Tribocorrosion Properties of PEO Coatings Produced on AZ91 Magnesium Alloy with Silicate- or Phosphate-Based Electrolytes

In this work, the tribocorrosion behavior of plasma electrolytic oxidation (PEO)-coated AZ91 samples was studied. In particular, two different coatings were produced and compared. One was obtained with an alkaline electrolyte containing sodium phosphate, whereas the other one was produced with an alkaline electrolyte containing sodium silicate. The coatings were characterized with SEM-EDS and XRD techniques, and after the tribocorrosion tests, the wear scars were analyzed with SEM-EDS. The tribocorrosion behavior was evaluated measuring the OCP during a pin on disk test performed in an aggressive environment. Moreover, potentiodynamic polarization and electrochemical impedance spectroscopy tests were performed, to evaluate the corrosion resistance of the different samples in the absence of wear phenomena. The behavior of all the PEO-treated specimens was compared with the one of the untreated sample. A remarkable increase in the tribocorrosion performances after the PEO treatments was observed. Moreover, the samples obtained with the electrolyte containing silicates showed higher tribocorrosion performances

Unveiling the impact of laser power variations on microstructure, corrosion, and stress-assisted surface crack initiation in laser powder bed fusion-processed Ni-Fe-Cr alloy 718

Corrosion and stress-corrosion related failures often compromise the integrity of critical metallic components during their service, raising significant concerns. It is crucial to comprehend the crack initiation mechanism and the impact of alloy microstructure on this crack initiation process. It is known that the introduction of unique microstructures through metal additive manufacturing brings new challenges. This study aims to investigate, for the first time, the effects of microstructural alterations resulting from fluctuations in laser power during laser powder bed fusion on the surface cracking initiation mechanism and electrochemical behaviour of Ni-Fe-Cr alloy 718, which is widely used in applications that require exceptional strength and corrosion resistance. To carry out this investigation, microcapillary electrochemical methods were combined with high-resolution techniques (TEM, SEM, AFM). The findings emphasize the existence of an optimal range of process parameters that effectively mitigate corrosion and crack initiation susceptibility. This work demonstrated that slight deviations in laser power from this optimal value result in diverse alterations at the micro and submicron scales. These alterations include increased subgrain width, porosity, dislocation density, density of nanovoids, and distribution of carbides. Importantly, these changes, particularly in dislocation and nanovoid densities caused by minor variations in process parameters, significantly affect the material's susceptibility to corrosion initiation and stress-assisted surface cracking

Tin deoxidation by atmospheric pressure plasma treatment.

Treatment of materials by atmospheric-pressure plasma technology is used in all sectors of industrial production and is a lower cost technology. Among the possible applications, the reduction of metallic oxides by plasma is often needed to prepare the surface of the substrates before treatment or to deoxidise materials which contain metals. In this work the deoxidation of metallic Sn by atmospheric pressure plasma treatment whit N2-H2 gas mixture was investigated. The particles of Sn metallic are very thermal sensitive and the use of atmospheric pressure plasma whit gas containing H2 for the removal of surface oxide is a promising technique to increase the soldering process efficiency. For pure tin, SnO grew faster under humid conditions than in dry air and a very thin layer of SnO2 was formed on the top surface under humid conditions; the mixture of both SnO and SnO2 grew for oxidation at 150\ub0C. Electrochemical reduction analysis is a relatively precise technique to measure quantitatively both the type and the thickness of oxides on metal surface. The surface of tin was degreased and polished according to the standard metallographic techniques and the test samples was fully reduced electrochemically at a high current density before being placed in an furnace. The oxidized specimens are treated whit a rotating jet atmospheric plasma which works whit a frequency of 16-20 kHz and generate a plasma whit a maximum power of 1 kW. This plasma is created in a jet through the action of a stepped, high-frequency, pulsed current that turns the gas (N2 99% - H2 1%) into a beam of plasma. The plasma generated at atmospheric pressure (1 bar) with low power consumption (E = 290 V) is directed onto the tin surface trough a torch which can be moved on the material\u2019s surface at different rates. The electrochemical reduction analysis was carried out in a borate buffer solution: a constant cathodic current (-20 microA) is applied between the surface and an inert counter electrode (Pt). The change of cathode potential of the oxidized surface during reduction is recorded as a function of time relative to a reference electrode. The potential-time curve was performed by AMEL 551 potentiostat - galvanostat equipped with an AMEL 567 function generator using a saturated calomel electrode (SCE) as reference and consists of a series of potential durations, which is characteristic of each type of oxides and indicate the results achieved in the deoxidation

Influence of CO2 Laser Beam Welding on Microstructure of Aluminum Metal Matrix Composites Reinforced with Al2O3 Particles

The continuous CO2 laser beam welding of two aluminum metal matrix composites (6061 and 2618 reinforced with 20% of Al2O3) has been investigated with special attention to the influence of the base alloys, the filler material and the process parameters on the microstructure of the welding bead. In this work square butt welding has been obtained with different CO2 laser power, feed rate and shielding gases. The microstructure of the welding beads has been examined by optical and electronic microscopy. A migration of the Al2O3 reinforcement particles from the fusion zone (FZ) towards the heat affected zone (HAZ) has been detected and the particles agglomerate near the interface FZ-HAZ. An increase in both porosity and agglomerate sizes inside the welding bead has been observed as the feed rate has been reduced. The hardness of welding beads has been higher than the unaffected composites. A reduction of both the beads hardness and the Al2O3 migration and agglomeration has been obtained by the use of magnesium-rich filler material in the welding process, probably because magnesium is able to increase the reinforcement wettability, and to reduce the rate of formation of spinel phase MgAl2O4. The use of Ar as shielding gas was the most effective, both in order to avoid the development of porosity and to reduce the Al2O3 agglomeration

Temperature dependent properties and aggregation behaviour of FeCo nanoparticles produced sonoelectrochemically

The study describes synthesis of FeCo nanoparticles by using the pulsed sonoelectrochemical technique, a method which couples an electrochemical process with the employment of high power ultrasound. An ultrasonic horn is also used as the working electrode and is subjected to a pulsed galvanic current and pulsed out of phase ultrasound. Nanoparticles made of FeCo alloy were synthesized at different bath temperatures, in order to study and evaluate the influence of this parameter on process efficiency and nanoparticles\u2019 features. Produced material was characterized by X-EDS, X-Ray diffraction, and finally by transmission electron microscopy. Moreover characterization of nanoparticles\u2019 tendency to aggregation was performed with dynamic light scattering and by using a polyacrilate to stabilize the suspensions. Process efficiency was found to be strongly influenced by temperature, and from chemical analyses, a preferential deposition of iron was observed, due to the lower iron reduction overpotential. Structural characterization stated that FeCo nanoparticles showed a bcc structure and a mean grain size below 30 nm, which depended on synthesis temperature (T) and decreased with T to 5 nm. TEM characterization showed that nanoparticles exhibited the same mean dimensions like ones found from XRD analyses; this led to conclude that nanopowders are monocrystalline