JPH01177393A - Production of high corrosion resistant electrogalvanized steel sheet - Google Patents

Abstract

PURPOSE:To develop an electrogalvanized steel sheet having excellent corrosion resistance and workability by allowing specified amount of Ni ion to be incorporated in plating liquid in case of performing electrogalvanizing uniformly contg. the fine particles of specified metallic oxide on the steel sheet. CONSTITUTION:In case of producing an electrogalvanized steel sheet, corrosion resistance in high-corrosion environment is improved by codepositing the fine particles of metallic oxide such as Mo, Ti, V, Cr, Mn and W in a galvanized layer. Therefor one or two kinds or more selected from among the above- mentioned metals are added in the form of aqueous salt and furthermore NiSO4.6H2O, etc., are added as an Ni ion source in galvanizing liquid so that these are regulated to 1-20g/l concn. expressed in terms of Ni<2+>. The codeposition quantity of the fine particles of the above-mentioned metallic oxide is increased in the electrogalvanized film by the existence of Ni<2+> and the electrogalvanized steel sheet codeposited with the fine particles of metallic oxide which is excellent in corrosion resistance and workability in the corrosive environment is obtained by codeposition of large amounts of fine particles of metallic oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing highly corrosion-resistant electrogalvanized steel sheets in which fine metal oxide particles are uniformly eutectoided in a plating film.

(Prior art) Galvanized steel sheets are used in automobiles and home appliances due to the excellent anti-corrosion function of zinc, which not only has an electrochemical protective effect on the base steel but also forms a passive film on the surface in a general corrosive environment. Widely used in products and building materials. In recent years, surface-treated steel sheets for automobiles have been used in such applications.
In particular, considering the spraying of salts on roads in cold regions,
There is a growing demand for high corrosion resistance that can withstand harsh corrosive environments. However, conventional electrogalvanized steel sheets have insufficient corrosion resistance even if the coating weight is increased. Therefore, against this background, in recent years Zn-Ni and Zn-
Various zinc-based alloy electroplated steel sheets, typified by Fe, have been developed and are being put into practical use. More recently,
Various so-called dispersion electroplated steel sheets in which fine particles such as metal oxides are compositely eutectoided in the plating film have been developed, and many patents have been filed for these.

Regarding dispersion electroplated steel sheets, for example, Japanese Patent Publication No. 61-47919 discloses an electrogalvanized steel sheet in which alumina is compositely eutectoid;
The publication contains 1 of silica, titanium oxide, and zirconium oxide.
Electrogalvanized steel sheets with composite eutectoids of more than
Japanese Patent Publication No. 56-49999 discloses an electrogalvanized steel sheet in which 5i(h) is compositely eutectoid.

It is certainly possible to improve corrosion resistance by applying this dispersion electroplating method to electrogalvanized steel sheets or zinc-based alloy electroplated steel sheets. However, when these plated steel sheets subjected to dispersion electroplating are press-formed, there is a drawback that a so-called powdering phenomenon occurs in which the plating film peels off in the form of powder.

In the case of dispersion electroplating, some of the oxides eutectoided in the plating film are present at the interface between the plating film and the steel sheet, reducing the adhesion of the plating film. For this reason, powdering is likely to occur.

This powdering phenomenon becomes a big problem because it affects the quality of surface-treated steel sheets for automobiles. In other words, surface-treated steel sheets for automobiles always involve press forming, but if the powdering resistance is poor, it not only reduces workability and corrosion resistance after processing, but also causes pieces of peeled plating film to adhere to the press mold. This can also cause scratches on the plating film. Due to such problems, it is no longer possible to take measures such as increasing the amount of plating deposited to improve corrosion resistance.

As described above, conventional dispersion electroplating has problems in terms of quality as well as difficulties in manufacturing as described below.

The production of surface-treated steel sheets for automobiles mainly involves electroplating, which involves unwinding a cold-rolled steel sheet wound into a coil, continuously introducing it into a plating bath, and plating it by continuously passing a large current through it. equipment is used. For this purpose, the amount of plating solution used is extremely large, and the equipment used is large-scale. In conventional dispersion electroplating, the oxide to be dispersed and eutectoided into the film is directly suspended in the plating bath. In large-scale electroplating equipment that uses plating, it is extremely difficult to maintain the dispersed particles uniformly in the plating bath and to bring the dispersed particles close to the steel plate surface at a constant rate to uniformly eutectoid. Therefore, the plated steel sheet obtained cannot be said to be excellent in quality uniformity.

To address these problems, the present inventors have developed a method for dispersion electroplating in which the oxide to be eutectoided into the film is directly suspended in a plating bath, targeting Zn-Ni alloy electroplating. We have previously filed patent applications for different and improved dispersion electroplating methods (JP-A-62-50497 and JP-A-62-67197).

The invention of the earlier application is a method in which metal ions of the target oxide are added to a Zn-Ni alloy electroplating bath in the form of a soluble salt, and the oxide is generated from the ionic state by an electrode reaction during plating. According to this method, compared to the method of directly suspending the oxide in the plating bath, the oxide exists in an ionic state until just before it approaches the surface of the plated steel sheet, so it is easier to maintain it uniformly in the plating bath. There is little aggregation of the dispersed particles, and they are incorporated into the film in a very fine form. Therefore, the plated steel sheet obtained is of excellent quality without deterioration in workability.

(Problem to be Solved by the Invention) However, when dispersion electroplating is applied by applying the plating method of the prior application to electrogalvanization, a plated steel sheet with no reduction in powdering resistance can be obtained, but the plated steel sheet is not dispersion electroplated. However, the intended effect of improving corrosion resistance will not be achieved.

The purpose of the present invention is to apply a method of dispersion electroplating by adding a metal in the form of a soluble salt to produce an oxide to be compositely eutectoided in a film, which is commonly used in electrogalvanization. The object of the present invention is to provide a method for producing an electrogalvanized steel sheet that has high corrosion resistance and excellent workability.

(Means for Solving the Problems) The present inventors have discovered that when dispersion electroplating is performed using an electrogalvanizing bath to which a metal for generating oxides is added in the form of a soluble salt, corrosion resistance is improved. As a result of various investigations into the reasons why this was not possible, the following findings were obtained.

When we investigated the film structure of the plated steel sheet that was dispersion electroplated using the above-mentioned method, we found that the amount of eutectoid oxides in the film was higher on the electrogalvanized steel sheet than on the Zn-Ni alloy electroplated steel sheet. The reason for this is that in the case of an electrogalvanizing bath, unlike a Zn Ni alloy electroplating bath, the amount of oxide produced by the electric scraping reaction is small, so the amount of eutectoid in the film is not sufficient. There was found.

Based on this knowledge, the present inventors have investigated a method that can ensure a sufficient amount of oxides to be generated and eutectoid to the plating film even in electrogalvanizing baths, and as a result, they have attempted to eutectoid. By adding a small amount of Ni'' ions to the electrogalvanizing bath to which the base metal of the metal oxide is added in the form of a soluble salt, a sufficient amount of oxides and eutectoids can be obtained in the plating film. As a result, it was discovered that a highly corrosion-resistant electrogalvanized steel sheet with excellent workability could be produced, leading to the present invention.

Here, the gist of the present invention is as follows: ``Additionally 1 to 20 g/i!
A method for producing a highly corrosion-resistant electrogalvanized steel sheet, which is characterized in that electrogalvanizing is performed using a plating bath to which Ni" ions are added.

In the present invention, the type of metal used to generate the metal oxide to be compositely eutectoided as an oxide in the plating film is not particularly limited. Any metal may be used as long as it is eutectoidally dispersed in the plating film. Examples of such metals include Mo, ↑11■, Cr, Mn, and W.

Then, one or more of these metals is used in the form of a soluble salt, for example, if the metal force <Mo is used to generate a metal oxide, NatMoOa, (NH4) JOO
For n, if the same <Ti, Tit(SO4)z, and if ■, VCjl! t, VCL, and in the case of Cr, the same as CrCl 2 (M
For n, MnCj#, MnCj! t・4HzO
Similarly, in the case of W, WCf! z, WCI! ,,
By adding 0.1 to 30 g of aluminum to the electrolytic galvanizing bath, the effect of improving corrosion resistance can be obtained.

Furthermore, there is no significant difference in the effects depending on the type of these metals.

(Function) Hereinafter, the method for manufacturing a highly corrosion-resistant electrogalvanized steel sheet of the present invention will be explained in detail together with its function and effects.

In the above-mentioned dispersion electroplating that utilizes the electrode reaction of metal salts, the mechanism by which a metal changes from an ionic state to an oxide is as follows, taking titanium as an example. That is, the ions added in the form of Ti'' into the plating bath exhibit a reaction as shown in equation (1) and are anodized.

Ti"10zO-e --+TiO"+2H"...
(1) On the other hand, on the surface of the steel plate, which is the cathode, hydrogen is generated along with the precipitation of metal due to energization. For this reason, near the cathode interface (steel plate surface), 98 increases due to the consumption of H, and (
Tie'' ions generated by the reaction shown in equation (1) become oxides by the reaction shown in equation (2) below due to a shift in pH equilibrium near the surface of the steel sheet.

Tie” +H,O→Ti(h+H”...(2
) The amount of oxides produced by the reactions shown in equations (11 and (2) above) is large in the Zn--Ni alloy electroplating bath, whereas the amount produced is small in the electrogalvanizing bath. In Zn-Ni alloy electroplating baths, the pH increases near the cathode interface, which is well known as anomalous electrodeposition. In the galvanizing bath, the current efficiency is high, so the amount of hydrogen ions consumed is small, and the increase in pi near the cathode is suppressed, so the amount of oxides produced is small.
The generated oxide is fixed to the steel plate surface by the electrostatic force of the cations adsorbed on the surface, and is covered with electroplating metal (plating metal) and eutectoid; however, in the Zn-Ni alloy electroplating bath, oxidation While there are two types of cations, H" ions and Ni1 ions, which adsorb to the surface of objects and give surface charges, in electrogalvanizing baths, only 11' ions exist, so the amount of oxide eutectoid is reduced. For these reasons, in electrogalvanizing baths, Zn-N
Compared to the i-alloy electroplating bath, the amount of oxides produced and the amount of eutectoid deposited into the film are smaller.

However, even in such an electrogalvanizing bath, adding a small amount of Ni ions increases the interfacial pH and promotes the imparting of positive charges to the oxide surface, thereby increasing the amount of oxide produced and the amount of oxide in the plating film. The present inventors have discovered that the amount of eutectoid increases.

This effect is due to Ni!, which is usually added in Zn-Ni alloy electroplating baths! □Aeon! This can be achieved with an amount of 1g71 or more, which is infinitely smaller than (adding 50g/f or more).

If the amount of Ni1 ions added is less than 1 g/l, the interface p
The effect of increasing H and imparting a positive charge to the oxide surface cannot be obtained, and the amount of oxide produced is small and the amount eutectoided into the film is also small. Therefore, the effect of improving corrosion resistance is small. Ni
The more 2° ions are added, the more the above-mentioned effect is promoted and the amount of oxides produced and the amount of eutectoid increases, which is preferable. There is no difference from conventional Zn-Ni alloy electroplating.Zn-Ni alloy electroplating steel sheet is
Since the Zn-Ni alloy of the film is an intermetallic compound, it is brittle and has inferior powdering resistance compared to pure galvanized steel sheet. If the amount of Ni precipitated in the plating film exceeds 3.0 wt%, the powdering resistance deteriorates significantly, so Ni is added to the plating bath so that the amount of Ni precipitated in the plating film is 3.0 wt% or less. The upper limit of added amount of ions is 20g/l
Preferably H1! The amount of + ions added is 10 to 15 g/1.

A method of imparting a positive charge using Ni" ions has already been proposed in the above-mentioned Japanese Patent Application Laid-Open No. 54-159342, but the invention of this Japanese Patent Application Laid-Open No. 54-159342 involves suspending the oxide directly in the plating bath. Because it is a clouding method, the oxides aggregate into large particles, and the amount of positive charge is not sufficient, which only increases the amount of oxides adsorbed on the plating surface. Kosho 61
This is as pointed out in JP-A-479192.

In contrast, in the present invention, oxide metal ions are added in the form of soluble salts, so they do not aggregate in the plating bath to form large particles. Furthermore, since the oxide produced by the electrode reaction is in principle very fine, even a small amount of Ni2° ions can provide sufficient positive charge to the oxide. For this reason, the oxide is incorporated into the plating film homogeneously and in a sufficient amount.

The present invention will be further explained below with reference to Examples.

(Example) Cold rolled steel plate after degreasing and pickling (JIS 5PCC: plate thickness 0.
8+u+X width 100 mm) was electrogalvanized using the plating conditions shown below.

Note (1) Plating bath composition Zn5Oa ・IHzO: 400g/INazS
Oa: 100g/l.

based on soluble salts of the types shown in Table 1, and soluble salts and Ni"" ions (NiSO.

・Electrogalvanizing bath containing both (added in the form of 6H20).

(2) Plating bath pH: 1. o~3.0 (3) Plating bath temperature: 50~60°C (4) Current density: 60~100A/dm" (5)
Current application time = 13 to 70 seconds (6) Coating amount = 20 to loog/ffi'' Workability and corrosion resistance of each plated steel plate obtained were investigated.

The results and the amounts of soluble salts and Ni'' ions added are also shown in Table 1.

Workability was tested by taking a 90mmφ disc blank from a plated steel plate, deep drawing it into a 5On+mφ, 28mm deep cylindrical shape, and peeling off the plating film on the side wall surface with adhesive tape. Visually inspect the amount of peeling,
Evaluation was made on a five-point scale. The evaluations are: 5: No peeling at all, 4: Tape area with peeling pieces attached to the total area of the tape is less than 1%, 3: Also less than 5%, 2: Same < less than 10%, 1: Same <10% or more.

Corrosion resistance was evaluated by conducting a salt spray test using 5% salt water and determining the time until red rust appeared.

As is clear from Table 1, soluble salts and HiZ +
Comparative example N, which corresponds to conventional electrogalvanizing without ion additives
o, 1 has excellent workability but poor corrosion resistance. Conventional Zn in which the added amount of Ni2゜ ions is greater than the range specified in the present invention.
Comparative Example No. 2 corresponding to -Ni alloy electroplating and Comparative Example N which is less than this but more than the amount specified in the present invention.
No. 3 has excellent corrosion resistance but poor workability.

Comparative Examples No. 4, No. 5, in which the oxide was directly suspended in the plating bath without the addition of Ni"° ions, were inferior in workability and corrosion resistance. In addition, Comparative Examples No. 6 and No. 5, in which the oxide was directly suspended in the plating bath without the addition of Ni'' ions, Comparative Example No. 8, in which the number of ions No. 7 and 129 ions are smaller than the range defined by the present invention, has excellent workability but poor corrosion resistance. In contrast, present invention example No.
All of Nos. 1 to 7 are excellent in both workability and corrosion resistance. has. In addition, the inventive example is the comparative example No.
, 4 and No. 5, there is no decrease in workability. Furthermore, even if the amount of plating deposited is increased as in Invention Example No. 094, no deterioration in workability is observed.

(Effects of the Invention) As explained above, according to the method of the present invention, extremely fine metal oxides can be uniformly eutectoided into a sufficiently precious plating film, so that the resulting electrogalvanized steel sheet has good workability. It has excellent corrosion resistance as well as excellent corrosion resistance.

Therefore, the method of the present invention greatly contributes to improving the corrosion resistance of surface-treated steel sheets for automobiles.

Claims (1)

[Claims] A plating bath to which soluble salts of metals are added to generate metal oxides to be compositely eutectoided and dispersed in the plating film is further added with 1 to 20 g/l of Ni^2^+ ions. 1. A method for producing a highly corrosion-resistant electrogalvanized steel sheet, characterized in that electrogalvanizing is carried out using a method of electrolytic galvanizing.