Surface treatment method, and structure

Abstract

To provide a surface treatment method which improves base material protection performance by means of a protective layer remaining after removal treatment of a deteriorated portion.SOLUTION: The present invention discloses a surface treatment method for a metal material having a protective layer formed thereon. In the method, the surface of the protective layer is scanned while irradiating the surface with a laser and moving the irradiated area along the surface, and, in a state where the metal material-side layer of the protective layer remains covering the metal material, the layer of the protective layer on the opposite side to the metal material side is removed by laser irradiation.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a surface treatment method for a metal material having a protective layer formed thereon, and to a structure having a metal material having a protective layer formed thereon. [Background technology]

[0002] For example, for structures such as steel towers installed as ground facilities for wireless communication networks for mobile phones and the like, iron-based materials such as steel with a protective layer formed on the surface by, for example, hot-dip galvanizing are used to prevent deterioration such as rust and corrosion. Hot-dip galvanizing is a technique in which steel material is immersed in a bath of molten zinc at high temperature to form a zinc film on the surface. The protective layer formed by hot-dip galvanizing, which is primarily made of zinc, acts as a protective coating that prevents air and water from coming into contact with the steel and inhibits the formation of rust.Even if the zinc plating is scratched and the base metal is exposed, the surrounding zinc dissolves before the base metal, providing electrochemical protection and exhibiting a sacrificial anticorrosion effect that inhibits corrosion of the iron.

[0003] In a steel structure having a protective layer formed by hot-dip galvanizing, if salt or the like adheres to the surface of the protective layer, deteriorated areas such as so-called white rust, which is mainly composed of zinc oxide, may occur. Furthermore, in situations where the protective layer is damaged, such as by corrosion or scratches, the base steel may corrode and produce so-called red rust. As a conventional technique for maintaining steel structures on which a protective layer is formed by hot-dip galvanizing, for example, Patent Document 1 describes a method in which a deteriorated coating is removed by scraping and cleaning the surface of the steel material using a power tool such as a disk sander and hand tools such as a scraper and hammer, and then a base coating is formed using a base coating treatment liquid containing phosphoric acid, a silicon compound, and an alcohol-based solvent, and after drying, a rust-converting type anti-corrosion paint or the like is applied. Furthermore, as a conventional technique for cleaning steel structures and the like, for example, Patent Document 2 describes that an irradiation head that irradiates the object with laser light is provided with a wedge prism that deflects the laser light by a predetermined deflection angle, and by irradiating the laser light while rotating this wedge prism, the irradiation point scans the surface of the object while rotating, and old paint films, foreign matter, etc. adhering to the surface of the object are removed (cleaned). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-148163 [Patent Document 2] Patent No. 5574354 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a surface treatment method and a structure in which the base material protection performance is improved by the protective layer remaining after treatment to remove deteriorated portions. [Means for solving the problem]

[0006] The present invention solves the above-mentioned problems by the following means. The invention of claim 1 is a surface treatment method for a metal material having a protective layer formed thereon, characterized in that the surface of the protective layer is irradiated with laser light and the irradiation point is moved along the surface while scanning the surface, and the layer of the protective layer on the opposite side to the metal material side is removed by irradiating with the laser light while the layer of the protective layer on the metal material side remains. This allows the protective layer to be partially removed by irradiation with laser light and partially left on the metal material side, so that part of the protective layer can remain on the metal material side in the same bonded state as before the treatment. Therefore, even after the treatment to remove the deteriorated portion, the remaining protective layer can enhance the base material protection performance. In the present invention, it is preferable that the layer on the metal material side of the protective layer remains, for example, over substantially the entire area where the protective layer was formed before irradiation. However, even if the protective layer is removed by irradiation with laser light, leaving some areas where the metal material is exposed, the base material protection performance can be improved in other areas. Such laser light irradiation treatment, which allows a portion of the protective layer to remain on the metal material side, can be achieved by appropriately setting the energy density and irradiation time that the laser light imparts to the object to be treated according to the amount of deterioration such as white rust and the film thickness of the protective layer. Furthermore, unlike mechanical scraping processes that require direct access by tools, laser light irradiation can be performed from a relatively remote location, making it easy and reliable to remove deteriorated areas in narrow areas, such as recesses and around bolts and nuts. Furthermore, periodic irregularities are formed on the surface due to irradiation marks caused by the laser light, increasing the surface roughness, thereby ensuring the bonding strength of the coating film when the surface is repainted after application. It should be noted that the metal material to be treated in the present invention is not limited to those having a protective layer formed over the entire surface, but also includes, for example, those in which the protective layer has been lost in some areas due to corrosion, wear, etc., and those in which no protective layer was originally formed. Even in these cases, the present invention can be implemented in the area where the protective layer has been formed, and the effects of the present invention can be obtained in that area.

[0007] The invention according to claim 2 is the surface treatment method according to claim 1, characterized in that the scanning is performed in a state in which the irradiated area moves periodically along a predetermined pattern on the surface. For example, the irradiation spot may be configured to scan while rotating in an arc along the surface of the processing object. This allows the quality of construction to be ensured by appropriately controlling the irradiation time and irradiation period when focusing on a single point on the surface of the object to be treated, depending on the size of the pattern (e.g., the diameter of the rotation circle in an arc-shaped rotation irradiation) and the movement period of the irradiation point (beam spot) (e.g., the rotation speed in a rotation irradiation). Furthermore, by scanning along a predetermined pattern, the processing area per unit time can be increased, thereby speeding up construction.

[0008] The invention according to claim 3 is the surface treatment method according to claim 1 or 2, characterized in that the metal material is an iron-based metal, and the protective layer is a coating containing zinc as a main component. This makes it possible to carry out the removal process of the deteriorated portion while maintaining the sacrificial anticorrosion effect between the protective layer made of zinc and the base material, which is an iron-based metal, and thereby obtain the above-mentioned effects. Such a coating can be formed, for example, by hot-dip galvanizing. Hot-dip galvanizing forms an alloy layer with a concentration gradient of iron and zinc between a layer mainly made of iron-based metal and a layer mainly made of zinc, thereby providing a high bonding strength of the protective layer to the metal material. In the present invention, even if a portion of the protective layer is removed by irradiation with laser light, the alloy layer remains, thereby maintaining the anticorrosion effect even after surface treatment.

[0009] The invention of claim 4 is a surface treatment method according to any one of claims 1 to 3, characterized in that after irradiating the surface with the laser light, the surface is washed with a liquid containing water as a main component. This makes it possible to reduce the salt remaining on the surface even when the heat input due to laser irradiation is relatively small. Therefore, it is possible to reduce the energy, such as the power, required for laser irradiation and the construction time, while achieving good construction quality with little residual salt content. Such liquids include, for example, pure water, tap water, groundwater, river water, and water containing salt corrosion inhibitors containing surfactants.

[0010] The invention according to claim 5 is characterized in that the amount of salt adhering to the surface after the irradiation or the cleaning is 50 mg / m 2 The surface treatment method according to any one of claims 1 to 4, characterized in that: This ensures durability such as corrosion resistance after construction.

[0011] The invention of claim 6 is a structure in which a protective layer is formed on the surface of a metal material, characterized in that irradiation marks are formed on the surface of the protective layer where the surface is scanned by irradiating areas with laser light. A seventh aspect of the present invention is the structure according to the sixth aspect, characterized in that the irradiation marks are formed along traces of periodic movement of the irradiation points on the surface. An eighth aspect of the present invention is the structure according to the sixth or seventh aspect, characterized in that the metal material is an iron-based metal, and the protective layer is a coating containing zinc as a main component. The invention according to claim 9 is characterized in that the amount of salt attached to the surface is 50 mg / m 2 The structure according to any one of claims 6 to 8, characterized in that: In each of the inventions relating to these structures, the same effects as those of the invention relating to the surface treatment method described above can be obtained. [Effects of the Invention]

[0012] As described above, the present invention can provide a surface treatment method and a structure in which the base material protection performance is improved by the protective layer remaining after the treatment to remove the deteriorated portion has been performed. [Brief explanation of the drawings]

[0013] [Figure 1]1 is a cross-sectional view of an irradiation head used in an embodiment of a surface treatment method to which the present invention is applied. [Figure 2] FIG. 4 is a diagram illustrating an example of a trajectory of a beam spot in the embodiment. [Figure 3] 1A to 1C are diagrams showing schematic cross sections of the surface of an object to be treated in time series before and after a surface treatment method using a power tool according to a conventional technology. [Figure 4] 2A to 2C are diagrams showing schematic cross-sectional views of the surface of a treatment object in time series before and after the surface treatment method of the embodiment. [Figure 5] 1 shows photographs of the appearance after surface treatment in Example 1, Example 2, and Comparative Example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the surface treatment method and structure to which the present invention is applied will be described. The surface treatment method of the embodiment removes deteriorated areas such as so-called white rust of zinc oxides and the like, and salts derived from seawater and the like, from structures such as steel towers (for example, steel towers installed as ground facilities in wireless communication networks for mobile phones and the like) in which a protective layer is formed by hot-dip galvanizing a base material made of an iron-based metal such as steel. Furthermore, the structure of the embodiment is, for example, such a steel tower.

[0015] In the surface treatment method of the embodiment, a deteriorated layer such as zinc oxide (so-called white rust) formed on the surface of the protective layer is removed by irradiating it with laser light, along with a portion of the surface layer of the protective layer, so that the layer on the base material side of the protective layer remains, and then the salt content is reduced by a water washing process. Hereinafter, the portion of the structure that is the target of surface treatment will be referred to as a treatment target O in the following description. The laser light irradiation process involves rotating and scanning the irradiation point (beam spot BS) on the surface of the object to be treated O along a relatively large arc, for example, with a diameter of about 10 mm or more, to remove deteriorated layers such as zinc oxide (white rust), deposits such as salt and dirt, and part of the surface of the protective layer that make up the surface of the object to be treated O. Furthermore, if the object to be treated O is painted, the old paint film to be peeled off is also an adhering material to be removed. FIG. 1 is a cross-sectional view of an irradiation head in a laser irradiation device of the first embodiment.

[0016] The irradiation head 1 irradiates the processing object O with a continuous wave (CW) laser beam L transmitted from a laser oscillator (not shown) via a fiber (not shown). The irradiation head 1 is, for example, a handheld type that can be held by an operator and used to trace a predetermined irradiation path, but it can also be attached to a robot that can move the irradiation head 1 along a predetermined path. Alternatively, the irradiation head 1 may be fixed, and the processing object O may be displaced relative to the irradiation head.

[0017] The irradiation head 1 includes a focus lens 10, a wedge prism 20, a protective glass 30, a rotary cylinder 40, a motor 50, a motor holder 60, a protective glass holder 70, a housing 80, a duct 90, and the like.

[0018] The focus lens 10 is an optical element onto which the laser beam L, which is transmitted from the laser oscillator to the irradiation head 1 via a fiber, is incident after passing through a collimator lens (not shown). A collimating lens is an optical element that converts (collimates) the laser light emitted from the end of the fiber into a substantially parallel beam. The focus lens 10 is an optical element that condenses (focuses) the laser beam L emitted from the collimator lens at a predetermined focal position. The focus lens 10 may be, for example, a convex lens having a positive power.

[0019] The beam spot BS, which is the point irradiated by the laser beam L on the surface of the processing object O, is positioned either coincident with this focal position or in a close state within the focal depth (focus state), or spaced apart from the focal position (defocus state). The depth of focus means the range in the optical axis direction in which the beam diameter is equal to or smaller than the diameter of a predetermined allowable circle of confusion.

[0020] The wedge prism 20 is an optical element that deflects the laser beam L emitted by the focus lens 10 by a predetermined deflection angle θ (see FIG. 1) to make the optical axis angles on the incident side and the exit side different. The wedge prism 20 is formed in the shape of a plate whose thickness changes continuously so that the thickness on one side in a direction perpendicular to the optical axis direction on the incident side is greater than the thickness on the other side. The protective glass 30 is an optical element made of a flat glass or the like and arranged adjacent to the wedge prism 20 on the focal position side (the processing object O side, the beam spot BS side) along the optical axis direction.

[0021] The protective glass 30 is a protective member that prevents foreign matter such as peeled matter and dust that scatters from the processing object O side from adhering to other optical elements such as the wedge prism 20. The protective glass 30 is an optical element that is positioned closest to the focal position along the optical axis direction among the optical systems possessed by the irradiation head 1, and is exposed to the side of the object to be treated O through the space A and the inside of the duct 90 described below. The focus lens 10, wedge prism 20, and protective glass 30 are configured by applying coatings for the purpose of preventing reflection, surface protection, etc. to the surfaces of members made of a transparent material such as optical glass.

[0022] The rotary cylinder 40 is a cylindrical member that holds the focus lens 10 and the wedge prism 20 on its inner diameter side. The rotary cylinder 40 is formed concentrically with the optical axis of the focus lens 10 and the optical axis of the laser beam L incident on the focus lens 10 (the optical axis of the collimator lens). The rotary cylinder 40 is supported by a bearing (not shown) relative to the housing 80 so as to be rotatable about a central axis of rotation that coincides with the optical axis of the focus lens 10 . The rotary cylinder 40 is made of metal such as an aluminum alloy, engineering plastic, or the like.

[0023] The motor 50 is an electric actuator that rotates the rotary cylinder 40 relative to the housing 80 around the central axis of rotation. The motor 50 is configured, for example, as a ring-shaped motor that is concentric with the rotary cylinder 40 and provided on the outer diameter side of the rotary cylinder 40 . A stator (not shown) of the motor 50 is fixed to the housing 80 via a motor holder 60 (described later). A rotor (not shown) of the motor 50 is fixed to the rotary barrel 40 . The motor 50 is controlled by a motor drive device (not shown) so that the rotation speed of the rotating barrel 40 substantially coincides with a desired target rotation speed.

[0024] The irradiation head 1 is maintained in a position such that the rotational center axis of the rotating cylinder 40 is perpendicular to the surface of the object to be processed O near the irradiation point, and the motor 50 rotates the wedge prism 20 together with the rotating cylinder 40, so that the beam spot BS rotates and scans in an arc around the rotational center axis of the rotating cylinder 40 along the surface of the object to be processed O. In this state, when the irradiation head 1 is translated along the surface of the processing object O, the beam spot BS scans the surface of the processing object O while revolving in a circular shape (arc shape). As a result, when an arbitrary point on the processing object O is focused on, the laser beam L is intermittently incident thereon for only a short period of time, and rapid heating and rapid cooling are sequentially performed within a short period of time. At this time, the surface portion of the object to be treated O is crushed and scattered.

[0025] The motor holder 60 is a support member that holds the stator of the motor 50 in a predetermined position inside the housing 80. The main body of the motor holder 60 is formed in a cylindrical shape and is fixed to the housing 80 in a state where it is inserted into the inner diameter side of the housing 80 . The inner peripheral surface of the motor holder 60 is disposed opposite the outer peripheral surface of the motor 50 and is fixed to the stator of the motor 50 .

[0026] A purge gas flow path 61 through which purge gas PG flows is formed in part of the gap between the outer peripheral surface and the inner peripheral surface of the motor holder 60. The purge gas PG is a gas that is blown out toward the treatment object O from a space A that contacts the surface of the protective glass 30 on the treatment object O side inside an inner cylinder 91 of a duct 90, which will be described later, when the irradiation head 1 is in use (during irradiation). The surface of the protective glass 30 on the treatment object O side is exposed inside the space A. The purge gas PG has a function of preventing debris such as dust and foreign matter scattered from the treatment object O side from flying into the housing 80 and adhering to the protective glass 30.

[0027] The purge gas flow path 61 is an opening formed by penetrating a part of the motor holder 60 in the axial direction of the motor 50 . The purge gas PG discharged from the purge gas flow path 61 passes through a flow path provided in the housing 80 and is introduced into the inner diameter side of the inner cylinder 91 of the duct 90 .

[0028] The protection glass holder 70 is a member that is fixed to the inner diameter side of the housing 80 while holding the protection glass 30 . The protective glass holder 70 is formed, for example, in the shape of a disk with a circular opening formed in the center. The laser beam L passes through the opening from the wedge prism 20 side to the object to be processed O side. A recess into which the protective glass 30 is fitted is formed on the surface of the protective glass holder 70 facing the treatment object O. The protective glass 30 is held inside the housing 80 in a state where it is fitted into this recess.

[0029] The protective glass 30 is detachably attached to a protective glass holder 70 so that it can be replaced if it becomes contaminated or burned. The surface of the protective glass holder 70 opposite to the object to be processed O is disposed opposite to the end face of the motor holder 60 on the object to be processed O side with a gap therebetween. This gap constitutes part of a flow path (part of a fluid supply portion) that introduces the purge gas PG introduced from the purge gas flow path 61 of the motor holder 60 into the space A on the treatment object O side of the protective glass 30.

[0030] The housing 80 is a cylindrical member that constitutes the enclosure of the main body of the irradiation head 1. The housing 80 contains the above-mentioned focus lens 10, wedge prism 20, protective glass 30, rotating cylinder 40, motor 50, motor holder 60, protective glass holder 70, etc., as well as the end of the fiber on the irradiation head 1 side (not shown), a collimator lens, etc.

[0031] The duct 90 is a double-tubular member provided to protrude from the end of the housing 80 on the treatment object O side. The duct 90 includes an inner cylinder 91, an outer cylinder 92, a dust collector connection cylinder 93, and the like. The motor holder 60, the protective glass holder 70, and the housing 80 are made of metal such as an aluminum alloy, engineering plastic, or the like.

[0032] The inner cylinder 91 is formed in a cylindrical shape. The laser beam L passes through the inner diameter side of the inner cylinder 91 and is emitted to the processing object O side. The inner cylinder 91 has a small diameter portion 91a at its end on the housing 80 side, the small diameter portion 91a being stepped smaller than the other portions. A purge gas PG is introduced from the inside of the housing 80 into the space A inside the small diameter portion 91a.

[0033] At the end of the inner cylinder 91 on the side of the object to be treated O, a tapered portion 91b is formed so that the diameter on the side of the object to be treated O becomes smaller. The tapered portion 91b has the function of allowing the laser beam L to pass through, while throttling the flow of the purge gas PG to increase the flow rate.

[0034] The outer cylinder 92 is a cylindrical member that is arranged concentrically with the inner cylinder 91 and is provided on the outer diameter side of the inner cylinder 91 . A continuous gap is formed between the inner peripheral surface of the outer cylinder 92 and the outer peripheral surface of the outer cylinder 91 over the entire circumference. The outer cylinder 92 has a small diameter portion 92a at its end on the housing 80 side, the small diameter portion 92a being stepped smaller than the other portions. The small diameter portion 92a is fitted and fixed to the end of the housing 80 on the object to be treated O side. The edge of the end 92b of the outer cylinder 92 on the side of the object to be processed O is formed at an angle with respect to the rotational axis of the rotating cylinder 40 so that during normal use when irradiating with the rotational axis of the rotating cylinder 40 horizontal, the upper side is closer to the housing 80 than the lower side.

[0035] The dust collector connection tube 93 is a cylindrical body that protrudes outward from the outer tube 92 and is connected in a state of communication with the inner diameter side of the outer tube 92 near the end of the outer tube 92 on the side of the object to be treated O. The dust collector connection tube 93 is provided below the outer tube 92 during normal use as described above. The dust collector connection tube 93 is disposed at an angle relative to the outer tube 92 so as to approach the housing 80 from the treatment object O side and to move away from the outer tube 92. The other end of the dust collector connection tube 93 is connected to a dust collector (not shown) and is adapted to be vacuum-suctioned so that the inside is at negative pressure.

[0036] In this embodiment, by rotating the rotary cylinder 40 and the wedge prism 20 while emitting the laser beam L, the beam spot BS revolves along the surface of the object O to be processed on an arc of a predetermined radius. In this state, by moving the irradiation head 1 in a relative translational motion along the surface of the object to be processed O, it is possible to perform a process in which the beam spot BS scans the surface of the object to be processed O while the scanning pattern (circular rotation in the first embodiment) moves over the surface at a predetermined feed speed. As the beam spot BS passes, the surface of the object to be treated O is given a spike-like thermal history in which it instantly heats up and then cools down, and part of the surface layer, along with deteriorated materials such as white rust, is crushed or melted and scattered around, and is removed.

[0037] FIG. 2 is a diagram showing an example of the trajectory of the beam spot BS in the embodiment. As shown in FIG. 2, the beam spot BS rotates in response to the rotation of the wedge prism 20, and moves in the feed direction of the irradiation head 1 relative to the object O to be processed. As a result, when the wedge prism 20 rotates one revolution (360°), an offset occurs between the previously irradiated path P0 (the locus of the beam spot) and the currently irradiated path P1. Therefore, in this specification, claims, etc., the overlap ratio is defined as the ratio (w / d×100(%)) of the width w of overlap between the most recently irradiated path P0 and the latest path P1 to the diameter d of the beam spot BS.

[0038] The overlap ratio is a value that indicates the percentage of the path of the beam spot BS on the surface that overlaps with the path of the beam spot BS in the previous irradiation when the beam spot BS repeatedly passes through a predetermined location in the scanning pattern. Here, when the scanning pattern is a circular rotation as in this embodiment, the width w can also be defined as the scanning amount (the amount of feed of the irradiation head) during the period (during one cycle) in which the wedge prism 20 rotates 360°. In other words, the overlap ratio can be defined as the ratio of the beam spot diameter to the feed speed of the scanning pattern in one period of the scanning pattern. For example, in the regions on the left and right of the circular rotation trajectory in Figure 2, the overlap ratio according to this definition substantially coincides with the overlap ratio defined above.

[0039] A comparative example of the surface treatment method of this embodiment, as well as examples 1 and 2, will be described below. In Examples 1 and 2 and the Comparative Example, the object to be treated O was a steel tower member in which deterioration such as white rust had occurred due to salt damage caused by splashing seawater. The processing object O is a steel material on which a protective layer is formed by hot dip galvanizing. In such structures, the base steel material may already be partially exposed due to the thickness of the protective layer at the time of new construction and the conditions of use thereafter. However, in the following explanation, the test materials are those having a film thickness of, for example, 400 μm or more, where it is considered effective to leave the existing hot-dip galvanized layer. The protective layer, which is mainly made of zinc, is bonded via an alloy layer formed between the zinc and the steel. <Comparative Example> In the comparative example, a so-called scraping process was carried out in which the white rust and the protective layer formed by hot dip galvanization were scraped off by mechanical input using a cup wire, which is a power tool. In the comparative example, the thickness of the protective layer before cleaning was 412 μm, whereas the thickness after cleaning was 245 μm, which is a reduction of 167 μm. The surface roughness after scraping (the depth from the tip of the convex portion to the bottom of the concave portion) was 20.9 μm, and visual inspection confirmed that white rust remained.

[0040] In scraping with a power tool, the amount of material removed often varies depending on the contact state of the tool with the workpiece O, and if the protective layer is completely removed locally, exposing the steel, a separate rust prevention treatment using, for example, zinc-rich paint is required. However, unlike protective layers formed by hot-dip galvanizing, the protective layer formed by holding the zinc with an organic binder is not bonded via an alloy layer, and therefore is inferior in corrosion and rust prevention performance to protective layers formed by hot-dip galvanizing. In addition, the salt content on the surface of the treatment object O before cleaning was 883.9 mg / m 2The salt content after cleaning was 146.3 mg / m 2 It was. The salt content can be measured using a known surface salinity meter in accordance with, for example, JIS Z0313. In the comparative example, when the sample was washed with water afterwards, the salt content was reduced, but the salt content after washing was 51.4 mg / m 2 Therefore, in order to prevent corrosion when structures such as steel towers are used continuously for, for example, 10 to 20 years in the future, it is necessary to further reduce the amount of salt.

[0041] Example 1 In Example 1, the laser beam L was irradiated in two passes (the turning circle passed twice) under the following irradiation conditions. The irradiation conditions of the laser beam L are set so that the layer on the opposite side of the protective layer from the steel material side can be removed along with deteriorated areas such as white rust, and so that the layer on the steel material side of the protective layer (and, if an alloy layer is formed between the protective layer and the steel material, the alloy layer) remains covering the surface of the steel material on which the protective layer (and, if an alloy layer is formed, the alloy layer) is formed. For example, the irradiation conditions of the laser beam L are set so that the protective layer and the alloy layer are removed over at least a wide range (typically substantially the entire surface) of the processing object O, and the steel material is not exposed. Note that, like the protective layer, the alloy layer also functions as a layer that protects the steel material, which is the base material. The laser beam L was irradiated so that the overlap ratio was 20%. In Example 1, the thickness of the protective layer before irradiation was 456 μm, whereas the thickness after two passes of irradiation was 197 μm, meaning that the thickness was reduced by 259 μm. The surface roughness after scraping was 66.9 μm, and visual inspection revealed that the white rust had been completely removed, exposing the shiny zinc over the entire surface. Furthermore, no exposure of the base steel material was observed.

[0042] Furthermore, arc-shaped irradiation marks were observed on the surface after irradiation, following the rotation circle of the beam spot BS. These irradiation marks can be confirmed, for example, even if the surface has been repainted after construction, by removing the paint film with a solvent or chemical remover. The surface roughness is higher than that of the comparative example, which is thought to be largely due to the influence of irradiation marks from the laser beam L. However, this is thought to be advantageous in terms of adhesion with the coating when repainting the surface after treatment. In addition, the salt content on the surface of the treatment object O before irradiation was 883.9 mg / m 2 The salt content after two passes was 12.3 mg / m 2 In Example 1, the degree of rust removal and the amount of residual salt were significantly improved compared to the comparative example.

[0043] <Example 2> In Example 2, irradiation with the laser beam L under the same irradiation conditions as in Example 1 is performed only once, and washing with tap water is performed after the irradiation. The water washing is carried out by spraying tap water onto the object to be treated O using a high-pressure washer, for example. In Example 2, the thickness of the protective layer before irradiation was 496 μm, whereas the thickness after one pass of irradiation was 253 μm, which is a decrease of 243 μm. The surface roughness after scraping was 40.4 μm, and visual inspection showed that the white rust had been completely removed, exposing the shiny zinc over the entire surface. Furthermore, no exposure of the base steel material was observed.

[0044] As in Example 1, arc-shaped irradiation marks along the rotation circle of the beam spot BS were observed on the surface after irradiation. In addition, the salt content on the surface of the treatment object O before irradiation was 883.9 mg / m 2 The salt content after irradiation was 61.2 mg / m 2 It was. Furthermore, the salt content after washing with water was 11.4 mg / m 2 It was. In Example 2, the degree of rust removal and the amount of remaining salt are further improved compared to Example 1. Furthermore, the amount of protective layer removed is reduced, and the decrease in the remaining film thickness can be suppressed. Furthermore, by performing the irradiation process in one pass, the processing time and the energy required for the processing, such as electricity, can be reduced, and by performing water washing, the amount of salt can be reduced even further than in Example 1, which uses two passes.

[0045] FIG. 3 is a time-series diagram showing schematic cross sections of the surface of an object to be treated before and after a surface treatment method using a power tool according to the prior art (comparative example, etc.). 3(a) to 3(f) are schematic cross-sectional views of the surface of the object to be processed at the same location, shown in chronological order (the same applies to FIG. 4, which will be described later). As shown in FIG. 3(a), when the object to be treated is new, a protective layer 120 containing zinc as a main component is formed on the surface of a steel material 110 serving as a base material. The protective layer 120 is formed by hot dip galvanizing, which involves immersing the steel material 110 in molten liquid zinc at a temperature of, for example, about 440 to 460°C. The protective layer 120 is an η layer having a composition similar to that of the molten zinc bath.

[0046] Between the steel material 110 and the protective layer 120, an alloy layer 130 is formed during the hot dip galvanizing process. The alloy layer 130 is a zinc-iron alloy layer in which the iron concentration varies with a concentration gradient, for example, having a ζ layer with an iron content of about 6% and a δ1 layer with an iron content of about 7 to 11%. The alloy layer 130 is formed when the surface of the steel material 110 is heated during the hot-dip galvanizing process, causing the thermal vibration of atoms to become intense, causing iron atoms to diffuse from the surface of the steel material 110 into the zinc layer and zinc atoms to diffuse into the interior of the steel material 110.

[0047] If the object to be treated is a structure installed outdoors, such as a steel tower for a communications facility, as the structure is used, deposits 140 such as salty seawater will adhere to the surface of the protective layer 120, as shown in Figure 3(b). Thereafter, the deposits 140 corrode a portion of the protective layer 120, and white rust 150 is formed on the surface of the protective layer 120, as shown in FIG. 3(c). The white rust 150 is mainly composed of zinc oxide, which is produced by oxidizing the zinc in the protective layer 120. The white rust 150 also contains salt derived from the deposits 140. White rust 150 is likely to occur when the surface of the protective layer 120 is wet with water and exposed to an environment that does not dry easily, and is particularly prone to occur when salt is attached. If the white rust 150 is left as it is, the thickness of the protective layer 120 will be reduced due to corrosion, and there is a concern that the corrosion prevention effect of the protective layer 120 on the steel material 110 will be impaired.

[0048] In surface treatment using a power tool such as a cup wire, for example, as shown in Figure 3(d), it may not be possible to completely remove the white rust 150, and some of the white rust 150 may remain on the surface of the protective layer 120 (corresponding to the comparative example described above). Moreover, if the surface is polished excessively to prevent such residue, substantially the entire protective layer 120 may be peeled off, exposing the alloy layer 130 containing iron, as shown in FIG. 3(e). In such a case, if a primer layer 160 and a top coat layer 170 are formed by new painting, as shown in Figure 3(f), the adhesion of the primer layer 160 may be impaired by corrosion, causing the primer layer 160 to peel off from the base. In this case, it becomes difficult to ensure sufficient corrosion resistance of the steel material 110.

[0049] FIG. 4 is a time-series diagram showing schematic cross-sectional views of the surface of a treatment object before and after the surface treatment method of the embodiment. 4(a) to 4(c) are similar to the above-described FIGS. 3(a) to 3(c). In the state of irradiation with the laser beam L shown in Figure 4(d), substantially all of the white rust 150 and a portion of the layer of the protective layer 120 on the side opposite to the steel material 110 are melted, crushed, and removed by the heat input during irradiation with the laser beam L, etc. On the other hand, a portion of the protective layer 120 on the side of the steel material 110 and the alloy layer 130 remains in a state of being bonded thereto. On the surface of the remaining protective layer 120, unevenness is formed due to irradiation marks T formed by scanning with the beam spot BS. Furthermore, since the alloy layer 130 is maintained both before and after the surface treatment, the bond strength between the steel material 110 and the protective layer 120 remains substantially the same before and after the surface treatment. However, at this stage, salt may remain on the surface of the protective layer 120 and adhere thereto. Therefore, in the embodiment (Example 2), the surface of the protective layer 120 is washed with water to reduce the salt content. FIG. 4(e) shows the state after washing with water. FIG. 4(f) shows the state after rinsing with water and then coating with a primer layer 160 and a top coat layer 170 in that order.

[0050] In the embodiment shown in Figure 4, before the primer layer 160 is applied, substantially all of the white rust 150 and salt is removed from at least a portion of the surface of the protective layer 120, which prevents the primer layer 160 from peeling off from the protective layer 120 due to corrosion and improves the adhesion and peeling resistance of the primer layer 160. Furthermore, the unevenness caused by the irradiation marks T exerts an anchor effect that increases the adhesive strength of the undercoat layer 160 to the protective layer 120, thereby more reliably preventing peeling of the undercoat layer 160.

[0051] FIG. 5 is a photograph of the appearance after surface treatment in Example 1, Example 2 of the present invention, and Comparative Example. The comparative example, example 1, and example 2 are shown in order from the top. 5(a), it can be seen that white rust remains after scraping using the cup wire. While this state is undesirable, there is a concern that if scraping is performed using a larger mechanical force, an area will be formed in which the protective layer 120 is completely peeled off together with the alloy layer 130, significantly impairing the corrosion protection effect. When processing is performed using laser beam L as in Examples 1 and 2 shown in Figures 5(b) and 5(c), it can be seen that arc-shaped irradiation marks are formed along the path traveled by the beam spot BS. In Figures 5(b) and 5(c), no remaining white rust 150 can be seen, and it can be seen that the zinc metal surface is exposed over substantially the entire surface.

[0052] When the surface treatment method of this embodiment is carried out, even if the average film thickness is, for example, about 1 μm after treatment, as long as the protective layer 120 remains on the surface of the irradiated object O, a certain level of anti-corrosion effect can be obtained through sacrificial corrosion protection. However, it is preferable that the average film thickness be 28 μm or more, for example, in accordance with the JIS H8641 Type 1A standard for hot-dip galvanizing. More preferably, the average film thickness may be 69 μm or more, in accordance with the JIS H8641 Type 2, 50 standard required for steel materials and steel products with a thickness of more than 5 mm. Furthermore, when the structure is used in a severely corrosive environment (such as a seaside location), the average film thickness may be 76 μm or more in accordance with the JIS H8641 Type 2 55 standard.

[0053] The salt content is 50 mg / m according to the measurement method in accordance with JIS Z0313. 2 It is preferable that: This value is a general value that is acceptable as the amount of residual salt adhesion in, for example, scraping treatment before painting of steel materials that are susceptible to salt corrosion. Salt content of 50mg / m 2 By satisfying the following conditions, it is believed that corrosion resistance that does not pose any practical problems when the structure is in use can be ensured.

[0054] It is possible that in the object to be processed O, the protective layer 120 is lost in some areas due to corrosion, wear, etc. before irradiation with the laser beam L, and the steel material 110 is exposed when the white rust 150 is removed. However, even in such a case, if there are other areas where the protective layer 120 remains both before and after irradiation, it is possible to obtain the effect of increasing the corrosion resistance of the steel material 110 in these other areas. However, if the processing object O is a structural component such as a steel tower, if the protective layer 120 does not remain before irradiation with the laser beam L, there is a risk of cross-sectional loss, and therefore, replacement of the component may be necessary. Furthermore, in the region where the protective layer 120 existed before irradiation with the laser beam L, it is preferable that the layer of the protective layer 120 on the steel material 110 side remains over substantially the entire region after irradiation with the laser beam L. However, even if the protective layer 120 is removed by the laser beam L and there are some areas where the steel material 110 is exposed, in the regions excluding such areas, the effect of improving the corrosion resistance of the steel material 110 can be obtained compared to when the steel material 110 is exposed.

[0055] In conventional surface treatment methods, such as the technique described in Patent Document 1, when a deteriorated coating film or the like is mechanically removed using a disk sander or the like, the process is based on the premise that the base material, such as steel, will be exposed because the method is highly aggressive to the existing protective layer. Furthermore, the technology described in Patent Document 2 is not based on hot-dip galvanizing, but is based on removing deposits from the surface of the object to be treated and exposing the base material.

[0056] In contrast, according to this embodiment, the following effects can be obtained. (1) By irradiating the laser beam L to remove the layer of the protective layer 120 on the side opposite the steel material 110, so that the protective layer 120 remains on the steel material 110 side, it is possible to leave a portion of the protective layer 120 on the steel material 110 side in a bonded state similar to that before the treatment via the alloy layer 130, and the corrosion resistance of the steel material 110 due to the protective layer 120 can be ensured even after the white rust 150 removal treatment has been performed. Furthermore, unlike mechanical scraping processes that require direct access by a tool such as a cup wire, irradiation with the laser beam L can be performed from a relatively remote location by setting the focal length of the focus lens 10, making it possible to easily and reliably remove white rust 150 in narrow areas, such as recesses and the areas around bolts and nuts. Furthermore, irradiation marks T are formed on the surface of the object to be treated O by irradiating it with the laser beam L, which increases the surface roughness, thereby ensuring the bonding strength of the coating film when repainting the surface after application. In the techniques described in Patent Documents 1 and 2, when a protective layer containing zinc is newly formed with an organic binder, for example, using zinc-rich paint, after the protective layer is removed, there is a concern that the bonding strength between the protective layer and the base material may decrease compared to when the structure was newly constructed. However, in this embodiment, such concerns do not need to be taken into consideration in the area where the protective layer 120 remains. (2) By configuring the beam spot BS to rotate in an arc while scanning the surface of the object to be treated O, it is possible to appropriately control the irradiation time and irradiation cycle when focusing on a single point on the surface of the object to be treated O by adjusting the irradiation circle diameter and the rotation speed of the wedge prism 20, thereby ensuring the quality of the work. Furthermore, by performing such rotational scanning, the processing area per unit time can be increased, and construction speed can be increased. (3) By forming the protective layer 120 by hot-dip galvanizing, it is possible to perform the removal process of the white rust 150 while maintaining the sacrificial anticorrosion effect between the protective layer 120 made of zinc and the iron-based metal of the steel material 110, and the above-mentioned effects can be obtained. (4) After irradiation with the laser beam L, by washing the surface of the object to be treated O with tap water, the salt remaining on the surface can be reduced even if the heat input due to the laser irradiation is relatively small. Therefore, it is possible to reduce the energy, such as the power, required for laser irradiation and the construction time, while achieving good construction quality with little residual salt content. (5) The residual salt content on the surface after surface treatment is 50 mg / m2 By doing so, durability such as corrosion resistance after construction can be ensured.

[0057] (Variation) The present invention is not limited to the above-described embodiment, and various modifications and variations are possible, and these are also within the technical scope of the present invention. (1) The surface treatment method and the structure configuration are not limited to the above-described embodiment, and can be modified as appropriate. For example, the laser light irradiation conditions in the embodiments are merely examples and can be changed as appropriate depending on the state of the object to be processed. (2) In the embodiment, the object to be treated is, as an example, a steel material on which a protective layer is formed by hot-dip galvanizing, but the material and manufacturing method of the object to be treated are not limited to this and can be changed as appropriate. For example, a protective layer containing zinc as a main component may be formed by a manufacturing method other than hot-dip plating, such as thermal spraying. Also, a protective layer containing an element other than zinc as a main component, such as an aluminum-based material, may be formed. (3) In the embodiment, a rotating wedge prism is used to rotate the irradiation point (beam spot) in a circular pattern on the surface of the object to be processed, but the irradiation point may also be moved (typically rotated) periodically along other patterns. For example, a galvanometer mirror or a polygon mirror may be used as a means for deflecting a laser beam to form such a pattern. Alternatively, a configuration may be adopted in which a plurality of optical elements such as a wedge prism, a galvanometer mirror, and a polygon mirror are used in combination. (4) In the embodiment (Example 2), washing with water is performed after laser irradiation, but washing with other liquids containing water as the main component is not limited to water. For example, cleaning may be performed using an aqueous solution of a salt corrosion inhibitor made of a surfactant or other substance. [Explanation of symbols]

[0058] 1 Irradiation head 10 Focus lens 20 Wedge prism 30 Protective glass 40 Rotating cylinder 50 Motor 60 motor holder 61 purge gas flow path 70 Protective glass holder 80 Housing 90 Duct 91 Inner cylinder 91a Small diameter section 91b Tapered section 92 Outer cylinder 92a Small diameter section 92b End 93 Dust collector connection tube L Laser beam BS Beam spot PG Purge gas O Processing object 110 Steel 120 Protective layer 130 Alloy layer 140 Deposit 150 White rust 160 Undercoat layer 170 Top coat layer T irradiation marks

Claims

1. A surface treatment method for a metal material having a protective layer formed thereon, comprising: a laser beam is irradiated onto the surface of the protective layer, and the surface is scanned while the irradiated spot is moved along the surface, and the layer of the protective layer on the opposite side to the metal material side is removed by the irradiation of the laser beam while the layer of the protective layer on the metal material side remains. A surface treatment method characterized by:

2. performing said scanning with said irradiated spot moving periodically along a predetermined pattern on said surface; The surface treatment method according to claim 1, characterized by:

3. The metal material is an iron-based metal, The protective layer is a coating containing zinc as a main component.

3. The surface treatment method according to claim 1 or 2, characterized by:

4. After irradiating the surface with the laser light, the surface is washed with a liquid containing water as a main component. The surface treatment method according to any one of claims 1 to 3, characterized in that:

5. The amount of salt attached to the surface after the irradiation or the cleaning is 50 mg / m ２ Is less than or equal to The surface treatment method according to any one of claims 1 to 4, characterized in that:

6. A structure in which a protective layer is formed on the surface of a metal material, Irradiation marks are formed on the surface of the protective layer by scanning the surface with the laser light. A structure characterized by:

7. The irradiation marks are formed along the traces of the irradiation points periodically moving on the surface.

7. The structure of claim 6,

8. The metal material is an iron-based metal, The protective layer is a coating containing zinc as a main component.

8. The structure according to claim 6 or claim 7, characterized in that:

9. The amount of salt attached to the surface is 50 mg / m ２ Is less than or equal to 9. A structure according to any one of claims 6 to 8, characterized in that

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