Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-04-07
2002-10-08
Green, Anthony (Department: 1755)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C106S014120, C106S014210, C148S264000, C148S267000
Reexamination Certificate
active
06461449
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a composition and method for conversion coating a zinciferous surface, without any need for a preparatory flash treatment using Ni or Co, or the like, to make the zinciferous surface superior in its resistance to blackening and white rust. For this purpose, a zinciferous surface includes a surface of pure zinc, any zinc alloy that includes at least 50% by weight of zinc, and any alloy of zinc and aluminum that includes up to 55% aluminum. The invention is particularly advantageously used on zinc plated steel sheet such as is commonly made by depositing a zinciferous surface on a steel substrate by electrolysis or by coating the substrate with molten zinc or zinc alloy. The invention will be further described hereinafter in terms of its use on zinc-plated steel sheet, but it should be understood to be applicable, mutatis mutandis, to other zinciferous surfaces.
Sacrificial anti-corrosion protection of steel by means of zinc plating is highly effective and economical; currently, therefore, about 10,000,000 tons of steel, corresponding to 10% of the annual crude steel production of about 100,000,000 tons in Japan, is produced as zinc-plated steel sheets, and such sheets are used in a wide variety of fields such as construction materials, automobiles and home electrical appliances, etc. Sacrificial anti-corrosion protection by means of zinc is believed to operate as follows: an electrochemical cell is formed under conditions in which the two metals (zinc and steel) are in contact, with zinc, the baser of the two metals, acting as an anode, and iron acting as a cathode. By this means, anodic dissolution of iron caused by local cell formation when iron alone is exposed to a corrosive environment is suppressed, thus preventing corrosion of the steel. Accordingly, anti-rust effectiveness of the iron and zinc combination ends at the point in time when the zinc in contact with the steel is consumed, so that corrosion of the zinc layer must be suppressed in order to obtain an enduring protective effect from the zinciferous coating. The prior art means used to achieve this is a chromate treatment of the zinciferous surface following plating.
However, in the case of rust-proofing methods using a chromate treatment on zinc-plated steel sheets, the following problems arise: Although the generation of white rust is very effectively prevented by subjecting zinc-plated steel sheets to a chromate treatment, the plated surface is still blackened by the generation of black rust in a relatively short time in cases where the sheets are stored in a coiled state or in cases where the sheets are applied to roofs or walls, etc. (this is also referred to as, the phenomenon of “blackening”). It has been recognized that this phenomenon occurs more readily in steel sheets in which the plating surface has been activated by skin pass rolling following zinc plating, and in steel sheets plated with a molten zinc-aluminum alloy containing several % aluminum.
To avoid the problem of blackening following chromate treatment, a flash treatment (this refers to a treatment in which an extremely small amount of a metal is chemically deposited; see further below) using an aqueous solution which contains metal ions of Ni or Co as taught in Japanese Patent Application Kokai No. SHO59-177381 is currently viewed as being a powerful countermeasure. Specifically, the gist of the invention in the aforementioned patent is that prior to the chromate treatment of a steel plate plated with zinc or a zinc alloy, the surface of the steel sheet is subjected to a flash treatment using an aqueous solution which has a pH of 1 to 4 or 11 to 13.5, and which contains metal ions of Ni or Co, or both, as a means of suppressing the above-mentioned black rust. In this case, after the aforementioned metal ions have been is deposited in the form of a metal or oxide, the sheet is washed with water, after which a chromate coating film is formed. However, when such a flash treatment is performed prior to a chromate treatment, although black rust is inhibited, white rust tends to be generated.
Japanese Patent Application Kokai No. HEI 5-331659 and Japanese Patent Application Kokai No. HEI 7-54156 teach methods which can simultaneously suppress both blackening and the generation of white rust by means of a single process consisting only of a chromate treatment, without any need for a two-process treatment consisting of a preparatory flash treatment and a chromate treatment. However, user needs in recent years have required highly stable resistance to blackening and resistance to white rust over a longer period of time regardless of variation in the activity of the plated surface, and such requirements cannot be satisfied using the above-mentioned conventional techniques.
The problems which the present invention attempts to solve are one or more of the above-mentioned problems in the prior art. The object of the present invention is therefore to provide to zinc-plated steel sheets a high resistance to blackening and white rust over a long period of time, regardless of any variation in the activity of the plated surface of the zinc-plated steel sheet.
BRIEF SUMMARY OF THE INVENTION
The object of the invention is achieved by an aqueous conversion coating forming liquid treatment composition that has both of the following properties (A) and (B):
(A) the composition comprises, preferably consists essentially of, or more preferably consists of water and the following components:
(A.1) dissolved ions and/or molecules that contain hexavalent chromium atoms;
(A.2) dissolved trivalent chromium cations;
(A.3) dissolved nitrate ions; and
(A.4) dissolved cations of one or more metals selected from the group consisting of Ni, Co, Zn and Al;
and, optionally, one or more of the following components:
(A.5) dissolved phosphate ions;
(A.6) dissolved fluoride ions;
(A.7) dispersed silica and/or dispersed and/or dissolved silicates; and
(A.8) dispersed and/or dissolved polymer resins; and
(B) in the liquid treatment composition:
(B.1) the molar ratio of dissolved trivalent chromium ions to hexavalent chromium atoms is in a range from 0.11:1.0 to 1.0:1.0;
(B.2) the molar ratio of nitrate ions to total chromium atoms (i.e., the sum of hexavalent chromium atoms and trivalent chromium cations) is in a range from 2.1:1.0 to 5.5:1.0;
(B.3) the molar ratio of the total of dissolved metal cations that are selected from the group consisting of Ni, Co, Zn and Al to nitrate ions is in a range from 0.05:1.0 to 1.0:1.0;
(B.4) the molar ratio of dissolved phosphate ions
1
to total chromium atoms is not more than 0.3:1.0; and
(B.5) the molar ratio of dissolved fluoride ions
1
to total chromium ions is not more than 0.1:1.0.
1
If the concentration of these ions is so small that it can not easily be measured, the ratio assumed to be 0.0:1.0.
A process according to the invention comprises an operation in which a liquid film of a treatment composition as described immediately above is formed over a zinciferous substrate and then dried, thus forming an adherent dry coating with a chromium content of 10 to 100 milligrams of chromium per square meter of surface coated, this unit of areal density or coating weight being hereinafter usually abbreviated as “mg/m
2
”.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
The molar ratio of trivalent chromium ions to hexavalent chromium ions contained in a chromate containing treatment solution used in the present invention is preferably in the range of 0.25:1.0 to 0.67:1.0. If the molar ratio of trivalent chromium ions to hexavalent chromium atoms is less than 0.11:1.0, the elution of chromium ions in a corrosive environment is excessive, leading to environmental contamination, which is undesirable. On the other hand, if the molar ratio of trivalent chromium ions to hexavalent chromium atoms exceeds 1.0:1.0, the corrosion resistance of the treated substrate is insufficient; this is also undesirable.
The preferred source of dissolved hexavalent chromium atoms for a treatment
Kato Akira
Matsubara Yuzuru
Green Anthony
Harper Stephen D.
Henkel Corporation
Ortiz Daniel S.
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