Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2001-11-20
2004-06-29
King, Roy (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
06755917
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 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 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, Cr
6+
, 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 “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.
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 Ce
3+
to Ce
4+
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 H
2
O
2
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®, 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, ref
Hardin Simon Gerard
Hughes Anthony Ewart
Nelson Karen Joy Hammon
Wittel Klaus Werner
Commonwealth Scientific and Industrial Research Organisation
Jacobson & Holman PLLC
King Roy
Oltmans Andrew L.
LandOfFree
Process and solution for providing a conversion coating on a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process and solution for providing a conversion coating on a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process and solution for providing a conversion coating on a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3350598