Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1999-10-13
2001-03-13
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S328000, C428S469000, C428S470000, C428S659000, C428S621000, C428S623000, C428S689000, C148S259000, C148S260000, C148S261000, C148S273000, C148S274000, C148S275000, C148S283000, C106S014120, C106S014130, C106S014140, C106S014150, C106S014210, C106S218000, C106S287320, C106S287350, C106S287290
Reexamination Certificate
active
06200672
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a surface treated metal sheet and a treatment solution therefor and, especially, it relates to a surface treated metal sheet with excellent corrosion resistance and having a coating layer containing absolutely no hexavalent chromium, and to a treatment solution for the same.
BACKGROUND ART
Rust-proof properties have conventionally been imparted to cold-rolled steel sheets, galvanized steel sheets, zinc-based alloy-plated steel sheets and aluminum-plated steel sheets used for automobiles, electrical appliances, building materials and the like, usually by coating their surfaces with chromate layers. Chromating treatment includes electrolytic chromating and application chromating. Electrolytic chromating is accomplished, for example, by using a bath composed mainly of chromic acid and also containing other anions such as sulfate, phosphate, borate and halogens, for treatment of the metal sheet by cathodic electrolytic treatment. Application chromating is designed in consideration of the problem of elution of chromium from chromated metal sheets, and it thus involves preparation of a treatment solution by adding an inorganic colloid or inorganic anion to a solution with a portion of the hexavalent chromium portion reduced to trivalent chromium beforehand or to a solution with a specified ratio of hexavalent chromium to trivalent chromium, and immersing the metal sheet therein or spraying the metal sheet with the treatment solution.
Of such chromate layers, those chromate layers formed by electrolysis cannot be said to have sufficient corrosion resistance despite the low elution of hexavalent chromium, and there is particular loss of corrosion resistance in cases where considerable layer damage occurs during working, etc. On the other hand, while metal sheets coated with application chromated layers have high corrosion resistance and especially high excellent corrosion resistance of worked sections, elution of hexavalent chromium from the chromate layers has been a problem. Elution of hexavalent chromium can be considerably reduced by coating with organic polymers, but this is still inadequate. Although an improvement in reducing elution of hexavalent chromium can generally be achieved by a method known as resin chromating treatment, such as disclosed in Japanese Unexamined Patent Publication No. 5-230666, it is still impossible to avoid trace elution.
Thus, in order to completely inhibit elution of hexavalent chromium it is necessary to develop a corrosion-resistant layer that uses absolutely no hexavalent chromium but has functions equivalent to conventional chromate layers containing hexavalent chromium.
One previous anti-corrosion technique for incorporating absolutely no hexavalent chromium is a method under development which uses an organic-based corrosion inhibitor. As such organic-based corrosion inhibitors there are known carboxylates such as benzoates, azelates, etc. and compounds containing —S—, —N— which readily interact with metal ions, as well as complexes thereof.
As techniques for including organic-based corrosion inhibitors in layers there have been proposed, for example, the hydrooxime complex of zinc disclosed in Japanese Unexamined Patent Publication No. 62-23989, the metal chelate compounds of Mg, Ca, Ba, Zn, Al, Ti, Zr, Sn, Ni, etc. disclosed in Japanese Unexamined Patent Publication No. 3-183790 and Japanese Unexamined Patent Publication No. 2-222556, the alkali earth metal salts, transition metal salts and transition metal complexes of organic compounds disclosed in Japanese Unexamined Patent Publication No. 6-321851, and the titanium and zirconium complexes of carboxylic acids disclosed in Japanese Unexamined Patent Publication No. 8-48916. These corrosion inhibitors, however, have weak anti-corrosion effects due to the metal elements forming the complexes and thus have failed to provide the same function as hexavalent chromium. In particular, almost no corrosion resistance can be expected at damaged sections or at the locations of layer defects produced during working.
Japanese Unexamined Patent Publication No. 7-188951 discloses a rare earth metal-organic chelate compound for the purpose of inhibiting corrosion of metals that contact solutions, such as radiators or pipes. This corrosion inhibitor was designed as a water-soluble compound, to allow continuous provision of the corrosion inhibitor to corrosion sites by circulation of the solution. Consequently, although the strong anti-corrosion effect of the rare earth metal element is utilized, with layers on metal sheets wherein the absolute amount of corrosion inhibitor onto the corrosion sites is limited by the coating coverage, elution occurs out of the layer in humid atmospheres so that long-term corrosion resistance comparable to chromate layers cannot be achieved.
DISCLOSURE OF THE INVENTION
In light of these problems, it is an object of the present invention to provide a surface treated metal sheet that has excellent corrosion resistance, particularly when the layer undergoes damage due to working or damage and which can substitute for a chromate layer, as well as a treatment solution therefor.
As a result of much diligent research by the present inventors in designing general purpose chemical treatment layers for use in systems containing absolutely no hexavalent chromium in place of the existing chromating treatment, and for the purpose of solving the problems mentioned above, it has been found that corrosion of a metal sheet can be effectively inhibited by using a rare earth metal element as a complex and/or salt with an organic compound for mixed dispersion in a layer on the metal sheet. As mentioned above, the prior art techniques are limited to water-soluble types wherein a complex of a rare earth metal element is used by being added to circulated water or the like, and no long-term corrosion resistance can be expected even when it is mixed in that form into a layer formed on a metal sheet. The present invention employs a rare earth metal element having a powerful anti-corrosion effect as a complex and/or salt of an organic compound to minimize the chemical interaction between the layer matrix component and other additives, while the structure of the matrix component is designed based on the solubility of the rare earth metal element as a complex and/or salt of an organic compound in water in the neutral range, thereby effectively bringing out the original functions of the components making up the layer.
In particular, for applicability to various different matrix components, it is designed such that the rare earth metal complex and/or salt is poorly water soluble in the neutral range so that elution out of the layer is inhibited to provide long-term corrosion resistance. It was found that if the rare earth metal complex and/or salt is designed to be water-soluble in the acidic range, the rare earth metal complex and/or salt will dissolve in response to pH drops at the sites where corrosion has occurred, so that a function is imparted which selectively repairs the corroding sections, such as worked sections or damaged sections.
In addition, by selecting the type of functional groups of the organic compound forming the complex and/or salt, a corrosion inhibiting function is imparted to the organic compound itself, thus reinforcing the anti-corrosion performance of the layer as a whole.
The gist of the present invention is as follows.
(1) A surface treated metal sheet characterized by being coated with a layer comprising, as main components, a complex and/or salt between a rare earth metal element and an organic compound having in the molecule one or more functional groups selected from among —O—, ═O, —OH, —COOH, —NH
2
, ═NH, ═N—, —SH, —SO
3
H and phosphoric groups, and a matrix capable of physically holding said complex and/or salt and having adhesive power for metal sheets.
(2) A surface treated metal sheet according to (1) above, characterized in that the solubility of the complex and/or salt in water at pH 6-7 is no greater tha
Sakashita Masao
Shoji Hiromasa
Tadokoro Kenichiro
Jones Deborah
Kenyon & Kenyon
Miranda Lymarie
Nippon Steel Corporation
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