluants toxiques et gênants.

Other disadvantages lie in the problems my questions to the user:

• - due to the presence of quantities of phosphate sludge which, although reduced in certain cases, still remain significant and which form during the use of the bath and,
• - By the presence of chromium VI ions, brought by the passivation step and which constitute toxic and annoying pollutants.

It is to the credit of the Applicant Company to have developed a new chemical conversion solution which better meets those which already exist for the various requirements of the technique.

• - un polyphosphate, soluble dans l'eau et de formule (X P0₃)_n dans laquelle n > 3 et dans laquelle X est un métal alcalin, alcalino-terreux ou l'ammonium,
• - un agent chélatant organique et
• - l'ion zinc,

le pH pouvant être amené à la valeur désirée à l'aide d'un acide minéral choisi dans le groupe comprenant les acides sulfurique, chlorhydrique et nitrique, l'acide nitrique étant préféré en raison de son caractère oxydant qui favorise l'initiation de la réaction de conversion.The conversion solution according to the invention is acidic and comprises:

• - a polyphosphate, soluble in water and of formula (X P0 ₃ ) _n in which n> 3 and in which X is an alkali, alkaline-earth metal or ammonium,
• - an organic chelating agent and
• - the zinc ion,

the pH can be brought to the desired value using a mineral acid chosen from the group comprising sulfuric, hydrochloric and nitric acids, nitric acid being preferred because of its oxidizing nature which promotes the initiation of conversion reaction.

The polyphosphate forming part of the solution in accordance with the invention can be chosen in particular from sodium trimeta-, tetrameta- and hexametaphosphate, sodium hexametaphosphate or HMPP being preferred.

• - l'EDTA (ou acide éthylène-diamine-tétracéti- que), le NTA (ou acide nitrilo-triacétique), le DTPA (ou acide diéthylène-triamine-penta- cétique),
• - les acides polycarboxyliques, tels que l'acide citrique, l'acide oxalique, l'acide malique, l'acide glutamique, l'acide tartrique, l'acide aspartique, l'acide glutarique, l'acide malonique et leurs sels,
• - les acides polyhydroxycarboxyliques tels que l'acide gluconique, l'acide glucoheptonique et leurs sels,
• - les acides polyhydroxypolycarboxyliques tels que l'acide glucarique ou l'acide galactari- que et leurs sels.

The chelating agent forming part of the solution in accordance with the invention can be chosen from:

• - EDTA (or ethylene-diamine-tetracetic acid), NTA (or nitrilo-triacetic acid), DTPA (or diethylene-triamine-pentacetic acid),
• polycarboxylic acids, such as citric acid, oxalic acid, malic acid, glutamic acid, tartaric acid, aspartic acid, glutaric acid, malonic acid and their salts,
• - polyhydroxycarboxylic acids such as gluconic acid, glucoheptonic acid and their salts,
• - polyhydroxypolycarboxylic acids such as glucaric acid or galactic acid and their salts.

Glucoheptonic acid and very particularly gluconic acid or their salts are preferred.

The zinc ion can be brought in any suitable way and in particular in the form of its salts, such as nitrate or sulphate or its oxide.

The amounts of polyphosphate, chelating agents and zinc ion present in the solutions according to the invention are at least 0.2 mmol, 0.3 mmol and 0.15 atg, respectively. per liter.

However, the best results are obtained when the zinc is brought in combined form with the chelating agent, preferably in the form of citrate, tartrate, glucoheptonate and, most particularly, gluconate.

• - de l'hexamétaphosphate de sodium,
• - du gluconate de zinc et
• - un acide minéral choisi parmi les acides sulfurique, chlorhydrique et nitrique, l'acide nitrique étant préféré en raison de son carac-

tère oxydant.A particularly preferred chemical conversion solution therefore comprises:

• - sodium hexametaphosphate,
• - zinc gluconate and
• a mineral acid chosen from sulfuric, hydrochloric and nitric acids, nitric acid being preferred because of its charac-

oxidizing tere.

The amounts of polyphosphate and zinc salts of at least one of the above-mentioned chelating agents present in the solutions according to the invention are respectively at least 0.2 mmol and 0.3 mmol per liter; in the case of HMPP and zinc gluconate, these lower limit quantities are 0.122 and 0.136 g / l respectively.

The upper limits of the quantities of polyphosphate of chelating agent and of zinc salt of the chelating agent forming part of the solution in accordance with the invention do not constitute critical data; theoretically, they are imposed only by the solubility limits; in practice, however, the quantity of polyphosphate is chosen low enough so that the quantities of sludge formed are not annoying.

Given these considerations, the chemical conversion solution according to the invention comprises from 0.25 g / l to 150 g / l of the composition consisting of the polyphosphate and the zinc salt of the chelating agent; preferably, this amount is from 2 to 100 g / l and, more preferably still, from 10 to 80 g / l.

In the case of the above preferred solution, the weight ratio between zinc gluconate and sodium hexametaphosphate is between approximately 10/1 and 1/7, preferably between approximately 8/1 and 1/4 and, more preferably again, between about 5/1 and 1/3.

Still in the case of the above preferred solution, the amount of zinc gluconate is from 10 to 60 g / l and the amount of hexametaphosphate from 2 to 30 g / l.

Still in the case of the aforementioned preferred solution, the pH of the solution is initially, that is to say before implementation, brought to a value less than 2, preferably between approximately 0.7 and 1, 7.

The chemical conversion process according to the invention is characterized in that it comprises the implementation of the chemical conversion solution according to the invention by spraying onto the metal substrate to be treated or by immersion of the substrate in the solution , immersion being preferred.

• - une étape de dégraissage,
• - une étape de rinçage,
• - une étape acide de décapage et/ou d'activation
• - une étape facultative de rinçage, notamment si le bain acide est constitué par une solution d'acide nitrique,
• - l'étape de conversion proprement dite,
• - une étape de rinçage, puis une étape facultative de séchage qui est fonction de la peinture destinée à être éventuellement appliquée dans une phase ultérieure.

The implementation of the conversion solution in accordance with the invention, in the context of the conversion process in accordance with the invention, is situated in a set of treatment steps preferably comprising:

• - a degreasing step,
• - a rinsing step,
• - an acid pickling and / or activation step
• - an optional rinsing step, in particular if the acid bath consists of a nitric acid solution,
• - the conversion stage proper,
• - a rinsing step, then an optional drying step which depends on the paint intended to be possibly applied in a later phase.

From the comparison with the succession of steps of the conversion methods of the prior art, it follows that the method according to the invention can be simplified by in particular the elimination of the passivation and "lubrication" steps.

Indeed, the objects treated by implementing the method according to the invention can be stored in the open air, without any protective treatment (for example lubrication) beforehand and without any degradation phenomena appearing. layer.

The temperature of the solution is between around 40 and 100 ° C, more particularly above 60 ° C and preferably between 65 and 98 ° C.

The contact between the solution and the mé substrate metal is maintained for a time varying, in practice, from 2 seconds to 60 minutes.

When it is the preferred treatment solution comprising zinc gluconate and sodium hexametaphosphate which is used on a metal substrate, it is found that the pH value, initially between approximately 0.7 and 1.7 , increases as a function of the number of objects treated or of the surface treated, by describing a curve which has two characteristic zones similar to landings. Most generally, the first of said zones is in a pH range between about 1.9 and 2.6 and the second is in a pH range between about 2.2 to 3.5, this depending in particular on treated surfaces and treatments prior to the actual conversion step.

It can be seen that the objects which are treated when the pH value corresponds to one of the zones or stages have particularly advantageous qualities.

It has thus been found, for example, that exceptionally high layer weights can be achieved, and this under normal processing conditions. For example, layer weights of the order of 40 to 60 g / m ² were obtained by immersing steel plates in a conversion solution according to the invention for a period of 15 to 25 minutes and at a temperature of 90 ° C.

Nevertheless, it can be pointed out that the weight of the conversion layer obtained at a pH value corresponding to the first level (or layers of "first zone") is less than that of the layers obtained at a pH value corresponding to the second level (or layers "second zone"). "Second zone" layers have exceptional corrosion resistance; but even the "first zone" layers have resistance to the horn significantly higher than that of the conversion layers obtained by conventional phosphating processes.

This remarkable resistance to corrosion, unknown to date, could result from a synergy between the polyphosphate and the chelating agent, including in particular gluconic acid. The Applicant Company has observed that in the presence of the conversion solution according to the invention, the increase in the concentration of sodium hexametaphosphate led to an increase in the thickness of the conversion layer. Without wishing to be bound by theory, it is believed that, during the implementation of the conversion solution, various complexes appear, the nature of which is not yet known and which allow the very homogeneous deposition of insoluble salts on the surface. of metal.

When it is the preferred solution which is used in the context of the process according to the invention, the pH of this solution is first brought to an initial value of approximately 0.7 to 1.7 by means of one of the above-mentioned mineral acids and then, before the actual application, the bath is ripened, in particular by contacting with metallic iron, so as to bring the pH of the solution to a treatment value corresponding to one or the other of the two aforementioned stages, that is to say comprised either between approximately 1.9 and 2.6, or between approximately 2.2 and 3.5.

The choice between one and the other of these levels is made according to the quality criteria sought for the conversion layer.

The pH can be maintained at this level if necessary by adding sufficient quantities of one of the above-mentioned mineral acids.

According to a preferred embodiment of the process according to the invention, the pH of the chemical conversion solution is changed from the initial value the between approximately 0.7 and 1.7 up to a value corresponding to the first and / or second level by adding to the solution a sufficient quantity of iron filings, generally from 0.5 to 4 g and, more preferably , from 0.75 to 3 g per liter of solution; the solution thus "matured" is implemented by immersion or spraying.

The presence in the solution of an ap ~ propriée amount of iron filings, which plays the role of a conversion accelerator, results in a considerable increase in the resistance of the objects treated to the test called "salt spray".

The contact time between the bath and the metal object to be treated can be reduced, from a value commonly situated between 60 and 30 minutes in the absence of filings, to a value of 15 minutes and even 5 minutes.

This accelerating effect of iron filings can be further increased by the addition of a small amount of P0 ₄ H ₃ and in any case much less than the amount of HMPP present in the solution.

• - des agents mouillants,
• - des agents régénérateurs (composés aminés, acide borique et autres),
• - des agents améliorant la conversion à la surface de corps creux tels que certaines pièces de carrosseries automobiles (composés de titane, tels que par exemple TiCl₄),
• - des agents accélérateurs de conversion autres que le fer (tels que le manganèse, le nickel, le cuivre et autres) amenés sous forme de nitrates, nitrites, fluorures, chlorates, sulfures, molybdates ou leurs acides.

In addition to the above-mentioned constituents, the conversion solution according to the invention can advantageously comprise:

• - wetting agents,
• - regenerating agents (amino compounds, boric acid and others),
• agents improving the conversion on the surface of hollow bodies such as certain parts of automobile bodies (titanium compounds, such as for example TiCl ₄ ),
• - conversion accelerating agents other than iron (such as manganese, nickel, copper and others) brought in the form of nitrates, nitrites, fluorides, chlorates, sulfides, molybdates or their acids.

It is pointed out that, among the conversion accelerators, manganese nitrate is particularly preferred and makes it possible to considerably improve the rate of crystallization of the deposit.

The effectiveness of the manganese nitrate is illustrated by the fact that treatment with the solu- _tion according to the invention containing Mn (N0 _{3) 2} gives rise to a crystalline deposit of expansion similar to that which is obtained in the absence of manganese nitrate but in the presence of iron filings and after baking at 135 ° C for 15 minutes. This observation can be made by comparative examination with a scanning electron microscope.

The preferred manganese concentration is between 0.5 and 1.5 g / 1 and, more preferably, between 0.75 g / 1 and 1.25 g / 1.

In addition to the accelerating effect, the presence of manganese contributes to improving the stabilization of the pH at the preferred values, which offers, unlike the phosphating methods according to the prior art, a much greater reproducibility of the tests.

The exceptional mechanical qualities of the conversion layers obtained by implementing the process according to the invention have been demonstrated by mechanical bending tests on a mandrel which have shown that the layer can undergo significant deformation without leaving the slightest trace. of discontinuity by delamination, even for high layer weights.

In addition to their excellent resistance to aggressive media and in particular to salt spray, the conversion layers obtained by implementing the process in accordance with the invention constitute an excellent bonding base for all organic coatings of the glycerophthalic, vinyl, epoxide, polyurethane type. , water-soluble alkyd, air or oven dried, as well than for all metallic coatings such as zinc, cadmium, tin and the like.

These organic or metallic coatings can be applied by brush, by dipping or by pneumatic gun or by spray gun under high pressure without air or also by electrostatic or by electrodeposition by anodic or cathodic way, on the chemical conversion layers obtained. beforehand.

In addition, at the concentrations and for the preferred ratios indicated above, the baths obtained using the conversion solutions in accordance with the invention do not give rise to the formation of the amounts of sludge encountered in the baths of the prior art , thus eliminating the problems of pollution and guaranteeing excellent stability without renewal, the baths of the prior art requiring, for their part, frequent renewals.

Another advantage lies in the fact that the preferred conversion solution according to the invention is essentially based on biodegradable products.

The invention can be understood even better with the aid of the examples which follow and which relate to advantageous embodiments.

EXAMPLE 1

Comparison between the results obtained, on the one hand, with a solution based on zinc gluconate and sodium hexametaphosphate and, on the other hand, with solutions based respectively on zinc gluconate alone and sodium hexametaphosphate only.

To do this, metallic test pieces of steel E 24 - 1 (0.22 X of carbon - 0.075 X of phosphate - 0.062 Â of sulfur) of dimensions approximately equal to 9.5 x 6.5 cm, having previously undergone a cold chemical pickling in 6N hydrochloric medium, are immersed for 60 minutes in one liter baths based three conversion solutions maintained at 95 ° C (solutions A, B, C).

Solution A contains 0.25 g / 1 of zinc gluconate or GZ dihydrate (the concentration is expressed without taking into account the two molecules of water of crystallization).

Solution B contains 0.25 g / 1 of sodium hexametaphosphate or HMPP.

Solution C is obtained by mixing equal volumes of solutions A and B.

The pH of each of solutions A, B and C is adjusted to the value of 2 by adding a few ml of nitric acid.

The effectiveness of the conversion treatment is assessed visually, the results being presented in Table I.

These results show that, under the conditions of the experiment, no conversion takes place with zinc gluconate alone or hexametaphosphate alone.

On the other hand, it seems that a start of uniform conversion of the metal surface appears by the implementation of solution C, thus showing a synergy between the two constituents.

A new series of tests was therefore carried out with increasing concentrations of the composition GZ + HMPP.

EXAMPLE 2

Influence of the concentration of the solution of conversion into zinc salt of the chelating agent, on the one hand, into polyphosphate, on the other hand.

We always use sodium hexametaphosphate or HMPP and zinc gluconate or GZ.

For the tests, as in example 1, the metallic test pieces of steel E 24 - 1 of dimensions approximately equal to 9.5 x 6.5 cm, having previously undergone a cold chemical pickling in hydrochloric medium at 1/2 (6 N), are immersed for 60 minutes in a liter of conversion solution, maintained at 95 ° C.

humidité de 85 à 90 % - pression = 0,9 bar.To assess the degree of chemical conversion, the various test pieces thus treated are exposed to the aggressiveness of a salt spray, obtained using a salt spray test apparatus. The conditions of these tests were as follows:

humidity from 85 to 90% - pressure = 0.9 bar.

• - Solution D contenant 2,5 g/1 de GZ et 2,5 g/1 d'HMPP
• - Solution E contenant 5 g/1 de GZ et 5 g/1 d'HMPP
• - Solution F contenant 10 g/1 de GZ et 10 g/1 d'HMPP
• - Solution G contenant 22,5 g/1 de GZ et 22,5 g/1 d'HMPP.

Four conversion solutions were tested:

• - Solution D containing 2.5 g / 1 of GZ and 2.5 g / 1 of HMPP
• - Solution E containing 5 g / 1 of GZ and 5 g / 1 of HMPP
• - Solution F containing 10 g / 1 of GZ and 10 g / 1 of HMPP
• - Solution G containing 22.5 g / 1 of GZ and 22.5 g / 1 of HMPP.

The visual observations made at the end of the treatment show that a uniform layer of grayish tint appeared on all the samples treated, and that the intensity of the grayish tint as well as its uniformity increases as the bath concentration in GZ + HMPP rises.

To verify that the tint intensity is in relation to the degree of conversion, the treated plates were subjected to the salt spray test for 24 hours.

Before exposure, the edges of the test pieces were protected with an adhesive and a V-shaped scarification was made at the bottom of the plates.

The results obtained are presented in Table II.

It is observed on examining the results gathered in Table II that a concentration of 45 g / l of the composition in the conversion bath makes it possible to obtain excellent protection.

The corrosion resistance of the plates treated with solution G was then tested for several times of exposure to salt spray, and was compared with that obtained with plates treated in a conventional manner, by conventional phosphating with iron and zinc, commercially available.

The results appear in Table III.

It can be seen that the protection obtained by treating the metal plates with the conversion solution containing 45 g / l of the composition sodium hexametaphosphate - zinc gluconate is much greater than that obtained by conventional phosphating, with iron and zinc. This is all the more remarkable since the plates treated in accordance with the invention have not been subjected to a chromating passivation, unlike the phosphate plates according to the prior art.

The concentration of zinc gluconate in the conversion solution remaining fixed at 22.5 g / l, the concentration of sodium hexametaphosphate was established at different values, in order to better determine its influence on the quality of the conversion.

• On immerge successivement des plaques d'acier E 24 - 1, préalablement décapées chimiquement, dans le bain de conversion maintenu à 95°C, dont le pH initial est amené à une valeur de 1 par addition d'acide nitrique, et contenant 1 g/1 de limaille de fer. Après un temps de traitement de 30 minutes, les plaques sont rincées puis séchées à l'air libre. L'épaisseur de la couche de conversion obtenue est alors mesurée à l'aide d'une jauge d'épaisseur du type DIAMETER SM commercialisée par la Société ERICHSEN. Puis, la résistance à la corrosion est déterminée comme précédemment, par mesure de la tenue au brouillard salin.

The experimental method used was as follows:

• E 24 - 1 steel plates, chemically etched beforehand, are successively immersed in the conversion bath maintained at 95 ° C., the initial pH of which is brought to a value of 1 by addition of nitric acid, and containing 1 g / 1 iron filings. After a treatment time of 30 minutes, the plates are rinsed and then dried in the open air. The thickness of the conversion layer obtained is then measured using a thickness gauge of the DIAMETER SM type sold by the company ERICHSEN. Then, the corrosion resistance is determined as above, by measuring the resistance to salt spray.

Correlatively, the pH of the conversion bath is measured after the treatment of each of the plates. To do this, a pH meter of the type 601 A / Digital IONALYSER, marketed by the company ORION RESEARCH _' , is used, provided with a high temperature electrode and calibrated at 95 ° C.

This makes it possible to follow the variation in the thickness of the conversion layer, the resistance to salt spray and the evolution of the pH as a function of the number of plates treated.

Three conversion baths were tested, containing respectively:

• - l'évolution du pH (courbes C₁H, C₁ ^I et C₁ ^J sur les figures 1, 2 et 3) en fonction du nombre n de plaques traitées,
• - l'évolution de l'épaisseur, en µ, de la couche de conversion (courbes C₂ ^H, C₂ ^I et C₂ ^J sur les figures 1, 2 et 3) en fonction du nombre n de plaques traitées,
• - l'évolution de la résistance, en heures h, au brouillard salin (courbes C₃ ^H; C₃ ^I et C₃ ^J sur les figures 1, 2 et 3) en fonction du nombre n de plaques traitées.

For each of the solutions H, I and J, the graphs in FIGS. 1, 2 and 3 are shown respectively:

• the evolution of the pH (curves C ₁ H, C ₁ ^I and C ₁ ^J in FIGS. 1, 2 and 3) as a function of the number n of plates treated,
• - the change in thickness, in µ, of the conversion layer (curves C ₂ ^H , C ₂ ^I and C ₂ ^J in FIGS. 1, 2 and 3) as a function of the number n of plates treated,
• - the evolution of the resistance, in hours h, to salt spray (curves C ₃ ^H ; C ₃ ^I and C ₃ ^J in FIGS. 1, 2 and 3) as a function of the number n of plates treated.

When examining the graphs of FIGS. 1, 2 and 3, it can be seen that the pH changes as the number of plates is treated and that, whatever the concentration chosen for HMPP, the most favorable, if we take the resistance to salt spray as a selection criterion, seems to be between 2.5 and 2.9 under these conditions.

It is indeed in this zone (it is the second zone, the first being around pH 2 ^± 0.1) that the thickest thickness of the conversion layer is obtained as well as the best resistance to salt spray (which moreover evolve correlatively with respect to each other).

There is better resistance to salt spray of the layers corresponding to the second pH zone. But whatever the zone considered, the thickness of the layers and their resistance to fog increase at the same time as the concentration of HMPP.

Under the best conditions, the following resistances to salt spray are obtained, as results from the curves:

These corrosion resistance values can be considered remarkable and no phosphating currently known allows such performances to be achieved.

But the increase in HMPP concentration, if it has a favorable effect on the thickness of the conversion layer and on the resistance to salt spray, is on the other hand troublesome from the point of view of the formation of sludge.

Thus, after exhaustion of each bath, if one proceeds to a filtration, one can note an almost total absence of sludge when the HMPP concentration is 5 g / 1, an amount of sludge of 5 g / 1 (expressed in dry matter) for a concentration of 10 g / 1 of HMPP and an amount of sludge of 20 g / 1 (expressed in dry matter) for a concentration of HMPP of 30 g / 1.

The best compromise between corrosion resistance and sludge formation therefore seems to be achieved under these conditions when the concentration of sodium hexametaphosphate is between 5 g / l and 10 g / l.

EXAMPLE 3

Comparison between the results of a conversion obtained with, on the one hand, a solution based on zinc sulfate and sodium hexametaphosphate and, on the other hand, with a solution based on zinc gluconate and hexametaphosphate sodium.

The experimental method used is identical to that described in Example 2 (influence of the HMPP concentration), except for the treatment time which is limited to 15 minutes.

The concentration of the conversion solutions is maintained whatever the composition.

In other words, whether it is zinc sulphate or zinc gluconate, the concentrations of zinc cations are equivalent.

The HMPP concentration is unchanged regardless of the combination studied.

The solutions tested are therefore:

• - l'évolution du pH (courbes C₁ ^N, C₁ ^O) en fonction du nombre n de plaques traitées,
• - l'évolution de l'épaisseur, en µ, (courbes C₂ ^N et C₂ ⁰) en fonction de n et
• - l'évolution de la résistance à la corrosion, en heures h (courbes C₃ ^N et C₃ ⁰) en fonction de n.

The results are shown in the curves of FIGS. 4 and 5, namely as in Example 2, which show:

• - the evolution of the pH (curves C ₁ ^N , C ₁ ^O ) as a function of the number n of plates treated,
• - the change in thickness, in µ, (curves C ₂ ^N and C ₂ ⁰ ) as a function of n and
• - the evolution of the corrosion resistance, in hours h (curves C ₃ ^N and C ₃ ⁰ ) as a function of n.

• - 60 heures au brouillard salin avec la solution 0 et seulement
• - 8 heures au brouillard salin avec la solution N.

It is very clearly observed, following these tests, that the best composition is indeed that based on zinc gluconate since, if the optimal conditions are obtained for obtaining a satisfactory corrosion resistance, we have :

• - 60 hours in salt spray with solution 0 and only
• - 8 hours in salt spray with solution N.

On the other hand, it can be seen that there are no well-pronounced pH levels when the conversion bath does not contain any complexing agent.

EXAMPLE 4

This example illustrates the use of citric acid as a chelating agent.

The experimental method used is identical to that described in Example 3.

The solution tested has the following composition:

• - l'évolution du pH (courbe C₁ ^R) en fonction du nombre n de plaques traitées,
• - l'évolution de l'épaisseur, exprimée en µ (courbe C₂ ^R) en fonction de n et
• - l'évolution de la résistance à la corrosion en heures h (courbe C₃ ^R) en fonction de n.

The results are shown by the curves in Figure 6; these curves show:

• - the change in pH (curve C ₁ ^R ) as a function of the number n of plates treated,
• - the change in thickness, expressed in µ (curve C ₂ ^R ) as a function of n and
• - the evolution of the corrosion resistance in hours h (curve C ₃ ^R ) as a function of n.

The comparison of these results with those obtained with solution N of Example 3 shows the advantage of adding a complexing agent.

EXAMPLE 5

This example also shows the advantage of introducing the zinc ion in the form of the salt of the chelating agent.

The performances obtained are compared, on the one hand, with a solution based on sodium gluconate plus zinc nitrate in mixture with hexametaphosphate and, on the other hand, with a solution based on zinc gluconate and sodium hexametaphosphate.

The experimental conditions used are those described in Example 2 with regard to the study of the influence of the HMPP concentration. The test pieces analyzed are those treated at a pH value corresponding to the second pH zone.

The concentrations of gluconate anions and zinc cations are equivalent in the two baths studied.

The concentration of hexametaphosphate is the same in both cases.

The composition of the two solutions studied is as follows:

Table IV shows the appearance of the test pieces after treatment and their resistance to salt spray expressed in hours.

From the examination of these results, the interest in using the zinc ion in the form of the salt of the chelating agent becomes clear.

EXAMPLE 6

This example illustrates the advantage of using the polyphosphate ion in place of the phosphate ion provided by sodium dihydrogen phosphate.

The performances obtained are compared, on the one hand, with a composition based on zinc gluconate and sodium dihydrogen phosphate and, on the other hand, with a composition based on zinc gluconate and HMPP in the chemical conversion d '' 9.5 x 6.5 cm steel E 24 - 1 specimens.

The operating mode is that described above.

The immersion time is 30 minutes.

The pH of the bath was successively brought directly to 2, 2.5 and 3 with nitric acid and iron filings.

• - 22,5 g/1 de GZ
• - ₅ g/1 d'HMPP ou de NaH₂P0₄.

The concentration of the two conversion solutions is:

• - 22.5 g / 1 GZ
• - ₅ g / 1 of HMPP or NaH ₂ P0 ₄ .

The results from the point of view of the thickness and of the resistance to salt spray are combined in table V in which "BS (h)" designates the resistance to salt spray expressed in hours and "Ep (p)" the thickness of the conversion layer expressed in p.

The results obtained with HMPP are clearly superior to those obtained with sodium dihydrogen phosphate from the point of view of resistance to salt spray, and this despite thinner thicknesses for the conversion layers.

EXAMPLE 7

Advantage resulting from the addition of a conversion accelerating agent, in particular from the point of view of the crystallinity of the coating.

1 g / l of either iron filings or Mn in the form of Mn nitrate is added to a conversion bath containing 22.5 g / l of zinc gluconate and 5 g / l of sodium hexametaphosphate.

The samples are degreased with acetone then with trichlorethylene and then pickled in an aqueous solution of 6N hydrochloric acid for 5 minutes at 40 ° C.

The treatments are carried out in the same way as in the previous examples.

• - dans le cas du bain au fer, le dépôt est plus ou moins cristallisé ; le dépôt améliore la topographie mais ne masque pas totalement le support ;
• - dans le cas du bain au manganèse, le dépôt est beaucoup plus cristallisé que dans le cas du bain précédent.

On examination with a scanning electron microscope of two sets of three samples treated respectively in the iron and manganese baths, it is found that:

• - in the case of an iron bath, the deposit is more or less crystallized; the deposit improves the topography but does not completely hide the support;
• - in the case of the manganese bath, the deposit is much more crystallized than in the case of the previous bath.

Therefore, Mn nitrate, as an accelerator, acts more sensitively than iron filings on the crystallization of the deposit.

It is noted that, in order to obtain an identical crystallization with iron, the plates must be incubated for 15 minutes at 130 ° C.

EXAMPLE 8

It is shown that the treatment of conversion of metal test pieces with a solution based on zinc gluconate and sodium hexametaphosphate does not alter the adhesion of the paints, compared to conventional surface treatments of the zinc or iron phosphating type.

The method used consists in measuring the force necessary to tear off a pad with a surface area of 3.14 cm ² , glued to a film-forming coating deposited on the surface of a test piece.

The measurements are carried out with an Instron type dynamometer.

The film-forming coatings examined are industrial type paints, applied in a single layer with an automatic film applicator.

• - simplement dégraissées,
• - traitées avec la solution à base de GZ et de HMPP,
• - traitées par une phosphatation classique au zinc,
• - traitées par une phosphatation classique au fer.

Four sets of test tubes are chosen respectively:

• - simply degreased,
• - treated with the GZ and HMPP solution,
• - treated with conventional zinc phosphating,
• - treated by conventional iron phosphating.

Various binders are applied to these test pieces and, after drying, the above pull-out test is carried out, which gives the force F expressed in kgf for the surface of 3.14 cm ² .

The results are combined in Table VI below, F being the average obtained from 5 successive measurements.

• - que la nature de la conversion chimique ne modifie pas l'adhérence des peintures du type glycérophtalique à base d'huile de lin, alkyde hydrosoluble, acrylique,
• - qu'il y a une diminution de l'adhérence des peintures du type polyuréthane sur les tôles traitées avec la composition GZ + HMPP, comparativement aux tôles phosphatées au fer ou au zinc, ou simplement dégraissées,
• - qu'il y a un comportement identique des peintures vinyliques sur les tôles traitées avec GZ + HMPP et sur les tôles phosphatées au fer,
• - qu'il y a une meilleure adhérence des peintures du type époxyde sur tôles traitées au GZ + HMPP, comparativement aux tôles phosphatées au fer ou au zinc.

Examination of the results gathered in Table VI shows:

• - that the nature of the chemical conversion does not modify the adhesion of paints of the glycerophthalic type based on linseed oil, water-soluble alkyd, acrylic,
• - that there is a decrease in the adhesion of polyurethane type paints on the sheets treated with the composition GZ + HMPP, compared with sheets phosphated with iron or zinc, or simply degreased,
• - that there is an identical behavior of vinyl paints on sheets treated with GZ + HMPP and on iron phosphated sheets,
• - that there is better adhesion of epoxy type paints on sheets treated with GZ + HMPP, compared to sheets phosphated with iron or zinc.

Claims (16)

1. Acid solution for the chemical conversion of metallic substrates, characterized in that it comprises: - a polyphosphate, soluble in water and of formula (X PO ₃ ) _n in which n ≥ 3 and in which X is an alkali, alkaline-earth metal or ammonium, - an organic chelating agent and - the zinc ion,
the pH can be brought to the desired value using a mineral acid chosen from the group comprising sulfuric, hydrochloric and nitric acids. 2. Solution according to claim 1, characterized in that the polyphosphate entering into its constitution is chosen from trimeta-, tetrameta-and sodium hexametaphosphate. 3. Solution according to one of claims 1 and 2, characterized in that the chelating agent entering into its constitution is chosen from: - EDTA (or ethylene-diamine-tetracetic acid), NTA (or nitrilo-triacetic acid), DTPA (or diethylene-triamine-pentacetic acid), polycarboxylic acids, such as citric acid, oxalic acid, malic acid, glutamic acid, tartaric acid, aspartic acid, glutaric acid, malonic acid and their salts, - polyhydroxycarboxylic acids such as gluconic acid, glucoheptonic acid and their salts, - polyhydroxypolycarboxylic acids such as glucaric acid or galactic acid and their salts. 4. Solution according to claim 1, character laughed at by the fact that polyphosphate is sodium hexametaphosphate. 5. Solution according to claim 1, characterized in that the chelating agent is chosen from hydroxy-carboxylic acids. 6. Solution according to claim 1, characterized in that the zinc is brought in combined form with the chelating agent. 7. Solution according to claim 1, characterized in that it comprises: - sodium hexametaphosphate, - zinc gluconate and - a mineral acid chosen from sulfuric, hydrochloric and nitric acids. 8. Solution according to one of claims 1 to 7, characterized in that it contains at least 0.2 mmol per liter of polyphosphate, 0.3 mmol per liter of chelating agent and 0.15 at.-g . zinc ion. 9. Solution according to one of claims 1 and 7, characterized in that it comprises from 0.25 to 150 g / 1, preferably from 2 to 100 g / 1 and, more preferably still, from 10 to 80 g / 1 of the composition consisting of the polyphosphate and the zinc salt of the chelating agent. 10. Solution according to claim 7, characterized in that the weight ratio between zinc gluconate and sodium hexametaphosphate is between approximately 10/1 and 1/7, preferably between approximately 8/1 and 1/4 and, more preferably still, between approximately 5/1 and 1/3. 11. Solution according to claim 7, characterized in that it comprises from 10 to 60 g / 1 of zinc gluconate and from 2 to 30 g / 1 of sodium hexametaphosphate. 12. Process for the chemical conversion of metallic substrates, characterized in that it com carries the implementation by spraying or by immersion of the chemical conversion solution according to one of claims 1 to 11. 13. The method of claim 12, characterized in that one implements the solution according to claim 7 after having brought its pH initially to a value between about 0.7 and 1.7 and then changes this pH by contacting with metallic iron up to a treatment value chosen either between approximately 1.9 and 2.6, or between approximately 2.2 and 3.5. 14. The method of claim 13, characterized in that the contacting with metallic iron is carried out by the addition of iron filings in an amount of 0.5 to 4 g, preferably from 0.75 to 3 g per liter of solution. 15. Method according to one of claims 12 to 14, characterized in that the temperature of the solution as used is 40 to 100 ° C, preferably greater than 60 ° C and, more preferably still, of 65 to 98 ° C. 16. Method according to one of claims 12 to 15, characterized in that the contact between the solution and the metal substrate is maintained for 2 seconds to 60 minutes.

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