The role of hydrogen in corrosion fatigue of high purity Al-Zn-Mg exposed to water vapor
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I.
INTRODUCTION
ALUMINUMalloys, and in particular the high strength A1Zn-Mg alloys, are susceptible to environmentally assisted crack growth. The first mechanistic models developed to explain stress corrosion cracking (SCC) of A1-Zn-Mg alloys were based on either stress assisted dissolution of the highly strained plastic zone at the crack tip or on passive film rupture and dissolution at active slip steps, t1"2]The localization of the crack path to the grain boundaries was thought to be due to either segregation of alloying elements or impurities to the grain boundaries or to preferential slip in the soft precipitate free zones, t1'2]Later, it was proposed that stress corrosion cracking could be caused by a hydrogen embrittlement process resulting from the cathodically evolved hydrogen which invariably accompanies the corrosion process on aluminum alloys. [34" 5] Hydrogen gas bubble nucleation and growth at grain boundaries has been observed in the transmission electron microscope.t6] However, Ford t7'81demonstrated that a calculation of anodic dissolution rates can quantitatively explain the observed crack growth rates, while hydrogen embrittlement, at least by the gas bubble nucleaton mechanism of Zapffe and Sims, tgl can not. Studies on pre-exposure of A1-Zn-Mg alloys to environments containing water and water vapor have shown that the alloys are embrittled by exposure to either liquid or vapor phase water, t~~ Tests have shown that there is a strong relationship between pre-exposure, test environment, strain rate, and ductility, t1~ Holroyd and Hardie ]151tested an A1-ZnMg alloy [7049] in dry air at varying strain rates after a three day pre-exposure to seawater, and found a minimum in ductility at a strain rate of 1 x 1 0 - 4 m / ( m 9 s). They attributed the minimum in ductility to increasing embrittlement with lower strain rates until the rate of hydrogen desorption during testing resulted in the loss of the embrittling species before failure, t151They reported that storage of the samples in vaccuum was not effective in restoring the ductility and concluded that dislocation motion was required to remove R. E. RICKER is Metallurgist, Institute for Materials Science and Engineering, National Bureau of Standards, Gaitherburg, MD 20899. D.J. DUQUETTE is professor, Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12181. Manuscript submitted August 24, 1987. METALLURGICAL TRANSACTIONS A
hydrogen. I151Swann et al. [16.17]found that pre-exposure to moist gases resulted in embrittlement and hydrogen absorption as detected by straining in a vacuum chamber equipped with a mass spectrometer. In contrast to SCC where cracking is virtually always intergranular, transgranular cracking or mixed intergranular and transgranular cracking is usually observed for corrosion fatigue (CF) of aluminum alloys although the same mechanisms have been proposed to explain the role of the environment in the fracture process, t18-211To distinguish between anodic dissolution and hydrogen embrittlement mechan
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