Cinthya Dinorah Arrieta González

The electrochemical behavior of the Ni5Al coating deposited by an electric arc was evaluated. Its behavior was evaluated in a saline solution as a function of temperature (20 °C, 40 °C, and 60 °C). The variation as a function of time of the Ni5Al coating’s values of open circuit potential, linear polarization resistance, and the evolution of its electrochemical impedance spectra were analyzed. With this deposition technique, the rapid formation of thick coatings is achieved, which reduces the presence of interconnected porosity. Even though the microstructural characteristics showed the presence of trapped oxides, the electrochemical measurements carried out showed excellent corrosion resistance of the coating. The coating quickly develops a passive layer that allows it to rapidly reach thermodynamic equilibrium with the corrosive medium.

The interest in studying the synthesis of an Al–Cr alloy system by non-equilibria processes is due to the formation of metastable or quasicrystalline phases when rapid solidification has been utilized. Similarly, the formation of quasicrystals has been reported to a much lesser extent when the mechanical alloying technique was applied. In the present research, a mixture of powders of Cr and Al (both elements with a purity of 99.99%) with compositions of Al-5 and 7.5 at. % Cr was subjected to a ball milling process. Afterwards, the powder mixture was subjected to a consolidation process, conducted by pressing and sintering processes. The X-ray diffraction analyses revealed that during 20 h of milling there was no formation of metastable or quasicrystalline second phases detected. In addition, the X-ray diffraction peaks revealed that as milling time increased, the nanometric grain size decreased, and once the sintering treatment was applied, the crystallite size decreased following the same tendency. The dislocation density was estimated using the size of nanometric grains; this computation revealed that the dislocation density grew throughout the ball milling process; even after sintering, the multiplication of dislocations prevailed following the same tendency.

This study reports the behavior of the Ni20Cr alloy in molten nitrate salts. Its behavior was evaluated in the eutectic mixture called Solar Salt (binary salt) and in a ternary mixture (90% Solar Salt and 10% lanthanum nitrate). The addition of lanthanum nitrate was performed to determine if the presence of the La³⁺ cation could act as a corrosion inhibitor. Through mass loss and potentiodynamic polarization studies, the effects of both electrolytes on the corrosion resistance of the alloy at 300, 400, and 500 °C and at exposure times of 250, 500, 750, and 1000 h were determined. The results showed an increase in the corrosivity of the ternary salt, due to a decrease in its melting point and an increase in the concentration of nitrate ions. However, it was observed that the La³⁺ cations formed a protective layer (La₂O₃) on the alloy surface. In both corrosive media, the Ni20Cr alloy showed excellent corrosion resistance, due to its ability to form protective layers of Cr₂O₃, NiO, and NiCr₂O₄, in addition to the presence of a layer of La₂O₃ in the case of the ternary salt.

Aluminum-based alloys have been considered candidate materials for cathodic protection anodes. However, the Al-based alloys can form a layer of alumina, which is a drawback in a sacrificial anode. The anodes must exhibit uniform corrosion to achieve better performance. Aluminum can be alloyed with Zn to improve their performance. In this sense, in the present research, the electrochemical corrosion performance of Al-xZn alloys (x = 1.5, 3.5, and 5 at.% Zn) exposed to 3.5 wt.% NaCl for 24 h was evaluated. Polarization curves, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS) were used to identify the electrochemical behavior. The microstructure of the samples before the corrosion assessment was characterized by means of X-ray diffraction analyses (XRD) and scanning electron microscopy (SEM). In addition, microstructures of the corroded surfaces were characterized using X-ray mappings via SEM. Polarization curves indicated that Zn additions changed the pseudo-passivation behavior from what pure Al exhibited in a uniform dissolution regime. Furthermore, the addition of Zn shifted the corrosion potential to the active side and increased the corrosion rate. This behavior was consistent with the proportional decrease in polarization resistance (R_p) and charge transfer resistance (R_ct) in the EIS. The analysis of EIS was done using a mathematical model related to an adsorption electrochemical mechanism. The adsorption of chloride at the Al-Zn alloy surface formed aluminum chloride intermediates, which controlled the rate of the process. The rate constants of the reactions of a proposed chemical mechanism were evaluated.

ZnO-based ceramic varistors have shown excellent electrical and dielectric properties due to their characteristics microstructures represented by the arrangement of their grains and grain boundaries that allow the absorption and flow of energy when subjected to an electrical surge. Their properties and characteristics depend on their chemical compositions and processing routes. Typical processing routes involve several stages of grinding and precalcination—which are time consuming processes. Because of this, this study proposes a simpler and cheaper alternative route for processing ceramic varistors. The alternative process proposed is the mixing of the precursor oxides by means of a hydrothermal treatment. The characteristics and properties of the synthesized ceramic varistors were evaluated by means of scanning electron microscopy, X-ray diffraction and impedance spectroscopy, considering the effect of the addition of rare earth oxides (La₂O₃, CeO₂ and Nd₂O₃). The results showed that the mixing of the oxides through hydrothermal treatment produces ceramic varistors with characteristics and properties similar to those obtained by other processing routes. Furthermore, it was observed that the addition of rare earth oxides affects the characteristics and properties of the ceramic varistor depending on the type of rare earth oxide added, its concentration and ionic radius.

In this work, the effect of the addition of noble metals on the order–order disorder process of the L12 structure corresponding to the intermetallic Ni₃Al is analyzed. Stoichiometric, nonstoichiometric, and quasi-binary compositions doped with noble metals such as Ag, Au, Pd, and Pt (1 at%) were analyzed. It was observed that depending on the composition, there is a modification in the activation energies calculated from the two time constants that characterize the disorder process. The statistic of atomic jumps was typified based on the configuration of the window to be crossed and, with this, it was identified that the origin of the negative activation energy of the long disorder process is due to an increase in the corresponding energy of the Al_Al-Ni jump through unnatural windows.

Fe–Al intermetallic compounds have been considered excellent candidates as alternative alloys for various applications in corrosive environments compared to other Fe-based alloys. Their excellent corrosion resistance is due to the development of an Al-based passive layer. The performance of the passive layer can be improved by adding a third alloy element. Therefore, in this study the electrochemical performance of the Fe40Al intermetallic alloy modified by the addition of a third alloy element (Cr, Ti, Co, Ni) is evaluated. The corrosion resistance of intermetallic alloys has been evaluated by electrochemical tests (potentiodynamic polarization curves, and measurements of open circuit potential, linear polarization and electrochemical impedance) in artificial saliva. The performance of intermetallic alloys was compared with that of Ti. The results obtained showed that the addition of Ni and Ti substantially improves the corrosion resistance of the base intermetallic. The corrosion resistance shown is comparable or greater than that shown by Ti. However, the addition of Co reduces the corrosion resistance of the base intermetallic.

In this study, the performance evaluation of lanthanum compounds as corrosion inhibitors of vanadium salts was performed. The inhibitors tested were lanthanum acetate and La₂O₃. The performance of the inhibitors was tested using sodium metavanadate (NaVO₃) as a corrosive medium at 700, 800, and 900 °C. The corrosion inhibitory effect was evaluated on the corrosion process of 304H stainless steel. The corrosion rate of the steel was determined by the mass loss technique after 100 h of immersion in the corrosive salt with and without the addition of the corrosion inhibitor. The results show that lanthanum compounds act as corrosion inhibitors of vanadium salts. The inhibitory effect increases by increasing the concentration and tends to decrease when increasing the test temperature. Lanthanum compounds act as excellent corrosion inhibitors due to their ability to stabilize vanadium cations. Vanadium is stabilized by forming a new compound, lanthanum vanadate (LaVO₄), with a melting point much higher than the compounds formed when Mg or Ni compounds are used as corrosion inhibitors.