JPH01129978A - Production of hot-dip plated steel sheet having excellent resistance to blackening and corrosion - Google Patents

Abstract

PURPOSE:To develop the title Zn-Al alloy hot-dip plated steel sheet with less elution of hexavalent Cr and having excellent resistance to blackening and corrosion by spraying the aq. soln. of a specified metal salt onto the surface of a Zn-Al alloy hot-dip plated steel sheet, and further chromating the surface. CONSTITUTION:The aq. soln. of a salt such as the nitrate and oxide of Co or Fe is sprayed onto the surface of the steel sheet hot-dip plated with a Zn-Al alloy consisting, by weight, of 4-53% Al, 0-0.5% Mg, 0-10% Si, 0.002-0.3% Pb, and the balance Zn while the plating surface is still kept at a high temp. of >=170 deg.C, and thermally decomposed by the temp. of the steel sheet to form the film of the oxides of Co, Fe, etc. A chromate film is formed at the Cr deposition amt. of 30-120mg/m<2> by the processing soln. contg. SiO2 as the silica sol in 1-4 molar ratio of Si/Cr<3+>, expressed in terms of Si, and PO in 0.02-0.38 molar ratio of P/total Cr, expressed in terms of P, and wherein the molar ratio of Cr<3+>/total Cr is controlled to 0.40-0.60. The film is not washed with water but heated at 150-250 deg.C, and dried.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention involves hot-dipping a steel plate with a zinc-aluminum alloy, then spraying an atomized metal salt aqueous solution to form a metal oxide film by thermal decomposition. In the 91 method of forming a blackening-resistant hot-dip plated steel sheet, which is then subjected to amate treatment,
The present invention relates to a method for reducing the elution of hexavalent chromium from a chromate film.

(Prior art) In recent years, hot-dip zinc-aluminum alloy plated steel sheets have been used in the fields of home appliances, building materials, equipment, etc. because they are inexpensive, have excellent corrosion resistance, and
It is attracting attention because of its beautiful appearance.

This hot-dip galvanized steel sheet is usually ^1:4~53%, Ng
: 0-0.5%, Si: 0-10%, Pb: 0.00
It is manufactured by hot-dip plating in a plating bath consisting of 2 to 0.3% Zn and the balance being Zn and unavoidable impurities, but in order to improve corrosion resistance after plating, chromate treatment is performed during manufacturing.

However, if chromate treatment is applied directly to the plating layer, the surface will turn black during storage and the product will lose its value. For this reason, the applicant proposed that as a method to prevent this blackening in advance,
A method is provided for forming an oxide film of cobalt or iron or both before chromate treatment.

This method involves hot-dipping zinc-aluminum alloy onto a steel sheet, adjusting the coating weight, and then applying the coating layer to a temperature of 170°C or higher.
An aqueous solution containing cobalt salts or iron salts (e.g. nitrates, chlorides, etc.) or both at a metal ion concentration of 0.1 to 209/e, heated above ℃ and atomized on the surface of the plating layer, is 1 to 209/e in terms of metal. Spray 100 mg7m2,
This method forms an oxide film of cobalt, iron, or both on the surface of the plating layer by thermally decomposing salt.

According to this method, blackening can be prevented, but in recent years, corrosion resistance in corrosive environments has become important in the above field, and thick chromate coatings have become widely used as a ring. .

Conventionally, this thick chromate film was formed by immersing a hot-dip galvanized steel sheet in a chromate-F treatment solution made by adding sulfuric acid, etc. to a highly concentrated chromate aqueous solution, and varying the treatment time and temperature. This was done by a method of attaching 1 and then drying without washing with water.

(Problems to be solved by the invention) However, the chromate film formed by this method contains a large amount of water- or alkali-soluble hexavalent chromium, which is likely to be eluted during handling, molding, or after processing. There was a problem.

For example, when molding is performed, press oil and machine oil are cleaned after processing, and when painting is performed, further degreasing, water washing, hot water washing, etc. are performed, but in these cases, hexavalent chromium is eluted, Corrosion resistance will decrease. Furthermore, if chromium is eluted, the waste liquid used for cleaning and degreasing will contain chromium, so the waste liquid must be treated to prevent pollution. Furthermore, even after the molding process, if the parts assembled in the PIi device come into contact with condensed water or the human body, hexavalent chromium will be eluted, causing problems such as food pollution and environmental contamination.

Therefore, the present invention is intended to solve this problem of elution of hexavalent chromium.

(Means for solving the problem) The present invention includes 5iOz and PO,"-, and the molar ratio of trivalent chromium to total chromium is trivalent chromium/total chromium = 0.4.
By applying a chromic acid aqueous solution having a concentration of 0 to 0.60 to a galvanized l-steel plate at a chromium adhesion amount of 30 to 120 l1g/+62, and heating and drying it at a plate temperature of 150 to 250°C without washing with water. Elution of hexavalent chromium was prevented.

Here, the molar ratio of trivalent chromium and total chromium is trivalent chromium/
The reason for setting total chromium = 0.40 to 0.60 is that if it is less than 0.40, the curing speed of the film will be slow and hexavalent chromium will easily be eluted, and it will also be difficult to prevent cobalt and iron oxide films from forming. This is because wettability decreases, and if it exceeds 0.60, the performance of the chromate film itself will not be a problem, but 3
This is because it condenses with chromium hydroxide, chromium phosphate, etc. that are normally added as chromium agents, impairing the stability of the aqueous solution.

The reason why SiO□ is added to water'lBn is to improve the wettability and corrosion resistance of the oxide film. In other words, if the amount of trivalent chromium is increased, the ``i'' release acid concentration decreases, which weakens the etching effect and reduces the wettability of the aqueous solution to the oxide film. When blended, the wettability of the chromic acid aqueous solution improves, and becomes better as the amount increases.

In addition, when SiO
Condenses with valent chromium (chromium hydroxide) and silane to strengthen the chromate film.

Therefore, it is necessary to determine the blending amount of 5in2 by considering the amount of trivalent chromium and the amount of ROM. It is preferable to set the Si/Cr" ratio to 1.0-4.0. If the molar ratio of Si is less than 1.0, wettability and corrosion resistance will be insufficient, and 4.0 is less than 1.0. If SiO□ normally exists as a silica sol in aqueous solutions and is easy to bulge, it will aggregate into a huge 70g, and when an aqueous solution is applied, it will cause roving and spot patterns, which will spoil the appearance. .In addition, as this SiO□,
Usually, silica sol is used.

In addition, PO, is added by etching the surface of the oxide film with phosphoric acid to form cobalt phosphate and iron, and also by forming a condensate of trivalent chromium and chromium phosphate to form a chromate film. This is to fix the oxide film to the oxide film. In addition, when po4a- is contained, the film becomes organic matter,
It has greater affinity with organic solvents, improves paint film adhesion and post-painting corrosion resistance, and the chromate film made of only chromic acid has a bluish tinge, making it less noticeable in appearance. It disappears.

The blending amount of PO,''- is the number of moles converted to P, and the molar ratio is P/(total chromium) = 0.02 to the number of moles of total chromium.
It is preferable to set it to 0.38. If the molar ratio is less than 0.02, the blending effect will be poor, and if it exceeds 0.38, there will be no problems with corrosion resistance, yellow discoloration, improvement in paintability, etc., but chromium elution from the film will easily occur, making cleaning difficult. Chromium is contained in the waste liquid during degreasing.

For applying the chromium aqueous solution to the steel plate, any known method such as a roll squeeze method, a chemical coater method, a roll coater method, an air knife method, a dipping method, or a spray method may be used as appropriate.
The amount of coating is 30 to 120 tng/+a in terms of chromium deposition amount.
Make it 2. If it is less than 30 +sg/m', the corrosion resistance will be insufficient, and if it is more than 120 mg7m', yellowing and color unevenness will be noticeable and the appearance will be unfavorable.

After coating, it is heated and dried without washing with water, and as mentioned above, the drying is carried out at a temperature of 150 to 250° C. in order to cause dehydration condensation between the hydroxyl group of 5i02 and trivalent chromium. Second
As shown in the thermal analysis diagram in the figure, the temperature must be within this range in order to cause an endothermic reaction (dehydration reaction). Further, when dried in the above temperature range, as shown in the fjS3 diagram, less chromium is eluted. Note that if the film is dried for a long time at a temperature higher than 250° C., oxidation cracks will occur in the film and the chromium elution resistance will decrease.

Conventional chromate films cause microcracks when heated to about 70°C, but the chromate film of the present invention does not have such problems even when heated to the above temperatures.

(Example) After hot-dipping a steel plate in the Zn-^1 alloy plating bath shown in Table 1, the coating weight was adjusted and Co(NO
s)2・61120 wo59/e*L l:
An aqueous solution in which 109/e of kFe(NO,)2.911zO was dissolved was atomized and sprayed.

Thereafter, a chromic acid aqueous solution prepared by blending chromic acid anhydride with silica sol and phosphoric acid, and a conventional 7 chromate treatment solution were prepared, and the steel plate was treated to form a chromate film. Table 2 shows the results.

In addition, blackening resistance, chromium elution property, and corrosion resistance were investigated as follows.

(1) Resistance to blackening A test piece was immersed in deionized water for 5 hours, and the degree of blackening was evaluated using the following criteria.

◎ No blackening, almost inconspicuous blackening ∆ Blackening is slightly noticeable × Blackening is significant (2) Elution of chromium The elution amount when immersed in hot water (tap water, 90°C) for 3 minutes was measured.

05 mg7m2 or less Δ 11-19mg/m2
06~10 B/m2X 20 B/+” or more (3)
The occurrence of white rust after 120 hours of corrosion resistance salt spray test vk was evaluated based on the following criteria.

◎ White rust occurrence area less than 10% ○ White rust occurrence area 10 to 30% Δ White rust occurrence area 30 to 70% X White rust occurrence area 70% or more (effect of the invention) Even if the chromate film is made thicker, the elution of hexavalent chromium can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the amount of SiO□ added to an aqueous chromic acid solution and the wettability of the aqueous solution to an oxide film. FIG. 2 is a thermal analysis graph when the chromic acid aqueous solution coated film of the present invention is heated and dried. FIG. 3 is a graph showing the relationship between the heating drying temperature of the chromic acid aqueous solution coated film of the present invention and the amount of HA eluted from the film.

Claims (4)

[Claims] (1) After hot-dipping zinc-aluminum alloy on a steel sheet, adjust the coating amount, and then reduce the coating layer to 1 layer.
Cobalt or In a method for producing hot-dip galvanized steel sheets in which an oxide film of iron or both is formed on the surface of the plating layer and then chromate treatment is performed, the chromate treatment is performed on SiO_2 and PO_4^3^-
Contains trivalent chromium and total chromium molar ratio is trivalent chromium/
Using a chromic acid aqueous solution with a total chromium content of 0.40 to 0.60, apply chromium in an amount of 30 to 120 mg/m^2, and then coat the plate at a temperature of 150 to 25 without washing with water.
A method for producing a hot-dip galvanized steel sheet with excellent blackening resistance and corrosion resistance, characterized in that the coating is performed by heating and drying at 0°C. (2) SiO_2 is a silica sol, and the amount thereof is Si equivalent moles and the molar ratio is Si/Cr^3^+ = 1.0 to 4.0 with respect to the moles of trivalent chromium. A method for manufacturing a hot-dip galvanized steel sheet with excellent blackening resistance and corrosion resistance as set forth in claim 1. (3) The amount of PO_4^3^- is the number of moles converted to P, and the molar ratio is P/total chromium = 0.02 to the number of moles of total chromium.
0.38. The method for producing a hot-dip galvanized steel sheet with excellent blackening resistance and corrosion resistance according to claim 1. (4) Al: 4-53%, Mg: 0-0.5%, Si:
0-10%, Pb: 0.002-0.3%, balance Zn
The method for producing a hot-dip plated steel sheet with excellent blackening resistance and corrosion resistance according to claim 1, characterized in that the steel sheet is hot-dipped in a zinc-aluminum alloy plating bath that contains unavoidable impurities.