Tip generation /substrate collection mode (TG/SC) TG/SCmethod is similar to the feedback mode of the SECM (Fernandez and Bard, 2003).In the TG/SC mode, the collector is the substrate while the tip is used tolocally generate the species which is examined. Here, the ME tip is held at apotential where an electrode reaction occurs and the substrate is held at adifferent potential. Here, the product of the tip reaction will response andthus be collected 66. In 2013, Leonard and Bard 78 had taken an initiative to studyon multireactional electrochemical interfaces using TG/SC mode.
This had beendone by modifying TG/SC mode where the two reactions occur on the tipelectrode, and the substrate electrode is held at a potential to collect onlyone of the products, allowing the determination of the individual partialcurrents. From the experiments, researchers were able to determine an exchangecurrent density for proton reduction on Mn in strong acid. 2.2.
3Ion-Selective Microelectrode (ISME) and Potentiometric Inorder to expand the functions of SECM, there is clearly an essential to developnew types of sensors as a probe, e.g., potentiometric sensors or in other name,ion-selective microelectrodes (ISMEs). ISMEs have been developed forpotentiometric examination of the local activity of targeted ions. Inpotentiometric mode, a potential difference between the ion-selectivemicroelectrode (ISME) tip and a reference electrode is examined under zerocurrent conditions.
This ISME tip does not affect significantly local corrosionevents. It is only detects variations of specific ion activity within thecorrosion sites. The ISME tip must be calibrated before and after measuring ionactivity.
In general, the potential of the ISME tip, Ei, depends on activity ofa specific ion in solution, ai, as provided by thewellknown Nernst equation: xxxxx In2007, Lamaka et al 79 had developed anMg-selective microelectrode (SME) and applied to study the corrosion activityin small defects of a coated magnesium alloy for the first time. It been usedfor mapping local activity of Mg2+ and H+ (by scanning ion-selective electrodetechnique, SIET) as well as local ionic current density measurements (byscanning vibrating electrode technique, SVET). Thus, it been reported that dueto the same principle of SIET and potentiometric mode of SECM, suggestedMg2+-SME can be applied as a probe for SECM and other methods exploiting thepotentiometric mode 54. Meanwhile,Kiss et al, 80 in his studies oncorrosion of a (Mg/Al)/Fe model galvanic pair were able to record Mg2+ ionconcentration profile before and during coupling.
Besides, the researchers alsostated that the combination of application of ISME and SECM can provide usefulinformation concerning on corrosion of metals. 2.2.4Application of SECM in corrosion studies Thereare few researchers whose studied in the background of SECM of Mg 55,59,81–83. Mostof the studies were applying SG/TC mode as their prominent mode, where the tipis held at -0.05V vs Ag/AgCl reference electrode and consequently H2bubbles could release form the Mg surface and formed corrosion on it. Thereaction that took place is: H2 à 2H+ + 2e- ThePt tip under such a potential when rastered across an Mg sample, canconsequently ‘image’ the cathodically active sites (H2 evolution sites).
Forexample, Tefashe, et al. in his studies towards AM50 alloys shows the SECMprofile of pits on the Mg surface (after 5 min), and high rates of H2 evolutionafter an hour. It is noted that a one-to-one correspondence between the Mgmicrostructure and H2 evolution from the surface cannot be directly.
Other thanthat, the studies also found that the H2 oxidation currents detected by the Pttip rise by two orders of magnitude after about an hour. In other case, a studyhave recently been performed by Jamali, et al. 76 on the Mg alloy AZ31,however it was challenging to link the one-to-one correspondence between thealloy microstructure, and the local electrochemical kinetics. Recently,local measurements on Mg using Mg2+ ion selective electrode beenperformed 75,84.
An Mg2+ ion-selective micro-tippedelectrode (ISME) had been developed by Izquierdo et al. It is able to measureMg2+ ion distributions over an Mg-based alloy in aqueouschloride-containing solution. Consequently, in 2013, Izquierdo and hisco-researchers 59,81, once again developedand fabricated faster, robust, solid contact, micropipette-based magnesiumion-selective electrode.
For this time, they constructed by using carbon fibercoated by polymer as internal contact. As aresult, it successfully employed to perform in situ experiments on corrodingsystem. This solid-contact ISME is greater in stability and capable to collectimages with a high spatial resolution. Besides, it’s been observed to read apotential change which is attributes to Mg2+ concentration andcorrosion current 81. 2.
2 Ion-Selective Microelectrodes (ISME) Ion selectivemicroelectrodes are a kind of analytical potentiometric electrochemical sensorthat can offer a suitable and low-cost method for ion measurements in liquidsample. Traditional solid-state ISMEpossess an internal filling solution that will be contributed to the overallelectrochemical potential of the electrode. However, a frequent error in thedetermination of selectivity is that it is determined when a non-Nernstianelectrode slope is detected for ions that exhibit less affinity towards theionophore. Normally, such complications arise when discriminated ion solutionscarry impurities of primary ions. A major source of contamination is the fluxof primary ions from the membrane to the sample 85,86. Consequently, theresulting selectivity coefficients may be biased by many orders of magnitude. Furthermore,it could limit the temperature range in sensor operation and contribute toundesired ionic fluxes in the membrane.
Research groups have focused on replacingthe inner filling solution and the internal reference electrode with a solidelectron conducting substrate since the early 1970s 87–90. 2.3.1 Component of solid contact ISME Conducting PolymerDueto the unique electrochemical properties of conducting polymer, such aselectron conductivity, redox capacitance and ion exchange capability, it hasbeen introduced as potential solid contacts in ISME. The reversible redoxreactions in the CP are called p-doping (oxidation) and n-doping (reduction).
Theoxidized/reduced of CP is complemented by a doping anion/cation. The material ionexchange properties can be improved considerably by changing the doping ion. Therefore,in the early reports, CPs had been suggested as a candidates or alternative toan ion selective membrane. However, this approach was unsuccessful due to some interferencecaused by the presence of redox reactants in solution, sample pH changes andpoor selectivity 89. Subsequently, researchers were proposed to electrodepositCP onto the electrical conductor. The ion selective membrane is then been gluedto the surface of the CP. The selectivity is directed by the membrane and CPacts as the ion-to-electron transducer in ISME.
Here, CPs demonstrates welldefined ion-to-electron transduction pathways it was found that high redoxcapacitance of the CP leads to a stable electrode potential. As a result, thesolid contact electrodes based on CPs with a combination of ion selectivemembranes have been the most effective technique to fabricate the ISME 90. Thefirst single piece all solid state ion selective electrode idea was introducedby Bobacka et al. 91 in 1995. By usingpolyaniline (PANI) doped with bis (2-ethylhexyl) hydrogen phosphate (DiOHP), aCa+ selective ion-selective electrode had been fabricated. A near Nernstianresponse ISE with stable standard potential was obtained due to the formationof this conducting polymer (CP). Years later, Bobacka and his researchers 92 proposed an advancedmodel of potentiometric ion-sensor response, which is a presence time-dependentcharacteristic of sensor response. The model considered is represented byfollowing scheme: sample| ion-sensitive membrane/ film | internal contact (e.
gsolution, gel, solid contact). Neutral or chargedcarrier (ionophores) ISEsbased on neutral or charged carrier (ionophores) as an ion-sensitive membraneis one of the most successful sensors applied in wide areas bobacka 2002. The new improved Mg2+ selectiveliquid membrane microelectrode based on new synthetic neutral carrier (ETH5214) had been designed by Hu et al. x in1989.
ETH 5214 is a modified ion carrier of N, N”-octamethylenebis(N’-heptyl-N’-methylmethylmalonamide), synthesised in 2-Nitrophenyl octyl ether(o-NPOE) plasticizer for intracellular magnesium activity measurements 43. Thisneutral ion carrier as a class of ionophore is electrically neutral lipophiliccomplexing agents, which have a capability to reversibly bind ions andtransport them through the organic membranes. The development of these ionophoresfor ISE had been extensively studied since 80’s centuries pretsch dan kene cari lagi. A molecular modellingtechnique is applied in order to design a stable reproducible sensor pretsch. Recently, ETH 5214 andETH1117 have been successfully used in Mg2+ ISME by Lamaka 33 and Javier x to image Mg2+ emanates from Mgcorroded samples. 1. 2.
3.2 Application of ISME incorrosion research ISMEhad been successfully used in SECM due to its advantages such as highsensitivity and low detection limit bard dr bobacka. In 2007, solid contactammonium-selective microelectrode had been fabricated for SECM measurements gyetvai. Recently, in corrosion studies area, asolid-contact Mg2+ ISME was developed and reported by Souto et al. 40-41. It had been used to map Mg2+ iondistribution in localized corrosion systems using SECM.
Tocalibrate the ISME, standard solutions containing a known concentration of targetedions had been used and an experimental relation is derived between the ISMEpotential (versus the reference electrode) and p(M+) –log10M+concentration . The relationship between the M+ion concentration and the ISME potential should ideally follow the Nernstequation (Eq. 1) Eq.1Where E is the potential, K is constant, F isthe Faraday constant, zi and ?i are theionic charge and activity of ion, respectively. When the ISME is used as aprobe in the SECM, a potential distribution across the Mg surface is monitored.The potentials are then converted to pMg2+ (using the practicalequation from experimental ISME calibration) which reveals the Mg2+ concentrationdistribution arising from the corroding Mg surface souto.
In 2017, Kiss et al, x in his study on galvanic corrosionusing ISME had been explained on effect of strong electric field during SECMmeasurements. Result of the potential difference between the surface and thegalvanic had been affected by the electric field and consequently influencedthe potential of the sensing ISME. Therefore, it should be consider that measuredpotential is the sum of these two contributions kiss.One of the alternatives suggested by Kiss is to bring the referenceelectrode very close the ISME and thus electric field experienced by these two electrodesis equal, and as a result, it will cancel it out.