Extremely high corrosion resistance in amorphous Cr–B alloys

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Abstract
Amorphous Cr–B alloy films containing 40 to 80 at. % Cr are prepared by rf and dc sputtering onto liquid nitrogen and water cooled substrates. Amorphous Cr–B alloys with Cr ≥60 at. % do not corrode in concentrated solutions of HCl (12N), 3/1 volume ratio HC1/H2O2, 1/1 HNO3/HCl, 1/1 H2O/H2SO4, and NH4 OH. For example, amorphous Cr80B20 and Cr60B40 films (∼4000 Å) can be immersed in concentrated (12N) HCL for over 3 months with no detectable change in thickness. By comparison, pure polycrystalline Cr dissolves at ∼700 μm/day. Corrosion properties of these new materials are investigated by electrochemical and immersion techniques as a function of alloy composition and thermal annealing. The corrosion tests are supplemented by measurements of electrical resistivity, and hardness and scanning electron microscopy observations. The main results of these studies are (1) high corrosion resistance is observed in 12N HCl only when Cr content is ≥60 at. %, (2) the corrosion rate is strongly dependent on surface smoothness and/or microstructural size only when Cr concentration is ≥60 at. %, and (3) at potentials above 1 V (SCE) the crystallized Cr–B alloy films corrode by pitting, while their amorphous counterparts exhibit slow and uniform dissolution. These results indicate that it takes both high Cr content and a smooth surface, as provided by the amorphous Cr–B films, to form a continuous, uniform passive oxide which is essential to the extremely high corrosion resistance in concentrated acids.