Cylinder block for outboard motor and method of manufacturing the same

Abstract

To provide a cylinder block for an outboard motor and its manufacturing method capable of forming a screw hole having a predetermined inner diameter even if the cylinder block for the outboard motor is subjected to anodic oxidation treatment, preventing corrosion of a base material due to remaining of treatment liquid, and further improving corrosion resistance to cooling water such as seawater.SOLUTION: Then, the temporary hole 31 is threaded to remove the anodic oxide film in the temporary hole and form the screw hole 34 in which the aluminum alloy is exposed to the inner peripheral surface, and then the chemical conversion film 35 is formed on the surface of the sealed anodic oxide film 32 and the surface of the aluminum alloy exposed to the inner peripheral surface of the screw hole 34.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a cylinder block for an outboard motor and a method for manufacturing the same. [Background technology]

[0002] In industrial internal combustion engines, cooling water is circulated to maintain performance and control the temperature of the engine. The cooling water typically used in automobiles contains water with additives such as ethylene glycol, and if the amount of water decreases, it must be replenished. On the other hand, engines such as outboard motors use seawater (salt water), which is readily available in the operating environment, as cooling water.

[0003] However, because the various components of outboard motors are made up of many different metallic materials, differences in the metallic materials of the components that come into contact with seawater create potential differences, and metallic materials with low natural potentials dissolve in seawater, which can cause corrosion. Aluminum alloys, which are used in components such as cylinder blocks for outboard motors, are particularly susceptible to corrosion compared to other materials.

[0004] To prevent such corrosion from occurring, for example, Patent Document 1 describes a method for preventing such corrosion in a cylinder block body for an outboard motor having a cylinder bore and a water jacket surrounding the cylinder bore. The region of the inner surface of the cylinder bore that encompasses the boundary between the cast iron of the cast-encased cylinder sleeve and the aluminum alloy base material is pressed with an elastic jig, and the cylinder block body is anodized in this state to form an anodized film on the surface of the aluminum alloy.A sealing process is then performed to seal the pores in the anodized film, thereby forming a sealed anodized film on the joining surface to be joined to the cylinder head and on the inner surface of the water jacket. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2023-72944 Summary of the Invention [Problem to be solved by the invention]

[0006] However, outboard motor cylinder blocks have numerous threaded holes, due to the screws used to fasten components and wiring, such as exhaust manifolds, oil pans, and various sensors. Therefore, when anodizing is performed on an outboard motor cylinder block, an anodic oxide film is formed inside the threaded holes, changing the inner diameter of the threaded holes, making it difficult to fasten the screws, and forcing the assembly can lead to damage to the screws. Furthermore, the threaded holes are arranged in various directions, making it difficult to thoroughly clean all of the numerous threaded holes, even with water rinsing or air blowing during anodizing. If anodizing solution remains in the threaded holes, it can cause corrosion of the base material within just a few days.

[0007] Therefore, an object of the present invention is to provide an outboard motor cylinder block and a manufacturing method thereof that can form a screw hole with a specified inner diameter even when the cylinder block is anodized, can prevent corrosion of the base material due to residual processing liquid, and can further improve corrosion resistance against cooling water such as seawater. [Means for solving the problem]

[0008] In order to achieve the above object, one aspect of the present invention is a cylinder block for an outboard motor, the cylinder block having, at a joining surface where it is joined to a cylinder head, a cylinder bore and a water jacket around the cylinder bore, and having, on its outer peripheral surface, threaded holes for fastening the outboard motor cylinder block to a member to be fastened, the peripheral surfaces of the openings of the threaded holes and the inner peripheral surface of the water jacket being covered with an anodized aluminum alloy film, the pores in the anodized film being sealed with a sealing product, the sealed anodized film being further covered with a chemical conversion film, and the inner peripheral surfaces of the threaded holes being covered with the chemical conversion film.

[0009] In another aspect, the present invention provides a method for manufacturing an outboard motor cylinder block, the method comprising the steps of: casting a cylinder block body made of an aluminum alloy; the cylinder block body having a plurality of threaded holes on its outer peripheral surface for fastening the outboard motor cylinder block to a member to be fastened; the cylinder block body having cylinder bores, a water jacket formed around the cylinder bores, and temporary holes having an inner diameter smaller than that of the threaded holes at locations where the threaded holes are to be formed, at a joining surface where the cylinder block body is joined to a cylinder head; forming an anodized film on the surface of the aluminum alloy portion of the cylinder block body by anodizing; sealing the surface of the anodized film by sealing; threading the temporary holes to remove the anodized film on the inner peripheral surface of the temporary holes and form the threaded holes so that the aluminum alloy is exposed on the inner peripheral surface of the threaded holes; and forming a chemical conversion film on the surface of the sealed anodized film and on the surface of the aluminum alloy exposed on the inner peripheral surface of the threaded holes. [Effects of the Invention]

[0010] Thus, according to the present invention, the aluminum alloy portion serving as the base material of an outboard motor cylinder block is covered with a sealed anodic oxide coating, and this sealed anodic oxide coating is further covered with a chemical conversion coating. Meanwhile, the aluminum alloy base material on the inner circumferential surface of the screw hole is covered with a chemical conversion coating. As a result, it is possible to form a screw hole having a predetermined inner diameter, prevent corrosion of the base material due to residual treatment liquid, and further improve corrosion resistance to cooling water such as seawater. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a perspective view of a cylinder head side showing an example of an outboard motor cylinder block. FIG. [Figure 2] 2 is a perspective view showing a gasket interposed between the cylinder block for an outboard motor shown in FIG. 1 and a cylinder head. FIG. [Figure 3]1 is a flowchart showing an embodiment of a method for manufacturing an outboard motor cylinder block according to the present invention; [Figure 4] 4 is a schematic cross-sectional view of a screw hole in the embodiment of the flow diagram shown in FIG. 3. FIG. [Figure 5] FIG. 6 is a flowchart showing another embodiment of the method for manufacturing an outboard motor cylinder block according to the present invention. [Figure 6] 6 is a front view schematically showing a transport jig and an outboard motor cylinder block body used in the embodiment of the flow chart shown in FIG. 5. FIG. [Figure 7] 6 is a perspective view showing an example of a first temporary hole and a second temporary hole in the outer peripheral surface of the cylinder block body for an outboard motor after casting S1 in the flowchart shown in FIG. 5. FIG. [Figure 8] 6 is a perspective view showing an example of a first temporary hole and a second screw hole in the outer peripheral surface of the cylinder block body for an outboard motor after machining S2 in the flowchart shown in FIG. 5. FIG. [Figure 9] 6 is a perspective view showing an example of installing an energized bolt or the like in a second screw hole in the outer peripheral surface of the cylinder block body for an outboard motor during anodizing treatment S3 in the flow diagram shown in FIG. 5. [Figure 10] 10 is a cross-sectional view schematically showing a second screw hole, a conductive bolt, and the like shown in FIG. 9. FIG. [Figure 11] 6 is a perspective view showing an example of a first screw hole and a second screw hole in the outer peripheral surface of the cylinder block body for an outboard motor after machining S4 in the flowchart shown in FIG. 5. FIG. [Figure 12] 6 is a perspective view showing a typical example of use of a second screw hole in the outer peripheral surface of the cylinder block body for an outboard motor after chemical conversion treatment S5 in the flow diagram shown in FIG. 5. FIG. [Figure 13] 1 is an image showing a cross section of the inside of a screw hole in Example 1. [Figure 14] 10 is an image showing a cross section of the inside of a screw hole in Comparative Example 1. [Figure 15] 1 is an SEM image showing the coating surface of Example 1 before chemical conversion treatment. [Figure 16] 1 is a graph showing the results of a component analysis in the depth direction of the coating of Example 1 before chemical conversion treatment. [Figure 17] 1 is an SEM image showing the coating surface after chemical conversion treatment in Example 1. [Figure 18] 1 is a graph showing the results of a component analysis in the depth direction of the coating after chemical conversion treatment in Example 1. [Figure 19] 1 is an SEM image showing the coating surface of Comparative Example 2 before chemical conversion treatment. [Figure 20] 1 is a graph showing the results of a component analysis in the depth direction of the coating of Comparative Example 2 before chemical conversion treatment. [Figure 21] 1 is an SEM image showing the coating surface after chemical conversion treatment in Comparative Example 2. [Figure 22] 1 is a graph showing the results of a component analysis in the depth direction of the coating after chemical conversion treatment in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a cylinder block for an outboard motor and a method for manufacturing the same according to the present invention will now be described with reference to the accompanying drawings.

[0013] First, we will explain the cylinder block body, which is the target of the manufacturing method of the cylinder block for outboard motors according to the present invention. As shown in Figures 1 and 2, a cylinder block body 10 has a joining surface 14 where it is joined to a cylinder head (not shown), and a plurality of cylinder bores 11 and a water jacket 15 formed around these cylinder bores 11. Also, a cylindrical cylinder sleeve 12 is installed on the inner circumferential surface of each cylinder bore 11. The cylinder block body 13 is made of an aluminum alloy, but the cylinder sleeve 12 may be made of either the aluminum alloy or cast iron.

[0014] As shown in Figure 2, the cylinder block body 10 is joined to the cylinder head via a gasket 16. Although these figures show the cylinder block body 10 with three cylinder bores 11 aligned horizontally, an actual outboard motor (not shown) will have the cylinder bores 11 aligned vertically in a cylinder block for an outboard motor. The number of cylinder bores 11 may be one for a single-cylinder engine.

[0015] Additionally, the outer peripheral surface of the cylinder block body 10 has a plurality of screw holes (not shown) for fastening components such as an exhaust manifold, an oil pan, and various sensors, as well as wiring, to the outboard motor cylinder block 10. Although the cylinder block body 10 has a complex shape as shown in Fig. 1, it is generally hexahedral, and the outer peripheral surface refers to these six faces, but as the cylinder head is positioned on joining surface 14 via a gasket 16 as shown in Fig. 2, screw holes are formed on the other five faces.

[0016] 3, one embodiment of the method for manufacturing an outboard motor cylinder block according to the present invention includes a casting step S1 in which an aluminum alloy cylinder block body is cast, the cylinder block body having a temporary hole with a smaller inner diameter than the screw hole where the screw hole is to be formed; an anodizing step S3 in which a sealed anodic oxide film is formed on the surface of the aluminum alloy portion of the cylinder block body; a finishing step S4 in which the temporary hole is threaded to remove the anodic oxide film on the inner surface of the temporary hole to expose the aluminum alloy on the inner surface of the screw hole; and a chemical conversion treatment step S5 in which a chemical conversion film is formed on the surface of the anodic oxide film and on the surface of the aluminum alloy exposed on the inner surface of the screw hole. Each step is described in detail below.

[0017] The casting step S1 can be performed using a conventional casting method for outboard motor cylinder blocks, such as die casting, to obtain a cylinder block body that will be processed in subsequent steps. The material for the cylinder block body is not particularly limited as long as it is an aluminum alloy, and it may contain various alloy components such as silicon and copper. When casting the cylinder block body, a cylinder sleeve may be cast into the casting, as described in Patent Document 1.

[0018] The cylinder block body obtained by the casting process S1 is produced with a temporary hole 31 having an inner diameter smaller than the screw hole to be finally produced in the mounting portion 30 of the fastened member, as shown in Figure 4(a).

[0019] Next, in the anodizing step S2, the cylinder block body is immersed in a treatment solution for electrolysis, forming a porous anodized film on the surface of the cylinder block body. The electrolysis dissolves the aluminum alloy on the surface of the cylinder block body, and the dissolved aluminum combines with oxygen in the treatment solution to form an anodized film primarily composed of aluminum oxide on the surface of the cylinder block body.

[0020] The treatment liquid for the anodizing treatment may be an acidic bath such as sulfuric acid, oxalic acid, phosphoric acid, or chromic acid, or a basic bath such as sodium hydroxide, sodium phosphate, or sodium fluoride. The electrolytic treatment is performed by applying a voltage between the cylinder block body 10 as the anode and an electrode plate (not shown) made of titanium, carbon, or the like as the cathode.

[0021] The thicker the anodic oxide film formed by anodizing treatment, the more its corrosion resistance tends to improve, but it also becomes more susceptible to a problem known as film burn, in which dielectric breakdown occurs in the film, significantly reducing the corrosion resistance of the affected area. Therefore, the thickness of the anodic oxide film is preferably, for example, 1 to 60 μm, and more preferably 3 to 20 μm.

[0022] In anodizing, when the aluminum in the base material of the cylinder block body 10 is oxidized to form a film, the aluminum changes to aluminum oxide, causing a volume expansion. That is, about half of the thickness of the anodized film that is formed is a penetration film that penetrates deep into the surface of the aluminum alloy base material, and about the remaining half is a growth film that grows outward from the surface of the base material.

[0023] If a cast iron cylinder sleeve 12 is cast into the inner circumferential surface of the cylinder bore 11, the cast iron cylinder sleeve 12 may corrode if exposed to the treatment solution. In such cases, because there are boundaries between the aluminum alloy and the cast iron at the cylinder head end and the crankshaft end of the inner circumferential surface of the cylinder bore 11, it is preferable to perform the anodizing process while pressing both boundaries with an elastic jig (not shown), such as an air-expandable elastic bag. This prevents the treatment solution from penetrating into the cast iron cylinder sleeve 12, allowing the anodizing process to be performed without dissolving the cast iron. Alternatively, the inner circumferential surface of the cylinder bore 11 may be coated with a masking agent.

[0024] The electrolysis method for anodizing treatment can be DC electrolysis or AC / DC superimposed electrolysis. The anodic oxide film formed by either electrolysis method is an anodic oxidation reaction that includes a permeation film and a growth film, as described above, but the properties of the resulting anodic oxide film differ.

[0025] Anodized films formed by DC electrolysis have cells that grow linearly while eroding perpendicularly to the base material surface. When there are many impurities or additives (such as silicon) in the aluminum alloy, cells do not grow around the impurities or additives near the surface, and depressions appear on the surface where the impurities or additives are precipitated, resulting in an anodized film with high surface roughness. In addition, an anodized film with large variations in film thickness is formed.

[0026] Anodized films formed by AC / DC superimposed electrolysis have a structure in which cells form a series of nearly continuous spheres or ellipses with a height less than twice the cell diameter, and these cells gather together to form a cluster of grapes. Therefore, anodized films formed by AC / DC superimposed electrolysis have a low ratio of the volume of the pores contained within the cells to the cell wall. In contrast, anodized films formed by DC electrolysis have a high ratio of the volume of the pores contained within the cells to the cell wall, because the cells are formed in a continuous cylindrical shape.

[0027] Furthermore, an anodic oxide film formed by AC / DC superimposed electrolysis avoids and incorporates impurities or additives that inhibit cell growth during cell growth, preventing cell growth from being inhibited by the impurities or additives. This allows the anodic oxide film to have a substantially uniform film thickness on the surface of the base material. Because the cell growth direction of an anodic oxide film formed by AC / DC superimposed electrolysis is minutely bent in random directions relative to the surface of the base material, resistance to water penetration is provided at the points where the direction changes, preventing water from reaching the base material. This provides higher corrosion resistance than an anodic oxide film formed by DC electrolysis.

[0028] After anodizing the cylinder block body, it is preferable to wash it with water with few impurities, such as ion-exchanged water or pure water. Washing with water can remove the anodizing solution adhering to the surface of the cylinder block body and also reduce the concentration of the remaining solution even in narrow places that are difficult to clean, such as the inside of the temporary hole 31.

[0029] In the anodizing step S3, a sealing treatment is then performed to close the pores in the anodized film. The sealing treatment is performed by immersing the cylinder block body on which the anodized film has been formed in a sealing solution or by applying a sealing solution to the cylinder block body. This seals the pores in the porous anodized film, improving the corrosion resistance of the anodized film.

[0030] The sealing treatment can be performed using known methods such as hydrothermal, boiling water, nickel acetate, low-temperature sealing, and lithium hydroxide. The low-temperature sealing method will be described as an example of the sealing treatment. The sealing solution used in the low-temperature sealing treatment contains, for example, chromium and / or zirconium and fluoride ions, and may further contain cobalt, calcium, zinc, or the like. In the low-temperature sealing treatment, negatively charged fluoride ions are adsorbed and reacted with the positively charged gel portion of the pores in the anodized film to form metal hydroxides such as chromium hydroxide. Aluminum fluoride and the metal hydroxide then co-precipitate through a series of reactions, thereby sealing the pores. The temperature of the sealing solution is preferably in the range of 5 to 70°C, the pH is preferably in the range of 2 to 7, and the treatment time is preferably in the range of 1 to 900 seconds.

[0031] The sealing treatment solution used in this embodiment will be described in detail later, but from the viewpoint of corrosion resistance, it is preferable that it contains chromium or zirconium. As the chromium source, trivalent chromium salts such as chromium nitrate, chromium sulfate, chromium chloride, chromium phosphate, chromium acetate, and chromium hydroxide can be used. As the cobalt source, cobalt nitrate, cobalt sulfate, cobalt chloride, etc. can be used. As the fluoride ion source, hydrogen fluoride, ammonium fluoride, potassium fluoride, sodium fluoride, etc. can be used. As the zirconium source, zirconium oxychloride, zirconium sulfate, zirconium nitrate, zirconium oxide, etc. can be used.

[0032] When anodizing is performed with both ends of the cylinder sleeve pressed with the elastic jig, it is preferable to also perform sealing with both ends of the cylinder sleeve pressed with the elastic jig, and remove the jig after sealing.

[0033] After the sealing treatment, it is preferable to wash the cylinder block body with water containing few impurities, such as ion-exchanged water or pure water, in the same way as after the anodizing treatment. If the cylinder block body, which has a complex shape, is to be dried naturally without using a drying oven after the sealing treatment, washing the cylinder block body with hot water at 50°C or higher after the water washing facilitates the natural drying.

[0034] 4(b) shows the fastened member mounting portion 30 of the cylinder block body on which a sealed anodized film has been formed. A sealed anodized film 32 is formed on the surface 30a of the fastened member mounting portion 30 and on the inner circumferential surface of the temporary hole 31 formed therein. Even if water rinsing is performed after both the anodizing treatment and the sealing treatment, there is a possibility that the treatment liquid 33 will remain inside the temporary hole 31.

[0035] In the finishing process S4, as shown in FIG. 4(c), the inner surface of the temporary hole 31 is threaded to form a threaded hole 34. At this time, the anodized film 32 on the inner surface of the temporary hole 31 is removed, exposing the base material, an aluminum alloy, on the inner surface of the threaded hole 34. Threading can be performed using a wide variety of threading methods, such as cutting, that are generally used to process aluminum alloys. The anodized film 32 is formed on the inner surface of the temporary hole 31, and the anodized film 32 is harder than the base material. Therefore, a cutting tool that is commonly used in the past or one suitable for cutting harder materials may be used.

[0036] By performing the finishing process S4 in this manner, even if processing liquid remains inside the temporary hole 31, the inner diameter of the temporary hole 31 is small and the amount of processing liquid remaining there is small, so it can be removed during thread cutting. Note that thread cutting can be done by wet or dry methods, but wet thread cutting is preferred because the remaining processing liquid can be washed away with cutting fluid in the wet method.

[0037] The cylinder block body thus finished is then subjected to a chemical conversion treatment step S5. By performing the chemical conversion treatment, as shown in Figure 4(d), a chemical conversion coating 35 is formed on the inner circumferential surface of the screw hole 34 where the aluminum alloy is exposed, and the chemical conversion coating 35 is also formed on the sealed anodized coating on the surface 30a of the mounting portion 30 of the fastened member (i.e., the peripheral surface of the opening of the screw hole 34). The aluminum alloy is exposed on the inner circumferential surface of the screw hole 34 due to the threading process, but by covering this exposed aluminum alloy with the chemical conversion coating 35, the corrosion resistance of the inner circumferential surface of the screw hole 34 can be improved.

[0038] While a wide variety of known chemical conversion treatments can be used for the chemical conversion treatment, it is preferable to use a chemical conversion treatment solution containing the same main components as the sealing treatment solution used in the sealing treatment step. For example, when a low-temperature sealing treatment method is used in the sealing treatment step, a chemical conversion treatment solution containing chromium or zirconium, as used in the sealing treatment solution, is particularly preferable. As the chromium source, a trivalent chromium salt can be used, as in the sealing treatment solution. Furthermore, as the zirconium source, either organic sources such as zirconium tetraethoxide and zirconium tetraisopropoxide, or inorganic sources such as zirconium oxide chloride, zirconium hydroxide, zirconium sulfate, and zirconium carbonate can be used. For example, an Alsurf treatment manufactured by Nippon Paint Surf Chemicals can be used as such a chemical conversion treatment. By using a chemical conversion treatment solution containing the same main component as the sealing treatment solution, the same type of component exists continuously on the surfaces of the chemical conversion coating 35 and the anodized coating 32, and therefore the chemical conversion coating 35 is formed in a good compatibility with the anodized coating 32, exhibiting high barrier properties and enabling a more significant improvement in corrosion resistance.

[0039] The thickness of the chemical conversion coating formed in the chemical conversion treatment step S5 is preferably less than 1 μm, for example, but is not particularly limited as it depends on the type of chemical conversion treatment.

[0040] It is preferable to degrease and rinse the finished cylinder block body before chemical conversion treatment. After chemical conversion treatment, it is preferable to clean the surface of the cylinder block body with water and then dry it.

[0041] Furthermore, a chemical conversion coating is formed on the inner peripheral surface of the cylinder bore 11 by the chemical conversion treatment, and in order to remove this chemical conversion coating, for example, honing can be performed to remove the chemical conversion coating on the inner peripheral surface of the cylinder bore 11. A conventional method can be used for honing, and the chemical conversion coating can be removed by inserting a honing tool (not shown) into the cylinder bore 11, rotating it, and grinding the inner peripheral surface of the cylinder bore 11 with the grinding stone of the honing tool.

[0042] According to the manufacturing method for an outboard motor cylinder block of this embodiment, which includes these steps, it is possible to obtain an outboard motor cylinder block in which the surface of the cylinder block body, for example, the surface 30a of the mounting portion 30 of the fastened member (the peripheral surface of the screw hole 34) is covered with a pore-sealing anodized coating 32, which is an aluminum alloy base material, and this pore-sealing anodized coating 32 is further covered with a chemical conversion coating 35, while on the inner peripheral surfaces of the screw holes 34, the aluminum alloy base material is covered with a chemical conversion coating 35. Because the chemical conversion coating is thin, with a thickness of less than 1 μm, there is little change in the inner diameter of the screw hole 34, allowing the screw to be fastened without strain.

[0043] Furthermore, the inner peripheral surface of the water jacket 15 is also treated in the same manner as the peripheral surfaces of the screw holes 34, so that the aluminum alloy base material is covered with a sealed anodic oxide film 32, and this sealed anodic oxide film 32 is further covered with a chemical conversion film 35, forming the inner peripheral surface of the water jacket 15. The inner peripheral surface of the water jacket 15 is close to the combustion chamber, is exposed to high temperatures, and is in contact with or likely to be in contact with cooling water (seawater), making it a location prone to corrosion. Therefore, by forming an additional chemical conversion film on the sealed anodic oxide film, corrosion resistance can be improved.

[0044] In the finishing process S4, the anodized coating on the joining surface 14 of the cylinder block body may be removed until the base aluminum alloy is exposed. Such processing may be performed by any method as long as it can smooth the joining surface 14. For example, a milling machine using a rotating flat milling cutter or a grinding machine using a flat grinding machine may be used. If the anodized coating on the joining surface 14 is removed in this manner, a chemical conversion coating is preferably formed on the joining surface 14 where the aluminum alloy is exposed in the chemical conversion treatment process S5. The joining surface 14 of the cylinder block may also be susceptible to corrosion if even a small amount of cooling water (seawater) seeps in between the gasket and the joining surface 14. However, forming a chemical conversion coating on the finishing joining surface 14 maintains the smoothness of the joining surface 14, providing a good seal with the gasket. Furthermore, since the chemical conversion coating is formed integrally with the inner circumferential surface of the water jacket 15, penetration is prevented, improving corrosion resistance. In particular, when inexpensive ADC or AC materials are used as aluminum alloys, corrosion resistance becomes a concern, and therefore, such a configuration is highly advantageous.

[0045] The method for manufacturing an outboard motor cylinder block of the present invention is not limited to the flow diagram shown in Fig. 3. For example, as shown in Fig. 5, a machining step S2 in which some of the temporary holes are threaded to form threaded holes may be performed between the casting step S1 and the anodizing step S3. The machining step S2 will be described in detail below.

[0046] In order to immerse the cylinder block body in the treatment solution during the anodizing treatment and sealing treatment in the anodizing step S3 and the chemical treatment in the chemical treatment step S5, it is preferable to attach the cylinder block body to a transport jig 20 as shown in Fig. 6. Transport jig 20 has a control unit 21 at the top in the vertical direction and a hanging unit 24 at the bottom. A hook of a transport device such as a hoist can be attached to control the movement of transport jig 20 and the elevation and lowering of hanging unit 24.

[0047] The hanging unit 24 has three horizontal members 26 arranged in the vertical direction, and is configured so that the cylinder block body 10 is attached between the horizontal members 26. The control unit 21 and the hanging unit 24 are fixed with connecting members 25. By raising and lowering the hanging unit 24, the cylinder block body 10 can be immersed in the treatment liquid (not shown) of various treatment tanks.

[0048] The cylinder block body 10 is attached to a horizontal member 26 from above and below using fixing members 27a, 27b. The horizontal member 26 is conductive and also functions as an anode bus bar. For anodizing the cylinder block body 10, two screw holes (not shown) in the cylinder block body 10 are fastened to the anode bus bar (horizontal member 26) via spacers 28 using energized bolts (not shown). This allows electricity to be applied to the cylinder block body 10 during the anodizing process. The two screw holes are preferably located at the same height on the transport jig 20.

[0049] FIG. 7 shows the anode busbar mounting portion 40 for fastening the cylinder block body 10 to the anode busbar, and the temporary hole 41 formed in its tip surface. FIGS. 1 and 2 also show the position of the anode busbar mounting portion 40 on the cylinder block body 10. The temporary hole 41 in the anode busbar mounting portion 40, like the temporary hole 31 formed in the tip surface of the mounting portion 30 of the workpiece, is formed in the casting process S1 and has a smaller inner diameter than the screw hole that will ultimately be formed. Multiple screw holes are formed in the cylinder block body 10. As described below, there are two types of screw holes that are formed through different processes. Therefore, the screw hole formed in the mounting portion 30 of the workpiece is referred to as the first screw hole 34, and the screw hole formed in the anode busbar mounting portion 40 is referred to as the second screw hole 42. The temporary hole for the first screw hole 34 is referred to as the first temporary hole 31, and the temporary hole for the second screw hole is referred to as the second temporary hole 41.

[0050] In the machining step S2, the second temporary holes 41 in the anode bus bar mounting portions 40 are threaded to form second screw holes 42 in the tip surfaces of the anode bus bar mounting portions 40, as shown in Fig. 8. The threading can be performed in the same manner as the threading in the finishing step S4, and therefore a description thereof will be omitted here.

[0051] 9 and 10, energized bolts 29 are screwed into the second screw holes 42 to fasten the anode bus bar mounting portions 40 and the horizontal member (anode bus bar) 26 via the spacers 28. During the anodizing treatment in the anodizing step S3, the hanging portion 24 of the transport jig 20 is lowered to immerse the cylinder block body 10 in the treatment solution, and electrolysis can be performed via the horizontal member (anode bus bar) 26. After the anodized film is formed, the hanging portion 24 is raised to lift the cylinder block body 10 out of the treatment solution. While the sealing treatment does not require energization of the cylinder block body 10, the sealing treatment may be performed using the transport jig 20 to ensure smooth immersion in the treatment solution. By performing the anodizing step S3 in this manner, a sealed anodized film is formed on the inner periphery of the first temporary hole 31 in the mounting portion 30 of the fastened member, but no anodized film is formed on the inner periphery of the second screw hole 42 in the anode busbar mounting portion 40.

[0052] Then, in the finishing process S4, the energized bolts 29 are removed from the second screw holes 42 in the anode bus bar mounting portions 40, and the cylinder block body 10 is removed from the transportation jig 20. After that, the first temporary holes 31 in the fastened member mounting portions 30 are threaded to form first screw holes 34. As described above, this exposes the aluminum alloy base material on the inner circumferential surfaces of the first screw holes 34. Then, in the chemical conversion treatment process S5, the cylinder block body 10 is subjected to a chemical conversion treatment, so that the aluminum alloy exposed on the inner circumferential surfaces of the first screw holes 34 in the fastened member mounting portions 30 and the second screw holes 42 in the anode bus bar mounting portions 40 can be covered with a chemical conversion coating.

[0053] As described above, the second screw holes 42 of the anode bus bar mounting portion 40, like the first screw holes 34 of the mounting portion 30 for the fastened member, are simply coated with a thin chemical conversion coating. Therefore, as shown in FIG. 12 , for example, the second screw holes 42 can be used as screw holes for bolts 44 for fastening the anode bus bar mounting portion 40 to an engine hook 43 for holding an outboard motor cylinder block. [Example]

[0054] [Example 1] Test pieces with screw holes (temporary holes) were prepared using ADC12 aluminum alloy, and after degreasing, they were anodized by direct current electrolysis to form a 3 μm thick anodic oxide film. The anodizing treatment was carried out by immersing the cylinder block in a sulfuric acid bath with a concentration of 200 g / L at a temperature of 20°C and a current density of 1.5 A / dm 2 A voltage was applied for 20 minutes at 40°C. After rinsing with water, the specimens were subjected to a low-temperature sealing treatment (40°C, 3 minutes) using a compound containing fluorine, chromium, and zirconium as the main components. Next, the inner surfaces of the screw holes were finished using cutting fluid to remove all of the anodized film on the inner surfaces of the screw holes. The entire test piece was then degreased in the same manner as before the anodizing treatment, and the test pieces were then subjected to a chemical conversion treatment (50°C, 2 minutes) using a chromate chemical conversion treatment solution (product name: Alsurf).

[0055] [Comparative Example 1] Test materials were prepared in the same manner as in Example 1, and then anodized, sealed, and chemically treated, except that the inner surface of the screw hole was finished before the anodizing treatment, rather than between the sealing treatment and the chemical treatment.

[0056] [Example 2] Test materials were prepared in the same manner as in Example 1, except that a sealing treatment using nickel acetate as the main component was carried out instead of the low-temperature sealing treatment, and then anodized, sealed, and chemically treated.

[0057] The inside of the screw holes in the test materials of Example 1 and Comparative Example 1 were observed. As the observation results, images of the cross sections of the screw holes after several days are shown in Figs. 13 and 14. As shown in Fig. 13, no corrosion was observed on the inner periphery of the screw hole in Example 1, but corrosion was observed on the inner periphery of the screw hole in Comparative Example 1. It was confirmed that this was due to the effect of removing the anodizing treatment solution by performing machining after anodizing treatment.

[0058] The coating surfaces of the test materials of Examples 1 and 2 before and after chemical conversion treatment were subjected to observation of the microstructure of the coating surface using a field emission scanning electron microscope (FE-SEM) and component analysis in the depth direction of the coating using glow discharge optical emission spectroscopy (GDOES). The results for Example 1 before chemical conversion treatment are shown in Figures 15 and 16, and the results for Example 1 after chemical conversion treatment are shown in Figures 17 and 18. The results for Example 2 before chemical conversion treatment are shown in Figures 19 and 20, and the results for Example 2 after chemical conversion treatment are shown in Figures 21 and 22.

[0059] In Example 1, the SEM images in Figures 15 and 17 show no change in the coating surface, and the graphs in Figures 16 and 18 showing the mass fraction of each element show no sign of a decrease in sealing components such as chromium and zirconium. In contrast, in Example 2, the SEM images in Figures 19 and 21 show that the coating surface after chemical conversion treatment was dissolved and roughened. Furthermore, the graphs in Figures 20 and 22 showing the mass fraction of each element show a decrease in the content of nickel, a sealing component, at a depth of approximately 0 μm. These comparison results confirm that the effects on the coating can be suppressed by using similar sealing components and chemical conversion treatment components. [Explanation of symbols]

[0060] 10 Cylinder block body 11 Cylinder bore 12 Cylinder sleeve 13 Base material 14 Cylinder head interface 15 Water Jacket 16 Gasket 20 Transport jig 21 Control Unit 24 Hanging part 25 Connecting material 26 Horizontal member (anode bus bar) 27a, 27b fixing members 28 spacer 29 energized volts 30 Mounting portion of fastened member 31 Temporary Hole (First Temporary Hole) 32 Anodic oxide film 34 screw hole (first screw hole) 35 Chemical coating 40 Anode bus bar mounting part 41 Second Temporary Hole 42 Second screw hole 43 Engine Hook 44 volts

Claims

1. A cylinder block for an outboard motor, the cylinder block having a cylinder bore and a water jacket around the cylinder bore at a joining surface where the cylinder block is joined to a cylinder head, and having screw holes on an outer circumferential surface for fastening the cylinder block to a member to be fastened, the aluminum alloy is covered with an anodized coating on a peripheral surface of an opening of the screw hole and an inner peripheral surface of the water jacket, pores in the anodized coating are sealed with a sealing product, and the sealed anodized coating is further covered with a chemical conversion coating, In the cylinder block for an outboard motor, the inner peripheral surface of the screw hole is covered with a chemical conversion coating made of the aluminum alloy.

2. 2. The cylinder block for an outboard motor according to claim 1, wherein said chemical conversion coating contains a main component of said sealing product.

3. 3. The cylinder block for an outboard motor according to claim 2, wherein the main component of said sealing product is chromium and / or zirconium.

4. A method for manufacturing an outboard motor cylinder block, the outboard motor cylinder block having a plurality of screw holes on its outer peripheral surface for fastening the outboard motor cylinder block to a fastened member, A process for casting a cylinder block body using an aluminum alloy as a base material, wherein the cylinder block body has a cylinder bore, a water jacket formed around the cylinder bore, and a temporary hole having an inner diameter smaller than that of the screw hole at a joining surface where the cylinder block body is joined to a cylinder head. forming an anodized film on the surface of the aluminum alloy portion of the cylinder block body by anodizing; a step of sealing the surface of the anodized coating by a sealing treatment; a step of threading the temporary hole to form the screw hole so that the anodized film on the inner peripheral surface of the temporary hole is removed and the aluminum alloy is exposed on the inner peripheral surface of the screw hole; forming a chemical conversion coating on the surface of the sealed anodized coating and on the surface of the aluminum alloy exposed on the inner circumferential surface of the screw hole; A method for manufacturing an outboard motor cylinder block, comprising:

5. the plurality of screw holes include first screw holes and second screw holes, and a first temporary hole for the first screw hole and a second temporary hole for the second screw hole are formed in the cylinder block body in the casting process; The method further includes a step of threading the second temporary hole to form the second screw hole before the step of forming the anodic oxide film, 5. The method for manufacturing an outboard motor cylinder block according to claim 4, wherein in the step of forming the anodic oxide film, energized bolts are fastened into the second screw holes so as to fasten the cylinder block body and the anode bus bar together, and electricity is passed through the cylinder block body via the anode bus bar to perform the anodic oxidation treatment.

Images