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In this study the microstructure evolution and corrosion resistance of NiTiCo and Co-free NiTi shape memory alloys (smart alloys) were investigated under various aging conditions. Cobalt NiTi alloy contains 4 at% in addition to 52 at% Nickel and 44 at% Titanium, while in Co- free alloy Titanium content increased to 48 at%. Optical and scanning electron microscopes were used to investigate the microstructure evolution of NiTi alloys. Potentiodynamic polarization measurements were used to study the corrosion behavior of the tested alloys in 1.0 M HCl solution at 25 °C. The results show that martensite phase, B19’, is predominant phase in the aged microstructures of the investigated alloys. Moreover, there is an intermetallic compound phase, Ti₂Ni, that precipitated in various densities and sizes in microstructures depending on the aging heat treatment conditions. Polarization measurements show that the corrosion rate of the NiTi alloys increases as the aging temperature increase. Also, potentiodynamic polarization shows that the addition of 4% of cobalt as alloying element decreases the corrosion rate.