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 10w-carbon and stai nless steels. The resulting s urfa ces were 0 .12 to 0 .76 volt 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.
The mechanism of s tre?s-co rrosion crt;tcking of annealed AZ31B magnesium a lloy in an a queous ~aClI(2 CrO . solutIOn was lI1vestlgated . Cracking was predom ina ntly an electrochem ical pro.cess a nd was s hown to be dependent on t he rate of stra in i n t he sp ecim en f01l0w-tng load lll g ] 11 tensIOn. It ] S postu lated t ha t cracks develop if t he protective fi lm on t he m etal surface IS r u ptu red (over narrow segments of t he specimen) a t a greater rate than it is repa ired 1Il t he corroding medium . T his wou ld expose film-free m etal t hat is anodic t o the fi l med m eta l.
The rates of corrosion of many m etals and alloys are increased by the presence of tensile stresses in the materials. In order to de termine t he effect of tensil e stress on the behavior of magnesium alloys exposed to corrosive media, stress-corrosion tests were made on seve ral magnesium base alloys at two weather exposure sites, and in the laborator y by continuou s immersion in a NaCl + K 2 CrO, solution and by inte rmittent immersion in a dilute NaCI solutioll.The periods of time to failure of specimen s exposed under tensile stress in the atmosphere at the National Bureau of Standards were less than those for the same m ate ri a ls expo ed in a marine atmosphere at Hampton Roads, Va.The Thll clad AZ31X-h sheet alloy proved more resistant to stress corrosion t han bare AZ31X-h, AZ51X, or AZ6IX sheet alloys and AZ80X extruded alloys.Prediction s of relati ve s usceptibility of mate ri a ls to stre s corrosion fr om intermi tte nt imm ersion laboratory tests in a O.OI-percent NaCl solu tion were in good agreement with t he results obtained from weather exposure tests at this Bureau.
The cr ystallographic planes bounding intercrystalline stress corrosion cracks in two large-grained Alpha brasses and those followecl by t ranscrystalline st ress-corros ion cracks in large-grained beta brass were determined b y X-ray diffract ion m cthods . Intercr ystalline stress-co rrosiOD cracking in t he alpha brasses originated bet weeD grains t hat, because of their relative orientatioD, had high iDterfacial e nm·gy contents. It is the relative orie ntation, Dot the cr ystallographic type, that de termines the s usceptibility to strcss-corrosion cracki ng of alpha brass in grain boundaries approximately Dormal to t he appli ed stress. Transcryst alline st ress-corrosion cracking of beta brass occu rred in grai ns t ha t were orie nte d m ost favorably for slip, and in plancs t hat were appro xi ma tely normal to t hc applied stress. No plane or famil y of planes in beta brass was found to bc pa rtic ularly s usceptiblc to stres3-corrosion cr acking. It is post ulated that in each case cracking progresscd by a film ruptu re mechanism .
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