NO771743L - PROCEDURES FOR PHOSPHATING METALS - Google Patents

Description

It is known that the phosphates metal surfaces, as

they are treated with an acid phosphating agent. This brings the metal surface into contact with an aqueous solution, which usually contains heavy metal ions and phosphate ions. However, acidic alkali phosphate solutions can also be used. This is usually done using the dipping or spraying method.

During the phosphate reaction, the phosphate solution is now enriched with heavy metal ions, which are released from the metal surface. These heavy metal ions usually lead to a solid adherent crust in the phosphating plant. This crust formation on spray devices, nozzles and heating units is particularly unfortunate. The crust formation finally leads to the extent that the performance of the phosphating solution is lowered in the nozzle system so that a steady supply of phosphating solution is no longer ensured after some time. There will then be disturbances in the formation of the phosphate coating. In addition, crust formation on the heating elements causes an insufficient heat transfer, which makes it difficult to maintain an even temperature. Phosphating plants must therefore only be kept in good operating condition with a high level of staffing.

There has certainly not been a lack of attempts to master these difficulties. Thus it is from DOS 2.402. 702 known a method for the phosphating of metals, where crust formation is to be reduced by the addition of a solution of a naphthalene sulfonic acid-formaldehyde condensation product or its salts. Also in DOS 2,240,867 the problem of reduction of crust formation by phosphating of metals is treated and there the addition of carbohydrates is proposed for the treatment of metals with a zinc phosphate solution.

The invention now relates to a method for reducing crust formation when phosphating metals with a metal ion-containing aqueous phosphating solution which has a pH value between 3.0 and 6.0, the method being characterized by adding an alkali or ammonium salt of poly-a to the solution -oxyacrylic acid with formula

wherein R-^ and means a hydrogen atom or an alkyl group or a carbon number between 1 and 3, M means' an alkali metal or an ammonium group and n means an integer between 3 and a' higher value, until the compound is still soluble in water.

Generally, it is not sufficient when the amount of poly-α-oxyacrylate in the phosphating solution amounts to less than 0.005 g/liter. On the other hand, however, it is also not necessary to add more than 1 g/litre. Very good results are always obtained when amounts between 0.01 to 0.2 g/litre are used. The molecular weight of the polymer used, as determined by the ratio of Flory (compare L. Manderkern and P.J. Flory, J. Chem. Physik, 1952, 20, pages 212 to 214) should generally be between 5,000 and 140,000. This means that in the case of sodium poly-α-hydroxyacrylate, n should have. a value in the aforementioned formula between 45 and 1275- Preferably the molecular weight is not less than 10,000 (n = 90).

The addition of poly-α-oxyacrylates does not completely prevent the deposition of metal crusts, however, a remarkable change in crust formation is evident. The deposit is now no longer formed as a solid crust, but in the form of an easily removable precipitate or sludge. Admittedly, even now from time to time removal of the sludge is necessary, however, this cleaning is carried out in a very simple way, for example by simply rinsing the surfaces that have the deposits with the help of a hose.

From the German Offenlegungsschrifter 2,161,727, 2,136,672 and 2,346,500, the use of poly-a-oxyacrylates as a complexing additive for metal ions in the detergent and cleaning agent is known. However, it could not be predicted

1

that the ability of poly-α-oxyacrylates to form complexes with metal ions in acid phosphating solutions instead of hard crusts would form an easily rinseable sludge.

A particular advantage of the method according to the invention is that the poly-a-bks<y>acrylates can be added to desired phosphating solutions in the OH range between 3.0 and 6.0, which contain metal ions and phosphate ions and are applicable for phosphate layer formation.n. The poly-a-oxyacrylates can also be used in single concentrations, which are diluted before use to produce a working solution. Such concentrates can be aqueous solutions as well as powdered products. In addition, however, the aforementioned polymerizate can also be added directly to the phosphating bath. The quantity of the poly-α-oxyacrylate to be added refers each time to the volume of the working solution. In addition to metal and phosphate ions, the phosphating solution can also contain various other additives. Their selection and concentration in the solution is based on the prescribed purpose of use and is common for professionals. However, a typical phosphating solution contains at least the following ions in the concentrations indicated below:

As a modifying agent, there is e.g. taking into account molybdenum cations (approx. 0.001 to 1 g/litre), p-nitro-benzoate (approx. 0.1 - 0.6 g/litre) or also anionic ones. or non-ionic surfactants (approx. 0.05 - 5 g/litre).

This working solution is normally brought into contact with the metal surfaces at temperatures between 40 to 90°C, preferably at 55 to 75°C. Aluminum, iron or steel surfaces are primarily considered as metal surfaces to be phosphated.

Example 1.

The following phosphating solutions were compared to each other:

The solutions (each time 1 litre) were adjusted with phosphoric acid to a pH = 4.0 and left at 60°C for 2 hours with 7.0 g of fine steel wool each time.

Next, 200 cm^ of each solution in a beaker was slowly evaporated in a drying cabinet at 140°C air temperature and then the inner walls of the beaker were assessed with regard to the deposit. With such an experimental device, it is possible to produce the conditions present in practice.

This resulted in the following results:

Resolution 1: Many point-shaped hard par- tickles.

Resolution 2: A thin, hard deposit film formed. Solution 3: No deposits appeared.

From this it appears that when a poly-a-oxyacrylate is added, there is a considerable reduction in crust formation, also compared to a phosphating solution containing sodium carboxymethyl cellulose.

Example 2.

In hard water (25° dH) a phosphate solution was prepared which contained 8 g/litre NaH2POi|, 0.1 g/

liter of molybdate, 1 g/litre of non-ionic surfactants and adjusted with phosphoric acid to pH = 4.0. With this resolution, it became i

■. the spray process treated at 65 o C et '! sheet steel. As a result, sludge formation occurred during use and firmly attached crust formations formed.

The procedure was then repeated, however, 0.2 g/litre of sodium poly-α-oxyacrylate was added to the solution. It turned out that now with the same duration of the procedure deposits also formed, which, however, could be very easily removed by simply flushing with a hose.

Claims (3)

1. Method for reducing crust formation when phosphating metals with a metal ion-containing aqueous phosphating solution, which has a pH value between 3.0 and 6.0, characterized by adding an alkali or ammonium salt of poly-a-oxyacrylic acid to the solution with formula in which Rj and means a hydrogen atom or an alkyl group with a carbon number between 1 and 3, M means an alkali metal or an ammonium group and n means an integer between 3 and a higher value, until the compound is still soluble in water. 2. Method according to claim 1, characterized in that the poly-a-oxyacrylate is added in an amount from 0.005 to 1 g/litre.... 3. Method according to claim 1, characterized in that the molecular weight of the poly-α-oxyacrylate according to Flory is between 5,000 and 140,000.