JPS6063385A - Production of electrogalvanized steel sheet having resistance to blackening - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to a method for producing electrogalvanized steel sheets that prevents the blackening phenomenon that occurs in electrogalvanized steel sheets that are stacked in a high temperature and humid environment. be.

(Prior Art) Galvanized steel sheets are manufactured by being subjected to chromate treatment in order to improve corrosion resistance to prevent white rust and improve adhesion. However, when electrogalvanized steel sheets are stacked and stored in a highly humid and corrosive environment, part or all of the surface of the steel sheets sometimes turns black or brown (blackening phenomenon), which significantly reduces the product value. It happened often. This blackening phenomenon is thought to be caused by corrosion caused by oxygen concentration cells that have entered the minute gaps between stacked electrogalvanized steel sheets. It tends to occur in some areas. In addition, the blackening phenomenon occurs due to increases in temperature and humidity during storage of steel sheets, as well as anions (So; 2. Cd-, etc.) in the electrolytic galvanizing bath or impurity ions (So, 2. Cd-, etc.) in the chromate bath that remain on the surface of the steel sheet. "2.
C6- etc.).

Therefore, as a measure to prevent blackening of electrogalvanized steel sheets, measures have been taken to strengthen water washing after electrogalvanizing or to remove impurities in the chromate bath. However, the preventive effect of such blackening prevention measures is not necessarily sufficient, and problems remain in storage in hot and humid environments or regions.

(Purpose and Structure of the Invention) The present inventors aimed to solve this problem and manufacture an electrical Ill lead-plated steel sheet that is less likely to cause blackening phenomenon (7
Black Hen U7. After careful consideration of the many factors that affect elephants, we found that 7'1. It was found that the influence of 111 was large.

However, it has been reported that Pb f, (:I4〒
It is already known that the chemical processability of electrolytic galvanizing, that is, the adhesion of the coating film, etc., can be reduced by 1° by zI.

Figure i191 N1 shows the influence of impure anastomotic ions contained in the electrobent plating bath on the blackening of the steel plate. In the figure, an electric +11i lead-plated steel sheet manufactured using a plating bath containing various impurities was subjected to chromate treatment, and after each condensation period was stored at 70°C for 24 hours, the surface of the plated steel sheet was treated with JIS Z8730. Adopts the L'a'b' color system based on
”) was evaluated.

That is, the blackening phenomenon of electrical IIF lead-plated steel sheets is improved by significantly reducing the amount of lead ions in the plating bath. The present invention was developed based on this knowledge, and its gist is to eliminate the presence of lead impurities by adding, for example, strontium carbonate, zinc powder, or other barium compounds in the electrogalvanizing method of steel sheets. .Electrogalvanize the steel plate in a plating bath reduced to less than 2 p.
This is a method for producing black-resistant electrogalvanized steel sheets that are then subjected to chromate treatment.

The present invention will be explained in detail below.

The steel plate that serves as the plated plate is activated through the usual steel surface cleaning pretreatment processes such as degreasing, water washing, pickling, and water washing, and then is treated with a zinc sulfate-conductive salt (sodium sulfate, ammonium sulfate, etc.) system. In a plating bath in which a soluble zinc electrode is immersed in a plating solution, pneumatic zinc plating is performed. The present invention
The present invention is not limited to such 'positive galvanizing baths. However, most of the lead contained as an impurity in the electrogalvanizing bath comes from the electrodes. Generally, high-purity electrolytic zinc metal is used for soluble zinc electrodes, and 50 ppp
It contains less than m (usually 10 to 30 ppm) of lead.

Furthermore, even if the highest purity zinc metal is used, 20ppm
It contains less than (usually 5 to 10 ppm) lead.

Even if lead is diluted in a plating bath, it cannot be suppressed to less than 0.2 ppm, and lead cannot be suppressed to less than 0.2 ppm. Oppm is mixed into the plating layer. In the present invention, in order to reduce the amount of lead contained in the plating layer of the electrogalvanized steel sheet to 0.2 ppm or less,
Add strontium carbonate or l14j lead powder to the plating bath. Reducing the lead content to 0.2 pp+n or less improves the resistance to blackening as explained in Figure 1, and also changes the preferred crystal orientation as shown in Figure 2.
containing lll1. As shown in comparison with a lead-plated steel sheet, (0002) parallel to the surface of the steel sheet is increased and the 11111 eating property after chromate treatment is improved. Adding strontium carbonate to the plating bath can be used to perform chemical conversion treatments such as chromate treatment and phosphoric acid treatment on electrogalvanized steel sheets.
This is a conventional technique used to improve the adhesion and paint density of treated films and to prevent uneven gloss on plating surfaces caused by heavy metal ions such as lead and arsenic. However, since the precipitate that has adsorbed heavy metal ions is reconstituted into the plating bath, the lead content is 0.3 to 0.5.
Electrolytic galvanized steel sheets containing lead on the order of ppm are manufactured. Therefore, as in the present invention, lead is 0.2 p.
When producing electrogalvanized steel sheets with a p+i+ (preferably 0, 1 ppm) or less, the precipitates generated during plating are immediately removed using a separator line such as a filter, and then added to the circulating plating solution. It is necessary to prevent recontamination.

Displacement plating using zinc metal powder is also effective for removing lead ions. Furthermore, the larger the amount of strontium carbonate or zinc powder added, the greater the lead ion removal effect, but due to the problem of precipitate removability and the occurrence of indentation scratches, the precipitate particle size should be adjusted to 0.5 to 10 μm, respectively. 0.3~50&/l and 1~50 g/13 are preferable-1
: [7.

After the steel sheet electrogalvanized as described above is washed with water, it is subjected to a chromate treatment using an aqueous solution containing Cr''-6.

This chromate treatment is carried out for the purpose of improving the corrosion resistance of the Zn-plated steel sheet, particularly preventing the formation of white rust when exposed to a corrosive environment, and improving paint adhesion.

In the present invention, the chromate treatment conditions &:L
(For example, a romate bath may be used as shown below, although it is not specified in the above.) 5 to 50
El/1Cry3 bath, (+31 (10~l 00 f
l/1) C101-(10~3o g/13) Silica sol (*Product name: Snowtex) Unclean (c) (10~100 J/l') Ammonium chromate-water soluble (01 fat (e.g. polystyrene) A treatment bath such as a copolymer of maleic acid, etc.) is used.

'ILp The steel plate that has been oxidized is subjected to immersion treatment, spray treatment, and electrolytic treatment using these chromate baths at the same treatment temperature. It is carried out at 1l111-80°C.

After this treatment, in order to remove the predetermined amount of Cr-coated NJa', use roll squeezing or high-pressure gas to remove excess chromium.・
A chromate deposition control process is performed to remove the bath.

Next, shake it directly or wash it with water at 50°C to 180°C.
(It is dried at a temperature of 1 degree and made into a product.

As described above, when the electrogalvanized steel sheets manufactured by the method of the present invention are stored in a stacked state in a high temperature and humid atmosphere, the blackening phenomenon on the surface thereof is significantly reduced.

(Example) In order to clarify the effects of the method of the present invention, the following examples are shown below.

Example 0.8 A cold-rolled steel plate with a thickness of 1.8 mm was degreased in 3-stroke sodium orthosilicate at a current density of 75 A/dm2 for 3 seconds, washed with water, and then treated with 7.5% H2SO for 5 seconds at room temperature using a bath. A pickling treatment was carried out.

After the above-described pre-activation treatment that is normally carried out in electrolytic Zn plating operations, the method of the present invention was carried out under the following conditions, and its performance was compared with a comparative example using a conventional manufacturing method.

As shown in Table 1, the performance improvement effect achieved by the method of the present invention is remarkable.

711, - pk Bankaku Naro et al.

[Brief explanation of drawings]

Figure 1 (A), (B), (C), (Impure anastomotic metal contained in DJ electrogalvanizing bath shows the influence on black discoloration, Figure 2 (A), (Bl is electrogalvanized PB content in the bath (0.5pp+n and 0.08
pprn) on the preferred crystal orientation of the deposited zinc layer 9 (
X-ray diffraction results) are shown. 1st Kuni (Kyu) + 23'/LI! (B) >1 degree (ppm) Figure 1 (C) IL degree (pprr:) Cd) ps2+ 71 degrees (ppm) Figure 2'',, Ij Continuation of page 1 [phase ] Inventor Ken Asagawa - Kitakyushu City Hata Steel Works, Hatto Ward

Claims (1)

[Claims] Inclusion of impurity lead in electrolytic galvanizing of steel sheets
A method for producing a blackening-resistant electrogalvanized steel sheet, which is characterized by electrogalvanizing a steel sheet in a plating bath with i't, f reduced to 0.2 ppm or less, followed by chromate treatment.