Perforated pitting prevention of 316L stainless steel using hafnium dioxide-aluminum oxide nanolaminate films

Hafnium dioxide (HfO2)--aluminum oxide (Al2O 3) nanolaminate films are explored as a method to control the perforated pitting of 316L stainless steel (SS) in a Hanks' balanced salt solution electrolyte. Uncoated 316L SS, and 316L coated with single layer HfO2, single layer Al2O3, and HfO2-Al2O 3 nanolaminates are examined. The HfO2 layer thickness in the nanolaminates is held constant at 7.3 nm and the Al2O 3 layer thickness is varied from 2.5 nm to 10.5 nm. The HfO2/Al 2O3 volume fraction in the nanolaminates is varied in this manner. The electrochemical behavior of the film-substrate composite is studied using direct current cyclic polarization (DCP). Sample morphology is studied by scanning electron microscopy with in situ electron energy dispersive spectroscopy for chemical analysis. SEM/EDS data show that both uncoated 316L SS and 316L SS coated with single layer HfO2 develop perforated pits that become autocatalytic, consistent with the observed positive DCP hysteresis. On the other hand, 316L SS coated with single layer Al 2O3 does not develop perforated pits, consistent with an observed negative DCP hysteresis. The Al2O3 coating promotes pit cover rupture at an early stage of pit growth, so that bulk and pit electrolyte can freely mix and the pit interior can repassivate. However, the Al2O3 coating extensively cracks and loses its integrity which limits its use as a protective coating against perforated pitting.;HfO2-Al2O3 nanolaminates show diverse electrochemical behavior. Nanolaminates with 0.5 HfO2-0.5 Al2O3 volume fraction offer excellent protection against 316L SS perforated pitting through controlled pit cover rupture before autocatalysis. On the other hand, nanolaminates with greater than 0.5 Al2O 3 volume fraction tend to crack in a brittle mariner. Nanolaminates with greater than 0.5 HfO2 volume fraction preserve a pit cover so that an autocatalytic pit geometry can be established.