Surface Film Formation Research Articles

Constructing dilute Mg-1.0In binary alloy: A pathway to enhanced anode performance in Mg-air battery

Magnesium anode is a critical material in Mg-air battery; however, the contradiction between mitigating self-corrosion and promoting activation poses a significant obstacle to the rapid advancement of primary magnesium batteries. Herein, this issue is addressed by constructing a dilute Mg-1.0In binary alloy with 1 wt% indium as the anode material. The equiaxed dendrites and moderate indium concentration gradient in the as-cast anode facilitate the formation of a protective surface film, leading to a low corrosion rate of 0.20 mm y−1 and enhancing the stability prior to discharge. Moreover, during the battery discharge process, this unique microstructure enables the redeposition of metallic indium and indium hydroxide, thereby dramatically accelerating the shedding of oxidation products and suppressing side hydrogen evolution reaction. The Mg-1.0In anode in a Mg-air battery delivers a discharge capacity of 1587 mA h g−1 and an average voltage of 1.46 V at 10 mA cm−2, outperforming most of previously reported magnesium anodes and the control samples with various indium contents. This work would inspire the achievement of superior Mg-air battery performance through the use of binary magnesium anode system in as-cast state, as opposed to a more complex and time-consuming multi-component anode design involving heat treatment and plastic deformation.

Motional Resistance as Highly Selective Descriptor to Probe Dynamic Formation of Surface Films on Zinc Anode

AbstractZinc anodes are expected as a promising alternative to lithium‐based anodes in energy storage systems due to their low cost, high theoretical capacity, and environmental friendliness. However, the development of efficient and stable zinc anode requires a fundamental understanding of the interfacial processes occurring during zinc deposition and dissolution cycling. In this study, we employed electrochemical quartz crystal microbalance (EQCM) analysis to investigate the potential‐dependent formation and decomposition of surface films on zinc metal anodes in sulfate‐based aqueous electrolytes. Changes in frequency and motional resistance served as complementary descriptors, with motional resistance being a highly selective indicator for probing dynamic surface film formation driven by side reactions at the zinc anode. While the frequency change provided the overall changes in the mass of both zinc metal and surface films, changes in the motional resistance selectively reflected the amount and nature of the visco‐elastic interface that comprise the surface films. The two descriptors provide quantitative and complementary means to discover the complex interfacial processes such as the formation of surface visco‐elastic films, guiding to the development of more stable and efficient zinc‐based electrochemical systems.

Investigation of in-situ ion release and surface film formation of hcp Mg-Li thin films

In this work, the dissolution process of magnetron sputtered Mg-Li thin films was investigated by in-situ flow cell/ICP-MS measurements and ex-situ ICP-MS measurements after longer immersion and additional XPS measurements. High Li concentrations are released due to a Li rich carbonate layer formed in air. The depletion of Li leads to preferred Mg release before preferred Li release occurs due to the higher activity of Li and incorporation of Mg in corrosion products. This data provides a baseline for developing release profiles for medical application, more generally, it unravels details of the corrosion mechanism of lightweight MgLi alloys.

Enhancing concrete bond strength with silicate-based curing agents: A focus on surface damage repair

In this study, the effects of varying concentrations of silicate-based curing agents on damage repair and bond strength between concrete surfaces were investigated. The results show that these agents interacted with the concrete to produce calcium silicate hydrate (C-S-H), which filled the surface pores and thus facilitated damage repair. Furthermore, the interfacial bonding strength (IBS) of concrete was assessed through direct tensile tests. Scanning electron microscopy (SEM) was used for microstructural analysis, and pigment identification technology (PIT) was used to quantify the shedding area. Results indicate that a dosage of 24 mol/L significantly reduced the IBS by 40 %, primarily due to the formation of surface film. However, when the concentration of curing agents was reduced to 1.5 mol/L, the IBS increased by 17.6 % compared to untreated surfaces, indicating an improvement in bond strength with decreasing concentrations of curing agents.

Effect of high-energy shot peening on the corrosion behavior and chloride threshold concentration of AISI 316L stainless steel rebar in simulated concrete pore solution

The corrosion behavior and chloride threshold concentration of AISI 316L stainless steel reinforcement bar (rebar), subjected to high-energy shot peening (HESPed), are compared with its solution-annealed (SAed) counterpart in simulated concrete pore solution (CPS). X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that HESP significantly reduces the surface grain size to the ultrafine/nanometric range and increases the density of various dislocation structures, including dislocation walls and cells. This augmented presence of grain boundaries and dislocations, acting as high diffusivity paths, contributed to the formation of a thicker surface film with higher Cr content and lower donor/acceptor density in the HESPed sample compared to the SAed. Moreover, corrosion behavior analyses in CPSs with NaCl concentrations ranging from 0.0 to 2.0 M demonstrated that HESPed samples exhibit broader passivation ranges, more positive corrosion potentials, and higher polarization resistance under equivalent chloride concentrations. Additionally, threshold chloride concentrations obtained from the potentiodynamic polarization tests show values between 0.5 and 1.0 M for the SAed and between 1.0 and 1.5 M for the HESPed samples. Based on diffusion equations, a physical model is proposed to elucidate the improved behavior resulting from the HESP treatment.

Effect of Al3+ on the surface microstructure and film formation mechanism of Zr-based conversion coatings with improved corrosion resistance performance on cold rolled steel

Most Zr conversion coatings are prepared on Al alloys, with Al3+ from the substrate contributing significantly to the film growth process. Thus, the effect of Al3+ on the electrochemical characteristics, surface microstructure and film-formation mechanism of Zr-based conversion coatings on old- rolled steel (CRS) substrates was investigated by growing the coatings with and without Al3+ as an additive. Electrochemical tests indicated that the anticorrosion performance of the Zr-Al coatings prepared in the presence of Al3+ was greatly superior to that of Zr conversion coatings obtained in the absence of Al3+. Field-emission scanning electron microscopy and X-ray photoelectron spectroscopy analysis indicated that adding of Al3+ in the conversion bath changed the surface microstructure of the Zr coating, greatly promoted the deposition of Zr and F on the CRS substrate and increased the film thickness and density. Both coatings were embedded into the etched CRS substrates. The presence of Al3+ additive induced the formation of different coating components. The Zr coating mainly consisted of ZrO2 and some FeFx, whereas the Zr-Al coatings comprised of ZrO2, ZrOxFy, Al2O3, AlFx and some FeFx. The present study results confirm the key role of Al3+ as an additive in the film formation and growth process of Zr coatings and its effect on enhancing the anticorrosion performance of CRS substrates in NaCl solutions.

Probing the Electrode-Electrolyte Interface of Sodium/Glyme-Based Battery Electrolytes.

Sodium-ion batteries (NIBs) are promising systems for large-scale energy storage solutions; yet, further enhancements are required for their commercial viability. Improving the electrochemical performance of NIBs goes beyond the chemical description of the electrolyte and electrode materials as it requires a comprehensive understanding of the underlying mechanisms that govern the interface between electrodes and electrolytes. In particular, the decomposition reactions occurring at these interfaces lead to the formation of surface films. Previous work has revealed that the solvation structure of cations in the electrolyte has a significant influence on the formation and properties of these surface films. Here, an experimentally validated molecular dynamics study is performed on a 1 M NaTFSI salt in glymes of different lengths placed between two graphite electrodes having a constant bias potential. The focus of this study is on describing the solvation environment around the sodium ions at the electrode-electrolyte interface as a function of glyme chain length and applied potential. The results of the study show that the diglyme/TFSI system presents features at the interface that significantly differ from those of the triglyme/TFSI and tetraglyme/TFSI systems. These computational predictions are successfully corroborated by the experimentally measured capacitance of these systems. In addition, the dominant solvation structures at the interface explain the electrochemical stability of the system as they are consistent with cyclic voltammetry characterization.

Computational Modelling and Comparative Analysis of aSchiff Base Ligand and Its Analog as Inhibitors Against Mild Steel Corrosion in 1M HCl

Schiff bases, alternative anticorrosive additive wassynthesized, characterized and investigated for the inhibition of mild steel corrosion in 1M Hydrochloric acid at concentrations of 20 ppm, 40 ppm, 60 ppm, 80 ppm and 100 ppm using weight loss (WL) and electrochemical methods. The novel Schiff base ligands obtained were characterized by Ultraviolet-visible and Fourier-Transform Infrared Spectroscopy. The elemental analysis data for the Schiff base ligands were used to confirm the general formula of the Schiff bases. Fourier-Transform Infrared spectroscopy provided evidence of formation of a complex surface film due to adsorption of the Schiff bases on the mild steel surface. Maximum inhibition efficiency for Schiff base ligand 1 (SBL1) and Schiff base ligand 2 (SBL2) obtained were respectively 76.92% and 86.21% at concentrationof100ppm,andthetrendfollowedSBL2>SBL1, indicating the effectiveness of SBL2 in corrosion prevention as compared SBL1. Potentiodynamic polarization(PDP) measurements showed that the Schiff bases acted as mixed type inhibitors. Electrochemical Impedance Spectroscopic (EIS) measurement revealed that the corrosion process was controlled by charge transfer process. Inhibition efficiency values obtained from the different techniques were comparable. Quantum chemical parameters such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital(LUMO) and energy gap (ΔE) were obtained using Hartree-fock Density Functional Theory by Hamiltonian method. The results showed that SBL2 was more reactive than SBL1. In conclusion, the inhibition of mild steel corrosion was due to adsorption of active molecules leading to formation of a protective layer on surface of mild steel.

Two-in-one strategy: Discretely functionalized electrolyte additive for stable lithium metal batteries coupled with Ni-rich cathode

Li metal batteries (LMBs) coupled with Ni-rich cathode are regarded as a holy grail to achieve high energy density for the application in electric vehicles. However, the practical applications of LMBs coupled with Ni-rich cathode are limited by the severe Li dendrite growth and structural deterioration of the Ni-rich cathode. Among the various strategies to address these undesirable issues, introducing a dual-functional electrolyte additive is considered as an attractive approach because it can construct beneficial interface on both electrodes with a single agent. Herein, the two-in-one electrolyte additive, calcium tetrafluoroborate (Ca(BF4)2), is proposed for the rational design of interfacial layers on both Li metal anode and Ni-rich cathode. The preferential reduction of Ca2+ formed the Ca-based SEI layer which prevents electrolyte decomposition. In particular, Li-Ca alloy with superior lithiophilicity suppresses the growth of Li dendrite by reducing the nucleation polarization. Concurrently, the preferential oxidation of BF4− in Ca(BF4)2 results in the formation of B-based surface film on the Ni-rich cathode, which protects the Ni-rich cathode from structural deterioration, including micro-crack and dissolution of transition metal ion. As a result of unique characteristics of Ca(BF4)2 as an electrolyte additive, outstanding electrochemical performance can be accomplished in LMBs configured with Ni-rich cathode in carbonate electrolyte.

Electrochemical intercalation of anions into graphite: Fundamental aspects, material synthesis, and application to the cathode of dual-ion batteries.

In this review, fundamental aspects of the electrochemical intercalation of anions into graphite have been first summarized, and then described the electrochemical preparation of covalent-type GICs and application of graphite as the cathode of dual-ion battery. Electrochemical overoxidation of anion GICs provides graphite oxide and covalent-fluorine GICs, which are key functional materials for various applications including energy storage devices. The reaction conditions to obtain fully oxidized graphite has been mentioned. Concerning the application of graphite for the cathode of dual-ion battery, it stably delivers about 110 mA h g-1 of reversible capacity in usual organic electrolyte solutions. The combination of anion and solvent as well as the concentration of the anions in the electrolyte solutions greatly affect the performance of graphite cathode such as oxidation potential, rate capability, cycling properties, etc. The interfacial phenomenon is also important, and fundamental studies of charge transfer resistance, anion diffusion coefficient, and surface film formation behavior have also been summarized. The use of smaller anions, such as AlCl4 -, Br- can increase the capacity of graphite cathode. Several efforts on the structural modification of graphite and development of electrolyte solutions in which graphite cathode delivers higher capacity were also described.

Inhibition of Acid Descaling and Pickling Effects on API 5CT Carbon Steel Using Schiff Base Ligand (C24H21N5O2) in 1 M H2SO4 Solution

The objective of this paper was to investigate the inhibition of acid descaling and pickling effects on API 5CT carbon steel using Schiff base ligand (C24H21N5O2) in 1 M H2SO4 solution using Potentiodynamic Polarization (PDP), Electrochemical Impedance Spectroscopy (EIS) and Weight Loss (WL) techniques. FTIR spectroscopy shows that there was a strong adsorption of SFBL on the carbon steel surface due to formation of a complex surface film. Corrosion rate of carbon steel decreased exceedingly from 0.0155 to 0.0002 while inhibition efficiency of SFBL rose from 78.8 % to 98.9 % between 20 ppm and 100 ppm respectively. PDP measurements revealed a mixed type inhibitor. EIS measurement reveals that the increasing charge transfer resistance was directly proportional to the increase inhibitor concentration and the double layer capacitance dropped from 1.98 to 0.61 indicating a stronger inhibition.

Surface Reconditioning of Lithium Metal Electrodes by Laser Treatment for the Industrial Production of Enhanced Lithium Metal Batteries

AbstractIncorporating lithium metal anodes in next‐generation batteries promises enhanced energy densities. However, lithium's reactivity results in the formation of a native surface film, affecting battery performance. Therefore, precisely controlling the chemical and morphological surface condition of lithium metal anodes is imperative for producing high‐performance lithium metal batteries. This study demonstrates the efficacy of laser treatment for removing superficial contaminants from lithium metal substrates. To this end, picosecond‐pulsed laser radiation is proposed for modifying the surface of lithium metal substrates. Scanning electron microscopy (SEM) revealed that different laser process regimes can be exploited to achieve a wide spectrum of surface morphologies. Energy‐dispersive X‐ray spectroscopy (EDX) confirmed substantial reductions of ≈80% in oxidic and carbonaceous surface species. The contamination layer removal translated into interfacial resistance reductions of 35% and 44% when testing laser‐cleaned lithium metal anodes in symmetric all‐solid‐state batteries (ASSBs) with lithium phosphorus sulfur chloride (LPSCl) and lithium lanthanum zirconium oxide (LLZO) solid electrolytes, respectively. Finally, a framework for integrating laser cleaning into industrial battery production is suggested, evidencing the industrial feasibility of the approach. In summary, this work advances the understanding of lithium metal surface treatments and serves as proof of principle for its industrial applicability.

ВЛИЯНИЕ ГИПОМАГНИТНОГО ПОЛЯ НА ХАРАКТЕР РОСТА И МОРФОЛОГИЮ КЛЕТОК БАКТЕРИЙ РОДА BACILLUS

Biological effects induced by hypomagnetic field (HMF) were investigated using g. Bacillus as a model. HMF was found to affect the growth of bacterial population and morphological structure of prokaryotic cells. Cultivation of B. subtilis and B. licheniformis in HMF was characterized by a more active growth and formation of a dense surface film in contrast to a uniform medium turbidity in the geomagnetic field (GMF). Scanning electron microscopy was used to visualize segmentation disorders and morphological modifications of the bacterial cells characterized by changes in the form and size due to, probably, destructive transformation of peptidoglycane. In HMF, sub-inhibiting concentrations of the ultraviolet chemical analog (4-NQO) in the medium have a synergic effect on the microbial growth, and also produce distinct morphological transformations.

Development of an Al-Zn-Bi alloy sacrificial anode for the protection of steel in artificial seawater: an electrochemical analysis

The Al-Zn sacrificial anodes are widely used for cathodic protection in marine steel structures. This study evaluates the impact of bismuth addition on the electrochemical properties of the Al-Zn sacrificial anode in artificial seawater. The microstructure analysis confirms the presence of uniformly distributed intermetallic β-AlFeSi and spherical Bi within the α-Al matrix. The open circuit potential (OCP) comparison between Al-Zn-Bi and carbon steel reveals a potential difference of approximately 400 mV, indicating sufficient cathodic protection for the steel. Electrochemical impedance measurements indicate the initial hindered dissolution of the anode due to surface film formation, which later dissociates due to the aggressive attack of Cl− species in the electrolyte. The sufficiently negative surface potential (−0.875 Vvs. Ag/AgCl) observed at 10 mA cm−2 demonstrates the suitability of anode for fulfilling the cathodic protection criteria of steel structures.

Quantifying the Inactivation of Battery Electrode Material Particles

Lithium ion batteries, like any other battery technology, tend to lose capacity with continued charging and discharging. One of the effects contributing to the decline in capacity is the inactivation of positive electrode material particles. The electrodes typically consist of polydisperse active material particles, a conductive agent and a binder. By charging and discharging of the battery the active material particles are lithiated and delithiated. If a particle becomes inactive, e.g. by losing the electronical contact or by the formation of ion blocking surface films, it stops following the lithiation and delithiation of the rest of the electrode. Analyses of the degree of lithiation (DOL) of single particles therefore provide valuable information on the capacity loss attributed to inactive particles.[1-3]Since TOF-SIMS and synchrotron-based techniques which are usually used to determine the lithium content of single electrode particles only analyze small areas of the electrodes and are not well suited for measuring large numbers of samples, a method has been developed applying common ICP-OES instrumentation. For the determination of the DOL of single particles, the most important advantage of many ICP-OES instruments is the simultaneous detection of several elements. In case of common positive electrode materials such as Li(NixMnyCoz)O2 (x+y+z=1) (NMC) the ratio of e.g. Li to Mn signal intensities can be used to determine the DOL. As an aqueous sample introduction cannot be used because these materials would undergo Li+/H+ exchange reactions, a sample introduction using argon gas that furthermore enables a classification of particle sizes was developed.This presentation will focus on the method development and application of single particle ICP-OES for battery materials. It will be shown how this method helps to elucidate the mechanisms of capacity fading, and the advantages and disadvantages of ICP-OES will be compared with those of alternative methods for this application. Different parameters affecting the inactivation of poly- and single-crystallin NMC particles will be discussed.[1] T. N. Kroger, S. Wiemers-Meyer, P. Harte, M. Winter, S. Nowak, Analytical Chemistry 2021, 93, 7532-7539.[2] T.-N. Kröger, P. Harte, S. Klein, T. Beuse, M. Börner, M. Winter, S. Nowak, S. Wiemers-Meyer, J. Power Sources 2022, 527, 231204.[3] Kröger, T.-N.; Wölke, M. J.; Harte, P.; Beuse, T.; Winter, M.; Nowak, S.; Wiemers-Meyer, S. Chemsuschem 2022, 15, e202201169. Figure 1

New Liquid Electrolytes Consisting of Li Salts and Urea Derivatives for Li-Ion Batteries

Deep eutectic solvents are mixtures that exhibit melting points much lower than those of the endmembers due to the molecular interactions. Deep eutectic solvents attracted attention because high melting point compounds are available as flame-resistance solvents.1 However, the stable operation of Li-ion batteries (LIBs) with deep eutectic electrolytes (DEEs) is still challenging probably due to their high protonic nature and consequent narrow potential window.2 To develop Li-ion DEEs with a wide potential window, we investigated DEEs based on lithium bis(fluorosulfonyl)amide (LiFSA) and a series of urea derivatives to clarify the correlation between compound structure and electrolyte properties. Furthermore, we examined the electrode performances of the Li4Ti5O12 (LTO) negative electrode and LiNi1/3Co1/3Mn1/3O2 (NCM) positive electrode in the DEEs.To prepare DEEs, urea derivatives, such as urea, 1-methylurea, 1,3-dimethylurea, 1,1-dimethylurea, and tetramethyl urea are mixed with LiFSA at molar ratios of 1:2 or 1:4 at room temperature. The oxidation and reduction stability was evaluated by linear sweep voltammetry (LSV) in three-electrode cells with Pt foil as the working electrode. Galvanostatic charge-discharge tests were performed in three-electrode cells with LTO or NCM working electrodes and activated carbon counter electrodes.The prepared LiFSA:urea derivative (1:4) mixtures (LiFSA:urea, LiFSA:1,3-dimethylurea, and LiFSA:tetramethylurea) became transparent liquids at room temperature. Moreover, LiFSA:1,3-dimethylurea and LiFSA:tetramethylurea were clear liquid at even in 1:2 mixtures. Using the five DEEs, we evaluated their electrolyte properties. Figure 1 shows the LSV curves of the DEEs. Among the five deep eutectic liquids, LiFSA:1,3-dimethylurea (1:2) exhibited the widest potential window of 4.49 V, showing an even wider potential window than electrolytes with non-protonic tetramethyl urea.Figure 2a displays the charge-discharge curves of LTO electrodes in the DEEs. The LiFSA:urea (1:4) electrolyte cell exhibits a large irreversible capacity, and its initial reversible capacity is 228 mAh g-1, significantly larger than the theoretical capacity. This indicates that side reactions possibly takes place in addition to the LTO charge-discharge reaction. On the other hand, the LiFSA:1,3-dimethylurea (1:2) electrolyte demonstrates stable and reversible charge-discharge for 50 cycles, with an initial discharge capacity of 179 mAh g-1, which is close to the theoretical capacity. The LiFSA:tetramethylurea cell showed lower cycle stability than the LiFSA:1,3-dimethylurea cell, possibly due to electrolyte precipitation during cycling because LiFSA:tetramethylurea (1:2) electrolyte is an almost saturated solution.Using the LiFSA:1,3-dimethylurea (1:2) DEE, we further evaluated the electrochemical performance of the NCM electrode as shown in Figure 2b. The NCM electrode showed reversible charge-discharge behavior over 10 cycles with moderate capacity of 75.7 mAh g-1. These results demonstrate the potential applicability of the DEE to 3 V-class LIBs. In the presentation, we will also discuss the optimization of LiFSA concentration to improve battery properties and the observation of the formation of surface films on the electrodes in DEEs.

Scanning photoelectron microscopy discloses the role of Cr and Mo in the selective corrosion of hardmetal grades with Co-Ni binders

Advanced applications of hardmetal (HM) in chemically aggressive ambients call for the improvement of their corrosion resistance, combined with optimal abrasion and toughness properties. This goal is chiefly achieved with binder alloying. In particular, Cr and Mo additions to Co-Ni matrices have proved effective in many cases, but the physico-chemical mechanisms underlying this functional improvement are poorly understood. This situation, on the one hand does not enable the fine-tuning of the composition, and, on the other hand, does not allow to reliably extrapolate the performance of these grades to new ambients. The aim of this work is to propose a molecular-level approach to the understanding of the electrochemical corrosion of HM, based on space-resolved compositional and chemical-state analysis of the surface of HM subjected to controlled electrochemical corrosion. This type of analysis is enabled by synchrotron-based scanning photoelectron microscopy (SPEM) and photoelectron microspectroscopy (μ-XPS). Specifically, we studied a WC-based grade with 15 w% binder with 4.67 Co/Ni ratio, alloyed with 0.78 w% Cr and 0.94 w% Mo, corroded potentiostatically in neutral sulphate aqueous solution in the pseudopassive and transpassive ranges. SPEM and μ-XPS allowed to locate and distinguish the chemical state of the binder and carbide elements in the respective phases and to distinguish the surface stoichiometry brought about by corrosion in different conditions. In particular, we found that pseudopassive conditions lead to the formation of surface films, enriched in W(VI) and Cr(III). Transpassive conditions cause notable surface enrichment in Cr(III) as well as to the formation of mixed Cr-W oxides in the WC regions. Mo is leached from the binder at all potentials investigated, while Mo(0) is present in the WC region under pseudopassive conditions, while transpassivity leads to its oxidative removal. Chloride in the electrolyte favours dissolution of oxidized metals, in particular causing the leaching of Cr(III), while Mo(III) is found at the surface of WC grains.

Reinforcement of Positive Electrode-Electrolyte Interface without Using Electrolyte Additives Through Thermoelectrochemical Oxidation of LiPF6 for Lithium Secondary Batteries.

Owing to the limited electrochemical stability window of carbonate electrolytes, the initial formation of a solid electrolyte interphase and surface film on the negative and positive electrode surfaces by the decomposition of the electrolyte component is inevitable for the operation of lithium secondary batteries. The deposited film on the surface of the active material is vital for reducing further electrochemical side reactions at the surface; hence, the manipulation of this formation process is necessary for the appropriate operation of the assembled battery system. In this study, the thermal decomposition of LiPF6 salt is used as a surface passivation agent, which is autocatalytically formed during high-temperature storage. The thermally formed difluorophosphoric acid is subsequently oxidized on the partially charged high-Ni positive electrode surface, which improves the cycleability of lithium metal cells via phosphorus- and fluorine-based surface film formation. Moreover, the improvement in the high-temperature cycleability is demonstrated by controlling the formation process in the lithium-ion pouch cell with a short period of high-temperature storage before battery usage.

Improving Corrosion and Stress Corrosion Cracking Performance of Machined Biodegradable Alloy ZX20 by HF-Treatment

The treatment with hydrofluoric acid (HF-treatment) was suggested to be an effective way of improving the corrosion resistance of Mg alloys, including Mg-Zn-Ca (ZX) ones used for biodegradable implants. However, the effect of the HF-treatment on the stress corrosion cracking (SCC) susceptibility of ZX alloys has not been reported yet, although this phenomenon can induce premature brittle failures of the metallic medical devices, and thus, it is critical for their in-service structural integrity. In the present study, the effect of the HF-treatment on the microstructure, cytotoxicity, corrosion rate, mechanical properties, and fracture and side surface characteristics of the as-cast ZX20 alloy were investigated with the use of scanning electron microscopy, immersion, and slow-strain rate tensile testing in Hanks’ solution and indirect cell viability tests. It is found that the HF-treatment exerts no cytotoxic effect and results in a significant reduction in corrosion rate (up to 6 times of magnitude) and SCC susceptibility indexes (up to 1.5 times of magnitude). The observed improvement of corrosion and SCC performance of the alloy by the HF-treatment is found to be attributed to three effects, including (i) formation of the protective surface film of MgF2, (ii) removal of surficial contaminations originating from sample preparation procedures, and (iii) dissolution of surficial secondary phase particles. The mechanism of corrosion and SCC in the specimens before and after the HF-treatment are discussed.

Unraveling Compacted and Nodular Cast Iron Porosity: Case Studies Approach

Porosity is the culprit for a large fraction of scrap in cast iron foundries, resulting in significant environmental and productivity losses. The present work focuses on characterizing and explaining porosity defects in industrial compacted and nodular graphite cast iron components, utilizing current literature for reference. The goal is to identify existing knowledge gaps in the field, fostering further research work. Complex-shaped castings were sampled from three foundries, weighing between 100 and 300 kg. These were carefully selected to capture recurring defects during stable production. The mechanisms behind these defects were discussed, and the findings were compared to the literature. Scanning electron microscopy (SEM) was used to investigate the inner surfaces of the pores with secondary electron imaging. The surrounding microstructure was captured with optical microscopy in combination with image analysis, where panoramic images and nodularity maps were built. Ultimately, etching based on Si segregation was employed. The results suggest that the understanding of pore surface film formation remains limited, particularly regarding graphite film formation. Notably, the observations reveal a multitude of previously unreported graphite structures within the pores, some with particles in their centers containing Ce, Ca, La and S. These novel structures can provide additional insights regarding pore formation chronology.