DE10162756A1 - Schwarzpassivierungsverfahren - Google Patents

Abstract

The invention relates to a mixture of substances, a method and a use thereof in the chrome(IV)-free black passivation of zinc alloys. According to said invention, an aqueous acid solution with chrome(III) ions is used and passivation is activated by addition of phosphate ions. Zinc alloys in particular containing nickel, with a nickel proportion of at least 10-12%, are used to form a black chrome layer containing nickel. The phosphate, added as an activator in a low quantity, permits a reaction activation, the removal of nickel ions from the alloy and the homogeneous deposit thereof in the form of black nickel(III) oxide. Nitrate ions and transition metal ions are also used as additional constituents and activators of the chromate bath.

Description

The invention relates to a mixture of substances, a method and a Use of fabric for black passivation of zinc alloys.

It is known the corrosion resistance of metallic surfaces for example from zinc and its alloys by treatment with to increase chromate-containing solutions. Depending on the composition of this Solutions are created blue, yellow, olive or black Surfaces that differ both in their visual impression and in their Make a strong distinction between corrosion protection. For zinc alloys such as zinc / nickel, zinc / iron or zinc / cobalt are stored with a suitable composition of the Chromating solution nickel, iron or cobalt oxide as a black pigment in the Chromate layer, because the second metal contained in the zinc alloy on the metal surface due to the action of acid also in the ionic Form transferred and thus involved in the formation of the metal oxide layer becomes.

The known methods of black chromating are considerable Amounts of chromium (VI) built into the chromate layer. The Corrosion protection of undamaged black chromations based on chromium (VI) amounts to 50 to 150 h in the salt spray test (PCT / DE 97/00800) according to DIN 50 021 SS.

In the corrosion test mentioned, a coated and passivated Part at 35 ° C and 100% humidity a 5% sodium chloride mist exposed. The time required is 5 to 10% of the area to oxidize to white rust.

Because of the toxicity and carcinogenic effects of chromium (VI) has been trying for years to use anti-corrosion agents based on the less toxic Establish chromium (III). Blue chromating processes were identified Basis of chromium (III) developed, however, usually with functional Disadvantages are.

For blue chromating, US Pat. No. 4,263,059 and US Pat DE 30 38 699 A1 known chromate-containing passivation solutions known from a blue solution of trivalent chromium, an acid like formic; Acetic or propionic acid, nitric, sulfuric, hydrochloric or Hydrofluoric acid and fluoride ions exist. The trivalent chrome is from hexavalent chromium by adding a reducing agent such as an aldehyde or alcohol, an alkali metal sulfite, bisulfite, metabisulfite, iodide, hydrogen peroxide, sulfur dioxide or an iron (II) Salt formed. Since the conversion to chromium (III) is not complete, it is possible that chromium (VI) is built into the passivation layer, which especially when using such coatings in the Food industry can lead to dermatoses.

Furthermore, an alternative approach has long been tried Blue passivation using only chromium (III) manufacture. The acidic chromating solutions used for this often contain additional oxidizing agents (US Pat. No. 4,171,231) and other additives such as silicates and / or other metal ions (U.S. Patents 4,384,902; 4,359,347; 4,367,099), organophosphorus compounds (U.S. Patent 4,539,348) or carboxylic acids (U.S. Patent 4,349,392). By the addition of oxidizing agents, however, is in turn not guaranteed that the layers produced are actually free of chromium (VI). Will continue by the oxidizing agent the wastewater treatment by the pH Increase in the sedimentation basin considerably more difficult. The so The layers produced result in decorative blue or yellow chromated Surfaces, but these only offer weak protection against corrosion (maximum 6 hours to 10% white rust) and, moreover, cannot be subsequently colored.

In the area of black chromating, especially with zinc alloys is of great importance, special problems arise because Black chromating in principle with the known reaction solutions for other types of chromating can be carried out, but frequently unsatisfactory results are achieved, which z. B. inhomogeneous or spotty covers.

EP 0 479 289 A1 describes a process in which phosphoric acid and Hydrofluoric acid in a certain ratio to the chromium (III) ions in the Solution available and to a passivation surface of improved Lead alkali resistance.

DE 38 12 076 A1 describes a process for blue chromating under Use an acidic chromate-containing passivation solution that only Chromium (III), but contains no oxidizing agents or strong complexing agents. The reaction solution used also contains nitrate ions, fluoride and Hydrochloric or nitric acid to adjust the pH. It is in the Approaches always contain at least one specific anion that is are nitrate, sulfate, phosphate, chloride, bromide, fluoride or iodide can. This anion is an unspecific lead ion because the anion is interchangeable here, added in high concentration will, and no advantageous concentration ranges are given. The corrosion protection achieved with this bath is relatively weak and lies at about 44 to 50 h according to DIN 50 021. A special application for Black chromating is not described.

U.S. Patent 5,393,354 describes a method of making iridescent chromating layer on zinc-nickel alloy with 8% Nickel content using phosphate ions.

The object of the invention is therefore a mixture of substances and a method for To provide black passivation, which is the formation of a chrome (VI) - free, homogeneous protective film of high corrosion resistance.

This task is accomplished by the mixture of substances named in the main claim and the preferred embodiments of the subclaims and by claimed method and the claimed use solved.

The invention is based on the use of an aqueous acidic solution Black passivation, in which the chromium ions contained in the trivalent form and passivation by adding phosphate ions is activated on a zinc-nickel surface with at least 10%, preferably 12% nickel.

The chromium (III) ions are required to form the protective layer. This process is time-dependent and therefore takes about 30 to 90 seconds. At the same time, the alloy layer is loosened. During this The dissolving process also removes nickel from the alloy and oxidizes it. The The resulting black nickel (III) oxide serves in the resulting Protective layer as a black color pigment. Is the attack on the alloy too strong, the color pigment forms too large. this leads to a powdery black coating on the surface. The height Nickel oxide concentration continues to prevent the formation of a homogeneous protective layer, d. H. the corrosion protection decreases. The attack is too slowly, a good passive layer forms with too little black pigment, the surface therefore appears spotty gray. So that's the speed attack on the alloy layer and the nickel content of the layer of great importance. There must be so much nickel in the period of training Passive layer are available as needed to form sufficient color pigment.

Tests have now shown that layers with over 12% nickel and using small amounts of phosphate (activator to attack the zinc-nickel alloy) these conditions to form a black passive layer can be achieved.

Omission of the phosphate ions leads to a blue layer (lack of attack, no formation of black color pigments) while the use of 8% nickel (not enough nickel for training of black color pigment) leads to an iridescent layer. It is both known in the art.

According to the invention, the chromium (III) ions can be present in the solution in concentrations between 200 mg / l up to the solubility limit. In a preferred embodiment, chromium (III) ions are contained in a concentration of about 3 to 6 g / l. A large number of soluble chromium (III) salts are suitable for adjusting the chromium (III) concentration. An example of such a salt is potassium chromium sulfate (chrome alum), KCr (SO ₄ ) ₂ × 12 H ₂ O. Alternatively, chromium nitrate, chromium sulfate or chromium chloride can be used.

The nitrate concentration can be between 1.5 and 73 g / l. In a In a preferred embodiment, the nitrate ion concentration is approximately between 9 and 30 g / l. Various soluble nitrate salts such as z. B. serve alkali metal nitrates or ammonium nitrate.

The phosphate content is preferably in a concentration range between 400 and 900 mg / l. It can be used as a source of phosphate ions Phosphoric acid, but also a soluble phosphate salt can be used. In a further preferred embodiment, the reaction solution Adjustment of the phosphate ion concentration between 0.01 and 5 ml Phosphoric acid (75%) / l added. This corresponds to a concentration range from about 15 mg to 7.5 g phosphate ions / l.

Transition metal ions from the group of cobalt, iron, nickel, molybdenum, manganese, lanthanum, cerium or other lanthanides and aluminum ions can be used as a supporting activator. In a preferred embodiment, cobalt (II) ions are contained in a concentration of about 3.6 g / l. Cobalt sulfate, Co (SO ₄ ) × 6 H ₂ O, is a suitable source of the cobalt (II) ions. Alternatively, cobalt nitrate, cobalt sulfate or cobalt chloride can be used.

For the course of the reaction is an aqueous acidic solution with a preferred pH range between 1.8 and 2.3 is provided. The setting of the acidic pH can be adjusted with nitric acid, but also with any other suitable Mineral acid, such as hydrochloric acid or sulfuric acid. Doing so the exact pH with a suitable acid / base pair such as Sulfuric acid / sodium hydroxide solution.

The passivation reaction as such runs in under standard conditions a temperature range from 20 ° C to the boiling point and at a Dive time from 60 to 180 seconds. The reaction preferably takes place at a Reaction temperature of 40 ° C with a diving time of 90 to 120 seconds the passivation, the treated object is rinsed with water and dried.

The present passivation process and the one used for it Reaction solution enable a chromium (VI) -free chromating, which one in Very high compared to other chromium (III) passivations Corrosion protection enabled. This corrosion protection lies with measurements according to the Salt spray tests according to DIN 50 021 SS at about 120 hours with regard to the White rust formation. The treated object receives one homogeneous, continuous black passivation layer of appealing Look.

Furthermore, the passivation process described offers advantages in With regard to implementation: it is already at low Reaction temperatures of about 40 ° C feasible and allows passivation in one Work step that is not due to subsequent treatments such as Rinsing can be completed or supplemented with a silicate solution have to.

Furthermore, in contrast to the previous ones, this method allows described blue chromating a subsequent coating with a top Coat that further strengthens the protection against corrosion, but also the coloring.

In the following, the chromating process and the Composition of the reaction solution provided for this based on Test results and a specific embodiment are explained in more detail.

Concentrates were first produced. 200 ml portions of the concentrate were made up to 1000 ml with deionized water. This diluted solution represents the actual reaction solution. Table 1 Composition of the concentrates (g / l)

Table 2 Concentrations in the finished reaction solution (g / l)

Table 3 maximum concentration ranges in the reaction solution

Unless otherwise noted, 0.3 ml of phosphoric acid was added per liter. This corresponds to a phosphate concentration of approximately 400 to 450 mg / l in the reaction solution. Table 4 Test results when treating zinc / nickel surfaces at different pH values and immersion times. The reaction temperature was in each case 40 ° C. Table 4

An example of an advantageous composition of the reaction solution is described in Tables 1 to 2 for Approach IV. To do this, first a Concentrate from the amounts given in Table 1 Individual components manufactured. 200 ml of this concentrate are with deionized water made up to 1000 ml. Then 0.3 ml Phosphoric acid (75%) added as an activator (corresponding to approx. 400 to 450 mg phosphate / l). With sulfuric acid / sodium hydroxide solution, a pH of 2.2 set. The object to be treated is then made of zinc-nickel at a reaction temperature of 40 ° C for 120 sec Chromating bath immersed. After that, the item with water rinsed and dried at room temperature. The received Passivation layer is almost black and slightly iridescent and has one Corrosion protection from 120 h against the formation of white rust. By Subsequent application of a top coat gives the item a deep black and glossy surface.

The minimum amount of phosphate required in this example is very high low and is usually well below 2.0 g / l.

Claims (20)

1. Mixture of substances for the black passivation of zinc-nickel alloy with at least 10% nickel content based on an aqueous acidic solution containing chromium (III) ions, characterized in that the passivation is activated by adding phosphate ions. 2. A mixture of substances according to claim 1, characterized in that the Concentration of the dissolved chromium (III) ions in a range of approximately 200 mg / l up to the solubility limit. 3. A mixture of substances according to claim 2, characterized in that the Concentration of the dissolved chromium (III) ions in a range of approximately 3 to 6 g / l. 4. A mixture of substances according to claim 1, characterized by nitrate ions in a concentration between 1.5 to 73 g / l. 5. A mixture of substances according to claim 4, characterized by nitrate ions in a concentration between 9 to 30 g / l. 6. A mixture of substances according to claim 1, characterized by fluoride ions in a concentration of about 450 mg to 23 g / l. 7. A mixture of substances according to claim 6, characterized by fluoride ions in a concentration of about 900 mg to 2.6 g / l. 8. Mixture of substances according to one of the preceding claims, characterized characterized in that phosphate ions in a concentration range of 400 up to 900 mg / l are added before the passivation reaction. 9. Mixture of substances according to one of the preceding claims, characterized characterized in that metal ions from the group of cobalt, iron, nickel, Molybdenum, manganese, lanthanum, cerium, another lanthanoid or Aluminum is included as additional activators. 10. A mixture of substances according to claim 9, characterized in that the Solution contains cobalt (II) ions in a concentration of about 3.6 g / l. 11. A mixture of substances according to one of the preceding claims, characterized characterized in that the pH of the solution is approximately 1.8 to 2.3. 12. A mixture of substances according to claim 1, characterized by the addition of 0.01 to 5 ml / l phosphoric acid (75%) to the reaction solution. 13. A mixture of substances according to claim 1, characterized in that the Passivation takes place at a reaction temperature of 40 ° to 50 ° C. 14. A mixture of substances according to claim 1, characterized in that the Diving time for passivation is 60 to 90 seconds. 15. Process for black passivation, characterized by the Preparation of a reaction solution with chromium (III) ions, immersion of the treating object in the reaction solution or application the reaction solution on the object to be treated, Activation of the passivation with phosphate ions, subsequent rinsing of the passivated object with water and final drying. 16. A method for black passivation according to claim 15, characterized by the addition of 0.01 to 5 ml / l phosphoric acid (75%) to the Reaction solution. 17. A method for black passivation according to claim 15, characterized by a reaction temperature of 40 ° to 50 ° C. 18. A method for black passivation according to claim 15, characterized by a diving time of 60 to 90 sec. 19. A method for black passivation according to claim 15, characterized by a subsequent treatment of the passivated Item with a top coat. 20. Use of phosphate to activate Passivation reactions in reaction solutions containing chromium (III) ions.