JPS62180097A - Surface-treatment of galvanized steel sheet - Google Patents

Abstract

PURPOSE:To carry out continuous surface treatment giving superior corrosion resistance by supplying AC to a Zn or Zn alloy plated steel sheet in an electrolytic soln. contg. sexivalent Cr and phosphate ions to form a surface film contg. specified amounts of Cr and P. CONSTITUTION:AC is supplied to a steel sheet plated with Zn or a Zn alloy contg. >=60% Zn in an electrolytic soln. contg. sexivalent Cr and phosphate ions to form a surface film by electrolysis in plural stages. At this time, <=10V voltage and 1-60Hz frequency are used in the first stage and 7-50V voltage and 10-400Hz frequency in the final stage. A surface film consisting of 10-500mg/m<2> Cr and <=1,000mg/m<2> P is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for surface treatment of galvanized steel sheets.

[Prior art]

Constant current and constant voltage electrolysis, such as electrolytic chromate treatment, is generally used for continuous electrolytic treatment of galvanized or zinc-based alloy coated steel sheets. This is because the formed film is conductive, so there is almost no need to change the current or voltage. The object of the present invention is a method of continuously forming a resistive coating. On the other hand, there is U.S. Patent No. 3,011,958 as a processing method for forming a film with high resistance, but this process uses constant current processing while increasing the voltage in batch processing for parts. Processing time is long and it is not suitable for continuous processing.

In addition, when forming this film, if constant current or constant voltage electrolysis is performed using one electrode or a set of electrodes using the general constant current method described above, low current or voltage will take too long and is not suitable for continuous treatment. However, at high current or voltage, the formed film is resistive, so current concentration occurs on the electrode entry side of the strip where no film is formed, resulting in pinholes and a high-quality film cannot be formed.

[Problem to be solved by the invention]

The present invention is a continuous surface treatment method for continuously forming a surface film on a zinc or zinc-based plated steel sheet in a short time and with good quality.

[Means for solving problems]

In the present invention, alternating current is applied to a zinc-plated steel sheet or a zinc alloy-plated steel sheet containing 60% by weight or more of zinc in an electrolytic solution containing hexavalent chromium and phosphate ions, and the amount of chromium is 1.
0-500 mg/rdt 10 as phosphorus amount
00mg/n (a treatment method characterized by continuously forming a surface film consisting of: applying an alternating current in the solution;
When performing multiple stages of electrolytic treatment to form a surface film,
The first stage voltage is 10 V or less, two frequencies are 1 to 60 Hz, the final stage voltage is 7 to 50 V, and the frequency is 10 to 400 Hz, and the amount of chromium is 10 to 500 mg/rrt.
The present invention relates to a surface treatment method for a galvanized steel sheet, which is characterized by forming a surface film having a phosphorus content of 1000 mg/rd or less.

This coating was applied to AC 5 as described in U.S. Patent No. 3,011,958.
The biggest problem with constant current processing at 0 or 60 Hz is that the processing time is too long; for continuous processing, the processing time must be within 10 seconds.

According to the research of the present inventor, the film formation mechanism is estimated as follows, and from this, current density, electrolytic waveform,
It was discovered that there are frequency, electrolytic voltage, offset amount, and duty ratio. Electrolytic treatment solution ((NH4) 2Cr
y, Bog/Q, (NH4), HPo, 15g
/ Q p N H4F 1 g / Q w C
rO310g/Q, 60°C], the equivalent circuit for forming a film by applying alternating current is as shown in FIG.

Figure 1 shows an equivalent circuit of the state near the interface between the plating layer and the electrolyte. 1 is the plating layer, 2 is the electrolyte, the upper part of the figure is the equivalent circuit for anode reaction, and the lower part is the cathode reaction. The reaction equivalent circuit is shown below. 3 is the dissolution reaction resistance of zinc, 4 is the hydrogen generation reaction resistance, 5 is the reduction reaction resistance of chromium, 6 is the electric double layer capacity, and 7 is the whirlpool gain pedance caused by the fluctuation of the interface. Electric double layer capacitance impedance and whirlpool gain pedance increase as frequency decreases. Reaction resistance is independent of frequency.

In order to easily form a film, the current flowing through the reaction resistor in the above equivalent circuit diagram should be made as large as possible, and the whirlpool gain pedance caused by the fluctuation of the interface should be made as small as possible.

First, regarding the current density, as the current density increases, the current flowing through the reaction resistor also increases, and the processing time is shortened.
/di or more, local discharge occurs and pinholes occur,
A suitable film will not be formed and if it is less than 10 A/dm, it will take a long time to process, which is not a good idea. Regarding the electrolytic voltage, electrolysis with a voltage between about 2 V and about 50 V is effective for film formation, but if the initial voltage is high, the current flowing will be large and the processing time will be shortened, but it is difficult to suddenly apply a voltage higher than IOV. If it is added, local discharge will occur, pinholes will occur, and an appropriate film will not be formed. Regarding the frequency, the lower the frequency, the larger the current flowing through the reaction resistor and the shorter the processing time, but IH
If it is less than z, the whirlpool gain pedance will be large and zinc will be actively dissolved and an appropriate film will not be formed.
In addition, the quality and density of the low-frequency film decreases. On the other hand, if the frequency is high, the quality and precision will improve, but the current flowing through one reaction resistor will decrease, and the processing time will increase, so it is not a good idea to use higher frequencies than 103 orders of magnitude. Further, within the above frequency range, it is possible to control the film components, and as the frequency is increased, the reduction current of chromium decreases, the amount of chromium in the film decreases, and the amount of phosphorus increases.

Regarding the electrolytic waveform, a triangular wave, a sine wave, and a rectangular wave can be applied, but at the same frequency and average current density, the processing time is shortened in the order of triangular wave, sine wave, and rectangular wave, which have higher low frequency component strength. For the offset amount and duty ratio, see Hibi 9.
If the value is within /1, there is an effect of shortening the treatment time, but if the value is larger than this and there are many positive values, zinc will be dissolved and no film will be formed, and if the value is larger than this value and there are many negative values, no treatment will be performed. Furthermore, it is possible to control the film-forming components by adjusting the offset amount and duty ratio, and as the amount increases on the negative side, the amount of chromium increases.

Furthermore, in order to form a continuous strip of coating, a single step process that combines the above effects does not provide a sufficient coating in a short time.

This is because the film is resistive and requires an increase in voltage for sufficient film formation.

Therefore, this problem was solved by using the above-mentioned effects and a method of performing electrolytic treatment in multiple stages.

The method is to first conduct electrolysis at a low frequency initially, increase the current flowing through the reaction resistor to speed up the formation of the film, and as the film forms, increase the frequency appropriately to reduce the resistance due to whirlpool gain pedance. As a result, the quality of the film (appearance, film density) is improved.

Furthermore, since the film is a resistive film, the interelectrode voltage increases as the film is formed.

FIG. 2 shows a schematic diagram of an example of a process in which two or more stages of electrolysis are carried out continuously to form a film. 8 is an inlet coil, 9 is an outlet coil, 10 is an energizing roll, 11 is an electrode, 1
2 to 15 indicate electrolytic power supplies, in order: 3V, 5Hz; 5V
, 10Hz; '7v, 30Hz; IOV, 50H
z, 16 is an electrolytic tank, 17 is a water washing tank, 18
is a drying oven.

Figure 2 shows a double-sided galvanized steel sheet or an alloy galvanized steel sheet containing 60% by weight or more of zinc at a line speed of 60 m/min after the usual pretreatment, alkaline degreasing, water washing, and drying in a chromic acid-phosphoric acid mixed electrolytic bath. [Composition (NH4)2Cr
0451g/Q+ (NH4)2HP0. 13g/
Q, Cr0.10g/I2. Four tanks of NH4F 1g/Q, bath temperature 60°C were used, and the length facing the strip was 20cm.

Using electrodes with a distance of 2 cm, the applied current was 3 in order from the previous stage.
V 5 Hz, 5 V 10 Hz, 7 V
30Hz.

It is a schematic diagram of a method of electrolyzing with an AC sine wave of 10 V 50 Hz, washing with water, and drying.

In the first stage, if a voltage higher than IOV is applied, local discharge may occur and pinholes may occur, damaging the appearance of the film.Furthermore, if the frequency is less than IHz, unevenness in appearance is likely to occur during continuous processing, and if the voltage is greater than 60 I-f z. and the initial film may be difficult to form.

In the final stage, if the voltage is lower than 7V, the film may not be formed sufficiently, and if the voltage is higher than 50V, the film will be destroyed due to discharge. Further, if the frequency is less than 10 Hz, the whirlpool gain pedance becomes large and discharge tends to occur, and unevenness tends to occur in continuous processing. If the frequency is 400 Hz or higher, it becomes sufficiently difficult to form a film.

The waveform and offset amount can be appropriately adjusted within the above range depending on the needs of the quality of the film, control of the amount of chromium in the film, etc. Further, although the electrolysis frequency and voltage vary depending on the area of the strip, the threading speed, and the distance between poles, electrolysis within the above range is effective. In FIG. 2, the strip is grounded for safety, but this process can also be carried out without grounding. Further, from the viewpoint of electrolytic equipment, it is not a good idea to have 11 or more different electrolytic stages from the viewpoint of line length and cost.

It is also possible to provide this processing equipment immediately after the plating line. Further, although FIG. 2 shows one electrode per tank as an example, it is also possible to subdivide the electrodes and provide two or more electrodes in one tank.

In addition, a plated steel material in which the formation of such a surface film is observed must contain 60% by weight or more of zinc. It can be applied to electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc sprayed steel sheets, zinc vapor-deposited steel sheets, etc. regardless of the plating method.

Further, alloyed hot-dip galvanized steel sheets and electrically alloyed steel sheets having a surface layer containing 60% by weight or more of zinc, such as zinc-iron and zinc-nickel alloy-plated steel sheets, can also be treated.

Other elements such as Mn, Cr, Sn, Pb, AQ,
Mo+ W, Go.

Even if Tin P, Si, Na, Ca, Mg, O, C, etc. are contained, it is applicable as long as zinc is contained in an amount of 60% by weight or more.

If necessary, pretreatments such as pickling and surface grinding can also be applied.

The amount of chromium in the film component is l Q m g / rd
Corrosion resistance is not sufficient below 500 mg/
m or more is not economically advisable. Regarding the amount of phosphorus,
If more than a trace amount of phosphorus is present, zinc will be less likely to adhere to the mold during processing and corrosion resistance will improve;
00mg/rr? Above this, the film is likely to peel off during processing by the user, and welding may become impossible, making it unsuitable for use as a plated steel sheet.
It is preferably 0 mg/rd or less.

〔Example〕

Next, examples of the present invention will be listed together with comparative examples.

[Effects of the Invention] According to the present invention, corrosion resistance is improved and the function as a surface-treated steel sheet can be fully exhibited. Further, it brings about excellent effects such as improved film density and formability of the surface-treated steel sheet.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of alternating current electrolysis, and FIG. 2 is a schematic diagram showing an example of continuous processing. Patent applicant Nippon Steel Corporation Representative Patent attorney Yasushi Furushima Figure 1

Claims (2)

[Claims] (1) Alternating current is applied to a zinc-plated or zinc alloy-plated steel sheet containing 60% by weight or more of zinc in an electrolyte solution containing hexavalent chromium and phosphate ions, and the amount of chromium is 10 to 10% by weight.
500mg/m^2, 1000mg/m^ as phosphorus amount
A method for surface treatment of a galvanized steel sheet, characterized by forming a surface film consisting of 2 or less. (2) When forming a surface film by applying alternating current to zinc or a galvanized steel sheet containing 60% or more of zinc in an electrolytic solution containing hexavalent chromium and phosphate ions and performing multiple stages of electrolytic treatment, voltage 10V or less, frequency 1~60H
z, processed at a final stage voltage of 7 to 50 V and a frequency of 10 to 400 Hz to form a surface film having a chromium content of 10 to 500 mg/m^2 and a phosphorus content of 1000 mg/m^2 or less, Surface treatment method for galvanized steel sheets.