JPH02243772A - Chromating solution ensuring high corrosion resistance for galvanized steel sheet and chromating method - Google Patents

Abstract

PURPOSE:To prevent the leaching of hexavalent Cr and the leaving of fingerprints on a Zn film and to render lubricity by specifying the amt. of total Cr, the molar ratio of trivalent Cr to total Cr and the amts. of ZrF6, SiO2, PO4<3-> and org. resin in a chromating soln. CONSTITUTION:This chromating soln. ensuring high corrosion resistance for a galvanized steel sheet has a compsn. contg. 5-100g/l total Cr in 0.30-0.70 molar ratio of trivalent Cr to total Cr, 0.5-7.0g/l ZrF6 in 0.05-0.10 molar ratio of ZrF6 to total Cr, 10-200g/l SiO2, 0.5-5g/l PO4<3-> and 10-200g/l water-soluble or water- dispersible org. resin. This chromating soln. is applied to a galvanized steel sheet by 30-120mg/m<2> (expressed in terms of Cr) and the steel sheet is dried by heating to 150-250 deg.C without washing to form a chromate coating film.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention is designed to prevent the elution of hexavalent chromium and the adhesion of fingerprints.
This invention relates to a highly corrosion-resistant chromate treatment solution for galvanized steel sheets that forms a chromate film with lubricating properties, and a chromate treatment method using the treatment solution.

(Prior art) Galvanized steel sheets such as electrogalvanized steel sheets and vapor-deposited galvanized steel sheets have been used in large quantities in fields such as home appliances, building materials, and leisure equipment because they are inexpensive, have excellent corrosion resistance, and have a beautiful appearance. has been done. Particularly in recent years, corrosion resistance under corrosive environments has become important, and thick chromate coatings have come into widespread use.

Conventionally, to form this thick chromate film, a chromate treatment solution made by adding sulfuric acid, etc. to a highly concentrated chromate aqueous solution is used, and the galvanized island steel sheet is immersed in this treatment solution, and the treatment time, temperature, etc. This was done by adjusting the amount of chromium to reach the target amount, and then drying without rinsing with water.

When using galvanized steel sheets treated with this treatment liquid to process them into parts for the above products, apply lubricating oil before pressing etc., as the plating layer of electrogalvanized steel sheets and vapor-deposited galvanized steel sheets is soft. The method used was to remove the oil and then remove the fat.

(Problem to be solved by the invention, α) However, applying lubricating oil and degreasing are laborious operations, and hexavalent chromium is eluted during degreasing, making it necessary to dispose of waste fluid to prevent pollution. Ta. Furthermore, since fingerprints are likely to adhere to the film, if care is not taken when handling the film after degreasing, fingerprints will adhere thereto and impair the appearance.

One possible way to solve this problem is to form a thin organic resin film of 1 to 3 μm on top of the chromate film, but the addition of this step increases costs.
It also makes grounding and resistance welding difficult.

As described above, conventional thick chromate treatment liquids have had various problems, and the present invention provides a chromate treatment liquid that solves these problems and a chromate treatment method using the treatment liquid.

(Means for Solving the Problems) The present invention provides a chromate treatment solution having a total chromium of 5 to 1009
/e, molar ratio of trivalent chromium and total chromium is trivalent chromium/total chromium = 0.30-0.70, ZrFa 0.5-7
．． 09/e, ZrF7 total chromium = 0.05-0.1
0, Sif□10-2009/e, PO4”-0,5
-59/e Oyo V Water-soluble or water-dispersible W1 Resin 1
The treatment liquid contained 0 to 2009/e. As for the treatment with this treatment liquid, if the treatment liquid is applied to a galvanized steel plate at a chromium deposition amount of 30 to 120 B/m2 and then heated and dried at a plate temperature of 150 to 250°C without washing with water, it is possible to We have discovered that it is possible to form a chromate film that does not have the following problems.

The components of the treatment liquid will be explained below.

First, the total chromium is set to 5 to 1009/g, and the molar ratio of trivalent chromium to total chromium is set to trivalent chromium/total chromium = 0.
30 to 0.70, total chromium is 59/
It is difficult to form a thick chromate film if it is less than 10
This is because if it exceeds 09/e, the film becomes more likely to be colored. In addition, if the molar ratio of trivalent chromium to total chromium is less than 0.30, corrosion resistance will be insufficient, and if it exceeds 0.70, the film will be colored when the total chromium concentration is increased, and This is because it condenses with chromium hydroxide, chromium phosphate, etc., impairing the stability of the treatment liquid.

ZrFa is added to ensure firm adhesion of the film. Since ZrFs is an etching agent, it dissolves the zinc in the plating layer and firmly adheres the chromium in the chromate film. , *Zr in ZrF6 reacts with the resin component and precipitates and adheres to the plating layer, thereby strengthening the adhesion of the resin to the plating layer. If this etching agent is not included, a film of chromium-silica-phosphoric acid-resin compound will be formed, but a compound that binds the resin and the plating layer will not be formed.

Note that ZrF6 is used as an etching agent because
Due to the following reasons. That is, when an etching agent such as sulfuric acid or phosphoric acid is added to the chromate treatment solution, the etching action of the etching agent causes the chromium in the film to adhere quite firmly, but it does not have the effect of strongly adhering the chromate. For this reason, when a strong process such as a drawbead is applied, the film cannot withstand the process. In addition, when using a heptadide as an etching agent, conventionally, acids such as heptatonic acid, sodium heptate, etc. This is because these materials do not have the effect of firmly adhering the plating layer and the organic resin.

This ZrF composition is less than 0.591, and ZrF is less than 0.591.
If Fa/total chromium = less than 0.05, the film adhesion effect will be weak, and if it exceeds 7.5 to 7.0 g/l, and Zr
If Fs/total chromium=R exceeds 10, the plating layer will be excessively dissolved and the corrosion resistance of the plating layer will be impaired.

5i02 is blended to improve the wettability of the treatment liquid to the steel plate and to improve the fingerprint resistance of the film. When the molar ratio of trivalent chromium to total chromium is set as above, the wettability of the treatment liquid to the steel plate decreases, but when SiO□ is added, the wettability improves. On the other hand, when 5in2 is force-dried at 150 to 250°C, the hydroxide group of 5in2 undergoes sil/al condensation with trivalent chromium (chromium hydroxide), resulting in liquid repellency and making fingerprints less likely to adhere.

If the amount of SiO□ is less than 109/e, the effect of improving wettability and liquid repellency is small, and if it exceeds 2009/g,
It agglomerates into a huge 70-piece, and when a treatment liquid is applied using the roll squeeze method, chemical coater method, air knife method, etc., it causes roving and spot patterns, which impairs the appearance. This aggregation of SiO2 occurs because SiO2 exists as a silica sol in the treatment liquid and has a strong tendency to agglomerate. Note that silica sol is used for 5in2.

The formulation of PO18- etches the galvanized surface with phosphoric acid to form zinc phosphate, and also forms a condensate of trivalent chromium and chromium phosphate to adhere the chromate film to the surface of the galvanized steel sheet. It's for a reason. Also,
When PO18- is contained, the film has a greater affinity with organic substances and organic solvents, improving paint film adhesion and post-painting corrosion resistance, and the chromate film made of only chromic acid is yellow, but has a bluish tinge. The yellow color is reduced and becomes less noticeable in appearance.

This PO18- uses phosphoric acid or a phosphate salt, but if the blending amount is less than 0.59/e, the blending effect will be poor, and if it exceeds 59/e, the yellow color will disappear and the paintability will deteriorate. Although there are no problems with improvement, corrosion resistance decreases, chromium is more likely to be leached from the film, chromium is included in waste liquid during cleaning and degreasing, and fingerprints become more noticeable.

The organic resin is blended to impart lubricity to the film and improve corrosion resistance. This organic resin is blended in the form of an emulsion with a water-soluble or water-dispersible resin, for example, one having a terminal group such as a carboxyl group, a hydroxyl group, or an ami-7 group. These may be homopolymers or copolymers. It may also be a blend of other resins.

If the blending amount is less than 109i, the lubricity that can withstand strong processing such as drawbead processing will not be obtained, and it will be necessary to apply lubricating oil during processing, and if it exceeds 2009/e, the film will become an obstacle. This makes grounding and resistance welding difficult.

The treatment solution is applied to galvanized steel sheets using the roll squeeze method,
Known methods such as the chemical coater method, roll coater method, air knife method, dipping method, and spray method may be used as appropriate, but the coating amount is 30-120 mg/+2 in terms of chromium deposition amount.
Make it. If it is less than 30 mH/m2, the corrosion resistance is insufficient, and if it is more than 120 mg/v2, the film becomes thick (so it is easy to peel off during processing, and the color tone is also Δb on a color difference meter.
The more the number exceeds 10, the more yellow it becomes, and although chromium does not elute, it gives the impression that chromium is eluting.

After application, it is heated and dried without washing with water, but the drying process involves dehydration condensation between the hydroxyl group of 5i02 and chromium hydroxide.
1 to improve fingerprint resistance and harden organic resins.
It is necessary to carry out the process at a temperature of 50°C or higher. However, 250℃
Drying at a higher temperature for a longer period of time will cause oxidation cracks in the film, resulting in decreased chromium elution resistance and fingerprint resistance, as well as decomposition of the organic resin.

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) ZrF, silica sol, phosphoric acid, chromic anhydride aqueous solution,
A chromate treatment solution with the composition shown in Table 1 was prepared by blending an emulsion of a mixture of polyacrylic acid and styrene, and the stability of the treatment solution was investigated. Electrolytic galvanized steel sheets were treated and the chromate film was removed. We investigated the performance. 1st
The results are shown on the right side of the table.

The stability of the treatment solution and the performance of the chromate film are investigated by
It was done as follows.

(1) Stability of treatment solution The treatment solution was left standing in a constant temperature bath at 40° C., and the number of days until it turned into a delta was counted.

0 No precipitation for more than 15 days △　Delled in 10-14 days × Delivered within 1 day (2) Appearance of chromate film The presence or absence of processing unevenness was visually observed.

◎ No processing unevenness ○ Slight streaks (roving) ∆ Moderate processing unevenness (Tap water, 90°C) for 3 minutes and the elution amount was measured.

◎ 5×6g/II+2 or less Δ 11~19 gloss g
/m206-10mg/m2X 20mg/+2 or more (4) Fingerprint-resistant artificial fingerprint liquid (sodium chloride 79/℃, urea 19/e,
Lactic acid 49/e, remainder') 1:1/9/-l) was completely deposited, and the change in color tone (Δb) before and after deposition was measured using a color difference meter.

◎ Δb 0.5 or less Δ Δb 1.1~2.
00 Δb 0.6 to 1.0× Δb 2.1 or more (
5) Corrosion resistance (a) Flat part salt spray test The state of white rust occurrence after 100 hours was evaluated based on the following criteria.

◎ No white rust O Very slight white rust Δ Within 10% of white rust area × 11% or more of white rust area
After processing with /cI112, a salt spray test was conducted, and 14
The occurrence of white rust after 8 hours was evaluated using the same standards as those for the flat part.

(6) Painting performance After painting the acrylic resin paint [Superlac F50, manufactured by Nippon Paint Co., Ltd.] according to the specifications, we conducted the goblin test, Elixe 26mm extrusion test, and DuPont impact test. The test area was investigated by forcibly peeling it off with cellophane tape. For evaluation, the total score of each test was set to a perfect score of 20 (no peeling of the paint film), and the peeling state of the paint film was evaluated according to the following criteria.

020 points Δ 10-14 points ○ 15-19 points × 9 points or less (Effects of the invention) As described above, if a galvanized steel sheet is treated using the chromate treatment solution of the present invention, the hexavalent and 4M elution of the film will be reduced. and fingerprint adhesion are improved, and lubricity is also imparted.

Claims (2)

[Claims] (1) Total chromium 5 to 100 g/l, molar ratio of trivalent chromium to total chromium: trivalent chromium/total chromium = 0.30 to 0.7
0, ZrF_60.5-7.0g/l, ZrF_6/
Total chromium = 0.05-0.10, SiO_210-20
0 g/l, PO_4^3^-0.5 to 5 g/l, and a water-soluble or water-dispersible organic resin of 10 to 200 g/l. (2) Applying the chromate treatment solution according to claim 1 to a galvanized steel plate in an amount of 30 to 120 mg/m^2 of chromium, and heating and drying at a plate temperature of 150 to 250°C without washing with water. A highly corrosion-resistant chromate treatment method for galvanized steel sheets.