JPH0192379A - Highly corrosion-resistant chromate treatment of aluminized steel sheet - Google Patents

Abstract

PURPOSE:To form a chromate film improved in the elution of Cr<6+> and the peeling of film by using an aqueous solution of chromic acid containing Cr<6+>, total Cr, Si, and P in specific ratios and successively carrying out the plating treatment and drying by heating of a steel sheet under respectively specified conditions. CONSTITUTION:An aqueous solution of chromic acid containing silica sol and phosphoric acid and satisfying Cr<3+>/(total Cr)=0.4-0.6, Si/Cr<3+>=0.5-5.0, and further P/(total Cr)=0.02-0.50 is prepared. Subsequently, this aqueous solution is applied to an alminized steel sheet so that coating weight is regulated to 20-250mg/m<2>, which is dried by heating to 150-250 deg.C sheet temp. By this method, highly corrosion-resistant chromate treatment improve in the elution of Cr<6+> and the peeling of film can be applied to the Al-plated steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a highly corrosion-resistant chromate treatment method for forming a chromate film on an aluminum plated steel sheet from which hexavalent chromium is difficult to elute.

(Conventional technology) Conventional home appliances, building materials, 8! ! In fields such as appliances, hot-dip aluminized steel sheets are inexpensive, have excellent heat resistance and corrosion resistance, and are
Because of its beautiful appearance, it is used in large quantities. In addition, a method for easily electroplating high-purity aluminum on steel sheets using a non-aqueous solution (for example, a molten salt bath of aluminum helodenide and fulkylpyrinonium helodenide) has also been developed, and its performance has also attracted attention. There is. Regarding these aluminum-plated steel sheets, in recent years, importance has been placed on corrosion resistance in corrosive environments, so those with thick amate coatings have become widely used.
Conventionally, the formation of this thick chromate skin was achieved by immersing an aluminum-plated steel sheet in a chromate treatment solution made by adding sulfuric acid, etc. to a highly concentrated chromate aqueous solution, and changing the treatment time and temperature to achieve the target chromium adhesion. This was done by drying without washing with water.

(Problems to be Solved by the Invention) However, since the chromate film formed by these methods contains a large amount of hexavalent chromium, the hexavalent chromium easily dissolves and the film easily absorbs moisture. Therefore, the film was easily peeled off. In particular, the peeling phenomenon of the film due to moisture absorption was more pronounced as the amount of the film increased.

Therefore, the present invention aims to improve the elution of hexavalent chromium and the peeling of the coating in a highly corrosion-resistant chromate coating on an aluminum-plated steel sheet.

(Means for Solving the Problems) The present invention includes silica sol and phosphoric acid, and the molar ratio of trivalent chromium to total chromium is trivalent chromium/total chromium = 0.4 to 0.
．． 6, and the molar ratio of trivalent chromium to Si in the silica sol is Si/Cr'◆=0.5 to 5.0, and the molar ratio of total chromium to P in phosphoric acid is P/total Kurumuro 00
A chromic acid aqueous solution with a concentration of 2 to 0.50 is applied to an aluminum plated steel plate at a chromium deposition amount of 20 to 250 IIIg/import 2
Apply and maintain the plate temperature at 150 to 250℃ without washing with water.
By heating and drying, it was possible to form a chromate film with less elution of hexavalent chromium and less peeling of the film.

The reason why the molar ratio of trivalent chromium to total chromium was set to trivalent chromium/total chromium = 0.4 to 0.6 is that if it is less than 0.4, the curing speed of the chromate film will be slow; Elution of hexavalent chromium increases. Furthermore, the wettability of the aqueous solution to the steel plate also deteriorates, making it difficult to form a uniform film. on the other hand,
If it exceeds 0.6, there will be no significant difference from the conventional chromate film, and it will condense with trivalent chromium sources (chromium hydroxide, chromium phosphate, etc.) contained in the aqueous solution, impairing the stability of the aqueous solution. .

The reason why silica sol is blended into the aqueous solution is to improve the wettability of the chromic acid aqueous solution to the steel plate. That is, in general, when the ratio of trivalent chromium to hexavalent chromium increases, the i-leaving concentration of chromic acid decreases, the etching effect on the aluminum plating layer decreases, and it becomes difficult to form a uniform film. However, when silica sol is blended, the silica/-ol groups react with aluminum and etch the surface of the plating layer, resulting in improved wettability and the formation of a uniform film. Further, when the silica sol-containing chromate film is later dried at a temperature of 150 to 250°C, the hydroxyl groups of the silica sol react with trivalent chromium to form a strong film.

Therefore, the amount of silica sol blended is limited in relation to trivalent chromium.

The amount of silica sol blended is Si exchanged with trivalent chromium so that the molar ratio is Si/Cr" = 0.5 to 5.0. This is because the molar ratio of Si is 0, If it is less than 5, there will be little effect of improving coating unevenness and corrosion resistance;
Even if the value is greater than 0, the effect of preventing uneven coating is the same as in the case of 5.0, but since silica sol has the property of being easily dispersed in water, the chromate film becomes easier to dissolve.
This is because it reduces corrosion resistance.

In addition, the addition of phosphoric acid involves etching the aluminum plating surface with phosphoric acid to form aluminum phosphate, and forming a composite of trivalent chromium and chromium phosphate to coat the aluminum plated steel sheet with chromate film. This is to make it stick to the surface.

The blended amount of phosphoric acid is P-exchanged, relative to the total chromium,
If the molar ratio of P/(total chromium) is 0.02 to 0.50, if the molar ratio is less than 0.02, the etching effect will be weakened, the adhesion of the film will be reduced, and trivalent chromium Since the condensation reaction also becomes insufficient, it becomes difficult to form a film with excellent corrosion resistance. On the other hand, when it exceeds 0.50, the film v! Although the adhesion is good, the etching effect is significantly increased, so if the aqueous solution is used in an immersion or circulation method, the solution will contain a large amount of aluminum phosphate.
The composition of the aqueous solution changes.

The application of chromium water-soluble a to aluminum plated 7gA plate is as follows:
Known methods such as the roll squeeze method, Chemi-Zuter method, Roll-Zooter method, air knife method, dipping method, and spray method may be used as appropriate, but the amount of coating should be 20
~250 mg7m2.

If it is less than 20mg/2, corrosion resistance is insufficient,
250 B7m2 (If the film is thicker, it will easily peel off during processing, and it will be colored yellow, impairing its appearance.

After application, it is heated and dried without washing with water, and drying is carried out at a temperature of 150 to 250"C. If the temperature is lower than 150C, the chromate film will not be sufficiently soluble or moisture-absorbent. Corrosion resistance and film q4 release properties do not improve, while 2
Drying at a temperature higher than 50° C. for an extended period of time causes oxidation cranks to occur in the film, resulting in a decrease in chromium elution. Figure 1 shows the relationship between drying temperature and amount of chromium eluted.

When conventional chromate films are heated to about 70° C., microcracks occur, but in the case of the adhesive amate film of the present invention, there is no such problem even when heated to the above-mentioned temperature.

(Example) Silica sol 1. in anhydrous chromium aqueous solution. A chromate treatment solution having the composition shown in Table 1 was prepared by blending phosphoric acid and chromium phosphate, and an aluminum plated steel plate was treated with the solution, followed by drying without washing with water.

The performance of the Chromate film is shown in Table 1. The performance evaluation was as follows: (1) The appearance of the Chromate film was visually observed for presence or absence of unevenness in treatment.

No unevenness in O treatment Δ Slight unevenness in treatment

0 5mg/plow 2 or less 2 or less Δ 11-1
9ron g/m 206-10 mackerel g/Zoz X
20 mg/bell 2 or more (3) IT Eating property The state of white rust generation after 100 hours was evaluated based on the large standard in a salt spray test according to JIS Z 2371.

. O No white rust Δ Slight white rust x Within 20% of white rust area The amount was determined and evaluated based on the following criteria.

0 2 So g/-2 or less Δ 2 Theory g/So 2 or less, 1 0 B7m2 or less
More than 10 mg/2 ゛ (Effects of the Invention) As described above, according to the present invention, it is possible to form a chromate film with improved hexavalent chromium elution and film peeling.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the heat drying temperature after applying the chromic acid aqueous solution of the present invention and the amount of chromium eluted from the film.

Claims (1)

[Claims] Contains silica sol and phosphoric acid, the molar ratio of trivalent chromium to total chromium is trivalent chromium/total chromium = 0.4 to 0.6, and the molar ratio of trivalent chromium to Si in the silica sol is Si/
Cr^3^+ = 0.5 to 5.0, and furthermore, the molar ratio of total chromium to P in phosphoric acid is P/total chromium = 0.02 to 0.
An aluminum-plated steel plate characterized by applying a chromic acid aqueous solution of No. 50 to an aluminum-plated steel plate at a chromium adhesion amount of 20 to 250 mg/m^2, and heating and drying the plate at a temperature of 150 to 250°C without washing with water. Highly corrosion-resistant chromate treatment method.