Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1997-07-24
2001-03-27
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S275000
Reexamination Certificate
active
06206982
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for forming a conversion coating on metal surfaces and a solution for use in said process. The invention extends to the conversion coated metal thus formed. The invention is particularly concerned with a process and solution for forming a conversion coating on aluminium or aluminium alloy, and the conversion coated aluminium or aluminium thus formed.
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 metal by the controlled chemical formation of a film. Oxides or phosphates are common conversion coatings. Conversion coatings are used on metals such as aluminium, iron, zinc, cadmium or magnesium and their alloys, and provide a key for paint adhesion and/or corrosion protection of the substrate metal. Accordingly, conversion coatings find application in such areas as the aerospace, architectural and building industries.
Known methods for applying conversion coatings to metal surfaces include treatment with chromate or phosphate solutions, or mixtures thereof. However, in recent years it has been recognised that the hexavalent chromium ion, Cr
6+
, is a serious environmental and health hazard. Phosphate ions can also be detrimental, particularly when they find their way into natural waterways and cause algal blooms. Consequently, strict restrictions have been placed on industrial processes and limitations have been placed on the release of such solutions 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. One prior conversion coating process has been described in Australian patent specification AU-A-14858/88 which is incorporated herein by reference. That conversion coating process comprises contacting a metal surface with a solution formed by an aqueous acidic solution containing cerium and H
2
O
2
in which some or all of the cerium has been oxidised to the +4 valence state. It is asserted in AU-A-14858/88 that an increase in the solution pH in the region of the metal surface to a sufficiently high value causes precipitation of a cerium containing coating on the metal surface.
There is, however, considerable room for improvement in the properties of prior rare earth element based conversion coatings, such as adhesion, 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 aluminium alloys, which coatings can be slow to deposit and have variable adherence or no adherence.
Accordingly, it is an object of the present invention to provide a process and solution for forming a conversion coating on a metal surface which overcome, or at least alleviate, one or more of the disadvantages or deficiencies of the prior art. It is also an object of the present invention to provide conversion coated metal surface formed by the process of the invention.
It has been discovered that 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.
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 “transition elements” or “transition metals” refers to the elements of the Periodic Table from scandium to zinc inclusively, yttrium to cadmium inclusively and lanthanum to mercury inclusively. Moreover, as used herein, the term “rare earth” elements, metals or cations refer to the elements of the Lanthanide series, namely those having the atomic number 57 to 71 (La to Lu), plus scandium and yttrium. In addition, the term “higher valence state” means a valence state above zero valency.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an aqueous, acidic solution for forming a rare earth element containing coating on the surface of a metal, said solution including effective quantities of:
(a) one or more rare earth element containing species, including at least one rare earth element capable of having more than one higher valence state; and
(b) one or more additives selected from the groups including:
(i) aqueous metal complexes including at least one peroxo ligand; and
(ii) metal salts or metal complexes of a conjugate base of an acid in which the metals are selected from Transition Elements and Group IVA elements of the Periodic Table.
The invention also provides a process for forming a coating on the surface of a metal, in which the metal surface is contacted with an aqueous, acidic solution including effective quantities of:
(a) one or more rare earth element containing species, including at least one rare earth element capable of having more than one higher valence state; and
(b) one or more additives selected from the groups including:
(i) aqueous metal complexes including at least one peroxo ligand; and
(ii) metal salts or metal complexes of a conjugate base of an acid in which the metals are selected from the Transition Elements and Group IVA of the Periodic Table.
The invention also extends to a metal surface having deposited thereon a conversion coating formed according to the process of the preceding paragraph.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will now be described with focus on its use for aluminium or aluminium containing alloys. However, a skilled addressee will understand that the invention is not limited to this use.
It may be appropriate for the process of the present invention to be preceded by the steps of degreasing and/or cleaning and deoxidising/desmutting the metal surface.
The degreasing step, if present, comprises treatment of the metal surface with any suitable degreasing solution to remove any oils or grease (such as lanoline) or plastic coating present on the metal surface.
The degreasing step, if present, preferably comprises treating the metal surface with a vapour degreasing agent such as tricholoroethane or an aqueous degreasing solution available under the trade name of BRULIN. A degreasing step may be necessary, for example, where the metal has been previously coated with lanoline or other oils or grease or with a plastic coating.
Subsequent to the degreasing step, the metal surface preferably undergoes a cleaning step in order to dissolve contaminants and impurities, such as oxides, from the surface of the metal. Preferably, the cleaning step comprises treatment with an alkaline based solution.
The alkaline solution is preferably a “non-etch” solution, that is, one for which the rate of etching of material from the metal surface is low. A suitable alkaline cleaning solution is that commercially available under the trade name RIDOLINE 53.
The treatment with an alkaline cleaning solution is preferably conducted at an elevated temperature, such as up to 80° C., preferably up to 70° C.
Treatment with an alkaline solution often leaves a “smut” on the surface of the metal. As used herein, “smut” is intended to include impurities, oxides and any loosely-bound intermetallic particles which as a result of the alkaline treatment are no longer incorporated into the matrix of the aluminium alloy. It is therefore preferable to treat the metal surface with a “desmutting” or deoxidizing solution in order to remove the smut from the metal surface. Removal of smut is normally effected by treatment with a desmutting (deoxidizing) solution comprising an acidic solution having effective amounts of appropriate additives. Preferably the desmutting solution also dissolves native oxide from the surface of the metal to leave a homogeneously thin oxide on the metal surface. The desmutting solution may be chromate-based. Alternatively, the desmutting solution may be phosphate based.
Alternative
Hughes Anthony Ewart
Nelson Karen Joy Hammon
Turney Terence William
Commonwealth Scientific and Industrial Research Organisation
McDermott & Will & Emery
Ottmans Andrew L.
Sheehan John
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