j Pergnmon Ekcwochimtca km. Vol. 40. No. I I, 1723 1734. 1995 pp. Copyright @; 1995 Elrvier Scienoc Ltd. Printed in Circa Britain. All rights reserved 0013 4686/95 59.50 + 0.00 zyxwvutsr AN X-RAY PHOTOELECTRON SPECTROSCOPY STUDY OF CHROMIUM-METALLOID A LLOYS-III T. P. MoFFAT,*t R. M. LATANISION and R. R. RUF~ tThe H. H. Uhlig Corrosion Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 flBM Thomas J. Watson Research Center, Yorktown Heights, N.Y. (Received 19 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE September 1994; in revisedform 29 November 1994) Abstract-X-ray photoelectron spectroscopy has been used to examine the surface of a series of Cr ,OO_xP, alloys (x = 0, 7, 13, 28) following a variety of electrochemical treatments in 1 mol/l H,SO,. At oxidizing potentials the passivating effect of the surface film formed on Cr,,,_,P, is similar to that found on chromium. Semi-quantitative analysis indicates a minor amount of oxidized phosphorus in the passive film formed on Cr,,P,,, while a much larger concentration of oxidized phosphorus is found in the passive film on Cr,,P,, and Cr,,P, Phosphorus is also slightly enriched in the metallic state beneath the oxide-based film. The thickness of the passivating overlayer is a monotonic function of the growth potential. At reducing potentials the surface of Cr,,P,, is enriched in both oxidized and metallic phos- phorus via selective dissolution of chromium. This layer inhibits further chromium dissolution and simul- taneously accelerates proton reduction. The coupling of these reactions is responsible for spontaneous passivation of Cr ,,,0 _ ,P, in reducing acids. Key words: metallic glass, Cr P 100-x x1 XPS, phosphorus enrichment, spontaneous passivation. INTRODUCTION The remarkable corrosion resistance of transition metal-metalloid metallic glasses such as (Fe, Ni, Co),,.. x - (P, Si, C, B),, is well established[l]. The unique chemical properties have been associated with alloys containing chromium and phosphorus. However, an understanding of the synergistic inter- action that occurs between these elements remains to be resolved. Consequently, the electrochemical behavior of a series of binary chromium-phosphorus (Cr P 100-x XI x = I, 13, 28) alloys has been investigatedC2, 31. For oxidizing conditions these materials are passive and exhibit film formation and dissolution kinetics quite similar to chromium. In reducing environments, chromium actively dissolves while Cr 1o0_ ,P, remains passive. Active dissolution of chromium generates Cr’+ and hydrogen, while passivation at higher potentials is associated with the formation of a CrOOH. H,O blocking layer[4, 5-J. In contrast, alloying with phosphorus inhibits Cr’+ production and simultaneously catalyzes proton reduction. The combination of these partial processes results in the spontaneous passivation of Cr,,,_,P, alloys[2, 31. This report will describe the results of an X-ray photoelectron spectroscopic examination of these surfaces following a variety of electrochemical treatments in 1 mol/I H,SO,. In addition, some preliminary results on the electro- chemistry of orthorhombic phosphorus and the *Present address: NIST, Bldg 224, 8166, Gaithersburg, Md 20899 and author to whom correspondence should be addressed. kinetics of redox reactions occurring on passivated Cr,,,-,P, will be presented. EXPERIMENTAL PROCEDURES M aterials The surface chemistry of glassy Cr,,P,, , Cr,,B,, , Cr,,P,, and dual-phase, glassy-nanocrystalline A15, Cr,,P,, were examined and compared to chromium. The synthesis and structural characterization of these alloys have been described previously[2]. In addition, the electrochemistry of orthorhombic black phosphorus was examined. Needles of orthorhombic phosphorus were produced from a melt of bismuth containing 2 wt% red phosphorus. The needles grew by solute rejection during solidification and were subsequently isolated by dissolving the bismuth matrix in nitric acid[6]. For the electrochemical experiments the needles were encapsulated in a con- centric series of epoxy coatings and then polished to nominally expose the (100) surface[7]. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemistry of chromium and chromium-phosphorus thin films after various treatments in 1 mol/l H,SO, Arc-melted chromium specimens were polished to a 0.03 pm y-Al,O, finish, while Cr ,OO_xPr thin-films were used in the as- deposited state. During the electrochemical pretreat- ment, the electrical contact to the electrode was masked with MiccroSuper XP 2000 Stop-Off Lacquer (Tobler Div., Pyramid Plastics, Inc., Michigan I723