JP2528944B2 - Method for producing Zn-based alloy electroplated steel sheet excellent in chemical conversion treatability and corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a method for producing a Zn-based alloy electroplated steel sheet,
In particular, it relates to a method for improving the chemical conversion treatment property and corrosion resistance of the plated surface.

<Prior Art> Since Zn-based electroplated steel sheets can achieve high corrosion resistance without deteriorating the material quality, their applications have expanded to a wide range of fields where cold-rolled steel sheets have been used, such as automobiles, home appliances, and building materials. It's starting.

In particular, in recent years, Zn-Ni, Zn-Fe for the purpose of improving corrosion resistance
Zn-based alloy electroplated steel sheets have been developed and are being actively applied to automobile bodies, resulting in a marked increase in production.

However, at present, the phosphate crystals generated during the phosphate chemical conversion treatment on the Zn-based alloy electroplated steel sheet are in the form of hopeite {Zn ₃
In _{(PO 4) 2 · 4H 2} O}, for the hopeite the adhesion of the paint is poor in comparison with _{phosphophyllite {Zn 2 · Fe (PO} 4) 2 · 4H 2 O} generated on cold-rolled steel sheet The main part of its use is the inner surface of the vehicle body, but there is a problem in using it outside the vehicle body.

Therefore, for example, as a method of forming a chemical conversion film having good coating adhesion with a dense crystal on the Zn-Ni plated surface, after applying a predetermined amount of Zn-Ni plating, non-immersion in a plating bath The method is disclosed in Japanese Patent Application Laid-Open No. 62-294198, and the present applicant has applied for a patent application No. 221955 (Japanese Patent Application Laid-Open No. 1-65289) in 1987. This method is a method of dissolving a coating film of zinc hydroxide, zinc oxide, etc. formed on the plating surface layer during electroplating of a Zn-based acid under acidless conditions with an acid, and commonly uses a plating solution as a dipping solution during acid dissolution. Therefore, it is only necessary to add a cell of Zn-based alloy electroplating equipment in terms of equipment, and it is simple in terms of equipment.Zinc dissolved during electroless immersion is effective again as a zinc source in electroplating without any treatment. In addition, the pH range of the plating solution for electroplating is the same as that of zinc hydroxide, zinc oxide, etc. for acid dissolution of the coating, so the pH can be adjusted only by adjusting the pH of the plating solution. It has the advantage of being able to

However, these methods have a problem that the corrosion resistance is deteriorated even though the chemical conversion treatability can be improved.

The reason is considered as follows. That is, in the case of alloy plating, if a zinc electrode is used, the zinc electrode dissolves and the balance of the plating bath composition cannot be maintained, so a lead-based insoluble electrode is generally used, and a metal suitable for the amount of plating deposition is used. The components are replenished from outside the system for electroplating with a predetermined alloy content and a predetermined adhesion amount, but in this case, lead, which is the main component of the electrode, elutes very little in the plating bath. Therefore, after performing a predetermined amount of Zn-Ni plating, if the plating solution is immersed under non-energized conditions, zinc in the plating surface layer dissolves in acid, but if lead ions are present in the plating solution, the Even if the amount is very small, substitutional precipitation occurs on the plating surface. Therefore, a galvanic cell is formed between lead and zinc, and Zn-Ni
The original corrosion resistance of plating cannot be obtained.

<Problems to be Solved by the Invention> The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a Zn-based alloy electroplated steel sheet having excellent chemical conversion treatability and corrosion resistance.

<Means for Solving the Problems> The present invention, after applying Zn-based alloy electroplating to a steel sheet using a lead-based insoluble electrode, the pH is 0.5 or more, 2.5 or less in the Zn-based alloy electroplating solution Electrolytically dipped to dissolve zinc in the plating surface layer with acid, then washed with water, and then Na, K, Li, Mg, Al, Ca, B
Anodic electrolytic treatment is performed in an electrolytic solution containing at least one of e, Ba phosphate, sulfate or oxyacid salt of 0.1 mol / min or more and having a pH of 4 or more, and substitution deposition is performed on the plating surface during the electroless immersion. Is a method for producing a Zn-based alloy electroplated steel sheet having excellent chemical conversion treatability and corrosion resistance, which is characterized by removing lead.

<Operation> The Zn-based alloy electroplated steel sheet in the present invention broadly includes Zn-based alloy plated steel sheet mainly containing Zn. For example, Zn-Ni-based, Zn-Mn-based, Zn-Cr-based, Zn-Fe-based alloy plating, and further P, Co, Cr, Sn, Sb, V, Fe, Ti, Ni, Mn, A
s, BiAl, Pb, Si, Tl and any Zn-based alloy or composite plating film, such as those intentionally added or inevitably mixed with one or more of oxides, carbides and nitrides thereof. It is plated with an acidic liquid such as a sulfuric acid bath, a chloride bath or a mixed bath thereof.

Usually, in order to industrially manufacture Zn-based alloy plated steel sheet,
After pretreatment such as electrolytic degreasing, washing with water, pickling, washing with water, etc., using an electrolytic solution of the specified composition, sequentially perform a cathodic treatment in a plating bath consisting of several to ten or more cells to contain the specified alloy. The amount and amount of electroplating is performed.

In the present invention, after performing electroplating with a predetermined alloy content and a predetermined adhesion amount, first, the zinc in the plating surface layer is acid-dissolved by electroless immersion in the same (or similar) plating solution as described above. After that, it is washed with water, and then subjected to an anodic electrolysis treatment to remove lead deposited by substitution on the plating surface during the electroless immersion. After finishing the anodic electrolytic treatment, the plated steel sheet is washed with water and dried to obtain a product.

Of the electroless immersion liquid performed before the anodic electrolytic treatment in the present invention
The pH is preferably 0.5 or more and 2.5 or less, and the immersion time is preferably 3 seconds or more and 20 seconds or less. The reason is that if the pH is less than 0.5, the plating layer will be excessively dissolved and the coating weight will be reduced, which is economically disadvantageous. Further, if the pH exceeds 2.5, the dissolving power is lowered, so that the dipping time is required to be long and the production capacity is lowered, which is not a good idea.

The immersion time is preferably 3 seconds or more and 20 seconds or less. Prolonging the immersion time beyond 20 seconds is not a good idea because it causes a decrease in productivity, and also causes a decrease in chemical conversion treatability. If it is less than 3 seconds, the effect of improving the chemical conversion treatment property is small.

It is desirable that the anodization has a current density of 1 A / dm ² or more. If it is less than 1 A / dm ² , the lead deposited by substitution and precipitation on the plating surface layer is not sufficiently removed, and the effect of improving the corrosion resistance is small.
It takes a long time in the process and is not economical. The upper limit of the current density is not particularly limited, but 150 A / dm ² or less is preferable in consideration of the power loss due to the increase in electrolysis voltage.

When the amount of electricity is 5 coulombs / dm ² or more, it is possible to remove lead deposited by substitution on the plating surface, and when it is less than 5 coulombs / dm ² , a sufficient effect cannot be obtained. Although there is no upper limit to the amount of electricity, when the amount of electricity increases, the amount of coating weight decreases due to metal dissolution, which is not economical and is preferably 500 coulomb / dm ² or less.

The pH of the electrolytic solution is not particularly limited, but is preferably 4 or more. If the pH is less than 4, the chemical dissolution of the plating layer will remarkably increase, and the coating weight will decrease, which is not economical.

As the electrolytic solution, if it is an electrolytic solution having a pH of 4 or more, it may be anything that is usually used as an anode electrolytic solution, and may contain any component, Na, K, Li, Mg, Al, Ca, Be, It is preferable to use at least one of phosphate, sulfate or oxyacid salt such as Ba as a main component, and it is desirable to adjust the pH using a phosphoric acid, sulfuric acid, oxyacid or sodium hydroxide solution. The reason for this is that the deposition potential of the metal ions is higher than the hydrogen reduction potential, so that electrolysis of the aqueous solution does not cause deposition of the metals. Therefore, the maintenance of the metal ion concentration in the bath and the maintenance of the cathode are easy, and the acid produced on the anode and the alkali produced on the cathode balance in an equivalence relation, which is also suitable for maintaining the bath pH. Because.

The concentration of the main component of the electrolytic solution is preferably 0.1 mol / mol or more in terms of total phosphoric acid concentration when an aqueous solution of phosphate is used. Also, when an aqueous solution containing a sulfate or an oxyacid salt as a main ingredient is used, the concentration of the electrolytic solution is preferably 0.1 mol / or more as the total sulfuric acid concentration or the total oxyacid concentration of the main ingredient, respectively. The reason for this is that if the component concentration of the electrolytic solution is less than 0.1 mol / liter, the effect of removing Pb which has been deposited by substitution on the plating surface is small. Further, the upper limit is preferably not higher than the solubility of phosphate, sulfate or oxyacid salt.

<Example> Next, the present invention will be described based on examples.

A cold-rolled steel plate having a plate thickness of 0.7 mm and equivalent to SPCC was used for plating. A lead-based insoluble electrode is used as the electrode, and the plating bath composition and plating conditions are as follows.

(1) Zn-Ni bath composition NiSO ₄・ 6H ₂ O 250g / ZnSO ₄・ 7H ₂ O 130g / Na ₂ SO ₄ 40g / Pb 0-0.01g / (2) pH 1.6 (3) Bath temperature 60 ℃ (4 ) Current density 50A / dm ² (5) Plating time 15sec (6) Plating basis weight 20g / m ² (7) Ni content 12% Immediately after plating, rinse with water, and continue to show in Table 1 with the plate surface wet. As described above, the electroless immersion treatment was performed, followed by washing with water, followed by anodic electrolytic treatment, and then washing with water and drying. The bath used for immersion was the plating bath, and the pH was adjusted with sulfuric acid and sodium hydroxide.

The amount of lead attached to the surface after electrolytic treatment was analyzed by glow discharge emission spectroscopy as integrated intensity from the surface layer to 500 Å.

The corrosion resistance was evaluated by a salt spray test and visually evaluated as follows.

○ …… The rate of red rust after 300 hours is less than 5% △ …… The rate of red rust after 300 hours is less than 5 to 10% × …… The rate of red rust after 300 hours is 10% or more Chemical conversion treatability Was subjected to chemical conversion treatment using a phosphating solution Granodine SD2500 (Dip method) manufactured by Nippon Paint. The chemical conversion treatability is evaluated by visually observing the external appearance and observing the chemical conversion crystals using a SEM (Scanning Elecron Microscope).

○ …… The appearance is uniform and uniform, and the crystals are uniform and dense △ …… The appearance is partially uneven and the crystals are slightly uneven × …… The appearance is uneven and the crystals are uneven <Invention Effect> As described above in detail, according to the manufacturing method of the present invention, a Zn-based alloy electroplated steel sheet excellent in chemical conversion treatability was obtained without deteriorating corrosion resistance.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Isamu Takasaki 2-88 Wakihama Kaigan-dori, Chuo-ku, Kobe-shi, Hyogo Kawasaki Steel Co., Ltd. Hanshin Works (56) Reference JP-A-58-210194 (JP, A) JP 58-45382 (JP, A) JP 63-266100 (JP, A) JP 56-1400 (JP, B2)

Claims (1)

(57) [Claims] 1. A Zn-based alloy electroplated on a steel sheet using a lead-based insoluble electrode, and then having a pH of 0.5 or more and 2.5 or less.
Electroless immersion in a system alloy electroplating solution, after dissolving zinc in the plating surface layer with acid, washed with water, and further Na, K, Li, Mg, Al, C
Anodic electrolytic treatment is performed in an electrolytic solution containing at least one of a, Be, Ba phosphates, sulfates or oxyacids of 0.1 mol / min or more and having a pH of 4 or more. A method for producing a Zn-based alloy electroplated steel sheet having excellent chemical conversion treatability and corrosion resistance, which is characterized by removing substitutionally precipitated lead.