JPS5822383A - Chromate treatment of plated steel plate - Google Patents

Abstract

PURPOSE:To form a chromate skin film having corrosion resistance, processability and close adhesiveness of a paint to a plated steel plate, by treating said plated plate with a treating liquid with a specific composition prepared by adding a silane coupling agent to chromic anhydride to reduce a part thereof. CONSTITUTION:A silane coupling agent is added to a chromic anhydride(CrO3) solution to reduce a part of chromic anhydride to adjust the concn. of chromic anhydride to 0.1-1mol/l and a mole ratio (Cr<+3>/Cr<+6>) to 0.05-0.7 in the composition of the liquid. This liquid is coated on the surface of a plated steel plate as a treating liquid by an appropriate method and dried thereon. By this method, chemical change such as crosslinking reaction is generated to form a strong chromate skin film. In addition, because this chromate treating method is a coating type, the treating liquid is not aged and, therefore, it is unnecessary to treat a waste liquid.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for chromate treatment of a plated steel plate for forming a chromate film having corrosion resistance, workability, and paint adhesion on the surface of the plated steel plate.

Conventionally, plated steel sheets plated with zinc or annealed aluminum have been used either unpainted, painted, or in combination. Therefore, the purpose of this product is to prevent rust and improve the adhesion and corrosion resistance of the paint film after painting.

Chromic acid treatment or phosphate treatment is mainly used. Chromate-based treatments to form a chromate film include chemical reactions involving etching reactions between the steel plate surface and the treatment solution, and hexavalent chromium by reducing a portion of hexavalent chromium to hexavalent chromium with a reducing agent. It can be divided into two types: one in which a treatment solution containing hexavalent chromium is applied to the surface of the steel plate, and one in which electrolytic treatment is used. Among these methods, in the method of forming a chromate film by a chemical reaction with the metal surface, the metal (Zn, AJ, etc.) on the surface of the base material is consumed by the hydrogen ions in the treatment solution at the same time as the metal ions. As a result, the free acid concentration in the solution decreases in proportion to the amount of treated base material, and eventually it reaches an appropriate level and no longer forms a romate film, and the treatment solution must be disposed of as waste and replaced with a new treatment solution. Naranakomi.

The treatment solution contains a large amount of hexavalent chromium and hexavalent chromium, which are pollution control substances, so if you do a non-pollution treatment to remove chromium using some method, it will be a waste. Yes, but
This waste liquid treatment is not only very troublesome but also requires a large amount of treatment cost.

Due to these problems in waste liquid treatment, there has recently been much effort to develop coating-type chromate treatment liquids that do not generate waste chromic acid. The coating type chromate treatment solution contains hexavalent chromium and hexavalent chromium produced as a result of reduction of hexavalent chromium by a reducing agent, or it may further contain inorganic polymers such as silica 11 alumina sol, alkyl silicate, polyvinyl alcohol, polyvinyl alcohol, etc. as a film forming agent. It is common to use water-soluble polymers such as acrylic acid and polyacrylic esters to improve corrosion resistance.

In addition, as reducing agents for reducing hexavalent chromium to trivalent chromium, organic reducing agents generally include glycols such as ethylene glycol, propylene glycol, and polypropylene glycol; monohydric alcohols such as methanol and ethanol; and pyrogallol, Polyhydric alcohols such as glucose, saccharose, methylcellulose, and mannitol; hydrogen peroxide as an inorganic reducing agent.

With these conventional reducing agents, it is difficult to maintain a constant ratio of hexavalent chromium to hexavalent chromium in the treatment solution and maintain it as a stable external treatment bath without precipitation of chromium hydroxide or gelation of the solution. Even if the paper or treatment bath could be maintained to a certain degree, on the other hand, the performance of the plate to be treated, such as corrosion resistance, workability, and coating adhesion, was insufficient.

As a result of intensive research to solve the above problems, the inventors of the present invention found that
By using a silane coupling agent as a reducing agent, excellent stability of the processing bath, which cannot be obtained with conventional known techniques, can be achieved.
Showing corrosion resistance, workability, coating adhesion, and not only galvanized steel sheets.

The present invention was completed by discovering that the present invention can also be applied to plated steel plates using other metals such as aluminum as the plated metal.

That is, 1. In the present invention, a silane coupling agent is added to a chromic anhydride (C,03) solution to reduce a part of the chromic anhydride, and the composition of the obtained solution is changed to 0.1 to 1 mo of chromic anhydride.
l/l, molar ratio of trivalent chromium to hexavalent chromium (Cr
The present invention relates to a method for chromate treatment of a plated steel sheet, characterized in that the surface of the plated steel sheet is treated with a treatment liquid having a ratio of 0.05 to 0.7.

The present invention will be explained in more detail below.

As the chromic anhydride used in the present invention, those conventionally used for surface treatment of steel plates are sufficient.

The silane coupling agent is represented by the general formula YR8i X3, where X represents a hydrolyzable group bonded to a silicon atom, such as an alkoxy group and an acetoxy group.

Further, Y represents an organic functional group, and examples thereof include a vinyl group, a methacryloxy group, a ranide group, a cyclic epoxy group, a glycidoxy group, a mercapto group, an amino group, and a diamino group.

The mechanism of reduction of hexavalent chromium by the silane coupling agent is as follows.

When a silane coupling agent is added to an aqueous chromic acid solution, these groups or atoms undergo hydrolysis to produce silanol and HX as shown in the following formula.

YR8iX3+ H2O-+ YR8i (OH)3
+3 HX For example, when X is a methoxy group or an ethoxy group,
HX becomes methanol and ethanol, respectively, and the by-produced alcohol reduces hexavalent chromium to trivalent chromium.

The processing solution used in the present invention contains 1 t"h of chromic anhydride and 0.1 to 1 mat/l of chromic anhydride, and converts trivalent chromium reduced by a silane coupling agent into hexavalent chromium and 6 valent chromium.
The molar ratio of valent chromium (C'+5/cr+6) is 0.05~
Adjust to include 0.7. The reason why the composition of the processing liquid is limited in this way is as follows. That is, the concentration of chromic anhydride -
If AEo is less than 1 m0171, a film with excellent corrosion resistance will not be formed in the short-time treatment of about 6 to 7 seconds that is carried out in industrial production processes, and if it exceeds 1 Wro171, the film will take on a yellowish tinge and the appearance will be impaired. Undesirable.

Furthermore, if the molar ratio of hexavalent chromium to hexavalent chromium is less than 0.05, no improvement in performance such as corrosion resistance or workability is expected;
This is because if the content exceeds 100%, the content of trivalent chromium becomes very high, and the corrosion resistance actually decreases.

The plated steel sheet to be treated in the present invention is a plated steel sheet that is hot-dip plated or electroplated with zinc-aluminum or the like. Also used is a plated steel plate that is subjected to alloying treatment by further heating at a high temperature after subplating (in the present invention, each steel plate to be treated is collectively referred to as a plated steel plate).

The method for surface treating such a plated steel plate with the treatment liquid used in the present invention is according to a commonly practiced method.
After degreasing the plated steel plate as necessary and cleaning the surface by washing with water, the plated steel plate is subjected to dipping, spraying, roll coating,
The treatment liquid is applied to the surface of the steel plate by a method such as a roll drawing method or an air knife drawing method.

Drying may be carried out to the extent that moisture is evaporated, and the board temperature is preferably 70 to 100°C.

When a silane coupling agent is used as a reducing agent, it is possible to obtain a number of excellent performances that cannot be obtained with conventional reducing agents.

The first consideration of such performance is the stability of the processing bath. In general, a chromate treatment solution containing hexavalent chromium tends to form a precipitate or gel when the pH exceeds 3. However, when a silica/coupling agent is used as a reducing agent, there is no problem with the stability of the liquid even if the pH is about 5.0.

The second feature is that the plated steel sheet has excellent corrosion resistance and workability. This is because the silanol groups generated by hydrolysis, which is the original function of the sila/coupling agent, are adsorbed onto the metal surface and C
It is conceivable that a chemical reaction such as a crosslinking reaction occurs with (OH) 3 or the like to form a strong cachromate film. On the other hand, since the silane coupling agent also has an organic functional group, it has excellent adhesion to alkyd, acrylic, and epoxy paints.

The chromate treatment method of the present invention significantly improves the corrosion resistance, workability, and coating adhesion of the plated steel sheet treated with it, and since the chromate treatment method of the present invention is a coating type, it requires no treatment solution. Coupled with the fact that it does not age and therefore does not require waste liquid treatment, it is extremely valuable industrially.

The present invention will be further explained below with reference to Examples and Comparative Examples.

Examples Table 1 shows detailed conditions such as the method of the present invention, its composition, pH 1 coating method, treatment method, film amount (chromium adhesion amount), drying temperature (plate temperature), etc. on the surface of various metal-plated steel sheets. Shown below.

Test pieces were taken from the chromate-treated steel plates obtained under the conditions shown in Table 1, and the salt spray resistance of the flat part, the Erichsen part as the processed part, and the bent part was evaluated for performance evaluation. As for film adhesion, tests were conducted using a cross-cut test, a DuPont impact test, and an Erichsen extrusion test. The test results are shown in Table 2.

Table 2 Performance evaluation The test method for each test is as follows.

Salt spray resistance test is based on JIS Z 2371. (Salt water spray test method), those that do not develop white rust after the salt water spray test are marked with ◎, those with less than 10 inches are marked with ○, and those with 10 or more rust are marked with 3
Those less than 0 inches were marked with Δ, and those with 60 inches or more were marked with X.

That is, Comparative Examples 1 to 5 in which no silane coupling agent was used. Comparative example 6°Cr”/cr+ where the concentration of chromic acid anhydride is too high
It can be seen that Comparative Examples 7 to 12, in which the ratio of 6 is outside the specified range of the present invention, are inferior to the Examples of the present invention.

Flat part: A flat test piece is used for the chisel test.

Evaluation was made after 160 hours of salt spray test.

The extruded product was tested and soaked in salt water. Evaluation was made after 120 hours of the spray test.

A test piece with a bent portion of 180° was subjected to a test and evaluated after 48 hours of a salt spray test.

The paint film adhesion test was conducted using an acrylic paint and using a painted test piece that had been dried at 200° C. for about 2 minutes, and was tested for each item as follows.

Goban test: Cut 1 mm square squares into the painted test piece with a cutter so that it reaches the steel base, apply cellophane tape and quickly peel it off, and place a weight of a different Kf on the painted surface at a height of 50 mm. The cellophane tape was quickly peeled off and the degree of abnormality on the painted surface was observed.

Erichsen extrusion test: 6 coated test pieces were prepared using an Erichsen extruder.
mm extrusion, the applied cellophane tape was quickly peeled off, and the degree of abnormality on the painted surface was observed.

The coating adhesion of each of the above items was classified into the following four levels depending on the degree of peeling of the coating.

◎: No paint film peeling ○g Paint film peeling less than 10 inches Δ: Paint film peeling 10 inches or more and less than 30 inches ×: Paint film peeling 60 inches or more Anal chromate reduced with a silane coupling agent as shown in Table 2 Embodiment 1 of the present invention in which surface treatment is performed with a treatment liquid
Comparative Examples 1 to 10 when using the chromate treatment method of 8.
It goes without saying that the chromate film formed has superior corrosion resistance on flat surfaces compared to when using a chromate treatment solution.

It was found that the corrosion resistance at the Erichsen extrusion part and the bent part was excellent, and the chromate film adhered extremely firmly to the plate to be treated, and therefore the workability was also excellent. It was also found that the inventive examples were superior to the comparative examples in terms of paint adhesion in the Goban, DuPont@Ning, and Kou V Kusen extrusion tests.

In addition, the chromate treatment solution reduced with a silane coupling agent has no problem in stability even at a pH of about 5.0, and is much easier to manage than conventional chromate treatment baths.

Claims (1)

[Claims] 1. The composition of the liquid obtained by adding silica/coupling agent to a chromic anhydride (CrOs) solution and reducing a part of the chromic anhydride is 0.1 to 1 Fl! of chromic anhydride! O／／／
, trivalent chromium, hexavalent chromium and o molar ratio (Cr"/C"
+6) 0.05 to 0.7 A method for chromate treatment of a plated steel sheet, characterized by surface treating the plated steel sheet with a Toshita treatment liquid. 2. The method for chromate treatment of a plated steel sheet according to claim 1, which uses a galvanized steel sheet that has been subjected to galvanization as the plated steel sheet to which the surface treatment is applied. 3. The method for chromate treatment of a plated steel sheet according to claim 1, which uses a galvanized steel sheet that has been subjected to galvanization and then further heated at a high temperature to undergo alloying treatment as the plated steel sheet to be subjected to surface treatment. 4. The method for chromate treatment of a plated steel plate according to claim 1, which uses an aluminum plated steel plate that has been subjected to aluminum plating as the plated steel plate to which the surface treatment is applied. 5. The method for chromate treatment of a plated steel sheet according to any one of claims 1 to 4, wherein the plated steel sheet to be surface-treated is plated by hot-dip plating. 6. The method for chromate treatment of a plated steel sheet according to claim 1 or 4, wherein the plated steel sheet to be surface-treated is plated by electroplating.