Effect of Carbon Nanotube Concentration on the Corrosion Behavior of Electroless Ni-B-CNT Coating

In this study, Ni-B-CNT composite was deposited on AISI 4140 steel using different concentrations of CNTs ranging from 0.2 to 1 gr/lit in an electroless bath. The phase analysis and surface morphology of the samples were characterized using x-ray diffraction and FESEM. The corrosion behavior of the coated samples was investigated in a 3.5 wt.% NaCl solution through potentiodynamic polarization, EIS, and electrochemical noise (EN) analyses. The corroded surface of the samples was further analyzed via FESEM observation after 10 days of immersion in corrosive medium. It was found that although adding 0.2 gr/lit CNT in electroless solution improves corrosion resistance, increasing CNT concentration from 0.2 up to 1 gr/lit increases the corrosion rate. The FESEM observations and EN results imply that the formation of micro-galvanic cells in samples with high concentration of CNTs leads to nucleation and growth of the pits. The results revealed that no passive layer was formed on the surface of the Ni-B sample, and the corrosion occurred through ionic diffusion into the cracks and crevices. The results of potentiodynamic test show that adding CNTs into electroless bath facilitates the formation of passive layer. However, results of EN test imply that increasing CNT concentration causes repetitive breakdown and recovery of passive film.