Understanding the operation and use of high temperature electrochemical corrosion rate probes Page: 4 of 10
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measured every 7 minutes to calculate the mass loss which was then divided by exposure time.
Qualitative Response of ECR Probes
Corrosion rates (mm/y) as a function of time, temperature, and water vapor content are shown in
Figure 3 for an ash-covered 4130 CS ECR probe exposed to air. The relatively high corrosion rate at the
beginning of the experiment is thought to be due to the abundance of water present in the slurry-added
ash. Note that after the temperature exceeded approximately 2000C the corrosion rate decreased rapidly
from 10 to approximately 0.02 mm/y. This is due to the drying of the ash. The corrosion rate in this
experiment appears to respond well to increases and decreases in temperature. Water concentration up to
10% did not seem to affect the measured corrosion rate in a positive manner, but there was a significant
decrease in corrosion rate when the water was unintentionally turned off at approximately 90 hours after
the start of the experiment. The addition of 15% water after the temperature reached 7000C appeared to
have a significant effect on the corrosion rate. It is well known that water vapor causes accelerated
corrosion in some high temperature environments8.
Figure 3 - Effect of time and temperature and the addition of water vapor on the corrosion
rate of mild steel.
A pitting or localized corrosion factor and the Stern-Geary constant (B) were measured for the
experiment shown in Figure 3. Pitting factor showed no correlation to any of the process variable
changes and was low, below 0.01, for the majority of the experiment. Interpretation of pitting and the
pitting factor at high temperatures is not routine and will require more research to determine what the
pitting factor represents in such cases. The average Stern-Geary factor measured for this experiment was
0.0117 V/decade, a value much lower than the 0.020 to 0.030 V/decade that is commonly assumed for
corrosion processes. One use for this measured value of B is to allow a more accurate calculation of
corrosion rate when using equation (1). The Stern-Geary constant and the Tafel constants that are used to
Corros ion Rate Tem e rature
-H20 H2O Off
10% H2O 15% H20
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Covino, Bernard S., Jr.; Bullard, Sophie J.; Cramer, Stephen D.; Holcomb, Gordon R.; Ziomek-Moroz, M.; Cayard, Michael S. (InterCorr International Inc.) et al. Understanding the operation and use of high temperature electrochemical corrosion rate probes, article, January 1, 2004; (https://digital.library.unt.edu/ark:/67531/metadc881055/m1/4/: accessed March 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.