KR100356162B1 - Method for producing chromate coating having excellent weldability and corrosion resistance - Google Patents

Abstract

The present invention relates to a method for producing a chromate film on a galvanized steel sheet; The purpose is to provide a method of improving continuous spot weldability by decreasing the chromate surface resistivity by increasing the electrical conductivity of the adsorbed cobalt ions by adding cobalt ions to the washing process after the chromate treatment.

In order to achieve the above object, the present invention provides a method for producing a chromate coating on a galvanized steel sheet, the weldability comprising: washing the steel plate on which the chromate coating is formed with a solution containing 5-50 g / l of cobalt ion; The technical gist of the method for producing a chromate coating having excellent corrosion resistance is described.

Description

Method for producing chromate coating having excellent weldability and corrosion resistance

The present invention relates to a method for producing a chromate coating on a galvanized steel sheet, and more particularly, to a zinc plated steel sheet on which a chromate coating is formed by washing with a solution containing Co ions to adsorb a cobalt component, thereby reducing surface resistivity. In addition to increasing the continuous spot weldability, and to improve the corrosion resistance by the corrosion barrier effect of the cobalt component.

Chromate treatment is a surface treatment technique for forming a chromate coating on the surface of the galvanized steel sheet to improve the corrosion resistance and surface appearance of the galvanized steel sheet. Such chromate coatings are classified into coating, reactive and electrolytic chromate treatments according to solutions and treatment methods. Among them, reactive chromate treatments capable of securing excellent surface appearance are widely used.

The chromate solution contains the main material (Cr ⁺⁶ ), which forms the chromate reaction film, and the auxiliary agent (sulfuric acid, nitric acid, hydrofluoric acid, etc.) that promotes the reaction, and each solution composition has its own know-how of the solution manufacturer and steel mill. Has been managed by. Specifically, Korean Patent Publication No. 93-01233 proposes a solution composed of T-Cr: 0.7-2.2g / L, nitrate group: 3.0-8.0g / L, and sulfate: 4.0-14.0g / L, and this solution Although it is claimed that excellent surface appearance can be obtained by applying, it is not mentioned about the continuous spot weldability of the chromate film formed from the solution.

Continuous spot weldability is the life of the electrode that is used continuously until the electrode is replaced by contamination of the electrode surface by the spark generated during welding when spot-treated steel sheets are welded with each other (copper electrode) of proper current density. It is about. Conventional surface-treated steel sheet has a characteristic that the surface resistivity is very high because the surface is formed of an amorphous inorganic material. That is, in the case of general carbon steel, the surface resistivity is almost zero as a complete conductor, while the surface resistivity of the phosphate-treated steel and chromate-treated steel sheet is increased by forming an inorganic material containing amorphous anhydride. . Therefore, the amount of current required for welding also increases, thereby increasing the contamination of the electrode surface by sparks generated during welding. The surface resistivity of the chromate-treated steel sheet is 100 ohm · cm, and the normal continuous spot weldability (current density 1.0-1.5 mA / m ² ) is known as 2500 spots. Is known as 3000 RBI. That is, when the chromate coating of the galvanized steel sheet has a problem that the specific resistance increases, the continuous RBI weldability is inferior. Therefore, in order to increase the continuous spot weldability of the chromate coating, it is required to reduce the specific resistance of the chromate coating.

The present invention provides a method for improving continuous spot weldability by reducing the chromate surface resistivity by increasing the electrical conductivity of adsorbed cobalt ions by adding cobalt ions to the washing process after chromate treatment in order to satisfy these requirements. There is this.

The present invention for achieving the above object is a method for producing a chromate coating on a galvanized steel sheet, comprising: washing the steel plate on which the chromate coating is formed with a solution containing 5-50 g / L of cobalt ion.

Hereinafter, the present invention will be described in detail.

The present invention improves continuous spot weldability by reducing the specific resistance of the chromate coating by allowing the adsorbed metallic material to act as a passage through which free electrons on the metal surface move by adsorbing a metallic material having good electrical conductivity during the washing process. Of course, the adsorbed metallic material exhibits the effect of shielding the external corrosion factor, it is characterized by improving the corrosion resistance.

The metallic material suitable for the present invention is excellent in electrical conductivity, can be easily adsorbed to chromate pores in a washing process, and more advantageously has affinity with oxygen to shield external corrosion factors. Such metallic material is cobalt.

When the cobalt metal is added to the flushing solution according to the present invention, the addition amount needs to be determined in consideration of continuous spot weldability, corrosion resistance and whiteness. In other words, to reduce the resistivity and improve the continuous spot weldability, and to improve the corrosion resistance, it is effective to contain cobalt ions in the washing solution of 5g / l or more, but when it exceeds 50g / l, the whiteness is detrimental. The amount of cobalt ions contained is preferably 5-50 g / l.

As the cobalt compound for supplying cobalt ions, various kinds such as cobalt nitrate and cobalt chloride may be used. More preferably cobalt nitrate is preferable because cobalt chlorine may adversely affect the corrosion resistance, but cobalt nitrate does not have a negative effect.

As described above, the present invention is to prepare a chromate coating with a water-containing solution containing Co, which is a metallic material, and other manufacturing conditions except this (washing treatment temperature and drying conditions) are commonly applied in the field of chromate coating. You can use Therefore, various changes and modifications are possible under the basic configuration of washing with the addition of a metallic material to the washing solution, and it is natural that such changes and modifications should be interpreted as the scope of the present invention without departing from the spirit or scope of the present invention.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

T-Cr: 1.0-3.3 g / l, nitrate: 5.0-10.0g / l, sulfide: 3.0-10.0g / A chromate coating having a Cr adhesion amount of 5-10 mg / m ² was formed with a reactive chromate solution of 1 L and free acidity (FA): 3.0-10.0. After the formation of the chromate film, water was treated with a solution in which the cobalt nitrate was added and the cobalt ion concentration was changed to 5-100 g / L, and the physical properties of the chromate thin film were measured and the results are shown in Table 1 below.

First, the specific resistance of the chromate film was measured using a conductivity meter using a four-point method, and continuous spot weldability was spot welded under the conditions of a voltage of 4.0-6.0KV and a current density of 1.0-1.5mA / m ² using a 25mm copper electrode. The continuous score was measured until the electrode was no longer welded. In addition, the corrosion resistance was evaluated according to ASTM B117 time required until the occurrence of 5% white rust. Whiteness was measured using a color difference meter. It is known that the higher the whiteness value, the better the surface appearance.

division Co concentration (g / ℓ) Resistivity (ohmcm) Continuous RBI Corrosion Resistance (hr) Whiteness Comparative example One 0 110-150 2500-2600 20 74-75 Inventive Example One 5 90-100 2600-2650 25 75-76 2 10 70-80 2600-2700 25 75-76 3 50 40-50 2650-3000 30 74-75 Comparative example 2 70 20-40 3000-3100 45 68-70 3 100 10-15 3000-3100 45 65-66

As shown in Table 1, the surface specific resistance was measured to be 110-150 when washed with a solution containing no cobalt ion after chromate coating treatment as in Comparative Example (1), and the continuous RBI was 2500-2600 and corrosion resistance. 20 hours and whiteness were measured at 75.

On the contrary, Inventive Example (1-3) showed that when the cobalt ion concentration in the washing solution increased, the surface resistivity decreased, the continuous RBI increased, and the corrosion resistance increased. For this reason, it was judged that cobalt ions contained in the washing solution were adsorbed on the chromate surface to reduce surface specific resistance, thereby improving weldability and improving corrosion resistance by the adsorbed cobalt component.

However, when the concentration of cobalt ion is increased to 70 or more, the surface resistivity decreases as in Comparative Example (2) and Comparative Example (3), thereby increasing continuous spot weldability and corrosion resistance but decreasing whiteness. For this reason, it is judged that the cobalt adsorbed excessively on the chromate film reduces the whiteness of the chromate film (looks black) by preventing the film from permeating scattering. As can be seen from this result, when cobalt ions are added to the washing process after chromate coating treatment, it is expected to increase the continuous spot weldability and to improve the corrosion resistance by decreasing the surface resistivity of the adsorbed cobalt ions. Was 5-50 g / l.

As described above, the present invention can improve the continuous spot weldability of the chromate film very simply by washing the chromate film with a flushing solution containing Co, which is a metallic material, and also has the effect of improving the corrosion resistance.

Claims (2)

A method of manufacturing a chromate coating comprising forming a chromate coating on a galvanized steel sheet and washing with water, wherein the weldability comprises washing the steel plate on which the chromate coating is formed with a solution containing 5-50 g / l of cobalt ion. And a chromate film production method excellent in corrosion resistance. The method of claim 1 wherein the cobalt ion is supplied from cobalt nitrate.