DE19740248A1 - Aqueous chromating or post-passivating solution - Google Patents

Abstract

An aqueous chromating or post-passivating solution, which contains trivalent and hexavalent chromium and acetate ions, has a pH of 3.9-5.0 and contains 0.1-10 g/l acetate ions. An aqueous chromating or post-passivating solution has a pH of 3.9-5.0, a total chromium content of 0.1-10 g/l, a Cr(III):Cr(VI) ratio of 1:1-4 and an acetate ion content of 0.1-10 g/l. Independent claims are also included for (i) an aqueous concentrate which, on dilution by a factor of 50-2000 with water, gives the above solution; and (ii) a chromating or post-passivating treatment, in which the metal surfaces are contacted for 1-120 secs. with the above solution at 20-60 deg C. Preferred Feature: The solution additionally contains one or more of up to 0.5 g/l fluoride ions, up to 1.0 g/l zirconium and/or titanium compounds, up to 0.5 g/l phosphate ions, up to 0.5 g/l nickel(II) ions and up to 0.5 g/l iron(III) ions.

Description

The invention relates on the one hand to the chromating of cleaned, but not further anti-corrosion pretreated metal surfaces. Through the Chromating is applied a corrosion protective layer, which as Can serve as a primer for subsequent painting. The invention relates equally the re-passivation of metal surfaces after a Cleaning already a conversion-protective pretreatment like for example iron or zinc phosphating or alkaline Passivation. The post-passivation will Corrosion protection effect of these upstream corrosion protection Treatment further intensified. The method is applicable to surfaces Steel, galvanized or alloy galvanized steel or aluminum and its Alloys. In further aspects, the invention includes that for the claimed Processable aqueous treatment solutions and their concentrates.

Chromating of metal surfaces or post-passivation already Corrosion-protective pre-treated metal surfaces with aqueous Cr-containing Solutions are widely used in the field of metal surface treatment. Acidic aqueous solutions, which are both trivalent as well as hexavalent chromium. Such solutions can, for example by dissolving suitable chromium (III) salts and chromic acid and / or Chromates are made in water. However, it is more economical from Chromic acid-containing solutions and a portion of the 6-valent Reduce chromium to a trivalent stage using suitable reducing agents. For this reduction is usually used industrially in methanol. Other  Reducing agents such as ethanol or higher alcohols and formaldehyde or however higher aldehydes are also suitable. The Reducing agent completely oxidized with the formation of water and carbon dioxide become. Often, however, the oxidation products remain, for example those with Use of ethanol-generated acetic acid or acetations in the Cr (III) / Cr (VI) containing solution. Examples of such solutions and their use for chromating metals are in EP-A-391 442 and WO 91/05078 contain. There is also shown that the corrosion protection effect of this Solutions through additional additives such as phosphate ions, Fluorozirconate ions, zinc ions, finely dispersed silica and organic Additives can be improved.

Such Cr (III) / Cr (VI) -containing solutions are used for metal surface treatment usually to a pH in the range between about 2 and about 3.8 set. The highest possible Cr (III) content has a favorable effect on the Layer formation and corrosion protection. However, Cr (III) salts are when in general, only in a sufficiently acidic environment in the required concentration soluble. At pH values above about 3.8, Cr (III) begins with the technical desired concentrations to precipitate as hydroxide. Then it stands on the one hand no longer available for the chromating process and must on the other hand as Waste is disposed of. However, one strives, if possible at the upper limit of the technically usable pH range, because this Chromating layers with particularly good anti-corrosive properties form, the pickling attack on the metal surface remains low or already Do not dissolve existing corrosion protection layers.

During the operation of a chromating bath, the Acid consumption due to the pickling attack on the metal surface the pH value of the bath. Until now, this had to be ensured by taking early bathroom care measures that the pH does not exceed the critical limit of about 3.8 and sludge containing chrome separates. The danger of one is particularly great Excessive alkalization of the Cr (III) / Cr (VI) bath, if this for Post-passivation of alkaline passivation layers is used. On the one hand  there is a risk that alkalinity from the passivation bath via the intermediate sink in the chrome-containing post-passivation bath is carried over. On the other hand, however acid also bound in the chrome-containing acidic passivation solution, that this reacts with the alkaline passivation layer. Here too the active substance content of the chromium-containing passivation solution decreases by the fact that as a result of the pH increase, chromium-containing sludges separate. There is therefore a need for chromate baths and chromium-containing ones Post-passivation solutions that work in the still acidic area, but they do are stabilized against Cr (III) precipitation at pH values up to about 5.

The invention therefore has as its object a solution for chromating or re-passivation of metal surfaces and a corresponding one To provide working procedures, including treatment solutions then the technically usual chrome contents and Cr (III) / Cr (VI) ratios if their pH range is in the still acidic area, but above of the currently technically usual area.

With regard to the treatment solution, this object is achieved by an aqueous solution for chromating or re-passivating surfaces made of galvanized or galvanized steel or of aluminum and its alloys, which contains trivalent and hexavalent chromium as well as acetates, characterized in that they
a pH in the range from 3.9 to 5.0,
a total chromium content between 0.1 and 10 g / l,
a ratio of Cr (III): Cr (VI) in the range from 1: 1 to 1: 4
and has a content of acetate ions in the range of 0.1 to 10 g / l.

Preferably the pH is in the range of about 3.9 to about 4.5. The preferred ratio Cr (III): Cr (VI) ranges from about 1: 2 to about 1: 3.

The higher the content of acetates, the higher the Total chromium content of the chromating or post-passivation solution. Here can be considered as a rule of thumb that a sufficient stabilizing effect then  is achieved when the acetate ion content in g / l is approximately 0.5 to 2 times that Total chromium content in g / l.

As is customary with chromate solutions, these solutions can contain further constituents which are built into the chromate layers or influence the layer formation. Examples of such optional components are:
up to 0.5 g / l fluoride ions,
up to 1.0 g / l zirconium and / or titanium compounds,
up to 0.5 g / l phosphate ions,
up to 0.5 g / l nickel (II) ions,
up to 0.5 g / l iron (III) ions.

In principle, such treatment solutions could be achieved by dissolving the individual Components in water in the desired concentration range at the location of the Insert are manufactured. In practice, however, it is common to use aqueous ones To provide concentrates that the individual components in the desired Quantity ratio included and from which by simply diluting on site the ready-to-use working solution can be manufactured. Accordingly, the Invention also an aqueous concentrate that when diluted with water around a ready to use factor of between about 50 and about 2,000 Treatment solution with the features mentioned above results. In the The preparation of these concentrates is usually carried out in such a way that one Cr (VI) -containing solution is reduced with a suitable reducing agent so far that the desired ratio of trivalent to hexavalent chromium sets. The starting compound containing Cr (VI) is preferably chosen Chromic acid or chromates. Since the solution according to the invention is sufficient Amount of acetic acid or acetations should have to the precipitation trivalent chromium compounds in the pH range to be selected according to the invention to prevent it is particularly efficient to reduce the hexavalent Chromium to use such compounds that are oxidized to acetic acid. A An obvious example of such a compound is ethanol. Can be beneficial  however, citric acid can also be used. However, it is not essential to which way the acetic acid or the Acetations in the required Concentrate in the treatment solution or its concentrate. So is it is also possible, for example, to use one that is partially reduced with methanol Chromic acid solution additionally by adding acetic acid or soluble Stabilize acetates. With such a separate addition, however, it is required that the Acetationen the chromic acid or chromate solution before the Reduction can be added. If you carry out the reduction in the pH according to the invention without the presence or direct formation of Acetations by, Cr (III) compounds fail, which is caused by subsequent The addition of acetate cannot be brought back into solution.

If in connection with this teaching of "Acetations or acetic acid" is spoken, it should be expressed that in the invention Range of pH values a balance between free acetic acid and acetate anions is present. Therefore, it is immaterial whether the solution is acetic acid or adding soluble acetate salts or whether these species are more suitable by oxidation Starting compounds such as ethanol can be formed. In which selected pH value is determined by the acid constant of the acetic acid predetermined balance by itself. Therefore, if only from "Acetations" is spoken, including the total amount of acetations and acetic acid understood, which are in equilibrium with each other.

In a further aspect, the invention relates to a method for chromating or re-passivating surfaces made of steel, galvanized or alloy-galvanized steel or of aluminum and its alloys, the metal surfaces being brought into contact with an aqueous solution for a period in the range from 1 to 120 seconds , which contains trivalent and hexavalent chromium and acetate ions and has a temperature in the range from 20 to 60 ° C, characterized in that the aqueous solution
a pH in the range from 3.9 to 5.0,
a total chromium content between 0.1 and 10 g / l,
a ratio of Cr (III): Cr (VI) in the range from 1: 1 to 1: 4
and has a content of acetate ions in the range of 0.1 to 10 g / l.

Here, an aqueous solution is preferably used, the pH of which is in the Range is from about 3.9 to about 4.5. This solution preferably has a Ratio Cr (III): Cr (VI) in the range from 1: 2 to 1: 3.

The higher the content of acetates, the higher the Total chromium content of the chromating or post-passivation solution. Here can be considered as a rule of thumb that a sufficient stabilizing effect then is achieved when the acetate ion content in g / l is approximately 0.5 to 2 times that Total chromium content in g / l.

The treatment solution which can be used in the processes mentioned can contain further constituents, the beneficial effect of which on layer formation and corrosion protection is known in the technical field concerned. Examples include:
up to 0.5 g / l fluoride ions,
up to 1.0 g / l zirconium and / or titanium compounds,
up to 0.5 g / l phosphate ions,
up to 0.5 g / l nickel (II) ions,
up to 0.5 g / l iron (III) ions.

Because of the advantage of the method according to the invention, even with only one weakly acidic treatment solution in the pH range between about 3.9 and to be able to work about 5, which in addition to hexavalent chrome one contains sufficient concentration of trivalent chromium, the increases Occupational safety when using chromate and chromate Post-passivation solutions. Pre-formed corrosion protection layers like for example crystalline or amorphous phosphate layers or the Protective layers after an alkaline passivation become less strong attacked, so that protective layers with improved corrosion protection arise and the economy of the overall process is improved. A  special aspect of the increased economy combined with a ecological advantage are the reduced amounts of waste to be disposed of chrome-containing sludges. If you set the inventive method as Chromating process, preferably using aqueous solutions a total chromium content in the range of about 2 to about 10 g / l. The applies Rule of thumb that the higher the concentration, the shorter the Treatment time is.

For example, the process according to the invention for chromating Use running metal belts. The treatment times are then in the On the order of 1 second and hardly exceed 5 seconds. Here the metal strips are preferably with the treatment solution whose Temperature is preferably in the range between about 40 and about 60 ° C, sprayed. After the specified response time, which results from the length of the Treatment line and the belt speed results, the Treatment solution either rinsed with water or with squeeze rollers squeezed. The treated goods are then dried as usual. In the Chromating of parts is longer treatment times up to about 2 minutes possible. Treatment can optionally be by spraying with the Treatment solution or by immersing in the solution. The Total chromium concentration can be near the lower end of the Concentration range from about 2 to about 10 g / l and for example be between about 2 and about 5 g / l. The temperature of the solution is also preferably in the range between about 40 and about 60 ° C. At the end the treatment solution is rinsed with water. Subsequently the parts are dried.

The solution according to the invention can also be used in a process for Post-passivation of pre-treated metal parts can be used. This Pretreatment can, for example, from iron or zinc phosphating or an alkaline passivation. The so pretreated Metal surfaces are brought into contact with an aqueous solution, the one Total chromium content in the range of about 0.5 to about 2 g / l. This can  by immersion in the treatment solution or by spraying with the Treatment solution take place. The temperature of the treatment solution is chosen preferably in the range from about 30 to about 50 ° C, particularly at about 40 ° C. For this post-passivation, a period of time is preferably chosen between about 30 and about 90 seconds, for example about 60 seconds. After that the treatment solution is rinsed with water. The following Exemplary embodiments show the advantages of the method according to the invention.

Embodiments

For comparative experiments 1 and 2, a commercial Chromating solution (Deoxylyte® 42, Henkel KGaA) used. This solution was also used for Examples 3, 5 and 6, with those in Tables specified amounts of acetic acid ("HAC") were added. All solutions two percent by volume were prepared in water. The Cr (VI) and Cr (III) and the pH are given as "start" values in the tables. The The stability of the solutions with an increase in pH was tested by that the solutions have different amounts of an alkaline Passivation solution were added. The different ones The pH value which adjusts the amount is given in the tables. Was a When the pH reached 5 or less, the experiment was terminated. As The results of the experiment were used to determine the amounts of the separated sludge. For the tests according to Table 1, the residual contents were also given 6-valent and 3-valent chromium in the solution after raising the pH and Filter the sludge determined. The experiments show that the unstabilized comparison solutions even with a slight increase in pH strong sludge development occurs. The amount of sludge after the end of the test is significantly higher in the comparison solutions than in the inventive solutions stabilized solutions. The residual content of trivalent chromium after the end of Alkaline addition is significantly lower in the comparison solutions than in the solutions stabilized according to the invention.  

For Examples 1, 2 and 4, a chromic acid solution with a Chromium content of 120 g / l reduced in different ways, at the same time Acetations were formed. For example 1, 114 g / l of citric acid added and the temperature of the solution kept at 85 ° C for 2 hours. At Example 2 was also added 114 g / l citric acid. The temperature of the The mixture rose to 80 ° C. The solution was opened after the addition Let it cool down at room temperature. The theoretical acetic acid content of the Solutions of Examples 1 and 2 is about 4.5 g / l. For example 4, ethanol was used used as a reducing agent. A degree of reduction of 28% was sought, d. H. 28% of the chromium content should be reduced to the trivalent level become. All 3 solutions were made by a 2 volume percent approach made in water chromate solutions. The contents of Cr (VI), Cr (III) and the initial pH values are entered in the tables as "start" values. Then the stability against alkali entry was as above described tested. The results are shown in the tables.  

Table 1

Table 2

Claims (9)

1. Aqueous solution for chromating or re-passivating surfaces made of steel, galvanized or alloy-galvanized steel or of aluminum and its alloys, which contains trivalent and hexavalent chromium and acetates, characterized in that they
a pH in the range from 3.9 to 5.0,
a total chromium content between 0.1 and 10 g / l,
a ratio of Cr (III): Cr (VI) in the range from 1: 1 to 1: 4
and has a content of acetate ions in the range of 0.1 to 10 g / l. 2. Aqueous solution according to claim 1, characterized in that it a pH in the range of 3.9 to 4.5 and a Cr (III): Cr (VI) ratio in the range from 1: 2 to 1: 3. 3. Aqueous solution according to one or both of claims 1 and 2, characterized in that it additionally contains one or more of the following components:
up to 0.5 g / l fluoride ions
up to 1.0 g / l zirconium and / or titanium compounds,
up to 0.5 g / l phosphate ions,
up to 0.5 g / l nickel (II) ions,
up to 0.5 g / l iron (III) ions. 4. Aqueous concentrate that when diluted with water by a factor between 50 and 2000 an aqueous solution according to one or more of the Claims 1 to 3 results. 5. A method for chromating or re-passivating surfaces made of steel, galvanized or alloy-galvanized steel or of aluminum and its alloys, the metal surfaces being brought into contact with an aqueous solution which is trivalent and for a period of time in the range from 1 to 120 seconds Contains hexavalent chromium and acetate ions and has a temperature in the range of 20 to 60 ° C, characterized in that the aqueous solution
a pH in the range from 3.9 to 5.0,
a total chromium content between 0.1 and 10 g / l,
a ratio of Cr (III): Cr (VI) in the range from 1: 1 to 1: 4
and has a content of acetate ions in the range of 0.1 to 10 g / l. 6. The method according to claim 5, characterized in that the aqueous solution a pH in the range of 3.9 to 4.5 and a Cr (III): Cr (VI) ratio in the range from 1: 2 to 1: 3. Process according to one or both of claims 5 and 6, characterized in that the aqueous solution additionally contains one or more of the following constituents:
up to 0.5 g / l fluoride ions
up to 1.0 g / l zirconium and / or titanium compounds,
up to 0.5 g / l phosphate ions,
up to 0.5 g / l nickel (II) ions,
up to 0.5 g / l iron (III) ions. 8. A method for chromating according to one or more of claims 5 to 7, characterized in that the metal surfaces with an aqueous Contact solution that has a total chromium content in the range of 2 to 10 g / l. 9. A method for post-passivation according to one or more of claims 5 to 7, characterized in that the phosphated or alkaline passivated metal surfaces in contact with an aqueous solution brings that has a total chromium content in the range of 0.5 to 2 g / l.