Method for forming corrosion-resistant coating layer on zinc-based alloy or zinc-plated steel sheet

Abstract

To provide a method for forming the corrosion-resistant coating layer of a zinc base alloy or galvanized steel sheet by reducing a processing time of forming a CHZ coating having a level required for improving corrosion-resistance.SOLUTION: A method for forming the corrosion-resistant coating layer of a zinc base alloy or galvanized steel sheet comprises the steps of: preparing a zinc base alloy or galvanized steel sheet used as a target forming a corrosion-resistant coating layer; preparing a calcium containing solution for immersing the zinc base alloy or galvanized steel sheet; enhancing the saturation dissolved concentration of the dissolved oxygen amount of the calcium containing solution as compared with the atmosphere of normal temperature and normal pressure; immersing the zinc base alloy or galvanized steel sheet into the calcium containing solution in the state of maintaining the dissolved oxygen amount of the calcium containing solution in a state same as the enhanced saturation dissolved concentration or equal to or more than a saturation dissolved concentration in the atmosphere of normal temperature and normal pressure; and determining whether or not the state of a calcium hydroxy zincate film covering the zinc base alloy or galvanized steel sheet is sufficient as the corrosion-resistant coating layer of the zinc base alloy or galvanized steel sheet.SELECTED DRAWING: Figure 1C

Description

Technical Field

[0001] The present invention relates to a method for forming a corrosion-resistant coating layer on a zinc-based alloy or a galvanized steel sheet.

Background Art

[0002] In order to protect steel structures used outdoors from deterioration due to corrosion, galvanized steel with a zinc-based plating (hereinafter referred to as galvanizing) applied to the surface of the steel material is often used. Technologies for improving the corrosion resistance of steel materials by galvanizing are reported, for example, in Non-Patent Documents 1-3 and Patent 1-3. Galvanizing protects the base steel material from corrosion by the following two-step action. · Blocking contact between the base steel material and the atmospheric environment When the steel material is exposed to the atmospheric environment, it corrodes due to water, oxygen, and chloride ions in the atmospheric environment. Galvanizing covers the surface of the steel material and suppresses the contact of the steel material with the atmospheric environment, thereby protecting the base steel material. · Corrosion suppression by sacrificial anticorrosion action When the galvanized layer deteriorates due to corrosion or scratches, the galvanized coating peels off and the base steel material is exposed to the atmospheric environment. In this case, the galvanized layer and the base steel material are in an electrically short-circuited state through water in the atmospheric environment. In this state, since zinc is electrochemically more base than iron, zinc becomes the anode and iron becomes the cathode, and the dissolution of iron does not occur. That is, the base steel material is protected from corrosion by zinc corroding (= sacrificing) instead of iron.

[0003] As described above, the base steel material is protected by zinc plating. However, since zinc itself is more prone to corrosion than iron, the service life of zinc plating in the atmospheric environment is about 10 years, for example, especially in areas with severe salt damage or heavy industrial areas where corrosion is intense. To improve the corrosion resistance of zinc plating, Zn-Al plating, Zn-Al-Mg plating, and Zn-Ni plating with aluminum, magnesium, or nickel added to zinc plating have been developed and commercialized. However, in any of these platings, the outermost surface of the plating in contact with the atmospheric environment is a pure zinc layer enriched with zinc, and corrosion occurs on the outermost surface of the plating, so the deterioration of the plating layer progresses. To improve the corrosion resistance of the outermost surface of the plating, trivalent chromate treatment is generally performed. However, it requires pretreatment with nitric acid, pH control of the chromate treatment bath, there is concern about future price increases because chromium contained in the solution belongs to rare metals, and a large amount of treatment solution containing chromium ions is required for treating large steel materials, and disposal is also necessary. Therefore, a surface treatment method to replace chromate treatment is desired.

[0004] As a surface treatment method for improving the corrosion resistance of zinc, the formation of calcium zincate hydroxide (Ca(Zn(OH)3)2·2H2O, hereinafter referred to as CHZ) on the zinc surface has been reported. Since CHZ has high corrosion resistance, it can be expected as a corrosion protection film that suppresses the dissolution of zinc. An example of using CHZ coating as a corrosion protection technology is the use of zinc-plated steel bars in reinforced concrete. In concrete, calcium ions are supplied from calcium hydroxide, which is the main component of concrete, so CHZ is spontaneously formed on the zinc surface. Therefore, zinc-plated steel bars with zinc plating on the surface of the steel bars are used as anticorrosion steel bars. However, it takes several weeks to several months for CHZ to cover the zinc surface. In the case of concrete, there is a curing period of about one month before it is used in the actual environment, so CHZ can be formed during that period. However, for industrially coating CHZ on a bare zinc-plated steel structure and using it in the actual environment, the accelerated formation of CHZ is essential.

[0005] To enable the accelerated formation of CHZ, first consider the electrochemical and chemical production processes of CHZ. The formation process of CHZ is as follows. Zn → Zn 2+ + 2e - ···(1) O2 + 2H2O + 4e - → 4OH - ···(2) Zn 2+ + 4OH - → Zn(OH)4 2- ···(3) 2Zn(OH)4 2- + Ca 2+ + 2H2O → Ca(Zn(OH)3)2·2H2O + 2OH - ···(4) Equation (1) is an anodic reaction that generates electrons, and the same amount of electrons is always generated simultaneously with the cathodic reaction of Equation (2) that consumes electrons. The zinc ions, hydroxide ions, calcium ions, and water contained in the solution generated in the electrochemical reactions of Equations (1) and (2) chemically react as shown in Equations (3) and (4), and CHZ is formed on the zinc surface. Here, if Zn 2+ is supplied, Equations (3) and (4) will occur spontaneously. Next, in a saturated Ca(OH)2 solution with a high-alkali environment of about pH 12.5, the dissolution of zinc is rate-determined by the oxygen reduction reaction, so Equation (1) is rate-determined by Equation (2). That is, it can be said that Equation (2) rate-determines the entire CHZ formation reaction. In other words, it is considered that CHZ can be formed at an accelerated rate by accelerating the oxygen reduction reaction in Equation (2).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in an open-air state, it takes about one month to form a CHZ coating that shows an improvement in corrosion resistance. However, there is a problem that the processing time until this CHZ coating is long, and shortening of the processing time has been desired for industrial use.

Means for Solving the Problems

[0009] The inventor considered that the formation treatment of CHZ on the zinc surface can be accelerated by promoting the oxygen reduction reaction that controls the dissolution of zinc, and thus arrived at the present invention. Then, by immersing zinc in a saturated Ca(OH)2 solution with an increased oxygen supply amount, an attempt was made to accelerate the formation of CHZ in the solution, which requires several weeks to several months to form a CHZ coating film in a processing environment of normal temperature and pressure, and the present invention was completed.

[0010] [1]The method for forming a corrosion-resistant coating layer on a zinc-based alloy or zinc-plated steel sheet of the present invention includes a step of preparing a zinc-based alloy or zinc-plated steel sheet to be formed with the corrosion-resistant coating layer, a step of preparing a calcium-containing aqueous solution in which the zinc-based alloy or zinc-plated steel sheet is immersed, a step of increasing the dissolved oxygen amount of the calcium-containing aqueous solution compared to the case of normal temperature and pressure in the atmosphere, and maintaining the dissolved oxygen amount of the calcium-containing solution at the same as the increased saturated dissolved concentration or at a state not less than the saturated dissolved concentration in the atmosphere of normal temperature and pressure, immersing the zinc-based alloy or zinc-plated steel sheet in the calcium-containing aqueous solution, and a step of determining whether the state of the calcium hydroxydizincate film covering the zinc-based alloy or zinc-plated steel sheet is sufficient as the corrosion-resistant coating layer of the zinc-based alloy or zinc-plated steel sheet. Here, the state of maintaining a state not less than the saturated dissolved concentration in the atmosphere of normal temperature and pressure is considered in view of the fact that CHZ formation acceleration is possible at an oxygen pressure higher than atmospheric exposure. That is, when taking a method of maintaining the increased oxygen pressure during actual CHZ formation, for example, even if oxygen decreases by up to 50% of the saturated dissolved concentration during oxygen pressurized supply, compared with oxygen supply (bubbling), it is considered that CHZ formation is promoted 2.5 times.

[0011] [2]In the method for forming a corrosion-resistant coating layer on a zinc-based alloy or zinc-plated steel sheet of the present invention, preferably, the zinc-based alloy or zinc-plated steel sheet is any one of pure zinc, zinc-plated steel, Zn-Al plated steel, Zn-Al-Mg plated steel, and Zn-Ni plated steel. [3]In the method for forming a corrosion-resistant coating layer on a zinc-based alloy or zinc-plated steel sheet of the present invention, preferably, the zinc-plated steel sheet is a zinc-plated steel sheet treated by hot-dip zinc plating or electrolytic zinc plating. [4]In the method for forming a corrosion-resistant coating layer on a zinc-based alloy or zinc-plated steel sheet of the present invention, preferably, the calcium-containing solution has a calcium ion concentration of 0.1 ppm or more and calcium saturation or less. More preferably, the calcium ion concentration and the composition of the solution are close to a saturated calcium hydroxide aqueous solution (calcium ion concentration 0.17%). [5] In the method for forming a corrosion-resistant coating layer on a zinc-based alloy or a galvanized steel sheet of the present invention, preferably, in the step of increasing the saturated dissolved concentration of dissolved oxygen in the calcium-containing aqueous solution, at normal temperature and normal pressure, the oxygen gas concentration in contact with the calcium-containing aqueous solution is preferably set to exceed 0.02 MPa and be 0.1 MPa or less in terms of oxygen partial pressure. [6] In the method for forming a corrosion-resistant coating layer on a zinc-based alloy or a galvanized steel sheet of the present invention, preferably, in the step of increasing the saturated dissolved concentration of dissolved oxygen in the calcium-containing aqueous solution, at normal temperature, the oxygen gas concentration in contact with the calcium-containing aqueous solution is preferably set to exceed 0.02 MPa and be 2 MPa or less in terms of oxygen partial pressure within the range of the supplied oxygen pressure. [7] In the method for forming a corrosion-resistant coating layer on a zinc-based alloy or a galvanized steel sheet of the present invention, preferably, the immersion period of the zinc-based alloy or the galvanized steel sheet in the calcium-containing aqueous solution is preferably 1 hour or more and 30 days or less. This period corresponds to the range of the CHZ formation period. [8] In the method for forming a corrosion-resistant coating layer on a zinc-based alloy or a galvanized steel sheet of the present invention, preferably, the criterion for determining whether the calcium hydroxyzinckate film coating the zinc-based alloy or the galvanized steel sheet is sufficient as the corrosion-resistant coating layer of the zinc-based alloy or the galvanized steel sheet is that the film thickness of the calcium hydroxyzinckate film is preferably 5 μm or more and 10 μm or less. [Effect of the Invention]

[0012] According to the method for forming a corrosion-resistant coating layer on a zinc-based alloy or a galvanized steel sheet of the present invention, hydroxycalcium zinckate (Ca(Zn(OH)3)2·2H2O, abbreviated as CHZ), which exhibits high corrosion resistance, is acceleratedly coated on the surface of the zinc-based alloy or the galvanized steel sheet by supplying oxygen. The corrosion rate of zinc coated with CHZ is about 1 / 5 times that of untreated zinc, and it is expected to contribute to the reduction of the corrosion cost of galvanized steel used as a structural material and the protection of zinc resources. [Brief Description of the Drawings]

[0013]

Figure 1

Figure 2A

Figure 2B

Figure 3A

Figure 3B

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0014] Hereinafter, the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram showing an embodiment of a coating treatment apparatus used for a method of forming a corrosion-resistant coating layer on a zinc-based alloy or a zinc-plated steel sheet of the present invention. (A) shows air exposure as a comparative example, (B) shows an oxygen supply treatment apparatus, and (C) shows an oxygen pressurized supply treatment apparatus.

[0015] In the figure, samples 10a, 10b, and 10c are zinc-based alloys or zinc-plated steel sheets to be subjected to the formation treatment of the corrosion-resistant coating layer. Here, the case of three sheets is shown, but it is not limited to three sheets, and it may be one sheet, two sheets, or four or more sheets. The calcium-containing solution 20 desirably supplies raw material elements such as calcium necessary for the formation treatment of the corrosion-resistant coating layer and promotes the oxygen reduction reaction that rate-determines the dissolution of zinc. The calcium-containing solution 20 can be appropriately selected from distilled water (calcium ion concentration 0.1 ppm) to a saturated aqueous calcium solution based on the calcium ion concentration. However, from the perspective of industrial use of shortening the formation treatment time of the corrosion-resistant coating layer and forming a uniform corrosion-resistant coating layer, a calcium ion concentration and solution composition close to a saturated calcium hydroxide aqueous solution (calcium ion concentration 0.17%) are preferable.

[0016] The transparent inorganic glass container (beaker) 30 is used for air exposure as a comparative example and houses the calcium-containing solution 20 and the immersed samples 10a, 10b, and 10c. Here, it is a glass container for the purpose of comparing treatment conditions and for the convenience of observation. However, it is not limited to this, and in industrial applications, for example, an opaque container made of metal or plastic may be used. When using an opaque container, the progress of the formation treatment of the corrosion-resistant coating layer of the samples 10a, 10b, and 10c may be observed after taking them out of the calcium-containing solution 20. As the solution evaporation prevention lid 32, a resin film for preventing solution evaporation, such as a transparent polyethylene film, which covers the upper opening of the transparent inorganic glass container (beaker) 30, may be used.

[0017] The acrylic container 34 is used in the oxygen supply treatment apparatus shown in Example 1, and houses the calcium-containing solution 20 and the samples 10a, 10b, and 10c immersed therein. Here, for the purpose of comparing treatment conditions and for the convenience of observation, it is a transparent acrylic resin container. The lid portion 34a covers the upper opening of the acrylic container 34, and an oxygen gas supply port 36 and an oxygen gas discharge port 38 are attached thereto. An oxygen gas supply facility (not shown) is connected to the oxygen gas supply port 36. An exhaust pump (not shown) is connected to the oxygen gas discharge port 38.

[0018] The pressure-resistant container 40 is used in the oxygen pressurized supply treatment apparatus shown in Example 2, and houses a transparent inorganic glass container (beaker) 30 that houses the calcium-containing solution 20 and the samples 10a, 10b, and 10c immersed therein. The gantry 42 is a support member such as a shelf for supporting the transparent inorganic glass container (beaker) 30 inside the pressure-resistant container 40. The lid portion 40a covers the upper access opening of the pressure-resistant container 40, and a pressure gauge 44, an oxygen gas supply port 46, and an oxygen gas discharge port 48 are attached thereto. The pressure gauge 44 is a sensor for measuring the internal pressure of the pressure-resistant container 40. A pressurized oxygen gas supply facility (not shown) is connected to the oxygen gas supply port 46. An exhaust facility (not shown) is connected to the oxygen gas discharge port 48. In addition, if the pressure-resistant container 40 is dedicated equipment for forming a corrosion-resistant coating layer of a zinc-based alloy or a zinc-plated steel sheet, in order to house the calcium-containing solution 20 and the samples 10a, 10b, and 10c immersed therein, the transparent inorganic glass container (beaker) 30 may be omitted and the pressure-resistant container 40 may be used directly.

[0019] Subsequently, the procedure for forming a corrosion-resistant coating layer of a zinc-based alloy or a zinc-plated steel sheet using the apparatus configured as described above will be described. <CHZ Accelerated Formation Evaluation> For samples 10a, 10b, and 10c, pure zinc plates with dimensions of 15 mm × 50 mm × 1 mm (Nilaco Corporation) were used. After polishing the surfaces of these samples with SiC waterproof abrasive paper up to #800, they were ultrasonically cleaned in isopropanol for 5 minutes. For the formation treatment of calcium zincate (CHZ), a saturated calcium hydroxide aqueous solution was used as the test solution 20, which is a calcium-containing solution.

[0020] CHZ was formed on the surface by immersing the sample in the saturated calcium hydroxide aqueous solution for a predetermined time, and the formation conditions of CHZ were changed by varying the amount of oxygen supplied into this aqueous solution. The oxygen supply conditions in Example 1, Example 2, and the Comparative Example were open to the atmosphere (no oxygen gas supply, oxygen partial pressure 0.02 MPa), oxygen gas supply (using 100% oxygen gas, oxygen partial pressure 0.1 MPa), and pressurized oxygen gas supply (using 100% oxygen gas, oxygen partial pressure 0.5 MPa), respectively. Hereinafter, the test conducted under the open-to-atmosphere condition is denoted as open to the atmosphere, the test conducted with an oxygen gas supply of 0.1 MPa is denoted as oxygen supply, and the test conducted with an oxygen gas supply of 0.5 MPa is denoted as pressurized oxygen supply.

[0021] In the case of open to the atmosphere as the Comparative Example, the test solution 20 and the sample 10 were placed in the beaker 30 and the upper part was covered with the wrap 32 to prevent evaporation of the solution (Fig. 1(A)). In the case of the oxygen supply treatment apparatus as Example 1, the test solution 20 and the sample 10 were placed in the acrylic sealed container 34, and 100% oxygen gas was supplied by bubbling (Fig. 1(B)). In the case of the pressurized oxygen supply treatment apparatus as Example 2, the beaker 30 containing the test solution 20 and the sample 10 was placed in the titanium pressure-resistant container 40, and 100% oxygen gas was supplied to be 0.5 MPa (Fig. 1(C)).

[0022] The test periods were 1 day, 3 days, and 7 days. After the test, the sample was taken out from the test solution and ultrasonically cleaned in distilled water and isopropanol for 5 minutes each and then air-dried. Surface photographs and cross-sectional observations after the test, measurement of the weight change before and after the test, and XRD analysis of the compound formed on the sample surface were performed. Also, samples as-received after polishing were prepared as comparison materials and used for the potentiodynamic polarization measurement and the dry-wet repeated corrosion test described later. The CHZ formation conditions in Example 1, Example 2, and the Comparative Example are summarized in Table 1.

[0023]

Table 1

[0024] <Dry-wet cyclic corrosion test> To evaluate the corrosion resistance of the samples with CHZ formed, a dry-wet cyclic corrosion test was conducted. Before the dry-wet cyclic corrosion test, 240 μL of a mixed solution of an aqueous NaCl solution and ethanol (mixing ratio 1:4) was dropped so that the amount of salt adhered to the sample surface became 1 g / m 2 Then, a dry-wet cyclic corrosion test consisting of 4 cycles (total 24 hours) with a wet state (relative humidity 95%, 4 hours) and a dry state (relative humidity 30%, 2 hours) as one cycle was conducted. During this test, the test temperature was maintained at 50 °C. After the test, photographs of the sample surface were taken and the weight change of the sample before and after the test was measured.

[0025] <Potentiodynamic polarization measurement> To investigate the electrochemical properties of the samples with CHZ formed, potentiodynamic polarization measurement was carried out. A 1 wt.% NaCl solution was used as the test solution. A three-electrode method was used with an Ag / AgCl electrode as the reference electrode, a Pt wire as the counter electrode, and the sample as-polished or the sample with CHZ formed as the working electrode. After immersing the sample in the test solution, it was swept from -1.2 V to -0.7 V. Note that all potential notations in Examples 1, 2 and the comparative example are the potentials of the silver-silver chloride electrode (vs. Ag / AgCl). The sweep rate was 20 mV / min.

[0026] <Results of CHZ accelerated coating treatment on zinc surface> Figure 2A shows surface photographs of the sample as polished and the sample after CHZ formation for a predetermined time under each oxygen supply condition. When the test was conducted under open atmosphere, almost no change was observed on the sample surface after 1 day and 3 days of the test. After 7 days of the test, a film was faintly formed on the sample surface and the gloss of the sample was lost. On the other hand, for the samples with oxygen supply and oxygen pressurized supply, the entire sample surface was covered with a film after 1 day of the test. Looking at the weight changes before and after the test shown in Figure 3A, under the open atmosphere condition, the weight continued to increase proportionally to the time until 7 days of the test, but the increase amount was small. On the other hand, with oxygen supply, there was a weight increase after 1 day of the test that was more than that after 7 days or more under the open atmosphere condition. After that, the inclination of the weight increase gradually decreased while weight increase was observed until 7 days of the test. With oxygen pressurized supply, a significant weight increase was observed after 1 day of the test, and thereafter, a decrease in the inclination of the weight increase and weight increase were observed in the same manner as in the 0.1 MPa oxygen supply.

[0027] Figure 2B shows a cross-sectional SEM image of the sample coated with CHZ. It can be seen that a CHZ layer with a thickness of about several μm is formed on the zinc substrate under any condition. For the samples with oxygen supply and oxygen pressurized supply, CHZ uniformly covers the entire sample surface, while under the open atmosphere condition, the zinc substrate is exposed and portions where the CHZ coating is not completed can be seen everywhere. Figure 3B shows the average thickness of CHZ under each condition. In the measurement of the average thickness of CHZ, only the portions where CHZ was formed were targeted for analysis. The average thickness of CHZ increased with the increase in the test period under the open atmosphere condition and showed about 7 μm at 7 days of the test period. With oxygen supply, it showed about 6 μm at 1 day of the test period, then increased slightly with the increase in the test period and showed about 8.5 μm at 7 days of the test period. With oxygen pressurized supply, it showed about 8.5 μm at 1 day of the test period, and thereafter, almost no change was observed in the CHZ average thickness.

[0028] From these results, it was revealed that the maximum value of the average thickness of CHZ obtained by immersion in a calcium hydroxide solution is about 8.5 μm, and thereafter the growth in the thickness direction of CHZ is suppressed. CHZ has been reported to be insulating, and it is considered that the dissolution of zinc is suppressed under CHZ having a certain thickness, so it no longer grows in the thickness direction. The measurement method of the average film thickness of CHZ may be determined by, for example, the following method. (A) Take cross-sectional SEM images as shown in Fig. 2B for 5 samples each. (B) Randomly select one location from the area where CHZ is formed in the taken cross-sectional SEM image and measure the thickness on the PC screen. (C) Convert to the actual thickness using the scale bar taken at the same magnification. (D) Perform the above operations for all the taken cross-sectional SEM images, and take the average value of 5 locations under the same conditions as the thickness of CHZ under that condition. Note that the number of samples is not limited to 5, and it may be an appropriate number. Also, when determining the thickness of CHZ from the taken cross-sectional SEM image, it is not limited to the mode of measuring the thickness on the PC screen. It may also be measured by printing and taking measurements, or the CHZ region may be extracted from the digital image information to obtain the average film thickness. From the above surface observation and thickness information, as conditions for CHZ showing corrosion resistance, considering the variation, the surface coverage rate is 90% or more and the average thickness is 5 μm or more and 10 μm or less. Since it can be seen that it grows up to about 10 μm partially due to the variation in the test, the maximum value of the CHZ film thickness is set to 10 μm.

[0029] Figure 4 shows the XRD spectra of the sample surfaces after one day of atmospheric exposure, oxygen supply, and oxygen pressurized supply. Only the peaks of Zn and CHZ were detected from all the samples. The Zn peak is considered to be derived from the zinc substrate, and the CHZ peak is considered to be the peak of the film formed on the sample surface. It was shown that the film formed by immersion in a saturated Ca(OH)2 solution is CHZ. Also, since only the peaks of Zn and CHZ were detected even in the samples with oxygen supply and oxygen pressurized supply, it can be said that no film other than CHZ is formed by the supply of oxygen. That is, the CHZ formation process is the same for atmospheric exposure, oxygen supply, and oxygen pressurized supply. Also, looking at the peak intensity ratio of Zn and CHZ for each sample, the Zn peak is stronger than the CHZ peak in the sample with atmospheric exposure, but the CHZ peak is larger than the Zn peak in the samples with oxygen supply and oxygen pressurized supply. From this, it can also be seen that a larger amount of CHZ covers the sample surface. From the above, it was shown that CHZ can be accelerated and coated on the zinc surface without changing the reaction process by oxygen supply and oxygen pressurized supply.

[0030] <Corrosion resistance evaluation of zinc coated with CHZ> Figure 5 shows the photographs of the sample surfaces after the dry-wet repeated corrosion test. The sample as polished was severely corroded by the dry-wet repeated corrosion test, and white corrosion products of zinc adhered to the sample surface. Severe corrosion and adhesion of white corrosion products were also observed in the sample with atmospheric exposure, but the amount of adhered corrosion products decreased as the CHZ formation period became longer. In the sample with atmospheric exposure, it is considered that the amount of CHZ formed increased with the increase in the test period, leading to an improvement in corrosion resistance. On the other hand, in the samples with oxygen supply and oxygen pressurized supply, almost no corrosion and adhesion of corrosion products were observed regardless of the test period. In the samples with oxygen supply and oxygen pressurized supply, it is considered that high corrosion resistance was exhibited because the formation of CHZ on the entire sample surface was completed on the first day of the test.

[0031] Figure 6 shows the weight changes of the samples before and after the wet-dry repeated corrosion test. After the test, since the weight change measurement was carried out with the corrosion products attached, the weight change showed a positive value for all samples. It can be seen that the samples as polished and those exposed to the atmosphere for 1 and 3 days had the largest weight changes and were the most corroded. Among the samples exposed to the atmosphere, the weight change of the sample exposed to the atmosphere for 7 days was slightly smaller. On the other hand, for the samples with oxygen supply and oxygen pressurization, they showed smaller weight changes than any of the samples exposed to the atmosphere from 1 day after CHZ formation. For the samples with oxygen supply, the longer the CHZ formation period, the smaller the weight change. For oxygen pressurized supply, the weight change was even smaller, and the samples with oxygen pressurized supply for 3 and 7 days showed the smallest weight changes. From these results, it became clear that the accelerated formation of CHZ improves the corrosion resistance of zinc.

[0032] To electrochemically evaluate the corrosion resistance of the samples with CHZ formed, potentiodynamic polarization measurements were carried out. Figure 7 shows the polarization curves from -1.2 V to -0.7 V under each condition. First, focusing on the polarization curve of the as-polished sample, a region where the current due to the reduction reaction showed almost a constant value from -1.2 V to near the natural potential was observed. The reaction in this potential region is the oxygen reduction reaction (cathodic reaction), and since it showed a constant current value independent of the potential, this current is the oxygen diffusion-limiting current, indicating that the corrosion of zinc is rate-determined by this value. On the other hand, in the potential region nobler than the natural potential, the current continued to increase as the potential increased. In this region, the dissolution reaction of zinc (anodic reaction) occurred. Next, for the samples exposed to the atmosphere, there was almost no difference in the polarization curves between the sample with CHZ formed for 1 day and the as-polished sample. As the CHZ formation period became longer, the oxygen reduction limiting diffusion current and the dissolution current of zinc were slightly suppressed. This is considered to be the result of the suppression of the oxygen reduction reaction on CHZ (suppression of the cathodic reaction) because CHZ is insulating, and at the same time, it is the result of the decrease in the area of the zinc substrate in contact with the solution due to CHZ covering the sample surface (suppression of the anodic reaction). From these results, the improvement in the corrosion resistance of the samples that formed CHZ was also electrochemically demonstrated. In the samples supplied with oxygen and the samples supplied with pressurized oxygen, the limiting diffusion current of oxygen reduction and the dissolution current of zinc were suppressed in the samples after 1 day of the test compared to the samples exposed to the atmosphere for 7 days, indicating that the improvement in the corrosion resistance of zinc by oxygen supply and pressurized oxygen supply can be achieved in a much shorter time than by exposure to the atmosphere.

[0033] To quantitatively evaluate the corrosion resistance of the CHZ-coated samples, the corrosion current in the open-circuit state of each sample was determined using the Tafel method, which can obtain the corrosion current at the natural potential. The results are shown in Fig. 8. The horizontal axis represents the CHZ formation period, and the vertical axis represents the corrosion current at the natural potential obtained by Tafel plotting. The larger the value on the vertical axis, the larger the corrosion current, that is, the lower the corrosion resistance. In the samples exposed to the atmosphere, the value of the corrosion current gradually decreased in proportion to the increase in the test period. However, in the oxygen-supplied samples, the corrosion current decreased rapidly on the first day of CHZ formation, and then decreased gradually in the subsequent period. Furthermore, in the samples supplied with pressurized oxygen, the corrosion current decreased rapidly on the first day of CHZ formation, and almost no change in the corrosion current was observed in the subsequent period.

[0034] From these facts, it became clear that the corrosion current when CHZ completely covered the entire sample was about 5 mA / cm 2 and it was possible to reduce the corrosion current to about one-fifth compared to the polished sample (corrosion current: about 25 mA / cm 2 ). In the samples exposed to the atmosphere, it would take approximately 25 days to reduce the corrosion current to this level when obtained from an approximate straight line. In the samples supplied with pressurized oxygen, since the entire surface of the CHZ sample was completely covered in about 1 day, it became clear that the formation of CHZ could be accelerated by up to about 25 times.

[0035] In Examples 1 and 2 and the Comparative Example, for the purpose of improving the corrosion resistance of a zinc-based alloy or a zinc-plated steel sheet, a pure zinc plate was immersed in a saturated Ca(OH)₂ aqueous solution, and oxygen gas was supplied to accelerate the formation of calcium hydroxyzinckate (Ca(Zn(OH)₃)₂, CHZ) that exhibits high corrosion resistance on the zinc surface. The findings obtained are shown below. (A) Compared with the air-exposed condition, under the conditions of oxygen supply and oxygen pressurized supply, the formation of CHZ can be accelerated in proportion to the supplied oxygen pressure. The acceleration rate of CHZ formation shown in Examples 1 and 2 and the Comparative Example is up to 25 times. (B) All the compounds formed by oxygen supply and oxygen pressurized supply consist of CHZ, and oxygen supply does not change the reaction process on the zinc surface. (C) From the results of the dry-wet repeated corrosion test, zinc coated with CHZ exhibits higher corrosion resistance compared to as-polished zinc. When the surface is completely coated with CHZ, the corrosion current is suppressed to about one-fifth compared to as-polished zinc.

Industrial Applicability

[0036] According to the method for forming a corrosion-resistant coating layer of the zinc-based alloy or the zinc-plated steel sheet of the present invention, by using a calcium-containing solution with a higher oxygen concentration compared to oxygen at normal temperature and pressure, for example, the time for CHZ coating can be shortened from about one month to one day in the prior art, and the industrial utility value is great. The corrosion rate of zinc coated with CHZ is about 1 / 5 times that of untreated zinc, and it is expected to contribute to the reduction of the corrosion cost of zinc-plated steel used as a structural material and the protection of zinc resources.

Explanation of Reference Numerals

[0037] 10, 10a, 10b, 10c: Samples 20: Calcium-containing solution (test solution) 30: Transparent inorganic glass container (beaker) 32: Solution evaporation prevention lid (wrap) 34: Acrylic container 34a: Lid part 36: Oxygen gas supply port 38: Oxygen gas discharge port 40: Pressure-resistant container 40a: Lid part 42: Stand 44: Pressure gauge 46: Oxygen gas supply port 48: Oxygen gas discharge port

Claims

1. A step of preparing a zinc-based alloy or a galvanized steel sheet to be the object of forming a corrosion-resistant coating layer; A step of preparing a calcium-containing aqueous solution for immersing the zinc-based alloy or the galvanized steel sheet; A step of increasing the saturated dissolved concentration of the dissolved oxygen amount of the calcium-containing aqueous solution as compared with the case in the atmosphere at normal temperature and normal pressure (however, except for the case where the oxygen partial pressure is 0.1 MPa or less); A step of immersing the zinc-based alloy or the galvanized steel sheet in the calcium-containing aqueous solution in a state where the dissolved oxygen amount of the calcium-containing aqueous solution is maintained at the same as the increased saturated dissolved concentration or in a state of being equal to or higher than the saturated dissolved concentration in the oxygen partial pressure of 0.1 MPa at normal temperature; A step of determining whether the state of the calcium hydroxyzinckate film coating the zinc-based alloy or the galvanized steel sheet is sufficient as a corrosion-resistant coating layer of the zinc-based alloy or the galvanized steel sheet; A method for forming a corrosion-resistant coating layer of a zinc-based alloy or a galvanized steel sheet, comprising the above steps.

2. The method for forming a corrosion-resistant coating layer of a zinc-based alloy or a galvanized steel sheet according to Claim 1, wherein the galvanized steel sheet is any one of galvanized steel, Zn-Al galvanized steel, Zn-Al-Mg galvanized steel, and Zn-Ni galvanized steel.

3. The method for forming a corrosion-resistant coating layer of a galvanized steel sheet according to Claim 2, wherein the galvanized steel sheet is a galvanized steel sheet treated by hot-dip galvanizing or electrolytic galvanizing.

4. The method for forming a corrosion-resistant coating layer of a zinc-based alloy or a galvanized steel sheet according to any one of Claims 1 to 3, wherein the calcium-containing aqueous solution has a calcium ion concentration of 0.1 ppm or more and calcium saturation or less.

5. In the step of increasing the saturated dissolved concentration of the dissolved oxygen amount of the calcium-containing aqueous solution, at normal temperature, the oxygen gas concentration in contact with the calcium-containing aqueous solution is set as a supply oxygen pressure range of exceeding an oxygen partial pressure of 0.1 MPa and being 2 MPa or less in oxygen partial pressure. The method for forming a corrosion-resistant coating layer of a zinc-based alloy or a galvanized steel sheet according to any one of Claims 1 to 4.

6. The method for forming a corrosion-resistant coating layer of a zinc-based alloy or a galvanized steel sheet according to any one of Claims 1 to 5, wherein the immersion period of the zinc-based alloy or the galvanized steel sheet in the calcium-containing aqueous solution ranges from 1 hour to 30 days.

7. The criterion for determining whether the state of the calcium hydroxyzinckate film coating the zinc-based alloy or galvanized steel sheet is sufficient as a corrosion-resistant coating layer for the zinc-based alloy or galvanized steel sheet is that the film thickness of the calcium hydroxyzinckate film is 5 μm or more and 10 μm or less. The method for forming a corrosion-resistant coating layer of the zinc-based alloy or galvanized steel sheet according to any one of claims 1 to 6.

Images