Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Patent
1994-03-08
1996-04-23
Zimmerman, John
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
428935, B32B 1518
Patent
active
055101968
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention of the subject application relates to corrosion resistant steel sheets that satisfy the various properties required of the corrosion resistant steel sheets for use on automobiles, etc., which include not only high corrosion resistance but also either one of high resistance to cosmetic corrosion, good formability, high chipping resistance, high corrosion resistance in the as-formed state, strong water resistant secondary adherence of coating and high perforation corrosion resistance.
BACKGROUND ART
Automotive corrosion resistant steel sheets commercially used today include electrogalvanized steel sheets, steel sheets with electroplated Zn--Ni alloys, steel sheets with electroplated Zn--Fe alloys, hot-dip galvannealed steel sheets and various other types, all of which are Zn base plated steel sheets. These make use of the self-sacrificial corrosion preventing action of Zn for steels. The most straightforward way to improve corrosion resistance is by increasing the coating weight of plating (hereunder referred to as "coating weight") but the increase in coating weight is accompanied by deterioration in formability, weldability and other quality factors.
Attempts have therefore been made to alloy Zn with other elements so that smaller coating weights than that of pure Zn will suffice for providing comparable degree of corrosion resistance. Potential effects of alloying include, for example, bringing the corrosion potential of the alloy even closer to steel so that the corrosion rate of the plating layer per se is allowed down, and stabilizing the corrosion product. However, the contribution of alloying to the improvement in corrosion resistance has been still unsatisfactory in the conventional Zn base alloy plated steel sheets. Under the circumstances, attempts have been made in recent years to add Cr as an alloying element to the Zn base plating layer. Examples of such attempts have been proposed in Japanese Patent Application (kokai) Nos. Hei 1-191797, 3-120393, etc. It is true that as far as the corrosion resistance in the bare state is concerned, increasing the percent Cr content contributes to the formation of a Zn--Cr alloy plating that exhibits better corrosion resistance than the conventional Zn base alloy plating.
As an example, a salt spray test was conducted in accordance with JIS Z 2371 and the number of days to 2% red rust development was checked. The results are shown in FIG. 1. Motorcar bodies are normally formed before use, so the test specimens were those which had been subjected to 17% stretch. In the following description, values of coating weight are sometimes indicated with the symbol for unit of its measure (g/m.sup.2) being omitted. For example, a coating weight of 30 g/m.sup.2 may be indicated as coating weight 30. In FIG. 1, EG 30 designates a commercial electrogalvanized steel sheet with coating weight 30; GA 60 is a commercial hot-dip galvannealed steel sheet with coating weight 60; and Zn--Ni 30 designates a commercial Zn--Ni alloy plated steel sheet with coating weight 30 and 13% Ni content. For all Zn--Cr specimens, the coating weight of the plating was 20 g/m.sup.2.
One can see from FIG. 1 that the corrosion resistance of the Zn--Cr alloy plated steel sheet in the bare state improves almost linearly with the increase in the percent Cr content of the alloy. It can also be seen that even with coating weight 20, the samples have better corrosion resistance in the bare state than EG 30 and GA 60 of higher coating weight if Cr/(Cr+Zn) is 2 wt % or more. Thus, the Zn--Cr alloy plated steel sheet exhibits better corrosion resistance in the bare state and this would be because in a corrosive environment, the surface oxide film of Cr suppresses the dissolved oxygen reducing reaction by a marked degree to reduce the corrosion current density, or retard the corrosion rate.
The experimental result under consideration is that of a test assuming corrosion that occurs principally in a site such as where the inner surface of an automotive body is electrodeposite
REFERENCES:
patent: 3822118 (1974-07-01), Fukuzuka et al.
patent: 4877494 (1989-10-01), Kanamaru et al.
patent: 5188905 (1993-02-01), Shindou et al.
patent: 5272643 (1993-12-01), Hasegawa et al.
Fujimura Tohru
Katayama Michio
Mochizuki Kazuo
Morito Nobuyuki
Nakamaru Hiroki
Kawasaki Steel Corporation
Miller Austin R.
Zimmerman John
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