Cylinder block for outboard motor and manufacturing method thereof

Abstract

To provide an outboard motor cylinder block and a method of manufacturing the same in which a main body having a cylindrical cylinder sleeve composed of cast iron is cast and wrapped in an aluminum alloy, which can be anodized and sealed without melting the cast iron of the cylinder sleeve.SOLUTION: Of an inner circumferential surface 11S of a cylinder bore 11, a region including the boundary 11B between a cast iron of a wrapped cylinder sleeve 12 and an aluminum alloy of the base material is pressed with an elastic jig 20, and in this state a cylinder block body 10 is anodized to form an anodized film on the surface portion of the aluminum alloy. Then, pores in the anodized film are sealed by a sealing treatment. This makes it possible to obtain an outboard motor cylinder block with a sealed anodized film on a bonding surface 10T to be bonded to a cylinder head and an inner circumferential surface of the water jacket 13.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, for example, Patent Document 1 describes the use of a metal such as zinc, which has an even lower potential than aluminum alloys, as a sacrificial anode in the area of ​​the cylinder block that comes into contact with seawater, such as a cooling water flow path called a water jacket.

[0005] On the other hand, anodizing, which forms an anodic oxide film on the surface of an aluminum alloy, has been known as a method for improving the corrosion resistance of aluminum alloys. In anodizing, aluminum is oxidized to form a porous anodic oxide film, but this porosity contributes to a decrease in corrosion resistance, so a sealing treatment is performed after anodizing to close the pores in order to improve corrosion resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-232614 Summary of the Invention [Problem to be solved by the invention]

[0007] The area around the combustion chamber of an outboard motor, where the cylinder block is located, is subject to extremely high temperatures, making it extremely susceptible to corrosion. Therefore, it is desirable to form an anodized coating not only on the inner surface of the cylinder block's water jacket, but also on the joint surface between the cylinder block and the cylinder head.

[0008] However, in an outboard motor cylinder block, a cylindrical cylinder sleeve made of cast iron is cast into the aluminum alloy base material on the inner periphery of the cylinder bore. When the entire cylinder block is immersed in an anodizing solution to anodize such a cylinder block, the cast iron of the cylinder sleeve dissolves, resulting in pitting corrosion. Therefore, the inventors performed an anodizing process by masking the entire inner periphery of the cylinder bore with a masking agent, but this did not solve the problem of partial peeling of the masking agent, which dissolves the cast iron underneath.

[0009] In view of the above problems, the present invention aims to provide a cylinder block for an outboard motor that can be anodized and subsequently sealed without dissolving the cast iron of the cylinder sleeve, and a method for manufacturing the same. The cylinder block has a cylindrical cast iron cylinder sleeve cast into an aluminum alloy. [Means for solving the problem]

[0010] In order to achieve the above object, one aspect of the present invention is a method for manufacturing an outboard motor cylinder block, wherein an outboard motor cylinder block body has a cylinder bore and a water jacket formed around the cylinder bore at a joining surface where a cylinder head is joined, and a cylindrical cylinder sleeve made of cast iron is cast into an aluminum alloy base material on the inner peripheral surface of the cylinder bore, and the aluminum alloy is exposed on the joining surface side of the inner peripheral surface of the cylinder bore, with a boundary between the cast iron and the aluminum alloy being present, and the manufacturing method includes the steps of: pressing an area of ​​the inner peripheral surface of the cylinder bore that includes the boundary with an elastic jig; anodizing the area while it is pressed, thereby forming an anodized coating on the surface of the aluminum alloy portion of the outboard motor cylinder block body; and sealing the area while it is pressed, thereby sealing the anodized coating.

[0011] In another aspect, the present invention provides 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 a cylinder head is joined, and a cylindrical cylinder sleeve made of cast iron cast into an aluminum alloy base material on the inner circumferential surface of the cylinder bore, the aluminum alloy being exposed on the joining surface side of the inner circumferential surface of the cylinder bore, with a boundary between the cast iron and the aluminum alloy being present, and an anodized coating covering the joining surface and the inner circumferential surface of the water jacket, with pores in the anodized coating sealed with a sealing product. [Effects of the Invention]

[0012] Thus, according to the present invention, an anodizing treatment and subsequent sealing treatment can be performed on an outboard motor cylinder block body having a cylindrical cylinder sleeve made of cast iron cast-into an aluminum alloy without dissolving the cast iron of the cylinder sleeve. [Brief explanation of the drawings]

[0013] [Figure 1]1 is a perspective view showing an example of an outboard motor cylinder block body to be processed in a manufacturing method for an outboard motor cylinder block according to the present invention; [Figure 2] 2 is a cross-sectional perspective view showing the inside of a cylinder bore of the cylinder block body for an outboard motor shown in FIG. 1. FIG. [Figure 3] 2 is a cross-sectional front view showing the inside of a cylinder bore of the cylinder block body for an outboard motor shown in FIG. 1. [Figure 4] 1 is a partial cross-sectional front view of an outboard motor cylinder block body for explaining a first embodiment of a method for manufacturing an outboard motor cylinder block according to the present invention. [Figure 5] 10 is a partial cross-sectional front view of an outboard motor cylinder block body for explaining a second embodiment of a method for manufacturing an outboard motor cylinder block according to the present invention. [Figure 6] 10 is an image of an outboard motor cylinder block showing test results of an example and a comparative example of a manufacturing method for an outboard motor cylinder block according to the present invention. [Figure 7] 7 is an enlarged image of the portion indicated by A in FIG. 6. DETAILED DESCRIPTION OF THE INVENTION

[0014] 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.

[0015] The method for manufacturing an outboard motor cylinder block of this embodiment includes a pressing step in which an area including the boundary between the cast iron and the aluminum alloy on the inner surface of the cylinder bore of the outboard motor cylinder block body is pressed with an elastic jig; an anodizing step in which the area is anodized while being pressed to form an anodized film on the surface of the aluminum alloy portion of the outboard motor cylinder block body; and a sealing step in which the area is sealed while being pressed to seal the anodized film.

[0016] First, we will explain the cylinder block body for an outboard motor, which is the subject of each process in this method. As shown in Figures 1 to 3, a cylinder block body 10 for an outboard motor has a plurality of cylinder bores 11 and a water jacket 13 formed around these cylinder bores 11 at a joining surface 10T where the cylinder head (not shown) is joined. Note that these figures show the cylinder block body 10 with three cylinder bores 11 aligned horizontally, but in an actual outboard motor (not shown), the cylinder block body 10 is mounted with the cylinder bores 11 aligned vertically. Also, the number of cylinder bores 11 may be one for a single-cylinder engine.

[0017] A cylindrical cylinder sleeve 12 made of cast iron is cast-inserted into the aluminum alloy base material on the inner circumferential surface of each cylinder bore 11. In such a cast-insertion method, the entire inner circumferential surface of the cylinder bore 11 is not usually covered by the cylinder sleeve 12, and the aluminum alloy base material is exposed on the inner circumferential surface at the end 11A on the joining surface 10T side. Therefore, on the inner circumferential surface of the cylinder bore 11, there is a boundary 11B between the different metal materials, the aluminum alloy and the cast iron.

[0018] In this embodiment, when anodizing this cylinder block body 10, a pressing step is first performed in which an elastic jig is used to press the area including the boundary 11B between the cast iron and the aluminum alloy on the inner circumferential surface of each cylinder bore 11. By pressing the area including the boundary 11B in this manner and then performing the next anodizing step in this state, it is possible to prevent the treatment solution from seeping into the boundary 11B between the cast iron and the aluminum alloy.

[0019] The region encompassing the boundary 11B between the cast iron and the aluminum alloy is preferably, for example, the region from the end of the inner circumferential surface of the cylinder bore 11 on the joining surface 10T side to the boundary 11B between the cast iron and the aluminum alloy, and more preferably the region from the end of the inner circumferential surface of the cylinder bore 11 on the joining surface 10T side to a position 500 mm behind the boundary 11B.

[0020] The elastic jig may be made of any material that is elastic enough to apply pressure uniformly around the entire inner circumferential surface of the cylinder bore 11, such as rubber or a thermoplastic elastomer. Examples of rubber include silicone rubber, nitrile butadiene rubber (NBR), styrene rubber (SBR), butyl rubber (IIR), and fluororubber (FKM). Examples of thermoplastic elastomers include polyester (TPC), polyurethane (TPU), and polyvinyl chloride (TPVC). In particular, since the elastic jig will come into contact with the processing solution in the subsequent anodizing process, it is preferable that it be made of a material with excellent chemical resistance, and it is more preferable to use silicone rubber, for example.

[0021] As for the dimensions of the elastic jig, the outer diameter is preferably larger than the inner diameter of the cylinder bore 11, and is preferably such that it can be attached to and detached from the inner circumferential surface of the cylinder bore 11. The length is preferably longer than the length of the end 11A of the aluminum alloy on the joining surface 10T side of the inner circumferential surface of the cylinder bore 11, i.e., the length from the end on the joining surface 10T side to the boundary 11B, and may be longer than the length of the inner circumferential surface of the cylinder bore 11. As long as the dimensions are as described above, various types of elastic jigs can be used.

[0022] Furthermore, to prevent the cast iron cylinder sleeve 12 in the cylinder bore 11 from coming into contact with the treatment solution during the subsequent anodizing process, the entire surface of the cylinder sleeve 12 may be covered with an elastic jig, or a different jig may be used. For example, the joining surface 10T side of the cylinder bore 11 may be covered with an elastic jig, and the opposite side may be covered with a similar elastic jig or a different jig. Furthermore, a masking agent may be applied to the inner surface of the cylinder bore 11 before pressing with the elastic jig. A commercially available masking agent for metal surface treatment may be used as the masking agent.

[0023] The pressing step may, for example, use an elastic plug as the elastic jig and insert this elastic plug into cylinder bore 11 from the joining surface 10T side to press the area including boundary 11B (first embodiment), or may use an elastic bag (or balloon) equipped with a gas supply unit as the elastic jig and place this elastic bag inside cylinder bore 11. Gas is supplied from the gas supply unit to inflate the elastic bag, and the entire inner circumferential surface of cylinder bore 11 is covered with the elastic bag, thereby pressing the area including boundary 11B (second embodiment). As the elastic bag, for example, a jig that is commercially available under the name of an air picker, in which an elastic bag portion can be expanded and contracted by gas pressure, may be used.

[0024] The anodizing process involves immersing the cylinder block body 10 in a treatment solution for electrolysis, forming a porous anodized film on the surface of the cylinder block body 10. The electrolysis dissolves the aluminum alloy that is the base material of the cylinder block body 10, and the dissolved aluminum combines with oxygen in the treatment solution to form an aluminum oxide film on the surface of the aluminum alloy portion of the cylinder block body 10.

[0025] 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.

[0026] The thickness of the anodized film formed in the anodizing treatment step is not particularly limited, but is preferably 1 to 60 μm, and more preferably 3 to 20 μm, for example.

[0027] 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 permeation film that penetrates deep into the surface of the aluminum alloy base material, and about the remaining half is a growth film that grows from the surface of the base material.

[0028] The anodizing process when the pressing process is performed in the first embodiment will be described with reference to Fig. 4. As shown in Fig. 4, the elastic plug 20 is inserted into the cylinder bore 11 from the joining surface 10T side, thereby pressing the area including the boundary 11B. The length of the elastic plug 20 needs only to be the length from the end of the cylinder bore 11 on the joining surface 10T side to a position behind the boundary 11B, which allows sufficient pressing against the end 11A of the aluminum alloy.

[0029] The entire cylinder block body 10 with the elastic plug 20 inserted is then submerged in a treatment tank (not shown) containing treatment liquid 40, with the mating surface 10T facing upward. Although the elastic plug 20 is inserted only into the opening of the cylinder bore 11 facing the mating surface 10T, an air pocket forms in the closed space formed by the bottom surface 20B of the elastic plug 20, the inner surface 11S of the cylinder bore 11, and the liquid level 40S of the treatment liquid 40. The air pressure in the air pocket suppresses the rise of the liquid level 40S of the treatment liquid 40, preventing the liquid level 40S of the treatment liquid 40 from reaching the cylinder sleeve 12. By applying a voltage in this state, an anodized film (not shown) is formed on the mating surface 10T of the cylinder block body 10 and the inner surface of the water jacket 13 without dissolving the cast iron of the cylinder sleeve 12.

[0030] Furthermore, even if the elastic plug 20 is inserted on the mating surface 10T side of the cylinder bore 11, it does not mean that the treatment liquid 40 will not at all seep into the gap between the inner circumferential surface 11S of the cylinder bore 11 and the elastic plug 20. However, even if an electrolytic reaction occurs at the end 11A of the aluminum alloy on the mating surface 10T side of the cylinder bore 11, this part is covered with aluminum alloy and is therefore anodized, and a film grows on the elastic plug 20 side, eliminating any small gaps. This further suppresses the seepage of the treatment liquid 40 and prevents the treatment liquid 40 from seeping across the boundary 11B between the aluminum alloy and the cast iron into the cylinder sleeve 12 and dissolving the cast iron of the cylinder sleeve 12.

[0031] In FIG. 4 , the cylinder block body 10 is immersed in the treatment liquid 40 with the joining surface 10T facing upward, creating an air pocket in the cylinder bore 11 and preventing the liquid level 40S of the treatment liquid 40 from rising to the position of the cylinder sleeve 12 within the cylinder bore 11. However, the present invention is not limited to this. For example, an air pocket can be created in the cylinder bore 11 by inserting an elastic plug 20 or other jig into the end of the cylinder bore 11 opposite the joining surface 10T. In this case, the joining surface 10T does not need to face upward when the cylinder block body 10 is immersed in the treatment liquid 40. Furthermore, compared to the case of FIG. 4 , splashing of the treatment liquid 40 onto the inner surface of the cylinder sleeve 12 during immersion and anodizing can be prevented.

[0032] The anodizing process when the pressing step is performed in the second embodiment will be described with reference to Fig. 5. As shown in Fig. 5, an elastic bag 30 equipped with a gas supply unit (not shown) is used, and this elastic bag 30 is loaded in a contracted state into the cylinder bore 11. By supplying gas from the gas supply unit, the elastic bag 30 can be expanded as indicated by the dashed arrow. The elastic bag 30 has a size that allows it to press at least the area including the boundary 11B between the aluminum alloy and the cast iron in its expanded state, and it is particularly preferable that the elastic bag 30 has a size that allows it to come into close contact with and press against the entire inner circumferential surface of the cylinder bore 11 in its expanded state, as shown in Fig. 5.

[0033] Then, with the elastic bag body 30 inflated to cover the entire inner surface of the cylinder bore 11, the cylinder block body 10 is immersed in a treatment tank (not shown) and a voltage is applied, thereby forming an anodized film (not shown) on the joint surface 10T of the cylinder block body 10 and the inner surface of the water jacket 13.

[0034] On the other hand, because the inner circumferential surface 11S of the cylinder bore 11 is covered by the elastic bag 30, it does not come into contact with the treatment liquid and is therefore able to prevent dissolution of the cast iron of the cylinder sleeve 12. Furthermore, even if the treatment liquid seeps into the joint surface 10T side of the cylinder bore 11 between the elastic bag 30 and the cylinder bore 11 and an electrolytic reaction occurs at the end 11A of the aluminum alloy on the joint surface 10T side of the cylinder bore 11, the part is anodized because it is covered with aluminum alloy, and a film grows on the elastic plug 20 side, eliminating any small gaps. This further suppresses the seepage of the treatment liquid 40 and prevents the treatment liquid 40 from penetrating beyond the boundary 11B between the aluminum alloy and the cast iron into the cylinder sleeve 12 and dissolving the cast iron of the cylinder sleeve 12.

[0035] To further explain the anodizing treatment process, direct current electrolysis and alternating current / direct current superimposed electrolysis can be used as the electrolysis method. The anodized film formed by either electrolysis method is an anodizing reaction that includes a permeation film and a growth film, as described above, but the properties of the resulting anodized film differ.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] The sealing treatment is performed by immersing the cylinder block body 10 having the anodized coating formed on its surface in a sealing treatment liquid or by applying a sealing treatment liquid. This seals the pores in the porous anodized coating, improving the corrosion resistance of the anodized coating. Before the sealing treatment, it is preferable to perform a pretreatment such as washing with water to prevent the anodized coating from contaminating the sealing treatment liquid and to remove any remaining anodized coating liquid from the pores in the anodized coating.

[0040] The sealing treatment step can employ known methods such as a hydrothermal method, a boiling water method, a nickel acetate method, and a lithium hydroxide method. As an example, the lithium hydroxide method, which exhibits a self-repair function, will be described. An aqueous solution containing lithium ions is used as the sealing treatment solution. Lithium ions or chemicals that serve as lithium ion sources include lithium hydroxide, lithium sulfate, lithium chloride, lithium silicate, lithium nitrate, lithium carbonate, lithium phosphate, and lithium hydroxide. Among these, lithium hydroxide, lithium carbonate, and lithium silicate are preferred, as their aqueous solutions are basic. However, lithium silicate is highly toxic and poorly soluble in water, making it impractical. Therefore, lithium hydroxide and lithium carbonate are more preferred.

[0041] In the lithium hydroxide method, the surface and pores of the anodized film are dissolved slightly by the sealing solution, and then the dissolved film components react with the sealing solution components to precipitate a sealing product. This sealing product blocks the interior of the pores in the film, and the surface layer of the film is transformed into a dense sealing product layer, completely blocking the pores in the film. Therefore, a sealing product of lithium, aluminum, and oxygen is present on the surface and inside the pores of the anodized film.

[0042] By performing the pressing, anodizing, and sealing processes in this manner, it is possible to manufacture an outboard motor cylinder block having a sealed anodized coating on the cylinder block joint surface 10T and the inner circumferential surface of the water jacket 13 without dissolving the cast iron of the cylinder sleeve 12. The cylinder block joint surface 10T and the inner circumferential surface of the water jacket 13 are close to the combustion chamber, exposed to high temperatures, and likely to come into contact with cooling water (seawater), making them prone to corrosion. Therefore, forming a sealed anodized coating can prevent corrosion in the outboard motor cylinder block. In particular, when inexpensive aluminum alloys such as ADC and AC are used, the benefits of forming a sealed anodized coating are significant because they have poor corrosion resistance. [Example]

[0043] Examples and comparative examples of the present invention will be described below, but the present invention is not limited to these examples.

[0044] A cylinder block made of aluminum alloy ADC12, with a cast iron cylinder sleeve cast into it, was prepared. The aluminum alloy base material was exposed on the inner circumferential surface of each cylinder bore on the joining surface side, confirming the existence of a boundary between the cast iron and the aluminum alloy. A masking agent (MS8-01ST, manufactured by Kakoki Shoji Co., Ltd.) was applied to the entire inner circumferential surface of each cylinder bore by brushing. An elastic stopper with a maximum diameter larger than the inner diameter of the cylinder bore was also prepared. An elastic stopper was inserted into one cylinder bore from the joining surface side, and the area from the joining surface to the area beyond the boundary was pressed with the elastic stopper (Example 1). Another elastic stopper was inserted into another cylinder bore from the joining surface side, pressing the area including the boundary as described above, and a separate elastic stopper was inserted from the opposite side of the joining surface (Example 2). For comparison, one cylinder bore was not inserted with an elastic stopper, and only a masking agent was used (Comparative Example).

[0045] The cylinder block was then anodized using AC / DC superimposed electrolysis, in which the cylinder block was immersed with the joining surface facing upward in a sulfuric acid bath with a concentration of 200 g / L at a temperature of 20°C, and a voltage of 40 V positive, 2 V negative, and a frequency of 10 kHz was applied for 10 minutes. As a result, an anodized film of 5 to 15 μm was formed on the joining surface of the cylinder block and the inner surface of the water jacket.

[0046] Next, the cylinder block was washed with water and then immersed for 1 minute in a sealing solution with a lithium ion concentration of 1.7 g / L, a pH of 13, and a temperature of 25°C for a sealing treatment. After washing with water again, all of the elastic plugs were removed from the cylinder bores.

[0047] The inner circumferential surface of the cylinder bore of a cylinder block having a sealed anodic oxide film formed on the joining surface of the cylinder block and the inner circumferential surface of the water jacket was observed. The results are shown in Figures 6 and 7.

[0048] In the photograph of the cylinder block in Figure 6, the left side shows a cylinder bore with an elastic plug inserted from the joining surface side (Example 1), the center side shows a cylinder bore with an elastic plug inserted from both the joining surface side and the opposite side (Example 2), and the right side shows a cylinder bore with only a masking agent without an elastic plug inserted (Comparative Example). Figure 7 is an enlarged photograph of area A near the joining surface on the inner surface of the cylinder bore (Comparative Example) on the right side of Figure 6. As shown in Figures 6 and 7, peeling of the masking agent B occurred over a wide area on the inner surface of the cylinder bore of the Comparative Example, including the boundary between the cast iron and the aluminum alloy. Furthermore, it was confirmed that the cast iron had melted in the area of ​​the cast iron cylinder sleeve where the masking agent had peeled off.

[0049] On the other hand, neither peeling of the masking agent nor dissolution of the cast iron was observed in the cylinder bores of Examples 1 and 2, in which the elastic plugs were inserted. In Example 1, when the cylinder block was submerged in the treatment solution with the joining surface facing upward and anodizing treatment was performed, it was confirmed that air pockets were formed inside the cylinder sleeve, preventing the liquid level of the sulfuric acid solution from reaching the cylinder sleeve portion inside the cylinder bore.

[0050] In addition, anodizing and sealing tests were conducted on an aluminum alloy cylinder block cast-in with a cast iron cylinder sleeve under the same conditions as above, except that DC electrolysis was used instead of AC / DC superimposed electrolysis. The conditions for DC anodizing were immersion in a sulfuric acid bath with a temperature of 20°C and a concentration of 200 g / L, and a current density of 1.5 A / dm 2 for 20 minutes to form an anodic oxide film of 5 to 15 μm.

[0051] When the cylinder blocks obtained in the DC anodizing test were examined, it was confirmed that, as with the AC / DC superimposed electrolysis test, the masking agent had peeled off over a wide area, including the boundary between the cast iron and the aluminum alloy, on the inner surface of the cylinder bores where no elastic plugs had been inserted, and that there were areas where the cast iron of the cylinder sleeve had dissolved.On the other hand, neither peeling of the masking agent nor dissolution of the cast iron was observed in the cylinder bores where the elastic plugs had been inserted.

[0052] As described above, the inner circumferential surface of the cylinder bore in the comparative example had a boundary between the cast iron and the aluminum alloy. Although the masking agent was applied over a wide area across this boundary, peeling of the masking agent and dissolution of the cast iron occurred. The following reasons are presumed to have occurred: The sulfuric acid solution used in the anodizing treatment seeped into the inner circumferential surface of the cylinder bore from the edge where the cylinder block joins, anodizing the aluminum alloy and causing volume expansion, which is thought to have caused the masking agent to peel off. In contrast, in Examples 1 and 2, the elastic plug pressed against the inner circumferential surface of the cylinder bore, and the anodizing treatment caused a film to grow toward the elastic plug 20, eliminating the slight gap. This is thought to have prevented further penetration of the sulfuric acid solution.

[0053] Therefore, in addition to the method of inserting an elastic plug into the cylinder bore in the example, it is believed that the penetration of the sulfuric acid solution and the anodization of the aluminum alloy can be sufficiently suppressed by pressing the area on the inner surface of the cylinder bore that includes the boundary between the cast iron and the aluminum alloy with an elastic jig such as silicone. Note that, although a masking agent was used in all of Examples 1 and 2 and the Comparative Example, for the reasons described above, it was found that the penetration of the sulfuric acid solution could also be suppressed by pressing the area on the inner surface of the silicone bore that includes the boundary with an elastic jig without using a masking agent. [Explanation of symbols]

[0054] 10 Cylinder block body 10T Cylinder head joint surface 11 Cylinder bore 11B The boundary between aluminum alloy and cast iron 12 Cylinder sleeve 13 Water Jacket 20 Elastic plug 30 Elastic bag 40 Anodizing treatment solution

Claims

1. A method of manufacturing an outboard motor cylinder block, comprising: The cylinder block body for an outboard motor has a cylinder bore and a water jacket formed around the cylinder bore at a joining surface where the cylinder block body is joined to a cylinder head, and a cylindrical cylinder sleeve made of cast iron is cast into an aluminum alloy base material on the inner peripheral surface of the cylinder bore, and the aluminum alloy is exposed on the joining surface side of the inner peripheral surface of the cylinder bore, and a boundary between the cast iron and the aluminum alloy is present, pressing an area of ​​the inner circumferential surface of the cylinder bore that includes the boundary with an elastic jig; performing anodizing treatment while pressing the area to form an anodized film on the surface of the aluminum alloy portion of the outboard motor cylinder block body; a step of performing a sealing treatment while pressing the region to seal the surface of the anodized coating; A method for manufacturing an outboard motor cylinder block, comprising:

2. 2. The method for manufacturing an outboard motor cylinder block according to claim 1, wherein in the step of pressing with the elastic jig, the elastic jig is an elastic plug, and the region is pressed by inserting the elastic plug into the cylinder bore from the joining surface side.

3. 3. The method for manufacturing an outboard motor cylinder block according to claim 2, wherein in the step of forming the anodic oxide coating, the outboard motor cylinder block body is submerged in a treatment tank containing a treatment liquid with the joining surface facing upward, and a voltage is applied to perform the anodic oxidation treatment in a state in which air is trapped in a closed space formed by the bottom surface of the elastic plug, the inner surface of the cylinder bore, and the surface of the treatment liquid.

4. 2. The method for manufacturing an outboard motor cylinder block according to claim 1, wherein in the step of pressing with the elastic jig, the elastic jig is an elastic bag body equipped with a gas supply unit, the elastic bag body is placed in the cylinder bore, gas is supplied from the gas supply unit to the elastic bag body to inflate the elastic bag body, and the entire inner surface of the cylinder bore is covered with the elastic bag body, thereby pressing the area.

5. 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, a cylindrical cylinder sleeve made of cast iron cast into an aluminum alloy base material on the inner peripheral surface of the cylinder bore; the aluminum alloy is exposed on the joining surface side of the inner circumferential surface of the cylinder bore, and a boundary between the cast iron and the aluminum alloy is present; an anodized coating that covers the joining surface and the inner circumferential surface of the water jacket; The pores of the anodic oxide film are sealed with a sealing product.

6. 6. The cylinder block for an outboard motor according to claim 5, wherein the aluminum alloy is an ADC material or an AC material.

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