Durable white inorganic finish for aluminium articles

Abstract

There is disclosed a method of forming a high luminosity inorganic coating on an aluminium or aluminium alloy article, wherein the article is immersed in an electrolyte and subjected to a plasma anodising process, wherein the coating has a luminosity L*≥80.0% and comprises at least 50 wt % gamma alumina. Also disclosed are inorganic coatings formed by the method, and aluminium or aluminium alloys coated by the method.

Claims

1. A white inorganic coating on an article made of aluminum alloyed with at least one element selected from the group of elements that have non-white oxides, the coating being formed by way of a plasma electrolytic oxidation process in an electrolyte, and wherein the coating has a surface lightness L*≥80, includes micropores of diameter >1 μm, has a porosity of at least 20%, and an anodic breakdown voltage of less than 300V, and comprises at least 50 wt % gamma alumina, with non-white oxides and other compounds of the at least one element formed during the plasma electrolytic oxidation process being retained within a lattice structure of the gamma alumina so as not to impair the surface lightness of the coating.

2. The coating as claimed in claim 1, wherein the at least one element comprises a metal.

3. The coating as claimed in claim 1, wherein the at least one element comprises a transition metal.

4. The coating as claimed in claim 1, wherein the at least one element comprises a metalloid and/or a non-metal.

5. The coating as claimed in claim 1, wherein the coating has a porosity of at least 25%.

6. The coating as claimed in claim 1, wherein the coating has a thickness of at least 10 μm.

7. The coating as claimed in claim 1, wherein the coating has a thickness of at least 15 μm.

8. The coating as claimed in claim 1, wherein the coating has a surface lightness L*>80.

9. An article made of aluminum alloyed with at least one element selected from the group of elements that have non-white oxides, coated with a white inorganic coating as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 is an X-ray diffraction spectrum for a first coating of an embodiment of the present disclosure; and

(3) FIG. 2 is a graph showing the elemental composition of contaminants in the coating of the FIG. 1 embodiment.

DETAILED DESCRIPTION

Example 1

(4) An article of aluminium 6082 alloy was placed in an electrolyte bath containing a solution of 2 wt % ammonium phosphate, 1 wt % acetic acid, 1 wt % potassium hydroxide and 1 wt % sodium tetrafluoroborate. Anodic voltage pulses were applied with a voltage of 480V, the discharge power was maintained below 15 W, and the process was continued until the coating reached 40 μm thickness. At the end of the process, the anodic breakdown voltage of the coating in the bath was measured to be 290V.

(5) X-ray diffraction was performed in Bragg-Brentano geometry, from 5-90° 2q, with 2 second, 0.02° steps in a 40 kV, 40 mA Phillips PW1830 Diffractometer. The resulting spectrum (FIG. 1) shows that the coating consists primarily of gamma phase alumina with a small amount of amorphous material. The coating had a white appearance and luminosity was measured on a Konica Minolta spectrometer using the CIE L*a*b* colour space to be 82.6%.

(6) The elemental composition of the finish was measured using electron dispersive spectroscopy (FIG. 2). The coating was found to comprise 95 wt % aluminium and oxygen with the remaining constituents being those found in the substrate alloy and in the approximate proportions of the substrate alloy. A typical spark anodised coating formed on 6082 would have a cream or brown appearance.

Example 2

(7) An article of 2219 alloy, containing nominally 6 wt % Cu, was placed in a bath containing a solution of 3 wt % trisodium phosphate, 1 wt % ammonium hydroxide, 1 wt % citric acid and 0.5 wt % sodium fluoride and anodic pulses of 350V were applied, alternated with cathodic pulses of 100V. Discharge power was maintained below 15 W by adjusting the duration of the pulses. The process was continued until the coating thickness reached 15 μm. At the end of the process, the anodic breakdown voltage of the coating in the electrolyte was measured to be 195V. The resulting coating was X-rayed and found to comprise over 90 wt % gamma alumina. Typical plasma anodising of such an alloy would produce a dark grey to black coating due to the high amount of copper in the alloy. However, in this case, luminosity (L*) was measured at 82.2%.

Example 3

(8) An article of 1050 alloy was placed in the same bath as Example 2 and bipolar pulses with 400V anodic voltage and 100V cathodic voltage were applied. Discharge power was maintained below 15 W by adjusting the duration of the pulses. The process was continued until the coating thickness reached 20 μm. At the end of the coating process, the anodic breakdown voltage of the coating was measured to be 192V. Luminosity (L*) was measured at 87.1%.

(9) Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(10) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

(11) The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.