JPS58204195A - Manufacture of steel plate electroplated with ni-zn alloy and provided with superior workability and corrosion resistance - Google Patents

Abstract

PURPOSE:To manufacture the titled plated steel plate by plating a steel plate at high current density in a plating bath having specified concns. of Zn<2+> and Ni<2+> and a pH below an upper limit value determined in accordance with the relative speed of the bath to the plate and the composition of the bath. CONSTITUTION:A plating bath contg. >=20g/l Zn<2+> while satisfying Ni<2+>/Zn<2+>+ Ni<2+>)=0.55-0.90 and having a pH below a valve defined by the equation [where v is the relative speed (m/sec) of the bath and a steel plate, Zn<2+> is the concn. of Zn<2+>, and Ni<2+> is the concn. of Ni<2+>] is used. A steel plate electroplated with an Ni-Zn alloy and provided with superior workability and corrosion resistance can be obtd. by carrying out plating in said plating bath at high current density.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention provides Ni-Zn with excellent workability and corrosion resistance.
The present invention relates to a method for manufacturing an alloy electroplated steel sheet.

Steel, the mainstay of industrial materials, is required to have improved corrosion resistance from the perspective of resource conservation. Particularly in the automobile industry, which accounts for a large proportion of steel demand, corrosion of car bodies caused by snow-melting agents such as rock salt, which are sprayed in winter to prevent roads from freezing in snowy regions such as North America and Northern Europe, has become a major problem. Therefore, it is necessary to strengthen the rust prevention ability of the car body.

The use of surface-treated steel sheets is rapidly increasing as a measure to strengthen car body rust prevention. The main performance requirements for surface-treated steel sheets in order to strengthen the rust prevention of car bodies are the corrosion resistance and workability of the painted parts, seam rings, and parts that remain unpainted.

Conventionally, galvanized steel sheets have been used as the above-mentioned surface-treated steel sheets, but since zinc plating has a high corrosion rate, thick plating is required. This causes problems in scrap disposal, and also causes blistering of the paint film due to corrosion reaction under coating, making galvanized steel sheets insufficient as a rust-preventing material for car bodies.

Under such circumstances, Ni-Zn, Fe-zn
, co-cr-Zn and other alloy electroplated steel sheets have been shown to be materials with excellent performance. In particular, in the unpainted areas, 5 to 10% of the conventional galvanized steel sheet
Ni-
Zn alloy electroplated steel sheets are attracting attention as a material suitable for strengthening the rust prevention ability of car bodies.

In Ni-Zn alloy electroplated steel sheets, the γ phase is formed as a recrystallized structure on the relationship between N1 and IO.

It is known to exhibit excellent bare corrosion resistance. Therefore, a Ni-Zn alloy electroplated steel sheet intended for high corrosion resistance is manufactured to have the above composition.

From the viewpoint of productivity of electroplated steel sheets, it is desirable to be able to operate at high current density. Conventionally, the maximum current density used for Ni-Zn alloy electroplating is about 30 A/dm'1, which is still lower than the current density for normal electrogalvanizing, and productivity3- cannot be said to be sufficient. do not have.

A method of performing electroplating at high current density while maintaining high corrosion resistance is disclosed in, for example, Japanese Patent Laid-Open No. 152194/1983. This method maintains the relative speed between the plating bath and the steel plate at 20 mpm or higher, thereby maintaining the plating film composition within a range with excellent corrosion resistance.
A/drr? This method performs electroplating at a high current density of approximately

However, simply increasing the relative speed between the plating bath and the steel plate to 20 m
It is clear that it is not possible to make the plating film composition excellent in high corrosion resistance simply by increasing the plating film composition to pm or more.
Not only the flow rate of the plating bath, but also the composition and pH of the plating bath affect the composition of the plating film.

Another method is to use “Tetsu to Hagane” No. 67 published by the Japan Iron and Steel Institute.
Volume Nα48-333 has plating conditions with a current density of 10
40A/(1??) by considering the range of ~40A/cl and selecting the plating bath composition and plating bath flow rate appropriately.
It is stated that the γ phase can be obtained in the current density range up to +'. However, it is not specified where the permissible upper limit of the yen is 4- + PH permissible upper limit and Ni /Z
It is completely unclear whether there is any relationship with n, and as will be described later, the workability of the plating film is not guaranteed.

As a manufacturing technology that takes into account the workability of plating films, there is a Japanese patent publication 1973. -32172 publication PH=4.0~4.5
A method has been described in which the internal stress of the plating film is lowered by increasing the ductility, but the current density is as low as 4.3 to 10.8 h/dd.

As described above, the technology for manufacturing Ni--Zn alloy electroplated steel sheets at high current density while taking both corrosion resistance and workability into consideration has not been sufficiently established.

From the above-mentioned viewpoint, the inventors of the present application have determined that the P of the plating bath is
We have extensively studied the effects of plating conditions such as the relative speed between the H1 plating bath and the steel sheet and the composition of the plating bath on the corrosion resistance and workability of Ni--Zn alloy electroplated steel sheets. As a result, the following findings were obtained.

(1), Ni2+/(Zn2+tenNi2+) in the plating bath
) shall be 0.55 or more.

[2] The pH of the plating bath is set to 5 relative to the plating bath and the steel plate.
- 30 A/d- or more if plating is performed under the conditions that the PH is below the upper limit of PH depending on the speed and plating bath composition, and (3) the Zn concentration of the plating bath is 20 f/ or more. It becomes possible to manufacture a Ni-Zn alloy electroplated steel sheet with excellent corrosion resistance and workability at a high current density of .

This invention was made based on the above knowledge.

In the method of manufacturing a Ni-Zn alloy electroplated steel sheet, PH is equal to or less than PHL expressed by the following formula.

However, y: relative speed (m/S) between the plating bath and the steel plate.

Zn2+ is 2 Of// or more, and -N12y
(2) The plating is performed at a high current density using a plating bath in which (Zn2++Ni2+) is in the range of 0.55 to 0.90.

This invention will be explained in detail.

=6= Ni-Zn alloy electroplating baths can be obtained from sulfuric acid baths, chloride baths, sulfamic acid baths, etc., but both self-soluble anodes and insoluble anodes can be used, and sulfuric acid A salt-based plating bath is practical.

Table 1 shows Ni-Z electrodeposited from a sulfuric acid bath under the following conditions.
The results of an investigation into how the workability of an n-alloy plating film is affected by current density are shown.

Condition.

Plating bath composition Zinc sulfate: 15011° Nickel sulfate: 350 f/l.

Anhydrous sodium sulfate: 60 f/l (conductivity aid).

Plating bath pH: 2.0° Relative speed between plating bath and steel plate: O,'i'm/S.

Plating bath temperature: 50°C.

Amount of plating: 2 Of//-0 7- Table 1 The workability of the plating film was evaluated by visually evaluating the degree of peeling with Sellotape after peeling off the plated steel plate at 1800°C so that the plating film was exposed to the outside. The evaluation criteria are as shown in Table 2.

In the explanation given later in Table 2, the workability of the plating film is as follows: 1. All of the workability was tested and evaluated using the above method.

8- As is clear from Table 1, the plating current density is 30A/
drrr? It can be seen that when the value exceeds , the workability of the plated film becomes low. This is considered to be due to the following reasons.

That is, (1) Generally, the deformability of intermetallic compounds is smaller than that of pure metals, and therefore their workability is poor.

According to the phase diagram, the γ phase of the Ni-Zn alloy is N1 at room temperature.
The N1 content ranges from 13.5% to about 20%, and as a stoichiometric intermetallic compound, N1 content is 17.6%.
i, Zn21 has been shown to exist. N
Since vacancies exist in the crystal lattice of Nip Zn2H when the 1 content is 17.6% or less, it is presumed that metal atoms can move to some extent during processing. For this reason, among the γ phases, those with a low N1 content are relatively easy to process, but as the Ni content increases, the processability decreases and peeling of the plating film occurs.

(2) As will be described later, when a high current density is applied, the PH1 of the plating bath at the interface between the iron base (steel plate) and the plating bath
That is, the interface pH increases. When the interface pH is 5.1 or more, Zn(OH)2 is generated, so Zn(oH)2 becomes 9
- Possibility of analysis. zn(oH)2 precipitated at the interface.

This is because it blocks the metallic bond between the iron base and the Ni-Zn alloy.

When the coverage of the iron substrate surface by zn(OH)2 becomes high, the bonding force between the iron substrate and the plating film becomes weak. When force from processing is applied, peeling occurs at interfaces with weak bonding strength.

The N1 content of the plating film in Ni-Zn alloy electrodeposition increases as the pH of the plating bath increases.
It is thought that the eutectoid of (OH)2 is also promoted, but the electrode reaction actually occurs at the interface between the steel sheet and the plating bath.
It is the pH of the interface that determines the Ni content and the eutectoid amount of zn(oH)2. The interface pH is determined by the balance between the supply of H+ from outside the diffusion layer of the plating bath and the generation of hydrogen gas. The supply of ribs is a matter of mass transfer, and if the diffusion layer is made thinner by increasing the relative velocity between the cathode (steel plate) and the plating bath, the supply of H+ is easily reduced, which tends to lower the interface PH.

Furthermore, according to research by the present inventors, Ni
- When Zn alloy plating is performed, Zn2+ preferentially precipitates.
The supply of 10- to the cathode is insufficient, and the cathode reaction corresponding to the shortage becomes a hydrogen gas generating reaction, and the interface pH increases.

From the above, the results shown in Table 1, in which the workability of the plated film decreases with an increase in current density, are due to the increased interface PH due to insufficient supply of H+ and Zn'' from the bulk side of the plating bath to the interface. However, the Ni content in the plating film and Zn
It can be understood that this is due to the increase in the amount of (OH)2 eutectoid. Based on this idea, the inventors of the present application have determined that the interface PH
We believe that suppressing the increase in Zn/(zn +Ni
), we have discovered a method to prevent deterioration in the workability of the plating film and ensure sound workability.

Figures 1 to 4 show the workability of the plating film when Ni-Zn alloy electroplating is performed under the following conditions and plating is performed at high current density, and the relative relationship between the PH1 plating bath and the steel plate. speed, N12+/(Zn2+ in the plating bath
10N12+) is shown. In each figure, the ○ mark indicates that there is no or very little peeling of the plating film.
・The mark indicates that the plating film has peeled off.

Conditions Zinc sulfate Nickel decasulfate: 500 g/1° Anhydrous sodium sulfate: 60 f//l.

Nl/(Zn+N1): 0.55, 0.6
0, 0.65, 0.70° Plating bath temperature: 50°C1 Current density: 50 A/dn1”.

Plating amount: 20? /n? .

As is clear from Figures 1 to 4, (1) a decrease in the pH of the plating bath, (2) an increase in the relative speed between the plating bath and the steel plate, and (3) Zn2+/(Zn2+1N12+).
-1-A pond in which the increase in Ni”/(Zn2+++Ni”+) contributes to preventing deterioration of workability of the plating film,
It can be seen that these factors (1) to (3) are not independent but are interrelated and influence workability.

Therefore, the inventors of the present invention related the factors (1) to (3) above and obtained the following experimental formula, which serves as a boundary condition for good workability and poor workability.

However, PHL: Upper limit of plating bath PH.

y: Relative speed (m/S) between the plating bath and the steel plate.

Zn, Zn concentration (μ).

Ni, Nl concentration (y/l).

That is, if plating is performed in a plating bath with a pH that satisfies the above formula, a plated film with good workability can be obtained.

In order to investigate the influence of Zn2+ on the interface PH in more detail, -N12+/(Zn2+10N12+)-〇, 65 was set constant, and the workability of the plated film was investigated by varying the Zn''+ concentration. The results are shown in Table 3. Shown below.

As mentioned above, the influence of Zr]2+ on the interfacial pH is caused by the insufficient supply of Zn2+ to the cand interface, so it is thought that the Zn concentration is also related to FHL, but as shown in Table 3, If the PH is smaller than the PHL when the Zn concentration is low, the workability of the plating film cannot be ensured. Therefore, it is clear that the above-mentioned formula for PHL holds true when the Zn''+ concentration is above a certain concentration, and experiments have shown that when Zn2+ is 20?/ or above, PHL
It was found that the equation H,- holds true.

As is clear from the above explanation, N1-
It can be seen that a ZNN alloy plating film can be obtained from a plating bath in which the PR of the plating bath is less than PHL and the Zn2+ is more than 20 f/, but only if excellent corrosion resistance is obtained in addition to workability. The excellent properties of Ni-Zn alloy electroplating are guaranteed.

It is well known that the corrosion resistance of Ni-Zn alloy electroplated steel sheets is determined by the N1 content in the plating film, but according to research by the inventors of the present invention, the corrosion resistance of Ni-Zn alloy electroplated steel sheets is determined by the N1 content in the plating film. It has become clear that the electroplated Ni-Zn alloy steel sheet also exhibits good corrosion resistance when the Ni content is in the range of 6 to 16%, and particularly shows extremely excellent corrosion resistance in the range of 10 to 14%.N1 Content rate is 10
-14% Ni-Zn alloy electroplated film, according to the research of the present inventors, can be obtained from a plating bath containing Zn and N1 with Ni2+/(zn + N1) of 0.55 or more and 0.90 or less. I found out that it can be done.

Table 4 shows the results of simultaneous evaluation of corrosion resistance and workability. Corrosion resistance was evaluated by testing an unpainted Ni-Zn alloy electroplated steel plate using a clay water spray test specified in JIS Z 2371, and evaluating the red rust occurrence time. The fifth evaluation criterion
Shown in the table.

The following can be seen from Table 5 and Table 4. That is, (1), if the Ni content in the plating film is less than 10%, the workability is good but the corrosion resistance is reduced.

(2) When the N1 content in the plating film is 14% or more, both workability and corrosion resistance tend to decrease.

(3) The N1 content in the plating film is 10% or more.

By performing plating under conditions that provide a plating film with excellent workability, a Ni-Zn alloy electroplated film with excellent corrosion resistance and workability can be obtained.

(4) If the pH of the plating bath is below PHL, the workability is good.

Summarizing the matters (1) to (4) above, Ni-Zn alloy electroplated steel sheets with excellent corrosion resistance and workability are
From the following conditions (a) to (-c), it is possible to manufacture while applying a high current density.

(a) The pH of the plating bath is lower than the PHL expressed by the following formula.

(b) Is the Zn2+ concentration in the plating bath 20? / or more.

(c), N12+/(Zn2+++Ni2+) is 0.55
-0.90.

Next, embodiments of the invention will be described.

Example 1 After pretreatment of electrolytic degreasing and pickling, Ni-Zn alloy electroplating was performed on a steel plate under the following plating conditions.
The corrosion resistance and workability of plated steel sheets were investigated. During plating, the relative speed between the steel plate and the plating bath was adjusted by flowing the plating bath.

(1) Composition of plating bath N12+/(Zn2+10N12+): 0.65 Zinc sulfate: 175 y/L.

Nickel sulfate: 325 fi! /l.

Anhydrous sodium sulfate: 60? /1 (2), PH of plating bath: 2. O.

(3), Relative speed: O ~ 1.5 m/S.

(4), Plating bath temperature: 50°C1 (5), Current density: 50 A/d-0.
Shown in the table.

As is clear from Table 6, when the pH of the plating bath is Nα4 to 6, which is below PHL, the plating amount is 202/might.

It can be seen that in any case of 30 g/d, a steel plate having a Ni-Zn alloy electroplated film having excellent corrosion resistance and workability can be manufactured.
1 Example 2 Zinc sulfate 150 t/l, nickel sulfate/silica 35 Of!
/l, anhydrous sodium sulfate 60%, sodium acetate 27%
A plating bath consisting of 2μ (PH buffer) was pH-adjusted with sulfuric acid.
= 1.8 and 2.4 respectively, and the corrosion resistance and workability of the electroplated steel plate were investigated under the following conditions: 0 (1), Plating current density: lO ~ 7 o A/drr?
.

Relative speed: 1.0 m1F3・ 21- Plating bath temperature: 50°C.

The results are shown in Table 7.

As is clear from Table 7, PH has a higher value than +PHL.
= 2.4, the current density is 10
At a low current density such as A/d-, a steel plate with a plated film with good corrosion resistance and workability can be obtained, but this performance is lost when the current density is 3 OA/(1-7Z' or higher). On the other hand, it can be seen that in a plating bath with a pH of 1.8, which is lower than PHL, it is possible to produce a steel plate with a plating film with good corrosion resistance and workability up to a high current density of '70 A/dtr? .

As explained above, the present invention brings about extremely useful effects such as being able to efficiently manufacture Ni-Zn alloy electroplated steel sheets with excellent corrosion resistance and workability by applying high current density.

[Brief explanation of drawings]

1 to 4 are diagrams showing the relationship between the relative speed between the plating bath and the steel plate and the pH of the plating bath. Applicant Nippon Kokan Co., Ltd. Agent Keitabe Tsutsumi (1 person Ike) 23-

Claims (1)

[Claims] In the method of manufacturing a Ni-Zn alloy electroplated steel sheet, PR is PHL expressed by the following formula, hereinafter, Zn2+
or 20 y/1 or more, and N1/(Zn + Ni
) is within the range of 0.55 to 0.90, and plating is carried out at high current density to produce a Ni-Zn alloy electroplated steel sheet with excellent workability and corrosion resistance. Method. 1-