JPH06306630A - Yellow powder generation preventing white chromate treating method excellent in corrosion resistance and coating adhesion and chromate treating solution - Google Patents

Abstract

PURPOSE:To provide a yellow powder generation preventing white chromate treating method excellent in corrosion resistance and coating adhesion and to provide a chromate treating soln. CONSTITUTION:A galvannealed steel sheet in which total >=0.1% of Pb, Sb and Sn in a plating layer is brought into reaction and coated with a chromate treating soln. prepd. by adding 4 to 180g/l colloidal silica (expressed in terms of SiO2), 3 to 15g/l phosphoric acid and 0.005 to 0.10 normal of one of >= two kinds among hydrochloric acid, nitric acid and sulfuric acid to partially reduced chromic acid in which the concn. is regulate to 2 to 30% expressed in terms of the Cr content and the reducing ratio Cr<3+>/(Cr<3+>+Cr<6+>) is regulated to 0.1 to 0.7, and drying is executed under heating to form a chromate film of 5 to 100mg/m<2> as the Cr content on the surface of the steel sheet. Thus, the effect that the chromate treated steel sheet with a white appearance in which the generation of yellow powder is securely prevented and furthermore excellent in bare corrosion resistance and coating adhesion or the like can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to home appliances, steel furniture,
The present invention relates to a chromate treatment method and a chromate treatment liquid which have a white appearance and are excellent in bare corrosion resistance, paint adhesion and yellow powder resistance when used for building materials and the like.

[0002]

2. Description of the Related Art Galvanized steel sheets used for home electric appliances, building materials, etc. are used in a bare or manufactured state, but in any case, long-term rust prevention is required. Therefore, chromate treatment is applied to these steel sheets for the purpose of improving bare corrosion resistance and coating adhesion. Chromate treatment is classified into coating chromate treatment, electrolytic chromate treatment, and reactive chromate treatment depending on the treatment method, but industrially, coating chromate treatment, which has low equipment cost and running cost, is predominant. In the coating chromate treatment, basically, a predetermined amount of a chromate solution is applied to a steel plate and then dried to form a chromate film in which Cr ⁶⁺ and Cr ³⁺ coexist on the steel plate surface. It is considered that Cr ⁶⁺ in the chromate film suppresses the oxidation of zinc by the self-repair effect and suppresses the generation of rust, and Cr ³⁺ which forms the chromate film forms a stitch structure together with oxygen, It is believed that by firmly bonding with the plating layer and also with the coating material applied on the chromate film, strong coating adhesion and coating corrosion resistance are exhibited.

However, when the chromate-treated steel sheet is used naked, the appearance is particularly important. Conventional chromate-treated steel sheets have a yellowish appearance color tone due to the influence of Cr ⁶⁺ contained in the chromate film, but in recent years the chromate-treated steel sheets have a white appearance, no chromate coating unevenness, and pre-processing finish. It is required that no dusty yellow powder (yellow powder) is formed on the surface of the steel sheet even during long-term stacking at the time of hanging or pre-painting, especially during rainy season in spring or long rain in autumn. In order to improve the yellow powder resistance and coating adhesion of the chromate film, for example, a method of improving coating adhesion by adding colloidal silica and phosphoric acid to partially reduced chromic acid (JP-A-62-2).
No. 70781) and a method of improving yellowing powder resistance by adding colloidal silica, phosphoric acid and fluorine ions to partially reduced chromic acid (JP-A-4-27297). By the method of
It is possible to improve the corrosion resistance, paint adhesion, and yellow powder resistance to some extent.

[0004]

However, the above-mentioned conventional method cannot completely improve the deterioration of coating adhesion due to the addition of phosphoric acid, and when a treatment liquid containing fluorine ions is used, However, since silica in the liquid easily gels, long-term use is impossible. Therefore, it is required to provide an inexpensive chromate treatment method and a chromate treatment liquid which have excellent corrosion resistance, coating adhesion, and yellow powder resistance in zinc-based plated steel sheets and have a white appearance.

[0005]

The present invention has been made to advantageously solve the above-mentioned problems, and its features are: (1) The total amount of Pb, Sb, and Sn in the plating layer is 0.1%.
In the hot-dip galvanized steel sheet contained above, the concentration of Cr is 2 to 30 g / l, and the reduction ratio is Cr ³⁺ / (Cr ³⁺ + C
r ⁶⁺ ) is 0.1 to 0.7 in partially reduced chromic acid, colloidal silica is ₄ to 180 g / l in terms of SiO ₂ , phosphoric acid is 3 to 15 g / l, and hydrochloric acid, nitric acid or sulfuric acid is used as a strong acid. Any one kind or two or more kinds in total of 0.005
A chromate treatment solution prepared by adding 0.10 N was applied by contact, dried by heating, and the amount of Cr was 5 to 5 on the surface of the steel sheet.
A yellow powder-resistant white chromate treatment method having excellent corrosion resistance and coating adhesion, which is characterized by forming a 100 mg / m ² chromate film.

(2) The concentration is 2 to 30 g / Cr as the amount of Cr.
1, and the reduction ratio Cr ³⁺ / (Cr ³⁺ + Cr ⁶⁺ ) is 0.1 to 0.
Partially reduced chromic acid of No. 7 and SiO ₂ with colloidal silica
4 to 180 g / l in conversion, phosphoric acid to 3 to 15 g / l, and corrosion resistance and coating by adding 0.005 to 0.10 N in total as hydrochloric acid, nitric acid, or sulfuric acid as a strong acid. A yellow powder resistant white chromate treatment liquid with excellent adhesion.

[0007]

The present inventors have studied to promote the reduction reaction of Cr ⁶⁺ to Cr ³⁺ during the chromate treatment in order to whiten the appearance of the chromate-treated steel sheet. By using partially reduced chromic acid which has already been partially reduced as chromic acid, the amount of Cr ⁶⁺ that must be reduced to Cr ³⁺ in the reduction reaction was reduced and the amount of reduction reaction was reduced. However, since a white appearance cannot be obtained by chromate treatment with only partially reduced chromic acid, various etching agents for promoting the reduction reaction of Cr ⁶⁺ were investigated. As a result, the present inventors have found that the addition of phosphoric acid provides a good white appearance and bare corrosion resistance. The appearance whitening mechanism is as follows: Part of the phosphoric acid added to the partially reduced chromic acid dissociates protons, resulting in a reduction reaction of Cr ⁶⁺ (2CrO ₄ ^2- + 3Z
n + 16H ⁺ → 2Cr ³⁺ + 3Zn ²⁺ + 8H ₂ O) is supplied with necessary H ⁺ to accelerate the reduction reaction,
It is considered that the appearance is whitened by reducing the colored Cr ⁶⁺ to colorless Cr ³⁺ .

As a mechanism for improving the bare corrosion resistance,
Cr ³⁺ produced by the reduction reaction of the phosphate ions involved in the reaction
It is considered that these form salts with Zn and Zn ²⁺ and remain in the chromate film, and these salts are colorless and strongly bond with the chromium chromate film to increase the film strength, thereby improving bare corrosion resistance. The above-mentioned measures are also effective in suppressing the generation of yellow powder. The present inventors have found that the yellow powder undergoes drastic changes in temperature and humidity, and that it occurs when dew condensation water is easily formed on the steel sheet surface, and microscopically, the yellow powder is in the grain boundary pits of the hot dip coating or It was discovered that the dendritic growth pattern was generated in the recess. From this, yellow powder is dissolved in dew condensation water with unreacted chromate remaining in the grain boundary pits or in the recesses of the dendrite growth pattern, and reacts with Zn to form reactive corrosion products containing Cr ^6+. It is considered that the product formed a substance and was blown out from the pit or the concave portion during the drying.

Therefore, the inventors of the present invention conducted a reproduction test of treating a plated steel sheet by chromic acid melting with various amounts of phosphoric acid added, and as a result, if 3 g / l or more phosphoric acid was added, the generation of yellow powder was completely completed. It was found that it can be suppressed. As a mechanism for suppressing the yellow powder, the addition of phosphoric acid increases the etching ability of the zinc-based plated steel sheet treatment, and the etching form is changed from local etching to form pits or recesses to full-scale etching, thereby eliminating pits or recesses and unreacted chromium. It is considered that the acid could not locally remain. Thus, the addition of phosphoric acid can suppress the generation of yellow powder and whiten the appearance, but phosphoric acid is a weak acid and has a low degree of proton dissociation. It is difficult to consume it, and unreacted phosphoric acid remains in the chromate film. This residual phosphoric acid deteriorates the film strength and corrosion resistance due to its hygroscopicity, and reduces the coating adhesion. In order to obtain good corrosion resistance and paint adhesion, when phosphoric acid is added,
It is important to completely consume phosphoric acid in the chromium reduction reaction by heating and drying during the chromate treatment.

Furthermore, the present inventors have found that the oxide film on the plated steel sheet is greatly involved as a factor that inhibits the reduction reaction accompanied by the consumption of phosphoric acid. That is, in the case of a plated steel sheet that has been degreased with an alkali to remove the oxide film in advance as a chromate-treated original plate, the chromium fixation rate is improved, and the bare corrosion resistance and coating adhesion are also improved, even under the same chromate treatment conditions, as compared to the undegreased material. all right. In order to mass-produce chromate-treated plated steel sheets at low cost, it is difficult to remove the oxide film from the plated steel sheet before chromate treatment.Therefore, during the chromate treatment, this oxide film is destroyed and removed, and etching with phosphoric acid is promptly performed. It is necessary to add a reaction accelerating aid to the chromate treatment solution to accelerate the reaction, and by adding a strong acid, a high chromium fixing rate can be obtained even immediately after chromate treatment, and the added phosphoric acid is completely consumed. It was found that yellowing resistance, white appearance, and chromate treatment with excellent bare corrosion resistance and coating adhesion can be performed.

This is because the strong acid added to the chromate treatment solution dissociates to release protons, and these protons destroy the oxide film on the plated steel sheet during the chromate treatment, allowing the phosphoric acid to rapidly perform the etching reaction on the steel sheet. This is because it made it possible. In this way, the addition of phosphoric acid suppresses yellow powder,
It was confirmed that by performing whitening and further adding a strong acid, performances excellent in bare corrosion resistance and paint adhesion can be secured without impairing the yellow powder resistance and white appearance. Further, in order to improve the corrosion resistance, addition of colloidal silica (silica sol) is effective. However,
Among strong acids, those containing fluoride ions impair the stability of the solution by decomposing and gelling the colloidal silica coexisting in the chromate treatment solution, so strong acids include one or two of hydrochloric acid, nitric acid, and sulfuric acid. It is necessary to be above.
It should be noted that even if a strong acid is added, it is difficult to whiten the appearance of the steel sheet with a chromate treatment solution without addition of phosphoric acid, but it is essential to add phosphoric acid to the whitening of the appearance.

Examples of the hot-dip galvanized steel sheet used in the present invention include hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet. Hot-dip galvanized steel sheets are usually added with Pb and Sb as spangle forming elements and Sn to prevent blackening, and the above object cannot be achieved unless 0.1% or more is added in total. On the other hand, these elements segregate at the zinc crystal grain boundaries during the solidification process after hot dipping. The noble metals Pb, Sb, Sn, etc. form a local battery with Zn, which promotes the reduction of residual Cr ⁶⁺ by Zn during storage after chromate treatment. It was found that the corrosion product turned into a yellow powder. That is, the generation of yellow powder becomes remarkable when the total amount of Pb, Sb, and Sn is 0.1% or more. Since the present invention exerts effects on the properties relating to the formation of the chromate film, such as white appearance, bare corrosion resistance, and paint adhesion, it can be applied regardless of the zinc-based coating weight. For example, hot-dip galvanized spangle materials, minimum or zero spangle materials, and alloyed hot-dip galvanized steel sheets have segregation of the above-mentioned noble metals and generate yellow powder, and are effective when applied to these.

The reduction ratio of the partially reduced chromate solution used is in the range of 0.1 to 0.7, preferably 0.3 to 0.
It is 5. When the reduction ratio is less than 0.1, Cr on the treated steel sheet
The reduction reaction does not proceed sufficiently and a sufficient white appearance cannot be obtained.
On the other hand, if it exceeds 0.7, the amount of Cr ⁶⁺ in the chromate film decreases, and even if colloidal silica is added, the corrosion resistance of the chromate-treated steel sheet is not sufficient, and the stability of the chromate solution itself deteriorates. Partially reduced chromic acid can be obtained by reducing chromic anhydride with various organic reducing agents (starch, methanol, sucrose, glycol, ethanol, etc.) or inorganic reducing agents (oxalic acid, hydrogen peroxide, hypophosphorous acid, etc.) .

The concentration of the partially reduced chromate solution varies depending on the coating method, but the Cr concentration in the solution is 2 to 30 as the amount of Cr.
Adjust to g / l. If it is less than 2 g / l, a sufficient amount of deposited Cr cannot be obtained and corrosion resistance cannot be ensured. Three
If it exceeds 0 g / l, it is difficult to secure an appropriate Cr adhesion amount described later,
It is difficult to obtain a uniform reaction film. In addition, the stability of the chromate solution itself also decreases. 5-10 as the Cr deposition amount
0 mg / m ^2, preferably it is preferable to manage the range of 20 to 60 mg / m ^2. If it is less than 5 mg / m ² , sufficient corrosion resistance cannot be secured, and if it exceeds 100 mg / m ² , unreacted C
Since the amount of r ⁶⁺ increases and the fixing rate decreases, the coating adhesion decreases.

Addition of phosphoric acid enhances the etching ability of the treated steel sheet, changes the etching form from local etching when not added to full-scale etching, helps prevent the formation of pits or recesses that cause the generation of yellow powder, and Cr ^{6 +} reduction reaction _{^{(CrO 4 2- + 8H + +}} 3e - → Cr 3+ + 4H
By contributing to the proton supply in ₂ O), the fixing rate is improved and the appearance is whitened. The appropriate addition amount is preferably controlled in the range of 3 to 15 g / l with respect to the concentration of the reduced chromate solution. If it is less than 3 g / l, the Cr ⁶⁺ reduction reaction on the surface of the steel sheet is delayed, and as a result, etching becomes insufficient, the generation of yellow powder cannot be suppressed, and the appearance cannot be whitened. If it exceeds 15 g / l, the reduction reaction of chromic acid accompanying the etching reaction proceeds too much, and C in the obtained chromate film is increased.
Even if r ⁶⁺ is too small and colloidal silica is added, sufficient corrosion resistance cannot be secured.

Colloidal silica strengthens the chromate film by virtue of its polar silanol groups and contributes to the improvement of corrosion resistance. It also improves the adhesion to the coating film. The silica fine particles may be in the form of particles, rods, or the like, but the particle size or minor axis is preferably several μm to several tens μm.
The amount of addition needs to be 4 to 180 g / l in terms of SiO ₂ . Desirably, it is 15 to 60 g / l.
If it is less than 4 g / l, it is difficult to secure excellent bare corrosion resistance.
If it exceeds 80 g / l, the long-term stability of the liquid cannot be ensured, and the coating adhesion also deteriorates.

The total amount of strong acids (one or more of hydrochloric acid, nitric acid and sulfuric acid) added is 0.005 to 0.10.
Need to be regulated. Below 0.005 regulation,
It is difficult to secure excellent coating adhesion, and if it exceeds 0.10 normality, the reduction reaction of chromic acid accompanying the etching reaction will proceed too much,
Cr ⁶⁺ in the obtained chromate film is too small, and sufficient corrosion resistance cannot be secured. There is no problem in adding a resin to the treatment liquid to further improve the corrosion resistance.

[0018]

EXAMPLES Examples of the present invention will be described below. A minimum spangle hot-dip galvanized steel sheet is used as the hot-dip galvanized steel sheet, and the chromate treatment is carried out by spray-applying the chromate solution shown in Tables 1 to 3 and then the amount of adhesion is adjusted with a roll squeeze. Dry to 0 ° C. As chromic acid, those partially reduced with hydrogen peroxide and chromic anhydride were used. Commercially available products were used for phosphoric acid, colloidal silica, hydrochloric acid, nitric acid, and sulfuric acid. The total Cr deposition amount (mg / m ² ) of the treated steel sheet was measured by fluorescent X-ray. The following experiments were carried out on the various treated steel sheets obtained as described above.

The results are summarized in Tables 1 to 3. As is clear from the table, the Cr concentration of the present invention is 2 to 30 g.
/ L, partially reduced chromic acid having a reduction ratio of 0.2 to 0.7, colloidal silica in the range of 4 to 180 g / l in terms of SiO ₂ , phosphoric acid in the range of 3 to 15 g / l, and further 0.005 to 0.10. The hot-dip galvanized steel sheet treated with a chromate solution containing a specified strong acid (either one of hydrochloric acid, nitric acid, or sulfuric acid in a total amount of two or more) exhibits excellent yellow powder resistance and white appearance, and It can be seen that it has excellent bare corrosion resistance and paint adhesion.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

1) Bare corrosion resistance End face and back face sealing was performed, and salt spray test (JIS Z
2371) was carried out and the white rust generation rate after 72 hours was evaluated, and less than 5% was judged to be good. 2) Coating adhesion Kansai Paint's Aramic # 1000 dried film thickness 25μ
2mm after applying as target and baking at 120 ℃ for 20 minutes
100 cross-cuts that reach the square base steel sheet were drawn and evaluated by the number of peeled coating films when peeled off with a cellophane tape, and a peeled number of 3 or less was judged to be good.

3) Appearance color tone measurement X, Y and Z were measured with a commercially available color difference meter, and the yellowness (YI
The value) was calculated by the following formula. YI = 100 × (1.28 × X
-1.06xZ) / Y. The smaller the YI value, the less yellowish the image was, and the YI value of 0 or less was judged to be white.

4) Situation of yellow powder generation The evaluation surfaces are superposed on each other, and the temperature is 40 ° C. and the humidity is 60 Wet.
%, 4hr dry environment, temperature 25 ℃, humidity 95Wet
%, A stack test (torque pressure 70 kgf × cm ² ) was repeated for 7 days in a humid environment of 4 hr. Four-stage visual evaluation of the yellow powder generation after unpacking (4: No yellow powder generation, 3:
A part of yellow powder was generated (small degree), 2: Whole surface yellow powder was generated (small degree), 1: Whole surface yellow powder was generated (large degree), and a score of 4 was judged to be good. 5) The composition of the plating layer is
It consists of n.

Claims (2)

[Claims] 1. A hot-dip galvanized steel sheet containing 0.1% or more of the total of Pb, Sb and Sn in the plating layer has a Cr content of 2 to 30 g / l and a reduction ratio of Cr ³⁺ / (Cr. ³⁺
+ Cr ⁶⁺ ) in a partially reduced chromic acid of 0.1 to 0.7, colloidal silica in an amount of 4 to 180 g / l in terms of SiO ₂ , phosphoric acid in an amount of 3 to 15 g / l, hydrochloric acid, nitric acid as a strong acid,
A total of 0.005 of any one or more of sulfuric acid
Approximately 0.10 N of the chromate treatment liquid added and mixed is applied by contact, dried by heating, and the amount of Cr is 5 on the surface of the steel sheet.
A yellow powder-resistant white chromate treatment method excellent in corrosion resistance and coating adhesion, which comprises forming a chromate film of up to 100 mg / m ² . 2. The concentration of Cr is 2 to 30 g / l,
Element ratio Cr³⁺/ (Cr ³⁺+ Cr⁶⁺) Is 0.1 to 0.7
Partially reduced chromic acid, SiO 2 with colloidal silica₂In conversion
4-180g / l, phosphoric acid 3-15g / l, stronger
Any one or more of hydrochloric acid, nitric acid, and sulfuric acid as the acid
Corrosion resistance added 0.005 to 0.10 N in total above
Yellow powder resistant white chromate treatment liquid with excellent paint adhesion.