JPH01191797A - Zinc-chromium alloy electroplated steel sheet - Google Patents

Abstract

PURPOSE:To improve corrosion resistance by forming a Zn-Cr alloy layer having a prescribed structure on a steel sheet by electroplating or by further forming an iron-based layer by electroplating. CONSTITUTION:A Zn-Cr alloy layer consisting of 5-40% Cr and the balance Zn and contg. no eta phase is formed on a steel sheet by electroplating. The alloy layer has an alloy phase having 2.15-2.12Angstrom , 2.29-2.19Angstrom and 2.33-2.36Angstrom spacing (d) measured by X-ray diffraction and one or more peaks. An iron-based layer or a layer consisting of 60% Fe and the balance Zn may further be formed on the alloy layer by electroplating.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rust-preventing electroplated steel sheet with excellent corrosion resistance and corrosion resistance after painting, which is used for automatic machines, home appliances, building materials, etc.

(Prior Art) Electroplated steel sheets containing Cr in Zn or Zn-based alloy plating include, for example, Japanese Patent Publication No. 61-36078, Japanese Patent Publication No. 58-56039, and Japanese Patent Application Laid-open No. 61-2.
70398, etc. are known, but Crv
The eutectoid rate is an extremely small amount of 0.005 to 5%,
The effect of Cr on corrosion resistance can only be incidental.
Therefore, for the purpose of improving corrosion resistance, a higher content of C'
It is strongly desired to eutectoid the

(Problems to be Solved by the Invention) However, conventionally there has been no Zn-Cr electroplating technology capable of increasing the Cr eutectoid rate. In other words, simply increasing the trivalent Cr ion concentration in the final bath does not result in a normal plating with good adhesion, and there are obstacles such as a sudden decrease in current efficiency, so it is difficult to increase the Cr eutectoid rate industrially. This was extremely difficult.

In view of the above problems, the present invention provides an electroplated steel sheet with excellent corrosion resistance and corrosion resistance after electrical installation.

(Means for solving problems) The gist of the invention is as follows.

(2) A zinc-chromium alloy electroplated steel sheet consisting of more than 5% to 40% Cr, the balance being Zn, and containing no sliver phase.

■ Plane spacing d = 2.15 to 2.12 people, d by X-ray diffraction
The zinc-chromium alloy electroplated steel sheet according to (1) above, comprising an alloy phase having one or more peaks of d = 2.29 to 2.19 and d = 2.33 to 2.36.

(2) A two-layer electroplated steel sheet in which the lower layer is coated with the zinc-chromium alloy electroplating described in (1) or (2) above, and the upper layer is coated with iron-based electroplating or electroplating consisting of 60% or more Fe and the balance Zn.

(Function) The present inventors developed a Zn-Cr electroplating method with a high Cr content, and obtained the prospect of dramatically improving corrosion resistance.

Further research revealed that corrosion resistance and post-painting corrosion resistance differed not only by the Cr content but also by the structure of the plating layer. They discovered that the structure of the plating layer could be controlled by electroplating under specific conditions, leading to the present invention.

The C content in Zn-Cr plating 8 has the greatest effect on corrosion resistance and corrosion resistance after painting, and the Cr content in the plating layer should be more than 5%.If it is less than 5%, it has a slight effect. However, the tendency for red rust to occur remains, and the corrosion resistance is not sufficient.If the corrosion resistance exceeds 5%, the occurrence of red rust will be suppressed in salt water spray tests, for example, and an epoch-making effect will appear. High corrosion resistance is achieved by conventionally known Zn plating or Z
n-F e.

This cannot be achieved with alloy plating such as Zn-Ni.

Cr does not become passivated when co-deposited with Zn, but participates in the sacrificial anticorrosion effect together with Zn, and the corrosion products of Cr deposit a poorly soluble protective film on the corroded areas, thereby suppressing corrosion. This is thought to be the reason why it exhibits high corrosion resistance.

Although high corrosion resistance can be maintained even if the Cr content exceeds 40%, there is a tendency for the so-called bagging property, in which the plating layer peels off during processing such as pressing, to deteriorate. Therefore, the Cr content is set to 40% or less.

There are no known intermetallic compounds that are stable in the Zn-Cr binary system.However, when a plating layer obtained by electroplating is analyzed by Several types of peaks with unknown interplanar spacing d values that cannot be identified as either Cr phase or Cr phase are observed. These are presumed to be some kind of Zn-Cr alloy phase. Typical X-ray diffraction patterns of the plating layer are shown in Figs. 1 to tjS5.

FIG. 1 is an X-ray diffraction diagram of a Zn--Cr plating layer containing 9% Cr. Peak A (d=2.1σ people) rising Peak B (d=2,47 people) It is a beam phase. peak C
(d=2.21 people) is estimated to be one type of Zn-Cr alloy phase. In addition, the peak of d=2.03 is the peak of 0-Fe originating from the base steel plate.

Figure 2 shows the XA of the Zn Cr plating layer containing 7% Cr.
It is a first diffraction diagram. Peak C (d=2.27 people) is Zn-
It is estimated to be a type of Cr alloy phase. No clear peaks were observed in the vicinity of d = 2.10 people and d = 2.47 people, and it can be determined that this plating layer does not substantially contain a gold phase.

FIG. 3 is an X-ray diffraction diagram of a Zn--Cr plating layer containing 12% Cr. Peak C (d=2.21) and peak D (d=2.14) are estimated to be IFI of the ZnCr alloy phase. No clear peaks were observed in the vicinity of d=2.10 and d=2.47, and it was concluded that this sludge layer did not substantially contain a sludge phase.

FIG. 4 is an X-ray diffraction diagram of a Zn--Cr plating layer containing 15% Cr. Peak D (d=2.13 people) and peak E (d=2.35 people) are estimated to be one type of Zn-Cr alloy phase. d=2.10 and Vd=2.47
No clear peak was observed in the vicinity of A, and it can be determined that this plating layer does not substantially contain a gold phase.

FIG. 5 is an X-ray diffraction diagram of a Zn--Cr plating layer containing 27% Cr. Peak D (d=2.12 people) is Zn-
It is estimated to be a type of Cr alloy phase. No clear peaks were observed in the vicinity of d=2.10 and 2.47, and it can be determined that this plating layer does not substantially contain a gold phase.

The structural examples of the Zn--Cr plating layer shown in FIGS. 1 to 5 are related to the Cr content of the plating layer, but are essentially determined by the plating conditions.

Measurement of the interplanar spacing d by X-ray diffraction involves a certain degree of error. Furthermore, since the interplanar spacing d of the alloy phase generally includes elements that vary slightly depending on the composition, it was defined as follows.

D peak: d=2.15-2.12 AC peak: d
= 2.29-2.19 people E peak: d = 2.36-2.
Regarding the C peak, it is unclear whether the peak in Figure 2 and the peak in Figure 3 belong to the same crystal structure, but since both are presumed to be alloy phases, both are considered here. It was assigned to the C peak.

It is practical to determine the presence or absence of a corrosion phase by whether or not a clear peak is observed near d=2.10 or d=2.47, and this is associated with corrosion resistance performance.

In addition, in FIGS. 1 to 5, the vertical axis represents the X#1 intensity (arbitrary scale), and the horizontal axis represents the 2θ value in Cu tarded. The interplanar spacing d value of each peak was calculated and described.

A peak: Plane spacing d=2.10 people's l phase B peak:
It d=2.47 people a-Fe: #
The manufacturing conditions for each Zn-Cr plated steel plate with 6-Fe phase or higher of d=2.03 base steel plate are based on Example Note 1).

If the Cr content is less than 5%, Zn-Cr alloy plating without a beam phase cannot be produced; if the Cr content exceeds 5%, an alloy phase appears, but depending on the plating conditions, the beam phase may be mixed (Fig. 1). There may be cases where the Cr content reaches 40% and the dust phase does not disappear.

Zn with a Cr content exceeding 5% and a composition up to 40%
- As mentioned above, the corrosion resistance of Cr-plated steel sheets and the corrosion resistance after painting are extremely excellent in salt spray tests, etc., but when exposed to even harsher corrosive environments for long periods, for example, salt spray,
When accelerated corrosion is carried out in a complex corrosive environment that combines a wet environment and a dry environment, the structure of the plating layer as shown in FIGS. 1 to 5 has an effect.

That is, compared to a Zn-Cr plated steel sheet that contains a phase as shown in FIG. 1, a Zn-Cr plated steel sheet that does not contain a phase as shown in FIGS. The Cr-plated steel sheet has further improved corrosion resistance, especially corrosion resistance after painting.

The above-mentioned ability of Cr to form a protective film under corrosion maintains the adhesion of the paint film by suppressing excessive local battery action even in the case of shoulder corrosion under the paint film, but when the adhesive phase is present, the local battery suppression effect is completely suppressed. This is thought to be because it is not.

Note that a small amount of FesN1 is contained in the Zn-Cr plating layer.
zCo, Mn, Sn%Cd, Pb%Cu%C%0. S,
P.

Even if B, Na, etc. are contained, it is permissible as long as the X-ray diffraction pattern is not essentially changed.

Next, there is a method for manufacturing a Zn-Cr alloy plated steel sheet without a phase.
．． Plating is carried out at a current density of 50 A/dm or higher using an acidic plating bath containing 01 to 20 g/l of a polyoxene alkylene derivative. By adding a polyoxyfluorene derivative, Zn-Cr plating can be performed with an advantageous current efficiency. At the same time, it is possible to obtain a plating layer that does not contain a wax phase and consists essentially of an alloy phase.

Polyoquine alkylene derivatives are generally Rm-0-(R
+O)n-H and or R*-(R+ 0)n-H.

Here, R1: Alkylene group R*: H, fluorinated group, phenyl group, naphthyl group and or their derivatives n = 1 to 2000 The plating layer of the present invention is not limited to the case where it is a uniform single layer. , I having the above plating layer composition or phase structure
In the plating layer, those that are dispersed or structured in a layered manner within the plating layer, or those that have a concentration gradient in the depth direction, exhibit good performance as well as a single layer, and therefore are included in the scope of the present invention. do.

Two-layer plated steel sheets, in which iron fan electroplating or a plating layer consisting of 80% or more Fe and the balance Zn, are applied on top of the Zn-Cr plating layer that does not have a glue phase. It is particularly suitable for cationic electrodeposition coating applications because it can prevent the formation of zinc phosphate coatings and form a zinc phosphate film with excellent coating adhesion.

In this way, even when the above upper layer is applied to Zn-Cr plating 8M, the corrosion resistance after painting is the same as the corrosion resistance after painting when no upper layer plating is applied.
It is thicker due to the plating layer (and is even better because there is no adhesion phase).

The appropriate amount of upper layer plating is 1 to 10 Fi/m". The amount of lower layer plating is 10 to 50 [1/-2
It is possible to ensure sufficient corrosion resistance etc.

(Example) Examples of the present invention are shown in Table 1 along with comparative examples.

Note 1) For Zn-Cr plating in Examples and Comparative Examples, cold-rolled steel sheets were plated under the conditions shown in Table 2. In addition, Zn
Plating and upper layer Fe-Zn plating were performed by well-known methods. The X-ray diffraction patterns of the obtained plating layer correspond to FIGS. 1 to 5, respectively.

Note 2) X#1 diffraction: Measured using C'u tarded using a sample rotation method on a plated plate. The measurement conditions were a voltage of 45 kV, a current of 150 mÅ, and a scan speed of 2 deg/sin.

Main peak: The peak with the highest intensity Presence: No other peaks: Not clearly recognized Note 3) Corrosion resistance of I-plated steel sheet ■ Salt spray test (based on JIS Z 2371) 7
% area of red rust after 20 hours ■ Combined cycle test Wet (50℃, 85% relative humidity) 16 hours → Dry (70
°C) Open at 3 o'clock → Soak in salt water (SO, 5% NaC1) for 2 hours → Leave indoors for 2 hours → Salt water spray (50 °C, others are JIS)
Z 2371) Maximum erosion depth (-1) after 672 hours of exposure to a cyclic corrosive environment consisting of 1 hour → Note 4) I! Corrosion resistance of i-plate After immersion type zinc phosphate treatment, a cross-cut was made in a test piece which was coated with 20 μm of cationic electrodeposition coating and subjected to a corrosion test.

■Salt spray Ka (JIS Z 23711: compliant)
One side paint film blisters after 600 hours! l! (ms) ■Combined cycle test Salt water spray (50℃, others comply with JIS Z 2371) 17 hours → Drying (70℃) 3 hours → Salt water immersion (50℃)
Maximum erosion depth after 2,016 hours of exposure to a cyclic corrosive environment consisting of 2 hours → 2 hours of standing indoors (Sinzeng) Note 5) Cation electrodeposition coating m membrane exterior immersion type zinc phosphate treatment The sample was subjected to cationic electrodeposition coating at 300 μm, and its appearance was observed.

○: No craters occurred Δ: Less than 10 craters/d-2 ×: 10 craters/d-2 or more (effects of the invention) As described above, the corrosion resistance of the bare plate! ! ! A rust-proof steel plate with extremely excellent corrosion resistance after mounting is obtained, and is particularly suitable for rust-proof steel plates for automobiles in salt-damaged areas with severe corrosive environments, building materials for coastal areas, etc.

[Brief explanation of the drawing]

Figure 1 is the Xi diffraction diagram of Comparative Example 1; Figure 2 is an X-ray diffraction diagram of Example 1; Figure 3 is an X-ray diffraction diagram of Example 2; FIG. 4 is an X-ray diffraction diagram of Example 3, FIG. 5 is an XIIA diffraction diagram of Example 4.

Claims (3)

[Claims] (1) More than 5% to 40% Cr, the balance being Zn,
A zinc-chromium alloy electroplated steel sheet containing no η phase. (2) Interplanar spacing d = 2.15-2.12 Å by X-ray diffraction
, d=2.29-2.19 Å, d=2.33-2.36
The zinc-chromium alloy electroplated steel sheet according to claim 1, comprising an alloy phase having one or more peaks of Å. (3) The lower layer is coated with the zinc-chromium alloy electroplating according to claim 1 or 2, and the upper layer is coated with iron-based electroplating or Fe6.
Two-layer electroplated steel sheet with electroplating consisting of 0% or more and the balance being Zn.