Nonlinear Electrochemical Impedance Spectroscopy (NLEIS) is a method of characterizing an electrochemical systems by applying a large amplitude sinusoidal perturbation signal. This article presents a comprehensive review of existing literature on NLEIS analysis development and applications. Two types of NLEIS reports available in the literature, viz. one in which only the fundamental impedance is studied and the other in which both the fundamental and higher harmonic responses are analyzed, are reviewed. Early reports on the development of NLEIS methodology are compiled. Recent advances in the application of NLEIS, in particular in the area of fuel cells, are summarized. In addition, applications of related techniques such as total harmonic distortion and Volterra kernel are included in the purview of this article. There is a large scope for applying NLEIS to understand electrochemical processes. The roadblocks that need to be overcome to enable wider application of NLEIS are identified, and directions for further research are suggested.

We investigated the anodic dissolution of polycrystalline titanium rotating disc electrode in 0.1 M hydrofluoric acid in active and passive regions using potentiodynamic polarization and electrochemical impedance spectroscopy. In the active region, complex plane plots of the impedance spectra exhibited three capacitive loops indicting the presence of at least two adsorbed intermediates. In the passive region, they exhibited a negative resistance and a low frequency inductive loop. The high frequency loops exhibit constant phase element behavior indicating that a heterogeneous 2D film with partial surface coverage is present in the active and passive regions. The impedance data was fit to a reaction model and a four step mechanism with two adsorbed intermediate species is proposed to explain the observed trends in the active and passive regions. The change in the surface coverage of the adsorbed intermediate species, with the overpotential is estimated. This model describes the dissolution of Ti via two parallel paths, viz. a chemical step and an electrochemical step. The onset of passivation ensues when the rate of TiO 2 film formation is more than that of its dissolution. Transpassive dissolution becomes dominant when the electrochemical dissolution from the bare surface becomes negligible. Titanium metal and its alloys are widely used in engineering applications and medical implants, due to their low density, high corrosion resistance and high strength.1 These materials possess excellent corrosion resistance in acidic media, through the formation of protective oxide film on the metal surface. In order to facilitate the attack of the passive oxide layer, solutions containing fluoride ions are used for the anodic dissolution of titanium. [2][3][4] The corrosion studies of titanium show that at low overpotentials, the dissolution is in active region, and at moderate overpotentials, surface passivation occurs. At high overpotentials, transpassive dissolution is observed.The dissolution mechanism of titanium in solutions, with and without fluoride ions have been characterized using various techniques. The dissolution of Ti in 10 M H 2 SO 4 was investigated using polarization and impedance techniques 5 and it was reported that the dissolved species is Ti 3+ . To the best of our knowledge, the earliest report on electrochemical studies of Ti dissolving in acidic fluoride ions was published by Kelly.6 Polarization studies show clear active, passive and transpassive regions. 6 The impedance spectrum was acquired in the transpassive region and was analyzed to show that the film thickness increased with the potential. 6 The impedance of Ti in solutions containing strong acids (H 2 SO 4 and HNO 3 ) along with 0.1 mM -10 mM hydrofluoric acid was analyzed 4 and a four step reaction mechanism involving Ti + , Ti 2+ and Ti 3+ adsorbed intermediate species and Ti 3+ dissolved species was proposed to explain the results. In each case, the spectra were acquired at only one potential. The confidence in the model identification wo...

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Anodic dissolution of Ti metal in varying concentrations of hydrofluoric acid (10-1000 mM HF) was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. Polarization curves obtained at all concentrations of HF solution showed that current initially rises with potential, and decreases at higher potentials. Complex plane plot of the impedance spectra of Ti dissolution in 25 and 50 mM HF solutions exhibited three capacitive loops in the active region and a capacitive behavior with negative resistance in the passive region. The surface was characterized using X-ray photoelectron spectroscopy. A four step mechanism with chemical and electrochemical dissolution steps was employed to model Ti dissolution. The analysis shows that the simulated results match the polarization trends satisfactorily in all the solutions investigated. The model predicts that HF − 2 and remaining HF are the species that influence chemical and electrochemical dissolution steps respectively. The estimates of surface coverage values of the adsorbed species showed that with increasing over potential, the metal surface was progressively covered with oxide film, while an increase in nominal HF concentration leads to a decrease in fractional surface coverage. Titanium is used as engineering material in medical implants, aerospace applications, and in industrial applications by virtue of its low density, high strength and high corrosion resistance.1 Due to the presence of a tenacious passive oxide layer on its surface, Ti is highly corrosion resistant in most of the harsh chemical environments. 2,3However, the oxide layer readily dissolves in acidic fluoride medium. The anodic dissolution phenomena of Ti in fluoride media is important not only in corrosion but also in several technologies and industrial applications such as pickling of Ti, 4 manufacture of Ti based automobile parts, 5 and formation of self-organized TiO 2 nano tubes. 2,3,6Ti dissolution in fluoride medium has been studied extensively, and its dissolution in acidic fluoride media depends mainly on the concentrations of various species formed by HF dissolution, pH of the solution and temperature.3,6-8 Straumanis and Chen measured the volume of hydrogen produced during Ti dissolution in HF and proposed that the reaction order is 0.66 when the HF concentration is below 0.2 N and unity when it is more than 0.2 N.9 Ti dissolution in a solution of 0.1 N HF + 1 M HCl was measured at various temperatures, and the order was reported to be 0.76 while the activation energy was 6.9 kcal/mol. 10 HF, in combination with HNO 3 and a few other additives such as acetic acid, H 3 PO 4 , dodecyl benzene sulfonic acid, carbamide, sodium benzoate or sodium nitrate, have been used for chemical milling of In Ti pickling, addition of nitric acid is used to prevent hydride formation at the metal surface, which can lead to hydrogen embrittlement. 16 Ti dissolution in HF + HNO 3 solution is modeled using a mechanistic approach with Ti 3+ and TiO 2 as intermediate species. 1...