Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-06-26
2003-05-13
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Composite
Of metal
C428S328000, C428S425900, C428S469000, C428S699000, C428S701000
Reexamination Certificate
active
06562474
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a coated steel sheet having excellent corrosion resistance and a method for producing the same.
BACKGROUND ART
Steel sheets for household electric appliances, for buildings, and for automobiles have widely used zinc based or aluminum based plated steel sheets treated on their surface with chromating using a solution consisting mainly of chromic acid, bichromic acid, or their salts in order to improve the corrosion resistance (white-rust resistance and rust resistance). The chromate treatment is an economical treatment method providing excellent corrosion resistance under a relatively simple procedure.
The chromate treatment uses hexavalent chromium which is a substance under regulation of pollution control laws. The hexavalent chromate substantially does not pollute environment nor attack human bodies, because it is perfectly handled in a closed system over the whole treatment process, thus it is completely reduced and recovered within the process. Therefore, it is never emitted to natural environment, and the sealing function of organic coatings reduces the chromium elution from the chromate coatings to nearly zero. Nevertheless, recent concern about global environment increases the independent movements to diminish the use of heavy metals including hexavalent chromium. In addition, there has begun a movement to eliminate or minimize the heavy metals in products to avoid contamination of environment when shredder scrap of wasted products is discarded.
Responding to those movements, many non-polluting treatment technologies without applying chromating have been introduced to prevent generation of rust and white-rust on zinc base plated steel sheets. Among them, several methods using organic compounds and organic resins have been proposed. Examples of these methods are the following.
(1) A method using tannic acid, (for example, JP-A-51-71233), (the term “JP-A” referred to herein signifies “Unexamined Japanese Patent Publication”).
(2) A method using a thermosetting paint prepared by blending an epoxy resin, an amino-resin, and tannic acid, (for example, JP-A-63-90581).
(3) A method using chelation force of tannic acid, such as a method using a mixed composition of a water-type resin and a polyhydric phenol carboxylic acid, (for example, JP-A-8-325760).
(4) A method using surface treatment to coat an aqueous solution of a hydrazine derivative onto the surface of tin plate or zinc plate, (for example, JP-B-53-27694 and JP-B-56-10386), (the term “JP-B” referred to herein signifies “Examined Japanese Patent Publication”).
(5) A method using an inhibitor containing an amine-added salt which is prepared by adding amine to a mixture of acylsarcosine and benzotriazole, (for example, JP-A-58-130284).
(6) A method using a treatment agent prepared by blending tannic acid with a heterocyclic compound such as benzothiazole compound, (for example, JP-A-57-198267).
These conventional technologies, however, have problems described below.
The methods (1) through (4) have a problem in corrosion resistance and other characteristics. That is, the method (1) gives insufficient corrosion resistance, and fails to give uniform appearance after the treatment. The method (2) does not aim to form a rust-preventive coating in a thin film form (having thicknesses of from 0.1 to 5 &mgr;m) directly on the surface of zinc base or aluminum base plating, thus the method fails to attain sufficient corrosion-protective effect even it is applied in a thin film form onto the surface of zinc base or aluminum base plating. The method (3) also gives insufficient corrosion resistance.
The method (4) is not applied to a zinc base or aluminum base plated steel sheet. Even if the method (4) is applied to those types of steel sheets, the formed coating has no network structure so that it has no satisfactory barrier performance, which results in insufficient corrosion resistance. JP-B-23772 (1978) and JP-B-10386 (1081) disclose blending of an aqueous solution of hydrazine derivative with a water-soluble polymer (such as polyvinylalcohol, a maleic acid ester copolymer, an acrylic acid ester copolymer) aiming to improve the uniformity of coating. However, that kind of mixture which simply blends an aqueous solution of hydrazine derivative with a water-soluble polymer compound cannot give satisfactory corrosion resistance.
Also the methods (5) and (6) do not aim to form a rust-preventive coating on the surface of a zinc base or aluminum base plated steel sheet in a short time. And, even when a treatment agent is applied onto the surface of the plated steel sheet, lack of barrier performance against corrosive causes such as oxygen and water fails to provide excellent corrosion resistance. The method (6) describes also about the additives in terms of mixing with resins (such as epoxy resin, acrylic resin, urethane resin, nitrocellulose resin, and polyvynilchloride resin). However, simple mixture of resin with a heterocyclic compound such as a benzothiazole compound cannot give sufficient corrosion resistance.
Under a practical condition that alkaline degreasing is applied at a pH range of from about 9 to about 11 using spray method or the like to remove oil which was applied onto the surface during press-working or other steps, all of the methods (1) through (6) have a problem that the alkaline degreasing induces peeling or damaging the coating, thus failing to sustain corrosion resistance. Therefore, all of these methods referred above are not suitable for practical uses as a method for forming rust-preventive coatings.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a coated steel sheet which does not contain heavy metals such as hexavalent chromium within the coating, and which is safe and non-toxic in manufacturing process and on using thereof, while attaining excellent corrosion resistance, and to provide a method for manufacturing thereof.
To attain the object, firstly, the present invention provides a coated steel sheet having excellent corrosion resistance, which comprises a zinc or a zinc alloy plated steel sheet or an aluminum or an aluminum alloy plated steel sheet, a composite oxide coating formed on the surface of the plated steel sheet, and an organic coating having a thickness in a range of from 0.1 to 5 &mgr;m formed on the composite oxide coating.
The composite oxide coating contains:
(&agr;) fine particles of oxide;
(&bgr;) at least one metal selected from the group consisting of Mg, Ca, Sr, and Ba (including the case that the metal is in a form of compound and/or composite compound); and
(&ggr;) phosphoric acid and/or phosphoric acid compound.
The composite oxide coating has a thickness in a range of from 0.005 to 3 &mgr;m, or has a total coating weight of the component (&agr;), the component (&bgr;) converted to metal concerned, and the component (&ggr;) converted to P
2
O
5
, in a range of from 6 to 3,600 Mg/m
2
.
The organic coating contains a product of reaction between a film-forming organic resin (A) and an active-hydrogen-laden compound (B), a part or whole of which compound (B) comprises a hydrazine derivative (C) containing active hydrogen.
Secondly, the present invention provides a coated steel sheet having excellent corrosion resistance, which comprises a zinc or a zinc alloy plated steel sheet or an aluminum or an aluminum alloy plated steel sheet, a composite oxide coating formed on the surface of the plated steel sheet, and an organic coating having a thickness in a range of from 0.1 to 5 &mgr;m formed on the composite oxide coating.
The composite oxide coating contains:
(&agr;) fine particles of oxide; and
(&bgr;) phosphoric acid and/or phosphoric acid compound.
The composite oxide coating has a total coating weight of the component (&agr;) and the component (&bgr;) converted to P
2
O
5
, in a range of from 5 to 4,000 mg/m
2
.
The organic coating contains a product of reaction between a film-forming organic resin (A) and an active hydrogen-laden compound (B), a part or whole of which compound (B) comprises a hydra
Ando Satoru
Furuta Akihiko
Haruta Yasuhiko
Kubota Takahiro
Matsuki Hiroyasu
Frishauf Holtz Goodman & Chick P.C.
NKK Corporation
Paulraj Christopher
Thibodeau Paul
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