KR20000041024A - Method for producing electrically zinc-plated steel sheet of excellent surface appearance - Google Patents

Abstract

PURPOSE: A method is provided to produce electrically zinc-plated steel sheet of excellent surface appearance by restraining elution of iron from steel sheet due to low pH and by preventing charge of iron in a plated layer when plating with zinc electrically. CONSTITUTION: A zinc-plated steel sheet is produced by using a horizontal electrical plating facility using insoluble anode of bath in sulfate of zinc. Sodium iodide and potassium iodide of 0.0003mol/l-1mol/l is added to the plating bath to restrain melting of iron ions from the steel sheet. The difference of whiteness due to partial charge of iron occurring in phosphate treatment is restrained. Thereby, electrically zinc-plated steel sheet of excellent surface appearance is obtained.

Description

Manufacturing method of galvanized steel sheet with excellent surface appearance

The present invention relates to a method for producing an electrogalvanized steel sheet having excellent surface appearance by suppressing the occurrence of black pattern caused by the difference in whiteness after phosphate treatment of an electroplated steel sheet in a zinc sulfate bath, and more particularly, horizontal electroplating. In the zinc sulfate bath of the insoluble anode used in the tank (LCC-H (Liquid Cushion Cell-Horizontal)), iron and zinc were coated on the plated steel plate by suppressing the elution of iron by the strong acid (pH 1.0 to 1.4) of the plating bath. In order to prevent black pattern caused by phosphate treatment due to vacancies, sodium iron and potassium iodide were added to the plating bath, and the concentration of iron present as an impurity in the zinc sulfate bath was changed by the difference in whiteness after phosphate treatment after electrogalvanization. Manufacturing method of electrogalvanized steel sheet with excellent surface appearance in zinc sulfate bath, characterized in that it is reduced to the range where black pattern is not generated Relate to.

In general, sulfuric acid bath plating using a horizontal cell has many different aspects from a conventional radial hydrochloric acid plating bath. Unlike conventional hydrochloric acid baths, zinc ions are not supplied by dissolution of the anode, and thus zinc zinc balls are used. Since it is dissolved in the plating solution and supplied, the working pH of the plating solution is controlled to be low in the range of 1 to 1.4 so that zinc ions can be replenished as quickly as the amount of zinc deposited on the steel sheet. Therefore, the iron ions eluted from the steel sheet due to the low plating solution pH are vacant in the plating layer during electro zinc plating, where the difference in plating and voltage becomes uneven due to the local flow rate difference over the entire width of the steel sheet, and thus the vacancy in the plating layer during electro zinc plating. The amount is different. At this time, the difference in the amount of vacancies is not shown as a difference in the surface appearance of the plating layer only by electro galvanization, but when the phosphate treatment is performed, the black pattern contour is more clearly displayed due to the color difference of the phosphate coating due to the difference in the iron content of the plating layer. It hurts the appearance. In order to solve this problem, there is a method of removing iron ions by chelating reagent to selectively remove only iron ions among ions in the plating solution, but there is a problem in that it is not applied due to high cost and low efficiency.

The present invention has been made to solve the above problems, in the sulfuric acid bath using an insoluble anode using a horizontal cell, it is suppressed that iron is eluted from the steel sheet due to the low pH to prevent the iron vacancy in the plating layer during electro-galvanization It is an object of the present invention to provide a method for producing an electrogalvanized steel sheet having excellent surface appearance after phosphate treatment.

In order to achieve the above object, the present invention provides a method for producing a galvanized steel sheet using a horizontal electroplating apparatus using an insoluble anode of zinc sulfate bath, the sodium bath and potassium iodide 0.0003mol / l in the plating bath By providing less than 1 mol / l to suppress the dissolution of iron ions from the steel sheet to provide a method for producing an electrogalvanized steel sheet excellent in surface appearance that can suppress the difference in whiteness due to partial vacancy of iron generated during phosphate treatment after plating It is characterized by.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the iron dissolution inhibition rate of steel sheet according to the amount of sodium iodide and potassium iodide added in zinc sulfate plating solution.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention uses an inorganic inhibitor such as sodium iodide and potassium iodide to chemically adsorb the steel sheet used as a cathode in the sulfuric acid bath of electro-zinc plating to inhibit the corrosion of the steel sheet to suppress the elution of iron ions of the steel sheet Way.

In general, zinc is plated on the surface of the steel plate to protect the base iron by anticorrosive action, and since the plating is easy, it has been used for a long time in automotive steel sheet or automotive steel sheet. The electroplating bath can be divided into horizontal, vertical, and radial type, and it can be divided into hydrochloric acid bath and sulfuric acid bath depending on the plating solution and soluble and insoluble anode depending on the dissolution of the anode. Among them, the insoluble anode, which has excellent plating structure and surface appearance, and has the advantage of high current density work by reducing the distance between poles, and the sulfuric acid bath-based equipment dissolves metal zinc balls with plating solution to supply zinc ions. The system is adopted. Therefore, by increasing the dissolution rate of the metal zinc in the pH of the plating bath 1.0-1.4 it can supply zinc ions more than the amount of zinc precipitated on the steel sheet.

However, since the pH of the plating solution is low, the dissolution rate of the plated steel sheet increases with the plating solution, and iron ions are eluted from the steel sheet, and thus they exist as impurities in the solution.

Such iron ions are about 0.32V precious metals compared to the precipitation potential of zinc, so they are easily precipitated and are vacant on the steel plate simultaneously with zinc in the plating bath in which zinc and iron ions are present.

At this time, the amount of vacancies in the steel sheet is different according to the local plating of the steel sheet and the voltage difference, and the difference in the amount of vacancies is caused by the plating pattern flow pattern black pattern defect due to the severe whiteness difference after phosphate treatment of the electroplated steel sheet. Will appear.

However, the steel sheet is a metal with high adsorption energy and has high surface energy. When a small amount of sodium iodide or potassium iodide is added to the electroplating sulfuric acid bath, iron and iodine chemically form surface bonds on the surface of the steel sheet. In addition to preventing Fe ²⁺ from diffusing into the solution, the rate of dissolution of the steel sheet is very slow due to the effect of H ⁺ ions preventing access from the plating solution to the metal surface. Under these conditions, the cations do not adsorb potassium and sodium to the steel sheet, but act as a conduction aid in the sulfuric acid bath, which affects the phoresis of ions and does not affect the speed of the electrode process. This sodium iodide and potassium iodide selectively dissolve and inhibit the metal zinc ball so that there is no problem in dissolving the metal zinc ball.

Hereinafter, an Example is given and this invention is demonstrated in detail.

(Example)

In the experimental procedure of the present invention, in order to measure the rate at which the steel sheet elutes into the plating solution, the specimen having a thickness of 0.7 mm was uniformly cut into 33ψ, and then washed and degreased three times with ultrasonic waves. Change the amount of sodium sulfide and potassium iodide to 0.0001 ~ 1M (mol / l), place it in a beaker of 1L of plating solution with glass cover, and soak for 30 minutes, and then use the weight loss difference before and after the test. The inhibition of dissolution was determined by

Melt inhibition rate (%) = (1- W˚ / W) x100 (1)

Where W = weight loss of the specimen in a solution free of sodium iodide and potassium iodide

W˚ = specimen weight loss in solution containing sodium iodide and potassium iodide

In addition, while varying the addition amount in the solution composition of Table 1, it was examined whether the plating and coating properties of the table 2 in a constant plating at 120A / dm ² using a large plating bath affects the appearance and adhesion of the plating.

The size of the negative electrode specimen used for plating was 100x200mm, the coating weight was 20g / m ² , and the gap distance between the positive electrode and the negative electrode was 10mm.

Zinc (g / l) PH Temperature (℃) 60 1.2 60

Temperature (℃) PH Current density (A / dm ² ) Coating Weight (g / dm ² ) Flow rate (m / sec) 60 1.2 120 20 1.5

FIG. 1 shows the dissolution inhibition rate for the case where the sodium iodide and potassium iodide were not added to the existing plating solution and when they were added. Even if only 0.1-0.6 M / l of sodium iodide and potassium iodide are present in the solution, it can be seen that the iron dissolution inhibition rate reaches 90% or more compared to the existing plating solution.

Table 3 shows the results of surface appearance and adhesion after plating depending on the amount of sodium iodide and potassium iodide added to the plating solution.

division Addition amount (mol / l) Whiteness Glossiness Adhesion Black pattern occurrence Comparative example 0 One One One Occur 0.0002 One One One Occur Inventive Example 0.0003 One One One Not Occurred 0.001 One One One Not Occurred 0.1 One One One Not Occurred 0.5 One One One Not Occurred One One One One Not Occurred

* Rating: Good ← 1 ~ 6 → Poor

In the comparative example, when the addition amount is 0.0002 mol / l, there is no problem in the surface appearance and adhesion of the steel sheet after plating, but as shown in FIG. 1, the dissolution inhibition rate is remarkably low, so that iron ions are dissolved in the steel sheet and may be vacated together with zinc during plating. High, showing slight black pattern after phosphate treatment. Thus, 0.0003 to 1 mol / l of sodium iodide and potassium iodide as described in the present invention was added to the sulfuric acid bath to significantly suppress the solubility of iron in the steel sheet, and plated iron present as an impurity of the plating bath during electroplating. By suppressing partial vacancy of the steel sheet, it is possible to suppress the generation of black patterns that may occur in the ginseng treatment after plating, and at the same time, there is no effect on plating efficiency and plating quality.

As described above, according to the present invention, sodium iodide and iodine in the plating bath to suppress the iron dissolution of the steel sheet generated in the low pH operating conditions of the plating bath in the electroplating equipment using an insoluble anode of zinc sulfate bath. By suppressing iron dissolution by adding potassium sulfide to more than 0.0003mol / l and less than 1mol / l, it prevents the vacancy of iron in the plated steel sheet and prevents black pattern generation after phosphate treatment. An electrogalvanized steel sheet having excellent surface appearance can be produced.

Claims (1)

In the method for producing a galvanized steel sheet using a horizontal electroplating apparatus using an insoluble anode of zinc sulfate bath, the steel sheet is added to the plating bath by adding sodium iodide and potassium iodide at least 0.0003 mol / l and less than 1 mol / l. Method for producing an electro-galvanized steel sheet having excellent surface appearance, characterized by suppressing the dissolution of iron ions from the white ions due to partial vacancy of iron generated during phosphate treatment after plating.