How to check and correct non-stationary EIS measurements using EC-Lab^® (part 1) Corrosion – Application Note 69-1

Abstract
This application note presents the various tools available in EC-Lab® that can be used to check and correct the non-stationarity of your EIS measurements. Using galvano control, the proprietary Non-Stationary Distorsion indicator, and the instantaneous impedance analysis tool Z Inst, users can ensure that EIS measurements are correctly interpreted and fitted. This application note shows the use of such tools on data obtained on a corroding electrode.

Introduction

For valid Electrochemical Impedance Spectroscopy (EIS) measurements, the system under investigation should be linear, stable, causal, and stationary [1,2]. In this note, the term “stationarity” comprises steady-state and time-invariance.

Steady-state is the state of a system after its transient state. For example, an R/C circuit submitted to a potential or current step is in a transient state and sees its response change after a certain amount of time has passed.

Time-variance refers to a system where parameters define its transfer function change with time. As an example, a corroding electrode whose polarization resistance changes over time, either because of corrosion or of passivation, is a time-variant system.

The two properties may be difficult to separate.

The most classical use of EIS in corrosion is for the determination of the polarization resistance R_p using the Stern or Wagner-Traud relationship [3-7]. A corroding system is a non-stationary system specifically after the first instant of immersion. The change in parameters can greatly affect the impedance data, especially at lower frequencies [8].

In this first part of application note 69, we will present various tools implemented in EC-Lab® to help check and correct the time-variance of measurements. These tools will be applied to measurements on a corroding mild steel electrode. The second part will apply these tools to a discharging battery [9].