GB2084614A - Processes and compositions for the treatment of aluminium surfaces - Google Patents

Abstract

A coating on which paint may be applied without intervening chemical treatment can be formed on aluminium by contacting the surface with an aqueous alkaline (pH 9-12.5) solution containing a dissolved complexing agent, such as NTA or EDTA, or other compound that has a nitrogen atom with a lone pair of electrons or a carboxylate group or both. The solution may further contain an oxidising agent such as a peracid salt or peroxide, and a betaine surfactant, but must not contain any fluoride, chromate or inhibiting agents (eg alkali metal phosphate or silicates).

Description

SPECIFICATION Process and compositions for the treatment of aluminium surfaces This invention relates to the treatment of aluminium surfaces, by which we mean surfaces consisting of aluminium or of alloys formed mainly of aluminium and which thus can be treated in a similar manner to aluminium.

It is well known to clean aluminium surfaces by contact with aqueous alkaline cleaning solutions. If the solution is free of ingredients that will inhibit alkaline attack on aluminium the pH of the cleaners is generally from 9 to 10, while if the solution ingredients that will inhibit alkaline attack of aluminium the pH may be 12 or 13. Examples of materials that will inhibit alkaline attack on aluminium are sodium phosphate and sodium silicate. It is known to include complexing agents or similar materials in alkaline cleaners.

It is often necessary to form a coating on a clean aluminium surface, for instance that has been cleaned by treatment with one of these cleaners, and traditional coating compositions for aluminium include solutions containing hexavalent chromium and fluoride, usually with additions of accelerators such as ferricyanides or molybdates. Although these solutions form coatings which provide a good base for paint they do suffer from severe environmental problems in that both fluoride and hexavalent chromium are undesirable on health and safety grounds. For instance effluent containing hexavalent chromium must be treated before discharge. The solutions are acidic and so the apparatus by which they are applied must be resistant both to acid and fluoride.

Alkaline processes for forming coatings are known but they involve the application of hot solutions containing hexavalent chromium and so still present environmental problems.

More recently processes for forming coatings without hexavalent chromium have been developed.

These therefore avoid the toxic hazards of hexavalent chromium but they are generally still acidic and still contain fluorides and so still create environmental problems and the need for apparatus which is resistant to acid and fluoride. Such solutions contain, for instance, tannin, titanium or zirconium, and flouride.

It has been our object to devise a processforform- ing a coating on aluminium that is a satisfactory base for paint and which does not involve the disadvantages of the use of hexavalent chromium, fluoride, or acidic solutions.

We have found that a coating on which paint may be applied without intervening chemical treatment can be formed on an aluminium surface by contacting the surface with an aqueous solution that is free of fluoride, chromate or inhibiting ingredients and that has a pH of between 9 and 12.5 and that contains a dissolved compound that has a nitrogen atom with a lone pair of electrons or a carboxylate group of both.

The invention includes the process of forming the coating, solutions by which the coating can be formed, and a process in which the coating is formed, is optionally rinsed and dried, and a paint coating is then formed on it, without any intervening chromate or other coating step. Most aqueous alkaline cleaning solutions simply clean the aluminium, but we have observed in some experiments that a slight coloured film is formed when some particular solutions are applied under unusual conditions, for instance when they contain a particulartype of complexing agent and are applied much hotter than usual. Initially we thought this film to be undesirable or, at best, unimportant.Thus we found that this film did not interfere with the subsequent conventional coating formation steps, such as treatment with hexavalent chromium and fluoride and so presumably the coloured film formed by the cleaner was destroyed during the formation of the coating that was to serve as the base for paint. We subsequently discovered that this film could be increased and improved so as to form a thicker coating and that we could rely upon this coating alone as the base for paint. As a result of the invention it is at least possible to form on aluminium a coating that will serve as a good base for paint without having to use acidic solutions, fluoride solutions or chromate solutions.

The coating that is formed is preferably thicker than any of the films we observed in our experimental work using cleaners even under unusual conditions. For instance the films had immeasurably low weights but in the invention the weight is generally at least 0.05 gums, often 0.1 to 1.0 glum2.

The solution must contain a dissolved compound that has at least one nitrogen atom with a lone pair of electrons or at least one carboxylate group. This may be in the form of a free acid or an acid salt. Such compounds include, for instance, ethylene diamine, ammonia, urea, acetic acid, citric acid, oxalic acid and their alkali metal or other water soluble derivatives. Preferably however the compound used in the invention is a metal complexing agent which contains both at least one nitrogen atom with a lone pair of electrons and at least one carboxylate group. Preferred complexing agents are amino carboxylic acids such as alanine, glycine, ethylenediamine tetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) as well as their alkali metal salts or other water soluble derivatives.

The amount of complexing agent or other dissolved compound is generally between 0.5 and 20 g/l.

When the compound is an amino carboxylic acid the amount, measured as the sodium salt, is preferably from 1 to 10 g/l. If the amounts are too low satisfactory coating formation does not occur and increasing the amount beyond the upper limits proposed does not give any advantage.

The pH of the solution must be between 9 and 12.5. If it is below 9 satisfactory coatings are not formed and if it is above 12.5 there is increased ten deny to etch the metal. Generally the pH is between 9.5 and 12, most preferably 10 to 11.5. The pH is achieved by including in the solution a sufficient amount of an appropriate alkaline compound. This compound must not be one that will result in inhibi tion of the attack ofthe solution on the aluminium and thus many of the alkaline compounds often used in cleaners, such as sodium phosphate or sodium silicate, cannot be used for the pH adjustment.

Whether or not a compound is an inhibiting ingre dient can be readily observed experimentally since its inclusion in the solution significantly reduces the effectiveness of the solution if it is an inhibiting ingredient. pH adjustment is preferably achieved using sodium hydroxide as the alkaline material but other alkaline materials that do not introduce inhibiting ions can be used, for instance sodium carbonate or ammonium hydroxide. Generally the alkaline material is one that introduces only ions that are present in any event in the tap water from which the solution may be prepared.

As mentioned, the solution must be free of inhibitors such as sodium phosphate or sodium silicate and another inhibitor that should be avoided is quaternary ammonium salt surfactant. Also the solution should of course be free of fluoride, chromium compounds or any of the other coating ingredients that are conventionally used when forming coatings on metal, for instance phosphates. Preferably indeed the solution consists solely of water, the complexing agent or other dissolved compound, alkali for pH adjustment and, optionally, non-inhibiting surfactant and, optionally, oxidising agent. The inclusion of surfactant is sometimes desirable and in particular may cause the colour of the coating to be deeper.

The preferred surfactants are betaines but noninhibiting non-ionic or anionic surfactants may also be used. Cationic surfactants are generally inhibit ing, and so cannot be used.

It is particularly preferred to include an oxidising agent since without an appropriate oxidising agent the coating formation is very slow and may require contact between the solution and the metal surface for more than, for instance, four hours, eight hours being a typical time. However with an appropriate oxidising agent good coating formation can be achieved much more quickly. The oxidising agent that is in the solution is preferably a per-salt oxidising agent and this may be introduced into the solution by adding sodium or hydrogen peroxide, a water soluble per-salt, such as sodium persulphate, or some other compound that will release peroxide in solution, for example perborate. Compounds such as persulphates are preferably introduced as alkali metal or ammonium compounds, for instance sodium, potassium or ammonium persulphate.

Other oxidising agents, such as nitrite, nitrate and chlorate, do not appear two be as effective as per-salt oxidising agents.

The amount of oxidising agent depends upon the particular material being used. If it is too low the processing time necessary to achieve a good coating will not be reduced adequately below the four hours or more necessary in the absence of oxidising agent.

If it is too high the solution may be rather unstable.

Generally the amount is such that adequate coatings are achieved in less than two hours. When the oxidising agent is introduced as a persulphate the amount should be from 1 to 50 gull, measured as sodium persulphate most preferably 3 to 20 gull. Simi paramounts of peroxide, measured as sodium peroxide, may be used. When the oxidising agent is hydrogen perioxide the amount is preferably similar measured as "100 volume" hydrogen peroxide.

The temperature of the solution during contact between the solution and the metal surface is generally between ambient and 50"C, preferably 25 to 45"C. Temperatures below ambient (e.g. below 20"C) result in slow coating formation and temperatures above 50"C lead to increased instability of the solution if an oxidising agent is present.

Contact between the solution and the surface can be achieved in any convenient manner, for instance by immersion or spray and is conducted for the duration necessary to achieve the desired coating. It is always less than 24 hours and is generally less than 2 hours. It normally has to be at least one minute and preferably is two to thirty minutes.

After the formation of a coating it is generally well rinsed with water and may then be dried. A paint coating may then be applied in conventional manner without the application of any prior chromate or other coating treatment. The paint may be a lacquer.

As an example, a solution was prepared by dissolving in tap water2 g/l trisodium NTA, 10 gil sodium persulphate and sufficient sodium hydroxide to bring the pH to 11. The solution was brought to a temperature of 35"C and a cleaned panel of S1C grade aluminium was immersed in the solution for 15 minutes. A brown coating was formed. The panel was then rinsed with water and dried and then painted with a stoving acrylic paint. The resultant paint coating was subjected to an acetic acid accelerated salt spray test according to ASTM B287 specification with cross scoring. After 500 hours the panel was removed and the score taped. No paint removal occurred, thus showing that the coating was a very satisfactory base for paint.

When the sodium persulphate is replaced by potassium persulphate a similarly coloured coating is obtained out when ammonium persulphate is used a multi-coloured coating is obtained which may be blue or purple and iridescent. The use of higher amounts of ingredients, e.g. 8 g/l trisodium NTA and 40 g/l sodium persulphate leads to a heavier coating and that is less coloured and is iridescent.

Claims (22)

1. A process in which a coating is formed on an aluminium surface by contacting the surface with an aqueous solution substantially free of inhibiting ingredients, fluoride or chromate and that has a pH of from 9 to 12.5 and that contains a dissolved compound that has a nitrogen atom with a lone pair of electrons or that has a carboxylate group or both.

2. A process according to claim 1 in which the coating has a coating weight of at least 0.05 glum2.

3. A process according to claim 1 or claim 2 in which the coating has a coating weight of 0.1 to 1.0 glm2.

4. A process according to any preceding claim in which the said dissolved compound is an amino carboxylic acid or a water soluble derivative thereof.

5. A process according to claim 4 in which the said dissolved compound is ethylenediamine tetracetic acid or nitrilotriacetic acid or an alkali metal salt thereof.

6. A process according to any preceding claim in which the solution contains a non-inhibiting nonionic or anionic surfactant.

7. A process according to claim 6 in which the surfactant is a betaine surfactant.

8. A process according to any preceding claim in which the sloution contains an oxidising agent.

9. A process according to claim 8 in which the oxidising agent is a persalt or peroxide.

10. A process according to claim 9 in which the oxidising agent is an alkali metal or ammonium persulphate or sodium peroxide or hydrogen peroxide.

11. A process according to claim 9 or claim 10 in which the solution contains 3 to 20 g/l oxidising agent, 0.5 to 20 g/l amino carboxylic acid or water soluble derivative and has a pH of 10 to 11.5.

12. A process according to any preceding claim in which the solution has a temperature from ambient to 50"C.

13. A process according to any preceding claim in which the coated surface is rinsed with water, dried and then coated with paint without application of a previous coating.

14. A process according to claim 1 substantially as herein described.

15. A solution suitable for use in a process according to any of claims 8 to 11 and which is an aqueous solution substantially free of inhibiting ingredients, flouride or chromate, that has a pH of 9 to 12.5, that contains an oxidising agent, and contains a dissolved compound that has a nitrogen atom with a lone pair of electrons or a carboxylate group or both.

16. A solution accordingtoclaim 15 in which the oxidising agent is a persalt or peroxide.

17. A solution according to claim 15 or claim 16 in which the oxidising agent is an alkali metal or ammonium persulphate or sodium peroxide or hydrogen peroxide.

18. A solution according to any of claims 15 to 17 in which the dissolved compound is an amino carboxylic acid or a water soluble derivative thereof.

19. Asolution accordingtoclaim 18 in which the dissolved compound is ethylenediamine tetraacetic acid or nitrilotriacetic acid or an alkali metal salt thereof.

20. A solution according to any of claims 15 to 19 containing 3 to 20 g/l oxidising agent and 0.5 to 20 g/l amino carboxylic acid and having a pH of from 10 to 11.5.

21. A solution according to any of claims 15 to 20 including a non-inhibiting non-ionic or anionic surfactant.

22. A solution according to claim 21 including a betaine surfactant.