The rise in electrochemical noise (EN) as a corrosion monitoring technique has resulted in unique problems associated with the field application of this method. Many issues relate to the EN probe design. The ability of an electrochemical noise monitoring system to identify and discriminate between localized corrosion mechanisms is related primarily to the capability of the probe to separate the corrosion cell anode from the corresponding cathode. Effectiveness of this separation is largely determined by the details of and the proper design of the probe that is in the environment of interest. No single probe design or geometry can be effectively use in every situation to monitor all types of corrosion. In this paper we focus on a case study and probe development history related to monitoring corrosion in an extremely hostile environment using EN. While the ultimate application of EN was and continues to be successful, the case study shows that patience and persistence was necessary to meet and properly implement the monitoring program. Other possible source of problems and frustration with implementing EN are also discussed.
INTRODUCTION AND BACKGROUND
The rise in electrochemical noise (EN) as a corrosion monitoring technique has resulted in unique problems associated with this method. Electrochemical noise systems use a three electrode configuration to make simultaneous, instantaneous measurements of zero resistance ammeter (ZRA) current between two electrodes (WE1 and WE2) and potential difference between the ZRA coupled electrodes and reference electrode (Figure 1). While other electrochemical methods also may also use three electrodes, electrochemical noise is fundamentally different from other corrosion monitoring techniques because its strength lies in its ability to identity and discriminate between localized corrosion phenomena.
Figure 2 shows schematically the objective of an EN probe design. The objective is to simultaneously measure electrochemical current noise (ECN) and electrochemical potential noise (EPN) using a zero resistance ammeter and high impedance voltmeter, respectively. The ability of an electrochemical noise monitoring system to identify and discriminate between localized corrosion mechanisms is related primarily to the capability of the probe to separate the corrosion cell anode from the corresponding cathode. The effectiveness of this separation is largely determined by the details of and the proper application of the corrosion surrogate (i.e. the probe) which is inserted into the environment of interest.
Selecting the location within the process or the plant for the probe is the first step in designing the corrosion monitoring system. Knowing where to put the probe is at least as important as knowing that you need a probe. Knowledge of plant process and previous corrosion failures or other corrosion monitoring data will help. Unfortunately, in many new installations, probe location is dictated by existing process penetrations and is not dictated by the corrosion professional. ?
Detailed design of the probe is the second step. EN probes come in many shapes and sizes (Figures 3 - 4). No single probe design or geometry can be effectively use in every situation to monitor any type of corrosion. Probe design and selection begins based
