GB2140033A - Sealing aluminum and aluminum alloys following anodization - Google Patents

Abstract

The anodized aluminum or alloy thereof is sealed in a bath at 15-30 DEG C containing an aqueous solution of nickel salts at a concentration of 0.1 to 50 g/liter and a non-ionic surfactant which lowers the surface tension of the cited bath to 30 to 35 dynes/cm range at a concentration of 25 to 200 ppm range. The non-ionic surfactant may be a fluorocarbon ester, a fluorinated ether or a polysiloxane.

Description

SPECIFICATION Method and bath for fixing aluminum and aluminum alloys following anodization This invention relates to fixing aluminum and aluminum alloys following anodization.

The most widely used industrial treatments of anodized aluminum directed to fixing tbe porous layer of anodic oxide by plugging the pores therein may be divided into two separate classes: (a) high temperature treatments, wherein fixing is effected either by immersion in deionized water at the boiling point, or by immersion in a hot solution of a nickel salt; (b) low temperature treatments, wherein the anodized aluminum is immersed in an aqueous solution containing organic solvents and metal salts, such as nickel fluoride.

The high temperature methods under (a) are highly disadvantageous in that they involve a high energy consumption for heating the bath and cannot provide, on completion of the treatment, surfaces which are free of the so-called "dust' '-the removal whereof requires aftertreatment of the workpieces with oils or paraffins. The low temperature methods under (b), as disclosed for example in the published Japanese patent application SHO-54-15856, while providing a reasonably good solution to the problems inherent in energy consumption for heating the bath, require the use of considerable amounts of organic solvents, some of which are potential sources of pollution and are sometimes difficult and hazardous to handle because they are inflammable or corrosive.

As an alternative, Italian patent application No. 41546 A/81 proposes the use of nickel salts dissolved in a simple aqueous solution, but experimental results have confirmed that such a fixing bath fails to provide satisfactory results in the treatment of anodized aluminum.

The present invention provides a fixing method and bath for low-temperature fixing of aluminum and aluminum alloys following anodization, which can improve the resistance of the surface layer of aluminum oxide resulting from anodic oxidation to attack by chemical substances and/or weathering.

The invention improves the quality of the fixing over that to be obtained with currently applied low-temperature fixing treatments, and also obviates the formation of the surface "dust" referred to above, thereby affording a decreased consumption of fixing agent and probable elimination or reduction of after-treatments with oils or paraffins.

The invention lowers the level of hazard of the fixing bath as compared with low-temperature processes using inflammable or polluting organic solvents.

According to the present invention, aluminum and alloys thereof which have been anodized are fixed by immersing the aluminum or alloy, for 5 to 30 minutes, in a fixing bath at 15-30"C containing an aqueous solution of at least one nickel salt in a concentration of 0.1 to 50 g/liter and a non-ionic surfactant at a concentration of 25 to 200 ppm effective to lower the surface tension of the fixing bath to 30 to 35 dynes/cm.

The aforesaid fixing bath itself is within the scope of the invention.

This invention is based upon the use, in the fixing of anodized aluminum, of an aqueous bath comprising a nickel salt and a particular surfactant capable of lowering the surface tension of the bath to or below the critical value of 35 dynes/cm.

The surfactants used in the present invention are, according to a first preferred embodiment, fluorocarbon esters of the general formula F-CH2-(CH2)n-COOR or fluorinated ethers of the general formula F~CH2~(CH2)n~(0~CH2)m~cH2 - cH2 - oH wherein n is an integer of from 1 to 17, m is an integer of from 5 to 8, and R is a straight or branched chain aliphatic radical with up to 4 carbon atoms.

In an alternative preferred embodiment, the surfactants used in the present invention are polysiloxanes of the general formula

wherein x and y ranges each of from 5 to 10.

As a silicone surfactant of the type set forth above, a dimethylpolysiloxane is preferred having a viscosity of from 300 to 350 centistokes.

Amongst the surfactants useful in the present invention, the preferred ones are the fluorocarbon esters such as the product available commercially as FC 430 Fluorad from 3M ITALIA or as AG 650 from Tessilchimica.

The surfactant is used in the fixing bath at a concentration from a minimum value of 25 ppm to a maximum value of 200 ppm. The preferred fluorocarbon surfactant types cited above already provide the fixing performance according to this invention at the very low concentrations of 25-50 ppm, at which the surface tension of the aqueous bath is considerably lowered.

Suitable nickel salts for use in this invention include, preferably, nickel fluoride, alone or admixed with one or more additional nickel salts, such as nickel acetate, formate, sulphate, and sulphamate. The overall concentration of the nickel salt(s) in the fixing bath is from 0.1 to 50 g/liter.

Advantageously, the fixing bath is adjusted to a pH of 5.5 to 6.5 by using appropriate proportions of the various component nickel salts and/or surfactants.

In the method of this invention the bath is operated at a temperature of 15-30"C for a duration of immersion which may vary from 5 to 30 minutes.

Optionally, the fixing process may be completed, as in conventional techniques, with "ageing" during the first few hours following withdrawal from the fixing bath, e.g. for 0 to 24 hours.

The following Examples illustrate the invention.

EXAMPLE 1 A UNI 3569 alloyed aluminum sample is used, which has been oxidized by anodization in a sulphuric acid solution at a concentration of 180 g/l, at 1 5 Volt, and at a temperature of 18"C, until an oxide thickness of approximately 1 5 microns has been formed.

Following rinsing, the sample is immersed for fixing in an aqueous solution having the following composition: Nickel fluoride trihydrate 6 g/l Nickel acetate 1.5 g/l Nickel formate 1.0 g/l Nickel sulphate heptahydrate 3.0 g/l Fluorocarbon ester-based surfactant (FC 430 Fluorad from 3M) 25 ppm The fixing bath is maintained at a temperature of 22"C, at pH = 6, for 15 minutes.

The fixing quality test is carried out in accordance with ISO 3210 standard by measuring the loss in weight after immersion of the anodized aluminum sample in a phospho-chromic solution for 1 5 minutes. The weight loss measured one hour after withdrawal from the solution was 18.1 mg per dm2 of fixed surface area.

Other samples prepared in a similar manner have given the following results, respectively: After 2 hours, loss = 13.4 mg/dm2 After 6 hours, loss = 9.2 mg/dm2 After 24 hours, loss = 6.8 mg/dm2 EXAMPLE 2 An aluminum alloy sample, also to UNI 3569 standard, was oxidized in a sulphuric acid solution at 180 g/l, at 1 5 Volts DC, and a temperature of 18"C until the oxide layer had a thickness of about 1 5 microns.

After rinsing, the sample was electrocolored in an aqueous solution of SnSO4 (15 g/l) with H2SO4 (18 g/l), at 13 Volts AC for 5 minutes, so as to obtain a dark bronze color.

After rinsing, the sample was immersed in a fixing solution with the following composition: Nickel fluoride trihydrate 6 g/l Nickel acetate 1.5 g/l Nickel formate 1.0 g/i Nickel sulphate heptahydrate 3.0 g/l Fluorocarbon ester based surfactant (AG 650 from Tessilchimica) 25 ppm Fixing is carried out at a temperture of 22"C for 15 minutes at pH = 6.

The fixing quality is again tested by assessing the loss of weight following immersion in a phosphochromic solution, in accordance with ISO 3210 standard.

The loss of weight after one hour was 19.9 mg per dm2.

Samples prepared similarly to the above have provided, by extending the test over longer periods of time, the following weight loss values: After 3 hours, loss = 16.37 mg/dm2 After 6 hours, loss = 9.4 mg/dm2 After 24 hours, loss = 8.22 mg/dm2 For comparison, the values given below were obtained with a conventional fixing bath comprising a nickel salt (which forms in situ) and an organic solvent but not the surfactant as used in this invention.

In this case also, the sample of the material to be treated was represented by a UNI 3569 aluminum alloy, which had been oxidized in the same conditions as described above to give an anodic oxide layer having a thickness of about 1 8 microns.

After rinsing, the sample was immersed in a solution having the following composition: Nickel acetate 6.5 g/l Sodium fluoride 2.0 g/l Isobutanol 50.0 g/l for 10 minutes at 35"C. The fixing quality was again tested according to ISO 3210 standard.

After two hours, a loss in weight equal to 34 mg per dm2 was measured.

The treatment described in Example 1 showed, in addition to a decreased loss of weight, two particular features. First, a reduction in the surface dust was observed which imparts to surfaces treated in a conventional fixing bath a milky white appearance. Secondly, it has been observed that the drastic reduction in surface tension, brought about by the presence of the surfactant in the fixing bath, enables a lower adsorption of nickel at the outer surface of the anodic oxide layer. An excessive adsorption is particularly harmful as it hinders complete filling of the pores in the anodic oxide layer. In particular, when using FC 430 Fluorad in water, the surface tension drop is about 20 dynes/cm at 25-50 ppm concentration.

The following experiment illustrates the aforementioned decreased nickel adsorption caused by the presence of the surfactant. Two aluminum alloy samples were treated, respectively, in a fixing bath like that described in Example 1 and a similar fixing bath containing no fluorocarbon surfactant.

Then the oxide present on the sample surfaces in consequence of the anodic oxidation treatment was dissolved in a phosphochromic mixture at 85"C.

The amounts of nickel in the two attacking solutions, one for each sample, were measured by atomic absorption, and found to be, in the attacking solution of the first sample, i.e. the one treated with a surfactant, 51 mg Ni+ +, which corresponds to 7.14 mg Ni+ +/dm2; and, in the attacking solution of the second sample, 66 mg Ni + +, corresponding to 7.84 mg/dm2 Ni+ + .

Both treated samples had a surface area of 200 cm2.

The increased adsorption of nickel by the surface of the sample fixed in a bath without surfactants is thus apparent.

The advantages provided by the application of the fixing method and bath of the invention will be also apparent. In particular, an increased resistance of the oxidized surface layer to attack by chemical substances and weathering is achieved which refiects an improved fixing quality over conventional products.

The utilization of anions of the fluoride (F-) type, which accelerate the absorption of nickel by the aluminum oxide layer, as well as the use of acetate and formate ions, which are effective to maintain both within the impregnating solution of the bath and within the pores present in the oxidized aluminum layer a pH which tends to become alkaline, enables the formation of salts of the A1203.NiO.nH2O type, or the like, to be accelerated which form the material which fills the said pores during the fixing process.

In particular, it is especially advantageous, from the point of view of the fixing quality obtained, to use bath residence times of 5 to 30 minutes, according to the thickness of the oxide layer formed in the anodization process.

As may be appreciated from the Examples given. the process reaches completion within the first few hours following withdrawal of the workpiece from the fixing bath (0 to 24 hours at the most, if a correct procedure has been followed), and successive changes resulting from ageing of the workpiece may be observed.

Claims (14)

1. A method for fixing aluminum or an alloy thereof following anodization, which comprises immersing the said aluminum or alloy for 5 to 30 minutes in a fixing bath at 15-30"C containing an aqueous solution of at least one nickel salt in a concentration of 0.1 to 50 g/l and a non-ionic surfactant in a concentration from 25 to 200 ppm effective to lower the surface tension of the fixing bath to 30 to 35 dynes/cm.

2. A method according to Claim 1, in which the fixing bath contains one or more of nickel fluoride, nickel acetate, nickel formate, nickel sulphate, and nickel sulphamate.

3. A method according to Claim 1, in which the fixing bath is maintained at a pH of 5.5 to 6.5.

4. A method according to Claim 1, in which the non-ionic surfactant is selected from a fluorocarbon ester of the general formula F-CH1-(CH,),-COOR a fluorinated ether of the general formula F~CH2~(CH2)n~(0~CH2~CH2)m~CH2~CH2~0H or a polysiloxane of the general formula

wherein n is an integer ranging of from 1 to 17, R is a straight or branched chain aliphatic radical with up to 4 carbon atoms, m is an integer ranging of from 5 to 8, and x and y are integers ranging of from 5 to 10.

5. A method according to Claim 4, in which the said surfactant is FC 430 Fluorad, AG 650, or a dimethylpolysiloxane.

6. A method according to any of the preceding claims, in which the aluminum or alloy thereof is aged after immersion in the said bath.

7. A bath for fixing aluminum or an alloy thereof following anodization, comprising an aqueous solution of at least one nickel salt in a concentration of 0.1 to 50 g/l and a non-ionic surfactant in a concentration of 25 to 200 ppm effective to lower the surface tension of the said fixing bath to 30 to 35 dynes/cm.

8. A fixing bath according to Claim 7, in which the nickel salt is one or more of nickel fluoride, nickel acetate, nickel formate, nickel sulphate, and nickel sulphamate.

9. A fixing bath according to Claim 7 or 8, in which the non-ionic surfactant is a fluorocarbon ester or a silicone compound.

10. A fixing bath according to Claim 9, in which the surfactant is FC 430 Fluorad, AG 650, or a dimethylpolysiloxane.

11. A fixing bath according to any of Claims 7 to 10, having a pH of 5.5 to 6.5.

12. A method according to Claim 1 substantially as defined in the foregoing Examples.

1 3. A fixing bath according to Claim 7 substantially as hereinbefore described.

14. Anodized aluminum and alloys thereof fixed by the method of any of Claims 1 to 6 or 12.