The microstructure, mechanical and electrochemical properties of 3D printed alloys with reusing powders.

CoCrMo and Ti6Al4V are widely used in medical, dental and 3D printing technology, allowing the accurate fabrication of geometrically complicated structures. In order to reduce the costs of printed objects, the reuse of powder is common daily practice. When using 3D printing technology, the direct impact of elevated temperatures and the influence of the laser beam may change the properties of the powder when it is reused, thus affecting the final properties of the printed object. The main aim of the present study was to investigate the impact of reused powder on the mechanical, microstructural and electrochemical properties of 3D printed objects. 3D printed objects fabricated from virgin and reused powder of both alloys were analyzed by metallographic observation, computed tomography, XRD and electrochemical methods. The main finding of the study was that the use of reused powder (recycled 3 times) does not detrimentally affect the mechanical and corrosion integrity of 3D printed CoCr and Ti6Al4V alloys, especially for the purpose of applications in dentistry.

Tribocorrosion Susceptibility and Mechanical Characteristics of As-Received and Long-Term In-Vivo Aged Nickel-Titanium and Stainless-Steel Archwires.

To evaluate the effect of long-term in-vivo aging on orthodontic archwires, we aimed to assess the triboelectrochemical and mechanical characteristics of as-received and in-vivo aged nickel-titanium (NiTi) and stainless-steel (SS) orthodontic archwires. Four consecutive tribocorrosion cycles on six NiTi and six SS archwires, as-received and in-vivo aged, were performed on a reciprocal tribometer. Electrochemical noise and friction coefficient measurements, three-dimensional surface profiling, and hardness measurements were performed. Repassivation times of as-received archwires were longer than of the in-vivo aged; however, were shorter for NiTi. Friction coefficients were higher for NiTi than for SS archwires. Sudden major current drops concomitant with inverse potential shifts and friction coefficients' fluctuations, were seen for as-received (last cycle) and in-vivo aged (last three cycles) NiTi archwires. More pronounced tribocorrosion damage was observed on in-vivo aged NiTi than on other archwires. Hardness was generally higher inside the wear track of archwires. Long-term in-vivo exposure decreases the corrosion susceptibility of archwires, more evidently for the NiTi ones. Sudden major fluctuations in electrochemical current, potential, and friction coefficient detected for NiTi archwires, might be related to localized residual parts of the oxide layer persisting due to increased surface roughness or to phase transformations of the alloy's crystal structure.

Characterizing Steel Corrosion in Different Alkali-Activated Mortars.

Alkali-activated materials (AAMs) present a promising potential alternative to ordinary Portland cement (OPC). The service life of reinforced concrete structures depends greatly on the corrosion resistance of the steel used for reinforcement. Due to the wide range and diverse properties of AAMs, the corrosion processes of steel in these materials is still relatively unknown. Three different alkali-activated mortar mixes, based on fly ash, slag, or metakaolin, were prepared for this research. An ordinary carbon-steel reinforcing bar was installed in each of the mortar mixes. In order to study the corrosion properties of steel in the selected mortars, the specimens were exposed to a saline solution in wet/dry cycles for 17 weeks, and periodic electrochemical impedance spectroscopy (EIS) measurements were performed. The propagation of corrosion damage on the embedded steel bars was followed using X-ray computed microtomography (µXCT). Periodic EIS measurements of the AAMs showed different impedance response in individual AAMs. Moreover, these impedance responses also changed over the time of exposure. Interpretation of the results was based on visual and numerical analysis of the corrosion damages obtained by µXCT, which confirmed corrosion damage of varying type and extent on steel bars embedded in the tested AAMs.

Sensor Development for Corrosion Monitoring of Stainless Steels in H2SO4 Solutions.

Equipment made of different stainless steels is often used in the hydrometallurgical processing industry. In this study, an electrical resistance sensor was developed for monitoring corrosion in acidic solutions at high temperature. Two types of stainless steel were used as the electrode materials, namely grade 316L stainless steel (EN 1.4404) and grade 2507 duplex stainless steel (EN 1.4410). The materials and sensors were exposed to a 10% H2SO4 solution containing 5000 mg/L of NaCl at various temperatures. Results from the sensors were verified using electrochemical techniques and postexposure examination. Results showed that the microstructure played an important role in the interpretation of corrosion rates, highlighting the importance of using an appropriate stainless steel for the production of sensors. Electrochemical tests and postexposure examination both showed that the grade 2507 had a significantly lower corrosion rate compared to the grade 316L. Under industrial­process conditions, the results for the grade 2507 sensor were promising with respect to sensor durability and performance, despite the extremely harsh operating environment.