Caracterização eletroquímica de ligas amorfas e cristalinas dos sistemas metálicos Ni-Nb e Ni-Nb-B

Abstract

Nickel based alloys are used in many industrial applications because of their excellent corrosion resistance. Most of these alloys are currently processed with crystalline structure. Amorphous metallic alloys generally exhibit higher corrosion resistance than their crystalline counterparts, which makes the study of amorphous metallic alloys based on nickel important. In this work, amorphous alloys of Ni62Nb38, Ni59.24Nb37.76B3.00 and Ni58.1Nb38.9B3.0 compositions, with crystalline and amorphous structures, produced by arc melting and melt spinning techniques were studied in corrosion tenting. The vitreous metals were analyzed by X-ray diffraction and differential scanning calorimetry to prove their amorphous nature. Images of optical and atomic force microscopy revealed the presence of intermetallic in the microstructure of the crystalline alloys. All the alloys were characterized by electrochemical techniques in 3.5% m/v NaCl at temperatures of 25 and 45 °C. The electrochemical impedance spectroscopy and polarization curves data revealed the superiority of the amorphous alloys compared to the crystalline ones for the same chemical composition. Passivation current density values were lower in the vitreous metals. The polarization resistance of the amorphous alloy reaches 20 times that of its corresponding crystalline. Both structures showed a reduction in corrosion resistance with increasing temperature. In the amorphous alloys, the presence of boron provided the samples higher resistant to corrosion at both temperatures. In addition, the higher percentage of niobium, among ternary compositions, also improved properties in relation to corrosion. In the crystalline alloys, the presence of boron made the samples less resistant to corrosion at 25 ºC. However, this element allowed the ternary alloys greater resistance to the thermal effect, improving their properties at 45 ºC. Finally, from the Mott-Schottky analysis revealed that the passive films formed in all the alloys exhibited n-type semiconductor behavior for a wide range of potentials. On the other hand, in higher potentials, there was a change in the nature of the major charge donors, so that the films started to behave like p-type semiconductors. Using the range of potentials in which the oxide behaved as n-type semiconductor, it was possible to estimate the number of major charge donors in the films. These values showed a magnitude of 10²0 cm-³. In the amorphous alloys, the concentration of these defects was slightly higher than in the crystalline ones. In addition, increasing the temperature reduced the amount of charge donors in the passive films of all samples.