Corrosion Behavior of Aluminum in Dilute Fluoride and Sulfate Solutions for Use as Bipolar Plate of PEFC
Authors: Alam Md. Ashraful , Jahan Aklima , Suzuki Eiichi , Yashiro Hitoshi 
Published: July 2021

ABSTRACT

In polymer electrolyte membrane fuel cell (PEFC), bipolar plate (BP) is a vital multifunctional component which constitute more than 80% of the total stack weight and about 30% of the materials cost.[1-3] Among many materials act as BP, aluminum (Al) is one of the most alternative for fulfilling the requirements that mentioned above for its special qualities such as mechanical properties, low cost and low density.[4-5] It is also generally required for a BP to be corrosion resistant even in sulfuric acid solution with a pH around 3.0, but it would be difficult for BP like Al to sustain under this condition. To fulfil this condition, a new type of BP is proposed by Yashiro et al. [6] that made of two sections: a flow path that directly contacts with MEA and a gas isolation plate. The gas isolation plate contacts with the generated water which is near to pure water and bare Al can also be used for this section. Therefore, in our previous works, the effects of fluoride and sulfate ions on pure Al in the simulated PEFC environment was studied at 80°C.
In this study, the corrosion behavior of Al in dilute sodium fluoride and sodium sulfate solutions with different concentrations at 40°C was evaluated. Open circuit potential (OCP), scanning electron microscopic (SEM) images, energy dispersive x-ray (EDX) spectra and cross-sectional images were also taken to check the corrosion potential, surface morphology, elemental composition and other analyses.
Pure Al having a thickness of 1 mm and area of 10 cm2 were used as the test samples for immersion process. 0.1ppm F- (pH 5.70), 0.1ppm SO4
2- (pH 5.64), mixture of 0.1ppm F- and 0.1ppm SO4
2- (pH 5.66), mixture of 0.1 ppm F- and 1ppm SO4
2- (pH 5.57) and distilled water (DSW) were used as the test solutions using NaF and Na2SO4. Firstly, Al was cut into desired sizes and polished by emery paper down to #2000, rinsed and immersed in around 90 cm3 aerated aqueous solutions. The immersion process was carried out at 40°C for a period of 24h, 48h, 72h, 96h and 120h. After the immersion test, weight was increased in all the test samples with increasing time regardless of the solutions. It is considered that the oxide film was formed in Al surfaces because of the oxidized aluminum at the time of immersion process. It is considered that the oxide films formation occurred because of the corrosive properties of the test samples with test solutions.
A significant amount of corrosion of pure as-polished Al immersed in different solutions for 24 to 120h at 40°C was found. The results show that corrosion amount is higher in 0.1ppm F- solution and lower in 0.1ppm SO4
2- solution. It is considered that the inhibition of corrosion was occurred by sulfate ions, and the amount of corrosion was decreased. However, the corrosion amount was increased more when a mixture of 0.1ppm F- and 0.1ppm SO4
2- was used, and vice versa in case of DSW. Furthermore, when the concentration of sulfate ions was increased around 10 times using 1ppm SO4
2- with 0.1ppm F-, the lowest amount of corrosion was found among the used solutions.
Fig. 1 shows the SEM images of pure as-polished Al of before and after immersion for 24h. Some irregular shaped particles of different sizes are observed in the Al surfaces that indicate the presence of aluminum oxide as corrosion products. In addition, thick oxide layers along with many cracks are found in the case the Al samples immersed in 0.1ppm F- and 0.1ppm F- + 0.1ppm SO4
2- solutions. Besides, very low amount of cracks are found in the case of DSW and 0.1ppm SO4
2- solution. Therefore, it is concluded that the presence of F- ions were confirmed which act as corrosion promoter and SO4
2- ions were act as corrosion inhibitor when Al is used as bipolar plates in the PEFC environment.

Acknowledgements

This work was supported by JSPS KAKENHI grant number JP19K05091.

References

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Figure 1