JPH0680200B2 - Chromate treatment method for galvanized steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a method for chromate-treating a Zn-plated steel material, and in particular, for unpainted corrosion resistance, paint adhesion, and adhesion of vinyl chloride and the like with an adhesive. Excellent adhesion
The present invention relates to a chromate treatment method for Zn-based plated steel material.

<Prior art and its problems> As a chromate treatment method for a Zn-based plated steel material, a reactive chromate treatment method is generally performed. The white rust generation time by the salt spray test (JIS-Z2371), which evaluates the corrosion resistance of the chromate film, is about 24 to 100 hours, but this is greatly influenced by the reactivity between the base metal and the chromate solution, and the corrosion resistance is improved. For this reason, if the amount of deposited Cr is increased, Cr ⁶⁺ increases and the film becomes yellow, which is not preferable in appearance. Moreover, since the film thickness increases, the paint adhesion and adhesiveness deteriorate.

On the other hand, there is a coating type or electrolytic chromate treatment method as a method for performing chromate treatment without being much affected by the underlying metal. The coating type has a corrosion resistance of 200 hours or more in a salt spray test, and there are methods such as adding colloidal silica as a film-forming agent and applying a chromate solution containing an organic resin with a roll coater. It is difficult to make the film uniform and the process unevenness is likely to occur. In addition, since it is a thick film, it lacks paint adhesion and processability.

On the other hand, the electrolytic chromate treatment method is a method to generate a chromate film electrically, and the thickness of the chromate film can be adjusted by adjusting the amount of electricity, and since it is not easily affected by the underlying metal, Zn-based plating Easy to apply to steel materials. Also, the paint adhesion is superior to the reactive type and the coating type. However, according to the known method (Japanese Patent Publication No. 47-44417), the chromate layer is difficult to grow even if the amount of electricity is increased, and therefore the corrosion resistance is poor.

<Purpose of the Invention> An object of the present invention is to provide a chromate treatment method for a Zn-based plated steel material, which solves the problems of the prior art and obtains a chromate film excellent in unpainted corrosion resistance, paint adhesion and adhesiveness with an adhesive. I will try.

<Structure of the Invention> In the chromate treatment, Cr ⁶⁺ + 3e ⁻ → Cr ³⁺ 2H ⁺ + 2e ⁻ → H ₂ Zn → Zn ²⁺ + 2e ⁻ occurs as the main reaction, and therefore Zn ²⁺ is eluted in the chromate bath. However, this becomes a hindrance factor to the film formation, resulting in uneven chromate treatment and adversely affecting quality (especially corrosion resistance). In addition to these, SO ₄ ²⁻ and Cl ⁻ may also act as interfering factors.

Therefore, it does not adversely affect the corrosion resistance of the chromate film,
In addition, as a result of examining the treatment conditions to obtain a good appearance without adversely affecting the paint adhesion and adhesion, the chromate film quality was limited by limiting the concentration of the following ions in the chromate treatment liquid to the following range. The present invention has been accomplished by finding that it can be improved.

Cr ⁶⁺ 2 ~ 80 g / l F ^- 0.05 ~ 5.0 g / l SiO ₂ 0.5 ~ 30 g / l Zn ²⁺ 0.001 ~ 10 g / l SO ₄ ^2- 0.001 ~ 5 g / l Cl ^- 0.001 ~ 10 g / l That is, the present invention, Zn-plated steel material as a cathode, a liquid containing the following components Cr ⁶⁺ 2 ~ 80 g / l F ^- 0.05 ~ 5.0 g / l SiO ₂ 0.5 ~ 30 g / l Zn ²⁺ 0.001 ~ 10 g / l SO ₄ ^2- 0.001 ~ 5 g / l Cl ^- 0.001 ~ 10 g / l, current density 1 ~ 50 A / dm ² , electricity 1 ~ 100 C / dm ²
Provided is a chromate treatment method for zinc-based plated steel material, which is characterized in that electrolytic treatment is carried out with.

The constitution of the present invention will be described in detail below.

As described above, the feature of the present invention is to limit the specific ion species and the like in the chromate treatment liquid to a specific concentration, and to subject the Zn-plated steel material as a cathode to electrolytic treatment in an appropriate current density and electric quantity range. is there.

This is due to the results of the chromate film performance test for various ion concentrations in the chromate treatment liquid shown below.

The experimental conditions and test conditions were the conditions shown in the examples.

(1) Cr ⁶⁺ Fig. 1 shows the relationship between the Cr ⁶⁺ concentration in the chromate bath and the amount of chromate film in terms of the amount of Cr deposited.

Cr ^6+, as shown in FIG. 1, the 80 g / l, greater than the etching force of Cr ⁶⁺ is strong, resulting in a decrease of the sudden Cr deposition amount,
If it is less than 2 g / l, the concentration is too dilute and the chromate film is not sufficiently formed.

As the Cr ⁶⁺ source, CrO ₃ , chromate, dichromate, etc. can be used, but the most common source is
It is better to use CrO ₃ .

(2) Regarding F ⁻ , FIG. 2 shows the relationship between the F ⁻ concentration in the chromate bath and the amount of chromate film.

As shown in FIG. 2, F ⁻ has a constant Cr deposition amount in the range of 0.05 to 5.0 g / l and exhibits a uniform appearance effect, but outside this range, a poor appearance occurs. This is because the F ^- ion is Na ₂ SiF ₆ , N
Even if added with a ₃ AlF _6, etc., if the F ⁻ ion amount is in this range, the same effect is exhibited.

The fluorine _{^{compound, AlF 6 3-, SiF 6 2-}} , BF 4 -, F - is the Na or K salts, and the like typically, F ^-
If the same amount is added as the concentration, they may be used alone or in plural.

(3) Regarding SiO ₂ , FIG. 3 shows the relationship between the amount of SiO ₂ in the chromate bath and the amount of chromate film.

When SiO ₂ was less than 0.5 g / l, a sharp decrease in the Cr deposit was observed, and when it was more than 30 g / l, the Cr deposit was increased and the liquid coagulated.

When SiO ₂ is added as colloidal silica, a thick chromate film can be obtained due to the adsorptivity of colloidal silica or the three-dimensional structure. Representative examples of colloidal silica include Snowtex O and C for anion type and Snowtex AK and BK for cation type (all manufactured by Nissan Chemical Industries). In particular, when a cationic type colloidal silica is used, the colloidal silica is promoted to be transported to the plating surface because the plating side is the cathode. Further, the CrO ₃ in aqueous solution to the colloidal silica surface
Adsorbs anions such as Cr ₂ O ₇ ²⁻ and CrO ₄ ²⁻ and is adsorbed on the cathode, forming a film with good corrosion resistance that contains a hexavalent chromium component that functions as a self-repairing function in the electrolytic chromate film. be able to.

(4) Regarding Zn ²⁺ , FIG. 4 shows the relationship between the amount of Zn ²⁺ in the chromate bath and the amount of chromate film.

FIG. 5 shows the relationship between the amount of Zn ²⁺ in the chromate bath and the white rust generation time of the formed chromate film in a salt spray test.

The following can be seen from FIGS. 4 and 5.

When Zn ²⁺ exceeds 10 g / l, a barrier layer of Zn (OH) ₂ is formed, so that supply of Cr ^{6+ to} the film formation is insufficient and it becomes difficult to form a chromate film. If Zn ²⁺ is less than 1 mg / l, the corrosion resistance is poor.

(5) Regarding SO ₄ ²⁻ , Fig. 6 shows that the chromate film is formed by changing the concentration of SO ₄ ²⁻ in the chromate bath, and the ratio of Cr ⁶⁺ , Cr ³⁺ to total chromium in this film is ESCA ( Electron Spectroscopy for Chemical
Analysis) shows the result measured.

FIG. 7 shows the relationship of the chromate film amount with respect to the SO ₄ ²⁻ concentration in the chromate bath.

Although SO ₄ ²⁻ also has a film-forming effect, as can be seen from the results of ESCA, SO ₄ ²⁻ has an effect of reducing Cr ⁺³ to metallic Cr, which not only darkens the appearance but also deteriorates corrosion resistance. SO ₄ ^2- is 0.001 ~
If it is within the range of 5 g / l, the film-forming effect will be exerted preferentially and the chromate film will be obtained with high quality.

(6) Regarding Cl ⁻ , FIG. 8 shows the relationship between the amount of chromate film and the concentration of Cl ⁻ in the chromate bath.

Cl ⁻ also has a film-forming effect similar to SO ₄ ²⁻ , but since it also has a function of incorporating a large amount of Cr ⁶⁺ into the film, if it is too large, it becomes yellowish and causes poor appearance. However, if Cl ^- is in the range of 0.001 to 10 g / l, the film-forming action is preferentially performed, resulting in high quality.

(7) FIG. 9 shows the change in corrosion resistance of the chromate film by the salt spray test when the current density was changed and the amount of electricity was changed.

The current density has different current efficiencies even with the same amount of electricity, and the composition ratios of Cr ⁶⁺ , Cr ³⁺ , and metallic Cr in the film are different.
The range of 1 to 50 A / dm ² is good because it affects the corrosion resistance.

(8) Fig. 10 shows the relationship between the amount of electricity for chromate treatment and the amount of chromate film.

From FIG. 10, the amount of electricity was set in the range of 1 to 100 C / dm ^{2 in} order to obtain the amount of adhesion from the thin chromate film to the thick chromate film level. This range is shown by hatching in FIG.

(9) In addition, since Cr ³⁺ in the chromate bath increases the electrolysis voltage as the concentration increases, it is better that the concentration is not too high, but it did not affect the quality up to about 10 g / l.

With regard to Fe ions and Ni ions, too, no adverse effect on quality was observed.

Further, the lower the pH of the bath, the more easily the film is formed.

The liquid flow rate does not affect the quality so much, but the higher the speed, the more the surface layer tends to dissolve.

In the above bath, the anode is an insoluble anode, for example, Pb-S
Treatment is carried out using a suitable electrode such as n (Sn 5%), preferably at a bath temperature of 30-60 ° C.

The zinc-based plated steel material to which the chromate treatment method of the present invention is applied is a Zn-plated steel sheet, a Zn-Ni-based alloy plated steel sheet, a Zn-Fe-based alloy plated steel sheet, a Zn-Co-based alloy plated steel sheet, a Zn-Mn-based alloy plated steel sheet. , Zn-Al-based alloy plated steel sheet, or any Zn-based plated steel sheet such as composite plated steel sheet in which silica, alumina, etc. are dispersedly contained in the Zn-based plated layer,
Further, a zinc-based plated steel material having any shape other than a steel plate may be used.

<Examples> The present invention will be specifically described below with reference to Examples.

(Examples and Comparative Examples) The treatment conditions of the test material were electrogalvanized steel sheet (Zn basis weight).
20g / m ² ), Cr ⁶⁺ is CrO ₃ , F ⁻ is Na ₂ SiF ₆ , SiO ₂ is colloidal silica, Z as a bath composition.
n ²⁺ is ZnCo ₃ , SO ₄ ²⁻ is H ₂ SO ₄ , Cl ⁻ is NaCl, and the current density is 1 to 50 A / dm ² , the quantity of electricity, and 1 to 100 C / dm ² are selected. A predetermined amount of deposited Cr was obtained under the above condition.

The following tests were conducted on the above test materials, and the results are shown in Table 1.

In addition, Zn-Ni plated steel sheet (plating amount 20 g / m ² , Ni content 12
%) Was subjected to chromate treatment under the same conditions as in the above bath and Table 1, and the results are shown in Table 2.

(1) The salt spray test (SST) was performed based on JIS-Z2371.

In the case of Zn plating, it was judged as: ◎ 72 hours or more white rust did not occur ○ 48 hours or more white rust did not occur △ 24 hours or more white rust did not occur × less than 24 hours white rust occurred.

In the case of Zn-Ni plating, ◎ White rust was not generated for 300 hours or more ○ White rust was not generated for 200 hours or more △ White rust was not generated for 100 hours or more × Less than 100 hours White rust was generated.

(2) Regarding paint adhesion, after applying Magicron # 1000 manufactured by Kansai Paint to 25 μm, the primary and secondary adhesion was
The amount of coating film peeling was measured by a DuPont impact test (weight 1 kg, punch diameter 3/8 inch, and 1/2 inch respectively dropped from a height of 50 cm) and cross-cut Erichsen (1 mm cross-cut 6 mm Erichsen extrusion).

Primary adhesion: Adhesion after coating Secondary adhesion: Adhesion after immersion in boiling water for 30 minutes In addition, the judgment is ◎ coating film peeling area 5% or less ○ coating film peeling area 10% or less △ coating film peeling area 20% or less × The coating peeling area was set to more than 20%.

<Effects of the Invention> According to the present invention, Cr ⁶⁺ , SiO ₂ , F ⁻ , Zn in the chromate bath are used.
^The following effects were obtained by limiting the concentration of ²⁺ , SO ₄ ²⁻ and Cl ⁻ within a predetermined range and limiting the conditions of cathodic electrolysis treatment.

The time until the occurrence of white rust in the salt spray test is, for example, 72 hours or more at the thick chromate level on Zn plating, and 300 hours or more in the case of Zn-Ni plating.

There is no visible unevenness in the chromate treatment.

The amount of film is stable.

The amount of deposited Cr and the amount of electricity have a correlation with little variation.

Therefore, the operation stability of the chromate treatment of the Zn-based plated steel material can be ensured.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the Cr ⁶⁺ concentration in the chromate bath and the Cr deposition amount. FIG. 2 is a graph showing the relationship between the F ⁻ concentration in the chromate bath and the Cr deposition amount. FIG. 3 is a graph showing the relationship between the SiO ₂ concentration in the chromate bath and the Cr deposition amount. FIG. 4 is a graph showing the relationship between the Zn ²⁺ concentration in the chromate bath and the Cr deposition amount. FIG. 5 is a graph showing the relationship between the Zn ²⁺ concentration in the chromate bath and the white rust generation time in the salt spray test. FIG. 6 is a graph showing the chromium composition ratio (ESCA) in the chromate layer due to the difference in SO ₄ ²⁻ concentration in the chromate bath. FIG. 7 is a graph showing the relationship between the concentration of SO ₄ ^2- in the chromate bath and the amount of deposited Cr. FIG. 8 is a graph showing the relationship between the Cl ⁻ concentration in the chromate bath and the Cr deposition amount. FIG. 9 is a graph showing the relationship between the amount of electricity due to the difference in current density and the white rust generation time in the salt spray test. FIG. 10 is a graph showing the relationship between the amount of electricity and the amount of Cr deposited.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Toshiro Ichida 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division

Claims (1)

[Claims] 1. A as a cathode the Zn-based plating steel, a liquid comprising the following components ^{Cr 6+ 2 ~80g / l F -} 0.05 ~ 5.0g / l SiO 2 0.5 ~30 g / l Zn 2+ 0.001~10 g / l SO ₄ ^2- 0.001 to 5 g / l Cl ^- 0.001 to 10 g / l, current density 1 to 50 A / dm ² , electricity 1 to 100 C / dm ²
A method for chromate treatment of galvanized steel material, characterized in that electrolytic treatment is carried out.