Page 153 - Corrosion Engineering Principles and Practice

128 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 129

Visual examination of the time record trace may provide
indications as to the type of corrosion processes that are occurring.
The following example illustrates how a simple examination of EN
measurements could reveal the corrosivity of various points of an
industrial gas scrubbing system where highly corrosive thin-film
electrolytes are known to form [23]. These conditions arise when gas
streams are cooled to a temperature below the dewpoint. The resulting
thin electrolyte layer (moisture) is often highly concentrated in
corrosive species.
The corrosion probe used in this example is illustrated in Figs. 5.32
and 5.33. A retractable probe with flexible depth was selected, in order
to mount the sensor surface flush with the internal scrubber wall
surface. The close spacing of the carbon steel sensor elements was
designed to work with a discontinuous thin surface electrolyte film.
This corrosion sensor was connected to a handheld multichannel data
recorder by shielded multistrand cabling Fig. 5.34. As the ducting of
the gas scrubbing tower was heavily insulated, no special precautions
were taken to cool the corrosion sensor’s surface.
Potential noise and current signals recorded during the first
hour of exposure at the conical base of the gas scrubbing tower are
presented in Fig. 5.35. According to the operational history of the
plant, condensate had a tendency to accumulate at this location
where highly corrosive conditions had been noted. The high levels
of potential noise and current noise in Fig. 5.35 are indicative of a
massive pitting attack which is consistent with the operational
experience. It should be noted that the current noise is actually off-
scale for most of the monitoring period, in excess of 10 mA. The
high corrosivity indicated by the electrochemical noise data from
this sensor location was confirmed by direct evidence of severe

FIGURE 5.32 Corrosion sensor and access fitting used for thin film corrosion
monitoring. (Courtesy of Kingston Technical Software)