JPH0788598B2 - Multi-layer zinc-chrome plated steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a multi-layer plated steel sheet for rust prevention, which is used for automobiles, home appliances, building materials, etc. and which has excellent corrosion resistance and chemical conversion treatability.

(Prior Art) Examples of electroplated steel sheets containing Zn or Zn-based alloy plating with Cr include, for example, Japanese Patent Publication No. 61-36078, Japanese Patent Publication No. 58-56039, and Japanese Patent Publication No. 61-270398. However, the eutectoid ratio of Cr is very small, from 0.005 to 5%, and the effect of Cr can only be incidental to the corrosion resistance. Therefore, for the purpose of improving the corrosion resistance, it is strongly desired to co-deposit a higher content of Cr.

(Problems to be Solved by the Invention) However, there has been no conventional Zn-Cr electroplating technique capable of increasing the Cr eutectoid ratio. That is, even if the trivalent Cr ion concentration in the plating bath is simply increased, normal plating with adhesion cannot be obtained, and there are obstacles such as a sharp decrease in current efficiency.
It was extremely difficult to increase the eutectoid rate.

The present inventors have proposed a Zn-Cr-based electroplating method having a high Cr content, and have obtained a use for dramatically improving corrosion resistance. However, as the Cr content increases, in the chemical conversion treatment commonly used as a pretreatment for coating, since the phosphate film is formed only incompletely, although the corrosion resistance after coating is slightly improved by the effect of the plating layer itself, I couldn't expect such a big improvement.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a plated steel sheet with greatly improved corrosion resistance after coating.

(Means for Solving the Problem) In the present invention, the zinc-chromium plating layer containing Cr of 7 to 40% is used as a lower layer, and the zinc-chromium plating layer containing Cr of 1% or more and less than 7% is used as an upper layer to form a multilayer coating. It is a zinc-chromium plated steel sheet.

(Operation) The corrosion resistance of the present double-layer plated steel sheet is mainly due to the operation of the lower layer. The corrosion resistance of the Zn-Cr system plating layer improves as the Cr content increases. When Cr is 5% or more, a remarkable corrosion resistance improving effect appears, and when Cr is 7% or more, for example, even in a salt spray test for one month, surprising corrosion resistance is observed to the extent that red rust hardly occurs.

Such high corrosion resistance, conventionally known Zn plating, or
This cannot be achieved at all by alloy plating such as Zn-Fe and Zn-Ni.

Cr does not passivate in the coexistence with Zn, participates in sacrificial anticorrosion action together with Zn, and suppresses corrosion by depositing the corrosion product of Cr as a sparingly soluble protective film on the corroded part. It is considered that this is the reason why high corrosion resistance is exhibited.

When the Cr content exceeds 40%, although it has corrosion resistance, the so-called loose powdering property that the plating layer peels off during processing such as press working deteriorates, so that it is practically difficult to apply.

However, when the Cr content is 7% or more, the phosphate film crystals become coarse when the chemical conversion treatment is performed, and bare parts where no film is formed are exposed between the crystals. When the Cr content is further increased, a phosphate film is finally not formed at all.

Phosphate treatment is generally used as a pretreatment for coating, and has the effect of ensuring the adhesion of the coating film and improving the corrosion resistance after coating. It is desirable to form a healthy phosphate film in order to extend the corrosion resistance life after painting.

It was found that the phosphate coating was densely formed in the chemical conversion treatment in the Zn-Cr plated layer having a composition range of Cr content of 1% or more and less than 7%.

However, the corrosion resistance of the Zn-Cr plating layer in this composition range is pure.
Compared to Zn plating, it is good but not sufficient. Therefore, the upper layer of the multi-layer plating layer and the lower layer of Zn7-40% Zn
By forming the Cr-plated layer, a plated steel sheet having both corrosion resistance and chemical conversion treatability can be obtained.

The upper layer preferably has a thickness of at least 0.5 g / m ² . If it is less than 0.5 g / m ² , there is a risk that the upper layer will be completely dissolved and the lower layer will be exposed during the chemical conversion treatment, and the phosphate film will locally become coarse. The thickness of the upper layer can be further increased, for example, about 15 g / m ² . This is because there is almost no potential difference between the upper layer and the lower layer, and therefore there is no factor that deteriorates the corrosion resistance of the entire multilayer structure even if the upper layer is thickened.

Applicable chemical conversion treatment is spray type or immersion type phosphate treatment, and the effect is recognized as a treatment agent which reacts with the upper layer to form a film during the treatment.

In addition to Zn and Cr, small amounts of Fe, Ni, Co, Mn, Sn and P are used in both upper and lower layers.
It is acceptable to include b, Cd, Al, Cu, H, B, C, P, O, Si, Ti, Zr, Sb, Mo, Ce and the like. Next, the manufacturing mode will be described.

In the electroplating method, both upper and lower layers are Zn ions and trivalent
From an acidic plating bath containing Cr ions and Cr precipitation accelerator,
It can be electrodeposited.

The upper layer is preferably electrodeposited with a current density of 30 A / dm ² or more, and the lower layer is electrodeposited with a current density of 50 A / cm ² or more.

As the Cr precipitation promoter, for example, 0.01 to 20 g / polyoxyalkylene derivative such as polyethylene glycol, alkyl polyethylene ether, polyoxymethylene derivative, α-ethoxylated naphthol and its sulfonic acid derivative is used. In another embodiment, oxide colloids such as Si, Al, Zr, Ti, Sb, Sn, Cr, Mo and Ce can be used in an amount of 2 to 200 g / addition. Further, these may be deposited in the plating layer.

In addition, alkali metal ions such as Na ⁺ , K ⁺ and NH ₄ ⁺ can be added as a conductivity imparting agent. As the anion, either a sulfuric acid bath or a chloride bath can be applied, and a pH of 0.5 to 3 is suitable.

In addition to inevitable coexisting ions such as hexavalent chromium ions, iron divalent and trivalent ions, Ni, Co, Mn, Cu, Ag, Sn, Sb, Cd,
Ions such as Pb, Mo, Al, and Mg may be added if necessary and may be deposited in the plating layer.

The flow velocity of the plating solution is 30 to 200 m / m as a relative velocity with the steel strip.
In, the appropriate plating temperature is 40-70 ℃.

As a special embodiment, the upper layer and the lower layer may be electrodeposited from the same plating bath at different current densities. Besides the electroplating method, vapor deposition plating, thermal spray plating, etc. can also be applied. In these methods, the Zn-Cr composition can be easily controlled in both the upper layer and the lower layer by projecting Zn and Cr from different radiation sources while controlling the projection speed.

(Example) Next, an example of the present invention is shown in Table 1 together with a comparative example.

Note 1) Lower layer plating conditions Zn ²⁺ ion 56g /, Cr ³⁺ ion 44g /, Na ⁺ ion 15g /
As a sulfate, bath, polyethylene glycol (molecular weight
1500) 1 g / addition pH = 1.2, 50 ° C. plating bath, cold-rolled steel sheet (0.7 mm thickness) with a plating solution flow rate of 60 m / min and current density of 100 A / dm ² Other examples were manufactured under different conditions.

Note 2) Upper layer plating conditions After lower layer plating, test pieces washed with water were electroplated under the following conditions.

Zn ²⁺ ion 85g /, Cr ³⁺ ion 50g /, Na ⁺ ion 12g /
As a sulfate, bath, polyethylene glycol (molecular weight
1500) 1g / added pH = 1,50 ℃ plating bath, and the plating solution flow rate 5m /
The upper layer of Example 1 was plated at a current density of 10 A / dm ² . Other examples were manufactured under different conditions.

The multi-layer plated steel sheet thus produced was evaluated as shown in Table 2.

Note 3) Corrosion resistance of bare galvanized steel sheet Indicates the area% of red rust after 720 hours by salt water spray test (according to JIS Z2371).

Note 4) Phosphate treatability Evaluate the crystalline state of the film in the immersion type phosphate treatment.

○: Dense crystals △: Coarse crystals Note 5) Corrosion resistance of coated plate Immersion type phosphate treatment, cationic electrodeposition coating 20 μm applied, intermediate coating, water polishing, top coating total coating thickness 80 μ
m, and the test piece containing the cross cut was exposed outdoors. A 5% saline solution was sprinkled on the sample once a week during the exposure period, and the rust width of the cross cut portion after 10 weeks was evaluated (mm).

(Effects of the Invention) As described above, according to the present invention, it is possible to obtain a multi-layer electroplated steel sheet for rust prevention, which has high corrosion resistance and excellent corrosion resistance after coating which is excellent in chemical conversion treatment.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Sachi Arai 5-3 Tokai-cho, Tokai-shi, Aichi Pref., Nippon Steel Co., Ltd. Nagoya Works (72) Inventor Shinichi Suzuki 5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Co., Ltd. Nagoya Steel Works

Claims (1)

[Claims] 1. A zinc-chromium plated steel sheet having a zinc-chromium plated layer containing 7 to 40% of Cr as a lower layer and a zinc-chromium plated layer containing 1% to less than 7% of Cr as an upper layer.