GB2530805A

GB2530805A - A method of colouring titanium or titanium alloy

Info

Publication number: GB2530805A
Application number: GB1417528.5A
Authority: GB
Inventors: Gregor Filipic; Kristina Elersic; Darij Kreuh; Janez Kovac; Uros Cvelbar; Miran Mozetic
Original assignee: Institut Jozef Stefan
Current assignee: Institut Jozef Stefan
Priority date: 2014-10-03
Filing date: 2014-10-03
Publication date: 2016-04-06
Anticipated expiration: 2034-10-03
Also published as: SI24851A; GB2530805B; GB201417528D0

Abstract

A method is described whereby titanium or titanium alloys are colored in different colours .A processing chamber, such as a quartz tube 1, may be used which is sealed with two gate valves 2,3. The work pieces are arranged in the processing chamber on a sample holder 4. A vacuum is then applied to the chamber 1. Oxygen is then passed into the processing chamber and a magnetic field applied. The method utilises an oscillating magnetic field in a low pressure oxygen-containing atmosphere to initiate titanium oxidation. The oxide layer so produced is thick enough to obtain chosen colour through interferometric effects. Colours range from yellow with the thinnest oxide layer, through to purple, blue, gold, violet, green to grey. No deposition of coatings or anodic oxidation occurs. This can be used for various applications, including: decorative purposes, labelling, protecting the surfaces made of titanium or titanium alloys, and increasing the biocompatibility.

Description

Title: A method of colouring titanium or titanium alloy

FIELD OF THE INVENTION

S The present invention concerns a method of colouring products made from titanium or titanium alloys. Against this background the present invention does not involve deposition of different coatings nor anodic oxidation.

BACKGROUND OF THE INVENTION

Products made from titanium or titanium alloys are widely used in various applications.

One important application addressed in this document is in medicine. Currently many devices such as bone screws are made from titanium or titanium alloys since titanium or titanium alloys exhibit high durability and almost excellent biocompatibility. The biocompatibility is reflected as almost no interaction is seen between products made from titanium or titanium alloys and body tissues and body liquids. Furthermore there is a low probability of inflammatory reaction upon insertion of products made from titanium or titanium alloys into human bodies.

Different applications require bone screws of different size and shapes. In current medical praxis it is not always trivial to choose the optimal pair of bone screws and nuts for fixing particular bones. It would be useful if the bone screws and nuts were marked somehow in order to choose the right pair for particular application.

Marks could be made by mechanical modifications like needle inscribing but such mechanical deformations might change the biocompatibility of the products made from titanium or titanium alloys. An alternative to mechanical marking is dyeing the products made from titanium or titanium alloys with an appropriate colour Painting or deposition of a dye is not suitable since the deposits will change the original biocompatibility of products made from titanium or titanium alloys.

An alternative to painting is synthesis or deposition of an oxide film on the surface of products made from titanium or titanium alloys. It is known that oxide films on products made from titanium or titanium alloys will assume different colours depending on the thickness of the oxide film. Consequently, methods for oxidation of products made from titanium or titanium alloys have been invented. The most common one is anodic oxidation.

Anodic oxidation involves the formation of a thin oxide film on the surface of products S made from titanium or titanium alloys upon treatment in a suitable chemical bath with simultaneous application of anodic potential. The drawback of anodic oxidation is ecological considerations. The chemical baths currently applied contain corrosive chemicals which are often poisonous and thus represent a serious ecological risk.

An alternative to anodic oxidation is deposition of suitable oxide films onto the surface of products made from titanium or titanium alloys. Deposition is often performed using the technology of plasma assisted chemical vapour deposition (PACVD). The drawback of the PACVD is a rather poor adhesion and flakiness of the oxide film.

Products made from titanium or titanium alloys covered with an oxide film prepared by t5 PACVD are not suitable for medical applications since any peeling of the coating will result in post-surgical complications.

STATE OF THE ART

Coloured surfaces of the titanium or titanium alloy are achieved in one of two ways.

Either to deposit a dye material on the surface (classical painting), or using the interferometric effect by applying a thin layer of optically different material on top of the titanium or titanium alloy. This layer has to be of a certain thickness for a specific colour.

Deposition of foreign material (dye) is in many cases not acceptable due to medical reasons, biocompatibility, physical proprieties such as abrasion and corrosion resistance, etc. However, the titanium oxide was proven to improve all the previously mentioned properties of titanium and titanium alloys, and thus its use is wide spread.

There are a few methods that can achieve titanium oxide layer on the titanium or titanium alloys. Most common are electrochemical oxidation methods -anodisation.

They typically use an electrolytic solution into which titanium or titanium alloy is dipped together with another electrode; then a potential is applied between them. By using diluted phosphoric or sulphuric acid, sulphamic acid or ammonium sulphate or sodium bicarbonate solutions and appropriate potential the titanium oxide layer is formed on the titanium or titanium alloy and thus its colour changes (EP1199385). Using a different electrolytic solution similar control of the oxide layer thickness is described in (US3338865). Another anodisation procedure is described in (US3075896) where S control of the thickness of titanium oxide layer is achieved through selection of high potential (above 1000V) and time of treatment. The goal of the patent is to prevent the corrosion of the surface of titanium or titanium alloy, although it is possible that using longer times the thickness of the oxide layer would be sufficient for colour change.

Similarly, the goal of patents (EF0520721 and EP1930479) was passivation of the surface, and photo-catalytic crystalline surface, respectively, but procedures might be used for the colouring as well. In the latter, an additional step is introduced prior to anodisation. Titanium nitride is formed by heating the titanium or titanium alloy in nitrogen or ammonium atmosphere.

A double step anodisation is introduced in (EP0406620) where the anodisation occurs at constant current condition and the current is temporarily switched off. The current that is applied the second time determines the colour tone.

Electrochemical procedures can also be used to deposit titanium oxide on surfaces of conductors (including titanium and titanium alloys). In (US3864224) the solution comprising titanium ions was then precipitated on the electrodes in form of oxide layer.

Adding ions of different metals (US3346469) also colours and colour tones were achieved which are otherwise not possible to obtain.

One of alternative methods to anodisation concerns use of oxygen plasma for direct titanium oxidation. In (W02008091053) plasma at low pressure from oxygen or oxygen containing gas was used to oxidise titanium. Then it was annealed for short time at high temperature.

Using plasma for physical vapour deposition (PVD) titanium dioxide coating was achieved by sputtering from bulk titanium dioxide (EP 0871792, DE 10359508).

Plasma can also be used for titanium oxide deposition on the surface of materials. In (US 20100075510) the inventors used low pressure radio-frequency (RF) pulsed plasma derived from a titanium tetraisopropoxide gas and a gas containing oxygen. In the glow discharge deposition was done at a low substrate temperature.

To overcome the need for a high vacuum, near atmospheric condition plasma can be used. In plasma enhanced chemical vapour deposition (PECVD) a metal oxide S precursor and an oxidising agent are lead through corona discharge or dielectric barrier discharge onto the surface of the substrate where metal oxide is deposited at low temperatures (W02005113856). Improved versions of dielectric barrier discharge with premixed precursor and oxidising gases were utilised in (W02004013376).

Titanium tetrachloride and alkoxides of titania were leaked in a laminar fashion through the plasma at atmospheric pressure to form titanium oxide on the surface of the substrate.

A pure chemical vapour deposition (CVD) can be used to deposit titanium oxide to a surface of substrate from a mixture of an organic titanium compound and an oxygen t5 containing compound (W02004085701, EP1608793). The temperature of the substrate must be sufficiently high to form the titanium oxide coating (above 400°C).

Preferred compounds are titanium isopropoxide and ethyl acetate.

Titanium oxide coating can be achieved also through thermal spraying (EP0893513, W02003022741). A substrate is heated under atmospheric conditions and sprayed by vaporized titanium alkoxide together with an inert gas as a carrier The result is a film which comprises of crystalline titanium oxide nanoagglomerates.

These methods might be used also to achieve the preferred colour of titanium or titanium alloys but there are great limitations on biocompatibility. All the mentioned methods that use deposition might have some remaining unwanted chemicals in the oxide layer which is a cause for concern. Anodic oxidation, which is used currently, produces coloured surface of titanium or titanium alloys at no expense on biocompatibility. However, the chemicals used are typically very corrosive and also poisonous and great care must be invested in waste management. The procedures of dyeing the surface using the anodisation are also not very straightforward.

Direct plasma oxidation with thermal annealing might not only colour the surface of the titanium or titanium alloys, but also preserve the biocompatibility. However, the high vacuum demand and the additional annealing make this process expensive in time and funds and it prevents use in a production line.

BRIEF DESCRIPTION OF THE INVENTION

S According to the methods of the invention drawbacks of anodic oxidation and deposition of oxide films by PACVD are effectively avoided using an alternative method.

Hence, the present invention provides a method of colouring products made from titanium or titanium alloys by oxidation of said products made from titanium or titanium alloys in the presence of strong oscillating magnetic fields. The products made from titanium or titanium alloys are exposed to an oxygen-containing atmosphere and simultaneously subjected to a strong oscillating magnetic field. In a preferred embodiment the strong oscillating magnetic field is homogeneous thus resulting in a uniform colour of the entire product made from titanium or titanium alloys.

The method comprises arranging products made from titanium or titanium alloys in a processing chamber, evacuating the chamber to a low pressure, supplying oxygen into the chamber, establishing an oscillating magnetic field in the chamber, allowing the oscillating magnetic field created in the processing chamber in an oxygen atmosphere to interact with the products made from titanium or titanium alloys, turning off the said oscillating magnetic field, and venting said treatment chamber once the products made from titanium or titanium alloys have assumed a chosen colour.

A product made from titanium or titanium alloy for use in the present method preferably contains at least 20 weight per cent of titanium, preferably at least 40 weight per cent titanium, more preferably at least 50 weight per cent and most preferably at least 70 weight per cent titanium, optionally at least 85% titanium. The product made from titanium or titanium alloy may contain any other metal or semiconductor including aluminium, iron, vanadium, nickel, magnesium, gallium, molybdenum, tantalum, niobium, manganese, cobalt, copper, silicon, tin at any concentration.

A preferred product is made from a biocompatible alloy such as Ti-6A1 -4V.

According to methods of invention, products made from titanium or titanium alloys are treated in a processing chamber containing oxygen at a low pressure. A product made from titanium or titanium alloys is mounted into a processing chamber which is hermetically tight. The chamber is first evacuated by means of a vacuum pump. The evacuation is performed in order to minimize the influence of gases other than oxygen which are present in air. Suitably the processing chamber is evacuated to a pressure S below 100 Pa, preferably below 50 Pa, more preferably to 1 Pa or below.

As the processing chamber is evacuated below the pressure where the density of gaseous molecules is low enough not to disturb the oxidation process, oxygen is leaked into the said processing chamber during continuous pumping. Whilst the oxygen may be of any level of purity, preferred embodiments employ oxygen having a purity of at least 95% by volume and more preferably 99% or more by volume.

Simultaneous leakage of oxygen on one side and pumping of the processing chamber on the other side allow for a negligible concentration of gases other than oxygen in the processing chamber during treatment procedure. Once such conditions are met the t5 oscillating magnetic field is applied. At this stage the pressure in the chamber is preferably from 1 Pa to 1000 Pa, suitably from 10 Pa to 500 Pa, more preferably from Pa to 300 Pa and most preferably from 50 Pa to 200 Pa.

In a preferred embodiment the oscillating magnetic field is substantially homogeneous over the range of volumes where the products made from titanium or titanium alloys are placed. In this respect a magnetic field may be regarded as "substantially homogeneous" if the magnetic field strength does not deviate from a selected value by more than 10%, and preferably does not vary by more than 7%, more preferably by no more than 5% of the selected value, along the whole surface of the titanium product to be treated.

Such a homogeneous oscillating magnetic field may be established by various means.

In the preferred embodiment a coil is mounted relative to the processing chamber in such a way that the products made from titanium or titanium alloys remain within the coil during the entire period of treatment. The coil may be powered with an AC power supply.

Preferably the magnetic field is selected to have a magnitude above lxlft3Tesla, suitably above 2x1&3Tesla, and more preferably at or above 1x102Tesla. Conveniently the magnetic field has a magnitude below 1 Tesla and preferably no more than 0.5 Tesla. The oscillating magnetic field may be of sinusoidal form with one or more harmonics. Preferably the field oscillates at a basic frequency above 10 kHz, more preferably above 1 MHz and conveniently at 10 MHz or above.

S Oxygen does not interact with products made from titanium or titanium alloys in the absence of the oscillating magnetic field, but starts forming an oxide film on the surface of products made from titanium or titanium alloys, providing the frequency as well as the density of the magnetic field are favourable. Once the products made from titanium or titanium alloys assume a chosen colour, the oscillating magnetic field is turned off and the products made from titanium or titanium alloys are collected from the chamber. The products may optionally be left in the oxygen atmosphere in the chamber to cool for a period of time before the chamber is vented. In particular, if the products are to be removed from the chamber manually they may be left in the oxygen atmosphere to cool to a temperature at which they can be handled, such as 50°C or below.

Preferably the oscillating magnetic field is applied for a period from 105 to 5000s, more preferably from lOOs to bOOs.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of embodiments the invention will become apparent upon reading the following descriptions and accompanying drawings in which: Figure 1 shows a schematic of the processing chamber with the position of a bone screw.

Figure 2 shows a photo of a bone screw before the treatment according to the methods of the invention (as received).

Figure 3 shows the range of pressures and densities of oscillating magnetic field where titanium or titanium alloy assumes a chosen colour.

Figure 4 shows a photo of a bone screw after the treatment according to the methods of the invention. The bone screw has a blue colour on its surface.

Figure 5 shows a XPS spectrum of a bone screw after the treatment according to the methods of the invention at optimal conditions, i.e. oxygen pressure of 100 Pa and density of oscillating magnetic field with magnitude of 2x102 Tesla.

Figure 6 shows an XPS depth profile of a bone screw after the treatment according to the methods of the invention at optimal conditions, i.e. oxygen pressure of 100 Pa and density of oscillating magnetic field with magnitude of 2x102 Tesla.

Figure 7 shows a XPS spectrum of untreated bone screw Figure 8 shows an XPS depth profile of an untreated bone screw.

DEFINITION OF MATERIALS AND MEDIA

t5 -sccm stands for standard cubic centimetre per minute. It is a volumetric measure unit of flow defined at standard conditions: temperature of 273,15 K and pressure of 1013 mbar.

-Ti-6A1-4V: titanium alloy with chemical composition in weight parts: 6% aluminium, 4% vanadium, iron less than 0.25%, less than 0.2% oxygen, and the remainder titanium -XPS: X-ray photoelectron spectroscopy. The commercial device by Physical Electronics was applied.

DETAILED DESCRIPTION OF PREFFERED EMBODIMENTS OF THE INVENTION

Titanium or titanium alloys form different compounds upon interaction with different gases. ben allowed to interact with hydrogen, titanium will form hydrates, when interacting with nitrogen, titanium will form nitrides, while when interacting with oxygen it will form oxides. The purity of these compounds often depends on the concentration of different gases in the gas mixture. Best results in terms of uniformity of the compound are obviously obtained when only one gas is used. In practical applications such conditions are realized using different processing systems and one type of a system often used is a chamber which is allowed to be evacuated to low pressure. If the chamber is evacuated to such a low pressure that the density of gases of residual atmosphere is negligible the processing with the selected gas may be highly controlled. That is why there is a need to make the processing chamber vacuum compatible. Additionally to satisfying one-gas-condition atmosphere, a reduced S pressure from atmospheric is needed during the treatment of the titanium or titanium alloy product to enable activation of oxide layer growth on the surface the said titanium or titanium alloy product by oscillating magnetic field. The said oscillating magnetic field applied to the titanium or titanium alloy products in the oxygen atmosphere needs to be uniform to ensure uniform oxide layer growth and with it uniform colouring of the titanium or titanium alloy products. The desired colour of the titanium or titanium alloy product may be achieved by appropriate selection of pressure, oxygen gas flow, strength of oscillating magnetic field, and time of treatment, for example.

The processing chamber can be a part of a production line and involves differential t5 pumping and/or shutter gates with vacuum pumps mounted to the chamber, both of which are already standard industrial equipment.

An example of utilisation of the invention is a processing chamber made in a way that the vacuum compatibility is realised by careful selection of materials used for construction of the processing chamber as well as connection tubes, gaskets, sample holders and alike. The said processing chamber is quartz tube (1 in Figure 1) sealed with two gate valves, one on each side of the said tube (2,3 in Figure 1). The products made from titanium or titanium alloys are arranged into the processing chamber through a gate valve (2 in Figure 1) on a sample holder (4 in Figure 1) in the middle of the said tube. After arranging the products, the gate valve is closed and a vacuum pump is connected to the tube through gate valve (5 in Figure 1) and applied in order to evacuate the processing chamber to such a low pressure that the measured value is below the detection limit of a vacuum gauge. In the methods of invention an absolute gauge with the sensitivity of 1 Pa is used and the pressure after successful evacuation of the processing chamber is preferably below this value.

In the next step, oxygen is leaked into the processing chamber through the connection in a gate valve (6 in Figure 1) during continuous pumping. The leakage is arranged using a precise mass flow controller. According to embodiments of the method of the invention a range of oxygen flows may be applied in order to define the optimal processing conditions. Since the processing chamber is pumped continuously at one side and oxygen is simultaneously leaked at the other side of the processing chamber a working pressure of oxygen is established in the processing chamber.

S An appropriate oxygen flow required in order to achieve a desired oxygen pressure in the chamber will be dependent upon various factors such as the throughput (evacuation power) of the vacuum pump, the volume of the processing chamber and the degree of vacuum tightness of the chamber. Typically a flow rate within the range of 10 sccm to 1000 sccm would be appropriate.

In the preferred embodiment, the processing chamber is a long cylindrical tube wrapped in a copper coil (7 in Figure 1). The coil is connected to a source of AC voltage at high frequency (8 in Figure 1), preferably 13.56 MHz. The oscillating voltage of the source of AC voltage causes oscillating electrical current through the copper coil t5 which in turn causes formation of the oscillating magnetic field (9 in Figure 1) inside the coil. Since the aspect ratio of the copper coil is large the oscillating magnetic field inside the copper coil is essentially homogeneous. This fact is important when considering the uniformity of treatment of products made from titanium or titanium alloys. These products are arranged into the processing chamber in such a way to assure that all products are treated in homogeneous oscillating magnetic field.

Schematic of the processing chamber in the preferred embodiment is shown in Figure 1.

A photo of a typical product made from a titanium alloy is shown in Figure 2. Since embodiments of the methods of invention are intended to be used for medical applications we selected a bone screw made from the biocompatible alloy of Ti-6A1-4V.

The product presented in Figure 2 is selected only as an example. The methods of the invention could be applied for any other product use at any other application as long as the products are made from titanium or titanium alloys. The colour of the product presented in Figure 2 is essentially grey. This colour is typical for many metals and their alloys. Neither the composition nor the surface finish of the product presented in Figure 2 could be deduced from its visual appearance.

In many practical cases it is beneficial to distinguish products made from essentially the same materials by colouration. According to the methods of invention the colouration is realized by treatment of a product made from titanium or titanium alloys with an oscillating magnetic field in an oxygen atmosphere. Such treatment results in products made from titanium or titanium alloys of a chosen colour provided that the processing parameters are chosen carefully.

S

The colour is due to formation of an oxide film upon exposure of products made from titanium or titanium alloys according to the methods of invention. Each colour forms in a limited range of oxide film thicknesses. In order to assure optimal conditions the treatment of the entire product made from titanium or titanium alloys or a group of products made from titanium or titanium alloys is essentially performed in a homogeneous oscillating magnetic field. Appropriate selection of both oxygen pressure and oscillating magnetic field of the right density is necessary as oxygen would otherwise not interact with products made from titanium or titanium alloys to form a

suitable oxide film.

According to the article Med. Eng. Phys. 23, 329-346 (2001): The electrochemical oxide growth behaviour on titanium in acid and alkaline electrolytes; the general relationship between the oxide layer thickness and the colour is generally as follows: nm/ yellow; 50 nm/ purple; 80 nm/ blue; 125 nm/ yellow-gold; 175nm/ scarlet; 240nm/ green; and 360 nm/grey; although other factors are relevant such as surface roughness, for example.

The surface mobility of atoms is low at room temperature so many metals do not form oxide films of substantial thickness. In the present method, as the products made from titanium or titanium alloys are subjected to strong oscillating magnetic field, the surface mobility of atoms is increased and oxidation is ignited. (It is not necessary to use an additional heat source to raise the temperature in the processing chamber).The surface temperature increases further since oxidation of products made from titanium or titanium alloys is a strongly exothermic process. The higher temperature assures for even more intensive oxidation so the film thickness increases with treatment time in the methods of the present invention. The driving force of oxidation is thermo-electrical migration which is a self-limiting process. This is an important effect which results in self-extinguishing of the oxidation process as soon as the oxide film on the surface of products made from titanium or titanium alloys assumes a certain thickness.

The film thickness depends on the rate of the supply of oxygen as well as the density of the oscillating magnetic field. The range of oxygen pressures and densities of oscillating magnetic field where titanium or titanium alloy can assume a chosen colour are shown in Figure 3. The evolution of the oxide film growth is monitored with a naked eye or a more sophisticated technique sensitive to the material colour

S

EXAMPLE 1

Methods of the invention have been applied to colour a medical bone screw from native titanium-grey colour to blue. The material of the screw (Figure 2) is titanium alloy Ti-6A1-4V. The screw was placed in a treatment tube in a region encompassed by a coil. Then the tube was evacuated through a port leading to a vacuum pump to an ultimate pressure below 1 Pa. Then pure oxygen gas was leaked into the treatment tube at the flow of 80 sccm. The pressure in the tube was 100 Pa. After establishing the atmosphere for oxidation, the voltage supply was turned on and a sinusoidal t5 oscillating magnetic field with first harmonic frequency of 13.56 MHz and magnitude of 2xlcY2 Tesla was formed. After 600 s the voltage generator was turned off. After an additional 120 s the sample cooled to a temperature below 50°C. The oxygen supply was turned off and chamber opened. The treated medical bone screw was then blue (Figure 4).

To confirm that the colour is due to a titanium oxide layer a XPS (Figure 5) and XPS depth profile (Figure 6) measurements were performed on the treated medical bone screw. For comparison, also XPS (Figure 7) and XPS depth profile (Figure 8) measurements of non-treated medical bone screw were performed. Figures 5 and 6 show the presence of higher levels of oxygen at the surface of the treated medical bone screw.

The method of the invention may be used to colour a product made from a titanium or titanium alloy for various uses or applications including: (i) A product made from titanium or titanium alloy is selected bone screws and nuts used for fixing broken or otherwise damaged human or animal bones.

(ii) a product made from titanium or titanium alloy which is required to withstand high temperatures, pressures and be friction resistant such as fan blades, discs, rings, airframes, fasteners, components of wings, pumps; ciii) a product made from titanium or titanium alloy which is used for decoration or art, such as statues, computer and phone housings, jewellery; (iv) a product made from titanium or titanium alloy which is a bracelet for spoils watches; and (v) a product made from titanium or titanium alloy which is constructed for use in contact with salt water, such as a hull of ship and pails for desalination plants.

S

Claims

CLAIMS1. A method of colouring a product made from titanium or titanium alloy, the method comprising the following steps: -arranging the product in a processing chamber; -evacuating the chamber; -supplying oxygen into the chamber; -establishing an oscillating magnetic field in the chamber; -allowing the product to interact with the oxygen in the chamber under theinfluence of the oscillating magnetic field;-turning off the magneticfield; and-venting the chamber.
2. A method according to claim 1, wherein the product made from titanium or titanium alloy contains at least 20 weight per cent of titanium, preferably at least 40 weight per cent titanium, more preferably at least 50 weight per cent and most preferably at least 70 weight per cent titanium.
3. A method according to claim 1 or 2, wherein the product made from titanium or titanium alloy contains any other metal or semiconductor selected from aluminium, iron, vanadium, nickel, magnesium, gallium! molybdenum, tantalum, niobium, manganese, cobalt, copper, silicon, tin at any concentration.
4. A method according to any preceding claim, wherein the processing chamber is evacuated to a pressure below 100 Pa, preferably below 1 Pa.
5. A method according to any preceding claim, wherein the processing chamber is filled with oxygen of any purity, preferably purity of at least 99 volume per cent and any pressure, preferably between 1 Pa and 1000 Pa, most preferably between 50 and 200 Pa.
6. A method according to any preceding claim, wherein the oscillating magnetic field is substantially homogeneous at a magnetic field density above 1x103 Tesla, preferably above lxi 3.2 Tesla.
7. A method according to any preceding claim, wherein the oscillating magnetic field oscillates in a sinusoidal form with one or more harmonics.
8. A method according to any preceding claim, wherein the oscillating magnetic field oscillates at the basic frequency above 10 kHz, preferably above 1 MHz.
9. A method according to any preceding claim, wherein the interaction time between the product and the oxygen under the influence of the magnetic field is between lOs and 5000 s, preferably between 100 s and 1000 s.
10. A method for colouring a product made from titanium or titanium alloy according to any one of claims 1 to 9, wherein the product made from titanium or titanium alloy is selected from bone screws and nuts used for fixing broken or otherwise damaged human or animal bones.
11. A method for colouring a product made from titanium or titanium alloy according to any one of claims 1 to 9, wherein the product made from titanium or titanium alloy is required to withstand high temperatures, pressures and be friction resistant such as fan blades, discs, rings, airframes, fasteners, components of wings, pumps.
12. A method for colouring a product made from titanium or titanium alloy according to any one of claims 1 to 9, wherein the product made from titanium or titanium alloy is used for decoration or art, such as statues, computer and phone housings, jewellery.
13. A method for colouring a product made from titanium or titanium alloy according to any one of claims 1 to 9, wherein the product made from titanium or titanium alloy is a bracelet for sports watches.
14. A method for colouring a product made from titanium or titanium alloy according to any one of claims 1 to 9, wherein the product made from titanium or titanium alloy is constructed for use in contact with salt water, such as hulls of ship, parts for desalination plants.