Corrosion behaviour of high chromium white iron hardfacing alloys based on chromium carbides
Creators: name: Marimuthu, Varmaa
Publisher: name: Charles Darwin University
Year: 2016

description: High Chromium White Irons (HCWI) hardfacing alloys are used extensively in the mining and other processing industries. Hardfacing alloys are mainly meant for combating the wear of components and structures. However, these alloys are also used under conditions that involve corrosive fluids and only limited studies are available on the corrosion behaviour of these alloys. The hardfacing alloys are mainly based on carbides. The most popular abrasion resistance alloy is based on chromium carbides. In the first stage of the thesis, Pourbaix diagrams developed for chromium carbides and other strong carbide forming elements such as titanium, niobium and tungsten at 298K using thermodynamic data of the stable species, equilibrium reduction reactions and thermodynamic calculations. The Pourbaix diagrams were useful in predicting and explaining the corrosion behavior of the hardfacing alloys. In addition, experiments were designed in the regions of interest such as a combination of pH and potential. In the second stage of this research, experimental work was carried out to investigate the corrosion behaviour of welded HCWI hardfacing alloys with strong carbide forming elements in acidic, alkaline and neutral environments. The HCWI selected are as described in the Australian Standards AS2576, which is probably the most advanced standard in the world as it defines not only the hardness but also its microstructures. The HCWI microstructure consists of primary and eutectic carbides and eutectic austenitic/martensitic matrix. The potentiodynamic and potentiostatic tests were used to study the corrosion behaviour of HCWI in all the three environments. In alkaline pH 14 and neutral pH 7 buffer solutions, corrosion of HCWI occurs at carbides in preference to eutectic austenite matrix. In acidic pH 1.5 and neutral pH 7 NaCl solutions, corrosion of HCWI occurs at eutectic martensitic matrix in preference to carbides. Whereas, in acidic solution of pH 2, the corrosion occurs at eutectic carbides in preference to eutectic austenite matrix. From these experimental results, the corrosion trend of HCWI in acidic, alkaline and neutral environments was studied using the superimposed Pourbaix diagram of carbides. From the study of superimposed Pourbaix diagrams of carbides found that in all the three environments, corrosion of eutectic austenite/martensitic matrix or carbides depended upon the formation of passive film on the surface of eutectic austenite/martensite. The corrosion of eutectic martensitic matrix in neutral pH 7 NaCl solution is mainly due to removal of passive layer by the aggressive chloride ions in the solution. The Pourbaix diagram of chromium carbides revealed that stability of Cr23C6 eutectic carbides were found to be less stable than the Cr7C3 primary carbides. The superimposed Pourbaix diagrams of carbides and hardness tests also showed that dissociation of carbides reduces the wear resistance of HCWI hardfacing alloys significantly. The results of this work provide significant contribution in designing better corrosion resistant HCWI hardfacing alloys for wear resistance applications.