Microgalvanic Cells Research Articles

Illustrating the influence of anodic/cathodic second-phases on dissolution mode via Mg–Ca–Sn alloy anodes for primary Mg-air cells

Two second-phases with an opposite potential difference to the α-Mg matrix may affect the discharge mechanism of magnesium (Mg) anodes for primary Mg-air cells through different micro-galvanic behaviors. Herein, using Mg–Ca-xSn alloys anode with Mg2Ca and CaMgSn phases, we investigated their influence on the dissolution mode of Mg-anode during discharge state via in-situ scanning vibrating electrode technique (SVET) technology. Firstly, the peak surface positive potential of Mg–1Ca alloy only containing Mg2Ca phase and Mg–1Ca–1Sn alloy with CaMgSn/Mg2Ca phase increases multiplied by 10 during the discharge process compared to that of OCP, exhibiting accelerated anodic substrate dissolution. Moreover, Mg2Ca-phases can be attacked firstly while promoting the discharge of the α-Mg matrix surrounding them, leading to the preferential dissolution of grain boundaries and “chunk effects (CE)” occurrence. More CaMgSn/α-Mg couples are activated to form dispersed micro-galvanic cell islands, which also boosts the dissolution of the α-Mg matrix but reduces the area of high-intensity dissolution reaction due to CaMgSn acting as a cathode. However, excessive CaMgSn-phase generally fails to discharge effectively to reduce anode efficiency, although it can improve the self-peeling capacity of the film layer through physical detachment; in addition, with Sn content increasing to 3 wt%, the open circuit voltage of Mg–1Ca–3Sn significantly declines. Mg–1Ca-0.2Sn exhibits the highest cell voltage and anodic efficiency by introducing a tiny amount of dispersed short rod-like CaMgSn, due to the adjusted dissolution mode, enhanced electrochemical activity and inhibited CE. Finally, our study provides a universal mechanism for fabricating Mg-anode with high performance via phase composition modulation.

Understanding the Effect of Corrosion Resistance in Welded AA2198‐T8 Alloy: Microstructural‐Electrochemical Insights

ABSTRACTIn this study, we explore the corrosion resistance of AA2198‐T8 alloy in friction stir‐welded zones, focusing on microstructure changes and resulting corrosion susceptibility. Macrogalvanic coupling effects are observed in weldment under corrosive conditions, particularly in the welding joint (WJ), heat‐affected zones, and base metal (BM). Severe localized corrosion is prevalent in anodic zones, revealing intricate trenching patterns around micrometric particles in cathodic domains. Investigating isolated zones exposes SLC across all welding‐affected regions and BM, underscoring the involvement of microgalvanic cells in corrosion. The novel utilization of a syringe cell as an electrochemical tool proves to be instrumental in delineating the electrochemical characteristics of welding testing zones. Notably, stir zone within WJ emerges as the region depicting the lowest corrosion resistance, a critical finding substantiated by localized electrochemical impedance spectroscopy. These insights into the interplay of electrochemical phenomena and microstructural attributes have implications for advancing corrosion mitigation strategies and alloy engineering in materials science.

Corrosion behavior of novel Ti6422 alloy with equiaxed structure and lamellar structure in HCl solution

This study systematically investigated the electrochemical corrosion and static immersion corrosion behavior of the novel Ti6422 (Ti-6Al-4V-2Mo-2Fe) alloy with equiaxed (EM) and lamellar structure (LM) in HCl solution. The differences in corrosion performance between EM and LM samples were analyzed based on the characteristics of the passive film and substrate microstructure. Results indicated that the LM sample contained higher contents of TiO₂, Al₂O₃, and MoO₃ with higher stability compared with the EM sample, and the thickness of passive film was approximately twice that of the EM sample. Additionally, the donor density of LM sample was lower than EM sample. These findings indicate that the passive film on LM samples is thicker, with fewer defects and higher density. Electrochemical corrosion results revealed that the corrosion current of LM samples was lower and the polarization resistance was higher than these of EM samples, both suggesting superior corrosion resistance of LM samples. Immersion corrosion further confirmed that the LM sample had a lower corrosion rate. The enhanced corrosion resistance in LM samples was primarily due to higher content of corrosion-resistant β phase. Conversely, the numerous micro-galvanic cells between αs precipitates and adjacent β phase in EM samples weakened the corrosion resistance. Overall, these results demonstrated that regulating microstructure through heat treatment is an effective strategy for improving corrosion resistance of titanium alloys.

MWCNT/Al–Bi–NaCl nanocomposites for enhanced H2 generation

H2 generation through hydrolysis of Al-composites has received eclectic attention for on-demand H2 applications such as low-power proton exchange membrane fuel cells. In this line, we prepared Multi-Walled Carbon Nanotubes (MWCNT)/Al–Bi–NaCl nanocomposites and tested them for enhanced H2 generation. These nanocomposites are prepared using high-energy ball milling followed by uni-axial cold compaction. H2 generation by these nanocomposites was tested using tap water at room temperature. An excellent maximum H2 generation rate of 13.33 mLg−1s−1 with an H2 yield of 98% and an induction time of less than 10 s is achieved. Analysis of different observations shows that adding an optimum percentage of MWCNT to the Al–Bi–NaCl composite results in flaky morphology, which facilitates easily accessible water pathways, enhancing the H2 generation rate of the nanocomposites. Also, an enhanced number of MWCNT/Al–Bi microgalvanic cells, optimum utilization of electrical conductivity of MWCNT, and dissolution of NaCl favored the H2 generation.

Corrosion behaviour of SiC particulate reinforced AZ31 magnesium matrix composite in 3.5 % NaCl with and without heat treatment

Magnesium is a lightweight structural material widely utilised in automotive applications. To enhance its mechanical properties, ceramic particulate reinforcement can be incorporated, particularly for wear resistance and high-temperature applications. However, the addition of ceramic particles to magnesium can compromise its corrosion resistance due to microgalvanic cell formation at the interfaces between the Mg matrix and the second phase. This reduces the chemical protection provided by the passive film. In this study, the corrosion properties of AZ31 and AZ31-5SiC samples were investigated, with a focus on the effect of heat treatment. Detailed microstructural and electrochemical analyses revealed that the AZ31 cast sample forms an effective passive film, resulting in improved corrosion resistance. However, the addition of SiC particles to AZ31 increased the corrosion rate, with corrosion mechanisms evolving over time. To mitigate these effects, a heat treatment process was employed to dissolve β-Mg17Al12 eutectic and Al8Mn5 intermetallic phases. The heat-treated AZ31 with SiC exhibited an improvement in corrosion resistance. These findings highlight the potential for heat treatment to enhance their corrosion resistance, thereby broadening their application prospects.

Effect of Si Content on Microstructures and Electrochemical Properties of Al-xSi-3.5Fe Coating Alloy

Hot-dip aluminum alloy is widely used in the engineering fields. However, during the aluminum plating process, Fe inevitably enters and reaches a saturation state, which has a significant impact on the corrosion resistance and microstructure of the coating. Currently, adding Si during the hot-dip aluminum process can effectively improve the quality of the coating and inhibit the Fe-Al reaction. To understand the effect of Si content on the microstructure and electrochemical performance of Al-xSi-3.5Fe coating alloys, the microstructure and post-corrosion morphology of the alloys were analyzed using SEM (Scanning Electron Microscope) and XRD (X-ray Diffraction). Through electrochemical tests and complete immersion corrosion experiments, the corrosion resistance of the coating alloys in 3.5 wt.% NaCl was tested and analyzed. The results show that the Al-3.5Fe coating alloy mainly comprises α-Al, Al3Fe, and Al6Fe. With the increase in Si addition, the iron-rich phase changes from Al3Fe and Al6Fe to Al8Fe2Si. When the Si content reaches 4 wt.%, the iron-rich phase is Al9Fe2Si2, and the excess Si forms the eutectic Si phase with the aluminum matrix. Through SKPFM (Scanning Kelvin Probe Force Microscopy) testing, it was determined that the electrode potentials of the alloy phases Al3Fe, Al6Fe, Al8Fe2Si, Al9Fe2Si2, and eutectic Si phase were higher than that of α-Al, acting as cathode phases to the micro-galvanic cell with the aluminum matrix, and the corrosion form of alloys was mainly galvanic corrosion. With the addition of silicon, the electrode potential of the alloy increased first and then decreased, and the corrosion resistance results were synchronous with it. When the Si content is 10 wt.%, the alloy has the lowest electrode potential and the highest electrochemical activity.

Effect of Cr content on micro-galvanic dissolution behavior and corrosion characteristics of Cu-Cr alloy laser cladding layer in a simulated seawater environment

Marine biofouling is a key issue that deteriorates the service performance and lifespan of ships and underwater engineering structures. The development of effective and long-term antifouling coating material is significant but still poses a great challenge. Herein, we have proposed the Cu-Cr cladding layers containing submicron Cr-rich precipitated phases were prepared via laser cladding technology. The cladding layers were aimed to ensure the continuous release of copper ions through the micro-galvanic cells by promoting Cu2(OH)3Cl flaking, and thus achieve effective and long-term antifouling performance. The effect of Cr content on corrosion behavior and antifouling performance of Cu-Cr cladding layers in a simulated seawater environment was investigated. The results demonstrated that corrosion and copper ions release rates increased while the P. tricornutum coverage rate decreased with the increase in the Cr content. It was associated with the increase of Cr-rich precipitates accelerating the corrosion of Cu-rich matrix within micro-galvanic cells. Importantly, the increased accumulation of Cr2O3-rich particles intensified the loose and cracked Cu2(OH)3Cl flaking, ensuring the long-term continuous release of copper ions from the cladding layer and thus achieving effective antifouling performance.

Corrosion performance of wire arc additively manufactured NAB alloy

Nickel–aluminum bronzes (NAB) are vital alloys, known for biofouling resistance, crucial for marine and shipbuilding industries. This study examined corrosion performance of NAB samples fabricated by wire arc additive manufacturing (WAAM) in as-built and heat-treated conditions. Microstructural analysis revealed the WAAM-NAB parts primarily consisted of the α-phase (copper) and three types of κ-phases: κII (spherical Fe3Al), κIII (Ni–Al in lamellar shape) within the interdendritic areas, and iron-rich κIV particles dispersed throughout the matrix. In contrast, casting-produced NAB showed the formation of a rosette-like κI phase as well. Corrosion behavior comparisons between the two NAB fabrication methods were also assessed. The microstructural characterizations revealed a rise in the size of the κIV particles after heat-treated at 350 °C for 2 h (HT1). Heat treatment at 550 °C for 4 h (HT2) resulted in a needle-like κV, coarsening of κII, partial spheroidization of κIII, and reduced κIV precipitation. When heat-treated to 675 °C for 6 h (HT3), κII and κV were coarsened, κIII was completely spheroidized, and κIV precipitation was significantly reduced. These microstructural features in HT2 and HT3 conditions steeply decreased their corrosion resistance compared to the WAAM as-built part. The as-built WAAM sample showed superior corrosion resistance in chloride solution, attributed to fewer κ-intermetallic phases and a finer microstructure. The κ-phases, irrespective of morphology, act as the cathodic areas versus the α-dendritic matrix, fostering microgalvanic cell formation. Consequently, precipitation of all cathodic κ-phases draws a higher galvanic current of the anodic α-phase, meaning a lower corrosion resistance.

Microstructure, mechanical properties, and corrosion behavior of a biodegradable Zn–1.7Mg–1Ca alloy processed by KoBo extrusion

The present study investigated the effects of Mg and Ca additions to pure Zn on its microstructure, mechanical properties, and corrosion resistance, as well as the influence of severe plastic deformation via KoBo extrusion on these properties for both pure Zn and the ternary Zn–1.7Mg–1Ca (wt.%) alloy. The microstructural analysis conducted using scanning electron microscopy and electron backscatter diffraction revealed that the addition of alloying elements led to the formation of Mg2Zn11 and CaZn13 intermetallic compounds. The equivalent strain generated via KoBo extrusion was calculated using FORGE® NxT software. The mechanical testing results revealed that the ternary Zn alloy had 186% higher hardness (100 ± 2.6 HV), 210% higher YS (31 ± 3.5 MPa), and 254% higher UTS (85 ± 7.4 MPa) compared to pure Zn. Moreover, the application of KoBo extrusion was found to be beneficial for further enhancing the mechanical properties of both pure Zn and the ternary Zn alloy. Furthermore, the results of the potentiodynamic polarization test performed in Ringer's solution for evaluating the corrosion resistance of the fabricated alloys revealed that the addition of alloying elements increased the corrosion rate by 11% compared to pure Zn (from 145 ± 9 to 161 ± 8 μm/year) due to the micro-galvanic cell formation between different phases, and the KoBo extrusion process increased the corrosion rate of pure Zn but decreased the corrosion rate of the ternary Zn alloy.

Efficient nanocatalysis of Ni/Sc2O3@FLG for magnesium hydrolysis of hydrogen generation

Magnesium is one of the most promising candidates of light metal materials for hydrogen production by hydrolysis due to its efficient and economical properties. Various modification methods have been investigated to improve the hydrolytic properties of Mg. However, the direction of the design of efficient catalysts is unclear and needs to be guided by a richer catalytic mechanism of hydrolysis. In this work, a simple approach was used to synthesize Few Layer Graphene (FLG)-loaded ultra-fine highly dispersed Ni/Sc2O3 nanocatalyst, which achieves impressive catalytic hydrolysis results. Here, the addition of 4 wt% Ni/Sc2O3@FLG catalyst allows Mg to produce 833 mL g–1 of H2 in 20 s at 30 °C. There is an initial hydrogen release rate as high as 5942 mL g–1 min–1, a final hydrogen yield of 859 mL g–1 (99.13%), and almost complete conversion of Mg to Mg(OH)2. Furthermore, surprisingly, even with only 0.2 wt% catalysts added, Mg still has an initial hydrogen generation rate of 3627 mL g–1 min–1, which is over 20 times faster than that of Mg. It also produces 690 mL g–1 of H2 in 30 s at 30 °C. Hydrolysis kinetic curves and microscopic morphology tests show that FLG could shape and hold Mg into thin sheets, giving them an ultra-high hydrolysis rate and conversion rate. The formation of micro-galvanic cells between Ni and Mg accelerates the electrochemical corrosion of Mg and greatly enhances electron transfer during hydrolysis. This work provides a new strategy for the preparation of efficient nanocatalysts, which is expected to make “Mg-efficient catalyst” the most ideal light metal-based material for hydrogen production by hydrolysis.

Characterization on corrosion and antibacterial properties of a new functionally graded porous Ti–Mo–Cu alloys with improved cytocompatibility

Biomaterial-associated infection for Ti–Mo alloy needs to be solved imminently, a functionally-graded porous Ti–15Mo-xCu alloys (x = 3, 7, 11, and 15 wt %) were developed by powder metallurgy. The effects of the Cu content on corrosion resistance, antibacterial properties, and cytocompatibility of the Ti–15Mo-xCu alloys were investigated. The results show the Ti–15Mo-xCu alloys were composed of β-Ti, α-Ti, and Ti2Cu phases. With the addition of Cu element, the Ti2Cu phase of the Ti–15Mo-xCu alloys mainly distributed at the grain boundaries, generating a micro-galvanic cell effect, which led to a decrease in the corrosion resistance of the Ti–15Mo-xCu alloys, compared with the Ti–15Mo alloy. Stable passivation was induced by the oxide films consisting of TiO2, MoO3, and Cu2O on the surface of the alloy during the corrosion process. The antibacterial properties increased with increasing Cu content. The antibacterial rate of the Ti–15Mo-xCu alloy with 7–15 wt % Cu content was greater than 90% against Escherichia coli, revealing good antibacterial properties. Moreover, the biocompatibility of the Ti–Mo–Cu alloy exhibited a downward trend after the culturing when the Cu content ranged from 11 to 15 wt %. Overall, the Ti–15Mo–7Cu alloy exhibited good corrosion resistance, antibacterial properties, and cytocompatibility, revealing its great potential in clinical applications.

Corrosion behavior and mechanism of 4004/Al-Mn-Cu-Mg-Si/4004 aluminum brazing sheet in sea water acidified accelerated test

The microstructure and corrosion properties of a novel 4004/Al-Mn-Cu-Mg-Si/4004 aluminum brazing sheet were investigated in comparison with the traditional 4004/3003/4004. It revealed that, the microstructure after brazing of two aluminum brazing sheets from outer to center were both residual Al-Si eutectic, precipitate-free zone (PFZ), band of dense precipitates (BDP) and core material, respectively. However, the alloying element distribution and the types and amounts of second phases were quite different. The sea water acidified accelerated test showed that 4004/Al-Mn-Cu-Mg-Si/4004 exhibited typical exfoliation corrosion, while the corrosion quickly spread deep into the core material of 4004/3003/4004. The formation of Cu concentration gradient and the strip distribution of Al-Mg-Cu-Si precipitates in the BDP region contributed to the exfoliation corrosion of 4004/Al-Mn-Cu-Mg-Si/4004. The corrosion behaviors of Al-Mn-Cu-Mg-Si and AA3003 core materials are both in form of intergranular corrosion (IGC)，and the later was more severe than the former. For Al-Mn-Cu-Mg-Si core material, high-density fine dispersed cathodic α-Al(Mn, Cu)Si precipitates promoted the formation of a profusion of micro-galvanic cells in the Al matrix, which would retard any effects between the grain boundary precipitations and the adjacent relatively PFZs. Hence, the IGC susceptibility of Al-Mn-Cu-Mg-Si core material was decreased. Besides, less large-sized α-AlFeMn(Cu)Si phases in core material also reduced the localized corrosion attack. At last, the corrosion in depth direction of 4004/Al-Mn-Cu-Mg-Si/4004 was retarded, compared to its counterpart.

Effect of heat treatment on mechanical and corrosion properties of an Al0.8CrFe2Ni2 alloy processed by laser powder bed fusion

This study presents an Al0.8CrFe2Ni2 high entropy alloy processed by laser powder bed fusion using prealloyed powder. Heat treatment of the near-single-phase face-centered-cubic microstructure, found in the as-built material, led to a fine dual-phase microstructure consisting of face-centered-cubic grains and Al-, Ni-rich body-centered-cubic B2 precipitates. Microstructure characterization, mechanical and corrosion testing revealed the effect of heat treatment on the properties of the novel alloy. The results suggested that tailoring the mechanical properties is possible with heat treatment, by controlling the precipitate size. Refinement of the precipitates led to enhanced strength without significant ductility loss. Corrosion tests in 3.5 % NaCl solution showed that the as-built samples provided an overall better corrosion resistance when compared to heat-treated samples. The presence of the precipitates enhanced the formation of micro-galvanic cells and influenced the corrosion behavior of the alloy in the heat-treated condition.

Highly Selective Reduction of Nitrate by Zero-Valent Aluminum (ZVAI) Ball-Milled Materials at Circumneutral pH: Important Role of Microgalvanic Cells for Depassivation of ZVAl and N2-Selectivity.

The passivation of zero-valent aluminum (ZVAl) limits its application in environmental remediation. Herein, a ternary composite material Al-Fe-AC is synthesized via a ball-milling treatment on a mixture of Al0, Fe0, and activated carbon (AC) powders. The results show that the as-prepared micronsized Al-Fe-AC powder could achieve highly efficient nitrate removal and a nitrogen (N2)-selectivity of >75%. The mechanism study reveals that, in the initial stage, numerous Al//AC and Fe//AC microgalvanic cells in the Al-Fe-AC material could lead to a local alkaline environment in the vicinity of the AC cathodes. The local alkalinity depassivated the Al0 component and enabled its continuous dissolution in the subsequent second stage of reaction. The functioning of the AC cathode of the Al//AC microgalvanic cell is revealed as the primary reason accounting for the highly selective reduction of nitrate. The investigation on the mass ratio of raw materials manifested that an Al/Fe/AC mass ratio of 1:1:5 or 1:3:5 was preferable. The test in simulated groundwater suggested that the as-prepared Al-Fe-AC powder could be injected into aquifers to achieve a highly selective reduction of nitrate to nitrogen. This study provides a feasible method to develop high-performance ZVAl-based remedial materials that could work in a wider pH range.

Unraveling the neglected role of elemental sulfur in chromate removal by sulfidated microscale zero-valent iron

Elemental sulfur (S0), as an oxidation product of low-valent sulfur, is widely believed to inhibit the reactivity of sulfidated zero-valent iron (S-ZVI). However, this study found that the Cr(VI) removal and recyclability of S-ZVI with S0 as the dominant sulfur species were superior to those FeS or iron polysulfides (FeSx, x > 1) dominated ones. The more S0 directly mixed with ZVI, the better Cr(VI) removal obtained. This was ascribed to the formation of micro-galvanic cells, the semiconductor properties of cyclo-octasulfur S0 with sulfur atom substituted by Fe2+, and the in situ generations of highly reactive iron monosulfide (FeSaq) or polysulfides precursors (FeSx,aq). The Cr(VI) sequestration of FeSx,aq was 1.2–2 times that of FeSaq, and the reaction rate of amorphous iron sulfides (FexSy) in the removal of Cr(VI) by S-ZVI was 8- and 66-fold faster than that of crystalline FexSy and micron ZVI, respectively. The interaction of S0 with ZVI required direct contact and needed to overcome the spatial barrier caused by FexSy formation. These findings reveal the role of S0 in Cr(VI) removal by S-ZVI and guide the future development of in situ sulfidation technologies to utilize the highly reactive FexSy precursors for field remediation.

Comparative corrosion study of HVOF coated Wc-10Co-4Cr, NiCrBSi and NiCr industrial waste coatings on AiSi304 stainless steel

The increasing demand for the high-velocity oxy-fuel (HVOF) thermal sprayed coated steel for boiler tube applications is the driving force behind this research work. Following the needs of the industry, the present study is aimed at understanding and comparing the corrosion resistance behavior of the WC-10Co-4Cr, NiCrBSi, and NiCrIW coating on AISI 304 stainless steel. A detailed microstructural characterization of the coatings before and after the corrosion experiment by FESEM, phase identification by XRD, and porosity determination by image analysis was performed. Corrosion behaviors of the coatings are examined by the electrochemical measurement technique after their immersion in a solution of 4 % NaCl for 1 hr, 12 hr, 36 hr, 102 hr, 200 hr, and 250 hr. Electrochemical impendence spectroscopy and Potentiodynamic polarization test results show that the porosity of the coatings and interface between unmelted particle and matrix (Formation of micro-galvanic cell) are potential sites for corrosion initiation. Electrochemical impedance spectroscopy (EIS) data indicates that the corrosion resistance of all types of coating initially decreases with increasing immersion time, but after some time corrosion resistance of the coatings again increases with increasing immersion time. This is because of the local accumulation of corrosion products in the defects hindering penetration of the electrolyte easily. Both the corrosion studies revealed that corrosion resistance of the coatings is in the following order: NiCrBSi > WC-10Co-4Cr > NiCrIW.

In-situ aging treatment by preheating to obtain high-strength ZK60 Mg alloy processed by laser powder bed fusion

In this study, a ZK60 Mg alloy fabricated by laser powder bed fusion (LPBF) is subjected to in-situ aging treatment by preheating the substrate. The effects of preheating temperature on the microstructure, mechanical properties, and corrosion resistance of the LPBF-processed ZK60 alloy are systematically analyzed. The results show that at the preheating temperatures of 35, 90, and 180 °C, the average grain size of the samples does not change much, ranging between 7.0 and 8.2 μm. As the preheating temperature increases, the mechanical properties of the alloy increase. The specimens treated at a preheating temperature of 180 °C exhibit a yield strength (YS) of 201 ± 5 MPa, ultimate tensile strength (UTS) of 291 ± 7 MPa, and elongation (EL) of 14.7 ± 0.8%. This is attributed to the precipitation of a large amount of β’ 1 phase in the alloy during the aging state in the LPBF process. However, the corrosion resistance first improves and then deteriorates with the increase in the preheating temperature. Under a preheating temperature of 90 °C, the corrosion resistance is improved due to the reduction in the galvanic corrosion area by the precipitated phases. When the preheating temperature is increased to 180 °C, the precipitated phases and α-Mg form a large number of microgalvanic cells, which increases the active center for corrosion and deteriorates the corrosion resistance. These findings indicate that the desired mechanical properties and corrosion resistance of the LPBF-processed ZK60 alloy can be obtained by selecting an appropriate preheating temperature. • The cracks significantly reduce in LPBF-processed ZK60 alloy by substrate preheating. • The mechanical properties increase with increasing preheating temperature. • The corrosion resistance first improves and then deteriorates. • β’ 1 phase is beneficial to mechanical properties but deteriorates corrosion resistance.

Microstructural Transformation and Hydrogen Generation Performance of Magnesium Scrap Ball Milled with Devarda's Alloy.

A method for magnesium scrap transformation into highly efficient hydroreactive material was elaborated. Tested samples were manufactured of magnesium scrap with no additives, or 5 and 10 wt.% Devarda's alloy, by ball milling for 0.5, 1, 2, and 4 h. Their microstructural evolution and reaction kinetics in 3.5 wt.% NaCl solution were investigated. For the samples with additives and of scrap only, microstructural evolution included the formation of large plane-shaped pieces (0.5 and 1 h) with their further transformation into small compacted solid-shaped objects (2 and 4 h), along with accumulation of crystal lattice imperfections favoring pitting corrosion, and magnesium oxidation with residual oxygen under prolonged (4 h) ball milling, resulting in the lowest reactions rates. Modification with Devarda's alloy accelerated microstructural evolution (during 0.5-1 h) and the creation of 'microgalvanic cells', enhancing magnesium galvanic corrosion with hydrogen evolution. The 1 h milled samples, with 5 wt.% Devarda's alloy and without additives, provided the highest hydrogen yields of (95.36 ± 0.38)% and (91.12 ± 1.19)%; maximum reaction rates achieved 470.9 and 143.4 mL/g/min, respectively. Such high results were explained by the combination of the largest specific surface areas, accumulated lattice imperfections, and 'microgalvanic cells' (from additive). The optimal values were 1 h of milling and 5 wt.% of additive.

Identification and development of a new local corrosion mechanism in a Laser Engineered Net Shaped (LENS) biomedical Co-Cr-Mo alloy in Hank’s solution

In this work, the microstructure and the corrosion behavior of a biomedical grade Co-Cr-Mo (CCM) alloy manufactured by laser engineered net shaping (LENS) was characterized. Electron microprobe maps identified the micro-segregation of Cr and Mo to the cell boundaries in the microstructure of the LENS alloys. Cyclic polarization in Hank’s solution and study of the corroded surface under scanning electron microscopy (SEM) revealed the development of micro-galvanic cells and new crack-like features which served as active sites of localized dissolution of the alloy. Based on this evidence, a new corrosion mechanism that constituted of selective dissolution of Cr/Mo-depleted regions and development of crack-like features at junctions between the various cell types was proposed for LENS CCM alloys.

Influence of SiC/TiB2 Particles Addition on Corrosion Behavior of As-Cast Zn-Al-Cu Alloy Hybrid Composites

In this work, the silicon carbide (SiC) and titanium diboride (TiB2) reinforced as-cast Zn-Al-Cu alloy-based hybrid composites were produced with ultrasonic-assisted stir casting. The corrosion behavior of as-cast Zn-Al-Cu/TiB2/SiC hybrid composites with varying 0, 5, 10, and 15 weight percent of SiC + TiB2 particulates in Zn-Al-Cu matrix alloy was investigated in this study, The effect of additional reinforcements content on the corrosion behavior of hybrid composites with respect to Zn-Al-Cu alloy was investigated. The corrosion rate of hybrid composites was taken by using potentiodynamic polarization equipment (with 3.5 wt.% NaCl solution). The potentiodynamic polarization test findings showed that the hybrid composites’ uniform and localized corrosion resistances were enhanced by the addition of dual reinforcements. As the weight fraction of the SiC/TiB2 reinforcement increases, weak microgalvanic cells that form between the constituent phases of the fabricated as-cast Zn-Al-Cu hybrid composites weaken, improving their resistance to uniform and localized corrosion. The results revealed that the corrosion rate values were reduced to 28.88% at 15% of reinforcement when compared to the base alloy.