Corrosion rate of construction materials in hot phosphoric acid with the contribution of anodic polarization

Abstract

The paper is focused on selection of a proper material for construction elements of water electrolysers, which make use of a 85% phosphoric acid as an electrolyte at temperature of 150 °C and which might be loaded with anodic polarization up to 2.5 V versus a saturated Ag/AgCl electrode (SSCE). Several grades of stainless steels were tested as well as tantalum, niobium, titanium, nickel alloys and silicon carbide. The corrosion rate was evaluated by means of mass loss at free corrosion potential as well as under various levels of polarization. The only corrosion resistant material in 85% phosphoric acid at 150 °C and at polarization of 2.5 V/SSCE is tantalum. In that case, even a gentle cathodic polarization is harmful in such an acidic environment. Hydrogen reduction leads to tantalum hydride formation, to loss of mechanical properties and to complete disintegration of the metal. Contrary to tantalum, titanium is free of any corrosion resistance in hot phosphoric acid. Its corrosion rate ranges from tens of millimetres to metres per year depending on temperature of the acid. Alloy bonded tantalum coating was recognized as an effective corrosion protection for both titanium and stainless steel. Its serviceability might be limited by slow dissolution of tantalum that is in order of units of mm/year.

Advanced Construction Materials for High Temperature Steam PEM Electrolysers

“…CVD tantalum coating on stainless steel showed an outstanding corrosion resistance, with the CRs being similar to earlier published data on this material [40]. The SEM image of the CVD-tantalum coated sample and the corresponding EDX spectra are shown in Figure 12 and Table 4 correspondingly.…”

PEM steam electrolysis at 130 °C using a phosphoric acid doped short side chain PFSA membrane

“…For comparison, the corrosion current density of Ta under these conditions was determined to 6.3$10 À5 mA cm À2 which gives a corrosion rate of under 1$10 À3 mm year À1 [28]. Similar extremely low corrosion rates of less than 1$10 À2 mm year À1 for Ta have been determined at temperatures up to 150 C in 85 wt% PA and at potentials as high as 2.27 V vs. SHE [29]. The excellent corrosion resistance of tantalum in aggressive acidic media is due to a protective oxide surface layer, Ta 2 O 5 , that is naturally formed on the surface but only in very thin layers (3 nm [30]).…”

Contact Resistance of Tantalum Coatings in Fuel Cells and Electrolyzers Using Acidic Electrolytes at Elevated Temperatures

“…Operating potentials are below 2 V and furthermore low corrosion rates have been reported for tantalum in phosphoric acid (<0.01 mm year −1 up to 150 • C). 5 The superior corrosion stability of tantalum is generally ascribed to formation of a passive oxide layer. Scanning electron microscopy images and X-ray diffraction patterns of tantalum layers before and after anodization (not shown) did not reveal such a layer.…”

“…[2][3][4] Our group and partners have thoroughly investigated the stability of various construction materials including several grades of stainless steels, tantalum, niobium, titanium, nickel alloys and silicon carbide in hot phosphoric acid. 2,5 Corrosion rates of various construction materials were evaluated under anodic and cathodic polarizations in phosphoric acid at 150 • C. With a corrosion rate of less than 1 mm year −1 only tantalum had sufficient stability under these conditions. We reported improvements of cell design by using tantalum coated stainless steel gas diffusion layers (GDLs) in HT-PEM electrolyzers.…”