Laser radiation enhancement of the corrosion resistance of an amorphous ribbon alloy
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Bruce M. Clemens Physics Department, Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090 (Received 22 September 1986; accepted 1 December 1986) The effects of pulsed laser radiation on the corrosion resistance, surface morphology, and composition of liquid-quenched amorphous Fe 32 Ni 36 Cr 14 P l2 B 6 (Allied Corporation Metglas® 2826A) are reported. Scanning electron microscopy, Auger depth profiling, and x-ray diffraction were used to characterize the surface, while the corrosion resistance was determined by anodic polarization in H2SO4. The surface of the as-received melt-spun ribbons exhibited many defects, including cracks, compositional irregularities, and microcrystals of Ni 5 P 4 . These microcrystals differ from those found upon bulk crystallization. Melting and rapid solidification by radiation with a Q-switched Nd-YAG laser [30 ns full width at half maximum (FWHM) ] modified the surface morphology (leaving composition constant), removing the microcrystals and cracks and reducing the carbon and oxygen contamination. The reduction of these surface defects resulted in improved corrosion resistance of the Metglas® 2826A ribbon. For example, spontaneous passivation is observed for the laser-treated samples, as opposed to critical current densities of 10 ft A/cm 2 and 1000//A/cm 2 for the as-received amorphous and crystallized Metglas® alloy, respectively.
I. INTRODUCTION Since the discovery of the first metallic glass in 1960 by Duwez et al.,1 work on liquid-quenched amorphous alloys has concentrated on the bulk properties of these ribbons. Corrosion resistance, a surface-sensitive phenomenon, has been studied under the assumption that the surface morphology and composition of the ribbon is identical to that of the bulk.2 Only recently has the surface structure been investigated and found to be different than the bulk.3 The surface has been found to contain numerous defects that could affect surface properties. This gives us an opportunity to study the corrosion resistance of two different surface morphologies at constant surface composition because, as we will show, it is possible to remove these defects by rapid solidification. We can rapidly melt the surface with a high-power 20 ns laser pulse and achieve a higher cooling rate than was achieved in the splat-cooling process used in fabricating the ribbon. We have chosen to work with liquid-quenched amorphous Fe 32 Ni 36 Cr 14 P 12 B 6 (Allied Corporation Metglas® 2826A) because it is known for its extremely high corrosion resistance.4'5 The chemical composition of Metglas 2826A and its chemically homogeneous single phase nature (i.e., the absence of crystalline defects such as grain boundaries, dislocations, or stacking faults) are believed to be responsible for its high corrosion resistance. The absence of these crystalline defects, 46
J. Mater. Res. 2 (1), Jan/Feb 1987
http://journals.cambridge.org
which are known to act as chemically active sites for corrosion, allows the formation of a uniform defect-free pass
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