EP0006046A1 - Use of a solution for forming protective layers on zinc surfaces by electrolysis - Google Patents

Abstract

The invention relates to the protection of zinc surfaces by depositing a protecting coating. The process of the invention comprises the formation of a pyrophosphate coating by electrolytic oxidization of the articles treated in contact with a solution containing pyrophosphate ions or ions capable of giving pyrophosphates. The invention is particularly useful for protecting galvanized objects in contact with aqueous environments.

Description

The invention relates to a method for forming protective coatings on zinc surfaces. It also relates to compositions used for the implementation of this process and to objects having a coating as obtained by said process.

Zinc surfaces and particularly surfaces obtained by galvanizing ferrous metals are sensitive to corrosion. Particularly in prolonged use, natural waters constitute aggressive environments with regard to galvanized objects.

To protect the zinc surfaces and in particular the surfaces obtained by galvanization, but also, if necessary, to facilitate certain subsequent treatments, it is known to form on the zinc a protective surface layer of insoluble phosphates.

This coating, in the techniques known up to now, is produced by contacting the parts with a treating solution containing phosphate ions. The deposition of insoluble salts from this solution results from the chemical shift of an equilibrium in which, of course, the constituents of the solution participate, but also the metal of the treated surface. In this operation, the parameters which influence the result obtained are multiple. The main ones are of course linked to the composition of the solution (nature and concentration), to its pH, but other factors such as temperature, duration of treatment, etc. are also involved.

In general, the treating solutions for the formation of a protective deposit of phosphates contain one or more phosphates soluble in acid solution. This solution, in contact with the metal surface, produces a light attack of the metal and the pre cipitation of insoluble zinc phosphates.

These known techniques are difficult to implement. To function satisfactorily, as we have indicated, solutions must have a series of well-defined characteristics. However, the use of these solutions quickly leads to a modification of these characteristics, so that the user is required to control and permanently adjust their composition. Failing to place themselves in the right conditions, either the deposit does not form, or it is accompanied by undesirable phenomena, and in particular significant precipitation of sludge which affects the quality of the coating and leads to an unnecessary depletion of the solution. .

Another type of difficulty is linked to the fact that, for corrosion treatments, the time required to form the coating is usually several. hours or even several days. The length of the treatment is all the more troublesome since it is most often carried out at temperatures above ambient temperature and requires permanent heating.

For all these reasons, the known methods are inconvenient and relatively expensive to implement.

Whatever measures are taken to maintain the treatment solution in the best conditions, the results obtained are not always satisfactory, one of the defects most often encountered being the lack of uniformity of the coating on the same batch of objects, or even to different parts of the same object. We know, in the field of corrosion protection, the importance of achieving perfectly uniform coatings otherwise the corrosion phenomena are concentrated where the coating is lacking, thus leading to rapid deterioration of the object under consideration. One reason for this type of fault is the difficulty in obtaining homogeneous contact conditions of the solution with the surface to be treated. Also, to limit the risk of such defects, an effort is made to ensure agitation of the objects or circulation of the solution, hence an additional burden for the implementation of these methods.

In view of what has just been indicated, the applicant has endeavored to find a process which would eliminate, or at least reduce, the drawbacks observed so far in treatments for protecting zinc surfaces, and in particular surfaces galvanized.

This could be obtained thanks to the invention which consists of a method of forming a coating of zinc pyrophosphates on a zinc surface, characterized in that the objects whose surfaces are to be coated are subjected to electrochemical oxidation with within an electrolyte containing pyrophosphate ions or phosphate ions capable of giving pyrophosphates by displacements of chemical equilibria.

The oxygenated acid species of phosphorus at the degree of oxidation V and the corresponding ions are very varied. These are in particular orthophosphates, pyrophosphates, metaphosphates. Also known are associated acids in macromolecular form, with an average composition intermediate between that of pyrophosphoric and metaphosphoric acids. All these species in aqueous solution give rise to equilibria and, depending on the overall concentration, the pH, the temperature, the proportions of each of them can vary.

It is preferable, according to the invention, to choose a solution for which the phosphorus condensation of the phosphate ions present is close to that which corresponds to the pyrophosphates, or even is greater than this condensation.

In the following description, we will therefore designate, for simplicity, a solution of phosphates or else of pyrophosphates a solution in which different species can coexist.

Likewise, in the following description and examples, the protective coating formed is designated as being zinc pyrophosphate. Analysis of the coating indeed shows that zinc pyrophosphate is the essential constituent of the deposit formed under the preferred implementation conditions of the process according to the invention. However, other insoluble zinc phosphates can be precipitated with the pyrophosphate.

To prepare the electrolyte, it is advantageous to use a solution of soluble pyrophosphates and consequently containing phosphate ions and in particular pyrophosphates.

It is also possible to prepare the electrolyte used for the formation of the pyrophosphate coating from other polyphosphates or from a mixture of polyphosphates capable of giving pyrophosphates under the conditions which are those of use of this electrolyte. As we have already indicated, it is well known in the prior art in particular that the polymetaphosphate ions possibly mixed with orthophosphate ions can lead, in solution and by equilibrium reactions, to mixtures of ions containing in particular pyrophosphates.

Consequently, what is indicated in the following description, referring for convenience to pyrophosphate solutions, also applies to solutions of other phosphates, and in particular of polymetaphosphate optionally in admixture with orthophosphates, since they can lead to pyrophosphates under the conditions of use described.

The solutions can also be prepared from ", orthophosphates by dehydration using phosphoric anhydride P ₂ O ₅ .

Pyrophosphates are on the whole very poorly soluble, and in practice to obtain solu sufficiently concentrated, we are led to choose those whose solubility is the highest possible. For this reason, use is preferably made, according to the invention, of a solution of alkaline pyrophosphates, in particular those of sodium or potassium.

The content of polyphosphate ions, and in particular pyrophosphates, in the solution is limited in value by the solubility characteristics. This content can be increased without leading to the formation of precipitate by modifying for example the pH conditions of the solution. Conversely, the presence of cations capable of leading to very poorly soluble compounds, in particular of Zn ²⁺ ions, reduces the admissible content of pyrophosphate ions.

In other words, the conditions of solubility of the phosphates of cations deemed insoluble are a function not only of the respective ionic concentrations of phosphates and cations but also of the pH range of the solution, it being understood that, when one speaks of insoluble phosphates, one refers to solutions that are neutral or that do not stray too far from neutrality.

Furthermore, it is advantageous to operate with a solution whose pyrophosphate content is as high as the solubility conditions allow in order to facilitate and accelerate the formation of the deposit during the treatment according to the invention.

Advantageously, a solution is used whose overall phosphate content expressed as phosphorus is between 0.6 and 12.5 g / l. Typically, when the solution is prepared using sodium pyrophosphate, 0.01 to 0.2 mole / l, and preferably about 0.1 mole / l, or 44.6 g / l, is used. from ^N a ₂ ^P ₂ ⁰ ₇ '10 ^H ₂ 0.

According to the invention, it is preferable to use a weakly acidic electrolyte. Indeed, a basic solution could lead to a modification of the structure of the deposit by the formation of basic salts or zinc hydroxides, and, ultimately, could lead to the dissolution of the object's zinc. It is advantageous to operate with an electrolyte whose pH is less than 6.

- A very strongly acidic solution is also not desirable. Such a solution, by promoting the solubility of zinc phosphates, would delay the formation of the desired coating. Furthermore, if the solution is very acidic, it can attack the zinc surface to be coated inappropriately. According to the invention, the electrolytes used advantageously have a pH which is not less than 2.5.

In addition, depending on whether the electrolysis is carried out in direct current or in alternating current, which will be specified below, the preferred pH conditions vary a little. Advantageously, if one operates in direct current, the pH is between 2.7 and 3, and if one operates in alternating current, between 4.5 and 5.

The choice of acid used to adjust the pH is not critical. The most advantageous is to acidify by means of phosphoric acids and in particular orthophosphoric acid, these acids contributing to the establishment of the desired phosphate ion content, but it is also possible to use other acids, in particular sulfuric acid.

In addition to the pyrophosphate ions (or the ions giving pyrophosphates) which are essential for the formation and implementation of the electrolyte used in the process according to the invention, it is possible to add other compounds for the purpose to improve or facilitate the achievement of results. It is thus advantageous to introduce into the electrolyte compounds releasing Zn ²⁺ ions. In fact, as already indicated, the precipitation of zinc phosphate on the treated surface results from the imbalance caused in contact with the surface itself by the Zn ²⁺ ions formed by electrochemical oxidation. The presence of Zn ²⁺ ions in the solution, before the oxyda electrochemical action, facilitates the formation of the precipitate by bringing the solution closer to the limit conditions of solubility, and the necessary electrochemical oxidation of the treated surface is all the more limited as the composition is closer to the zinc and phosphate contents. beyond which precipitation would begin.

It goes without saying that the addition of compounds releasing zinc ions is limited so that a precipitate does not occur before the electrochemical oxidation of the surface has been started. The permissible content of Zn ²⁺ ions depends mainly on both the content of phosphate ions and the pH of the solution.

Under the conditions of phosphate content and of pH indicated above, and provided that the solubility conditions which we have seen are respected, an electrolyte is advantageously used, the Zn ²⁺ content of which can range up to 8.10 ^{- 3} ion g / 1.

As an indication, taking into account the influence of pH, for the same content of pyrophosphate ions of 0.1 ion g / 1, advantageously used, at pH 3, 2.5 × 10 ion g / 1 of Zn, and 6 x 10 g / 1 ion when the pH is lowered to 2.7.

As a compound introduced into the electrolyte to release Zn ²⁺ ions therein, it is advantageous to use zinc chloride or zinc oxide. When using zinc oxide, it is necessary to take into account its own basicity and to modify the quantities of acid used to maintain the pH of the solution within the preferred values.

The temperature at which the process according to the invention is carried out is not critical. This temperature is not, however, indifferent. An increase in temperature results in two antagonistic effects, on the one hand, an increase in the solubility of phosphates, which consequently is contrary to the formation of the precipitate, on the other hand, an increase in cine chemical tick of the process corresponding to the shift of equilibria, which promotes the formation of the precipitate. Overall, however, it seems that the second effect predominates and that an increase in temperature accelerates the formation of the deposit.

The operation is advantageously, according to the invention, at a temperature between 20 and 70 ° C., and preferably at around 60 ° C.

In practice, the electrolytic baths having a tendency to heat up by Joule effect, when they are in service, one can operate at the temperature which establishes itself, without it being necessary to heat them or cool them.

In the implementation of the method according to the invention, electrolysis is carried out in the traditional way, with an electrolyte as defined above.

Electrolysis can be carried out by direct or alternating current. In both cases, a deposit of zinc pyrophosphates generated by the surface anodic oxidation of the zinc of the treated object is obtained.

When operating with direct current, the treated object is placed at the anode of the electrolysis device. The electrolytic yield is all the better as the electrolyte used is closer to the conditions for precipitation of insoluble phosphates. Even very limited oxidation of the anode, by modifying the equilibrium conditions of the electrolyte, then allows the desired deposit to be formed.

Still when operating in direct current, it is necessary to avoid that the cathodic reaction modifies the equilibrium conditions of the electrolyte, and in particular it is necessary to avoid depositing zinc. For this purpose, the anode and cathode compartments are separated using a membrane which does not allow the Zn ²⁺ ions to pass. Membranes of this type are well known in the fields of industrial electrochemistry; they may in particular be asbestos membranes or anion exchange resins.

The mechanisms which lead to the formation of the phosphate coating are not known in detail with certainty when operating with alternating current and the surface of the treated zinc successively plays the role of anode and cathode. Previous studies for other systems have nevertheless shown that, in alternating current electrolysis, the formation of an "insulating" coating tends to be accompanied by a half-wave rectifying effect which does not generate coating formation.

In addition to the advantage of using a current which does not need to be rectified, it is noted that the operation in alternating current can lead to a more regular and finer grain deposition.

One can reasonably think that in the present case, when the electrolysis is carried out with a zinc electrode on which the deposition of pyrophosphate behaves as an insulator, the reaction mechanisms are indeed of this nature. Anyway, we obtain the formation of the desired pyrophosphate coating.

An additional advantage provided by the use of alternating current according to the invention is that it becomes superfluous to use a diaphragm. This is particularly advantageous when the treatment envisaged requires the use of devices (tanks, electrodes, etc.) of complex shapes.

The cathode is chosen from a material such that it remains stable in contact with the electrolytic solution and, consequently, does not modify the composition of the latter. Cathodes of steel, lead or graphite can in particular be used.

To carry out the electrolysis forming part of the process according to the invention, the only condition necessary for the electrical parameters is that the voltage applied to the electrodes is sufficient, under the operating conditions, for the electrochemical oxidation to be super zinc of the treated objects occurs. This voltage depends on many factors: nature of the electrolysis, nature and configuration of the electrodes, arrangement of the electrodes, resistance of the diaphragm, etc. In practice, the voltage is fixed so that there is oxidation of the zinc of the objects treated regularly, which can be controlled by the density of the anode current.

The applied voltage should not exceed that at which one would lead to the electrolysis of the water of the solution used, failing which, in addition to detrimental modifications of the bath, the faradaic yield of the operation would drop significantly. Preferably, continuously, current densities of the order of 40 A / m are used. It is of course possible to operate with lower current densities, but the formation of the deposit is then slower and the treatment is longer for a deposit of the same size.

As an indication, potential differences applied to experimental electrolytic systems, and which have made it possible to obtain values of current densities indicated above, are between approximately 2 and 4 volts. However, as has been said, these values are not limiting since they depend on factors which can be modified at will without departing from the scope of the process which is the subject of the invention.

In alternating current, the current densities and effective voltage applied are higher. Advantageously, the current density is adjusted to values of 40 to 100 A / m ^2. As previously, for information, to establish current densities of the order of those which have just been indicated and for the installations used, the applied voltages are between 5 and 10 V.

The duration of treatment varies according to the size of the deposit sought, but also according to many factors such as current density, temperature. rature, the concentration of the solution, etc.

When the treatment according to the invention aims at the complete formation of a protective coating, given as we have indicated the insulating nature of the layer of pyrophosphates deposited, the treatment is continued until the necessary voltage applied becomes too large, risking the electrolysis of water or the "breakdown" of the protective coating.

By way of example continuously, by applying an initial current density of 40 A / m ² with an electrolyte meeting the preferred conditions defined above, the protection is completed in approximately 30 minutes.

Treatments leading to thinner coatings can also be practiced thanks to the invention.

It is also possible to combine the method according to the invention with other treatment methods, and in particular with purely chemical methods, for the formation of protective coatings. The advantage of electrolytically treating, according to the invention, objects having previously undergone protection by chemical deposition of insoluble phosphates is that the deposit obtained by electrochemical means will be located at the precise points where the first coating is missing or insufficient. Thus, by a limited additional treatment, the defects are healed and a very uniform coating is obtained, which is essential to guarantee effective protection.

Optionally, the order of the electrochemical treatments according to the invention and chemical treatments according to traditional methods can be reversed.

In general, objects of the most diverse forms can be treated by the method according to the invention. As far as possible, it is preferable to operate in such a way that the electric field lines are evenly distributed at the surface of the treated object. This can be promoted by traditional means such as, for example, the use of associated electrodes having particular shapes.

The effectiveness of the protection conferred by the treatment according to the invention was checked on samples of galvanized sheet metal. These tests were carried out, on the one hand, by determining the polarization curves or intensity-potential curves of the treated samples compared to those of control samples, and, on the other hand, by subjecting treated and control samples to accelerated corrosion tests by contact with aerated solutions containing cupric and chloride ions.

The results obtained show the excellent resistance to corrosion of the treated samples compared to the control samples.

Claims (17)

1. A method of forming a coating mainly consisting of zinc pyrophosphate on a zinc surface, characterized in that the objects whose surfaces are to be coated, placed in contact with an electrolyte containing pyrophosphate ions or capable phosphate ions to give pyrophosphates, are subjected to a surface electrolytic oxidation of zinc resulting in the precipitation of zinc pyrophosphate. 2. Method according to claim 1, characterized in that the electrolyte is formed by dissolution of one or more soluble phosphates from the group comprising pyrophosphates, metaphosphates, polyphosphates, or mixtures of the above and orthophosphates, or acids correspondents. 3. Method according to claim 2, characterized in that the content of phosphate ions in the solution, expressed as phosphorus, is between 0.6 and 12.5 g / 1. 4. Method according to any one of the preceding claims, characterized in that the electrolyte is adjusted to a pH of between 2.5 and 6. 5. Method according to claim 4, characterized in that the electrolyte further contains Zn2 + ions up to a content of 8.10 ^-3 ion g / 1 .. 6. Method according to any one of the preceding claims, characterized in that the temperature of the electrolyte is between 20 and 70 ° C. 7. Method according to any one of the preceding claims, characterized in that the electrolysis is continued until a deposit insulating the surface of the treated zinc is formed. 8. Method according to any one of the preceding claims, characterized in that, when operating in direct current, the treated objects are placed at the anode of the electrolysis device. 9. Method according to claim 8, characterized in that the cathodic reduction of the ions is prevented zinc by separating the cathode from the electrolyte by a membrane preventing the passage of these ions. 10. Method according to any one of claims 1 to 7, characterized in that one operates on alternating current. 11. Method according to any one of the preceding claims, characterized in that the electrolysis is carried out by applying a voltage sufficient to cause a surface oxidation of the zinc of the object treated, and is less than that which would be accompanied of the electrolysis of the water in the solution. 12. Electrolytic solution for implementing the method according to any one of the preceding claims, characterized in that it is formed by a solution of one or more soluble phosphates from the group comprising pyrophosphates, metaphosphates, polyphosphates, or mixtures of the above and orthophosphates, or of the corresponding acids, and that it has a pH of between 2.5 and 6. 13. Solution according to claim 12, characterized in that, overall, the phosphorus condensation of the phosphate ions in solution is close to that corresponding to or greater than the pyrophosphates. 14. Solution according to claim 12 or claim 13, characterized in that the phosphate content, expressed as phosphorus, is between 0.6 and 12.5 g / 1. 15. Solution according to any one of claims 12 to 14, characterized in that it also contains Zn ²⁺ ions at a content of up to 8.10 ^-3 ion g / 1, but remaining less than that corresponding to the solubility limit value, taking into account the phosphate concentrations and the pH of said solution. 16. Product having a zinc surface coated with a protective layer of pyrophosphates having the characteristics of that obtained by placing implementing the method according to any one of claims 1 to 11. 17. Product according to claim 16, characterized in that the protective layer is formed on a galvanized surface.