Surface-treated Steel Plates High In Anticorrosiveness

Abstract

A surface-treated steel plate high in anti-corrosiveness which plate has been produced by coating a steel plate with an aqueous solution containing the nitrate or acetate of Ni as the principal component in conjunction with such metals as Cr, Mn, Zn and Al as selective components, followed by heating to produce a thermodecomposing reaction so as to cause a strong film containing metallic nickel and at least one metallic oxide to form on the surface of the steel plate. The thus-produced surface-treated steel plate is especially adapted for making cans.

Parent Case Text

This is a division of application Ser. No. 32,439, filed Apr. 27, 1970, now U.S. Pat. No. 3,677,797.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of making surface-treated steel plates by thermodecomposing plating.

2. Description of the Prior Art

As is well known, tin-plated steel plates are primarily used for producing cans. However, tin has certain drawbacks in that it is expensive, it is not endurable at high temperatures, it is weak against corrosion in the atmosphere and further it turns black, depending on the contents of the can. Furthermore, tin is in such short availability in the world, that its supply is unstable. As a result of making researches on surface-treated steel plates, the present inventors have discovered that in order to produce surface-treated steel plates which are devoid of the above defects, it is necessary to form a film on the surface of the steel plate, which film is more stable than the steel plate to be plated. The film formed herein is based on a technical idea different from that of plating, such as tin-plating to electrochemically protect the steel plate in a corrosive liquid.

Further, when using metal-plated surface-treated steel plates, including tin-plated steel plates, as the material for making cans, it is usual to coat the metal-plating layer with well known lacquers of various kinds such as an epoxy resin series paint to prevent the metal-plating layer from being corroded and the metal ions from being formed by such contents in the can as, for example, a beverage, liquor, oil or fat.

Therefore, a metal-plated surface-treated steel plate should satisfy certain conditions. It should not only be high in antirusting properties in the atmosphere, but also, when coated with a lacquer and dipped in the above described corrosive liquid, the lacquer coating film should not peel off, and, moreover, it should not interfere with the can-manufacturing operations; for instance, it should be easy to solder and mold.

To exemplify the peeling-off of such lacquer film, the case of an Al- or Zn-plated steel plate coated with a lacquer will be explained. When dipping said steel sheet in the above described corrosive liquid, the corrosion of the Al- or Zn-plating layer by said corrosive liquid proceeds faster than the corrosion of the iron base material on account of an anodic protective action of Al or Zn on the steel sheet, resulting in the peeling-off of the lacquer film.

As opposed to the case of an Al- or Zn-plated steel plate, there occurs no peeling-off of a lacquer from the surface of a Cr- or Ni-plated steel plate when dipped in a corrosive liquid because Cr or Ni is inherently high in antirusting properties and is more stable than the iron base; consequently the lacquer adheres firmly to the surface of the Cr- or Ni-plated steel plate. However, because the plating layer is thick, this plating method is not economical.

SUMMARY OF THE INVENTION

On the basis of such knowledge, as is described above, the present inventors have developed a novel process for obtaining a surface-treated steel plate high in anticorrosion by forming, on a steel plate, a film more stable than the iron base.

An object of the present invention is to provide surface-treated steel plates which are so high in anti-corrosiveness that they can sufficiently serve in place of tin-plated steel plates. The surface-treated steel plates are novel and economical and can be used as a material for manufacturing tin-free cans. These plates may also be used as materials for automobiles, various constructions and toys.

In order to attain the object of the present invention, the following methods are provided, which are characterized by the following features: that is, a method for obtaining surface-treated steel plates having excellent anticorrosiveness, wherein an aqueous solution of nitrate and/or acetate of Ni is applied on the surface of the steel plates, previously subjected to a surface-cleaning treatment, and the surface treated steel plates are then heated in a nonoxidative gas atmosphere, so as to cause a thermodecomposing reaction, to thereby form a film containing metallic nickel on the surface of the steel plate; a method for forming a further improved film, that is, a method for obtaining surface-treated steel plates having excellent anticorrosiveness wherein an aqueous solution of nitrate and/or acetate of Ni with the addition of one or more nitrates and acetates of Cr, Mn and Zn is applied on the surface of steel plates, previously subjected to a surface-cleaning treatment, and then heated in a nonoxidative gas atmosphere, so as to cause a thermodecomposing reaction, to thereby form a film containing metallic nickel and one or more of oxides of Cr, Mn and Zn on the surface of the steel plates; and further a method for forming a more excellent film, that is, a method for obtaining surface-treated steel plates having particularly excellent anticorrosiveness, wherein an aqueous solution of a nitrate and/or acetate of Ni with the addition of one or more of nitrates and acetate of Cr, Mn and Zn and with a further addition of a nitrate and/or acetate of Al is applied on the surface of a steel plate, previously subjected to a surface-cleaning treatment, and then heating the thus-treated steel plates in a nonoxidative gas atmosphere, so as to cause a thermodecomposing reaction, to thereby form a film containing metallic nickel and one or more of oxides of Cr, Mn, Zn and Al on the surface of the steel plates.

The present invention is particularly economical because it is possible to utilize a heating cycle of annealing conditions for the thermodecomposing reaction, and the method of the present invention is therefore very high in practical value.

Moreover, the thus obtained treated film is not only excellent in anticorrosive activity but is also very excellent in its paint adhesiveness and mechanical workability. Therefore, the present invention is particularly adapted as a method of making steel plates for making cans.

The method of the present invention shall be detailed in the following.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The essence of the present method is to form a very thin film of metallic nickel on the surface of a steel plate by applying an aqueous solution, for example, of nickel nitrate to coat the said surface of the steel plate and then heating it in a reductive or inert non-oxidative atmosphere so as to cause a thermodecomposing reaction of the treating solution and the reduction (complete or partial) of nickel nitrate to metallic nickel.

Nickel nitrate or nickel acetate, which is an essential component of the treating aqueous solution of the present invention, may be used individually or in admixture. However, Ni compounds (particularly oxides) other than the aforementioned compounds should be avoided, since they involve difficulties in forming films of satisfactory anticorrosiveness, paint adhesiveness and workability.

In the following, for the convenience of the explanation, the aqueous solution containing a Ni-salt prepared as described above shall be called a treating solution.

It is difficult to uniformly apply some of the treating solutions of the present invention to coat steel plates. However, in such case, an improvement can be obtained by adding a proper amount of a surface active agent, such as a nonionic active agent. The deposited amount of the treating solution differs remarkably, depending on the method of coating the surface of the steel plates, for example, by blowing a spray of the treating solution on the surface of the steel plates, applying the treating solution with a roller, or dipping the steel plate in the solution. The concentration of the treating solution is difficult to uniformly define but, if the effective quantitative range is to be defined in relation to the viscosity of the solution or the uniformity of the decomposing reaction, Ni ions are to be contained in an amount of 0.5 to 100 g./l. or preferably 1 to 20 g./l., the later described Cr.sup.+.sup.+.sup.+ ions are present in amounts less than 20 g./l. or preferably 10 g./l., Al ions are present in amounts less than 20 g./l. or preferably less than 10 g./l., Zn ions are present in amounts less than 40 g./l. or preferably less than 10 g./l. and Mn ions are present in amounts less than 20 g./l. or preferably less than 10 g./l.

Further, in order to improve the characteristics of the treated film, a nitrate or acetate of Mg, Ca or K may be added in an amount of about 1 g./l.

It is wise to prevent, as much as possible, the pH of the treating solution of the present invention from becoming excessively acidic to avoid an exchanging reaction with Fe. Therefore, in industrial practice, it is desirable to make the pH of the treating solution 3 to 4.

In regard to films which are heavily colored in their appearance after being heat-treated, there is observed a tendency of these films to reduce the adhesiveness of the lacquer and also to reduce the adhesion of the film itself to the base. Therefore, it is desirable to control the deposited amount of the film so that the appearance after heat-treatment may have a transparency to such a degree as to not substantially impair the luster of the cold-rolled steel plate or to control the deposited film so that it has a light tone to such a degree as to be barely recognizable with the naked eye (that is, the metal thickness may be less than 1 .mu. or, if possible, about 0.1 .mu. or less).

Now, in the method of the present invention, the above described treating solution is applied and is then quickly heated to cause a thermodecomposing reaction at a temperature of 200 to 750.degree.C. in a furnace of a nonoxidative gas atmosphere such that it has, for example, a H.sub.2 content of 2 to 20% and wherein the remaining component is essentially N.sub.2 (as DX or NX gas) used as a brightly annealing gas to form a strong metallic nickel film. When the content of H.sub.2 in the gas atmosphere is less than 2%, the Ni salt is so hard to reduce that there is formed an undesirable film because the film containing metallic nickel becomes lusterless and blackish. On the other hand, when the content of H.sub.2 is more than 20%, there is a great economic loss. In addition, such a high content of H.sub.2 is not desirable, particularly in the method wherein salts of Al, Cr, Mn and Zn are added to a salt of Ni, because the reduction of salts of these metals proceeds too far with the result that such a metallic nickel-plated film containing oxides of these metals, as is intended in the present invention, can not be obtained.

The above described nonoxidative gas many contain a slight amount of carbon dioxide, carbon monoxide or water. However, the heating atmosphere must not be oxidative (i.e. contain oxygen) to such a degree as to have an adverse influence on the reduction of the salt of nickel to metallic nickel; otherwise the salt of nickel applied to the surface of a steel plate may not be reduced to metallic nickel to the required amount during the annealing time and may remain as nickel oxide on the surface of the steel plate. The latter effect would not be in keeping with the object of the present invention.

When the heating temperature is less than 200.degree.C., no effective thermodecomposition takes place. On the contrary, when it is more than 750.degree.C., there are produced undesirable results, such that all the metallic nickel produced by the thermodecomposition, will alloy with the iron base to form a hard film which is not only low in adhesiveness but also low in the anticorrosiveness, and further even impair the mechanical properties of the steel plate itself.

In the foregoing, an example of applying a treating solution composed of only a Ni-salt has been explained. The surface film obtained by this method is thought to be a two-layer film high in anticorrosiveness, consisting of an Fe-Ni alloy in the lower layer and metallic nickel in the upper layer. As a result of making further investigations, however, the present inventors have succeeded in developing an improved method of forming, on the surface of a steel plate, a film which is more excellent than the above described film. This film is thought to be formed of two layers, wherein the lower layer is made of an Fe-Ni alloy and the upper layer is a mixture mainly composed of oxides of Cr, Mn and Zn and metallic nickel.

The improved method involves applying to the steel sheet, an aqueous solution (which shall be called a treating solution with additive for the convenience of the explanation hereinafter) obtained by adding one or more of nitrates and acetates of Cr, Mn and Zn to a nitrate and/or acetate of Ni and heating the thus-coated steel sheet in a nonoxidative gas atmosphere furnace to form a film consisting of one or more of oxides of Cr, Mn and Zn, and metallic nickel. The above-mentioned treating solution with additive may be prepared by adding one or more members consisting of carbonates, oxalates and hydroxides of Cr, Mn and Zn to an aqueous solution of Ni nitrate and/or acetate.

In the foregoing, there have been shown examples of dissolving a nitrate or acetate of Ni and carbonates, oxalates, hydroxides and oxides of Cr, Mn and Zn in an aqueous solution of nitric acid and/or acetic acid. However, the present invention is not limited to them except for the Ni salts. Any salts of Cr, Mn and Zn may be used, if they leave no anion in an aqueous solution of nitric acid or acetic acid when dissolved in such solution.

As to the method of applying said treating solution with additive, any of the methods adopted when applying the above-mentioned treating solution may be used, such as the spraying method, the roller-coating method and the dipping method. If necessary, also a surface active agent may be further added.

Also, the heating can be carried out under exactly the same conditions and in the same manner, that is, in a temperature range of 200 to 750.degree.C. in a furnace of a nonoxidative gas atmosphere in which, for example, the H.sub.2 content is 2 to 20% and the rest is mostly N.sub.2. However, the thus obtained film is higher in anticorrosiveness than in the case of the above mentioned treating solution composed of the Ni salt only.

As a result of making further investigations, the present inventors have discovered that when an Al salt is further added to the above-mentioned treating solution with additive, a film having more excellent anticorrosiveness and workability can be obtained.

For example, when a treating solution prepared by adding an Al salt to the above-mentioned treating solution with additive (which contains one or more salts of Cr, Mn and Zn in addition to Ni) is subjected to the thermodecomposition in a temperature range of from 200 to 750.degree.C. in a furnace of a nonoxidative gas atmosphere and a H.sub.2 content is 2 to 20%, there can be formed on the surface of a steel plate a very thin film. This film has proven to be all the more improved in anticorrosiveness and workability, as illustrated in the following examples.

As will be understood from the foregoing, the present invention provides for a method of inexpensively producing a steel plate having a strong inactive film on the surface thereof, which surface is just like that of stainless steel. The thus-produced steel sheets are particularly suitable as a material for manufacturing cans therefrom because they are high in anticorrosiveness and excellent in paint adhesiveness.

The specific examples of the present invention shall be detailed below.

EXAMPLE 1

A cold-rolled steel plate of a thickness of 0.26mm., which had been cold-worked by a well known method, that is, by using a continous strip rolling apparatus, but had not yet been annealed, was subjected to a well known pretreatment, for example, such surface adjustments as alkali-defatting and sulfuric acid-pickling, was dipped in an aqueous solution of nickel nitrate or nickel acetate so that the nickel salt might be deposited on the surface, and was immediately thereupon heated at a temperature of 600.degree.C. in an annealing gas atmosphere of a H.sub.2 content of 6% and the rest being N.sub.2, so as to effect a strain-removing annealing, and at the same time to form a nickel film, and was then rolled for refining at reduction rate of 1%. A comparison of the performances of this steel was made, as mentioned below, in respect to the antirusting properties, corrosion resistance below the coating film and paint adhesiveness. The samples Nos. 1, 2 and 3 in the below-mentioned performance test comparison table are steel plates having a film composed of metallic nickel, all prepared by the above-mentioned method. On the other hand, a referential sample No. 13 in the same table was prepared by subjecting a steel to a cold-rolling with the above-mentioned continuous strip rolling apparatus, thereupon to alkali-defatting and pickling and then to an annealing in the above-mentioned gas atmosphere (this referential steel shall be called a nontreated steel plate hereinafter for the convenience of the explanation).

In the rusting test, by an indoor exposure, the non-treated steel plate rusted within one month, while samples Nos. 1, 2 and 3 did not rust in 3 to 5 months in the same indoor exposure. Further, in the corrosion test, in which samples were dipped in a carbonic acid beverage after being coated with an epoxy resin series paint, samples Nos. 1, 2 and 3 showed results much superior to the nontreated sample in anticorrosiveness, but somewhat inferior in paint adhesiveness.

__________________________________________________________________________ Performance test comparison table __________________________________________________________________________ Performance tests Sample Treating solution composition Treating process 1) Antirusting 2) Corrosion 3) Paint ad- No. property resistance hesiveness the coating __________________________________________________________________________ film 1 Nickel nitrate (60 g./l.) .degree. Dip-coating 5.0 3 4 2 Nickel acetate (30 g./l.) .degree. Heating atmos- 4.0 3 4 phere H.sub.2 : 6% 3 Nickel nitrate (10 g./l.)+ N.sub.2 : rest 2.5 2 4 nickel acetate (10 g./l.) .degree. Heating temper- ature: 600.degree.C. 4 Nickel nitrate (20 g./l.)+ .degree. Roller-coating 7.0 4 5 chromium acetate (10 g./l.) .degree. Heating atmos- 5 Nickel acetate (20 g./l.)+ phere manganese nitrate (10 g./l.) H.sub.2 : 10% 6.0 3 4 N.sub. 2 : rest 6 Nickel nitrate (20 g./l.)+ .degree. Heating temper- 4.5 3 4 zinc nitrate (10 g./l.) ature: 600.degree.C. 7 Nickel acetate (20 g./l.)+ - chromium acetate (5 g./l.)+ 9.5 5 5 manganese nitrate (5 g./l.) 8 Nickel nitrate (20 g./l.)+ manganese nitrate (5 g./l.)+ 8.5 4 3 zinc nitrate (5 g./l.) 9 Nickel nitrate (20 g./l.)+ .degree. Dip-coating 11.5 5 5 aluminum nitrate (5 g./l.)+ chromium acetate (5 g./l.) .degree. Heating atmos- phere 10 Nickel nitrate (20 g./l.)+ H.sub.2 : 6% aluminum nitrate (5 g./l.)+ N.sub.2 : rest 10.0 5 5 zinc nitrate (5 g./l.) .degree. Heating temper- ature: 250.degree.C. 11 Nickel acetate (20 g./l.)+ aluminum nitrate (5 g./l.)+ 9.0 5 5 zinc nitrate (5 g./l.) 12 Nickel acetate (10 g./l.)+ aluminum nitrate (5 g./l.)+ chromium nitrate (5 g./l.)+ 12.5 5 5 manganese nitrate (5 g./l.) 13 (nontreated steel plate) 0.5 1 5 __________________________________________________________________________

EXAMPLE 2

The same cold-rolled steel plate as in Example 1 was defatted with an alkali and pickled with sulfuric acid, was then coated with a treating solution prepared by adding one or more of chromium acetate, manganese nitrate and zinc nitrate into an aqueous solution of nickel acetate or nickel nitrate by using a roller and was then immediately heated at a temperature of 600.degree.C. in a brightly annealing gas atmosphere of a H.sub.2 content of 10%, the rest being N.sub.2, to form a film. The samples Nos. 4 to 8 in the above-mentioned performance test comparison table were steel plates prepared by the above-mentioned process. It is apparent that the films obtained by applying a treating solution containing one or more salts of Cr, Mn and Zn in addition to a Ni salt shown properties which are performances more excellent than that of the film prepared in Example 1. That is, they showed no rusting in 4 to 10 months in the rusting test by indoor exposure and were very high in their corrosion resistance below the coating film when placed in a carbonic acid beverage, after being coated with a lacquer.

EXAMPLE 3

A cold-rolled steel plate of a thickness of 0.26mm. which had been cold-worked by a well known method, for example, by using a continuous strip rolling apparatus, thereafter alkali-defatted, sulfuric acid-pickled and then annealed in a reductive atmosphere was once more subjected to an alkali-defatting and sulfuric acid-pickling surface treatment, and was then dip-coated with a treating solution prepared by adding one or more of chromium acetate, manganese nitrate and zinc nitrate into an aqueous solution of a mixture of nickel nitrate and aluminum nitrate and was then immediately heated at a temperature of 250.degree.C. in a heated gas atmosphere of a H.sub.2 content of 6%, the rest being N.sub.2, to form a film.

The samples Nos. 9 to 12 in the above-mentioned performance test comparison table which correspond to this example, did not rust in 9 to 13 months in a rusting test by indoor exposure, were not corroded at all below the lacquer-coated film, when placed in a carbonic acid beverage, after being coated with a lacquer and were high also in paint adhesiveness.

In all the Examples 1, 2 and 3, the heating time was very short, amounting to several seconds, though there was a slight difference according to the temperature in the range of 200 to 750.degree.C.

The numerals in the above-mentioned performance test comparison table are defined as follows:

1. Antirusting property

The period until a rust recognizable by observation with the naked eye was generated on the surface of the steel plate by the indoor exposure test was represented by the number of months.

2. Corrosion resistance below the coating film

The steel plate was coated with an epoxy resin series lacquer, and then scratched, so that the scratch mark had a width of 0.1mm. and was dipped for one month in a cola series carbonic acid beverage, kept at such fixed temperature as 38.degree.C. in the example, and then the degree of corrosion below the coating film in the scratched part was observed. The numerals in the table were defined, as follows, by dividing the evaluations into five grades:

5--When no coating film peeling was recognized at all.

4--When a coating film peeling of about 0.1 to 0.2mm. was recognized.

3--When a coating film peeling of about 0.2 to 0.4mm. was recognized.

2--When a coating film peeling of about 0.4 to 0.8mm. was recognized.

1--When a coating film peeling of about 0.8 to 2.0mm. was recognized.

3. Paint adhesiveness

A lacquer was applied and was then bonded with a binder; then the tensile strength was measured and was evaluated by making the maximum value 5 and the minimum value 1 as the scale of lacquer adhesiveness.

Claims

What is claimed is:

1. An iron or steel product having, on the surface thereof, a film of a thickness of less than 1.mu. consisting essentially of nickel and at least one oxide selected from the group consisting of oxides of Cr, Mn and Zn with a coating of an organic resin paint on the surface of said film.

2. An iron or steel product according to claim 1 wherein the organic resin paint is an epoxy resin paint.