CA2041891C

CA2041891C - Process of aftertreating conversion layers

Info

Publication number: CA2041891C
Application number: CA002041891A
Authority: CA
Inventors: Thomas Kolberg; Werner Rausch; Peter Schubach; Thomas Wendel
Original assignee: Metallgesellschaft AG
Current assignee: Chemetall GmbH
Priority date: 1990-05-29
Filing date: 1991-05-06
Publication date: 2001-07-31
Anticipated expiration: 2011-05-06
Also published as: DE4017187A1; DE59100650D1; AU634652B2; JPH04231480A; CA2041891A1; JP3083873B2; ZA914086B; EP0459550A1; BR9102174A; AU7618091A; EP0459550B1; ES2047370T3

Abstract

The invention teaches that conversion layers formed an surfaces of aluminum or aluminum alloys by a treat-ment with solutions based on titanium, zirconium and/or haf-nium ions should be aftertreated with aoueous solutions which contain aluminum, zirconium and fluoride and have been ad-justed to a pH value ~ 5.
In the solutions used for aftertreatino the to-tal concentration of A1 + Zr + F preferably lies between 0.1 and 8.0 g/l, preferably between 0.2 and 5.0 g/l, and the Al : Zr : F mole ratios have been adjusted to (0.15 to 8.0) :
1 : (5 to 52), particularly to (0.15 to 2.0) : 1 : (5 to 16) and most desirably to (0.15 to 0.67) : 1 : (5 to 7).
The aftertreating solutions should be at a temperature of 20 to 80°C, preferably of 20 to 50°C, and are preferably applied for 1 to 30 seconds.
The process in accordance with the invention is particularly used to pretreat the surfaces before a succeeding painting, film laminating or adhesive coating operation.

Description

This invention relates to a process of post rinsing conversion layers which have been formed on surfaces of aluminum or aluminum alloys by a treatment with solutions based on titanium, zirconium and/or hafnium ions, and to the use of that process for a pretreatment preceding a painting, film laminating or adhesive coating operation.
The production of conversion layers on surfaces of aluminum or aluminum alloys, particularly as a pretreatment before the application of an organic coating, is industrially performed on a large scale. Such conversion layers will prevent or oppose the occurrence of corrosion and under an organic coating will improve the bond strength of the coating which has been applied. Whereas said requirements are sufficiently met, as a rule, by the widely employed chromate layers, the production of chromium-free conversion layers is increasingly preferred for reasons of work place hygiene and environmental protection or because the treated material is intended to be used for specific purposes, e.g., for packaging foodstuffs. The treating solutions for forming chromium-free conversion layers are generally adjusted to an acid pH value and they contain, e.g., titanium, fluoride, phosphate and tannin (U. S. Patent 4,017,334) or zirconium, fluoride and boron (U. S. Patent 3,964,936). A treating solution which contains hafnium and fluoride has been described in French Patent Specification 2,417,537.
The properties of the conversion layers which have been formed can further be improved by a succeeding passivating aftertreatment. For the reasons stated, it is preferred to effect the aftertreatment with chromium-free treating solutions, which contain organic active substances, as a rule. Such an aftertreating solution based on polyvinylphenol has been described in US-A 4,376 000.
But that solution will pollute the sewage, particularly because there is a large demand for oxygen required to decompose the organic component.
It is an object of the invention to provide for the passivating post rinsing of chromium-free conversion layers on surfaces of aluminum or aluminum alloys a process which does not have the disadvantage of post rinsing solutions that contain organic components as regards the need for a treatment of the waste water but will result in an improvement of the corrosion resistance and the bond strength of paint at least to the same degree.
The object is accomplished in that the process which is of the kind described first hereinbefore is carried out in such a manner in accordance with the invention that the conversion layers are aftertreated with aqueous solutions which contain aluminum, zirconium and fluoride and have been adjusted to a pH value <_ 5.
More particularly, the present invention proposes the a process of aftertreating conversion layers which have been formed on surfaces of aluminum or aluminum.alloys by a treatment with solutions containing titanium, zirconium and/or hafnium ions, characterized in that the conversion layers are aftertreated with aqueous solutions which contain aluminum, zirconium and fluoride and have been adjusted to a pH value <_ 5; and wherein the Al: Zr: F mole ratios of the aqueous solutions have been adjusted to (0,15 to 8) . 1: (to 52).
The process in accordance with the invention can be used for a passivating aftertreatment of conversion layers that have been formed on surfaces of aluminum or aluminum alloys, which materials may consist of massive aluminum or massive aluminum alloy or of objects which have been plated therewith, e.g., by hot dip coating. Suitable elements which can be alloyed with aluminum are particularly silicon, manganese, magnesium zinc and copper.
The conversion layers formed on such surfaces by a treatment with solutions based on Ti, Zr and/or Hf distinguish in that the thickness of the layer is distinctly below 1 Vim, that the layers are amorphous at least in part, and that they do not contain chromium. In addition to titanium, zirconium and/or hafnium ions the treating solutions employed to form the conversion layers contain additional film-forming and/or pickling components, such as fluorides, phosphates and compounds of boron, and 3a optionally contain passivating components, such as tannins.
Suitable treating solutions have been described in the patent specifications mentioned herein before.
The surface on which the conversion layer has been formed is suitably rinsed with water.
The post rinsing solution may be applied, e.g., by dipping, flooding, spraying or roller coating.

A preferred feature of the invention resides in that the post rinsing solution employed contains aluminum, zirconium and fluoride in a total A1 + Zr + F concentration between 0.1 and 8 g/l, preferably between 0.2 and 5 g/l.
The Al . Zr . F mole ratios should desirably be adjusted to (0.15 to 8) . 1 . (5 to 52), particularly to (0.15 to 2.0) . 1 . (5 to 16). In a particularly preferred embodiment of the process the A1 . Zr . F ratios in the post rinsing solution are (0.15 to 0.67) . 1 . (5 to 7).
According to a further desirable feature of the invention the pH value is adjusted to 2 to 5.
The post rinsing solutions used in the process in accordance with the invention contain, inter alia, acid aluminum fluoride zirconates and in case of a surplus of aluminum additionally contain other salts of aluminum, such as fluorides, terafluoroborates, nitrates. They may be produced, e.g., in that zirconium metal or zirconium carbonate is initially dissolved in aqueous hydrofluoric acid to form complex fluorozirconium acid. Aluminum metal or aluminum hydroxide or an aluminum salt, such as the nitrate, fluoride, tetrafluoborate, formate, acetate, is then added, preferably in dissolved form, and is optionally dissolved. A possible slight cloudiness of the solution will not adversely affect its effectiveness. Whereas the preparation is preferably effected on the described route, the solutions may alternatively prepared in a different manner.
The pH value of the solution is preferably adjusted with rations of volatile bases. These include particularly ammonium, ethanolammonium and di- and triethanolammonium. The adjustment particularly to relatively high pH values in the stated pH range and to relatively high concentrations in the range stated for the total concentration of A1 + Zr + F may result in a cloudiness of the solution but this will not adversely affect the effectiveness of the process.
According to a further desirable feature of the invention the surfaces provided with a conversion coating are aftertreated with an aqueous solution which additionally contains at least one of the anions benzoate, caprylate, ethyl hexoate, salicylate, preferably in a total concentration of 0.05 to 0.5 g/l. This will further increase the bare corrosion protection. The anions may be introduced by means of the corresponding acids or their salts.
According to a further desirable feature of the invention the post rinsing solution is applied for between about 1 and 120 seconds, particularly for between 1 and 30 seconds: The solution may be applied at a temperature between 20°C and about 80°C. Temperatures between 20 and 50°C are preferred.
Deionized water or low-salt water is preferably used to prepare the post rinsing bath. Water having a high salt content is less suitable for the preparation of the bath.
After the passivating aftertreatment the surface may be dried, e.g., on the air or in an oven and optionally may be rinsed before with deionized water. According to a preferred embodiment of the invention the surface which has been subjected to the passivating aftertreatment is subjected to an accelerated drying, e.g., with hot air or by infrared radiation.

The process in accordance with the invention serves primarily to pretreat surfaces of aluminum or aluminum alloys before they are painted, laminated with a film or coated with an adhesive. Suitable organic coating materials include, e.g., polyesters, silicone-modified polyesters, polyvinylidenefluorides, acrylates, epoxides, epoxy-phenol resins, plastisols or organosols (e.g., or PVC
or acrylates).
The advantage afforded by the process in accordance with the invention resides particularly in that the bond strength of the organic films on the metallic substrate is increased, as is reflected, e.g., by correspondingly good results achieved in the T-Bend Test (ISO 1519-1973) or in the Feathering Test (bond strength of paint during an opening of can closures). Besides, the corrosion resistance of the organic films to a corrosive action, is also increased, e.g., in a test in which an aqueous condensate is formed under constant air conditions (DIN 50 017 KK) or a sterilization test. The process in accordance with the invention also increases the corrosion resistance of the surface which has not been provided with an organic coating, as is reflected, e.g., in a test for well water blackening (no blackening in boiling water).
The process in accordance with the invention will be explained more in detail and by way of example with reference to the following example.
Example To form a conversion coating on cleaned and degreased aluminum sheets they were dipped for l0 seconds into a solution which was at a temperature of 50°C and had the following composition:

Ti 0.17 g/1 F 1.24 g/1 P205 0.09 g/1 NH4 0.91 g/1 tannin 0.11 g/1 Na 0.003 g/1 biocide 0.10 g/1 Thereafter the sheets were rinsed with water and were then subjected to a passivating aftertreatment. To that end the sheets were dipped into the post rinsing solution for 5 seconds and were subsequently squeeged for a removal of surplus solution. After a drying at 60°C in an oven operated with circulating air for 0.5 minute the sheets were painted with a two-layer coating that is approved for use with food-stuffs. The first layer of said coating consists of an epoxyphenol resin and the second layer consists of an organosol. The coating had a total thickness between 10 and 15 ~,m.
Circular blanks 60 mm in diameter were subsequently blanked from the treated sheets, which had a thickness of about 0.25 mm, and the blanks were deep drawn to form cups which were 26 mm in diameter and had a height of 25 mm.
Said cups were subjected to a sterilization test, in which they were exposed in a pressure container to the action of an aqueous solution of 3% common salt, 1% citric acid and 0.5% lactic acid at 121°C for 40 minutes. The defects subsequently detected on the cups (delamination of paint, blisters) were rated with reference to a scale from 1 (delamination of the paint throughout the shell of the cup = unusable) to 15 (no paint defect = excellent.) To prepare the post rinsing solution to be employed, 1.6 g/1 and 3.2 g/1 respectively of an aqueous concentrate, which contained 0.855% by weight Al and 8.62%
by weight Zr and 10.7% by weight F, was diluted with deionized water to yield post rinsing solution A and post rinsing solution B.
Both solutions were adjusted with ammonia to a pH
value of about 3.6.
For comparison, an aftertreatment was effected with a polyvinylphenol solution having a concentration of 0.6 g/1 and a pH value of about 5 (post rinsing solution C) .
The post rinsing solutions employed in the process in accordance invention had the following with the compositions:

Post rinsing solution A: Post rinsing solution B:

A1 0.014 g/1 A1 0.028 g/1 Zr 0.14 g/1 Zr 0.28 g/1 F 0.17 g/1 F 0.34 g/1 NH4 0.016 g/1 NH4 0.03 g/1 Test results obtained in sterilization test (Rating scale: 1 = unusable to 15 = excellent) Post rinsing Rating solution A (invention) 11 B (invention) 10 C (invention) 6 A comparison of the tabulated data reveals that the process in accordance with the invention produces distinctly better results than the comparison example using an post rinsing solution based on polyvinylphenol.

Claims

1. A process of aftertreating conversion layers which have been formed on surfaces of aluminum or aluminum alloys by a treatment with solutions containing titanium, zirconium and/or hafnium ions, characterized in that the conversion layers are aftertreated with aqueous solutions which contain aluminum, zirconium and fluoride and have been adjusted to a pH value ~ 5; and wherein the Al: Zr: F
mole ratios of the aqueous solutions have been adjusted to (0.15 to 8) : l : (5 to 52).

2. A process according to claim 1, characterized in that the conversion layers are aftertreated with solutions which contain aluminum, zirconium and fluoride in a total Al + Zr + F concentration between 0.1 and 8 g/l.

3. A process according to claim 2, charac-terized in that said total Al + Zr + F concentration is between 0.2 and 5 g/l.

4. A process according to claim 3, charac-terized in that the Al: Zr: F mole ratios are adjusted to (0.15 to 2.0) : l : (5 to 16).

5. A process according to anyone of claims 1 to 4, characterized in that the conversion layers are aftertreated with solutions in which the Al: Zr: F mole ratios have been adjusted to (0.15 to 0.67): l : (5 to 7).

6. A process according to anyone of claims 1 to 5, characterized in that the conversion layers are aftertreated with solutions which have been adjusted to a pH value of 2 to 5.

7. A process according to anyone of claims 1 to 6, characterized in that the conversion layers are aftertreated with solutions in which the pH value has been adjusted with cations of volatile bases.

8. A process according to anyone of claims 1 to 7, characterized in that the conversion layers are aftertreated with solutions which additionally contain at least one of the anions benzoate, caprylate, ethyl hexoate, salicylate in a total concentration of 0.05 to 0.5 g/l.

9. A process according to anyone of claims 1 to 8, characterized in that the conversion layers are aftertreated for 1 to 120 seconds.

10. A process according to anyone of claims 1 to 9, characterized in that the conversion layers are aftertreated with solutions which are at temperatures of 20 to 80°C.

11. A process according to anyone of claims 1 to 10, characterized in that the surface is dried when it has been aftertreated.

12 12. A process according to claim 7, charac-terized in that said bases are choosen from ammoniums, ethanolammonium, di- or triethanolammonium.

13. A process according to claim 9, charac-terized in that the conversion layers are aftertreated for 1 to 30 seconds.

14. A process according to claim 10, charac-terized in that said solutions are at temperatures of 20 to 50°C.

15. The use of the process according to anyone of claims 1 to 14, as a pretreatment before a painting, film-laminating or adhesive coating operation.