Surface treatment method

Abstract

In a first step S101 of the present invention, the work function of an oxide layer, which is formed on the surface of a structure composed of metal by laser irradiation, is determined for each laser irradiation condition. In a second step S102, determined is a laser irradiation condition under which an oxide layer having the maximum work function is formed. In a third step S103, the surface of the structure is irradiated with a laser under the determined laser irradiation conditions.

Description

Surface treatment method

[0001] The present invention relates to a surface treatment method.

[0002] Generally, when structures such as steel towers and bridges deteriorate significantly, surface preparation is performed. In the case of steel structures, Type 1 surface preparation involves completely removing mill scale, rust, and old paint to expose a clean structural surface, and then applying paint.

[0003] However, over time, water could penetrate the paint film, and when this water reached the surface of the steel, corrosion would begin beneath the paint film, leading to the deterioration of the structure.

[0004] This invention was made to solve the above-mentioned problems and aims to suppress corrosion on the surface of a metal structure on which a coating film has been formed.

[0005] The surface treatment method according to the present invention includes a first step of determining the work function of an oxide layer formed on the surface of a metal structure by laser irradiation for each laser irradiation condition; a second step of identifying the laser irradiation conditions under which the oxide layer with the maximum work function is formed; and a third step of performing laser irradiation on the surface of the structure under the identified laser irradiation conditions.

[0006] As described above, according to the present invention, since the laser irradiation conditions that result in the formation of an oxide layer with the maximum work function are determined, corrosion on the surface of a metal structure on which a coating film is formed can be suppressed.

[0007] Figure 1 is a flowchart illustrating a surface treatment method according to an embodiment. Figure 2 is a configuration diagram showing the configuration of a laser irradiation device. Figure 3 is an explanatory diagram illustrating electron transfer in a chemical reaction in a redox system consisting of metal, water, and oxygen. Figure 4 is a characteristic diagram showing the relationship between the work function change ΔΦ and the laser irradiation time. Figure 5 is a characteristic diagram showing the relationship between the laser irradiation time and the iron oxide formation ratio. Figure 6 is an explanatory diagram illustrating the location where the iron oxide formation state formed by laser irradiation was measured by Raman spectroscopy. Figure 7 is a characteristic diagram showing the calculated work function of iron oxide formed by laser irradiation. Figure 8 is a characteristic diagram showing the change in surface free energy due to the terminal structure of α-Fe2O3. Figure 9A is an explanatory diagram showing a model of α-Fe2O3 with a terminal structure of "Fe-O3-Fe-". Figure 9B is an explanatory diagram showing a model of α-Fe2O3 with a terminal structure of "O-Fe-Fe-". Figure 9C is an explanatory diagram showing a model of α-Fe2O3 with a terminal structure of "Fe-Fe-O3-".

[0008] The surface treatment method according to an embodiment of the present invention will be described below with reference to Figure 1.

[0009] This surface treatment method first involves determining the work function of the oxide layer formed on the surface of a metal structure by laser irradiation in the first step S101, for each laser irradiation condition. In the first step S101, the laser is irradiated onto the exposed surface of the structure (metal structure). For example, in the first step S101, the laser is irradiated onto the surface of the structure that has been exposed by surface preparation.

[0010] For example, the laser irradiation device shown in Figure 2 can be used. The laser irradiation device focuses a laser 102 emitted from a laser light source 101 using a focusing optical system 103, such as a lens, and irradiates the surface of a steel material 104, which is a metal structure. This process allows for the formation of an oxide layer at the irradiated area of ​​the steel material 104. When the focused laser 102 is irradiated, laser annealing causes the irradiated area of ​​the steel material 104 to rapidly heat up and melt, and then it is rapidly cooled in the atmosphere. During this cooling, iron oxide is formed on the surface of the steel material 104 by a reaction with oxygen in the air. Note that if a beam-shaped laser can be output from the laser light source, a focusing optical system is not necessary.

[0011] Next, in the second step S102, laser irradiation conditions are identified that result in the formation of an oxide layer with the maximum work function. Then, in the third step S103, laser irradiation is performed on the surface of the structure under the identified laser irradiation conditions. Note that the above method can be applied even if mill scale or rust has formed (remains) on the surface of the target structure. Furthermore, the above method can be applied even if a coating film has formed.

[0012] The following provides a more detailed explanation.

[0013] Metallic materials are used in various structures around us, and a major cause of deterioration in these materials is corrosion due to oxidation reactions on the surface. In the atmosphere, metals react with water and oxygen to oxidize (corrode), and this corrosion reaction continues in a chain reaction, causing an oxide layer to grow. Taking iron as an example, the reaction varies depending on the atmospheric environment and the structure of the iron surface, but examples of possible oxidation reactions are shown in equations (1) to (4).

[0014] Fe+1 / 2O2+H2O→Fe(OH)2...(1) Fe(OH)2+1 / 2O2+H2O→2Fe(OH)3...(2) Fe(OH)3→FeOOH+H2O...(3-1) 2Fe(OH)3→Fe2O3+3H2O...(3-2) 6FeOOH+Fe→2Fe3O4+Fe(OH)2+2H2O...(4)

[0015] In chemical formula (1), it is Fe; in chemical formula (2), it is Fe(OH)2; in chemical formulas (3) and (4), it is Fe(OH)3; and in chemical formula (5), it is FeOOH and Fe. Because the surface reactivity differs, the rate of corrosion, and therefore the rate of oxide layer growth, also differs.

[0016] Regarding the susceptibility of metal surfaces to corrosion, there is a study that considers the work function as a parameter, as shown in Reference 1. According to this study, for metals with a work function greater than approximately 3 eV, the anode current (corrosion current) tends to decrease as the work function increases, meaning that the metal is less susceptible to corrosion (Fig. 2 in Reference 1).

[0017] The work function is the minimum energy required to extract one electron from a solid surface. The work function is expressed as the difference between the vacuum level and the Fermi level, and is influenced by the type of material, surface orientation, and strain. Because the work function represents the height of the Fermi level, it serves as an indicator of the direction of electron movement and thus as an indicator of chemical reactivity. For example, in a redox system consisting of a metal and water / oxygen, as shown in Figure 3, when the two materials come into contact, their Fermi levels are at different heights, causing electrons to move in an attempt to align the Fermi levels.

[0018] In the case shown in Figure 3, the Fermi level of the metal is higher than that of the redox system (assuming water and oxygen), so electrons flow from the metal to the redox system, meaning that an oxidation reaction of the metal occurs. Note that in Figure 3, D M occ D is the density of occupied states. redox UNO The occupies the density of unoccupied states (occ: occupied, uno: unoccupied). Also, D M (E) is the density of states of the metal, D redox (E) is the density of states of the redox system.

[0019] Generally, structures such as steel towers and bridges undergo surface preparation when they deteriorate significantly. In the case of steel structures, if the surface preparation level is Grade 1, iron rust such as α, β, and γ type FeOOH and old paint films are removed, exposing the steel surface before painting. However, over time, water permeates the paint film, and the permeated water reaches the steel surface, initiating corrosion beneath the paint film and causing rust to grow.

[0020] In recent years, laser cleaning technology, a surface preparation method that removes paint, rust, and corrosion from the surface of materials such as metals by irradiating them with a laser, has been attracting attention. Compared to the power tools and blasting methods currently mainly used for surface preparation, it is gaining attention because of its advantage of a higher rate of corrosion removal.

[0021] With power tools and blasting methods, there is little to no heat transferred to the structure during operation, or only minor heat such as frictional heat. In contrast, when a laser is irradiated onto metal in the atmosphere, the temperature reaches several hundred degrees Celsius to over 1000 degrees Celsius, and it is known that metal oxides are formed on the surface by laser annealing.

[0022] When surface preparation is performed, the bare steel material is exposed, revealing a highly reactive, and therefore easily degradable, surface. If this surface is painted, water will penetrate and the steel will corrode rapidly. In contrast, it is known that the corrosion resistance of the surface is improved by dense rust composed of iron oxide. For example, weathering steel is a type of steel that utilizes the corrosion resistance of dense iron oxide. Using weathering steel has the advantage that dense rust forms on the surface several years after the structure is installed, slowing down surface corrosion. However, weathering steel is not used in all steel structures, and is rarely used in transmission towers. Therefore, for existing structures, it is important to improve the corrosion resistance of the exposed metal surface, for example, during surface preparation.

[0023] By irradiating structures, primarily composed of metal, with a laser, metal oxides are formed. This process utilizes the change in the work function relative to the original structure surface, resulting from the formation of the metal oxides. By forming metal oxides and increasing the work function, surface stability is improved, thereby enhancing corrosion resistance.

[0024] Next, let's explain the change in the work function. Using steel as an example, although steel contains carbon and other elements, these are present in trace amounts compared to Fe, so the band structure of steel is thought to be similar to that of Fe, and the Fermi level is located within the band.

[0025] On the other hand, many iron oxides have a band gap, and the Fermi level is located within this gap. Therefore, they have a different work function than Fe. Consequently, the work function changes due to iron oxide formation by laser irradiation, i.e., a change in surface material.

[0026] Next, we will explain how to maximize the degree of surface stability improvement. To stabilize the surface, since the experimental work function of Fe is approximately 4.6 eV, it is important to increase the work function above that of Fe (Reference 1). Note that the work function of steel is smaller than that of Fe, ranging from 4.1 to 4.2 eV. Therefore, in the case of steel, it is important to increase the work function above 4.1 to 4.2 eV. Based on these considerations, to maximize the degree of surface stability improvement, it is necessary to identify the iron oxide that has the highest work function among the iron oxides formed by the laser, and to identify that state.

[0027] For example, one can use a method to numerically calculate the work function of each iron oxide for a model with a fixed atomic arrangement, such as in first-principles calculations. Another method involves growing a single crystal with precisely arranged atoms and measuring its work function using ultraviolet photoelectron spectroscopy. Any of these methods is acceptable, and the method for specifying the state of iron oxide and the formation conditions that maximize the work function is not specified.

[0028] The iron oxide and its structure that maximizes the work function can be identified by one of the following methods. After this identification, the degree of improvement in surface stability can be maximized by optimizing the laser irradiation conditions so that iron oxide of this structure can be formed. Laser irradiation conditions include, for example, the laser mode (continuous wave or pulsed), wavelength, power, focused spot diameter, and irradiation time.

[0029] By laser annealing a metal surface, a metal oxide is formed on the surface, thereby changing the work function. By optimizing the laser irradiation conditions to form iron oxide with the highest work function, the surface stability is maximized.

[0030] [Examples] The following will be explained using examples.

[0031] A CW laser with a central wavelength of 1070 nm was used. Further, a focusing optical system with a focal length of 300 mm was used to focus and irradiate the surface of the steel material with a spot diameter of 50 μm. The laser power was set to 300 W. The irradiation time was set to 0.5 to 10 seconds. Further, SS400 was used as the steel material. The steel material was 15×15×3.2 (mm 3 ) in a plate shape. Further, the irradiated surface was mirror-polished.

[0032] Next, the work function of the laser-irradiated portion was evaluated by a Kelvin probe. The amount of increase was evaluated from the difference between the work function in the non-laser-irradiated state and the work function in the laser-irradiated state. The measurement system had a humidity of 50%, a temperature of 40°C, and a cylindrical probe tip with a diameter of 2 mm was used. Further, the average value of 100 measurements was evaluated. However, by scanning the Kelvin probe relative to the steel material used as the sample, mapping measurement was performed in the pre-laser-irradiated region to find the coordinate where the work function was maximum. Thereafter, the probe tip was fixed at the coordinate where the work function was maximum, and the work function was measured.

[0033] Next, the quantity evaluated by the Kelvin probe will be described. The quantity measurable by the Kelvin probe is the contact potential difference ΔV between the tip and the sample c (V), where the work function of the tip is denoted as Φ tip (eV), and the work function of the sample is denoted as Φ sample (eV), then "Φ sample = ΔV c + Φ tip " holds. Note that the scales of the units V and eV are the same. Here, since it is desired to measure the change in work function due to laser irradiation, the work function change amount ΔΦ = Φ laser - Φ pristine is compared for each laser irradiation time. Here, Φ laser is the work function of the sample after laser irradiation. Further, Φ pristine is the work function of the sample that has not been laser-irradiated. Unlike the case where the absolute value of the work function is obtained, since ΔΦ is to be evaluated, Φ tip does not need to be measured.

[0034] The relationship between the work function change amount ΔΦ described above and the laser irradiation conditions was investigated.

[0035] FIG. 4 shows the relationship between the work function variation ΔΦ and the laser irradiation time. Although the work function variation does not increase monotonically with the irradiation time, it was found that the work function variation ΔΦ tends to increase as the irradiation time becomes longer. However, it is not known whether the condition of an irradiation time of 5 s is the condition that maximizes ΔΦ. ΔΦ can be maximized by optimizing the type of substance to be formed and its surface state.

[0036] Accordingly, the degree of improvement in surface stability is maximized, that is, an investigation is conducted to determine what type and state of iron oxide formed on the surface gives the maximum work function. For this investigation, work function calculation based on the first principle was performed.

[0037] For the sake of simplicity this time, among the iron oxides confirmed to be formed by laser irradiation as shown in FIG. 5, α-Fe₂O₃, FeO, and Fe₃O₄, which had high formation ratios, were selected as calculation targets. Further, Fe was also selected as a target for comparison. Note that the resolution of the Kelvin probe in the xy plane is about 2 mm. Therefore, Raman spectroscopic measurement was performed at 8 locations in a region within 1 mm from the center point irradiated with the laser, as indicated by the vertical arrows in FIG. 6. The value obtained by averaging the formation ratios of the iron oxide at each of the 8 locations obtained by the Raman spectroscopic measurement was defined as the iron oxide formation ratio at each laser irradiation location.

[0038] The calculation conditions are as follows. The work function on the (001) or (0001) surface of each substance was calculated based on the first principle. Regarding the surface structure, for Fe₃O₄, two types of structures terminated with Fe atoms that are tetrahedrally coordinated or octahedrally coordinated to O atoms are conceivable. Therefore, the work functions of those structures were calculated respectively. For α-Fe₂O₃ having various termination structures, three termination structures with low surface free energy were adopted (Reference 2).

[0039] For the first-principles calculations, we first used a plane-wave basis simulation package (Vienna ab-initio simulation package; VASP) based on density functional theory. For the electron-nucleus interaction, we used the Projector Augmented Wave potential. For the exchange-correlation term, we used the GGA-PBE potential. All calculations were performed within the GGA+U framework. Here, we incorporated effective on-site Coulomb interaction and exchange interaction terms. The on-site Coulomb interaction term was set to 5 eV. The exchange interaction term was set to 0.5 eV. The cutoff energy was set to 400 eV. The k-point sampling rate was set to <0.013 nm in the xy-plane direction. 1 The number of k points (approximately 6 to 18 points per direction) was set to achieve the desired result. Furthermore, the number of k-point samples was set to 1 in the z direction, and calculations were performed taking magnetism into consideration.

[0040] All first-principles calculations were performed using a GUI called MedeA (MedeA® version 3.0. MedeA is a registered trademark of Materials Design, Inc., San Diego, CA, USA).

[0041] Figure 7 shows the calculated work functions for FeO, Fe3O4, and α-Fe2O3. The work function value for Fe is also shown in Figure 7.

[0042] In this calculation, since α-Fe2O3 and Fe3O4 have multiple structures, their ranges are represented by boxes as models. For example, the surface free energy of α-Fe2O3 changes depending on the terminal structure, as shown in Figure 8 (Reference 2). In Figure 8, the horizontal axis is the relative chemical potential, and the vertical axis is the surface free energy. For example, α-Fe2O3 with a terminal structure of "Fe-O3-Fe-" is modeled as shown in Figure 9A. α-Fe2O3 with a terminal structure of "O-Fe-Fe-" is modeled as shown in Figure 9B. α-Fe2O3 with a terminal structure of "Fe-Fe-O3-" is modeled as shown in Figure 9C. In Figures 9A, 9B, and 9C, the larger sphere (circle) represents Fe atoms, and the smaller sphere (circle) represents O atoms.

[0043] Comparing the maximum calculated work function values ​​for each model, as shown in Figure 7, the work function of α-Fe2O3 with the structure of Figure 9B was the highest. Therefore, if α-Fe2O3 with the structure of Figure 9B can be formed on the surface of the steel material by laser irradiation, the work function can be maximized. In other words, if α-Fe2O3 with the structure of Figure 9B can be formed on the surface of the steel material, the degree of improvement in surface stability can be maximized. Note that since elements other than Fe are present in the steel material, the work functions of steel and Fe are different, with Fe having a larger work function. However, calculations show that, compared to Fe, when α-Fe2O3 with the structure of Figure 9B is formed by laser irradiation, the work function can be increased by up to 2.5 eV.

[0044] Finally, let's discuss the effect of being able to increase the work function from Fe by 2.5 eV. For example, the oxidation reaction of Fe: Fe → Fe 2+ At +2e-, the reaction rate of Fe can be expressed as the time change of the Fe concentration [Fe] (mol / L) "-(d[Fe]) / dt". This can also be expressed as the current density during oxidation.

[0045] Since the current density is due to electrons emitted from Fe, i = -(F / 2) × {(d[Fe]) / dt} × 10 3 (A / m 3This can be expressed as ) where F is the Faraday constant. Therefore, the effect of the change in surface stability due to the increase in the work function can be evaluated from the current density.

[0046] From Fig. 2 in the left column on page 245 and equation (5) in the right column on page 244 of Reference 1, when the work function increases by 2.5 eV from Fe, the value on the vertical axis of Fig. 2 corresponds to an increase of approximately 7.5 V from Fe's value of -1.0 V. Here, as shown by the solid line in Fig. 2 of Reference 1, it is assumed that the vertical axis increases linearly when the work function is approximately 4 eV or more, and that the rate of increase is 3. Also, assuming that only the second term of equation (5) of Reference 1 changes, the current density of α-Fe2O3 with the structure of Figure 9B is 4.5 × 10⁻⁶ compared to Fe. 181 It was found that the size doubled, making oxidation reactions less likely to occur. Note that this value was obtained assuming a rearrangement coefficient α of 0.5 (a standard value) and a temperature T = 300 K.

[0047] However, in this calculation, the surface orientation was fixed, and no distortion was applied to the lattice. Instead, the iron oxide that maximized the work function was investigated by focusing on the type of material and the terminal structure. Therefore, by considering all structural states for all iron oxides, it may be possible to identify iron oxides that yield an even greater work function. In the example, mirror-polished steel was used as an example, but the surface treatment method according to the embodiment can be applied even if black scale or rust is formed on the surface of the target structure, and similar effects can be obtained.

[0048] As described above, according to the embodiment, the laser irradiation conditions that result in the formation of an oxide layer with the maximum work function are determined, thereby suppressing corrosion on the surface of the metal structure on which the coating film is formed.

[0049] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be implemented within the technical concept of the present invention by those with ordinary skill in the art.

[0050] [Reference 1] Saburo Shimohira, "On the Relationship between Work Function and Corrosion Rate of Metals," Journal of the Japan Institute of Metals, Vol. 23, No. 4, pp. 243-246, 1959. [Reference 2] Manh-Thuong Nguyen et al., "Water adsorption and dissociation on α-Fe2O3(0001): PBE+U calculations," The Journal of Chemical Physics, vol. 138, no. 19, 194709, 2013.

[0051] 101...Laser light source, 102...Laser, 103...Focusing optical system, 104...Steel material.

Claims

1. A surface treatment method comprising: a first step of determining the work function of an oxide layer formed on the surface of a metal structure by laser irradiation for each laser irradiation condition; a second step of identifying the laser irradiation conditions under which the oxide layer with the maximum work function is formed; and a third step of performing laser irradiation on the surface of the structure under the identified laser irradiation conditions.

2. A surface treatment method according to claim 1, wherein the first step is a surface treatment method comprising irradiating the exposed surface of the structure with a laser.

3. A surface treatment method according to claim 2, wherein the first step is a surface treatment method comprising irradiating a laser onto the exposed surface of the structure that has undergone surface preparation.

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