Fundamentals of the Techniques

The measurement of polarisation resistance is known and shall here not be discussed further, as well as the measurement of wall potentials with respect to a reference electrode. Both methods are so far helpful to indicate the state of a surface as "passive" or "active", and the overall corrosion rate can be determined.

The additional evaluation of the electrochemical noise gives some advantages:

The noise spectrum is characteristically for different corrosion processes. So, the nature of the corrosion process can be determined.

Different methods are described in the literature showing how to get information from the noise spectra5. A drawback of these methods is that some additional assumptions have to be made concerning the relation of noise current density to total current density.

Strong peaks indicate onset of pitting.
Low noise of passive state (note the intensity scale).
AC measurements Frequency spectra
Figures above:
Potential signals of a duplex - electrode (1.4462) in chloride containing process medium in a distillation column.

To avoid these drawbacks, the NDU combines the methods of noise measurement, potential measurement, galvanic current and polarisation resistance measurement.

So, four different methods allow to cross-check the respective data. Even wrong data from a drifting reference electrode now will not longer produce wrong alarms or - worse - pretend good state of the apparatus when rapid corrosion is developing.

Alarm thresholds are set according to the plant's properties. The programme is secure against unauthorised changes.

The amount of information allows a description of the state of the corroding surface. On the other hand, the operator in the plant it is essential to get information in terms of "good" or "alarm". For this purpose, the software consists of two main parts: Part one cares for data recording and screening the signals for alarm states. If alarm states are detected, the complete set of data will be stored. If no alarm occurs, only once a day one set of data is stored for evaluation purpose. Part two is a capable programme which enables the engineer to interpret the data measured, if alarms occur.

Both alarm data and raw data can be transferred via a network for remote control.

 

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