EP0213590B1 - Process for forming conversion layers on zinc and/or zinc/aluminium alloys - Google Patents

Abstract

An aqueous acidic treating solution for forming a chromium-containing passivate coating on the surface of zinc or zinc-aluminium alloys contains chromium ions of which the predominant proportion are in the hexavalent state, preferably in an amount 2-20 g/l., nitrate ions, preferably in an amount 0.5-6 g/l., and phosphate ions, preferably in an amount 1-15 g/l. The solution may also contain fluoride ions, molybdate ions and tungstate ions. The phosphide may be an alkali metal or ammonium phosphate, a polyphosphate or phosphoric acid.

Description

The invention relates to a process for the production of conversion layers on surfaces made of zinc and / or zinc-aluminum alloys by means of aqueous acidic chromating solutions which contain chromium VI, nitrate and phosphate ions.

It is known to treat surfaces obtained by hot-dip galvanizing or electrolytic galvanizing of steel with chromating solutions, in particular to improve corrosion protection and to prevent discoloration and white rust formation. In addition to processes for green chromating, i.e. For chromating with chromating solutions containing phosphate, especially those for yellow chromating have become established (Handbuch der Galvanotechnik, Vol. 3 (1969), pages 135 ff.). Chromating solutions that contain chromate, boric acid and fluoride as essential components serve primarily for this purpose.

Although the latter chromating solutions give good results in the treatment of pure zinc surfaces, difficulties arise when the surface to be treated consists of zinc-aluminum alloys. These types of alloys have recently become increasingly important as a coating material for steel sheet or strip for the replacement of conventional hot-dip galvanizing. An example of such a zinc-aluminum alloy is the alloy commercially available under the name Galvalume®, which contains 55% by weight. Contains aluminum, 43.5% by weight zinc and 1.5% by weight silicon. Conventional chromating solutions used for the treatment of pure zinc surfaces fail with such zinc alloys because the conversion layers produced with them cannot prevent blackening or discoloration of the zinc-aluminum alloy under normal atmospheric influences.

From Patent Abstracts of Japan, Volume 8, No. 222, 1984, page 5, C 246 and Kokai No. 59 104 483 a chromating process of surfaces of zinc alloys is known. The chromating solution contains 0.1 to 15 g / l Cr⁶⁺, 1.0 to 12 g / l Cl⁻, 0.5 to 2.0 g / l F⁻, 0.5 to 2.0 g / l SO₄²⁻ , 0.5 to 3.0 g / l Zn²⁺ and optionally 0.05 to 1.0 g / l PO₄³⁻ and 0.005 to 1.0 g / l NO₃⁻. Using this solution, a zinc alloy containing 1 to 30% by weight of aluminum is treated to form chromate.

US Pat. No. 4,531,978 discloses a process for producing chromate conversion coatings on aluminum surfaces, the aluminum surface being immersed in an acidic, aqueous solution which contains hexavalent chromium. In addition, the aluminum surface is subjected to an ultrasound treatment to avoid the commonly used accelerator components.

From Patent Abstracts of Japan, Volume 9, No. 17, 1984, page 134, C 262 and Kokai No. 59 166 678 a treatment method of steel sheets is known, which are coated with iron-zinc alloys.

The sheets are placed in an aqueous solution containing 0.005 to 1 mol / l chromium anhydride, 0.001 to 0.5 mol / l nickel, cobalt and iron as nitrate, sulfate, phosphate, fluoride or as a fluorocomplex.

The object of the invention is to provide a process for producing conversion layers which, particularly when used on zinc-aluminum alloys, gives good results in terms of corrosion resistance, but is also universally applicable insofar as conventional "pure" zinc surfaces are provided with a conversion layer can be.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the surfaces are brought into contact with a chloride-free chromating solution which contains 2 to 20 g / l of chromium ions (calculated as Cr) and 1 to 15 g / l phosphate ions (calc. as PO₄) and 0.5 to 6 g / l nitrate ions (calc. as NO₃) contains.

The process is for the treatment of zinc surfaces, i.e. Workpieces made of solid zinc, or galvanized metals, such as galvanized steel, which contain practically no or only minor alloy components, but in particular for surfaces made of zinc-aluminum alloy, e.g. of the aforementioned quality.

The active constituents of the chromating solution are introduced in a manner known per se.

The chromium VI ions are expediently added via chromic acid, alkali metal or ammonium chromate or dichromate. In addition to the chromium VI ions, the chromating solution generally also contains chromium III ions, which can be added as such or can be generated by reducing some of the chromium VI ions. However, the major part of the chromium ions dissolved in the chromating solution is in the hexavalent state.

The nitrate ions are most conveniently introduced via nitric acid, alkali and / or ammonium nitrate. The use of nitric acid is preferred.

The phosphate ion content is adjusted in the simplest manner by adding alkali metal or ammonium phosphate or polyphosphate or else by adding phosphoric acid.

The chromating solution works in the acidic range. It is particularly advantageous to bring the surfaces into contact with a chromating solution whose pH is below 2. The pH can be adjusted if individual of the aforementioned bath components are added in the form of free acid. However, other suitable acids can also be used.

In particular in the treatment of zinc-aluminum alloys, such as galvalume, it is advantageous to add activator ions to the chromating solution in order to accelerate the formation of the conversion layer and to increase its quality.

A further preferred embodiment of the invention therefore provides for the surfaces to be brought into contact with a chromating solution which additionally contains an amount of fluoride, preferably 0.5 g / l, or molybdate, preferably 0.5 to 0.7 g, which accelerates the formation of the layer / l, or tungsten, preferably about 0.2 g / l. The fluoride is expediently added in the form of hydrofluoric acid, molybdate in the form of the alkali or ammonium salt, tungstate in the form of alkali tungstate.

The treatment solution is applied to the previously cleaned surfaces in the usual way, for example by spraying, dipping, flooding, brushing or roller application. Excess solution can be removed, for example by using squeeze rollers. In this way, a liquid film with a certain thickness is created on the surface, which - when it dries - is responsible for the desired layer weight. According to a preferred embodiment of the invention, the layer weight should be set to approximately 10.8 mg / m 2 (calculated as Cr). In processes in which the layer is formed in contact with the chromating solution, the layer weight obtained depends on the duration of the treatment, the temperature of the chromating solution and the concentration of the active ingredients.

The application temperature of the chromating solution is generally in the range from 37.8 to 87.8 ° C. A temperature in the interval of 54.4 to 71.1 ° C is preferred. A treatment temperature of 60 ° C. is particularly favorable.

The treatment time can be between 0.5 sec. And 1 min. lie. If particularly high layer weights are desired, the treatment time can also be over 1 min. lie.

The process for the production of galvanized and chromated steel strip usually proceeds as follows:
The steel strip is first cleaned with an organic solvent to remove anti-rust oil, then subjected to cleaning with, for example, an alkaline cleaner. This is followed by water rinsing, drying and galvanizing in a zinc-aluminum alloy melt. After the alloy layer has solidified, the strip can be brought into direct contact with the chromating solution.

The chromating solution can be prepared directly in a usable form using the effective bath components. In general, however, it is more expedient to prepare the treatment solution using a concentrate. Such a suitable concentrate contains, for example, 30.4% by weight chromic acid (approx. 219 g / l chromium ions); 4.35% by weight nitric acid (100% by weight; approx. 59 g / l nitrate ions), 10.65% by weight phosphoric acid (100% by weight; approx. 142 g / l phosphate ions), Rest of water. It has a density of 1.383 g / cm³.

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

example 1

Steel sheet coated with the zinc-aluminum alloy Galvalume® was freed from rust protection oil with an organic solvent and then cleaned for 10 seconds by spraying at 54.4 ° C. with an alkaline cleaner (Parco ^(R) Cleaner 338 from Parker Chemical Company) . After thorough water rinsing at 49 ° C was treated with a chromating solution, the
0.71% by weight chromic acid (= 3.7 g / l Cr)
0.15% by weight nitric acid (68% by weight; 1 g / l NO₃)
0.33% by weight phosphoric acid (75% by weight; 2.4 g / l PO₄)
contained. The treatment temperature was 60 ° C. The liquid film applied was dimensioned by squeezing in such a way that a layer weight of 10.8 mg / m 2 (calculated as Cr) resulted after drying at room temperature.

The sheet obtained in this way was cut into sheet pieces measuring 9 x 19 cm and oiled with an anti-rust oil (Castrol Rustillo® DW-924 HF).

The pieces of sheet metal were then subjected to the industry standard test to determine weather resistance. For this purpose, the rust protection oil was first removed with petroleum solvent. Then were the sheet metal pieces were placed upright in glass dishes filled with distilled water in a damp room which had been heated to 37.7 ° C. During the test, the appearance of the surface was checked and assessed at regular intervals with regard to darkening or discoloration. The evaluation was carried out on a scale ranging from 1 to 8, with 8 meaning no discernible and 1 complete darkening. The results are shown in the table below.

Example 2

A second series of sheet metal pieces was treated and tested as indicated in Example 1. A chromating solution was used to produce the conversion layer
0.65% by weight chromic acid (= 3.4 g / l Cr)
0.14% by weight of nitric acid (68% by weight; approx. 0.93 g / l NO₃)
0.33% by weight phosphoric acid (75% by weight; approx. 2.4 g / l PO₄)
0.07% by weight hydrofluoric acid (70% by weight; approx. 0.5 g / 1 F)
0.09% by weight sodium molybdate dihydrate (= 0.6 g / l MoO₄)
0.03% by weight sodium tungstate (= 0.22 g / l WO₄)
contained.

The results obtained are also shown in the table.

Example 3 (comparative example)

A third series of sheet metal pieces was treated and evaluated as indicated in Example 1. A conventional chromating solution, the
0.38% by weight chromic acid
0.11% by weight boric acid
0.11% by weight of hydrofluoric acid (70% by weight)
contained.

The results obtained are in the table.

Example 4 (comparative example)

A fourth series of sheet metal pieces was treated and evaluated as described in Example 1, but treatment with a chromating solution was omitted.

The results are also given in the table for this.

The results show that the merely cleaned sheet metal pieces (Example 4) were completely dark after a test period of 168 hours. In contrast, the sheet metal pieces treated according to Example 1 had a rating of 7.5 after a test duration of 1100 hours. The sheet metal pieces treated in accordance with Example 2 even received the grade 8 corresponding to no discernible discoloration. Even with a test duration of 2164 hours, the sheet metal pieces with marks of 7.3 (example 1) or 7.8 (example 2) were still of excellent quality.

The comparative tests, for which conversion layers with conventional chromating solutions were used, show that severe discoloration occurs after a test duration of 1100 hours.

Claims (7)

1. A process for producing conversion layers on surfaces of zinc and/or zinc-aluminum layers by means of aqueous acidic chromating solutions which contain chromium(VI) and phosphate and nitrate ions by contacting the surfaces with a chromating solution, characterized in that the chloride-free chromating solution contains
   from 2 to 20 g/l of chromium ions (calc. as Cr),
   from 1 to 15 g/l of phosphate ions (calc. as PO₄), and
   from 0.5 to 6 g/l of nitrate ions (calc. as NO₃).
2. The process according to claim 1, characterized in that the surfaces are contacted with a chromating solution, the pH value of which is below 2.
3. The process according to claims 1 to 2, characterized in that the surfaces are contacted with a chromating solution which additionally contains an amount of fluoride, preferably 0.5 g/l, accelerating the completion of the layer formation.
4. The process according to one or more of claims 1 to 3, characterized in that the surfaces are contacted with a chromating solution which additionally contains an amount of molybdate, preferably from 0.5 to 0.7 g/l, accelerating the completion of the layer formation.
5. The process according to one or more of claims 1 to 4, characterized in that the surfaces are contacted with a chromating solution which additionally contains an amount of tungstate, preferably about 0.2 g/l, accelerating the completion of the layer formation, preferably about 0.2 g/l.
6. The process according to one or more of claims 1 to 5, characterized in that the surfaces are contacted with a chromating solution at a temperature of from 37.8 °C to 87.8 °C, and preferably from 54.4 °C to 71.1 °C.
7. The process according to one or more of claims 1 to 6, characterized in that the surfaces are contacted with a chromating solution in such a manner that, upon drying-up, there results a layer weight of about 10.8 mg/m² (calc. as Cr).