JP3307267B2 - Chromate treated steel sheet with excellent blackening resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a galvanized steel sheet or a zinc alloy-coated steel sheet (hereinafter, referred to as a "zinc-based steel sheet") and a chromate film formed on the surface thereof, and has excellent blackening resistance. It relates to a chromate treated steel sheet.

[0002]

2. Description of the Related Art When a steel sheet is galvanized, the corrosion resistance of the steel sheet is improved by the sacrificial corrosion prevention effect of zinc. That is, when a steel sheet is galvanized, zinc and iron come into contact with each other to form a battery, and zinc, which is a more noble metal, becomes an anode, and iron, which is a noble metal, becomes a cathode. Therefore, in the galvanized steel sheet, the corrosion resistance of the steel sheet is improved, unlike the case of iron corrosion due to anodic dissolution of the local battery when iron alone is used.

In such a galvanized steel sheet, when the zinc in contact with the iron disappears due to corrosion, the anticorrosion action by the galvanization ends. Therefore, in order to maintain the sacrificial corrosion protection effect of the galvanizing described above for a long period of time, it is necessary to suppress the corrosion of the galvanized layer. As a countermeasure, a galvanized steel sheet is subjected to a chromate treatment.

[0004] Chromate-treated electrogalvanized steel sheet is excellent in corrosion resistance and fingerprint resistance.
Widely used in various fields. Chromate treatment methods applied to electrogalvanized steel sheets are roughly classified into three types: coating type, reaction type, and electrolytic type.

[0005] The coating type chromate treatment involves coating a chromate treatment solution on the surface of a plated steel sheet with a roll coater or the like, and then baking and drying it without washing with water.

[0006] In the reactive chromate treatment, a galvanized steel sheet is brought into contact with a chromate treatment solution containing hexavalent Cr ions by dipping or spraying to dissolve a metal zinc plating layer, and at the same time, reduce and produce a trivalent chromium. Chromate composed of Cr ions and hexavalent Cr ions is deposited in a gel state, and thus a chromate film is formed on the galvanized layer of the galvanized steel sheet.

[0007] In the electrogalvanized steel sheet subjected to the above-mentioned chromate treatment, although the generation of white rust of zinc is remarkably prevented, black rust is generated in such a chromate-treated steel sheet, and there is a problem relating to deterioration in appearance. Occurs. The occurrence of such black rust is called a blackening phenomenon, and in particular, in a steel sheet for use in which appearance is important, the commercial value is significantly reduced. Moreover, since this blackening phenomenon does not appear immediately after the production of the chromate-treated steel sheet, but appears during storage or transportation, it is desired to develop a means for solving such a troublesome blackening phenomenon. Was rare.

As a method for preventing the above-described blackening phenomenon of a chromate-treated steel sheet, Japanese Patent Application Laid-Open No. 60-152680 discloses an electrogalvanizing bath in which the amount of Pb ^{2+ as} an impurity is limited to 0.2 ppm or less. It discloses that the blackening phenomenon is prevented by performing galvanization and then performing a chromate treatment (hereinafter referred to as “prior art 1”).

Japanese Patent Publication No. 2-8374 discloses that the amount of Pb ^{2+ as} an impurity is limited to 0.5 ppm or less, Ni ²⁺ in the range of 100 to 300 ppm is added, and P
The ratio of the amount of Ni ^{2+ to} b ²⁺ (Ni ²⁺ / Pb ²⁺ ) is 50
It discloses that galvanizing is performed using an electrogalvanizing bath having a value of 0 or more, and then a chromate treatment is performed to prevent a blackening phenomenon (hereinafter, referred to as “chromating”).
"Prior art 2").

[0010] Japanese Patent Application Laid-Open No. 7-268592 discloses that
The blackening phenomenon is a phenomenon in which zinc on a chromate base is oxidized.To prevent oxidation of zinc, it is effective to spray a water-soluble metal salt onto a zinc plating layer immediately after galvanizing. (Hereinafter referred to as “prior art 3”).

[0011]

As apparent from the prior art 1-3, as a countermeasure against the blackening phenomenon, the amount of Pb in the galvanized layer is reduced, the amount of Ni is increased, and the oxidation of zinc is reduced. In view of the prevention, it has been proposed to spray a water-soluble metal salt immediately after galvanization.

However, the lower the Pb content of the zinc plating bath is, the lower the better, the better.
In order to maintain the blackening resistance of zinc plating by controlling the lead content, it is necessary to exclude Pb ions and monitor lead ions with high sensitivity of 1 ppm or less, which is disadvantageous in cost. In particular, in electrogalvanizing using a self-fluxing electrode of zinc, it is impossible to completely remove lead from the zinc electrode, so lead must always be removed from the plating bath.

As described in JP-A-5-279889, when Ni ions are added to a zinc plating bath, the blackening resistance of chromate is improved, but the zincate is prepared using the same zinc plating bath. It has been known that when a galvanized steel sheet is subjected to a phosphate treatment, the appearance is significantly reduced. Therefore, it is necessary to avoid producing a galvanized steel sheet for phosphate treatment immediately after producing a nickel-added galvanized steel sheet for chromate treatment with excellent blackening resistance using the same production equipment. Adjustment is required, which adversely affects the productivity of the entire equipment.

Furthermore, spraying a water-soluble metal salt onto the zinc plating layer immediately after the zinc plating and then subjecting the zinc plating layer to a chromate treatment is disadvantageous in terms of cost because it requires changing manufacturing equipment. In view of this point, the prior art 3 is not feasible.

[0015] Accordingly, an object of the present invention is to solve the above-mentioned problems embraced by prior arts 1 to 3, that is, strict control of Pb ions in a zinc plating bath, addition of Ni ions, and production equipment. An object of the present invention is to provide a chromate-treated steel sheet that can prevent the occurrence of a blackening phenomenon without making any change and that has a low manufacturing cost.

[0016]

According to a first aspect of the present invention, there is provided a chromate-treated steel sheet comprising a galvanized steel sheet and a chromate film formed on the surface thereof, wherein the Auger electron beam on the surface of the chromate film is provided. Atomic ratio of Zn to Cr (Z
(n / Cr) is in the range of 1.2 to 3.0.

[0017]

Next, a chromate-treated steel sheet according to an embodiment of the present invention will be described in detail. The chromate-treated steel sheet of the present invention has a basic structure including a zinc-based plated steel sheet and a chromate film formed on the surface thereof. A feature of the chromate-treated steel sheet of the present invention is that, in the above basic structure, Z with respect to Cr, determined by Auger electron spectroscopy, on the surface of the chromate film
That is, the atomic ratio of n (Zn / Cr) is maintained in the range of 1.2 to 3.0.

As is clear from the above description, the amount of Zn atoms in the chromate film of the chromate-treated steel sheet of the present invention is larger than that of Cr atoms as compared with the existing chromate film. Therefore, even if the chromate-treated steel sheet of the present invention and the conventional chromate-treated steel sheet have the same Cr adhesion amount, in the former, a large amount of Zn is incorporated in the chromate film, The chromate film becomes thick and very dense. At the same time, the composite oxide film of Zn and Cr has a complicated structure, and as a result, blackening resistance is improved.

The chromate-treated steel sheet of the present invention has a basic structure consisting of a galvanized steel sheet and a chromate film formed on the surface thereof. A feature of the chromate steel sheet of the present invention is that, in the above basic structure, the atomic ratio of Zn to Cr (Zn / Cr) on the surface of the chromate film determined by Auger electron spectroscopy is 1.2.
From 3.0 to 3.0. Such a chromate film differs from a normal chromate film in that the number of Zn atoms in the chromate film is sufficiently larger than the number of Cr atoms. When such a chromate film has the same amount of chromium as that of a normal chromate film, the chromate film is thick and extremely Become dense. At the same time, the composite oxide film of Zn and Cr has a complicated structure, and as a result, blackening resistance is improved.

The atomic ratio of Zn to Cr (Zn / Zn), determined by Auger electron spectroscopy of the chromate coating.
When Cr) is less than 1.2, a blackening phenomenon occurs unless lead in the galvanizing bath is reduced or nickel is added as is well known. This is the same as the comparative example in the example described later. In the case of such an atomic ratio of Zn to Cr (Zn / Cr), the effect of the present invention is that the number of Zn atoms in the chromate film is sufficiently larger than the number of Cr atoms, so that the chromate film is thick and dense. Does not appear.

On the other hand, on the surface of the chromate film, Z with respect to Cr, determined by Auger electron spectroscopy.
If the atomic ratio of n (Zn / Cr) exceeds 3.0, the properties of the chromate film become stronger than the properties of zinc and chromium oxides, and the properties of zinc oxide become stronger.
When the atomic ratio of Zn to Zn (Zn / Cr) is 5.0 or more, a brittle film such as white rust is formed, and the corrosion resistance is deteriorated.

Therefore, on the surface of the chromate coating,
The atomic ratio of Zn to Cr (Zn / Cr), as determined by Auger electron spectroscopy, should be maintained in the range of 1.2 to 3.0.

Quantified by Auger electron spectroscopy,
The above-described chromate film in which the atomic ratio of Zn to Cr (Zn / Cr) is maintained in the range of 1.2 to 3.0 is, for example, a Cr ion reduction ratio (Cr) in a chromate bath.
It can be obtained by controlling ^{3 +} / total Cr ions (sum of ^{Cr3 +} and ^{Cr6 +} ), Zn ion amount, and free acidity (FA).

According to the present invention, Pb in the zinc plating bath
Without introducing a very difficult technique of controlling the amount of ions and Ni ions, and without introducing new equipment, using a normal zinc plating bath and a chromate bath, Zn in the chromate film, By maintaining the atomic ratio (Zn / Cr) in the range of 1.2 to 3.0, a chromate film containing a large amount of Zn can be obtained as described above. An excellent chromate-treated steel plate can be manufactured at low cost.

[0025]

Next, the chromate-treated steel sheet of the present invention will be described in more detail with reference to examples and comparative examples.

EXAMPLE A galvanized steel sheet having a galvanizing amount of 20 g / m ² was used as a test piece, degreased and washed with a weak alkaline degreasing agent, washed with water, and further heated with hot air. Dried. Next, a chromate treatment liquid is sprayed onto the thus dried electrogalvanized steel sheet to perform a chromate treatment, and then the water is washed with water to thereby obtain a chromate treated steel sheet within the scope of the present invention (hereinafter, referred to as “ No. 1 treated steel sheet of the present invention
To 5 ").

With regard to the chromate treatment liquid, in order to change the quality of the chromate coating film, in particular, the amount of Zn in the chromate coating film, the Cr ion reduction ratio (Cr ³⁺ / total Cr ion (Cr ³⁺ And Cr ⁶⁺ ),
The amount of Zn ions and the free acidity (FA) are controlled as parameters, and the appropriate amounts of sulfate ions, fluorine ions and chloride ions are blended to reduce the total amount of Cr ions to 5000 ppm.
And maintaining the Zn ion amount at 2500 ppm,
SO ₄ ²⁻ , NO ₃ ⁻ , and F ⁻ were appropriately added as anions.

The steel sheet treated according to the present invention, Treatment time, chromate treatment solution free acidity (FA) and Cr ion reduction rate (Cr ³⁺ / total C
The r ions (sum of Cr ³⁺ and Cr ⁶⁺ ) are shown in Table 1 below.

[0029]

[Table 1]

The treated steel sheet No. 1 of the present invention prepared as described above was used. For each of 1 to 5, the amount of Cr attached was measured by the atomic absorption method. C measured in this way
The amount of r adhesion is shown in Table 1.

Further, the treated steel sheet No. For each of 1 to 5, the lightness L value was measured in order to evaluate the blackening resistance. Next, considering that the chromate-treated steel sheet is likely to be blackened when stored in a place with high humidity, the treated steel sheet No. 1 of the present invention was placed in a box having a temperature of 50 ° C. and a humidity of 95%. 1 to 5 were stored for 10 days to perform a blackening acceleration test, and then the lightness L value was measured again. The difference ΔL between the lightness L value before the black discoloration acceleration test and the lightness L value after the black discoloration acceleration test thus determined was determined,
It was used as an index of blackening resistance. Table 1 also shows the value of ΔL as an index of blackening resistance thus obtained. Since the lightness after the accelerated blackening test is lower than the lightness before the accelerated blackening test, ΔL is a negative value. The smaller the absolute value is, the better the blackening resistance is. It means that the blackening resistance is inferior.

Further, Auger electron spectrometer PHI-54
0A, the treated steel sheet No. of the present invention. Quantifying the number of atoms of each of Cr and Zn from each surface of 1 to 5;
In the chromate film, the atomic ratio of Zn to Cr (Z
n / Cr). The atomic ratio of Zn to Cr (Zn / Cr) thus determined is also shown in Table 1.

Regarding the determination of the number of atoms of each of Cr and Zn described above, the treated steel sheet No. With respect to each of 1 to 5, the average value of the measured values at arbitrary 10 positions on the surface was employed while the chromate treatment was performed. The acceleration voltage of the incident electrons was 3 keV, and the measured Auger peaks were Cr-LMM and Zn-LMM.
Met. In such quantification, it is necessary to use a relative sensitivity coefficient in order to have a unique Auger peak intensity for each device and each element. Therefore, in this embodiment, the relative sensitivity coefficient is 0.320 for Cr, Z
Quantification was performed using 0.170 for n.

As far as the measurement is performed under the above-mentioned conditions, judging from the viewpoint of the chromate coating film thickness and the Auger electron escape length predicted from the Cr adhesion amount, the obtained numerical value is at least the exact Zn to Cr content of the chromate film. It is clear that Zn in the underlying zinc plating layer does not contribute to Auger strength.
Regarding these measurement methods, general methods, for example,
It goes without saying that the same value as the above-mentioned quantitative value can be obtained as long as the quantitative value is determined by a method described in Kyoritsu Shuppan “Auger electron spectroscopy for users”.

The treated steel sheet No. In each of 1 to 5, the value of ΔL as an index of blackening resistance was plotted in FIG. FIG. 1 is a graph showing the relationship between the amount of Cr attached and ΔL as an index of blackening resistance. As is apparent from FIG. 1, the treated steel sheet No. 1 having a Cr adhesion amount of about 11 to 79 mg / m ² was obtained. From 1 to 5, the value of ΔL was in the range of about -1.8 to -2.9, and the blackening resistance was excellent.

Further, the treated steel sheet No. The atomic ratio of Zn to Cr (Zn / Cr) in the chromate coating for each of 1 to 5 is plotted in FIG. FIG.
Is a graph showing the relationship between the atomic ratio of Zn to Cr (Zn / Cr) in the chromate film and ΔL as an index of blackening resistance. As is clear from FIG. 2, the atomic ratio of Zn to Cr in the chromate film (Zn / C
r) within the range of 1.2 to 3.0, the treated steel sheet No. of the present invention. From 1 to 5, the value of ΔL was in the range of about -1.8 to -2.9, and the blackening resistance was excellent.

[Comparative Example] For comparison, the above-described embodiment was used.
As in the above, as a test sample, 20 g / m ^TwoZinc
Use an electrogalvanized steel sheet with a plating amount
After degreasing and washing with a weak alkaline degreasing agent,
It was washed and dried with hot air. Then dry like this
Chromated by spraying on galvanized steel sheet
Inject the treatment liquid, perform the reaction chromate treatment, and then
By washing this with water, it is out of the scope of the present invention.
Chromate treated steel sheet (hereinafter referred to as “Comparative treated steel sheet No. 1
To 4 ").

With respect to the chromate treatment liquid, the Cr ion reduction ratio (Cr ³⁺ / total Cr ions (Cr ³⁺⁾ of the chromate treatment liquid is changed in order to change the film quality of the chromate film, in particular, the amount of Zn in the chromate film. And Cr ⁶⁺ ),
The amount of Zn ions and the free acidity (FA) are controlled as parameters, and the appropriate amounts of sulfate ions, fluorine ions and chloride ions are blended to reduce the total amount of Cr ions to 5000 ppm.
And the Zn ion content was maintained at 2500 ppm.

The comparative treated steel sheet No. Treatment time, chromate treatment solution free acidity (FA) and Cr ion reduction rate (Cr ³⁺ / total C
The r ions (sum of Cr ³⁺ and Cr ⁶⁺ ) are also shown in Table 1 below.

The comparative treated steel sheet No. 1 prepared as described above was used. For each of Examples 1 to 4, the amount of Cr attached was measured by the fluorescent X-ray method as in the above-described examples. Table 1 also shows the Cr adhesion amounts measured in this manner.

Further, the comparative treated steel sheet No. For each of 1 to 4, the lightness L value was measured in order to evaluate blackening resistance. Next, a blackening acceleration test was performed under the same conditions as in the above-described examples, and then the lightness L value was measured again. The difference ΔL between the lightness L value before the blackening acceleration test and the lightness L value after the blackening acceleration test thus measured.
And used as an index of blackening resistance. Table 1 also shows the value of ΔL as an index of blackening resistance thus obtained.

Further, under the same conditions and using the same Auger electron spectroscopy apparatus as in the above-described embodiment, the comparative treated steel sheet No. 1 The number of atoms of each of Cr and Zn from each of the surfaces 1 to 4 was quantified, and
The atomic ratio of Zn to Cr (Zn / Cr) was determined.
The atomic ratio of Zn to Cr (Zn / Cr) thus determined is also shown in Table 1.

The treated steel sheet No. The value of ΔL as an index of blackening resistance in each of 1 to 4 was also plotted in FIG. As is apparent from FIG.
Comparative treated steel sheet N having a Cr adhesion amount of 4 mg / m ²
o. From 1 to 4, the value of ΔL is from about -5.5 to-
8.2, which was inferior to blackening resistance. In particular,
Comparative treated steel sheet No. In Examples 1 to 4, it was confirmed that the blackening resistance decreased as the amount of deposited Cr increased.

Furthermore, the comparative treated steel sheet No. The atomic ratio of Zn to Cr (Zn / Cr) in the chromate coating for each of 1 to 4 was also plotted in FIG.
As is clear from FIG. 2, the atomic ratio of Zn to Cr (Zn / Cr) in the chromate film is from 0.3 to 1.
0, which is in the range of No. 0 for comparison. From 1 to 4, the value of ΔL was in the range of about -5.5 to -8.2, and the blackening resistance was poor.

In the above-described embodiment, the reaction type chromate treatment was applied as the chromate treatment. However, the same results as described above were obtained by applying the coating type and the electrolytic type chromate treatments.

[0046]

As described above, according to the present invention, strict control of Pb ions in a zinc plating bath,
It is possible to provide a chromate-treated steel sheet that can prevent the occurrence of black discoloration phenomenon and that can be manufactured at low cost without adding Ni ions and without changing manufacturing equipment, and thus has a useful effect. Is brought.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of Cr attached to a chromate film and ΔL as an index of blackening resistance.

FIG. 2 is a graph showing the relationship between the atomic ratio of Zn to Cr (Zn / Cr) in a chromate film and ΔL as an index of blackening resistance.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yoshiharu Sugimoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-9-67682 (JP, A) JP-A-5 JP-A-57-63680 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 22/00-22/86

Claims (1)

(57) [Claims] 1. A chromate-treated steel sheet comprising a galvanized steel sheet and a chromate film formed on the surface thereof, wherein the number of Zn atoms relative to Cr on the surface of the chromate film is determined by Auger electron spectroscopy. A chromate-treated steel sheet having excellent blackening resistance, wherein the ratio (Zn / Cr) is in the range of 1.2 to 3.0.