Passivity of a Nanostructured Directionally Solidified NiAl-Re Alloy as Substrate for Electrodeposition of Gold

Directionally solidified NiAl-Re eutectic alloys can be employed as substrates for the formation of gold nano- and microelectrodes by electrodeposition of this metal in the nanofibers present in the eutectic structure. An initial polarization of the eutectic is required in order to passivate the NiAl matrix and thus confine the electrodeposition process to the fibrous phase. The effect of the applied polarization potential as well as the duration of the treatment were investigated and related to the nucleation process during electrodeposition. It was found that polarization at low anodic potentials (0 V) resulted in a poor passivation of the NiAl matrix, thus yielding gold deposits randomly distributed over the whole surface of the sample. Polarization at high anodic potentials (0.7 V) is essential to ensure that the matrix is fully passive, with the fibrous phase remaining as the only electrochemically active area. The impedance measurements carried out on the samples after polarization at 0.7 V for longer time intervals showed a decrease in resistance after a sufficiently long passivation, attributed to the formation of a porous oxide film and partial corrosion of the sample through the pores created during the process. The polarization time must therefore be confined to 15-20 min to avoid undesirable corrosion. Under these conditions, electrodeposition of gold takes place along the pores left by the dissolution of the fibrous phase and originates arrays of aligned gold microspheres.