ABSTRACT

Due to its non-invasive nature and today's improved data acquisition electrochemical noise (ECN) techniques are about to become a reliable monitoring tool in laboratory and industry. The newly developed CoulCount method, based on current noise measurements will surely contribute to this due its versatile applicability. The time-related cumulated noise charges extracted from the noise signals between two ,identical' electrodes yield a direct quantitative easy-to-interpret information on time-related corrosion intensifies. The multitude of advantages of the new software-assisted method are outlined and demonstrated in a failure analysis on the cause of crevice corrosion at spigots of automotive cooling water pumps.

INTRODUCTION

Electrochemical noise signals can be taken as the language of corrosion systems. However, not in all cases is it easy to understand this language. The analysis of electrochemical noise data aims at getting detailed information on time-related uniform or localized corrosion activity on the surface of the coupled electrodes. The signal analysis is performed via different and more or less sophisticated data evaluation methods which have been summarized many times in the literature, e.g. in reference [ 1 ], and shall not be discussed in this paper. The present paper will discuss the results of efforts to evaluate electrochemical noise data between freely corroding ,identical' electrode pairs in a way that they are easy to interpret and can be used as a reliable tool for online monitoring of corrosion systems under laboratory and service.

THE COULCOUNT METHOD

In a Faraday cage a pair of electrodes as ,identical' in material and surface properties as possible is exposed under free corrosion conditions in the selected corrosive environment. In the CoulCount- Method only current noise and not potential noise is meassured (Figure 1). This avoids the use of a reference electrode which always presents problems in long term monitoring under service conditions due to fouling, potential drifts and degradation.

Specifically in systems with low corrosion rates the current fluctuations between the pair of electrodes have to be meassured with a highly sensitive and accurate current measuring system. For this purpose a signal acquisition card (SAC) has been developed which allows to measure small currents with a low external noise level and to convert this signal into a proportional voltage signal. The SAC is also positioned in the Faraday cage. The voltage signal is led to the data acquisition card in the PC where the raw data are stored with a specially developed data acquisition software. The aim is to collect (count) on a given DC level the total amount of noise charges exchanged between the pair of electrodes in a given time interval. The counting of noise charges occurs regardless of which electrode released the electrons. This typical function of counting of noise electrons lead to naming this kind of data evaluation the CoulCount Method (Counting Coulombs).

From the raw noise signals to the evaluation of time-related amounts of noise charges there are three steps" 1. scanning of the current signals, 2. elimination of the DC part, 3. summarizing of the time- related current levels. Scanning of Current Signals

The analog signal is digitalized with a scanning rate of 20 Hz using a 16 bit data acquisition card in the PC. The data acquisition can occur continuously o r - in case of long time monitoring - in measuring intervals (e.g. every 60 min in a intervals of 15 min).

Elimination of the DC Part

The raw current noise signal contains a DC part and a noise part (Figure 2). The DC part is the part of the signal wit

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