JPS6028918B2 - Post-treatment method for non-plated side of single-sided zinc-based electroplated steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a post-treatment method for removing discoloration substances generated on the non-plated surface of a single-sided zinc-based electroplated steel sheet and improving the phosphating properties of the non-plated surface.

Single-sided zinc-based electroplated steel sheets are mainly used as exterior panel materials for automobiles to extend the life of the vehicle body.

Conventional double-sided galvanized steel sheets have poor performance after painting, especially blister J resistance, and paint deterioration occurs early. As described above, galvanized surfaces cause paint deterioration, so the exterior panels of automobiles are coated with a non-plated surface to prevent the above-mentioned paint deterioration, and the inner surface is coated with zinc-based electroplating to prevent steel plates. Single-sided galvanized steel plate is used to prevent rust. On the other hand, there is a tendency for thick plating to be required in order to further improve the corrosion resistance of the plated surface.

In electroplating, the thicker the plating, the greater the amount of current required. Therefore, when manufacturing thickly plated steel plates on a continuous plating line, it is necessary to reduce the line speed as the plating becomes thicker due to the electric capacity of the rectifier of the plating line. Because electroplating liquid is electrically stable,
A sulfuric acid aqueous solution is mainly used, but when electroplating is performed at such low speeds, complex oxides are formed on the iron surface in the plating solution on the unplated surface (hereinafter referred to as the non-plated surface or iron surface). The color changes to dark brown or blackish brown (hereinafter referred to as black discoloration). These discolorations not only significantly impair the appearance of the non-plated surface of a single-sided zinc-based electroplated steel sheet, but also impair the phosphating properties of the iron surface and deteriorate the coating performance after coating. In order to solve these problems, [1] Tokkan Sho 53-1
22631, Tokukan Sho 53-122632 A method of covering the non-plated surface with a synthetic resin sheet to eliminate contact with the plating solution.

■ There was a method of protecting the non-plated surface of Mochiko Sho 55-46470 by repeatedly using the non-plated surface as a cathode and then an anode and then applying electricity.

The mth method requires major modification of the conventional plating line, and the synthetic resin sheet that comes into contact with the various rolls of the plating line is easily damaged, and the airtightness between the non-plated surface and the synthetic resin sheet cannot be completely ensured. There were problems such as difficulty in maintaining it.

Method #2 seems to be effective in preventing blackening, but the iron surface is dark and
Because of the positive ambipolar property, the surface oxide film layer on the iron surface, which has traditionally had good phosphate treatment properties, deteriorates and degrades the phosphate treatment properties.

In addition, the current distribution within each electrolytic cell is due to uneven electrode consumption and uneven flow velocity within the tank, so that the plating layer plated at the cathode cannot be uniformly melted at the anode even if the same amount of electricity is passed through the anode as at the cathode. It is difficult to protect the steel surface, and there are problems such as parts of the plating layer remaining and the steel surface melting, making it impossible to completely protect the steel surface. Therefore, the present inventors used conventional electroplating equipment as is to remove discoloration substances from iron surfaces industrially and easily and without deteriorating the quality or loss of the plated surface, and improved the appearance of the iron surface by phosphate treatment. Attempted to improve performance.

As a result, after performing zinc-based electroplating on one side of a steel plate in a sulfuric acid aqueous solution, the total phosphoric acid concentration was 0.0 on the non-plated side.
By electrolytically treating the anodic current density of 2A/d or more in a solution with a pH of 4 or more and 7 or more, the black discoloration on the non-plated surface can be removed and at the same time iron phosphate of 0.05 mole or less is removed. It has been found that the processability of phosphoric acid seams can be improved by forming them on non-plated surfaces.

Each condition of the present invention will be described below. In the method of the present invention, if the gutter property is anodic, it is possible to electrolytically remove black discoloration, but if it is cathodic, it is difficult to remove black discoloration.

The black discoloration can be removed at a current density of 2 A/d or higher, but
If the power is less than A/d, it will be difficult to remove black discoloration, and at the same time the amount of iron phosphate produced will be small.
Since it takes a long time, 2A/d or more is required.

Considering power loss due to increase in electrolytic voltage, 200A/
A value below d is desirable. Regarding the pH, if the pH is 7 or more, it is difficult to remove black discoloration, and if the pH is less than 4, the melting of the steel sheet will increase and the production of iron phosphate will be inhibited, so no improvement in phosphate treatment properties can be expected.

Therefore, the pH was set to a range of 4 or more and less than 7. As for the electrolyte components, it is possible to remove black discoloration with a neutral salt electrolyte that does not contain phosphate ions, such as sodium sulfate, sodium chloride, sodium fluoride, etc. within the above pH range.
There is no effect of improving phosphate treatment properties, and acid components such as sulfuric acid, phosphoric acid, nitric acid, and perchloric acid can remove black discoloration, but they do not have any effect of improving phosphate treatment properties.

When the total phosphoric acid concentration is 0.05 mol/more, removal of black discoloration and phosphate treatment properties are improved;
If the amount is less than 0.05 mol/s, it will be difficult to produce iron phosphate, and the effect of improving phosphate treatment properties will be reduced.

Further, the upper limit is preferably equal to or lower than the solubility of the phosphate. As the phosphate, sodium phosphate is suitable, and it is desirable to adjust the pH using phosphoric acid or sodium hydroxide solution. In order to increase the electrolyte of the electrolyte, Na2S04 (
A strong electrolyte salt such as NH4)2S04 may be added, but
In order to obtain the effect of improving phosphate treatment properties, the anion concentration is preferably 1'2 equivalent or less of the total phosphoric acid concentration.

However, the concentration of halogen ions such as chlorine ions is 0.01
It is necessary to use less than mol/mole; if it exceeds this, yellowing will occur on the iron surface after electrolytic treatment, and the phosphate treatment properties will deteriorate. When the amount of iron phosphate produced exceeds 0.05 m/l, the amount of oxide film on the non-plated surface increases, resulting in poor phosphate treatment properties.

Therefore, in the method of the present invention, the amount is set to 0.05 evening/meter or less. The reason why the black discoloration removal and phosphate treatment properties of the present invention are improved is not clear, but it is thought to be as follows.

In the present invention, the potential during electrolysis on the iron surface is about 1.5 V or more with respect to the sweet bundle reference electrode, and the iron surface becomes a hyperpassive state.

The electrode reaction in the hyperpassive region is expressed by the following equation.

20H-→02 2H+10 ・・・・・・・・・
...mFe.

→Fe300$・・・・・・・・・・・・
...'2) is e. V+XLO→↑Fe203 Ten mother H+++Sagi……{3}FeQ TenP04-3→FeP04
・・・・・・・・・・・・・・・{3-' Oxygen generation reaction formula ○) is the main reaction, and the base iron dissolution reaction formula ■ and film formation reaction formula If it exists, the expression ■ becomes the expression {3}'. The reason for the removal of black discoloration in this hyperpassive state is as follows: 1. The black discoloration film is peeled off due to a slight dissolution of the base metal, and the removal of the black discoloration film is further promoted by the mechanical action caused by the generation of oxygen gas.

'2} According to formula [1}, hydrogen ions increase at the anode interface, the pH decreases, and the blackened film dissolves.

Adsorption of glue anions (phosphate ions in the case of the present invention) lowers the surface tension, turning the blackened film into a colloid, and the blackened film is destroyed by being exposed to electrostriction when placed at a high potential. The black discoloration is removed by the above reactions individually or in combination.

Next, we will discuss the improvement in phosphate treatment properties. An extremely stable oxide film is likely to be formed on a steel surface from which an oxide film formed on the surface of a steel sheet formed during an annealing process has been removed by pickling or the like, resulting in poor phosphate treatment properties.

The present invention is electrolysis in a hyperpassive state, and since it contains phosphate ions, a film-forming reaction similar to the formula occurs and FePO
A J membrane is formed which serves as a nucleus during phosphate treatment to form proper phosphate crystals. Next, details of the present invention will be explained based on examples.

Embodiment 1 FIG. 1 shows an equipment layout diagram of a single-sided zinc-based electroplating apparatus according to the prior art, in which a strip uncoiled from an uncoiler 1 is degreased in a degreasing tank 4, washed with water in a washing tank 5, and then transferred to a pickling tank. After pickling in step 6 and washing with water in washing tank 7, one side is plated in electrogalvanizing tank 8, washed with water in washing tank 9, dried in dryer 10, and rolled up in recoiler 12.

In the figure, 2 and 11 are shears, and 3 is a welding machine. In this way, in the continuous electrogalvanizing apparatus shown in Fig. 1, a steel strip (120 ribs in the middle, plate thickness 0.8 side) was plated on one side with 80 coats/gal at a line speed of 40 m/leg.
A black-brown discoloration occurred on the unplated iron surface at the outlet of the plating bath. The plating solution was ZnS04, 7th 20th and 2nd S04.
and Na2S04 dissolved in water are used. FIG. 2 shows the equipment layout for single-sided zinc-based electroplating according to the present invention. An electrolytic tank 13 and a washing tank 14 are provided after the washing tank 9 in FIG. 1, which is an equipment layout diagram of a conventional single-sided electrogalvanizing apparatus. A detailed diagram of one example of the electrolytic cell 13 is shown in FIG. The electrode 15 is placed opposite to the iron surface. 16 is a presser roll, and 17 is an energizing roll.

The following table shows the results of Examples and Comparative Examples of the present invention.

The plating solution is ZnS04/7th 20200 evening/ku, sun 2S
0425 evening/evening, Na2S04100 evening/evening, pH 1.
0, temperature 6000. The appearance evaluation of the iron surface after the electrolytic treatment was ◎ metallic color, 0 slightly yellowish metallic color, △ black discoloration partially remaining, × black discoloration remaining.

For the phosphate treatment conditions, a chemical manufactured by Nippon Parker Rising ■ was used.

Fine Cleaner-4349 degreaser, concentration 20/
In the evening, the treatment temperature was 560℃, the treatment time was 12 minutes, and the free acidity was 0 using Bonderite 3118 after spray degreasing and washing with water.
．． Using a solution adjusted to 5 to 0.7 points, total acidity 14 to 15 points, and accelerator concentration 1.5 to 2.0 points,
After spraying between 12 inkstones and sand, the amount of phosphate crystals attached and the crystal grain size were measured. As shown in the table, A indicates the phosphate treatment property of general cold-rolled steel sheets, and it can be said that this value is appropriate.

B shows the phosphating property of conventional single-sided plated steel sheet, A
Compared to Chopsticks, it is significantly inferior. Comparative Example C other than phosphate has improved appearance but poor phosphate treatment properties. Comparative Examples D, E, and F, in which acids and pH were less than 4, had improved appearance but poor phosphate treatment properties. Comparative example G with negative polarity
，The black discoloration does not come off during the day. In Comparative Example P, the sulfate ion concentration exceeds the equivalent of the phosphate ion concentration, which slightly deteriorates the phosphate treatment properties. Comparative Example Q has a low phosphate ion concentration and deteriorates phosphate treatment properties. Comparative Example R has a low current density, and Comparative Example S has a high pH and is poor in both appearance and phosphate treatment properties. Invention example 1, J, K, L, M, N, 0 are comparative examples B, C, D
, E, F, G, JP, P, Q, R, and S, they exhibit better appearance and phosphate treatment properties.

Example 2 In the manufacturing equipment for single-sided zinc electroplated steel sheet shown in FIG. 2, the plating bath composition was ZnS04/7 ratio 0200.
Tanok, Sun 2S0410 Evening/So, Na2S04100 Evening/So, NiS04・Thin 20160 Evening/So, FeS041
7th, 2060 evening/evening, C [2 (S04) 320 evening/The plating grade was used with its sulfuric acid aqueous solution at pH 1.8 and temperature 60 qo.

The evaluation of the iron surface after the electrolytic treatment, the phosphate treatment conditions, and the evaluation criteria were the same as in Example 1. Invention examples T and U in the following table
, V, and W are excellent in iron surface appearance and phosphate treatment properties.

Satoshi Next, the advantages of the present invention will be described.

(1) Since the pH is relatively high, elution from the plated surface is extremely small, and there is no loss of zinc surface or performance deterioration.

'2} Elution from the iron surface is significantly small, and there is little deterioration of the electrolyte. (3' Since there is less iron melting on the plated surface, there is less exposure to the electrode material, which is the drinking electrode.

{No yellowing phenomenon after pickling seen in 4' pickling.

{5) Neutral salts containing phosphate ions have a strong buffering effect and are less susceptible to pH changes due to electrolysis. '6- No toxic gas is generated due to electrolysis.

The present invention uses a sulfuric acid acidic aqueous solution to coat zinc-based alloy single-sided electroplated steel sheets, such as Zn-Ni, Zn-Ni-Co, Zn-Fe, Zn-Ni-Fe, and Zn-Ni-F.
e -Cr series, Zn-Mn series, Zn-Mg series, Zn-B
It can also be applied to non-plated surfaces such as i-type.

[Brief explanation of the drawing]

Figure 1 is an equipment layout diagram of a normal single-sided electroplating equipment, Figure 2
3 and 3 are equipment layout diagrams of an apparatus according to an embodiment of the present invention. 1... Uncoiler, 2... Shear, 3... Welding machine, 4
... Degreasing paste, 5 ... Washing tank, 6 ...
... Pickling tank, 7... Washing tank, 8...
Electroplating tank, 9... washing tank, 10... dryer, 11... shear, 12...・Recoiler,
13... Electrolytic cell, 14... Washing tank, 15...
... Electrode, 16 ... Presser roll, 17 ...
・Electrification roll. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1 After performing zinc-based electroplating on one side of a steel plate, electrolyze the non-plated side in a solution with a total phosphoric acid concentration of 0.05 mol/l or more and a pH of 4 or more and less than 7 at an anode current density of 2A/dm^2 or more. Non-plating of single-sided zinc-based electroplated steel sheet, characterized in that the process removes black discoloration on the non-plated surface and at the same time generates an iron phosphate film of 0.05 g/m^2 or less on the non-plated surface. Surface post-processing method.