Complexity of Products of Tungsten Corrosion: Comparison of the 3D Pourbaix Diagrams with the Experimental Data
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RODUCTION
TUNGSTEN is a transition metal and it exhibits various oxidation states from 2 to +8.[1] Interaction of tungsten with water is therefore quite complex; it leads to a cascade of chemical reactions which are difficult to control and identify.[1–3] However, one can take advantage of this weak corrosion resistance of tungsten to use it for shaping tungsten specimens using anodic dissolution, also called electropolishing. In this case, tungsten specimen is set as an anode. Upon the voltage application, tungsten is removed to form a complex-shaped article.[4–8] To suppress the evolution of hydrogen in the electrochemical cell, anodic dissolution of tungsten is normally carried out at a high anodic potential.[9–11] However, understanding of interaction of tungsten with aqueous solutions at low voltages opens up new doors in technological developments and applications requiring synthesis of complex tungsten oxides.[12] In this paper, the thermodynamic limitations of tungsten stability in different aqueous solutions are discussed. This problem is of general importance not only for materials science, but also for the environmental and life sciences as well. Tungsten is used in living organisms, and recent alarming reports on the potential harmful effects of tungsten compounds in the development of cancers[13,14] require understanding of the conditions of thermodynamic stability of different tungsten compounds. In general, it is believed that low- and high-voltage anodizations of metals follow different thermodynamic
MARYANA I. NAVE and KONSTANTIN G. KORNEV are with the Department of Materials Science and Engineering, Clemson University, Clemson, SC, 29634. Contact e-mail: [email protected]. Manuscript submitted April 4, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
pathways resulting in different products.[15,16] In particular, two distinct regimes of tungsten anodic dissolution in aqueous solutions of potassium hydroxide were discovered.[11,17–19] In the regime of a high-voltage anodic dissolution, when the applied potential is greater than 2 V, one observes necking of the tungsten wire. The neck radius decreases with time and finally the wire breaks onto two pieces each having sharp tips. In the regime of a low-voltage anodic dissolution, when the potential is less than 2 V, anodic dissolution proceeds through the formation of a solid porous shell wrapping up the tungsten wire,[17,19] Figures 1(a through c and e). This shell damps the flow of the reaction products near the wire surface. It also completely suppresses the development of morphological instabilities on the wire surface. As a result, electrochemical removal of tungsten occurs along the wire from the top to the bottom.[17,19] Practical importance of this low-voltage regime of tungsten anodic dissolution is that it is controllable, provides the nanometer sharp tips, and small taper (Figure 1(d)). In our earlier publications,[17–19] it was shown that the shell consists of the complex tungsten-based compounds. Therefore, it confirmed that the electrochemical
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