GB2148942A

GB2148942A - Process for treating aluminium surfaces

Info

Publication number: GB2148942A
Application number: GB08427550A
Authority: GB
Inventors: Shinichi Tsubaki; Yokichi Satoh; Harumi Terada
Original assignee: Nihon Parkerizing Co Ltd
Current assignee: Nihon Parkerizing Co Ltd
Priority date: 1983-10-31
Filing date: 1984-10-31
Publication date: 1985-06-05
Also published as: JPS6210304B2; GB2148942B; JPS6096773A; GB8427550D0

Abstract

Solutions for cleaning aluminium-magnesium alloy or other aluminium surfaces prior to conversion coating contain fluoride ions and surfactant and are alkaline.

Description

SPECIFICATION Process for treating aluminium surfaces This invention relates to processes for cleaning aluminium surfaces and to solutions for effecting such cleaning processes.

Aluminium for use in manufacturing containers for use in construction works is generally treated so as to increase its corrosion resistance and often to provide a decorative or protective surface coating, for example, of a resin. The treatment of aluminium generally comprises cold rolling the metal to form a strip, heating the strip, washing the surface with an alkali or a solvent, optionally rinsing with water, optionally etching with alkali and washing with water, providing a chemical conversion coating and then providing a top coat of, for instance, a paint or a resin.

Until recently, the cold rolling of the aluminium was followed by a box annealing step, in which the aluminium was heated in a type of batch process. The box annealing step has recently been replaced by a continuous heat treatment process, which is more economical to run. The method, however, suffers from the disadvantage that it encourages the formation of deposits of metal oxides on the surface of the aluminium. This is a particular problem in aluminium alloys, such as magnesium aluminium alloys, since magnesium oxide is impossible to remove satisfactorily by the conventional cleaning processes. If the oxide is not removed from the surface, the subsequent conversion coating does not appear to adhere to the aluminium surface satisfactorily.

According to the invention, an aqueous alkaline solution for cleaning metal surfaces comprises fluoride ions and a surface active agent.

The solution according to the invention is useful for treating aluminium surfaces and the invention accordingly includes a process for treating aluminium surfaces comprising the steps of continuously heat-treating the aluminium, contacting the surface of the aluminium with a aqueous alkaline solution containing fluoride ions and providing the resulting surface with a conversion coating. Generally the aluminium surface is the surface of an aluminium alloy, usually an aluminium-magnesium alloy.

The aqueous cleaning solution generally has a pH of between 8 and 14, a fluoride ion concentration of 0.01 to 0.5 g/l and a concentration of surface active agents in the range of from 0.5 to 3.0 g/l. The cleaning treatment is generally carried out at a temperature of between 40 and 80"C and is generally by dipping or spraying.

When the concentration of the fluoride ions in the cleaning solution is below 0.01 g/l the solution may have poor cleaning effect. Above a concentration of 0.5 g/l the effect of the solution does not improve. The fluoride ions may be provided by dissolving in solution for example NaF, NH4F, KF, NaHF2, KHF2 or NH4HF2.

Alkalis used to maintain the pH at a value of at least 8 may be one or more compounds selected from alkali metal hydroxides, sulphates, hydrogen carbonates, carbonates, phosphoric acid salts, phosphorous acid salts and condensed phosphates. Sufficient alkali should be added to maintain the pH above 8, since below that value the solutions are ineffective as cleaners.

Surface active agents should be present in the cleaning solution at concentration of at least 0.5 g/l, and generally less than 3.0 g/l, since increasing concentration above this value does not improve the properties and values below 0.5 g/l may give poor results. Many surface active agents may be used in the solutions, but preferred surface active agents are non-ionic.

The cleaning solutions of the invention may contain other conventional additives, such as antifoaming agents.

The cleaning steps should generally be carried out at a temperature of at least 40"C and generally below 80"C, since above that temperature, the process is not improved. Generally the temperature is below 70do.

Treatment with the cleaning solutions may be by dipping or by spraying. Contact of the solution with the aluminium surfaces is generally for a period of between one and twenty seconds, generally between one and ten seconds.

The invention is illustrated in the following examples.

EXAMPLES 1-3 A plate A5052 and a plate A5082 were cleaned with the cleaning solutions containing a nonionic surfactant Nonipole 85 (Sanyo Kasei) and under the conditions indicated in the accompanying Table 1, chemical conversion-treated with a phosphoric acid chromate agent (a registered trade mark, Bonderite K702) at 50"C for ten seconds, washed with water, spray treated with de-ionized water of quality of less than 3yS/cm for three seconds, and then dried in a hot air at 102"C. The chemical conversion treated plates were coated with an acrylic containing coating for coloured aluminium to provide a film of thickness of 1 5-1 7 microns, baked at 210"C for three minutes and then let stand for twenty four hours.After that, the plates were tested for their coatings by the salt spray test and the folding test. The salt spray test for corrosion resistance is carried out according to JIS Z-2371 and the results indicate the time take before cross-cut parts start to corrode when subjected to salt spray.

In the folding test for adhesion a plate of the same thickness as that of a sample plate is laid over the sample plate. The pair of plates are folded by 180 with the sample plate outside. The results indicate the ratio of the area of remaining film of the folded part to the original area of the part.

COMPARATIVE EXAMPLES 1C-3C Similar A5052 plates and A5082 plates to those used in Examples 1-3 were cleaned with comparative cleaning solutions 1 C-3C obtained by omitting the fluorine compound from each of cleaning solutions 1-3 under the same cleaning conditions as in Examples 1-3, and then chemical conversion treated and subsequently coated by the same methods and compounds as in Examples 1-3. The three resulting samples were samples of comparative examples 1 C-3C respectively.

The samples were subjected to the same tests as the samples from Examples 1-3 Table 1 shows the ingredients and process conditions for the cleaning step of Examples 1-3 and 1 C-3C and also the results of the test for corrosion resistance and adhesion for the resultant samples. The amount of film in Table 1 is a measure of the weight of the chromate conversion coating on the plate. It can be seen that Examples 1-3 show greatly improved results over those of Examples 1 C-3C in both tests.

XE 1 1 2 3 lC 2C 3C NaHCO3 g/l - 1.0 - - 1.0 Na2CO3 g/l 3.0 - - 3.0 - NaCH - - 10.0 - - 10.0 NaHPO4 g/l - 10.0 - - 10.0 Na2HPO3 g/l 3.5 - - 3.5 5 Na5P30l0 g/l - 0.5 - - 0.5 Na2EDTAg/l 0.5 - - 0.5 Nagluconate - - 3.0 - - 3-0 NaF g/l 1.0 - - - - KF g/l - 0.1 - HF g/l - .- 0.2 - - Surfactant g/l 2.0 0.7 1.0 2.0 0.7 1.0 pH 9.0 8.6 13.0 Temp. OC 65 65 70 65 65 70 Duration of spray(s) 10 10 5 10 10 5 A5052 Amount of film 2) 20 20 20 20 20 20 (irglm Salt spray hr. 2000 2000 2000 1200 1200 1200 Folding % 90 100 100 20 20 20 A5082 Amount of film (mg/m2) 20 20 20 20 20 20 Salt spray 2000 2000 2000 1200 1200 1200 Folding % 90 90 100 20 20 20 EXAMPLES 4-5 A5052 plates and A5082 plates were cleaned with cleaning solutions containing a non-ionic surfactant (Sedran FF-110, Sanyo Kasei) under the cleaning conditions indicated in Table 2 and then chemical conversion treated by spraying a non-chromate chemical conversion agent containing zirconium for aluminium (registered trade mark Bonderite 3756) at 50"C for ten seconds, washed with water, treated with de-ionized water of quality of less than 3/1S/cm for three seconds and then dried in a hot air at 120"C These chemical conversion treated plates were coated with alkyd melamine containing coating to provide a film of thickness of 19-21 microns, baked at 120"C for forty minutes and then let stand for twenty four hours. After that, these resin-coated plates were tested for their coatings by the salt spray test (as Examples 1-3) and the "checkers" test.

The "checkers" test for adhesion is carried out according to the method described in "Testing methods for coatings" by Japan Coatings Industry Society. The result shows the percentage of 1 mm2 checkers remaining unpeeled at the end of the test.

COMPARATIVE EXAMPLES 4C-5C Similar A5052 plates and A5082 plates as those used in Examples 4-5 were cleaned with comparative cleaning solutions 4C-5C obtained by omitting the fluorine compounds from each of the cleaning solution 4-5 under the same cleaning conditions as in Examples 4-5, and then chemical conversion treated and subsequently coating treated by the same methods as in Examples 4-5. The two resulting resin coated samples 4C and 5C were tested by the salt spray test and checkers test as above for corrosion resistance and adhesion.

Table 2 shows the ingredients of the cleaning solutions used in Comparative Examples 4, 5, 4C and 5C and the results of the tests carried out on the products of those examples. The value of the film weight indicates the weight of the zirconium containing conversion coating on the plates. From the test results it can be seen that surfaces cleaned by the process of the present invention have far superior corrosion resistance and adhesive properties to those cleaned by the comparative conventional alkaline cleaning solutions.

TABLE 2 EXAMPLE 4 5 4C 5C Na2SO4 g/l 3.0 - 3.0 SNO3 g/l - 2.0 - 2.0 Na2CO3 g/l 10.0 - 10.0 K2HPO4 g/l 7.0 - 7.0 - NaOH g/l - 3.0 - 3.0 Na3N17 g/l 3.0 - 3-0 EDTANa2 g/l - 1.0 - 1.0 2 g/l 1.0 - - NaF g/l - 0.7 - Surfactant g/l 2.0 3.0 2.0 3.0 pH 8.8 11.7 Temp. C 60 60 60 60 Time spray s. 10 5 10 5 A5052 Film weight 2 mg/m2 15 15 15 15 Salt spray hr. 300 300 200 200 Checkers test % 100 100 50 50 A5082 Film weight 2 mg/m2 15 15 15 15 Salt spray hr. 300 300 200 200 Checkers test % 99 100 50 50

Claims

1. An aqueous alkaline solution for cleaning metal surfaces comprising fluoride ions and surface active agent.

2. An aqueous solution as in claim 1 comprising fluoride ions at a concentration in the range of from 0.01 to 0.5 g/l.

3. A solution as in either claim 1 or claim 2 having a pH in the range 8-14.

4. A solution as in any one of claims 1 to 3 in which the surface active agent is non-ionic.

5. A solution according to any one of claims 1 to 4 in which the concentration of the surface active agent is in the range of from 0.5 to 3.0 g/l.

6. A solution according to any one of claims 1 to 5 containing one or more compounds selected from alkali metal hydroxides, sulphates, hydrogen carbonates, carbonates, phosphoric acid salts, phosphorous acid salts and condensed phosphates.

7. A solution according to any one of claims 1 to 6 comprising an anti-foaming reagent.

8. A process for treating aluminium surfaces comprising the steps of contacting the surface of aluminium that has been continuously heat treated with a solution according to any one of claims 1 to 7 and conversion coating the treated surface.

9. A process according to claim 8 in which the aluminium surface comprises an aluminiummagnesium alloy.

10. A process according to either claim 8 or claim 9 in which the solution is at a temperature in the range of from 40 to 80"C.

11. A process according to claim 10 in which the temperature of the solution is less than 70"C.

1 2. A process according to any one of claims 8 to 11 in which the solution is sprayed onto the surface.

1 3. A process according to any one of claims 8 to 1 2 in which the solution is contacted with the surface for a period of from one to ten seconds.

14. A process according to any one of claims 8 to 1 3 in which the conversion coating is a chromate coating or a phosphate coating.

1 5. A process according to any one of claims 8 to 1 5 comprising the additional step of coating the conversion coated surface with a resin.

1 6. A process according to any one of claims 8 to 15 substantially as hereinbefore described.