Influence of Chloride on Nanoscale Electrochemical Passivation Processes

Abstract

Nucleation and growth of metastable oxides on polycrystalline, solid solution Ni-22 wt % Cr alloy were studied in slightly acidic chloride solution and compared to sulfate solutions at the same pH. A combination of ac and dc electrochemical methods, photoelectron spectroscopy, and atomic force microscopy was utilized to characterize the passivation and local breakdown of oxides, yielding information on oxide morphology at the nanometer scale, thickness, composition, and global rates of formation. Oxide nucleation occurs in the form of islands, which form on newly passivating surfaces within seconds of the establishment of an electrochemical potential driving force. Oxides tend to grow first as islands and coalesce to form nm-scale coverages on the NiCr alloy surface, which is consistent with the sharp decrease in oxidation rates commensurate with thin film-field-driven passivation. At the same time, it is found that chemical driving forces favor simultaneous oxide dissolution, especially in the case of Ni 2+ oxides. The balance between these two processes is argued to govern morphologies and thickness evolution. Roughness and topographical heterogeneity evolve with time and depend critically on anion identity. Cl − is shown to enhance the spread in oxide island radii, heights, and Cr/Ni ratio compared to SO 4 2− . The possible impacts of morphology and chemical composition on oxide instability and breakdown are discussed.

Corrosion Resistance, Composition, and Stratification of Passive Films: Ni-22Cr and Ni-22Cr-6Mo Alloys Passivated and Exposure Aged in Acidic Chloride Solutions

“…The comparison of the oxide thickness calculated by the Z″ method with the thickness obtained from XPS has been reported 55 and they show a reasonable agreement with a difference in thicknesses within a factor of 1.3-2.0 times between the methods. This variation in thickness between the two methods is common, and has been reported by other corrosion studies for passive films on Cr-containing 24,42,44 and Al-containing alloys. 56,57 Figure 10 summarizes the passive film composition as it relates to electrochemical processing for both alloys using bulk and surface The passive layer composition for the ternary alloy is near constant, and aging mostly influences hydroxide and Mo distributions throughout the film as discussed in Figs.…”

“…AR-XPS requires relatively smooth surfaces to allow for a meaningful interpretation of the data, and previous studies on Ni-22Cr in pH 4 NaCl solution, shows sub-nanometer root mean square (RMS) roughness values which is sufficient for the current work. 44 Table I. Summary of experimental conditions and corresponding sample labels.…”

Effect of Cations on the Oxidation and Atmospheric Corrosion of Iron Interfaces to Minerals

“…The mechanism of corrosion begins with pitting of the surface with typically chloride ions, leading to breakdown of the native oxide surface layers, dissolution of the metal surface (acting as the anode), followed by metal oxidation, and results in the growth of inorganic mineral scale. , The chemical composition of these minerals are influenced by the type of ions present in aqueous solutions and exposure to oxidizing gases, such as oxygen, water, and carbon dioxide. − The presence of ions, particularly chloride, is known to accelerate or catalyze the corrosion, due to increased ionic strength affecting charge transfer in the redox reaction.…”

“…11,20 The chemical composition of these minerals are influenced by the type of ions present in aqueous solutions and exposure to oxidizing gases, such as oxygen, water, and carbon dioxide. 21−25 The presence of ions, particularly chloride, 26 is known to accelerate or catalyze the corrosion, due to increased ionic strength affecting charge transfer in the redox reaction.…”

A High Throughput Aqueous Passivation Testing Methodology for Compositionally Complex Alloys Using a Scanning Droplet Cell

“…These exact details can be elucidated in follow-on high fidelity studies which can address many of these questions. 42,[44][45][46][47][48][49] Moreover, a similar HTp approach can be developed in sulfate and chloride with a better emphasis on chloride breakdown. Some indicative metrics might differ while others such as self-healing are likely of similar value toward guiding the discovery of improved alloys.…”