Patent Publication Number: US-2009223601-A1

Title: Chemical conversion treatment agent and surface-treated metal material

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a chemical conversion treatment agent and a surface-treated metal material. 
     2. Background Art 
     A metal material, such as a galvanized steel sheet, is widely used in vehicle bodies and others. In such cases, the metal material is generally subjected to a coating process, such as a cationic electrodeposition coating process. Further, in advance of the cationic electrodeposition coating process, the metal material, i.e., a workpiece to be coated, is subjected to a chemical conversion treatment as a pretreatment. In this chemical conversion treatment, a chemical conversion treatment agent comprising a primary component of zinc phosphate (i.e., zinc phosphate-based treatment agent) is often used as a chemical conversion treatment agent. The workpiece subjected to the chemical conversion treatment using the zinc phosphate-based treatment agent makes it possible to improve the quality of an electrodeposition coating film to be obtained by the cationic electrodeposition coating process (hereinafter also referred to simply as “coating film”). That is, in the cationic electrodeposition coating process, excellent electrodeposition coatability (film thickness characteristic of a coating film) can be obtained. 
     However, the zinc phosphate-based treatment agent has a problem that phosphate ions thereof cause eutrophication. Moreover, the chemical conversion treatment using the zinc phosphate-based treatment agent involves a problem about production of sludge to be wasted (i.e., waste sludge). With a view to solving these problems, there has been proposed a metal oxide-type chemical conversion treatment agent which comprises: at least one selected from the group consisting of zirconium, titanium and hafnium; fluorine; and a water-soluble resin (see, for example, JP 2004-218074A (U.S. Patent Application Publication No. 20040163735)). 
     A chemical conversion treatment agent comprising a primary component of a zirconium compound is currently under development. This zirconium compound-based treatment agent is not only capable of solving the above problems with the zinc phosphate-based treatment agent, but also relatively excellent in terms of cost and quality. 
     However, when a workpiece is subjected to a chemical conversion treatment using the zirconium compound-based treatment agent, a chemical conversion film will be formed in which the number of local low-resistance areas each having an insulation resistance value of 5000Ω or less is 20 or less/100 μm 2 . That is, a chemical conversion film having a relatively small number of the local low-resistance areas i.e., a relatively low electrical conductivity, as compared with a chemical conversion film formed using the zinc phosphate-based treatment agent, will be formed on a surface of the workpiece. 
     Then, in an electrodeposition coating process, a relatively high voltage will be applied between an anode, and a portion of the workpiece adjacent to the anode (in a vehicle body, an outer panel), whereas a relatively low voltage will be applied between the anode, and a portion of the workpiece far from the anode (in the vehicle body, an inner panel), as a phenomenon specific to the electrodeposition coating process. In this situation, if a chemical conversion film having a relatively low electrical conductivity is formed on the workpiece, a deposition amount of coating film is liable to decrease in the portion of the workpiece far from the anode and belonging to a low voltage-applied region. 
     Therefore, a workpiece having a chemical conversion film formed using the zirconium compound-based treatment agent involves a problem that a deposition amount of coating film decreases in a portion of the workpiece far from an anode and belonging to a low voltage-applied region (in a vehicle body, an inner panel), as compared with a workpiece having a chemical conversion film formed using the zinc phosphate-based treatment agent, as will be described later (see  FIG. 2 ). 
     SUMMARY OF THE INVENTION 
     In view of the above circumstances, it is an object of the present invention to provide a chemical conversion treatment agent capable of forming a chemical conversion film which allows a high-quality coating film to be formed by a subsequent cationic electrodeposition coating process, even if it contains a film-forming component to form only a relatively small number of local low-resistance areas in the chemical conversion film. That is, it is an object of the present invention to provide a chemical conversion treatment agent capable of improving electrodeposition coatability in a portion of a workpiece belonging to a low voltage-applied region, even if it forms a chemical conversion film having a relatively small number of local low-resistance areas. 
     It is another object of the present invention to provide a surface-treated metal material having a chemical conversion film formed using the above chemical conversion treatment agent. 
     In order to achieve the above objects, according to one aspect of the present invention, there is provided a chemical conversion treatment agent which comprises: a film-forming component to form a chemical conversion film in which the number of local low-resistance areas each having an insulation resistance value of 5000Ω or less is 20 or less/100 μm 2 ; and at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles. 
     In the first aspect of the present invention, the chemical conversion treatment agent can form a chemical conversion film which allows a high-quality coating film to be formed by a subsequent cationic electrodeposition coating process. Specifically, the chemical conversion treatment agent can form a chemical conversion film which allows a formation (deposition) of a coating film in a subsequent cationic electrodeposition coating process to be accelerated so as to improve electrodeposition coatability in a portion of a workpiece belonging to a low voltage-applied region. The reason is considered to be that, even if a chemical conversion film having a relatively small number of local low-resistance areas is formed by the chemical conversion treatment agent, the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles is contained in the chemical conversion film, and the number of local electrical-conductive areas can be increased during voltage application in the electrodeposition coating process, by utilizing a tunneling effect based on the fine particles. 
     According to a second aspect of the present invention, there is provided a surface-treated metal material which comprises a chemical conversion film formed using a chemical conversion treatment agent. The chemical conversion treatment agent includes: a film-forming component to form a chemical conversion film in which the number of local low-resistance areas each having an insulation resistance value of 5000Ω or less is 20 or less/100 μm 2 ; and at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles. In the surface-treated metal material, the chemical conversion film is codeposited with the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles. 
     In the second aspect of the present invention, the surface-treated metal material can have a high-quality coating film formed by a cationic electrodeposition coating process. Specifically, the surface-treated metal material has a coating film excellent in electrodeposition coatability even in a portion of a workpiece belonging to a low voltage-applied region. The reason is considered to be that, even if a chemical conversion film formed on a surface of the surface-treated metal material has a relatively small number of local low-resistance areas, the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles is contained in the chemical conversion film, and the number of local electrical-conductive areas can be increased during voltage application in an electrodeposition coating process, by utilizing a tunneling effect based on the fine particles. 
     These and other objects, features, aspects and advantages of the invention will become more apparent upon reading the following detailed description along with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram schematically showing a cationic electrodeposition coating process. 
         FIG. 2  is a graph showing a film thickness characteristic of a coating film formed on each of a conventional ZrO 2  film and a zinc phosphate film. 
         FIG. 3  is an explanatory diagram conceptually showing low-resistance areas of a zinc phosphate film. 
         FIG. 4  is an explanatory diagram conceptually showing deposition of a coating film on the low-resistance areas of the zinc phosphate film. 
         FIG. 5  is a top plan view conceptually showing an initial stage of deposition of a coating film on the low-resistance areas of the zinc phosphate film. 
         FIG. 6  is a top plan view conceptually showing an intermediate stage of the deposition of the coating film on the low-resistance areas of the zinc phosphate film. 
         FIG. 7  is a front view conceptually showing a last stage of the deposition of the coating film on the low-resistance areas of the zinc phosphate film. 
         FIG. 8  is an explanatory diagram conceptually showing low-resistance areas of a ZrO 2  film. 
         FIG. 9  is an explanatory diagram conceptually showing deposition of a coating film on the low-resistance areas of the ZrO 2  film. 
         FIG. 10  is a top plan view conceptually showing an initial stage of deposition of a coating film on the low-resistance areas of the ZrO 2  film. 
         FIG. 11  is a top plan view conceptually showing an intermediate stage of the deposition of the coating film on the low-resistance areas of the ZrO 2  film. 
         FIG. 12  is a front view conceptually showing a last stage of the deposition of the coating film on the low-resistance areas of the ZrO 2  film. 
         FIG. 13  is a graph showing a film thickness characteristic of a coating film formed on each of a ZrO 2  film codeposited with n-type ZnO, a conventional ZrO 2  film, and a zinc phosphate film. 
         FIG. 14  is an explanatory diagram conceptually showing deposition of a coating film on the ZrO 2  film codeposited with n-type ZnO. 
         FIG. 15  is a graph showing a current density distribution during non-voltage application, in each of the conventional ZrO 2  film, and the ZrO 2  film codeposited with n-type ZnO. 
         FIG. 16  is a graph showing a current density distribution during voltage (1 V) application, in the conventional ZrO 2  film. 
         FIG. 17  is a graph showing a current density distribution during voltage (1 V) application, in the ZrO 2  film codeposited with n-type ZnO. 
         FIG. 18  is a table showing an influence of a content ratio of n-type ZnO (semiconductor fine particles) to the ZrO 2  film codeposited with n-type ZnO, on a film thickness characteristic (electrodeposition coatability) and corrosion resistance. 
         FIG. 19  is an explanatory graph showing a technique of determining an upper limit of an amount (mass %) of n-type ZnO in view of corrosion resistance. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, the present invention will now be described based on one embodiment thereof. 
     Firstly, a cationic electrodeposition coating process will be described. 
     Typically, in a coating process for vehicles, a chemical conversion film is formed on a workpiece by a chemical conversion treatment (a surface-treated metal material is formed), and then a cationic electrodeposition coating film (undercoating film) is formed on the surface-treated metal material. 
       FIG. 1  is an explanatory diagram schematically showing a cationic electrodeposition coating process. As shown in  FIG. 1 , in the cationic electrodeposition coating process, a workpiece (e.g., vehicle body) W is firstly immersed in a cationic electrodeposition coating material (cationic electrodeposition paint) contained in a tank T. Then, a voltage is applied between the tank T and the workpiece W under a condition that the tank T and the workpiece W are set as an anode and a cathode, respectively. As a result, a coating film is formed (deposited) on a surface of the workpiece W. The cationic electrodeposition coating film can be improved in terms of electrodeposition coatability and adhesion as well as corrosion resistance by a chemical conversion film formed on a surface of the workpiece W before the cationic electrodeposition coating process. 
     The chemical conversion film is formed by a chemical conversion treatment using a chemical conversion treatment agent. 
     The chemical conversion treatment agent according to this embodiment of the present invention comprises: a film-forming component to form a chemical conversion film in which the number of local low-resistance areas each having an insulation resistance value of 5000Ω or less is 20 or less/100 μm 2 ; and at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles. 
     Preferably, the film-forming component contains a compound having at least one selected from the group consisting of Zr, Ti, Hf and Si, as a primary component, and further contains an etching agent, such as fluorine, and a water-soluble resin, as a secondary component. Based on this chemical conversion treatment agent, the chemical conversion film will comprise a primary component consisting of an oxide of at least one selected from the group consisting of Zr, Ti, Hf and Si. 
     More specifically, in this embodiment, H 2 ZrF 6  as a zirconium compound is used as the primary component of the film-forming component, and a chemical conversion film comprising a primary component of a zirconium oxide (hereinafter expressed as “ZrO 2 ”) will be formed on a workpiece to be coated (the chemical conversion film will hereinafter be referred to as “ZrO 2  film”). That is, the workpiece is formed as a surface-treated metal material having a ZrO 2  film on a surface thereof. 
     Through a chemical conversion treatment using the above chemical conversion treatment agent, the workpiece (metal blank (steel sheet)) is dissolved (etched) by acid, so that hydroxide ions will be formed on a surface of the workpiece to increase a pH value of the surface. Along with the increase in pH value, zirconium hydroxide is deposited, and converted to ZrO 2  through dehydration/condensation reactions. 
     For example, in the chemical conversion treatment, reactions expressed as the following reaction formulas (1) to (6) are developed: 
       Fe→Fe 2+ +2e −   (1) 
       2H 2 O+2e − →2OH − +H 2    (2) 
       ZrF 6   2− +4OH − →Zr(OH) 4 +6F −   (3) 
       Zr(OH) 4 →ZrO 2 +2H 2 O   (4) 
       2H 2 O+2F 2 →4HF+O 2    (5) 
       HF+H 2 O→H 3 O + +F −   (6) 
     For example, the above reaction formulas can be collectively expressed as the following formulas (7) and (8): 
     Etching of steel sheet 
       Fe+3HF→FeF 3 +3/2H 2    (7) 
     Deposition of film 
       H 2 ZrF 6 +2H 2 O→ZrO 2 +6HF   (8) 
     The reason for using the ZrO 2  film as the chemical conversion film is that the ZrO 2  film can prevent eutrophication and suppress the production of waste sludge associated with the chemical conversion treatment while ensuring corrosion resistance, as compared with a zinc phosphate film formed using a zinc phosphate-based treatment agent. More specifically, there has heretofore been known a zinc phosphate film formed using a zinc phosphate-based treatment agent, as a chemical conversion film excellent in corrosion resistance, adhesion of a coating film, etc. However, the use of the zinc phosphate film involves problems that phosphate ions of the zinc phosphate-based treatment agent cause eutrophication, and waste sludge is produced along with the chemical conversion treatment. Therefore, in this embodiment, a chemical conversion treatment agent is selected in such a manner as to form the ZrO 2  film as a chemical conversion film capable of preventing the above problems while ensuring corrosion resistance, etc. In this regard, it is known that the ZrO 2  film has a property of being formed as a continuous noncrystalline (amorphous) film, and thereby the number of local low-resistance areas each having an insulation resistance value of 5000Ω or less tends to decrease, specifically, become 20 or less/100 μm 2 . 
     In this embodiment, the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles is contained in (codeposited with) the ZrO 2  film as the chemical conversion film formed using the chemical conversion treatment agent. The reason is that there is a need for correcting the following disadvantage of the ZrO 2  film. While the ZrO 2  film can prevent the problems about eutrophication and sludge production, a film thickness characteristic (electrodeposition coatability) of a coating film based on the ZrO 2  film itself is inferior to that of the zinc phosphate film (see  FIG. 2 ), because the ZrO 2  film has a relatively small number of the local low-resistance areas, as mentioned above. 
     The present invention will be more specifically described below. 
     In a cationic electrodeposition coating process, as one characteristic thereof, a relatively high voltage will be applied between an anode (in  FIG. 1 , the tank T) and a portion of a workpiece W adjacent to the anode (in a vehicle body, an outer panel), whereas a relatively low voltage will be applied between the anode and a portion of the workpiece W far from the anode (in the vehicle body, an inner panel), as shown in  FIG. 1 . Thus, formation (deposition) of a cationic electrodeposition coating film is initiated from the portion of the workpiece W adjacent to the anode. The deposited coating film has electrical insulation properties, and therefore an electrical resistance of the deposited coating film becomes higher as an amount of the deposited coating film is increased along with progress of the deposition of the coating film. Consequently, the deposition of the coating film onto the portion having the deposited coating film will be gradually reduced, and instead deposition of the coating film onto a portion having no deposited coating film will be initiated. 
     During a course of the cationic electrodeposition coating process, as shown in  FIG. 2 , if a conventional ZrO 2  devoid of any of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles is formed on a workpiece (e.g., cold-rolled steel sheet), a film thickness of a cationic electrodeposition coating film is liable to become excessively small in a low voltage (about zero to 70 V)-applied region, and to become excessively large in a high voltage (70 V or more)-applied region, as compared with a workpiece having a zinc phosphate film formed thereon.  FIG. 2  is a graph showing a relationship between a film thickness of a cationic electrodeposition coating film and an applied voltage, in each of two types of workpieces on which a conventional ZrO 2  film and a zinc phosphate film are formed, respectively, as a chemical conversion film. In  FIG. 2 , the vertical axis represents a film thickness (μm) of a cationic electrodeposition coating film, wherein a larger film thickness indicates a capability to obtain more excellent corrosion resistance, and the horizontal axis represents a voltage (V) to be applied to a surface-treated metal material, wherein a low voltage-applied region, i.e., a region applied with a relatively low voltage may be considered to be a portion of the workpiece (surface-treated metal material) far from an anode, e.g., an inner panel of a vehicle body, and a high voltage-applied region, i.e., a region applied with a relatively low voltage may be considered to be a portion of the workpiece adjacent to the anode, e.g., an outer panel of the vehicle body. 
     As seen in  FIG. 2 , in a portion of a workpiece adjacent to the anode and belonging to the high voltage-applied region (in a vehicle body, an outer panel), a film thickness of a coating film in the workpiece having the conventional ZrO 2  film becomes larger than that of a coating film in the workpiece having the zinc phosphate film. Further, in a portion of a workpiece far from the anode and belonging to the low voltage-applied region (in a vehicle body, an inner panel), the film thickness of the coating film in the workpiece having the conventional ZrO 2  film becomes smaller than that of the coating film in the workpiece having the zinc phosphate film. Thus, the workpiece having the conventional ZrO 2  film is inferior in throwing power of a cationic electrodeposition coating film, to the workpiece having the zinc phosphate film. 
     Through various researches on the above controversial phenomenon, the inventors have formulated the following hypothesis. 
     (1) When a surface of a steel sheet S (a surface of a workpiece W) is treated with a zinc phosphate-based treatment agent, a crystalline zinc phosphate film  1  having a large number of pointed-shaped portions lying side-by-side is formed to define a large number of low-resistance areas (lower regions of boundary spaces between respective adjacent ones of the pointed portions)  2 , as shown in  FIG. 3  which is an explanatory diagram conceptually showing a configuration, etc., of the low-resistance areas of the zinc phosphate film. 
     Thus, electrons transfer to each of the low-resistance areas  2 , so that electrolysis occurs on the surface of the steel sheet S to generate hydroxide ions, and acid giving water solubility to a coating material is neutralized by the hydroxide ions. During this process, H 2  is generated. 
     Based on the neutralization of the acid, a coating film F is deposited on the surface of the steel sheet S, as shown in  FIG. 4  which is an explanatory diagram conceptually showing deposition of a cationic electrodeposition coating film in the low voltage-applied region of the workpiece having the zinc phosphate film formed thereon. Thus, formation of the coating film F on the surface of the steel sheet S is promoted even in a portion of the workpiece (steel sheet S) far from the anode and belonging to the low voltage-applied region. 
     In contrast, when a steel sheet S is subjected to a chemical conversion treatment using a zirconium compound-based treatment agent, a ZrO 2  film  21  is formed as a flat continuous noncrystalline film, as shown in  FIG. 8  which is an explanatory diagram conceptually showing low-resistance areas of the conventional ZrO 2  film. Although a local low-resistance area  22  is formed in the conventional ZrO 2  film  21 , the number of the local low-resistance areas  22  is extremely small. Thus, the conventional ZrO 2  film  21  has a relatively low electrical conductivity, and thereby an amount of coating film to be deposited on a portion of the workpiece (steel sheet S) far from the anode and belonging to the low voltage-applied region becomes smaller. 
     (2) A resistance in each of the few local low-resistance areas of the conventional ZrO 2  film  21  is greater than that in each of the low-resistance areas of the zinc phosphate film  1 . Therefore, no current flows through the conventional ZrO 2  film  21  unless a certain level or more of voltage is applied thereto. Thus, as shown in  FIG. 9  which is an explanatory diagram conceptually showing deposition of a cationic electrodeposition coating film in the low voltage-applied region of the workpiece (steel sheet S) having the conventional ZrO 2  film formed thereon, it is more difficult to deposit a coating film F on a portion of the workpiece far from the anode and belonging to the low voltage-applied region, as compared with the workpiece having the zinc phosphate film  1  (see  FIG. 4  for comparison). That is, in the workpiece having the conventional ZrO 2  film  21 , a cationic electrodeposition coating film is unlikely to be deposited on a portion of the workpiece other than the local low-resistance areas  22 , as shown in  FIG. 9 . 
     (3) Further, a resistance in a maximum-resistance area (an area having a maximum film thickness of about 50 nm: see  FIG. 8 )  23  of the conventional ZrO 2  film  21  is less than that in a maximum-resistance area (a pointed area having a maximum film thickness of about 1 to 2 nm: see  FIG. 3 )  3 . Therefore, in the high voltage-applied region, a coating film F is more widely deposited on the conventional ZrO 2  film  21  than on the zinc phosphate film  1 . Thus, in a portion of a workpiece adjacent to the anode and belonging to the high voltage-applied region (in a vehicle body, an outer panel), a film thickness of the coating film F in the workpiece having the conventional ZrO 2  film  21  becomes fairly greater than that of a coating film in the workpiece having the zinc phosphate film  1 . 
       FIGS. 5 ,  6 ,  10  and  11  conceptually show the above hypothesis. Specifically,  FIG. 5  is an explanatory diagram conceptually showing an initial stage of deposition of a cationic electrodeposition coating film in the high voltage-applied region of the workpiece having the zinc phosphate film formed thereon, and  FIG. 6  is an explanatory diagram conceptually showing an intermediate stage of the deposition of the cationic electrodeposition coating film in the high voltage-applied region of the workpiece having the zinc phosphate film formed thereon.  FIG. 10  is an explanatory diagram conceptually showing an initial stage of deposition of a cationic electrodeposition coating film in the high voltage-applied region of the workpiece having the conventional ZrO 2  film formed thereon, and  FIG. 11  is an explanatory diagram conceptually showing an intermediate stage of the deposition of the cationic electrodeposition coating film in the high voltage-applied region of the workpiece having the conventional ZrO 2  film formed thereon. 
     (4) A size (spatial size) of each of the low-resistance areas  2  of the zinc phosphate film is relatively small. Thus, electrolysis occurs in each of the low-resistance areas  2  to generate hydroxide ions, and acid giving water solubility to paint is neutralized by the hydroxide ions. During this process, H 2  is generated. That is, H 2  is continuously generated from the initial stage. Then, when a coating film F is deposited, (the space of) each of the low-resistance areas  2  is easily filled with the coating film F, as shown in  FIG. 7  which is an explanatory diagram conceptually showing a last stage of the deposition of the cationic electrodeposition coating film in the high voltage-applied region of the workpiece having the zinc phosphate film formed thereon. 
     In contrast, each of the few low-resistance areas  22  of the conventional ZrO 2  film  21  is thinner and larger (wider) than the low-resistance area  2  of the zinc phosphate film  1 . Thus, although a coating film F is deposited through concentration of electric charges in the large low-resistance area  22 , generation of hydroxide ions and neutralization of acid giving water solubility to paint by the hydroxide ions (a process of generating H 2 ), the large low-resistance area  22  is not easily filled with the coating film F, as shown in  FIG. 12  which is an explanatory diagram conceptually showing a last stage of the deposition of the cationic electrodeposition coating film in the high voltage-applied region of the workpiece having the conventional ZrO 2  film formed thereon. Therefore, a resistance is not increased along with the deposition of the coating film on the workpiece (steel sheet S) to allow the coating film F to be continuously deposited, so that a film thickness of the coating film F becomes fairly greater than that of a coating film in the workpiece having the zinc phosphate film  1 . This makes it difficult to allow electrons to transfer to a portion of the workpiece far from the anode (in a vehicle body, an inner panel) to which electrons essentially hardly transfer, and thereby no coating film is deposited thereon. 
     Based on the above hypothesis, the inventors have formed a chemical conversion film using the chemical conversion treatment agent according to this embodiment, in such a manner that a ZrO 2  film  21  is employed as a matrix, and at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles is codeposited within the ZrO 2  film  21 . 
     The reason for employing the ZrO 2  film  21  as a matrix is to ensure basic functions, such as corrosion resistance. Further, the reason for codepositing the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles within the ZrO 2  film  21  is to reduce a rate of ZrO 2  so as to reduce an amount of coating film F to be deposited on a portion of a workpiece adjacent to an anode (in a vehicle body, an outer panel), and increase the number of electrical-conductive areas each capable of conducting a current therethrough only when a given value or more of voltage is applied thereto, with a focus on an electron tunneling effect. Based on reducing a rate of ZrO 2 , an amount of coating film F to be deposited on a portion of a workpiece adjacent to an anode (in a vehicle body, an outer panel) can be reduced to prevent excessive deposition of the coating film F. Further, based on electrical conductivity (tunneling effect) of the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles, deposition of the coating film F can be promoted in a portion of the workpiece far from the anode (in a vehicle body, an inner panel). This makes it possible to allow the ZrO 2  film  21  to have a film thickness characteristic (electrodeposition coatability) of a coating film close to that based on a zinc phosphate film  1 . In addition, the ZrO 2  film  21  codeposited with the at least one kind of fine particles can achieve satisfactory corrosion resistance and electrodeposition coatability, as well as being able to prevent the problems about eutrophication and sludge production. 
     As a specific example of the fine particles, the metal fine particles may be fine particles of at least one of Mg, Al, Ca, Co, Ni, Cu and Zn. Preferably, the semiconductor fine particles may be fine particles of an oxide semiconductor, such as ZnO or TiO 2 . Further, an oxide of Ti, Zn, etc., which exhibits semiconductor properties, may be used in the form of ions. The electrically conductive organic fine particles may be polyaniline fine particles or fine particles of a metal protected by an organic matter. The fine particles preferably have an average particle size of 40 nm or less, more preferably 20 to 40 nm. 
     As for the problem with the conventional ZrO 2  film that a film thickness of a coating film in a workpiece having the conventional ZrO 2  film becomes fairly larger than that of a coating film in a workpiece having the zinc phosphate film, in a portion of the workpiece adjacent to the anode and belonging to the high voltage-applied region, and becomes fairly smaller than that of the coating film in the workpiece having the zinc phosphate film, in a portion of the workpiece far from the anode and belonging to the low voltage-applied region, it is contemplated that a size of each of the low-resistance areas  22  of the conventional ZrO 2  film devoid of any of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles is reduced in some way to prevent electric charges from concentrating in the low-resistance area  22 . 
     However, if the size of each of the low-resistance areas  22  is reduced in the above manner, a thickness of the ZrO 2  film will be increased, and no coating film will be deposited unless a voltage for initiating deposition of a coating film is set at a higher value. In contrast, in the ZrO 2  film  21  codeposited with the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles, it is considered that, although each of the low-resistance areas  22  is relatively large, the number of electrical-conductive areas is increased during voltage application by a tunneling effect based on the at least one kind of fine particles, so as to avoid concentration of electric charges in the large low-resistance area  22 . In this respect, the ZrO 2  film  21  can have a film thickness characteristic of a coating film close to that based on a zinc phosphate film  1 , while preventing the aforementioned problems. 
     In order to verify the above hypothesis, a chemical conversion film formed using the chemical conversion treatment agent according to this embodiment, i.e., a ZrO 2  film codeposited with the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles, is compared with the conventional ZrO 2  film and the zinc phosphate. The following description will be made based on one example where the chemical conversion film formed using the chemical conversion treatment agent according to this embodiment is a ZrO 2  film codeposited with n-type ZnO which is semiconductor fine particles. 
       FIG. 13  shows a film thickness characteristic of a coating film formed on the ZrO 2  film codeposited with n-type ZnO (semiconductor fine particles), as chemical conversion film, for the purpose of supporting the above hypothesis. Specifically,  FIG. 13  is a graph showing a relationship between a film thickness of a cationic electrodeposition coating film and an applied voltage, in each of three types of workpieces on which the ZrO 2  film codeposited with n-type ZnO, the conventional ZrO 2  film and the zinc phosphate film are formed, respectively, as a chemical conversion film. In  FIG. 13 , the vertical axis and the horizontal axis are the same as those in  FIG. 2 . 
     In this example, a content ratio of the n-type ZnO to the chemical conversion film was 5.6 mass %, and the n-type ZnO had the following composition and characteristics.
         Composition: Ga-doped ZnO   Volume resistivity: 20 to 100 (Ω·cm)   Specific surface area: 30 to 50 (m 2 /g)   Average particle size (primary particle size): 20 to 40 (nm)       

     The chemical conversion film containing the n-type ZnO in an amount of 5.6 mass % was formed using a chemical conversion treatment agent which has a composition consisting of 100 ppm of H 2 ZrF 6  as a metal equivalent to Zr, 7 of fluorine in a mole ratio to Zr, and 50 ppm of amino group-containing alkoxysilane as a water-soluble resin, in solid concentration, with the remainder being a solvent (consisting primarily of water). Each of the conventional ZrO 2  film and the zinc phosphate film was formed using a conventional chemical conversion treatment agent. 
     As seen in  FIG. 13 , the ZrO 2  film codeposited with the n-type ZnO as the semiconductor fine particles had a film thickness characteristic (electrodeposition coatability) of a coating film close to that based on the zinc phosphate film  1 . Specifically, in the case where the ZrO 2  film  21  codeposited with the n-type ZnO as the semiconductor fine particles is used as a chemical conversion film, it is considered that the number of local electrical-conductive areas are increased during voltage application (only one local electrical-conductive area is shown in  FIG. 14 ) to promote deposition of a coating film (resin) F on a surface of a steel sheet S, as shown in  FIG. 14  which is an explanatory diagram conceptually showing deposition of a cationic electrodeposition coating film in a low voltage-applied region of a workpiece (steel sheet S) having the ZrO 2  film codeposited with the n-type ZnO. In this case, an applied voltage is preferably set at a value greater than a corrosion potential (e.g., about 1 V), to increase the number of electrical-conductive areas. In  FIG. 14 , the reference code P indicates a coating material having water solubility given by acid. 
     In order to support the above hypothesis, a current density distribution on a surface of each of the conventional ZrO 2  film devoid of any of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles, and the ZrO 2  film codeposited with the n-type ZnO, was measured using a scanning vibrating electrode technique (SVET).  FIG. 15  is a graph showing a current density distribution during non-voltage application, in the ZrO 2  film codeposited with the n-type ZnO. In the measurement on the conventional ZrO 2  film, the same result as that in  FIG. 15  was obtained. That is, in either measurement, no current was detected during non-voltage application. 
       FIG. 16  is a graph showing a current density distribution during voltage (1 V) application, in the conventional ZrO 2  film, and  FIG. 17  is a graph showing a current density distribution during voltage (1 V) application, in the ZrO 2  film codeposited with the n-type ZnO. As seen in  FIG. 16 , even if a voltage of 1V was applied, no current was detected in the conventional ZrO 2  film. In contrast, as seen in  FIG. 17 , in response to applying a voltage of 1V, a current was detected in the ZrO 2  film codeposited with the n-type ZnO. This verified that the n-type ZnO contributes to an increase in the number of local electrical-conductive areas, so as to promote deposition of a coating film F. 
     Further, an influence of a content ratio of the semiconductor fine particles was checked. 
       FIG. 18  is a table showing an influence of a content ratio of the n-type ZnO (semiconductor fine particles) to the ZrO 2  film codeposited with the n-type ZnO, on a film thickness (electrodeposition coatability) of a coating film and corrosion resistance. As seen in  FIG. 18 , a film thickness (electrodeposition coatability) of a coating film becomes larger (better) along with an increase in amount (mass %) of the n-type ZnO, and a problem about corrosion resistance occurs when the amount (mass %) of the n-type ZnO is increased up to a given value or more although the corrosion resistance is in an allowable range when the amount is less than the given value. The corrosion resistance was evaluated based on measurement of a swelling rate (%) of a coating film F after 60 cycles of cyclic corrosion tests (CCTs) (1 cycle of the CCT≈JIS K5600-7-9 cycle A×3). Specifically, a smaller swelling rate (%) of the coating film F after 60 cycles of the CCTs indicates more excellent corrosion resistance. 
     Five types of chemical conversion treatment agents were used in this measurement to form chemical conversion films containing the n-type ZnO in respective amounts of zero mass %, 3.2 mass %, 5.6 mass %, 7.7 mass % and 11.3 mass %. Specifically, for example, a content of H 2 ZrF 6  in the aforementioned chemical conversion treatment agent for forming the chemical conversion film containing the n-type ZnO in an amount of 5.6 mass % was changed to allow a content of the n-type ZnO to be set at each of the above values. 
       FIG. 19  is a graph showing a content ratio of the semiconductor fine particles and a coating-film swelling rate, in a workpiece on which the ZrO 2  film codeposited with the n-type ZnO as the semiconductor fine particles is formed. In  FIG. 19 , the vertical axis represents a coating-film swelling rate (%) after 60 cycles of the CCTs (1 cycle of the CCT≈JIS K5600-7-9 cycle A×3), and horizontal axis represents a content ratio (mass %) of the n-type ZnO to the ZrO 2  film codeposited with the n-type ZnO. That is,  FIG. 19  shows a technique of determining an upper limit of an amount (mass %) of the n-type ZnO in view of corrosion resistance. Specifically, the relationship between the amount (mass %) of the n-type ZnO and the coating-film swelling rate (%) after 60 cycles of the CCTs in  FIG. 18  is plotted in  FIG. 19 , and an upper limit of the amount (mass %) of the n-type ZnO is determined based on a coating-film swelling rate of 30 (%) which is used as an allowable limit (reference value) of corrosion resistance. In this case, a coating-film swelling rate of 30 (%) is used as an allowable limit (reference value) of corrosion resistance. This is based on the following reason. A 12-year warranty against a rust hole of an outer panel of a vehicle body becomes mainstream, and it has been confirmed by past records that the warranty is satisfied when the coating-film swelling rate is less than 30 (%). As seen in  FIG. 19 , the amount (mass %) of the n-type ZnO at the allowable limit of corrosion resistance is 8.2 mass %. That is, it is necessary to set the amount of the n-type ZnO at 8.2 mass % or less in order to ensure corrosion resistance. 
     Although the above description has been made based on one example where the fine particles are the semiconductor fine particles, the same effects could be obtained using metal fine particles or electrically conductive organic fine particles. Specifically, as with the ZrO 2  film codeposited with the n-type ZnO as the semiconductor fine particles, a workpiece having a ZrO 2  film codeposited with the metal fine particles or electrically conductive organic fine particles allowed a high-quality coating film to be formed by a subsequent cationic electrodeposition coating process. That is, a formation (deposition) of a coating film in a subsequent cationic electrodeposition coating process can be accelerated to improve electrodeposition coatability in a portion of a workpiece belonging to the low voltage-applied region. Further, the metal fine particles or the electrically conductive organic fine particles had the same desirable range of the content ratio as that of the semiconductor fine particles. Specifically, when an amount of the metal fine particles or the electrically conductive organic fine particles is set at 8.2 mass % or less, a content ratio (volume %) of the metal fine particles or the electrically conductive organic fine particles to a chemical conversion film can be set to be equal to or less than a content ratio (volume %) of the n-type ZnO to the chemical conversion film, so as to ensure corrosion resistance. 
     As mentioned above in detail, according to a first aspect of the present invention, there is provided a chemical conversion treatment agent which comprises: a film-forming component to form a chemical conversion film in which the number of local low-resistance areas each having an insulation resistance value of 5000Ω or less is 20 or less/100 μm 2 ; and at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles. 
     In the first aspect of the present invention, the chemical conversion treatment agent can form a chemical conversion film which allows a high-quality coating film to be formed by a subsequent cationic electrodeposition coating process. Specifically, the chemical conversion treatment agent can form a chemical conversion film which allows a formation (deposition) of a coating film in a subsequent cationic electrodeposition coating process to be accelerated so as to improve electrodeposition coatability in a portion of a workpiece belonging to a low voltage-applied region. The reason is considered to be that, even if a chemical conversion film having a relatively small number of local low-resistance areas is formed by the chemical conversion treatment agent, the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles is contained in the chemical conversion film, and the number of local electrical-conductive areas can be increased during voltage application in the electrodeposition coating process, by utilizing a tunneling effect based on the fine particles. 
     Preferably, in the chemical conversion treatment agent of the present invention, the film-forming component contains, as a primary component, a compound having at least one selected from the group consisting of Zr, Ti, Hf and Si. Although a chemical conversion film formed using this film-forming component has a relatively small number of the local low-resistance areas, the film-forming component allows the chemical conversion film to have propertied capable of preventing eutrophication and suppressing sludge production associated with the chemical conversion treatment while ensuring corrosion resistance. More preferably, the film-forming component contains H 2 ZrF 6 . 
     Preferably, in the chemical conversion treatment agent of the present invention, the fine particles have an average primary particle size of 20 to 40 nm. In this case, the above advantages can be effectively obtained. 
     According to a second aspect of the present invention, there is provided a surface-treated metal material which comprises a chemical conversion film formed using a chemical conversion treatment agent. The chemical conversion treatment agent includes: a film-forming component to form a chemical conversion film in which the number of local low-resistance areas each having an insulation resistance value of 5000Ω or less is 20 or less/100 μm 2 ; and at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles. In the surface-treated metal material, the chemical conversion film is codeposited with the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles. 
     In the second aspect of the present invention, the surface-treated metal material can have a high-quality coating film formed by a cationic electrodeposition coating process. Specifically, the surface-treated metal material has a coating film excellent in electrodeposition coatability even in a portion of a workpiece belonging to a low voltage-applied region. The reason is considered to be that, even if a chemical conversion film formed on a surface of the surface-treated metal material has a relatively small number of local low-resistance areas, the at least one kind of fine particles selected from the group consisting of metal fine particles, semiconductor fine particles and electrically conductive organic fine particles is contained in the chemical conversion film, and the number of local electrical-conductive areas can be increased during voltage application in an electrodeposition coating process, by utilizing a tunneling effect based on the fine particles. 
     Preferably, in the surface-treated metal material of the present invention, the chemical conversion film includes an oxide which has, as a primary component, at least one element selected from the group consisting of Zr, Ti, Hf and Si. Although the chemical conversion film has a relatively small number of the local low-resistance areas, the above component allows the chemical conversion film to have propertied capable of preventing eutrophication and suppressing sludge production associated with the chemical conversion treatment while ensuring corrosion resistance. 
     Preferably, in the surface-treated metal material of the present invention, the fine particles are contained in the chemical conversion film in an amount of 8.2 mass % or less with respect to 100 mass % of the chemical conversion film. The fine particles contained in the chemical conversion film makes it possible to improve electrodeposition coatability in a portion of a workpiece belonging to a low voltage-applied region, while reliably preventing corrosion resistance from deteriorating beyond an allowable limit. 
     Preferably, in the surface-treated metal material of the present invention, the chemical conversion film comprises a primary component consisting of ZrO 2 . This makes it possible to provide a surface-treated metal material having a chemical conversion film comprising a more embodied primary component. 
     This application is based on Japanese Patent application No. 2008-053227 filed in Japan Patent Office on Mar. 4, 2008, the contents of which are hereby incorporated by reference. 
     Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.