Corrosion Behavior of SUS 304L Steel in Concentrated K2CO3 Solution
Authors: Nakagawa Takuma , Matsushima Hisayoshi , Ueda Mikito , Ito Hiroshi 
Published: December 2020

Introduction
Recently, interest in hydrogen energy has been increasing as it is an environmentally friendly energy source. Water electrolysis is one method to produce hydrogen and oxygen, and can be achieved with renewable energy such as wind and solar power.[1] The alkaline polymer electrolyte membranes (AEM) type electrolysis can be used with inexpensive metal oxides and catalysts such as metal and nitrogen-doped carbon and may be expected to be able to produce hydrogen at low cost. The electrolyte used in AEM type electrolysis is an alkaline solution such as NaOH or KOH with high pH. The K2CO3 solution is also expected as a high electrical conductivity electrolyte despite lower pH than NaOH and KOH.[2] Titanium is currently used for materials of electrolysis cells. However, since titanium is expensive, steel based materials are required. In this study, SUS 304L steel was selected as a commercial steel, and electrochemical measurements were carried out in a K2CO3 solution with pH = 12. After the electrochemical measurements, the steel surface was analyzed.
Experimental
The SUS 304L (18%Cr-10%Ni-Fe alloy) was used as a sample for electrochemical measurements, then polished with water-resistant polishing paper (# 400 to 2400) and further polished to a mirror surface using alumina powder. After the polishing, ultrasonic cleaning was performed in distilled water and acetone. An electrolyte was prepared by K2CO3 and distill water to pH = 12. A potentiostat (Hokuto Denko Co, HZ-7000) was used for the electrochemical measurements. The experimental cell was a cylindrical PTFE cell with a working electrode, counter electrode, and reference electrode attached to the lid. The SUS 304L was used for the working electrode, a platinum plate was used for the counter electrode, and Hg/HgO was used as the reference electrode. The electrolyte was controlled to 60 oC. Linear Sweep Voltammogram (LSV) measurements were performed from the open circuit potential to 1.0 V with a sweep rate of 5 mVs-1. Constant potential electrolysis was carried out at a potential of 1.0 V in the electrolyte with pH = 12 for 1, 10, and 100 hours. The surface roughness of the SUS 304L before and after the constant potential electrolysis were measured by an atomic force microscope (AFM) (Shimadzu Co: SPM-9700HT). The chemical states of Fe, Cr, and Ni on the surface of the specimens and depth profile before and after the constant potential electrolysis were analyzed by XPS (JEOL: JPS-9200) measurements.
Results and Discussion
In the LSV results, an increase of anodic current was observed from 0.76 V (vs. Hg/HgO) and attributed to oxygen evolution reaction. It was confirmed that the value of current at 1.0 V was higher than that of NaOH solution at pH = 13 and lower than that at pH = 14. In the constant potential electrolysis, gas evolution was occurred on the surface of the SUS 304L electrode during the electrolysis, without any change in the observed color of the electrolyte after the electrolysis.
From the AFM measurements, the roughness before the electrolysis was 4 nm. On the other hand, the roughness of the sample after electrolysis for 1, 10 and 100 hours were 8, 21 and 58 nm, respectively. Since this result does not follow the parabolic law between the roughness value and electrolysis time, it is considered that a film formation may proceed on the substrate.
In the XPS measurements of the SUS 304L surface, peaks of Cr, Fe and Ni metal and peaks of Cr and Fe oxide were detected before the electrolysis. After each electrolysis, peaks of Fe and Ni hydroxides were detected. However, after electrolysis for 100 hours, peak of Cr was hardly observed. Therefore, it is considered that Cr in the surface was dissolved to the solution during electrolysis, and Fe and Ni remained as hydroxide immediately after dissolution reaction.
Acknowledgement
Part of the experiments reported here are supported by the New Energy and Industrial Technology Development Organization(NEDO).
[1]B. Pivovar, N. Rustagi, and S. Satyapal, Electrochem. Soc. Interface, vol. 27, 47–52, 2018.
[2]H. Ito, N. Kawaguchi, S. Someya and T. Munakata, Electrochimica Acta., vol.297, 188-196, 2019.