JPH02118099A - Electrolytic chromating bath for metal plated steel sheet - Google Patents

Abstract

PURPOSE:To produce a plated steel sheet having superior resistance to corrosion by neutral salt spray test and superior coatability by adjusting a soln. contg. specified amts. of CrO3, SiO2, H2ZrF6, H2SO4 and F<-> to a specified pH and by using the resulting electrolytic chromating bath for a plated steel sheet. CONSTITUTION:A soln. contg. 10-60g/l CrO3, 1-20g/l SiO2, 0.5-10g/l H2ZrF6, 0.1-2g/l H2SO4 and 0.1-3g/l F<-> is adjusted to pH2.0-3.2 to prepare an electrolytic chromating bath for a metal plated steel sheet. By using this chromating bath, a plated steel sheet having superior resistance to corrosion by neutral salt spray test and superior coatability can be produced.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a method for applying white color with excellent corrosion resistance and paintability to metal-plated steel sheets such as zinc or tin alloy-plated steel sheets, aluminum or aluminum-based alloy-plated steel sheets, etc. This invention relates to an electrolytic chromate (treatment) bath for metal-plated steel sheets that forms an electrolytic chromate film with good appearance.

(Prior art and problems to be solved by the invention) The electrolytic chromate film maintains adhesion with paint and improves corrosion resistance.
It is applied to many types of metal-plated steel sheets as a protective coating. For example, Japanese Patent Publication No. 47-44417 states “
Chromic anhydride 15-130/l, sulfur 1/l 0.05-0
, a method for producing galvanized steel sheets with excellent corrosion resistance and secondary adhesion after painting by subjecting galvanized steel sheets to cathodic ``Ki-dissolving egg treatment'' in 1 volume solution containing 5 g/Q. However, in today's harsh environments, its corrosion resistance is insufficient and it exhibits a pale yellow appearance. The electrolytic chromate-treated steel sheet exhibiting such a pale yellow color has a problem in its appearance, which significantly impairs its commercial value.

In general, cathode electrolytic coatings compensate for corrosion resistance by increasing the amount of film formed (mainly the amount of chromium deposited). but,
Chromate coatings have the drawback of becoming colored when the amount of coating increases, impairing the appearance of the treated material and further reducing coating performance. .

It was difficult to form a film with excellent corrosion resistance and coating performance. Furthermore, these cathodic electrolytic methods have a problem in that when metal-plated steel sheets are continuously treated, 7 n2 + ions accumulate in the treatment solution, significantly reducing corrosion resistance.

Furthermore, a method for improving corrosion resistance, coating performance, and appearance is disclosed in Japanese Patent Application Laid-open No. 62-263997, entitled ``A method for modifying a chromate film in which a metal-plated steel sheet is cathodic electrolytically treated in a chromate bath, followed by anodic electrolytically treated.'' . In this method, a metal-plated steel sheet with a chromate film that satisfies the desired properties can be obtained in the initial chromate bath, but during long-term use, the anodic electrolytic treatment dissolves the plated metal according to Faraday's law and the bath There was a problem that a chromate film with stable properties could not be obtained due to changes in composition.Also, methods have been proposed in which CO gold metal cations are added to a chromate bath and applied or immersed. No method has been developed for stably incorporating metal ions into a chromate film to improve the electrical properties of the chromate film.

(Means for Solving the Problems) The present invention improves the above-mentioned drawbacks of the conventional electrolytic chromate treatment method and improves the uniformity of the chromate film.

An electrolytic chromate that can form an industrially stable chromate film with excellent corrosion resistance, white appearance, and paint adhesion on the surface of metal-plated steel sheets such as zinc or zinc-based alloy-plated steel sheets, aluminum or aluminum-based alloy-plated steel sheets. The purpose is to provide a (processing) bath. The gist is Cry: 10-60 g/Q I5102
: l~20g/Q+H2ZrFs
: 0.5~10g *Hz S 04 : O
-1~2g/12:F-ion: 0.1~3 gIQ
This is an electrolytic chromate bath for metal-plated steel sheets having a composition consisting of the following and having a pH adjusted to 2.0 to 3.2.

(effect) The present invention will be explained in detail below.

The CrO3 concentration in Rakuchu is as shown in Figure 1.

10-60g IQ is required. CrO, a degree is 10g
If it is less than /Q, the corrosion resistance of the formed film is insufficient,
Further, at a concentration exceeding 60 g/Q, the effect of the film becomes saturated and the appearance tends to take on a pale yellow tinge. 5in2 is supplied from colloidal silica as a component that improves the corrosion resistance and paint adhesion of the film, and as shown in Figure 2, its effect is small for the purpose of the present invention at Ig. The conductivity of the bath decreases as its efficacy reaches saturation.

There is a problem of increasing the amount of electricity required for film formation, therefore, in the present invention, the SiO, fi degree is 1 to 2 Q g
Limited to IQ. Ht Z r F is added to impart corrosion resistance and appearance whiteness to the film.As shown in Figure 3, the effect of addition is small at concentrations below 0.5 g/Q, and at concentrations exceeding 10 g/Q. In this case, the appearance whiteness is reduced. As shown in Figure 4, H2SO4 is added to improve the corrosion resistance of the coating, control P)t, suppress the generation of precipitates, and maintain bath stability. If it is less than 0.1g/Q, the effect is small;
In addition, if the concentration exceeds 2 g/Q, the plated metal of the steel plate to be electrolytically treated will dissolve and the corrosion resistance of the metal plated steel plate itself will deteriorate.

As shown in Figure 5, F- ions are added to impart uniformity and corrosion resistance to the film, and their effects are small at concentrations below 0.1 g/Q, and their effects are diminished at concentrations above 3 g/Q. Along with saturation.

The outer v1 whiteness of the film is reduced and it becomes pale yellow. For this reason, in the present invention, the 5 degree of F+ ion is set to 0.1~
It was limited to 3.0g/Q. Further, as shown in FIG. 6, the bath circle 1 has a corrosion resistance of 2.0 to 3.2 based on the external whiteness. If the pH is less than 2.0, the external whiteness of the film will decrease and it will become pale yellow, resulting in poor corrosion resistance and paintability, and
There is a tendency to increase the dissolution rate of plated metals such as L and Pb and reduce the corrosion resistance of metal plated steel sheets. Also 3,
If Pl+ exceeds 2, the stability of the bath decreases significantly and the corrosion resistance of the coating tends to deteriorate. In the figure, the whiteness (W') is a value obtained by measuring the stimulation values X, Y, and Z according to JIs K-7105 and using the following formula.

W' = 4 (0,847Z) -: 3V In an electrolytic chromate bath having the bath composition and pH as described above, cathodic electrolysis treatment is performed using a metal plated steel plate as a cathode.8 Note that the chromate treatment bath of the present invention The metal surface-treated steel sheet treated by this method is applied to the following steel sheets manufactured by electroplating, hot-dip plating, vacuum evaporation, etc. For example, (1) Zinc and zinc alloy plated steel sheet is a plated steel sheet containing Zn and 0.5% or less of A12 in Zn.

These include plated steel sheets containing a small amount of unavoidable impurities such as sb, Pb, and Cd, and AQ (3 to 60%) to Zn.
alloy-plated steel sheet containing 2% or less of one or more or two or more of Si, Mg, and Mitsushi metal;
Zn with one or more of Ni and Co (5-25%)
Alloy plated steel plate containing 10% or more of one or more of SiC, Cr, Z
Alloy-plated steel sheet containing F tr (8-90%) in n (2) What is aluminum and aluminum alloy-plated steel sheet?

Aluminized Ti plate consisting of AM and inevitable impurities, A
Q is an aluminum alloy plated steel sheet consisting of (1~15%) Sl and unavoidable impurities, AQ is (1~15% Sl and 3%
Alloy-plated steel sheets containing the following Mg (■ Buttons and lead alloy-plated steel sheets are PB-plated steel sheets consisting of PB and inevitable impurities; Gold-plated steel plate, Pb with (1-30%) Sn and 5% or less Sb, Zn, Cd
(1) Examples include alloy plated steel sheets containing one or more of two or more unavoidable impurities.

In addition, C generated by the reduction reaction during the cathodic electrolytic treatment of the present invention
It may be added to the treatment solution in advance as a redox reaction product between r'' ion and Or1' hydroxide, reconstituted or refined product, or Cr'' and an organic compound such as alcohol, starch, or tannic acid. Yet another '+? A small amount of these plated gold L]( will be eluted into the tFil chromate bath, but these plated metal ions may be added in advance in the form of oxides, hydroxides, sulfates, carbonates, etc.) .

Next, examples of the present invention will be described.

Various plated steel plates were subjected to electrolytic chromate treatment using the treatment bath of the present invention. The treatment conditions at that time, the performance of the obtained chromate-treated steel sheets, and 11 examples of their ratios are shown in the first section.
Shown in the table.

In Example 1 and Comparative Example 1, the materials to be treated were electrogalvanized steel sheets, and the treated materials of the present invention showed excellent performance in all the investigated items such as appearance salt spray corrosion resistance, paintability, and corrosion resistance after painting. . In Example 2 and Comparative Example 2, the material to be treated is a hot-dip galvanized steel sheet, in Example 3 and Comparative Example 3, the material to be treated is an alloyed hot-dip galvanized steel sheet, and in Example 4 and Comparative Example 4, the material to be treated is The material to be treated in Example 5 and its comparative example 5 was a +8 fusion aluminized steel plate, and the material to be treated was a fusion turn-plated steel plate.

In Example 6 and Comparative Example 6, electrolytic lead-plated steel plates were subjected to chromate treatment, and those subjected to chromate treatment using the treatment bath of the present invention obtained better performance than the comparative example.

The evaluation test method and evaluation criteria were based on the following methods.

(m) Appearance The plated surface after chromate treatment was evaluated by naked eye ILN according to the following criteria.

O No yellow coloration or uneven treatment O Slight yellow coloration or uneven treatment Tomorrow Δ Yellow coloration or uneven treatment is clear × Yellow coloration or uneven treatment is noticeable (2) Corrosion resistance According to the salt spray test method based on JIS-Z-2371.

The state of rust (both white rust and black spots due to partial dissolution of the plating layer) after a specific time was observed and evaluated using the following evaluation criteria.

@・・・・Rust less than 1% O・・・・ More than 1% to less than 5% △・・・・・・ More than 5% to less than 15%×・・・・・・ More than 15% (3 ) Coating Performance A commercially available melamine alkyd white paint was coated to a thickness of 25μ and its performance was evaluated.

? ! ! ! After cutting 100 base grids at 1 m intervals on the film, extrude it 7 m using an Erichsen tester, perform a peel test with cellophane tape, and visually judge the degree of peeling of the paint film.
◎Evaluated in 4 stages: 101Δ, X.

O is the best.

(3)-2 Dupont impact test Using a Dupont impact tester, one weight of 500g was
Falling from a height of CI11 and landing on the paint surface? #A blow was applied and the degree of peeling of the coating film was judged with the naked eye: ◎101Δ.

Evaluation on a four-point scale of X, with O being the best.

(3)-3 After coating, scratches reaching the plating surface were made, and evaluation was made by observing the blistering width of the coating film after 240 hours of salt spray test.

◎・・・Average blister width from scratch part 2IIW
11 or less O...Average blistering from scratch area I4] 2m
Super~4m or less Δ...Average blister width from scratch area 4III
More than I~7m or less×・・・Average blister width from scratch area 7 [I1
More than 1 (Effects of the invention) Electrolytic galvanized steel sheets, hot-dip galvanized steel sheets 1 Alloyed hot-dip galvanized steel sheets, hot-dip aluminized steel sheets, hot-dip turn-plated steel sheets, electrolytic lead treated with the electrolytic chromate bath for metal-plated steel sheets of the present invention All of the plated steel plates showed better results than conventional ones in terms of appearance, salt spray corrosion resistance, paintability, and corrosion resistance after painting.

[Brief explanation of the drawing]

FIGS. 1 to 5 are diagrams showing the relationship between the concentration of each additive agent, the corrosion resistance of the chromate-1 coating (salt spray type 1jI), and the whiteness of the surface in a chromate treatment bath. Figure 1 shows Cry,
Concentration and whitening, Figure 2 is SiO, 4 degrees, Figure 3 is HZ
Z r Fb 6 degrees, Figure 4 is H, So, Figure 5 is F
-Ion concentration, Figure 6 shows the influence of I〕1 (SST ()2H) Mill rust photogenicity (Z) Whiteness (w) SST (72H) Mill rust photogenicity (Z) White ffi (w') SST (72H) White rust completion rate (Z)

Claims (1)

[Claims] CrO_3: 10-60g/l, SiO_2: 1-20
g/l, H_2ZrF_6:0.5-10g/l, H_
2SO_4: 0.1-2g/l, F^-ion: 0.1
A solution of components consisting of ~3 g/l with a pH of 2.0-3.
An electrolytic chromate bath for metal-plated steel sheets, characterized in that the bath is adjusted to 2.