JPH01198340A - High corrosion resistance organic coated zinc-iron alloy plated steel material - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a zinc-iron (Zn-Fe) alloy plated steel material having excellent corrosion resistance and having an organic coating as the uppermost layer.

[Conventional technology]

In recent years, the environments in which steel materials are used have become increasingly harsh.For example, they are used in corrosive environments such as bag structures, mating parts, and lower parts of automobiles, as well as marine structures and bridges (wetting with salt water, drying, etc.). There is a need for steel materials that exhibit significantly high corrosion resistance under repeated conditions (especially under high humidity conditions).

To improve the corrosion resistance of steel materials, it is possible to improve the material itself or to form a protective film, but Zn-Fe
Alloy-plated steel materials have relatively good corrosion resistance, so they have traditionally been widely used in such severe corrosive environments.

This Zn-Fe alloy plating is performed by electroplating and hot-dip plating. In the electroplating method, Fig. 2 (
As schematically shown in a), a plating bath (e.g., sulfate bath) consisting of an aqueous solution containing Fe"" ions and Zn2" ions
A Zn--Fe alloy film is formed by simultaneously precipitating Zn and Fe and alloying them.

Electrochemically less base zinc is easier to precipitate than iron, so
Because the bath composition changes constantly during plating and the alloy composition changes due to the effects of variations in current density, etc., multiple Zn-Fe alloy phases coexist in the Zn-Fe alloy plating film produced by electroplating. It turns out.

On the other hand, in the hot-dip plating method, as shown in Figure 2 (bl),
A Zn--Fe alloy film is formed by heat-treating a galvanized layer obtained by hot-dipping pure zinc to mutually diffuse Zn and Fe in the base material to form an alloy.

This method is generally called alloyed hot-dip galvanizing method. According to this method, the Fe content is highest at the base steel/plating interface, and the Fe content increases toward the outer surface of the plating.
content decreases.

That is, when looking at the cross section of the plating skin 11Q, there is a Zn-Fe alloy phase that is the most electrochemically harmful at the base M/plating interface, and there are multiple Zn-Fe alloy phases that become electrochemically less noble toward the surface. It has a structure in which Zn-Fe alloy phases are sequentially arranged in layers.

[Problem to be solved by the invention]

The above-mentioned Zn-Fe alloy plated steel materials, regardless of the plating method, are particularly effective in suppressing corrosion of base steel materials. It cannot show sufficient corrosion resistance to meet the standard requirements.

An object of the present invention is to provide a highly corrosion-resistant steel material, particularly a highly corrosion-resistant wAtjA that can withstand use in severe corrosive environments such as steel materials used in automobiles, marine structures, bridges, etc.

[Means to solve the problem]

In order to achieve the purpose of "second speed," the present inventors focused on improving the corrosion resistance of the Zn-Pe alloy plating film itself of plated steel materials, and in the case of electroplating, various plating conditions and corrosion resistance. We attempted a wide range of basic studies on the relationship between the degree of alloying and the effects of overlapping plating layers with different compositions, as well as the relationship between the degree of alloying and corrosion resistance in the case of hot-dip plating. Furthermore, samples were prepared in which the various plated copper materials were subjected to chromate treatment as a lower layer coating, and then a paint was further applied as an upper layer coating, dried, and cured to form an organic polymer coating, and the corrosion resistance was investigated.

As a result, we found an interesting phenomenon in that although corrosion resistance generally improved with chromate treatment and organic coating, the corrosion resistance order in the unpainted case was reversed by chromate treatment and organic coating. In other words, the samples that exhibit outstanding corrosion resistance among painted materials with organic coatings have a common feature in their plating film structure, and the outermost layer of the plating film has a plating consisting of pure Zn or η phase (zinc-iron solid solution phase). There were layers. However, the plating film of this structure has poor corrosion resistance, as white rust occurs early when unpainted, leading to corrosion of the base steel material.

Zn having a layer consisting of pure Zn or η phase on the outermost layer
The remarkable high corrosion resistance that the -re plating film exhibits after chromate treatment and organic coating is not due to the effects of either the chromate layer or the organic coating layer, considering that the corrosion resistance of this plating film is poor when unpainted. Without,
It is thought that this was caused by a synergistic effect between the two.

As a result of further studies based on the knowledge obtained from this basic study, the present invention was completed.

Here, the gist of the present invention is to provide Z as a first layer on the surface of the steel material.
A plating layer consisting of a Zn-Fe alloy phase other than the η phase which is an n -Fe solid solution is 10 to 150 g/m, and a second layer thereon is a plating layer consisting of zinc or the η phase of 0.2 g/m.
A chromate lower layer coating with a Cr deposition amount of 10 to 300 mg/m and an organic polymer upper layer coating with a film thickness of 0.3 P or more on the plating surface of the two-layer plated steel material with ~20 g/g deposited. A highly corrosion-resistant organic coated Zn-Fe alloy plated steel material characterized by:

The first plating layer has a plurality of Zn-Fe alloys in which the electrochemically most responsible alloy phase exists in the lowermost layer at the interface with the base steel, and the higher the layer, the more electrochemically base the alloy phase becomes. Particularly excellent corrosion resistance can be obtained by constructing the Zn-Fe alloy plating film with a multilayer structure in which phases are stacked in layers. A Zn-Fe alloy plating film with such a multilayer structure can be obtained by the alloying melt-dip galvanizing method as described above.
A film having such a multilayer plating structure can also be obtained by electroplating by performing plating two or more times while appropriately varying the bath composition.

[Effect]

The cross-sectional structure of the coating in the plated steel material of the present invention is schematically shown in FIG. As shown in Fig. 1, the first N of the plating film contains a Zn-Fe alloy phase other than the η phase (Zn-Fe solid solution), that is, Z consisting of an intermetallic compound.
n -Fe alloy plating film and a second N layer on this first layer.
A pure zinc plating film or an η-phase Zn-Fe alloy plating film is provided as the plating film.

As a result of a more detailed study of the structure of this plating film, we found that high corrosion resistance is achieved when the coating weight of the second layer (outermost layer) of pure Zn or η phase is within a thin range of 0.2 to 20 g/m. It has become clear that this will be ensured. 0.
If the layer thickness is less than 28 m, the formation of the chromate film will be non-uniform, and the thin part of the film will become the starting point for corrosion, resulting in deterioration of corrosion resistance.

On the other hand, if it exceeds 20g/n, the corrosion resistance will deteriorate in the damaged parts of the paint film after painting.
．． A range of 5 to 10 g/m is suitable.

The amount of plating of the lower layer (first layer) relative to the second plating of the outermost layer is 10 to 150 g/1, which is relatively thick.
High corrosion resistance can be obtained within the range of m. It has been found that when this is less than 10 g/m, the time from the occurrence of white rust to the occurrence of red rust is very short, and the corrosion resistance against the base steel material is insufficient. On the other hand, 150g/n? Even though the corrosion resistance is saturated when using ultra-thin steel, problems such as deterioration of weldability and increase in weight occur, so it was judged that it is unsuitable for use as a practical plated steel material. The amount of second layer plating is 30 to 90g/
m is preferred.

The lower layer plating of the first layer can be formed by either electroplating or hot-dip plating. As already mentioned with reference to FIG. 2, the structure of the Zn-Fe alloy plating film differs depending on the plating method, but it is particularly suitable as a component of [lower layer plating, outermost layer plating + chromate + organic coating] according to the present invention. It was found that there were no significant differences in corrosion resistance due to differences in the coating structure and plating method. Therefore, even if the lower plating film is formed by electroplating, sufficient corrosion resistance can be ensured. However, since corrosion resistance is mainly affected by the amount of plating applied, it is preferable to form the lower plating film by alloyed zinc plating by hot-dip plating, which allows relatively thick plating to be easily achieved. In addition, in the case of a Zn-Fe alloy plating film formed by hot-dip plating, as mentioned above, the interface side with the steel substrate is the most electrochemically sensitive, and the cross-sectional structure is a layered structure that becomes progressively less noble toward the top. As will be described later, this structure is thought to contribute to further improvement in corrosion resistance.

When applying the lower layer plating by electroplating, the plating bath composition is selected so that the plating film is substantially composed of a Zn-Fe alloy phase other than the η phase, and after the formation of the lower layer plating film, - the above-mentioned outermost layer plating is applied. A film is formed separately.

When forming the lower plating film by alloying hot-dip galvanizing, the heating conditions for the normal alloying treatment are appropriately selected to ensure that the outermost layer of the plating film is pure Zn or η-phase Zn-Fe alloy. By doing so, a plating film having a two-layer structure of the lower layer plating and the outermost layer plating of the present invention can be formed at once. Alternatively, the heating conditions are selected such that the entire plating film is composed of a Zn-Fe alloy phase other than the η phase to form the lower plating film,
The two-layer plating film structure of the present invention can also be obtained by performing melt galvanizing or electrogalvanizing after the alloying treatment to form the outermost plating film.

According to the present invention, a chromate film is formed as a lower layer film and an organic polymer film is formed as an upper layer film on the above-described two-layer plating.

The chromate film can be formed by a conventional method using a conventional chromate treatment solution, and the composition is not particularly limited. Both coated and reactive chromate solutions can be used. Alternatively, electrolytic chromate may be used. However, in order to ensure sufficient corrosion resistance, it is necessary to
g/n (amount of chromium-1 film deposited is required. If this deposit amount exceeds 30On+g/n, the corrosion resistance after painting will deteriorate.The desired deposit amount is 40~2 as Cr).
00mg/n(.

The plated steel sheet subjected to the chromate treatment is then overcoated with a suitable paint (eg, enamel, face, braimer, etc.) to form an organic polymer upper coating. Various types of paint can be used as this paint, such as room temperature drying type, baking drying type, radiation curing type, etc., but baking drying type paint is particularly preferred. This is because bake-curable paints have excellent corrosion resistance, and the resin in the paint is oxidized during baking due to the small amount of hexavalent chromium present on the surface of the lower chromate film, causing the paint film and lower chromate film to oxidize. This is because the adhesion with the metal is improved and the corrosion resistance is further improved. Typical examples of bake-curable paints include epoxy resin paints, synthetic resin paints, polyamide resin paints, acrylic resin paints, and the like.

This paint is applied so that an organic polymer film having a film thickness of 0.3 μIn or more is obtained after drying. Film thickness is 0.3, u1
If it is less than 11, the effect of improving corrosion resistance by forming an organic film is insufficient. It's performance! There is no upper limit for the J thickness, but if it exceeds about 50 pJlj, the corrosion resistance due to this coating will become the main factor, and the effect of the two-layer plating in the present invention will become unclear. The thickness of the organic polymer upper layer film is preferably 0.
．． It is 7 to 30 μfil.

The reason why the present invention exhibits unexpectedly high corrosion resistance is thought to be as follows.

As schematically shown in FIGS. 3(1) and (II), 2
The second layer plating, which is the outermost layer in the layer plating, is electrochemically less base than the first layer Zn-Fe alloy plating layer, which mainly has the anticorrosive effect (b) on the substrate fM+J. On the other hand, cathodic protection (Al) is performed.The melting of the surface plating that functions as a sacrificial anode is suppressed to the minimum ratio because the plating surface layer is maintained by the chromate lower layer film and the organic upper layer film (C).In addition, The second layer plating on the outermost layer is not directly plated on the base steel material, which has a large potential difference, but on the Zn layer in the lower layer, which has a relatively small potential difference.
The fact that it is formed on the -Fe alloy plating will also be a factor that suppresses the dissolution of the outermost layer plating. It is thought that the fact that the outermost layer plating exhibits cathodic corrosion protection extremely efficiently is the main reason for the high corrosion resistance. In addition, Fig. 3 (1) shows the case where the lower layer plating film consists of a single alloy phase or a plurality of mixed alloy phases like electroplating, and the same figure (■) shows the case where the lower layer plating film consists of a single alloy phase or a plurality of mixed alloy phases as in electroplating. This shows the case of a plurality of layered alloy phases. Also,
In these figures, the outermost layer of the second N plating is pure Zn.
This is represented by plating, but this is Zn-Fe in the η phase.
The same situation applies to alloy plating.

When the lower layer alloy plating is formed by the alloyed zinc plating method, as schematically shown in FIG. 3 (TI), a plurality of Zn-
The Fe alloy phases (1) to (III) are arranged in layers, and the phase (III) located at the interface with the base steel has the highest Fe content and is the most electrochemically responsible, and the upper phase (1) As the temperature increases, the Zn content increases and becomes electrochemically less noble. However, Z, which forms the top layer of the lower layer plating,
Since the n-Fe alloy phase (I) is other than the η phase, η
It is electrochemically nobler than the second phase surface layer plating consisting of Zn--Fe alloy plating or pure Zn plating. Configuring the lower layer plating in this way also induces a cathodic corrosion protection effect between the base metal phases, that is, the electrochemically base alloy plating layer closer to the surface layer exerts a corrosion protection effect on the alloy plating layer immediately below. Therefore, it is thought that this contributes to further improvement of corrosion resistance. Such a Zn-Fe alloy plating film having a multilayer structure can be electroplated by performing electroplating two or more times by appropriately varying the bath composition, as shown in 4 and 5 of Examples below. It can also be formed by a method.

Next, the present invention will be explained in more detail with reference to Examples.

〔Example〕

A cold-rolled steel sheet with a thickness of 0.7 mm whose surface has been cleaned by a conventional method is coated with Zn in the lower layer by electroplating under the conditions shown in Table 1.
-A Fe alloy plating film (two-layer film for No. 4 and 5) and an upper layer of pure Zn or η phase Zn-Fe alloy plating film are formed, and then Cr0z: 20g/
A chromate film was formed using a chromate solution containing I-,1 (3PO4: 2g/β).A high molecular weight phenoxy resin paint (trade name: Bakelite PKHH, manufactured by Union Carbide Co., Ltd.) was applied on top of the chromate film. An organic coated Zn-Fe alloy plated steel sheet was prepared by applying Zn-Fe alloy coated steel sheet to a predetermined dry film thickness and baking it at a board temperature of 250°C for 60 seconds to harden the film. Tested.

The test pieces used for the corrosion resistance test were an unprocessed flat plate test piece and a cylindrical drawing test piece with a diameter of 50111111 mm.
It is a kind. The die shoulder of the cylindrical drawing was cleaned with trichloroethane and polished with No. 120 emery paper each time so that its surface roughness remained constant. These specimens were treated with salt spray (5χNaC1, 35°
C) 2 hours - drying (50°C) 2 hours - relative humidity 9
Corrosion resistance was evaluated by observing the occurrence of red rust on the test pieces through an accelerated corrosion resistance test in which the test pieces were left in a humid atmosphere of 5% or more (50°C) for 4 hours for 4 hours. The test results are summarized in Table 1 below, together with the conditions for two-layer plating and the amount of Cr deposited, the amount of Cr deposited in the lower layer chromate, and the film thickness of the upper organic film. The evaluation criteria for corrosion resistance are as follows: ◎: No red rust is observed even after 200 cycles, ○: Red rust begins to occur around 150 cycles, △:
Red rust was observed within 100 cycles. ×: Red rust was observed within 50 cycles.

(Hereinafter, blank space) Table 1 (Continued from next vane) Table 1 (Continued) [Effects of the invention] As is clear from Table 1, the organic coated Zn − of the present invention
Fe alloy plated steel exhibits high corrosion resistance in severe accelerated corrosion resistance tests. On the other hand, as shown in Example Teeth 6 to 12, if one or more of the structure of the two-layer plating, its adhesion amount, chromate adhesion amount, and organic coating thickness deviate from the conditions specified in the present invention, the corrosion resistance deteriorates. is inferior.

In this way, by using Zn-Fe alloy plating for the lower layer, a gentle potential gradient is generated from the outermost plating layer to the base steel sheet, and the plating on the surface layer is always more moderate than the plating on the inner layer. Since it does not exert sacrificial corrosion protection, the dissolution of the plating film is suppressed and the corrosion protection continues. Moreover, the lower chromate film and the upper organic polymer film provided on the outermost plating effectively protect this outermost layer, resulting in excellent corrosion resistance even in highly corrosive environments. be. The high corrosion resistance achieved by this two-layer plating can only be achieved by providing a chromate film and an organic film on top, and since the corrosion resistance is significantly inferior when unpainted, the synergistic effect of the film structure of the present invention is recognized. be.

As described above, the organic coated plated steel material of the present invention can sufficiently withstand over a long period of time in highly corrosive environments such as bag structures, joint parts, and lower body parts of automobiles, as well as marine structures and bridges. It has excellent corrosion resistance.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the plating film structure according to the present invention, FIG. (TI) is a schematic explanatory diagram of the corrosion mechanism utilized in the present invention.

Claims (2)

[Claims] (1) As a first layer, 10 to 1 plating layer consisting of a zinc-iron alloy phase other than the η phase, which is a zinc-iron solid solution, is applied to the surface of the steel material.
The amount of Cr deposited is 10 on the plating surface of a two-layer plated steel material, on which a plating layer consisting of zinc or the above-mentioned η phase is deposited as a second layer of 50 g/m^2 and 0.2 to 20 g/m^2. A highly corrosion-resistant organic coated zinc-iron alloy plated steel material, characterized by having a chromate lower layer coating of ~300 mg/m^2 and an organic polymer upper layer coating of 0.3 μm or more in thickness. (2) The first plating layer is composed of a plurality of layered alloy phases, in which the electrochemically noblest alloy phase directly coats the steel surface, and becomes less noble toward the surface layer. The highly corrosion-resistant organic coated zinc-iron alloy plated steel material according to claim 1, further characterized in that it has a multilayer structure in which alloy phases are sequentially arranged.