Investigation of Corrosion Behaviour of FeCoNiAlTa Shape Memory Alloy

Abstract

FeNiCoAlTa shape memory alloy (SMA) possesses both large superelastic strain and high yield strength. This makes it a potential candidate for industrial applications, such as actuators and pipe couplings. The corrosion behaviour of this alloy since is mostly unknown. The SMA was tested by means of electrochemical methods in 0.6 molar (M) NaCl solution at four different temperatures (25 oC, 40 oC, 60 oC, and 80 oC) and three levels of solution pH (3, 6, and 10). At 80 oC, the corrosion resistance of the alloy was lower than that at the lower temperatures. This was indicated by higher corrosion rates and lower open circuit potentials. Low corrosion resistance in acidic solutions, good corrosion resistance in alkaline solutions in comparison to near neutral 0.6 M NaCl solution were observed. The SMA did not passivate in the 0.6 M NaCl solution, but suffered localized corrosion in the form of corrosion pits. The effect of heat treatment on corrosion properties of the alloy and its microstructure was also investigated. It was found that aging heat treatment caused β - Ni3Al phase to precipitate at the grain boundaries, resulting in chemical segregation between the grains and grain boundaries. The segregation degraded the corrosion resistance of the alloy, and caused intergranular corrosion in 0.6 M NaCl solution at 25 oC. Furthermore, the corrosion resistance of the alloy was determined in 0.5 M NaOH and 0.5 M H2SO4 solutions at 25 oC. It showed good corrosion resistance to the NaOH solution, but suffered severe corrosion in the H2SO4 solution. The alloy was coupled to UNS G10180 (AISI 1018), UNS S30400 (AISI 304), UNS S31603 (AISI 316L), and UNS S32750 (AISI 2507), and its galvanic corrosion behaviour was investigated electrochemically. The test was conducted in 0.6 M NaCl solution at 25 oC, 40 oC, and 60 oC. For each couple, the specimen tested at 60 oC showed increased localized and pitting corrosion, an increase in galvanic current density, and a lower galvanic potential compared to couples tested at the other temperatures. The results show that when the alloy was coupled to UNS G10180, it acted as the cathode, whereas when coupled to stainless steels, it acted as the anode, and suffered localized corrosion attack with the formation of large pits and its overall corrosion increased.