An Evaluation of Carbon Steel Corrosion in Nitrate Solution Using Electrochemical and Solution Analysis Methods
Authors: Zakeri Masoud , Joseph Jiju M , Yong Otto , Wren Jungsook Clara 
Published: May 2020

Nuclear power reactors offer a long term, cost efficient and sustainable method to produce electricity with very low greenhouse gas emissions. As nuclear reactors age and their lifetimes are extended, accurate assessment of the integrity and longevity of the reactor structural materials is increasingly important. Corrosion in the vicinity of a nuclear reactor core happens in the presence of high energy, ionizing radiation, which needs to be considered. In particular, the potential for accelerated (galvanic) corrosion attack on carbon steel (CS) adjacent to the dissimilar metal weld between CS (SA 36) and stainless steel (SS) (Type 304L) at the periphery of the annular gap must be addressed.
The initial environment inside the gap of structural support for End Shield Cooling (ESC) System could be humid due to trapped water in its annular gap. The pH of the ESC System water is adjusted to around 10.4, but when it condenses onto the weld region the pH might change. The oxygen level, the humidity, and many other factors will change with time. In addition, ionizing radiation decomposes water into a range of redox active species ranging from highly oxidizing (e.g., •OH and H2O2) to highly reducing (e.g., •eaq
-) whose concentrations evolve with time (1-4). Humid-air radiolysis produces nitric acid (HNO3), that can dissolve into the water (5). The radiation products will lower the pH of the water in the droplet.
In this work, corrosion of CS is evaluated carefully, and the effect of all solution reactions on interfacial charge transfer reactions is considered.
The effect of [NO3
-] and solution pH is studied by using potentiodynamic polarization experiments and corrosion potential measurements (Figure 1), along with solution analysis. The results of polarization measurements in the presence of 0.01 M and 0.1 M [NO3
-] (with initial pH 2.0) show that the oxidation reactions are implicitly influenced by the mass transfer of metal cations from the CS surface into the solution. Also, the comparison of the rate of corrosion measured using Tafel extrapolation method and those obtained from ICP measurements suggests that the Tafel extrapolation method enormously underestimate the corrosion rate (in this case by about 100 times for 8 h corrosion) and must be applied more carefully.
In order to study the effect of pH, we have compared the electrochemical results in pH 2.0 and pH 6.0 obtained in different concentrations of nitrate. The results show that solution pH is a rate controlling factor and the metal oxidation is limited by the metal transfer even at potentials close to/at the corrosion potential, and this influence the rate of corrosion.
The results of this work imply that solution reactions can alter the corrosion behavior of carbon steel and hence, its galvanic coupling with stainless steel.

References:
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4. P. A. Yakabuskie, J. M. Joseph and J. Clara Wren, Radiat. Phys. Chem., 79, 777 (2010).
5. R. S. Wittman, Radiolysis Model Sensitivity Analysis for a Used Fuel Storage Canister, in, p. Medium: ED; Size: PDFN, ; Pacific Northwest National Lab. (PNNL), Richland, WA (United States) (2013).

Figure 1