The effect of test conditions on the sensitivity and resolution of SRET signal response

Abstract

Purpose -The aim of this paper is to study and analyse the advantages and limitations of the scanning reference electrode technique (SRET) to detect and assess localised electrochemical activity based on an evaluation of the influence of the principal test parameters on the sensitivity and resolution of the technique. Design/methodology/approach -Measurements of Ohmic potential gradients induced by ionic flux close to a point current source (PCS) were carried out using a scanning reference electrode technique (SRET) instrument, which comprised a vertical rotating working electrode and a scanning probe formed by a pair of platinum electrodes of approximately 200 mm of diameter. Ionic flux was induced by anodic polarisation applied to a gold micro-disc electrode, which acted as the PCS. Measurements were conducted in electrolytes of ten different conductivities, using different scanning probe tips to sample surface distance and different working electrode rotation rates. The range of conductivities used included most of the possible electrolytes to which metallic materials can be in contact under real service conditions. Findings -The SRET signal sensed from a polarised PCS showed a strong dependence on the rotation rate of the working electrode for electrolytes of low conductivity but a minimal effect on electrolytes with conductivities higher than 50 mS/cm. Originality/value -This work presents the effect of wide variations on the electrochemical and operational conditions on the sensitivity and resolution of SRET signal response and discusses the limitations of the technique to assess localised electrochemical activity due to the effect of high conductivity electrolytes, large separation distance between the SRET scanning probe and developing dissolution and scanning rate of a the localised site.

Quantitative determination by SRET measurements of pit dissolution rate of AISI 304 stainless steel in natural sea water

“…The difference between SRET and material removal measurements can be attributed mainly to the proper nature of the microelectrode scanning technique. 15,18,23 Scanning reference electrode technique measurements for the quantitative assessment of localised corrosion under the experimental conditions used in the present work can be considered an acceptable approach for real time results. In a study of hydrogen evolution during anodic polarisation of pure Mg and AZ31 Mg alloy in chloride containing electrolyte, Williams et al applied numerical integration of local current distributions obtained from scanning vibrating electrode technique measurements providing quantitative results of localised electrochemical activity.…”

“…A previous determination of the dissolution current of a pit from SRET measurements was carried out using equation (1), obtained from a calibration procedure. 23 with V MSD as the maximum output signal detected by SRET in mV, I is the current emanated from the pit in mA, whereas a (28.092) and b ( − 0.016) are constants that depend on the electrolyte conductivity, the rotation rate of the working electrode and the distance between the SRET probe tip and the working electrode surface.…”

“…Scanning reference electrode technique measurements can be converted into quantitative data through specific relationships between the current generated by a site of localized activity and the magnitude of the potential gradient detected by the SRET (the calibration routine of the SRET system). 17,19,23…”

“…The nature of the metal–electrolyte system in which localised corrosion takes place, as well as the sensitivity and resolution of the equipment, has great effect on the quantitative assessment of localised dissolution rate through SRET measurements. 19,23 Once the effect of test conditions on the sensitivity and resolution of SRET signal response are established, 23 direct real time measurements of localized dissolution can be carried out from at least a semiquantitative approach.…”

Recent Researches in Corrosion Evaluation and Protection

“…For the case of studies of localised corrosion in diluted electrolytes, this is a useful finding. It is of paramount importance an accurate understanding of the effects that operating parameters of the SRET equipment has on the measured output signal (Dzib-Pérez et al, 2009). Studies of pitting corrosion of AISI 304 stainless steel in natural seawater were conducted performing SRET measurements on samples under cyclic anodic potentiodynamic polarisation from the open circuit potential to the repassivation potential as shown in Figure 15, (Dzib-Pérez, 2009;González-Sánchez, 2011).…”