The Effect of Phosphorous on the Corrosion Resistance of Glassy Copper-Zirconium Alloys
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THE EFFECT OF PHOSPHOROUS ON THE CORROSION RESISTANCE OF GLASSY COPPER-
ZIRCONIUM ALLOYS T.D. BURLEIGH AND R.M. LATANISION The H.H. Uhlig Corrosion Laboratory, Department of Materials Science and Engineering, Room 8-202, Massachusetts Institute of Technology, Cambridge, [A 02139 ABSTRACT Phosphorous has been shown to have a beneficial effect on reducing the corrosion rate of various metal alloys. This research was undertaken to determine by what nechanism the nhosrhorous is able to improve the corrosion resistance of metals. The amorphous cooper-zirconium system was chosen for exoerimentation because the amorphous structure eliminates the complications due to crystalline defects. The zirconium forms zirconia (ZrO2), which is normally a protective surface oxide. Copper-zirconium alloys with and without phosphorous were melt-spun, and were tested in various electrolytes. In comparing Cu-42Zr-2P with Cu-4OZr, the results show that the corrosion rate of Cu-42Zr-2P is lower in IN sulfuric acid, and significantly lower in 3.5 ./o NaCl, and in lINHCI. ESCA analysis shows that phosphorous is present in the zirconia film, and has a valence of +5. Photocurrent experiments suggest that phosphorous alters the semiconductor properties of the zirconia film. INTRODUCTION Phosphorous has a beneficial effect in reducing the corrosion rate of numerous metallic alloys. It is used in corrosion inhibitors, surface treatments, and also is an alloying element in weathering steels. In glassy alloys, the metalloids P, B, C, and Si are used because of their glass forming properties. However, phosphorous has in addition the major effect of improving corrosion resistance [1]. Phosphorous is most effective in alloys which contain a passive film forming alloy addition (e.g., Cr or Ti). The passive film is metal-oxide or oxyhydroxide film which forms on the surface and protects the alloy from further corrosion. The purpose of this research is to determine the mechanism by which phosphorous alters the passive film to make it more protective. EXPERIMENTAL The glassy copper-zirconium alloy system was chosen in order to eliminate the complications of inhomogeneities due to grain boundaries, precipitates, and other crystal defects. Cu-Zr binary is a relatively simple system electrochemically [2]. The zirconium is important because it forms a passive film, zirconia (ZrO2 ) (Figure 1). The alloys were melt-spun at MIT, starting with Cu (99.999%), Zr (99.9%), and Cu3 P (99.5%). The original composition of the phosphorous containing alloy was Cu-37Zr-5P, but due to formation of zirconium phosphide dendrites and to phase separation in the melt, the final composition of the melt-spun ribbon was Cu-42Zr-2P. The final ribbon compositions were determined by electron microprobe X-ray analysis. The ribbons were Mat.
Res. Soc.
Symp. Proc. Vol.
28 (1984)
Published by Elsevier Science Publishing Co.,
Inc.
228
Aqueous Solution -30
C Z-()Al loy Figure 1. A passive film of zirconia forms on the alloy surface. determined to be glassy by X-ray diffraction and
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