JPH10237668A - Production of chromate treated extra-low lead hot dip galvanized steel sheet excellent in white rust resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a chromate treated hot dip galvanized steel sheet using an extra-low lead hot dip galvanized steel sheet having a uniform appearance as the stock and showing excellent white rust resistance over a long period without damaging the stability of a chromate treating soln. SOLUTION: An extra-low lead hot dip galvanized steel sheet applied with plating by a hot dip galvanizing bath contg., by weight, <=0.01% Pb and 0.1 to 0.3% Al is coated with a chromate treating soln. contg. sexivalent chromium ions, trivalent chromium ions and nitric acid ions, in which the molar ratio of trivalent chromium ions/sexivalent chromium ions is regulated to 1/9 to 1/1, and the molar ratio of nitric acid ions to the total chromium ions (sexivalent chromium ions + trivalent chromium ions) is regulated to 0.1 to 1.6, which is dried at a sheet temp. of 40 to 250 deg.C without washing to form chromate coating with 5 to 50mg/m<2> coating weight expressed in terms of metallic chromium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a chromate treated ultra-low lead galvanized steel sheet having excellent white rust resistance by chromate treatment.

[0002]

2. Description of the Related Art Since sacrificial corrosion protection by galvanizing steel sheets is the most effective and economical corrosion protection method, at present, 10 million tons, which is about 10% of the annual crude steel production in Japan (about 100 million tons), is used. Produced as galvanized steel sheet, it is used in a wide range of fields, such as building materials, automobiles and home appliances. The sacrificial corrosion protection mechanism using zinc is based on the formation of a battery in a situation where two metals, zinc and iron, are in contact with each other. It suppresses anode dissolution and prevents corrosion of iron. Since such rust prevention action ends when the zinc in contact with iron disappears, it is necessary to suppress the corrosion of the zinc layer in order to maintain the action effect for a long period of time. It is widely performed to perform a chromate treatment later.

[0003]

Hitherto, many rust-prevention methods by chromate treatment have been proposed in which the corrosion resistance is improved by adding phosphoric acid to a chromic acid-based treatment solution. . However, the chromate treatment solution to which phosphoric acid has been added has the following problems. That is, phosphate ions react with trivalent chromium ions to form chromium phosphate, but some unreacted phosphoric acid remains on the plated steel sheet surface after the treatment.
Although this portion exhibits hygroscopicity, the plated steel sheet subjected to the chromate treatment has a remarkably improved white rust resistance in a relatively short period of time, but cannot be expected to have a long-term effect of suppressing the generation of white rust. Further, since chromium phosphate is partially generated in the chromate treatment liquid and becomes sludge in the treatment liquid, removal of the sludge is a serious problem.

Accordingly, an object of the present invention is to provide a chromate-treated hot-dip galvanized steel sheet having a uniform appearance and exhibiting a long-term excellent white rust resistance without impairing the stability of the chromate-treating solution. It is to provide a method capable of manufacturing a galvanized steel sheet.

[0005]

Means for Solving the Problems The present inventors have conducted various experiments on methods for improving the white rust resistance and chromate treatment solution stability of a chromate-treated ultra-low lead galvanized steel sheet during production. As a result of the examination, the steel plate after plating was subjected to chromate treatment with a chromate treatment solution containing a specific composition containing nitrate ions, and thereby excellent white rust resistance without impairing the stability of the chromate treatment solution. It has been found that a chromate treated ultra-low lead hot-dip galvanized steel sheet can be manufactured industrially stably. The present invention has been made based on such findings, and the features thereof are as follows.

[1] Pb: 0.01% by weight or less, Al:
An ultra-low-lead hot-dip galvanized steel sheet plated in a hot-dip galvanizing bath containing 0.1 to 0.3% by weight contains a hexavalent chromium ion, a trivalent chromium ion, and a nitrate ion, The molar ratio of the hexavalent chromium ion is from 1/9 to 1
/ 1, the molar ratio of nitrate ion to total chromium ion (hexavalent chromium ion + trivalent chromium ion) is 0.1 to 1.6.
Is applied at a plate temperature of 40 to 250 ° C. without washing with water to form a chromate film having an adhesion amount of 5 to 50 mg / m ^{2 in} terms of chromium metal. A method for producing chromate treated ultra-low lead galvanized steel sheet with excellent white rust resistance.

[2] In the production method of the above [1], the chromate treatment solution contains one or more kinds of metal ions selected from cobalt, nickel, strontium and barium, and the total chromium ion (6 Chromium ion +3
A method for producing a chromate treated ultra-low lead galvanized steel sheet having excellent white rust resistance, wherein the total molar ratio of the metal ions to the chromium ions is 0.04 to 0.2. [3] In the production method of the above [1], the chromate treatment solution contains cobalt ions, and the molar ratio of cobalt ions to all chromium ions (hexavalent chromium ions + trivalent chromium ions) is 0.04 to 0.2. A method for producing a chromate treated ultra low lead galvanized steel sheet with excellent white rust resistance.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a method for producing Pb:
The following is a method of chromate treating a very low lead hot-dip galvanized steel sheet plated in a hot-dip galvanizing bath containing 0.1 to 0.3% by weight of Al. Such an ultra-low lead hot-dip galvanized steel sheet is required to have a uniform appearance without forming a clear spangle. Since the extremely low lead galvanized steel sheet has a very low lead content in the coating layer, it is difficult to form minute local batteries starting from lead,
For this reason, it is characterized by exhibiting excellent blackening resistance. Further, since a local battery is hardly formed as described above, the starting point of corrosion is small, and therefore, the corrosion resistance is excellent. However, it is not possible to maintain sufficient corrosion resistance as it is, and it is necessary to perform a chromate treatment.

In order to solve such a problem, according to the present invention, when a very low lead hot-dip galvanizing is brought into contact with a chromate treatment solution containing nitric acid, zinc on the surface of the plating is oxidized, and passivation of chromium proceeds. It has been found that white rust resistance can be effectively suppressed. The effect of improving the white rust resistance due to the oxidation of zinc on the plating surface and the passivation of chromium is particularly remarkable when a chromate treatment solution containing nitrate ions is used. Even if phosphoric acid, sulfuric acid, hydrofluoric acid or the like is contained, a sufficient effect of improving white rust resistance cannot be obtained. Further, even if the conventionally known chromic acid or a partially reduced chromic acid solution is used, such an effect is hardly obtained.

The chromate treatment using the treatment solution containing nitric acid includes hexavalent chromium ion, trivalent chromium ion and nitrate ion, and the nitrate ion is contained in a molar ratio of 0.1 to 0.1% in total chromium ion. It was found that by performing the chromate treatment with the chromate treatment solution prepared in 1.6, even when used for a long period of time, sludge was not generated due to contamination of impurities such as zinc, and the composition of the chromate treatment solution could be stably maintained. In addition, it was found that there was almost no sludge generation because there was no reaction / precipitation in the chromate treatment liquid, and thus the composition of the chromate treatment liquid could be stably maintained.

The details of the present invention and the reasons for limitation will be described below. The hot-dip galvanized steel sheet to be subjected to the chromate treatment in the present invention has a steel sheet having a Pb: 0.01% by weight or less, A
1: Very low lead hot-dip galvanized steel sheet obtained by plating in a hot-dip galvanizing bath containing 0.1 to 0.3% by weight. If the lead content in the plating bath exceeds 0.01% by weight, spangles are formed on the surface of the hot-dip galvanized coating, and a beautiful zero spangle cannot be sufficiently obtained. On the other hand, if the aluminum content in the plating bath is less than 0.1% by weight, sufficient plating adhesion cannot be obtained. On the other hand, if the aluminum content exceeds 0.3% by weight, blackening occurs when exposed to a high-temperature and high-humidity environment after chromate treatment. It is not preferable because it becomes easy to perform.

In the method of the present invention, hexavalent chromium ion, trivalent chromium ion and nitrate ion are contained, and the molar ratio of trivalent chromium ion / hexavalent chromium ion and the molar ratio of nitrate ion to all chromium ions are predetermined. The chromate treatment solution prepared in the range is applied to form a chromate film. The trivalent chromium ion and hexavalent chromium ion contained in the chromate treatment solution are 1/9 to 1/1, preferably 1/4 to 2/3, in a molar ratio of trivalent chromium ion / hexavalent chromium ion.
And If the molar ratio of trivalent chromium ion / hexavalent chromium ion is less than 1/9, elution of chromium in a corrosive environment becomes excessive, so that long-term corrosion resistance cannot be maintained, and environmental pollution due to elution of chromium is not preferred. . On the other hand, when the molar ratio of trivalent chromium ion / hexavalent chromium ion is 1
If it exceeds / 1, the effect of improving corrosion resistance by hexavalent chromium ions cannot be sufficiently obtained.

The mechanism of preventing the formation of white rust by the chromate film is generally understood to be an inhibitory effect of suppressing the corrosion of zinc by hexavalent chromium ions. It is considered that the self-repairing action of repairing the damaged part also contributes to the improvement of white rust resistance. However, the chromate film obtained only with hexavalent chromium ions causes excessive elution of hexavalent chromium ions due to the presence of moisture and causes the formation of white rust. It is widely practiced to incorporate chromium ions. In the present invention, for the same purpose, trivalent chromium ions and hexavalent chromium ions are contained in the chromate treatment solution in the above molar ratio.

Generally, in an aqueous solution containing only trivalent chromium ions and hexavalent chromium ions, it is necessary that trivalent chromium ions exist in the aqueous solution as ions without settling.
The molar ratio of valent chromium ion / hexavalent chromium ion needs to be 2/3 or less. On the other hand, in the case of extremely low-lead galvanized steel sheets that are zero spangles, the appearance color tone is often regarded as important.In such cases, if the amount of chromium attached is large, the appearance color tone becomes yellow and the appearance color tone is impaired, Even if corrosion resistance is sacrificed to some extent, the amount of chromium deposited must be suppressed. In order to solve such a problem, the chromate treatment solution used in the present invention contains nitric acid as an acid component other than chromic acid. The molar ratio of chromium ions / hexavalent chromium ions can be increased to 2/3 or more (provided that the upper limit of the molar ratio of 1/1 described above is limited), whereby the desired coloring can be achieved without reducing the amount of chromium adhering. The prevention effect can be obtained.

In order to supply chromium ions to the chromate treatment solution, a solution obtained by partially reducing an aqueous solution of chromic anhydride with a known reducing agent can be used, and chromium nitrate is used to supply trivalent chromium ions. Can also. The nitrate ions contained in the chromate treatment solution are all chromium ions (6
(Chromium ion + trivalent chromium ion), that is, the molar ratio of nitrate ion / total chromium ion is 0.1 to 1.6, preferably 0.4 to 1.2. If the molar ratio of nitrate ion / total chromium ion is less than 0.1, the effect of improving corrosion resistance is insufficient, while if the molar ratio of nitrate ion / total chromium ion exceeds 1.6, the etching power of the chromate solution becomes high. It is not preferable because sludge is generated due to contamination of contaminant ions such as Zn and Al, and high corrosion resistance cannot be stably maintained. The nitrate ions in the chromate solution can be supplied by nitric acid, chromium nitrate, cobalt nitrate, zinc nitrate or the like.

Further, the chromate treatment liquid contains cobalt,
One or more metal ions selected from the group consisting of nickel, strontium and barium are used in a molar ratio to all chromium ions (hexavalent chromium ion + trivalent chromium ion), that is, a metal ion / total chromium ion molar ratio. In the range of 0.04 to 0.2. It is considered that these metal ions form a hardly soluble compound with the chromate ion, and this compound enhances the barrier property of the chromate film to improve the corrosion resistance. If the amount of metal ions added is less than 0.04 in terms of molar ratio to the total chromium ions, the effect of improving corrosion resistance is poor,
On the other hand, if it exceeds 0.2, the chromate treatment liquid is liable to precipitate, and the liquid stability decreases, which is not preferable. The chromate treatment solution contains Zn, Al, and Pb that are inevitably mixed from the plating components due to the etching action.
In some cases, such metal ions may be contained, but the incorporation of these metal ions does not affect the effects of the present invention.

Further, among the above-mentioned metal ions, the most remarkable effect of improving white rust resistance is obtained when cobalt ions are added. As a result of the experiments by the present inventors, the chromate film obtained by the chromate treatment solution containing cobalt ions has a lower hexavalent chromium content in the film than the chromate film obtained by the treatment solution containing no cobalt ions. Was almost the same, but it was found that the hexavalent chromium content in the outermost layer of the chromate film was large. Therefore, it is considered that the chromate film containing cobalt ions has higher self-repairability with hexavalent chromium ions than the chromate film containing no cobalt ions, and the white rust resistance is remarkably improved. Therefore, the cobalt ion is most preferable as the metal ion added to the chromate treatment liquid.

The metal ions in the chromate treatment liquid can be supplied by a basic carbonate, carbonate, nitrate or the like. In addition, colloidal silica such as silica gel or fumed silica, an aqueous resin, or the like can be added to the chromate treatment liquid as needed. After applying the above chromate treatment solution to the surface of the plated steel sheet, 40
Dry at a maximum plate temperature range of ~ 250 ° C. At a plate temperature of less than 40 ° C, water remains, resulting in a chromate film that is easily dissolved. The film becomes low molecular,
These are not preferred because they cause a reduction in corrosion resistance.

The amount of chromium deposited on the chromate film coated and dried in this manner is 5 to 50 mg / chromium equivalent.
m ² , preferably 10 to 30 mg / m ² . If the attached amount of the chromate film is less than 5 mg / m ^{2 in} terms of metallic chromium, the corrosion resistance is insufficient, while if it exceeds 50 mg / m ² , the coloring becomes remarkable and the beautiful appearance of the extremely low lead galvanized steel sheet is impaired. Therefore, it is not preferable. The method of applying the chromate treatment liquid is arbitrary, and for example, a known application method such as a method of performing roll squeezing or gas squeezing after spraying or dipping, or a roll coating method can be applied.

[0020]

EXAMPLE An ultra-low lead hot-dip galvanized steel sheet (a coating weight: 90 g / m ² , zero spangle material) plated in a hot-dip galvanizing bath containing Pb: 0.003% by weight and Al: 0.2% by weight ) Sprayed with tap water (10 seconds)
Then, after drying, a chromate treatment solution having the composition shown in Tables 1 and 2 was applied by a roll coating method, and the furnace temperature was 300 ° C.
Maximum plate temperature 4 in a hot air drying oven with a 2m / sec air velocity
It dried in the range of 0-270 degreeC, and manufactured the test material of the chromate processing extremely low lead galvanized steel sheet.

The obtained test materials were evaluated for film appearance and corrosion resistance. In these evaluations, for all of the examples of the present invention and a part of the comparative examples, the test material manufactured at a stage shortly after the start of the chromate treatment and the chromate treatment proceeded to some extent (with respect to 1 L of the treatment liquid). 20m for test material
^{After two} consecutive treatments), the test was carried out for both the test materials subjected to the chromate treatment with the treatment liquid in which Zn was dissolved. Further, with respect to the treatment liquid having undergone chromate treatment to some extent, the presence or absence of sludge generation after one month storage at 50 ° C. was examined. The results are shown in Tables 1 and 2 together with the composition of the chromate treatment solution, the amount of chromium deposited, and the like.

[Evaluation of Performance] (1) Appearance of film The Δb (b value of the test material−b value of the untreated material) of the test material and the untreated material was measured by a color difference meter, and the film appearance was evaluated according to the following evaluation criteria. Was evaluated. ：: Δb is less than 1 ○: Δb is 1 or more and less than 3 Δ: Δb is 3 or more and less than 5 ×: Δb is 5 or more (2) Treatment liquid stability 20 m ² continuous treatment of test material with 1 L of treatment liquid After that, the treatment liquid was kept at 50 ° C. for one month, and the state of sludge generation was visually determined. The evaluation criteria are as follows. ◎: No sludge generation ○: Extremely slight sludge generation △: Clearly sludge generation ×: Significant sludge generation

(3) Corrosion resistance A plurality of test pieces of 70 mm × 150 mm were cut out from each test material, and the test pieces were subjected to a salt spray test specified in JIS Z 2371. Was visually determined. The evaluation criteria are as follows. ：: No white rust generated ○: White rust generated area rate less than 5% △: White rust generated area rate 5% or more, less than 25% ×: White rust generated area rate 25% or more

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

As described above, according to the present invention, it is possible to stably produce a chromate treated extremely low lead galvanized steel sheet having excellent white rust resistance without impairing the stability of the chromate treatment solution. Can be.

Claims (3)

[Claims] 1. Pb: 0.01% by weight or less, Al: 0.1% by weight.
An ultra-low-lead hot-dip galvanized steel sheet plated in a hot-dip galvanizing bath containing 1 to 0.3% by weight contains trivalent chromium ion, trivalent chromium ion and nitrate ion, The molar ratio of the valent chromium ion is 1/9 to 1 /
1. A chromate treatment solution in which the molar ratio of nitrate ions to total chromium ions (hexavalent chromium ions + trivalent chromium ions) is adjusted to 0.1 to 1.6, and coated at 40 to 250 ° C without washing with water A method for producing a chromate-treated ultra-low lead galvanized steel sheet having excellent white rust resistance, comprising drying at a temperature and forming a chromate film having an adhesion amount of 5 to 50 mg / m ^{2 in} terms of metal chromium. 2. The chromate treatment liquid contains one or more selected from the group consisting of cobalt, nickel, strontium, and barium metal ions, and is used for all chromium ions (hexavalent chromium ions + trivalent chromium ions). The method according to claim 1, wherein the total molar ratio of metal ions is 0.04 to 0.2. 3. The chromate treatment solution contains cobalt ions, and the molar ratio of cobalt ions to total chromium ions (hexavalent chromium ions + trivalent chromium ions) is 0.04 to 3.
2. The method for producing a chromate-treated ultra-low lead galvanized steel sheet having excellent white rust resistance according to claim 1.