JPS6293383A - Surface treated steel material having superior corrosion resistance and its manufacture - Google Patents

Abstract

PURPOSE:To provide superior corrosion preventing properties to a steel material by forming a porous Fe-Zn alloy film on the surface of the steel material and further forming an aqueous chromate film on the porous films so as to fill the pores in the porous film and to coat uniformly the surface of the porous film. CONSTITUTION:A blasting material made of an aggregate of separate particles each obtd. by depositing Zn or a Zn alloy around an Fe or Fe alloy nucleus with an Fe-Zn alloy layer in-between is projected on the surface of a steel material to form a porous Fe-Zn alloy film. An aqueous mixture contg. 0.1-5g/l chromic acid compound and 0.01-5g/l sulfuric acid is brought into contact with the porous film and the contact is kept for 0.5-8sec to carry out chromating. By this chromating, an aqueous chromate film is formed so as to fill the pores in the porous film and to coat uniformly the surface of the porous film.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention relates to a surface-treated steel material that has excellent long-term corrosion resistance and a method for producing the same.

<Prior Art> The most common surface treatment for steel materials for the purpose of corrosion prevention is plating with zinc or zinc-based alloy. Hot-dip plating and electroplating are common methods for plating this type of metal (or alloy), but recently, an epoch-making method that can be called a dry method has been developed and proposed. (Special Publication No. 59-9i312). This was done by none other than one of the non-applicants, and the method was as follows. In other words, a blast material (hereinafter referred to as iron-zinc) is made up of an aggregate of independent particles with iron or iron alloy as a core and zinc or zinc alloy coated around the core through an iron-zinc alloy layer. (referred to as blasting material)
Mechanical plating is a method of projecting onto the surface of iron or iron alloy (hereinafter referred to as mechanical plating).
This method has absolute advantages such as low equipment costs, low energy consumption, and low environmental pollution.

By the way, in general, zinc-based plating is poor, and if it is used alone, it is difficult to avoid rusting at a relatively early stage, and to prevent this, it is necessary to combine it with some kind of thin film treatment.
Film produced by the above MP method (iron-zinc alloy film; hereinafter referred to as MP
(as a film) is no exception.

Means and Effects for Solving the Problems The present inventors investigated the compatibility of various thin film treatments with the MP film in detail in order to find a thin film treatment that achieves particularly excellent corrosion resistance in combination with the above-mentioned MPgl film. As a result of investigation and research, we have discovered that by subjecting the MP film to chromate treatment using an aqueous treatment solution (aqueous chromate treatment), extremely good corrosion resistance performance can be ensured. Water-based chromate treatment has traditionally been carried out on galvanized films (bath dip plating, electroplating), but the corrosion resistance improvement effect of the same treatment on MP films is greater than that of the above-mentioned ordinary galvanized films. This is completely different from what was seen in 2015, and it far exceeds common sense predictions. This is due to the characteristics of the MP film itself; in other words, the film is porous, unlike general plating films.
When water-based chromate treatment is applied to this, the chromate film penetrates not only the surface of the film but also the small pores and crevices inside the porous structure, which is effective in preventing corrosion, as will be explained in detail later. It acts on

Also, although there are several types of water-based chromate treatment as described below, for MP coating,
At the same time, the inventors have also discovered through experiments that a method of contacting a strongly reactive treatment liquid containing a relatively small amount of a strong acid for a short period of time is particularly effective.

That is, the gist of the present invention is as follows.

1) Excellent corrosion resistance characterized by providing a porous iron-zinc alloy film on the surface of the steel material and further applying an aqueous chromate treatment film that penetrates into the voids inside the porous material and uniformly covers the film surface. surface treated steel.

2) Cast the iron-zinc blasting material on the surface of the steel material.

1. A method for producing surface-treated steel materials with excellent corrosion resistance, which comprises: forming a porous iron-zinc alloy film by irradiating the film, and then subjecting the film to an aqueous chromate treatment.

8) As water-based chromate treatment, chromic acid compound 0.1
50 g/l of sulfuric acid and 0.01 to 5 g/l of sulfuric acid for 0.5 to 8 seconds.

Hereinafter, the present invention will be explained specifically and in detail.

・Section 1) Invention First of all, this invention is applicable to hot-rolled steel plates, shaped steel, bolts/nuts, springs, building materials, etc. (steel materials are:
(This is a general term for these.)

In the present invention, a porous iron-zinc alloy film is first provided on the surface of such a steel material, but this porous film can be considered to refer to substantially the entire MP film.

The MP coating is produced by a so-called blasting process, and the coating structure consists of minute pieces of iron-zinc alloy (■) layered in many layers on the surface of the steel material (2), as shown in the schematic diagram in Figure 1. It has a porous overall shape.

Here, the amount of deposited iron-zinc alloy film is preferably 8 mm or more, preferably about 6 mm. 8g/
If it is less than rn', no substantial effect on corrosion resistance can be obtained;
Even if the amount is less than 6, the effect of the combination with the water-based chromate treatment film described below will be small. Figure 2 shows experimental data showing this tendency (amount of water-based chromate treatment film: 100 m depth, results of salt spray test (SST, J Engineering 5-Z-2871)). The time until red rust appears shows a rapid decreasing trend. On the other hand, even if the amount of this adhesion exceeds 80 g/r, there is no problem in terms of corrosion resistance, but the treatment will take a long time, so considering economic efficiency, it is preferable to set it to 80 g/r or less. .

Next, in the present invention, a water-based chromate treatment film is applied to such a porous iron-zinc alloy film, but this chromate treatment film is Of course, it is necessary to cover the surface uniformly, but it is also essential that it penetrates into and fills the microscopic voids inside the porous structure. That is, by being impregnated with the iron-zinc alloy film in this way, the corrosion resistance is dramatically improved.

The reason for this is considered as follows.

The films obtained by general bath dip plating and electroplating are dense, and those that are subjected to chromate treatment are
As shown in Figure 8, both the lower layer plating film and the two-layer chromate film have a thin plate-like shape. The film functions to elute preferentially and protect the underlying plating film. However, if the flaw is wide or deep, the chromate film does not function effectively and cannot be repaired sufficiently, which becomes the starting point for deterioration of corrosion resistance.

On the other hand, when a chromate film is formed based on the above-mentioned porous film and penetrates into the micropores inside the film, even if a large flaw that reaches the substrate occurs, the chromate film inside the porous film will be significantly damaged. It is thought that it will not be destroyed and will function effectively, repairing the flaw in a sufficient manner and maintaining good corrosion resistance.

As the chromate treatment film, a water-based treatment film will be adopted. There are eight types of water-based treatment films: baking type, strongly reactive type, and weakly reactive type. A description of each of these processing methods is given in the second invention section.

Non-aqueous chromate treatment films are also known, but in the present invention, we excluded these and limited to aqueous chromate treatment films. The reason for this is that non-aqueous chromate treatment uses a weak acid such as an organic acid as a catalyst acid, which will be explained later, so chromate reactivity is weak and corrosion resistance is inferior to aqueous chromate treatment. Because it will be.

Various chromate coatings, including the non-aqueous ones mentioned above, can be applied to porous iron.
The corrosion resistance when combined with a zinc alloy film is as shown in Table 1.

1st surface inspection ◎: No red rust, ○: Slight occurrence, ×: All over red rust Chromate elution property: Based on the Galvanized Iron Sheet Association chromium elution test method for chromate film.Evaluation: MP film 12 and chromate film 110m. In the table, in all cases, the aqueous chromate-treated films achieved better performance in SST and chromate bathing properties than the non-aqueous chromate-treated films. However, in the case of the special strong reaction type, which will be described in detail later, extremely excellent corrosion resistance is ensured.

In the present invention, the amount of the aqueous chromate treatment film is not particularly limited, but in practice it is 80" g/n1
It is preferable to set the length to 1 or more, and more preferably about 60 to 800 m. If it is less than 33m, the contribution to corrosion resistance is low, and even if it is less than 60m, sufficient effects cannot be obtained. Figure 4 shows experimental data showing this tendency (MP film adhesion amount = 10 g/m', SST results). On the other hand, if the amount of adhesion exceeds 800 mg/m', the rate of chromium elution increases, which is particularly disadvantageous in terms of pollution.

・Inventions in Items 2) and 3) First, the invention in Item 2) is based on the above-mentioned MP method.
This is a method for producing steel products according to the invention in item 1), in which a zinc alloy film is formed and then subjected to a water-based chromate treatment. As a condition for implementing the MP method to obtain an iron-zinc alloy film, the size of each individual particle of the iron-zinc plastic material used varies depending on the properties of the surface to be projected, but in general, it is appropriate that it is about 16 mesh or less. It is. Further, as the material of the plastic material, approximately 50 to 70 wt % iron and 30 to 501 wt % zinc are suitable, depending on the performance of the produced film and the conditions of the blast treatment.

In carrying out this MP method, it is preferable to clean the surface of the steel material to be treated in advance by descaling and removing dirt by pickling or mechanical treatment.

Next, regarding the aqueous chromate treatment performed on the porous iron-zinc alloy film obtained in this way, as mentioned earlier, there are generally three types of aqueous chromate treatment (baking, type, strong reaction type, and weak reaction type). In the present invention, any of these eight types can be used. A brief explanation of each is as follows.

-Baking type chromate: The treatment liquid is a mixture of a water-bathable chromic acid compound, a reducing agent, and water, which is applied to the surface of the object to be treated (by roll coater, dipping, etc.) and then baked and heated. By heating, the water-bathable chromic acid compound is reduced and a chromate film is produced.

O strongly reactive chromate: A treatment solution consisting of a water bathable chromic acid compound, strong acid, and water is used, and this is applied to the surface to be treated (by dipping, spraying, etc.) to dissolve the zinc on the surface and remove chromium. A reduction reaction occurs to form a chromate film. After treatment, washing with water is performed. Generally employed treatment liquids and treatment conditions (excess, contact time) are shown in the table below.

Table 2: Weakly reactive chromate: The treatment liquid is a mixture of a water-soluble chromic acid compound, a weak acid such as an organic acid, and water.

This is applied to the target surface (spray, dip, roll coater), and then the water is dried and evaporated to form a chromate film.

In addition to the above, there are non-aqueous chromate treatments, but for reference purposes, these can be briefly summarized as follows: ・Non-aqueous (solvent type) chromate dichromate compound and organic solvent C Mainly halogenated A solution containing a hydrocarbon solvent) and alcohol-1v as a solubilizer as the main components, and also a stabilizer and a reaction accelerator is used as a treatment liquid, and a film is formed by drying after immersion treatment.

By the way, in the case of non-invention, basically any water-based chromate treatment can be used as described above, but each of the eight conventionally known types has some disadvantages. It cannot be denied that there is. Namely, Q-baking type chromate: Requires a baking furnace as equipment, and baking energy is also required for running.

・Strongly reactive chromate film: Because the porous iron-zinc alloy film has a large surface area, it tends to undergo a lot of dissolution during the treatment process, and the formation of the chromate film tends to be somewhat unstable. Another disadvantage is that a large amount of wastewater containing chromium is produced during the subsequent washing process.

・Weakly reactive chromate: Since the iron-zinc alloy film is more inert than ordinary zinc alone, we cannot expect much reaction during the treatment process, and the formed chromate film is likely to re-elute, resulting in poor corrosion resistance. I feel like it's a little lacking.

Taking the above points into consideration, it can be said that it is most preferable to employ a chromate treatment under the following conditions, which is different from conventionally known types. That is, chromic acid compound 0.1-50 g/
This is a method in which a water mixture containing 0.01 to 5 l of sulfuric acid is used as a treatment liquid, and this is brought into contact for 0.5 to 8 seconds. This is done by taking into consideration the compatibility with the porous iron-zinc film.
This is a condition that was discovered through experimentation by a non-inventor. this is,
The type of treatment liquid basically belongs to the strongly reactive type, but compared to general ones, the amount of strong acid is much smaller, and the contact time is also extremely short. It is characteristic. If this method is adopted, there is no need for baking treatment, so there is no need for equipment or energy for that, and the elution of the iron-zinc alloy film during the treatment process is suppressed compared to the general strong acid type. Not only is the formation of a chromate film stable, but the short-time treatment produces fewer reaction products, so subsequent washing with water can be omitted. What is noteworthy is that the chromate film is shown as a special strong reaction type in Table 1 above. As mentioned above, in combination with a porous iron-zinc alloy film, extremely excellent corrosion resistance is achieved, and this advantage is extremely large.

Here, to explain each treatment condition in this special strong reaction type, treatment liquid: First, the amount of chromic acid compound is 0.1 to 50 g/i
However, if it is less than 0.1 gz'2, the amount of chromium deposited is small and the corrosion resistance is also low. If it exceeds 50 g/l, the chromate reaction will be accelerated and the MP film will dissolve. The most preferable range for the amount of chromium oxide is 1.
It is 0 to 10 g//l.

Next, as for the amount of sulfuric acid, if it is less than 0.01%, the chromate film will be easily eluted, whereas if it exceeds 5 g/l, the chromate reaction will be accelerated and the MP film will dissolve, which is not preferable. Most preferably, the amount of sulfuric acid is in the range of 0.1 to 1 g/2.

In addition to these main components, the treatment solution may also contain nitric acid, hydrofluoric acid, etc. in an amount that does not exceed the amount of sulfuric acid mentioned above. It has an effect on generation.

Contact time: 0.5 to 8 seconds, but if it is less than 0.5 seconds,
The chromate solution does not penetrate sufficiently into the MP film layer. In addition, if the time exceeds 8 seconds, porous iron may become
The reaction and dissolution of the zinc alloy film progresses, accelerating the deterioration of the treatment solution, which is disadvantageous.

In addition, methods of contact include dipping, spraying,
Needless to say, it is also possible to use a roll coater or the like.

In addition, the amount of chromate film produced (adhered amount) is 80 mg/m' or more, as in the case of the general water-based treatment described above.
More preferably, it is in the range of 60 to 800111 g/m2.

<Example 1> A hot-rolled steel plate is used as the material, and after descaling it by shot blasting, the above-mentioned iron-zinc blasting material is projected onto the surface under the following conditions to form an MP coating (main coating). C non-inventive example) in which four types of aqueous chromate treatment, baking type, strong reaction type (general type), weak reaction type, and special strong reaction type, were performed under the following conditions.
. For comparison, in the above example, the main coating layer is a hot-dip galvanized film or an electroplated film, and the water-based chromate treatment is also changed to the following non-aqueous solvent-based chromate (the above is a comparative example). ), were also created.

・Blast (MP) conditions 1) Projection method Invera type projection method (Tangera type)
2) Projection speed 64. m/S 3) Projection amount 54 kg/m1n 4) Control the adhesion amount by adjusting the projection time 5) Blast material Iron 70wt% Zinc 80wt% Chromate treatment conditions Baking type: Kansai Paint Acomet-C treatment liquid used Weak Reaction type - Cr○J (1g7z) Decaacetic acid (0,5
g/l) treatment liquid ■Fast reaction type...Chromic anhydride 250g/nitric acid 100g
■Special strong reaction 5- Cr03 (5fE/, l) +HaSOa (
0.5g/l, ) treatment solution for 2 seconds → Natural drying Solvent type: 100 parts of tert-butanol, 20 parts of water
1 part, 0.7 part of chromic acid, 0.02 part of hydrogen fluoride, and a treatment liquid containing methylene chloride.
1 was conducted, and the occurrence of white rust or red rust was investigated over time. The results are shown in Table 3.

In the table above, in examples 1 to 4 of hot-dip plating or electroplating films, white rust occurs within just 120 hours regardless of whether it is aqueous or non-aqueous chromate treatment, and this changes to red rust in a short period of time. did.

In addition, Example 5 is an MP film, but the chromate treatment is solvent-based.
Also, relatively early red rust is observed.

On the other hand, in the non-inventive examples that combine water-based chromate with MP coating, even the one using chromate with less than 60% reaction, which has the worst condition, takes 500 hours to develop red rust, and the other ones take more than 1000 hours. Intervals 9 to 13 using special strong reaction type chromate are 2000 Hr or more, especially 1I
Ill-112~13 recorded over 5000Hr,
This clearly shows the effectiveness of the present invention.

<Example 2> Water-based chromate treatment under the following conditions (special strong reaction type)
Surface-treated steel sheets (invention examples, comparative examples (bath melting or electrogalvanized film adopted) were prepared in the same manner as in Example 1 except that
), and each steel plate was subjected to a salt spray test to evaluate its corrosion resistance. The results were as shown in Table 4.

・Chromate treatment conditions: 0r031~5og7' sulfuric acid 0.1~5 g/l Immersed for 8 seconds in various treatment solutions → Dry From the results in the table above, combine special strong reaction type chromate treatment with MP film. It is clear that extremely good corrosion resistance performance can be ensured by this method. Further, in comparison with the results of the comparative example, it is recognized that the corrosion resistance performance far exceeds the mere sum of the corrosion resistance of the MP coating and the corrosion resistance of the special strong reaction type chromate treatment coating.

<Effects of the Invention> As is clear from the above description, the present invention combines a porous iron-zinc alloy film by the MP method and an aqueous chromate treatment, and has a synergistic effect that improves the conventional zinc plating film and chromate film. The corrosion resistance effect in combination with
It has the effect of dramatically extending the service life of any steel used in a corrosive environment.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional schematic diagram showing the internal laminated structure of the porous iron-zinc alloy film produced by the MP method, and Fig. 2 shows the adhesion amount and S of the porous iron-zinc alloy film in the surface-treated steel material according to the present invention.
A diagram showing the relationship between ST and time until red rust occurs. Figure 8 is a schematic cross-sectional diagram of a general galvanized film + chromate film.
FIG. 4 is a diagram showing the relationship between the amount of chromate film deposited and the time until red rust occurs in SST in the surface-treated steel material of the present invention.

Claims (3)

[Claims] (1) Corrosion resistance characterized by having a porous iron-zinc alloy film on the surface of the base steel material, and further having an aqueous chromate treatment film impregnated inside the porous film and uniformly covering the surface. Excellent surface treatment steel material. (2) A blasting material consisting of an aggregate of independent particles made of iron or iron alloy as a core and zinc or zinc alloy coated around this core through an iron-zinc alloy layer is projected onto the steel surface. 1. A method for producing a surface-treated steel sheet with excellent corrosion resistance, which comprises forming a porous iron-zinc alloy film and then subjecting the film to an aqueous chromate treatment. (3) Water-based chromate treatment uses chromic acid compounds from 0.1 to
50 g/l of sulfuric acid and 0.01 to 5 g/l of sulfuric acid for 0.5 to 8 seconds. manufacturing method.