WO2002081784A1

WO2002081784A1 - Method for anodising magnesium and magnesium alloy components or elements

Info

Publication number: WO2002081784A1
Application number: PCT/NZ2002/000052
Authority: WO
Inventors: Jiangnan Yuan; Matthew Neil Henderson
Original assignee: Industrial Research Limited
Priority date: 2001-04-03
Filing date: 2002-03-28
Publication date: 2002-10-17
Also published as: NZ510922A

Abstract

The invention comprises a method for anodising magnesium or magnesium alloy components or elements, comprising or including the steps of: (1) providing an aqueous electrolytic solution consisting of an alkali or alkali earth metal hydroxide, and aluminate salt and a phosphate which is not a monophosphate, where the electrolyte solution does not contain ammonia of chromate or fluoride species; (2) providing an inert cathode in the solution; (3) providing a magnesium or magnesium alloy anode in the solution; and (4) passing a current between the anode and cathode through the solution, to deposit a film on the magnesium or magnesium alloy anode. Further, products of the method of the invention are claimed.

Description

'METHOD FOR ANODISING MAGNESIUM AND MAGNESIUM ALLOY COMPONENTS OR ELEMENTS"

FIELD OF INVENTION

The present invention relates to a method for anodising magnesium and magnesium alloy components or elements.

BACKGROUND OF INVENTION In recent years, the world market has seen a tremendous surge in the demand for magnesium alloys in electronic, automobile, and aerospace industries due to their high strength/weight ratio, high structural stability, and high castability. Magnesium and magnesium alloys are the most chemically active engineering materials and surface treatments such as anodising, chemical conversion, and painting are required before their applications. Surface treatments, such as anodising and conversion processes, have been developed to protect magnesium alloys from corrosion. However, traditional anodising treatments do not normally confer sufficient corrosion resistance for the alloys to be used without further protective measures, such as painting. Furthermore, many of the better treatments involve solutions containing environmentally damaging chemicals, such as cl romates and fluorides, which found very limited special applications such as military and aerospace.

In a magnesium anodising process, the magnesium alloy parts or elements are immersed as the anode in an electrolyte containing the chemical species that largely determines the compositions and structures of the anodising coating. A voltage or current is applied between the anode and the cathode, which normally is made from stainless steel or other inert materials. The anodising film is formed on the magnesium alloy surface through a number of complicated reactions including anodic dissolution of magnesium alloy, formation of passive film or barrier film, dielectric breakdown of the film formed, microplasmic discharge, etc. Magnesium anodising is actually a process in which insoluble chemical compounds are formed. For magnesium the element, a limited number of insoluble or low solubility magnesium compounds exist, which include oxide, chromate, fluoride, silicate, aluminate, phosphate, etc. The environmental legislation around the world allows even less choice among these compounds for the targeted anodising coating compositions and structures. The commercially available Mg anodising processes, which are environmentally friendly, actually form one or the combination of these compounds together with other minor additives.

Recently several processes have been developed, which are claimed to be environmentally friendly. However, there are still some problems associated with these processes as compared with the well-established aluminium surface treatment, such as high cost, volatile chemicals involved which make the processes difficult to maintain, and give rise to dark tints that are not flexible for colouring.

The rapidly increasing demand of magnesium and magnesium alloys in the industries, particularly the electronic and automobile industries requires that the anodising treatment processes be environmentally friendly, low in cost, and able to provide improved surface properties, such as corrosion resistance, wear resistance, and surface decoration. The present invention will provide a magnesium process that is able to meet these requirements. This new process is entirely environmentally friendly and able to deposit a coating that is low in cost and flexible for colouring.

Previously work has been conducted in relation to Magnesium anodising, which involves, the use of anodising baths without ammonia, chromate or fluoride and using an alkali or alkaline earth metal hydroxide in combination with either a monophosphate or an aluminate salt (PCT/NZ02/ 00012). This study provides a thin dark shade coating which is not flexible for colouring. OBJECTION OF THE INVENTION

It is an objection of the invention to provide an improved method for anodising magnesium over the abovementioned method and/or to provide a useful alternative.

SUMMARY OF INVENTION

According to the first aspect of the invention there is provided a method for anodising magnesium or magnesium alloy components or elements, comprising or including the steps of: 1) providing an aqueous electrolytic solution consisting of an alkali or alkali earth metal hydroxide, and aluminate salt and a phosphate which is not a monophosphate, where the electrolyte solution does not contain ammonia of chromate or fluoride species. 2) providing an inert cathode in the solution, 3) providing a magnesium or magnesium alloy anode in the solution, and

4) passing a current between the anode and cathode through the solution, to deposit a film on the magnesium or magnesium alloy anode.

Preferably the electrolyte further includes boric acid.

Typically the alkali or alkali earth metal hydroxide will be sodium hydroxide or potassium hydroxide.

Preferably the phosphate is a polyphosphate.

Preferably the polyphosphate will be of formula Na(PO₃)n or K(PO₃)n where n=l,2,3... and typically will be a mixture of polyphosphates with n > 3. Alternatively tripolyphosphate can also be used.

Alternatively or additionally the phosphate may be a diphosphate. Preferably the concentration of the phosphate in the electrolyte is in the range 0.1 to 10 g/litre. More preferably the concentration of the phosphate is in the range 0.5 to 5 g/litre.

Preferably the concentration of the alkali hydroxide in the electrolyte is in the range 0.01 to 1 mol/litre. More preferably the concentration of the alkali hydroxide is in the range 0.02 to 0.5 mole/litre.

Preferably the concentration of aluminate salt or aluminium hydroxide in the electrolyte is in the range 0.01 to 1 mol/litre. More preferably the aluminium species is in the range of 0.02 to 0.5 mol/litre.

Preferably the concentration of boric acid in the electrolyte is in the range of 0.01 to 0.5 mol/litre. More preferably the boric acid is in the range of 0.02 to 0.2 mol/litre.

Preferably the electrolyte is stirred or agitated during anodising to maintain both temperature and electrolyte concentration at the anode surface, and aid in the removal of evolved oxygen from the anode surface.

Preferably the pH of the electrolyte is in the range 11 to 14. More preferably the pH of the electrolyte is in the range 12.0 to 13.5.

Preferably the temperature in the anodising bath is maintained in the range about 15 to about 30°C.

Preferably the current density in the anodising bath is in the range 50 to 1000 A/m²' Most preferably the current density in the anodising bath is in the range 100 to 400 A/m².

Preferably the voltage applied to the anodising bath is in the range about 150 to about 500V and more preferably in the range 200 to 390V. Preferably the voltage and current density are maintained sufficiently low together to achieve a "pore plugging" effect to form a less permeable and thus more corrosion resistant coating on the component or element.

It has been found that using the electrolyte system of the invention, films of good uniformity and thickness with good levels of film hardness and low pore size can be achieved. Typically the films are lightly coloured (the films are not white. More accurately described as silver-grey) and can thus be readily coloured by addition of dyes to the electrolyte. The process of the invention does not require the addition of ammonia to the electrolyte or the use of chromate or fluoride or other toxic or environmentally unfriendly species.

The invention also includes magnesium or magnesium alloy components or elements anodised by the method of the invention.

BRIEF DESCRIPTION OF DRAWING

The invention is further described with reference to the accompanying drawing by way of example only and without intending to be limiting which is a schematic diagram of an anodising bath used in the method of the invention.

DETAILED DESCRIPTION

Definition

Where in the specification the term "polyphosphate" is used, phosphates having more than two phosphate atoms are required. For example, in particular sodium polyphosphate is in the form (NaPO₃)_n .where n is approximately 25.

We have surprisingly found that the use of polyphosphate or diphosphate (rather than simple monophosphates) and also in combination with aluminates provides favourable results to the anodising of magnesium. The advantage of using a polyphosphate with the aluminate and alkali-hydroxide is that it produces thick coatings with better wear resistance compared to other studies. Coating thicknesses of 20 microns can be achieved. This is much thicker than a previous invention (PCT/NZ02/ 00012), which used simple phosphates in combination with alkali hydroxides and produced coating thicknesses of 3-4 microns.

Referring to the figure, bath 1 contains an electrolyte 2, anode 3 and cathode 4. The anode 3 and cathode 4 are connected to the positive and negative terminals respectively of power source 7 via connections 6 and 5. A magnesium or magnesium alloy component or element is placed in the bath as the anode 3. A PC computer 9 controls the current or voltage output of the electric power source 7. The area 8 of anode 3 surrounded by electrolyte 2 will be anodised when voltage is passed through the electrolyte.

Example 1

A sample of AZ91 magnesium die-casting alloy plate was immersed as the anode in an electrolyte containing 0.02 mol/litre sodium tripolyphosphate, 0.2 mol/litre sodium hydroxide, and 0.1 mol/litre aluminium hydroxide. Voltage was applied to the system and increased while keeping the current density at a value of approximately 200 A/m². Upon reaching a voltage of 200 V, the anodisation process was stopped. A thin light grey coat was formed on the sample.

Example 2

A sample of AZ91 magnesium die-casting alloy plate was immersed as the anode in an electrolyte containing 0.18 mol litre potassium hydroxide, 0.13 mol litre aluminium hydroxide and 2 g/litre sodium polyphosphate. Voltage was applied to the system and increased while keeping the current density at a value of approximately 85 A/m². Upon reaching a voltage of 433N, the anodisation process was stopped. A thin light grey coat of approximately 7.5 μm thickness was formed on the sample. Example 3

A sample of AZ31 magnesium wrought alloy plate was immersed as the anode in an electrolyte containing 2 g/litre sodium polyphosphate, 0.1 mol/litre sodium hydroxide, and 0.05 mol/litre aluminium hydroxide. A constant current of 150 A/m² was applied to the AZ31 magnesium anode. Anodisation was stopped upon the voltage reaching 350 V. A thin light grey coating was formed on the sample.

Example 4

A sample of AZ91 magnesium alloy die-casting plate was immersed as the anode in an electrolyte containing 2 g/litre sodium polyphosphate, 0.1 mol/litre sodium hydroxide, 0.05 mol/litre aluminium hydroxide, and 0.05 mol/litre boric acid. A constant current of 150 A/m2 was applied to the anode plate. Anodising was stopped upon the voltage reaching 380 V and a thin white grey coating was formed on the surface.

The foregoing describes the invention including a preferred form thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated with the scope hereof.

Claims

WHAT WE CLAIM IS:

1. A method for anodising magnesium or magnesium alloy components or elements comprising or including the steps of : i) providing an aqueous electrolytic solution consisting of an alkali or alkali earth metal hydroxide, and aluminate salt and a phosphate which is not a monophosphate, wherein the electrolytic solution does not contain ammonium or chromate of fluoride species, ii) providing an inert cathode in the solution, iii) providing a magnesium or magnesium alloy anode in the solution, and iv) passing a current between the anode and the cathode through the solution to deposit a film on the magnesium or magnesium alloy anode.

2. A method as claimed in claim 1 wherein the electrolytic solution also contains boric acid.

3. A method as claimed in claim 1 or 2 wherein the alkali or alkali earth metal hydroxide is sodium hydroxide or potassium hydroxide.

4. A method as claimed in claim 1, 2 or 3 wherein the phosphate is a polyphosphate.

5. A method as claimed in claim 4 wherein the polyphosphate is of general formula Na(PO₃)n or K(PO₃)n where n=l,2,3...

6. A method as claimed in claim 5 wherein the polyphosphate is a mixture of polyphosphates with n > 3.

7. A method as claimed in any one of claims 3 to 6 wherein the phosphate is, or includes additionally a diphosphate or tripolyphosphate.

8. A method as claimed in claim 7 wherein the concentration of the phosphate in the electrolyte is in the range 0.1 to 10 g/litre.

9. A method as claimed in claim 8 wherein the concentration of the phosphate is in the range 0.5 to 5 g/litre.

10. A method as claimed in claim 9 wherein the concentration of the alkali hydroxide in the electrolyte is in the range 0.01 to 1 mol/litre.

11. A method as claimed in claim 10 wherein the concentration of the alkali hydroxide is in the range 0.02 to 0.5 mole/litre.

12. A method as claimed in any one of the preceding claims wherein the concentration of aluminate salt or aluminium hydroxide in the electrolyte is in the range 0.01 to 1 mol/litre.

13. A method as claimed in 12 wherein the concentration of the aluminate salt is in the range of 0.02 to 0.5 mol/litre.

14. A method as claimed in 13 wherein the concentration of boric acid in the electrolyte is in the range of 0.01 to 0.5 mol/litre.

15. A method as claimed in claim 14 wherein the concentration of the boric acid is in the range of 0.02 to 0.2 mol/litre.

16. A method as claimed in any one of the preceding claims wherein the electrolyte is stirred or agitated during anodising to maintain both temperature and electrolyte concentration at the anode surface, and aid in the removal of evolved oxygen from the anode surface.

17. A method as claimed in any one of the preceding claims wherein the pH of the electrolyte is in the range 11 to 14.

18. A method as claimed in claim 17 wherein the pH of the electrolyte is in the range 12.5 to 13.5.

19. A method as claimed in any one of the preceding claims wherein the temperature in the anodising bath is maintained in the range about 15 to about 30°C.

20. A method as claimed in any one of the preceding claims wherein the current density in the anodising bath is in the range 50 to 1000 A/m²'

21. A method as claimed in claim 20 wherein the current density in the anodising bath is in the range 100 to 400 A/m².

22. A method as claimed in any one of the preceding claims wherein the voltage applied to the anodising bath is in the range about 150 to about 450V.

23. A method as claimed in claim 22 wherein the voltage applied to the anodising bath is in the range 200 to 380V.

24. A method as claimed in claim any one of the preceding claims wherein the voltage and current density are maintained sufficiently low together to achieve a "pore plugging" effect to form a less permeable and thus more corrosion resistant coating on the component or element.

25. Magnesium or magnesium alloy components or elements anodised substantially according to the method of any one of the preceding claims.