Development of a Liquid-Phase Ion Gun and Its Application for Sulﬁdation of Silver Surface

Asystemforsafegenerationofsulﬁdeionswasestablishedbyusingthemicroelectrodetechniqueinordertoinvestigatesulﬁdationofthemetalsurfaceofsilver.BothsilvermicroelectrodesulﬁdationandsilversulﬁdereductiononthemicroelectrodewerereversibleinNa 2 S solution and corresponded to Ag 2 S formation and HS − generation, respectively. Cathodic polarization of Ag 2 S, which covered the silver microelectrode, in pH 8.4 boric-borate buffer solution successfully generated HS − above a glass or silver substrate. Concentration of HS − in the vicinity of the substrate was dependent on the distance between the microelectrode and the substrate. The silver substrate was locally sulﬁdated by HS − generated from the microelectrode. However, at potentials higher than 0.14 V RHE , local sulﬁdation of the silver substrate was independent of the substrate potential. It is thought that mass transport of HS − is dominant for sulﬁdation of the silver substrate. 13 15 any

in environments containing sulfide ions and cause various types of 23 sulfide-induced corrosion such as general corrosion, localized corro-24 sion, and stress corrosion cracking. Considerable experience has been 25 acquired concerning corrosion behavior of metals in sulfide-induced 26 corrosion. [1][2][3][4][5] Many researchers have attempted to create a sulfide ion-27 containing environment by flowing H 2 S gas 6-8 or adding Na 2 S 9-11 into 28 aqueous solutions in order not only to control concentration of sulfide 29 ions but also to investigate sulfidation behavior of metals. However, 30 it was difficult to concentrate with an infinitesimal amount of sulfide 31 ions on a local area. Moreover, sulfide ions can produce H 2 S, which is 32 an extremely toxic gas and accelerates degradation of the experimen- mechanism of a passive film on iron by using a local chloride ion gen-53 eration system. 17 They also reported that depassivation susceptibility 54 of iron was dependent on applied potential and electric field as well as 55 solution pH. 18  A silver wire with a purity of 99.9% and a diameter of 500 μm was 69 embedded in a glass capillary with an outer diameter of 1 mm using 70 an epoxy resin. The cross section of the silver-glass capillary tip was 71 used as a silver microelectrode after mechanical polishing with SiC 72 papers down to 4000 grit and rinsing with distilled water. Figure 1 73 shows an optical microscopic image of the tip of the fabricated silver 74 microelectrode. A silver plate with a purity of 99.9% and a surface area 75 of 0.8 cm 2 was prepared as a substrate electrode. The silver substrate 76 was mounted in an epoxy resin mechanically ground with SiC papers 77 down to 800 grit and then rinsed with distilled water.

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Electrochemical experiments of using silver microelectrode and/or 79 the silver substrate electrode were carried out in a four-electrode 80 electrochemical cell of 100 cm 3 in volume with a platinum counter 81 electrode and an Ag/AgCl/sat. KCl reference electrode. However, 82 all the potentials in this study were with respect to the reversible 83 hydrogen electrode (RHE) potential. Cyclic voltammetry (CV) of the 84 silver microelectrode was conducted in a potential range between 0.38 85 and -0.06 V RHE in 0.1 mol dm -3 Na 2 S solution (pH 13.4) at a scan 86 rate of 20 mV s -1 . After a steady state had been obtained in CV, the 87 microelectrode was polarized at 0.3 V RHE in the same solution until the 88 electric charge of 10 mC, Q me.charge , was consumed. On the other hand, 89 potentiodynamic polarization of the silver substrate was performed in 90 a potential range from 0.7 to 1.1 V RHE at a scan rate of 1 mV s -1 in 91 pH 8.4 boric-borate buffer solution.
92 Figure 2 schematically depicts the experimental setup for liquid-93 phase ion gun. An optical microscope with a resolution of ca. 5 μm 94 and a stepping motor stage (SGSP20-35, Sigma Koki) with an incre-95 mental motion of 0.1 μm were used to control the distance between 96 the microelectrode and substrate of a grass plate or silver electrode. 97 The silver microelectrode was positioned above the substrate with a 98 distance of 125, 250, 500, 750, 1000 or 10000 μm. A bipotentiostat 99 (HAL-1512mM2, Hokuto Denko) independently controlled potentials 100 of the microelectrode and the substrate. The microelectrode potential, 101 E me , was initially kept at 0.4 V RHE for 100 s and then changed to 0.0 102 V RHE , whereas the silver substrate potential, E sub , was potentiostat-103 ically controlled at 0.04, 0.14, 0.24, 0.34, 0.54, 0.64 or 1.00 V RHE . 104 The silver substrate was also polarized at the same potential condition 105    The results demonstrate that the anodic reaction of silver and the 130 cathodic reaction of silver sulfide correspond to the following back-131 ward and forward reactions in an aqueous solution, respectively: 132  Figure 7a shows transients of currents I me and I sub of the Ag/Ag 2 S 185 microelectrode and silver substrate, respectively, in a pH 8.4 boric-186 borate buffer solution when potential of the microelectrode E me was 187 changed from 0.4 to 0.0 V RHE with potential of the substrate E sub being 188 kept at 1.00 V RHE . As discussed above, the generation of HS − indicates 189 a cathodic current flowing through the microelectrode, although the 190 current spike for charging is observed at the beginning. After 1-2 ks 191 from onset of the HS − generation, peaks are seen in both I me and I sub . 192 The value of I me is almost constant, while that of I sub is dependent on 193 the distance between the microelectrode and the substrate. Distance 194 independency of I me disagrees with the case on a glass substrate. Thus, 195 the HS − generation is affected by reaction of HS − with the substrate 196 as well as diffusion in the narrow space.
197 Figure 7b shows the electric charge Q sub consumed at the silver 198 substrate as a function of the electric charge Q me consumed at the 199 Ag/Ag 2 S microelectrode during the generation of HS − . The value of 200 Q sub increases linearly with increase in Q me . The slope of the linear 201 relation between Q sub and Q me increases with decrease in the distance 202 between electrodes. The slope at the distance of 125 μm is close to -1, 203 demonstrating that an anodic current equivalent to the cathodic current 204 for HS − generation flows through the silver substrate. It is thought that 205 the anodic reaction on the substrate is dominantly affected by HS − 206 generated from the microelectrode. From the larger space between 207 electrodes, a large amount of HS − can diffuse out to the solution bulk 208 instead of the silver substrate surface. The shortage of HS − diffusion 209 results in a decrease of the anodic current.  the silver wire in the microelectrode itself but with an outer diameter 216 of a microelectrode sheath. The products were confirmed to be Ag 2 S 217 from the XRD pattern (Fig. 8b). The silver substrate surface is locally 218 sulfidized by HS − ions generated from the microelectrode.  On the other hand, a protrusion of silver built up on the microelec-241 trode surface during HS − generation was observed. At a very close 242 distance, this may lead to the formation of a short circuit between 243 the electrodes, which are inappropriate for sulfidation of the specimen 244 surface using the Ag/Ag 2 S microelectrode in this study.
245 Figures 10a and 10b show transients of currents I me and I sub of 246 the Ag/Ag 2 S microelectrode and silver substrate, respectively, during 247 HS − generation by changing the microelectrode potential E me from 248 0.4 to 0.0 V RHE at a distance of 250 μm when the silver substrate 249 was polarized at various values of E sub . Regardless of the values 250 of E sub , the cathodic current for HS − generation shows almost the 251 same behavior, suggesting that HS − generation is not affected by the 252 silver substrate potential. However, the current flowing through the 253 silver substrate is strongly associated with the applied potential. As 254 seen in Fig. 6, the silver substrate is oxidized and this leads to the 255 flow of an anodic current at potentials higher than 0.73 V RHE , while 256 sulfide or water is reduced at lower potentials. Figure 10c shows 257 the relation between electric charges Q sub and Q me consumed at the 258 microelectrode and substrate, respectively. The slope corresponds to 259 sulfidation efficiency of the substrate at potentials higher than 0.64 260 V RHE . Thus, the efficiency is unity at 1.00 V RHE . However, negative 261 slopes at potentials lower than 0.54 V RHE suggest that a cathodic 262 reaction like reduction of contaminated oxygen is dominant.   Figure 11. Electric charge Q sub.end consumed at the silver substrate until HS − generation of 10 mC is completed as a function of the substrate potential E sub . The charge Q sub.end was subtracted from Q sub.end by that of Q sub.control consumed for oxidation.
Potential of LPIG as a sulfide ion generation apparatus.-In gen-279 eral experiments to investigate a sulfidation of silver, Ag 2 S layer forms 280 on the silver surface in the presence of sulfide ions-containing media 281 such as H 2 S, K 2 S and Na 2 S. 1,4,7,[25][26][27] The sulfidized layer in this study 282 by using Ag/Ag 2 S microelectrode (Fig. 8) is Ag 2 S. This result is not 283 new findings. However, we have successfully developed a very safe 284 sulfide ion generator with an amount of HS − of 5.2 × 10 -8 mol. A 285 generation of HS − is possible to concentrate with an infinitesimal 286 amount of HS − on a local area for the first time. Although the local 287 concentration HS − for sulfidation is estimated as 0.5 mol dm -3 , the 288 average concentration of HS − is lower than 0.5 ppm in the electro-289 chemical cell of 100 cm 3 in volume until the completion of HS − gen-290 eration. This is sufficiently smaller than the ceiling limits of H 2 S in air, 291 Figure 10. (a, b) Transients of currents I me and I sub , and (c) relation between electric charges Q sub and Q me when potential E me was changed from 0.4 to 0.0 V RHE with the substrate polarized at E sub = 0.04, 0.14, 0.24, 0.34, 0.54, 0.64, or 1.00 V RHE . The interelectrode distance was kept at 250 μm.
) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.81 Downloaded on 2018-07-18 to IP 20 ppm. 28 We consider that this experimental system is fairly safe for 292 an HS − generation apparatus.