Characteristics of the Oxides Films formed on Alloy C-22
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Characteristics of the Oxides Films formed on Alloy C-22 Alan W. Szmodis, Kelly L. Anderson, Joseph C. Farmer, Tiangan Lian, and Christine A. Orme Lawrence Livermore National Laboratory Chemistry and Material Science Division Livermore, CA 94550
ABSTRACT The passive corrosion rate of Alloy C-22 is exceptionally low in a wide range of aqueous solutions, temperatures and electrochemical potentials. Alloy C-22 contains approximately 22% chromium (Cr) by weight; thus, it forms a Cr-rich passive film in most environments. A study of the composition, thickness and other properties of the passive film was undertaken to better understand the role of the protective oxide in preventing corrosion. In general the oxide film is expected to be a function of solution pH, temperature and applied electrochemical potential. In this work we focus on the oxide films that form at pH=8.
INTRODUCTION Alloy C-22 (UNS# N06022) is a nickel (Ni) based alloy with nominally 22% chromium (Cr), 13% molybdenum (Mo), 3% tungsten (W) and 3% iron (Fe) as shown in Table 1. It is amongst the most corrosion resistant engineering materials available today and is thus being considered as a barrier material for nuclear waste containment by the Yucca Mountain Program. The passive corrosion rate of Alloy C-22 in a wide range of aqueous solutions, temperatures and electrochemical potentials is exceptionally low. The high chromium concentration present in Alloy C-22 allows a Cr-rich passive film to form in most environments. To gain a better understanding of the oxides that form we are studying its composition, thickness and morphology. The corrosion characteristics are tested in a series of multi-component waters derived from the geochemistry of the Yucca Mountain Site. This work focuses on the characteristics of oxide films formed in a saline solution with pH 8. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS) were used to characterize the transformation of the surface oxide as Alloy C-22 was potentiodynamically driven from the passive region to transpassive dissolution. Atomic force microscope images show that as the voltage is increased the surface is changed from a thin passive oxide to a bilayer structure that includes a thick (~300nm) porous overlayer. From x-ray photo-emission spectroscopy we show that the inner oxide layer is composed of chromium, nickel, iron, and molybdenum oxides whereas the outer layer is predominately iron and nickel oxide. We show that the pore structures and film thickness can be tuned using electrochemical parameters such as temperature and voltage. Table 1. The major constituents of nickel-based Alloy C-22 (UNS N06022) according to ASTM B5751.
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Element Nickel Chromium Molybdenum Iron Tungsten Cobalt
Min weight % (Except Ni balance) 63 20 12.5 2 2.5 0
Max weight % Measured weight % (Except Ni balance) 50.025 Remainder 22.5 22.0 14.5 13.6 6 4.4 3.5 3.0 2.5 2.3
EXPERIMENTAL A saturated Ag/AgCl reference electrode, platinum counter electrode and
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