Investigation of the electrochemical dissolution behavior of Inconel 718 and 304 stainless steel at low current density in NaNO3 solution

In electrochemical machining (ECM), current densities are typically 20–200 A cm−2. However, sometimes the workpiece is exposed to a low-current-density electric field (<5A cm−2), resulting in undesirable dissolution. This is known as stray corrosion. To investigate stray corrosion in ECM, this paper focuses on electrochemical dissolution at low current density in NaNO3 solution. The anodic polarization curves and current efficiencies of 304 stainless steel (SS) and Inconel 718 are obtained. Comparison of the ce–j curves reveals the unique dissolution behavior of Inconel 718 at low current density. Dissolution experiments are conducted at different corrosion times. The corroded specimens are examined and analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. The results indicate that unlike 304 SS, whose surface is well protected by a compact oxide film formed in NaNO3 solution at low current density, Inconel 718 suffers serious selective corrosion due to the formation of a porous film in NaNO3. The formation of solid black products is investigated and discussed. It is found that niobium plays an important role in the dissolution of Inconel 718 at low current density. Qualitative models have been assumed to illustrate the electrochemical dissolution behavior of 304 SS and Inconel 718 at low current density in NaNO3 solution.