This invention relates to a surface treated part with a conversion coating formed on a metallic surface and to a process for forming this conversion coating, to a liquid aqueous concentrate for the make-up for the replenishing of a conversion coating solution as well as to a solution for forming a conversion coating on surfaces of metallic materials.  The invention is particularly concerned with a conversion coating on aluminum, aluminum alloy, magnesium, magnesium alloy, zinc or zinc alloy and a process, a concentrate and a solution for the formation of a conversion coating on parts of these metallic materials. 
The term xe2x80x9cconversion coatingxe2x80x9d is a well known term of the art and refers to the replacement of native oxide on the surface of a metallic material by the controlled chemical formation of a film.  Oxides, chromates or phosphates are common conversion coatings.  Conversion coatings are used on metallic materials such as steel or aluminum, zinc, cadmium, magnesium and their alloys, and provide a key for paint adhesion and/or corrosion protection of the metallic substrate.  Accordingly, conversion coatings find application in such areas as aerospace, automotive, architectural, and packaging. 
Known methods for applying conversion coatings to metallic surfaces include treatment with chromate or phosphate solutions, or mixtures thereof.  However, in recent years it has been recognized that the hexavalent chromium ion, Cr6+, is a serious environmental and health hazard.  Similarly, phosphate ions pose a considerable risk, particularly when they find their way into natural waterways and cause algal blooms.  Consequently, strict restrictions have been placed on the quantity of these species used in a number of industrial processes and limitations have been placed on their release to the environment.  This leads to costly effluent processing. 
In the search for alternative, less toxic conversion coatings, research has been conducted on conversion coatings based on rare earth compounds.  However, there is considerable room for improvement in the adhesion and corrosion protection properties of prior rare earth element (hereinafter referred to as xe2x80x9cREExe2x80x9d) based conversion coatings and in the time required to deposit those coatings.  The need for improvement is particularly true for conversion coatings on certain metal alloys, such as 3000, 5000 and 6000 series aluminum alloys, which coatings can be slow to deposit and have variable adherence or no adherence. 
It is also very important to develop conversion coating solutions and processes which are compatible with existing coating apparatus and equipment used in the art.  In particular, the use of stainless steel containers to hold conversion coating solutions is prevalent in the conversion coating industry.  Typically much money and infrastructure has been invested in such equipment and it is often impractical and/or prohibitively expensive to replace it. 
WO 88/06639 teaches a process for forming a conversion coating on metal using a cerium containing conversion coating solution.  However, it has been found that said process does not produce acceptable coatings on alloys of the 3000, 5000 and 6000 series of aluminum alloys within the time needed for industrial coating, that means within much less than five minutes.  Moreover, this process requires a specified initial chloride content which increases in the bath over the course of the process.  It has been found that the initial and increasing chloride content in the bath adversely affects stainless steel containers by considerable corrosion attack. 
WO 96/15292 describes a REE containing conversion coating and a process for its formation using a solution containing REE and additives selected from (i) metal peroxo complexes in which the metal is selected from Groups IVB, VB, VIB and VIIB; and (ii) metal salts or complexes with a conjugate base of an acid in which the metal is selected from Transition Elements other than chromium especially copper, silver, manganese, zinc, iron, ruthenium and Group IVA elements, especially tin.  The solution preferably includes hydrogen peroxide.  Good results were obtained using the additive Cu alone or in combination with Mn, Ti-peroxo complexes and/or Mo-peroxo complexes.  However, it has been found that the use of two different accelerators creates difficulties in controlling the process, particularly when it is used on an industrial scale.  In all the other examples disclosed in W096/15292 a time for applying the solution was needed which was much longer than the typical time required in current industrial practice, i.e. from about 1 to 3 minutes.  Moreover, while anions other than chloride are mentioned in WO 96/15292, only chloride containing solutions were disclosed and the concentrations of chloride in those solutions have been found to cause corrosion attack of stainless steel equipment. 
Examples 13 to 15 of WO 96/15292 indicate in comparison to examples 7 to 12 and 16 to 27 that optimum results are obtained in a very narrow window of conditions, i.e. a pH value only of 2.3 and a relatively high copper content of about 100 ppm.  These optimum conditions however, are quite problematic.  The pH value of 2.3 is quite high with the result that the solution is close to the stability limit of the trivalent REE ions.  For example, the oxidation of Ce3+ to Ce4+ is pH dependent and is favoured at higher pH values.  If pH increases to 2.5 and above, formation of insoluble Ce(IV) compounds occurs.  This means that REE compounds are already precipitating out of solution, causing sludge in the bath and thus further costs are required to remove it.  Moreover, a copper content of about 100 mg/l causes the rapid catalytic decomposition of hydrogen peroxide to water and oxygen requiring replenishment of H2O2 which leads to increasing costs and a considerable dilution of the solution. 
Over the years there have been numerous attempts to replace chromating chemicals by ones less hazardous to health and the environment.  One major disadvantage of the replacement solutions is that they form colourless conversion coatings, e.g. Gardobond 764(copyright), which is based on zirconium fluoride.  Coloured conversion coatings are highly desirable from a practical point of view as they give a readily visible indication of the presence of a coating and its quality. 
Another major disadvantage of prior replacement solutions is that they have required very long treatment times, like the chemical oxidation process described in EP-A-0 769 080.  Zirconium and titanium based conversion coating processes have found some applications in certain market niches, but they have failed in the past 25 years to replace chromating as a pre-treatment prior to painting of aluminum, magnesium, zinc or their alloys. 
Accordingly, it is an object of the present invention to provide a conversion coating for the surface of a metallic material which overcomes, or at least alleviates, one or more of the disadvantages or deficiencies of the prior art.  It is also an object of the present invention to provide an aqueous, rare earth element containing conversion coating solution for use in providing a conversion coating on a metallic surface.  It is a further object to provide a process for forming a conversion coating on the surface of a metallic material which overcomes, or at least alleviates, one or more of the disadvantages of the prior art. 
Advantages of this invention include the provision of a process and a solution which can meet the industrial requirements of 1.  formation of the coating in a short time, 2.  the generation of coloured coatings of high adhesion and coating quality, and 3.  solutions which may be used in stainless steel containers. 
It has been discovered that the careful selection of additives, to the coating solution can assist in accelerating the coating process, improving the coating quality, and/or the adhesion of the conversion coating to the metal surface, without causing corrosion of stainless steel containers. 
Throughout the specification, reference will be to the CAS version of the Periodic Table, as defined in (for example) Chemical and Engineering News, 63(5), 27, 1985.  Furthermore, as used herein, the term xe2x80x9crare earthxe2x80x9d elements or ions, or xe2x80x9cREExe2x80x9d refers to the elements of the Lanthanide series, namely those having the atomic number 57 to 71 (La to Lu), plus scandium and yttrium.  Moreover, as used herein, the term xe2x80x9cperoxidic compoundxe2x80x9d refers to any of the group of peroxo acids and their salts or any peroxo containing compound such as hydrogen peroxide.  Also, the expression: xe2x80x9cmetals of Groups IB, IIB, IVA, VA, VIA, and VIII of the Periodic Tablexe2x80x9d refers to both metals and metalloids of each group.  It explicitly covers the elements Cu, Ag, Au, Zn, Cd, Hg, Si, Ge, Sn, Pb, As, Sb, Bi, Se, Te, Po, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd and Pt.  Further, the generic term xe2x80x9cpartxe2x80x9d is intended to cover any body or component of any shape or size having at least one metallic surface thereon. 
According to the present invention, there is provided an aqueous, acidic solution for forming a conversion coating on the surface of a metallic material, said solution containing at least one rare earth element (as herein defined) containing species, an accelerator additive selected from the group consisting of metals of Groups IB, IIB, IVA, VA, VIA and VII of the Periodic Table, a peroxidic species, and at least one acid selected from the group of mineral acids, carboxylic acids, sulphonic acids and phosphonic acids, wherein said solution contains no more than 20 mg/l each of fluoride and of phosphate, preferably no more than 10 mg/l each, and the solution is essentially free of chromate.  Preferably the amount of chloride containing species present in the coating solution is controlled so that the concentration of total chloride is within the range of from 50 to 1500 mg/l.
According to the present invention, there is also provided a process for forming a conversion coating on the surface of a metallic material including the step of contacting said surface with an aqueous, acidic conversion coating solution containing at least one rare earth element (as herein defined) containing species, an accelerator additive selected from the group consisting of metals of Groups IB, IIB, IVA, VA, VIA and VII of the Periodic Table, a peroxidic species, and at least one acid selected from the group of mineral acids, carboxylic acids, sulphonic acids and phosphonic acids, wherein said solution contains no more than 20 mg/l of each of fluoride and of phosphate, and the solution is essentially free of chromate.  Preferably, the amount of chloride present in the coating solution is controlled to be within the range of from 50 to 1500 mg/l.
The present invention also provides a surface treated part including a metallic material having a conversion coating thereon resulting from treatment with the aqueous, acidic conversion coating solution of the invention.  The treated part may additionally bear a coating of a paint, a lubricant and/or a sealant.  The treated part may be subsequently used in a process involving cold forming, glueing, welding and/or other joining processes.  The conversion coating preferably contains at least 5% by weight of a rare earth compound. 
The present invention also provides a liquid acidic aqueous concentrate for the make-up of a conversion coating solution according to the invention wherein the concentrate contains at least 80 g/l and preferably at least 100 g/l of total rare earth elements (as herein defined), an accelerator selected from the group consisting of metals of Groups IB, IIB, IVA, VA, VIA and VIII of the Periodic Table, and at least one acid selected from the group of mineral acids, carboxylic acids, sulphonic acids and phosphonic acids, wherein the concentrate contains no more than 100 mg/l each of fluoride and of phosphate and the solution contains essentially no chromate. 
The present invention also provides a liquid acidic aqueous concentrate for the replenishing of a conversion coating solution according to the invention, wherein the concentrate contains rare earth ions (as herein defined) and monovalent anions in a molar ratio of total rare earth ions:monovalent anions of from 1:200 to 1:6 and/or rare earth ions and divalent anions in a molar ratio of total rare earth ions:divalent anions of from 1:100 to 1:3 and/or the concentrate contains at least one metal selected from Groups IB, IIB, IVA, VA, VIA and VII, preferably from the group of Cu, Ag, Au, Cd, Hg, Ni, Pd, Pt, Co, Rh, Ir, Ru, Os, Sn, Pb, Sb, Bi, Se and Te and anions such that the molar ratio of the sum of the elements in this group: anions is in the range from 1:50 to 1:10,000. 
Preferably the accelerator additive is selected from the elements Cu, Ag, Au, Cd, Hg, Ni, Pd, Pt, Co, Rh, Ir, Ru, Os, Sn, Pb, Sb, Bi, Se and Te.  The most preferred accelerator additive is Cu. 
The at least one acid is preferably selected from the group comprising sulphuric acid, sulphamic acid, hydrochloric acid, nitric acid, perchloric acid, carboxylic acids, alkyl sulphonic acids, aryl sulphonic acids, alkyl phosphonic acids, and aryl phosphonic acids.