Micro-scale potentiodynamic polarisation curves of 316L stainless steel

Description

This database comprises 5 Potentiodynamic Polarisation (PP) datasets. Each dataset consists of a pair of CSVs: 1 file containing the values of the applied potential scan rate (mV/s); and 1 having the corresponding current density j (µA/cm²) values. This database was first deployed in the following scientific article, accepted for publication in Corrosion Science journal on 10 March 2023: "Probing the randomness of the local current distributions of 316L stainless steel corrosion in NaCl solution". Leonardo Bertolucci Coelho1,2,∗, Daniel Torres1, Miguel Bernal1, Gian Marco Paldino3, Gianluca Bontempi3, Jon Ustarroz 1,2 1 ChemSIN – Chemistry of Surfaces, Interfaces and Nanomaterials, Université libre de Bruxelles (ULB), Brussels, Belgium 2 Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel, Brussels, Belgium 3 Machine Learning Group (MLG), Université libre de Bruxelles (ULB), Brussels, Belgium *leonardo.bertolucci.coelho@ulb.be 955 PP curves were recorded on the same 316L sample using the Scanning Electrochemical Cell Microscopy (SECCM) in hopping-mode protocol. A total of 12 SECCM sessions were performed, comprising maps with up to 17x17 points (during up to 24 h of continuous measurement). Five different combinations of [NaCl] and voltammetric scan rates were employed: 0.005 M NaCl – 100 mV/s, 0.01 M NaCl – 100 mV/s, 0.01 M NaCl – 50 mV/s, 0.05 M NaCl – 100 mV/s, 0.05 M NaCl – 50 mV/s. The number of data samples (j Vs E curves) is 287, 377, 119, 125 and 47 for each dataset. These quantities of curves were considered representative of each testing condition, as the underlying j distributions were continuous. The datasets were sliced from 0.5 V Vs Ag/AgCl (considerably more positive than the OCP) upward to avoid the eventual and deleterious occurrence of negative currents. Positive j values were preferred for building histograms and obtaining defined log(j) numbers. The SECCM experiments were designed to detect low corrosion currents, and the trade-off for such a high signal sensitivity was the I saturation level at 10 nA. In case of missing data points in the anodic sweep data (most often caused by oversaturation at high applied overpotentials), these were replaced by data interpolation (Python pandas interpolate() method, spline). This data-filling procedure was used for a small proportion of the data populations (0%, 0%, 1%, 1% and 11%, respectively, for the datasets with increasing testing aggressiveness). The final shapes of the j Vs E curves were validated against similar curves with unsaturated signals at the relevant E ranges.

Steps to reproduce

The investigated stainless steel consisted of an industrial plate of 316L (Aperam, France) subjected to electropolishing (bath temperature = 65 °C, current densities between 20 and 40 A/dm2) followed by passivation (a proprietary process by Packo Inox, Belgium). For SECCM testing, one squared coupon (~1 cm²) was cut with a lever cutter and degreased in acetone (10 min in ultrasonic bath). In brief, the SECCM probe was brought into contact with the surface at randomly selected locations, and upon each approach, 1 potentiodynamic polarisation was recorded. During each approach of the nanopipet, the working electrode current was used as a signal to detect when the meniscus cell had made contact with the SS substrate (the magnitude of the threshold current used was ca. 6.5 pA, under an applied potential of −0.6 V Vs Ag/AgCl quasi reference counter electrode (QRCE)). Next, the probe retracted and moved to a new location automatically (in point grid fashion) to acquire the corrosion data. Single-barrel pipets (borosilicate) were pulled with a Sutter P2000 pipet puller to a final internal circular diameter of ~2 µm, and were used as SECCM probes. The droplet size is expected to be of similar dimensions as the probe, and this was confirmed with preliminary FESEM images of the SECCM footprints on the SS surface. Current density j was then calculated by normalising the measured current by 3.14 µm², assuming a consistent droplet size. Ag/AgCl QRCEs, well-known for their long-term stability, were inserted into pipets filled with the NaCl electrolyte. The data were recorded with an acquisition rate of 7.68 ms (sampling rate of 30 µs averaged 256 times). The time constant of the current amplifier was 100 µs. The starting E was -0.5 V, and the end anodic E was 1.355 V (Open Circuit Potential (OCP) values varied between 0 and 200 mV Vs Ag/AgCl). A custom-built potentiostat featuring current collectors with a sensitivity of 1 nA/V was employed, and the data collection system (PCIe-7856R FPGA card, National Instruments) was controlled by LabVIEW (2019, National Instruments) interface running Warwick’s software (WEC-SPM, www.Warwick.ac.UK/electrochemistry). To avoid any perturbation from sound, light or mechanical vibrations and reduce electrical noise, the experiments were performed inside a faraday cage (Thorlabs) equipped with acoustic insulation panels (Kevothermal) and aluminium heat sinks, mounted on top of an optical table (TMC Ametek) with automatic levelling.

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