Investigations on kinetics of electrode processes-studies on exponential transient technique to corrosion kinetics

Abstract

Research efforts are constantly directed towards development of reliable methods for estimation of corrosion rates (icorr) of metals in solution. The present study is an attempt in this direction. Following a brief presentation of the concepts of electrodics and corrosion kinetics, various electrochemical techniques for estimation of icorr have been critically reviewed. The potentialities of the accelerated Tafel plot (ATP) technique – a transient polarization technique – for application to corrosion kinetics are traced. Subsequently the experimental methodology to analyse then –t transient response through slope and intercept measurements proposed earlier for obtaining the kinetic parameters has been described. The results on icorr data obtained from ATP measurements for the chosen corrosion systems, viz., nickel, iron and steels in acids have been presented and compared with the icorr data collected from steady-state polarization and potential - sweep methods. Simulation studies on a model involving a faradaic reaction and double layer charging have been undertaken to delineate the role of double layer charging and understand the observed variation of icorr at low input time-constants in the ATP method. Based on these studies , two new methods for analyzing ATP data have been evolved, which are free from the drawbacks of the intercept-slope method. The ATP data for the corrosion systems under investigation have been subjected to analysis by the two new methods which yielded satisfactory results. Efforts to assess the ATP method for its suitability to study inhibited corrosion reactions are also presented. The inhibition efficiency data resulting for the corrosion of mild steel in sulphuric acid containing thiosemicarbazide indicated the usefulness of the technique. The efficacy of the two new methods of analysis has also been checked by applying to the redox system viz. ferric-ferrous on nickel. The exchange current data are consistent with expectations. Finally, the related exponential transient technique with linear polarization has been described and taken up for experimental confirmation of its utility using ferric-ferrous on nickel as a model system