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Fundamentals of Stress Corrosion Cracking: Light-Weight Materials Influence of Mg on the Corrosion of Al D. R. Baer, C. F. Windisch, Jr.,(a,b) M. H. Engelhard, M. J. Danielson,(a,b) R. H. Jones,(a,b) and J. S. Vetrano(a,b) Supported by Division of Materials Sciences, Office of Basic Energy Sciences. (a) EMSL User. (b) Environmental and Health Sciences Division. A series of experiments have been conducted to determine the influence of Mg on the corrosion and electrochemical behavior of Al. Lightweight strong materials are desired in order to allow increased automotive fuel efficiency without impacting safety. This need has led to an increased interest in Al alloys with properties adequate to serve as structural elements (Burger et al. 1995). Aluminum-magnesium alloys are candidates for components that require moderate strength, formability and weldability. As produced, Al-Mg alloy 5083 with 4.5 wt% Mg meets these goals. However, alloys with greater than »3 wt% Mg have shown (Speidel and Hyatt 1972) susceptibility to corrosion and stress corrosion cracking (SCC) following low-temperature heat treatments or thermal exposures as low as 90°C. This corrosion and SCC susceptibility has been related to the precipitation of the Al3Mg2 (b phase) at grain boundaries of Al-Mg alloys (Speidel and Hyatt 1972; Beck and Sperry 1969; Sprowls and Brown 1969). This overall program involves many different experimental and theoretical approaches to learn about important aspects of the SCC of these alloys. The EMSL capabilities have been used to assist in microstructural and microchemical characterization of grain boundaries and to examine the reactivity of surfaces that model grain-boundary composition. Auger electron spectroscopy and transmission electron microscopy measurements show that alloys having a distribution of Al3Mg2 (b phase) precipitates and segregated Mg on grain boundaries are more susceptible to cracking. To understand the roles of Mg on the cracking process, we compared the corrosion potential and film formation of pure Al, Al implanted with Mg, a 7 wt% Mg-Al alloy, and pure Al3Mg2 phase. These experiments use the EMSL specimen transfer capability and involve transfer of specimens from a surface spectrometer to an attached electrochemical cell. The surfaces of the specimens were cleaned and prepared in a surface analysis system and transferred in a vacuum transfer system to a corrosion cell. After solution exposure and electrochemical measurements, the specimens were returned to the spectrometer and analyzed by x-ray photoelectron spectroscopy. One objective of this study was to determine if segregated Mg significantly influenced the corrosion properties of the Al-Mg alloys and how surfaces with segregated Mg compared to the Al3Mg2 phase. First, the thickness and composition of the film formed on each of the samples is similar at the open circuit potential as shown in Figure 6.13. These results indicated that the Mg did not significantly alter the nature of the film that forms on these materials upon exposure to a salt solution. Furthermore, the open circuit potentials for Al, Mg implanted Al, and the 7% alloy were found to be nearly identical as shown in Figure 6.14. However, the corrosion potential for the b phase differed significantly. Figure 6.13. Oxygen sputter profiles for the four materials used in this study after exposure to a salt solution for 200 seconds. Figure 6.14. Variations in the OCP of the four materials used in this study as a function of time after immersion in the salt solution. SEM, AES, and TEM measurements show that the b-phase particles which form are not continuous, but cover less than 10% of the surface when cracking occurs. The differences in open circuit corrosion potential, the b-phase particles, and the Al- and Mg-containing materials suggests that the surface will have some mixed potential that will be anodic to the open circuit potential (OCP) of the b phase. If pure b-phase material is held at the potential of the pure Al, this phase rapidly corrodes and evolves hydrogen. The data to this point provide no evidence that small amounts of Mg in the alloy, or even some amount of segregated Mg at an Al surface, will alter properties that may enhance cracking. However, the Mg-rich b -phase particles in the mixed potential environment of a fracture surface appear to be readily oxidized and provide an opportunity for cracking, likely by the introduction of hydrogen. Since this phase appears to cover less than 10% of the grain-boundary surface when cracking occurs, it would not appear to be a mechanical property of the phase that is controlling the cracking. References Beck, A. F., and P. R. Sperry, in Fundamental Aspects of Stress Corrosion Cracking (National Association of Corrosion Engineers, Houston, Texas 1969) p. 513. Burger, G. B., et al., "Microstructural Control of Aluminum Sheet Used in Automotive Applications," Materials Characterization 35, 23-39 (1995). Speidel, M. O., and M. V. Hyatt, in Advances in Corrosion Science and Technology, Vol. 2, M. G. Fontana and R. W. Staehle, Ed. (New York, Plenum Press 1972) p. 115. Sprowls, D. O., and R. H. Brown, in Fundamental Aspects of Stress Corrosion Cracking (National Association of Corrosion Engineers, Houston, Texas 1969) p. 466. William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147