The passive state and adhesion

In the course of an investigation into the tensile strength of joints formed by thin films of pure chemical substances between surfaces of steel or copper (‘Roy. Soc. Proc.,’ A, vol. 118, p. 209 (1928)), a curious effect due to the production of passivity at a copper surface was noticed. In making such measurements it is, of course, essential that the metallic surfaces should be absolutely clean and dry. Owing to the ease with which copper surfaces acquire a tarnish film, their cleaning presented some difficulty. On one occasion the copper plate was boiled in absolute alcohol and plunged, whilst still hot, into water containing a few drops of concentrated nitric acid, in order that it might cool in a non-tarnishing medium. When taken out, the surface was found to have assumed a uniform dull reddish tint quite unlike the patchy appearance produced by the usual visible tarnish film. This appearance persisted when the plate was boiled in alcohol or water and cooled in air and also when it was left exposed to impure damp air for several hours,i. e. , it survived conditions under which a normal copper surface would have tarnished rapidly. Gentle polishing with very finely divided magnesia powder on silk did not remove the reddish-tint, although abrasion with fine emerypaper, by removing the upper surface layers did. Normality was most conveniently restored by boiling the plate in a 2-5 per cent. aqueous solution of ammonia when the surface tarnished and blackened in patches; this tarnish was removed by polishing with magnesia powder on a wet leather and the process repeated until the surface tarnished uniformly. The surface of the plate, after removal of this tarnish, was found to be normal.