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

C-(N)-S-H and N-A-S-H gels: Compositions and solubility data at 25°C and 50°C

Abstract Calcium silicate hydrates containing sodium [C–(N)–S–H], and sodium aluminosilicate hydrates [N–A–S–H] are the dominant reaction products that are formed following reaction between a solid aluminosilicate precursor (eg, slags, fly ash, metakaolin) and an alkaline activation agent (eg NaOH) in the presence of water. To gain insights into the thermochemical properties of such compounds, C–(N)– S–H and N–A–S–H gels were synthesized with compositions: 0.8≤Ca/Si≤1.2 for the former, and 0.25≤Al/Si≤0.50 (atomic units) for the latter. The gels were characterized using thermogravimetric analysis (TGA), scanning electron microscopy with energydispersive X-ray microanalysis (SEM-EDS), and X-ray diffraction (XRD). The solubility products (KS0) of the gels were established at 25°C and 50°C. Selfconsistent solubility data of this nature are key inputs required for calculation of mass and volume balances in alkali-activated binders (AABs), and to determine the impacts of the precursor chemistry on the hydrated phase distributions; in which, C–(N)–S–H and N–A–S–H compounds dominate the hydrated phase assemblages. KEYWORDS calcium silicate hydrate, cements, geopolymers, solubility, thermodynamic

Cooling-Rate Effects in Sodium Silicate Glasses: Bridging the Gap between Molecular Dynamics Simulations and Experiments

Although molecular dynamics (MD) simulations are commonly used to predict the structure and properties of glasses, they are intrinsically limited to short time scales, necessitating the use of fast cooling rates. It is therefore challenging to compare results from MD simulations to experimental results for glasses cooled on typical laboratory time scales. Based on MD simulations of a sodium silicate glass with varying cooling rate (from 0.01 to 100 K/ps), here we show that thermal history primarily affects the medium-range order structure, while the short-range order is largely unaffected over the range of cooling rates simulated. This results in a decoupling between the enthalpy and volume relaxation functions, where the enthalpy quickly plateaus as the cooling rate decreases, whereas density exhibits a slower relaxation. Finally, we demonstrate that the outcomes of MD simulations can be meaningfully compared to experimental values if properly extrapolated to slower cooling rates

Micro-Raman analysis on the combined use of ammonium oxalate and ammonium phosphate for the consolidation and protection of carbonate stone artifacts

Ammonium oxalate ((NH4)2C2O4, AmOx) and more recently di-ammonium phosphate ((NH4)2HPO4, DAP) are used as inorganic agents in the conservation of cultural heritage for protection and consolidation of carbonate stone artifacts. In this work, we carry out a Raman investigation on the extent of penetration provided by the combined use of them. In particular, AmOx followed by DAP, DAP followed by AmOx, and a DAP + AmOx mixture are applied on tablets of pure CaCO3 as well as on degraded marble samples. Then, cross-sections of samples are analyzed in depth from surface to bulk. Characteristic differences in penetration depth of these agents and distribution of their products of interaction with the substrate are detected and discussed. Homogeneous distribution of whewellite inside the substrates down to a depth of ~1 mm was detected, which became larger in highly degraded regions of marble substrate. Ca-phosphates in the form of hydroxyapatite were detected at greater depth (down to 2.5 mm), confirming better consolidating properties of DAP with respect to AmOx. Among the application methods tested in our investigation, the DAP followed by AmOx treatment appears the most effective. The discussion of results takes into consideration several aspects including solubility and interaction dynamics between reaction products as well as the peculiar morphological features of the artifact, which are evidenced to play a significant role in treatment choice

Prediction model for hardened state properties of silica fume and fly ash based seawater concrete incorporating silicomanganese slag

Growing concrete consumption has gradually depleted conventional resources. This research incorporates silicomanganese (SiMn) slag, marine sand and seawater as alternative concreting materials. The use of SiMn slag to replace limestone as coarse aggregate enhances sustainability, though reducing strength and durability of concrete. This research aims to enhance the SiMn slag concrete by incorporating silica fume (SF) and fly ash (FA). The interaction of SF and FA on strength, durability and workability of concrete is investigated by statistically evaluating the experimental result. In this regard, the polynomial function prediction model is developed using the Response Surface Method (RSM) for the optimization of SF and FA contents. Analysis of variance (ANOVA) using p-value at significance level of 0.05 showed that the models were statistically significant and had marginal residual errors. All models had high fitness with R2 value ranging from 0.853 to 0.999. Adequate precision of models was above 4, indicating that the models had a low prediction error and were fit for optimization. Optimization indicated that a combination of 11.5% SF and 16.3% FA produced concrete that met the optimization criteria. Experimental validation showed that the highest prediction error was 3.4% for compressive strength, 3.2% for tensile strength, 4.9% for sorptivity and 18% for chloride permeability. The optimized concrete exhibited compact microstructure with good bonding between aggregate and cement paste. By using the established linear equation with SiMn slag concrete, the models also predicted the compressive strength of limestone concrete containing SF and FA with an error of between 0.9% and 5.4%

Solution chemistry of cubic and orthorhombic tricalcium aluminate hydration

This paper presents a solution chemistry-focused analysis of orthorhombic and cubic tricalcium aluminate (orth-C3A and cub-C3A, respectively) hydration. It is shown that the different solubilities of cub- and orth-C3A influence the bulk aqueous Ca to Al concentration ratio and the C3A/solution interface chemistry. The results are consistent with the bulk solution chemistry controlling orth-C3A dissolution, and with cub-C3A dissolution controlled by the formation of an Al-rich leached layer and adsorption of Ca‑sulfur ion pair complexes onto this layer. The polynaphthalene sulfonate-based admixture used here is identified to modify the solution chemistry and retard cub-C3A dissolution. Strategies to inhibit C3A dissolution in Portland cement are discussed

Uptake of chloride and carbonate by Mg-Al and Ca-Al layered double hydroxides in simulated pore solutions of alkali-activated slag cement

Chloride ingress and carbonation are major causes of degradation of reinforced concrete. To enable prediction of chloride ingress, and thus to improve the durability of structural alkali-activated slag cement (AAS) based concretes, it is necessary to understand the ionic interactions taking place between chlorides, carbonates, and the individual solid phases which comprise AAS. This study focused on two layered double hydroxides (LDH) representing those typically identified as reaction products in AAS: an Mg-Al hydrotalcite-like phase, and an AFm structure (strätlingite), in simulated AAS pore solutions. Surface adsorption and interlayer ion-exchange of chlorides occurred in both LDH phases; however, chloride uptake in hydrotalcite-group structures is governed by surface adsorption, while strätlingite shows the formation of a hydrocalumite-like phase and ion exchange. For both Ca-Al and Mg-Al LDHs, decreased chloride uptakes were observed from solutions with increased [CO₃²⁻]/[OH⁻] ratios, due to the formation of carbonate-containing hydrotalcite and decomposition of AFm phases, respectively