Resistência à corrosão em meio salino e à oxidação em elevadas temperaturas de uma liga Cu-Al-Ni-Mn com efeito memória de forma, processada por conformação por spray

Abstract

The Cu-Al-Ni, Cu-Zn-Al, and Ni-Ti alloy systems belong to the class of shape memory alloys (SMAs). The interest in studying copper-based SMAs arises from the fact that these alloys are more cost-effective than Ni-Ti alloys and exhibit better properties for specific applications. In this work, the electrochemical properties of a Cu-11.85Al-3.2Ni-3Mn (wt.%) SMA processed by spray forming were evaluated using electrochemical impedance spectroscopy (EIS) in a 3.5% m/v NaCl aqueous solution. The SMA alloy studied was hot-rolled and heat-treated at 250 °C and 550 °C prior to electrochemical testing. The effect of corrosion on the sample surfaces was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). XRD analysis showed the formation of austenite after heat treatment at 550 °C. EDS analysis identified an enrichment of aluminum and manganese on the alloy surface, as these are the most active elements whose oxides have a higher absolute value of negative free energy of formation. The impedance of the untreated alloy and the alloy heat-treated at 550 °C increased by two orders of magnitude as immersion time increased up to 286 hours due to the formation of protective oxides. The alloy heat-treated at 250 °C exhibited the lowest corrosion resistance after 286 hours of immersion in a saline solution. The isothermal oxidation behavior in an oxygen atmosphere of the SMA was also studied in the temperature range of 500–900 °C using the thermogravimetric method (TG/DTA). The TGA curve showed that the alloy exhibits parabolic isothermal oxidation characteristics. The characterization of the oxidized alloy was carried out using SEM, EDS, and X-ray photoelectron spectroscopy (XPS). XPS analysis identified that the main corrosion products present were MnO2, MnO/Mn2O3, and Al2O3.