WO2014149002A1

WO2014149002A1 - Method of forming a coating on a metal substrate

Info

Publication number: WO2014149002A1
Application number: PCT/SG2014/000113
Authority: WO
Inventors: Jianhong Lu; Xianghua SONG; Xi Jiang YIN; Jianping Jiang
Original assignee: Singapore Polytechnic
Priority date: 2013-03-20
Filing date: 2014-03-07
Publication date: 2014-09-25
Also published as: SG2013020532A; CN105121711A

Abstract

There is provided a method of forming a coating on a metal substrate, the method comprising the steps of: a. contacting the substrate with an electrolyte solution containing silicate and hydroxide anions and being substantially free of at least one of fluorine compounds, chromate compounds and phosphate compounds; and b. passing a current through the electrolyte solution to form the coating on the metal substrate, wherein the molar ratio of silicate to hydroxide anions in the electrolyte solution is selected to be in a range to provide a corrosion resistance of said coating that is at least equal to or higher than the corrosion resistance of a coating formed using an electrolyte solution comprising at least one of fluorine compounds, chromate compounds and phosphate compounds. There is also provided a coating, optionally produced by the disclosed method, comprising metal silicate and metal oxide, said coating comprising interstitial spaces having a size of 5 µm or less.

Description

METHOD OF FORMING A COATING ON A METAL SUBSTRATE

Technical Field

The present invention generally relates to a method of coating a metal substrate. More specifically, the present invention relates to method of forming a coating on a metal substrate.

Background

In today' s global economy, there is a pressing need to reduce the carbon footprint in industries such as the automotive and aerospace industries. As a result, weight reduction by using light-weight metals, such as magnesium, aluminum, titanium and their alloys , has been one of the ways explored in a bid to reduce fuel consumption.

The demand for magnesium alloys has been and is expected to grow tremendously. For example, the worldwide production of magnesium alloys has reached approximately 610,000 tons in the year of 2006 and is increasing at a rate of 15% per year.

In the year of 2005, the global production of aluminum, which is the most widely used non-ferrous metal, was 31.9 million tons and the forecasted production for the year of 2012 is about 42 to 45 million tons.

In addition, about 186,000 tons of titanium metal sponge was produced in 2011.

In view of the increased use of light-weight metals, there is a need to provide methods of improving the properties of these metals. This is because metals, such as magnesium, and their alloys have a tendency towards corrosion and environmental degradation. Metallic surfaces coated with paints or enamel, while being fairly resistant to chemical attack, are subject to degradation at high temperatures. Further, paints or enamel may adhere poorly to the metal surface particularly during temperature variations .

A current method to improve the wear resistance, corrosion resistance and temperature resistance of lightweight metals is to treat their surfaces using an anodizing process. While anodizing of metal surfaces imparts a more effective and long-lasting coating than paints or enamel, anodic coatings often lack the desired degree of hardness, smoothness, durability, adherence or imperviousness required to meet the ever- increasing industrial and household demands.

The nature and property of anodic coatings formed on the metal surface depend, to a great extent, on the composition of the electrolyte solution used during the anodizing process. For example, known anodizing methods utilize chromate-, phosphate- or fluoride-containing electrolyte solutions to increase the corrosion resistance and compactness of the oxidized layer. Without being bound by theory, it is postulated that these compounds are prevalently used in known anodizing methods because they react readily with the metal substrate to form salts . These salts further demonstrate high packing density, which lead to the formation of compact coating layers with good corrosion resistance.

However, these electrolyte solutions are hazardous to health as well as the environment. Furthermore, there are increasingly tighter regulatory requirements, such as the Restriction of Hazardous Substances (RoHS) standard and other trade effluent regulations, that drive industries to explore greener process technology. In addition, the process conditions used during the anodizing process also contribute to the nature and quality of the coating. In this regard, the operating voltage in an anodizing process using prior art electrolyte solutions is typically less than 300 V. Ostensibly, this is to prevent violent sparking during the anodizing process, which would otherwise lead to unevenly formed coatings with large pores residing within the coating matrix. Such large pores in turn lead to a decrease in corrosion resistance and wear resistance.

Accordingly, there is a need to provide a method of forming a coating on metal substrates that overcomes, or at least ameliorates, one or more of the disadvantages described above. In particular, there is a need to provide an anodizing method for metal substrates which is compliant with regulatory standards, environmentally palatable, and yet at the same time, able to provide coatings with comparable or superior resistance properties.

There is also a need to provide a coating that overcomes, or at least ameliorates, one or more of the disadvantages described above.

Summary

In one aspect, there is provided a method of forming a coating on a metal substrate, the method comprising the steps of: a. contacting the substrate with an electrolyte solution containing silicate and hydroxide anions and being substantially free of at least one of fluorine compounds, chromate compounds and phosphate compounds; and b. passing a current through the electrolyte solution to form the coating on the metal substrate, wherein the molar ratio of silicate to hydroxide anions in the electrolyte solution is selected to be in a range to provide a corrosion resistance of said coating that is at- least equal to . or higher than the corrosion resistance of a coating formed using an electrolyte solution comprising at least one of fluorine compounds, chromate compounds and phosphate compounds.

In one embodiment, the electrolyte solution is substantially free of all of fluorine compounds, chromate compounds and phosphate compounds .

Advantageously, the disclosed method does not utilize hazardous chemicals and is thus environmentally friendly and safe. Furthermore, the absence of one or more of fluorine, chromate and/or phosphate compounds ensures that the disclosed method is compliant with regulatory standards (e.g. the RoHS standard and other trade effluent regulations) restricting the use and disposal of such compounds in industrial waste. The disclosed method is also cost effective as it does not require the use of costly reactants and chemicals.

Advantageously, the coating formed by the disclosed method exhibits a corrosion resistance that is at least equal to or higher than the corrosion resistance of a coating formed using an electrolyte solution comprising one or more of fluorine, chromate and/or phosphate compounds .

In embodiments, the coating comprises interstitial spaces having a size of about 5 μπα or less. Advantageously, the metal oxide and metal silicate particles formed within the coating have minimal interstitial spaces to thereby ensure that the coating possesses optimal corrosion resistance.

In embodiments, the corrosion current density of a current passing through the coated substrate is less than about 10^"7 A/cm² when an open-circuit potential of between about -0.3 V to about 0.3 V is being applied. The current density can be obtained by potentiodynamic polarization testing of the coated substrate and indicates the corrosion current or the corrosion resistance of the coated substrate. Advantageously, the corrosion current density of the coated substrate obtained from the disclosed method is about 2-3 orders of magnitude lower than the current density of the initial metal substrate, i.e. the uncoated substrate. In. embodiments, the molar ratio of silicate to hydroxide anions in the electrolyte solution is in the range of about 0.01 to about 50. The molar ratio of silicate to hydroxide anions is selected to ensure a balance between the metal oxide and metal silicate particulate phases formed within the coating. A balance between the metal oxide and metal silicate particulate phases advantageously ensures that the particles are well- packed and have minimal interstitial spaces. Further advantageously, when the metal oxide and metal silicate particulate phases are balanced, the coating formed may possess a corrosion resistance that is at least equal to or higher than the corrosion resistance of a coating formed using an electrolyte solution comprising at least one of fluorine compounds, chromate compounds and phosphate compounds .

In another aspect, there is provided a coating comprising metal silicate and metal oxide, said coating comprising interstitial spaces having a size of 5 μπι or less. In embodiments, the coating' includes, but is not limited to, metal silicate and metal oxide. In other embodiments, the coating may further include, but is not limited to, metal tungstate, metal molybdate, metal vanadate, metal zirconate and other compounds as disclosed herein.

In yet another aspect, there is provided a coating produced by the disclosed method, wherein the coating comprises metal silicate and metal oxide and comprises interstitial spaces having a size of 5 μτ or less.

Advantageously, the disclosed coating possesses a corrosion resistance that is at least equal to or higher than coatings comprising one or more of metal fluoride, metal chromate and/or metal phosphate.

Definitions

The following words and terms used herein shall have the meaning indicated:

The term "sparking" as used herein is the observance of micro-plasma arcs on the anodizing surface during the anodizing process, especially as small sparks, often small blue sparks similar to neon lights, e.g. of up to 3 mm in length each. Typically, the "sparking regime" is dependent on the electrical conditions, such as current density and voltage, together with the control of the chemical composition of the electrolyte solution.

The term "micro- sparking regime" means that the micro-plasma arcs do not provide significant break-downs in the anodizing coating, such as formation of large pores of more than about 5 μτη in diameter for example, which can have a negative influence on corrosion resistance.

The term "break-down of the coating" means a spot or area where the metallic surface was already at least ^•partially coated and where the anodizing caused at least partial destruction.

The term "fluorine compound" as used herein refers to a compound that comprises at least one fluorine atom, said compound being able to exist in molecular form or ionic form in solution, and wherein said ionic form may include salts, solvates, hydrates of said fluorine compound. An example of a fluorine compound includes, but is not limited to, sodium fluoride.

The . term "chromate compound" as used herein refers to a compound that comprises at least one chromium atom, said compound being able to exist in molecular form or ionic form in solution, and wherein said ionic form may include salts, solvates, hydrates of said chromate compound. Examples of chromate compounds include, but are not limited to, compounds containing chromate, dichromate and hexavalent chromium.

The term "phosphate compound" as used herein refers to a compound that comprises at least one phosphorus atom, said compound being able to exist in a molecular form or ionic form in solution, and wherein said ionic form may include salts, solvates, hydrates of said phosphate compound. A phosphate compound may also include organic phosphate compounds, such as, organophosphates .

The term "silicate" as used herein has its ordinary meaning as known to those skilled in the art and refers to silicon bearing compounds including, but not limited to, trioxygenated silicon species, or tetra-oxygenated silicon species, and may also include organosilicon or silane.

The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. The term "substantially free" when used to refer to compounds in the electrolyte solution means, unless otherwise indicated, that the electrolyte solution has an amount of compounds which is less than about 0.1 wt%, based on the non-volatile components of the solution, and preferably about 0 wt%.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically. +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely, for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3 , from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Disclosure of Optional Embodiments

Exemplary, non-limiting embodiments of a method of forming a coating on a metal substrate will now be disclosed. In one embodiment, there is provided a method of forming a coating on a metal substrate, the method comprising the steps of: a. contacting the substrate with an electrolyte solution containing silicate and hydroxide anions and being substantially free of at least one of fluorine compounds, chromate compounds and phosphate compounds; and b. passing a . current through the electrolyte solution to form the coating on the metal substrate, wherein the molar ratio of silicate to hydroxide anions in the electrolyte solution is selected to be in a range to provide a corrosion resistance of said coating that is at least equal to or higher than the corrosion resistance of a coating formed using an electrolyte solution comprising at least one of fluorine compounds, chromate compounds and phosphate compounds.

In an embodiment, the method is an anodic oxidation method. In another embodiment, the method is a micro-arc oxidation method.

In embodiments, the metal substrate is subjected to a current while the metal substrate is immersed in the disclosed electrolyte solution. The voltage applied may vary.

In embodiments, when the voltage applied during the disclosed method is below about 100 V, no sparks develop at the metal substrate-electrolyte interface. In such a situation, although oxidation may occur, the micro- sparking regime has not been reached and thus no coating was formed on the metal substrate .

When the voltage applied is increased to about 100 V and above, the micro-sparking regime may be reached. The thermal energy of the micro-sparks which develop at the metal substrate-electrolyte interface is used to form and build up the coating on the surface of the metal substrate.

An example of the reaction that occurs during the oxidation process when the metal substrate is magnesium is as follows.

40H^" - 4e^~ = 2H₂0 + 2 [0] (1), wherein OH^" represents the hydroxide anions from the electrolyte solution and [0] represents active oxygen.

2H₂0 - 4e^~ = 0₂ + 4 H⁺ (2) 2H⁺ + 2e = H₂ (3)

Mg - 2e = g²⁺ (4) , wherein the thermal energy of the micro-sparks frees the electrons from the magnesium atoms at the metal substrate- electrolyte interface.

Mg²⁺ + [O] = MgO (5)

In step (5) , the active oxygen reacts with the metal ions (in this case magnesium ions) at the metal substrate- electrolyte interface to form magnesium oxide.

2Mg²⁺ + 2Si0₃ ^2" = Si0₂ + Mg₂Si0₄ (6) In step (6) , the magnesium ions at the metal substrate-electrolyte interface also react with the silicate anions from the electrolyte solution to form magnesium silicate. Accordingly, in this embodiment, the coating is composed of magnesium oxide and magnesium silicate. An example of X-ray diffraction patterns of a coated magnesium substrate and of the initial (uncoated) magnesium substrate, respectively, is shown in Figs. 1(a) and (b) .

Advantageously, because the coating is integrally formed on the metal substrate, the coating has excellent adhesion to the metal substrate. The disclosed method thus provides an improved method of coating metal substrates.

Typically when the voltage applied in known oxidation processes is above about 300 V, the sparks become too large, causing a break-down in the coating formed. That is, large pores are formed in the coating. The interstitial spaces, leads to inferior properties such as low corrosion resistance and non-uniform thickness.

In embodiments, the coating formed according to the disclosed method comprises interstitial spaces having a size of about 5 μτ or less. In embodiments, the coating formed according to the disclosed method comprises interstitial spaces having a size of ^"less than about 5 μτη, less than about 4 μπι, less than about 3 μπι, less than about 2 μιτι, less than about 1.8 μπι, less than about 1.6 μη^η, less than about 1.4 μΐ , less than about 1.2 μπι, less than about 1.0 μπι, less than about 0.8 μπι, less than about 0.6 μτη, less than about 0.4 μιη, less than about 0.2 μπι, or less than about 0.1 μπι. In embodiments, the voltage applied during the disclosed method is greater than about 300 V. In embodiments, the voltage applied during the disclosed method may be up to about 800 V before a break-down in the coating occurs. In one embodiment, the voltage applied is from about 100 V to about 800 V. Advantageously, a wider range of voltages can be applied using the disclosed process.

In embodiments, the current density of a current passing through the coated substrate is less than about 10^" ⁵ A/ cm², less than about 10^"6 A/ cm², less than about 10^" ⁷ A/ cm², less than about 10^"8 A/ cm², less than about 10^" ⁹ A/cm², less than about 10^"10 A/cm², or less than about 10^" ¹¹ A/cm² during potentiodynamic polarization testing. The potentiodynamic polarization testing may be conducted with an open-circuit potential of between about -0.3 V to about 0.3 V. The scan rate may be about 1 mV/s.

In general, the lower the corrosion current density value of a substrate, the higher the corrosion resistance of the substrate. A lower corrosion current density value implies that a lower amount of metal from the metal substrate is ionized, i.e. corroded, and therefore a lower amount of current passes through a unit area of the substrate. Advantageously, a lower amount of ionized metal indicates that the substrate is less susceptible to corrosion. Hence advantageously, the coated substrate obtained from the disclosed method has higher corrosion resistance.

In embodiments, the corrosion current density is obtained by potentiodynamic polarization testing of the coated substrate. The current density values can be obtained from the potentiodynamic polarization curve of the substrate.

The potentiodynamic polarization testing compares the corrosion resistances of the uncoated metal substrate and the coated metal substrate. In embodiments, the uncoated metal substrate and the coated metal substrate are corroded using a corrosive medium, such as a NaCl solution. The substrate under evaluation will then be mounted to a cell with an area of the substrate sample contacting the corrosive medium. The reference electrode can be an Ag/AgCl electrode and the counter electrode can be a platinum electrode. In an embodiment, the corrosion resistances of the uncoated metal substrate and the coated metal substrate are evaluated using an AUTOLAB- POTENTIOSTAT electrochemical corrosion test system. The scan may be conducted at a potential range of about ±0.3 V based on an open-circuit potential (OCP) at a constant scan rate of about 1 mV/S.

In a particular embodiment where the metal substrate is magnesium alloy, when a pulsed current at 240 V to 320 V was used during the disclosed method, the corrosion current density of the coated Mg alloy substrate obtained is in the magnitude order of 10^~7 A/cm² during potentiodynamic polarization testing. In some instances, the corrosion current density of the coated Mg alloy substrate obtained may be less than about 10^"7 A/cm².

The molar ratio of silicate to hydroxide anions in the electrolyte solution may be selected to be about 0.01 to about 50, or about 0.1 to about 50, or about 0.5 to about 50, or about 1 to about 50, or about 5 to about 50, or about 0.01 to about 40, or about 0.01 to about 30, or about 0.01 to about 20, or about 0.01 to about 10, or about 0.01 to about 5, or about 0.1 to about 20, or about 0.1 to about 10, or about 0.1 to about 5, or about 0.5 to about 30, or about 0.5 to about 20, or about 0.5 to about 10, or about 0.5 to about 5, or about 1 to about 30, or about 1 to about 20, or about 1 to about 10, or about 1 to about 5.

Advantageously, when the molar ratio is within the disclosed ranges, the coating formed may possess a corrosion resistance that is at least equal to or higher than the corrosion resistance of a coating formed when the molar ratio is out of the disclosed range. Without being bound by theory, when the molar ratio is within the disclosed ranges, the metal oxide and metal silicate particulate phases will be balanced so that the interstitial spaces between the particles are minimal. However, if the molar ratio is below about 0.01, i.e., when the amount of the particles in the metal oxide phase is too high, or if the molar ratio is above about 50, i.e., when the amount of the particles in the metal silicate phase is too high, the resultant coated substrate will not possess effective corrosion resistance property. The term "effective" as used herein refers to a coated substrate having a corrosion current density of at least about 10^"5 A/cm² during potentiodynamic polarization testing.

In a particular embodiment, the molar ratio of silicate to hydroxide anions in the electrolyte solution is selected to be about 0.5 to about 5. Advantageously, when the molar ratio is within 0.5 and 5, the metal oxide and metal silicate particles formed are tightly packed with minimal interstitial spaces, thereby resulting in a coating with optimal corrosion resistance.

In an embodiment, the electrolyte solution is substantially free of all of fluorine compounds, chromate compounds and phosphate compounds. Instead, the fluorine, chromate and phosphate compounds are replaced by silicate which is more environmentally friendly. Yet the coating formed by the disclosed method has a corrosion resistance which is at least equal to or higher than the corrosion resistance of a coating formed using an electrolyte solution comprising at least one of fluorine compounds, chromate compounds and phosphate compounds .

Advantageously, the absence of one or more of fluorine, chromate and/or phosphate compounds ensures that the disclosed method is compliant with regulatory standards restricting the use and disposal of such compounds in industrial waste. In some instances, the electrolyte solution may comprise less than about 1 milligram/liter of chromate compounds or less than about 1 ppm of chromate compounds . In other instances, the electrolyte solution may comprise less than about 0.8 ppm, less than about 0.6 ppm, less than about 0.4 ppm, less than about 0.2 ppm or less than about 0.1 ppm of chromate compounds .

In some instances, the electrolyte solution may comprise less than about 5 milligram/liter of phosphate compounds or less than about 5 ppm of phosphate compounds . In other instances, the electrolyte solution may comprise less than about 4.5 ppm, less than about 4 ppm, less than about 3.5 ppm, less than about 3 ppm, less than about 2.5 ppm, less than about 2 ppm, less than about 1.5 ppm, less than about 1 ppm, less than about 0.5 ppm or less than about 0.1 ppm of phosphate compounds .

In some instances, the electrolyte solution may comprise less than about 15 milligram/liter of fluorine compounds or less than about 15 ppm of fluorine compounds. In other instances, the electrolyte solution may comprise less than about 10 ppm, less than about 8 ppm, less than about 5 ppm, less than about 4 ppm, less than about 3 ppm, less than about 2 ppm, less than about 1 ppm, less than about 0.5 ppm or less than about 0.1 ppm of fluorine compounds .

In some instances, the electrolyte solution comprises less than 1 ppm of chromate compounds, less than 5 ppm of phosphate compounds and less than 15 ppm of fluorine compounds. Advantageously, the disclosed method is compliant with the allowable limits of trade effluent discharge in countries, such as Singapore.

A current is passed through the electrolyte solution at an anodizing potential that is sufficient to reach the micro-sparking regime and yet does not cause a break-down in the coating formed.

The current that is passed through the electrolyte solution may be an alternating current. The current that is passed through the electrolyte solution may also be a direct current. Alternatively, the current that is passed through the electrolyte solution may be a pulsed current. Advantageously, pulsed currents permit savings in the electricity used as a current is passed only at intervals while still achieving a coating with the desired properties. Furthermore, pulsed currents confer process stability onto the disclosed method because the current and/or density may be controlled.

In the embodiments where direct current or pulsed current is used, the metal substrate is the anode. In the embodiment where alternating current is used, the metal substrate is used as an electrode.

For alternating currents or direct currents, the output model may be a constant current model or a constant voltage model . Examples of a direct current waveform and an alternating current waveform are shown in Figs. 2 and 3, respectively.

For the constant current model, the current density used may be anywhere from about 0.01 A/dm² to about 5 A/dm². In one embodiment, the current density used is about 2 A/dm . In another embodiment, the current density used is about 5 A/dm². The voltage applied varies from about 0 V to about 800 V or from about 100 V to about 800 V.

For the constant voltage model, the voltage used may be anywhere from about 100 V to about 800 V. In one embodiment, the voltage used is about 400 V. The current density used varies from about 0 A/dm² to about 5 A/dm² or from about 2 A/dm² to about 5 A/dm².

For pulsed currents, the pulse frequency may be from about 0.1 Hz to about 1000 Hz or from about 1 Hz to about 100 Hz. The pulse frequency is expressed mathematically as , wherein T_on is the amount of time when a voltage

is applied (V_H) and T_off is the amount of time when a voltage is not applied (V_L) . An example of a pulse current waveform is shown in Fig. 4.

The duty factor of the pulsed current is the percent of time that a current is passed and is expressed

T

mathematically as duty factor = -— . The duty factor used in the disclosed method may be from about 0.01 to about 0.95.

At T_on, the voltage applied (V_H) may be from about

100 V to about 800 V. At T_off, the background voltage (V_L) may be from about -100 V to about 100 V.

Advantageously, a wide range of electrical parameters, such as those disclosed above, can be used in the disclosed method without resulting in inferior coatings being formed. The disclosed method thus advantageously presents practical utility by providing flexibility in the choice of electric source desired.

Further advantageously, the disclosed method may be performed under a wide range of physical parameters. For example, the disclosed method may advantageously be performed at a wide temperature range anywhere from about 5°C to about 80°C.

The silicate anions in the electrolyte solution have at least one O-Si-0 group. In embodiments, the silicate anions may be derived from alkali metal silicates or alkaline earth metal silicates..

In one embodiment, the alkali metal silicate is selected from the group consisting of lithium silicate, sodium silicate and potassium silicate. In another embodiment, the alkali earth metal silicate is selected from the group consisting of magnesium silicate and calcium silicate.

In an embodiment, the silicate anions are derived from colloidal silica or silica gel. In yet another embodiment, the silicate anions are derived from the group consisting of potassium silicate, sodium silicate, colloidal silica, silica gel and mixtures thereof.

The hydroxide anions in the electrolyte solution may be derived from inorganic hydroxide compounds. In embodiments, the hydroxide anions are derived from water- soluble inorganic hydroxide compounds. In an embodiment, the hydroxide anions are derived from alkali metal hydroxides or alkaline earth metal hydroxides . In another embodiment, the hydroxide anions are derived from ammonium hydroxide .

In one embodiment, the alkali metal hydroxide is selected from the group consisting of lithium hydroxide, sodium hydroxide and potassium hydroxide. In another embodiment, the alkali earth metal hydroxide is selected from the group consisting of magnesium hydroxide, calcium hydroxide and barium hydroxide.

In a particular embodiment, the hydroxide anions are derived from the group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, barium hydroxide and mixtures thereof. The metal substrate to be coated may be magnesium, magnesium alloys, aluminium, aluminium alloys, titanium or titanium alloys. Accordingly, when the metal is magnesium or magnesium alloys, the particulate phases in the formed coating comprise magnesium oxide and magnesium silicate. When the metal is aluminium or aluminium alloys, the particulate phases in the formed coating comprise aluminium oxide and aluminium silicate. When the metal is titanium or titanium alloys, the particulate phases in the formed coating comprise titanium oxide and titanium silicate. In a particular embodiment, the metal is magnesium and the particulate phases in the formed coating comprise magnesium oxide and magnesium silicate.

The electrolyte solution may further comprise at least one stabilizer compound selected from the group consisting of an organic amine, an organic polyol, an alcohol amine and mixtures thereof. Advantageously, the stabilizer compound controls the intensity of the spark within the micro- sparking regime and prevents excessive sparking at the metal substrate-electrolyte interface. A stable electrolyte solution can thus be provided.

The organic amine may be a primary, secondary or tertiary amine. The organic amine may also be an aliphatic amine or an aromatic amine. In an embodiment, the organic amine is selected from the group consisting of ethylenediamine , aminoform, tripropylamine and mixtures thereof .

The organic polyol has at least two alcohol groups and may have three, four or more alcohol groups. The organic polyol may be an aliphatic alcohol or an aromatic alcohol. The aliphatic alcohol may have saturated or unsaturated alkyl groups. In an embodiment, the organic polyol is selected from the group consisting of aliphatic alcohols, aromatic alcohols, glycitol and. mixtures thereof.

The alcohol amine has at least one alcohol group and at least one amine group. The alcohol group may be an aliphatic alcohol group or an aromatic alcohol group. The aliphatic alcohol may be a saturated or unsaturated aliphatic alcohol. The amine group may be a primary, secondary or tertiary amine group. The amine group may be an aliphatic amine group or an aromatic amine group. In one embodiment, the alcohol amine is selected from the group consisting of ethanolamine , diethanolamine, triethanolamine, 2- (2-aminoethoxy) ethanol, 3-amino-l- propanol and mixtures thereof .

The concentration of the stabilizer compound in the electrolyte solution may range from about 0.5 g/L to about 300 g/L. In other embodiments, the concentration of the stabilizer compound in the electrolyte solution may range from about 1 g/L to about 300 g/L, or from about 5 g/L to about 300 g/L, or from about 10 g/L to about 300 g/L, or from about 0.5 g/L to about 200 g/L, or from about 0.5 g/L to about 150 g/L, or from about 1 g/L to about 150 g/L, or from about 5 g/L to about 150 g/L. In a particular embodiment, the concentration of the stabilizer compound in the electrolyte solution ranges from about 5 g/L to about 150 g/L.

If the concentration of the stabilizer compound is below 0.5 g/L, the intensity of the spark may not be adequately controlled within the micro-sparking regime, leading to large sparks being formed that will adversely affect the corrosion resistance of the oxide coating. Conversely, if the concentration of the stabilizer compound is above 300 g/L, the coating formed may be of inferior quality as the particulate phases of the formed coating may become unbalanced.

The electrolyte solution may comprise additives to improve the hardness of the formed coating. The additive may be a metal salt additive. The metal salt additive may be a transition metal salt additive. The metal salt additive may be a water-soluble transition metal salt additive.

In one embodiment, the metal salt additive is selected from the group consisting of tungstate compounds, molybdate compounds, vanadate compounds, zirconate compounds and mixtures thereof .

The tungstate compound may be selected from the group consisting of ammonium tungstate, potassium tungstate, sodium tungstate, lithium tungstate and sodium polytungstate . The molybdate compound may be selected from the group consisting of ammonium molybdate, sodium molybdate, potassium molybdate and lithium molybdate. The vanadate compound may be selected from the group consisting of ammonium vanadate, sodium vanadate, potassium vanadate and lithium vanadate. The zirconate compound may be selected from sodium zirconate, zirconium silicate and zirconium dioxide.

The concentration of the metal salt additive in the electrolyte solution may range from about 0.1 g/L to about 80 g/L. In other embodiments, the concentration of the metal salt additive in the electrolyte solution may range from about 0.5 g/L to about 80 g/L, or. about 1 g/L to about 80 g/L, or about 5 g/L to about 80 g/L, or about 10 g/L to about 80 g/L, or about 20 g/L to about 80 g/L, or about 0.1 g/L to about 50 g/L, about 0.1 g/L to about 30 g/L, about 0.1 g/L to about 20 g/L, or about 0.1 g/L to about 10 g/L, or about 0.1 g/L to about 5 g/L, or about 0.1 g/L to about 1 g/L, or about 0.5 g/L to about 50 g/L, or about 0.5 g/L to about 30 g/L, or about 0.5 g/L to about 20 g/L, ^'or about 0.5 g/L to about 10 g/L, or about 1 g/L to about 50 g/L, or about 1 g/L to about 30 g/L, or about 1 g/L to about 20 g/L, or about 1 g/L to about 10 g/L. In a particular embodiment, the concentration of the metal salt additive in the electrolyte solution ranges from about 1 g/L to about 30 g/L.

If the concentration of the metal salt additive is below about 0.1 g/L or above about 80 g/L, the formed coating may not possess sufficient hardness. Furthermore, if the concentration of the metal salt additive is out of the disclosed ranges, the corrosion resistance of the formed coating will decrease. In an embodiment, there is provided a coating comprising metal silicate and metal oxide, the coating comprising interstitial spaces having a size of about 5 μπι or less.

In embodiments, the coating includes, but is not limited to, metal silicate and metal oxide. In other embodiments, the coating may further comprise compounds including, but not limited to, metal tungstate, metal molybdate, metal vanadate, metal zirconate and other compounds disclosed herein. In another embodiment, there is provided a coating produced by the disclosed method, wherein the coating comprises metal silicate and metal oxide and comprises interstitial spaces having a size of 5 μτη or less. The interstitial spaces may have a size as disclosed herein. The disclosed coating may have a thickness of about

2 μπι to about 20 μιη. In embodiments, the thickness of the coating depends on the parameters of the method disclosed herein, including processing time, current density, applied voltage and current model, etc.

In embodiments, the interstitial spaces have a size of less than about 5 μπι, less than about 4 μπι, less than about 3 μπι, less than about 2 μπι, less than about 1.8 μτη, less than about 1.6 μπι, less than about 1.4 μτη, less than about 1.2 μιη, less than about 1 μιη, less than about 0.8 μπι, less than about 0.6 μτη, less than about 0.4 μτα, less than about 0.2 μτ^η, or less than about 0.1 μιη. In embodiments, the corrosion current density of a current passing through the coating is less than about 10^" ⁵ A/cm², less than about 10^"6 A/cm², less than about 10^" ⁷ A/cm², less than about 10^"8 A/ cm², less than about 10^" ⁹ A/cm², less than about 10^"10 A/cm², or less than about 10^" ¹¹ A/cm² during potentiodynamic polarization testing. In an embodiment, the current density of a current passing through the coating is less than about 10^"7 A/cm² during potentiodynamic polarization testing.

In embodiments, the metal is selected from the group consisting of magnesium, magnesium alloys, aluminium, aluminium alloys, titanium, titanium alloys and mixtures thereof. In an embodiment, the metal is magnesium.

Brief Description Of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention. Figs. 1(a) and (b) show X-ray diffraction patterns of a coated magnesium substrate and of the initial (uncoated) magnesium substrate, respectively. Fig. 2 is an example of a direct current waveform.

Fig. 3 is an example of an alternating current waveform.

Fig. 4 is an example of a pulse current waveform.

Fig. 5 shows field emission scanning electron microscopic (FESEM) images of the MAO treated magnesium alloy from Example 1 after 30 minutes at a pulsed current of 260 V.

Fig. 6 shows FESEM images of the MAO treated magnesium alloy from Example 1 after 30 minutes at a pulsed current of 320 V.

Fig. 7 shows the potentiodynamic polarization curves for (a) the untreated magnesium alloy, (b) the 260 V MAO treated magnesium alloy from Example 13, and (c) the 260 V MAO treated magnesium alloy from Comparative Example 1 at a molar ratio of silicate to hydroxide anions in the electrolyte solution of 70.

Fig. 8 shows the potentiodynamic polarization curves for (a) the untreated magnesium alloy, (b) the 300 V MAO treated magnesium alloy from Example 15, and (c) the 300 V MAO treated magnesium alloy from Comparative Example 2 at a molar ratio of silicate to hydroxide anions in the electrolyte solution of 70.

Fig. 9 shows the potentiodynamic polarization curves for (a) the untreated magnesium alloy, and (b) the 240 V MAO treated magnesium alloy from Example 12.

Fig. 10 shows the potentiodynamic polarization curves for (a) the untreated magnesium alloy, and (b) the 280 V MAO treated magnesium alloy from Example 14. Fig. 11 shows the potentiodynamic polarization curves for (a) the untreated magnesium alloy, and (b) the 320 V MAO treated magnesium alloy from Example 16.

Examples Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Examples 1-4

An AZ31B magnesium alloy was cleaned in a solution containing 1 molar sodium hydroxide (obtained from Sinopharm Chemical ISEC Reagent Co. Ltd, China) and 0.4 molar sodium carbonate (obtained from Sinopharm Chemical ISEC Reagent Co. Ltd, China) at a temperature of 30-60°C and was rinsed completely with tap water thereafter.

The cleaned alloy was then immersed in acidic solution to remove the surface oxide film, and was rinsed completely with tap water.

The pre-treated alloy was anodised by micro-arc oxidation (MAO) in an electrolyte solution at a temperature of 12-35 °C.

The above method was repeated with different electrolyte solutions. The components of the difference electrolyte solutions are shown in Table 1 below. Table 1

The hexamethylene tetramine, sorbitol and glycerol are provided by Sigma Aldrich, Missouri, USA, while the sodium silicate and triethanolamine are provided by Sinopharm Chemical ISEC Reagent Co. Ltd, China.

During the MAO treatment process, a fixed voltage in the range of 0 V to 400 V was applied with a fixed on-off pulse-time ratio of T_on/T_off = 4 ms : 1 ms, V_H = 240-400V, V_L = 0-lOOV, pulse frequency of 200Hz and duty factor of 80%. A graphical representation of the pulse current used is shown in Fig. 4.

Examples 5-7

Aluminium alloy 1050 (obtained from Aalco Metals, the United Kingdom) or AZ61B magnesium alloy was pre-cleaned in an alkaline solution of 2 M KOH (Sigma Aldrich, Missouri, USA) at a temperature of 25-60°C. Thereafter, the pre-cleaned alloy was etched in acid solution and rinsed completely with tap water.

The cleaned alloy was then anodised by MAO in an electrolyte solution at a temperature of 10-55°C.

The above method was repeated with different electrolyte solutions. The components of the difference electrolyte solutions are shown in Table 2 below.

Table 2

The potassium hydroxide, ethylene glycol, glycerol and sodium molybdate are provided by Sigma Aldrich, Missouri, USA, while the sodium silicate and triethanolamine are provided by Sinopharm Chemical ISEC Reagent Co. Ltd, China.

During the MAO treatment process, a constant voltage output model was used. The voltage was fixed in the range of 160-360 V and direct current was applied. A graphical representation of the direct current used is shown in Fig. 2. Examples 8-11

An AZ91 magnesium alloy was pre-cleaned in an alkaline solution at a temperature of 30-60°C. The pre- cleaned alloy was etched in acid solution and rinsed completely with^" tap water.

The cleaned alloy was then anodised by MAO in- an electrolyte solution at a temperature of 15-55°C.

The above method was repeated with different electrolyte solutions. The components of the difference electrolyte solutions are shown in Table 3 below.

Table 3

Missouri, USA, while sodium silicate, ethylenediamine, ethanolamine, 2- (2-aminoethoxy) ethanol and potassium tungstate are provided by Sinopharm Chemical ISEC Reagent Co. Ltd. , China.

During the MAO treatment process, a constant current output model was used. The voltage was fixed in the range of 160-360 V and alternating current was applied. The frequency was 50/60Hz and the current density value was set within 2 A/dm². A graphical representation of the alternating current used is shown in Fig. 3. Examples 12-16

An AZ3 IB magnesium alloy was prepared and anodized according to the procedure set out in Example 1, except that the anodizing temperature was 15°C and the electrolyte solution used was 0.14M of Na₂Si0₃, 0.09M of KOH, 0.4M of triethanolamine and 0.33M of glycerol. Accordingly, the ratio of M_Si₀2 to M₀H is 1.56.

During the MAO treatment process, different fixed voltages as shown in Table 4 below were applied in accordance with the procedure in Example 1 for 30 minutes. Table 4

Field emission scanning electron microscopic (FESEM) images of the MAO treated magnesium alloy from Example 13 at 260 V after 30 minutes are shown in Fig. 5, while FESEM images of the MAO treated magnesium alloy from Example 16 at 320 V after 30 minutes are shown in Fig. 6. It can be seen from Figs. 5 and 6 that the oxide surface is composed of substantially homogeneously distributed 0.5 μτη small pores and crater- like pores. When Figs. 5 and 6 are compared, the surface morphology of the oxide surface at an applied voltage of 260 V appears to be more homogeneous and compact with smaller crater- like pores of around 1-2 μιη as compared to the oxide surface at an applied voltage of 320 V. The cross-sectional morphology shown in Figs. 5 and 6 demonstrates the uniformity of the coating formed having a thickness of approximately 2 - 20 μτη. Comparative Examples 1 and 2

The AZ31B magnesium alloy was prepared and anodized according to the procedure set out in Example 12, except that the ratio of _si02 to M_0H is 70.

During the MAO treatment process, different fixed voltages as shown in Table 5 below were applied in accordance with the procedure in Example 12 for 30 minutes.

Table 5

The corrosion resistance of the Mg alloy substrate and the MAO coated substrate were evaluated using an AUTOLAB-POTENTIOSTAT electrochemical corrosion test system, in which a 3.5 wt % NaCl solution was used as a corrosive medium.

The coated substrate sample under evaluation was mounted to a cell with an area of 1 cm² of the sample being in contact with the corrosion solution. The reference electrode was a saturated Ag/AgCl electrode and a platinum counter electrode was used. The scan was conducted at a potential range of +0.3 V based on an open-circuit potential (OCP) at a constant scan rate of 1 mV/s.

The potentiodynamic polarization curves for the various voltages are shown in Figs. 7-11. The gradients of each curve are extrapolated respectively to meet at a point, which indicates the value of the current that is passed through the respective substrate. As mentioned above, the lower the current, the higher the corrosion resistance of the substrate. It can be seen from Fig. 7 which compares Example 13 (curve b) with Comparative Example 1 (curve c) at 260 V that the untreated magnesium alloy (curve a) is about 1000 times less corrosion resistant than (b) , while (c) is about 10 times less corrosion resistant than (b) .

It can be seen from Fig. 8 which compares Example 15 (curve b) with Comparative Example 2 (curve c) at 300 V that the untreated magnesium alloy (curve a) is also about 1000 times less corrosion resistant than (b) , while (c) is about 100 times less corrosion resistant than (b) .

It can be seen from Fig. 9 which compares Example 12 (curve b) at 240 V with the untreated magnesium alloy (curve a) that (a) is about 10 times less corrosion resistant than (b) .

It can be seen from Fig. 10 which compares Example 14

(curve b) at 280 V with the untreated magnesium alloy (curve a) that (a) is about 1000 times less corrosion resistant than (b) .

It can be seen from Fig. 11 which compares Example 16 (curve b) at 320 V with the untreated magnesium alloy (curve a) that (a) is also about 1000 times less corrosion resistant than (b) .

Applications

The disclosed method finds utility in the aviation and transportation industries, computers, communications and consumer electronic applications, tools and nursing apparatuses .

The disclosed method utilizes non-hazardous and environmentally friendly electrolyte solutions which is at least free of one of fluorine compounds, chromate compounds and phosphate compounds. Advantageously, the absence of one or more of fluorine compounds, chromate compounds and phosphate compounds ensures that the disclosed method is compliant with at least the RoHS standard.

Advantageously, the coating formed by the disclosed process possesses a corrosion resistance that is at least equal to or higher than the corrosion resistance of a coating formed using an electrolyte solution comprising one or more of fluorine compounds, chromate compounds and phosphate compounds .

The disclosed method may advantageously be more cost effective than prior art methods due to the absence of use of costly reactants and chemicals.

The coated metal substrate obtained from the disclosed method possesses significantly improved corrosion resistance and hardness.

The disclosed electrolyte solution is advantageously stable during the anodizing process in that the micro- sparking regime can be maintained.

The disclosed method is able to operate under a wider variety of process conditions, such as alternating currents, direct currents, pulse currents and wide temperature ranges .

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims .

Claims

A method of forming a coating on a metal substrate, the method comprising the steps of: a. contacting the substrate with an electrolyte solution containing silicate and hydroxide anions and being substantially free of at least one of fluorine compounds, chromate compounds and phosphate compounds ; and b. passing a current through the electrolyte solution to form the coating on the metal substrate, wherein the molar ratio of silicate to hydroxide anions in the electrolyte solution is selected to be in a range to provide a corrosion resistance of said coating that is at least equal to or higher than the corrosion resistance of a coating formed using an electrolyte solution comprising at least one of fluorine compounds, chromate compounds and phosphate compounds .

The method of claim 1, wherein the electrolyte solution is substantially free of all of fluorine compounds, chromate compounds and phosphate compounds .

The method of any one of the preceding claims, wherein the coating comprises interstitial spaces having a size of 5 μτη or less.

The method of any one of the preceding claims, wherein the current density of a current passing through the coated substrate is less than lO^"7 A/cm² when an open-circuit potential of between -0.3 V to 0.3 V is being applied.

The method of any one of the preceding claims, comprising the step of selecting the molar ratio of silicate to hydroxide anions in the electrolyte solution to be 0.01 to 50.

The method of claim 5, comprising the step of selecting the molar ratio of silicate to hydroxide anions in the electrolyte solution to be 0.5 to 5.

7. The method of any one of the preceding claims, wherein the current passed through the solution is an alternating current.

The method of any one of claims 1-6, wherein the current passed through the solution is a direct current .

The method of any one of claims 1-6, wherein the current passed through the solution is a pulsed current .

The method of any one of claims 7-9, wherein the voltage applied is from 100 V to 800 V.

11. The method of claim 10, wherein the voltage applied is greater than 300 V.

12. The method of any one of the preceding claims, wherein the electrolyte solution further comprises at least one stabilizer- compound selected from the group consisting of an organic amine, an organic polyol, an alcohol amine and mixtures thereof.

13. The method of claim 12, wherein the concentration of the stabilizer compound in the electrolyte solution ranges from 0.5 g/L to 300 g/L.

14. The method of any one of the preceding claims, wherein the electrolyte solution comprises a metal salt additive.

15. The method of claim 14, wherein the concentration of the metal salt additive in the electrolyte solution ranges from 0.1 g/L to 80 g/L.

16. The method of any one of the preceding claims, wherein the method is performed at a temperature range of from 5°C to 80°C.

17. A coating comprising metal silicate and metal oxide, said coating comprising interstitial spaces having a size of 5 μπι or less.

18. A coating produced by the method of any one of claims 1-16, wherein the coating comprises metal silicate and metal oxide and comprises interstitial spaces having a size of 5 μτα or less.

19. The coating of any one of claims 17 and 18, wherein the current density of a current passing through the coated substrate is less than 10^"7 A/cm² when an open-circuit potential of between -0.3 V to 0.3 V is being applied.

20. The coating of any one of ^"claims 17 to 19, wherein the metal is magnesium.