EP0603921B1 - Process for forming phosphate coatings - Google Patents

Abstract

In a process for forming phosphate coatings on metal surfaces of iron or steel by means of aqueous phosphating solutions based on magnesium phosphate, solutions are used which contain 0.2 to 4 g/l of magnesium, 1 to 20 g/l of phosphate (calculated as P2O5) and, as accelerator, a peroxide compound, in particular H2O2, and are free of those inorganic substances which cannot be precipitated with calcium hydroxide in the neutral or alkaline range. Preferably, phosphating solutions are used which additionally contain activators which can be precipitated by calcium hydroxide in the neutral or alkaline range.

Description

The invention relates to a method for producing phosphate coatings on metal surfaces made of iron or steel by non-layer-forming phosphating by means of aqueous phosphating solutions containing accelerators based on magnesium phosphate.

The process of producing phosphate coatings by means of aqueous phosphating solutions is widely used in the metalworking industry. The phosphate layers created with this process on the treated metal surfaces are used in particular for corrosion protection and as a lacquer primer as well as to facilitate non-cutting cold forming and sliding.

Such phosphating processes work, for example, with phosphating solutions which have a pH between about 1.8 and 3.8 and contain zinc and phosphate ions as process-determining components. In addition to the cation zinc, other cations, e.g. B. ammonium, calcium, cobalt, iron, potassium, copper, sodium, magnesium and manganese may be present. To accelerate the phosphate layer formation, the phosphating baths are generally oxidizing agents, e.g. B. bromate, chlorate, nitrate, nitrite, organic nitro compounds, perborate, persulfate, hydrogen peroxide are added. Oxygen-containing gas can also be used for the oxidation of iron (II) to iron (III).

Another type of phosphating process is the so-called iron phosphating or "non-layer-forming phosphating". She usually works with phosphating solutions based on alkali phosphate, but occasionally also with levels of magnesium, calcium and barium phosphate. Such methods are comparatively simple to implement and associated with low costs. They lead to phosphate coatings of sufficient quality for many cases, especially if the objects provided with them are not exposed to high corrosive influences.

The two types of process mentioned above have in common that the phosphating solutions are brought into contact with the workpiece surfaces to be treated in immersion, flooding or spraying. During the contact time, which can range from a few seconds up to approx. 15 minutes, firmly grown phosphate layers form through chemical reaction with the metal. Since residues of the phosphating solution remaining on the surface usually interfere with further processing, the water is thoroughly rinsed after the phosphating.

In a process of the so-called "non-layer-forming phosphating" it is known to apply a phosphate coating to improve the resistance of metal surfaces to corrosive attacks by treatment with solutions which contain predominantly phosphoric acid and / or a non-layer-forming phosphate (GB-PS 517 049). Alkali, ammonium and magnesium phosphate are mentioned as non-layer-forming phosphates. They are said to be advantageous because they are available cheaply and form little sludge compared to zinc, manganese, cadmium or iron phosphate. The solutions can also contain small amounts of nitrite, nitrate or sulfite and at most small amounts of layer-forming phosphates.

However, this process has the disadvantage that it is particularly due to the aforementioned nitrite contents,

Nitrate and sulfite, but also when using alkali or ammonium ion as a non-layer-forming cation, leads to waste water in the subsequent rinsing, which cannot be disposed of without extensive aftertreatment.

However, this consumes a considerable part of the advantage of the "non-layer-forming phosphating processes" which are intrinsically cost-effective.

The object of the invention is to provide a method for non-layer-forming phosphating which does not have the known, in particular the aforementioned disadvantages, and yet is inexpensive and simple to carry out and monitor.

The object is achieved in that the method of the type mentioned according to the invention is designed in such a way that the metal surfaces are brought into contact with phosphating solutions containing 0.2 to 4 g / l of magnesium, 1 to 20 g / l of phosphate (calc as P₂O₅) and as an accelerator contain peroxide compound and are free of such inorganic substances that cannot be precipitated with calcium hydroxide in the neutral or alkaline range.

The formulation "based on magnesium phosphate" used above is intended to express that magnesium phosphate or the cation magnesium constitutes the predominant proportion of the phosphate component or cations present in the phosphating solution.

Particularly suitable accelerators used in the process according to the invention are H₂O₂, perphosphate and percarbonate.

According to a preferred embodiment of the invention, the metal surfaces are brought into contact with phosphating solutions which contain H₂O₂ as accelerators. It is appropriate to choose the concentration such that the phosphating solution contains H₂O₂ in amounts of 0.02 to 0.2 g / l.

Another advantageous embodiment of the invention provides for the metal surfaces to be brought into contact with phosphating solutions which additionally contain activators. However, these activators must in turn be precipitated with calcium hydroxide in the neutral or alkaline range. The following activators with the concentrations specified in each case are particularly advantageous
0.01 - 0.2 g / l MoO₃
0.01 - 0.2 g / l WO₃
0.01 - 0.2 g / l VO₃
0.1-2 g / l F
0.01-0.2 g / l Ni
0.01-0.2 g / l Mn
0.01 - 0.2 g / l Zn
0.1 - 1 g / l Ca and / or
0.001 - 0.02 g / l Cu

Since the phosphating bath should be free of such components that cannot be precipitated with calcium hydroxide in the neutral or alkaline range, only those chemicals are suitable for the preparation and addition of the bath, through which no ions are introduced that lead to water-soluble salts. The use of bromate, chlorate, nitrate, nitrite or sulfite as an accelerator is also prohibited. Oxalates, sulfates and optionally also fluorides can be used in the Phosphating solution should be present. The admissibility of the individual bath components can be checked by bringing a bath sample to pH 8.5 with calcium hydroxide and then determining the salt content (anions and cations) in the supernatant water. The salt content in the supernatant water should then not be higher than in normal service water, ie not higher than about 500 mg / l. Just as no compounds that cannot be precipitated with calcium hydroxide in the neutral or alkaline range may be used to prepare the phosphating solution, care must also be taken when adding that no such substances are introduced. Accordingly, it is advisable to add at least a portion of the cation determining the phosphating system, magnesium, in the form of oxide, hydroxide and / or carbonate.

The application of the phosphating solution can be carried out in the usual way, e.g. B. by spraying, dipping or flooding. Depending on the form of application and the desired nature of the phosphate layer, e.g. B. in terms of layer thickness, the treatment time can be 5 seconds to a few minutes. The layer weight of the phosphate layer produced is generally between 0.1 and 1.0 g / m. Depending on the treatment conditions, higher layer weights can also be achieved. If a separate pH adjustment is required, it is generally done with phosphoric acid.

The usual treatment measures can be taken before and after the phosphating stage. For example, the metal surfaces can usually be subjected to a cleaning treatment to remove fat and Dirt, e.g. B. with the help of an alkaline cleaner. If necessary, pickling treatment to remove rust can follow. Between cleaning, possibly pickling, and phosphating, it is generally rinsed thoroughly with water.

If the workpieces to be phosphated are only moderately contaminated and therefore a separate cleaning stage can or should be dispensed with, phosphating solutions to be used in the process according to the invention can be used for surfactants or emulsifiers of various types, e.g. B. contain those that are also used in metal cleaning. Of course, they must be compatible with the phosphating solution.

After phosphating, there is generally a water rinse and an aftertreatment with any rinse aid, such as chromium (IV) / chromium (III) solution. After possibly rinsing the water again, the metal surface is finally dried.

The invention is illustrated by the following examples, for example and in more detail.

example 1

Zone 1:

Reinigen mit einem mildalkalischen Reiniger (5 g/l) bei 60°C durch 2 minN. langes Spritzen.

Zone 2:

30 sec. Spülen mit Wasser von Raumtemperatur.

Zone 3:

Phosphatieren durch 2 min. langes Spritzen mit einer Phosphatierungslösung von 60°C, enthaltend:
   0,55 g/l Mg
   3,3 g/l Phosphat (ber. als P₂O₅)
   30 mg/l H₂0₂
   pH-Wert der Phosphatierungslösung ca. 5.

Zone 4:

Spülen mit Wasser von Raumtemperatur.

Zone 5:

Nachspülen mit vollentsalztem Wasser, Trocknen.

RSt 1203 steel refrigerator cabinets were prepared for powder coating using the following five-zone process.

Zone 1 :

Clean with a mildly alkaline cleaner (5 g / l) at 60 ° C for 2 minN. long splash.

Zone 2 :

Rinse with water at room temperature for 30 sec.

Zone 3 :

Phosphate through 2 min. long spraying with a phosphating solution of 60 ° C, containing:
0.55 g / l Mg
3.3 g / l phosphate (calculated as P₂O₅)
30 mg / l H₂0₂
pH of the phosphating solution approx. 5.

Zone 4 :

Rinse with room temperature water.

Zone 5:

Rinse with deionized water, dry.

The phosphate layers produced by the process described above were uniformly opaque and formed a perfect base for the subsequent powder coating.

The rinse water in Zone 4, which has a pH of 5.3 and a conductivity of 320 µS. cm⁻¹ was prepared by adding as much slurry of Ca (OH) ₂ in water until the mixture had a pH of 9.0. The conductivity of the treated rinsing water after separation of the precipitation was 110 µS. cm⁻¹. It could be used in zone 4 for new flushing purposes.

Example 2

Zone 1:

Reinigen und Phosphatieren mit einer Lösung von 50°C Durch 2 min. langes Spritzen, pH-Wert der Lösung ca. 5.
   Die Lösung enthielt:
   0,55 g/l Mg
   3,3 g/l Phosphat (ber. als P₂O₅)
   30 mg/l H₂O₂
   0,5 g/l nichtionogenes Tensid

Zone 2:

30 sec. langes Spülen mit Wasser von Raumtemperatur.

Zone 3:

Passivierendes Nachspülen mit CrVI enthaltender Lösung, anschließendes Trocknen.

Refrigerator housings of the type mentioned in Example 1 were also treated for the subsequent powder coating using the three-zone process outlined below. Cleaning and phosphating were carried out with a solution in Zone 1.

Zone 1 :

Clean and phosphate with a solution of 50 ° C by 2 min. long spraying, pH value of the solution approx. 5.
The solution contained:
0.55 g / l Mg
3.3 g / l phosphate (calculated as P₂O₅)
30 mg / l H₂O₂
0.5 g / l non-ionic surfactant

Zone 2:

Rinse for 30 seconds with water at room temperature.

Zone 3:

Passivating rinsing with CrVI-containing solution, then drying.

The phosphate layers produced by this process were also uniformly covering and, in connection with the subsequent powder coating, provided excellent corrosion protection.

In addition to the treatment of the rinsing water in zone 2, which was carried out analogously to example 1, the cleaning / phosphating solution in zone 1 had to be prepared from time to time due to the absorption of grease, oil and other contaminants, and the used solution had to be prepared.

For this purpose, the used solution (pH approx. 4.9; conductivity 2480 µS. Cm⁻¹) was first subjected to ultrafiltration in order to remove the organic impurities. Then there was so much slurry of Ca (OH) ₂ added in water until a pH of 9.0 was reached (conductivity 140 µS. cm⁻¹). This measure not only removed the phosphating constituents of the solution, but also the impurities absorbed by them, so that after the precipitation had been separated off, the treated solution could be drained into the sewage system or used for rinsing purposes or to prepare a new cleaning / phosphating solution.

Claims (5)

1. Process for forming phosphate coatings on metal surfaces of iron of steel in a non-layer-forming phosphating manner by means of aqueous, accelerator containing phosphating solutions being based on magnesium phosphate, characterized in that the metal surfaces are brought in contact with phosphating solutions which contain 0,2 to 4 g/l magnesium, 1 to 20 g/l phosphate (calc. as P₂O₅) and as accelerator peroxide compound and are free of inorganic substances which are not precipitable with calcium hydroxide in the neutral or alkaline region.
2. Process according to claim 1, characterized in that the metal surfaces are brought in contact with phosphating solutions which contain as accelerator hydrogen peroxide.
3. Process according to claim 2, characterized in that the metal surfaces are brought in contact with phosphating solutions which contain hydrogen peroxide in an amount of 0,02 to 0,2 g/l.
4. Process according to claim 1,2 or 3, characterized in that the metal surfaces are brought in contact with phosphating solutions which additionally contain activators which are precipitable with calcium hydroxide in the neutral or alkaline region.
5. Process according to claim 4, characterized in that the metal surfaces are brought in contact with phosphating solutions which contain as activators.
      0,01 - 0,2 g/l MoO₃
      0,01 - 0,2 g/l WO₃
      0,01 - 0,2 g/l VO₃
      0,1 - 2 g/l F
      0,01 - 0,2 g/l Ni
      0,01 - 0,2 g/l Mn
      0,01 - 0,2 g/l Zn
      0,1 - 1 g/l Ca and/or
      0,001 - 0,02 g/l Cu