WO2008029979A1

WO2008029979A1 - Repair method of pitting damage or cracks of metals or alloys by using electrophoretic deposition of nanoparticles

Info

Publication number: WO2008029979A1
Application number: PCT/KR2007/001029
Authority: WO
Inventors: Chang-Kyu Rhee; Min-Ku Lee; Gyoung-Ja Lee
Original assignee: Korea Atomic Energy Research Institute
Priority date: 2006-09-09
Filing date: 2007-02-28
Publication date: 2008-03-13
Also published as: KR100753909B1

Abstract

Disclosed is a method of repairing pitting damage or cracks in metals or alloys using an electrophoretic deposition process. More particularly, this invention provides a method of repairing pitting damage or cracks in metals or alloys using an electrophoretic deposition process, comprising (a) preparing nano-powder particles of a metal or alloy that is the same as the metal or alloy having the pitted or cracked region; (b) adding the nano-powder particles of the metal or alloy in (a) to an organic solvent containing a dispersant, thus preparing a stabilized colloidal dispersion; (c) immersing the metal or alloy having the pitted or cracked region in the stabilized colloidal dispersion in (b); and (d) applying an electrical field to the metal or alloy having the pitted or cracked region via electrophoretic deposition to concentrate current density on the pitted or cracked region of the metal or alloy, such that the nano-powder particles of the metal or alloy having surface charges are intensively deposited in the pitted or cracked region of the metal or alloy, thereby forming a uniform layer. Compared to conventional repair methods, the repair method of the invention is advantageous because it has a short process time period, may be performed at low temperatures, has no influence on the metal around the pitting damage or cracks, and can form a continuous and dense bond, thus realizing superior mechanical strength in the repaired structure.

Description

REPAIR METHOD OF PITTING DAMAGE OR CRACKS OF

METALS OR ALLOYS BY USING ELECTROPHORETIC

DEPOSITION OF NANOPARTICLES

Technical Field

[1] The present invention relates to a method of repairing pitting damage or cracks in metals or alloys using an electrophoretic deposition process.

[2]

Background Art

[3] When using machine parts containing various metals or alloys, the parts may be corroded or damaged under a variety of natural environmental conditions, such as rain and snow, corrosive environments, or the operating conditions of the parts used, undesirably generating pitting damage or cracks in the parts. Further, the pitting damage or cracks thus generated cause the lifetime of the parts to be shortened and hasten the time for replacement with new parts. A consequence of the earlier part replacement time is the problem of increasing costs for maintenance and repair of apparatuses composed of the parts. In particular, in the case where parts, for example, heat exchangers, various condensers, heat transfer tubes of instrumental coolers, or various pipes, are used under poor operating conditions, they need to be maintained and repaired in proper time.

[4] As conventional methods of surface-repairing heat exchangers, various condensers, heat transfer tubes of instrumental coolers, or various pipes, a plating process for coating all surfaces thereof is typically used. However, since the parts should be coated all over in a plating process, this process is difficult or impossible to apply, depending on the type of part. In this regard, although most of methods for inspection and maintenance of nuclear steam generators in Korea are independently realized through domestic techniques, repair methods mainly depend on foreign techniques. Thereby, the maintenance and repair costs of the power plants are increased, and a sufficient period of time required for inspection and repair cannot be ensured, thus undesirably interrupting the supply of electrical power. Moreover, the other problems, in which workers' exposure is increased and the possibility of leaking radioactive material is increased, have arisen.

[5] As an example of methods of repairing such parts having pitting damage or cracks,

Korean Patent No. 266881 discloses a process for conducting surface- alloying on plated metal or alloy substrates or surface-repairing of damaged metal or alloy substrates using a laser beam, comprising forming a film or coat having a desired alloy composition on a metal or alloy substrate through wet plating, electroplating, or other processes and then melting the surface of the substrate using a laser beam to thereby form an alloy layer.

[6] In addition, Korean Patent No. 270193 discloses a heat tube repair method and corresponding apparatus for a large heat exchanger, the method comprising steps of fitting a sleeve for explosive expansion to the position where the heat tube is damaged; explosive expanding the sleeve for explosive expansion at the above position; removing the expanded residue and disposing a laser sleeving head at the above position; and applying a laser beam to the inner surface of the sleeve in a circumferential direction using the laser sleeving head, therefore welding the low- melting-point brazing metal of the outer surface of the sleeve to the heat tube.

[7] In addition, Korean Patent No. 425950 discloses a method of repairing an oil storage tank for preventing the leakage of oil due to corrosion, in which a strip having a predetermined width is continuously brought into contact with the inner surface of a corroded oil storage tank and the joint therebetween is welded to thus manufacture a double-walled oil storage tank, thereby completely repairing the oil storage tank.

[8] In the present invention, an electrophoretic deposition process is a combination of techniques of electrophoresis and deposition. The electrophoretic deposition process is a technique in which particles are induced to aggregate and thus deposited at predetermined positions by controlling the movement of charged particles by appropriately combining direct current and alternating current. However, the above- mentioned conventional techniques do not describe a method of repairing parts by intensively depositing a nano-metal colloid in a partially cracked region using an electrophoretic deposition process.

[9]

Disclosure of Invention

Technical Problem

[10] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a method of repairing pitting damage or cracks in metals or alloys using an electrophoretic deposition process, which can rapidly repair localized damage or corrosion pits occurring in the surface of a metal or alloy substrate, and can also form a repaired structure that is closely continuous and dense, without a change in the microstructure of the metal around the damage or pits, thus exhibiting superior mechanical strength of the repaired structure.

[H]

Technical Solution [12] In order to accomplish the above object, the invention provides to a method of repairing pits or cracks in metals or alloys using an electrophoretic deposition process.

[13]

Advantageous Effects

[14] According to the present invention, the repair method is advantageous because it has a short process time period, can be performed at low temperatures to thus have no influence on the microstructure of the metal around the pits or cracks, and can form a continuous and dense bond to assure superior mechanical strength of the repaired structure than when using conventional repair methods. Ultimately, the repair method of the present invention is expected to be effectively applied to techniques for repairing various kinds of metals or alloys having diverse pits or cracks.

[15]

Brief Description of the Drawings

[16] FIG. 1 schematically illustrates the aggregation of nano-metal colloid particles using an electrophoretic deposition process;

[17] FIG. 2 illustrates the (a) potentiodynamic polarization curve of Ni metal plate with a scan rate of 0.5 mV s in aqueous 0.1 M NaCl solution and (b) potentiostatic anodic current transient obtained from Ni metal plate at the applied anodic potential of 0.9 V (vs. Ag/ AgCl) in aqueous 0.1 M NaCl solution;

[18] FIG. 3 illustrates SEM micrographs of pit morphology on the surface of Ni metal plate subjected to a constant electric field of 20 VCt for (a) 0 sec, (b) 120 sec and (c) 1200 sec, respectively, in Ni-dispersed solution;

[19] FIG. 4 illustrates SEM micrographs of pit morphology on the surface of Ni metal plate subjected to a constant electric field of 100 VCt for (a) 0 sec, (b) 90 sec and (c) 180 sec, respectively, in Ni-dispersed solution;

[20] FIG. 5 illustrates binary SEM images transformed from the digitized SEM images of pitted Ni metal specimens (a) A, (b) B, (c) C and (d) D. Specimens A, B, C and D were made just after the fresh Ni metal plates were subjected to a constant anodic potential of 0.9, 0.8, 0.7 and 0.6 V (vs. Ag/ AgCl) above the pitting potential, respectively, for 60 sec;

[21] FIG. 6 illustrates the dependence of perimeter P and area A obtained from the binary SEM images of pitted Ni metal specimens (a) A, (b) B, (c) C and (d) D and

[22] FIG. 7 illustrates the plots of (a) electrophoretic current Ip versus applied electric field E experimentally determined from pitted Ni metal specimen A and (b) electrophoretic current Ip versus surface fractal dimension (d F,surf ) for pitted Ni metal specimens A (O), B(Δ), C(D) and D(V) under applied electric field E of 20 YU. Best Mode for Carrying Out the Invention

[24] The present invention provides a method of repairing pitting damage or cracks in metals or alloys using an electrophoretic deposition process, comprising (a) preparing nano-powder particles of a metal or alloy that is the same as the metal or alloy having the pitted or cracked region; (b) adding the nano-powder particles of the metal or alloy in (a) to an organic solvent containing a dispersant, to thus prepare a stabilized colloidal dispersion; (c) immersing the metal or alloy having the pitted or cracked region in the stabilized colloidal dispersion in (b); and (d) applying an electrical field to the metal or alloy having the pitted or cracked region via electrophoretic deposition to concentrate current density on the pitted or cracked region of the metal or alloy such that the nano-powder particles of the metal or alloy having surface charges are intensively deposited in the pitted or cracked region of the metal or alloy, thereby forming a uniform layer.

[25] In the present invention, the repair method using an electrophoretic deposition process functions to repair the cracked or pitted surface region of structural parts using electrophoresis, that is, using the electrohydrodynamic flow of colloidal metal or alloy particles having surface charges through the application of an electrical potential to metal or alloy powder, having a particle size set on the nano-scale through a nano- technique, dispersed in an appropriate dispersion medium. Further, based on an electrochemical principle in which current is selectively increased in a cracked region, as is apparent from FIG. 1, current density is concentrated on the pitted or cracked region in order to enable the selective coating and/or repair of the cracked region.

[26] In the repair method of the present invention, (a) is a step of preparing nano-power particles of a metal or alloy that is the same as a metal or alloy having a pitted or cracked region.

[27] The nano-powder particles of the metal or alloy in (a) may be prepared through levitation-gas condensation. Such a levitation-gas condensation is a technique well- known in the art, in which a base material is heated and evaporated through high- frequency induction heating to thus produce powder, which is then condensed with an inert gas, for example, nitrogen or argon, thereby preparing nano-powder, but the present invention is not limited thereof.

[28] As the metal or alloy, any metal ion or alloy thereof may be used, examples of which include, but are not limited to, metals such as Ni, Zn, Fe, W, and Sn, alloys thereof, and alloys based thereon. As such, the nano-powder particles of the metal or alloy may be prepared to a size ranging from 10 nm to 40 nm. This is because, within the above size range, it is easy to disperse the particles in a colloidal solution and also to aggregate the particles using an electrical field to thus efficiently repair the pitted or cracked region.

[29] The nano-colloid prepared using the nano-powder particles of the metal or alloy may have a size ranging from 10 to 100 nm. If the size of the prepared nano-colloid is less than 10 nm, the ability to aggregate the nano-metal colloid in the locally damaged or corroded region using the current density generated by the predetermined electrical field may be deteriorated. On the other hand, if the size exceeds 100 nm, the nano- metal colloid may be easily precipitated in the dispersion medium due to its weight.

[30] In the present invention, (b) is a step of adding the nano-powder particles of the metal or alloy in (a) to an organic solvent containing a dispersant, thus preparing a stabilized colloidal dispersion.

[31] Examples of the organic solvent include C1-C5 alcohols, and examples of the dispersant include, but are not limited to, polyvinylpyrrolidone, polyacrylamide, poly- acrylonitrile, polyethyleneimine, polyglycine, polyacrylic acid, polymethacrylic acid, poly(3-hydroxybutyric acid), poly-L-leucine, poly-L-methionine, poly-L-proline, poly- L-serine, poly-L-tyrosine, poly(vinylbenzenesulfonic acid), poly(vinylsulfonic acid), etc.

[32] In the case where the nano-powder particles are composed of Ni, the dispersant is polyvinylpyrrolidone and the organic solvent is a C1-C5 lower alcohol.

[33] In the present invention, (c) is a step of immersing the metal or alloy having the pitted or cracked region in the stabilized colloidal dispersion in (b).

[34] The stabilized colloidal dispersion, in which the metal or alloy having the pitted or cracked region is immersed, may be used as an electrolytic solution for the efficient flow of an electrical field in a subsequent step (d).

[35] In the present invention, (d) is a step of applying an electrical field to the metal or alloy having the pitted or cracked region via electrophoretic deposition to concentrate the current density on the pitted or cracked region of the metal or alloy such that the nano-powder particles of the metal or alloy having surface charges are intensively deposited in the pitted or cracked region of the metal or alloy, thereby forming a uniform layer.

[36] The electrical field may be used at 10-120 YU for 100- 1 ,500 sec. If the electrical field is less than 10 YU, it is difficult for the nano-metal colloid to have sufficient current density at the damaged or corroded region, and thus the nano-metal or alloy colloid is not efficiently induced to aggregate. On the other hand, if the electrical field exceeds 120 VtJ, the nano-metal or alloy colloid does not further aggregate, despite the application of the above electrical field.

[37] The electrophoretic deposition process may be performed, for example, using an Ni or Ni alloy having a pitted or cracked region as a working electrode, a platinum electrode as a counter electrode, and a colloidal dispersion having Ni nano-powder particles dispersed therein as an electrolyte.

[38] The electrophoretic deposition for the pitted or cracked region of the metal or alloy may be realized through lyosphere distortion. According to a lyosphere distortion phenomenon, in the particles of a double layer consisting of a van der Waals attractive component and an osmotic pressure component, polarization is decreased and thus van der Waals attraction dominates, whereby polarization induced between the particles leads to aggregation of the particles.

[39] In the present invention, after the formation of the uniform layer, a heat treatment step may be further included. Through such heat treatment, mechanical strength may be additionally increased.

[40] A better understanding of the present invention may be obtained through the following examples and experimental examples, which are set forth to illustrate, but are not to be construed as the limit of the present invention.

[41]

Mode for the Invention

[42] <Examples 1~2> Repair of Nickel Metal Plate using Electrophoretic Deposition

[43] 0.01-1 wt% of poly vinylpyrrolidone as a dispersant and 0.01-1 wt% of nickel powder particles were dissolved in 1 L of ethanol, after which the mixture was subjected to ultrasonication for 3 hours to thus prepare a nano-nickel colloid solution. The zeta potential of the nano-nickel colloid thus prepared was measured using a 90 plus particle size analyzer, available from Brookhaven Instruments, USA.

[44] The Ni metal plate, having a locally damaged or corroded region in the surface and

Pt plate were used as the working electrode and the counter electrode, respectively. The nano-nickel colloid was used as the electrolyte. The area of Ni metal plate exposed to the electrolyte amounted to 1 cm2. Since the zeta potential of nano-nickel colloid was measured to be - 48.5 mV in value, Ni metal plate having the locally damaged or corroded region was connected with a anode and Pt plate was connected with an cathode for electrophoretic deposition.

[45] Thereafter, Ni metal plate, which was immersed in the nano-nickel colloid, was treated with a predetermined electrical field of 20 Vtf using a PE 1649 DC power supply, available from Philips, Belgium, for 120 sec and 1200 sec, respectively, in order to subject it to electrophoretic deposition.

[46]

[47] <Examples 3~4> Repair of Ni metal plate using Electrophoretic Deposition

[48] The sample was treated as in the same manner as in Example 1, with the exception that the Ni metal plate immersed in the nickel dispersion was treated with a predetermined electrical field of 100 Vtf and allowed to react for 90 sec and 180 sec, re- spectively.

[49]

[50] Experimental Example 1> Electrochemical Polarization Test for Formation of

Pitting Damage

[51] Change in current density as a function of potential applied to the Ni metal plate was observed. As the application potential was increased in a positive (+) direction, a thick oxide film was formed on the Ni sample via repassivation. In addition, as the potential was continuously further increased, the oxide film was locally damaged to thus form pits having various sizes and shapes.

[52] In order to determine the pitting potential, the potentiodynamic polarization experiment was made on Ni metal plate in the applied potential range of -0.5 to 1.5 V (vs. Ag/ AgCl) with a scan rate of 0.5 mV s in aqueous 0.1 M NaCl solution by using a Potentiostat/Galvanostat (EG&G Model 263A). The results are shown in FIG. 2(a).

[53] As shown in FIG. 2(a), it can be expected that the formation and growth rates of a stable pit may increase above the pitting potential Epit, ca. 0.4 V (vs. Ag/ AgCl). The increase in current density above Epit implies that the overvoltage for the anodic dissolution of a pit becomes lower, i.e., the formation and growth of the stable pits on the surface occurs more easily. Hence, in the present experimental example, four kinds of pitted Ni metal specimens A, B, C and D were prepared by applying anodic potentials of 0.9, 0.8, 0.7 and 0.6 V (vs. Ag/ AgCl), respectively, to Ni metal plate for 60 sec in aqueous 0.1 M NaCl solution.

[54]

[55] Experimental Example 2> Measurement of Change in Surface Current Density by

Pitting Damage

[56] The Ni metal plate was immersed in a 0.1 M NaCl solution at a potential of 0.9 V

(vs Ag/ AgCl) for 60 sec to prepare the pitted Ni metal specimen having a damaged surface. In the specimen, change in current density as a function of the time of exposure to the solution was measured. The results are shown in FIG. 2(b).

[57] It can be seen from Fig. 2(b) that the pitting process is categorized into the two stages: a decrease in anodic current density up to time necessary for pit embryo formation tpit, i.e., the first stage of passivation, an increase in anodic current density after tpit, i.e., the second stage of pit formation and growth. The fall of current density in the first stage indicates the thickening of oxide film due to the repassivation process on the bare surface. The increase in current density in the second stage is attributable to the film breakdown caused by the formation and growth of the stable pits.

[58]

[59] Experimental Example 3> Observation of the change in pit morphology using

Scanning Electron Microscopy [60] In order to evaluate the degree of deposition of nano-nickel colloid depending on the electrophoresis time, the following experiment was performed.

[61] Morphological changes in the nickel metal plate of Examples 1-4 were observed using a scanning electron microscope (SEM), XL 30 SEFG. The results are shown in FIGS. 3 and 4.

[62] As shown in FIGS. 3 and 4, an electrical field of 20 Vtf was applied to the Ni metal plate having the locally damaged or corroded region in the presence of the nano-nickel colloid for 120 sec and 1200 sec respectively in Examples 1 and 2, and the electrical field of 100 Vtf was applied for 90 sec and 180 sec respectively in Examples 3 and 4. In these cases, it can be seen that the nano-nickel colloid was intensively deposited in the pitted region of the Ni metal plate. In particular, in the case of Example 2, in which the electrical field of 20 VCt was applied for 1200 sec, it can be seen that the repaired structure having a very smooth surface was formed at the locally damaged or corroded region.

[63] Thereby, when the electrical field is applied to the pitted Ni metal plate, the current density of the locally damaged or corroded region is higher than the outer surfaces of the pitted Ni metal plate, such that the nano-nickel colloid is intensively deposited in such a region to thus self-repair the pitted Ni metal plate.

[64]

[65] Experimental Example 4> Quantitative analyses of pit morphologies of Ni metal plates based upon the fractal theory

[66] The surface fractal dimension (hereinafter, d F.surf ) characterizes the surface ir- regularity: the larger the value of d F,surf is, the more irregular and the rougher becomes the surface. The values of d 2 and 3 mean a perfectly flat surface and a very rough

F.surf surface, respectively. Now, let us determine the value of d F.surf which characterizes the surface roughness and irregularities of pitted Ni metal plates based upon fractal theory. [67] Fig. 5(a-d) present the binary SEM images transformed from the digitized SEM images of pitted Ni metal specimens A, B, C and D, respectively. The white portions that look bright are considered to be the pit of the specimen and dark portions are regarded as the outer surfaces of the specimen. The d for four kinds of pitted

F.surf specimens were determined by using perimeter-area method. [68] The perimeter- area method is based upon the fact that the intersection of a plane with a self-affine fractal surface generates self-similar lakes. It is well known that area A and perimeter P of the self-similar lakes relate to their

F, ss by [69] MathFigure 1 / 2

[70] where βis a proportionality constant. The surface fractal dimension of the 3-D pit surface d F.surf is related to

F,ss by

[71] MathFigure 2

[72] After measuring and of each self-similar lake in Fig. 5, P is plotted against on a logarithmic scale, as depicted in Fig. 6. In order to confirm the reproducibility of the data, the measurement of d was performed more than five times on each specimen

F.surf using the different SEM images of the pits for the same specimen.

[73] Fig. 6(a-d) demonstrates on a logarithmic scale the dependence of perimeter P on area A of the pit obtained from the binary SEM images of specimens A, B, C and D, respectively, given in Fig. 5. For all the specimens, one can find clearly a linear relationship between log P and log A. From the slopes of the linear regions in Fig. 6(a-d), the values of d were estimated from Eqs. 1 and 2 to be 2.324, 2.264, 2.216

F.surf and 2.180 for specimens A, B, C and D, respectively. [74] [75] Experimental Example 5> Relation between d F.surf of the pitted Ni metal specimen and electrophoretic current during electrophoresis [76] Fig. 7 (a) plots the electrophoretic current I versus the applied electric field E exper-

P imentally measured on specimen A. It was found from Fig. 7(a) that the value of I is p linearly proportional to the value of E. This means that the higher the value of E is, the more move Ni nano-particles, resulting the rapid self -repairing of the pit.

[77] For larger E, the deposition rate correspondingly becomes higher but the surface of the repaired pit consists of more agglomerates of Ni nano-particles. While, for smaller E, the deposition rate is correspondingly lower but the repaired pit exhibits a smoother surface with less agglomerates of Ni nano-particles. As a result, it is strongly suggested that the applied electric field is an important external parameter in controlling the electrophoretic deposition rate and the surface property of the repaired pit.

[78] Fig. 7(b) demonstrates the plot of electrophoretic current I p against the d F,surf for four kinds of pitted specimens A (O), B(Δ), C(D) and D(V) during electrophoretic deposition under E of 20 Vtf. Here, the value of I was determined by taking the current

P value within the deposition time of 180 sec at which the current exhibits nearly the constant value. The increase in d F,surf im ^plies the more surface irreg °ularities, which leads to the enhancement of the electrochemical active area Aac. Since the elec- trophoretic current I is proportional to the electrochemical active area Aac, the higher

P the value of d F,surf is, the higher value exhibits I p . That is, the surface irregularities of the pit help the facilitation of the deposition of Ni nano-particles on pitted Ni metal specimen during electrophoretic deposition. [79]

Industrial Applicability

[80] As described above, the present invention provides a method of repairing pitting damage or cracks in metals or alloys using an electrophoretic deposition process. Compared to conventional repair methods, the repair method of the present invention is advantageous because it has a short process time period, may be performed at low temperatures to thus have no influence on the microstructure of the metal around the pitting damage or cracks, and can form a continuous and dense bond to realize superior mechanical strength of the repaired structure. Consequently, the repair method of the invention can be efficiently used in repair techniques of various kinds of metal or alloy having diverse pits or cracks, therefore being industrially applicable.

Claims

[1] A method of repairing pitting damage or cracks in metals or alloys using an elec- trophoretic deposition process, comprising:

(a) preparing nano-powder particles of a metal or alloy that is the same as a metal or alloy having a pitted or cracked region;

(b) adding the nano-powder particles of the metal or alloy in (a) to an organic solvent containing a dispersant, to thus prepare a stabilized colloidal dispersion;

(c) immersing the metal or alloy having the pitted or cracked region in the stabilized colloidal dispersion in (b); and

(d) applying an electrical field to the metal or alloy having the pitted or cracked region via electrophoretic deposition to concentrate current density on the pitted or cracked region of the metal or alloy such that the nano-powder particles of the metal or alloy having surface charges are intensively deposited in the pitted or cracked region of the metal or alloy, thereby forming a uniform layer.

[2] The method according to claim 1, wherein the nano-powder particles of the metal or alloy in (a) are prepared via levitation-gas condensation.

[3] The method according to claim 1 or 2, wherein the nano-powder particles of the metal or alloy have a size from 10 nm to 40 nm.

[4] The method according to claim 1, wherein the metal or alloy is at least one selected from among Ni, Zn, Fe, W, Sn, alloys thereof, and alloys based thereon. [5] The method according to claim 1, wherein the organic solvent in (b) is at least one selected from among C ~C alcohols.

1 5

[6] The method according to claim 1, wherein the dispersant in (b) is at least one selected from among polyvinylpyrrolidone, polyacrylamide, polyacrylonitrile, polyethyleneimine, polyglycine, polyacrylic acid, polymethacrylic acid, poly(3-hydroxybutyric acid), poly-L-leucine, poly-L-methionine, poly-L-proline, poly-L-serine, poly-L-tyrosine, poly(vinylbenzenesulfonic acid), and poly(vinylsulfonic acid).

[7] The method according to any one of claims 1 and 4 to 6, wherein the dispersant is polyvinylpyrrolidone and the organic solvent is a C ~C lower alcohol when the nano-metal powder in (b) is Ni.

[8] The method according to claim 7, wherein the electrical field in (d) is maintained at 10-120 Vd for 100-1,500 sec.

[9] The method according to claim 7, wherein the electrophoretic deposition in (d) is performed using an Ni or Ni alloy having a pitted or cracked region as a working electrode, a platinum electrode as a counter electrode, and a colloidal dispersion having Ni nano-powder particles dispersed therein as an electrolyte. [10] The method according to claim 1, further comprising heat treatment, after forming the uniform layer in (d).