Modification of 27Cr cast iron with alloying yttrium for enhanced resistance to sliding wear in corrosive media
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I. INTRODUCTION
HIGH Cr cast iron is widely used in many fields, such as mining, oilsand, mineral processing, and construction,[1,2] e.g., for slurry pumps, brick molds, coal grinding mills, shot blasting equipment, and components for quarrying, hardrock mining, and milling.[3] The high Cr iron usually consists of hard M7C3 carbides and martensitic matrix (after heat treatment) or austenitic matrix (as cast).[4] Excellent wear resistance of the material results from its hard M7C3 carbides and tougher austenitic matrix or martensitic matrix with residual austenitic phase,[5] which is resistant to propagation of microcracks. In addition, when austenite transforms to martensite under external force, the phase transformation consumes additional deformation energy or impact energy, thus further retarding crack propagation.[6] Another merit of this material is its good corrosion resistance due to the existence of a protective passive film.[7] The high content of Cr increases the stability of the passive film, thus leading to enhanced corrosion resistance. The Cr has high passivation capability, and its anodic passivation can be established even when the surface of Cr is scratched at very high-speed.[8] Other elements have also been alloyed to high Cr cast iron to reduce the cost or to meet specific requirements.[9,10] It has been observed that yttrium addition can further improve the strength of cast iron[11] and its corrosion resistance.[12] Recent studies have shown that yttrium addition can also markedly diminish corrosive wear. Liu and Li[13] demonstrated that the corrosive wear of stainless steel and aluminum alloys in a mixture of lubricant and sulfuric acid can be decreased by adding a small amount of yttrium powder to the lubricant containing the acid. The present authors have also observed that yttrium is very beneficial to the resistance of stainless steel and aluminide coatings to wear in different corrosive media.[14,15] The improved TIANCHENG ZHANG, Postdoctoral Fellow, and D.Y. LI, Associate Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Contact e-mail: [email protected] Manuscript submitted May 14, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
performance of the passive alloys is largely attributed to the enhancement of their passive films when yttrium is added.[16] It is expected that yttrium may also play the same role when alloyed into high-Cr cast white iron, a widely used industrial wear-resistant material. In this work, the performance of 27Cr-cast white iron alloyed with yttrium during sliding wear in various corrosive solutions was evaluated, in comparison with Y-free cast 27Cr iron. Mechanical, physical, and electrochemical properties of passive films on the Y-containing and Y-free cast irons were investigated using a nanomechanical probe, a scanning Kelvin probe, and an electrochemical system. II. EXPERIMENTAL PROCEDURE The chill-cast white iron used for the present study contained the following elements (w
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