Protection Mechanisms of Inorganic Inhibitors for Localized Corrosion of Stainless Steels at Anodic Polarization
Authors: Ueyama Meiko , Tsukioka Chinatsu , Hoshi Yoshinao , Shitanda Isao , Itagaki Masayuki , Takata Satoshi , Kogure Tomonari 
Published: February 2020

Nowadays, metal separators have been used for automotive polymer electrolyte fuel cell (PEFC). Stainless steel (SUS) is one of the suitable materials for metal separators [1]. However, localized corrosion could occur due to the potential change during PEFC operations because stainless steel as a separator contacts with acidic solution containing small amounts of chloride and fluoride ions. It is well known that a few percent of molybdenum as an alloy element in SUS316 and SUS447 inhibits pitting corrosion. Hashimoto et al. [2] reported that molybdenum in stainless steel forms molybdenum oxy-hydroxide or molybdate on the active surface sites which are defects formed in the passive film, and this action could bring a homogeneous passive film. The present work has been carried out to discuss the protection mechanisms of molybdenum alloy element and molybdate ions for the stainless steel and to propose the theory of inhibitor development for the stainless steel separators in PEFC.
Measurements of anodic polarization curves of SUS304, SUS316 and SUS447 were carried out. The working electrode (WE) was polished with diamond paste as a pretreatment and its minimum particle size was 1 μm. After that, it was ultrasonically cleaned in methanol and distilled water. The surface area of the WE was 0.28 cm2. The counter electrode was platinum and the reference electrode was a saturated KCl/Ag/AgCl (SSE) electrode. Polarization curves were measured by scanning the working electrode potential at 100 mV min-1 from open circuit potential to 1.5 V vs. SSE. The electrolyte solution was 1 mol/dm3 H2SO4aqueous solution (pH 3) which contained 10 ppm NaCl and 30 ppm NaF.
Figure 1 shows polarization curves of SUS447 in electrolytes which contain (a) no inhibitor, (b) 1 mmol/dm3 Na2MoO4 and (c) 1 mmol/dm3 Na2MoO4 + 1 mmol/dm3 Na2WO4 as inhibitors. Passive and transpassive regions are observed in all polarization curves and the boundary between passive and transpassive region is approximately 0.8 V vs. SSE. The cathode of PEFC in practical use is between 0.7 V and 0.8 V vs. SSE. It is considered that the potential of cathode is normally in the passive region. However, the current density in the transpassive region should be small to avoid contamination of metallic ions dissolved from stainless steel. In Fig 1, all current density is approximately 5 μA cm-2 in the passive region. The potential of current density in the solution of (c) is ten times smaller than that of (a) in the transpassive region, indicating that the coexistence of Na2MoO4 and Na2WO4has significant inhibitor effect for transpassive dissolution of stainless steel. In this study, protection mechanisms of inorganic corrosion inhibitors are proposed on the basic of thermodynamic data.

Reference
s

[1] J. Imamura, Y. Tarutani, NIPPON STEEL
& SUMITOMOMETAL
Technical Report, 396 (2013).
[2] K. Hashimoto, K. Asami, M. Naka and T. Masumoto, Corros
. S
ci., 19, 857 (1979).

Figure 1