Effect of Mo Addition on the Corrosion Behavior of Al-40Cr- x Mo Coatings on Type 316L Stainless Steel
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DUCTION
PROTON exchange membrane fuel cells (PEMFCs) have been considered as one of the most promising devices for clean energy production due to their high energy efficiency, low operating temperature, high power density, quick startup, and minimal pollutant emission.[1,2] A key component in PEMFCs is bipolar plate (BPP), which serves multiple functions, such as distributing reactant gases and coolants, conducting electricity, supporting the membrane electrode assembly (MEA), and removing the produced water.[3–5] Conventionally, graphite and graphite polymer matrix composites were used as BPP materials due to their excellent electrical conductivity and corrosion resistance.[6–8] However, they exhibit inherent drawbacks, such as high manufacturing cost and poor mechanical properties, adding to the low power density of PEMFCs. The latter arises due to the fact that graphite BPPs occupy major space of the PEMFCs. Further, high-volume production AVINASH V. INGLE, V.S. RAJA, and J. RANGARAJAN are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India. Contact email: [email protected] P. MISHRA is with the Materials Processing and Corrosion Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. Manuscript submitted on 5 August 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
of metallic BPPs is significantly easier and more cost effective than that of the graphitic counterparts. Metallic BPPs can be easily stamped into metallic sheets (~ 0.2 mm thick) to form flow channels for transporting fuel, oxygen, and water.[9,10] Therefore, in recent years, metallic BPPs have been considered as an alternate for graphite BPPs. Notwithstanding these advantages, the corrosion resistance of widely considered type 316L stainless steel (SS) BPPs is poor in the humid and acidic (pH 4 to 5) environment of fuel cells operating at 70 to 80 C.[11–13] Furthermore, the passive film formed on SS BPPs significantly increases the interfacial contact resistance (ICR) between BPPs and gas diffusion layers (GDLs).[14,15] What can aggravate further the PEMFCs’ performance in using type 316L SS is that the leached ions, such as Fe, Cr, and Ni, can poison the Pt catalyst, decrease the membrane proton conductivity, and thereby degrade the performance and durability of PEMFCs.[16–18] Therefore, surface modifications and protective coatings are essential to increase the interfacial conductivity and corrosion resistance of SS BPPs.[4] Moreover, during the operation of PEMFCs, water is continuously produced due to the electrochemical reduction of oxygen at the cathode and has to be drained for efficient operation of the fuel cell. If the water with acidic ions is not removed in time, it will not only block the channel, in which the reactant gases access the electrodes, but also accelerate the corrosion of BPPs.[19] Therefore, the hydrophobic
property of coatings is an important parameter to sustain the performance of PEMFCs. To combat corrosion and to lower I
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