EP0285535A1 - Chemical surface treatment process of aluminum products before phosphating - Google Patents

Abstract

This process for treating surfaces of coated products by soaking in a bath of molten zinc and aluminum, and intended to be phosphated, is characterized by an operation, prior to phosphating, of pickling with a strongly alkaline solution of high pH and containing zinc in solution. The process is mainly applied to products with a high proportion of aluminum coating, particularly for galvanized sheets, which it facilitates phosphating. Application to the field of sheet metal in the automotive industry.

Description

The present invention relates to the chemical surface treatment of a metallurgical product made of aluminum alloy, or even of technically pure aluminum. More particularly, the invention relates to such products forming a coating on a substrate which, most often, is a steel sheet.

We know indeed that the current market, that of the automobile in particular, is very captive of this kind of steel sheets coated on at least one face, generally called "galvanized", and this mainly because of their resistance property corrosion, which authorizes their use in the state. It also happens that we want to paint them, in which case a preliminary phosphating treatment is necessary to ensure good adhesion of the paint.

Although the invention can generally be applied to monolithic products, such as aluminum sheets, we will consider hereinafter, for the sake of clarity, only the case, moreover probably the most frequent, steel sheets coated with a layer of a compound of the "zinc-aluminum" type.

It is known that these aluminous compounds have on the surface a thin film of oxide of these metals, and particularly of alumina, which forms naturally. The presence of these oxides on the surface disrupts the subsequent surface treatment operations to which the sheet may be subjected, and in particular the phosphating operation.

In particular, it has been found that it was difficult to obtain correct phosphating, and even sometimes that phosphating was impossible, when the aluminum content of the coating was high.

For example, it has been observed that sheet steel coated with a Zn-Al alloy containing 5% aluminum can only be properly phosphated if fluoride ions are added to the phosphating bath (French patent application no. ° 2,575,188).

In the case of a zinc-aluminum coating with a higher aluminum content, of the order of 55% and beyond for example, phosphating was not carried out at all in a fluorine-free bath, and in a bath which contained it, the phosphate layer formed on the aluminum-rich coating was very heterogeneous and porous.

French patent application 2,567,158 describes a process for preparing a hot-dip galvanized steel product by soaking in a bath containing zinc or a Zn-Al alloy, and intended to be phosphated. This process consists in forming on the product an electrolytic deposit of a metal chosen from a determined group of metals, this metal possibly, in particular, being zinc. According to this document, the zinc layer thus deposited on the oxide film formed on the surface of the coating, ensures better adhesion of the phosphate layer deposited later.

The process of electrolytic deposition in an acid medium, described above, has the disadvantage, on the industrial level, of requiring a modification of the whole of the usual phosphating chain, due to the introduction of a treatment. additional involving the use of an electrolytic tank, and its power supply.

Even more, as the aforementioned document indicates, the electrolytic deposition is preceded by an electrolytic pickling bath (electrochemical polishing) necessary to remove the layer of oxides formed on the surface without polluting the zinc deposition bath. It is easily understood that, because of the two electrolytic baths, this process is particularly greedy in electrical energy, which represents a big disadvantage from the economic point of view.

Another document, patent application GB 2 152 955, also relates to a process for treating sheets coated with a zinc-aluminum alloy in order to facilitate a subsequent phosphating operation. According to this process, the sheet is treated in an alkaline solution intended to strip the aluminum existing on the surface. However, as the examples described reveal it, the coating of the treated sheets is relatively poor in aluminum (less than 10%).

In addition, the solution used contains ions of at least one of the following metals: nickel, iron, cobalt, manganese. These metals can deposit on the surface of the coating and, although they are likely to facilitate subsequent phosphating, they form with the zinc of the coating, a heterogeneous surface layer. Within this layer can be created electrochemical phenomena (phenomena of batteries between different metals) leading to a subsequent degradation of it.

The present invention aims to facilitate the phosphating of an aluminous coating, and more particularly of a coating with a high aluminum content, while avoiding the phenomena described above.

Another object is to improve the bonding and the behavior of the layer of phosphates deposited subsequently during phosphating.

Yet another object is to provide pretreatment before phosphating at a reduced cost.

More specifically, the object of the invention is to produce on the coating intended to be phosphated, a deposit of metal other than aluminum and in particular zinc, having properties for bonding phosphates, this deposit being sufficient to ensure a good adhesion of the phosphate layer deposited later.

With these aims in mind, the present invention relates to a process for the surface treatment of an aluminous product intended to be subjected to a phosphating operation, process during which said product is covered with a layer of metal other than the aluminum, and having phosphate bonding properties, and characterized in that to produce this layer of metal, an etching treatment of said product is carried out by means of a strongly alkaline solution containing said metal in solution.

According to a particular characteristic of the invention, the alkaline solution has a pH greater than 11, and preferably at least equal to 12.5 /

According to another characteristic, the alkaline solution is a caustic soda lye which contains at least five moles of NaOH per liter of water.

According to an additional characteristic, the treatment is carried out at a temperature of approximately 10 to 65 ° C., and preferably at ambient temperature (ie of the order of 15 to 20 ° C.).

According to yet another characteristic, the treatment is carried out for a time sufficient to obtain a deposit of metal in the form of a substantially continuous layer, the latter preferably having a thickness of approximately 1 μm.

The invention also relates to the application of the method described above to the phosphating of coated sheets by dipping in a bath of molten metal comprising a high proportion of aluminum. In this case, after a possible first degreasing of the sheet with solvents, it is passed, in accordance with said process, into a strongly alkaline degreasing bath containing the metal in solution and maintained at ambient temperature, then it is passed through a cold rinsing bath, followed by a refining bath containing titanium or nickel, then in the phosphating bath.

As will no doubt have been understood, a decisive advantage of the method according to the invention is that it consumes no electrical energy, which makes it economically particularly advantageous. In addition, no significant modification is necessary to adapt a conventional phosphating chain to the process, since only the composition of the alkaline degreasing bath is modified.

Indeed, the inventors discovered, during a study on the influence of surface treatments of galvanized coatings on the adhesion of paints, that, while the problems of phosphating of sheets coated with Zn-Al alloys high proportion of Al were well known, there was no need to resort, to solve these problems, expensive methods of electrolytic deposition of zinc. The inventors have in fact discovered that, unlike the electrolytic deposition of zinc in an acid bath, the deposition of zinc can be carried out by simple passage of the coated sheet metal to be phosphated in a strongly basic pickling bath containing zinc in solution.

The results of the tests carried out have shown that the deposition rate and the thickness of the zinc layer thus formed essentially depend on the NaOH concentration of the alkaline bath, the deposition being facilitated when the pH of the bath is high. The tests carried out also made it possible to show that the zinc concentration of the bath influences the characteristics of the deposit: an increase in the quantity of zinc in solution promotes the deposit, and makes it possible to compensate, to a certain extent, for the effects of a possible reduction in the pH of the bath.

It should of course be understood, as explained moreover in the document FR 2 567 158 mentioned above, that the result targeted by the invention can be achieved by a surface deposit of zinc metal on the coating of the sheet, this metallic zinc making it possible to bond the layer of phosphates, which themselves are generally zinc phosphates. Attention must however be paid to the fact that phosphating takes place in an acid medium (which would naturally tend to eliminate the deposit of zinc), and that to avoid an overly pronounced attack of said deposit leading to the exposure of the 'underlying aluminum, the zinc deposit must have a sufficient thickness. Furthermore, the deposit must not be too thick either, the deposited zinc then having a tendency to be powdery, which would harm the adhesion of the phosphating layer. Taking into account some irregularities due to the presence of the two metals: zinc and aluminum, as an undercoat, we can consider as optimum an average thickness of the order of 1 µm.

It can also be admitted that the deposited zinc layer may not be absolutely continuous, that is to say that if slight imperfections of this layer appear, they will not disturb the germination of the phosphates, during the phosphating operation, these- these are inserted into any imperfections in the zinc layer. This tolerance should not, of course, be extended, otherwise the anti-corrosion properties of the product may deteriorate rapidly.

The inventors have tried to understand and explain the surprising effect of the deposition of zinc in solution in an alkaline pickling bath on a Zn-Al substrate.

The hypothesis is that the alkaline pickling bath (also called degreasing), insofar as its pH is sufficiently high, chemically attacks the substantially continuous surface layer of alumina present on the surface of the Al-Zn alloy, thus as the immediately underlying layer of zinc and aluminum oxides, until the substrate surface is completely deoxidized. Metallic zinc and aluminum then appear, which form with pickled zinc and aluminum in solution in ionic form, two red-ox couples. Due to the differences in respective potentials of these two pairs, the zinc in solution has a strong tendency to redeposit on the substrate and in particular on the aluminum visible on its surface, thus quickly forming a continuous layer of zinc. The dissolved aluminum remains in solution or can be complexed and deposited in the form of sludge at the bottom of the bath.

One might think that the quantity of zinc dissolved by basic attack on its oxide is sufficient to reform the surface layer by redepositing on the substrate. In fact, as already indicated above, the addition of zinc to the base solution of the pickling bath facilitates and accelerates the deposition and compensates for the effects of a possible decrease in pH.

A more in-depth study of the chemical reactions during deposition would however be necessary to fully elucidate the phenomena observed.

It will be noted that, although the method according to the present invention uses the pickling of surface aluminum with an alkaline solution notably revealed by the document GB 2 152 955 already cited, it differs greatly in principle. It is important to remember beforehand that, at high concentrations of alkaline agent, the pH of a solution is not representative of said concentration. In addition, it is likely to vary greatly during treatments such as those considered here, this being due in particular to the reactions occurring in the solution during the treatment and also to carbonation, resulting from contact of the solution with the ambient medium and the carbon oxides it contains.

The inventors have been able to establish that, at the concentrations referred to in document GB 2 152 955 (1 to 60 g of sodium hydroxide per liter), the solution used for pickling the coating would see its pH drop rapidly during treatment, greatly reducing this does the pickling efficiency. These concentrations may however prove to be sufficient to surface-pickle the surface alumina and allow the deposition on the remaining alumina of metals, such as Ni, Cr, Mn, contained in the solution in ionic form.

On the other hand, the presence of residual alumina after a partial pickling would prohibit the deposition of zinc thereon. The inventors have in fact discovered that it is possible, thanks to a solution having a very high concentration of alkaline agent (normality at least equal to 5), to strip the coating until removing the alumina and not allowing to remain, besides zinc coating, than metallic aluminum, which was not achievable by the aforementioned prior methods. This then authorizes the deposition of the zinc in solution (zinc provided in the form of salt or dissolved zinc following the pickling of the coating). In particular, it is possible to use a solution containing at least 200 g of NaOH per liter.

A primary advantage of this process is to provide a surface layer containing only zinc, which avoids the problems of dissolution caused by the electrochemical phenomena described in the first part of this thesis.

We will now describe more precisely, by way of example, the phosphating process of a sheet, of the "Galvalume" R type, coated by hot immersion of the steel sheet. in a bath of Zn-Al alloy at 55% Al and 1.6% Si. As specified above, this coating, with a high proportion of aluminum, presents difficulties in surface treatment. However, this product, whose main current applications are in the building sector, and, to a lesser extent, in that of automobile construction, could see its use increase significantly in these sectors of activity as material of appearance, provided that it can be phosphated correctly to guarantee good adhesion of the paints and lacquers.

It will be noted in particular that the method according to the invention makes it possible to dispense with the operations of protecting a front face galvanizing, or eliminating by scraping the layer deposited during the galvanizing, operations which were necessary to produce the sheets. galvanized on one side, which are used in this field of activity.

To this end, a phosphating process including the surface treatment method according to the invention comprises the following steps:
- the sheet goes into a degreasing bath of the solvent type to remove most of the surface oils or greases;
- then, after a rinsing bath, the sheet passes into an alkaline pickling bath (commonly called "degreasing" bath, this bath being strongly basic, with a pH greater than 12, such as, for example, a sodium hydroxide solution 400 g of NaOH per liter of water. This bath contains an aqueous solution of zinc introduced in the form of zinc powder or zinc oxide, at the rate of a few grams per liter. Contrary to usual practice, avoid Agitate the bath by injecting air, which promotes carbonation of the bath and therefore a decrease in its pH. Mechanical agitation is preferred, for example. free from the bath, just as we will try to minimize the carbonate content in the bath water. Similarly, we may prefer to choose a low bath temperature, which will thus avoid carbonation of the bath and the concomitant decrease in pH. The duration of the treatment in the alkaline bath influencing the thickness of the zinc layer deposited, the treatment will be carried out for a time sufficient to obtain the deposition of a substantially continuous layer. In the case of a strongly basic bath (400 g of caustic soda per liter of water), this time could for example be around 45 s;
- the sheet is then rinsed cold in water;
- Then it passes into a surface conditioning bath, called refining bath, containing titanium or nickel in solution, intended to facilitate the subsequent germination of the phosphates;
and finally, it passes into a phosphating bath, then the various rinsing baths with water, chromic rinsing, steaming, etc.

Obviously, the process described above is not limitative of the invention, and numerous variants can be applied without departing from the field covered by the invention. In particular, the invention can be applied to all products or coatings comprising an alloy of zinc and aluminum, from low Al percentages of the order of 5%, up to practically 100%, i.e. pure aluminum. It is obvious, however, that the main interest of the method is revealed for alloys comprising a high proportion of aluminum, greater than about 50%.

The characteristics of the alkaline bath used can also vary within the limit where it remains strongly alkaline. One can, for example, consider not using a degreasing bath upstream of the alkaline bath, or introducing therein nonionic surfactants, this solution can however present the risk of rapid pollution of the degreasing bath (or pickling) alkaline.

Although the treatment at low temperature, close to ambient, has advantages in maintaining a high pH, by avoiding carbonation of the bath, it will nevertheless be possible to increase the temperature to around 60 or 65 ° C without risk of unacceptable carbonation.

Claims (10)

1) Process for the chemical surface treatment of an aluminous product with a view to its phosphating, in particular of a product forming a coating on a steel sheet, process during which said product is covered with a layer of metal, other than aluminum, having phosphate bonding properties, characterized in that, to produce said layer of metal, an etching treatment of said product is carried out by means of a strongly alkaline solution containing said metal in solution. 2) Method according to claim 1, characterized in that the alkaline solution is of normality at least equal to 5. 3) Method according to claim 2, characterized in that the alkaline solution is an aqueous sodium hydroxide solution. 4) Method according to claim 1, characterized in that the covering metal is zinc. 5) Method according to claim 1, characterized in that the treatment is carried out at a temperature of 10 to 65 ° C. approximately. 6) Method according to claim 1, characterized in that the zinc is introduced into the solution in the form of zinc powder or zinc oxide. 7) Method according to claim 1, characterized in that the treatment is carried out for a time sufficient to obtain a zinc deposit in the form of a substantially continuous layer. 8) Method according to claim 7, characterized in that the deposited zinc layer has a thickness of about 1 µm. 9) Method according to claim 1, characterized in that the alkaline solution contains nonionic surfactants. 10) Application of the method according to any one of the preceding claims to the phosphating of coated sheets by soaking in a bath of molten metal comprising a high proportion of aluminum, characterized in that, after a possible degreasing with solvents, pass the sheet in a strongly alkaline degreasing bath containing zinc in solution and maintained at room temperature, then it is passed through a refining bath containing titanium or nickel, then in the phosphating bath.