EP0111246B1 - Process for phosphatizing zinc-electroplated metal objects - Google Patents

Abstract

A process for phosphating zinc-plated metal articles, particularly electrolytically zinc-plated steel sheets, by treatment thereof for a short period with an acidic phosphating solution which, in addition to zinc and phosphate ions, can contain other metal cations and/or anions of oxygen-containing acids having an accelerating effect, wherein the phosphating solution has a content of Zn2+-cations of between about 1 and 2.5 g/l, a free acid content in the range from 0.8 to 3 points, and the acid ratio of total acid to free acid in the range from 5 to 10. The phosphating treatment is carried out for a period of time not in excess of about 5 seconds.

Description

The invention relates to an improved method for producing closed, finely crystalline zinc phosphate layers 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 sheets.

The processes currently used in practice for the phosphating of, for example, electrolytically galvanized steel strips still have restrictions, the elimination of which is desirable. In order to ensure adequate protection against corrosion, it is considered necessary to form surface-related masses of the phosphate coatings of approximately 2 to 3 g / m ² . The result of these comparatively high area-related masses is often an unsatisfactory to poor adhesion of subsequent coatings, particularly when phosphated and coated material is deformed. The duration of the phosphating is above 5 seconds in the processes used in practice. Your reduction - for example by increasing the belt speed from 60 to 120 m / min. - is very difficult or impossible with the methods used to date. Sources of error include, for example, non-closed layers of the phosphate coating and thus poorer protection against corrosion as well as unsatisfactory deformation and paint adhesion properties. Treatment times in the phosphating stage of less than 5 seconds cannot be achieved with the known methods used in practice.

DE-B-19 55 002 relates to a method for applying thin, corrosion-resistant and adhesive zinc phosphate coatings on metal surfaces, for example on those made of galvanized or hot-dip galvanized steel. Here, the metal surfaces are treated with acid phosphating solutions to which at least one carbohydrate consisting of starch, a starch derivative or a polysaccharide, which has been produced by acidic decomposition of starch or a starch derivative, has been added. The acidic, aqueous phosphating solutions also contain zinc and phosphate ions, nickel cations, nitrate and also fluorides. With such phosphating solutions, galvanized or hot-dip galvanized steel plates are sprayed at temperatures in the range from 40 to 70 ° C. either for a period of 3 to 10 seconds or treated in a dipping process for a period of 5 to 30 seconds. Here, adherent zinc phosphate coatings with a uniform appearance and a low coating weight of 1.2 to 1.8 g / m ^{2 are} formed. If, on the other hand, the same phosphating solutions are used without additives, the result is substantially coarser, less dense and less uniform coatings, the coating weight of which is 3.5 to 4.0 g / m ² .

For practical use, there are considerable difficulties in using the starch, starch derivatives or polysaccharides in the phosphating bath. The incorporated organic components are decomposed by the temperature and the pH value with increasing bath life. The initially low phosphate requirements increase significantly. The decomposition products lead to strong odor nuisance. The extremely strong sludge formation is also particularly undesirable.

FR-A-2033717 (corresponding to GB-A-1237996) describes a method for phosphating electrolytically galvanized steel surfaces, which results in phosphate layers which have a layer weight of up to 21.5 mg / dm ² (corresponding to 2.1 g / m ² ), preferably in the range from 5.3 to 16.1 mg / dm ² (corresponding to 0.5 to 1.6 g / m ² ). The phosphating solutions used for this purpose contain 0.12 to 0.50 percent by weight zinc ions, 0.55 to 2.20 percent by weight phosphate ions and 0.009 to 0.036 percent by weight nickel ions; the total acid content is in the range from 7.5 to 30 points, the temperature of the phosphating solution is 38 to 60 ° C. Of particular importance is the nitrate ion content of the phosphating solutions, which should be 0.001 to 0.02 percent by weight, preferably 0.005 to 0.015 percent by weight. It is said that a nitrate content of more than 0.02 percent by weight causes layer weights of more than 21.5 mg / dm ² . The duration of treatment in this procedure should be between 2 to 45 seconds; the example speaks of a treatment duration of 15 seconds.

For the formation of thin, coherent phosphate coatings with a layer weight of <1.0 g / m ² , DE-A-21 00 021 suggests treating the metal surfaces with solutions which essentially contain nickel ions as layer-forming cations. In addition to the nickel ions, layer-forming cations of a further divalent metal, in particular zinc ions, can also be present. However, the molar ratio of nickel ions to the other divalent metal cations is clearly above 1. It should be in the range from 1: 0.001 to 1: 0.7. Essentially nickel phosphate layers are deposited here. The formation of the zinc phosphate layers required in practice is not successful. The thin nickel phosphate layers according to this proposal of the prior art are also subject to drastic restrictions. So they always require a subsequent layering with other coating agents to ensure lasting protection.

The demand for improved corrosion protection has led to an increased use of electrolytically galvanized steel in many industrial goods. At the same time, the practice endeavors to close existing systems at higher speeds for economic reasons drive. In the phosphating processes for electrolytically galvanized steel that are still common today, these shortened treatment times result in a significant deterioration in the phosphate layer produced.

Accordingly, the invention is based on the object of forming high-quality and improved zinc phosphate layers on electrolytically galvanized materials, in particular ferrous metals, despite substantially reduced processing times in the phosphating stage. The invention deliberately intends to accept thin layers of the phosphate layers without, however, having to give up the uniform covering of the galvanized material with a fine crystalline, firmly adhering, self-contained zinc phosphate layer. With the method according to the invention, it is possible, for example, to form uniform, closed phosphate layers on electrolytically galvanized steel sheets with a duration of the phosphating step of a maximum of about 5 seconds, which at the same time guarantee corrosion protection that at least approximately achieves that of "thick-layer phosphating", but has other properties even significantly different from the known thicker phosphate layers. For example, the adhesion of organic coatings during and after deformation steps - for example when folding, deep drawing, flanging and the like - is improved compared to the results that have been achievable so far. The invention furthermore wants to make it possible, for example, to be able to drive electrolytic galvanizing lines with subsequent phosphating at very different strip speeds, without any significant differences in the phosphate coating being associated therewith. The belt speeds that occur in practice today are, for example, in the range between 20 and 120 m / min. According to the invention, a constant quality of the phosphate coating is achieved, in particular also in the range of high belt speeds, that is, for example, in the range from 100 to 120 m / min. enables.

• a) die Dauer der Behandlung 5 s nicht übersteigt,
• b) die Phosphatierung im Temperaturbereich von 50 bis 70°C durchgeführt wird und die Phosphatierungslösungen den folgenden Bedingungen entsprechen:
• c) Gehalt an Zn^2+-Kationen = 1 bis 2,5 g/l,
• d) Gewichtsverhältnis Zn²⁺/H₂PO₄ - = 1:(1 bis 8),
• e) Gewichtsverhältnis Ni2+ IZn2+ = 1:(2 bis 20),
  welches dadurch gekennzeichnet ist, dass für die Phosphatierungslösungen ferner die folgenden Bedingungen gelten:
• f) Gewichtsverhältnis Zn²⁺/NO₃ = 1: (1 bis 8),
• g) Gewichtsverhältnis PO₄ ^3-/NO₃ = 1: (0,1 bis 2,5),
• h) Gehalt an freier Säure = 0,8 bis 3 Punkte,
• i) Säureverhältnis Gesamtsäure/freie Säure = 5 bis 10

The invention accordingly relates to a process for the phosphating of electrolytically galvanized metal products, in particular electrolytically galvanized steel strips, with the formation of zinc phosphate layers with a mass per unit area below 2 g / m ² , preferably from 0.6 to 1.9 g / m ² and in particular from 1.2 to 1.4 g / m ² , by short-term treatment with acid phosphating solutions which, in addition to zinc and phosphate ions, contain nickel cations and, as accelerators, nitrate ions - but are free from fluorides -,

• a) the duration of the treatment does not exceed 5 s,
• b) the phosphating is carried out in the temperature range from 50 to 70 ° C. and the phosphating solutions meet the following conditions:
• c) content of Zn ^{2 +} cations = 1 to 2.5 g / l,
• d) weight ratio Zn ²⁺ / H ₂ PO ₄ - = 1: (1 to 8),
• e) weight ratio Ni2 + IZn2 + = 1: (2 to 20),
  which is characterized in that the following conditions also apply to the phosphating solutions:
• f) weight ratio Zn ²⁺ / NO ₃ = 1: (1 to 8),
• g) PO ₄ ^3- / NO ₃ = 1: (0.1 to 2.5) weight ratio,
• h) free acid content = 0.8 to 3 points,
• i) Acid ratio total acid / free acid = 5 to 10

This process of the invention produces zinc phosphate coatings of 0.6 to 1.9 g / m ² , which have a closed, finely crystalline structure and give the electrolytically galvanized sheet a desired uniform, light gray appearance. An electrolytically galvanized steel strip which has been phosphated in this way can also be processed without subsequent coating. The thin phosphate layers produced by the method according to the invention behave more favorably in many shaping processes than the phosphate layers of a higher mass per unit area produced with the previous conventional methods. However, organic coatings applied subsequently also show significantly improved adhesion compared to the prior art, both during and after deformation processes.

The free acid content of the phosphating bath used according to the invention is preferably in the range from 1.2 to 1.8 points. The preferred acid ratio of total acid to free acid is in the range from 6 to 8. For the definitions of free acid, total acid and the primary phosphates mentioned below in phosphating baths, reference is made to the relevant prior art in writing, in particular to the publication Christian Ries «Monitoring of phosphating baths» Galvanotechnik, 59 (1968) No. 1, pages 37 to 39 (Eugen G. Leuze Verlag, Saulgau (Württ)). In this publication, both the terms of the parameters mentioned here and their determination are described in detail. The free acid score is accordingly defined as the number of milliliters n / 10 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 n / 10 NaOH required to titrate 10 ml bath solution using phenolphthalein as an indicator until the first pink color.

Accordingly, the combination of the following parameters is essential for the method according to the invention: The concentration of Zn ^{2 +} ions is kept in a low-limited range. This is an important prerequisite for the inventive design of the desired thin but nevertheless homogeneously closed layers. At the same time, the bath solution has a comparatively high content free acid is used, which, as indicated, is in the range from 0.8 to 3.0 points, preferably in the range from 1.2 to 1.8 points. Finally, the duration of the treatment is deliberately chosen to be short. 5 seconds are not exceeded. A treatment period of 2.5 to 5 seconds is generally used.

Investigations into the history of origin and transformation of the phosphate layer to be formed according to the invention have shown the interesting fact that due to the bath and process parameters chosen according to the invention - presumably in particular under the influence of the high content of free acid - the zinc phosphate layer initially builds up very quickly, which but then proportionally reduced again within the short process period selected according to the invention. The mass of the phosphate coating in the new process apparently passes through a maximum value in the initial phase, which decreases again in the later process sections - that is to say in the period of 3 to 5 seconds. The thin zinc phosphate coatings produced in this way meet the requirements of the task according to the invention.

The zinc phosphate layers produced by the new process are preferably in a range from 0.6 to 1.9 g / m ² , the range from 1.2 to 1.4 g / m ^{2 being} particularly preferred.

The additional components to be used in the phosphating baths of the invention result from the knowledge of the prior art. For example, nitrate is also used as the anion of an oxygen-containing acid with an activating effect. The weight ratio of Zn ²⁺ to NO ₃ - is in the range from 1 to (1 to 8). The phosphate and nitrate content of the phosphating bath is adjusted to one another in such a way that the weight ratio of PO ₄ ^3- to NO ₃ ^{- is} in the range from 1 to (0.1 to 2.5). Furthermore, the ratio of zinc cations to primary phosphate is chosen such that weight ratios of Zn ²⁺ to H ₂ P0 ₄ - are kept in the range from 1 to (1 to 8) in the treatment bath.

In addition to zinc, nickel cations are also used in the process according to the invention. However, these are used in minor amounts, but the zinc ion content always predominates. Mixing ratios of 20 to 2 parts by weight of Zn ^{2 +} ions to one part by weight of Ni ^{2 +} ions are used according to the invention. It is interesting in this context that nickel cannot generally be detected analytically in the zinc phosphates deposited by the process according to the invention. At best, it is present in traces in the phosphate coating that are below the detection limit.

The phosphating itself takes place at moderately elevated temperatures in the temperature range from about 50 to 70 ° C. The temperature range from 60 to 65 ° C. can be particularly suitable. Any technically useful way of applying the treatment solution is suitable. In particular, it is therefore possible to carry out the new method both by means of 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 continuously operating conveyor systems. If the surface of the electrolytically galvanized strip is oiled for storage and corrosion protection, this oil must be removed by means of known, suitable means and processes prior to phosphating. The water-wettable electrolytically galvanized metal surface is then expediently subjected to an activating pretreatment known per se before the phosphating solution is applied. Suitable pretreatment processes are described in particular in DE-A-20 38 105 and 20 43 085. Accordingly, the metal surfaces to be subsequently phosphated are treated with solutions which contain, as activating agent, essentially titanium salt and sodium phosphate together with organic components such as gelatin or alkali metal salts of polyuronic acids. Soluble compounds of titanium such as potassium titanium fluoride and in particular titanyl sulfate can preferably be used as the titanium component. Disodium orthophosphate is generally used as the sodium phosphate, but can be replaced in whole or in part by other sodium phosphates such as monosodium orthophosphate, trisodium orthophosphate, tetrasodium pyrophosphate and sodium tripolyphosphate. Titanium-containing compounds and sodium phosphate are used in such proportions that the titanium content is at least 0.005% by weight, based on the weight of the titanium-containing compounds and the sodium phosphate.

As described in the prior art, for example DE-A-21 00 021, it can also be advantageous for the process according to the invention or the zinc phosphate layers produced thereafter to passivate the phosphate layers produced in a subsequent process step. Such passivation can take place, for example, with dilute chromic acid and / or phosphoric acid. the concentration of chromic acid and / or phosphoric acid is generally between 0.01 and 1.0 g / l. It is possible to post-treat the protective layers with dilute chromic acid, which contains chromium (III) ions. In general, the application concentrations of the hexavalent chromium are between 0.2 and 4.0 g / l Cr0 ₃ and those of the trivalent chromium between 0.5 and 7.5 g / l Cr ₂ 0 ₃ . It is expedient to rinse with water between the phosphating and the aftertreatment step. However, this rinsing is not absolutely necessary and can be omitted in particular when working with squeeze rollers.

The following examples describe the procedure according to the invention.

example 1

• 1,1 g/l Zn²⁺
• 0,4 g/I Ni²⁺
• 7,4 g/l PO₄ ^3-
• 2,1 g/I N0₃ ^-
• 3 mg/l Fe²⁺

An electrolytically galvanized surface was treated at 30 ° C. with a solution which contained a titanium-containing activating agent, as described in DE-A-20 38 105, in an amount of 3 g / l. The activated surface was then treated at 60 ° C. with a solution which had the following composition:

• 1.1 g / l Zn ²⁺
• 0.4 g / l Ni ²⁺
• 7.4 g / l PO ₄ ^3-
• 2.1 g / I N0 ₃ ^-
• 3 mg / l Fe ²⁺

The free acidity was 1.3 points and the total acidity was 10.8 points. (The points free acid and total acid mean the ml of 0.1 N NaOH, which are required to titrate 10 ml of bath solution against bromophenol blue or phenolphthalein as an indicator.) After a phosphating time of 3.5 seconds, the sheet was rinsed with water and then passivated with a solution containing Cr-3 + / Cr-6 + at 50 ° C and dried.

The mass per unit area of the phosphate coating was 1.6 g / m ² . The corrosion protection test according to SS DIN 50021 was comparable to layers that had been produced using conventional methods and had a mass per unit area of 2.4-2.6 g / m ² .

Example 2

• 1,80 g/I _Zn ²⁺
• 0,35 g/I Ni²⁺
• 5,50 g/I PO₄ ^3-
• 4,8 g/I NO₃ ^-

A phosphating solution was prepared, with which an electrolytically galvanized steel sheet was treated at 63 ° C. The bathroom had the following composition:

• 1.80 g / _{l Zn} ²⁺
• 0.35 g / l Ni ²⁺
• 5.50 g / I PO ₄ ^3-
• 4.8 g / I NO ₃ ^-

The total acidity of the bath was 9.9 points and the free acidity was 1.4 points. An electrolytically galvanized sheet was phosphated with this solution for 5 seconds. The sheet had a closed, light gray phosphate layer with a mass per unit area of 1.3 g / m ² .

With a subsequent bend of 90 ° and 180 ° there were no cracks or flaking of the phosphate layer.

A sheet metal sample was painted and, after drying at elevated temperature, provided with a cross cut according to DIN 53151. The adhesion value was perfect both without and with 8 mm Erichsen cupping.

Example 3

• ^{2,0 g/l} Zn²⁺
• 0,4 g/l Ni²⁺
• 4,95 g/l PO₄ ^3-
• 6,0 g/l NO₃ ^-

A freshly electrolytically galvanized steel sheet was activated at 45 ° C. with a solution which contained 1.5 g / l of a titanium-containing component and had a pH of 8.5 in demineralized water. The galvanized surface was then phosphated with a solution of the following composition at 60 ° C. for 4 seconds:

• ^{2.0 g / l} Zn ²⁺
• 0.4 g / l Ni ²⁺
• 4.95 g / l PO ₄ ^3-
• 6.0 g / l NO ₃ ^-

The free acidity of the bath was 2.1 points and the total acidity 11.3 points. In this case, too, the tape had a uniform, light gray appearance. The phosphate layer formed was closed and had a mass per unit area of 1.1 g / m ² . The paint adhesion on a sample of this sheet was good.

A sheet electrolytically galvanized and phosphated by a conventional method with a mass per unit area of 2.3 g / m ² was coated with the same paint and subjected to the same deformation operation. The paint adhesion values were significantly poorer than in the case of the sheet phosphated using the method according to the invention.

Claims (4)

1. A process for the phosphating of electrolytically zinc-plated metal products, more especially electrolytically zinc-plated steel strip, with formation of zinc phosphate coatings having a weight per unit area of less than 2 g/m², preferably from 0.6 to 1.9 g.m² and more preferably from 1.2 to 1.4 g/m² by brief tratment with acidic phosphating solutions which, in addition to zinc and phosphate ions, contain nickel cations and, as accelerators, nitrate ions, but are free from fluoride ions, with the proviso that

a) the treatment time does not exceed 5 seconds,

b) phosphating is carried out at a temperature of 50 to 70°C and the phosphating solutions satisfy the following requirements:

c) content of Zn²⁺ cations = 1 to 2.5 g/I,

d) ratio by weight of Zn²⁺ to H₂PO₄ = 1: (1 to 8),

e) ratio by weight of Ni²⁺ to Zn²⁺ = 1 :(2 to 20), characterized in that the phosphating solutions also satisfy the following requirements:

f) ratio by weight of Zn²⁺ to NO₃ - = 1: (1 to 8),

g) ratio by weight of P0₄ ^3- to N03 = 1: (0.1 to 2.5),

h) free acid content = 0.8 to 3 points,

i) acid ratio of total acid to free acid = 5 to 10.

2. A process as claimed in claim 1, characterized in that the phosphating solutions have a free acid content of from 1.2 to 1.8 points and an acid ratio of total acid to free acid of from 6 to 8.

3. A process as claimed in claims 1 and 2, characterized in that phosphating is carried out at a temperature in the range from 60 to 65°C.

4. A process as claime in claims 1 to 3, characterized in that electrolytically zinc-plated ferrous metals which have been subjected to an activating pretreatment known per se, more especially with titanium-containing activating solutions, are used in the process.