WO2003083171A1

WO2003083171A1 - Conversion coating solution

Info

Publication number: WO2003083171A1
Application number: PCT/GB2002/003669
Authority: WO
Inventors: Kevin Richard Baldwin
Original assignee: Seamless Plating (Uk) Limited
Priority date: 2002-04-02
Filing date: 2002-08-09
Publication date: 2003-10-09
Also published as: GB0207527D0; AU2002367829A1

Abstract

A conversion coating solution comprises at least one precursor agent, at least one activator agent, and water solvent, in which on provision to a solution suitable for the deposition of a substantially metallic coating, provides an in-situ conversion coating to the surface of the substantially metallic coating.

Description

Conversion Coating Solution

The present invention related to a conversion coating solution, a process that provides conversion coatings to substantially metallic coatings, and articles treated using the solution and process.

Coatings that consist substantially of metallic elements are provided to substrate materials such as steels on an industrial scale. The most usual method is through electrodeposition from aqueous solutions, known in the art as electroplating. A further method for the deposition of substantially metallic coatings is electroless plating, also familiar to those practiced in the art. Metallic coatings are often not deposited as a pure metal, due to side reactions such as the deposition of hydrogen, the uptake of precipitated substances, or the intentional inclusion of non-metallic elements, and it is more -accurate to employ the term 'substantially metallic' .

Substantially metallic coatings are used to obtain properties that cannot be afforded by the substrate material alone. These properties include appearance, corrosion protection, and engineering or physical properties such as improved wear resistance. Chromium is used for decorative effects, and is usually deposited over an undercoat of copper or nickel. Others include gold, silver, platinum, nickel, copper, and rhodium. For applications where improved engineering or physical properties are required, chromium also finds extensive use, as do nickel, tin, lead, and silver. In applications where sacrificial corrosion resistance is required, zinc, and to a lesser extent, cadmium, are the preferred choices. In the case of zinc, it can be electrodeposited either alone, or in combination with other metals to form alloy coatings, for example, zinc-cobalt, zinc- iron, and zinc-nickel. In some cases, particles are incorporated into metallic deposits to improve properties such as wear resistance, known in the art as composite coatings. There is a vast range of applications for all of these coatings .

In particular, the electrodeposition of substantially metallic coatings, takes placed on a global scale. It is known to those practised in the art that the electrodeposition of substantially metallic coatings requires the following: an external circuit, consisting of a source of electrical current, means of conveying current to the plating cell, and associated instruments such as ammeters, and means of regulating the voltage and current; the negative electrodes or cathodes, which are the articles to be coated, along with a means of positioning the article in the plating solution so that contact is made with the current source; the plating solution itself, almost always aqueous, and contained within a tank; and the positive electrodes, the anodes, usually of the metal being plated.

The electrolyte most often comprises an aqueous solution containing ions of the metal to be deposited. Sometimes the solution also contains ions of a different metal, for example, when an alloy deposit is desired. On the passage of electrical current through the cell, the metal ions are attracted to the article, where they are reduced to form a coating of that metal. The current flow is terminated when the required thickness of coating has been achieved. The coated article is then most usually rinsed several times in water, to remove surplus solution, brought into contact with a neutralising solution to counteract remaining traces of the electrolyte, and again rinsed in water. If the coated article is to be subject to conversion coating treatment, it is most often then brought into contact with an acid rinse to activate the surface. This basic sequence of rinses and treatments may be modified according to the nature of the metallic coating.

The various treatment solutions, including rinse water, are most often contained within steel tanks often lined with plastic materials. The electrodeposition of steel strip involves continuously moving the strip through the various solutions. For small items such as fasteners, jigging is avoided through the use of rotating barrels that contain electrolyte solution. The various treatment solutions must be maintained at a constant temperature, and heating is usually provided through the use of electrical elements . Large volumes of clean water are required especially for the numerous rinse stages. The treatment solutions, especially those used to deposit the metallic coating, require regular chemical analysis to ensure that the formulation is within specification. If the solution is outside specification, the appropriate chemicals are added in the required amounts to the solution, thus returning the solution to within specification. Other solutions, such as the acid rinse, gradually become contaminated and must be drained from the tank and replenished at regular intervals.

The multiple treatment tanks also produce 'drag out' . This refers to small amounts of water or solution that adheres to the article surface following removal from the tank. This becomes 'drag in' when the article is then brought into contact with the next solution in the sequence. This solution is thus contaminated with the 'drag in' . A great deal of effort is expended in minimising the effects of 'drag out' , which depletes solution, and 'drag in' , which can lead to serious contamination problems. One approach is to use continuous filtration of the most important solutions to remove contaminating ions .

Before the article is subject to the application of coating treatments, it is understood by those practiced in the art, that appropriate cleaning procedures must be carried out.

For steel articles, solvent degreasing may be used to remove oils and greases, followed by contact with solutions that render the article surface in a chemically clean condition, such that if a water film were applied to the surface, no break in that water film would be observed.

Solutions that are suitable for use in coating deposition are known widely in the art. For electrodeposition, proprietary solutions are available for the full range of substantially metallic coatings. For other purposes, the compositions of suitable solutions are widely published in the open literature, and are well known by those practiced in the art. The solutions all contain metal ions of the metals that are to be deposited onto the article. For acid solutions these are most often .salts of the metals that are sufficiently soluble in water. For alkaline solutions, lower concentrations of the metal salts are added to concentrated solutions of hydroxide salts such as sodium hydroxide.

The remaining additions to the electroplating solution wholly depend on the qualities that are desired of the substantially metallic coating, although some general points can be made. Wetting agents are widely used to reduce the surface tension of the solution, which has the effect of minimising the formation of pores in the deposit. A common example of a wetting agent is sodium lauryl sulphate. The addition to the solution of complexing agents is sometimes required. These form complexes with the metal ions in order to facilitate the deposition of the metal, without which their deposition might not be possible. The addition of complexing agents is of particular importance with alkaline solutions. The throwing power of acid solutions is often improved through the addition of salts that increase conductivity, for example, salts of alkali metals or ammonium compounds. For acid solutions, it is often necessary to stabilise the pH through the addition of buffers such as boric acid or acetic acid. For all solutions, the physical form of the deposit can be modified or regulated through the addition of levelling agents, which assist in the formation of uniform deposits, or brightening agents, which promote the deposition of bright coatings . Other chemical additions may be required to aid in the dissolving of anodes, and to modify other properties, either of the solution or of the deposit, depending on the specific case.

Many substantially metallic coatings consisting substantially of chromium, nickel, rhodium, platinum and gold, possess inherent tarnish resistance, and can be put straight into the use for which they are intended. For others, the coating deposit formed on the article requires further treatment to impart the range of properties demanded by the end user. These include zinc, cadmium, silver, ^ tin, and copper, and alloys such as zinc-cobalt, zinc-nickel, and zinc- iron. In the case of those coatings consisting substantially of zinc, the surfaces are often dull and susceptible to staining, and possess inadequate corrosion resistance. Moreover, their tendency to oxidise in the atmosphere renders them unfit for use, and the formation of loosely adherent corrosion products prevents the successful application of paints .

For coatings containing substantial quantities of metals such as zinc, cadmium, silver, tin, and copper, the further treatment consists of the application of a conversion coating. This is carried out in a series of treatment tanks separate from those involved in electrodeposition of the coating onto substrate materials such as steels. A conversion coating is known in the art to consist of a coating produced by chemical or electrochemical treatment on a substantially metallic surface, which gives a superficial layer containing compounds of the metals present.

To those practiced in the art, a conversion coating is known not to be necessarily identical to the films provided to substantially metallic coatings known as passivation treatments. Conversion coatings may act as a barrier layer, in which case the underlying metallic surface is not rendered to a passive state. Other conversion coatings, for example those containing appreciable amounts of chromate, however, are known to change the surface of some metallic coatings from an active state to a more passive state, in which case they are sometimes referred to as passivation films .

The most widely used conversion coating treatments are those based on chromates or phosphates, or combinations of chromates and phosphates. These are widely available as proprietary solutions that are formulated for specific substantially metallic coatings. For those coatings containing appreciable amounts of cadmium, copper, and zinc, as well as other metals, treatments containing chromates are the preferred option. Phosphates are also widely used, for example, on zinc-iron alloys, where they improve paint adhesion. There are different types of phosphate treatment available, including iron-phosphates and zinc-phosphates as treatments before painting, and manganese phosphates, which provide additional protection against wear.

Chromate conversion coatings are most often imparted to the surface of substantially metallic coatings through immersion or spraying of aqueous solutions of hexavalent chromate salts. The solutions are most often strongly acidic and the contact time with the coating is usually no more than 30s. This brief contact is sufficient to polish and smooth the otherwise dull coating surface to a brilliant and specular finish. The chromate solutions also remove haze or other surface films that would otherwise interfere with specular reflection . In most countries, there are many proprietary chromate treatment solutions available. They have been used routinely since the early years of the twentieth century. They are formulated to impart characteristic coloured finishes to articles coated with substantially metallic deposits, and are often called up in specifications. For maximum corrosion resistance, the yellow chromates are the preferred option, since these conversion coatings contain the highest concentrations of chromate. These conversion coatings are used on coated steel articles from fasteners on aircraft, to automobile bodies, through to large steel structures. Olive drab is used on military equipment. Iridescent chromates are less corrosion resistance but impart a distinctive decorate finish. There are other chromates available including black, which are often .used on automobile parts . Chromate finishes can also be dyed for colour coding purposes . Chromate conversion coatings are also applied onto metallic coatings as a base for paints and other organic finishes. The chromate conversion coating provides 'anchor points' to which the paint film can adhere; it also retards the spread of corrosion if the paint is scratched or damaged.

The colour and lustre of chromate conversion coatings vary according to the nature of the substantially metallic coating, the composition of the treatment solution, method of application, and the treatment conditions such as temperature, time and acidity. It is essential that the conversion coating treatments impart the required colour and reflective brilliance, lest the treated article be rejected.

Chromate conversion coatings are formed by a chemical reaction between the substantially metallic coating surface and the hexavalent chromium in solution. The metals present in the surface of the metallic coating are 'converted' to their oxides, and the hexavalent chromium is reduced to trivalent chromium. The conditions at the surface become more alkaline and this causes the trivalent chromium to precipitate in the form of a gel, which entraps some of the hexavalent chromium solution, thus generating the conversion coating. Chromate conversion coatings are in essence amorphous gels and are considered to 'harden' over time. This arises from gradual dehydration of - the conversion coating, leading to the formation of micro cracks. Under normal circumstances, this is not considered to be detrimental, although excessive heat can dehydrate the gel and damage corrosion resistance.

The deposition of substantially metallic coatings followed by the application of a conversion coating, has formed established practice for many years. There are two drawbacks inherent with the established practice. Firstly, the treatment by the conversion coating requires the use of numerous rinsing and other treatment stages following electrodeposition. Secondly, the use of hexavalent chromates is extremely undesirable because they are known to be carcinogenic and harmful to the environment. The chromate solutions, including the treatment solution, and rinse water used to remove surplus solution, requires extensive treatment before it can be recovered for subsequent reuse, or disposed of. Those containing phosphates are substantially less harmful, however, the treatment is of a similarly multi-stage nature .

The industry has attempted to replace chromate solutions with less toxic alternatives. This approach has fallen down on two counts, firstly, the alternative solutions do not match the overall performance provided by chromates, and, secondly, they are invariably more expensive than chromates.

The use of chromates or phosphates as conversion coatings treatments currently requires the use of many treatment stages, and extensive ancillary equipment such as filtration plant and chemical analysis facilities. Separate plant is required for the treatment of water contaminated with chromates. Any solid chromate waste must be stored or taken to licensed landfill sites . Others employ ion exchange columns to separate out the chromates, allowing the cleaned water to be reused. These multiple treatments were once acceptable when the raw materials required to service the many treatment stages were relatively inexpensive, and the treatment of chromate waste less regulated by governments. This picture has changed in most developed countries, and water, electricity, equipment, and factory space are going up in cost. This is exacerbated by the continued use of chromates, and there is currently no answer to the threat posed by national and international legislation that is likely to prohibit their use .

Accordingly a first aspect of the present invention is directed to a conversion coating solution comprises at least one precursor agent, at least one activator agent, and water solvent .

Preferably, the conversion coating solution further comprises at least one modifying agent.

Accordingly .a second aspect of the present invention is directed to a process for coating an article, comprising the step of providing a conversion coating solution comprising at least one precursor agent, at least one activator agent, and water solvent to a solution suitable for the deposition of substantially metallic coatings, which imparts a conversion coating layer to the substantially metallic coating.

Accordingly a third aspect of the present invention is directed to a process for coating an article comprising the steps of providing a conversion coating solution comprising at least one precursor agent, and water solvent to an aqueous solution suitable for the deposition of substantially metallic coatings, bringing an article into contact with the solution, such that a substantially metallic coating is formed on the article, providing at least one activator agent to the solution, such that a conversion coating layer is imparted to the substantially metallic coating.

Accordingly a fourth aspect of the present invention is directed to a process for coating an article comprising the steps of providing a conversion coating solution comprises at least one precursor agent, and water solvent to an aqueous solution suitable for the electrodeposition of substantially metallic coatings, bringing an article into contact with the solution, passing an electrical current to the article such that a substantially metallic coating is deposited onto the article, substantially reducing or halting the current, providing at least one activator agent to the solution, such that a conversion coating layer is imparted to the substantially metallic coating.

Accordingly a fifth aspect of the present invention is directed to a process for coating an article wherein a conversion coating solution comprises at least one activator agent, and water solvent to an aqueous solution suitable for the electrodeposition of substantially metallic coatings, bringing an article into contact with the solution, passing on electrical current to the article such that a substantially metallic coating is deposited onto the article, substantially reducing or halting the current, providing at least one precursor agent to the solution, such that a conversion coating layer is imparted to the substantially metallic coating.

Advantageously the coating solution is substantially cleared of activator agent before it is reused.

The solution suitable for the deposition of a substantially metallic coating is employed to provide a metallic coating to the surface of an article, which can then be provided the conversion coating.

To those skilled in the art, a solution suitable for the deposition of a substantially metallic coating will be recognised as any that wherein use provides a substantially metallic coating to an article. These solutions are widely available either as laboratory formulations or on a proprietary basis from any supply house.

Preferably, the solution suitable for the deposition of substantially metallic coatings is an electroplating solution.

Preferably, the electroplating solution is suitable for the deposition of substantially metallic coatings comprising the following metals: Zn, Fe, Ni, Co, Ag, Sn, Cu, Mn. Preferably, the electroplating solution is suitable for the deposition of substantially metallic coatings comprising the following metals: Zn, Fe, Ni, Co.

Preferably the electroplating solution is acid, alkaline, or substantially neutral. Especially suitable are acidic solutions containing counter ions of sulphates, chlorides, or combinations of sulphates and chlorides .

The provision of the conversion coating is mostly independent from the provision of the metallic coating. The requirement is that the conversion coating solution and process of the present invention does not act to interfere with the deposition of the substantially metallic coating.

The conversion coating solution comprises at least two parts, the at least one precursor agent and the at least one activator agent .

The at least one precursor agent is a chemical species that on contact with the at least one activator agent, reacts to form a solid precipitate.

Preferably, the at least one precursor agent is chosen from a salt of the following: Y³⁺, La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺. Suitable salts are chlorides or sulphates, or mixtures of the two.

More preferably, the at least one precursor agent is selected from Y³⁺, La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺.

Preferably, the at least one activator agent is peroxide (0₂ ^2~) , peroxonium salts (H₂OOH)⁺, hydroperoxide (OOH)^~, or ozone analogues of peroxide (HOOOH) , or nitrate salts.

More preferably, the at least one activator agent is hydrogen peroxide .

The modifying agent is a chemical species that is entrapped during the formation of the solid precipitate, arising from the -reaction between the precursor and activator agents. This serves to impart additional properties on the conversion coating, for example, those relating to appearance such as colour or reflection, and properties such as corrosion resistance.

Preferably, the at least one modifying agent is an anion of non-metals selected from: silicate, oxalate, iodate, formate, phosphate, or carbonate.

More preferably the at least one modifying agent is silicate or phosphate. Preferably, the at least one precursor agent, the at least one activator agent, and the at least one modifying agent are dissolved in a solvent prior to being provided to the plating solution.

Preferably, the solvent used to dissolve the at least one precursor agent, the at least one activator agent, and the at least one modifying agent is water.

Preferably, the concentration of the precursor agent is in the range O.OOlg/L to saturation.

Preferably, the concentration of the activator agent is in the range O.OOlg/L to saturation.

The concentration of hydrogen peroxide requires especially careful control lest the rate of conversion coating formation is excessive, interfering with the initiation and growth of the metallic coating, and causing the growth of macroscopic dendrites .

Preferably, the concentration of the modifying agent is in the range O.OOlg/L to saturation. Preferably, the precursor, activator, and modifying agents are provided separately to the solution suitable for the formation of metallic deposits .

More preferably, the precursor agent and modifying agents are provided to the solution prior to the provision of the activator agent .

The present invention provides the advantage of the provision of conversion coatings from the same solution that provides the metallic coating onto an article. This substantially reduces the number of treatment stages required in order to provide a conversion coating to a metallic coating. This brings about major advantages over existing practices, including reduced water consumption, reduced energy consumption, and -the need for much fewer treatment tanks. All of the costs and risks currently associated with chromates are eliminated.

The plating solution once provided with the at least one precursor agent, the at least one activator agent, and the at least one modifying agent, can be re-used until the aforementioned additions are exhausted, after which they may be replenished so that their concentrations are sufficient to permit conversion coating formation. It has been found advantageous to remove the activator agent at the end of each ___ _ plating/conversion coating cycle as this avoids any dendrite production .

The present invention provides conversion coatings that are colourless, iridescent, or coloured. The formulations and processes can be chosen to provide conversion coatings that are identical in appearance as those provided by chromates . The process of the present invention may be used to treat article substrates that can be treated using traditional chromate or phosphate conversion coating processes.

For the provision of coloured conversion coatings that are identical in appearance to those provided by chromates, purely for illustrative purposes, the precursor agent might comprise Ce³⁺, the activator agent, H₂0₂. The precursor agent is provided to a zinc sulphate plating bath, and the article brought into contact with the bath. Electrical current is passed through the solution such that zinc coating is provided to the article, and the current substantially reduced or switched off. The peroxide is provided to the solution. This reacts with Ce³⁺ to form a sparingly soluble precipitate of hydrated cerium oxide on the surface of the metallic deposit, which is gelatinous in substance and yellow in colour. The precipitate forms a continuous coating layer of approximately

2μm in thickness that is visually indistinguishable from those

provided by chromates . The activator agent may be provided to the solution before, during, or after, the formation of the substantially metallic coating.

The precursor agent, the activator agent, and modifying agent, require periodic replenishment. Where the activator constitutes peroxide, this substance decomposes naturally, and must be replenished frequently.

Preferably, the electrical current to reduce the metal ions in the plating bath, thereby providing a substantially metallic coating to the article, is direct current. Also suitable is pulsed current, or current with AC superimposed.

Preferably, the conversion coating once imparted is rinsed in cold water to remove surplus solution.

Preferably, the rinsed conversion coating is further

rinsed in water in the temperature range 20-80° C, but more

preferably in the temperature range 30-55° C.

Preferably, the hot rinse solution contains the at least one precursor agent used in the prior provision of the conversion coating, in the concentration range O.OOlg/l to saturation. Preferably, to the rinsed conversion coating is imparted a sealing agent.

Preferably, the sealing agent is an organic substance.

More preferably, the sealing agent is a silane. Suitable silanes are those of the general formula R-Si(OX)₃, where R is a functional group and X is an alkoxy group or other organo- functional group, which are hydrolysed to silanols of the general formula R-Si(OH)₃. Other suitable sealing agents are metallo-siloxanes, silicates, silicate esters, polyesters, polyoxyethylenes, titanates, thioglycollates, trithioglycollates, and acrylics.

Preferably, the article material is a metal such as those consisting substantial quantities of iron, titanium, or aluminium.

Preferably, the article material is non-metallic, such as a plastic or carbon fibre, onto which a thin substantially metallic coating has been added.

More preferably, the article material is steel.

Examples of conversion coating according to the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of a current state of the art conventional electroplating process;

Figure 2 shows a schematic diagram of a surface treatment process according to the present invention;

Figure 3 shows a schematic diagram of a plating cell suitable for the provision of the conversion coating solution of the present invention; and

Figure 4 shows a profile of the operation of a surface treatment process according to one aspect of the present invention .

Figure 1 shows a conventional electroplating process. Each box represents a separate treatment stage, through which the cleaned article is moved in a sequential manner. The electrodeposition stage provides the substantially metallic coating to the article. Two rinsing stages remove surplus solution, remaining solution is neutralised, and neutralising solution removed by water rinsing. An acid rinse removes surface films. The coated article is immersed in a bath to provide the chromate conversion coating. Two rinsing stages remove surplus solution. The treated article is rinsed in hot water and then dried in warm air. The treatment cycle is completed and the treated article is inspected and stored in readiness for dispatch.

Figure 1 shows the flow of clean water to the rinse stages for the removal of surplus solution following electrodeposition, annotated (1) on the diagram. The water becomes contaminated with metals from the electrodeposition stage through the effects of drag-in. The rinse water runs into a general waste stream (2) . The water may be sent to a treatment plant, where it is cleaned or disposed. For disposal, the water may be piped to open tanks where the water is allowed to evaporate. The preferred option is to recover the water through removing contamination, using techniques such as reverse osmosis or ion exchange. It is then suitable for reuse and returned to the clean water stream (-3) .

Figure 1 shows the flow of clean water to the rinse stages for the removal of surplus solution following chromate treatment (4) . Once again the water becomes contaminated but this time with hexavalent chromate, and is run into a separate waste stream (5) . This requires special treatment due to the extreme toxicity of chromate. One approach is to run the water into evaporation tanks, reduce the hexavalent chromate to trivalent chromium through chemical additions, which then precipitates out. The precipitate is allowed to settle in the tanks to form sludge. The sludge is taken to landfill whilst the water is then treated further to remove all traces of ___ _^ chromate, using techniques such as ion exchange. The treated water is then returned to the water stream (6) .

Figure 2 illustrates a process of the present invention to allow comparison with traditional electroplating practices as described above. The conversion coating solutions are held in a series of automatic titrators (7) that provide the solutions to the electroplating solution when required. This enables the conversion coating to be formed in-situ, alongside the electrodeposition stage. The water stream (8) is required only for a cold-water rinse to remove surplus solution, and a hot rinse. For maximum corrosion resistance the optional no- rinse sealing solution is employed. The sealing solution provides a thin transparent film to the coated article. The coated article is dried in a stream of air. The wastewater stream (9) contains metal ions from the drag-in, and requires no special treatment. The metal ions may be removed from the waste stream through the use of recovery techniques such as ion exchange. The treated water is then returned to the water stream (10) . The metal ions can be separated, or precipitated, and reused. The present invention can be arranged such that it forms a closed loop.

Figure 3 illustrates a cell suitable for the operation of one aspect of the present invention. The article to be coated

(12) is held on a conducting bar (13) and immersed in plating solution (14). An electrical heater (15) maintains the plating solution temperature at a ^'constant value. An anode material (16) is present in the plating solution, and connected to a power supply. Air is introduced into the cell through a frit (17), creating small bubbles and vigorous agitation of the solution. The solutions comprising the conversion coating solution are held in reservoirs above automatic titration devices (18). These possess valves that release pre-set quantities of the solutions into the plating cell. The titrators are connected to ion selective electrodes (19) or other detection apparatus that determine the concentrations of the different species in solution, and, if they fall below the required limits, activate the titrators to release additional solution .

Figure 4 shows the profile of the process according to one aspect of -the present invention. This shows four consecutive treatment cycles. In cycle A, the voltage is increased sufficiently to permit the electrodeposition of the substantially metallic coating. This is followed by a soak period (20) where the conversion coating is formed. Cycles B and C are identical. In Cycle D, additions of the conversion coating formulation are made (21) in order to maintain their concentration in solution, since some of the solution would have been consumed in Cycles A, B and C.

Example 1. An electroplating-conversion coating solution comprising cerium sulphate, Ce₂(S0₄)₃, lg/L, hydrogen peroxide, H₂0₂ (30vol) , 0.5ml/L, and zinc sulphate heptahydrate, ZnS0₄.7H₂0, 220g/L, in distilled water was provided.

The solution was contained in a plastic vessel into which a platinised titanium anode was placed. The plastic vessel was

heated by an electrical element to a temperature of 25°C. The

solution was aspirated with air. A cleaned steel article was connected to the negative terminal of a direct current power supply, whilst the anode was connected to the positive terminal . The steel article was immersed in the solution thus forming the cathode in the cell, and current passed through the cell to provide a cathodic current density of 2A/dm². A zinc coating was formed on the surface of the steel article that was light grey in appearance, and after 20mins, the

thickness achieved was lOμm. The current was switched off and

the conversion coating formation took place, through reaction of the cerium salt and peroxide, forming an adherent and continuous layer on the coated article. After 2mins the article was removed from the solution, rinsed in distilled

water at ambient temperature , rinsed in water at 60°C, and

then dried in a stream of air. The conversion coating was light yellow with a brilliant lustre. The conversion coating was visually identical to traditional yellow chromate.

The adherence of the conversion coating was determined using a tape test. Adhesive tape was pressed onto the surface of the conversion coating and pulled off in one rapid movement. The tape was examined and there were no signs of the conversion coating, indicating no loss of adhesion.

Example 2

The electroplating-conversion coating solution employed in Example 1 was reused after 30mins. The sequence of treatment stages detailed in Example 1 was repeated. The conversion coating obtained was identical to that obtained in

Example 1.

Example 3

After a period of 24hrs the solution employed in Example 2 was reused. The sequence of treatment stages detailed in Example 1 was repeated, however, an additional aliquot of 0.5ml/L 30vol H₂0₂ was provided to the solution immediately following the electrodeposition stage. The conversion coating obtained was identical to that obtained in Example 1. __ _„

Example 4

Solutions were prepared identical to those in Example 1, except that H₂0₂ was excluded. The same sequence of treatment stages was employed, except that immediately following the electrodeposition stage, 0.2ml/L of H₂0₂ was added to the solution. The conversion coating was formed on the surface of the zinc coating. After 5mins, the coated article was removed, and subject to rinsing identical to that in Example 1. The conversion coating obtained was identical to that obtained in Example 1.

Example 5

Solutions were prepared identical to that in Example 1, and the same treatments used, except that following the electrodeposition stage, the current was reduced such that the cathode voltage increased to a value of -0.8V, from -1.2V during electrodeposition. The conversion coating was formed and after 4mins, the current flow was switched off. The conversion coating thus obtained was identical to that obtained in Example 1.

Example 6

An electroplating-conversion coating solution comprising cerium sulphate, Ce₂(S0 )₃, lg/L, and hydrogen peroxide, H₂0_2/ 30vol, lml/L, dissolved in distilled water was provided. This was added to a solution comprising zinc sulphate heptahydrate, ZnS0₄.7H₂0, 220g/L and nickel sulphate hexahydrate, NiS0₄.6H₂0, 90g/L.

heated by an electrical element to a temperature of 25°C. The

solution was aspirated with air. A cleaned steel article was connected to the negative terminal of a direct current power supply, whilst the anode was connected to the positive terminal. The steel article was immersed in the solution thus forming the cathode in the cell, and current passed through the cell to provide a cathodic current density of 2A/dm². A zinc alloy coating, _.containing 10 weight per cent nickel, was formed on the surface of the steel article that was light grey in appearance, and after 20mins, the thickness achieved was

lOμm. The current was switched off and the conversion coating

formation took place, forming an adherent and continuous layer on the coated article. After 2mins the article was removed from the solution, rinsed in distilled water at ambient temperature, and then dried in a stream of air.

The conversion coating was light yellow with a brilliant lustre. The conversion coating was visually identical to traditional yellow chromate. Example 7

A solution and process identical to that described in Example 6 was prepared and followed, except that cobalt chloride, CoCl₂, 50g/L, was substituted for NiS0₄.6H₂0. The metallic coating obtained was a zinc alloy containing 0.8 weight per cent cobalt. The conversion coating was light yellow with a brilliant lustre. The conversion coating was visually identical to traditional yellow chromate.

Example 8

A solution and process was employed identical to that in Example 1. Following the formation of the conversion coating and rinse stages, the coated article was immersed in a proprietary silane sealing solution to which was added

Ce₂(S0₄)₃, lg/L. The solution was maintained at 30°C and the

contact time was 2mins .

The sealing solution provided a thin transparent film to the surface of the yellow conversion coating.

The corrosion resistance of the sealed conversion coating was determined through the used of a standard neutral salt spray test. Mild steel panels treated using the method described in this example were placed in the test chamber with chromated zinc coatings of the same thickness. The time to the formation of white corrosion products, namely the corrosion products of zinc, was estimated to determine the corrosion resistance of the conversion coatings. The time to the formation of red corrosion products, namely the corrosion products of zinc, was estimated to determine the corrosion resistance of the overall coating treatment in its ability to protect steel.

The table provides the corrosion data obtained in hours

Example 8 The corrosion test demonstrates that the conversion coating of the present invention affords the same degree of corrosion protection to the zinc coating as do traditional chromate finishes, and are more protective to the steel substrate .

Example 9 An electroplating-conversion coating solution comprising cerium sulphate, Ce₂(S0 )₃, lg/L, sodium silicate, Na₂Si0 , lg/1, and hydrogen peroxide, H₂0₂, 30vol, Iml/L, dissolved in distilled water was provided. This was added to zinc sulphate heptahydrate, ZnS0₄.7H₂0, 220g/L.

The solution was contained in a plastic vessel into which a steel anode was placed. The plastic vessel was heated by an

electrical element to a temperature of 25°C. The solution was

aspirated with air. A cleaned steel article was connected to the negative terminal of a direct current power supply, whilst the anode was connected to the positive terminal . The steel article was immersed in the solution thus forming the cathode in the cell, and current passed through the cell to provide a cathodic current density of 2A/dm². A zinc coating was formed on the surface o^'f the steel article that was light grey in

appearance, and after 20mins, the thickness achieved was lOμm.

The current was switched off and the conversion coating formation took place, forming an adherent and continuous layer on the coated article. After 2mins the article was removed from the solution, rinsed in distilled water at ambient

temperature , rinsed in water at 60°C, and then dried in a

stream of air.

Example 10 An electroplating-conversion coating solution comprising zinc sulphate heptahydrate, ZnS0₄.7H₂0, 220g/L, yttrium chloride, YC1₃, lg/L, NdCl₃ lg/L and hydrogen peroxide, H₂0₂, 30vol, 1.5ml/L, dissolved in distilled water was provided.

electrical element to a temperature of 25°C. The solution was

aspirated with air. A cleaned steel article was connected to the negative terminal of a direct current power supply, whilst the anode was connected to the positive terminal . The steel article was immersed in the solution thus forming the cathode in the cell, and current passed through the cell to provide a cathodic current density of 5A/dm². A zinc coating was formed on the surface of the steel article that was light grey in

appearance, and after 20mins, the thickness achieved was lOμm.

temperature, rinsed in water at 60°C, and then dried in a

stream of air.

The conversion coating was iridescent with a brilliant lustre. The conversion coating was visually identical to traditional iridescent chromate. Example 11

An electroplating-conversion coating solution comprising zinc sulphate heptahydrate, ZnS0₄.7H₂0, 220g/L, praseodymium sulphate, Pr₂(S0₄)₃, 2g/L, and hydrogen peroxide, H₂0₂, 30vol, 0.3ml/L, dissolved in distilled water was provided.

heated by an electrical element to a temperature of 25°C. The

solution was aspirated with air. A cleaned steel article was connected to the negative terminal of a direct current power supply, whilst the anode was connected to the positive terminal. The steel article was immersed in the solution thus forming the cathode in the cell, and current passed through the cell to provide a cathodic current density of 2A/dm². A zinc coating was formed on the surface of the steel article that was light grey in appearance, and after 20mins, the thickness achieved was lOμm. The current was switched off and

the conversion coating formation took place, forming an adherent and continuous layer on the coated article. After 2mins the article was removed from the solution, rinsed in distilled water at ambient temperature, rinsed in water at 60°C, and then dried in a stream of air. The conversion coating was light yellow with a brilliant lustre. The conversion coating was visually identical to traditional yellow chromate.

Example 12

An electroplating-conversion coating solution comprising zinc sulphate heptahydrate, ZnS0₄.7H₂0, 220g/L, cerium sulphate, Ce(S0₄)₂, 2.5g/L, lanthanum chloride, LaCl₃, lg/L, and hydrogen peroxide, H₂0₂, 30vol, lml/L, dissolved in distilled water was provided.

The solution was contained in a plastic vessel into which a platinised anode was placed. The plastic vessel was heated

by an electrical element to a temperature of 25°C. The

thickness achieved was lOμm. The current was switched off and

the conversion coating formation took place, forming an adherent and continuous layer on the coated article. After 2mins the article was removed from the solution, rinsed in distilled water at ambient temperature, rinsed in water at

60°C, and then dried in a stream of air.

The conversion coating was a golden yellow with a brilliant lustre.

Example 13

An electroplating-conversion coating solution comprising zinc sulphate heptahydrate, ZnS0₄.7H₂0, 220g/L, neodymium chloride, NdCl₃, lg/L, and sodium nitrate, NaN0₃, lg/L, dissolved in distilled water was provided.

heated by an electrical element to a temperature of 25°C. The

solution was aspirated with air. A cleaned steel article was connected to the negative terminal of a direct current power supply, whilst the anode was connected to the positive terminal. The steel article was immersed in the solution thus forming the cathode in the cell, and current passed through the cell to provide a cathodic current density of 2A/dm². A zinc coating was formed on the surface of the steel article that was light grey in appearance, and after 20mins, the

thickness achieved was lOμm. The current was switched off and the conversion coating formation took place, forming an adherent and continuous layer on the coated article. After 2mins the article was removed from the solution, rinsed in distilled water at ambient temperature, rinsed in water at 60°C, and then dried in a stream of air.

The conversion coating was light blue with a brilliant lustre .

Further experimental studies have shown that the presence of 30-Vol hydrogen peroxide in the electrolyte can lead to the formation of dendrites during electroplating, due to the reaction between lanthanide ions and peroxide. As this blocks off sites where the zinc ions can be reduced on the cathode surface, leading to localised electrodeposition phenomena, and the growth of zinc dendrites .

A method of tackling such growth is to remove the peroxide from the electroplating solution after each plating run. Thus for electroplating, the solution might contain Zn₂ ⁺ and Ce₃ ⁺, (Solution A) . Following electroplating, H₂0₂ is added to the electrolyte (Solution B) , thus stimulating the formation of the coloured passivation film on the surface of the zinc electroplated coating. The current is either off, or on, but at a low voltage or current density. Before the next plating run is carried out, the peroxide is removed, or significantly reduced. This is achieved by pumping the electrolyte through a column containing material that decomposes H₂0₂ using a peristaltic pump, and returning it to the plating bath in a closed loop. As only low concentrations of H₂0₂ are involved, this cleansing is achieved within a few minutes, and certainly within an acceptable time frame for industrial operation. In doing so Solution B substantially returned to the condition of Solution A.

The materials tested in the columns to remove as much H₂0₂ as possible include granular activated carbon (GAC) , alone and with transition metals, including iron, copper, and manganese. Acceptable results have been obtained with GAC alone. GAC has not been found to have any detrimental effect on the other species added to the baths .

Using this cycling approach, creation of dendrites is substantially avoided and the coatings produced are of a very good quality.

Further it has been found that the best quality films obtained from the plating-passivation solution are those that contain more than one lanthanide ion. The experiments were based on the solution concentrations listed in Example A, namely lg/L total of lanthanide salt (as sulphate), 0.5mi/L

H202, and 220g/L zinc sulphate (as heptahydrate) . Analar or equivalent grades of chemicals were used where appropriate.

Four samples were produced from each solution. The first one was discarded as it was produced from ^" Solution B " before the electrolyte had been treated through activated carbon. The remaining three samples were seen as being more representative of the actual operational process. After the plating-^" passivation process had been carried out, the samples were immersed in static 3.5% NaCl solution at room temperature.

The time taken for red rust to appear, namely the corrosion products of the underlying 2 " x 2^" mild steel test panels, was then estimated from daily examinations and averaged for each set of samples. There are a large number of possible permutations in terms of lanthanide ion combinations, but some of the results obtained were as follows:

Lanthanide Time to red rust (hours) " Average of three samples Unless otherwise stated _γ Y₃3+⁺ 168

L Laa33⁺⁺ 168

C Cee33⁺⁺ 288

N Ndd33⁺⁺ 216

P Prr33^TT 196 (2 samples tested)

C Cee33⁺⁺ Y Y33"" 384

C Cee33⁺⁺ L Laa33⁺⁺ 476

C Cee33⁺⁺ N Ndd33⁺⁺ 452 (1 sample)

C Cee33⁺⁺ P Prr33⁺⁺ 428

N Ndd33⁺⁺ P Prr33⁺⁺ 336

Ce3⁺ La3⁺ Pr3⁺ 524 (1 sample)

Ce3⁺ La3⁺ Nd3⁺504

Note that the total lanthanide salt concentration was lg/L, thus for two lanthanide ions, the concentration of each was 0.5_g/l, and for three 0.33 g/L. The thickness of the underlying zinc coatings was 15 microns, and the filming treatment time (namely the time of exposure to H₂0₂) was 60- 120 sec.

In most cases of two or three samples the error was +/- 24 hours.

The experiments demonstrate that for two lanthanide ions, improvements in corrosion resistance were obtained (over one alone, and with three, there were further gains in resistance to red rust formation.

This is important at lower grade commercially available, lanthanide salts contain known ratios of other lanthanides than the chief lanthanide. Otherwise using three different lanthanides may not be economic, but two would be more acceptable .

Claims

1. A conversion coating solution comprises at least one precursor agent, at least one activator agent, and water solvent, in which on provision to a solution suitable for the deposition of a substantially metallic coating, provides an in-situ conversion coating to the surface of any substantially metallic coating.

2. A solution according to claim 1, in which the solution further comprises at least one modifying agent.

3. A solution according to claim 1 or claim 2, in which the at least one precursor agent is chosen from Y³⁺, La³⁺, Ce .4"+^τ, P .3^J+ 3+

Sm ,^JT , τ Eτ,u,3^J+^T,

Tb .3^J+^T, Dy .3^J+⁺, Ho ,3^J+⁺ , Er^J+ , Tm³⁺ , Yb³⁺, Lu³⁺ .

4. A solution according to claim 3 in which the at least one precursor agent is chosen from Y 3+, La .3+, Ce .3+, Ce , Pr .3+,

A solution according to any preceding claim, in which the at least one activator agent comprises a peroxide, peroxonium salt, a hydroperoxide, ozone analogue of peroxide, or nitrate.

6. A solution according to any one of claims 1 to 4, in which the activator agent is peroxide.

7. A solution according to any preceding claim upon addition to the solution suitable for deposition, in which the solution suitable for the deposition of a substantially metallic coating comprises at least one of the following: Zn²⁺, Fe²⁺, Fe⁺, Ni²⁺, Co²⁺, Ag⁺, Sn²⁺, Sn³⁺, Cu⁺, Cu²⁺, Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺.

8. A solution according to claim 7, in which the solution suitable for the deposition of a substantially metallic coating comprises at least one of the following: Zn²⁺, Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺.

9. A solution according to claim 7 or claim 8, in which the solution suitable for the deposition of a substantially metallic coating is an electroplating solution.

10. A solution according to claim 9, in which the solution suitable for the electrodeposition of a substantially metallic coating is a proprietary electroplating solution .

11. A solution according to any preceding claim further comprises a conversion coating modifying agent, comprising a non-metallic anion.

12. A solution according to claim 11, in which the non- metallic anion comprises one of silicate, oxalate, iodate, formate, phosphate, or carbonate.

13. A solution according to any preceding claim, in which the solvent is acid or alkaline.

14. A solution according to any preceding claim, in which the solvent is water.

15. An article treated using a solution according to any previous claim.

16. A process for coating an article, comprising the step of providing a- conversion coating solution comprising at least one precursor agent, at least one activator agent, and water solvent to a solution suitable for the deposition of substantially metallic coatings, which imparts a conversion coating layer to the substantially metallic coating.

17. A process for coating an article comprising the steps providing a conversion coating solution comprising at least one precursor agent, and water solvent to an aqueous solution suitable for the deposition of substantially metallic coatings, bringing an article into contact with the solution such that a substantially metallic coatings, is formed on the article, providing at least one activator agent such that a conversion coating layer is imparted to the substantially metallic coating.

18. A process for coating an article comprising the steps of providing a conversion coating solution comprising at least one precursor agent, and water solvent to an aqueous solution suitable for the electrodeposition of substantially metallic coatings, bringing an article into contact with the solution, passing on electrical current to the article such that a substantially metallic coating is deposited onto the article, substantially reducing or halting the current, providing at least one activator agent to the solution, such that a conversion coating layer is imparted to the substantially metallic coating.

19. A process for coating an article comprising the steps of providing a conversion coating solution comprising at least one activator agent, and water solvent to an aqueous solution suitable for the electrodeposition of substantially metallic coatings, bringing an article into contact with the solution, passing an electrical current to the article such that a substantially metallic coating is deposited onto the article, substantially reducing or halting the current providing at least one precursor agent to the solution, such that a conversion coating layer is imparted to the substantially metallic coating.

20. A process according to any one of claims 15 to 19, in which the solution is reused after the further step of substantially removing the activator agent.

21. A process according to any one of claims 15 to 20, in which the activator agent comprises a peroxide or nitrate.

22. A process according to any one of claims 15 to 21, in which the precursor agent comprises Y³⁺, La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺.

23. A process according to claim 22, in which the at least one precursor agent is chosen from Y³⁺, La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺.

24. A process according to any one of claims 15 to 23, in which the solution suitable for the deposition of a substantially metallic coating comprises at least one of the following: Zn²⁺, Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺, Ag⁺, Sn²⁺, Sn³⁺, Cu\ Cu²⁺, Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺.

25. A process according to claim 24, in which the suitable for the deposition of a substantially metallic coating comprises at least one of the following: Zn²⁺, Fe²⁺, Fe³⁺, Ni , Co

26. A process according to any one of claims 15 to 25, in which the solution suitable for deposition of a substantially metallic coating is an electroplating solution .

27. A process according to claim 26, in which the solution suitable for the electrodeposition of a substantially metallic coating is a proprietary electroplating solution .

28. A process according to any of claims 15 to 27, in which the solution further comprises a modifying agent.

29. A process according to claim 28, in which the modifying agent is a non-metallic anion comprising at least one of silicate, oxalate, iodate, formate, phosphate, or carbonate .

30. A process according to any of claims 15 to 29, in which the solvent in acid or alkaline.

31. A process according to any of claims 15 to 30, in which the solvent is water.

32. A process according to any of claims 15 to 31, in which the activator is provided to the solution prior to deposition or electrodeposition.

33. A process according to any of claims 15 to 31, in which the activator is provided to the solution during deposition or electrodeposition.

34. A process according to any one of claims 15 to 31, in which the activator is provided to the solution following deposition or electrodeposition.

35. A process according to any one of claims 15 to 34, in which the electrical current provided to reduce metal ions in the plating bath to their metallic state is direct current.

36. A process according to any one of the claims 15 to 34, in which the electrical current provided to reduce metal ions in the plating bath to their metallic state is pulsed current.

37. A process according to any one claims 15 to 34, in which the electrical current provided to the plating bath is direct current superimposed with alternating current.

38. A process according to any claims 15 to 37, in which the substrate is a metal.

39. A process according to claim 38, in which the substrate is steel.

40. A process according to any of claims 15 to 39, where the substrate is a non-metal.

41. A process according to claim 40, in which the substrate is a plastic or carbon fibre.

42. An article treated using a process according to any of claims 15 to 41.