Corrosion behaviour of a Ti-base nanostructure-dendrite composite

It has been reported that the presence of a ductile second phase can significantly improve the mechanical properties of nanocrystalline alloys. The usual low ductility of homogeneous nanocrystalline alloys is greatly enhanced in a promising, newly developed Ti-base bulk alloy with a dendritic phase dispersed in a nanostructured matrix. The corrosion behaviour of this alloy with outstanding mechanical properties has been studied in the present work to determine its suitability for long-term applications. Polarization tests of Ti60Cu14Ni12Sn4Nb10 copper mould cast specimens indicate good corrosion resistance in acidic media (0.5N H2SO4), medium resistance in alkaline (0.5N NaOH) but the alloy undergoes pitting in chloride media (1, 0.1 and 0.01N NaCl). Detailed examination of the surface of specimens after corrosion test by high resolution scanning electron microscopy (HRSEM) disclosed preferential dissolution of the nanocrystalline matrix is cathodic. Contrarily, observations of the uncorroded surface by scanning Kelvin probe force microscopy (SKPFM) suggest that the nature of the matrix is cathodic with respect to the dendrites. This apparent contradiction in results throws some light on the open question of whether the Volta potential difference measured in air between the phases present on the surface of a specimen can be used to determine their relative susceptibility to corrosion in solution under anodic polarization conditions. (c) 2004 Elsevier Ltd. All rights reserved.