Electrophotographic charging member

A charging member for amorphous silicon photoreceptors which can be supplied with a uniform and stable resistivity, exhibits an excellent durability and is insusceptible to variation of resistivity against environmental conditions, pinhole leak and contamination on the photoreceptors, showing a good quality and a long life. An electrophotographic charging member for charging a photoreceptor (preferably, mainly composed of amorphous silicon), which comprises a porous anodized aluminum film formed by anodically oxidizing a support the surface of which is made of aluminum or aluminum alloy. In the charging member, it is preferred that the pores in the porous anodized aluminum film be filled with a metal or an electrically conductive material made of an oxyacid salt of transition metal or pure water be attached to the inner wall of the pores in the porous anodized aluminum film. The charging member of the present invention may comprise a surface protective layer on the porous anodized aluminum film. The surface protective layer preferably is made of an organic high molecular compound or inorganic high molecular compound having electrically conductive fine particles dispersed therein. Alternatively, the surface protective layer may comprise an abrasive dispersed therein.

FIELD OF THE INVENTION
 The present invention relates to a charging member for electrophotographic
 apparatus.
 BACKGROUND OF THE INVENTION
 An electrically conductive member to be incorporated in a charging member
 for charging an electrophotographic photoreceptor is required to exhibit
 electrical conductivity of 10.sup.3 to 10.sup.9 .OMEGA. as calculated in
 terms of resistivity (hereinafter determined by means of an electrode
 having an area of 1 cm.sup.2). In general, it is provided on a metallic
 shaft and its external surface as an electrically conductive layer such as
 electrically conductive rubber layer. In order to make the electrically
 conductive member fully functional as a charging member, it is considered
 preferred that its resistivity level be in the range of 10.sup.3 to
 10.sup.9 .OMEGA. as defined above. If there occurs a locally low
 electrical conductivity, it causes a phenomenon of passage of excess
 current through defective portions on the photoreceptor, i.e., so-called
 pinhole leak, resulting in image defects. Therefore, in order to inhibit
 pinhole leak, the lower limit of the resistivity of the electrically
 conductive rubber layer is preferably in the range of 10.sup.6 to 10.sup.9
 .OMEGA.. However, if the resistivity of the electrically conductive rubber
 layer exceeds 10.sup.9 .OMEGA., no discharge occurs. Thus, the charged
 potential of the photoreceptor is not sufficient, causing the image to be
 entirely fogged (i.e., ghost).
 In order to eliminate this defect, a function separation structure in which
 the resistivity of the electrically conductive rubber layer is kept low
 and a resin having a high resistivity is provided on the surface of the
 rubber layer as a resistive layer has been employed (JP-A-1-79958 (The
 term "JP-A" as used herein means an "unexamined published Japanese patent
 application")). However, this structure is disadvantageous in that
 different environmental conditions give different resistivities and hence
 different image densities. Another inevitable problem is that as the
 photoreceptor is repeatedly charged, the resin layer is unevenly worn,
 causing uneven discharging, or discharge products or part of toner
 constituent materials is attached to the surface of the resin layer,
 causing image defects. Further, the resin or rubber is worn while being
 brought into contact with the photoreceptor, producing rubber tailings
 that can be transferred to images.
 The foregoing electrically conductive rubber layer is normally made of an
 electrically conductive rubber composition comprising a synthetic rubber
 such as EPDM rubber or silicone rubber with a powder of an electrically
 conductive material or electrically conductive fiber (carbon black,
 metallic powder, carbon fiber, etc.) incorporated therein. In order to
 allow the electrically conductive rubber layer to have an electrical
 conductivity as 10.sup.3 to 10.sup.9 .OMEGA., it is necessary that the
 powder of an electrically conductive material or electrically conductive
 fiber be uniformly dispersed within the plane. However, reproducibility
 and mass producibility problems such as resistivity variation within the
 plane and from lot to lot make it difficult to obtain sufficient
 properties.
 In order to eliminate these difficulties, a charging roll has been proposed
 which is provided with an ionically conductive rubber layer that makes the
 use of the inherent ionic conductivity of synthetic rubber or an ionically
 conductive rubber layer obtained by adding a high dielectricity liquid or
 ionic substance to a synthetic rubber so that its ionic conductivity is
 increased (JP-A-2-199163). In this case, the ionically conductive rubber
 layer is a uniform dispersion system and thus can provide a uniform
 resistivity. However, it is disadvantageous in that when charging is
 repeated with a charging roll having the ionically conductive rubber layer
 being brought into direct contact with the surface of the photoreceptor,
 low molecular components contained in the rubber layer are transferred to
 the photoreceptor, causing image defects.
 The charging roll is rotated while being pressed against the external
 surface of the exposing drum so that discharging occurs in the vicinity of
 the contact to charge the external surface of the exposing drum. Thus, the
 charging roll is required to be rigid enough to give no stress to the
 photoreptor. Thus, it is desired to replace the electrically conductive
 rubber layer by a rigid material such as semiconducting material. In this
 respect, a ceramic roller having a semiconducting substance free of
 elastic material is disclosed (JP-A-50-843). However, this ceramic roller
 can easily cause uneven discharging and thus cannot provide stable
 charging. This ceramic roller is also disadvantageous in that when it is
 rotated while being pressed against a photoreceptor made of a polymer
 material such as organic photoreceptor, it suffers from abrasion scratch
 or the like, resulting in the deterioration of the photoreceptor.
 Therefore, the state-of-the-art charging roll has an electrically
 conductive rubber roller.
 As mentioned above, charging members such as charging roll which have
 heretofore been proposed all have problems in characteristics such as
 contamination on the photoreceptor, pinhole leak and deterioration due to
 the attachment of foreign matters or abrasion. Further, these charging
 members find difficulty in controlling the resistivity variation within
 the plane. Thus, these prior art charging members leave something to be
 desired.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide a charging
 member for charging photoreceptors (preferably, amorphous silicon
 photoreceptors) which can be supplied with a uniform and stable
 resistivity, exhibits an excellent durability and is insusceptible to
 variation of resistivity against environmental conditions, pinhole leak
 and contamination on the photoreceptors, showing a good quality and a long
 life.
 The foregoing and other objects of the present invention will become more
 apparent from the following detailed description and examples.
 The present invention concerns an electrophotographic charging member for
 charging a photoreceptor, which comprises a porous anodized aluminum film
 formed by anodically oxidizing a support the surface of which comprises
 aluminum or aluminum alloy.
 In the charging member of the present invention, it is preferred that the
 pores in the porous anodized aluminum film be filled with a metal or an
 electrically conductive material made of an oxyacid salt of transition
 metal or pure water be attached to the inner wall of the pores in the
 porous anodized aluminum film.
 The charging member of the present invention may comprise a surface
 protective layer on the porous anodized aluminum film. The surface
 protective layer preferably comprises an organic high molecular compound
 or inorganic high molecular compound having electrically conductive fine
 particles dispersed therein. Alternatively, the surface protective layer
 may comprise an abrasive dispersed therein.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention will be further described hereinafter.
 FIGS. 1 and 2 each illustrate a sectional view of a typical example of the
 charging member of the present invention. In FIGS. 1 and 2, the reference
 number 1 indicates an aluminum support on which a porous anodized aluminum
 film 2 is formed. The porous anodized aluminum film 2 has a surface
 protective layer 3 formed thereon. In FIG. 1, the pores in the porous
 anodized aluminum film are filled with a deposited metal 4, and the
 surface protective layer has electrically conductive fine particles 5
 dispersed therein. In FIG. 2, an electrically conductive material 6 made
 of an oxyacid salt of a transition metal is attached to the inner wall of
 the pores in the porous anodized aluminum film, and the surface protective
 layer has electrically conductive fine particles 5 and an abrasive 7
 dispersed therein.
 FIGS. 3 and 4 each illustrate a schematic diagram of an electrophotographic
 duplicating machine employing a charging member of the present invention.
 In FIG. 3, a charging member in the form of charging roll is used. In FIG.
 4, a blade type charging member is used. Referring to FIG. 3, a charging
 roll 10 comes into contact with a drum-shaped photoreceptor 11. A voltage
 having a DC voltage superimposed on an AC voltage from a power supply 12
 is applied to the charging roll so that the surface of the photoreceptor
 is uniformly charged. A latent image is formed on the surface of the
 photoreceptor by a means of exposure such as LED (Laser Emitted Diode) and
 LD (Laser Diode) while being rotated in the direction of the arrow. The
 latent image is then developed by a means of development 14. The image
 thus developed is then transferred to a paper by a means of transfer 15 to
 form an image thereon. The photoreceptor is then cleaned by a means of
 cleaning 16 to prepare for the subsequent operation. In FIG. 4, a charging
 blade 17 comes into contact with a drum-shaped photoreceptor 11. A DC
 voltage from a power supply 18 is applied to the charging blade so that
 the surface of the photoreceptor is uniformly charged. The reference
 number 19 indicates a means of discharging. Like remaining numerals refer
 to similar elements in FIG. 3.
 The charging member of the present invention may be in the form of roll or
 blade. Referring to roll-shaped charging member, a porous anodized
 aluminum film is formed on a pipe-shaped support having a diameter of 5 to
 50 mm. The porous anodized aluminum film is preferably configured such
 that a metal is deposited to a predetermined height in the pores or an
 electrically conductive material made of an oxyacid salt of a transition
 metal or pure water is attached to the inner wall of the pores.
 As the support employable in the present invention there may be used a
 material made of aluminum or aluminum alloy at least in the surface
 thereof (hereinafter referred to as "aluminum-surfaced support"). In order
 to obtain an anodized aluminum film having excellent properties, as the
 aluminum material there may be used pure aluminum as well as aluminum
 alloy material such as Al-Mg alloy, Al-Mg-Si alloy, Al-Mg-Mn alloy, Al-Mn
 alloy, Al-Cu-Mg alloy, Al-Cu-Ni alloy, Al-Cu alloy, Al-Si alloy, Al-Cu-Zn
 alloy, Al-Cu-Si alloy, Al-Mg-Cu-Zn alloy and Al-Mg-Zn alloy. The aluminum
 material contains aluminum in an amount of 90 wt % or more, preferably 95
 wt % or more. These materials may be properly selected. Further, a
 double-structured material having an aluminum alloy layer on stainless
 steel can be used.
 Further referring to the anodization for forming a porous anodized aluminum
 film on a support, the support is planished (mirror finished), and then
 degreased to completely remove oil contents therefrom. Subsequently, the
 support is anodized to form a porous anodized aluminum film thereon.
 The anodization can be carried out as follows. In some detail, an
 electrolytic solution (anodizing solution) is charged into an electrolytic
 bath (anodizing bath) made of stainless steel or hard glass to a
 predetermined liquid level. As the electrolytic solution there is used a 1
 to 30 wt. % acidic aqueous solution of an inorganic polybasic acid
 selected from the group consisting of sulfuric acid, phosphoric acid and
 chromic acid or an organic monobasic or polybasic acid selected from the
 group consisting of oxalic acid, malonic acid and tartaric acid. Examples
 of pure water to be used as a solvent include distilled water and
 ion-exchanged water. In particular, it is necessary that impurities such
 as chlorine content be thoroughly removed to inhibit the corrosion or
 pinholing of the anodized aluminum film and hence obtain a good quality
 film.
 In the electrolyte are then dipped the foregoing aluminum-surfaced support
 as an anode and a stainless steel plate or aluminum plate as a cathode
 with a predetermined distance therebetween. The distance between the two
 electrodes is properly determined to 0.1 to 100 cm. A DC power supply is
 then prepared. The positive (plus) terminal of the DC power supply is
 connected to the aluminum-surfaced support while the negative (minus)
 terminal is connected to the cathode plate. A voltage is then applied
 across the anode and the cathode in the electrolyte. The electrolysis is
 normally effected by a constant current process or constant voltage
 process. The voltage to be applied may consist of DC component or DC
 component and AC component which are superimposed to one another.
 The current density during anodization is predetermined to 0.1 to 10
 A.dm.sup.-2. In the light of rate of film growth and cooling efficiency,
 it is preferably predetermined to 0.5 to 3.0 A.dm.sup.-2. The anodizing
 voltage is normally in the range of 3 to 150 V, preferably 7 to 100 V. The
 liquid temperature of the electrolyte is predetermined to -5 to 90.degree.
 C.
 In the light of production efficiency, production rate and film properties,
 one of the most preferred embodiments of the present invention comprises
 the anodization in a 10 to 20 wt. % aqueous solution of sulfuric acid at a
 temperature of 5 to 25.degree. C.
 The passage of electric current under the foregoing conditions allows the
 formation of a porous anodized aluminum film on the aluminum surface of
 the support as the anode.
 The porous anodized aluminum film has a rate of pore area (rate of the
 total pore area per unit area) of 10 to 70%, preferably 20 to 60%, and has
 an average pore size of 2 to 90 nm, preferably 5 to 50 nm.
 The thickness of the porous anodized aluminum film can be controlled by
 varying the electrolysis time. It is normally predetermined to 1 to 100
 .mu.m, preferably 10 to 50 .mu.m. If the film thickness falls below 1
 .mu.m, a uniform resistivity can hardly be obtained or pinholes can easily
 occur. On the contrary, if the film thickness exceeds 100 .mu.m, it raises
 the production cost and easily produces nonuniformity on the surface of
 the film. The anodized aluminum film thus formed is then washed with pure
 water.
 Subsequently, a secondary electrolysis is effected so that a metal is
 deposited in the pores in the porous anodized aluminum film. The
 electrical conductivity of the metal thus deposited contributes to the
 control of the resistivity, making the charging member more functional.
 The metal with which the pores are filled preferably comprises at least
 one selected from the group consisting of Fe, Ni, Co, Sn, Cu and Zn. In
 order to electrodepositing these metals, as the electrolyte there is used
 a solution containing a salt of at least one selected from the group
 consisting of Fe, Ni, Co, Sn, Cu and Zn and an inorganic or organic ion
 which serves as a complexing agent with these metals. An AC current or
 equivalent electric current is used as cathodic current component as
 viewed from the specimen to effect electrolysis.
 As the metal salts to be incorporated in the electrolyte, sulfates such as
 ferric ammonium sulfate, nickel sulfate, cobalt sulfate, stannous sulfate,
 copper sulfate and zinc sulfate are economically advantageous. Any metal
 salts which dissociate into the foregoing metallic ions can be used.
 Examples of the inorganic ion-forming substance which serves as a
 complexing agent with the foregoing metals include boric acid, sulfamic
 acid, and ammonium sulfate. Examples of the organic ion-forming substance
 which serves as a complexing agent with the foregoing metals include
 citric acid, tartaric acid, phthalic acid, malonic acid, and malic acid.
 The electric resistivity of the charging member can be controlled by the
 filling depth of the deposited metal from the bottom of the pore in the
 porous anodized aluminum film. The filling depth of the deposited metal
 from the bottom of the pore is in the range of 1/100 to 1/2, preferably
 1/50 to 1/2 of the depth of the pore.
 The porous anodized aluminum film thus formed is then washed with
 ion-exchanged water or pure water.
 In the case where an electrically conductive material made of an oxyacid
 salt of a transition metal is used, a support on which a porous anodized
 aluminum film has been formed may be dipped in an aqueous solution of an
 oxyacid salt of a transition metal for 1 minute to 10 hours, preferably 5
 minutes to 5 hours, optionally followed by the reduction of the attached
 substance, to allow the electrically conductive material to be attached to
 the inner wall of the pores.
 In the case where pure water is attached to the inner wall of the pores,
 the support may be dipped in deionized water or pure water for 1 minute to
 10 hours, preferably 5 minutes to 5 hours.
 As the oxyacid salt of a transition metal to be used to cause an
 electrically conductive material to be attached to the inner wall of the
 pores there may be preferably used an oxyacid salt of at least one
 selected from the group consisting of W, Mo, Cr and Mn. The oxyacid salt
 may be in the form of hydrogen salt of oxyacid, ammonium salt of oxyacid
 or alkaline metal salt of oxyacid. The dipping temperature is preferably
 in the range of 10 to 70.degree. C.
 The porous anodized aluminum film to which an electrically conductive
 material made of an oxyacid salt of a transition metal has been attached
 is then thoroughly washed with ion-exchanged water or distilled water so
 that the dipping solution is not brought to the subsequent step. The
 porous anodized aluminum film may be then optionally dipped in an aqueous
 solution containing a reducing agent. As the reducing agent there may be
 used a stannous solution, L-ascorbic acid solution or the like.
 Subsequently, a surface protective layer may be formed on the porous
 anodized aluminum film as necessary. The surface protective layer
 comprises an organic high molecular compound or inorganic high molecular
 compound having electrically conductive fine particles or an abrasive
 dispersed therein.
 Examples of the organic high molecular compound in which electrically
 conductive fine particles are to be dispersed include polyamide resins,
 polyacrylate resins, polyester resins, phenolic resins, acrylic resins,
 polyurethane resins, epoxy resins, silicone resins, urethane rubbers,
 silicone rubbers, NBR (nitrile butadiene rubber), CR (chloroprene rubber),
 polystyrenes such as SBR (styrene-butadiene rubber), polyisoprene, natural
 rubber, polybutadiene, EPDM (ethylene-propylene-diene polymer), RB
 (butadiene resin) and SBS (styrene-butadiene-styrene elastomer),
 thermoplastic elastomers such as polyolefin, polyester, polyurethane and
 PVC, polyurethane, polystyrene, PE (polyethylene), PP (polypropyrene), PVC
 (polyvinyl chloride), styrene-vinyl acetate copolymer,
 butadiene-acrylonitrile copolymer resin, vinyl acetate emulsion, vinyl
 acetate latex, acryl emulsion, natural rubber latex, isoprene rubber
 latex, butadiene rubber latex, and styrene-butadiene rubber latex.
 Examples of the inorganic high molecular compound in which electrically
 conductive fine particles are to be dispersed include glass, silicon
 oxide, zirconium oxide, titanium oxide and aluminum oxide. The inorganic
 high molecular compound can be formed by a gas phase method with such as
 plasma CVD and sputtering, or by coating a hydrolysate of a compound
 containing a hydrolyzable group such as alkoxy group of organic silicon
 compounds, organic zirconium compounds, organic titanium compounds and
 organic aluminum compound, followed by curing.
 As the electrically conductive fine particles to be dispersed in the
 surface protective layer there may be preferably used one having a
 particle size of not greater than 5 .mu.m and a specific volume
 resistivity of not greater than 10.sup.9 .OMEGA..cm. For example, fine
 particles of a metal oxide such as tin oxide, titanium oxide, zinc oxide,
 CeO.sub.2, ZrO.sub.2 and InO.sub.3 or alloy thereof, or particulate
 BaSO.sub.4 or TiO.sub.2 coated with such a metal oxide, or carbon black
 may be used. The resistivity control by the electrically conductive fine
 particles can prevent the resistivity of the surface protective layer from
 being varied with the environmental conditions to obtain stable
 properties. The electrically conductive fine particles may be used in an
 amount of 5 to 95 wt %, preferably 40 to 80 wt % based on the total amount
 of the surface protective layer. Further, an insulating abrasive such as
 alumina, silica, clay, kaolin, SiC, Si.sub.3 N.sub.4, BaSO.sub.4,
 CaCO.sub.3, MgCO.sub.3 and FeO.sub.2 may be incorporated in the surface
 protective layer to provide the surface of the roll with unevenness and
 hence reduce the burden developed during friction with the photoreceptor,
 making it possible to the abrasion resistance of the roll with the
 photoreceptor. On the contrary, the photoreceptor may be positively worn
 to inhibit dilation (blurred image). The abrasive may be used in an amount
 of 5 to 95 wt %, preferably 40 to 80 wt % based on the total amount of the
 surface protective layer.
 The surface protective layer may further comprise fluorinic or siliconic
 resins or grains incorporated therein to render the surface thereof
 hydrophobic, preventing foreign matters from being attached to the surface
 of the roll. Moreover, the surface protective layer may comprise a
 silicone oil for inhibiting the dilation of the photoreceptor. In order to
 enhance its adhesivity to the porous anodized aluminum film, the surface
 protective layer may comprise a coupling agent incorporated therein.
 The thickness of the surface protective layer is preferably in the range of
 5 to 3,000 .mu.m. The surface protective layer may be constituted of two
 or three layers.
 The surface protective layer can be formed by an ordinary method. For
 example, a method can be employed which comprises dispersing an
 electrically conductive material such as metal oxide (e.g., SnO.sub.2) and
 carbon black and an abrasive such as Al.sub.2 O.sub.3 in a solution of a
 polyamide resin in a solvent such as methanol to make a coating solution,
 applying the coating solution to the external surface of the porous
 anodized aluminum film, and then drying the material.
 The conventional charging members comprise a laminate of an electrically
 conductive rubber layer and a resistive layer formed on a metallic shaft.
 The electrically conductive rubber layer is used to compensate for the
 nonuniformity in the contact with the photoreceptor due to the
 nonuniformity in the molding of the metallic shaft. Since the charging
 member of the present invention is essentially free of rubber layer, a
 surface protective layer is preferably formed on the surface of the
 charging member to compensate for the nonuniformity in the contact with
 the photoreceptor. The thickness of the protective layer is in the range
 of 1 to 300 .mu.m, preferably 10 to 150 .mu.m, most preferably 50 to 130
 .mu.m. A photoreceptor which can be charged by the charging member
 according to the present invention may be any of an azo type,
 phthalocyanine type, squarylium type or perylene type organic
 photoreceptor and an inorganic photoreceptor such as Se, CdTe, CdSe, a-Si
 and A-C. Amorphous silicon can be advantageously used as a photoreceptor
 to enhance the maximum allowable contact pressure against the charging
 member.
 The present invention will be further described in the following examples
 and comparative examples, but the present invention should not be
 construed as being limited thereto.
 Example 1
 A 12-mm diameter aluminum rod made of an Al-Mg alloy was washed with an
 aqueous solution of a degreasing agent and then with pure water.
 As a primary aqueous electrolyte there was prepared an aqueous solution of
 150 g of H.sub.2 SO.sub.4 and 25 g of Al.sub.2 (SO.sub.4).sub.3.14 to
 18H.sub.2 O in 1,000 ml of water. A 1.0 A constant DC voltage (10 V) was
 applied across the aluminum rod and the aluminum cathode for 10 minutes to
 effect electrolysis. As a result, a porous anodized aluminum film having a
 thickness of 5 .mu.m was formed.
 Example 2
 In the same manner as in Example 1, a 12-mm diameter aluminum rod made of
 an Al-Mg alloy was washed with an aqueous solution of a degreasing agent
 and then with pure water.
 As a primary aqueous electrolyte there was prepared an aqueous solution of
 180 g of H.sub.2 SO.sub.4 and 30 g of Al.sub.2
 (SO.sub.4).sub.3.multidot.14 to 18H.sub.2 O in 1,000 ml of water. A 1.0
 constant DC voltage (10 V) was applied across the aluminum rod and the
 aluminum cathode for 30 minutes to effect electrolysis. As a result, a
 porous anodized aluminum film having a thickness of 15 .mu.m was formed.
 Subsequently, the aluminum rod was thoroughly washed with distilled water.
 The aluminum rod was then subjected to secondary electrolysis with respect
 to a carbon electrode. In some detail, as a secondary aqueous electrolyte
 there was prepared an aqueous solution of 60 g of
 CoSO.sub.4.multidot.7H.sub.2 O, 24 g of H.sub.3 BO.sub.3 and 6 g of
 (NH.sub.4).sub.2 SO.sub.4 in 1,000 ml of water. A 20 V AC voltage was then
 applied across the two electrodes for 30 minutes. In this manner, Co was
 deposited in the pores.
 50-.mu.m thick surface protective layer was then formed on the porous
 anodized aluminum film thus treated. The surface protective layer was
 formed as follows. In some detail, to 45 parts by weight of a copolymer
 nylon, 55 parts by weight of electrically conductive fine particles having
 a particle size of 0.5 .mu.m comprising particulate BaSO.sub.4 coated with
 tin oxide (PASTRAN, available from Mitsui Mining & Smelting Co., Ltd.),
 and 5 parts by weight of an abrasive Al.sub.2 O.sub.3 was added methanol
 as a solvent. The mixture was then subjected to dispersion by means of a
 sand grinder mill for about 1 hour to obtain a resin solution for the
 formation of a surface protective layer. The resin solution was then
 viscosity-modified. The resin solution was then charged into a dipping
 tank as a dipping solution. In the dipping solution was then dipped the
 aluminum rod so that the aluminum rod was coated with the resin solution.
 The material was dried at a temperature of 130.degree. C. for 10 minutes,
 and then desolvented to form a surface protective layer on the porous
 anodized aluminum film. Thus, a desired charging member was obtained.
 Example 3
 A charging member was prepared in the same manner as in Example 2 except
 that instead of depositing a metal by secondary electrolysis, the porous
 anodized aluminum film was dipped in an aqueous solution of 10 g of
 (NH.sub.4).sub.6 Mo.sub.7 O.sub.2.4H.sub.2 O in 1,000 ml of water for 20
 minutes so that molybdic ions (VI) were attached to the inner wall of the
 pores in the film.
 Example 4
 A charging member was prepared in the same manner as in Example 2 except
 that the charging member thus prepared was dipped in distilled water for
 10 minutes, and then dried at room temperature.
 Comparative Example 1
 A charging member was prepared in the same manner as in Example 1 except
 that a surface protective layer was directly formed on an aluminum rod in
 the same manner as in Example 2 without forming an anodized aluminum film
 thereon.
 Comparative Example 2
 A charging member was prepared in the same manner as in Example 1 except
 that the anodized aluminum film was replaced by an electrically conductive
 elastic layer obtained by uniformly winding an EPDM rubber composition on
 the aluminum rod and a surface protective layer was formed on the
 electrically conductive elastic layer in the same manner as in Example 2.
 These charging members were each mounted on a electrophotographic printer.
 An AC voltage having a DC voltage component superimposed thereon was then
 applied across the charging member while being rotated together with an
 amorphous silicon photoreceptor drum in contact therewith so that the
 photoreceptor was charge. In this manner, an image was repeatedly formed.
 This test was intermittently conducted in an atmosphere of high
 temperature and high humidity (28.degree. C./80% R.H.) and in an
 atmosphere of low temperature and low humidity (10.degree. C./15% R.H.).
 As a result, the use of the charging members of Examples 1 to 4 resulted
 in the formation of a good quality image even in the 20,000th sheet. In
 particular, the charging member of Example 4 exhibited a small
 environmental dependence and provided a predetermined potential and a good
 quality image at a low current in any environment. While its mechanism is
 unknown, it is thought that water ion takes part in the phenomenon of
 charging, enabling the simultaneous occurrence of discharging and
 injection.
 On the contrary, when the charging member of Comparative Example 1 was
 used, an image defect (black band) due to pinhole leak occurred
 continuously after several copies. It is thought that the surface
 protective layer cannot completely cover up metal protrusions and the
 voltage applied leaks through the metal protrusions to cause
 shortcircuiting. On the other hand, when the charging member of
 Comparative Example 2 was used, it provided a normal image in an
 atmosphere of high temperature and high humidity. In an atmosphere of low
 temperature and low humidity, however, it provided an image having a low
 contrast at some portions (particularly at edges). The measurement of the
 resistivity of the electrically conductive rubber layer showed that the
 rubber layer has a high resistivity at some portions. This possibly causes
 the low contrast.
 As mentioned above, the charging member of the present invention comprises
 a porous anodized film formed on the surface of a support. Therefore, no
 fluctuations of resistivity occur within the plane and from lot to lot
 during the preparation. The charging member of the present invention can
 hardly be worn, causing no foregoing matters to be attached to the
 photoreceptor. Thus, the charging member of the present invention exhibits
 an excellent durability and a long life. It is also insusceptible to
 pinhole leak. Further, even if the environmental conditions are altered,
 the charging member of the present invention exhibits a stable
 resistivity, causing no contamination on the photoreceptor. Accordingly,
 the charging member of the present invention can be advantageously used to
 charge amorphous silicon photoreceptors.
 While the invention has been described in detail and with reference to
 specific embodiments thereof, it will be apparent to one skilled in the
 art that various changes and modifications can be made therein without
 departing from the spirit and scope thereof.