The Influence of Electrolyte Movement on Measured Electrochemical Noise

Legat, Andraz (Slovenian National Building)

OnePetro 

ABSTRACT
In the present study the influence of movement of the solution on the characteristics of measured electrochemical noise (EN) has been investigated. For this reason the measurements were performed in still solution, as well as in movement of electrolyte. In order to relate measured EN to the development of corrosion processes, digitized images of the electrodes were recorded continuously during these measurements. It was found that the characteristics of EN, in general, change significantly with strong movement of the solution: stirring, or laminar flow. The results of this study confirmed that main source for this change is transformation of corrosion processes: in a still solution the corrosion process tends to be localized, whereas during strong movement of the electrolyte this tendency is oriented towards uniform corrosion. It was established that the direct effect of the electrolyte movement on measured EN (modulation of signals due to spatio-temporal flow disturbances) is small.

INTRODUCTION
Electrochemical noise (EN) consisted of potential and current fluctuations spontaneously generated by corrosion reactions °4). These fluctuations can be measured in freely corroding systems, and therefore the measurements and analysis of EN is one of the most promising methods for the detecting various types of corrosion: uniform corrosion, metastable pitting, pitting corrosion, crevice corrosion, stress-corrosion cracking C4t5). Several attempts were made to characterize these corrosion processes by means of measurements and analysis of EN 14"5' 16-z5), but no unified theory still exists to explain the sources of EN.

In few of numerous study of EN for monitoring corrosion in various environments it was found that stirring of solution may represent additional source of potential and current fluctuations: it was indicated that the amplitude of EN increased modestly as the flow rate was increased C26"27). It was established during previous investigations (2s) that the nature of EN measured with stirring of the electrolyte is rather different than without it. In the case of stirring the high-frequency part was ordinarily quite high, whereas EN measured in the still solution usually consisted of low-frequency fluctuations of electrochemical potential and current. The influence of different types of electrolyte movement on measured EN, however, has not yet been exactly determined.

Since the EN is generated by corrosion reactions, the influence of the electrolyte flow on corrosion processes has to be considered as a first. It was established (27-32) that corrosion processes may involve a number of mass transport stages which can be influenced by solution flow: a) delivery of reactants to the anodic site, b) removal of products from the anodic site, c) analogous mass transfer at the cathodic site, property changes due to anodic film repair or breakdown. The superimposed effect of aeration and consequent oxygen reduction was also confirmed in certain systems. These observations could explain the changes in measured EN due to electrolyte flow mentioned above (2s'26), but some direct effect of the electrolyte hydrodynamics (modulation of EN signals due to spatio-temporal flow disturbances in electrolyte) could also be possible.

The main aim of this study was to investigate the influence of various types of solution movement on the characteristics of measured EN. In order to determine the main source of certain types of fluctuations in EN, digitized images of electrode surfaces were recorded at the same time as the measurements of EN were made. The emphasis was made also to assess the eventual direct influence of the solution movement on measured EN: it was assumed that the turb