EP0219779B1 - Phosphatizing process for electrolytically galvanized metal objects - Google Patents

Abstract

A method for phosphate coating zinc or nickel-zinc alloy electrolytically coated metal surfaces, in which they are exposed to a phosphating solution containing zinc, manganese, and phosphate ions, and optionally cobalt and/or nitrate ions. The method is conducted at an elevated temperature for up to slightly more than 5 seconds.

Description

The invention relates to an improved method for producing fine crystalline, closed conversion layers, consisting predominantly of zinc phosphate, with a low mass per unit area in very short treatment times on electrolytically galvanized metal products, in particular ferrous metals, for example on electrolytically galvanized steel strips.

The process is not limited to "pure zinc layers". Steel strips that have been given a zinc alloy coating can also be treated. In addition to zinc, iron, nickel and cobalt are examples of alloy components.

The use of galvanized steel, especially electrolytically galvanized steel in strip form, has become very important in recent years. As a result, the pretreatment and phosphating processes for electrolytically galvanized steel strip have been increasingly processed.

Area-related masses of 2 to 3 g / m² were considered necessary in order to provide adequate corrosion protection in the subsequent to achieve lacquered condition as well as only phosphated. These comparatively high layer runs led to a number of problems, such as unsatisfactory adhesion properties of subsequent coatings, especially in the case of deforming further processing. However, mass per unit area of over 2 g / m² is also a disadvantage when processing in the unpainted, only phosphated state during deformation and welding processes.

The treatment times were over 5 seconds; targeted belt speeds of 90 to 120 m / min, for example, were impossible or difficult to achieve and with quality losses, such as non-closed layers.

DE-B-19 55 002 proposes the use of acid phosphating solutions to which a carbohydrate obtained from starch, starch derivative or polysaccharide by acidic decomposition is added in order to produce thin, adherent and corrosion-resistant zinc phosphate layers on, for example, electrolytically galvanized steel surfaces. Zinc phosphate layers with mass per unit area of, for example, 1.2 to 1.8 g / m 2 should form in treatment times of 3 to 10 seconds.

However, the addition mentioned leads to difficulties in practice. The organic components are decomposed in acidic solutions at elevated temperatures and increasing bath life. The initially low area-related masses of the phosphate layers increase significantly. The decomposition products lead to unpleasant smells. Heavy sludge formation is also undesirable.

DE-A-21 00 021 proposes to form thin closed phosphate layers with mass per unit area of <1.0 g / m 2 to treat the metal surfaces with phosphating solutions which essentially contain nickel ions as layer-forming cations. In addition to the nickel ions, further metal ions, in particular zinc ions, can be present. The molar ratio of the nickel ions to the other divalent metal ions should be in the range from 1: 0.001 to 1: 0.7.

Essentially, nickel phosphate layers are deposited. In contrast to the desired zinc phosphate layers, they always require a subsequent coating with, for example, a varnish in order to achieve acceptable corrosion protection. This is a serious disadvantage.

As already mentioned, the demand for improved corrosion protection has led to an increased use of electrolytically galvanized steel in many industrial goods. However, this also required improved phosphating processes. An essential step in the desired direction is a method proposed in DE-A-32 45 411. A method for the phosphating of electrolytically galvanized steel strips is described. In treatment times of not significantly more than 5 seconds, usually in 5 or less, zinc phosphate layers with mass per unit area of less than 2 g / m², preferably 0.6 to 1.9 g / m², are deposited, which do not affect the required corrosion protection, both in the unpainted as well as in the lacquered state, which do not have disadvantages due to the high layer coverage. You work with acid phosphating solutions, which, in addition to zinc and phosphate ions, can contain further metal cations and / or anions of oxygen-containing acids with accelerating action. The content of Zn²⁺ cations is about 1 to 2.5 g / l, the free acid content is in the range of 0.8 to 3 points, the acid ratio (total acid / free acid) is kept in the range of 5 to 10 . It is preferred to work with phosphate solutions containing nitrate.

The weight ratio of Zn²⁺ / NO₃⁻ is kept in the range of 1 / (1 to 8), the weight ratio PO₄³⁻ / NO₃⁻ in the range of 1 / (0.1 to 2.5).

In addition to zinc, other cations can also be used, but usually in minor amounts. Thus, DE-A-32 45 411 uses nickel ions, the Zn²⁺ content preferably always predominating. Ratios of 20 to 2 parts by weight of Zn²⁺ ions to one part by weight of Ni²⁺ ions can be particularly useful. Most of the time, nickel cannot be detected analytically in the deposited layer, so it is only present in the layer in traces that remain below the detection limit. The phosphating takes place in the temperature range from 50 to 70 ° C., preferably in the range from 60 to 65 ° C. The process is suitable for both spray and dip application.

As mentioned, the use of electrolytically galvanized steel in the manufacture of a wide variety of industrial goods has increased significantly in recent years, including in the automotive industry. The reason, as mentioned, is the need for improved corrosion protection. For the same reason, the automotive industry has the priming job, particularly in body construction, from anodic to cathodic Electrodeposition coating changed. This necessitated phosphating processes which had a high iron content in the layer produced, ie in addition to zinc phosphate (hopeit), zinc iron phosphate (phosphophyllite) should be formed in the highest possible proportions. The iron required must be supplied from the substrate by an acid pickling reaction.

• 1.) Klaus Wittel, "Moderne Zinkphosphatierverfahren, Niedrig-Zink-Verfahren", Industrie-Lackierbetrieb 5/83, Seite 169 und Industrie-Lackierbetrieb 6/83, Seite 210.
• 2.) James W. Davis, "The pretreatment of steel and galvanized steel for cathodic electrodeposition paint systems". SAE Technical Paper Series 820336, SAE The Engineering Resource for Advancing Mobility, 400 Commonwealth Drive, Warrendale, PA 15096, USA.
• 3.) Harry E. Chandler, "Second Generation Zinc Phosphate Coating". Metal Progress May 1982, 1982 American Society for Metals.

It has also been shown to be extremely desirable that phosphate layers with a cube-like or cuboid-like structure are deposited before cathodic electrocoating. Needle-shaped crystals are undesirable. Both properties, the iron content in the layer and the changed layer morphology, can be achieved by suitable formulation of the phosphating solutions and by appropriate process control. References to these relationships can be found in the literature. The following should be mentioned:

• 1.) Klaus Wittel, "Modern zinc phosphating processes, low-zinc processes", industrial paint shop 5/83, page 169 and industrial paint shop 6/83, page 210.
• 2.) James W. Davis, "The pretreatment of steel and galvanized steel for cathodic electrodeposition paint systems". SAE Technical Paper Series 820336, SAE The Engineering Resource for Advancing Mobility, 400 Commonwealth Drive, Warrendale, PA 15096, USA.
• 3.) Harry E. Chandler, "Second Generation Zinc Phosphate Coating". Metal Progress May 1982, 1982 American Society for Metals.

The first requirement, the incorporation of iron into the layer, cannot, of course, be met on a zinc surface. In addition to many other areas of application, electrolytically galvanized steel strip is also increasingly being used in body construction; in many cases the phosphating is already applied in combined galvanizing and pre-treatment lines and the material is supplied as galvanized, "pre-phosphated" steel.

The phosphating should therefore also be suitable for subsequent cathodic electrocoating. Since iron cannot be built into the phosphate layer in the present case, layers with a cubic or cuboid structure must be created.

The object of the invention is a method described below which fulfills the requirement.

Acid phosphating solutions are used which, in addition to zinc, manganese and phosphate ions, can contain further metal cations and / or anions of oxygen-containing acids with accelerating action. The phosphate layers produced have mass per unit area of less than 2 g / m², the preferred range is 0.9 to 1.6 g / m². Layers in the range of 0.6 to 1.9 g / m² can be deposited. The acidic phosphating solutions are characterized in that the zinc cation content is in the range from 0.1 to 0.8 g / l, preferably 0.25 to 0.6 g / l. The content of manganese II cations is kept in the range of 0.5 to 2 g / l, preferably 0.75 to 1.25 g / l.

The free acid content is kept in a range from 4 to 8 points, preferably 5 to 7 points. The acid ratio (total acid / free acid) is kept in the range of 2.5 to 5 points, the preferred range is 2.8 to 4.5.

With regard to the definitions of free acid, total acid and the phosphates mentioned below in phosphating baths, reference is made to the prior art, in particular to the publication Christian Ries "Monitoring of Phosphating Baths", Galvanotechnik, 50, (1968), No. 1, Pages 37 to 39 (Eugen G. Leuze Verlag, Saulgau (Württ.)). The free acid score is defined as the number of milliliters of 0.1 N NaOH required to titrate 10 ml bath solution against dimethyl yellow, methyl orange or bromophenol blue. The total acid score is the number of milliliters of 0.1 N NaOH required to titrate 10 ml bath solution against phenolphthalein as an indicator until the first pink color.

The process according to the invention is further characterized in that the phosphating baths contain nitrate. The weight ratio of the sum of Zn²⁺ and Mn²⁺ cations to the nitrate ions is kept in the range (ZN²⁺ + Mn²⁺) / NO₃⁻ = 1: / (0.5 to 1.5), preferably in the range 1 / ( 0.7 to 1.25), while the ratio Zn²⁺ / Mn²⁺ in the range 1 / (1 to 3), preferably in the range 1 / (1.5 to 2.5), and the weight ratio H₂PO₄⁻ / NO₃ ⁻ In the range from (6 to 9) / 1, preferably at (7 to 8) / 1.

It is also essential for the phosphating solutions according to the invention that the weight ratio of Sum of Zn²⁺ and Mn²⁺ cations to primary H₂PO₄⁻ anions in the range 1 / (6 to 9) is kept.

It has also proven to be advantageous to drive the phosphating solutions according to the invention with a low cobalt content. The cobalt content, based on the Zn²⁺ and Mn²⁺ content, is preferably one part of cobalt per 100 to 150 parts of Zn²⁺ and Mn²⁺.

The treatment times are deliberately kept short in view of the modern systems for the electrolytic galvanizing and phosphating of steel strips, 90 to 120 m / min speed of the strip. 5 seconds are not significantly exceeded, treatment times of 2.5 to 5 seconds are generally used.

The phosphating is expediently carried out at elevated temperatures, in particular in the temperature range from 40 to 70 ° C., the temperature range from 45 to 55 ° C. being particularly suitable. Any technically useful way of applying the treatment solution is applicable. It is particularly interesting that the process according to the invention can be carried out both by spraying technology and by immersion.

Before the phosphating solution is applied, the electrolytically galvanized surface must be completely water wettable. This is usually the case in conveyor systems. If the surface of the electrolytically galvanized strip is oiled for the purpose of temporary corrosion protection, this oil must be removed by known, suitable means and methods before phosphating.

The water-wettable electrolytically galvanized metal surface is expediently treated with activating solutions known per se before phosphating. The activators essentially contain titanium salts and phosphates, together with organic components. Information on suitable activation methods can be found in DE-A-20 38 105 and DE-A-20 43 085.

According to the state of the art, e.g. described in DE-A-21 00 021, it can also be advantageous for the process according to the invention to passivate the deposited conversion layers with dilute chromic acid and / or phosphoric acid. The chromic acid concentration is generally between 0.01 and 1 g / l. It is also possible to passivate the protective layers with dilute chromic acid, which contains chromium III ions. The concentrations generally used are 0.2 to 4.0 g / l CrO₃ (hexavalent chromium) and 0.5 to 7.5 g / l Cr₂O₃ (trivalent chromium).

From the acidic phosphating solutions according to the invention, phosphate layers are produced on electrolytically galvanized steel, which clearly show a cuboid or cuboid structure. This is proven by scanning electron microscope images. This was not possible with the previously known method, including the method described in DE-A-32 45 411. Crystals with a needle-like shape were deposited. The method according to the invention thus achieves the object of providing a conversion layer suitable for the subsequent cathodic electrocoating on electrolytically galvanized steel produce. The type of layer described is also achieved on steel galvanically coated with a zinc-nickel alloy.

In comparison to the previously known methods, a lighter colored conversion layer is produced. This is particularly desirable when the electrolytically galvanized and phosphated steel is used without further coating. In this case, phosphating is expected to significantly delay or suppress both the occurrence of the so-called "white rust" (formation of zinc corrosion products) and the "red rust" (iron corrosion products).

The layers deposited from the acidic phosphating solutions according to the invention fulfill this task significantly better than layers deposited from conventional treatment baths.

This can be proven by a comparative test according to DIN 50 021 SS (salt spray test).

For the combined adhesion and corrosion testing of electrolytically galvanized, phosphated and steel coated with a cathodic electrodeposition coating as primer, water storage tests have recently been increasingly carried out. In such tests too, the layers deposited from the acid phosphating baths according to the invention prove to be superior to the layers produced by conventional methods.

Because of the high content of free acids, the acidic phosphating baths according to the invention work very low in sludge. This is a not insignificant advantage for practical use.

The comparatively low possible bath temperatures of 45 to 55 ° C meet the pursuit of energy savings.

As mentioned at the beginning, the deposited layers consist predominantly of Zn phosphate. The small amounts of cobalt contained in the phosphating baths lead one to expect that, given the low surface masses deposited according to the invention, cobalt can no longer be detected in the layers, since the content is below the detection limit. It is surprising, however, that manganese is only found in very small amounts in the layers deposited from the acid phosphating solutions according to the invention.

The values found in samples are 25 to 125 mg / m² manganese. The exact mechanism of the layer formation from the solutions according to the invention is not yet fully understood. However, it can be assumed that zinc levels in the solutions are only required for the start. If the baths are to stand for a longer period of time, the zinc required for the layer formation is supplied by the acidic phosphating solutions pickling on the electrolytic galvanizing. This view is supported by the results of extensive throughput tests, in which the baths were operated with "low-zinc" to zinc-free resharpening solutions.

The following examples describe the mode of operation according to the invention:

example 1

An electrolytically galvanized surface was treated at 30 ° C. with a solution which contained an activating agent containing titanium, as described in DE-A-20 38 105, in an amount of 3 g / l. The activated surface was then treated with a solution of the following composition at 50 ° C:
1.1 g / l Mn²⁺,
0.50 g / l Zn²⁺,
0.01 g / l Co²⁺,
11.2 g / l H₂PO₄⁻,
1.5 g / l NO₃⁻.

The free acidity was 6 points and the total acidity was 19.6 points. After a phosphating time of 3.5 seconds, the sheet was rinsed with water and then passivated with a solution containing Cr⁶⁺ + Cr³⁺ and dried. The mass per unit area of the phosphate coating was 1.15 g / m².

After 55 h of salt spray test (DIN 50021), the unpainted, phosphated sample showed white rust only on 10 to 20% of the surface; There was no red rust.

A sheet treated in the same way was cathodically electrocoated and provided with a filler and topcoat that are common in the automotive industry. The painted surface was bombarded with steel granules under defined conditions and then stored at 40 ° C. for 40 h in a 5% saline solution. Then the sheet was again bombarded with steel granules.

The size of the surface on which the paint build-up is destroyed by this test, ie the surface is exposed, can be expressed by a characteristic value:
largest possible area = worst test result = characteristic value 10
smallest possible area = best test result = characteristic value 1

The sheet according to Example 1 was given characteristic values 3 to 4.

Example 2

The activation of the sheets was carried out as shown in Example 1, as was the passivation following the phosphating. The phosphating time and the temperatures also correspond to example 1. The same amounts g / l as in example 1 were used, but the solution contained no cobalt. Free acid and total acid as in Example 1. The weight per unit area of the phosphate coating was 1.3 g / m².

After a 55-hour salt spray test, the phosphated, unpainted sample showed white rust on approximately 40% of the surface and red rust on approximately 10% of the surface. The water storage test of the sheet painted and tested as in Example 1 gave a characteristic value of 6.

Test sheets made using conventional methods, e.g. according to the method proposed in DE-A-32 45 411, show a significantly poorer behavior after the tests described.

Claims (12)

1. A process for phosphating electrolytically zinc-coated metal goods, preferably electrolytically zinc-coated steel goods, more especially electrolytically zinc-coated steel strip, by brief treatment for not much longer than 5 seconds with acidic phosphating solutions which, in addition to zinc, manganese and phosphate ions, contain other metal cations and/or anions of oxygen-containing acids having an accelerating effect to form coatings consisting predominantly of zinc phosphate and having a weight per unit area of less than 2 g/m², characterized in that the phosphating solutions used are acidic phosphating solutions of which the content of zinc cations (Zn²⁺) is from 0.1 to 0.8 g/l and their content of manganese cations (Mn²⁺) from 0.5 to 2.0 g/l, while the free acid content is from 4 to 8 points and the acid ratio (total acid to free acid) in the range from 2.5 to 5.
2. A process as claimed in claim 1, characterized in that the content of Zn²⁺-cations is in the range from 0.25 to 0.6 g/l.
3. A process as claimed in claims 1 and 2, characterized in that the content of Mn²⁺-cations is in the range from 0.75 to 1.25 g/l.
4. A process as claimed in claims 1 to 3, characterized in that the free acid content is from 5 to 7 points and the acid ratio is in the range of from 2.8 to 4.5.
5. A process as claimed in claims 1 to 4, characterized in that coatings predominantly consisting of zinc phosphate are desposited in a thickness of from 0.6 to 1.9 g/m².
6. A process as claimed in claims 1 to 5, characterized in that nitrate-containing phosphating baths are used, the ratio by weight of Zn²⁺ + Mn²⁺ to NO₃⁻ being in the range of 1/(0.5 to 1.5).
7. A process as claimed in claims 1 to 6, characterized in that the ratio of Zn²⁺ to Mn²⁺ is in the range of 1/(1 to 3).
8. A process as claimed in claims 1 to 7, characterized in that the ratio by weight of H₂PO₄⁻ to NO₃⁻ is in the range of (6 to 9)/1.
9. A process as claimed in claims 1 to 8, characterized in that the ratio of (Zn²⁺ + Mn²⁺) to H₂PO₄⁻ is in the range of 1/(6 to 9).
10. A process as claimed in claims 1 to 9, characterized in that the baths used contain small quantities of cobalt, the cobalt content, based on the content of (Zn²⁺ + Mn²⁺) being 1 part of cobalt to 100 - 150 parts (Zn²⁺ + Mn²⁺).
11. A process as claimed in claims 1 to 10, characterized in that phosphating is carried out at a temperature of from 40 to 70°C.
12. A process as claimed in claims 1 to 11, characterized in that electrolytically zinc-coated strip steel which has been subjected to an activating pretreatment known per se with titanium-containing activating solutions is phosphated.