In electrochemistry, a valve metal is a metal which passes current in only one direction. Usually, in an electrolytic cell, it can function generally as a cathode, but not generally as an anode because a (highly resistive) oxide of the metal forms under anodic conditions.[1] Valve metals include commonly aluminium, titanium, tantalum, and niobium. Other metals may also be considered as valve metals, such as tungsten, chromium, zirconium, hafnium, zinc, vanadium, bismuth or antimony.[2]
Original source: https://en.wikipedia.org/wiki/Valve metals.
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