Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-12-17
2004-01-13
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S251000, C148S253000, C148S255000, C148S256000, C148S257000, C148S258000, C148S263000, C148S264000, C148S265000, C148S267000, C148S268000, C148S273000, C148S274000, C148S275000, C427S372200, C427S384000, C427S407100, C427S409000, C427S419100, C428S623000, C428S626000, C428S628000, C428S658000, C428S659000, C428S472000, C428S472300, C428S689000, C428S925000
Reexamination Certificate
active
06677053
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a surface-treated steel sheet having a high corrosion resistance and a method for producing the same.
DESCRIPTION OF THE RELATED ARTS
Conventionally, chromate treatment films have been widely used in primary anticorrosion treatment. The chromate treatment film is formed on a surface of a zinc-base-plated steel sheet to protect the surface from corrosion until a consumer uses the steel sheet. In recent years, however, even after a product has been fabricated using such a steel-sheet material, the steel-sheet material is still required to maintain the corrosion-resisting function.
Among zinc-base-plated steel sheets, a Zn—Al-base-alloy-plated steel sheet has a relatively high corrosion resistance. The resistance is higher than that of the zinc-base-plated steel sheet. The Zn—Al-base-alloy-plated steel sheet is therefore enjoying increasing demands in industrial fields, particularly in the field of building materials.
In the recent building-material field, however, the severity of requirements is increasing for the durability of the corrosion resistance and maintenance-free properties of materials. With this background, the appearance of the surface of the Zn—Al-base-alloy-plated steel sheet is required to be durable for a longer period in various environments. Inherently, the appearance of the surface is required to be maintained in the fabrication of products in various shapes. As such, additional functions are required for the conventional chromate treatment film formed by applying a primary-rust-preventing treatment onto a Zn-5% Al-alloy-plated steel sheet that contains about 5 wt % Al to protect corrosion in a period until the steel sheet is used by a consumer. The required functions are as follows:
(a) a function (processed-portion corrosion resistance) of providing a high corrosion resistance even after the steel sheet fabricated into an intended product in a corrosive environment; and
(b) a function of inhibiting a blacken phenomenon in which the plated surface of the sheet material is blackened when the sheet material is stored outdoors for several days prior to fabrication.
Furthermore, the following functions are required for a Zn-55% Al plating alloy that contains about 55 wt % Al:
(a) a function of providing a high corrosion resistance even after the steel sheet fabricated into an intended product in a corrosive environment (processed-portion corrosion resistance); and
(b) a function of inhibiting a blacken phenomenon in which the plated surface of the sheet material is blackened in a humid environment (producing antiblackening resistance).
Chromate treatment films are broadly grouped into the following three types. They are an electrolysis-type chromate treatment film, a reaction-type chromate treatment film formed of a principal component of a trivalent-chromium compound, and a coating-type chromate treatment film formed of a compound of trivalent chromium and hexavalent chromium.
In these chromate-treatment films, the refractory trivalent chromium works as a barrier against corrosion-introducing factors, such as chloride ions and oxygen. That is, the refractory trivalent chromium provides barrier effects against the corrosion factors. On the other hand, in the coating-type chromate treatment film, the hexavalent chromium is dissolved out to a damaged portion of the chromate treatment film, and passivates the damaged portion. Thereby, the hexavalent chromium forms the film with corrosion-inhibiting effects (which hereinbelow will be referred to as “self-healing effects”).
For the above-described reasons, the coating-type chromate treatment film is applied for coating in many cases in which the processed-portion corrosion resistance is required. However, since the hexavalent chromium has high oxidizeability, it is prone to be reduced to trivalent chromium as time passes. In addition, since the hexavalent chromium is water-soluble, it is prone to be dissolved out of the compositional system. Hence, in many cases, when the film is damaged, a phenomenon occurs in which the film loses residual hexavalent chromium sufficient to allow the film to impart self-healing effects. Thus, the provision of a sufficient processed-portion corrosion resistance cannot be insured.
In this field, there are known technical methods proposed to solve the above-described problems. The methods can be broadly grouped into two types (1) and (2) described as follows.
(1) Methods of a type for reducing the extent of damage on a film
For example, JP-A-2-34792, (the term “JP-A” referred herein signifies the “unexamined Japanese patent publication”). discloses a method in which a fluorine-based resin is added in a chromate treatment film to have lubricity. Another example method of the type (1) is disclosed in JP-A-10-1762809. In this method, a thermoplastic elastomer is included in a film to impart ductility to the film. Concurrently, this method reduces the extent of a damaged film portion caused by sliding operation in, for example, press-forming. This enables the self-healing effects to be obtained with a relatively small amount of hexavalent chromium.
(2) Methods of a type for minimizing the dissolution amount of hexavalent chromium contained in a film
For example, one of the methods of the captioned type is disclosed in Domestic Republication of PCT International Publication for Patent Application No. 9-800337. According to the disclosed method, refractory chromium hydrochloric acid is dispersed within a film to inhibit chromium from being dissolved out in sound film portions. On the other hand, in a damaged film portion, corrosion reaction (the pH value increases) is used as a trigger to dissolve hexavalent chromium. This enables the film to impart self-healing effects.
In the methods of the type (1) above, a reduction can be achieved in regard to the extent of film damage caused in fabrication operations, such as press-forming and bending. This surely enables the film to impart a certain degree of the self-healing effects. However, reduction effects cannot be obtained for film damage caused when the film is in contact with, for example, a sharp-edged metal piece. In addition, since the resin to be included in the film is expensive, the method is problematic in both the economy and productivity.
In the methods of the type (2), the durability of the self-healing effects is improved in comparison to the case where water-soluble chromium acid is applied onto the steel-sheet surface to form the film. On the other hand, however, the water-solubility restricts the level of the self-healing effects that can be obtained. Even when the highest possible level of the self-healing effects is obtained in the above method, the level is equivalent to the level that can be achieved immediately after water-soluble chromium compound is included in the film. In addition, generally, films including the chromium hydrochloric acid tend to be discolored. The discoloration significantly reduces the value of products, particularly, products that are used without coating.
In a Zn—Al-base-alloy-plated steel sheet, a sacrificial anticorrosion action and a passivation-film forming action work in a synergetic manner. This causes a high corrosion resistance to be imparted. Nevertheless, however, since the aluminum (Al) has inherent properties to form an active metal, when a passivation film is damaged, a blackened phenomenon easily occurs in a humid environment.
The following describes four methods proposed for inhibiting the blackening behavior of a Zn—Al-base-alloy-plated steel sheet that contains 4 to 25 wt % Al:
(1) Method in which treatment is performed after plated is performed using solution that contains Ni ions and Co ions (according to JP-A-59-177381);
(2) Method in which a heat treatment is performed after skin-pass rolling (according to JP-A-55-131178);
(3) Method in which plated surfaces are cleaned using alkali water solution (according to JP-A-61-110777); and
(4) Method in which post-plating blasting is performed prior to a chromate treatment (JP-A-63
Hamada Etsuo
Matsuzaki Akira
Morita Kenji
Yamaji Takafumi
Yamashita Masaaki
Frishauf Holtz Goodman & Chick P.C.
Koehler Robert R.
NKK Corporation
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