Chemistry of inorganic compounds – Sulfur or compound thereof – Oxygen containing
Patent
1993-12-20
1995-04-25
Niebling, John
Chemistry of inorganic compounds
Sulfur or compound thereof
Oxygen containing
423557, 205105, 205173, 205253, C01G 1900
Patent
active
054096855
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to manufactured tin(II) sulfate granules for the electrolytic coloring of anodized aluminum with metal salts, to a process for their production and to their use for the electrolytic coloring of anodized aluminum with metal salts.
STATEMENT OF RELATED ART
It is known that, on account of its base character, aluminum becomes covered with a natural oxide coating generally below 0.1 .mu.m in thickness (Wernick, Pinner, Sheasby, The Surface Treatment and Finishing of Aluminum and its Alloys, ASM International).
Considerably thicker oxide coatings can be obtained by electrolytic oxidation of aluminum. This process is known as anodizing. Sulfuric acid, chromic acid or phosphoric acid is preferably used as the electrolyte. Organic acids, such as for example oxalic acid, maleic acid, phthalic acid, salicylic acid, sulfosalicylic acid, sulfophthalic acid, tartaric acid or citric acid, are also used in some processes.
However, sulfuric acid is the most commonly used electrolyte. Depending on the anodizing conditions, layer thicknesses of up to 150 .mu.m can be obtained in this process. However, layer thicknesses of 20 to 25 .mu.m are sufficient for external applications, such as for example facade facings or window frames.
The anodizing process is generally carried out in 10 to 20% sulfuric acid with a current density of 1.5 A/dm.sup.2, at a temperature of 18.degree. to 22.degree. C. and over a period of 15 to 60 minutes, depending on the required layer thickness and the particular application.
The oxide coatings thus produced have a high absorption capacity for a number of organic and inorganic dyes.
Electrolytic coloring processes, in which anodized aluminum is colored by treatment with alternating current in heavy metal salt solutions, have been known since the middle of the thirties. The heavy metals used are, above all, elements of the first transition series, such as Cr, Mn, Fe, Co, Ni, Cu and, in particular, Sn. The heavy metal salts are generally sulfates, pH being adjusted to a value of 0.1 to 2.0 with sulfuric acid. The coloring process is carried out at a voltage of around 10 to 25 V and the resulting current density. The counter-electrode may either consist of graphite or stainless steel or of the same material which is dissolved in the electrolyte.
In this process, the heavy metal pigment is deposited in the pores of the anodic oxide coating in the half cycle of the alternating current in which aluminum is the cathode, the aluminum oxide coating being further thickened by anodic oxidation in the second half cycle. The heavy metal is deposited at the bottom of the pores and thus colors the oxide coating.
Tin salts in particular are used, colors varying from champagne via various bronze tones to black being obtained according to the procedure adopted.
However, one of the problems encountered where coloring is carried out in tin electrolytes is that the tin readily oxidizes so that basic tin(IV) oxide hydrates (stannic acid) are rapidly precipitated during the application and, in some cases, even during the storage of the Sn solutions. It is known that aqueous tin(II) sulfate solutions are oxidized to tin(IV) compounds simply by exposure to atmospheric oxygen. This is highly undesirable in the coloring of anodized aluminum in tin electrolytes because, on the one hand, it disrupts the process sequence (frequent renewal or topping up of the solutions rendered unusable by the formation of precipitates) and, on the other hand, leads to considerable extra costs because of the tin(IV) compounds which cannot be used for coloring. Accordingly, various processes have been developed, differing in particular in the means used to stabilize the generally sulfuric acid tin(II) sulfate solutions, for the electrolytic coloring of aluminum.
Phenol-like compounds, such as phenol sulfonic acid, cresol sulfonic acid or sulfosalicylic acid, are by far the most commonly used (S. A. Pozzoli, F. Tegiacchi; Korros. Korrosionsschutz Alum., Veranst. Eur. Foed. Korros. Vortr. 88th 1976,
REFERENCES:
patent: 5064512 (1991-11-01), Brodalla et al.
S. A. Possoli, F. Tegiacchi; Korros. Korrosionsschutz Alum., Veranst. Eur. Foed. Korros., Vortr. 88 th 1976, 139-45 (no month).
Beaujean Hans-Josef
Bode Jens
de Riese-Meyer Loert
Sander Joerg
Sander Volker
Henkel Kommanditgesellschaft auf Aktien
Jaeschke Wayne C.
Niebling John
Szoke Ernest G.
Wisdom, Jr. Norvell E.
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