Direct observation of pitting corrosion evolutions on carbon steel surfaces at the nano-to-micro- scales.

The Cl--induced corrosion of metals and alloys is of relevance to a wide range of engineered materials, structures, and systems. Because of the challenges in studying pitting corrosion in a quantitative and statistically significant manner, its kinetics remain poorly understood. Herein, by direct, nano- to micro-scale observations using vertical scanning interferometry (VSI), we examine the temporal evolution of pitting corrosion on AISI 1045 carbon steel over large surface areas in Cl--free, and Cl--enriched solutions. Special focus is paid to examine the nucleation and growth of pits, and the associated formation of roughened regions on steel surfaces. By statistical analysis of hundreds of individual pits, three stages of pitting corrosion, namely, induction, propagation, and saturation, are quantitatively distinguished. By quantifying the kinetics of these processes, we contextualize our current understanding of electrochemical corrosion within a framework that considers spatial dynamics and morphology evolutions. In the presence of Cl- ions, corrosion is highly accelerated due to multiple autocatalytic factors including destabilization of protective surface oxide films and preservation of aggressive microenvironments within the pits, both of which promote continued pit nucleation and growth. These findings offer new insights into predicting and modeling steel corrosion processes in mid-pH aqueous environments

Dissolution Amplification by Resonance and Cavitational Stimulation at Ultrasonic and Megasonic Frequencies

Acoustic stimulation offers a green pathway for the extraction of valuable elements such as Si, Ca, and Mg via solubilization of minerals and industrial waste materials. Prior studies have focused on the use of ultrasonic frequencies (20-40 kHz) to stimulate dissolution, but mega sonic frequencies (≥1 MHz) offer benefits such as matching of the resonance frequencies of solute particles and an increased frequency of cavitation events. Here, based on dissolution tests of a series of minerals, it is found that dissolution under resonance conditions produced dissolution enhancements between 4x-to-6x in Si-rich materials (obsidian, albite, and quartz). Cavitational collapse induced by ultrasonic stimulation was more effective for Ca- and Mg-rich carbonate precursors (calcite and dolomite), exhibiting a significant increase in the dissolution rate as the particle size was reduced (i.e. available surface area was increased), resulting in up to a 70x increase in the dissolution rate of calcite when compared to unstimulated dissolution for particles with d50\u3c 100 μm. Cavitational collapse induced by mega sonic stimulation caused a greater dissolution enhancement than ultrasonic stimulation (1.5x vs 1.3x) for amorphous class F fly ash, despite its higher Si content because the hollow particle structure was susceptible to breakage by the rapid and high number of lower-energy mega sonic cavitation events. These results are consistent with the cavitational collapse energy following a normal distribution of energy release, with more cavitation events possessing sufficient energy to break Ca-O and Mg-O bonds than Si-O bonds, the latter of which has a bond energy approximately double the others. The effectiveness of ultrasonic dissolution enhancement increased exponentially with decreasing stacking fault energy (i.e., resistance to the creation of surface faults such as pits and dislocations), while, in turn, the effectiveness of mega sonic dissolution increased linearly with the stacking fault energy. These results give new insights into the use of acoustic frequency selections for accelerating elemental release from solutes by the use of acoustic perturbation

Direct observation of pitting corrosion evolutions on carbon steel surfaces at the nano-to-micro- scales.

The Cl--induced corrosion of metals and alloys is of relevance to a wide range of engineered materials, structures, and systems. Because of the challenges in studying pitting corrosion in a quantitative and statistically significant manner, its kinetics remain poorly understood. Herein, by direct, nano- to micro-scale observations using vertical scanning interferometry (VSI), we examine the temporal evolution of pitting corrosion on AISI 1045 carbon steel over large surface areas in Cl--free, and Cl--enriched solutions. Special focus is paid to examine the nucleation and growth of pits, and the associated formation of roughened regions on steel surfaces. By statistical analysis of hundreds of individual pits, three stages of pitting corrosion, namely, induction, propagation, and saturation, are quantitatively distinguished. By quantifying the kinetics of these processes, we contextualize our current understanding of electrochemical corrosion within a framework that considers spatial dynamics and morphology evolutions. In the presence of Cl- ions, corrosion is highly accelerated due to multiple autocatalytic factors including destabilization of protective surface oxide films and preservation of aggressive microenvironments within the pits, both of which promote continued pit nucleation and growth. These findings offer new insights into predicting and modeling steel corrosion processes in mid-pH aqueous environments

Calcination-free production of calcium hydroxide at sub-boiling temperatures.

Calcium hydroxide (Ca(OH)2), a commodity chemical, finds use in diverse industries ranging from food, to environmental remediation and construction. However, the current thermal process of Ca(OH)2 production via limestone calcination is energy- and CO2-intensive. Herein, we demonstrate a novel aqueous-phase calcination-free process to precipitate Ca(OH)2 from saturated solutions at sub-boiling temperatures in three steps. First, calcium was extracted from an archetypal alkaline industrial waste, a steel slag, to produce an alkaline leachate. Second, the leachate was concentrated using reverse osmosis (RO) processing. This elevated the Ca-abundance in the leachate to a level approaching Ca(OH)2 saturation at ambient temperature. Thereafter, Ca(OH)2 was precipitated from the concentrated leachate by forcing a temperature excursion in excess of 65 °C while exploiting the retrograde solubility of Ca(OH)2. This nature of temperature swing can be forced using low-grade waste heat (≤100 °C) as is often available at power generation, and industrial facilities, or using solar thermal heat. Based on a detailed accounting of the mass and energy balances, this new process offers at least ≈65% lower CO2 emissions than incumbent methods of Ca(OH)2, and potentially, cement production

Revealing How Alkali Cations Affect the Surface Reactivity of Stainless Steel in Alkaline Aqueous Environments.

Stainless steel is a ubiquitous structural material and one that finds extensive use in core-internal components in nuclear power plants. Stainless steel features superior corrosion resistance (e.g., as compared to ordinary steel) due to the formation of passivating iron and/or chromium oxides on its surfaces. However, the breakdown of such passivating oxide films, e.g., due to localized deformation and slip line formation following exposure to radiation, or aggressive ions renders stainless steel susceptible to corrosion-related degradation. Herein, the effects of alkali cations (i.e., K+, Li+) and the interactions between the passivated steel surface and the solution are examined using 304L stainless steel. Scanning electrochemical microscopy and atomic force microscopy are used to examine the inert-to-reactive transition of the steel surface both in the native state and in the presence of applied potentials. Careful analysis of interaction forces, in solution, within ≤10 nm of the steel surface, reveals that the interaction between the hydrated alkali cations and the substrate affects the structure of the electrical double layer (EDL). As a result, a higher surface reactivity is indicated in the presence of Li+ relative to K+ due to the effects of the former species in disrupting the EDL. These findings provide new insights into the role of the water chemistry not only on affecting metallic corrosion but also in other applications, such as batteries and electrochemical devices

Revealing How Alkali Cations Affect the Surface Reactivity of Stainless Steel in Alkaline Aqueous Environments

Stainless steel is a ubiquitous structural material and one that finds extensive use in core-internal components in nuclear power plants. Stainless steel features superior corrosion resistance (e.g., as compared to ordinary steel) due to the formation of passivating iron and/or chromium oxides on its surfaces. However, the breakdown of such passivating oxide films, e.g., due to localized deformation and slip line formation following exposure to radiation, or aggressive ions renders stainless steel susceptible to corrosion-related degradation. Herein, the effects of alkali cations (i.e., K+, Li+) and the interactions between the passivated steel surface and the solution are examined using 304L stainless steel. Scanning electrochemical microscopy and atomic force microscopy are used to examine the inert-to-reactive transition of the steel surface both in the native state and in the presence of applied potentials. Careful analysis of interaction forces, in solution, within ≤10 nm of the steel surface, reveals that the interaction between the hydrated alkali cations and the substrate affects the structure of the electrical double layer (EDL). As a result, a higher surface reactivity is indicated in the presence of Li+ relative to K+ due to the effects of the former species in disrupting the EDL. These findings provide new insights into the role of the water chemistry not only on affecting metallic corrosion but also in other applications, such as batteries and electrochemical devices

Mineral Dissolution under Electric Stimulation

Although mineral dissolution and precipitation have been extensively studied, the role of electric stimulation on these processes remains unclear. We reveal the effects of subcritical electric potential (i.e., lower than the breakdown potential of water) on the bulk dissolution rates of calcite (carbonate; CaCO3) using a custom-built three-electrode cell. The effects of applied potential depend on the pH, ionic strength, and temperature. For calcite, the enhancement in dissolution rates-under isothermal conditions-is explained by enhanced ion transport. Thus, at acidic to near-neutral pH (pH 4-6) wherein calcite\u27s dissolution is mass transfer or mixed mode controlled, dissolution rates increase with increasing potential. But, under alkaline conditions (pH 10), wherein surface reactions limit calcite\u27s dissolution, its dissolution rate is unaffected by electric potential. This suggests that subcritical applied potentials do not appear to alter the distribution of charged surface sites within the inner Helmholtz plane (IHP) of the electric double layer (EDL) at the mineral-solution interface. Rather, applied potential acts to enhance transport-controlled dissolution by enhancing the ion flux in to and out of the diffusion boundary layer (DBL). This results in a reduction in the activation energy of mineral dissolution in proportion to the applied potential, indicating a Butler-Volmer mechanism of dissolution stimulation. This mechanism is confirmed by comparison to orthoclase (KAlSi3O8), which dissolves exclusively under surface control-whose dissolution is unaffected by applied potential. As a result, applied potentials are only effective when the mass transfer of ions is the rate-limiting step of mineral dissolution

Evolution of Strain in Heteroepitaxial Cadmium Carbonate Overgrowths on Dolomite

The evolution and accommodation of lattice strain in an epitaxial mineral film grown on an isostructural substrate were observed as a function of film thickness. Cadmium carbonate films (approximately CdCO₃ (otavite) in composition) were grown on the (104) surface of CaMg­(CO₃)₂ (dolomite) from aqueous solutions that were supersaturated with respect to both pure otavite and Cd-rich (Cd_1–<i>x</i>Ca<i>_x</i>)­CO₃. Specular and nonspecular X-ray reflectivity (XR) revealed that the structure and strain of the otavite overgrowths evolved in a manner that is fully consistent with a Stranski-Krastanov growth mode. Otavite films initially grew as coherently strained films, up to an average thickness of ∼15 Å, with lateral compressive strains and an expansion of the vertical film lattice spacing resulting in a unit cell volume consistent with pure otavite. Thicker films (>15 Å) became incommensurate with the substrate, having lattice parameters that are indistinguishable from pure otavite. These results indicate that the evolution of these mineral films is controlled by epitaxy and are consistent with the growth of essentially pure otavite films. These results provide a foundation for understanding the stability of thin-film overgrowths in the natural environment

Effects of Irradiation on Albite’s Chemical Durability

Albite (NaAlSi₃O₈), a framework silicate of the plagioclase feldspar family and a common constituent of felsic rocks, is often present in the siliceous mineral aggregates that compose concrete. When exposed to radiation (e.g., in the form of neutrons) in nuclear power plants, the crystal structure of albite can undergo significant alterations. These alterations may degrade its chemical durability. Indeed, careful examinations of Ar⁺-implanted albite carried out using Fourier transform infrared spectroscopy (FTIR) and molecular dynamics simulations show that albite’s crystal structure, upon irradiation, undergoes progressive disordering, resulting in an expansion in its molar volume (i.e., a reduction of density) and a reduction in the connectivity of its atomic network. This loss of network connectivity (i.e., rigidity) results in an enhancement of the aqueous dissolution rate of albitemeasured using vertical scanning interferometry (VSI) in alkaline environmentsby a factor of 20. This enhancement in the dissolution rate (i.e., reduction in chemical durability) of albite following irradiation has significant impacts on the durability of felsic rocks and of concrete containing them upon their exposure to radiation in nuclear power plant (NPP) environments