JPH07292495A - Chromate-treated galvanized steel sheet excellent in corrosion resistance, fingerprint resistance and blackening resistance - Google Patents

Abstract

PURPOSE:To develop a chromate-treated galvanized steel sheet excellent in corrosion resistance, fingerprint resistance and blackening resistance by forming a chromate film having a compsn. contg. specified amounts of Ni and silica particles and contg. no metal Cr on the surface of a galvanized steel sheet. CONSTITUTION:The surface of a steel sheet is coated with Zn or a Zn alloy by electroplating, galvanizing or galvannealing. At the time of applying the surface with electrolytic chromate treatment, a chromate film having a compsn. contg. the oxide and hydroxide of Cr by 5 to 200mg/m<2> total Cr content expressed in terms of Cr so as to regulate the content of soluble Cr therein to 1 to 30wt.%, furthermore contg. metal Ni and the oxide of Ni by 10 to 500mg/m<2> Ni total content, contg. silica having 3 to 100nm average particle size by 100 to 1000mg/m<2> and substantially contg. no metal Cr is formed, by which the chromate-treated galvanized steel sheet excellent in adhesion, appearance, fingerprint resistance and blackening resistance can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromate-treated zinc-based plated steel sheet which is mainly used for home appliances and which is excellent in corrosion resistance, fingerprint resistance and blackening resistance.

[0002]

2. Description of the Related Art Chromate-treated electrogalvanized steel sheets are widely used for home electric appliances such as chassis and motor covers because of their excellent resistance to blackening and lubrication during processing.

Chromate treatment methods on electrogalvanized steel sheets are roughly classified into the following three methods. a) Coating chromate treatment b) Reactive chromate treatment c) Electrolytic chromate treatment

A) The coating type chromate treatment is a process in which a chromate film is formed by coating a chromate solution on an electrogalvanized steel sheet with an air knife or a roll coater and then baking and drying it in an oven without washing with water. is there. According to this method, since the amount of reaction between the chromate treatment liquid and the zinc plating layer is small, there is an advantage that the treatment liquid is less deteriorated by eluted zinc or the like and liquid management is easy. However, in order to form a sound chromate film, a baking step at a temperature of 100 ° C. or higher is indispensable, so that an oven is required, resulting in an increase in the cost required for equipment, which is not economical.

B) In the second reaction type chromate treatment, the galvanized steel sheet is immersed in or sprayed with a reaction type chromate solution containing hexavalent Cr ions to dissolve the metal zinc plating layer and simultaneously reduce it. A chromate film is formed on the plating by depositing the generated chromium chromate composed of trivalent Cr ions and hexavalent Cr ions in a gel form. Unlike coating type chromate, high temperature drying after chromating is not required, so that the production cost can be reduced, and reactive type chromate-treated electrogalvanized steel sheets with various Cr deposits are produced depending on the purpose and application. However, since the galvanized layer is dissolved in the film formation, Z in the treatment liquid
Accumulation of n-ions was unavoidable, and liquid management was a problem, and quality variation due to changes in processing liquids was a problem. Furthermore, since a chromate film is formed by a chemical reaction, it is difficult to control the amount of adhesion, which is a problem.

C) The third electrolytic chromate treatment is to form a chromate film by electrolytically treating a Zn-plated steel sheet as a cathode in a chromate treatment liquid containing hexavalent Cr ions. is there. According to this method, the elution of zinc into the processing solution is reduced, the solution management is easy, and the amount of applied electricity can be controlled by the amount of electricity applied, but the functionality such as corrosion resistance and fingerprint resistance is improved. Since it does not have, it was used only for painting bases and general chromates.

As a steel sheet which has improved such a problem and provided electrolytic chromate with high functionality, Japanese Patent Publication No. 30475/1992 has a chromium oxide layer and a layer composed of silica and chromium oxide on a metal Cr layer. A chromate-treated Zn-plated steel sheet is also disclosed in JP-A-1-255690 as C.
A chromate-treated zinc-based plated steel sheet having a chromate film layer composed of a metal layer of r and Ni and an oxide layer of Cr and Si is further disclosed in JP-A-4-304397.
A chromate-treated zinc-based plated steel sheet having a chromate film layer composed of a metal layer of i and an oxide layer of Cr, Ni, and Si is disclosed.

However, according to these known steel sheets, although the corrosion resistance is slightly improved by the effect of silica, the corrosion resistance of the high corrosion resistance chromate steel sheet by the coating type chromate treatment is far lower than that. Further, in these steel sheets, when placed in a high temperature and high humidity environment, the so-called black resistance, which is discoloration to black, was often inferior.

[0009]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is intended that the chromate film contains predetermined silica, metallic nickel, nickel, and does not contain metallic chromium. Corrosion resistance,
An object of the present invention is to provide a chromate-treated zinc-based plated steel sheet having excellent fingerprint resistance and blackening resistance.

[0010]

To achieve the above object, according to the present invention, at least one side of a steel sheet is plated with zinc or a zinc-based plating, and a chromate film is formed on the chromate-treated zinc. Chromate film, chromium oxide, chromium hydroxide,
Provided is a chromate-treated zinc-based plated steel sheet excellent in corrosion resistance, fingerprint resistance, and blackening resistance, which is composed of metallic nickel, nickel oxide, and specific silica particles, and is substantially free of metallic chromium in the film. To be done.

Here, it is desirable that the total chromium amount in the chromate film is 5 to 200 mg / m ² in terms of Cr and the soluble Cr amount is 1 to 30 wt%.
The average particle size of silica in the chromate film is 3
˜100 nm, silica amount 100-1000 mg
/ M ² is desirable. Further, it is desirable that the amount of nickel in the chromate film is 10 to 500 mg / m ² .

[0012]

The present invention will be described in more detail below.

As the starting material of the present invention, it is preferable to use a thin steel plate having a plate thickness of 3.2 mm or less. After undergoing heat treatment to obtain the required material, electrogalvanizing plating is performed. Electrogalvanic plating is generally performed in a plating bath used for industrial production. That is, it is composed of an additive containing Zn ions and added as needed for the purpose of improving conductivity and imparting gloss. As a counter ion of Zn ion, a generally used sulfate ion or chlorine ion may be used, and a bath in which both are mixed may be used.
Further, alloy plating may be carried out by adding metal ions such as Ni ²⁺ , Co ²⁺ , Fe ²⁺ , Mn ²⁺ to the bath, for example, dispersion plating in which Cr ³⁺ -Co ²⁺ -alumina is added. Good.

As the plating tank, any of carrousel plating which is one-sided sequential plating, horizontal cell which is both-sided simultaneous plating, and vertical type cell such as Gravitel may be used. In these plating baths, a predetermined amount of zinc-based plating is applied.

As the zinc-based plating, not only electrogalvanized plating but also hot-dip galvanized or galvannealed steel sheet may be used. However, when these plated steel sheets are used, the oxides of Zn or Al may be formed in the surface layer by the heat treatment, and it is effective to remove them by the alkali treatment prior to the chromate treatment.

In the present invention, the steel sheet thus plated with zinc is subsequently subjected to electrolytic chromate treatment as chromate treatment.

The action of the chromate of the present invention will be described in comparison with a conventional electrolytic chromate film. Generally, at the time of electrolytic chromate treatment, a cathode is applied to a Zn-plated steel sheet in a solution containing chromium ions to form a current circuit with the anode in the bath. At this time, the following reactions occur at the cathode, and a chromate film is formed by the combined action of these reactions. ^{Cr 6+ + 3e - → Cr 3+} (1) 2H + + 2e - → H 2 (2) Cr 3+ +3 (OH) - → Cr (OH) 3 (3) 2Cr (OH) 3 + Cr 2 _{^{O 7 2- + 2H + → Cr}} (OH) 3 Cr (OH) CrO 4 + 2H 2 O (4) Cr 6+ + 6e - → Cr 3+ + 3e - → Cr 0 (5)

In order to add further functionality, the above-mentioned 3
In the prior art disclosed in the two publications, silica is added to the film. However, the conventional electrolytic chromate-treated zinc-based plated steel sheet thus formed has a problem in that it is slightly improved in corrosion resistance but inferior in blackening resistance. According to the study of the present inventors, it has been found that the cause of the blackening resistance is due to the metallic chromium present in the chromate film. That is, in the conventional electrolytic chromate treatment, metallic chromium is deposited during cathodic electrolysis according to the above formula (5). Since the metal chrome layer is very hard, a crack is formed, and an electrochemical microcell is formed between the zinc plating layer and the metal chrome exposed at the crack portion, which causes black rust. In the present invention, in order to suppress the black discoloration due to the occurrence of such black rust, it is an essential condition to remove metallic chromium from the chromate film.

Now, a method of detecting the presence or absence of metallic chromium in the chromate film will be described. The amount of metallic chromium in the present invention is measured by state analysis by X-ray photoelectron spectroscopy (ESCA). As a specific measurement example, using ESCA Lab5 manufactured by VG, sputtering ion species Ar, acceleration energy 3 keV, current density 400 nA / m ² , measurement area 3 mmφ, vacuum degree 3 to 4 ×.
The sample is sputtered under the condition of 10 ⁻⁸ until reaching the base metal, and 100 K, 1486.6 eV of Al Kα ray is irradiated for detection. The absence of metallic chromium in the chromate film in the present invention means that the sample of metallic chromium is not subjected to a penetration analysis under such conditions until the base metal is reached and a peak of metallic chromium is not detected. Usually, since the analytical limit is 1 atomic%, for example, if metallic chromium is not detected in the chromate film mainly composed of Cr (0H) ₃ Cr (OH) CrO ₄ and the amount of Cr is 50 mg / m ² , metallic chromium Is less than 1 mg / m ² even if present.

In the present invention, the constituent components of the chromate film are chromium oxide, chromium hydroxide, metallic nickel, nickel oxide and specific silica particles.

Of these, chromium oxide and chromium hydroxide
The compound is a component forming the basic skeleton in the chromate film.
The total amount of chromium in the chromate film is 5 to 2 in terms of Cr.
00 mg / m²And the amount of soluble Cr is 1 to 30w
It is preferably t%. Cr amount 5 mg / m²Is less than
Then the chromate film is not enough to cover the entire plating layer
Because the required corrosion resistance and fingerprint resistance cannot be obtained.
200 mg / m ²No more corrosion resistance for adhesion exceeding
It is uneconomical because it cannot be expected
This is because the fingerprint property and blackening resistance are impaired.

In the present invention, the soluble Cr content is the Cr weight per unit area eluted in boiling water when the sample is immersed in pure water boiled at atmospheric pressure for 30 minutes, and the unit area of the sample before immersion. It is defined as the percentage divided by the Cr deposit weight per unit. If the soluble component is less than 1 wt%, the so-called self-repairing action of the chromate film disappears, and the corrosion resistance is extremely deteriorated. If the soluble component exceeds 30 wt%, chromium elution becomes excessive, On the contrary, not only the corrosion resistance is impaired, but the environmental pollution due to hexavalent chromium becomes a problem.

In the present invention, the average particle diameter of silica in the chromate film is 3 to 100 nm, and the amount of silica is 100 to 100 nm.
It is preferably 1000 mg / m ² . Silica in the chromate film is necessary for improving corrosion resistance and imparting fingerprint resistance. High corrosion resistance is brought about by the silanol groups on the silica surface trapping Zn ions eluted during corrosion. Therefore, the high corrosion resistance improves as the surface area of silica increases, that is, as the particle size of silica decreases.

Therefore, in the present invention, the upper limit value of 100 nm is specified as the average particle diameter at the limit of obtaining good corrosion resistance. Zn that elutes when the average particle size exceeds 100 nm
The number of ion trap sites is small and the corrosion resistance is poor.
More preferably, the average particle size of 20 nm or less ensures good corrosion resistance. If the average particle size is less than 3 nm, silica aggregates and a stable chromate film cannot be formed. Therefore, the lower limit value is 3 nm. Also,
This is because when the amount of silica is less than 100 mg / m ² , the corrosion resistance is poor, and when it exceeds 1000 mg / m ² , the silica in the film is easily peeled off and the adhesion is poor.

The amount of nickel in the chromate film is 10 to 10.
It is preferably 500 mg / m ² . To obtain the electrolytic chromate film of the present invention, N in an electrolytic chromate bath is used.
i-ions are added. The Ni ions act as a carrier for uniformly co-depositing silica and chromium oxide in the chromate film. The metallic nickel and nickel oxide in the coating are coating constituents that are inevitably and inevitably incorporated into the coating. None of them contributes to the improvement of corrosion resistance and fingerprint resistance. When this amount is less than 10 mg / m ² , or when metallic nickel does not exist, it does not function as a carrier during electrolytic chromating, and the element distribution in the film becomes nonuniform. On the other hand, when it exceeds 500 mg / m ² , the appearance is bluish, and not only the original color tone of the chromate steel sheet cannot be obtained but also the blackening resistance is sometimes impaired.

The method for producing a chromate film thus formed is usually carried out by cathodic electrolysis in a vertical electrolytic cell. A vertical cell is desirable as the electrolytic cell.
This is to eliminate hydrogen gas generated during cathode electrolysis. Although the composition of the electrolytic bath is not necessarily limited, for example, chromic anhydride 30 g / l, nickel nitrate (hexahydrate) 20 g / l, colloidal silica sol 100 g / l having an average particle size of 7 nm, isopropyl alcohol 2 g
/ L, and the pH of an aqueous solution containing less than 0.05 g / l of sulfate ions contained as impurities in the bath was adjusted to pH 3.6 with sodium hydroxide, and electrolytic zinc was added to the bath heated to 50 ° C. The steel sheet of the present invention can be obtained by subjecting the plated steel sheet to cathodic electrolysis at 10 A / dm ² .

The pH of the electrolytic bath must be 3.0 or higher and the presence of trivalent chromium ions is required. Metallic chromium is deposited by the reduction reaction from hexavalent chromium represented by the above formula (5), and at this time consumes hydrogen ions as represented by the formula (2). 5)
It becomes possible to suppress the reaction of. Further, the presence of trivalent chromium ions in the bath promotes the expressions (3) and (4) and suppresses the precipitation of metallic Cr in the film. Further, it is desirable that the amount of sulfate ion contained as an impurity in the bath be less than 0.05 g / l. This is because the sulfate ions in the bath promote the precipitation of metal ions. The bath temperature in the bath is preferably in the range of normal temperature to 80 ° C.

[0028]

EXAMPLES The effects of the present invention will be specifically described below based on examples.

(Example 1) As a material, a plate thickness of 0.7 mm
The cold-rolled steel plate SPCC of was used. The electrogalvanization was performed in the following plating bath using a fluidized bed cell with platinum as a counter electrode. Current density of electrogalvanizing is 100A
/ Dm ² and the deposition amount was 20 g / m ² .

Plating bath: ZnSO ₄ · 7H ₂ O 430 g / l K ₂ SO ₄ 2 g / l Na ₂ SO ₄ 4 g / l pH 1.5 (adjusted with H ₂ SO ₄ ) Bath temperature 60 ° C., electrolytic degreasing → 5 % Sulfuric acid pickling → electroplating → water washing →
The order is drying → electrolytic chromate → washing → drying.

The corrosion resistance after the chromate treatment was evaluated by observing the appearance after 100 hours of a salt spray test (hereinafter SST, JIS Z2371 compliant). ◎: White rust less than 5% ○: White rust 5% or more and less than 30% △: White rust 30% or more and less than 80% ×: Red rust occurred

The resistance to blackening is measured by a wet test (50 ° C., 95% R
H). The appearance discoloration degree after the test for 240 hours was evaluated by visual observation. ◎: No discoloration ○: Minor discoloration △: Severe black discoloration ×: Full black discoloration

Fingerprint resistance is TC-180 manufactured by Nippon Denshoku Industries Co., Ltd.
0 After measuring L value, a value, and b value using SZ-Σ80 type, white petrolatum was applied, wiped with a waste cloth, L value, a value, and b value were measured again, and ΔE = √ as color difference ΔE
(L ² + Δa ² + Δb ² ) was calculated and evaluated. ⊚: ΔE <1.0 ○: 1.0 ≦ ΔE <2.0 Δ: 2.0 ≦ ΔE <4.0 x: ΔE ≧ 4.0 Table 1 shows the results.

The adhesion of the film was evaluated by visually observing the degree of discoloration of the tape after the cellophane tape was attached and the tape was peeled off. ◎: No discoloration ○: Fine discoloration △: Medium discoloration ×: Large discoloration

[0035]

[Table 1]

As is clear from Table 1, all the chromate-treated electrogalvanized steel sheets of the present invention exhibited good corrosion resistance, fingerprint resistance, blackening resistance, and film adhesion.

[0037]

As described in detail above, since the chromate-treated zinc-based plated steel sheet of the present invention is constituted as described above by the electrolytic chromate treatment method described above, it has excellent corrosion resistance, fingerprint resistance, and It exhibits blackening resistance and film adhesion, and has the effect of being widely used in home appliance chassis and motor covers.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Nobuo Totsuka, Inventor No. 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Division (72) Nobuyuki Mori-To Chuo-ku, Chiba, Chiba Kawasaki Town No. 1 Kawasaki Steel Corporation Technical Research Division

Claims (4)

[Claims] 1. A chromate-treated zinc-based plated steel sheet, wherein at least one side of a steel sheet is plated with zinc or a zinc-based material, and a chromate film is formed on the zinc- or zinc-based plated steel sheet. A chromate-treated zinc-based plated steel sheet excellent in corrosion resistance, fingerprint resistance, and blackening resistance, which is composed of nickel, nickel oxide, and silica particles, and has substantially no metallic chromium in the film. 2. The total amount of chromium in the chromate film is C
The chromate-treated galvanized steel sheet according to claim 1, which has an r-converted amount of 5 to 200 mg / m ² and a soluble Cr amount of 1 to 30 wt%. 3. The average particle size of silica in the chromate film is 3 to 100 nm, and the amount of silica is 100 to 10.
The chromate-treated steel sheet according to claim 1, wherein the chromate-treated steel sheet is 00 mg / m ² . 4. The total amount of nickel in the chromate film is
It is 10-500 mg / m < ² >, The chromate-treated steel plate of Claim 1 characterized by the above-mentioned.