JPH0820900A - Removing method of iron component in zinc-alumina silica based dispersion electroplating bath - Google Patents

Abstract

PURPOSE:To provide an economical removing method of Fe component in an electroplating bath containing each on of Zn, Co, Cr and further containing alumina and/or silica. CONSTITUTION:The plating bath is filtered at pH >=3, the filter residue is backward washed with an acid at pH <=2.5, the Fe component is deposited and removed by electrolyzing the backward washing solution at a constant potential and the filtrate and the backward washing solution are returned to the plating bath. As a result, Fe ion concn. is easily and economically reduced to about >=2000ppm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing Fe in a Zn-alumina / silica-based dispersion electroplating solution for producing highly corrosion-resistant electroplated steel sheets for automobiles and home appliances.

[0002]

2. Description of the Related Art Conventionally, pure Zn electroplated steel sheets and Zn-based alloy electroplated steel sheets have been generally used. However, the applicant of the present invention has remarkably superior bare corrosion resistance, corrosion resistance after painting, and bath contact resistance. As a Zn-based plated steel sheet with various properties such as Al, Si oxide and / or hydroxide, and Co, Cr in the electric Zn plating layer
A highly corrosion-resistant Zn-based composite plated steel sheet obtained by co-depositing the above is provided (JP-A-60-125395, JP-A-61-130498). These high-corrosion-resistant Zn-based composite plated steel sheets have been established with a manufacturing method mainly based on a chloride bath or a sulfuric acid bath and the quality thereof, and it is recognized that both the manufacturing method and the product quality are industrially excellent. ing.

By the way, when the raw steel sheet is electroplated by the usual method, Fe content due to the melting of the raw steel sheet is accumulated in the plating bath, and when the Fe ion concentration exceeds 2000 ppm, the plating quality is impaired and red rust is generated. It becomes faster and the bare corrosion resistance deteriorates.

[0004]

As a conventional Fe content removing technique, there is a method of increasing the pH and precipitating and filtering the Fe content as ferric hydroxide. In this method, effective alumina in the dispersion plating bath is used. And silica are removed at the same time. In addition, in the conventional Fe content removal technology by potentiostatic electrolysis, effective cobalt deposition potential (−0.
28V, SHE: Saturated hydrogen electrode) iron deposition potential (-0.44)
Since it is lower than V, SHE), there is a disadvantage that cobalt is simultaneously precipitated.

An object of the present invention is to provide a method for solving the above problems, minimizing fluctuations in the bath composition of the plating bath, and economically advantageous removal of Fe.

[0006]

According to the present invention, a plating bath containing Zn, Co and Cr ions and further containing alumina and / or silica is filtered at a pH of 3 or more, and the filtration residue is kept at a pH of 2.5 or less. Fe-precipitation is removed by performing a constant potential electrolysis of this back-cleaning solution, and then this back-cleaning solution is returned to the original plating bath. Zn-alumina, silica-based dispersion electroplating. This is a method for removing Fe in the bath.

[0007]

The function of the present inventors is that the Fe content in the dispersion plating bath has a pH of 3
It has been found that the particles are adsorbed on dispersed particles of alumina or silica in the above cases. That is, when filtered in this state, most of Zn, Co and Cr ions and a small amount of dispersed particles are contained in the filtrate. This filtrate can be directly reduced to the plating bath.

On the other hand, the filtration residue contains most of the dispersed particles of alumina or silica, the adsorbed Fe component and a part of Cr ions, and almost no Co ions. When this filter residue is back-washed with diluted sulfuric acid or diluted hydrochloric acid having a pH of 2.5 or less, the Fe content adsorbed on the dispersed particles of alumina or silica and a part of
Cr ions dissolve. This backwash solution is, for example, -0.5
By conducting constant potential electrolysis with V and SHE, only Fe content can be deposited and removed. There is no Co ion in this backwash solution,
Since the precipitation potential of Cr is -0.74 V and SHE does not precipitate, only Fe can be selectively precipitated.

Further, the pH obtained by removing iron from the above-mentioned filtrate having a pH of 3 or more
By mixing a backwashing solution of 2.5 or less, neutralizing it, and returning it to the plating bath, the plating bath can be economically managed with little fluctuation in pH and bath composition. It should be noted that FIG. 1 shows a flow diagram of a system for removing Fe from the plating bath of the present invention.

[0010]

EXAMPLES The effects of the present invention will be specifically described below with reference to examples. Example 1 Plating bath composition Zinc sulfate (heptahydrate) 400 g / l Cobalt sulfate (heptahydrate) 100 g / l 40% Chromium sulfate aqueous solution 70 g / l 20% Alumina sol solution 40 g / l Impurity Fe concentration 3 g / l The above plating bath The Fe concentration could be maintained at 500 ppm or less by performing the Fe content removal system shown in FIG.

Example 2 Plating bath composition Zinc sulfate (heptahydrate) 400 g / l Cobalt sulfate (heptahydrate) 100 g / l 40% Chromium sulfate aqueous solution 70 g / l 20% Silica sol solution 40 g / l Impurity Fe concentration 3 g / l The Fe concentration could be maintained at 500 ppm or less by performing the Fe content removal system shown in FIG. 1 using the above plating bath.

Example 3 Plating bath composition Zinc sulfate (heptahydrate) 400 g / l Cobalt sulfate (heptahydrate) 100 g / l 40% Chromium sulfate aqueous solution 70 g / l 10% Alumina sol solution 20 g / l 10% Silica sol solution 20 g / l Impurity Fe concentration 3 g / l Using the above plating bath, the Fe content removal system shown in FIG. 1 was performed, whereby the Fe concentration could be maintained at 500 ppm or less.

[0013]

According to the present invention, the Fe ion concentration in the Zn-alumina, silica-based dispersion electroplating bath can be easily and economically reduced to 2000 ppm or less, so that the deterioration of plating quality due to the occurrence of red rust can be prevented. I was able to do it.

[Brief description of drawings]

FIG. 1 is a flow chart showing a system for removing Fe in a plating bath.

Claims (1)

[Claims] 1. A plating bath containing Zn, Co and Cr ions and further containing alumina and / or silica is filtered at a pH of 3 or more, and the filter residue is back-washed with an acid having a pH of 2.5 or less. Fe content is deposited and removed by constant potential electrolysis of the backwash solution, and then this backwash solution is returned to the original plating bath. Zn-alumina, Fe-based removal method for silica-based dispersion electroplating bath .