Surface Coatings For Metal Objects Aluminium Alloys

Abstract

An electrolyte composition is disclosed for use in forming a protective coating on a metallic specimen by way of plasma electrolytic deposition, comprising an aqueous solution containing a charge carrier component together with an effective amount of carbamide, melamine, ammonium nitrate and / or similar nitrogen source. A system and method for applying a protective coating on a metallic specimen utilises a controllable power supply adapted to apply, for a predetermined period of time, a pulsed DC voltage of selected frequency and power characteristics sufficient to generate plasma electrolytic deposition on the surface of the object while the object is, in use, coupled to the power supply as the anode and immersed in the electrolyte solution. Advantageously, a sodium metaphosphate, such as sodium hexametaphosphate, can be used as the charge carrier component, wherein the electrolyte solution has a substantially neutral pH.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to surface coatings for oxide-forming alloys, together with processes and compositions and systems for creating such surface coatings.BACKGROUND

[0002] Lightweight oxide-forming metal alloys are very useful and versatile materials that can be used to fabricate a wide variety of items. Such materials include alloys of aluminium, magnesium, titanium, brass and zinc, as well as other valve metals, including zirconium, niobium and tantalum. Depending on the item and its circumstances of use, oftentimes, it is desirable to provide the surface with a protective layer against corrosion and the like.

[0003] Various techniques have already been adopted to fabricate corrosion-protective layers on metallic components, including line-of-sight processes such as painting, powder coating and flame spray technologies, which attach extra material on top of the substrate, forming protective layers.

[0004] Electrochemical techniques, however, use electrolytic reactions to generate a protective oxide surface layer from the substrate material itself, meaning that surface modifications can occur wherever the substrate material is in contact with the process electrolyte. Thus, such processes are particularly well suited for components with complex internal geometries, such as heat exchangers. Electrochemical processes include anodising and plasma electrolytic deposition (PED). Both processes operate on the same principle: using the aluminium (or other oxide-forming alloys) component as an anode in an electrolyte bath with electrical current passing through the circuit via a cathode. The processes differ in the electrolyte composition and concentration, as well as the electrical parameters, where PED processes tend to use a significantly higher voltage than anodising processes. A key feature of the PED process that differs from anodising is the occurrence of plasma discharging at the metal-coating interface when employing high potentials. When the applied potential exceeds a certain critical breakdown point, a number of discrete short-lived micro-discharges will appear and will move across the metal surface to form a surface film. This process can be used to grow ceramic coatings on metal substrates. Because these surface coatings can provide high hardness and a continuous barrier, they can offer protection against wear, corrosion or heat, as well as electrical insulation.

[0005] Conventionally, the surface coating generated by plasma electrolytic deposition techniques is actually a chemical conversion of the substrate metal into its oxide (plasma electrolytic oxidation, or PEO). During the micro-arcing process, the oxides grow both inwards and outwards from the original metal surface. Because of this conversion process, the coatings have strong adhesion to the substrate metal, compared to conventional deposited coatings. Through adjusting electrolyte composition, the metal surface can be saturated by non-metallic elements such as O, C, N, B and a combination of these. These elements can form a vapour envelope along the metallic substrate surface and diffuse inward to the metal in a PED process. The diffusing species are the chemicals that can be negatively ionised in the electrolyte. The plasma will vaporise these species to form a vapour envelope. The electric field applied by PED will accelerate across the voltage drop to bombard the substrate surface through interstitial and grain-boundary diffusion.

[0006] Modifying the PEO electrolyte to include a significant nitrogen source can result in a significant nitride surface coating. Such a process is known as plasma electrolytic nitriding, or PEN. Compared to PEO, the resulting nitride surface coating from the PEN process has higher density, lower porosity, improved mechanical properties, improved adhesion to sharp geometric features, better thermal conductivity and improved corrosion resistance properties similar to other coated layers.

[0007] Traditional line-of-sight coatings such as painting, powder coating and flame spray technologies are limited in that only areas within direct sight of the applicator can be coated, meaning internal surfaces cannot be treated. Issues with electrochemical anodisation include poor corner coating followed by crack formation, low hardness (relative to PED) and inability to create continuous, dense coatings on common high-silicon aluminium alloys due to the insulating silicon interfering with anodic layer formation.

[0008] Pain points of known electrolytic protective coating techniques include high costs due to multi-step process requirements and / or low-density coatings, resulting in less than optimised corrosion protection. Additionally, the coating of high-silicon aluminium alloys has not been widely reported. Moreover, known processes tend to use harmful electrolytes that are often toxic and / or corrosive, such as potassium hydroxide.

[0009] Within this context, there is a need for improved plasma electrolytic deposition techniques and compositions that can overcome or avoid one or more of the limitations of known processes, or can at least provide the public with a useful choice.SUMMARY OF THE INVENTION

[0010] In accordance with one aspect of the present invention, there is provided an electrolyte composition comprising an aqueous solution containing a charge carrier component together with an effective amount of carbamide, melamine, ammonium nitrate and / or similar nitrogen source for use in forming a protective coating on a metallic specimen by way of plasma electrolytic deposition.

[0011] The charge carrier component is preferably substantially PH neutral. In embodiments of the invention, the charge carrier component comprises a sodium metaphosphate. For example, the charge carrier component may comprise sodium hexametaphosphate. In embodiments, the electrolyte composition may include up to about 5 wt. % of sodium hexametaphosphate.

[0012] In embodiments of the invention, the electrolyte composition may include up to about 40 wt. % carbamide.

[0013] In embodiments of the invention, the electrolyte composition may include up to about 5 wt. % melamine.

[0014] In embodiments of the invention, the electrolyte composition may include up to about 20 wt. % glycerol, acting as an electrolyte conductivity modulator.

[0015] In embodiments of the invention, the electrolyte composition solution has a substantially neutral pH level.

[0016] One particular embodiment of the invention provides an electrolyte composition for plasma electrolytic deposition comprising an aqueous solution having about 1 wt. % sodium hexametaphosphate and about 30 wt. % carbamide.

[0017] One particular embodiment of the invention provides an electrolyte composition for plasma electrolytic deposition comprising an aqueous solution having about 1 wt. % sodium hexametaphosphate and about 3 wt. % melamine.

[0018] One particular embodiment of the invention provides an electrolyte composition for plasma electrolytic deposition comprising an aqueous solution having about 1 wt. % sodium hexametaphosphate, about 40 wt. % carbamide and about 20 wt. % glycerol.

[0019] The present invention also provides a method for forming a protective coating on a metallic object, comprising:

[0020] immersing the object in an electrolyte solution according to a composition defined above;

[0021] electrically coupling the object as the anode in a power supply circuit that also includes a cathode in contact with the electrolyte solution;

[0022] controlling the power supply circuit to apply, for a predetermined period of time, a pulsed DC voltage of selected frequency and power characteristics sufficient to generate plasma electrolytic deposition on the surface of the object.

[0023] In embodiments, the method may include inducing relative movement between the object and the electrolyte solution during immersion to ensure that all surfaces of the object to be coated are in contact with the electrolyte solution during the plasma electrolytic deposition operation. Relative movement between the object and the electrolyte solution may be provided by mechanical agitation of the electrolyte solution or specimen, or both. The method may include inducing flow of the electrolyte solution to ensure that the electrolyte sufficiently wets all surfaces of the object, and all surfaces are continually replenished with electrolyte during the plasma electrolytic deposition operation.

[0024] The method may include determining a surface area of the object and controlling characteristics of the power supply circuit according to the determined surface area.

[0025] In embodiments, the predetermined period of time is selected according to a desired plasma electrolytic deposition layer thickness.

[0026] The present invention also provides a system for forming a protective coating on a metallic object, comprising:

[0027] an electrolyte bath containing an electrolyte solution according to a composition defined above;

[0028] a cathode that is in contact with the electrolyte solution, in use;

[0029] a controllable power supply adapted to apply, for a predetermined period of time, a pulsed DC voltage of selected frequency and power characteristics sufficient to generate plasma electrolytic deposition on the surface of the object while the object is, in use, coupled to the power supply as the anode and immersed in the electrolyte solution.

[0030] The present invention also encompasses a metallic object formed with a protective coating according to the method as defined above and / or utilising the system as defined above.

[0031] Coatings formed using the novel electrolyte composition and electrical operating parameters as disclosed herein have a number of advantages. For instance, embodiments of the present invention provide an ability to grow nitride coatings on high-silicon aluminium alloys. A greater range of alloys to apply coatings to means greater material choices for design engineers. Moreover, PEN coatings formed according to embodiments of the invention exhibit:

[0032] Higher corrosion resistance than PEO coatings;

[0033] Higher thermal conductivity than PEO coatings;

[0034] Higher coating hardness than PEO coatings;

[0035] Greater adhesion to sharp geometric features than PEO coatings.

[0036] The PEN process disclosed herein addresses issues associated with the prior art in the following ways:

[0037] Complex internal or non-line-of-sight regions can be coated as components are submerged in bath with electrolyte contacting all internal and external surfaces.

[0038] Significantly better adhesion to sharp corner features than anodising and higher strength to mitigate crack formation and propagation.

[0039] The micro-discharge process results in the ability to create coatings on high-silicon aluminium alloys.

[0040] Single step process due to the novel composition of the electrolyte and electrical parameters.

[0041] Therefore, our developed PEN-coated parts by using the novel electrolyte and operating parameters could create low-porosity surface coatings on oxide-forming alloys that improve corrosion resistance while maintaining sufficient thermal conductivity for heat transfer applications.

[0042] Compared with prior art processes, electrolytes disclosed herein are vastly more environmentally friendly and non-toxic since a sodium polymetaphosphate is used as a charge carrier. It is also close to neutral, with ~6.5 pH, meaning it is not acidic or basic, and thus, not corrosive.

[0043] Further aspects, features and advantages of the present invention will be apparent to those of ordinary skill in the art from the accompanying description and drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order that the invention may be more fully understood, the following detailed description of particular embodiments is presented, by way of example only and with reference to the accompanying drawings, in which:

[0045] FIG. 1 is a flow-chart diagram outlining a process according to embodiments of the invention for applying a protective surface coating to a specimen;

[0046] FIG. 2 is a schematic diagram of a system for creating a PEN coating on specimen according to embodiments of the invention;

[0047] FIG. 3 is an image of a PEN coated specimen;

[0048] FIG. 4 is a cross-sectional microscopy image showing coating density and thickness (3-4 μm) and attachment to metallic substrate;

[0049] FIG. 5 is a schematic diagram of material phases present through coating thickness;

[0050] FIG. 6 illustrates the thermal diffusivity of AlSi10Mg specimens coated with three variations of PEN coating processes according to embodiments of the invention, versus an uncoated specimen;

[0051] FIG. 7 illustrates Vickers hardness of AlSi10Mg specimens coated with three variations of PEN coating processes according to embodiments of the invention, versus an uncoated specimen;

[0052] FIG. 8 illustrates flat-cell corrosion results (corrosion current density versus corrosion potential) of three coated AlSi10Mg specimens versus the uncoated specimen; and

[0053] FIG. 9 illustrates a representative composition-depth profile of a PEN surface coating according to an embodiment of the invention.DETAILED DESCRIPTION

[0054] Disclosed herein are compositions, processes, apparatus and techniques for applying protective coatings to metal components. The developed coatings can be applied to oxide-forming alloy components that are manufactured by forming, forging, casting, fabricating, machining and / or additive manufacturing (3D-printing).

[0055] Such components may include simple geometries such as brackets or housings, complex parts such as heat exchangers or other parts featuring highly complex geometric features such as lattice structures, honeycombs, gyroids, etc.

[0056] Coated components can be put into service in corrosive environments where their service life would be severely reduced without protective coatings. Such environments may be in marine, energy sector, outdoor plant equipment, industrial processing, etc.

[0057] Corrosion resistance is increased as the nitride layers generated by the PEN process disclosed herein greatly reduces the formation of pitting under the influence of the negative potential and presence of aggressive species such as chloride ions present in corrosive solutions of the prior art. Erosion resistance is also improved as the fabricated ceramic layer can increase surface hardness, improving mechanical protection.

[0058] A process 100 for applying a protective surface coating according to embodiments of the invention is illustrated in the form of a flowchart diagram in FIG. 1, beginning at operation 102. The process can be performed using a system 10 for applying a PEN coating such as that shown schematically in FIG. 2. The system 10 comprises an electrolyte bath in the form of an electrolyte solution 20 in a container 15 suitable for holding the specimen 50 to be coated. A hanger 25 or other suitable means may be provided to support the specimen while submerged in the electrolyte bath. The specimen 50 is electrically connected to the positive polarity output of a controllable electrical power supply 40, while a cathode member 30 is connected to the negative polarity output. The cathode 30 is formed from a suitable corrosion-resistant conductive metal material and, in use, is submerged in the electrolyte solution along with the specimen 50 which comprises the anode. For example, the cathode may comprise a 316L-Grade stainless steel mesh.

[0059] According to an embodiment of the invention, the controllable electrical power supply 40 comprises a direct current (DC) plasma generator with the following capabilities and characteristics:

[0060] Pulsed square wave DC output

[0061] Up to 1000V DC

[0062] Current (Amps) requirement dependant on specimen surface area

[0063] Power output (Watts) dependant on current: Watts=Voltage×Current

[0064] Frequency up to 30 kHz

[0065] Referring again to FIG. 1, since the current required to be delivered by the power supply is dependent on the surface area of the specimen to be coated, operation 104 involves determining the surface area of the specimen 50. This information can be extracted from CAD model data, for example, or may be calculated manually for simpler components.

[0066] Operation 106 entails preparing the electrolyte solution for the electrolyte bath. To create a PEN coating according to embodiments of the invention an electrolyte composition having a source of nitrogen is used. Table 1 below provides a guide to the functional components of an electrolyte composition according to embodiments of the present invention.ElectrolyteConcentrationcomponentsrange (wt. %)Role of each componentCarbamide0-40Nitrogen sourceGlycerol0-20Reducing the electrolyteconductivity to avoid formingtroublesome sparksMelamine0-5 Nitrogen sourceSodium0-5 Creating the conductivehexametaphosphatemedium to transfer electrons

[0067] For example, aqueous solutions may be mixed to provide transparent conductive solutions with the following compositions:ElectrolyteCompositionPW1 wt. % sodium hexametaphosphate (NaPO3)6 +30 wt. % carbamide (H2NCONH2) + deionised water (bal.)PM1 wt. % sodium hexametaphosphate (NaPO3)6 +3.2 wt. % melamine + deionised water (bal.)P201 wt. % sodium hexametaphosphate (NaPO3)6 +20 wt. % glycerol (HOCH2CHOHCH2OH) +40 wt. % carbamide (H2NCONH2) +deionised water (bal.)

[0068] Sodium hexametaphosphate (SHMP) is a form of sodium metaphosphate, in particular a sodium polymetaphosphate, that is a benign substance which has applications in a wide variety of industries, including as a food additive. In the electrolyte compositions disclosed herein the sodium hexametaphosphate acts as charge carrier, facilitating conduction of electrons into the specimens being coated. Sodium metasilicate has also been used for this function in testing, but results indicate the production of a lower quality coating (higher porosity) as compared to SHMP. Testing also indicates that, to some degree, it is possible to reduce the current density required by increasing sodium hexametaphosphate concentration.

[0069] Ammonium nitrate and other similar nitrogen sources can be added to the electrolyte as well. Other nitrogen sources are also possible, but qualities of the coating may vary due to different number of anions and cations in solution due to different chemistry. A different porosity may result as the coating would develop at different applied voltage.

[0070] The inclusion of glycerol in the electrolyte can reduce the incidence of hot-spots (a.k.a. “troublesome sparks”) on the surface of the specimen during coating, by controlling electrolyte conductivity. The result of glycerol addition is increased control of surface porosity via desired small, consistent sparks at the surface.

[0071] An electrolyte with a composition as described above is mixed to provide a transparent conductive solution. Once prepared, the electrolyte solution 20 is added to process container 15. The particular electrolyte composition can affect characteristics of the resulting coating such as thermal diffusivity, corrosion resistance and hardness, which is discussed hereinbelow with reference to FIGS. 6, 7 and 8.

[0072] With the electrolyte bath prepared, the power supply is connected to cathode 30 and the specimen 50 (operation 108), both of which are submerged in the electrolyte solution (operation 110). For components with complex geometries, including voids, closely packed features and / or internal passages such as those of a heat exchanger, relative movement between the electrolyte and coating specimen may be required to achieve optimal coating. This movement may be provided by mechanical agitation of the electrolyte solution or specimen or by way of a flow-inducing system to ensure that the electrolyte sufficiently wets all surfaces of the specimen, and all surfaces are continually replenished with electrolyte during the coating operation.

[0073] Electrical operating parameters are entered into plasma DC generator controller at operation 120, for example:

[0074] Current density: 0.05-0.3 A / cm2 (note: required electrical current [Amps] increases with increasing surface area).

[0075] Voltage: 200-400 V (note: dependent on electrolyte chemistry—by increasing the amount of sodium hexametaphosphate within the electrolyte, the plasma may be formed at a lower voltage)

[0076] Frequency: 5-30 kHz (note: dependent on electrolyte chemistry)

[0077] By altering the frequency at the constant pulse time, it is possible to tune the porosity and the quality of the protective coating.

[0078] If the frequency is increased with constant pulse-width, the resulting coating can be of higher quality. However, there comes a point in increasing frequency when plasma is no longer generated at the surface, and thus, the coating does not materialise.

[0079] The coating operation 112 involves applying the selected electrical operating parameters to the power supply so that the plasma DC generator 40 controls current flow through electrolyte bath and specimen for a specified duration. Increased duration results in increased coating thickness. In trials, duration in the range of about 3-15 minutes has proved successful. However, the coating time can vary based on the type of electrolyte, parameters and customer requirements. Coating time is not dependent on surface area.

[0080] Current flow causes electrical micro-discharges to occur at the surface of the specimen resulting in localised melting of the substrate, accompanied by the reconstruction of the inorganic layer throughout its thickness via melt flow, solidification, sintering and densification of the mostly crystalline formed nitride.

[0081] After the prescribed duration, the coating operation is stopped and the specimen is removed from the bath (operation 114). The specimen is ultrasonically cleaned in deionised water to remove residual electrolyte (operation 116). After the specimen is dried, the coating process is complete (operation 118).Process Example

[0082] The aluminium-containing sample such as AlSi10Mg, as the anode, and a stainless-steel mesh, as the cathode, were connected to a pulse DC power generator operating at a unipolar pulsed regime with 1.8 A current, the mean power of 2 KW, and a frequency of 20 kHz (pulse time=12.5 μs) within the electrolyte to generate plasma. Regarding the electrolyte composition, an aqueous solution of PM, containing 0-3.2 wt. % melamine (C3H6N6) and 0-5 wt. % sodium hexametaphosphate (NaPO3)6 with the neutral pH, was mixed. The treatment time was selected to be 6 min to provide a desired coating thickness, using cooling and circulating in which the electrolyte temperature is maintained below 30° C. during the process. After the process, the specimen was washed with distilled water and dried. The finished coated samples showed improved corrosion resistance in comparison to the reference alloy.

[0083] FIG. 3 is an image showing a specimen 51 which has undergone an electrolytic coating process according to an embodiment of the present invention using the electrolyte composition designated as ‘P20’ (see above). FIG. 4 is a cross-sectional microscopy image showing density, thickness and attachment of the coating to the aluminium substrate. The coating duration for the specimen in this image was 6 minutes, and the protective coating as shown has a thickness of about 3-4 μm.

[0084] FIG. 5 is a schematic diagram of material phases present though the coating thickness. As shown in this diagram, the coating comprises a dense interface layer that integrates with the metal alloy, a thick and solidified oxide layer, and a porous outer layer. A representative composition-depth profile for such a coating is illustrated in FIG. 9, which indicates the relative amounts of significant chemical components in the coating according to depth from the coating surface. As can be seen, at the coating surface the composition is dominated by the presence of nitrogen and oxygen with a relatively small amount of aluminium.

[0085] FIGS. 6, 7 and 8 are charts illustrating some performance parameters for specimens of aluminium alloy coated using the three electrolyte compositions disclosed above, compared with an uncoated sample. FIG. 6 shows good thermal diffusivity for the coated samples (~58-63 mm2 / S), although somewhat lower than the uncoated sample (~75 mm2 / S) as expected. FIG. 7 charts a measure of hardness, which shows two of the three coated specimens (P20, PW) exhibiting significantly increased hardness as compared to the uncoated sample. FIG. 8 illustrates a measure of corrosion resistance, which shows that the coated specimens all substantially outperform the uncoated sample.

[0086] Testing indicates that corrosion resistance is increased as the nitride layers generated by the PEN process greatly reduces the formation of pitting under the influence of the negative potential and presence of aggressive species such as chloride ions in corrosive solutions. Erosion resistance is also improved as the fabricated ceramic layer can increase surface hardness, improving mechanical protection.

[0087] Embodiments of the invention have been described herein by way of example, with reference to various possible components, compositions, operations and control characteristics. Such embodiments are intended to be illustrative rather than restrictive. It should be understood that embodiments include various combinations and sub-combinations of features described herein, even if such features are not explicitly described in such a combination or sub-combination.

[0088] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0089] Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

1. An electrolyte composition comprising an aqueous solution containing a charge carrier component together with an effective amount of carbamide, melamine, ammonium nitrate and / or similar nitrogen source for use in forming a protective coating on a metallic specimen by way of plasma electrolytic deposition.

2. The electrolyte composition of claim 1, wherein the charge carrier component comprises a sodium metaphosphate.

3. The electrolyte composition of claim 1, wherein the charge carrier component comprises sodium hexametaphosphate.

4. The electrolyte composition of claim 3, including up to about 5 wt. % sodium hexametaphosphate.

5. The electrolyte composition of claim 1, including up to about 40 wt. % carbamide.

6. The electrolyte composition of claim 1, including up to about 5 wt. % of melamine.

7. The electrolyte composition of claim 1, further including up to about 20 wt. % glycerol.

8. An electrolyte composition for plasma electrolytic deposition according to claim 1, comprising an aqueous solution having about 1 wt. % sodium hexametaphosphate and about 30 wt. % carbamide.

9. An electrolyte composition for plasma electrolytic deposition according to claim 1, comprising an aqueous solution having about 1 wt. % sodium hexametaphosphate and about 3 wt. % melamine.

10. An electrolyte composition for plasma electrolytic deposition according to claim 1, comprising an aqueous solution having about 1 wt. % sodium hexametaphosphate, about 40 wt. % carbamide and about 20 wt. % glycerol.

11. A method for forming a protective coating on a metallic object, comprising:immersing the object in an electrolyte solution according to the composition defined in claim 1;electrically coupling the object as the anode in a power supply circuit that also includes a cathode in contact with the electrolyte solution;controlling the power supply circuit to apply, for a predetermined period of time, a pulsed DC voltage of selected frequency and power characteristics sufficient to generate plasma electrolytic deposition on the surface of the object.

12. The method of claim 11, including inducing relative movement between the object and the electrolyte solution during immersion to ensure that all surfaces of the object to be coated are in contact with the electrolyte solution during the plasma electrolytic deposition operation.

13. The method of claim 12, wherein relative movement between the object and the electrolyte solution is provided by mechanical agitation of the electrolyte solution or specimen, or both.

14. The method of claim 12, including inducing flow of the electrolyte solution to ensure that the electrolyte sufficiently wets all surfaces of the object, and all surfaces are continually replenished with electrolyte during the plasma electrolytic deposition operation.

15. The method of claim 11, wherein the predetermined period of time is selected according to a desired plasma electrolytic deposition layer thickness.

16. The method of claim 11, including determining a surface area of the object and controlling characteristics of the power supply circuit according to the determined surface area.

17. A system for forming a protective coating on a metallic object, comprising:an electrolyte bath containing an electrolyte solution according to the composition defined in claim 1;a cathode that is in contact with the electrolyte solution, in use;a controllable power supply adapted to apply, for a predetermined period of time, a pulsed DC voltage of selected frequency and power characteristics sufficient to generate plasma electrolytic deposition on the surface of the object while the object is, in use, coupled to the power supply as the anode and immersed in the electrolyte solution.

18. A metallic object formed with a protective coating according to the method claim 11.

19. A metallic object formed with a protective coating according to utilizing the system as set forth in claim 18.

20. The electrolyte composition of claim 1, wherein the charge carrier component comprises sodium hexametaphosphate and wherein the electrolyte composition further comprises:up to about 5 wt. % sodium hexametaphosphate;up to about 40 wt. % carbamide;up to about 5 wt. % of melamine; andup to about 20 wt. % glycerol.

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