Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-11-09
2003-04-08
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S629000, C428S632000, C428S633000, C428S469000, C428S472000, C428S472100, C428S472300, C428S696000, C428S697000, C428S702000, C428S908800
Reexamination Certificate
active
06544666
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a chemically processed steel sheet remarkably improved in corrosion resistance by generation of a converted layer with a self-repairing faculty on a surface of a zinc plating layer.
2. Description of Related Art
Zn or its alloy-coated steel sheets (hereinafter referred to as “zinc-coated steel sheet”) have been used as corrosion-resistant material. But, when the zinc-coated steel sheet is held as such in a humid atmosphere, exhaust gas or an environment subjected to dispersion of sea salt grains for a long time, its external appearance is worsened due to generation of white rust on the plating layer. Generation of white rust is conventionally inhibited by chromating.
A conventional chromate layer is composed of complex oxides and hydroxides of trivalent and hexavalent Cr. Scarcely-soluble compounds of Cr(III) such as Cr
2
O
3
act as a barrier against a corrosive atmosphere and protects a steel base from corroding reaction. Compounds of Cr(VI) are dissolved as oxoatic anions such as Cr
2
O
7
2−
from the converted layer and re-precipitated as scarcely-soluble compounds of Cr(III) due to reducing reaction with exposed parts of a steel base formed by working or machining. Re-precipitation of Cr(III) compounds automatically repairs defective parts of the converted layer, so that corrosion-preventing faculty of the converted layer is still maintained after working or machining.
Although chromating effectively inhibits generation of white rust, it obliges a big load on post-treatment of Cr ion-containing waste fluid. In this consequence, various methods using chemical liquors, which contains titanium compound, zirconate, molybdate or phosphate instead of chromate, have been proposed for generation of Cr-free converted layers.
As for generation of a molybdate layer, JP 51-2419 B1 proposed a method of dipping a steel member in a chemical liquor containing magnesium or calcium molybdate, and JP 6-146003 A1 proposed a method of applying a chemical liquor, which contains a partially reduced oxide of Mo(VI) at a ratio of Mo(VI)/total Mo to 0.2-0.8, to a steel member. As for generation of a titanium-containing layer, JP 11-61431 A1 proposed a method of applying a chemical liquor, which contains titanium sulfate and phosphoric acid, to a galvanized steel sheet.
These converted layers, which have been proposed instead of the conventional chromate layer, do not exhibit such a self-repairing faculty as the chromate layer.
For instance, a titanium-containing layer does not exhibits a self-repairing faculty due to insolubility, although it is uniformly generated on a surface of a steel base in the same way as the chromate layer. As a result, the titanium-containing layer is ineffective for suppression of corrosion starting at defective parts formed during chemical conversion or plastic deformation. The other Cr-free converted layers are also insufficient for corrosion prevention due to poor self-repairing faculty.
A chemical liquor, which is prepared by mixing phosphoric acid to an aqueous titanium sulfate solution, is easy to generate precipitates. Once precipitates are generated, it is difficult to uniformly spread the chemical liquor to a surface of a steel base, resulting in generation of an ununiform converted layer. When precipitates are included in the converted layer, adhesiveness of the converted layer and external appearance of the processed steel sheet are worsened. Corrosion resistance of the converted layer would be degraded due to residual sulfate radical. Moreover, composition of the chemical liquor is often varied to a state unsuitable for generation of a converted layer with high quality due to the precipitation.
A manganese-containing converted layer, which is generated from a phosphate liquor, is relatively soluble, and dissolution of the converted layer occurs in a humid atmosphere. In this regard, an effect of the converted layer on corrosion resistance is inferior, even if the converted layer is thickened. Furthermore, the phosphate liquor shall be intensively acidified due to poor solubility of manganese phosphate. The acidified liquor violently reacts with a zinc plating layer, and loses its validity in a short while.
SUMMARY OF THE INVENTION
The present invention aims at provision of a processed zinc-coated steel sheet remarkably improved in corrosion resistance by generation of a converted layer, which contains insoluble or scarcely-soluble compounds useful as a barrier for insulation of a steel base from an atmosphere and soluble compounds with a self-repairing faculty for repairing damaged parts of the converted layer.
The present invention proposed a new processed zinc-coated steel sheet comprising a steel base coated with a Zn or its alloy plating layer and a chemically converted layer, which contains at least one complex compound of Ti and Mn, generated on a surface of the plating layer. The complex compound is selected from oxides, phosphates, fluorides and organic acid salts of Mn and Ti. The organic acid salts preferably have carboxylic groups.
A chemical liquor for generation of such a converted layer contains one or more of manganese compounds, titanium compounds, phosphoric acid or phosphates, fluorides and organic acids. The organic acid preferably has a carboxylic group. The chemical liquor is adjusted at pH 1-6.
The present invention proposes another new processed steel sheet comprising the same steel base and a converted layer, which contains both of at least one oxide or hydroxide and at least one fluoride of valve metals, generated on a surface of a Zn or its alloy plating layer. The valve metal is an element, whose oxide exhibits high insulation resistance, such as Ti, Zr, Hf, V, Nb, Ta, Mo or W. A self-repairing faculty of the converted layer is apparently noted by incorporation of a fluoride in the converted layer at an F/O atomic ratio not less than 1/100.
The converted layer may further contains one or more of soluble or scarcely-soluble metal phosphates or complex phosphates. The soluble metal phosphate or complex phosphate may be a salt of alkali metal, alkaline earth metal or Mn. The scarcely-soluble metal phosphate or complex phosphate may be a salt of Al, Ti, Zr, Hf or Zn.
After the chemical liquor is spread to a zinc-coated steel sheet, the steel sheet is dried as such at 50-200° C. without washing to generate a converted layer on a surface of a plating layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Manganese compounds and valve metal fluorides are effective components other than chromium compound, which give a self-repairing faculty to a converted layer, since these compounds are once dissolved to water in an atmosphere and then re-precipitated as scarcely-soluble compounds at defective parts of the converted layer.
A manganese compound present in a converted layer is partially changed to a soluble component effective for realization of a self-repairing faculty. Accounting the feature of the manganese-containing converted layer, the inventors experimentally added various kinds of chemicals and researched effects of the chemicals on corrosion resistance. In the course of researches, the inventors discovered that addition of titanium compound to a chemical liquor for generation of a manganese compound converted layer effectively suppresses dissolution of the converted layer without weakening a self-repairing faculty.
Improvement of corrosion resistance by addition of titanium compound is supposed by the following reasons, and confirmed by the under-mentioned examples.
A converted layer, which is generated from a manganese phosphate liquor on a surface of a zinc plating layer, is relatively porous. The porous layer allows permeation of corrosive components therethrough to a steel base, resulting in occurrence of corrosion.
On the other hand, when a converted layer is generated from a titanium-containing chemical liquor, pores of the converted layer are filled with titanium compounds precipitated from the chemical liquor. The titanium compoun
Ariyoshi Yasumi
Izumi Keiji
Matsuno Masanori
Morikawa Shigeyasu
Nakano Tadashi
Koehler Robert R.
Nisshin Steel Co. Ltd.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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