Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2001-09-21
2004-09-21
Oltmans, Andrew L. (Department: 1742)
Stock material or miscellaneous articles
Composite
Of metal
C148S243000, C148S244000, C148S272000, C148S273000, C148S275000, C427S419200, C427S419300, C428S471000
Reexamination Certificate
active
06794046
ABSTRACT:
The present invention relates to an article made of magnesium or its alloys which has a conversion coating produced by passivating the surface, to a process for producing such an article, and to its use.
Magnesium and its alloys are the lightest but also least noble metallic construction materials (standard potential of Mg −2.34 volts) and therefore have a very strong tendency to corrode. To counter this disadvantageous property, magnesium and its alloys are treated in aqueous passivating electrolytes. The redox procedure which occurs during this treatment (without an external current source) forms a conversion coating consisting of oxides of the magnesium material and oxide-type reaction products originating from the constituents of the aqueous passivating electrolyte.
The term “conversion coating” refers here and below to a coating which is formed not by application to a surface but rather by chemical transformation (conversion) of the metallic surface and of various constituents of the aqueous passivating electrolyte (cf. H. Simon, M. Thoma “Angewandte Oberflächentechnik für metallische Werkstoffe”, Carl Hanser Verlag, Munich (1985) p. 4).
For example, the chromation of articles made of magnesium or its alloys is known. The corresponding processes are described in particular in the MIL M3171 type I to type III. In that case, chromic acid or its salts are used for passivation. The use of sodium dichromate in combination with potassium permanganate has also been described (Dow Chemical Treatment, No. 22). Chemical passivation using aqueous passivating electrolytes containing chromium(VI) is simple to carry out. However, it has the serious disadvantage that the chromate substances, which are also present in the resulting conversion coatings, are carcinogenic.
Furthermore, the recyclability of chromated articles made of magnesium or its alloys represents a considerable problem, since because of their heavy metal content such articles require considerable effort in order to be recyclable to what are known as “high-purity” materials.
On grounds of environmental protection and workplace safety, a concern among manufacturers and processors of passivated articles made of magnesium or its alloys is to find a substitute for the conventional chromation, using chromate-free, aqueous passivating electrolytes.
Known chromate-free aqueous passivating electrolytes for the passivation of articles made from magnesium or its alloys are stannate-based aqueous passivating electrolytes which are marketed, for example, by the company Dow Chemical. However, it has been found that the corrosion protection effect of the resulting conversion coating is lower in comparison to the chromated magnesium materials.
U.S. Pat. No. 5,743,971 describes a process for forming corrosion protection coatings on metals such as Zn, Ni, Ag, Fe, Cd, Al, Mg and their alloys.
In this process, these metals are immersed in a solution which comprises an oxidizing agent, a silicate and at least one cation from the group consisting of Ti, Zr, Ce, Sr, V, W and Mo. The pH of this solution is, in particular, in a range between 1.5 and 3.0.
The oxidizing agent is exclusively selected from the group of the peroxo compounds. A potassium permanganate oxidizing agent is not mentioned. Nor does this citation reveal the actual improvements provided by the process it describes for magnesium or its alloys in comparison to conventional chromations.
Moreover, the phosphation of articles made of magnesium or its alloys is also known (cf. Dow Chemical Treatment No. 18). Phosphating with simultaneous use of potassium permanganate is described in D. Hawk, D. L. Albright, “A Phosphate Permanganate Conversion Coating for Magnesium”, Metal Finishing, October 1995, pp. 34-38. Here again, the corrosion protection obtained using these aqueous passivating electrolytes is significantly lower in comparison to a chromated coating.
A further possibility for chemical passivation is described by CHIBA Institute of Technology, Japan (published in the conference material INTERFINISHING 96 World Congress, Birmingham, UK, 10-12 Sep. 1996, pp. 425-432), according to which a solution of potassium permanganate, alone or in combination with small amounts of acids (HNO
3
, H
2
SO
4
, HF), is present in an aqueous passivating electrolyte. The aqueous passivating electrolyte temperature required for the chemical passivation is between 40 and 84° C.
The conversion coating obtainable in this way exhibits a good protective effect; however, the stability of the aqueous passivating electrolyte is inadequate for an industrial application of this process. Thus, after a short time, magnesium dioxide (MnO
2
) is precipitated, rendering the aqueous passivating electrolyte unusable for the further passification of magnesium materials.
An object of the invention is to provide a chemically passivated article made of magnesium or its alloys whose conversion coating is obtainable by an electrolytic, current-free process which is simple to apply and is transferable to the industrial scale. The corrosion protection effect of such a conversion coating, moreover, should not be poorer than that of the known, chromated articles made of magnesium or its alloys.
This object is achieved in accordance with the invention by means of an article made of magnesium or its alloys, some or all of whose surface has a conversion coating, characterized in that the conversion coating comprises MgO, Mn
2
O
3
and MnO
2
plus at least one oxide from the group consisting of vanadium, molybdenum and tungsten.
The conversion coating of the invention may be obtainable by passivating the article using an aqueous passivating electrolyte which comprises potassium permanganate and at least one alkali metal salt or ammonium salt of an anion from the group consisting of vanadate, molybdate and tungstate.
The object on which the invention is based is likewise achieved by means of a process for producing a conversion coating on an article made of magnesium or its alloys, characterized in that the article is subjected to passivation using an aqueous passivating electrolyte which comprises potassium permanganate and at least one alkali metal salt or ammonium salt of an anion from the group consisting of vanadate, molybdate and tungstate.
The conversion coating of the invention has a golden brown to greyish brown, iridescent colour and comprises MgO, Mn
2
O
3
, MnO
2
and at least one oxide from the group consisting of vanadium, molybdenum and tungsten.
Investigations have shown that the corrosion protection effect of this conversion coating is no lower than that of a conventional chromate coating.
Especially in view of the fact that, in comparison with the chromate ions, the anions used in accordance with the invention have when considered individually a lower oxidizing power than chromate ions, it is clear that only through combination of the permanganate ions with the corresponding vanadate, molybdate and/or tungstate ions is a synergistic effect achieved which leads to the formation of a corrosion-inhibiting conversion coating on articles made of magnesium or its alloys.
This is of particular significance since the prior art aqueous passivating electrolytes comprising potassium permanganate can only achieve such an oxidizing power of the electrolyte solution by a reduction in pH and/or increase in temperature.
One possible explanation for this synergistic effect may lie in the formation of very strong, so-called heteropolyacids in the form of their soluble ammonium or alkali metal salts.
A particular advantage of the process of the invention is the fact that, even after a relatively long time in use, the aqueous passivating electrolyte remains stable without the precipitation of magnesium dioxide in an amount which would render the aqueous passivating electrolyte unusable for the passivation of articles made of magnesium or its alloys.
Accordingly, it is possible with the present process in a simple manner simply to top up the chemicals which have been consumed following a prolonged time in use without the ne
B{overscore (a)}nerjee Dora
Kohler Marco
Kruger Ulrike
Kurze Peter
AHC Oberflachenechnik GmbH & Co. OHG
Connolly Bove & Lodge & Hutz LLP
Oltmans Andrew L.
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