JP2017115166A - Anodic oxidation method for aluminum based member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an anodic oxidation method for an aluminum based member capable of forming a really thick anode oxidation film in a short time.SOLUTION: Provided is an anode oxidation method for an aluminum based member comprising an electrolytic process where, while contacting the face to be treated made of pure aluminum or an aluminum alloy with an electrolytic solution, the face to be treated is subjected to energization, and an anode oxidation film is formed on the face to be treated. The electrolytic process is performed by current control of energizing AC current of periodically repeating the maximum current density and the minimum current density. The minimum current density is preferably -0.4 to 0.3 A/cm. The frequency of the AC current is preferably 2 Hz to 9 kHz. The maximum current density is preferably 1 A/cmor more. By using the anode oxidation method in this invention, a thick heat insulation layer made of an anode oxidation film can be efficiently formed at least on a part of the tip of the piston for an internal combustion engine.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an anodizing method for forming an anodized film on at least a part of a surface of an aluminum-based member.

Aluminum-based members are often anodized for the purposes of corrosion resistance, wear resistance, insulation, and the like. Anodizing treatment is an oxidation treatment performed by immersing a treated portion of an aluminum-based member in an electrolytic bath (sulfuric acid bath, oxalic acid bath, etc.) and energizing the treated portion as an anode. As a result, an anodic oxide film (alumite film) made of aluminum oxide (Al ₂ O ₃ or the like) generated by oxidizing the base material is formed on the surface (surface to be processed) of the processed portion. Thus, the anodizing treatment is different from the plating treatment or the like in that it involves oxidation of the substrate itself.

  The anodic oxide film thus formed usually comprises a dense and thin (several tens of nm) barrier layer (active layer) and a porous layer grown on the barrier layer. The thickness of a general anodic oxide film is several to several tens of μm or more, and most of it consists of a porous layer. In addition, since the porous layer is usually composed of a large number of straight tubular fine holes opened on the surface side, a sealing process for sealing the porous layer or a sealing process for filling the entire hole is often performed.

  By the way, in order to efficiently perform the anodizing treatment, various energization methods have been conventionally proposed. For example, there are descriptions related to the following patent documents.

JP-A-62-253797 JP 2000-282294 A JP 2004-35930 A JP 2006-83467 A JP 2007-204831 A JP 2009-235539 A JP2015-124400A

  In Patent Document 1, a single-phase commercial frequency (60 Hz) alternating current and a positive (plus) rectangular pulse waveform current of 12 Hz are superposed and energized. However, although Patent Document 1 has a description regarding the current density, there is no specific description about each current value or each voltage value to be superimposed. Moreover, the alternating frequency shown there is only the commercial frequency.

  In Patent Document 2, the AC voltage and the DC voltage are superimposed and energized so that a negative (minus) voltage is not applied to the sample (object to be processed). However, in Patent Document 2, there is no specific description regarding the AC frequency, and there is merely a description that the higher frequency side (100 Hz or higher) is preferable ([0022]).

  In Patent Document 3, an AC voltage and a DC voltage are superposed and energized so that a substantially positive voltage is applied to the object to be processed (so that the lower limit value of the superimposed voltage is substantially 0 V), The AC frequency is set relatively high (200 to 2000 Hz). However, the deposition rate of the anodic oxide film described in Patent Document 3 is very small and is only less than 2 μm / min (see Table 1).

  In Patent Document 4, energization is performed to apply a positive voltage and remove charges (apply a negative voltage) at a high frequency (10 kHz or more recommended). However, even if such energization is performed for 4 minutes, the thickness of the anodic oxide film obtained is only about 15 to 20 μm (4 to 5 μm / min in terms of film formation speed) (see [0061]).

In Patent Document 5, AC / DC superposition energization is performed to change the base current density from the start of electrolysis to increase → decrease → constant, and an anodic oxide film having a thickness of 150 μm or more is formed. Specifically, an anodic oxide film having a thickness of about 300 μm is formed by processing for 150 minutes. However, the deposition rate at that time is only about 2 μm / min. This is presumably because the maximum current density is as small as 11 A / dm ² (= 0.11 A / cm ² ) (see [0041], [0042], FIG. 5 etc.). As described above, in the anodic oxidation methods as disclosed in Patent Documents 1 to 5, the film forming speed obtained is only a few μm / min.

In Patent Document 6, after energization to apply a DC voltage to the vicinity of the burn generation voltage, the negative peak current value is 10% or less of the positive peak current value in a high frequency range (5 to 15 kHz). It is energized with AC and DC superimposed. Specifically, the voltage control, the first 10 seconds DC application (current density: 100A / dm ^2), and subsequent 5 seconds high frequency current (frequency to the DC: 15 kHz / current density: 120A / dm ² ) Is superimposed and energized. It is described that the film forming speed at this time is 65 μm / min. However, in Patent Document 6, only an extremely short time (15 seconds) until the occurrence of burns is performed, and the thickness of the obtained anodic oxide film is only 16.2 μm at most ([0033 ]reference).

  Patent Document 7 proposes an anodic oxidation method capable of obtaining a thick anodic oxide film in a short time by setting the voltage ratio between the DC component and AC component to be AC / DC superimposed and the frequency of the AC component within a specific range. doing. According to the present inventor's recent research, it is possible to obtain a thick anodic oxide film in a short time even outside the conditions defined in Patent Document 7 and without performing complicated voltage control. I understood it.

  The present invention has been made in view of such circumstances, and an aluminum-based film which can form a large anodic oxide film in a short time by performing electrolysis under conditions different from those of conventional anodic oxidation methods. An object is to provide a method for anodizing a member.

  As a result of earnest research to solve the above-mentioned problems, the present inventor conducted an anodic oxide film having a very large thickness relatively easily by passing an alternating current having a minimum current density and frequency within a predetermined range. However, it was newly found that it can be formed in a short time. By developing this result, the present invention described below has been completed.

<< Anodic oxidation method for aluminum-based members >>
(1) The anodizing method for an aluminum-based member of the present invention (simply referred to as “anodic oxidation method” as appropriate) is applied to the surface to be processed while contacting the surface to be processed made of pure aluminum or an aluminum alloy with the electrolytic solution. An anodizing method for an aluminum-based member comprising an electrolysis process for forming an anodic oxide film on the surface to be processed, wherein the electrolysis process is an alternating current that periodically repeats a maximum current density and a minimum current density The minimum current density is −0.4 to 0.3 A / cm ² and the frequency of the alternating current is ² Hz to 9 kHz.

(2) According to the anodic oxidation method of the present invention, a high film formation rate can be stably obtained, so that it is not necessary to energize by complicated voltage control, and the surface to be treated (base material surface) of the aluminum-based member A thick anodic oxide film can be reliably formed in a short time. The mechanism etc. by which the anodic oxidation method of the present invention exhibits such excellent film forming properties are not necessarily clear, but at present, it is considered as follows.

  First, it is known that the anodic oxide film is an insulator as well as a dielectric, and a pseudo circuit (equivalent circuit) comprising an electric resistance (R) and a capacitance (C) is provided in the vicinity of the anodic oxide film. It is thought that it is formed. For this reason, when anodizing is performed by energizing alternating current (simply referred to as “alternating current energizing”), as the frequency increases, the impedance near the anodized film decreases, and the current density flowing through the surface to be processed increases. It can be considered that the deposition rate of the anodized film is improved.

  However, as a result of diligent research by the present inventors, it has been surprisingly revealed that the film formation rate exhibits a maximum when AC current is applied in a considerably lower frequency range than before. The reason for this is not clear, but is affected by the potential dependence of oxygen ion adsorption sites and the presence of an electric double layer in the vicinity of the interface between the anodized film formed on the surface to be treated and the electrolyte. It is guessed.

Incidentally, the electric double layer referred to here is a cation in which the anions (OH ⁻ , SO 4 ^2−, etc.) are arranged in layers on the anodic oxide film side in the electrolyte solution in the vicinity of the above-mentioned interface, and form a pair opposite thereto. (H ⁺ etc.) means a state in which the layers are aligned. The electric double layer formed by interposing the electrolytic solution as described above is also considered as a pseudo circuit (equivalent circuit) including an electric resistance (R ′) and a capacitance (C ′), as in the case of the anodic oxide film. be able to. That is, the vicinity of the surface to be processed can be grasped as an equivalent circuit in which the first pseudo circuit configured in the vicinity of the anodized film and the second pseudo circuit configured of the electric double layer are connected. Therefore, as described above, the film formability of the anodic oxide film cannot be grasped only by the first pseudo circuit in the vicinity of the anodic oxide film, and the presence or absence of the second pseudo circuit by the electric double layer and the characteristics thereof are also taken into consideration. For the first time, it will be possible to obtain an overall understanding.

  By the way, in the electrolysis process according to the present invention, the minimum current density of the alternating current to be energized and the frequency thereof are limited within a specific range. In this case, it becomes difficult to maintain the electric double layer, and the concentration and movement of each ion (especially anion) are close to the desired state in the vicinity of the interface between the anodic oxide film and the electrolytic solution, and as a result, efficient energization is possible. Thus, it is presumed that a high film forming speed can be stably obtained.

  Incidentally, even if only the minimum current density is within the above-mentioned range, the excellent film formability as in the present invention cannot be obtained. The reason is considered as follows. The electric double layer is formed of ion particles having a mass much larger than that of electrons. When a high-frequency alternating current is passed through such an electric double layer, ion particles having a large mass can hardly follow the frequency change. In this case, the electric double layer remains substantially remaining, and an improvement in the film formation rate cannot be expected. In any case, in the case of the anodic oxidation method of the present invention, a very excellent film forming property has been exhibited by cooperation or synergy between the minimum current density and the frequency of the alternating current to be applied. Certainly.

<< Aluminum-based parts >>
The present invention can be grasped not only as an anodic oxidation method but also as an aluminum-based member having an anodic oxide film formed thereby.

<Others>
(1) The term “alternating current” as used in this specification means that the current value or voltage value fluctuates periodically, and the waveform, amplitude value, median value (average value), peak value, etc. are not limited. “Direct current” means other than the alternating current, and the current value or voltage value may be constant or may change over time if it is not periodic, and its waveform, polarity (positive or negative), etc. are not questioned.

  The “alternating current component” is a current component or a voltage component that varies periodically, and the “direct current component” is a bias current or a bias voltage. When AC / DC superimposition is performed, an average value of periodically changing current values or voltage values is set as a DC component. The “average value” here is not an effective value, but an arithmetic average value of upper and lower peak values (upper and lower limit values) of a current value or a voltage value. Since the electrolysis process according to the present invention is performed by current control, alternating current (component) or direct current (component) is related to current unless otherwise specified.

  In the present invention, “current control” means that the minimum current density (or minimum current value), maximum current density (or minimum current value), and frequency are maintained while maintaining a predetermined value or a predetermined range within a predetermined range. AC current is applied between the anode (base material side) in contact with the liquid and the cathode. The minimum current density (and the maximum current density) is not necessarily constant as long as it is within the range defined by the present invention. For example, when the DC component to be superimposed on the AC component changes with time (in a linear function), the minimum current density can also change.

  Considering such a case, the minimum current density or the maximum current density in the present invention is an arithmetic average value of peak values (minimum value or maximum value) in each cycle during the energization period. When the frequency of the alternating current fluctuates during the electrolysis process, an average value obtained by dividing the total number of cycles during the energization period by the energization time is defined as the frequency referred to in the present invention.

  Incidentally, the “current density” in the present specification is obtained by dividing the applied current by the area of the surface to be treated (anode electrode surface) in contact with the electrolytic solution. The area is determined based on actual measurement or a design value of an aluminum-based member.

(2) In the anodic oxidation method of the present invention, the film formation speed and the film thickness of the anodic oxide film are not questioned. Speaking daringly, the film formation rate may be, for example, 50 μm / min or more, further 65 μm / min or more, and the film thickness may be, for example, 50 μm or more, 75 μm or more, further 100 μm or more. Further, the processing time (electrolysis time) necessary for obtaining such a thick anodic oxide film may be, for example, about 1 to 10 minutes, 1.5 to 5 minutes, or even about 2 to 3 minutes.

  The film thickness can be obtained by observing the cross section of the anodized film with a scanning electron microscope (SEM) or by measuring it with an eddy current film thickness meter. Unless otherwise noted, the measured value of the eddy current film thickness meter based on. The film thickness can be specified by SEM observation by observing a cross section orthogonal to the film formation surface and determining the distance from the interface between the anodized film (aluminum oxide) and the base material (pure aluminum or aluminum alloy) to the outermost surface. Measure and do.

  The film formation speed is obtained by dividing the film thickness by the processing time (energization time). However, when the film thickness itself is small, the film forming speed tends to be calculated to be large. Therefore, the film forming speed according to the present invention is a value when an anodic oxide film having a film thickness of 40 μm or more is formed.

(3) Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. A range such as “a to b” can be newly established with any numerical value included in various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value.

It is a graph which shows the relationship between the minimum current density of alternating current, and the film-forming speed | rate. It is a graph which shows the relationship between the frequency of alternating current, and the film-forming rate. It is a graph which shows the relationship between the frequency of the rectangular wave current which adjusted the duty ratio, and the film-forming speed | rate. It is a graph which shows the relationship between a maximum current density and a duty ratio.

  The contents described in this specification can be appropriately applied not only to the anodizing method of the present invention but also to an anodized aluminum-based member. One or more components arbitrarily selected from the present specification may be added to the above-described components of the present invention. In this case, the components related to the method are fixed (for example, it is impossible or impractical to directly identify the “object” by structure or characteristics (impossible / unpractical circumstances), etc.) ), It can also be a component for “things” as a product-by-process. Which embodiment is the best depends on the target, required performance, and the like.

《Anodic oxidation method》
(1) AC Current The electrolysis process according to the present invention is performed by applying an AC current whose current density and frequency are controlled within a predetermined range. Minimum current ^{density, -0.4~0.3A} / cm ^2, preferably -0.3~0.2A / cm ² and even more are -0.2~0.1A / cm ^2. Whether the minimum current density is too low or too high, the deposition rate can be reduced. In particular, when the minimum current density is excessive, so-called burn (burning) is likely to occur in the anodic oxide film.

Maximum current density is, for example, 1A / cm ² or more, the 1.5A / cm ² or more further is a 1.8A / cm ² or more preferred. If the maximum current density is too low, the deposition rate cannot be improved. If the maximum current density is excessive, burns (burns) are likely to occur in the anodic oxide film.

The frequency of the alternating current is preferably 2 Hz to 9 kHz, 5 Hz to 1 kHz, 10 to 1000 Hz, 20 to 900 Hz, and further 30 to 200 Hz. Whether the frequency is too low or too high, the deposition rate can be reduced. By the way, if the pseudo circuit formed near the interface of the anodized film is considered as a series AC circuit, the impedance (Z) is Z = {R ² + (ωL−1 / ωC) ² } ^1/2 (R : Resistance, L: inductance, C: capacitance, ω: angular frequency (= 2πf), f: frequency of alternating current). Considering this, when the frequency is too low, the impedance increases due to the capacitive component associated with the generation of the reaction layer and the electric double layer, and when the frequency is excessive, the impedance increases due to the induced component associated with the anodic oxidation reaction. In any case, it is explained that the current value can be reduced and the film formation can be suppressed or stagnated. Although it is not easy to specify specific R, L, and C, based on the above formula, when ω = (1 / LC) ^1/2 , that is, f = (1 / LC) ^1/2 / 2π. In this case, Z can be a minimum value (minimum value). Conversely, the existence of f ₀ that minimizes Z is expected, and a frequency within the above range is considered to give such f ₀ .

  In addition, the optimum value of the current density and frequency according to the present invention depends on the situation change such as the material of the base material, the concentration of the electrolytic solution, the temperature (bath temperature), the fluidity in the bath (for example, the degree of stirring), There may be some variation within the above range. However, the basic idea (principle) according to the present invention is not changed, and as long as the current density and the frequency are within the numerical ranges described above, anodization with excellent film forming properties can be performed.

  Various waveforms of the alternating current applied in the electrolysis process are conceivable, such as a rectangular wave, a triangular wave, a sawtooth wave, and a pulse wave in addition to a sine wave. When an alternating current is obtained by superimposing (combining) an alternating current component and a direct current component, the waveform can be adjusted by the alternating current component, and the average current (density) of the alternating current can be adjusted by the direct current component (bias current). In particular, the electrolysis process is preferably performed using an alternating current in which an alternating current component having a constant frequency and peak current value (minimum current value and maximum current value) and a direct current component having a constant current value are superimposed.

In particular, the alternating current has a duty ratio (τ / T) of 0.05 to 0.5, 0.1 to 0.3, which is a ratio of the energization time (τ) at which the maximum current density is reached in one cycle (T). Furthermore, a rectangular wave current of 0.15 to 0.25 is preferable. If the duty ratio is too small, the film forming speed cannot be improved. However, by reducing the duty ratio relatively, the film forming speed can be improved without causing defects such as burns. In particular, a good anodic oxide film can be formed at high speed by increasing the maximum current density while reducing the duty ratio. For example, while the duty ratio to 0.5 or less, the maximum current density 2A / cm ² or more, or equal to 3A / cm ² or more further 4A / cm ² or more. In addition, when the frequency of the alternating current at that time is 20 to 900 Hz, further 50 to 500 Hz, the film formation rate can be further improved.

(2) Electrolyte Solution Regardless of the type of the electrolyte solution (anodizing treatment solution), for example, an inorganic acid solution such as a sulfuric acid aqueous solution, a phosphoric acid aqueous solution, or a chromic acid aqueous solution, or an organic acid solution such as an oxalic acid aqueous solution may be used. However, the electrolytic solution is preferably a sulfuric acid aqueous solution from the viewpoint of economy and the like. The sulfuric acid aqueous solution preferably has a concentration of about 5 to 40% by mass, more preferably about 10 to 30% by mass. If the concentration is too low, a sufficient film formation rate cannot be obtained, and if the concentration is too high, the corrosion resistance of the anodic oxide film may be lowered. The temperature (bath temperature) of the electrolytic solution is preferably 0 to 40 ° C, more preferably about 10 to 30 ° C. If this temperature is too low or too high, a sufficient film formation rate cannot be obtained. Incidentally, a platinum electrode, a graphite electrode, or the like is usually used as the counter electrode provided in the electrolytic solution, but an electrode made of another conductive material may be used.

(3) Post-treatment The anodized film according to the present invention may be anodized as it is, but post-treatment such as sealing treatment, sealing treatment, heat treatment, and coating may be appropriately performed. For example, when the sealing treatment is performed, the pores of the anodized film (porous layer) are sealed, and the corrosion resistance of the aluminum-based member can be improved. Incidentally, the sealing treatment method is well known, for example, performed by exposing the anodized film to boiling water or high-pressure steam. In addition, when an anodic oxide film is used as the heat shield film, for example, a sealing layer made of high heat-resistant silica, alumina, or the like may be formed on the surface of the anodic oxide film to perform the sealing process. Specifically, for example, a reinforcing layer formed by applying polysilazane, polysiloxane or the like on the surface of the anodic oxide film and firing it to convert to silica is formed in the porous layer or on the outermost surface thereof and sealed. You may stop processing.

<< Aluminum-based parts >>
(1) Anodized film The anodized film is mainly composed of aluminum oxide (Al ₂ O ₃ ), but its form (particularly the form of the porous layer) can be changed depending on the conditions of the anodizing treatment. An appropriate anodic oxide film may be selected according to the application and required specifications. For example, a dense anodic oxide film is preferable for the purpose of wear resistance, but a sparse anodic oxide film may be used for the purpose of heat shielding.

  Furthermore, the anodized film formed on the combustion chamber surface of the internal combustion engine (for example, the top surface of the piston for the internal combustion engine or the combustion chamber wall surface of the cylinder head) has fine pores (pores) separately from the porous layer. It is preferable that it is positively included. An anodic oxide film having a high porosity exhibits low heat conductivity in addition to the inherent heat resistance, and is therefore effective as a heat insulating layer for members exposed to high temperature environments. Moreover, since the anodic oxide film has a thickness (film thickness) of several tens μm to several hundreds μm at most and a heat capacity is sufficiently small, the temperature followability is excellent. The thermal conductivity and the temperature followability are collectively referred to as “swing characteristics” (see Japanese Patent No. 5642640, Japanese Patent Application Laid-Open No. 2015-3226, etc.).

(2) Substrate The aluminum-based member according to the present invention has a substrate made of pure aluminum or an aluminum alloy at least in part, and an anodized film is formed on at least a part of the surface (surface to be treated). Anything that is formed is sufficient. The composition and structure of the substrate can be various, but the closer the substrate is to pure aluminum, the easier it is to obtain a dense anodic oxide film. On the other hand, in the case of forming an anodic oxide film containing pores different from the porous layer, the base material is preferably made of an aluminum alloy containing Si or Cu. When Si (crystal grains or the like) is present on the surface to be processed, the anodic oxide film is difficult to grow in the vicinity thereof. Further, Cu compound particles and the like on the surface to be processed are easily eluted. Thus, if Si or Cu is present on the surface to be processed, vacancies are easily formed in the anodized film. Furthermore, if Si (crystal grains or the like) or Cu (compound grains or the like) is finely dispersed in the vicinity of the surface to be processed, the vacancies in the anodized film tend to be finely dispersed. Fine pores are almost spherical, so they are unlikely to be a starting point for fracture. Therefore, when the base material is made of an aluminum alloy containing Si or Cu, it is possible to form an anodic oxide film having a high porosity that is excellent in heat insulation (low thermal conductivity), temperature followability, mechanical properties (strength, toughness, etc.), etc. It becomes possible.

  In such a case, the aluminum alloy according to the present invention preferably contains 5 to 60%, 8 to 58%, 11 to 55%, 12 to 50%, 13 to 40%, and further 17 to 30% of Si. The aluminum alloy preferably contains 0.5 to 10%, 1.5 to 9%, 2.5 to 8%, 3 to 7%, 4 to 6.5%, and further 5 to 6% of Cu. . Of course, the aluminum alloy may contain Si and Cu simultaneously. Si and Cu are preferably dispersed independently in the aluminum alloy as crystal grains and compound grains.

  The aluminum alloy can contain various alloy elements in addition to Al, Si, and Cu. For example, Mg may be included in an amount of 0.5 to 3%, further 0.7 to 2%. In addition, the aluminum alloy has a small amount (for example, about 0.01 to 1%) of other modifying elements that improve mechanical properties and a finer that refines the metal structure (especially Si crystal grains or Cu compound grains). May be included. For example, in the case of a hypereutectic aluminum alloy containing 13% or more of Si, it is preferable to contain P as a finer.

  The aluminum alloy used as the base material may be any of a cast material, a processed material subjected to plastic processing such as forging and extension processing, a thermal spray material, and an adhesive material. As the thermal spray material, for example, an aluminum alloy powder containing Si: 40 to 70% can be used. As the adhesive material, for example, a JIS standard or AA standard # 4000 series aluminum alloy for extension (for example, Si content: 8 to 20%, or 10 to 17%) can be used.

<Application>
The use of the aluminum-based member or the anodic oxide film according to the present invention is not limited. For example, a heat shield film that covers the top surface part (including the piston ring land part and groove part) of an internal combustion engine piston (aluminum member), an insulating film that covers the outer surface of a coil or wiring (aluminum member), etc. The anodic oxide film of the present invention is preferably used. The piston for the internal combustion engine is, for example, Si: 5 to 24%, further 11 to 13%, Cu: 0.5 to 4%, further 0.8 to 1.3%, Mg: 0.7 to 2% Furthermore, it consists of an aluminum alloy for casting containing 1 to 1.3%. Of course, various modifying elements (for example, 3% or less or 1% or less in total) may be further included, and the remainder other than these is Al and inevitable impurities. Examples of the modifying element include Ti or B for refining crystal grains (particularly α-Al grains) of the base material, Sr, Na or Sb for refining (spheroidizing) Si grains, and Zn for improving heat resistance. Fe, Mn, Ni, Pb, Sn or Cr.

  Assuming that a thick anodic oxide film is formed on the top surface of the piston for an internal combustion engine, an electrolysis process by current control was performed using alternating currents having different minimum current densities and different frequencies. In this way, a large number of samples in which an anodized film was formed on the surface (surface to be treated) made of an aluminum alloy were manufactured. Based on these samples, the relationship between the deposition conditions (minimum current density, frequency) of the anodized film and the deposition rate was clarified. The present invention will be described in more detail below with specific examples.

<Production of sample>
(1) Base material As a base material (aluminum-based member) for forming an anodized film, a test material made of an aluminum alloy for casting (JIS AC8A / Al-12% Si-1% Cu-1% Mg) was prepared. .

(2) Anodizing treatment (electrolysis process)
A test material (surface to be treated) was immersed in an aqueous sulfuric acid solution (electrolytic solution), and the sample was energized with the anode and the platinum electrode as a cathode. At this time, the other surface of the test material excluding the surface to be processed was masked so that current was supplied between the surface to be processed and the platinum electrode. The electrolytic solution was sulfuric acid concentration (mass%): 20% and temperature (bath temperature): 10 ° C. The energization was performed while stirring the electrolytic solution.

  A current-controlled alternating current (rectangular wave current) was used for energization. This rectangular wave current has a constant current value (current density) every half cycle (duty ratio: 0.5), an AC component composed of a rectangular wave in which the positive side waveform and the negative side waveform are symmetrical, and the current value ( It was formed by superimposing a DC component of a linear waveform with a constant current density.

  Anodization was performed by changing the maximum current density, minimum current density or frequency. Thus, a large number of samples on which an anodized film was formed were obtained. The current density was obtained by dividing the applied current value by the surface area of the test material.

(3) The test material after completion of electrolysis was thoroughly washed with distilled water after being taken out from the electrolytic solution, sprayed with compressed air to remove moisture, and then sufficiently dried in the atmosphere. And the film thickness of the anodic oxide film currently formed on the to-be-processed surface of each test material was measured with the eddy current type film thickness meter. The film thickness of each sample exceeded 45 μm.

  Thus, the relationship between the film formation conditions (minimum current density or frequency) and the film formation speed (film thickness of the anodic oxide film / energization time: μm / min) for each sample was clarified.

<Effect of minimum current density>
(1) The maximum current density of alternating current: 2 A / cm ² (constant), the frequency: 20 Hz, the minimum current density was variously changed, and the above-described anodizing treatment was performed for 60 seconds. Using the sample thus obtained, the relationship between the minimum current density of the alternating current to be applied and the film formation rate was clarified. The results are shown in FIG. In FIG. 1, “試 料” indicates that the sample has burned in the anodic oxide film.

(2) As can be seen from FIG. 1, it can be seen that there is a critical region where the deposition rate of the anodic oxide film reaches a peak without causing burns in the vicinity where the minimum current density is near zero. Specifically, when the minimum current density was −0.4 to 0.3 A / cm ^{2, or} −0.2 to 0.2 A / cm ² , the film formation rate peaked.

<< Effect of frequency >>
(1) The maximum current density of alternating current: 2 A / cm ² (constant), the minimum current density: 0.1 A / cm ² (constant), the frequency was variously changed, and the above-described anodizing treatment was performed for 80 seconds. . Using the sample thus obtained, the relationship between the frequency of the alternating current to be applied and the film formation rate was clarified. The results are shown in FIG.

(2) The maximum current density of alternating current: 2 A / cm ² (constant), the minimum current density: −0.1 A / cm ² (constant), the frequency was changed variously, and the above-described anodizing treatment was performed for 80 seconds. It was. Using the sample thus obtained, the relationship between the frequency of the alternating current to be applied and the film formation rate was clarified. The results are shown in FIG.

(3) As can be seen from FIG. 2 and FIG. 3, the frequency is 2 Hz to 5 kHz, 5 Hz to 1 kHz, 10 to 500 Hz, 15 to 200 Hz, or 20 to 90 Hz even if the current varies slightly depending on the current density of the applied alternating current. It can be seen that there is a critical region in the vicinity where the deposition rate of the anodic oxide film peaks. On the other hand, it can be seen that at least when the frequency is 1 Hz or less or 10 kHz or more, the deposition rate is significantly reduced.

<Effect of maximum current density>
(1) The minimum current density of alternating current: 0 A / cm ² (constant), the frequency: 20 Hz, the duty ratio of the alternating current and the maximum current density were variously changed, and the above-described anodizing treatment was performed for 60 seconds. Using the sample thus obtained, the relationship between the duty ratio and maximum current density of the alternating current to be applied and the film formation rate was clarified. The results are shown in FIG. In FIG. 4, “x” indicates a sample in which a defect such as burn has occurred in the anodic oxide film. The line in FIG. 4 is a constant velocity line of the film formation speed drawn based on the film formation speed actually measured for each sample and the total amount of electricity (coulomb amount) energized in each sample. Incidentally, when the duty ratio (D) is 0.6 (the energization time (τ) of the minimum current density per cycle: 0.02 s), the maximum film formation speed (Vm): 60 μm / min, D: 0.5 (τ : 0.025 s), Vm: 65 μm / min, and D: 0.3 (τ: 0.035 s), Vm: 75 μm / min.

(2) As can be seen from FIG. 4, it can be seen that the smaller the duty ratio is, the fewer defects are generated. It was found that even if the duty ratio is reduced, a good anodic oxide film can be obtained at a high deposition rate by increasing the maximum current density. In particular, it can be said that it is preferable that the duty ratio is 0.5 to 0.1, more preferably 0.3 to 0.2, and the maximum current density is 1.5 to 5 A / cm ^2, further ² to 4 A / cm ² .

  From the above results, by using an alternating current within the range specified in this specification, the film formation speed of the anodic oxide film can be greatly improved, and a thick anodic oxide film can be effectively formed in a short time. It was confirmed that a film could be formed on the substrate.

Claims (9)

An anodizing method for an aluminum-based member comprising an electrolysis step of energizing the surface to be processed made of pure aluminum or an aluminum alloy in contact with an electrolytic solution and forming an anodized film on the surface to be processed Because
The electrolysis step is performed by current control for passing an alternating current that periodically repeats the maximum current density and the minimum current density,
The minimum current density is −0.4 to 0.3 A / cm ² ,
The method of anodizing an aluminum-based member, wherein the frequency of the alternating current is 2 Hz to 9 kHz. The method for anodizing an aluminum-based member according to claim 1, wherein the maximum current density is 1 A / cm ² or more.   The method according to claim 1, wherein the alternating current is formed by superimposing an alternating current component and a direct current component.   The AC current is a rectangular wave current having a duty ratio (τ / T) of 0.05 to 0.5, which is a ratio of energization time (τ) at which the maximum current density is reached in one cycle (T). Item 4. The method for anodizing an aluminum-based member according to any one of Items 1 to 3.   The method for anodizing an aluminum-based member according to claim 1 or 4, wherein the frequency of the alternating current is 20 to 900 Hz.   The method for anodizing an aluminum-based member according to any one of claims 1 to 5, wherein in the electrolysis step, the deposition rate of the anodized film is 50 µm / min or more.   The method for anodizing an aluminum-based member according to claim 6, wherein the electrolysis step is performed for 1 to 10 minutes.   The said to-be-processed surface consists of an aluminum alloy containing Si: 5-60% or Cu: 0.5-10% by making the whole alloy 100 mass% (it is only called "%"). A method for anodizing an aluminum-based member according to claim 1.   The method for anodizing an aluminum-based member according to claim 8, wherein the surface to be treated is at least a part of a top surface of a piston for an internal combustion engine.

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