The Role of Grain Refinement in Micro-scales on Passive and Semiconducting Properties of 304 Stainless Steel in an Acidic Solution

In this research, grain refinement process of AISI 304 stainless steel (304 SS) was successfully performed by an advanced thermomechanical operation. Then, the role of grain refinement on the electrochemical response of 304 SS in an acidic solution (0.1 M HNO3 solution) was investigated. Microstructural observations depicted that thermomechanical operation resulted in reduction in the grain size from about 37 µm to about 380 nm by a factor of ~100. All the electrochemical experiments concerning the passive and semiconducting properties evaluated by potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S) measurements. PDP and EIS plots revealed that grain refinement led to superior conditions for forming the passive layers. M–S tests showed that grain-refining process cannot change the semiconductor properties (n- and p-type semiconductors) of the passive layers. Moreover, M–S tests depicted that the calculated acceptor and donor densities (NA and ND) declined with grain refinement. These responses were consistent with the observations of the PDP and EIS tests, evidencing that the passive response enhanced with decreasing the grain size. In conclusion, all the electrochemical experiments depicted that the grain refinement in microscales can significantly enhance the passive and semiconducting properties of 304 SS by creation of more stable and compact passive layer.