Passivity determines corrosion resistance and stability of highly-alloyed stainless steels, and passivity breakdown is commonly believed to occur at a fixed potential due to formation and dissolution of Cr(VI) species. In this work, the study of a 25Cr-7Ni super duplex stainless steel in 1 M NaCl solution revealed that the passivity breakdown is a continuous degradation progress of the passive film over a potential range, associated with enhanced Fe dissolution before rapid Cr dissolution and removal of the oxide. The breakdown involves structural and compositional changes of the passive film and the underlying alloy surface layer, as well as selective metal dissolution depending on the anodic potential. The onset of passivity breakdown occurred at 1000 mV/ Ag/AgCl , and Fe dissolved more on the ferrite than the austenite phase. With increasing potential, the passive film became thicker but less dense, while the underlying alloy surface layer became denser indicating Ni and Mo enrichment. Rapid Cr dissolution occurred at ≥1300 mV/ Ag/AgCl .
The
anodic corrosion behavior of 50 Å thick single-crystalline
IrO2(110) films supported on slightly bulk-reduced TiO2(110) single crystals is studied during acidic water splitting
by a unique combination of operando techniques, namely, synchrotron-based
high-energy X-ray reflectivity (XRR) and surface X-ray diffraction
(SXRD) together with highly sensitive inductively coupled plasma mass
spectrometry (ICP-MS). Corrosion-induced structural and morphological
changes of the IrO2(110) model electrode can be followed
on the atomic scale by operando XRR and SXRD that are supplemented
with ex situ scanning tunneling microscopy (STM) and X-ray photoelectron
spectroscopy (XPS), whereas with ICP-MS, the corrosion rate can be
quantified down to 1 pg·cm–2·s–1 with a time resolution on the second scale. The operando synchrotron-based
X-ray scattering techniques are surprisingly sensitive to Ir corrosion
of about 0.10 monolayer of IrO2(110) in ∼26 h, i.e.,
0.4 pg·cm–2·s–1. The
present study demonstrates that single-crystalline IrO2(110) films are much more stable than hitherto expected. Although
the dissolution rate is very small, ICP-MS experiments reveal a significantly
higher dissolution rate than the operando high-energy XRR/SXRD experiments.
These differences in dissolution rate are suggested to be due to the
different modi operandi encountered in ICP-MS (dynamic) and operando
XRR/SXRD experiments (steady state), a fact that may need to be considered
when hydrogen production is coupled to intermittent energy sources
such as renewables.
We have developed
a microscope with a spatial resolution of 5 μm,
which can be used to image the two-dimensional surface optical reflectance
(2D-SOR) of polycrystalline samples in
operando
conditions.
Within the field of surface science,
operando
tools
that give information about the surface structure or chemistry of
a sample under realistic experimental conditions have proven to be
very valuable to understand the intrinsic reaction mechanisms in thermal
catalysis, electrocatalysis, and corrosion science. To study heterogeneous
surfaces
in situ
, the experimental technique must
both have spatial resolution and be able to probe through gas or electrolyte.
Traditional electron-based surface science techniques are difficult
to use under high gas pressure conditions or in an electrolyte due
to the short mean free path of electrons. Since it uses visible light,
SOR can easily be used under high gas pressure conditions and in the
presence of an electrolyte. In this work, we use SOR in combination
with a light microscope to gain information about the surface under
realistic experimental conditions. We demonstrate this by studying
the different grains of three polycrystalline samples: Pd during CO
oxidation, Au in electrocatalysis, and duplex stainless steel in corrosion.
Optical light-based techniques such as SOR could prove to be a good
alternative or addition to more complicated techniques in improving
our understanding of complex polycrystalline surfaces with
operando
measurements.
We have developed an electrochemical cell for in situ 2-Dimensional Surface Optical Reflectance (2D-SOR) studies during anodization and cyclic voltammetry. The 2D-SOR signal was recorded from electrodes made of polycrystalline Al, Au(111), and Pt(100) single crystals. The changes can be followed at a video rate acquisition frequency of 200 Hz and demonstrate a strong contrast between oxidizing and reducing conditions. Good correlation between the 2D-SOR signal and the anodization conditions or the cyclic voltammetry current is also observed. The power of this approach is discussed, with a focus on applications in various fields of electrochemistry. The combination of 2D-SOR with other techniques, as well as its spatial resolution and sensitivity, has also been discussed.
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