The Passive State of Metals

Abstract

The phenomenon of passivity was probably first noted with iron,l when treated with concentrated nitric acid. The general property has later been found marked in the cases of aluminum, chromium, nickel, tin, and cobalt. Passivity* is also found in the cases of tungsten, molybdenum, silver, bismuth, platinum, zinc, magnesium, copper, lead, columbium, vanadium, niobium, ruthenium, gold, tantalum, uranium, manganese, and to some extent probably with most of the metals under extreme conditions. Attempted explanations of the facts of passivity have dealt largely with iron, consequently in this discussion iron will be studied more in detail than other cases of passivity. The other metals will be considered however. The passive state of iron is a condition, which it acquires when it is exposed to the action of oxidizing agents, whereby its surface becomes more noble, in other words its solubility is decreased. The single potential is a noble one, rather than the less noble one associated with iron itself. The measurement of this potential has been used to test the presence of the passive state.3 It bas led to many contradictions, however, on account of the fact that different solutions are used and different values obtained. Furthermore, too much depends on the method of measurement. The non-replace