Apparatus and process for applying electrodeposition painting by alternating current
United States Patent Tanaka et al.  
 [ APPARATUS AND PROCESS FOR APPLYING ELECTRODEPOSITION PAINTING BY ALTERNATING CURRENT [75] Inventors: Tadashi Tanaka, Ranishi-machi;  
 Tatsuro Obi. Tokyo; Yoshio Shindow, Kawasaki-shi, all of Japan [73] Assignee: Nippon Steel Corporation, Tokyo.  
 Japan [22] Filed: Sept. 12, 1972 211 Appl. No.: 288,400  
 [30] Foreign Application Priority Data Sept. 14, l97l .lapan 46-7l33l [52] US. Cl 204/181; 204/299 EC [51] Int. Cl. Blk /00; C23b 13/00 [58] Field of Search 204/181, 300 EC, 299 EC [451 Apr. 15, 1975 Wcigel 204/l8l Tsuk 204/18] FOREIGN PATENTS OR APPLICATIONS 482,548 3/1938 United Kingdom 2(l4/l8l Primary E.\&#39;amim&#39;rHoward S. Williams Attorney, Agent, or FirmTorcn, McGcady and Stanger [57] ABSTRACT [56] References Cited UNITED STATES PATENTS 23 Claims, Drawing Figures 3,392,10l 7/1968 Barrett et al 204/181 100 {Al material withOJlu oxidefilm} Coulombic co 0mm efiic&#39;enc U l mfio cmy efficiency Coulombic efficiency of AC. deposition x1000) ratio(&#39;l.) Coulombic efficiency of ac. deposition Al oxide film thicknessb FLTENTEIEAFR I 5375 878,078  
 sum 010$ 10 metal is positive -anode counter-electrode is negative Bath voltage *cathod e couter-electrode is positive Metal is negative cathode -anode (ii) r Current in the direction to the order of electrodeposition coating I paint precipitation l l l l I i l I I 1 Bath current c\ Current in reverse direction to electrodeposition coat|ng--- loss of current dueto paint redlssolution etc.  
  Current in the direction (m) to the order of electrodeposition coating Bath current 1 Current in reverse direction to electrodeposition coating FjTEFKKIAPRISEYS 3 7 078 SHEET L23? i0 FIG.2  
 Acrylic resin both Solid content 15% Coulombic efficiency ratio of D. C. electrodeposition=13 3 Coulombic efficiency &#34;%,ul 1 0 Thickness of To oxide film (A&#39;) FIG.3  
 Coulombic igftigiegcy 50 282212 3 Coulombic efficiency of A.C. deposition U rotio( Coulombic effciency of DC. deposition Thickness of To oxide film (A) i I L i LL- SHEET 3 0F 1 O {Al material with 012; oxide film} Coulombic Both mainly composed of ocr lic resin efficiency ui Solid content 15% y i0 Couiombic efficiency ratio of DC.  
 eii-:*ctrodeposition=18.8 mi  
 Al oxide thickness 1 00 {Al material with 012, oxide film} col-jlpmbic Coulombic efi cie ncy efficiency Coulornbic efficiencyof A.C. depositi X100? 50 rcitio(/.) Coulombic efficiency of ac. deposition 0 2 1. e 8 i0&#34; so 100 150 Al oxide film ihicknessU m1: sssz&#39;s 3 878,978  
 sum on HF 10 Alkyd resin both pH= 8.50 Solid contentl5ln Coulombic efficiency 12. 8 &#34;$6 Amount of Deposited point &#34;%,,,2  
 Electro deposition Coating Time(Sec) Alkyd resin both pH: 8.50 Sohd content 15% Coulombic elf|ciency12.8&#39;%,  
 Amount of Deposited point M Both Voltage (V) Coulombic efficiency .jiiitiil 3,878,078  
 SHEET E OF 1 0 Coulombic fluency of present process x100(/.) 30 efficiency ratio Coulombic efficiency of DC. process Acrylic resin Solid content 50 Bath temperature C pH=7.9O Bcith VoltagelOOV 5 second I electrodeposition To oxide film electrode Sine waved A.C. frequency (f) l Both Voltage 1 Triangle wove l 110v Bath Voltage \l Echelon wave Bath Current y W SHEET G70F1O FIGJO TyiTEiITii 3,878,078  
 SHEET C8 [IF 1O through metal 0 I to be coated (12) I I Bath VoltugeW) O t I I I I I I I I I I I I I I I 1 i i 0=in+i2 I I I 1 Total bath Current I /I I (10) Q( I i\ {\J I a oh I Qc I l I I I I I I I I I I I 1 I Third electrode I I t current (II) 0 M I I I l i I Current actually used I I I I i for electrodeposition Current passing /-I t I I I I l PffirHEEERZSETS 3878.078  
 SHEET E 9 5F 1 O 100 Metal to be coated(Substrate for plating) Before baking Number of pinholes/ z After baking including number) of foams 5 0(=lo (X area rudi&#39;o Surface Area of Third Electrode Surface Area of Metal to be Coated Metal to be coated (Substrate for plating) Coulombic Efficiency fioul) O l I 1 l 1 1 I =logl 04 Area radio: Surface Area of Third Electrode Surface Area of Metal to be Coated APPARATUS AND PROCESS FOR APPLYING ELECTRODEPOSITION PAINTING BY ALTERNATING CURRENT The present invention relates to a process for electrodeposition painting by alternating current or by using an a.c. source. This invention is fora process ofeleetrodeposition painting by an a.c. voltage to popularize the technique of electrodeposition painting and to permit a converter from a.c. to direct current (designated hereinafter as d.c.) to be omitted and therefore to simplify the necessary electric devices.  
  The present invention may be summarized to be a process for applying electrodeposition painting by a.c. comprising applying painting by an a.c. voltage in a resin bath for the electrodeposition painting using an oxide film electrode as counter-electrode of a filmforming metal such as Ta. Al. Ti. Zr and Nb and an alloy thereof.  
  In principle. processes for electrodeposition painting so far used comprises applying an external d.c. voltage between a metallic matter to be coated as anode and a counter-electrode or cathode in a bath for the electrodeposition painting containing an electrodeposition paint material. and painting the surface of the metallic matter with the deposited paint material. Most other processes are modifications of the above process.  
  Although some processes for electrodeposition painting using a.c. voltage have been devised in which an a.c. voltage is applied between a metallic matter to be coated and a counter-electrode to coat the surface of the former with the paint material deposited. these processes are of almost no practical use because of the following difficulties:  
  A special bath of paint material is used for the a.c. electrodeposition painting. In this case. the paint material is deposited on the counter-electrode. though it does not need to be coated. and a device is necessary to remove the paint material which. to the contrary. loses the merit of the electrodeposition coating method by a.c.  
  2. The amount of deposited paint material per unit time is so small that profitable application of the technique is questionable.  
  3. The coulombic efficiency, or amount of paint material deposited in mg/dm per unit coulomb. is small relative to that in the method using d.c.  
  4. A measure should be taken to the gas which is evolved at the cathode.  
  However. if the above difficulties could be solved. the process for the electrodeposition painting by a.c. is apparently more profitable in equipments and devices than the corresponding processes by d.c.  
  The present invention has succeeded in solving these difficulties profitably and developing a practical process for the electrodeposition painting by a.c.  
  Difficulties generally accompanying with the proeesses for the electrodeposition painting by a.c., such that a thick coating film is not obtained and that the coulombic efficiency is low. arise from the mode of treatment in which the a.c. voltage is directly applied between a metallic material to be coated and the counter-electrode, where the metallic material is alternatingly polarized as anode and cathode in accordance with the cycle of the a.c. voltage source so that the coating film that is formed when the material is anodic is inevitably dissolved or peeled in part when the polarity is reversed.  
  In this invention. however. the electrical resistance between both electrodes is made lower. hence the bath current larger. when the metallic material to be coated is anodic and the counter-electrode is cathodic. and therefore the coating process to the metal surface proceeds regularly. On the other hand. when the metallic material is cathodic and the counter-electrode is anodic. the resistance between both electrodes is made higher and hence the bath current smaller so that the deposited coating film on the surface of the metallic material is not easily dissolved or peeledl Furthermore. a more perfect process for .the electrodeposition painting has been completed by compensating the undesired current which flows when the metallic matter to be coated is cathodic and the counter-electrode is anodic.  
  The characteristic feature of the process of this invention is that all the requirements in realizing electrodeposition painting by an a.c. voltage have been met solely by modifying the counter-electrode without introducing a sophisticated set of apparatus.  
  Subsequently. performance of the counter-electrode of this invention as well as the consequence in which the process for the electrodeposition painting by a.c. has been realized will be explained in the following paragraphs.  
  The feature of the counter-electrode employed in the process of the present invention is that the electrical resistance and the electrostatic capacitance between the electrode and the paint material bath are large when the electrode is anodic while they are small when cathodic.  
  The present invention shall be described in details referring to the attached drawings.  
  FIG. I shows the relation between the bath voltage and the bath current when a sinusoidal a.c. voltage is used in the process of this invention, where (i) shows the bath voltage. (ii) the bath current depending on the polarity of voltage and (iii) the bath current when uneffective current is compensated with electrostatic capacity.  
  FIG. 2 shows the coulombic efficiency in the a.c. electrodeposition coating when the tantalum oxide film electrode is used.  
  FIG. 3 shows the ratio in coulombic efficiency of the a.c. electrodeposition painting with the tantalum oxide film electrode to the d.c. method.  
  FIG. 4 shows the coulombic efficiency in the a.c. electrodeposition painting when the aluminum oxide film electrode is used.  
  FIG. 5 shows the ratio in coulombic efficiency of the a.c. electrodeposition painting with the aluminum oxide film electrode to the d.c. method.  
  FIG. 6 shows the relation between the time for electrodeposition and the amount of paint material deposited in the period when a tin-plated iron plate is coated by the a.c. electrodeposition using the tantalum oxide film electrode.  
  FIG. 7 shows the relation between the bath voltage and the amount of paint material deposited when a tinplated iron plate is coated by electrodeposition using the tantalum oxide film electrode.  
  FIG. 8 shows the relation between the cycle of the sinusoidal alternating current and, the ratio in the coulombic efficiency.  
  FIG. 9 shows the relation between the bath voltage and the bath current in electrodeposition coating when alternating currents of various wave forms are used.  
  FIGS. 10 show a modification of the present invcntion. FIG. 10 shows the principle of the modification of the present invention. FIG. 1 I is a graph for explaining the bath current. FIG. 12 shows the relation between the area ratio of the third electrode and pin holes in the coating film. FIG. 13 is a graph for explaining the area ratio of the third electrode and the coulombic efficiency. FIG. 14 shows the position of the third electrode and FIG. 15 explains the behavior of the third electrode in relation to the current elements.  
  The electrical resistance between the counterelectrode and the paint material bath is high and low when the electrode is anodic and cathodic. respectively. As shown as (ii) in FIG. 1 when the metallic material to be coated is positive and the counter-electrode is negative or. in other words. the paint material is deposited on the surface of the metallic material. the resistance between the counter-electrode and the solution is low enough to allow a large bath or a forward current to flow. When the metallic material is negatively and the counter-electrode is positively polarized. respectively. high resistance between the counterelectrode and the solution causes the bath current or the current in the inverse direction to be low and suppress the re-dissolution and peeling of the paint coating films formed on the surface of the metallic material to be coated.  
  However. re-dissolution and peeling of the coating films which cause a loss. due to the current in the inverse direction. though small. can not be avoided.  
  The loss can be compensated by the electrostatic capacitance when the Namely. when the counter-electrode is anodic. the phase of the bath current leads the bath voltage due to the capacitance. As a result. as shown as (ii) in FIG. 1, while the counter-electrode is positive and the metallic material to be coated is negative according to the bath voltage (interval B on (i) or under the voltage favorable to re-dissolution of films). current flows in the direction to deposit the paint material for a time period on the metallic material (interval C on (iii). Thus. additional coating isapplied to the metallic matter to compensate the loss due to the current in the inverse direction.  
 In the above. the coulombic efficiency is emphasized.  
 but a different requirement should be met with the counter-electrode. If the paint material is deposited on the counter-electrode itself. a thick film will be formed on the surface which increases the electrical resistance between the electrode and the solution. requiring a higher bath voltage for the coating to proceed. A counter-electrode free from such trouble is desired.  
  In this connection, the counter-electrode of the present invention has an oxide film on the surface prevents the coating resin from completely depositing on the electrode surface. As an anode. the electrode forms a current leading in phase due to the electrostatic capacitance so that the coating film once formed is dissolved while acting as anode and. therefore. the loss in bath voltage due to the resistance of coating film can be kept low.  
  As has been explained. the counter-electrode having the specified properties outlined above permits electrodeposition painting by a.c. to be performed with much counter-electrode is anodic.  
 simpler apparatus in comparison with conventional processes.  
  In following the process and apparatus of this invention. the counter-electrode having the above properties is required. Such an electrode can be prepared from a film-forming metal such as Ta. Al. Ti. Nb. Zr. Y. Hf. Cr. Mo. W. Bi. Mg. Ge. V and Si and alloys thereof by forming an oxide film on the surface by a chemical or electrochemical manner.  
  For example. a tantalum plate. after a pre-treatment (immersing for about 10 sec. in a solution containing H 50 HNO and HF in the ratio 5 2 2). is anodically treated for l() to 120 min. by d.c. voltage using a carbon cathode in a 15 percent solution of boric acid at C. The oxide film formed on the surface has the thickness as shown in Table 1 depending on the voltage applied to the bath.  
  Table I shows the oxidation treatment of the tantalum and the film thickness.  
 Table l Voltage of anodic Film thickness treatment (V) (A) Color 30 about 500 dark pale blue 70 I000 golden 1300 reddish violet I00 I500 blue I30 I900 golden I60 2400 red 2700 green Using tantalum as a counter-electrode which was coated by an oxide film. an aluminum material (oxide film being 0.l2,u. thick) was coated by electrodeposition in a coating resin bath of the d.c. anodically deposition type by an a.c. voltage from the commercial source. If the tantalum electrode had been sufficiently treated to form the oxide film. the coulombic efficiency was almost equal to that in the d.c. electrodeposition painting process as shown in FIG. 2. The result illus- Similar property of the tantalum oxide film electrodementioned can be obtained by the anodic oxidation treatment in various electrolysis solutions such as H 50 CrO HNO NaOH. Na SO and many others.  
  When the counter-electrode is prepared from aluminum. the procedure is the same as for tantalum. The oxide film forming process is followed for 5 to 300 min. in an electrolysis solution containing substances of moderate dissolving power such as H 80. and under a 3.0 to 200 V d.c. or a.c. voltage. so as to achieve sufficiently high resistance between the electrode and the solution when the former is anodic. At the same time. the electrostatic capacitance is ensured therebetween. The coulombic efficiency of the electrode thus formed is close to that of the electrodeposition painting by a d.c. process if sufficient treatment of the oxide film formation has been performed. as shown in FIGS. 4 and 5.  
  ln other words. the ratio of the coulombic efficiency in the electrodeposition coating processes conducted in identical baths by d.c. and ac. approaches lUU percent.  
  This ratio can never be attained with the previously cited electrodes. namely of stainless steel (SL&#39;S 27). platinum. carbon and aluminum with less than 0.2;; thick oxide film). when these electrode are used as .ounter-electrode and an aluminum material (cm X iOcm. 0.12;.t thick oxide film) is coated by electrodeposition using an ac voltage.  
  An aluminum electrode can be provided with the property of the counter-electrode of this invention by treating to form an oxide film on the surface by many procedures. as is the case with tantalum. For example. electrolytic oxidation in solutions of sulfuric. chromic and boric acids. The methods are not restricted to those mentioned above.  
  Film-forming metals such as Ti. Zr and Nb and alloys thereof could provide the counter-electrode which have necessary properties required to achieve the process of this invention by being treated to form oxide films in fundamentally the same way as before.  
  in the oxide film electrodes of this invention. the oxide film should be especially stable and uniformly formed on the electrode surface. If otherwise. the oxide film may be deteriorated or broken to fall off due to evolution of hydrogen gas when serving as a cathode and likely deposition of resin on the electrode surface and deterioration of the electrostatic capacity may result when serving as an anode. anodic general. thickness of oxide films is important with the oxide films which is required to form the electrodes of the present invention. ln particular. for the metals of the first group. that is Ta and Nb. an oxide film thicker than 0.2a is sufficient and the film can be obtained with relative ease in an acid. neutral or alkaline solution. With the second group metals. that is Zr and Hf. oxide film thickness more than 0.35;; for Zr and 0.4;; for Hf is necessary. which could be prepared by an annodic treatment in almost any acid, neutral or alkaline electrolysis solution. except that of hydrofluoric acid. With film-forming metals of the third group. that is Al. Ti and others. a thickness of oxide films more than la is necessary. The electrode could be prepared by chemically treating in an electrolyte which is mostly acid having moderate dissolving power against the oxide films (dilute sulfuric, nitric, chromic and oxalic acids being contained). With the fourth group metals consisting of alloys of oxide film-forming metals such as Ta. Nb. Zr.  
 Hf. Al and Ti. the actual thickness of the oxide films should be greater than the summed products of thicknesses of oxide films and the contents with respect to all composing elements.  
  Finally with the fifth group metals which consist of alloys of one or more elements of the oxide filmforming metals belonging to groups 1 through 4 above such as Ta. Nb. Zr. Hf. Al and Ti as the major constituent with elements other than the oxide film-forming metals. the thickness ofthe oxide films should be larger than product of the reciprocal of content of the major constituent and the minimum film thickness of the filmforming metals. In these cases the procedure to treat the oxide films may be similar to that for the oxide filmforming metal contained as the major constituent.  
  In the above statement. the structure and quality of the counter-electrode were explained with regard to the coulombic efficiency. However. further typical requirements that follow should be met with the counterelectrode in order for the clectrodeposition painting process by ac. to be extensively applicable in industry.  
  I. The process should be applicable to various metals. When applied to Al. zinc-plated iron plate. tinplated iron plate. Fe. tin-free sheet and many other materials. it should exhibit excellent performance and high coulombic efficiency as a process for electrodeposition painting.  
  2. Any desired resin type coating materials such as acrylic. polyester. alkyd. epoxyester and maleic oil resins could be applied with the high coulombic efficiency as could be encountered in the electrodeposition painting process by d.c.  
  3. Time required for the electrodcposition painting could be varied as desired. In other words. a coating layer of the desired thickness could be obtained within a certain time interval which is predetermined as desired.  
  4. As ac voltage sources, not only the commercially supplied sinusoidal a.c. source of or c.p.s. but also any a.c. sources of higher or lower cycles could be used for the purpose. Further. sawtooth or square wave a.c. could be satisfactorily used for the coating.  
  All the requirements above have been met by the process of the present invention with satisfactory results as illustrated below.  
  I. Table 2 shows the performance of the process of the present invention as a coating process applicable to various metallic materials including Al. zinc-plated iron plate, tin-plated iron plate. Fe and tin free sheet.  
 Table 2 Coulombic efficiency at the electrt deposition coating of various metallic materials Electrode of the process of this invention Tantalum oxide Aluminum Stainless Steel Pure Al Platinum film electrode oxide film electrode electrode electrode electrode (SLS 27) (0.12,.  
 thick oxide film) Aluminum lUl) 9; lot) 1 l &#39;/r l /l 2 1r Zinc-plated iron plate lllll Z l l l Tin plate 100 I00 2 l l 1 Iron (for plating) I00 I00 2 l3 Tin free sheet I00 I00 I l4 1 [Counter-electrode (electrode of the present invention)] l. Tantalum oxide film electrode: 2700 (A) thick oxide film 2. Aluminum oxide film electrode: l(l.5 (/J.) thick oxide film [Conditions under which the electrodeposition coating was performed] A counter-electrode of the same dimension cm X 10cm as a metallic material to be coated was placed at a 10cm distance from the latter in an electrodeposition painting bath. The coating was conducted for 5 see. by the commercial a.c. (50 c.p.s. sinusoidal) of lOOV. and the coulombic efficiency was calculated. Subsequently. the corresponding coulombic efficiency for the d.c. coating was obtained when the same material was treated by electrodeposition painting process under application of 70V d.c. source in the same bath for 3 sec. using a stainless cathode. Ratio of the coulombic efficiencies shows the relative performance of the electrodeposition coating of each counter-electrode.  
 [Bath for the electrodeposition painting] Acrylic resin: the solid content of the bath being percent Bath temperature: C  
  The coulombic efficiency of the process of this invention was in all cases close to that of the d.c. method. and therefore the ratio in the coulombic efficiencies was lUO percent. Such values of coulombic efficiency were never obtained in the a.c. electrodeposition coating with electrodes other than the counter-electrode of the present invention.  
  2. As for the coulombic efficiency when a coating material of an arbitrary resin type was used, the same coulombic efficiencies as those by d.c. electrodeposition painting processes were obtained by the a.c. process for a varieties of coating materials including those containing as major constituent acrylic, polyester. alkyd. epoxyester and maleic oil resins, as shown in Table 3.  
 Table 3 that the deposition rate may be arbitrarily varied by changing the bath voltage to meet any requirement.  
  This is one of the important requirements which the process of this invention should meet in order to be generally used as a painting process for industrial purpose. The reason for it is self-evident. since a paint coating process to be used in industrial purposes should proceed at a rate comparative with that of other operations in a sequence. For example, in coating automobile bodies the coating should proceed at the same rate as that of body-forming operation prior to the coating. and in coil coating very rapid coating is required to attain a desired thickness of the coating film in a short time.  
  4. With regard to the a.c. sources to be applicable to the present invention, this invention exhibits the high performance in the electrodeposition painting, as shown in FIG. 8, by using a.c. of a very wide range of frequency and, in addition to sinusoidal waves, by sawtooth or square waves shown in FIG. 9 with higher coulombic efficiencies than in other a.c. electrodeposition painting processes.  
  According to a modification of the present invention, a third electrode is provided in addition of the metal to be painted by electrodeposition and the coupled electrode and is electrically connected to the metal to be painted by electrodeposition.  
  This modification of the present invention further enhances the advantages of the alternating current electrodeposition paint coating and particularly enhances the paint film quality. V  
  The third electrode in the modification of the present invention has its objects to prevent gas generation at the metal to be painted by electrodeposition and to effect precision control of the paint film thickness, and is an oxide film electrode&#34; or may be an ordinary electrode commonly used for electrolysis connected with a rectifying element of silicon, germanium selenium, etc.  
 in such a manner that the current flows in the direction from the electrode to the rectifying element.  
 Coulomhic efficiencies of a.c. electrodeposition coating in various types of resin baths A B C e Coulnm- Coulombic efli- A.c. electrodeposihic pH of efficiency ciency by this tion coating with bath by d.c. inventiontmg/coul) other counter- Resin electrodetmgl oull process Ta oxide Al oxide Stain- Pt Al (mg/coal) film film less electelectelectrode electrode SLS27) rode rode (0.2;mxide film) 1 Acrylic 7.90 ms is m7 0.2 0.2 0.: 2 Polyester 6.47 14.8 l-LX 14.8 ().2 U.2 02 3 Maleic oil 7.70 6.2 6.2 6.3 02 02 4 Epoxyester 9.l() l7.8 17.9 l7.7 ().2 ().3 ().2 5 Alkyd 8.50 l2.5 12.5 l2.5 ().2 ().2 02  
  3. As for the time required to complete the electrodeposition coating to obtain a desired thickness of the coated films. H68. 6 and 7, which illustrate the deposition rate of the coating materials of this invention, show The oxide film electrode referred to above is composed of metals which form an oxide film such as Ta. Al, Ti. Zr. Nb. etc. or their alloys on which an oxide film is formed.  
  The modification of the present invention shall be described in details referring to FIGS. 10 15.  
  In FlG. l0, 1 is a bath filled with clcctrodeposition paint. 2 is an oxide film electrode used as a coupled electrode. 3 is a metal to be painted by clcctrodeposition. connected to an alternating current source. 4 is another oxide film electrode used as the third electrode connected to the side of the metal to be painted through a resistor 5.  
  When the alternating current is supplied. the metal 3 to be paint coated and the third electrode are impressed with positive and negative voltages alternatively in respect to the counter-electrode 2. As an oxide film electrode is used for the counter-electrode. a large bath current passes under the bath voltage condition in which the counter-electrode is negative and the metal 3 to be paint coated and the third electrode 4 are positive. and clcctrodeposition paint coating is effected thereby. Since the third electrode is the oxide film electrode. the current is hard to pass and passes mainly through the metal to be paint coated. thus using most of the current for electrodeposition paint coating on the surface of the metal to be paint coated.  
 Next. when the counter-electrode 2 is positive and the metal to be coated 3 and the third electrode 4 are negative. at small amount of current flows in a direction reverse to that for effecting the clcctrodeposition paint coating. If this reverse current passes through the metal to be coated. not only the electrodeposited film is redissolved. but also hydrogen is generated by the cathodic electrolysis. ln the modification of present invention. this reverse current is made to pass through the third electrode. thus eliminating the above defect. Namely. because the third electrode is an oxide film electrode. the current easily passes when it serves as a cathode while the current does not easily pass through the surface ofthe metal to be coated because of the electrodepositcd coating film. Thus. the bath current passes mainly through the third electrode. Therefore. the hydrogen gas generation due to the cathodic electrolysis is caused on the surface of the third electrode.  
  In this way. pin holes in the paint coating film due to the gas generation on the surface of the metal to be paint coated, or foams in the coating film are remarkably reduced.  
  However. when the third electrode is used. a small amount of current passes from the third electrode when it serves as anode. and this current is of use for electrodeposition paint coating. thus lowering the amount of coating clectrodeposited per unit current passage. Thus there is an optinum range for the ratio of the surface area of the third electrode to the metal to be paint coated. FIG. 12 shows the relation between the ratio of the surface area a and the number of the pin holes in the coating film. The solid line represents the relation before the baking and the dotted line represents the relation after the baking. From this figure. it is clear that the ratio a should be log 10 5. The relation between the coulombic efficiency (the amount of electrodeposited coating (mg) pen unit current passage (per 1 coulomb)) and the area ratio of the third electrode is shown in FIG. 13. It is clear that the increased area ratio lowers the efficiency. and particularly when the ratio is larger than 1 (==log a rapid lowering is caused and the lowering is moderated when the ratio is 1(= log 10&#34;) or more. From a view point of industrial t) is better. thus the range of the value. log It) a area ratio is:  
  As shown in FIG. 10. the connection of the third electrode to the metal to be coated may be preferably done through a variable resistor for better current balance between the third electrode and the metal to be coated. and in some cases for precision control of the amount of the electrodeposited paint coating. The capacity of the resistor may be 0 200 K9 for presently available paints and it is enough if the resistor can flow the current corresponding to l0 percent of the bath current.  
  Lastly. regarding the shape and position of the third electrode. it is preferable from better efficiency that the third electrode is positioned up to 3/5 L(m) (corresponding to the slant lined portion in FlG. 14) from the metal to be coated supposing the distance between the metal and the counter-electrode is L(m). Regarding the shape of the third electrode. it is preferable that the electrode has some measures such as perforation because ifa plate-form electrode is positioned in parallel the current for clcctrodeposition is hindered. From a point of equipment efficiency. a round bar form of electrode is preferred for stability and draining of the bath solution at the suspension of the operation. FIG. 14 shows one example.  
  As explained above. with the use of the third electrode pin holes or foams are prevented from taking place in the paint coating film. thus solving one of the big problems of the a.c. clcctrodeposition paint coating.  
  Meanwhile. when an electrode commonly used for electrolysis is used as the third electrode. it is necessary that a rectifying element 107 such as of silicon. germanium. selenium. etc. and a variable resistor by orienting the rectifying element are connected is series in such a direction that the current flows in the direction 108 as shown in FIG. 15. In this case. the capacity of the rectifying element or the resistor should be large enough to pass the current corresponding to l/10 of the bath current.  
  The electrode commonly used for electrolysis referred to above means an electrode of a metal which does not react with water such as stainless steel. lead. nickel. chromium. iron. silver. platinum. etc. or of nonmctallic material such as carbon.  
  As has been explained above. the clcctrodeposition painting by a.c. of the present invention is a very excellent as well as practical process with a wide range of application.  
 EXAMPLE 1 A tantalum oxide film electrode having a 2700 A thick oxide film to be used in the process of this inven tion was prepared as follows: A 10cm X 10cm plate of tantalum was treated by immersing for 10 sec. in a mixed solution of H 80 HNQ, and HF in a ratio of 5 2 2. and an oxide film was formed on the surface by the anodic oxidation with a carbon cathode in a 15 percent boric acid solution at 70C. applying a voltage of about 5V at the beginning which was increased gradually up to V in 60 min. and maintained at the voltage for 20 min. Subsequently the plate was washed with water and then dried.  
  With the above tantalum oxide film electrode as counter-electrode. a 10cm X lOcm aluminum material (with one side sealed and having a 0.12,u. thick oxide film) placed at a distance of 10cm from the counterelectrode was coated by electrodeposition for 5 sec. using the commercial sinusoidal ac. voltage of 100V and 50 c.p.s. in a coating bath containing acrylic resin of the d.c. anode deposition type. As a result. the coating material was deposited by 58.1 mg/dm&#34; and the quantity of electricity involved in the treatment. as measured from the current form on an oscilloscope. was calculated according to the following expression. to be 3.09 couI/dm (see (iii) in FIG. 1).  
  Q Q&#34; Q, where 0 is the effective quantity of electricity. 0,, and 0,, are the quantity of electricity of the current in phase with the applied bath voltage. and 0,. is the quantity of electricity of the current in inverse phase with the applied bath voltage (or the quantity of electricity once charged in the electrode and then discharged).  
 0,, 3.10 (coul/dm 0,, 0.10 (coul/dm therefore. 0 3.10 0.10 0.11 3.09 (couI/dm&#34;) Consequently. the coulombic efficiency of the process of the present invention was 58.1 (mg/dm )/3.09 (couI/dm&#39;-) 18.8 (mg/coul). which was equal to the coulombic efficiency in the d.c. electrodeposition coatmg.  
 COATING BATH OF ACRYLIC RESIN 1. Acrylic resin was the major constituent with the solid content of the bath being 15 percent. Bath temperature 20C. Emulsion type.  
 2. The coulombic efficiency when an aluminum material having a 0.12p. thick oxide film was coated by electrodeposition for sec. using a 70V d.c. applied bath voltage was 18.8 mg/coul.  
  For the sake of comparison. a platinum plate. a stainless steel (SUS 27) and pure aluminum (having a 0.1211. thick oxide film) which had not the quality required for the process of this invention were used as counterclcctrode, and the electrodeposition painting was performed by ac. under the same conditions of bath as well as painting. In any case the coulombic efficiency remained below- 0.2 percent.  
 EXAMPLE 2 An aluminum oxide film electrode (having 10.511. thick oxide film) to be used in the present invention was prepared as follows: A cm 10cm pure aluminum plate was kept for 3 min. in a 5 percent oxalic acid solution with a 5V voltage applied until the current decreased to a steady state. The voltage was then elevated. under the precaution not to change the current seriously. to 100V in about 60 min. Kept standing for 10 min., washed with water and dried.  
  With the said aluminum oxide film electrode as counter-electrode. a 10cm 10cm aluminum material (with one side sealed and having a 0.12;. thick oxide film) placed oppositely at a distance of 10cm from the counter-electrode was coated by electrodeposition for 5 sec. using the commercial sinusoidal ac. voltage of 100V and 50 c.p.s. in a coating bath containing acrylic resin of the d.c. anode deposition type. As a result, the paint material was deposited by 60.3 mg/dm COATING BATH OF ACRYLIC RESIN Major constituent Acrylic resin Solid content of bath: 1571 State of bath Emulsion type pH of bath 7.90 Bath temperature 20C Coulomhic efficiency in this bath by the d.c.  
 method 18.8 mg/coul when an aluminum material (having 0.12;; thick oxide film) was coated by electrodeposition for 5 sec. at the d.c. applied bath voltage The quantity of electricity involved in the treatment, as measured from the current form on an oscilloscope, was calculated according to the following expression, to be 3.21 (coul/dm 0 0,, 0,, 0, (see (iii) in FIG. 1), where Q is the effective quantity of electricity, 0,, and Q, are the quantity of electricity of the current in phase of the applied bath voltage, and 0, is the quantity of electricity of the current in inverse phase of the ap- Coulomhic efficiencv in the process of this invention Coulomhic efficiency in the d.c. process K EXAMPLE 3 A titanium oxide film electrode to be used as counter-electrode of the process of this invention was prepared as follows: A 0.1 mm thick 10cm X 10cm foil of pure titanium was treated, after degreasing. with 2 percent hydrofluoric acid for 3 min. to dissolve impure oxide, electrolytically cleansed (with the current density 0.5A/dm in an aqueous solution containing 10 percent sulfuric, 7 percent phosphoric and 18 percent acetic acids. anodically oxidized by a current of 0.1A/dm in a 10 percent solution of phosphoric acid in glacial acetic acid, and then electrolytically oxidized by a current of 0.1A/dm followed by heating at 300C in the air to coat the surface with an oxide film. the film being 2 1. thick.  
  With the titanium oxide film electrode as a counterclectrode, a 10cm X 10cm aluminum material (with one side sealed and having a 0.12;; thick oxide film) placed oppositely at a distance of 10cm from the counter-electrode was coated by electrodeposition for 5 sec. using the commercial sinusoidal ac. voltage of 100V and 50 c.p.s. in a coating bath containing acrylic resin of the d.c. anode deposition type. As a result, the coating material was deposited by 46.3 mg/dm&#34;.  
 COATING BATH OF ACRYLIC RESIN Major constituent Acrylic resin Solid content of bath: 1592 State of bath: Emulsion type pH of bath 7.90  
 -Continued Bath temperature 20C Coulomhic efficiency in this bath by the dc method 18.8 mg/coul when an aluminum material (having a 0.12p. thick oxide film) was coated by electrodeposition for 5 sec. at the dc applied bath voltage 70\&#39;.  
  The quantity of electricity involved in the treatment. as measured from the current form on an oscilloscope. was calculated according to the following expression. to be 3.21 (coul/dm&#34;).  
 (see (iii) in FIG. 1). where is the effective quantity of electricity. 0,, and 0,, are the quantity of electricity of the current in phase of the applied bath voltage. and 0.. is the quantity of electricity of the current in inverse phase of the applied bath voltage. and  
 0,, 3.17. 0,, 0.10 and 0.. 0.06  
  Consequently. the coulombic efficiency in the electrodeposition coating by a.c. using the counterelectrode of this invention was 46.3 (mg/dm )/3.2l (coul/dm 14.4 (mg/coul).  
  The ratio of the coulombic efficiencies was calculated to be 76 percent by the following expression.  
 Ratio of coulombic efficiency Coulomhic efficiency in the process of this invention m0 Coulomhic efficiency in the dc. process A EXAMPLE 4 Using the tantalum oxide film electrode (with 0.27 thick oxide film) prepared according to Example 1 and a platinum electrode for the purpose of comparison counter-electrodes. electrodeposition painting was conducted by a.c. with the following four materials. Metal specimens to be coated by electrodeposition (10cm X 10cm. with one side sealed) 1. Bonde zinc-plated iron plate: Amount of bonde film (1.2 glm Amount of zinc plated (183 g/m 2. Tin-plated iron plate (ET No. 25): Commercial electro tin-plated iron plate Amount of tin plated (lb/BB) 3. Fe (substrate for plating): Cold rolled in steel plate, with oil application 4. Tin-free sheet: Amount of chromium plated (metallic chromium 1.00 mglm chromate chromium 4.0 mg/m The acrylic resin bath was used as the coating bath and the electrodeposition coating was performed for 5 see. with a distance of cm between electrodes by the commercial 100V and 50 c.p.s. sinusoidal a.c. source.  
 ACRYLIC RESIN COATING BATH Major constituent Acrylic resin Solid content of bath: 1571 State of bath Emulsion type pH of bath 7.90  
  The amount of coating material. the quantity of electricity required for the electrodeposition painting and the coulombic efficiency. or the amount of the coating material divided by the quantity of electricity. are shown in Tables 4 and 5 for the tantalum oxide film electrode and the platinum electrode. respectively, used as counter-electrode.  
  The quantity of electricity. the coulombic efficiency and the ratio in percentage of coulombic efficiencies are those obtained according to calculation in Examples l. 2 and 3.  
 Table 4 With a tantalum oxide film electrode as counter-electrode Bonderized Tin-plated Fe Tinzinc-plated iron plate (substrate free iron plate (ET No. for sheet 25) plating) Amount of coating material 60.0 62.0 (11.0 68.5 deposited (mg/dm&#34;) Quantity of electricity 3.19 3.30 3.24 3.614 (coul/d m&#34;) Coulomliic efficiency 18.8 18.8 18.8 18.8 (mg/coul) Ratio of coulombic lUU I00 100 efficiencies Table 5 With a platinum electrode as counterelectrode Bonderiled Tinplated Fe(subst- Tin- 7incplated iron plate rate for free iron plate (EIiNo. plating) sheet Amount of coating material 1 4 2.1 2 2 1.3 deposited (mgldm Quantitv of electricity 7.10 7.20 6.90 8.00 (coul/dm) Coulombic efficiency 0.2 0.3 (1.3 0.1 (mg/coul) Ratio of coulombic l 2 2 1 efficiencies (7r EXAMPLE 5 Using the aluminum oxide film electrode (with 10.5 1. thick oxide film) prepared according to Example 2. and a stainless steel electrode (10cm X 10cm. SUS 27) and a pure aluminum electrode (commercial material. no pretreatment. oxide film not thicker than 0.2a) for comparison purpose as counter-electrode. electrodeposition coating was conducted for 5 see. with an electrolytically tin-plated iron plate (commercially available tin-plated iron plate with plated tin by 0.24 lb/BB) of a dimension 10cm 10cm (one side sealed) placed at a 10cm distance from the counter-electrode by the commercial 100V. 50 c.p.s. sinusoidal a.c. voltage in a coating bath of acrylic resin.  
  l COATING BATH OF ACRYLIC RESIN Major constituent Acrylic resin Solid content of bath G diff t 2 v Sum: f hath 2 Emulsiun UPC crent baths f coating materials. pH of bath 7.90 Bath temperature 25C 5 Table 8 (oulomhic efficiency by the dc method in this hath 18.9 mg/coul when a tin-plated iron El &#34;t d is&#39;t&#39; zt&#39; b a. I 1 plate (ET No. 25) was coated by the electrou l zl lrl fl l lil t ih ld fillil el ecii ot jie deposition for 5 sec. at d.c. 7()\&#39;. i After the plate was baked for min.&#39; at 2l0C. the Mulm P Y Alkyd amount of the coating material deposited was mea- Coming sured and the coulombic efficiency was calculated T i (l .2  
 - I I from the quant ty of electricity passed. The coulombic efficiency only when the aluminum oxide film elec- Quantityof s 7 trode was used was equal to that for the dc. method. as observed in Table 6. l5 (oulomhic Table 6 Ac electrodeposition coating of a tin-plated iron plate (ET No. using various counter-electrodes Electrode of Electrode other than this invention of this invention Aluminum oxide Stainless steel Aluminum film electrode (SLS Z7Jelectrode (oxide film less than 0.2;; thickness) Coating material (10.9 .0 I18 deposited (mg/dm&#34;) Quantity olelectricity 3.22 7.0 6.0 teoul/dm l (&#39;oulomhic efficiency I85) 0.3 ll (mg/coal) Ratio olcoulombic HRH) 2 l I efficiency n EXAMPLE 6 efficiency 14.x 6.2 17.8 12.5 (mg/coul) Using the tantalum oxide film electrode (having a mmr 027p. thick oxide film) prepared according to Example 1 l as counter-electrode. an aluminum material (having a 0. I 2y. thick oxide film) ofa dimension l0cm X l0cm (one side sealed) placed oppositely at a l0cm distance EXAMPLE 7 from the counter-electrode was coated for 5 sec. by the Using the aluminum Oxide film deem-Ode (having a clectrodepositionin4different baths ofcoating matcri- 4s 5, thick oxide fil appearing in Example 2 as ills f&#34; Table 7 the commerclal 100V and counter-electrode. a l0cm l0cm (one side sealed) -P- smPsoldal h Plate aluminum material (commercially available material washed with water. baked for 20 mm. at 210 C and the with an Oxide m thicker h 012, m d ilmoulft 0f lheocoatmg mafenal depqslled was positely at a l0cm distance from the counter-electrode Sllfed g f The q y ofelccmclty P 50 was coated by electrodeposition for 5 sec. in a coating calculated as in Example l to obtain the coulombic efbath of a|kyd resin (Shown in Table 7) using the ymercial lOOV 50 c.p.s. sinusoidal a.c. voltage. The alu- Table 7 minu&#39;m plate after the above treatment was washed with water. baked for 20 min. at 210C and then the Baths of coatmg materials (in outlme) amount of the coating material deposlted was mealf I AIR) sured. The quantityof electricity passed was calculated as in Example 1. from which the coulombic efficiency Major Oil-free Maleated Epoxyester Alkyd was calculated constituent olvester p 5 ml mm min For the sake of comparison with the process of the i f m ff my; 15,; present invention, a stainless steel plate, a platinum State of bath Water- Water- Water- Water- Plate P an alummum plate (Von-treated p alum! Soluble Soluble Soluhlfi 1 22 num with an oxide film not thicker than 0.2a) of the pH l same dimension. l0cm X l0cm. were used as counter- Coulomhic efficiency by 14.8 6.2 17.24 13.5 electrode. and the electrodeposition coating by a.c. mvlllwd were performed under the identical condition to obtain the coulombic efficiency. Table 9 shows the result. As  
 Coulomhic efliciency by dc. method:  
 (oulombic cfficiency observed when an aluminum material (having a 0.12;; thick oxide film) was coated for 5 sec. by the electrodeposition using 7(l\&#39; d.c. voltage.  
  Table 8 shows the results. The same coulombic efficiencies as those by the dc. method are obtained for seen in the table. only the aluminum oxide film electrode of the process of this invention shows the same coulombic efficiency. 12.5 mg/coul. as by the dc. method.  
 Table 9 Present lnventive Electrode other than of the Electrode present invention Aluminum Stainless Platinum Aluminum oxide film steel electrode electrode electrode electrode Coating material deposited 52.0 1.0 1.0 1.0 (mg/dm&#34;) Quantity of electricity 4.1 8.2 8.4 7.3 (coal/din&#34;) Coulombic efficiency 1 .6 02 0.2 0.2 (mg/coal) EXAMPLE 8 Using the tantalum oxide film electrode (having a EXAMPLE 9 Using the tantalum oxide film electrode (having a 2700A thick oxide film) prepared in the manner as described in Example 1 as counter-electrode. a plate of electrolytically tin-plated iron (commercial material having tin plated in 0.24 lb/B.B) of a dimension 10cm X 10cm (one side scaled) placed oppositely against the counter-electrode at a 5cm distance was coated for 5 see. by the electrodeposition in an acrylic resin bath (the same as in Example 1 at a l00\&#39; a.c. voltage by means of a source which generated sinusoidal a.c. of 20. 150. 300 and 600 c.p.s. The plate was then washed with water. baked for 20min. at 210C. and the amount of the coating material deposited was measured in mg/dm On the other hand. the quantity of electricity passed was measured as in Example 1. which gave the coulombic efficiency. The results are shown in Table l l. Apparently. sufficiently high coulombic efficiencies in the practical sense are maintained regardless of varied frequencies.  
 Table l I Effect of frequencies of electric source on the process of the present invention 20 c.p.s. 150 c.p.s. 300 c.p.s. 600 cps.  
 Amount of coating material deposited (mg/(1m 58.3 59.2 ML] 58.} Quantity of electricity passed (coul/dm] 3.1) U3 3.18 3.13 Coulombic efficiency 18.3 18.9 18.) 18.6  
 0.27;/. thick oxide film) prepared according to the pro- EXAMPLE l0 cess in Example 1 counter-electrode. an electrolytically tin-plated iron plate (commercially available matcrial with an amount of tin-plated 0.24 lb/BB) of a dimension 10cm 10cm (one side scaled) placed oppositely at a distance of 10cm from the counter-electrode was coated by the elcctrodeposition for 5 sec. using the commercial I00. 200 and 300V a.c. voltage (50 c.p.s.) in an alkyd resin coating bath (bath temperature 20C. see Table 7). washed with water. baked for 20 min. at 210C. and then the amount of the coating material deposited (mg/dm was measured.  
  The quantity of electricity passed (coul/dm was calculated in the manner as explained in Example 1 to obtain the coulombic efficiency.  
  Table 10 shows the result. Obviously. the rate of deposition of coating material could be improved without lowering the coulombic efficiency by elevating the applied bath voltage.  
 Table 10 Rate of deposition of coating material controlled by the applied a.c. bath voltage Using the aluminum oxide film electrode (having a 10.5;1. thick oxide film) shown in Example 2 as counterelectrode. an aluminum material (commercially available untreated pure aluminum with an oxide film not thicker than 0.12p.) ofa dimension 10cm X 10cm (one side sealed) placed oppositely against the counterelectrode at a 10cm distance was coated by the electrodeposition for 5 sec. in an epoxyester resin bath (the same bath as shown in Example 1 by the saw-tooth a.c. voltage shown in FIG. 10. The aluminum was washed with water. baked for 20 min. at 210C. and the amount of the coating material deposited was measured to be 50.0 mg/dm The &#39;quantity of electricity passed Q was estimated as the difference of the quantity of electricity (0 0,,) due to current in phase with the applied bath voltage and the quantity of electricity Q,- due to current in inverse phase with the applied bath voltage. Thus.  
  (coul/dm The amount of the coating material deposited divided by the above quantity of electricity passed gave the coulombic efficiency,  
  50.0 (mg/dm +2.81 (coul/dm z 17.8 (mg/coul) this value being almost equal to the coulombic efficiency 17.8 by the dc. method.  
 EXAMPLE 11 A tantalum oxide film electrode having a 2.1a thick oxide film to be used in the process of this invention