Development of Evaluation Method of Corrosion Resistance and Long-Term Durability of Stainless Steel for PEMFC
Authors: Takahashi Shuntaro , Tsutsumi Yusuke , Kadowaki Mariko , Watanabe Hikari , Shitanda Isao , Katayama Hideki , Itagaki Masayuki 
Published: December 2024

In recent years, Proton Exchange Membrane Fuel Cell (PEMFC) have attracted attention as an important technology for achieving carbon neutral society due to high energy efficiency without carbon dioxide emission during power generation. In order to cost reduction of PEMFC, the use of cheaper structural materials, such as stainless steels, that possess excellent processability, has been studied in substitution for titanium for bipolar plate (separator). The practical environment in PEMFC is severely corrosive: high temperature, acidic, and fluoride-containing solution. Many study has been conducted on the development of materials that have sufficient corrosion resistance and low contact resistance enabling long-term durability even in such severe environment. However, present evaluation method is insufficient for diagnosis the lifetime of bipolar plate. The new evaluation technique, that accurately simulates the environment inside PEMFC and appropriately accelerates the corrosion degradation reaction, make it easier to predict the lifetime of products and precisely evaluate the performance of newly developed materials. Therefore, the purpose of this study is development of novel corrosion evaluation method with high reproducibility, high and rapid diagnosis performance via applying various electrochemical measurements simulating PEMFC environment and for elucidation the corrosion behavior in practical environment.

Firstly, several common corrosion resistance tests were performed in a specific solution (80 ℃, pH3, 10ppm Cl-, 3ppm F-) that simulated the internal environment of PEMFC. As a result, potentiostatic polarization (0.724 V vs. Ag/AgCl, 24 h) showed poor reproducibility because the corrosion behavior in later stage depends largely on the occurrence of localized corrosion initial stage of the test. In the case of potentiodynamic polarization(+1 mV/s sweep from open-circuit potential), risk of localized corrosion could be investigated, however, it was not suitable to predict long-term corrosion behavior. In the case of potentiodynamic cyclic polarization (cyclic voltammetry), it was found that the cathodic sweep, which never occur at in-sever PEMFC condition, inhibited the stable growth of the passivation film on the steel surface by reduction reaction, resulting in improperly accelerated excessive corrosion.

Then, new testing method of the intermittent polarization, which was a combination of potentiostatic polarization and open-circuit condition, was performed. It has the advantage in flexibility of testing conditions such as polarization potential and duty cycle in accordance with to the degree the corrosion acceleration. In fact, it was found that the localized corrosion caused by initial stage of potentiostatic polarization step was repassivated during the following open circuit step. Therefore, the reproducibility was improved and the passive film growth was properly evaluated by intermittent polarization test, therefore. It was expected to be promising technique to diagnose the long-term durability of PEMFC bipolar plate.

In order to demonstrate the effectiveness of the intermittent potentiostatic polarization method, a variety of type 304 stainless steel specimens with different corrosion resistance were used. The detailed results will be presented on the relevant session.