CA2303101A1

CA2303101A1 - Chromate treatment or postpassivation with stabilised solutions based on cr (iii)/(vi)

Info

Publication number: CA2303101A1
Application number: CA002303101A
Authority: CA
Inventors: Joerg Riesop; Heike Krautbauer
Original assignee: Individual
Current assignee: Henkel AG and Co KGaA
Priority date: 1997-09-12
Filing date: 1998-09-05
Publication date: 1999-03-25
Also published as: ZA988356B; DE19740248A1; EP1017878A1; WO1999014398A1

Abstract

The invention concerns a method for chromate treatment or postpassivation of a corrosion-resistant coating previously formed, and an aqueous solution for the chromate treatment or postpassivation of steel, zinc-coated or galvanized surfaces by depositing an alloy either of aluminium or of aluminium alloys, containing trivalent or hexavalent chromium, and acetate ions. Said aqueous solution is characterised it has a pH ranging between 3.9 and 5.0, a total chromium content ranging between 0.1 and 10 g/l, a ratio Cr (III): Cr (VI) of the order of 1: 1 to 1: 4 and an acetate ion content of the order of 0.1 to 10 g/l.

Description

' CA 02303101 2000-03-13 Chromatising or post-passivating using stabilised Cr(IIn/Cr(Vn-containing solutions This invention relates to chromatising metal surfaces which have been cleansed but have not received any other anti-corrosive protection. As a result of the chromatising process, a corrosion-protectivelayer is applied which may act as an adhesive substrate for subsequent lacquering. The present invention also relates to post~assivating metal surfaces which have already been subjected to corrosion-protective pretreatment after cleansing, such as iron or zinc phosphatising or alkaline passivation. As a result of post-passivation, the anti-corrosive action of this previous corrosion-protective treatment is further intensified. The process may be used for surfaces of steel, galvanised or alloy-galvanised steel or aluminum and alloys thereof. Other embodiments of the present invention include the aqueous treatment solutions which may be used for the present process and the concentrates thereof.
The chromatising of metal surfaces or post~assivation of metal surfaces which have already received corrosion-protective pretreatment using Cr-containing aqueous solutions is very widely applied in the field of metal surface treatment.
Acidic aqueous solutions are frequently used for this purpose, these containing both 3- and 6-valent chromium. These types of solutions may be prepared, for example, by dissolving suitable chromium(III) salts and chromic acid and/or chromates in water. It is more economical, however, to start with chromic acid-containing solutions and to reduce a proportion of the 6-valent chromium to the 3-valent state by means of suitable reducing agents. Methanol is conventionally used on an industrial scale for this reduction process. Other reducing agents such as ethanol or higher alcohols and formaldehyde or higher aldehydes, however, are also suitable. In this case, the reducing agents may be completely oxidised with the formation of water and carbon dioxide. Frequently, however, the oxidation products, for example the acetic acid or acetate ions produced when using ethanol, remain in the Cr(III)/Cr(VI)-containing solution. Examples of these types of solutions and the use thereof for chromatising metals are given in EP-A-391 442 and WO 91/05078. These documents also indicate that the anti-corrosive action of these solutions may be improved by further additives, such as phosphate ions, fluorozirconate ions, zinc ions, finely dispersed silica and organic additives.
These types of Cr(III)/Cr(VI)-containing solutions are conventionally adjusted to a pH
between about 2 and about 3.8 for metal surface treatment. This produces the highest possible proportion of Cr(III), which is beneficial for layer formation and corrosion-protection. Cr(III) salts, however, are soluble in the required concentration, if at all, only in sufficiently acid medium. At a pH above about 3.8, Cr(III) starts to precipitate as hydroxide at the concentrations which are required for industrial purposes.
On the one hand, it is then no longer available for the chromatising process and, on the other hand, it has to be disposed of as a waste material. Attempts are made to work as near to the upper limit of the industrially usable pH range as possible, however, because then chromatising layers having especially good corrosion-protective properties are formed, the corrosive attack at the metal surface remains at a low level or corrosion-protective layers which are already present are not dissolved.
During operation of a chromatising bath, the pH of the bath increases due to acid consumption as a result of corrosive attack at the metal surface. Hitherto, care has had to be taken, by means of practical measures taken at an early stage, to ensure that the pH does not exceed the critical limit of about 3.8 and that chromium-containing sludges do not settle out. The risk of excessive alkalinity in the Cr(III)/Cr(VI) bath is particulary great if this is used for post~assivating alkaline passivating layers. Firstly, there is the danger that alkalinity from the passivating bath may be carried over into the chromium-containing post-passivating bath via the intermediate rinsing stage.
Secondly, however, if acids are also present in the chromium-containing acid passivating solution, that these react with the alkaline passivating layer.
This also lowers the content of active substance in the chromium-containingpassivating solution so that chromium-containingsludges settle out as a result of the increase in pH. Thus, there is a need for chromatising bath and post-passivating solutions which, although they still work in the acid region, are stabilised against Cr(III) precipitation up to a pH
of about 5.
An object of the present invention is therefore providing a solution for chromatising or post-passivating metal surfaces and a suitable method of working, wherein the treatment solutions also still have industrially conventional chromium contents and Cr(III)/Cr(VI) ratios when, although still in the acid region, the pH range involved is higher than the current industrially conventional range.
With regard to the treatment solution, this object is achieved by an aqueous solution for chromatising or post-passivating surfaces made of steel, galvanised or alloy-galvanised steel or aluminum and alloys thereof, which contains 3-valent and 6-valent chromium and acetate ions, characterised in that it has:
a pH in the range 3.9 to 5.0, a total chromium content of between 0.1 and 10 g/1, a Cr(III) : Cr(VI) ratio in the range 1:1 to 1:4 and an acetate ion content in the range 0.1 to 10 g/1.
The pH is preferably in the range about 3.9 to about 4.5. The preferred Cr(III) Cr(VI) ratio is in the range 1:2 to about 1:3.
The higher the total chromium content of the chromatising or post-passivating solution, the higher the content of acetate ions should be adjusted. A useful rule of thumb is that an adequate stabilising effect is achieved when the acetate ion content in g/1 is about 0.5 to 2-times that of the total chromium content in g/1.
As is conventional with chromatising solutions, these solutions may contain other constituents which are incorporated in the chromatising layers or have an effect on layer formation. The following may be mentioned as optional components of this type:
up to 0.5 g/1 of fluoride ions, up to 1.0 g.l of zirconium and/or titanium compounds, up to 0.5 g/1 of phosphate ions, up to 0.5 g/1 of nickel(II) ions, up to 0.5 g/1 of iron(III) ions.
In principle, these types of treatment solutions may be prepared by dissolving the individual components in water in the desired concentration range when required. In practice, however, it is conventional to prepare aqueous concentrates which contain the individual components in the desired ratio by amounts and from which the ready-to-use working solution may be prepared by simple dilution as required.
Accordingly, the present invention also provides an aqueous concentrate which produces a ready-to-use treatment solution having the previously mentioned features on diluting with water by a factor of between about 50 and about 2,000. When preparing these concentrates, the conventional procedure is to reduce a Cr(VI)-containing solution using an appropriate reducing agent to the point where the desired ratio of 38valent to 6-valent chromium is achieved. Chromic acid or chromates are preferably selected as the Cr(VI)-containing starting compound. Since the solution according to the present invention is intended to contain an adequate amount of acetic acid or acetate ions, in order to inhibit precipitation of trivalent chromium compounds in the pH range selected in accordance with the present invention, it is particularly efficient to use compounds which are oxidised to give acetic acid for reducing the hexavalent chromium. One obvious example of this type of compound is ethanol. Citric acid, however, may also be used to advantage. The particular way in which the acetic acid or acetate ions are produced at the concentration required in the treatment solution or its concentrate, however, is not important. Thus, it is also possible, for example, additionally to stabilise a chromic acid solution partly reduced with methanol by adding acetic acid or soluble acetates. In the case of this type of separate addition, however, the acetate ions must be added to the chromic acid or chromate solution prior to the reduction process. If reduction is performed at a pH in accordance with the present invention, but without the presence of or direct production of acetate ions, Cr(III) compounds precipitate out and these cannot be taken back into solution by adding acetate later.
When "acetate ions or acetic acid" is mentioned in this context, this is intended to signify that there is an equilibrium set up between free acetic acid and acetate anions in the pH range in accordance with the present invention. Therefore, it does not matter whether acetic acid or soluble acetate salts are added to the solution or whether these species are produced by oxidising suitable starting compounds, such as ethanol.
At the pH selected, the preset equilibrium is automatically set up by the acidity constant of acetic acid. When, therefore, only "acetate ions" are mentioned in the following, this is to be understood to mean the total amount of acetate ions and acetic acid which are present in equilibrium with each other.
The present invention also provides a process for chromatising or post-passivating surfaces of steel, galvanised or alloy-galvanised steel or aluminum and alloys thereof, wherein the metal surfaces are placed in contact, for a period in the range 1 to 120 seconds, with an aqueous solution which contains 3-valent and 6-valent chromium and acetate ions and which has a temperature in the range 20 to 60PC, characterised in that the aqueous solution has:
a pH in the range 3.9 to 5.0, a total chromium content of between 0.1 and 10 g/1, a Cr(III) : Cr(VI) ratio in the range 1:1 to 1:4, and an acetate ion content in the range 0.1 to 10 g/l.
Here, an aqueous solution having a pH in the range about 3.9 to about 4.5 is used.

This solution preferably has a Cr(111) : Cr(VI) ratio in the range 1:2 to 1:3.
The higher the total chromium content in the chromatising or post-passivating solution, the higher the content of acetate ions is adjusted. A useful rule of thumb is that an adequate stabilising effect is achieved when the acetate ion content in g/1 is about 0.5 to 2 times that of the total chromium content in g/1.
The treatment solution for use in the process may contain other constituents which have a beneficial effect on layer formation and corrosion protectionrecognised in the relevant technical field. The following may be mentioned by way of example:
up to 0.5 g/1 of fluoride ions, up to 1.0 g.l of zirconium and/or titanium compounds, up to 0.5 g/1 of phosphate ions, up to 0.5 g/1 of nickel(II) ions, up to 0.5 g/1 of iron(III) ions.
Due to the advantage of the process according to the present invention, which is the ability to work with an only weakly acid treatment solution in the pH range between about 3.9 and about 5, but one which nevertheless contains an adequate concentration of 3-valent chromium in addition to 6-valent chromium, operational reliability is increased when using chromium-containing acid chromatising and post-passivating solutions. Previously applied corrosion-protective layers, such as crystalline or amorphous phosphate layers or protective layers after alkaline passivation, are less severely corroded so that protective layers having an improved corrosion-protective effect are produced and the economic viability of the overall process is improved. A
particular feature of the increased economic viability, associated with an ecological advantage, is the reduced quantity of chromium-containing sludge which has to be disposed of. If the process according to the present invention is used as a chromatising process, aqueous solutions having a total chromium content in the range from about 2 to about 10 g/1 are preferably used. A useful rule of thumb is that the concentrations should be selected to be higher, the shorter the treatment time.
By way of example, the process according to the present invention may be used to chromatise continuous metal strips. The treatment times are then of an order of about 1 second and rarely exceed 5 seconds. In this case, the metal strips are preferably sprayed with treatment solution, the temperature of the solution preferably being between about 40 and about 60°C. After the reaction time mentioned, which is given by the length of the treatment zone and the speed of the strip, the treatment solution is either rinsed off with water or squeezed off with squeegee rolls. The treated material is then dried in a conventional manner. When chromatising parts, longer treatment times of up to about 2 minutes are possible. Treatment may be performed, as desired, by spraying with treatment solution or by immersion in the solution. The total chromium concentration may thus be towards the lower end of the concentration range of about 2 to about 10 g/1 and is, for example, between about 2 and about 5 g/1.
The temperature of the solution is likewise preferably in the range between about 40 and about 60°C. After treatment, the treatment solution is rinsed off with water. The parts are then dried.
The solution according to the present invention may also be used in a process for post-passivating previously treated metal parts. This pretreatment may consist, for example, of an iron or zinc phosphatising process or alkaline passivation. The metal surfaces pretreated in this way are placed in contact with an aqueous solution which has a total chromium content in the range from about 0.5 to about 2 g/1. This may be achieved by immersion in the treatment solution or by spraying with the treatment solution. The temperature of the treatment solution is chosen to be in the range from about 30 to about 50°C, in particular up to about 40°C. The duration of this post-passivating process is preferably chosen to be between about 30 and about 90 seconds, for example about 60 seconds. The treatment solution is then rinsed off with water.
The following Working Examples illustrate the advantages of the process according to the present invention.
Workin~ples A commercial chromatising solution (Deoxylyte7 42, Henkel KGaA) was used for Comparison Examples 1 and 2. This solution was also used for Examples 3, 5 and 6, wherein the amounts of acetic acid ("HAC ") stated in the Tables were added.
All solutions were made up as 2 vol-% solutions in water. The contents of Cr(VI) and Cr(III) and the pH are given as "start" values in the Tables. The stability of the solutions with respect to an increase in pH was tested by adding different amounts of an alkaline passivating solution to the solutions. The pH reached for the different amounts being added is given in the Tables. When a pH of 5 or a little below that was reached, the test was terminated. The results of the test were taken as the amounts of sludge which settled out. For tests in accordance with Table 1, the residual contents of 6Bvalent and 3-valent chromium in solution after raising the pH and filtering off the sludge were also determined. The tests show that in the non~tabilised comparison solutions, even a small increase in pH caused the production of a great deal of sludge.
The amount of sludge after completing the test is much higher for the comparison solutions than for stabilised solutions according to the present invention.
The residual content of 3-valent chromium after completing the addition of alkali is significantly less in the comparison solutions than in stabilised solutions according to the present invention.
For Examples 1, 2 and 4, a chromic acid solution having a chromium content of g/1 was reduced in different ways, wherein acetate ions were simultaneously produced. In Example 1, 114 g/1 of citric acid were added and the temperature of the solution was held at 85°C for 2 hours. In Example 2, 114 g/1 of citric acid were also added. The temperature of the mixture then rose to 80~C. The solution was allowed to cool to room temperature after completing the addition procedure. The theoretical content of acetic acid in the solutions in Examples 1 and 2 was about 4.5 g/1.
In Example 4, ethanol was used as the reducing agent. Here, a degree of reduction of 28 was striven for, i.e. 28 % of the chromium content should have been reduced to the trivalent state. Chromatising solutions were prepared from all three solutions by making up at 2 vol-% in water. The contents of Cr(VI), Cr(III) and the initial pH are given as "start" values in the Tables. Then, the stability towards the introduction of an alkaline solution was tested in the same way as described above. The results are given in the Tables.

Table 1:
Example 1 Example Example 3 Comp. Ex.

Deoxylyte7 Deoxylyte742 42 +

2 g/1 HAC

Mixture: 2 vol-% 2 vol-% 2 vol-% 2 vol-%

CrVI start g/1 1.30 1.34 1.71 1.71 CrIII start 1.18 1.14 0.76 0.76 g/1 pH start 2.70 2.72 2.59 2.75 Added: pH of the pH of the pH of the pH of the solution solution solution 3.88 solution 3.27 3.51 + 1 ml 3~~ (heavy sludge concentrate deposit) of an alkaline passivating4.50 3.67 solution 4.88 4.52 3.95 4.01 n - 4.93 4.19 4.51 n - 4.55 5.00 n - 4.84 n Amount of sludge0.67 0.44 0.17 1.25 in g/1 CrVI end g/1 1.21 1.32 1.66 1.49 CrIII end g/1 1.04 1.11 0.56 0.0!

Table 2:
Example Example 5 Example Comp. Ex.

Deoxylyte7 Deoxylyte7 Deoxylyte742 42 + 42 +

0.3 g/1 HAC 0.9 g/1 HAC

Mixture: 2 vol-% 2 vol-% 2 vol-% 2 vol-CrVI start g/1 1.90 1.76 1.75 1.75 CrIII start 0.72 0.65 0.65 0.65 g/1 pH start 1.85 2.60 2.62 2.60 Added: pH pH pH pH

+3 ml concentrate4.07 4.48 4.28 4.45 of an alkaline (a lot of (a lot passivating sludge) of solution sludge) 4.73 4.63 + 1 ml concentrate of an alkaline 4.9 5.00 passivating solution Amount of sludge0.19 2.11 1.5 3.5 in g/1

Claims

1. An aqueous solution for chromatising or post-passivating surfaces of steel, galvanised or alloy-galvanised steel or aluminum and alloys thereof, which contains 3-valent and 6-valent chromium and acetate ions, characterised in that it has:
a pH in the range 3.9 to 5.0, a total chromium content of between 0.1 and 10 g/l, a Cr(III) : Cr(VI) ratio in the range 1:1 to 1:4, and an acetate ion content in the range 0.1 to 10 g/l, wherein the acetate ion content in g/l is 0.5 to 2 times the total chromium content in g/l.

2. An aqueous solution according to Claim 1 wherein it has a pH in the range 3.9 to 4.5 and a Cr(III) : Cr(VI) ratio in the range 1:2 to 1:3.

3. An aqueous solution according to one or both of Claims 1 and 2 wherein it also contains one or more of the following constituents:
up to 0.5 g/l of fluoride ions, up to 1.0 g.l of zirconium and/or titanium compounds, up to 0.5 g/l of phosphate ions, up to 0.5 g/l of nickel(II) ions, up to 0.5 g/l of iron(III) ions.

4. An aqueous concentrate which produces an aqueous solution according to one or more of Claims 1 to 3 on dilution with water by a factor of between 50 and 2000.

5. A process for chromatising or post-passivating surfaces of steel, galvanised or alloy-galvanised steel or aluminum and alloys thereof, wherein the metal surfaces are placed in contact, for a period in the range 1 to 120 seconds, with an aqueous solution which contains 3-valent and 6-valent chromium and acetate ions and has a temperature of 20 to 60°C, characterised in that the aqueous solution has:

a pH in the range 3.9 to 5.0, a total chromium content of between 0.1 and 10 g/l, a Cr(III) : Cr(VI) ratio in the range 1:1 to 1:4 and an acetate ion content in the range 0.1 to 10 g/l, wherein the acetate ion content in g/l is 0.5 to 2 times the total chromium content in g/l.

6. A process according to Claim 5 wherein the aqueous solution has a pH in the range 3.9 to 4.5 and a Cr(III) : Cr(VI) ratio in the range 1:2 to 1:3.

7. A process according to one or both of Claims 5 and 6 wherein the aqueous solution also contains one or more of the following constituents:
up to 0.5 g/l of fluoride ions, up to 1.0 g.l of zirconium and/or titanium compounds, up to 0.5 g/l of phosphate ions, up to 0.5 g/l of nickel(II) ions, up to 0.5 g/l of iron(III) ions.

8. A process for chromatising according to one or more of Claims 5 to 7 wherein the metal surfaces are placed in contact with an aqueous solution which has a total chromium content in the range 2 to 10 g/l.

9. A process for post-passivating according to one or more of Claims 5 to 7 wherein the initially phosphatised or alkaline passivated metal surfaces are placed in contact with an aqueous solution which has a total chromium content in the range 0.5 to 2 g/l.