Oxide Scale Analysis of the Carbon Steel Exposed to Bentonite after Electrochemical Test
Authors: Uyama Masao , Hitomi Takashi , Saito Hiroyuki , Aoshima Kazuki , Osawa Motoaki 
Published: February 2020

Introduction

Geological disposal is the most realistic method in high-level radioactive waste (Hereafter HLW) in Japan. To consider long-term stability for geological disposal, several types of material Engineer barrier with host rock is studied. In such studies, the combination of carbon steel and bentonite is proposed one of idea for use. Carbon steels show uniform corrosion in many cases1. Magnetite scale is formed on carbon steel when it is exposed to low-oxygen partial pressure. And the Magnetite scale may serve as corrosion resistant film. Corrosion under film, however, is known even if the film is a paint film2. Therefore we have to investigate the long-term corrosion tendency of carbon steel. For this purpose, the electrochemical test for accelerated corrosion.

Experimental

A sample composed of SS400 class carbon steel (ISO 630 equivalent) and bentonites (Kinigel V1: 21% pts. Mass% water and 1.6g/cm3 dry density) is prepared. Two carbon steel plates having t=2mm thickness and 60mm diameter are emplacement between both ends of bentonite column. One is used for a working electrode and the other is used for a counter. The sample is surrounded by an acrylic container to limit the oxygen diffusion. This is a model of HLW disposal container into the underground. Regarding oxide scale of the surface carbon steel, it is remained considering to in its natural state for Overpack made of carbon steel before the disposal. For measuring the potential of sample, we used the silver-silver chloride electrode.
At first, a polarization test is carried out. Scanned overvoltage is -250mV to 250mV. The scan rate was 1mV/s3.
After that, we carried out +100mV electrolysis of the same sample for 13 days. After electrolysis3, we analyzed the steel/ bentonite interface with SEM, EDX and Raman Spectroscopy.

Results and Discussion

The corrosion potential measured by polarization test was -0.6VvsSSE3. This is the mixed potential of steel and scale. Hara and co-researchers report that the corrosion potential of iron in supernatant of bentonite water solution is -0.4VvsSSE and that of magnetite, main composition of the oxide scale, is -0.8VvsSSE4. Therefore the potential we obtained is the mixed potential of iron and magnetite.
After electrolysis test, we obtained that the steel corroded under the oxide scale confirmed3. The interface of dummy canister shows same as the electrolysis test
This corrosion phenomenon was also found at Engineer barrier experiment test in Switzerland for study HLW disposal5.
Those results indicate that at least a part of the inner steel metal is connected to outside bentonite and corroded at mixed potential of magnetite and steel. This means that the steel metal may be a sacrifice anode.
The scale is confirmed to be FeOOH by micro Raman spectroscopy. The scale is trivalent iron. In the other case of a lab experiment, Hara et al. 4, carbon steel (SM400B) with Fe3O4 film into bentonite (Kunigel V1) showed that the corrosion product was a mixture of FeCO3 and Fe2(OH)2CO3. At first stage, Fe3O4 film-coated carbon steel in compressed bentonite is reacting as the self-reduction reaction. Next step, this reaction is a trigger to destroy Fe3O4. As a result, the internal iron is corroded and bivalent iron is created. The difference of corrosion product, trivalent or bivalent iron, is determined by the presence of the internal state oxygen during the test.
As the next step, the authors are planning to make a lab test under the conditions of oxygen and total current density controlled.

Reference

E. McCafferty, Introduction to Corrosion Science, Chapter1,Springer (2010)

Y. Akage, H.Saito, ECS transactions, 50(50),pp.7-11(2013)

H.Saito, M. Uyama,et.al., 67th ISE Proseeding (now printing)

T.Tsushima, N.Hara, K.Sugimoto,Zairyo-to-Kankyo,52,pp545-553(2003) in Japanese

Nagra Grimsel Test Site HP (FEBEX-DP), http://www.grimsel.com/gts-phase-vi/febex-dp/febex-dp-introduction