JPS62167898A - Steel sheet for rust prevention - Google Patents

Abstract

PURPOSE:To remarkably improve the corrosion resistance of the titled steel sheet obtd. by forming a coating layer dispersed with metallic Cr particles in a Zn (alloy) plating layer on the steel sheet. CONSTITUTION:An example is taken of the case in which Zn (alloy) plating is executed by an electroplating method. The metallic Cr particles of about <=200 mesh minus sieve are added to an acidic Zn (alloy) plating bath and while the bath is kept stirred, the steel sheet is subjected to plating, by which the coating layer taking the Cr particles of about 1mum diameter into the plating layer is formed thereon. The large particles of Cr settle down onto the bottom of the plating bath. The same effect is obtd. as well in the case of executing the above-mentioned plating by a hot dip coating method, ion plating method, thermal spraying method, etc. The double coating layers are formed by forming the above-mentioned coating layer as an inside layer and subjecting the extreme surface layer to Zn or Fe plating or alloy plating essentially consisting thereof. Such steel sheet is adequate for an automobile, etc.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to construction materials, ? ! ! This relates to rust-preventing steel plates used in gas equipment materials, automobiles, transportation materials, etc.

[Prior art]

Conventionally, zinc or zinc-based alloy plated steel sheets have been widely used for rust prevention purposes, and the sacrificial anticorrosion effect of zinc and zinc is the basis. However, as usage and corrosive environments become more severe.

In the current situation where there is an increasing demand for the durability of materials, steel plates with even higher corrosion resistance are required.

From the viewpoint of sacrificial corrosion protection, one solution is to increase the thickness of the zinc-based plating layer, but this has the disadvantage of impeding press workability and weldability.

Zinc and more dangerous metals such as nickel and iron
An alloy-plated steel sheet (Japanese Patent Application Laid-open No. 152662/1983) has also been proposed. By alloying with nickel, iron, etc., the potential difference between the plating layer and the steel base is reduced, and the excessive electrochemical activity of zinc is suppressed to some extent, thereby reducing the corrosion rate of plating in corrosive environments. The aim is to make it even smaller. However, it cannot be denied that there is a risk of reducing the sacrificial corrosion protection effect.

A coated steel sheet in which colloidal particles such as SiO□ and TiO□ are dispersed and composited in a zinc-based plating layer has also been proposed.

These dispersed phases are inert in a corrosive environment, and after the zinc is dissolved away due to corrosion, the basic effect of the remaining dispersed phase against intrusion of water, salt, etc. is merely the physical barrier effect, and it is still inactive. It is enough.

[Means to solve problems]

If sacrificial corrosion protection can be maintained in the steel base and the rate of erosion of the plating layer in a corrosive environment can be reduced, an ideal rust-proof steel plate can be obtained. The present invention was developed from this point of view, and its gist is a rust-preventing steel sheet having a coating layer containing metallic chromium particles dispersed in a zinc or zinc-based alloy plating layer. Also, the inner layer has a coating layer containing metal chromium particles dispersed in a zinc or alloy plating layer mainly composed of zinc, and the outermost layer is a zinc or iron plating layer, or an alloy plating layer mainly composed of them. It is a multi-layer anti-rust steel plate with

As a result of various basic studies, the present inventors have found that, in addition to the electrochemical activity of zinc, the corrosion rate of zinc or zinc-based alloy coated steel sheets is We focused on the protective effect of the product.

As is well known, although zinc is an electrochemically base metal, its corrosion rate is extremely low in a dry atmosphere due to the protective effect of its oxide film. However, in a humid environment, especially F! A: In the corrosive environment of automobiles in areas where snow salt is sprayed, the oxide film is easily eluted5 and the final corrosion product is considered to be basic zinc chloride.

This substance also has a relatively high solubility in a humid environment, and no protective effect can be expected at all in an acidic environment that corrodes the coating film.

Metals that have been conventionally proposed as zinc-based alloy plating, such as nickel, iron, magnesium, manganese, cobalt, copper, and cadmium, cannot be expected from the same point of view as zinc in protecting against corrosion products.

The present inventors focused on chromium. As is well known, metallic chromium is an extremely corrosion-resistant material that becomes passivated in the presence of oxygen and is not corroded even in dilute acids. However, when in contact with zinc, chromium becomes electrochemically in a base state similar to zinc, and it has been confirmed that it has a sufficient sacrificial corrosion protection effect on steel substrates. The corrosion product in a humid environment is presumed to be a basic chloride of trivalent chromium, but f@ is recognized as a type of poorly soluble polynuclear complex.

When applying metallic chromium to zinc-based plating, ensuring sufficient chromium content (for example, 0.1% by weight or more) to contribute to corrosion resistance as an alloy plating is achieved by electroplating, hot-dip plating, etc. In either case, it is virtually impossible. As another aspect, a plated steel sheet has been proposed in which the surface of a galvanized layer is coated with chromium oxide, metallic chromium, or both, but there is a drawback that once the surface coating is completely dissolved in a corrosive environment, it does not contribute to corrosion resistance.

Therefore, in the present invention, a layer is formed in which metallic chromium particles are dispersed in a zinc or zinc-based alloy plating layer. By doing this, as long as metallic chromium continues to be in contact with zinc, it participates in sacrificial corrosion protection against the steel substrate in a corrosive environment, and is uniformly corroded with the matrix (plating layer) at almost the same rate as zinc, and the chromium Corrosion products are deposited not only on the dispersed phase but also on the surface of the plating layer to form a protective film, thereby terminating the local corrosion. The so-called self-healing action reduces the rate of erosion of the plating layer without sacrificial corrosion protection. This is a durable rust-proof steel plate.

The amount of chromium contained in the plating layer is preferably 0.3 to 20% in order to produce a self-healing effect.

If it is less than 0.3%, there will be little effect on corrosion resistance, etc., and even if it is less than 20%, sufficient effects can be obtained. Also, the chromium particle size is 1
It is preferably 0 μm or less. Particles having a diameter of more than 10 μm are not preferable because they have poor corrosion interaction with the surrounding matrix and remain after the zinc has been dissolved, potentially promoting pitting corrosion on the steel substrate.

The matrix plating layer is zinc or zinc-based alloy plating. The latter is preferably an alloy plating containing 60% by weight or more of zinc. The reason for this is that the matrix itself has sufficient sacrificial corrosion protection, and at the same time, electrochemical interaction with the chromium dispersed phase is taken into account. In order to maintain the sacrificial anticorrosion effect of chromium,
The potential of the plating layer in contact with chromium must be close to the potential of chromium in its active state.

This is because it is considered that if the potential of the plating layer approaches a nobler potential, the potential of chromium will also become nobler. Based on these considerations, the alloying elements in the matrix include iron, nickel,
Manganese, chromium, cobalt, aluminum, magnesium, cerium, titanium, zirconium, cadmium,
1 selected from tin, thorium, lead, copper, carbon, phosphorus, etc.
A species or two or more elements can be applied. In the present invention.

Metal chromium particles include single metal chromium particles as well as Fe + Ni + Co I Cu + in metal chromium particles.
One or more of T1rAQ, Zn, P, etc. may be contained in appropriate amounts.

Next, an example of the manufacturing mode of the present invention will be described. Normally, electroplating, hot-dip plating, ion plating, thermal spraying, etc. are applied to galvanized or zinc-based alloy coated steel sheets, but the metal chromium dispersion method of the present invention can be applied to any of these methods. However, embodiments will be described by taking as an example the electroplating method, which is currently widely used and easy to apply.

Zinc plating or electroplating of zinc-based alloys often uses an acidic bath, but metallic chromium particles can exist stably by being passivated even in an acidic solution with a pH of around 1. Furthermore, by adsorbing ions in the solution onto the particle surface, a dispersed state can be maintained without aggregation. If necessary, ions may be added to the bath to promote charging. The volume occupancy of metallic chromium incorporated into the plating layer can be controlled by the dispersion concentration of metallic chromium particles in the bath, particle size, current density, and flow rate.

According to experiments conducted by the present inventors, metallic chromium particles are electrodeposited in a form embedded in the zinc or zinc-based alloy of the plating layer; In some cases, the plating layer is etched during the treatment process, exposing a portion of the chromium particles, and no chemical conversion coating is formed on that portion. If the chemical conversion coating is incomplete, sufficient coating adhesion cannot be ensured in a corrosive environment. To deal with this problem, for example, if the outermost surface is plated at 1 g/m or more, preferably at 2 g/m or more, with only a matrix metal that does not contain metallic chromium, this problem can be avoided. The thickness of the outermost surface layer can be determined from the amount of etching during the chemical conversion treatment.

As a more preferable embodiment, the outermost surface is 1 g/n? Above, preferably 2 g/rr? The above coating film vB has good visibility.

A plating layer may be applied on top of the metal chromium dispersed plating layer.

As a preferable example, iron or iron-zinc alloy plating containing 60% by weight of Fe may be applied.

This will be explained below using examples.

An example of manufacturing a plated steel sheet in an example described below will be given.

[Examples 1 to 2] Zn":=75g/Q+ M separation H2S O-" 30
500 g/Q of metal chromium particles under a 200 mesh sieve were added to an acidic zinc plating bath consisting of Na2So4 = 65 g/Q, and plating was performed under stirring. 7Ii current density is 10 A/d rrf ~l
00 A/di to adjust the basis weight and chromium content. Large particles of chromium sink to the bottom of the plating bath.

Most of the particles actually incorporated into the plating layer were below 1 μm. When the plating layer was dissolved with 10% HCQ to measure the basis weight, chromium was completely dissolved along with zinc. Note that the plating bath remained stable even after being left for several days, with no coloring due to Cr'+ being observed and no hydrogen gas being generated.

[Examples 3 to 6] 500 g/Q of metallic chromium particles were added to an acidic zinc-iron plating bath consisting of a total of Zn"+ and Fe"+ of 110 g/Q r free H2So4 = 15 g/Q, and stirred. Plating was performed in this state. Current density is 10A/drr
? It was varied at ~100 A/d, and the date amount and chromium content were adjusted. The iron/zinc alloy composition in the plating layer was adjusted by changing the Fe""/Zn2+ ion ratio in the bath. The iron/nitrous alloy composition in the plating layer is the Fe in the bath.
”/Zn2+ ion ratio was adjusted by changing. Large particles of chromium sank to the bottom of the plating bath.

Most of the particles actually incorporated into the plating layer were below 1 μm. When the plating layer was dissolved with 10% HCQ to determine the basis weight dl'l, it was completely dissolved together with the plating layer. The chromium particles were stable in the plating bath, and no changes such as hydrogen gas generation were observed.

[Example 7] The total of z n2+ and Ni'' is 100 g/Q, N
200 g/Q of total chromium particles were added to a sulfate bath of 80.100 g/Q, and plating was performed under stirring. Current density is IOA/d~100A/d
nr, and the basis weight and chromium content were adjusted. The nickel/zinc alloy composition in the plating layer is determined by the N in the bath.
It was adjusted by changing the x ”/Z n 24” ion ratio. Most of the chromium particles incorporated into the plating layer had a diameter of 1 μm or less, as described above.

When the plating layer was dissolved with heated 10% HCΩ to measure the basis weight, the chromium was completely dissolved along with the matrix. Note that the chromium particles in the plating bath were stable, and no changes such as hydrogen gas generation occurred.

(Test conditions) (Bare corrosion resistance of plated plate) Immersed in 5% NaCQ aqueous solution for 30 minutes and in the atmosphere at 60°C and humidity 6
It was carried out for 6 weeks with a 0% 30 minute repeat cycle.

(Corrosion caused by injection holes after painting) Samples facing each other with a gap of 3 mm were subjected to immersion chemical conversion treatment, and cationic electrodeposition was applied to the outside by 20 μm, followed by salt water spraying for 6 hours → drying at 70°C for 4 hours → moistening. 50℃95
A repeated cycle test of 4 hours at RH and then 4 hours at cooling at -20°C was conducted for 6 weeks.

(Hermeticity after painting) A cross-cut was made on a test piece coated with 20 μm thick cationic electrodeposition using dipping type chemical conversion treatment, and the maximum bulging width on one side from the cross-cut after 4 weeks of repeated cycle tests under the same conditions as in the previous section. measurement.

〔Effect of the invention〕

According to the present invention, the corrosion resistance of zinc or zinc alloy coated steel sheets can be significantly improved, and excellent effects can be obtained, such as being able to be applied to steel sheets for automobiles, etc., and achieving great industrial effects. It will be done.

Claims (2)

[Claims] (1) A rust-preventing steel sheet having a coating layer containing metallic chromium particles dispersed in a zinc or zinc-based alloy plating layer. (2) The inner layer has a coating layer containing metal chromium particles dispersed in a zinc or zinc-based alloy plating layer,
A multi-layer anti-corrosion steel plate with a zinc, iron, or alloy plating layer mainly composed of these on the outermost layer.