Film-rupture mechanism of stress corrosion

The at mospherically formed protective fi lms were r emoved by abrasion in an argon atmosphere from surfaces of an aluminum alloy, t wo braRses, a magnesium a lloy, fi nd 10wcarbon and stai nless steels.The resulting s urfa ces were 0 .12 to 0 .76volt more negative with respect to It calomel electrode t han surfaces prep ared and mea sured under norma l atmospheric conditions.Appreciable changes in electro chemical sol ution p ~ten tials of notched specimen.s,st ressed in tension occurred at or just above stresses at wInch the true stJ'es~-tl' u e stram curves devia ted fro;n the modu lus lines.These changes in p otential were caused by ruptur ing of t he protective fi l ms at the roots of the notches anc~ we;'e of t h e ord er of 0.1 6~0 0.70 v.olt at failure dependinO' on the material.Stress oOrroslOn 18 postulated to occ ur III corrOSlVe media, at stresses s~fficient to rupt1ll'e th e protective film, by electrolytic actio n between the filmed (c3thodi c) and film-free (anodic) areas.