Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Treating electrolytic or nonelectrolytic coating after it is...
Reexamination Certificate
2000-07-12
2001-09-04
Beck, Shrive P. (Department: 1762)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Treating electrolytic or nonelectrolytic coating after it is...
C205S271000, C205S291000, C427S433000, C427S436000, C427S437000, C427S438000, C427S406000, C427S320000, C427S321000, C427S343000, C427S383700
Reexamination Certificate
active
06284122
ABSTRACT:
FIELD OF THE INVENTION
The present invention refers to an improvement in the production of a zinc-aluminum alloy coating by immersion into molten metal baths and, more precisely, it refers to an improved process to discontinuously coat metallic bodies with zinc-aluminum alloys, by immersion in molten baths of said alloy.
BACKGROUND OF INVENTION
State of the Art
The discontinuous coating of metal bodies with a zinc-aluminum alloy is disclosed in the '049 application. Also as disclosed therein, drawbacks were encountered whereby uneven coatings or bare spots were obtained. Prior to the method disclosed therein, acceptable coatings were obtainable only with difficulty and by complicated, time consuming procedures.
The '049 application discloses a very good solution to such drawbacks, essentially comprising a method whereby the metal bodies to be discontinuously coated are preferably electrolessly pre-coated with a metallic pre-coating, before the immersion in the zinc-aluminum molten bath. The pre-coating is preferably a metal chosen from the group consisting of copper and nickel. Cobalt could be used, but is not preferred for a number of reasons, including cost and toxicity. The pre-coating forms a very thin coating, permitting a good adhesion of the zinc-aluminum alloy.
However, even if a pre-coating is used, the final layer of zinc-aluminum alloy may present a number of adhesion, compactness and appearance defects, attributed to the formation of metal oxides during air drying after the pre-coating and prior to the immersion of the pre-coated metal body in the Zn—Al bath. Such oxides prevent a proper formation of the final coating. This outer oxidation layer, particularly for baths containing 0.1-25% wt. % Al, is a physical barrier against the inter-action or reaction of the pre-coating metal and the Al in the bath.
Attempts were made to eliminate such superficial oxidation through mechanical polishing with emery papers aided by a final treatment with alumina impregnated cloths. Another treatment utilized was a surface activation by pickling in diluted hydrochloric acid, followed by water rinsing and drying. Neither of these approaches yielded consistently satisfactory results.
It is an object of the present invention to avoid those drawbacks, through a surface conversion treatment resulting in a compact, continuous and glossy coating.
DESCRIPTION OF THE INVENTION
According to present invention, after metal bodies are pre-coated with a thin protective metallic layer, but before they are immersed in a zinc-aluminum alloy molten bath, they undergo a surface activation treatment by immersion in a diluted solution containing hydrochloric acid. The objective of the activation treatment is to form a salt layer on the pre-coated surface which protects the surface from further oxidation prior to immersion in the Zn—Al bath. By immersing the pre-coated metal object in hydrochloric acid, a reaction between the pre-coating metal and the hydrochloric acid occurs, thereby forming a chloride salt. When the object is then removed from the hydrochloride acid solution, the acid solvent is allowed to evaporate leaving a dry protective salt layer on the surface. When treatment of the pre-coated surface with hydrochloric acid is followed by water washing, poor results can occur due to the washing away of the salt solution on the object surface. When the surface is then dried, oxides can form on the surface, which oxides interfere with the subsequent galvanizing step in the Zn—Al alloy bath.
Moreover, and as is disclosed in the '049 application, the metallic pre-coating either substantially completely reacts with the Al in the Zn—Al bath (such as is the case with a Ni pre-coating to form an interface Ni—Al compound) or dissolves in the bath (such as is the case with a Cu pre-coating), thereby exposing the surface of the metal object to the Zn—Al alloy. It is therefore necessary that the chloride salt layer created by the activation step have a melting point below the temperature of Zn—Al bath, such that the chloride salt melts in a relatively short time upon immersion of the object in the Zn—Al bath. When Cu is used as the pre-coating metal, the preferred chloride salt that is formed is CuCl. As noted in the Handbook of Chemistry and Physics (CRC Press, 77
th
Edition, 1996-1997, pp. 4-54 and 4-55), the melting point of CuCl is 430° C., which is sufficiently low to cause melting when the salt surface is immersed in a Zn—Al bath at a temperature above 430° C., e.g. 450° to 600° C. The melting point of CuCl
2
, on the other hand, is 630° C., too high for consistently good results. In both instances, the chloride reacts with the Cu pre-coating. It is therefore preferred that the reaction between Cu and Cl be controlled so that excess Cl does not cause the CuCl reaction product to further react and form substantial amounts of CuCl
2
. This is accomplished by controlling the Cl concentration in the hydrochloric acid bath, and/or by limiting the reaction time, for example by limiting the immersion time in the hydrochloric acid to a few seconds.
In general, the chloride salt or mixture of chloride salts should melt between about 300 and 600° C., depending upon the Zn—Al composition.
The activation bath may also contain an acid ionic or non-ionic surfactant, as well as one or more added chlorides of elements of groups IA, IIA, IB and IIB. The diluent for the hydrochloric acid is water or an alcohol chosen between methanol, ethanol, propanol, and the like, preferably ethanol and glycerol.
The formation of the thin pre-coating onto the metal body to be coated is obtained through electrolytic or electroless deposition or cementation. Cementation is preferred since it results in a very thin, monoatomic coating.
The concentration of the hydrochloric acid in the treatment solution preferably is between 5 and 20% vol., preferably between 10 and 15, while the added chlorides concentration preferably is between 10 and 100, preferably between 10 and 24, g/l.
Due to the complete evaporation of the solvent, the salts contained in said solution precipitate onto the pre-coated surfaces. In the case of pre-coating with copper, a mixture of cuprous chloride with the above salts is obtained, thus protecting from oxidation the copper surface and acting as fluxant, as they melt during immersion into the zinc-aluminum alloy bath, at a temperature between 430 and 600° C., depending upon the amount of aluminum in the alloy. This ensures a clear surface to be coated and thus a high quality coating.
The following Examples will show some preferred embodiments of present invention, without in any way limiting scope and objects of the invention.
REFERENCES:
patent: 4228224 (1980-10-01), Heredy et al.
patent: 4285995 (1981-08-01), Gomersall
patent: 4505958 (1985-03-01), Lieber et al.
patent: 5203985 (1993-04-01), Nishimura et al.
patent: 5827618 (1998-10-01), Oyagi et al.
Brevaglieri Bruno
Cecchini Michele
Memmi Massimo
Natali Stefano
Baker & Botts L.L.P.
Barr Michael
Beck Shrive P.
International Lead Zinc Research Organization, Inc.
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