Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2001-11-20
2004-08-10
Oltmans, Andrew L. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S272000, C148S273000, C148S275000, C106S014210, C252S387000
Reexamination Certificate
active
06773516
ABSTRACT:
FIELD OF THE INVENTION
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 or 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.
BACKGROUND OF THE INVENTION
The term “conversion coating” 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 the aerospace, automotive, architectural, can stock, instrument and building industries.
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, Cr
6+
, is a serious environmental and health hazard. Similarly, phosphate ions are 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 “REE”) 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.
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 within the time needed for industrial coating, that means within much less than five minutes.
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 of 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 also 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 such a composition leads to a significant decomposition of the peroxidic compound. Furthermore, 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 WO96/15292, a time for applying the solution was needed which was much longer than the typical times required in current industrial practice, i.e. from about 1 to 3 minutes.
Over the years there have been numerous attempts to replace chromate-based chemicals by ones less hazardous to health and the environment. The proposed alternatives suffer from the disadvantages of either forming colorless conversion coatings—for example Gardobond 764®, which is based on zirconium fluoride—or requiring very long treatment times, such as 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 chromate based solutions 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 the conversion coating on the metallic surface.
It has been discovered that the addition of one or more additives, having particular compositions, to the coating solution can assist in accelerating the coating process and/or improving adhesion of the conversion coating to the metal surface. Such coating solutions have the advantages of forming conversion coatings in a short period of time as required in industrial applications, and having a low rate of decomposition of peroxidic composition solution.
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 “rare earth” elements, metals or ions, or “REE” 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 “peroxidic compound” refers to any of the group of peroxo acids and their salts or any peroxo containing compound such as hydrogen peroxide. Also, the expression: “metal selected from Groups VA and VIA of the Periodic Table” is intended to cover both metals and metalloids of Groups VA and VIA, namely As, Sb, Bi, Se, Te and Po. Further, the generic term “part” is intended to cover any body or component of any shape or size having at least one metallic surface thereon.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an aqueous, acidic solution for forming a rare earth element containing conversion coating on the surface of a metal, said solution being chromate-free and including effective quantities of at least one rare earth element (as herein defined) containing species, an oxidant and at least one accelerator, comprising a metal selected from Groups VA and VIA of the Periodic Table.
According to the present invention, there is also provided a process for forming a conversion coating on the surface of a metallic material including contacting said surface with an aqueous, acidic conversion coating solution containing at least one rare earth element (as herein defined) containing species and a peroxidic species, said solution including at least one accelerator, comprising a metal selected from Groups VA and VIA of the Periodic Table, wherein the solution is essentially free of chromate.
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 aqueous, acidic conve
Hardin Simon Gerard
Hughes Anthony Ewart
Nelson Karen Joy Hammon
Wittel Klaus Werner
Commonwealth Scientific and Industrial Research Organisation
Jacobson & Holman PLLC
Oltmans Andrew L.
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