Pitting corrosion protection of stainless steel by sputter deposited hafnia, alumina, and hafnia-alumina nanolaminate films

316L stainless steel coated with sputter deposited HfO2, Al2O3, and HfO2-Al2O3 nanolaminate films were subjected to direct current cyclic potentiodynamic polarization (DCP) in Hanks' balanced salt solution electrolyte. Postexposure morphology was characterized by scanning electron microscopy (SEM) with in situ energy dispersive spectroscopy (EDS). SEM/EDS data show that bare steel and steel coated with single-layer HfO2 develop pits with perforated covers. These pits become autocatalytic, consistent with an observed positive DCP hysteresis. On the other hand, SEM/EDS data show that steel coated with Al2O3 and HfO2-Al2O3 nanolaminate films does not develop autocatalytic pits, consistent with an observed negative DCP hysteresis. However, Al2O3 splinters upon polarization whereas the HfO2-Al2O3 nanolaminate remains intact. The areas of worst damage in the nanolaminate correspond to pit cover rupture before autocatalysis, allowing pit and bulk electrolyte to mix and the newly exposed steel surface to repassivate. The films' diverse behavior is discussed in terms of a model for perforated pit growth that requires occlusion until an autocatalytic geometry is established. The authors conclude that the key property a film must have to arrest autocatalytic geometry development is the ability to rupture locally at an early stage of pit growth.