Circuit Transmission and Interference of Activation Waves in Passive Iron Wire.

Abstract

The pure commercial soft iron wire known as Armco,∗ when rendered passive in nitric acid of 70-80 v. %† transmits activity in the usual manner whenever the wire is activated locally by any means, such as scraping with glass or touching with zinc. The resulting activity in such strong acid is temporary, each area of the wire becoming active in its turn as the state of activity spreads, and then reverting to the passive state. Both the transmission and the repassivation are electrochemical processes; any area on becoming active forms the anode of a local electric couple, the adjoining passive area forming the cathode; the latter is then automatically activated through the destruction of the passivating surface film by electrochemical reduction; and by repetition of this process the state of activity spreads rapidly over the whole wire. The reversal or repassivation is a result of the reformation of the surface film by oxidation of the exposed metal; in strong acid (above 60 v.%) the reaction of the metal with the acid is self-limiting, by reason of the electrochemical oxidation which follows automatically in any area of metal as it becomes active and therefore anodal. This oxidation reforms the film and passivates the metal. Accordingly, in strong acid the reaction at any point of the wire lasts for only a brief time, which is the briefer the stronger the acid, e. g., a small fraction of a second in fully recovered Armco wires in 80 v.% HNO₃ at 20°.

When a passive Armco wire in strong acid is touched with zinc at any point a restricted area or “wave” of activity, indicated by a darkening of the metallic surface and a slight effervescence, is seen to pass along the wire in both directions from the contact. The velocity and length of this wave vary with the concentration of acid, the temperature and the interval since the previous activation. In HNO₃ of 78 to 80 v.% at 15°, under the conditions described below, the wave retains an approximately constant length of 12 to 15 cm. and travels at ca. 15 cm. per second. During the time (ca. 1 second) occupied by the reaction at each point the surface film is successively broken down by cathodic reduction and reformed by anodic oxidation. Alternate reduction and oxidation thus constitute the chemical cycle at each region of the surface as the activation wave passes.