the phenomenon that in an electrochemical process, aluminum and such metals as tantalum, niobium, manganese, titanium, zinc, cadmium, etc., can form an oxide layer which blocks an electric current from flowing in one direction but which allows current to flow in the opposite direction, was first observed in 1857 by the german physicist and chemist (1805–1878). it was first put to use in 1875 by the french researcher and founder eugène ducretet, who coined the term "valve metal" for such metals.