Corrosion Behavior of Rapidly Solidified Fe-Ti-P Alloys with Cu, Cr, and B Additions
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CORROSION BEHAVIOR OF RAPIDLY SOLIDIFIED Fe-Ti-P ALLOYS WITH Cu, AND B ADDITIONS
Cr,
C. R. SHASTRY,* R. M. LATANISION** and H. E. TOWNSEND* * Sheet Steels and Coated Products Division, Research Department, Bethlehem Steel Corporation,Bethlehem, PA 18016; ** H. H. Uhlig Corrosion Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139.
ABSTRACT Rapidly solidified ribbons of Fe67Ti13P(20-x)Bx (x= 0,7, 20 a/o) and Fe(85-y)Til5Py (y=5,10 a/o) alloys were produced by melt spinning, and their structure and anodic polarization behavior were examined by x-ray diffraction and potentiodynamic polarization measurements in deaerated I N H SO4, respectively. The only alloy that developed an amorphous structure on rapid solidification, Fe67Ti13B20, exhibited poor corrosion resistance, whereas crystalline alloys with 10 a/o or more P exhibited a tendency to passivate during anodic polarization. Substituting Cr for Ti led to significant improvement in corrosion resistance and promoted development of glassy structures in Fe67(TiCr)13P13B7 compositions. No such improvements were observed when Cu was substituted for Ti. It is concluded that alloy composition is more important than degree of crystallinity in determining corrosion resistance.
INTRODUCTION Extensive research in the last 20 years has shown that amorphous (that is, glassy or non-crystalline) structures can be produced in many alloy systems by rapid solidification. In others, metastable crystalline phases can be produced, solid solubility limits extended, and several other metallurgical modifications can be generally brought about by this technique. The absence of structural inhomogeneities such as grain boundaries and dislocations in the amorphous state and of chemical segregation in microcrystalline alloys is believed in some instances to lead to improved corrosion resistance. Detailed studies by Hashimoto and co-workersIl-l1] have shown that enhanced corrosion resistance of a number of rapidly solidified ironbase compositions can be attributed primarily to the presence of (a) a film forming element such as Cr, (b) a reactive base alloy which promotes rapid formation of Cr-rich passive film after an initial period of active dissolution, and (c) a highly uniformand adherent passive film formed on an amorphous substrate. Based on these observations it is expected that several corrosion-resistant compositions could be identified in alloy systems fulfilling the above requirements. The present work was undertaken to examine this possibility with a suitably selected alloy system. The FeTi-P system was chosen for investigation as it satisfies the first two requirements mentioned above, viz, (a) the presence of a passivating component (Ti) and (b) a reactive base due to the presence of P. Also, since the system combines transition metals with metalloids, amorphous structures are expected in alloys near the eutectic composition. EXPERIMENTAL WORK The nominal compositions of alloys investigated are listed in Table 1. The first three compositions combine the meta
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