Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-11-22
2003-02-11
Barr, Michael (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S330000, C427S419400
Reexamination Certificate
active
06517904
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of Invention
The present invention relates to a process for the enameling of aluminum or aluminum alloy surfaces. In particular, aluminum or aluminum alloy surfaces containing a relatively high amount of magnesium (Mg), copper (Cu), or both.
(2) Description of Related Art
Aluminum alloys with relative high Mg and Cu contents tend to be susceptible to spalling. Spalling, in the context of the invention, is defined as flaking or chipping off of the vitreous enamel layer from the aluminum substrate after firing. Thereby, destroying the aesthetic and/or functional qualities of the enameled surface. Standard ISO/DIS 13805 (1998) describes a method to test the spalling resistance of an enameled aluminum surface. According to the prior art, it is well known that aluminum alloys, with amounts of Mg>50 PPM and/or Cu>0.2 weight percent (wt. %), are certainly susceptible to spalling.
It is known that magnesium in aluminum alloys, if not “tied up” into Mg
2
Si precipitates when silicon is present, migrates to the surface during the firing of a vitreous enamel layer applied thereto, and later forms magnesium oxide (MgO). Because MgO is hardly soluble in aluminum enamels, an intermediate layer is formed between the aluminum alloy surface and the enamel layer, when the diffusion of magnesium occurs during firing. After diffusion, magnesium can easily be oxidized by the contact with oxygen and high temperatures during the firing. The change Mg+1/2O
2
→MgO induces an increase of the volume, e.g., increase in the lattice parameter. This intermediate layer with high MgO content provides very poor adherence between the aluminum alloy and the enamel layer. During weathering, exposure to chemical attack, or thermal changes of such enameled surfaces, the intermediate layer corrodes and induces spalling through stresses at the interface.
Several methods have been proposed in order to improve the adherence of vitreous enamel layers on aluminum alloy surfaces. A popular technique is chromatation of the aluminum alloy surface before enameling. U.S. Pat. No. 3,222,266, EP 0,611,834 A1,and EP 0,648,863 B describe a method involving the anodisation of the aluminum alloy surface before enameling. A disadvantage of these methods is that the require high strength aluminum alloys. Further, these methods are expensive and in the case of chromatation, dangerous.
U.S. Pat. No. 2,932,584 and GB A 840,469 describe a method to avoid migration of Mg to the surface by tying up all the Mg in the aluminum alloy as Mg
2
Si. A drawback of this method is that it involves adding silicon (Si) to the substrate in very precise ratios of Mg to Si.
EP 0,686,609 A1 describes adding iron oxide to the vitreous enamel slip during milling to improve adherence. A drawback of this method is that there exist aluminum alloy surfaces which cannot be successfully enameled with a standard vitreous enamel slip that only contains 1-10% by weight iron oxides as milling agent
In addition, all of these prior methods are not suitable to provide sufficient adherence of the vitreous enamel layer to the aluminum alloy surface containing an amount of Mg>50 PPM, and/or an amount of Cu>0.2 wt. %.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to improve the adherence between an aluminum or aluminum alloy surface and the vitreous enamel after filing and to solve the aforementioned problems in enameling processes of aluminum or aluminum alloy surfaces containing a high amount of Mg, Cu, or both.
A feature of the invention is the use of a vitreous enamel containing copper oxide (CuO) from the addition of CuO either directly into the composition of a standard vitreous enamel frit suitable for enameling on aluminum, or in the vitreous enamel slip prepared using a standard frit suitable for enameling on aluminum.
In an embodiment of the invention, a process for the enameling of aluminum or aluminum alloy surfaces comprises the step of using a vitreous enamel containing CuO. The surface layer is selected from a group consisting of: an aluminum cast alloy, an aluminum wrought alloy, a hot dip aluminum alloy, and a cladded aluminum alloy. The aluminum or aluminum alloy contains greater than 50 PPM Mg, greater than 0.2 wt. % Cu, or both. The process may include a degreasing, pickling, conversion treating, or a combination thereof, pretreatment. A content of CuO in the range of 0.5 to 20 wt. % is achieved by partly or completely adding CuO during a melting of a frit. Alternatively, CuO is added during enamel slip preparation.
In an another embodiment of the invention, the above process further comprises the step of adding 0.5 to 10 wt. % of an oxide selected from a group consisting of Ni, Fe, Co, Mn, Sb, Bi, Cr oxides, or a combination thereof, to the vitreous enamel. Alternatively, 10 to 40 wt. % TiO
2
may be added. These oxide additions are partly or completely added to the vitreous enamel frit during melting of the flit composition. Alternatively, the oxide additions are partly or completely added to the vitreous enamel slip during slip preparation.
In another embodiment of the invention, the above processes further comprise the step of preparing white and dark vitreous enamel layers as a single layer coating or as the first layer of a multi-layer coating, wherein a thickness of the layer is in the range of 0.5 and 100 &mgr;m.
An advantage of the invention is the improved adherence between the aluminum alloy surface and the vitreous enamel after firing. Another advantage of the invention is the ability to enamel aluminum alloys with relatively high contents of Mg and/or Cu, e.g., Mg>50 PPM, and/or an amount of Cu>0.2 wt. %.
Another advantage of the invention is the elimination of a special, expensive, and dangerous pre-treatment of the surface, e.g., chromatation or anodisation; the requirement of a special ratio of Mg to Si in order to precipitate Mg completely.
In addition, another advantage of the invention is that the addition of CuO provides better enameling than the addition of iron oxide as described in prior art EP 0 686 609 A1. Further, aluminum alloy surfaces, which cannot be successfully enameled with a standard vitreous enamel slip that contains 1-10% by weight iron oxides as milling agent, can be successfully enameled with a vitreous enamel slip made of frits containing CuO and/or by the use of CuO as a milling addition.
In addition to CuO, the present invention includes the use of any compounds which may result in the formation of CuO, particularly during the firing of the frit, and is not limited to the direct addition of CuO as such.
Other objects and advantages of the invention are set forth in part in the description which follows, and in part, will be apparent from this description, or may be learned from the practice of the invention.
DETAILED DESCRIPTION OF THE INVENTION
This invention depicts a method to enamel aluminum or aluminum alloy surfaces containing relatively high amounts of Mg and Cu. The surface layer is an aluminum cast alloy, an aluminum wrought alloy, a hot dip aluminum alloy or a cladded aluminum alloy. Further, these surfaces may be connected to other substrates. The invention is particularly suitable to enamel as defined above wherein the Mg content in the aluminum alloy surface is larger than 50 PPM, and in particular, larger than 100 PPM; wherein the Cu content in the aluminum alloy surface is greater than 0.2 wt. %, and in particular, greater than 0.4 wt. % by weight; or both.
Although the present invention allows enameling of aluminum or aluminum alloy surfaces as defined above without specific pretreatment, pretreatment of the surface according to the prior art may be administered in combination with the invention. For example, a simplified pretreatment of the surface may include degreasing and/or pickling only.
In an embodiment of the invention, a vitreous enamel slip is made of frits containing CuO and/or by the use of CuO as a milling addition. The amount of CuO in the vitreous enamel can be varied in a wide range. A preferred ran
Crevits Nacy
Humez Sebastien
Leveaux Marc
Lips Koen
Barr Michael
Hunton & Williams
Pemco Brugge N.V.
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