JPS61170576A - Production of galvanized and chromate treated steel sheet having excellent corrosion resistance, color tone and appearance - Google Patents

Abstract

PURPOSE:To subject the surface of a galvanized steel sheet to a chromate treatment in a manner as to have excellent corrosion resistance, color tone and appearance by activating the surface of said steel sheet by ultrasonic oscillation in an aq. soln. contg. an acid or surface active agent as a pretreatment in the case of chromating the surface of said sheet. CONSTITUTION:The galvanized steel sheet is dipped into the dilute chromic acid soln. contg. 1-10g/l chromic acid in terms of CnO3 or the aq. soln. of 30-70 deg.C contg. the nonionic surface active agent at 0.1-1g/l concn. and 20-50kHz/sec ultrasonic oscillation is applied thereto for 0.5-2sec at 0.1-1W/cm<2> energy to activate uniformly the galvanized surface in the stage of chromating said steel sheet. The steel sheet is thereafter chromated by a general treating liquid contg. CnO3, by which the chromate-treated film having the excellent color tone, appearance and corrosion resistance is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a pretreatment process for chromate treatment of galvanized steel sheets, and relates to a method for producing galvanized chromate-treated steel sheets having excellent corrosion resistance and color appearance.

<Prior art and its problems> The characteristics required for galvanized chromate-treated steel sheets are prevention of white rust, that is, high corrosion resistance, and coating HvE adhesion when used as a material for painting or laminating adhesives. Furthermore, color tone is also important for products made of chromate-treated steel sheets.

Zinc plating chromate treatment can be broadly divided into reactive type,
It can be divided into a coating type and an electrolytic type, but the reaction type is easier to obtain a balance between the above-mentioned characteristics of corrosion resistance and coating adhesion, and is cheaper to manufacture.

It is widely used because it is easy to maintain.

However, the 1-reaction type uses a chromate treatment solution consisting of hexavalent chromium and inorganic acid, and the acid corrodes the metal base such as zinc.
Part of the hexavalent chromium is returned to form an insoluble hydrated oxide containing trivalent chromium, which is deposited on the metal and forms a film. If the chromium concentration in the chromate treatment solution is increased in an attempt to obtain corrosion resistance, discoloration, partial etching, and chromate unevenness due to residual chromate may occur.

To solve these problems, etching agents and film-forming agents have traditionally been added.
No. 4 states that aluminum fluorine compounds are effective. However, these effects have limits, and some degree of coloring has been considered unavoidable in order to obtain high corrosion resistance with the reactive type.

<Object of the Invention> The object of the present invention is to solve these drawbacks, improve the pretreatment process for chromate treatment, and provide a method for producing galvanized chromate-treated steel sheets that have excellent corrosion resistance and color appearance. This is what we provide.

<Configuration of the Invention> The present invention is characterized in that as a pretreatment for chromate treatment of a zinc-based plated steel sheet, ultrasonic vibration is applied in a solution containing an acid and/or a surfactant to uniformly activate the surface. This is a method for producing galvanized chromate-treated steel sheets with excellent corrosion resistance and color appearance.

Before entering into the structure of the invention, the circumstances leading to the invention will be described.

There are the following problems with the corrosion resistance of chromate-treated, dampened steel sheets, and through research on these points, the present inventors have found a method that does not cause problems with unevenness on the surface of the plated steel sheet that should be chromated, and that does not damage the plating layer. The present invention was developed by focusing on the fact that the plating surface can be activated and the reactivity can be made uniform by performing a pretreatment step of chromate treatment.

When conducting a corrosion resistance test on chromate-treated steel sheets, it was found that those with a certain level of corrosion resistance did not develop white rust over a certain area, but white rust progressed starting from areas where the chromate film was missing. do. To prevent this, it is necessary to form a uniform chromate film, and to do so, the plating surface must have uniform reactivity when immersed or sprayed in the chromate solution. . .

Pickling and brushing can be considered to make the reactivity of the plated surface uniform, but the former is not preferable because the acid remains on the plated layer. It is not valid in some cases.

Therefore, in the present invention, the following treatment is performed before the chromate treatment.

In the present invention, an ultrasonic cleaning bath is provided before the chromate layer. An ultrasonic vibrator is installed in the ultrasonic cleaning tank.

A transmitter installed outside the ultrasonic cleaning tank generates high-frequency electrical energy, which is applied to the vibrator to emit ultrasonic waves. The frequency of ultrasonic waves is 20 Hz/s to 50 KHz/
It is preferable to use a transmitter with an output of 0.1 to 1 W/cm 2 to give vibration energy to the plate surface.

When the frequency is 20 KHz/s or less, the sound pressure is small and the cavillation effect is small, making it impossible to make the reactivity uniform on the surface of a steel sheet with irregularities such as an alloyed hot-dip galvanized steel sheet.

Moreover, when the frequency exceeds 50 KHz/s, the cavitation strength becomes weak even if the acoustic energy is increased, and the dissolving power decreases. Vibration energy is 0.1W/cm
If it is less than 2, the effect will be small, and if it is more than 1 W/cm2, the plated surface will become rough due to excessive dissolution, which is not preferable.

The cleaning liquid used for ultrasonic cleaning contains an acid and/or a surfactant. Various acids can be used, such as inorganic acids and organic acids, but considering the risk that the steel sheet will adhere to the steel plate and be carried into the chromate treatment solution during chromate treatment later, chromate should be used as the acid. Preferably, a dilute aqueous solution of chromic acid is used for the treatment.

Alternatively, an aqueous solution containing a surfactant may be used.
The cleaning power can be further increased by adding a surfactant to a dilute aqueous solution of chromic acid as a cleaning solution.

It is preferable to use a dilute solution of chromic acid with a concentration of 1-11-1O in terms of CrO3, and a surfactant with a concentration of 0.1-1 g/Jl.The concentration of chromic acid and surfactant is not above. If the cleaning power exceeds the above, acids and surfactants may be mixed in during the chromate treatment after pretreatment, and
Degrading the quality of plated steel sheets.

A nonionic surfactant is used as the surfactant. Ether type is a nonionic activator.

There are alkylphenol type, ester type, sorbitan ester type, etc., but polyoxyethylene sorbitan monostearate or polyoxyethylene sorbitan monolaurate is preferable in terms of water solubility, and the reason why it is limited to nonionic surfactants is as follows. This is because if an ionic activator is incorporated into a chromate film, it may reduce corrosion resistance.

The solution is processed at a temperature ranging from 30°C to 70°C. If the treatment temperature is low, the cleaning effect will not be improved, and if the treatment temperature is high, the surfactant may decompose. The treatment time may be 0.5 to 2 seconds, and the steel plate treated in the ultrasonic cleaning tank is subjected to chromate treatment by squeezing the liquid with a ringer roll and then placing it in a normal chromate bath.

Note that the term "zinc-based plating" as used in the present invention refers to zinc plating v) and other zinc-based alloy platings mainly consisting of zinc, such as Zn-old, Zn-Fe, and Zn-AIL.

Practical example Hereinafter, the present invention will be specifically explained with reference to examples.

-Table 1 shows the corrosion resistance and appearance evaluation results of galvanized chromate-treated steel sheets in which the chromate treatment pretreatment step of the steel sheet manufacturing method of the present invention was carried out by changing the CrO3 concentration and surfactant concentration in the cleaning solution. be. Corrosion resistance was evaluated by SST (salt spray test, JIS Z2371) based on the occurrence of 1% white rust. In addition, the appearance is determined by the b value (e) measured by a color difference meter.
It is shown as blue 0 yellow ■).

By performing a pretreatment process using ultrasonic cleaning before chromate treatment, we were able to obtain a product with excellent corrosion resistance, a uniform color tone throughout the plated steel sheet gold mine, and no yellowing.

For comparison, a sample without the pretreatment step was subjected to the same chromate treatment as in the example, and the same observation was made. The results are shown in Table 1.

<Effects of the Invention> In the present invention, before subjecting a galvanized steel plate to chromate treatment, the surface of the steel plate is uniformly activated by applying ultrasonic vibration in a solution containing an acid and/or a surfactant. In particular, it is possible to uniformly activate the surface of a galvanized steel sheet with an uneven surface, such as an alloyed hot-dip galvanized steel sheet.

By uniformly activating the surface of the steel sheet, the galvanized steel sheet that is then subjected to chromate treatment has much better corrosion resistance than those produced using conventional methods.

It is possible to obtain a galvanized steel sheet which has a uniform color tone throughout the plated steel sheet gold plate and has an excellent color tone without yellowing in terms of aromatic appearance.

Claims (1)

[Claims] As a pre-treatment for chromate treatment of galvanized steel sheets,
A method for producing a galvanized chromate-treated steel sheet with excellent corrosion resistance and color appearance, characterized by uniformly activating the surface by applying ultrasonic vibration in a solution containing an acid and/or a surfactant.