Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-08-21
2004-01-13
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S935000, C205S245000
Reexamination Certificate
active
06677057
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Zn—Co—W alloy electroplated steel sheet and an electrolyte for manufacturing the same, and more particularly to a Zn—Co—W alloy electroplated steel sheet with excellent corrosion resistance and weldability, and an electrolyte for manufacturing the same in a stable manner.
2. Description of the Prior Art
In recent years, zinc plated steel sheet has been favored over competing steel sheets having other corrosion resistant surfaces and has been widely used for automobiles, household electric appliances and construction materials because of its excellent corrosion resistance. However, new plated steel sheets with high corrosion resistance by coatings in the form of a thin film are now required in terms of energy and resource savings. To meet these requirements, a Zn—Fe—and a Zn—Ni— alloy electroplated steel sheet have been developed and now are commercially available. In addition, a Zn—Cr alloy plated steel sheet has been developed.
However, as for a Zn—Fe alloy plated steel sheet, iron is contained in a plating layer which is formed on the steel sheet. Therefore, when the steel sheet is exposed to a corrosive atmosphere, the plating layer protects the steel sheet by sacrificial anticorrosive reaction. However, the plating layer is dissolved and then iron therein is oxidized to produce a red corrosive product. Final consumers consider it as a rusted steel sheet and thus tend to avoid the use of the Zn—Fe alloy plated steel sheet. Furthermore, there is a disadvantage in that ferrous ion is oxidized to ferric ion, thereby forming sludge during manufacturing the Zn—Fe alloy plated steel sheet.
As for a Zn—Ni alloy plated steel sheet, it is widely used as a material of automobiles due to its excellent corrosion resistance. However, because humans have been known to experience an allergic reaction to nickel, use of nickel-plated products has been prohibited in Europe. This trend continues to spread all over the world.
A Zn—Cr alloy plated steel sheet is markedly better than a Zn—Fe or a Zn—Ni plated steel sheet in terms of corrosion resistance and can obtain desired corrosion resistance even though a thin film of plating is applied to the steel sheet. In spite of these advantages, plating efficiency is low and thus production cost is high. Furthermore, chromium is harmful in the human body and thus its use is prohibited according to the environmental acts and regulations. Accordingly, it is difficult to use it practically.
As for a chromate surface treated steel sheet to improve corrosion resistance of zinc plated steel sheets, it does not have sufficient corrosion resistance to be used as a steel sheet for automobiles. Besides, chromium on the surface of the steel sheet is vaporized during a process of manufacturing automobiles, thereby causing damage to the human body. Therefore, its use has now been prohibited.
Accordingly, there is need to develop new alloy plated steel sheets with excellent corrosion resistance which are less hazardous to the human body.
An alloy electroplated steel sheet developed for these purposes is disclosed in U.S. Pat. No. 3,791,801. The patent discloses an electroplated steel sheet with excellent corrosion resistance, in which 0.05-2 weight % of one or more of molybdenum oxides or tungsten oxides are present in a zinc plating layer, or 0.05-2 weight % of one or more of molybdenum oxides or tungsten oxides and 0.5-15 weight % of metals or oxides of Fe, Ni, Co, Sn, Pb, etc. are co-deposited in the zinc plating layer.
In the above U.S. Pat. No. 3,791,801, the molybdenum and tungsten are present as their colloidal oxides in an acidic electrolyte. The molybdenum and tungsten oxides are physically filled in or are adsorbed chemically on a plating layer during plating, thereby being present as oxides such as MoO
2
, Mo
2
O
3
, WO
2
and W
2
O
3
, or hydroxides in the plating layer. If the molybdenum or tungsten oxide is present in the plating layer, the oxide is effective to restrain the dissolution of zinc under corrosive conditions, thereby enhancing corrosion resistance. In addition, if the oxide is present on the surface of the plating layer, the oxide adheres tightly to the paint layer, ensuring excellent adhesion after painting
A zinc plated steel sheet comprising oxides such as MoO
2
, Mo
2
O
3
, WO
2
, W
2
O
3
, etc. in a plating layer, is known to be useful in electric household appliances requiring corrosion resistance and paintability. However, because Mo or W is not present as alloy with zinc but is individually present as an oxide, when spot welding is carried out, for example, in a car body, there is a problem in that the oxide present on the surface of the plating layer can inhibit the flow of current and thus lower weldability. In particular, recently, automobile manufacturing companies have used mainly a projection welding method, in which several electrode tips are mounted, thereby being capable of carrying out the spot welding at the several electrode tips at one time. In this case, electric resistance must be uniform throughout a steel sheet, so that uniform welding at all electrode tips is accomplished.
Accordingly, as for a steel sheet, in which oxide such as MoO
2
, Mo
2
O
3
, WO
2
or W
2
O
3
or the like is present at the steel sheet surface and a plating layer which is formed on the steel sheet, due to high electric resistance of the plating layer, it is necessary to apply higher current for normal welding. However, when higher current is applied, zinc is quickly vaporized, so as to shorten the life of electrode tips; at the same time, spatter, a phenomenon that small droplets are dispersed to adjacent areas, is generated.
Furthermore, when there are differences in oxide content among sections of the steel sheet, electrical resistance among the sections is different. As a result, current passes toward the electrode tips with low electrical resistance during projection welding, but does not pass toward the electrode tips with high electrical resistance. Consequently, welding is not achieved.
Still furthermore, when oxide such as MoO
2
, Mo
2
O
3
, WO
2
, or W
2
O
3
, or the like is present in a zinc plating layer, binding force of metallic zinc with the oxide is weak. As a result, a plating material is stripped off as fine powder during bending of the steel sheet. This phenomenon is what is called “powdering”. When powdering occurs, there is a problem in that after the bending, the sections where powdering occurs are not provided for the substrate protection, thereby lowering corrosion resistance.
Meanwhile, Japanese Patent Laid-Open Publication No. 57-114686 discloses a technique, by which citric acid, formic acid and tartaric acid are added to an acidic electrolyte, to prevent tungsten and molybdenum, etc. from forming colloidal oxide sludge in the acidic electrolyte. As described in the publication, a glossy zinc electroplated steel sheet is manufactured using an electrolyte containing zinc ion as a main component, one or more of Co, Mo, Ni, Fe, Cr, W, V, In, Sn, and Zr ions, and an organic additive. Addition of citric acid, formic acid, and tartaric acid to the electrolyte makes it possible to prevent sludge formation of the metal ions and thus to reduce the amount of the sludge. As a result, workability in plating is enhanced and loss of effective metal ions in the electrolyte is reduced.
As can be seen from the FIG. 1 and FIG. 2 in the publication, the amount of sludge was reduced in the electrolyte containing citric acid, formic acid, and tartaric acid, compared with the electrolyte in the absence of them. However, the formation of sludge was not completely prevented.
Generally, when a steel strip is continuously electroplated, as plating proceeds, the concentration of a variety of metal ions in an electrolyte is reduced. Therefore, the reduced amount must be replenished, so that a plating layer with uniform metal ion content can be stably formed on a steel sheet.
As for the above publication, in the same manner, as plating proceeds, the conce
Posco
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
Zimmerman John J.
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