JP3279190B2 - Method for producing chromated lead-containing hot-dip galvanized steel sheet with excellent resistance to blackening and white rust - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a chromate-treated lead-containing hot-dip galvanized steel sheet having excellent resistance to blackening and white rust due to chromate treatment.

[0002]

2. Description of the Related Art Since sacrificial corrosion protection by galvanizing steel sheets is the most effective and economical, at present, 10 million tons, equivalent to about 10% of Japan's annual crude steel production (about 100 million tons), are galvanized. It is produced as a steel sheet and 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 zinc in contact with iron disappears, it is necessary to suppress corrosion of the zinc layer in order to maintain the action effect for a long time,
As a countermeasure, chromate treatment is widely performed after plating.

[0003] However, when a galvanized steel sheet is chromate-treated, the corrosion resistance (white rust resistance) is remarkably improved, but on the contrary, a blackening phenomenon that greatly impairs the appearance of the steel sheet during storage or transportation occurs. There is. This blackening phenomenon is particularly likely to occur in a hot-dip galvanized steel sheet that has been skin-passed after plating, a hot-dip galvanized steel sheet manufactured in a plating bath containing lead, and a hot-dip zinc-aluminum-coated steel sheet that contains several percent of aluminum. It is known. This blackening phenomenon is characterized in that a particular crystal orientation in the spangle on the plating surface becomes particularly gray-black, and therefore, in order to suppress the occurrence of blackening, spangles may be minimized by a well-known method to some extent. It is valid. Further, lead particles are present in spangles in which black discoloration occurs, and these become active sites to promote the black discoloration phenomenon. Therefore, the composition of the plating bath is made extremely low lead (Pb: 0.01 wt.
% Or less), it is also known that blackening hardly occurs. However, many users prefer spangles for hot-dip galvanized steel sheets, and adding lead to the plating bath is inevitable.

As a method for preventing black discoloration after chromate treatment, Japanese Patent Application Laid-Open No. S59-177381 proposes a flash treatment (a treatment for chemically depositing a trace amount of metal) with an aqueous solution containing Ni ions or Co ions. In recent years, this flash processing is regarded as promising as a countermeasure against blackening after chromate processing. JP-A-59-177
No. 381 discloses an aqueous solution having a pH of 1 to 4 or 11 to 13.5 and containing one or two types of Ni ions and Co ions prior to chromate treatment of a zinc or zinc alloy plated steel sheet. The surface of the steel sheet is subjected to a flash treatment to precipitate the metal ions in the form of a metal or an oxide on the surface of the steel sheet, followed by washing with water, and then a chromate film is formed.

The mechanism by which such flash treatment with Ni or Co prevents blackening of a zinc-coated steel sheet after chromate treatment is not yet established, but is described in the abstract of the 60th Annual Conference of the Japan Society for Metal Surface Technology. Page 150 to 15
According to the description on page 1, a large amount of flashed metal is precipitated at the grain boundaries of zinc crystals, and the chromium compound attached by the subsequent coating-type chromate treatment is also distributed at the grain boundaries. It is presumed that there is some interaction (interaction) between the flashed metal and the chromium compound, and the chromium compound is adsorbed and fixed to the flash metal.

[0006] The blackening phenomenon of a zinc-based plated steel sheet is that basic zinc carbonate represented by (ZnCO ₃ ) x · [Zn (OH) ₂ ] y, like white rust, has a grain size of 400 to 700 nm in the visible light wavelength region. Because of the diameter, light is likely to be scattered and absorbed, and appears to be black. Blackening is a corrosion product under oxygen-deficient conditions, and is thought to be formed especially as corrosion progresses from the grain boundaries. Deters corrosion,
It is considered that this contributes to the prevention of black rust. Such flash treatment of Ni, Co, etc. prior to the chromate treatment can be an effective countermeasure against blackening of the galvanized steel sheet.

[0007]

However, when the above-mentioned flash treatment is performed prior to the chromate treatment, blackening is suppressed but white rust is more likely to occur. It became clear. This is due to the N deposited on the plating surface by flash treatment.
This is probably because i and Co form a local battery with zinc. Therefore, the object of the present invention is
It is an object of the present invention to provide a method capable of producing a chromate-treated lead-containing hot-dip galvanized steel sheet having excellent blackening resistance and white rust resistance without flash treatment.

[0008]

Means for Solving the Problems The present inventors have conducted various experiments and studies on a method capable of improving the blackening resistance and white rust resistance of a chromate-treated lead-containing hot-dip galvanized steel sheet. By performing chromate treatment with a chromate treatment solution having a specific composition containing nitrate ions, it is possible to industrially produce a chromate-treated lead-containing hot-dip galvanized steel sheet having excellent blackening resistance and white rust resistance. It was out have seen.

The present invention has been made based on such findings.
The characteristic configuration is as follows: Pb : 0.05-0.
3 wt%, Al: 0.1 to 0.3 in lead-containing galvanized steel sheet plated with molten zinc plating bath containing a weight percent, containing the hexavalent chromium ions and trivalent chromium ions and nitrate ions With strontium and burr
One or two selected from the metal ions of
Containing trivalent molar ratio of chromium ions / hexavalent chromium ions 1 / 9-1 / 1, before the molar ratio of nitrate ions 0.1 to 1.6, to the total chromium ions to total chromium ions
Apply a chromate treatment solution in which the total molar ratio of the metal ions is adjusted to 0.04 to 0.2 , and wash the solution without washing with water.
It is dried at a plate temperature of 0 to 250 ° C. and forms a chromate film having an adhesion amount of 5 to 50 mg / m ^{2 in} terms of chromium metal. This is a method for producing a hot-dip galvanized steel sheet .

[0010]

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a method for producing Pb: 0.05-0.
In this method, a lead-containing hot-dip galvanized steel sheet plated in a hot-dip galvanizing bath containing 3% by weight and Al: 0.1 to 0.3% by weight is chromate-treated. In such a lead-containing hot-dip galvanized steel sheet, the lead in the plating film is a component necessary for obtaining a beautiful spangle pattern, but this lead is concentrated on the plating surface, and thus the lead is concentrated on the plating surface. The presence of the converted lead makes the plating film electrochemically non-uniform, further promoting the blackening phenomenon.

In order to solve such a problem, according to the present invention, when a lead-containing hot-dip galvanized steel sheet is brought into contact with a chromate treatment solution containing nitric acid, lead concentrated on the plating surface is removed, and the blackening phenomenon is effectively prevented. We found that it could be suppressed.
Of course, not all of the lead on the plating surface is removed by such a chromate treatment, and lead remains on the plating surface. However, the effect of suppressing the blackening by the above chromate treatment is remarkable, and the chromate treatment It has been found that the formation of the film covers the surface of the active lead remaining on the plating surface with a large amount of chromium, thereby improving the corrosion resistance. The effect of removing lead concentrated on the plating surface is particularly remarkable when a chromate treatment solution containing nitric acid is used, and the chromate treatment solution contains other inorganic acids such as phosphoric acid, sulfuric acid, hydrofluoric acid, and the like. However, a sufficient removal effect cannot be obtained. In addition, a conventionally known chromic acid or a chromic acid solution in which a part thereof is partially reduced has almost no effect of removing lead.

Regarding the above chromate treatment,
It contains hexavalent chromium ion, trivalent chromium ion and nitrate ion, and has a nitrate ion in a molar ratio of 0.1 to the total chromium ion.
The use of the chromate treatment solution prepared at ~ 1.6 allows the lead to be effectively removed with an extremely small etching amount, and the use of such a chromate treatment solution for contamination of impurities such as zinc even after long-term use. It was found that no sludge was generated due to the above, and the composition of the chromate treatment solution could be stably maintained.

The details of the present invention and the reasons for limitation will be described below. The lead-containing hot-dip galvanized steel sheet to be subjected to the chromate treatment according to the present invention is prepared by subjecting the steel sheet to a hot-dip galvanizing bath containing Pb: 0.05 to 0.3% by weight and Al: 0.1 to 0.3% by weight. It is obtained by plating. As described above, such a lead-containing hot-dip galvanized steel sheet is particularly liable to cause blackening. If the lead content in the plating bath is less than 0.05% by weight, a beautiful regular spangle of the plated steel sheet cannot be sufficiently obtained, while if it exceeds 0.3% by weight, the effect is saturated and the economy is impaired. Not only that, the grain boundary corrosion is easily caused. 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.

The method of the present invention contains hexavalent chromium ion, trivalent chromium ion and nitrate ion (further described below.
(Contains a constant metal ion.) Apply a chromate treatment solution in which the molar ratio of trivalent chromium ion / hexavalent chromium ion and the molar ratio of nitrate ion to all chromium ions are within predetermined ranges to form a chromate film. I do. The molar ratio of trivalent chromium ion and hexavalent chromium ion contained in the chromate treatment liquid is 1/9 to 1/1, preferably 1/4 to 2/3 in a molar ratio of trivalent chromium ion / hexavalent chromium ion. . The molar ratio of trivalent chromium ion / hexavalent chromium ion is 1
If it is less than / 9, the elution of chromium in a corrosive environment becomes excessive, so that it is not possible to maintain corrosion resistance for a long period of time, and environmental pollution due to elution of chromium is not preferred. on the other hand,
The molar ratio of trivalent chromium ion / hexavalent chromium ion is 1/1
If it exceeds 3, the effect of improving corrosion resistance by hexavalent chromium ions cannot be sufficiently obtained.

The mechanism for preventing the formation of white rust by the chromate film is generally understood to be an inhibitory effect of inhibiting 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, a 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.

In general, in an aqueous solution containing only trivalent chromium ions and hexavalent chromium ions, it is necessary for the trivalent chromium ions to be present in the aqueous solution as ions without sedimentation.
The molar ratio of valent chromium ion / hexavalent chromium ion needs to be 2/3 or less. On the other hand, lead-containing hot-dip galvanized steel sheets with beautiful spangled patterns are often considered to have an important appearance color, and in such cases, a large amount of chromium adhering causes yellowish appearance and impairs the appearance color. In addition, even if the 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. Chrome ion / 6
The molar ratio of the chromium ion is increased to 2/3 or more (however,
(The upper limit of the molar ratio of 1/1 described above is a limit), whereby a desired anti-coloring effect can be obtained without reducing the amount of deposited chromium.

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 ion contained in the chromate treatment liquid has a molar ratio of nitrate ion / total chromium ion of 0.1 to 1.6, preferably 0.1 to 1.6.
4 to 1.2. When the molar ratio of nitrate ion / total chromium ion is less than 0.1, the effect of suppressing blackening is insufficient. On the other hand, when the molar ratio of nitrate ion / total chromium ion exceeds 1.6, the etching power of the chromate solution is high. Too much, Z
It is not preferable because it causes sludge generation due to contamination of contaminant ions such as n and Al, which makes it impossible to stably maintain high corrosion resistance. The nitrate ions in the chromate solution can be supplied by nitric acid, chromium nitrate, cobalt nitrate, zinc nitrate or the like.

Furthermore, strontium is used in the chromate treatment solution.
One or two selected from metal ions of chromium and barium are used in a molar ratio of 0.0
It is contained in the range of 4 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. The amount of metal ions added is 0.04 in molar ratio to all chromium ions.
If it is less than 0.2, the effect of improving the corrosion resistance is poor. On the other hand, if it exceeds 0.2, the chromate-treated solution tends to precipitate and the stability of the solution is undesirably reduced. In some cases, the chromate treatment solution contains metal ions such as Zn, Al, and Pb which are inevitably mixed from the plating components due to the etching action. The mixing of these metal ions affects the effect of the present invention. Do not give.

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 the above-mentioned chromate treatment solution is applied to the surface of the plated steel sheet, it is dried without washing with water at a maximum temperature of 40 to 250 ° C. If the plate temperature is lower than 40 ° C, moisture remains, resulting in an easily soluble chromate film.
On the other hand, if the temperature exceeds 250 ° C., hexavalent chromium ions effective for corrosion resistance are reduced to trivalent chromium ions, and the chromate film which has been polymerized is reduced in molecular weight, which causes a reduction in corrosion resistance. The amount of chromium deposited on the chromate film coated and dried in this way is 5 to 50 mg / m ² , preferably 10 to 30 m ^{2 in} terms of chromium metal.
g / m ² . If the amount of the chromate film adhered is less than 5 mg / m ^{2 in} terms of chromium metal, the corrosion resistance is insufficient, while if it exceeds 50 mg / m ² , the coloring becomes remarkable and the beautiful appearance of the lead-containing galvanized steel sheet is impaired. Not preferred. 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.

[0022]

[Example] Lead-containing hot-dip galvanized steel sheet plated in a hot-dip galvanizing bath containing 0.1% by weight of Pb and 0.2% by weight of Al (amount of plating: 90 g / m ² , regular spangle material) Spray water with tap water (10
Seconds) and dried, and then a chromate treatment solution having a composition shown in Table 1 is applied by a roll coating method, and a maximum reached plate temperature of 40 to 2 is obtained in a hot air drying oven having a furnace temperature of 300 ° C. and an in-furnace air velocity of 2 m / sec.
The sample was dried at a temperature of 70 ° C. to produce a test material of a chromate-treated lead-containing hot-dip galvanized steel sheet. The obtained test materials were evaluated for film appearance, blackening resistance 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). After the test material was continuously treated for 20 m ² ), the test was performed on both of the test materials subjected to the chromate treatment with the treatment liquid in which Zn was dissolved. Table 1 shows the results together with the composition of the chromate treatment solution, the amount of chromium adhering, etc.
To show.

[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 with 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) Blackening resistance A plurality of test pieces of 70 mm × 150 mm were cut out from each test material, and the test surfaces of the test pieces were faced to form a pair. Packed in paper, sandwiched between two stainless steel plates with a thickness of 10 mm with an acrylic plate attached on the inside, tightened the four corners with bolts and applied a load of 0.67 kgf · cm ^{2 with} a torque wrench. 5
After being kept in a humid box at 0 ° C. and a relative humidity of 95% for 240 hours, it was taken out, and the blackened state of the overlapped portion was visually judged. The evaluation criteria are as follows. ：: no blackening ○: extremely slight graying △: blackening occurred ×: remarkable blackening occurred

(3) Corrosion resistance A plurality of test specimens of 70 mm × 150 mm were cut out from each test material, and a salt spray test specified in JIS Z 2371 was performed on these test specimens. The rust generation area 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

[0026]

[Table 1]

[0027]

[0028]

As described above, according to the present invention, a hot-dip galvanized steel sheet containing lead excellent in film appearance, blackening resistance and white rust resistance can be stably manufactured.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masaaki Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-7-54156 (JP, A) JP-A-5 -331659 (JP, A) JP-A-7-180070 (JP, A) JP-A-5-65669 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 22/00- 22/86 C23C 28/00

Claims (1)

(57) [Claims] 1. Pb: 0.05-0.3% by weight, Al:
The lead-containing galvanized steel sheet plated with molten zinc plating bath containing 0.1 to 0.3 wt%, containing Then preparative and hexavalent chromium ions and trivalent chromium ions and nitrate ions
In addition, strontium and barium metal ions
One or two selected from the group consisting of: trivalent chromium ion / hexavalent chromium ion in a molar ratio of 1/9 to 1 /
1. The molar ratio of nitrate ions to all chromium ions is 0.1.
1 to 1.6 , the ratio of the metal ion to all chromium ions
A chromate treatment solution adjusted to a total molar ratio of 0.04 to 0.2 is applied and dried at a plate temperature of 40 to 250 ° C. without washing with water.
/ M ^2, a method for producing a chromate-treated hot-dip galvanized steel sheet excellent in blackening resistance and white rust resistance, characterized by forming a chromate film of / m ² .