JPH0637706B2 - High corrosion resistance chromate treatment method for galvanized steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention can be applied to a galvanized steel sheet without aging.
The present invention relates to a chromate treatment method capable of forming a chromate film in which valent chromium is less likely to be eluted and fingerprints are not attached.

(Prior Art) Galvanized steel sheets have been widely used in the fields of home appliances, building materials, equipment, etc. since they are inexpensive, have excellent corrosion resistance, and have a beautiful appearance. Particularly in recent years, corrosion resistance in a corrosive environment has been emphasized, and thick chromate coatings have been widely used.

Conventionally, this thick chromate film is formed by immersing a galvanized steel sheet in a chromate treatment solution in which sulfuric acid or the like is added to a high-concentration chromate solution, and changing the treatment time and temperature to achieve the target chromium deposition amount. Then, the method was followed by drying without washing with water.

(Problems to be Solved by the Invention) However, the chromate film formed by these methods has the following various problems.

(1) A large amount of water or alkali-soluble hexavalent chromium is contained, and it is easily dissolved during handling and molding. For example, when molding is performed, press oil or machine oil is washed after processing, and when coating is applied, further degreasing, water washing,
Although washing with hot water or the like is performed, in these cases, hexavalent chromium is eluted and corrosion resistance is reduced. Further, when chromium is eluted, the waste liquid used for cleaning and degreasing contains chromium, and therefore, the waste liquid must be treated for pollution.

(2) Since fingerprints are easily attached, they are attached during processing and subsequent handling, and the appearance is impaired.

(3) The film immediately after being formed is liable to elute by sweat and moisture due to handling, and is likely to fall off. Therefore, when the period from formation to processing is short, sufficient corrosion resistance is not exhibited. The chromate film immediately after formation generally has the property of being easily eluted by sweat or moisture, but when 3 to 4 months have elapsed after formation, the sol film becomes a gel film and the film does not easily fall off. This change is usually called aging, and although it depends on the type of chromate film composition, the loss of the chromate film is considerably improved.

However, in recent years, since the period from order receipt to delivery and processing has been considerably shortened, such an aging period cannot be provided. Also, when shipped in coils, the steel strip is wrapped tightly and the contact between the coating and the atmosphere is poor,
The progress of aging becomes extremely slow, and the film exfoliation property is not improved even if the aging period exists.

The present invention provides a chromate treatment method capable of forming a chromate film in which the elution of hexavalent chromium and fingerprint adhesion are improved, and the film is less likely to elute even without aging.

(Means for Solving Problems) The present invention contains SiO ₂ , PO ₄ ³⁻ and H ₃ BO ₃ and has a molar ratio of trivalent chromium to total chromium of trivalent chromium / total chromium = 0.40-0.
Chromic acid aqueous solution of 55 is applied to galvanized steel sheet in an amount of chromium adhesion of 30 to 120 mg / m ² and without washing with water,
The chromate film as described above was formed by heating and drying at 150 to 250 ° C. at the plate temperature.

The present invention was invented because the following phenomena were observed when trivalent chromium in a chromic acid aqueous solution had a high concentration.

(1) The film curing rate after chromate treatment is extremely high, and aging is unnecessary.

(2) Elution of hexavalent chromium is suppressed.

(3) It has liquid repellency that repels fingerprint liquid, making it difficult for fingerprints to adhere.

(4) The wettability of the aqueous solution with respect to the galvanized steel sheet is improved.

Here, the molar ratio of trivalent chromium to total chromium is defined as trivalent chromium /
It was less than 0.40 that the total chrome was 0.40 to 0.55.
If the effects of (1) to (4) above are insufficient, and if it exceeds 0.55, the performance of the chromate film itself will not be a problem, but it will condense with chromium hydroxide or chromium phosphate in the aqueous solution and stabilize the aqueous solution. The sex is lost.

Incorporating SiO ₂ into the aqueous solution improves the corrosion resistance, and when the molar ratio of trivalent chromium to total chromium is set as described above, the wettability of the aqueous solution with respect to the steel sheet decreases, so the wettability is improved. This is to make it better. That is, as shown in FIG. 1, when SiO ₂ is blended, the wettability of the chromic acid aqueous solution is improved and becomes better as the amount thereof is increased.

In general, SiO ₂ has a strong hygroscopic property as silica gel used as a desiccant. Therefore, when it is contained in the chromate film, it is considered that the chromate film has high hygroscopicity. However, 150-250 ℃
The hygroscopicity is suppressed by forced drying with. It is considered that this is because the hydroxyl group of SiO ₂ undergoes silanol condensation with trivalent chromium (chromium hydroxide) when heated to the above temperature in the drying step after applying the aqueous solution.

Therefore, it is necessary to determine the compounding amount of this SiO ₂ in consideration of the amount of trivalent chromium. In the case of the present invention, SiO ₂ is the number of moles of SiO ₂ in terms of Si, and Si / C in molar ratio
Make r ³⁺ = 1.0 to 4.0. This Si molar ratio is 1.0
If it is less than the above, as shown in FIG. 2, the change in color tone (Δb measured by a color difference meter) before and after the imprinting of the artificial fingerprint liquid (having the composition used in the examples) becomes large, and the fingerprint becomes conspicuous. On the other hand, when it exceeds 4.0, the fingerprint resistance is improved, but the coating property tends to be poor. Further, SiO ₂ is usually present in an aqueous solution as a silica sol, but it has a strong aggregation property, and when the Si molar ratio exceeds 4.0, it aggregates into a huge frog, which can be formed by a roll squeeze method, a chemicoater method, an air knife method, etc. When an aqueous solution is applied, it causes roping and speckled pattern and impairs the appearance. As the SiO ₂ , silica sol is usually used.

Further, to formulate PO ₄ ^3-, together to form a zinc phosphate by etching the galvanized surface with phosphoric acid, 3
This is because a chromate film is fixed to the surface of the galvanized steel sheet by forming a valent chromium and chromium phosphate-based condensate. Further, the inclusion of PO ₄ ^3-, coating organic material becomes large affinity with organic solvent, coating adhesion, as well as improved corrosion resistance after painting, chromate film of chromic acid Plain is yellow However, it becomes bluish and the yellow color is alleviated, making it less noticeable in appearance.

The amount of this PO ₄ ³⁻ compounded is preferably P / (total chromium) = 0.02 to 0.38 in terms of molar ratio with respect to the total chromium mole number in terms of P. If the molar ratio is less than 0.02, the compounding effect is poor, and if it exceeds 0.38, there is no problem with corrosion resistance, yellow discoloration, improvement of paintability, etc., but chromium elution from the film tends to occur, which may occur during cleaning or degreasing. Chromium is contained in the waste liquid, and fingerprints are also easily noticeable. The pH of the chromic acid aqueous solution is preferably 1.5 to 2.5.

As described above, SiO ₂ , PO ₄ ^3- was mixed with the chromic acid aqueous solution,
When the molar ratio of trivalent chromium to total chromium is adjusted as described above, elution of hexavalent chromium and fingerprint adhesion are improved, and the film is less likely to elute without aging.
When H ₃ BD ₃ is blended, the wettability with respect to the steel plate and the coating material can be further improved, and the elution of hexavalent chromium can be suppressed.

If the wettability of the aqueous solution to the steel sheet is not sufficient, when the chromate film is thickened, the entire surface of the steel sheet is not uniform, so a difference in film thickness occurs between the poorly wetted portion and the good portion, and Shading will occur. But H ₃ BD
_{When 3} is blended, as shown in FIG. 1, the wettability is improved, the color tone of the film is not generated, and the appearance is good.

Further, in the SiO ₂ -containing chromate film, since SiO ₂ has a cissing effect on the paint, when the amount of SiO _{2 is} increased, cissing occurs on the coating film depending on the paint. However, when H ₃ BD ₃ is blended, the wettability with respect to the paint is also improved, and the occurrence of cissing in the coating film is suppressed.

The reason why the wettability to the steel sheet is improved by the blending of H ₃ BD ₃ is considered to be that the silanol of SiO ₂ and the hydroxyl group of H ₃ BO ₃ are dehydrated and condensed, and the water repellency of SiO ₂ is lost. Also,
The cissing does not occur in the coating film, in addition to the dehydration condensation as described above, the dehydration condensation occurs between the silanol of SiO ₂ and the carboxyl group of the coating resin, and further the chromate film is formed.
It is considered that dehydration condensation also occurs between B-OH generated by crosslinking with H ₃ BO ₃ and an amino group such as a curing agent in the coating material.

Further, when H ₃ BO ₃ is blended, H ₃ BO ₃ becomes SiO ₂ when heated to 150 to 250 ° C. after applying the aqueous solution to the steel sheet,
Dehydrates and condenses with the chromium phosphate and the hydroxyl groups of the chromate film to vitrify and insolubilize the film. Therefore, even if washing or degreasing is performed, and even if aging is not performed, hexavalent chromium is not eluted or is insignificant.

The blending ratio of H ₃ BO ₃ is preferably B / (Si + P + total chromium) = 0.01 to 0.7 in terms of B equivalent mol number, based on the total molar number of Si, P and total chromium. The molar ratio of B is 0.01
If it is less than 1.0, improvement of wettability for steel sheet and paint and suppression of hexavalent chromium elution will be insufficient, and if it exceeds 0.7, condensation reaction will occur in the H ₃ BO ₃ molecule, resulting in SiO ₂ , chromium phosphate, and chromate film. The dehydration condensation with and becomes less, and the chromate film becomes a mixture film with a mere H ₃ BO ₃ condensate.

For the application of the aqueous chromium solution to the galvanized steel sheet, a known method such as a roll squeeze method, a chemi-coater method, a roll coater method, an air knife method, a dipping method, or a spray method may be appropriately used. ~ 120 mg / m
^Set to ² . If it is less than 30 mg / m ² , the corrosion resistance is insufficient, and if it is more than 120 mg / m ² , the film becomes thick, so it easily peels off during processing, and the color tone adheres to yellow as Δb exceeds 10 on a color difference meter, Although there is no elution of chromium, it gives the impression of elution.

After coating, it is dried by heating without washing with water, but the drying is performed by dehydration condensation with the hydroxyl group of SiO ₂ and chromium hydroxide to eliminate the hygroscopicity of SiO ₂ or H ₃ BO _{3 as described above.} And Si
In order to make the film vitrified insoluble by dehydration condensation with O ₂ , chromium phosphate, and hydroxyl groups of the chromate film, 15
It is carried out at a temperature of 0 to 250 ° C. As shown in the thermal analysis chart of FIG. 3, this temperature range must be set in order to cause an endothermic reaction (dehydration reaction). Further, as shown in FIG. 4, the one dried in the above temperature range has less elution of chromium. If the coating is dried at a temperature higher than 250 ° C. for a long time, oxidation cracks will occur in the film, and the chromium elution resistance and fingerprint resistance will be reduced.

When the conventional chromate film is heated to about 70 ° C., microcracks are generated, but in the case of the chromate film of the present invention, such a problem does not occur even if it is heated to the above temperature.

(Example) Silica gel, phosphoric acid, or boric acid was added to an aqueous solution of chromic anhydride to prepare a chromate aqueous solution having the composition shown in Table 1, and the stability of the aqueous solution was investigated.
The galvanized steel sheet was processed and the performance of the chromate film was investigated. Table 2 shows the results.

The stability of the aqueous solution and the chromate film performance are investigated by
It went as follows.

(1) Stability of aqueous solution The aqueous solution was allowed to stand in a constant temperature water bath at 50 ° C, and the number of days until gelation was investigated.

○ No precipitate was generated for 15 days or more. △ Gelation within 10 to 14 days × Gelation within 7 days (2) Appearance of chromate film The presence or absence of uneven treatment was visually observed.

◎ No treatment unevenness ○ Slightly streaky pattern (roping) △ Medium treatment unevenness × Significant treatment unevenness (3) Chromium elution When immersed in a 2% sodium orthosilicate aqueous solution (60 ° C) for 2 minutes and hot water (upper) The amount of elution when immersed in water (90 ° C.) for 3 minutes was measured.

◎ 5 mg / m ² or less ^{△ 11~19mg / m 2 ○ 6~10mg /} m 2 × 20mg / m 2 or more (4) anti-fingerprint artificial fingerprint liquid (sodium chloride 7 g /, urea 1 g /,
4 g of lactic acid / the remaining 1: 1 methanol) was adhered, and the change in color tone (Δb) before and after the adhesion was measured with a color difference meter.

◎ Δb 0.5 or less Δ Δb 1.1 to 2.0 ○ Δb 0.6 to 1.0 × Δb 2.1 or more (5) Corrosion resistance White rust occurrence at the time of 100 hours of salt spray test was evaluated according to the following criteria.

◎ White rust does not occur ○ Very slightly white rust occurs △ White rust occurrence area is within 10% × White rust occurrence area is 11% or more (6) Coating performance Acrylic resin paint [Superlac F50, manufactured by Nippon Paint Co., Ltd.] After coating, the coating appearance, coating adhesion and corrosion resistance were evaluated according to the following criteria.

(A) Appearance of coating ◎ Good △ Less cratering ○ Partially cratering × Many cratering (B) Cohesiveness of coating film After each test of the eyelet test, Erichsen 6 mm extrusion test and DuPont impact test, the test part is forcibly peeled off with a tape. Then
Evaluation was made on a scale of 20 (no peeling of coating film).

◎ 20 points △ 10 to 14 points ○ 15 to 19 points × 9 points or less (C) Corrosion resistance Put a cut reaching the surface of the steel sheet on the coating film, and the maximum blister width on one side of the coating film at the cut part after 500 hours of salt spray test Was measured.

◎ Less than 1mm △ 2.5 to 5.0mm ○ 1.5 to 2.0mm × 5.1mm or more (Effects of the Invention) As described above, according to the present invention, the elution of hexavalent chromium and the fingerprint adhesion are improved, and a chromate film that is difficult to elute without aging can be formed.

[Brief description of drawings]

Fig. 1 shows the case where SiO ₂ was added to the chromic acid aqueous solution, and
When blended with O ₂ and H ₃ BO ₃ is a graph showing the wettability of the relationship between the aqueous solution for the amount and the hot dip plated steel sheet of the.
FIG. 2 is a graph showing the relationship between the blending amount and the change in color tone when SiO ₂ is blended in the chromic acid aqueous solution. Figure 3 shows
It is a thermal-analysis graph at the time of heat-drying the chromic acid aqueous solution coating film of this invention. FIG. 4 is a graph showing the relationship between the heating and drying temperature of the chromic acid aqueous solution coating film of the present invention and the amount of chromium eluted from the film.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiromi Masuhara, 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture, Nisshin Steel Co., Ltd. (72) Inventor, Junji Ikeda, 5th Ishizu Nishimachi, Sakai City, Osaka Nisshin Steel Co., Ltd. Inside the Hanshin Works (72) Inventor Yasutaka Kawaguchi, 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Inside the Hanshin Works, Nisshin Steel Co., Ltd. (56) Reference JP-A-60-218483 (JP, A)

Claims (2)

[Claims] 1. A composition comprising SiO ₂ , PO ₄ ³⁻ and H ₃ BO ₃ in which the molar ratio of trivalent chromium to total chromium is trivalent chromium / total chromium = 0.4.
Zinc characterized by applying a chromic acid aqueous solution of 0.55 to 0.55 to a galvanized steel sheet at a chromium deposition amount of 30 to 120 mg / m ² and heating and drying at a plate temperature of 150 to 250 ° C. without washing with water. Highly corrosion resistant chromate treatment method for plated steel sheet. 2. The amount of H ₃ BO _{3 is} the number of moles in terms of B, and Si and PO of SiO _2
4 ^3- in molar ratio to the total number of moles of P and total chromium
B / (Si + P + total chromium) = 0.01-0.7, The high corrosion-resistant chromate treatment method of the galvanized steel plate of Claim 1 characterized by the above-mentioned.