WO2023099880A1

WO2023099880A1 - Use of chelating agents in plasma electrolytic oxidation processes

Info

Publication number: WO2023099880A1
Application number: PCT/GB2022/053023
Authority: WO
Inventors: Robin Francis; Ankit Khurana; Graham STEEL
Original assignee: Keronite International Limited
Priority date: 2021-12-03
Filing date: 2022-11-30
Publication date: 2023-06-08
Also published as: GB2613562A; GB202117544D0

Abstract

There is disclosed a plasma electrolytic oxidation process for generating a ceramic coating on a surface of a workpiece made of an alloy comprising a base metal and one or more secondary metals. A series of electrical current pulses are applied to the metallic workpiece in an electrolyte so as to generate plasma discharges at the surface of the metallic workpiece to form the ceramic coating. The electrolyte comprises at least one chelating agent selected to bind with ions of at least one of the one or more secondary metals that are released from the workpiece during the plasma electrolytic oxidation process. This helps to prevent or hinder a concentration of secondary metal compounds at an outer surface of the ceramic coating.

Description

USE OF CHELATING AGENTS IN PLASMA ELECTROLYTIC OXIDATION PROCESSES

[0001] This disclosure relates to the use of chelating agents as components in electrolytes for plasma electrolytic oxidation coating processes.

BACKGROUND

[0002] Plasma electrolytic oxide (PEO) coatings on metal substrates such as aluminium, magnesium, titanium, zirconium and zinc are widely used in numerous applications due to their chemical inertness, durability and high dielectric strength amongst other useful properties. PEO coatings are usually comprised principally of the oxides of the substrate material. However, there are often other secondary phases within the coatings, some of which contain other metals. For some applications it is critically important that the quantities of secondary metals on or near the surface of the coatings is extremely low, particularly with regard to transition metal elements such as Fe, Ni, Mn and Cr. Examples of such applications include semi-conductor processing equipment, space and satellite systems, and medical equipment.

[0003] Further details of various PEO coating processes and equipment may be found in the present Applicant’s earlier patent applications, including but not limited to WO99/31303, W003/083181 , WO2010/112914, W02015/008064 and WO2017/216577, the full contents of which are herein incorporated by reference.

[0004] A challenge is that secondary metals on the surface of PEO coatings can not only derive from external sources (e.g. the electrolyte, counter electrodes, or contact with other bodies) but can also derive from the substrate alloy itself. Applications such as those referred to above typically will use alloys containing significant quantities of alloying elements. For example one of the most widely used aluminium alloys is Al 6061 which contains up to 0.7% Cu, up to 0.4% Cu and up to 0.4% Cr in addition to other alloying elements. Since the PEO process proceeds at least partially via conversion of the substrate, these elements will tend to be found in the PEO coating.

[0005] The present Applicant has additionally found that the high energies of PEO processes can lead to a redistribution of the elements in the coating and that in some cases this can lead to a concentration of secondary metals at or near to the surface while depleting their concentration nearer the substrate. This represents a problem in the light of the requirement for low surface concentrations of these metals at the surface as described above. [0006] One option is to utilise alloys with lower concentrations of alloying elements. For example Al 6061 could be replaced by Al 6063, which has significantly lower amounts of copper. However, changing alloys is often not a feasible option due to other constraints such as mechanical performance, or due to pre-existing designs that cannot be changed.

[0007] Rogov, A.B.; “Plasma electrolytic oxidation of A1050 aluminium alloy in homogeneous silicate-alkaline electrolytes with edta⁴' complexes of Fe, Co, Ni, Cu, La and Ba under alternating polarization conditions”; Materials Chemistry and Physics 167 (2015) pp 136-144; Elsevier discloses bipolar PEO processes in which EDTA is added to the electrolyte in order to promote electrolyte homogeneity, and in particular to reduce the tendency of transition metals to form insoluble hydroxide precipitates in alkaline solution. In the Conclusions section, Rogov states “It is shown that metals from EDTA complexes during PEO accumulates in upper coating layer”, which is undesirable in the context of the present application. In Rogov, large quantities of secondary metals are specifically added to the electrolyte, with the EDTA chelating agent only being in a slight excess (page 137). Therefore any disruption or decomposition of the chelating agent (e.g. during PEO discharge) will lead to precipitation of the metals (as described on page 143, right hand column, item 6). In general terms, Rogov teaches that a high concentration of metals is held in solution by the chelating agent, and the metals are “pushed” into the coating.

BRIEF SUMMARY OF THE DISCLOSURE

[0008] Viewed from a first aspect, there is provided a plasma electrolytic oxidation process for generating a ceramic coating on a surface of a workpiece made of an alloy comprising a base metal and one or more secondary metals, wherein a series of electrical current pulses are applied to the metallic workpiece in an electrolyte so as to generate plasma discharges at the surface of the metallic workpiece thereby to form the ceramic coating, and wherein the electrolyte comprises at least one chelating agent selected to bind with ions of at least one of the one or more secondary metals that are released from the workpiece during the plasma electrolytic oxidation process, thereby to prevent or hinder a concentration of secondary metal compounds at an outer surface of the ceramic coating.

[0009] In the process of the present application, it will be understood that the secondary metals only come from the workpiece substrate. Since there is a vast excess of chelating agent relative to the amount of potential ejected metals, the ejected secondary metals (or at least a high proportion of them) are held in solution in the process of the present application. It may be considered that a large excess of uncomplexed chelating agent is used to “suck” the metals from the substrate, which is complete contrast to the process disclosed in Rogov above. [0010] The at least one chelating agent may preferably be at least one of: ethylenediamine tetramine (EDTA), nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, succinates, citrates, hydroxy-ethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA). Alternatively or in addition, the at least one chelating agent may be at least one of: citric acid, acetic acid, oxalic acid, salicylic acid, ethylenediamine, triethylenetetramine, nitriloacetic acid, gluconic acid, 2,3- dimercapto-1 -propanesulfonic acid (DPMS), thiamine tetrahydrofurfuryl disulfide (TTFD), and 2,3-dimercaptosuccinic acid (DMSA).

[0011] The at least one chelating agent may be present in the electrolyte at a concentration of 0.1 to 2.0 grammes per litre at room temperature and pressure (25°C, 100kPa) prior to the start of the PEO processing.

[0012] The electrolyte may have a pH of 10 to 14 at room temperature and pressure (25°C, 100kPa) prior to the start of the PEO processing.

[0013] The electrical current pulses may have a bipolar waveform with an anodic voltage of 400 to 700V and a cathodic voltage of 50 to 300V.

[0014] The base metal may be at least one of: aluminium, magnesium, titanium, zirconium, niobium, and tantalum.

[0015] The one or more secondary metals may be at least one of: copper, iron, chromium, manganese, zinc and nickel. The one or more secondary metals may be transition metals.

[0016] The at least one chelating agent may be selected to sequester sufficient of the one or more secondary metals such that an amount of the one or more secondary metals in a topmost 5pm layer of the outer surface of the ceramic coating is less than 1 atomic percent. In some embodiments, the amount of the one or more secondary metals in a topmost 5pm layer of the outer surface of the ceramic coating is less than 0.5 atomic percent. In some embodiments, the amount of the one or more secondary metals in a topmost 5pm layer of the outer surface of the ceramic coating is less than 0.1 atomic percent.

[0017] Viewed from a second aspect, there is provided an electrolyte for use in a plasma electrolytic oxidation process for generating a ceramic coating on a surface of a workpiece made of an alloy comprising a base metal and one or more secondary metals, wherein a series of electrical current pulses are applied to the metallic workpiece in an electrolyte so as to generate plasma discharges at the surface of the metallic workpiece thereby to form the ceramic coating, wherein the electrolyte comprises at least one chelating agent selected to bind with ions of at least one of the one or more secondary metals that are released from the workpiece during the plasma electrolytic oxidation process, thereby to prevent or hinder a concentration of secondary metal compounds at an outer surface of the ceramic coating.

[0018] The at least one chelating agent may preferably be at least one of: ethylenediamine tetramine (EDTA), nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, succinates, citrates, hydroxy-ethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA). Alternatively or in addition, the at least one chelating agent may be at least one of: citric acid, acetic acid, oxalic acid, salicylic acid, ethylenediamine, triethylenetetramine, nitriloacetic acid, gluconic acid, 2,3- dimercapto-1 -propanesulfonic acid (DPMS), thiamine tetrahydrofurfuryl disulfide (TTFD), and 2,3-dimercaptosuccinic acid (DMSA).

[0019] The at least one chelating agent may be present in the electrolyte at a concentration of 0.1 to 2.0 grammes per litre at room temperature and pressure (25°C, 100kPa) prior to the start of the PEO processing.

[0020] The electrolyte may have a pH of 10 to 14 at room temperature and pressure (25°C, 100kPa) prior to the start of the PEO processing.

[0021] Viewed from a third aspect, there is provided a plasma electrolytic oxidation ceramic coating on a surface of a workpiece made of an alloy comprising a base metal and one or more secondary metals, wherein an amount of the one or more secondary metals in a topmost 5pm layer of the outer surface of the ceramic coating is less than 1 atomic percent.

[0022] The amount of the one or more secondary metals in the topmost 5pm layer of the outer surface of the ceramic coating may be less than 0.5 atomic percent, or may be less than 0.1 atomic percent.

[0023] Chelating agents are chemical substances that bind strongly to ionic species in solution. They are typically multi-dentate organic species containing several functional groups with lone pairs of electrons or ionic sites. Examples include ethylenediamine tetramine (EDTA), nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, succinates, citrates, hydroxy-ethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA). Further examples include citric acid, acetic acid, oxalic acid, salicylic acid, ethylenediamine, triethylenetetramine, nitriloacetic acid, gluconic acid, 2, 3-di mercapto- 1 -propanesulfonic acid (DPMS), thiamine tetrahydrofurfuryl disulfide (TTFD), and 2,3-dimercaptosuccinic acid (DMSA). The present Applicant has surprisingly found that by using selected chelating agents in a PEO electrolyte, secondary metal species ejected during the PEO process can be sequestered into the electrolyte. The tightly-bound metals are not then able to be incorporated into the coating with a result that the concentration of secondary metals in the oxide coating is significantly reduced.

[0024] Plasma electrolytic oxidation is usually performed in an aqueous electrolyte, which typically consists of a dilute alkaline solution acting as a source of hydroxide ions. The appropriate chelating agent is dissolved in this solution and the pH of the electrolyte may be adjusted to an appropriate range.

[0025] Additional materials may be added to the electrolyte and may subsequently be incorporated into the coating to control the coating composition and microstructure.

[0026] The plasma electrolytical oxidation process proceeds via the application of a series of voltage pulses to the metal substrate. The pulses may be unipolar (positive only) or bipolar (alternating positive and negative voltages). The pulses may be applied in galvanostatic conditions (constant current) or potentiostatic conditions (constant voltage). By controlling the anodic and cathodic voltage and currents applied, and/or the duration of the pulses and/or their waveforms, it is possible to control the composition and microstructure of the resulting ceramic coating.

[0027] The base metals of the alloys suitable for PEO coating include aluminium, magnesium, titanium, zirconium, niobium and tantalum. Secondary metals include both minor alloying elements contained in the substrates, or metals derived from other elements of the plasma electrolytic process such as counter electrodes. Examples of such secondary metals include iron, copper, chromium, manganese, zinc and nickel. More broadly, examples of such secondary metals include or consist of transition metal elements.

[0028] Additional processes may be combined with the use of chelating agents to derive coatings with even lower surface concentrations of secondary elements. Examples of such processes include the use of alloys with inherently low concentrations of such elements such as Al 6063, or polishing post-coating to remove the top layers of the as- coated material.

[0029] Coatings formed by plasma electrolytic oxidation processes are typically principally the oxides of the substrate metal if pure, or oxides of the base or principal metal if an alloy. For example, coatings on 6000 series aluminium are typically >90% alpha and gamma phases of aluminium oxides. However, in some applications there may be significant quantities of other materials such as silicates and phosphates. Minor phases include the oxides, silicates, and phosphates of secondary alloying elements, mixed oxide phases, partially oxidised secondary elements, or elemental forms of the secondary elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

FIGURE 1 shows an exemplary bipolar pulse form;

FIGURE 2 shows an exemplary apparatus suitable for performing the plasma electrolytic oxidation process;

FIGURE 3 shows the weight percentages of Cu, Mg and Zn recorded on the Reference and Chelating articles described in Example 1 measured using Energy Dispersive Spectroscopy; and

FIGURE 4 shows the weight percentages of Fe, Cu, Cr, and Mn recorded on the Reference and Chelating articles described in Example 2 measured using Energy Dispersive Spectroscopy.

DETAILED DESCRIPTION

[0031] In it well known in the art that PEG processes can be used to convert the surface of a number of metals, principally aluminium, magnesium and titanium, but also metals such as zirconium, tantalum and niobium, to a ceramic coating. The coatings are usually principally oxides but depending on the composition of the electrolyte, other species such as silicates and phosphates can be present in appreciable quantities.

[0032] The coatings may be characterised by a number of advantageous properties including exceptional adhesion, high hardness, high thermal and chemical stability, corrosion resistance, and high dielectric strength. The coatings are widely utilised for applications such as wear protection, corrosion protection, and inert barrier coatings in semi-conductor processing equipment. Porosity may be evident in the coatings which enables other applications such as photocatalysis, chemical catalysis, and osseointegration.

[0033] Coatings formed by plasma electrolytic oxidation processes are typically principally the oxides of the substrate metal if pure, or oxides of the base or principal metal if an alloy. For example, coatings on 6000 series aluminium are typically >90% alpha and gamma phases of aluminium oxides. However, in some applications there may be significant quantities of other materials such as silicates and phosphates. Minor phases include the oxides, silicates, and phosphates of secondary alloying elements, mixed oxide phases, partially oxidised secondary elements, or elemental forms of the secondary elements. [0034] When the PEO process is applied to alloys, compounds (e.g. oxides) of the principle element in the alloy will predominate. However, the coatings will also contain compounds of secondary elements in the alloy. For example, Al 6061 contains up to 1.2% Mg, up to 0.7% Cu, up to 0.4% Cu and up to 0.4% Cr in additional to minor other alloying elements, with the remainder (>90%) being aluminium. When such an alloy is subjected to PEO in a dilute alkaline electrolyte, the coating is predominantly (>90%) crystalline aluminium oxide (mixture of alpha and gamma phases). However, the coatings will also contain small quantities of copper, nickel, chromium, zinc, manganese and/or magnesium compounds such as oxides, and hydroxides. Under some conditions, reduced phases such as lower oxides or even elemental forms of these metals may also be formed.

[0035] The PEO process proceeds via the formation of energetic discharges which can produce local temperatures of >10,000°C. These discharges form bubbles of vaporised material. Subsequently, rapid collapse of the discharge bubble occurs due to the presence of cold electrolyte surrounding the workpiece, causing condensation of metal compounds, ceramic phases and other chemical species. These processes occur on a rapid timescale, with bubble formation and collapse happening over a period of microseconds. A large number of thermochemical reactions take place during this process, including vaporisation of the substrate, oxidation of the substrate, melting of previously-formed coating, dissolution of species into the electrolyte, condensation of metal oxides, hydroxides and other species, and incorporation of materials from the electrolyte. When a bipolar waveform is applied, additional processes can occur during the cathodic half-cycle, including partial or complete reduction of chemical species in the electrolyte or the coating, and precipitation of further phases from the electrolyte.

[0036] The present Applicant has surprisingly found that the complex thermochemical processes happening during these processes can cause a redistribution of species across the coating. This can cause the concentration of secondary elements near the surface to be higher than in the base alloy. For example, for a 40 micron coating formed on Al 6061 in a hydroxide electrolyte, it has been found that copper-containing species form >1 atomic percent of the top 5pm of the ceramic coating versus an atomic percentage of 0.4% in the base alloy, while the lower layers are relatively depleted.

[0037] For some applications, the presence of these secondary phases, especially near the surface of the coatings, can be disadvantageous. For example, in medical applications, alloying elements could cause a cytotoxic response, while in semiconductor processing applications using ionised plasma, compounds of secondary elements could cause increased recombination of reactive radicals decreasing the effectiveness of processing units. For space, satellite and other applications where the thermo-optical properties of the materials are important, secondary elements can cause a darkening of the colour of coating and a negative impact on the absorptivity and emissivity properties.

[0038] In the present disclosure, coatings containing greatly reduced quantities of compounds of secondary alloying elements are produced using electrolytes containing chelating agents (chemical species which bind strongly to metal cations). In some embodiments, the coatings contain <1, <0.5, and in some cases <0.1 atomic percent of the secondary alloying elements in the top 5pm of coating as compared to >1% in equivalent cases where chelating agents are not used.

[0039] A wide variety of chelating agents can be used. In general, chelating agents are multi-dentate molecules containing several groups which form chemical bonds to metal ions. These groups can be ionic groups or polar groups containing lone pairs of electrons. Examples of chelating agents which can be used include ethylenediamine tetramine (EDTA), nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, succinates, citrates, hydroxy-ethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA). Further examples of chelating agents which can be used include citric acid, acetic acid, oxalic acid, salicylic acid, ethylenediamine, triethylenetetramine, nitriloacetic acid, gluconic acid, 2, 3-di mercapto- 1 -propanesulfonic acid (DPMS), thiamine tetrahydrofurfuryl disulfide (TTFD), and 2,3-dimercaptosuccinic acid (DMSA). Preferred examples of chelating agents include ethylenediamine tetramine (EDTA), hydroxy-ethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA). The chelating agents can be used in a large range of concentrations. However, preferred embodiments use the chelating agents in the range of 0.1 -2.0 g/L at room temperature and pressure (25°C, 100kPa) prior to the start of the PEO processing, optionally in the range of 0.1 to 1.0 g/L at room temperature and pressure (25°C, 100kPa) prior to the start of the PEO processing.

[0040] It will be appreciated by those skilled in the art that the sequestering ability (binding constant) of chelating agents for particular metal ions depends on pH, temperature, and the other constituents of the solutions. While local temperatures in the immediate vicinity of the plasma discharges can be in excess of 10,000°C, the bulk temperature of the electrolyte is typically in a range of 5°C to 50°C, or in a range of 10°C to 25°C. A further aspect of the present disclosure is therefore the selection of chelating agents which can effectively sequester target metal ions such as Fe, Cu, Ni, Mn, Zn and Cr under the conditions of typical PEO electrolytes which are often high pH (11-14). For example, the pH of the electrolyte will affect the protonation or deprotonation of chelating ligands such as amine and/or carboxylic acid groups. In turn, this affects the way that the chelating agent coordinates to a metal and the stability of the resulting complex. In the case of EDTA, it has been suggested (http://www.chm.bris.ac.uk/motm/edta/edtah.htm) that the most stable complex of EDTA and Fe is a 6-coordinate complex, where the EDTA molecule forms only five bonds to the Fe³⁺ ion, a molecule of H2O forms the sixth bond, and the non-coordinated carboxylic acid group is protonated. At high pH, the carboxylic acid group will be deprotonated, and this can destabilise the chelate complex. Accordingly, preferred chelating agents for high pH PEO processes are those that form stable complexes with the target metals when fully deprotonated.

[0041] Other components of the electrolyte include those that are typically used in PEO processes. In some embodiments, alkaline electrolytes are formulated via addition of metal hydroxides, such as sodium and/or potassium hydroxide. In other embodiments, acidic electrolytes are formulated by adding acids such as phosphoric or citric acid. In a preferred embodiment potassium hydroxide is used at concentrations of 0.5-3g/L.

[0042] Other components can optionally be added to the electrolyte to modify the chemical composition and microstructure. For example, metal silicates and/or phosphates can be added to the electrolyte which may lead to the incorporation of silicates or phosphates in the coating. It will be appreciated by those skilled in the art that the range of possible electrolytes that can used in PEO is very large, and the current disclosure is without limit in terms of the other components of the electrolyte that can be used together with the chelating agent.

[0043] It is well known to those skilled in the art that the composition and microstructure of coatings formed depends on the processing conditions in addition to the nature of the electrolyte. In particular, the nature of the applied waveform is of critical importance. Parameters such as applied voltage (anodic and cathodic), current density (anodic and cathodic), the duration of the pulses, frequency, and shape of the waveform are all important parameters. In general, the present disclosure is without limit with regard to the applied waveform. However, it is particularly applicable to bipolar waveforms in which the anodic voltages (and hence currents) can be controlled as separate parameters.

[0044] A wide variety of substrates can be used including aluminium, magnesium, titanium, zirconium, niobium and tantalum or alloys thereof. However, commercial applications of PEO are dominated by aluminium, magnesium and/or titanium due to the attractive material properties of both the substrate and coatings formed on them. Coatings on aluminium are particularly widely used due to the low cost and wide use of aluminium and the very high hardness and dielectric strength of aluminium oxide (particularly the alpha form, often called corundum or sapphire).

[0045] Figure 1 shows an exemplary time dependence of the form of the current pulses (positive and negative) passing in a circuit between the supply source and an electrolytic bath. Each current pulse has a steep front, so that the maximum amplitude is reached in not more than 10% of the total pulse duration, and the current then falls sharply, after which it gradually decreases to 50% or less of the maximum.

[0046] Figure 2 shows an exemplary apparatus suitable for performing the plasma electrolytic oxidation process of the present application. The apparatus comprises an electrolytic bath 1 and a power supply source 12, connected to each other by electrical busbars 15, 16.

[0047] The electrolytic bath 1 comprises a stainless steel vessel 2, containing an electrolyte 3 and at least one workpiece 4 immersed in the electrolyte 3. The electrolytic bath 1 is equipped with a transfer pump 5 and a filter 6 for coarse cleaning of the electrolyte 3.

[0048] An agitator 7 is fitted in the lower part of the vessel 2. A regulating valve 8 and a pressure gauge 9 are provided to regulate the pressure of the electrolyte 3 supplied to an input to the agitator 7. A valve 10 is provided to regulate the flow rate of the air going to the agitator 7. The electrolyte circulating system includes a heat exchanger or cooler 11 to maintain the required temperature of the electrolyte 3 in the course of the plasma electrolytic oxidation process.

[0049] The power supply source 12 may comprise a three-phase pulse generator 13 fitted with a microprocessor 14 to control the electrical parameters of the plasma electrolytic oxidation process.

[0050] During processing, the metallic workpiece 4 is immersed in a continuously agitated, re-circulated, cooled electrolyte 3, and an electrical connection is made to one output terminal of the power supply 12, with the vessel 2 acting as an inert (e.g. 316 stainless steel) counter electrode connected to the other output terminal.

EXAMPLES

Example 1

[0051] A workpiece article (hereafter in this example referred to as the “Reference” article) of aluminium 7075 alloy containing nominally 88% Al, 5.5% Zn, 2.5% Mg, 1.3% Cu (balance other minor alloying elements) was placed in an electrolyte bath containing an alkaline electrolyte comprising 1g/litre of potassium hydroxide. Bipolar pulses of 530V anodic and 250V cathodic with pulse durations of 500ps on both the anodic and cathodic parts of the cycle were applied to the part and the coating process continued until the coating thickness reached 40pm. An identical article (hereafter in this example referred to as the “Chelating” article) was placed in the same electrolyte save for the addition of 1 g/litre of diethylenetriaminepentaacetic acid and the same bipolar pulses applied until the coating reached 40pm.

[0052] X-ray diffraction was performed in the Bragg-Brentano geometry and showed that both coatings consisted primarily of gamma phase alumina, with small amounts of amorphous phases.

[0053] Energy Dispersive Spectroscopy was performed using an Oxford Microanalysis Group Energy 5431 unit attached to a Leo 14455VP Scanning Electron Microscope to determine the weight percentages of minor elements in the coating surface for both the Reference and Chelating articles. The resulting data, shown in Figure 3, shows that the Reference article contained approximately 0.59wt% Cu, 2.15wt% Mg, and 1.25wt% Zn at the coating surface (all other metals levels were <0.2wt%). These percentages are in the approximate proportions seen in the substrate alloy, although some reduction in the relative proportions of Cu and Zn is seen, likely due to ejection of these elements to the electrolyte. The data for the Chelating article showed that the coating surface contained approximately 0.2wt% Cu, 1.3wt% Mg, and 1.1 wt% Zn showing the effect of the chelating agent for reducing surface concentrations of secondary metals.

[0054] The lower quantity of secondary metals at the surface was reflected in the colour of the samples which was noticeably lighter for the Chelating article in comparison to the Reference article. Lightness in the CIE L*a*b* colour space was measured using a Konica Minolta spectrometer and found to be L*=40.8 for the Reference sample and L*=48.7 for the Chelating sample.

[0055] The dielectric breakdown voltage (DBV) for the parts were measured using Megger MIT520 insulation tester. This showed a somewhat higher value for the Chelating article, possibly due to the lower quantities of lower dielectric strength oxides. A DBV of 1210 ± 150V was recorded for the Reference article versus 1580 ± 210V for the Chelating article.

Example 2

[0056] A workpiece article (hereafter referred to as the “Reference” article) of aluminium 6061 alloy containing nominally 97% Al, 1% Mg, 0.5% Si, 0.4% Fe, 0.3% Cu, 0.2% Cr, 0.15% Mn (balance other minor alloying elements) was placed in an electrolyte bath containing an alkaline electrolyte comprising 1 g/litre of potassium hydroxide. Bipolar pulses of 510V anodic and 230V cathodic with pulse durations of 500ps on both the anodic and cathodic parts of the cycle were applied to the workpiece article and the coating process continued until the coating thickness reached 40pm. An identical workpiece article (hereafter referred to as the “Chelating” article) was placed in an electrolyte identical to that used for the Reference article, except that 0.5g/litre of hydroxyethylethylenediaminetriacetic acid (HEDTA) was added as a chelating agent. The same voltage applied until the coating thickness reached 40pm.

[0057] X-ray diffraction was performed in the Bragg-Brentano geometry, and this showed that both coatings consisted primarily of gamma phase alumina, with smaller amounts of alpha alumina and amorphous phases.

[0058] Energy Dispersive Spectroscopy was performed using an Oxford Microanalysis Group Energy 5431 unit attached to a Leo 14455VP Scanning Electron Microscope to determine the weight percentages of minor elements in the coating surface for both the Reference and Chelating articles. The resulting data, shown in Figure 4, shows that the Reference article contained approximately 0.26wt% Fe, 0.44wt% Cu, 0.12wt% Cr, and 0.13wt% Mn at the coating surface. These percentages are in the approximate proportions seen in the workpiece substrate alloy, although there is some enrichment of copper near the surface. In contrast, the data for the Chelating article showed that the coating surface contained approximately 0.14wt% Fe, 0.11wt% Cu, 0.08wt% Cr, and 0.07wt%Mn showing the effect of the chelating agent for reducing surface concentrations of secondary metals.

[0059] The lower quantity of secondary metals at the surface was reflected in the colour of the samples which was noticeably lighter for the Chelating article in comparison to the Reference article. Lightness in the CIE L*a*b* colour space measured using a Konica Minolta spectrometer and found to be L*=55.5 for the Reference sample and L*=63.4 for the Chelating sample.

[0060] The dielectric breakdown voltage (DBV) for the workpiece articles were measured using a Megger MIT520 insulation tester. This showed no significant difference between the DBV of the two articles. A DBV of 2280 ± 170V was recorded for the Reference article versus 2210 ± 230V for the Chelating article.

[0061] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. [0062] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0063] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS:

1. A plasma electrolytic oxidation process for generating a ceramic coating on a surface of a workpiece made of an alloy comprising a base metal and one or more secondary metals, wherein a series of electrical current pulses are applied to the metallic workpiece in an electrolyte so as to generate plasma discharges at the surface of the metallic workpiece thereby to form the ceramic coating, and wherein the electrolyte comprises at least one chelating agent selected to bind with ions of at least one of the one or more secondary metals that are released from the workpiece during the plasma electrolytic oxidation process, thereby to prevent or hinder a concentration of secondary metal compounds at an outer surface of the ceramic coating.

2. A process according to claim 1 , wherein the at least one chelating agent is selected from a group consisting of: ethylenediamine tetramine (EDTA), nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, succinates, citrates, hydroxyethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA).

3. A process according to claim 1 , wherein the at least one chelating agent is selected from a group consisting of: ethylenediamine tetramine (EDTA), nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, succinates, citrates, hydroxyethylethylenediaminetriacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA), citric acid, acetic acid, oxalic acid, salicylic acid, ethylenediamine, triethylenetetramine, nitriloacetic acid, gluconic acid, 2,3-dimercapto-1-propanesulfonic acid (DPMS), thiamine tetra hydrofurfuryl disulfide (TTFD), and 2,3-dimercaptosuccinic acid (DMSA).

4. A process according to any preceding claim, wherein the at least one chelating agent is present in the electrolyte at a concentration of 0.1 to 2.0 grammes per litre at room temperature and pressure prior to the start of the PEO processing.

5. A process according to any preceding claim, wherein the electrolyte has a pH of 10 to 14 at room temperature and pressure prior to the start of the PEO processing.

6. A process according to any preceding claim, wherein the electrical current pulses have a bipolar waveform with an anodic voltage of 400 to 700V and a cathodic voltage of 50 to 300V.

7. A process according to any preceding claim, wherein the base metal is selected from a group consisting of: aluminium, magnesium, titanium, zirconium, niobium, and tantalum.

8. A process according to any preceding claim, wherein the one or more secondary metals are transition metals.

9. A process according to any preceding claim, wherein the one or more secondary metals are selected from a group consisting of: copper, iron, chromium, manganese, zinc and nickel.

10. A process according to any preceding claim, wherein the at least one chelating agent sequesters sufficient of the one or more secondary metals such that an amount of the one or more secondary metals in a topmost 5pm layer of the outer surface of the ceramic coating is less than 1 atomic percent.

11. A process according to any one of claims 1 to 9, wherein the at least one chelating agent sequesters sufficient of the one or more secondary metals such that an amount of the one or more secondary metals in a topmost 5pm layer of the outer surface of the ceramic coating is less than 0.5 atomic percent.

12. A process according to any one of claims 1 to 9, wherein the at least one chelating agent sequesters sufficient of the one or more secondary metals such that an amount of the one or more secondary metals in a topmost 5pm layer of the outer surface of the ceramic coating is less than 0.1 atomic percent.

13. An electrolyte for use in a plasma electrolytic oxidation process for generating a ceramic coating on a surface of a workpiece made of an alloy comprising a base metal and one or more secondary metals, wherein a series of electrical current pulses are applied to the metallic workpiece in an electrolyte so as to generate plasma discharges at the surface of the metallic workpiece thereby to form the ceramic coating, wherein the electrolyte comprises at least one chelating agent selected to bind with ions of at least one of the one or more secondary metals that are released from the workpiece during the plasma electrolytic oxidation process, thereby to prevent or hinder a concentration of secondary metal compounds at an outer surface of the ceramic coating. 16

14. An electrolyte as claimed in claim 13, wherein the at least one chelating agent is selected from a group consisting of: ethylenediamine tetramine (EDTA), nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, succinates, citrates, hydroxyethylethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA).

15. An electrolyte as claimed in claim 13, wherein the at least one chelating agent is selected from a group consisting of: ethylenediamine tetramine (EDTA), nitrilotriacetic acid (NTA), iminodisuccinic acid (IDS), polyaspartic acid, succinates, citrates, hydroxyethylethylenediaminetriacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA), citric acid, acetic acid, oxalic acid, salicylic acid, ethylenediamine, triethylenetetramine, nitriloacetic acid, gluconic acid, 2,3-dimercapto-1-propanesulfonic acid (DPMS), thiamine tetrahydrofurfuryl disulfide (TTFD), and 2,3-dimercaptosuccinic acid (DMSA).

16. An electrolyte as claimed in any one of claims 13 to 15, wherein the at least one chelating agent is present in the electrolyte at a concentration of 0.1 to 2.0 grammes per litre at room temperature and pressure prior to the start of the PEO processing.

17. An electrolyte as claimed in any one of claims 13 to 16, wherein the electrolyte has a pH of 10 to 14 at room temperature and pressure prior to the start of the PEO processing.

18. A plasma electrolytic oxidation ceramic coating on a surface of a workpiece made of an alloy comprising a base metal and one or more secondary metals, wherein an amount of the one or more secondary metals in a topmost 5pm layer of the outer surface of the ceramic coating is less than 1 atomic percent.

19. A ceramic coating as claimed in claim 18, wherein the amount of the one or more secondary metals in the topmost 5pm layer of the outer surface of the ceramic coating is less than 0.5 atomic percent.

20. A ceramic coating as claimed in claim 18, wherein the amount of the one or more secondary metals in the topmost 5pm layer of the outer surface of the ceramic coating is less than 0.1 atomic percent.