EP0337411B1 - Process for preparing an acidic passivating bath for zinc, zinc alloys an cadmium surfaces, containing chromium III and fluoride - Google Patents

Abstract

An acidic chromium(III)-containing and fluoride-containing passivating bath for surfaces of zinc, zinc alloys and cadmium is prepared by mixing 20-200 g/l of a soluble chromium(III) compound, 20 to 600 g/l of a soluble nitrate, 5 to 100 g/l of a fluoride and of a sulphate, phosphate, chloride, bromide, fluoride or iodide anion, adding hydrochloric or nitric acid to a pH of 1.8 to 2.2 and heating to above 60 DEG C or adding a catalyst, the amount of nitrate being greater than the chromium(III) concentration. It contains one or more complex(es) of the formula (1): <IMAGE> (x = 1 to 3, n = valency of A and A = nitrate, sulphate, phosphate, chloride, bromide, fluoride or iodide).

Description

The invention relates to a method for producing an acidic chromium-containing and fluoride-containing passivation bath for zinc or cadmium surfaces.

It is e.g. an acidic chromium-containing passivation bath for zinc or cadmium surfaces is known from US Pat. No. 4,263,059 and DE-OS 30 38 699, which, in addition to a "blue solution" of trivalent chromium and an acid, such as formic, acetic or propionic acid or nitric, sulfuric, hydrochloric and hydrofluoric acid contains a fluoride with a "green solution" of hexavalent chromium, for example Chromium trioxide, alkali metal chromate and dichromate and a reducing agent such as an aldehyde or alcohol or an alkali metal sulfite, bisulfite, metabisulfite, iodide, hydrogen peroxide, sulfur dioxide or an iron (II) salt is formed.

As a result, surfaces made of zinc, zinc alloys, galvanically or by hot-dip galvanizing on iron or steel or zinc layers or cadmium surfaces are made more corrosion-resistant, which is indicated by a coloration that ranges from blue to black and yellow to olive and is also used for decorative purposes. In particular, such a passivation, for example, the so-called blue passivation, ie Generation of a light blue, very thin passivation layer with a good decorative appearance, formation of zinc corrosion products also called "white rust", greatly delayed.

The salt spray test according to DIN 50021 provides a measure of the corrosion protection of these chromates. A zinc-coated and passivated part is exposed to a 5% sodium chloride mist at 40 ° C and 100% humidity. The time required to oxidize 5 - 10% of the surface to white rust is then given.

In the case of blue passivation, the corrosion protection according to DIN 50021 should be 20 to 24 hours.

However, due to the presence of toxic chromium (IV) compounds in the passivation solutions of the prior art, it is possible for chromium (VI) to be incorporated into the passivation layer, which can lead to and also from dermatitis diseases, particularly when such layers are used Reasonable concern in food industry equipment. Wastewater treatment for the detoxification of passivation solution residues or rinsing water offers problems and the solutions consume quickly and can only be re-sharpened (regenerated) to a limited extent, but must soon be disposed of. Rather, the corrosion protection wears off after a short time, often after just one day. Other acidic solutions which contain chromium (III) ions and in addition to these also oxidizing agents (US Pat. No. 4,171,231) and further additives such as silicates and / or other metal ions (US Pat. No. 4,384,902, 4,359,347, 4,367 099) or organophosphorus compounds (US Pat. No. 4,359,348) or carboxylic acids (US Pat. No. 4,349,392) do have the property of forming decorative blue or yellow passivations, but they cannot be subsequently colored and their corrosion behavior in the Order of magnitude of a maximum of 6 hours on 10% white rust according to DIN 50021. Because of the presence of oxidizing agents, the layers are not free of chromium (VI) compounds, which also occur when the pH is increased in the sedimentation basin of wastewater treatment and make detoxification more difficult.

Although from J. Am. Chem. Soc. 74 (1952) Pages 3509 - 3512 Chromium (III) fluorine complexes of chromium (III) are known, the formation of which is accelerated by zinc, but there is no evidence of the blue passivation of zinc or galvanized in this investigation of equilibrium constants Iron surfaces, on where there is no chrome metal deposition, but a polymeric compound with divalent chromium is deposited.

In addition, a certain amount of iron is dissolved in the solution by means of acidic chromating solutions in the case of galvanized iron objects, as a result of which such solutions become unusable after a certain time, in particular if they contain Cr³⁺. This is particularly the case for objects that do not have a completely closed zinc layer for geometric reasons.

An acidic chromium-containing passivation bath which contains only chromium III compounds but no oxidizing agents and no strong complexing agents is known from US Pat. No. 4,705,576. However, such baths are characterized by an iron content such as is inevitably e.g. arises during the treatment of galvanized inner tubes. A process has now been found for producing a passivation bath which has a long service life, produces passivations which can be colored and allows organic polymers to be better adsorbed onto the passivation layer produced and does not become unusable due to iron which may have been dissolved in the bath.

This object is achieved according to the invention by a process for producing an acidic, chromium (III) -containing and fluoride-containing passivation bath for surfaces made of zinc, zinc alloys and cadmium by mixing
20 to 200 g / l of a soluble chromium (111) compound
20 to 600 g / l of a soluble nitrate (total nitrate)
5 to 100 g / l of a fluoride
and hydrochloric or nitric acid to a pH of 1.8 to 2.2 and an anion selected from the group consisting of sulfate, phosphate, chloride, bromide, fluoride and iodide, the soluble nitrate in greater concentration than the chromium (III ) Concentration (in g / l) is present, and the mixture is either heated to 60 ° C. or treated with a catalyst above 15 ° C. and the concentrate obtained is optionally brought to an application concentration of 2 to 20% by weight in water , dissolved, which is characterized in that it additionally contains an acidic ion exchanger loaded with Cr³⁺ and / or an agent for the insolubilization of Fe³⁺ ions in an amount of 0.1 to 100 g / l.

Treatment with a catalyst, in particular with activated carbon at 15 ° C. or above, for example at room temperature (20-25 ° C.) or brief heating, for example 30 seconds to 15 minutes. to at least 60 to 80 ° C, seems to promote the formation of the more stable chromium (III) complexes, which is surprising is because only the hexahydrate complexes of chromium are stable at room temperature, but they are unusable for passivation.

vor, worin x = 1 - 3 (Anzahl der Fluoridionen) n = Wertigkeit des Anions A und A = Nitrat, Sulfat, Phosphat, Chlorid, Bromid, Fluorid und Jodid, was durch Leitfähigkeitsmessungen an der Fluorid-Elektrode bestimmt werden kann.The passivation baths produced according to the invention contain complexes of the general formula

before, where x = 1-3 (number of fluoride ions) n = valence of the anion A and A = nitrate, sulfate, phosphate, chloride, bromide, fluoride and iodide, which can be determined by conductivity measurements at the fluoride electrode.

Passivation baths preferred according to the invention contain the following amounts in a concentrate which is usually used in a concentration of 2 to 20% by weight in water:
20-200 g / l chromium (III) compound, e.g. as chromium chloride or chromium nitrate,
20 - 600 g / l soluble nitrate, such as sodium, potassium or ammonium nitrate,
5 - 100 g / l fluoride, e.g. sodium, potassium, ammonium fluoride
Hydrochloric or nitric acid up to a pH of 1.8 to 2.2.

Passivation baths made from this concentrate show corrosion protection values of a blue passivation according to DIN 50021 from 44 to 50 h, the layers can be colored and good organic polymers are applied to the layers.

example 1

Das Bad wurde mit Wasser im Gew.-Verhältnis 1 : 10 gemischt und mit Salpetersäure auf den pH-Wert 2,0 eingestellt. Mit diesem Bad wurden feuerverzinkte Eisenstangen von 2 cm Durchmesser und 20 cm Länge blaupassiviert. Nach 3 sec wurden die Stangen aus dem Bad genommen und auf Korrosionsverhalten nach DIN 50021 geprüft. Der Korrosionsschutzwert betrug 48 h.A concentrate of the following composition was produced:
50 g / l chromium (III) chloride
125 g / l sodium nitrate
50 g / l sodium fluoride
The concentrate was heated to 65 ° C. It contained complexes of the formula

The bath was mixed with water in a weight ratio of 1:10 and adjusted to pH 2.0 with nitric acid. With this bath, hot-dip galvanized iron bars of 2 cm in diameter and 20 cm in length were blue passivated. After 3 seconds, the rods were removed from the bath and checked for corrosion behavior in accordance with DIN 50021. The corrosion protection value was 48 hours.

The durability of the bath was extended by a factor of 10 by adding 50 g / l of acetylacetone.

Example 2

Es wurde der gleiche Korrosionsschutzwert erhalten. Im Vergleichsversuch auf einer normalen Blauchromatierung (handelsüblich) wurden folgende Ergebnisse erhalten: mit
Cr VI (handelsüblich) : 15 mg Cr/m² Zink; 24^h Korrosionsschutz
Cr III (erfindungsgemäß : 30 mg Cr/m² Zink; 48^h Korrosionsschutz.A bath was out
60 g / l Cr (NO₃) ₃.9H₂O
100 g / l NaNO₃
40 g / l NaF HNO₃ ad pH 2.1
and without heating, but prepared by treating with granular activated carbon in a porous bag for 10 minutes and, after mixing with water in a weight ratio of 1:12, used for the same experiment. It contained complexes of the formula

The same corrosion protection value was obtained. The following results were obtained in a comparison test on normal blue chromating (commercially available): with
Cr VI (commercially available): 15 mg Cr / m² zinc; 24 ^h corrosion protection
Cr III (according to the invention: 30 mg Cr / m² zinc; 48 ^h corrosion protection.

The durability of the bath was extended by a factor of 5 by adding 20 g / l salicylic acid.

Example 3

There was a bath according to Example 1 with a strongly acidic ion exchanger,
which had been loaded with a 3 M chromium (III) nitrate solution, used in a bath with 1.0 g / l Fe³⁺ content. The same bath without ion exchanger was used for comparison.

The bath with the ion exchanger showed a perfect blue passivation, while the comparison bath was unusable after a short time. The addition of 10 g / l fumaric acid increases the life of the bath by a factor of 5.

Example 4

Four solutions of 100 ml each of a 10% passivation solution according to Example 1 were prepared (pH 1.6) and the amounts of a mixture of alkylamines and amides with 5 to 12 carbon atoms given in the table were added. 1mm thick iron sheet with an area of 1 dm² was inserted. After 12 hours, the iron contents of the solutions were determined analytically by atomic absorption spectrometry. The following results were obtained, which show the dependence of the iron content of the passivation solution on the amount of inhibitor added, ie means for solubilizing the dissolved Fe³⁺ ions. The addition of 1 g / l inhibitor showed practically only a slight difference in color and corrosion protection compared to the passivation solution without inhibitor. table Passivation solution ppm / 1 Fe without inhibitor 1000 + 1 g / L " 250 + 5 g / l " 60 +10 g / l " 30th

Claims (2)

1. Process for preparing an acid chromium (III)- and fluoride-containing passivation bath for surfaces of zinc, zinc alloys and cadmium by mixing of
      20-200 g/l of a soluble chromium (III) compound;
      20-600 g/l of a soluble nitrate (total nitrate);
      5-100 g/l of a fluoride,
   and hydrochloric or nitric acid up to pH value 1.8-2.2 as well as an anion selected from the group consisting of sulphate, phosphate, chloride, bromide, fluoride and iodide, in which the soluble nitrate is present in a higher concentration than the chromium (III) concentration (expressed in g/l), and the mixture is heated either to 60°C or is treated with a catalyst at a temperature above 15°C and the concentrate obtained is optionally brought to a use concentration of 2-20 wt.% in water, characterized in that further an acid ion exchanger loaded with Cr³⁺ and/or an insolubilizer for Fe³⁺ ions are added in an amount of 0.1-100 g/l.
2. Process according to Claim 1, characterized in that the insolubiliser for Fe³⁺ ions is selected from the group of acetylacetone, fumaric acid, gluconic acid, oxalic acid, lactic acid, cyclic and alicyclic, primary or secondary amine, amino acids and amides having 3-16 carbon atoms, which form compounds with Fe³⁺ ions in water that are barely soluble, respectively insoluble.