B. S. Covino, S. J. Bullard, S. D. Cramer, G. R. Holcomb, M. Ziomek-Moroz, US Department of Energy, Albany, OR; D. A. Eden, M. S. Cayard, InterCorr International, Houston, TX
High temperature corrosion occurs in different sections of fossil energy power plants due to a number of factors: ash deposition, coal impurities, thermal gradients, and low NOx conditions, among others. High temperature electrochemical corrosion rate (ECR) probes are rarely used at the present time, but if they were more fully understood, corrosion could become a process variable at the control of plant operators.
Research is being conducted to understand the effects of probe composition, ash composition, environment chemistry, and measurement technique on the accuracy, response, and longevity of electrochemical corrosion rate probes. The primary goal is to understand when ECR probes accurately measure corrosion rates and when they are simply qualitative indicators of changes in the corrosion processes. Research to date has shown that ECR probe corrosion rates and corrosion rates from mass loss coupons agree within a factor of 2. This good agreement was found to depend on the composition of the sensors, with the best results coming from more highly alloyed materials such as 316L stainless steel and poorer results from carbon steel sensors. Factors being considered to help explain the good or poor agreement between mass loss and ECR probe corrosion rates are: values selected for the Stern-Geary constant, the effect of internal corrosion, and the presence of conductive corrosion scales and ash deposits.
Corrosion rate probes can facilitate the use of corrosion as a process variable in high temperature fossil energy operations. Studies to date show that these probes can qualitatively predict changes in corrosion rate but that sensor composition may be a variable affecting the quantitative nature of the probes.
