Analysis of the Dissolved Surface of NiTi Shape Memory Alloy in Electrochemical Micromachining Process Using Potentiodynamic Polarization and EIS Techniques

Electrochemical micromachining, an unconventional machining method, has emerged as a promising approach for micro-feature fabrication on temperature-sensitive NiTi shape memory alloys. This study investigates NiTi dissolution in neutral aqueous electrolytes (NaCl, NaNO 3 , NaBr) using potentiodynamic polarization (PDP) techniques, linear sweep voltammetry, and cyclic voltammetry (CV). CV analysis reveals that NiTi dissolution in NaCl and NaNO 3 electrolytes is predominantly passive, whereas, in NaBr, it is active, as confirmed by the peak current (I pk ) order: NaCl &lt; NaNO 3 &lt; NaBr. Post-CV analysis shows NaNO 3 yielding the lowest average surface roughness ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>S</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 0.072 μm), reflecting uniform dissolution. Surface passivation proves instrumental in enhancing surface roughness, further emphasized by electrochemical impedance spectroscopy techniques like Nyquist and Bode plots, reflecting the dual nature of the oxide layer. X-ray photoelectron spectroscopy reveals the composition of the surface oxide layer, elucidating the role of stable metal complex sodium titanate (Na 2 TiO 3 ) in facilitating NiTi dissolution in NaNO 3 . Micro-grooves were attempted using all three electrolytes, but only NaNO 3 successfully fabricated them with the lowest surface roughness ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 0.86 μm), ensuring a uniform dissolution process.