Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonelectrolytic coating after forming nonmetal...
Patent
1997-09-08
1999-10-05
Gorgos, Kathryn
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonelectrolytic coating after forming nonmetal...
205200, 205201, 205229, 148272, 148274, 148275, 148276, C23C 2800, C23C 2282, C23C 2200, C25D 548
Patent
active
059618094
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a new chromium-free process by which the adhesion of paints, more particularly powder coatings, to metal surfaces subjected to thin-layer anodization to improve corrosion resistance is improved. The process sequence: thin-layer anodization--aftertreatment to improve paint adhesion--powder coating, is particularly suitable for suppressing so-called filiform corrosion.
TECHNICAL BACKGROUND AND RELATED ART
Filiform corrosion is understood to be the growth, under a layer of paint, of filament-like traces of corrosion which can occur under certain conditions of atmospheric moisture and electrolyte content. Painted architectural aluminium in costal regions is particularly susceptible to this form of corrosion. A discussion of the causes, manifestations and mechanisms of this form of corrosion can be found, for example, in: G. Steele, "Filiform Corrosion on Architectural Aluminium--A Review", Polymers Paint Colour J., 184 (4345), pages 90 to 95 (March 1994). Although this form of corrosion can be checked by chromating before painting, it cannot be certainly prevented. An effective alternative to chromating as a pretreatment is so-called thin-layer anodization which, although extremely effective against filiform corrosion, is often attended by the problem of poor paint adhesion.
The electrochemical anodic oxidation of metals in suitable electrolytes known in brief as "anodization" is a widely used process for forming corrosion-controlling and/or decorative coatings on suitable metals. These processes are briefly characterized, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. 9 (1987), pages 174-176. According to this literature reference, titanium, magnesium and aluminium and their alloys are anodizable, the anodization of aluminium and its alloys being of the greatest importance on an industrial scale. The electrolytically produced anodizing layers protect the aluminium surfaces against the effects of weathering and other corrosive media. In addition, anodizing layers are applied to obtain a harder surface and hence to achieve increased resistance to wear of the aluminium. Particular decorative effects can be obtained through the color of the anodizing layers or by adsorptive or electrolytic coloring. The anodization of the aluminium is carried out in an acidic electrolyte, sulfuric acid being the most widely used. Other suitable electrolytes are phosphoric acid, oxalic acid and chromic acid. The properties of the anodizing layers can be varied within wide limits through the choice of the electrolyte and its temperature and through the current density and the anodizing time. The anodizing process is usually carried out with direct current or with a direct current superimposed on alternating current.
Architectural aluminium is normally anodized in a layer thickness of up to about 20 micrometers that requires anodizing times of the order of 30 to more than 60 minutes. The anodizing layers thus applied are generally not painted, but are generally either left in their natural color or are adsorptively or electrolytically colored and then sealed. In this way, they offer protection against corrosion and have an aesthetic effect.
Relatively thin anodizing layers, for example with layer thicknesses of around 2 to at most 10 micrometers, can be produced by thin-layer anodization, but afford adequate protection against corrosion only in conjunction with subsequent painting. In principle, thin-layer anodization, which is occasionally also known as pre-anodization, may be carried out using the same electrolytes as employed for conventional anodization. The thin anodizing layers are obtained by applying reduced current intensities and/or reduced anodizing times. For example, anodizing layers around 5 micrometers thick can be obtained by the direct-current sulfuric acid process ("GS" process), providing aluminium panels are anodized in an electrolyte containing 180 g/l of sulfuric acid and 5 g/l of aluminium ions over a period of 5 minut
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Technischen Richtlinien der GSB International, p. 35B, Mar. 1994.
Nowak Andreas
Roland Wolf-Achim
Gorgos Kathryn
Henkel Kommanditgesellschaft auf Aktien
Jaeschke Wayne C.
Szoke Ernest G.
Wisdom, Jr. Norvell E.
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