The Effects of Testing Conditions on Corrosion Behaviours of SS316L for Bipolar Plate of Proton Exchange Membrane Fuel Cell

Abstract

Proton exchange membrane fuel cell (PEMFC) has been regarded as the most promising technology for solving problems, such as energy shortage, environmental pollution and global warming caused by fossil fuels due to its high efficiency, low operating temperatures, near zero emission and unlimited sources of hydrogen. However, large-scale commercial application of PEMFC is still hindered by high cost, limited performance and durability. As one of the key components, bipolar plate (BPP) has a great impact on performance, durability and cost of PEMFC. Stainless steel 316L (SS316L) has been considered to be one of the most appropriate candidates for automotive application because of its high thermal and electrical conductivity, superior mechanical properties, ease of manufacture and relatively low cost. However, release of metal ions and improvement of interfacial contact resistance (ICR) caused by corrosion of stainless steel BPP in PEMFC working environment still remains a big challenge. The corrosion of BPP may lead to deteriorate of performance and durability, or even the damage of PEMFC. Therefore, it is important to improve the performance, durability and lower the cost by seeking the appropriate base or coating materials for BPP. Because of the complexity of working conditions for BPP in a PEMFC, it is also critical to determine a reliable assessment method for evaluating corrosion resistance of materials for BPP. The typical working environment of PEMFC is normally considered to be weak acidic with pH in the range of 3 ~ 6 and contain different cations, such as F-1 and Cl-1 while the temperature is in the range of 60 ~ 80 ℃. However, most of the studies for measuring the corrosion resistance of BPP materials are conducted in a much harsher condition of 0.5 ~ 1 M H2SO4 (pH about 0.3 ~ 0) and 2 ppm HF with temperature in the range of 70 ~ 80 ℃. Although a few researches have been carried out to study individually the effects of pH or temperature on corrosion resistance of stainless steel for BPP, the combined effects of pH, temperature and bubbled gas on corrosion resistance of SS316L have not been investigated. In addition, the effects of high potential on corrosion behaviours of SS316L are neglected in former studies. In this work, the effects of temperature, pH, bubbled gas and potential on corrosion behaviours of SS316L are studied comprehensively. Moreover, SEM, EBSD and EDX are performed to reveal the mechanism of microstructure and chemical composition evolution of SS316L under different testing conditions mentioned above. The results reveal that temperature, pH and bubbled gas all have significant influences on corrosion potential (Ecorr.), corrosion current density (Icorr.) and passive current density (Ip) of SS316L. Besides, the influence of any one of the aforementioned factors on corrosion behaviour of SS316L is determined by the other two factors. Icorr. of SS316L in logarithm is generally proportional to pH at all temperatures in both argon or oxygen bubbled solutions. There are distinct passive regions for the Tafel curves of SS316L when potentiodynamic tests are performed in accelerated solutions with pH in the range of 0 ~ 1. On the contrary, pseudopassive behaviour would occur when SS316L are tested in the simulated conditions with pH in the range of 3 ~ 5. The results also indicate that severer corrosion would appear as higher potential is applied on SS316L. Pitting are shown to be the main corrosion for SS316L subjected to simulated solutions while the main corrosion mechanism is shifted to intergranular or uniform corrosion for SS316L under harsher testing conditions with pH in the range of 0 ~ 1. Figure 1