JPS60152680A - Manufacture of galvanized steel sheet capable of preventing blackening during storage - Google Patents

Abstract

PURPOSE:To prevent the surface blackening of galvanized steel sheets when the sheets are stored in piles, by galvanizing steel sheets in a galvanizing bath having a low Pb content and by chromating the galvanized steel sheets in a chromating bath having a specified SO4<-2> and Cl<-> content. CONSTITUTION:The surfaces of steel sheets are cleaned, pretreated, and galvanized in a galvanizing bath. At this time, strontium carbonate, Zn powder or a Ba compound is added to the bath to reduce the amount of Pb contained in the bath as an impurity to <=0.2ppm. Chromate films are than formed on the galvanized surfaces with a chromating soln. The total amount of SO4<-2> and Cl<-> contained in the soln. is regulated beforehand to <=about 300ppm. Even when the steel sheets are stored in piles at high temp. and humidity, no surface blackening is caused.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides an electrogalvanized steel sheet that prevents the blackening phenomenon that often occurs in electrogalvanized steel sheets when stored in stacks in a high temperature and humid environment. It concerns the manufacturing method.

(Prior Art) Electrogalvanized steel sheets are manufactured by being subjected to chromate treatment in order to improve corrosion resistance to prevent white rust and improve paint adhesion. However, electrogalvanized steel sheets
When stored in stacks in a hot, humid, corrosive environment,
There has often been a problem in which a portion or the entire surface of the steel plate turns black or brown (blackening phenomenon), which significantly impairs commercial value. This blackening phenomenon is said to be caused by corrosion caused by oxygen concentration cells that have entered the minute gaps between stacked electrogalvanized steel sheets, and is especially caused by the thin layer coating of chromate-treated electrogalvanized steel sheets. It's easy to happen. In addition, the blackening phenomenon occurs as the temperature and humidity increase during storage of steel sheets. , CJ!, -, 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 black discoloration prevention measures is not necessarily sufficient, and how to prevent the anions in the electrolytic galvanizing bath from being carried into the chrome-Y bath after plating is difficult. Even though it was strengthened, it was not enough.

Therefore, in order to prevent blackening when stored in such a hot and humid environment or region, drastic measures are required that can prevent blackening even if the permissible amount of anions in the chromate bath is large.

(Objective and Structure of the Invention) The present inventors have solved this problem and developed an electrogalvanized steel sheet that is less likely to cause blackening without having to strictly control the content of anions in the chromate treatment bath. We investigated various methods of manufacturing electrogalvanized steel sheets that can prevent the occurrence of blackening. As a result, they found that the lead contained in the electrolytic galvanizing bath had a large effect.

However, as shown in Japanese Patent Publication No. 5B-1194, it has been known to improve the chemical processability of electrogalvanizing, that is, the adhesion of the coating film, by removing lead from the electrogalvanizing bath. .

The present inventors also found that lead in the electrolytic galvanizing bath was reduced to 0, 2
An application was filed for a method to prevent blackening when electrolytic galvanized steel sheets after chromate treatment are stacked and stored in high temperature and humidity conditions by reducing the concentration to below ppm (% Application No. 168439/1982)
No.).

However, in the above-mentioned known technology, when the anions so, -2, C1- in the plating bath that remain on the surface of the electrolytic galvanized plate and washed with water, which is the object of the present invention, are brought into the chromate bath. There is no mention of blackening prevention technology.

In view of these points, the present invention has been developed to reduce the amount of 5o4-'', C
A method for producing electrogalvanized steel sheets with excellent resistance to blackening even when anions such as carbon dioxide are accumulated has been completed and developed in relation to the lead content in the electrolytic galvanizing bath. In particular, the present invention facilitates the management of anions in water washing and chromate treatment baths after electrolytic galvanizing, allowing industrial and
It is extremely advantageous economically.

Figure 1 shows the effects of impurity lead ions contained in the electrogalvanizing bath and 504-2,C,1,- ions in the chromate treatment bath on the black discoloration of electrogalvanized steel sheets. .

In the figure, electrogalvanized steel sheets manufactured using plating baths and chromate baths containing various impurities are subjected to chromate treatment, dew is allowed to form, and then stacked and stored at 70°C for 24 hours. The Lab color system is used to measure the degree of blackening by lightness ('L').
) was evaluated.

In other words, the blackening phenomenon of electrogalvanized steel sheets can be solved by significantly reducing lead ions in the plating bath, greatly increasing the amount of anions contained in the chromate treatment bath, and further improving the blackening resistance. do.

The present invention was constructed based on this knowledge.

The gist is that in the electrogalvanizing method of steel sheets, for example, a steel sheet is electrogalvanized in a plating bath to which strontium carbonate or zinc powder * and other barium compounds are added to reduce the content of impurity lead to less than O2ppm, and then 5
The total amount of one or both o4-2 and C1- is 300 pp
This is a method for producing electrogalvanized steel sheets with excellent blackening resistance, which is subjected to chromate treatment using a chromate bath containing less than m.m.

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 plated with a zinc sulfate-conductive salt (sodium sulfate, ammonium sulfate, etc.) system. Electrogalvanizing is performed in a plating bath in which a soluble zinc electrode is immersed in a solution. The invention is not limited to such electrogalvanizing baths. However, most of the lead contained as an impurity in the electrogalvanizing bath comes from the electrodes. Generally, a high-purity electrolytic zinc ingot is used for the soluble zinc electrode, and contains 50 ppm or less (usually 10 to 30 ppm) of lead.

Furthermore, even if the highest purity electrolytic zinc ingot is used, it still contains 20 ppm or less (usually 5 to 10 ppm) of lead. Therefore, the lead content in the plating bath is at least 0.2ppm or less in the plating bath immediately after preparation, and as the electrolysis time increases, the amount of lead contained in the plating bath is reduced to 0.2 ppm or less due to elution from the impurity lead contained in the electrode. Therefore, the plating bath usually contains 0.8 to 1.0 ppm.

In the present invention, the lead content in the electrogalvanizing bath is 0.2
Strontium carbonate or zinc powder is added to the plating bath in order to reduce the amount to ppm or less.

Reducing the lead content to 0.2 ppm or less improves blackening resistance as explained by presenting Figure 1, and
As shown in FIG. 2, the deposited zinc layer is preferentially deposited on the (0002) plane, which has excellent corrosion resistance.

As a result, when the amount of lead contained in the plating bath is large, zinc layers with different crystal orientations and corrosion resistance, such as the (1oi1) plane (LOIZ) plane other than the (0002) plane, are deposited irregularly. Ru.

Therefore, when the chromate treatment bath contains a large amount of 5o4-2°C1-, etc., the surface of the galvanized layer is etched by these anions at the same time as the chromate treatment.

In this case, it is necessary that the electrodeposited galvanized layer be etched uniformly from the viewpoint of preventing blackening in a high temperature and humid atmosphere. Therefore, as in the present invention, the crystal orientation of the electrodeposited galvanized layer is changed by reducing the lead content in the plating bath.
It is very effective to deposit the galvanized layer as an electrolytic galvanized layer having a certain crystal plane consisting almost mainly of the 002) plane.

That is, it is well known that the blackening phenomenon is caused by corrosion due to oxygen concentration batteries that are generated in response to the intrusion of oxygen in the gaps between electrogalvanized steel sheets when they are stacked in a high temperature and humid atmosphere.

In other words, the blackening phenomenon begins to occur in the anode part where there is a small amount of oxygen penetration, uneven parts of the chromate-treated galvanized surface that are easily corroded, parts that are easily corroded locally, etc., and eventually the entire surface turns black. reach.

Therefore, as in the case where lead is contained in the electroplating bath,
If the crystal orientation of the electrolytic galvanized layer is different and the chromate treatment bath contains anions such as So4-2 and (J-), each crystal orientation plane will be different when the chromate treatment is performed. The ease of etching differs, and the amount of chromate coating also differs slightly for each crystal orientation plane.

As a result, as mentioned above, the galvanized surface was etched a lot during the chromate treatment, and the surface was finely roughened to prevent corrosion caused by the formation of oxygen concentration batteries when stored in a stacked state in a high temperature and humid atmosphere. Blackening is extremely likely to occur because the activated crystal planes and the amount of chromate coating are different for each crystal plane, and the surface condition is non-uniform and easily corroded locally.

On the other hand, when the lead content in the electrolytic galvanizing bath is reduced to an extremely small amount (lead content ≦0.2 ppm), the crystal orientation is composed of a constant crystal orientation plane consisting of approximately (0002) plane. Therefore, even if a chromate bath containing anions such as So, C4-, etc. is used, it is formed with a constant crystal orientation mainly consisting of the (0002) plane, which has excellent corrosion resistance. The amount of etching is small, and the surface condition is uniform, such as the amount of chromate film being uniformly formed.

As a result, even if oxygen concentration cells occur when stored in a stacked state in a high-temperature and humid atmosphere, the surface has a chromate film with a nearly uniform thickness and has excellent corrosion resistance (0
002) Since the surface has a certain surface condition consisting mainly of a zinc electrodeposited layer, even if a part that is easily corroded locally, hair node parts and cathode parts are generated one after another, and the surface becomes black due to surface corrosion. This is extremely effective in preventing blackening, which is the objective of the present invention. Accordingly, in the present invention, strontium carbonate or zinc powder is added to the electrolytic galvanizing bath in order to reduce the lead content to 0.2 ppm or less.

As shown in Figures 1 and 2, reducing the lead content to less than CO2ppm means that even if a chromate bath containing So4-2, CJ-, etc. is used, the electrodeposited galvanized layer ( 0
002) By forming an electrodeposited galvanized layer with a certain crystal orientation mainly consisting of planes, surface corrosion due to etching during chromate treatment is uniformly carried out on the plated surface, and the amount of chromate coating is almost the same. The purpose of this structure is to prevent local corrosion on the surface, which is the main cause of black discoloration, and improve black discoloration under the corrosive conditions that occur in oxygen concentration batteries.

Therefore, it has been known to add strontium carbonate to the plating bath, but the conventional purpose was to remove the coating film when electrolytic galvanized steel sheets are subjected to chemical conversion treatments such as chromate treatment and phosphoric acid treatment. This is done to improve adhesion and coating performance, etc., or to prevent uneven gloss on the plating surface due to heavy metal ions such as lead and arsenic in zinc alloy plating, etc., but as described above, this is different from the purpose of the present invention. It is.

Furthermore, in order to achieve the objective, the present invention differs from the prior art in that the lead content in the plating bath is reduced to 0.2p1)m.
It is necessary to control the amount to an extremely low level (0.1 ppm) or less.

For this purpose, it is necessary to remove lead in a plating bath storage tank, etc. that is directly connected to the plating electrolytic bath.
It is necessary to remove the lead-collecting precipitates of strontium carbonate and zinc dust using a separation device such as a filter to prevent them from entering the circulating plating bath, ie, the electrolytic treatment tank.

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 effect of removing lead ions. So that each is 03~50? /L and 1 to 50 t/l are preferred.

The steel sheet electrogalvanized as described above is washed with water and then subjected to chromate treatment using an aqueous solution containing Cr+6.

This chromate treatment is carried out for the purpose of improving the corrosion resistance of electrogalvanized steel sheets, particularly preventing the formation of white rust when exposed to corrosive environments, and improving paint adhesion.

In the present invention, chromate bath composition, chromate treatment method, chromate treatment conditions such as drying temperature are not particularly defined, but for example, a chromate bath as shown below is used.

In the present invention, the chromate treatment conditions are not particularly defined, but for example, a chromate bath as shown below is used.

(a) 5-50 f/l CrO3 bath, (b) (10
~1ooVt) Yellow CrO3-(10-300t/l) Silica sol (8
A treatment bath such as a (trade name: Snowtex) bath (c) (10-100 t/l) ammonium chromate-water-soluble resin (for example, a copolymer of polystyrene maleic acid, etc.) bath is used.

Sob", C contained in these chromate baths
J, - anions are 50 ppm from the viewpoint of preventing blackening.
Hereinafter, it was necessary to control the content so that it is contained as little as possible, but in the present invention, blackening can be prevented by controlling the content to 300 ppm or less, preferably 200 ppm or less.

Therefore, the tolerable amount of anions that remain on the surface and are brought in after washing with water after electrogalvanizing is accumulated in the chromate bath can be significantly increased, which is extremely effective industrially.

Thus, electrogalvanized steel sheets are subjected to immersion treatment, spray treatment, and electrolytic treatment using these chromate baths at treatment temperatures ranging from room temperature to 80°C.

After this treatment, in order to obtain a predetermined amount of Cr deposited, a control process for the amount of chromate deposited is performed to remove excess chromate bath using roll squeezing or high pressure gas.

Next, the product is dried as it is or after washing with water at a temperature of about 50° C. to 180° C. to obtain 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 high humidity atmosphere, the blackening phenomenon on the surface is significantly reduced.

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

Example 0: A cold-rolled steel plate with a thickness of 8 m was treated with 3% orthosilicate salt.
A degreasing process was carried out for 3 seconds at a current density of 7.5 A/d- in a vacuum cleaner, and after washing with water, a pickling process was carried out for 5 seconds at room temperature using a 75% H2So bath.

After the activation pretreatment normally performed in electrogalvanizing work as described above, 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.

(Note) 1. Performance (blackening resistance) evaluation criteria ◎ Extremely good with no surface discoloration ○ No blackening observed, but slight white rust-like discoloration occurring, but relatively good △ Poor blackening with slight blackening x Extremely significant blackening Defective 2, ■: The performance evaluation results are shown below, with electrolytic galvanizing performed using the same treatment method as in the example, and chromate treatment as a comparative example.

[Brief explanation of drawings]

Figure 1 shows the influence of lead ions contained in the electrogalvanizing bath and 5o4-2 and C2- ions in the chromate treatment bath on the blackening resistance of electrogalvanized steel sheets. ) shows the effect of PB ions in the bath on the surface.

Claims (1)

[Claims] In the electrogalvanizing method for steel sheets, electrogalvanizing is performed in a plating bath in which the content of impurity lead is reduced to 0.2 ppm or less, and then 5O-2 in the chromate treatment bath is used.
, a method for producing an electrogalvanized steel sheet capable of preventing blackening during storage, characterized in that chromate treatment is performed using a chromate bath in which the total amount of CL- ions is 3 CI Oppm or less