JPH01234591A - Production of material electroplated with iron-zinc alloy - Google Patents

Abstract

PURPOSE:To form a layer plated with Fe-Zn alloy which is excellent in coating finishing properties, film homogeneity and appearance properties, etc., by adding a specified pH buffering agent and a nonionic surfactant into a sulfate-base Fe-Zn alloy plating bath and performing electroplating while moving the plating bath relatively to the material to be plated. CONSTITUTION:One or more kinds of citric acid, malonic acid, oxalic acid, acetic acid, tartaric acid or salts thereof are added at 10-500g/l total amount as a pH buffering agent to an Fe-Zn alloy electroplating bath in which Fe ion and Zn ion are dissolved in a sulfate form and one or more kinds of polyethylene glycol and polypropylene glycol or their derivatives having 100-10,000mol. wt. are added at the rate of 0.1-10ppm as a nonionic surfactant to the electroplating bath. While this plating bath is allowed to flow at the relative flow velocity not less than 0.5m/sec to the material to be plated, electroplating is performed.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention is an iron-zinc alloy (Fe-Zn
(Alloy) This relates to a method for producing electroplated materials.

<Prior art and its problems> Fe-Zn alloy plated steel sheets have become the mainstream of rust-proof steel sheets for automobiles in recent years because of their excellent paintability and corrosion resistance.

However, recently, it has become extremely important for these anti-corrosion steel sheets to have not just paintability and corrosion resistance, but also a high level of uniformity in surface appearance and plating film, and in particular, excellent ``appearance performance after painting'' when used for automobile body exterior panels. It has come to be required as an element.

On the other hand, in the Fe-Zn alloy plating, the deposited crystals tend to become coarse (crystal grain size is 0.5 to 1.0 μmφ).
The surface uniformity was not fully satisfactory, as not only was the gloss poor, but also the crystal grain size and coverage of the plating tended to change due to fluctuations in operating conditions.

By the way, in Pe-Zn alloy plating, it is known that the behavior of iron ions in the plating bath has a large effect on the surface appearance and plating uniformity.

That is, during Fe-Zn alloy plating work, F in the plating bath
e2" ions are oxidized due to air entrainment during liquid circulation (Fe"1!4l-fzo+!40z-Fe"+O
In addition to H), when an insoluble anode is used, it is also oxidized to Re'' ions by the oxygen generated from the anode.It is also oxidized by direct electrode reactions at the anode, so the amount of Fe3+ ions in the bath gradually increases. In addition to reducing the plating current efficiency, these PE3+ ions are also a factor that has a negative effect on the quality of the plating surface appearance and plating uniformity.In order to improve the appearance after painting, the PE2+ ions Although there has been a desire for a plating bath that suppresses anodic oxidation, satisfactory results have not yet been achieved.

However, in order to reduce the Fe" ions generated in the plating bath, metal Fe or metal Zn is also added to replenish the ions, but in this case, quality issues
Although it is economically desirable to have a high reduction efficiency of p e 3+ ions, a plating bath with sufficient reducing conditions has not yet been found.

On the other hand, in light of the above-mentioned situation of Fe-Zn alloy plating, various improvement measures on the paint surface have been considered in order to improve the finish and sharpness of the coating, but these are not sufficient at present. I could not say it.

For this reason, in order to improve the surface appearance and homogeneity of Fe-Zn alloy plating itself, 1) increase the line speed during plating, thereby increasing the relative plating solution flow rate, which is the so-called "high-speed high-speed method". Implement “current density operation” [“Tetsu to Hagane J, vol. 71 (1
985), page S-446, and vol. 72 (19
1986), page S-1324].

ii) 0.00005 to 0.1 mol/of a cyclic compound having one or more hydroxyl groups (-0)() and substantially no sulfonic acid groups (-SO3) to the plating bath;
Add A [JP-A-60-169587].

111) Total amount of 0.01 to log in the plating bath
/ Adding one or more types of polyoxyethylene derivative compounds of j2 [JP-A-60-1.556973
.

Methods such as the above have also been proposed, but the method 1) does not fully satisfy the current requirements for both coating finish and plating homogeneity.On the other hand, the methods 11) and 111) Although the effect of improving the surface appearance and suppressing the oxidation of iron ions in the bath was observed, a sufficient improvement effect could not be obtained regarding the finish quality (image clarity) of the coating.

<Means for Solving the Problems> From the above-mentioned viewpoints, the present inventor has created a plating film that has excellent uniformity and a good appearance, and has improved the finish quality after painting, especially the "sharpness" In order to find a method to stably produce even better Fe-Zn alloy electroplating materials,
As a result of intensive research, [normal sulfate-based Fe-Zn
When electroplating is carried out in an alloy plating bath, a small amount of nonionic surfactant is added to the plating bath in combination with a specific amount of pH buffering agent, and at the same time, it is necessary to By ensuring a relative flow rate of 100%, the anodic oxidation of Fe2+ ions present in the plating bath during energization can be sufficiently suppressed, and the metal When replenishing Fe and metallic Zn, effects such as reduction of Fe" ions in the bath and smooth dissolution of the metal are also added, making the deposited crystals finer and further improving the homogeneity of the plating. The findings showed that the paint finish (image clarity) was significantly improved.

This invention has been made based on the above findings, and includes the following: ``In addition to adding one or more pH buffers in a total amount of 10 to 500 g/β into a sulfate-based Fe-Zn alloy plating bath, , one or more nonionic surfactants are also added in a total amount of 0.1 to 1 oppm,
By performing electroplating at a plating solution relative flow rate of 0.5 m/sec or more, it is possible to stably produce a highly corrosion-resistant Fe-Zn alloy plated material with excellent image clarity after painting and fine uniformity of plating. It is characterized by the following points.

Here, as the pH buffering agent, citric acid, malonic acid,
Oxalic acid, acetic acid, tartaric acid, boric acid or lactic acid, and their salts, aluminum sulfate, ammonium sulfate, phosphoric acid, phosphates (sodium dihydrogen phosphate, potassium dihydrogen phosphate, etc.), and EDTA (ethylene diamine, dihydrogen phosphate, etc.)
It is preferable to add one or more of the following: (tetra acetate) and the like.

On the other hand, as the nonionic surfactant, it is preferable to add one or more of polyethylene glycol, polyethylene glycol derivatives, polypropylene glycol, and polypropylene glycol derivatives.

Here, the polyethylene glycol derivative is 2X-0-
(CH2-CH2-0)rl-H, and polypropylene glycol derivatives are represented by the formula, and the molecular weight is preferably 100 to 10,000 in view of the effect of addition (more preferably 500 to 10,000). ~2000 is recommended).

In the method of this invention, the amount of pH buffer added,
The reason why the addition ratio 1 of the nonionic surfactant and the relative flow rate of the plating solution were limited as described above is as follows.

a) pH buffering agent 1) If the addition rate of H buffering agent is less than 10g/A, the effect will not be shown, and on the other hand, if it is added in excess of 500g/A, the effect will not improve to the extent commensurate with the amount added. From, p
The addition ratio of H buffer was limited to 10 to 500 g/l.

b) Non-ionic surfactant If the addition ratio of non-ionic surfactant is less than 0,1 ppm, the effect of the addition will not be apparent, while if it is added in excess of 1 ppm, the powdering properties of the plating film will deteriorate. Since problems such as these may occur during processing, the addition ratio of the nonionic surfactant was limited to 0.1 to LOppm.

In addition, the molecular weight of this surfactant is 1
0 to 10,000 is preferable, more preferably 500 to 2
It is better to use 000.

C) Relative flow rate of plating solution The relative flow rate of the plating solution to the material to be plated is 0.5 m/s.
If it is less than ec, the effect of improving image clarity after coating is small, and in addition, in the plating bath according to the present invention, there is a risk of deterioration of powdering properties and plating burn (black discoloration) depending on the electrolytic conditions. Therefore, the relative flow velocity of the plating solution was determined to be Q, 5 m/sec or more.

Figure 1 is a graph showing the relationship between the relative flow velocity of the plating solution and the image clarity after painting. From this figure, it can also be seen that when the relative flow velocity is less than 0.5 m/sec, the image clarity sharply decreases. It can be seen that it deteriorates. The results shown in FIG. 1 were obtained based on the following plating conditions.

Thou art the master Na25Oa: 40g/j! .

re”: 50g/Il.

Zn”: 50g/β.

Polypropylene glycol: 5ppm.

Sodium citrate: 50g/β.

Plating bath pH pH1.8゜ Current density 80A/cl a.

Further, "image sharpness (PGD value)" in FIG. 1 was measured as follows. That is, the Fe-Zn alloy plated steel sheet obtained under the above conditions was treated with zinc phosphate (using PB-3080L C trade name manufactured by Nippon Parkerizing Co., Ltd.) ↓ Electrodeposition coating: 20 μm (U- 100
(Product name used) ↓ Middle and top coat: 80 μm (melamine alkyd resin) After treatment, P GD (portable
gloss meter of distinction)
This is the value obtained by measuring the image clarity with a meter.

By the way, as for the basic composition of the sulfuric acid bath-based Fe-Zn alloy plating bath to which the pH buffer and nonionic surfactant are added, and other plating conditions, any of those known so far may be adopted. Needless to say, it's a good thing.

'' As described above, the present invention involves electroplating at a relative flow rate of 0.5 m/sec or more in a Fe-Zn alloy plating bath containing a pH buffer and a nonionic surfactant as an organic additive. The reason why this improves the sharpness of the image after painting, the uniformity of plating, etc. is thought to be as follows.

<Function> Nonionic surfactants such as polyethylene glycol and polypropylene glycol are linear polymers, and during plating, they cause block formation near the interface with the base metal (plating metal) and slight adsorption to the base metal, resulting in a current drop. It is thought that by dispersing the particles, the crystal growth at the active sites is suppressed, and the precipitation and uniform adhesion of fine crystals is promoted.

Note that the cationic surfactants commonly used in Fe-Zn alloy plating are ten-charged, so they move quickly to the base metal by electrophoresis and are easily adsorbed, but they are difficult to incorporate into the plating film. It's inconvenient. On the other hand, in the case of nonionic surfactants, block formation near the interface is more dominant than adsorption, so they are not affected by irregularities on the surface of the base metal, resulting in fine precipitation of the plated metal. Enables improved uniformity.

On the other hand, the pH buffer is used at the cathode (part to be plated) during electrodeposition.
It exerts the effect of suppressing the pH increase due to OH- production.

Generally, in Fe-Zn alloy plating baths, Fe" ions in the bath tend to be oxidized to Fe"° ions by anodization, air oxidation, etc., but these ions bond with OH" ions and form F
The formation of e(OH)3 promotes non-uniform electrodeposition.

p) T buffer suppresses this Fe(OH)3 production reaction,
This reduces nonuniform electrodeposition caused by pe3+ ions.

It is presumed that due to this effect, a plated product with stable quality can be obtained even if the amount of naturally formed Fe3+ ions increases to some extent.

In other words, the finish quality (image clarity) after painting by the method of the present invention
The improvement effect is due to the above-mentioned effect of the nonionic surfactant combined with the effect of the H buffer described above, and the addition of the effects of uniformity and fineness of electrodeposition due to the high relative flow rate of the plating solution. It is thought that the synergistic effect is brought about by the morphological uniformity of the plated surface, good coverage, compositional uniformity, etc. that contribute to electrodeposition coating properties.

See, for example, FIG. FIG. 2 shows the plating bath composition Na25On: 40 g/6°Pe": 50 g/(1°Zn": 50 g/β.

Polypropylene glycol '5ppm + Sodium acetate (p
H buffer): various amounts.

Thou Yu Dong Kei Engineering and pH1.8゜ Current density 80A/d.

relative liquid flow rate 1.Qm/sec.

When manufacturing Fe-Zn alloy plated steel sheet under the following conditions,
This is a graph showing the relationship between the amount of pH buffer (sodium acetate) added and the paint sharpness, and from this figure 2 it can be seen that adding a nonionic surfactant and increasing the relative flow rate of the plating solution It is not possible to ensure a good appearance after painting with only
In particular, it can be seen that a Fe--Zn alloy plated mesh plate with excellent paint sharpness can only be obtained by simultaneously adding a pH buffering agent of 10 g/β or more.

Furthermore, the Fe-Zn alloy plating bath according to the present invention also works to reduce anodic oxidation of iron in the bath, and when metal Fe or metal Zn is added as ion supply to the bath, these metals are It has also been revealed through research by the present inventors that the reduction efficiency of Fe3+ ions is increased by the dissolution of Fe3+ ions.

Table 1 compares the amount of anodic oxidation and the metal reduction efficiency of Fe-Zn alloy plating baths with and without the additive according to the present invention.

The results in Table 1 were measured under the following conditions.

Plating bath conditions Bath composition: Fe”: 50g/I2゜Zn”:
50g/j! .

Na2S O4: 40g/ff.

Bath temperature: 50°C 0 bath pH: pH 1.8° Electrolytic conditions Current density: 80 A/d%.

Relative flow rate of liquid...l m/sec.

Plating film composition: Fe is 15iyt%.

Furthermore, the "amount of anodic oxidation" in Table 1 is 7000 c/j using the insoluble anode under the above conditions! This is the amount of anodic oxidation when electricity is applied, and the "metal reduction efficiency" is a value calculated using the following formula.

Table 1 It can also be confirmed from Table 1 above that in the FeZn alloy plating bath according to the present invention, anodic oxidation of iron is suppressed and the Fe'' reduction efficiency when metal Fe and metal Zn are added is high.

However, since the effect of this oxidation suppression is small when only a nonionic surfactant is added, this effect is also likely to be synergistic when combined with a pH buffer, as in the case of image clarity after painting. Conceivable.

Next, the present invention will be specifically explained with reference to Examples.

<Example> A cold-rolled steel plate with a thickness of 0.8 mm was prepared and subjected to Fe-Zn alloy plating under the electroplating conditions shown in Table 2.

The basic composition of the plating bath at this time was Na2SO4 except for the Fe ions and Zn ions shown in Table 2, the electrolytic current density was 80 A/drd, and the relative liquid flow rate was l.
m/sec.

The Fe content of the plating film obtained by such Fe-Zn alloy electroplating treatment, the amount of plating, the paint sharpness, the metal dissolution and reduction efficiency of the plating bath, and the F content in the plating bath.
The results of the investigation on the amount of e'' ions produced are also shown in Table 2.

In addition, "Metal dissolution reduction efficiency of plating bath" in Table 2
and ``amount of Fe'' ions produced in the plating bath'' were measured using the same method used to obtain the results in Table 1, and ``paint sharpness'' was measured using the results shown in Figure 1 or 2. As in the case obtained above, the value is measured using a PGD meter using a 3-coat material (film thickness: ~100 μm) (the larger the value, the better the image clarity).

As is clear from the results shown in Table 2, when plating treatment is carried out according to the conditions of the present invention, both air oxidation and anodic oxidation of Fe in the plating bath are suppressed, and metal Pe and metal It can be seen that the dissolution reduction efficiency due to Zn also increases, and as a result, a Fe-Zn alloy plated steel sheet with excellent surface gloss and uniformity can be stably obtained.

Summary of Effects> As explained above, according to the present invention, the plating film has excellent uniformity and has a good appearance, and the finish quality (image clarity, etc.) after painting is significantly improved. Excellent Fe −
Industrially extremely useful effects are brought about, such as making it possible to stably produce Zn alloy electroplated materials at low manufacturing costs.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between relative liquid flow rate and coating clarity. FIG. 2 is a graph showing the relationship between the amount of pH buffer added and paint clarity.

Claims (1)

[Claims] Add one or more pH buffers in a total amount of 10 to 500 g/l into the sulfate-based iron-zinc alloy plating bath, and add one or more nonionic surfactants. Production of an iron-zinc alloy electroplated material with excellent paint finish, characterized by adding 0.1 to 10 ppm in total amount and electroplating at a relative flow rate of plating solution of 0.5 m/sec or more. Method.