Abstract
The high Si containing X1CrNiSi18-15-4 stainless steel (SS) spontaneously forms a protective oxide film that is mostly composed of mixed chromium and silicon oxides. This film ensures a good durability of the industrial facilities the alloy was designed for, containing very acidic electrolytes such as hot and concentrated nitric acid, HNO3, in presence of oxidizing species. In the present work, the chemistry of the oxide formed and the passivation kinetics of the alloy in sulfuric acid, H2SO4, and for the first time in HNO3, were monitored by atomic emission spectroelectrochemistry (AESEC) over successive activation and passivation cycles of the material. X1CrNiSi18-15-4 SS was compared to a low Si containing SS, the X2CrNiN18-10 SS. It was found that a similar quantity and rate of passive film was formed during passivation, and dissolved during activation. Reproducible results were obtained over several active-passive cycles. The excess Cr was correlated with the dissolution rate decay during passivation. The Si/Cr ratio of the passive film was determined by X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy (performed using transmission electron microscopy), and AESEC giving similar results within experimental error. The EDX profile suggest that the passive film consists of a Si rich outer and Cr rich inner layer.
Highlights
Cu dissolution was detected in H2SO4 around the same potentials by Ogle et al.[35] and Ruel et al.[36] but in the present work, the dissolution occurred systematically in two peaks whose origin is still unclear
The direct observation by atomic emission spectroelectrochemistry (AESEC) of Fe, correspond to (Cr), and Si elemental dissolution during active – passive cycles is reported for two varieties of stainless steels in 4 mol dm−3 HNO3 and 2 mol dm−3 H2SO4
X-ray photoelectron spectroscopy (XPS) ex situ measurements performed in H2SO4 and HNO3
Summary
Wavelength/nm Detection limit in H2SO4 C3σ/μg dm−3 Detection limit in HNO3 C3σ/μg dm−3. Atomic emission spectroelectrochemistry.—The experimental set-up including data acquisition has been described in detail previously.[32] Briefly, the working electrode releases ions into the electrolyte in the flow cell which is continuously fed into the plasma of the ICP-AES where the emission intensities of the different ions are measured simultaneously. These emission intensities are converted into concentration using standard ICP-AES calibration techniques.
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