LPR is, at first sight, probably the easiest to implement but requires some methodology for minimizing the effects of noise in the corrosion process. It does require some default value of the Stern-Geary constant to be applied. Field proven. HDA has the advantages of allowing derivation of the Stern-Geary factor and corrosion rate directly, but at low corrosion rates, and in unstable systems, the value of the third harmonic (which is required for correct analysis) may be too low to measure accurately or may be corrupted. Field proven.
IMD requires the analysis of the distortion products which are products of the second harmonic (which will not exist if the anodic and cathodic coefficients are equal) and has not been field proven.
EIS difficult to automate interpretation and subsequent corrosion rate evaluation. Not field proven.
ECN is the only choice if detection of localized corrosion is an issue. Field proven.
Monitoring of some forms of localized corrosion (e.g. stress corrosion cracking) benefits from a probe configuration that reflects the physical attributes of the equipment being monitored (i.e. including applied stress or artificial crevice on the working electrode). Also, it is important to note that some forms of corrosion (e.g. erosion or cavitation) are 'physical' phenomena although the effects of them can be detected electrochemically.
4. Assuring data integrity Need to ensure that the data is valid by integrating automated system checks and calibration where necessary.
5. The response (or cycle) time of the measurement The measurement and analysis time frame determines the data update rate to the DCS. Ideally this should be on a similar time frame to other inputs such as temperature, pressure, flow etc, in order that the corrosion information may be correlated with process operations.
6. The ability to provide general corrosion rate information The instrumentation should provide high quality general corrosion information using techniques which will reject adventitious noise such as may arise from mains voltage power lines.
7. The ability to provide information regarding localized corrosion propensity localized corrosion is more insidious than general corrosion, since only a small area of the metal is undergoing metal loss. The localized corrosion rate may be orders of magnitude greater than that being reported by general corrosion measurement techniques, hence it is important to be aware of the transitions between general to localized corrosion.
8. Measurement and analyses automation One of the major difficulties with migrating the corrosion measurement techniques from the laboratory to the field is in automating the measurement and analysis routines. Currently this may be achieved using embedded processors and sophisticated algorithms residing in firmware which control timing and communications, along with measurement and analysis routines, with a high degree of accuracy and precision. The algorithms required for the electrochemical measurements are somewhat different to those required, for example, when measuring temperature or pressure, where the physics of the sensors used is quite well understood and defined. With the electrochemical methods we are trying to determine both the kinetics and mechanistics of the corrosion processes occurring at the metal surface.
9. Simplicity of installation Installing the instrument should require minimum effort in terms of its location, wiring and set-up.
10. Interfacing with the DCS system Ideally the corrosion information should be made directly available to the operator via the DCS to be viewed as another process variable. When integrated into the DCS and the plant historian database, the corrosion data can be analysed with respect to its interaction with other process variables.
11. Convincing the site personnel that the corrosion measurements are a true reflection of what is happening in the system Plant inspections are a relatively infrequent occurrence, usually coinciding with routine shutdown and maintenance. In the real world corrosion rarely takes place at a steady rate for prolonged periods, but usually occurs with some short periods of very aggressive attack and relatively long periods of little or no attack. For example, if the corrosion processes occur for 10% of the time, the actual corrosion rate instead of being say 10 mpy may peak at ~100mpy or much higher. These periods of high corrosion rate often go unnoticed until the events leading to the corrosion increase in frequency and ultimately lead to failure. Subsequent failure analysis finds “corrosion” to blame but does not give any clues as to what happened and when.
System Configuration: The configuration of the system can take many forms, depending upon the intended application. For laboratory studies, a relatively simple layout can be used with bench experiments and flow loops. In the case of field studies, a variety of configurations is available to suit plant installation (hardwire connection to computer or direct to control system) or remote applications where radio communications and/or solar power sources are required.
Probe Configuration: The corrosion probe design is all-important to the system as this is the component that interfaces directly with the process environment and must be both suitable for the installation location and enable representative corrosion measurements to be made. All too often, the quality and relevance of the corrosion data measured can be severely compromised by inappropriate probe design or even installation of the probe in a location where it is impossible to capture the process conditions that actually cause the corrosion.
Integrating with Plant Management Systems for Unified Corrosion and Process Control