Corrosion Behavior of SUS304L Steel in High pH NaOH Solution
Authors: Nakagawa Takuma , Matsushima Hisayoshi , Ueda Mikito , Ito Hiroshi 
Published: May 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 electrolysis of water can be achieved with little energy using renewable energy from sources such as wind and solar power.[1, 2] The AEM type water electrolysis using alkaline polymer electrolyte membranes (AEM) 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 with a high pH of 12 or 13. Titanium is currently used for electrolysis cells. However, because titanium is expensive, it is hoped that steel based materials could be used. In this study, SUS304L steel was used as a highly versatile commercially available steel, and electrochemical measurements were carried out in a NaOH solution with a pH of 12 or 13. After the electrochemical measurements, the steel electrode surface was analyzed.
Experimental
The SUS304L (18%Cr-10%Ni-Fe alloy) was used as a sample for electrochemical measurements, 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. As the electrolytic solution, an aqueous sodium hydroxide solution adjusted to pH = 12 and 13 were used. 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 SUS304L 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 temperature of the electrolytic solution was 25 and 60 ° C. Linear Sweep Voltammogram (LSV) measurements were performed from the open circuit potential to 1.2 V with a sweep rate of 5 mVs-1. Constant potential electrolysis was carried out at a potential of 1.0 V in the electrolytic solution at 60 ° C and with pH = 13 for 5, 10, or 20 hours. The surface roughness of the stainless steel before and after the controlled 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 before and after the constant potential electrolysis were determined by XPS (JEOL: JPS-9200) measurements.
Results and Discussion
In the LSV results, an increase of anodic current was observed from 0.79 V at a temperature of 25 oC with the pH of 12. At pH of 13, the increase was from 0.71 V. At 60 oC, a current increase was observed from 0.75 V and 0.65 V at pH 12 and 13, respectively. From these results, the constant potential electrolysis was carried out at 1.0V, 60 oC, and in a pH 13 NaOH solution. Gas generation was confirmed on the surface of the SUS304L sample during the electrolysis, without any change in the observed color of the NaOH solution after the electrolysis.
From the AFM measurements of the surface roughness before and after the constant potential electrolysis, the roughness was about 5 nm before the electrolysis, about 15 nm after electrolysis for 5 h and about 20 nm after 20 h of electrolysis. The XPS measurements of the sample surface detected Cr and Fe oxides before the electrolysis, and after electrolysis for 5 hours, the presence of CrOOH and FeOOH was confirmed, in addition to the oxides. These results suggest that the oxide film formed on the surface was dissolved by electrolysis, following this some Fe and Cr were also dissolved, and these formed the FeOOH and CrOOH via hydroxides on the surface.
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]P. Lettenmeier, R. Wang, R. Abouatallah, F. Burggraf, A. S. Gago, and K. A. Friedrich, J. Electrochem. Soc., vol. 163, No. 11, F3119–F3124, 2016.