Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1997-04-07
2000-02-08
Willis, Prince
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148273, 148275, 148276, 148277, 148285, 4284722, C23C 2282
Patent
active
060224258
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a conversion coating for metal surfaces and a process and a solution for forming a conversion coating on metal surfaces. The invention is particularly concerned with a conversion coating on aluminium or aluminium alloy and a process and a solution for the formation of a conversion coating on aluminium or aluminium alloy.
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, steel, 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, automotive, architectural, can stock, instrument 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.sup.6+, is a serious environmental and health hazard. Similarly, phosphate ions are a serious 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 based conversion coatings and in the time required to deposit those coatings.
Accordingly, it is an object of the present invention to provide a conversion coating for a metal surface and a process 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 an aqueous, rare earth element containing solution for use in providing a conversion coating on a metal surface.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a process for forming a conversion coating on the surface of a metal, including: to initiate growth of a metal oxide cell structure on the metal surface; the oxide and form an oxide containing layer of a desired thickness; and substantially seal the oxide containing layer.
The present invention also provides a conversion coated metal, wherein the conversion coating comprises a metal oxide containing layer which is impregnated with one or more rare earth elements.
The present invention further provides an aqueous, rare earth element containing solution for use in providing a conversion coating on a metal, said solution including a sufficient quantity of a rare earth element to impregnate and substantially seal a metal oxide containing layer formed on the surface of said metal.
The invention will now be described with particular reference to its use for aluminium or aluminium containing alloys. However, a skilled addressee will understand that the invention is not limited to this use and can be used in relation to other metals, such as zinc.
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 t
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Abstract--Bibber, J.W., "Corrosion resistant coating composition for aluminum and its alloys--contains alkali metal permanganate and has basic pH".
Abstract--Bibber, J.W., "Corrosion resistant vonversion coating prepn. for aluminum alloys--by typically treating alloy with aluminum and alkai metal nitrate compsn. then alkai metal permanganate compsn.".
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Abstract--Bibber, J.W., "Corrosion resistant coating composition for aluminum alloys--which comprises potassium permanganate and borax, etc. in aq. soln.".
Henderson Mark Julian
Hinton Bruce Roy William
Hughes Anthony Ewart
Nelson Karen Joy Hammon
Nugent Sally Ann
Commonwealth Scientific and Industrial Research Organisation
Ohmans Andrew L.
Willis Prince
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