A Comprehensive Numerical Model for One-Dimensional Pit Growth of 316L SS under a Salt Film

A comprehensive mathematical model, including electrode kinetics, hydrolysis and complexation reactions, salt film precipitation and pit interface movement, was developed to investigate pit growth of stainless steels 316L under a salt film. The new mathematical framework incorporates activity coefficients into the hydrolysis and complexation reaction calculations for the first time, using experimental results to parametrize the electrode kinetics in a saturated pit solution. The model was validated by 1D pit experiments and results documented in the literature. It can successfully estimate the transition potentials, salt film thickness, pit stability product, saturated pit concentration, and the pH at the pit base during pit propagation under the presence of a salt film. Moreover, the model can predict the Cr enrichment and Fe depletion in the saturated solution at the pit base, attributed to the higher diffusion coefficient of Fe and the lower Cr diffusivity.

Study of Cu Electrochemical Polishing Mechanism With Observation of Water Acceptor Diffusion

The salt-film and water acceptor mechanisms were generally accepted mechanisms for Cu electrochemical polishing (ECP) theory. These mechanisms of Cu ECP are still controversial for a long time. Conventional and new electrochemical analysis methods were used to investigate the mechanisms and behaviors of Cu electrochemical polishing. Two cases of Cu dissolution, with and without polishing, were classified by results of linear scan voltammetry (LSV) and scanning electron microscopy (SEM). The electrochemical impedance spectroscopy (EIS) results showed the main difference in these two cases was in the low-frequency region. However, it was hard to distinguish between the salt-film and water acceptor mechanisms by conventional electrochemical analysis. A scanning electrochemical microscopy (SECM) system, a new electrochemical analysis method that measures the electrolysis currents of the water acceptors along with a set distance from the substrate, was used to investigate the Cu ECP mechanism. Accordingly, the diffusion of the water acceptors was successfully confirmed for the first time. Finally, the mechanisms of the Cu ECP are definitively described by using all analysis results.

Improved Oxidation and Hot Corrosion Resistance of 1Cr11Ni2W2MoV Stainless Steel at 650 °C by a Novel Glass-Ceramic Coating

A novel glass-ceramic coating was applied onto the 1Cr11Ni2W2MoV stainless steel. The oxidation and corrosion behaviors of coated and uncoated steels were comparatively investigated in air and in the presence of NaCl + Na2SO4 eutectic deposits at 650 °C, respectively. Protective scales formed on the surface of stainless steel prevented the severe oxidation of the alloy. Catastrophic hot corrosion occurred on the steel when a salt film was attached, producing loose iron oxide layers and internal corrosion zone. The glass-ceramic coating acted as a barrier that effectively hindered the invasion of corrosive species during the oxidation and hot corrosion tests.

Effects of Sodium Phosphate and Sodium Nitrite on the Pitting Corrosion Process of X70 Carbon Steel in Sodium Chloride Solution

In this paper, effects of sodium phosphate (Na3PO4) and sodium nitrite (NaNO2) on the pitting corrosion of X70 carbon steel in 0.10 mol/L NaCl solution were investigated by potentiodynamic polarization technique, electrochemical impedance spectroscopy (EIS) method, scanning electron microscope (SEM) and scanning electrochemical microscope (SECM). The SECM equipment was used to observe the dynamic processes of the pitting corrosion in situ. Na3PO4 or NaNO2 in the sodium chloride solution decreased the local anodic dissolution and increased the pitting resistance of the specimen. By analysis and comparison, it can be concluded that the inhibition effect of Na3PO4 is mainly due to the formation of a salt film, while the corrosion inhibition of NaNO2 is principally attributed to a protective oxide film on the electrode surface.