TECHNICAL FIELD

The present invention relates to improvements in or relating to the surface treatment of magnesium and magnesium alloys.

The present invention may be particularly suitable for use in applications such as the colouring of magnesium and magnesium alloy containing articles. However, because the invention may have many uses and/or applications it is to be understood and appreciated that the invention is not to be limited to such use. The prior art and possible applications of the invention as discussed below are therefore given by way of example only.

BACKGROUND ART

Until recent times, magnesium anodising was a relatively unimportant method of treating magnesium articles. Several new processes are of commercial significance. These disclose magnesium anodising processes for magnesium that do not require heavy metals or other undesirable substances, and that produce smooth, hard coatings possessing relatively high corrosion resistance.

The advantages of these new anodising techniques means that they are now widely used. The properties that they impart to magnesium articles such as their resistance to corrosion has meant that these magnesium articles are becoming widely distributed in highly visible situations.

These new uses have lead to an interest in colouring the anodised articles to provide a suitable aesthetic appearance. There are processes available for colouring anodised magnesium articles. However, these processes utilise fabric dyes and/or other dyes. These processes do not have satisfactory results in that the coverage is inadequate, or patchy, and the colours are often not true, or non colourfast.

This is because the dyes used do not have properties that allow them to stick to the anodised articles adequately, irrespective of which ever anodising process was used. Further, at present each anodising process requires a different or colouring method.

It would be advantageous to have a colouring process that provided a desirable aesthetic appearance and possessed beneficial qualities such as being colourfast and resistant to weathering.

It would also be beneficial to have a colouring process which could be used regardless of the anodising process used for the magnesium article.

Additionally, it may be desirable to treat the surface of an article so that it displays surface characteristics that are not visible by the human eye. It may be that the characteristic imparted to the article surface is displayed if not in the human visible spectrum.

All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process.

It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

DISCLOSURE OF INVENTION

According to another aspect of the present invention there is provided a method of surface treatment of an anodised magnesium article including the steps of;

a) conditioning the article, and

b) rinsing the conditioned article, and

c) treating the surface, and

d) rinsing the surface treated article with water.

In the present specification the term anodised magnesium article should be understood to mean a magnesium or magnesium alloy containing article that has undergone a surface anodising treatment procedure. In preferred embodiments this magnesium article will have been treated using the anodising process known as Anomag2. However, this should not be seen as a limitation on the anodising process to be used. Other embodiments include the anodising process disclosed in United States Patent 6,286,598 or any other anodising process applied to a magnesium article.

In the present specification the term conditioning should be understood to refer to creating a strongly alkaline environment on the anodised article. The inventor has found that the way to do this is by only the application of a metal hydroxide to the surface of the article.

The application of a metal hydroxide to the anodised magnesium article prepares the article for the subsequent surface treatment. The anodising process results in the production of a thin film coating the magnesium article. The metal hydroxide alters the nature of this film so that it can receive and bond with the substance applied in the subsequent surface treatment step.

A metal hydroxide is used for the conditioning step because it has been found by the inventor to be the only class of substance energetically capable of preparing the film adequately to allow the surface treatment step to occur successfully.

The changes are similar to those when a dye forms a "lake" with aluminium hydroxide. It is the conversion of some of the outer layers of magnesium phosphate to a special form of magnesium hydroxide. These changes are well described in the literature.

There are three aspects to the conditioning step, these are: the metal hydroxide applied to the article, the temperature of the metal hydroxide applied to the article, and the duration of application of the metal hydroxide to the article. In preferred embodiments of the present invention the metal hydroxide is sodium hydroxide.

However this should not be seen as a limitation on the metal hydroxide to be used. Other embodiments envisioned for the metal hydroxide include an alkaline metal hydroxide, an alkaline earth metal hydroxide, or combinations thereof.

In preferred embodiments of the present invention the treatment period of the article will be 20-30 seconds. However, the treatment period can be a time depending on the subsequent surface treatment steps, preferably in the order of seconds and depending inversely on the strength of the metal hydroxide and the temperature.

In the preferred embodiment of the current invention the conditioning step will be undertaken at a temperature of 7O0C. However, the temperature of the conditioning step can be varied depending on the subsequent surface treatment step.

Temperatures envisaged in the current invention include a temperature range of 00C to 1500C. The actual range used depends on the strength of the metal hydroxide and the treatment period.

In preferred embodiments of the current invention the step of rinsing the conditioned article will be done with both water and acid.

Water is used because it is the only solvent capable of dissolving substances produced by treating the anodised article with a metal hydroxide.

The preferred embodiments for the conditioning step of the current invention have been chosen to provide an efficient method of treating a surface article. They allow a trade off between treatment time and efficiency. If a strong metal hydroxide is chosen then the treatment period only needs to be a short period of time. However, if a weak metal hydroxide is chosen then a longer treatment period is required. This leads to decreased efficiency in implementing the invention.

Additionally the temperature of the conditioning step allows further control of the conditioning step.

If a hot temperature is used with a strong metal hydroxide then an unworkable, short treatment period is necessary. To overcome this problem the treatment period, the strength of metal hydroxide used and temperature are varied to maximise efficiency and allow practical implementation of the invention.

In preferred embodiments of the current invention rinsing the article with water will occur in one or more separate rinse tanks.

Separate rinse tanks allow the article to go through a rinsing process that ensures removal of all the byproducts of the conditioning step.

The use of one or more separate rinse tanks provides a quality control process. It ensures the conditioned article is gradually rinsed by water containing less by-products of the conditioning process, and improves the efficiency of the rinsing process.

However this should not be seen as a limitation on the method of rinsing the article with water. Other embodiments envisaged include a spray rinse system or a static rinse bath system.

In preferred embodiments of the current invention the step of rinsing the article with water will take place at a temperature in the range of 250C to 350C. However, this should not be seen as a limitation on the temperature that the step of rinsing the conditioned article occurs at.

In preferred embodiments of the current invention the acid is acetic acid.

However this should not be seen as a limitation on the acid envisaged for use in this invention.

Other embodiments envisaged include an organic acid, ethanoic acid, or any other weak acid in conjunction with its alkali metal salt eg phosphoric acid with sodium di- hydrogen phosphate.

In preferred embodiments of the current invention the pH of the acid is 3.5. However, this should not be seen as a limitation on the pH envisaged in the current invention. Any pH less than 7 is envisaged.

In the preferred embodiment of the current invention the concentration of acetic acid is 0.1 %. However, this should not be seen as a limitation on the concentration of the acid envisaged in the current invention. Other concentrations envisaged include 0.01 % to 100% volume/volume of acid.

The step of rinsing the conditioned article with acid has the advantage of ensuring that all of the metal hydroxide is washed from the conditioned article. It ensures that the pH of the conditioned article is below 7 and is critical to ensuring that the subsequent surface treatment step occurs successfully. This will allow the subsequent surface treatment step to occur effectively.

In the current specification the term surface treatment should be understood to mean the treatment of the anodised, conditioned, and rinsed article with a suitable dye. There are four aspects to the surface treatment step, these are: The dye used to treat the conditioned magnesium article, the buffer solution to be used in the surface treatment step, the pH that the buffer solution maintains, and the duration of the surface treatment step.

Preferred embodiments for the dye include Sanadal black OA dye (an azo dye) and sanadal blue (also an azo dye). However, this should not be seen as a limitation on the dye envisaged for use with the current invention.

Other embodiments envisaged for the dye are an anthraquinone, pthalocyanin, and methine dyes, and combinations thereof.

These dyes are specifically designed for colouring anodised aluminium articles. Their use in colouring magnesium articles is desirable because they are commonly available, colour fast and possess strong wear characteristics. The current method has the advantage that it allows the use of these dyes to treat anodised magnesium articles.

Additionally, other embodiments envisaged for the surface treatment step include a dye that imparts characteristics to the surface which would not normally be in the visible spectrum for a human eye or other commercially available dyes.

The addition of a clear organic surface coat to improve corrosion resistance or the same to alter the smoothness of the surface. This can be achieved by dipping or spraying the organic surface coat onto the conditioned article. These additional coatings are usually transparent and thin.

In preferred embodiments of the current invention the concentration of the dye is 5 grams per litre (weight/volume). However, this should not be seen as a limitation on the concentration of dye envisaged. Other embodiments envisaged in the current invention include a concentration of dye in the range of 0 to 100 grams per litre.

In the current specification the term buffering solution should be understood to mean a solution whose pH resists change when amounts of acid or base are added.

In preferred embodiments of the current invention the buffer solution is a mixture of acetic acid and sodium acetate in a ratio of approximately 1:2.

However, this should not be seen as a limitation on the type of buffer solution envisaged. Other embodiments envisaged include a phosphate buffer, carbonate buffer, or fluoride ion buffer.

In preferred embodiments the pH of the buffer solution will be 5.4. However, this should not be seen as a limitation on the pH of the buffer solution which preferably has a pH of 7 or less.

In preferred embodiments of the current invention the duration of the surface treatment step will be 5 minutes.

However, this should not be seen as a limitation on the duration of the surface treatment step envisaged.

It has been found by the inventor that by altering the dye concentration, the treatment duration, and using mixtures of dyes in varying proportions the colour saturation and the dominant hue can be varied over a very wide range with no other alteration to the process.

I have no retained specific information to construct a table. All the most successful samples were processed according to the conditions shown for the examples shown below. Usually NaOH 20-30% w/v at 60-800C rinsed and followed by washing in a buffered solution of either acetic or phosphoric acid at a pH of 2.5-5.0 to suit the best performance range for the chosen dye bath. The wash time could vary from 5 min to 30 min.

BEST MODES FOR CARRYING OUT THE INVENTION:

The following is a description of the best method of performing the invention. Three specific examples are given and relate to different conditions for the conditioning step and surface treatment step to allow efficient colouring by different dyes.

1. An article anodised by the process disclosed in US Patent #6,280,598. This was rinsed then introduced to a solution containing 25% NaOH w/v, at 7O0C for 30 seconds. Following this treatment, the article was rinsed twice in water, the second time using a spray rinse, then once more in a solution containing 0.1% CH3COOH, at room temperature. The pH of the acetic acid solution was 3.5. The article was then introduced to a solution containing Sanodal® Black OA dye (an azo dye), in a concentration of 5 g/l (w/v). This had been buffered to a pH of 5.4 using a mixture of acetic acid and sodium acetate, in an approximate ratio of 1 :2. The concentration of NaOOCCH3 was approximately 2 g/l in this solution. After five minutes, the article was removed, and rinsed. It had a uniform deep black colour.

2. An article anodised as described above, was treated in a 10% solution of NaOH at 6O0C for 20 seconds, then coloured in the same dye using the same rinse techniques. The object emerged with a uniform grey tone.

3. An article was anodised in an anodising process described in New Zealand Provisional Patent Number 519071 and coloured as described in example number 1 supra, but the dye was Sanodal® Blue. The result was a pleasing, uniform shade of blue. Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope of the appended claims.