Microstructure and Corrosion Behavior of Ni52Ti48-xCox Shape Memory Alloys in 1.0 M HCl Solution

The effect of Co addition, as an alloying element, on the microstructure and corrosion behavior of Ni52Ti48-xCox (x = 0, 0.5, 1.5, and 4.0%) shape memory alloys (SMAs) was studied. The microstructure of Ni52Ti48-xCox SMAs consisted of B2 austenite phase as the matrix, and small percentages of B19' martensite phase. In addition, two types of NiTi intermetallic compounds were found in the microstructure. The first one was Ti2Ni and it can be seen in the all microstructures of the four tested SMAs to an extent depending on Co content in the tested SMA. The other type of NiTi precipitates was Ni2Ti, which found only in the microstructure of Ni52Ti48Co0 SMA, and completely eliminated upon introducing Co. Tafel polarization and impedance measurements were used to investigate the corrosion behavior of the tested SMAs in 1.0 M HCl solution at 25 degrees C. The variation with time (up to 24h) of the open circuit potential (OCP) of the tested SMAs was also studied in 1.0 M HCl solution at 25 degrees C. Results obtained revealed that alloying Ni52Ti48 SMA with Co, at the expense of Ti, improved its corrosion resistance. This improvement increased when the percentage of the alloyed Co was increased from 0.5% to 4.0%. The role played by Co in enhancing corrosion resistance of these advanced materials was discussed.