Passivation properties of ultrafine grain 316L stainless steels manufactured by ball milling and spark plasma sintering

Abstract

If the grain size reduction is known to enhance the yield stress of alloys, its impact on the corrosion resistance is still misunderstood. In this work, corrosion and passivation mechanisms of stainless steel 316L manufactured by ball milling and spark plasma sintering were studied in Na2SO4. The elaboration by powder metallurgy and fast sintering route produces dense samples with grain sizes in the ultrafine domain without crystallographic texture. Electrochemical properties were analysed by potentiodynamic polarization experiments. The samples display weak passivation current density, typically around a few μA/cm2, with a grain size reduction. This was related to an increase in polarization resistance of the free surfaces for smallest grain sizes. Moreover, a passivation of the milled powder in HNO3 before sintering has a beneficial impact on the stability of the passive layer, especially for the smallest grain sizes. The physical properties of the oxide layer were analysed by impedance spectroscopy and Mott-Schottky formalism. Capacitive properties and thickness of the oxide do not strongly depend of the grain size. Smallest grain size specimens manufactured with passivated powder exhibit the higher electrical resistance of the interface. Donor density also decreases with grain size, leading to most compact and less defective oxide layer.