JP2012251188A - Method for forming uniform anodic oxide film, and component with anodic oxide film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a uniform anodic oxide film improved in hardness and corrosion resistance even on an aluminum metallic component having such a precise shape (and/or complicated shape) as having a narrowed recess, an interior of fine pore and the like.SOLUTION: A method for forming an anodic oxide film includes lowering an application current to a low current having the value of 40-95% of the application current from 3-60 minutes before the completion of current application in an anodization process. There is provided the aluminum metallic component on which the uniform anodic oxide film is formed by the method.

Description

For the purpose of improving the surface hardness, corrosion resistance, electrical insulation, wear resistance, heat dissipation, etc. of aluminum metal including pure aluminum and aluminum alloy, the present invention attaches the aluminum metal to the anode and immerses it in an acidic solution. The present invention relates to an “aluminum metal anodizing technique” in which an electric current is applied to artificially promote oxidation on the surface of an aluminum metal to produce alumina (Al ₂ O ₃ ) as an oxide film.

  The anodizing process is performed by an electrolytic cell having a basic configuration as schematically shown in FIG. 1 and its peripheral devices. The workpiece (F) is attached and immersed in the anode (D) with the electrode jig (E) in the electrolytic solution (B) of the electrolytic cell (A) in which the cathode (C) is attached to the inner wall, and the DC power supply (G) The anodic oxide film is obtained by artificially accelerating the oxidation by applying a direct current and applying a direct current. At that time, Joule heat is generated in the work material. Therefore, the electrolytic solution (B) is cooled by the cooler (H), and the heat generated by the Joule heat is diffused by air stirring by the blower (I) to prevent damage to the work material. .

  In the conventional anodic oxidation technology, as shown in the photographs of FIGS. 2, 4, 6, 8, 10, and 12, the inside of narrow recesses and pores depending on the shape of the aluminum metal part to be processed, the process electrolysis When the distance from the cathode of the tank is far away, or when there is a shadow that does not directly face the cathode, there is a difference of about 5 to 30 microns in the thickness of the oxide film generated between each part in the part. Although it has been a cause of deteriorating stable corrosion resistance and electrical insulation, it has been overlooked as a limitation of anodizing technology because it is an inevitable problem due to the characteristics of current flow.

  Even if it is an excellent technique for generating a much thicker oxide film as compared with the conventional anodizing technique as in Japanese Patent No. 4069135, it is also possible to use “screw” as in Japanese Patent Application Laid-Open No. 2009-030736. By using an AC power supply for such a shape part having extremely sharp corners or narrow recesses, a DC current (pseudo) is generated by adjusting the waveform, suppressing the generation of Joule heat, and pulse-like Even with special technology that generates a composite hard anodic oxide film on a processed part using the flow of pressure current at the time of current rise, the difference in the thickness of the oxide film generated in the aluminum part, There was an inevitable phenomenon without touching the problem that it was difficult to form an oxide film in narrow recesses or pores. In addition, this method has the disadvantage that the current flow is interrupted, the film formation rate slows down, and the production efficiency decreases.

Patent No. 4069135 JP 2009-030736 A

  An object of the present invention is to provide a uniform anode having improved hardness and corrosion resistance even in an aluminum metal part having a fine shape (and / or a complicated shape) having a narrow concave portion, an inside of a fine pore, etc. It is to produce an oxide film.

The present invention produces an anodic oxide film characterized in that, in anodic oxidation processing, the applied current is reduced to a value of 40% to 95% from 60 minutes to 3 minutes before the end of current application to make the current low. Provide a way to do it.
The present invention provides an aluminum metal part, in particular a screw, having a uniform anodic oxide film.

According to the present invention, it is possible to generate a uniform anodic oxide film with improved hardness and corrosion resistance for aluminum metal parts. Even if the aluminum metal part has a fine shape, for example, a complicated fine shape, an anodic oxide film having a uniform thickness can be obtained.
According to the present invention, an anodic oxide film having a uniform thickness can be formed on an aluminum metal such as ADC12, which is conventionally difficult to anodicize.
A portion that has been conventionally low in corrosion resistance, such as a narrow portion in an aluminum metal part, has high corrosion resistance and electrical insulation.

Schematic of an anodizing tank and related equipment. The expanded sectional photograph of the state of the anodic oxide film which the A1100 material was processed by the prior art method, and the thickness was not uniform. The expanded sectional photograph of the state of the anodic oxide film with the uniform thickness produced | generated by the processing technique of this invention about A1100 material. The expanded cross-section photograph of the state of the anodic oxide film which thickness produced by processing A7075 material by the prior art method is not uniform. The expanded sectional photograph of the state of the anodic oxide film with the uniform thickness produced | generated by the processing technique of this invention about A7075 material. The expanded cross-sectional photograph of the state of the anodic oxide film which the ADC12 material was processed by the prior art method, and the thickness was not uniform. The expanded sectional photograph of the state of the anodic oxide film with uniform thickness produced | generated by ADC12 material with the processing technique of this invention. The enlarged cross-sectional photograph of the state of the anodic oxide film which was produced by processing the shape of the opening angle of 90 ° of A6063 material by a conventional method and having a nonuniform thickness. The enlarged cross-sectional photograph of the state of the anodic oxide film with the uniform thickness produced | generated by the processing technique of this invention about the shape with an opening angle of 90 degrees of A6063 material. The enlarged cross-sectional photograph of the state of the anodic oxide film which was produced by processing the shape of the A6063 material with the opening angle of 60 ° by the conventional technique. The enlarged cross-sectional photograph of the state of the anodic oxide film with the uniform thickness produced | generated by the processing technique of this invention about the shape of 6060 degrees of A6063 materials. The enlarged cross-sectional photograph of the state of the anodic oxide film having a non-uniform thickness produced by processing the shape of the A6063 material with an opening angle of 30 ° by the conventional method. The enlarged cross-sectional photograph of the state of the anodic oxide film with the uniform thickness produced | generated by the processing technique of this invention about the shape of 3060 degrees of A6063 materials. An enlarged cross-sectional photograph of a corroded state after 192 hours of alternate immersion of salt water in a non-uniform anodic oxide film of A7075 material. An enlarged cross-sectional photograph of an uncorroded state after 888 hours of alternate immersion in salt water in a uniform anodized film of A7075 material.

In the present invention, an anodizing treatment is performed on a base material made of aluminum metal to form an anodized film on the base material.
In the present invention, the part to be processed is attached to the anode, immersed in an acidic electrolyte, and energized to artificially generate an oxide film on the part surface to improve the corrosion resistance of the metal part. The current value applied between 60 minutes and 3 minutes before the current application scheduled end time is reduced to a value of 40 to 95%, and the reduced current value is held until the current application is completed, and the application end scheduled A current is applied until time to produce a uniform anodic oxide film over the entire component surface.

Anodizing is performed using an anodizing tank (ie, electrolytic cell) (A) as shown in FIG. The anodizing tank (A) includes an electrolytic solution (B), a cathode (C), and an anode (D), and further includes a cooler (H) and a stirring blower (I).
A member (F) to be processed is immersed in an anode (D) with an electrode jig (E) in an acidic electrolyte (B) mainly composed of sulfuric acid, and a direct current is applied with a direct current power source (G) to perform anodization. Processing. However, the anodizing tank used in the present invention is not limited to the shape and equipment shown in FIG.

The acidic electrolyte (B) is an aqueous solution of an acid such as sulfuric acid, oxalic acid, and other acids (for example, phosphoric acid, chromic acid, malonic acid). Sulfuric acid and succinic acid are preferably used. Furthermore, it is preferable to use a metal salt (for example, a metal salt of sulfuric acid or oxalic acid). Specific examples of the metal in the metal salt are iron, nickel, copper, zinc, silver and the like. Furthermore, organic substances, in particular, organic low molecular weight polymers, such as silicone, especially low polymerization (for example, molecular weight 200-5000) silicone, acrylic compounds, particularly low polymerization (for example, molecular weight 200-5000) acrylic resin compositions and these May be added. By adding oxalic acid, processing is performed at a high voltage current density, so that the wear resistance and generation rate of the anodized film are improved and the processing time can be shortened. In general, when oxalic acid is used, it is necessary to accept the fact that the production rate of the film slows down and the production rate decreases, and it is necessary to suppress the heat generation by using an alternating current. By adding salt or organic matter, functions such as film hardness and corrosion resistance caused by heat generation are not impaired even when a direct current is applied.
The acid concentration may be 50 to 2000 g / L, for example 100 to 1000 g / L. It is particularly preferable to use a combination of sulfuric acid having a concentration of 180 g / L to 300 g / L and oxalic acid having a concentration of 40 g / L to 70 g / L. The concentration of other components other than the acid may be 1 to 100 g / L.
The temperature of the electrolytic solution may be −10 to + 30 ° C., more preferably −5 ° C. to + 10 ° C.

The cathode (C) is made of a conductive material (particularly metal) that can be used as a cathode for anodizing treatment.
The anode (D) is aluminum. The anode aluminum is electrolyzed to form aluminum oxide.

  The electrode jig (E) is preferably a metal having a high electrical resistance value when a current flows from the member to be processed, and having a high corrosion resistance. The electrode jig (E) is preferably a titanium alloy. In some cases, a corrosion-resistant SUS material (stainless steel) may be used. Aluminum metal may be used, but aluminum metal of the same type as the aluminum member to be processed, or aluminum metal having a higher electric resistance value than the aluminum member to be processed is preferable. If a material with a low electrical resistance value or a material with low corrosion resistance is selected, when current is applied, oxidation starts before the member to be processed, and the current flow is obstructed, so that sufficient current is supplied to the processed member. Is not applied, and in addition, the electrode jig (E) begins to melt due to heat generation, and the desired effect cannot be obtained.

  The member (F) to be processed is made of aluminum metal. “Aluminum metal” means pure aluminum or an aluminum alloy. Pure aluminum is composed of almost 100% by weight of aluminum (for example, the purity of aluminum is 99.00% by weight or more, particularly 99.50% by weight or more). Aluminum alloys contain other metals in addition to aluminum. Examples of other metals are Fe, Mg, Si, Cu, Zn, Ni, Mn, Ag, Cr and Zr. Other metals may be one kind or a combination of two or more kinds. The amount of other metals included in the alloy may be 0.1 to 40% by weight, for example 0.5 to 14% by weight, based on the aluminum alloy.

Specific examples of the aluminum metal are as follows.
Pure aluminum (1000 series),
Aluminum-copper (Al-Cu) alloy (2000 series),
Aluminum-manganese (Al-Mn) alloy (3000 series),
Aluminum-silicon (Al-Si) alloy (4000 series),
Aluminum-magnesium (Al-Mg) alloy (5000 series),
Aluminum-magnesium-silicon (Al-Mg-Si) alloy (6000 series),
Aluminum-zinc-magnesium (Al-Zn-Mg) alloy (7000 series)

A DC voltage is applied by a DC power supply (G). An AC voltage may be applied as necessary. The applied voltage is preferably 5 to 200 volts, more preferably 10 to 80 volts. The applied current may be such that the current density is 1.0 A / dm ^{2 to} 50 A / dm ² , more preferably 1.5 A / dm ^{2 to} 30.0 A / dm ² with respect to the workpiece surface area. .

  In the present invention, anodization is first performed at a high current, and then anodization is performed at a low current. The low current is a value of 40% to 95% of the high current, for example 60 to 90%. For example, the low current value may be a current value that is 0.1 to 3.0 amps, 0.3 to 1.0 amps lower than the high current value. The time for flowing a high current is 5 to 120 minutes, for example 10 to 80 minutes, and the time for flowing a low current is 3 to 60 minutes, for example 5 to 30 minutes. The time for flowing the low current may be 1/8 to 1/2, for example, 1/5 to 1/3 with respect to the anodic oxidation time (that is, the current flowing time (generally 8 minutes to 180 minutes)). .

  In the conventional anodizing method, when current is applied, current tends to flow excessively to the convex portion due to the characteristics of the current, and oxidation tends to be promoted. In addition, the portion facing in parallel with the cathode or the portion close to the distance is accelerated in the same manner as the convex portion, and as a result, the oxide film may be formed thicker than the other portions.

  Therefore, the portion where oxidation is promoted increases the electrical resistance value as the oxide film becomes thicker, the voltage increases as long as it is required to continue to apply the specified current value, including the portion where the oxide film is difficult to generate, The oxide film is continuously grown while maintaining and amplifying the difference in thickness of the oxide film.

  Lowering the voltage by reducing the current value before the end of current application stops the current supply to the convex part where the oxide film is thick and the electrical resistance value is increased, and the electrical resistance value in the part is reduced. The current necessary for generating the oxide film is supplied only to the low part, that is, the thin part of the oxide film.

As a result, the current is mainly supplied to the narrow recesses where the oxide film is difficult to be formed or the part that was shaded from the cathode, so that the ion exchange is performed between the processed substrate and the electrolytic solution, so that the oxide film is formed. It will be generated.
When the electrical resistance value rises and the voltage reaches the value at the time when the current value is lowered, the current is again supplied to the portion where the current easily flows, such as the convex portion, and the anodic oxidation process is finished at that time. By doing so, a uniform oxide film is generated. The oxide film has excellent corrosion resistance over a long period of time, and does not cause stress corrosion cracking even after alternate immersion in salt water (for example, over 192 hours as shown in FIG. 15).

Moreover, the time for performing the anodic oxidation processing by lowering the current value is 3 to 60 minutes, for example 5 to 30 minutes, particularly 5 to 15 minutes. When the time for applying a low voltage (that is, low current) exceeds 60 minutes, the thick oxide film portion is not supplied with current and exposed to the oxidizing action of the electrolyte for a long time. Functions such as corrosion resistance required for oxide films are impaired. Incidentally, if current is supplied, ion exchange with the electrolytic solution is performed, and an oxide film formed on the aluminum metal as the base metal grows, and the required function is not impaired.
The current is reduced to a value of 40% to 95%, for example 60 to 90%. The voltage decreases almost in proportion to the decrease in current. If a low current continues to flow, the voltage gradually increases, but when the voltage reaches a high voltage just before the decrease, the energization is terminated and the anodizing process is terminated.
The thickness of the anodic oxide film to be formed is generally 3 to 200 micrometers, for example 10 to 150 micrometers.

  According to the method of the present invention, an alumina film can be formed on various aluminum metal parts. Even if the aluminum metal part has a fine and / or complicated shape, the alumina film can be formed with a uniform thickness. Aluminum metal parts include ice making and thawing trays, rice cookers, pots, kettles, kettles and other heating cookers, instantaneous water heaters, heat exchangers, air conditioners, refrigerators, refrigerators, oil heaters, radiators, cooling Fins, air-cooled and water-cooled engines (promotion of heat dissipation), aircraft wings (anti-icing prevention), semiconductor heat dissipation substrates, semiconductor packages, heat pipes, bearings, various sliding members, brake shoes, popcorn and ice cream makers, electrical equipment It may be a chassis, a casing such as a motor or a transformer, or a fastener (for example, a screw, a bolt and a nut). A screw is particularly preferable.

The present invention is effective for preventing galvanic corrosion between different metals after fastening by a fastener (for example, a screw), that is, electrolytic corrosion. If the ionization tendency of the metal is larger than H ⁺ , the metal surface is easily ionized, and if the metal oxide is water-soluble, the barrier layer such as a natural oxide film is also peeled off.
In addition, since there is an oxidation-reduction reaction at the center of metal corrosion, the part in contact with a different metal becomes a factor that accelerates corrosion in order to form a galvanic cell.

It is possible to propose an effective fastening method of magnesium alloy, which is an important issue demanded by the current industry. Magnesium alloy has a higher specific strength than aluminum in metal (ie, aluminum metal) and can be an important base metal with abundant supply. Galvanic corrosion is likely to occur due to the potential difference.
However, the “aluminum screw” manufactured according to the present invention has a galvanic corrosion problem that is a problem in fastening between magnesium alloys and fastening of dissimilar metals due to its surface hardness, stable corrosion resistance and electrical insulation. Can be wiped out.

  In addition, the “aluminum screw” manufactured by the present invention is used for fastening resin parts and resin parts, resin parts and metal parts, because the linear expansion coefficient due to heat shows a numerical value approximate to that of resin parts. It is difficult to cause distortion due to heat, and it is possible to maintain stable fastening force and improve the life of parts.

Examples and Comparative Examples Using the apparatus shown in FIG. 1, anodized films were formed on aluminum metal screws (length 25 mm, M5, pitch 0.8) and other aluminum metal parts. The following conditions and conditions shown in the table were used.
(Electrolyte)
Sulfuric acid: 280 g / L
Succinic acid: 45 g / L
Nickel sulfate: 24g / L
Acrylic composition (68% hydroxypropyl methacrylate, 10% neopentyl glycol dimethacrylate, 19.5% polypropylene glycol methacrylate, 1% 1,6-hexanediol diglycidyl ether, 1% butyl peroxyoctate, (Consisting of 500 ppm hydroquinone monomethyl ether and 0.3% dicyandiamide): 300 g / L
Electrolyte temperature: + 10 ° C

About the obtained anodic oxide film, the uniformity was evaluated by the enlarged photograph.
The results are shown in Table A1, Table A2, Table B, Table C, and FIGS.

The salt water alternate immersion test used in FIGS. 14 and 15 was conducted in accordance with JIS H8711 (stress corrosion cracking test method for aluminum alloy). The procedure of the alternate salt water immersion test is as follows.
(1) Use a stainless steel jig and nut that are anodized with a flat washer as a tightening sample.
(2) Tighten the screws with a tightening torque that provides an axial force of 90% of the yield load.
(3) Immerse the clamped sample in the test solution for 10 minutes and repeat holding and drying in the room for 50 minutes.
(4) Sodium chloride 3.5% ± 0.1% aqueous solution with ion exchange water was used as the test solution.

(Note) If the ratio value in the above table is 1, it means that it is completely uniform, and if the value exceeds 1, the corrosion resistance of the narrow part will improve and the resistance to stress corrosion will increase. Show things.

  Referring to Table C, the ratio of the narrow portion confirmation point film thickness to the reference point film thickness is compared between FIG. 2 and FIG. 3, between FIG. 4 and FIG. 5, between FIG. 6 and FIG. 10, FIG. 11, and FIG. 12 and FIG. 13, it can be seen that according to the present invention, a screw that is more uniform and / or superior in corrosion resistance than the conventional one can be obtained.

  When the electrical resistance value rises and the voltage reaches the value at the time when the current value is lowered, the current is again supplied to the portion where the current easily flows, such as the convex portion, and by finishing the processing at that point, 3, 5, 7, 9, 11, and 13 shown in Table A1 and Table A2, a uniform oxide film is generated, and the process takes a long time (for example, 888 hours as shown in FIG. 15). ) Even after alternate immersion in salt water, even corrosion that causes stress corrosion cracking does not occur.

  As shown in FIG. 14, in the case of an anodic oxide film that is not uniform as in the prior art, stress corrosion cracking has progressed due to poor corrosion resistance in a narrow recess. However, when a uniform anodic oxide film is formed as shown in FIG. 15, no stress corrosion cracking occurs in the same narrow recess.

  The verification conditions are based on a salt water alternate dipping method in which immersion is continued in a 3.5% salt water for 10 minutes and a 50 minute drying cycle. In addition, a tensile stress load of 90% of the proof stress was continuously applied to a narrow concave portion by using a normal SUS material, and was performed in a more severe corrosive environment in which electric corrosion was also added.

  In addition, as shown in Table A1, it is difficult to produce an anodic oxide film because of the large amount of aluminum metal A1100 (FIGS. 2 and 3) and Cu that can easily form an anodic oxide film, as shown in Table A1. An A7075 material called duralumin (FIGS. 4 and 5), and an aluminum die-casting ADC12 that is used for general purposes, but it is difficult to form an anodized film due to its extremely high Si content. A total of three types of test pieces (Figs. 6 and 7) were used, and the corrosion test was performed using A7075 material (Figs. 4 and 5) present at an intermediate position.

  The present invention is particularly effective for improving the surface hardness, corrosion resistance, and electrical insulation of parts having both an acute angle portion and a narrow concave portion in the same member, such as “screws”.

A ... Anodizing tank B ... Electrolyte C ... Cathode D ... Anode E ... Electrode jig F ... Member to be processed G ... DC power supply H ... Cooling machine I ... Stir blower

Claims (9)

In the anodizing process, a method for producing an anodized film is characterized in that the applied current is reduced to a value of 40% to 95% from 60 minutes to 3 minutes before the end of current application to make the current low. The method according to claim 1, wherein the time during which the low current is applied is 1/8 to 1/2 of the anodic oxidation time. The method of claim 1, wherein the low current value is 0.1 to 3.0 amperes lower than the high current value before the current drop. The method according to claim 1, wherein an aluminum alloy or a titanium alloy is used in an electrode jig for attaching and energizing a component to an anode. An aluminum metal part having a uniform anodic oxide film formed by the method according to claim 1. 6. The component according to claim 5, which has a uniform anodic oxide film on an aluminum metal which is pure aluminum or an aluminum alloy. The component according to claim 5 or 6, which has a fine shape. The aluminum according to any one of claims 5 to 7, wherein the shape of the part on which the uniform film is formed is a shape having features such as an acute angle portion, a narrow concave portion, a bag-like shape or a penetrating pore portion. Metal parts. Aluminum metal screw with uniform anodic oxide film.