KR20020034485A - Corrosion resistant zinc plating bath and products - Google Patents

Abstract

PURPOSE: A zinc plating bath is provided to increase corrosion resistance by forming proeutectic ξ(Zn solid solution) phase or proeutectic σ(MgZn2) phase in zinc-alloy plated layer. CONSTITUTION: The corrosion resistant zinc plating bath is composed of Al 1.5-3.0wt.%, Mg 2.0-6.5wt.%, and the balance Zn and other inevitable impurities wherein Al and Mg satisfies the following inequality: 1.6<=Mg/Al<=6.4. The zinc-alloy plated layer formed by the corrosion resistant plating bath has proeutectic ξ(Zn solid solution) phase or proeutectic σ(MgZn2) phase.

Description

Corrosion resistant Zinc plating bath and products}

The present invention relates to a hot-dip zinc alloy plated steel sheet for use in automobiles, home appliances, and building materials. More specifically, the present invention is based on the interaction of Al and Mg in a zinc plating bath to affect the expression of corrosion resistance. The present invention relates to a zinc plating bath capable of forming ξ (Zn solid solution) or primary σ (MgZn ₂ ) and a zinc alloy plated steel sheet produced by the plating bath.

Zinc-plated products protect zinc by sacrificial anti-corrosion acting as zinc in the zinc-plated layer with higher oxidation potential is first dissolved when part of the base metal is exposed, and corrosion products of zinc formed from it are densified. It acts to suppress. Corrosion protection mechanism (Mechanism) of the galvanized product is known to exhibit corrosion resistance of about 5 ~ 10g / ㎡ / year in the power supply zone, and corrosion resistance of about 20 ~ 30g / ㎡ / year in the coastal zone. In other words, as the amount of plating deposition increases, the service life of the steel increases. In order to extend the life of the zinc alloy plated steel sheet, a technique for improving the corrosion resistance of the plating layer itself by adding an ellipsoid to the hot dip plating bath has been generalized.

As an example, US Patent No. 3,505, 042 proposes a plating bath in which 1.5 to 5% by weight of magnesium and 0.05 to 1% by weight of aluminum are added to a hot dip galvanizing bath. This product has the disadvantage of poor corrosion resistance. In addition, since it is difficult to control the plating amount due to lack of fluidity of the plating bath, there is a disadvantage in weakening the plating conditions such as increasing the pressure of the air wiper for adjusting the plating amount.

As another example, Japanese Laid-Open Patent Publication No. 58-177447 has a composition of 0.1 to 25% by weight of aluminum, 0.05 to 2.0% by weight of magnesium, 0.005 to 1.5% by weight of antimony, 0.01% by weight of lead and the rest of zinc. A plating bath is proposed. This plating bath is also considered to be weak in securing corrosion resistance of zinc alloy plated steel sheets.

The present invention is to provide a zinc plating bath and a zinc alloy plated steel sheet prepared by the zinc plating bath to maintain the proper fluidity while improving the corrosion resistance of the zinc alloy plated steel sheet, the object thereof.

Figure 1 (a) (b) is a microstructure photograph of the plating layer formed according to the present invention,

Figure 1 (a) is a plated texture picture of the primary ξ (Zn solid solution) (A in the image) is formed

Figure 1 (b) is a plated structure picture formed with a primary σ (MgZn ₂ ) phase (A in the photo)

Figure 1 (c) is a microstructure photograph of the plating layer formed by a method deviating from the present invention

The galvanizing bath of the present invention for achieving the above object comprises 1.0 to 4.0% by weight of aluminum and 2.0-20.0% by weight of magnesium and the remaining Zn and other unavoidable impurities.

In addition, the zinc alloy plated steel sheet of the present invention has a zinc alloy plating layer containing 1.0 to 4.0% by weight of aluminum and 2.0-20.0% by weight of magnesium and the remaining Zn and other unavoidable impurities. Solid solution) or σ (MgZn ₂ ).

Hereinafter, the present invention will be described in detail.

The present inventors, in the course of research to improve the corrosion resistance of zinc alloy plated steel sheet, the primary ξ (Zn solid solution) or primary σ (MgZn ₂ ) appears depending on the content of Mg in the plating layer formed by the Zn-Mg-Al plating bath. It has been found that the corrosion resistance is greatly improved in the plating layer having a supernormal phase. In addition, it was confirmed that the ratio of the addition amount of Mg and Al also affected the corrosion resistance characteristics in the plating layer having these supernormal phases. In addition, it has been possible to find out a new effect that the amount of Al added in order to secure plating adhesion affects the flowability of the plating bath at 1.0% by weight or more. The present invention will be described by dividing the plating bath and the plated steel sheet manufactured by the plating bath.

[Molten zinc plating bath]

In the zinc plating bath of the present invention, Al and Mg are added to the Zn plating bath. The plating bath contains inevitable impurities of Fe, Pb, Sb, Sn, and Cu, but these impurities have little effect within the usual content range (about 0.02%). Although the compounding ratio of Zn, Mg, and Al was derived so that primary ξ (Zn solid solution) or primary σ (MgZn ₂ ) was formed in the plating layer obtained by this plating bath, there is a feature of the present invention. In the Zn-Al-Mg plating bath system, a superfine phase is formed according to Mg content. When the Mg content is 2-3%, a primary ξ (Zn solid solution) phase is formed, and when the Mg content is 3% or more. A primary σ (MgZn ₂ ) phase is formed. Importantly, these ultranormal phases have a close influence on the corrosion resistance of the plating layer.

Aluminum (Al) is preferably contained 1.0 to 4.0% by weight.

Al is known as an element to secure the adhesion of the plating layer. When aluminum is added to the molten zinc plating bath, the ternary alloy plating layer of iron-aluminum-zinc and the base iron are uniformly and continuously formed to improve adhesion. Al was added in an amount of about 0.2% by weight in order to secure adhesion.

However, the present inventors have found a new effect of affecting the corrosion resistance with Mg while controlling the fluidity in the plating bath to which Mg is added as the amount of Al is increased to 1.0% by weight. To this end, Al should be added more than 1.0% by weight, but even more than 4.0% by weight has no effect on the progress of fluidity. When the Al content is 3.0% by weight, the flowability is improved, so the Al content is most preferably added 1.5 to 3.0% by weight.

Magnesium (Mg) is preferably contained 2.0 to 20.0% by weight.

In the present invention, magnesium is an important element for improving the corrosion resistance. The zinc plating bath produces zinc hydroxide and zinc oxide, which are corrosion products, in the corrosive environment. When magnesium is added, it acts to suppress zinc oxide, which is an early corrosion product, and to stabilize zinc hydroxide. In addition, magnesium exists as an oxide in the pole surface layer of the plating layer, contributing to the improvement of corrosion resistance. For this purpose, it is preferable to add 2.0% by weight, but corrosion resistance is maintained even when added to 20.0% by weight. However, at 20.0% by weight or more, it is not preferable because it is difficult to deal with the plating bath containing excessively strong magnesium and cost is added. Magnesium content is greatly improved to the corrosion resistance up to 6.5%, Mg is preferably added at 2.0 to 6.5% because the effect of improving the corrosion resistance is slowed even if added more than 6.5%.

Ratio of Mg / Al

The inventors found out that 1.0 to 4.0% by weight of Al and 2.0 to 20.0% by weight of Mg are most effective in improving the corrosion resistance when the Mg / Al ratio is adjusted to 1.6 to 6.4. When the Mg / Al ratio is less than 1.6, it is disadvantageous to secure the fluidity due to the lack of Al content. When the Mg / Al ratio exceeds 6.4, there is no effect on the progress of corrosion resistance.

[Molten zinc alloy plated steel sheet]

When the alloy plating layer is formed on the steel sheet by the Zn-Al-Mg plating bath, the present inventors have formed a matrix structure of the alloy plating layer as a dendrite, and the primary crystal ξ (Zn solid solution), or primary σ ( Formation of MgZn2) was found to be effective in improving corrosion resistance. The mechanism of improving the corrosion resistance of the primary ξ (Zn solid solution) or primary σ (MgZn 2) is believed to stabilize the corrosion products of zinc hydroxide and zinc oxide, delay the formation of these corrosion products and improve the corrosion resistance. do.

FIG. 1 (a) is a structure photograph of a plated layer of a steel plate plated with a galvanizing bath containing 1.0 wt% aluminum and 2.0 wt% magnesium and consisting of the remaining Zn and other unavoidable impurities. As can be seen in Fig. 1 (a), the primary ξ (Zn solid solution) phase indicated by A is shown on the dendritic matrix.

FIG. 1 (b) is a structure photograph of a plated layer of a steel plate plated with a galvanizing bath including 3.2 wt% aluminum and 6.3 wt% magnesium and the remaining Zn and other unavoidable impurities. As can be seen from Fig. 1 (b), the σ (MgZn ₂ ) phase indicated by A is shown in the dendritic matrix.

In Fig. 1 (c), no photograph is shown in the dendritic matrix structure as a photograph of an alloy plating layer obtained by a plating bath composed of 0.5% by weight of aluminum and 0.12% by weight of Zn.

The alloy plating layers of FIGS. 1 (a) and 1 (b) had weight loss of 8.7 and 0.9 g / m 2 in the corrosion-promoting test by SST, respectively, whereas the alloy plating layer of FIG. 1 (c) had weight loss of 17.1 g / m 2. It was confirmed that the alloy plating layer formed with primary tablets was excellent in corrosion resistance.

The zinc alloy plated steel sheet produced according to the present invention may be subjected to skin pass milling, tension lwvelling, chromate or phosphate treatment or oiling in a subsequent process according to a conventional method. .

Hereinafter, the present invention will be described in detail through examples.

Example 1

Prepare specimens by removing ultrasonic contaminants and oil from the surface of the steel sheet by immersing them in acetone in a hard cold rolled steel sheet having a thickness and width of 0.8 mm x 220 mm, respectively. The prepared specimen is heat-treated to 700 ~ 820 ℃ to suit the mechanical properties required in the reducing atmosphere and then cooled to the temperature of the plating bath. At this time, the plating bath property to have each composition of the Examples and Comparative Examples described in Table 1. After the specimen was immersed in the plating bath for about 3 seconds, plating was performed by adjusting the plating deposition amount to about 150 g / m 2 (single side) with an air wiper.

The method of measuring physical properties, such as corrosion resistance and fluidity | liquidity, was implemented by the following method.

Corrosion resistance: Measure the weight of the plating specimens prepared in each plating composition. The corrosion promotion test by SST is then carried out for 120 hours. Subsequently, it is immersed in a 10% solution of ammonium chloride having a characteristic of dissolving only zinc corrosion products for 60 minutes to dissolve and remove the corrosion products formed on the surface, and then weigh again. The difference from the initial weight is called weight loss. The smaller the weight, the better the corrosion resistance. The target weight loss should be less than 10g / ㎡.

Adhesiveness: Detach the plated specimens with scotch tape by conducting 180 bend tests.

○: Good adhesion if no plating peeling is attached to the scotch tape

X: When the peeling of plating is attached to the scotch tape and plating peeling occurs, the adhesion is poor.

Flowability: Flow at the surface of plating when the steel plate specimen is immersed in the hot dip bath and then raised

(Circle): It flows smoothly at the plating surface.

(Triangle | delta): There exists a little bit of a working flow state of molten state on the surface.

X: Surface flow is bad, and unevenness | corrugation is severe as a flow trace.

division Plating bath composition (wt%) Corrosion resistance Adhesion liquidity Al Mg Zinc and Unavoidable Impurities Mg / Al Inventive Example 1 1.00 2.00 Remainder 2 8.7 ○ ○ Inventive Example 2 3.04 3.04 3.0 ○ ○ Inventive Example 3 4.58 4.58 4.4 ○ ○ Inventive Example 4 6.33 6.33 2.2 ○ ○ Inventive Example 5 11.98 11.98 3.5 ○ ○ Inventive Example 6 15.86 15.86 4.5 ○ ○ Inventive Example 7 20.0 20 4.0 ○ ○ Inventive Example 8 1.17 6.33 5.41 3.1 ○ ○ Inventive Example 9 3.20 6.33 1.97 0.9 ○ ○ Inventive Example 10 3.98 6.33 1.6 2.1 ○ ○ Comparative Example 1 - - - 90.0 × × Comparative Example 2 - 3.14 - 3.5 × △ Comparative Example 3 - 9.84 - 5.2 × △ Comparative Example 4 0.5 0.12 0.24 17.1 ○ △ Comparative Example 5 0.5 3.05 6.1 3.9 ○ △

As can be seen in Table 1, the steel plate plated with a plating bath of the present invention containing 1.00 to 3.98% by weight of aluminum and 2.00 to 20.0% by weight of magnesium is very excellent in corrosion resistance with a weight loss of 10 g / ㎡ , Adhesiveness and fluidity were also good. However, when aluminum is not contained and magnesium is contained in 3.14 to 9.94% by weight, plating peeling phenomenon occurs and plating adhesion is poor. In addition, when the aluminum content contains 0.5% by weight of aluminum, the flowability of the plating bath is slightly insufficient, so that there is a slight flow trace due to the plating flow.

According to the present invention, a method for producing a hot-dip galvanized steel sheet having excellent corrosion resistance by adding aluminum and magnesium to a hot-dip galvanizing bath is provided, thereby significantly improving the corrosion resistance of secondary steel products. In addition, when the plating having the same corrosion resistance can be performed, the plating deposition amount can be greatly reduced, and the effect of maximizing resource utilization is expected.

Claims (5)

A hot dip galvanizing bath comprising 1.0 to 4.0% by weight of aluminum and 2.0-20.0% by weight of magnesium and the remaining Zn and other unavoidable impurities. The hot dip galvanizing bath having excellent corrosion resistance according to claim 1, wherein the aluminum is 1.5 to 3.0% by weight and the magnesium is 2.0 to 6.5% by weight. The hot dip galvanizing bath having excellent corrosion resistance according to claim 1 or 2, wherein aluminum (Al) and magnesium (Mg) satisfy the following relationship: 1.6≤Mg / Al≤6.4. Including the 1.0~4.0% by weight of aluminum and magnesium of 2.0-20.0 wt% and the remainder Zn and other has a zinc alloy plating layer consisting of unavoidable impurities, the zinc alloy coating layer has invited ξ (Zn solid solution) or a Primary σ (MgZn ₂₎ Zinc alloy plated steel sheet with excellent corrosion resistance. The zinc alloy plated steel sheet having excellent corrosion resistance according to claim 4, wherein aluminum (Al) and magnesium (Mg) satisfy the following relationship: 1.6≤Mg / Al≤6.4.