Electrochemical noise has shown considerable promise for monitoring corrosion, and particularly localized corrosion, in environments of low conductivity such as those created when the corrosive electrolyte is simply due to surface coverage by thin films. A retractable probe was selected, in order to mount the sensor surface flush with internal scrubber wall surface. The close spacing of the carbon steel sensor elements was designed specifically for discontinuous thin surface electrolyte films.
The data acquisition unit and the computer/controller may be either an integrated sub-system, or separate devices. The sensing elements can be configured so as to measure either potential or current noise in isolation, or, as is more usual for simultaneous measurements. The sensing element array design depends upon the specific application and type of corrosion process being monitored.
A prime consideration for the sensor array is that the elements need to be electrically isolated from each other. Highly corrosive thin-film electrolytes can be formed in several industrial processes. These conditions arise when gas streams are cooled to below the dew point. The resulting thin electrolyte layer (moisture) is often highly concentrated in corrosive species.
This corrosion sensor was connected to a computer controlled miniaturized multi-channel corrosion monitoring system by shielded multi-strand cabling. As the ducting of the gas scrubbing tower was heavily insulated, no special precautions were undertaken to cool the corrosion sensor surfaces. Cooling of probes in similar applications is usually necessary for the corrosion sensor surfaces to attain the same temperature as the internal duct surfaces.
Potential and current noise mesurements were recorded at a conical section, at the base of the gas scrubbing tower. At this location, condensate tended to accumulate, and highly corrosive conditions were noted from the operational history of the plant. The high levels of potential noise and current noise were consistent with the operational experience. The high corrosivity indicated by the electrochemical noise data from this sensor location was confirmed by direct evidence of severe pitting attack on the sensor elements, revealed by scanning electron microscopy. In contrast, at a position higher up in the tower, where the sensor surfaces remained dry, the electrochemical noise levels remained at completely negligible levels.
