Environmental factors affecting localized corrosion of 7075-t7351 aluminum alloy plate
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INTRODUCTION
THET73 temper for alloy 7075, a temper designed specifically for high resistance to stress corrosion cracking (SCC), has exhibited excellent resistance to SCC in both laboratory evaluations and outdoor exposures. Despite its extensive use in the aerospace industry for the last 20 years, no instance of service failure by SCC has been reported. The T7 temper designation is given to alloys that have been aged beyond their maximum strength. The T7 aging for A1-Zn-Mg-Cu alloys is usually a two-step process. The first step involves aging at a temperature in the range used to develop T6 properties, namely, 100 ~ to 135 ~ This aging step reduces the zinc and magnesium from solid solution and produces GP zones along with small amounts of a transition precipitate 7/', which is partially coherent on (111) planes. The pitting potential in a 3.5 pct NaCI solution shifts 75 mV in the cathodic direction as a result of first step aging (1). The second step aging is carried out at temperatures above 160 ~ This causes a growth in GP zone size, a corresponding increase in the amount of 7/', and the formation of the equilibrium phase ~7. The second aging step reduces both zinc and copper from solid solution, and the pitting potential, in a 3.5 pct NaC1 solution, shifts about 25 to 30 mV in the anodic direction compared to the peak aged T6 temper. Sprowls and Brown 1 attribute the improved resistance to intergranular corrosion in the T73 temper to similar potentials of the grain interior and grain boundary regions. A previous investigation on the mechanism of localized corrosion of 7075 alloy plate in NaC1 solutions established that a composition gradient existed from grain interior to grain boundary regions, even in the resistant T7351 temper. 2 Based on the results obtained from potentiodynamic anodic polarization curves and constant potential exposure tests, it was inferred that the grain boundary region was made more anodic by the segregation of Zn and Mg atoms in solid solution. 2 Such an inference was consistent with the results of actual microanalytical examinations made on AI-Zn-Mg and A1-Zn-Mg-Cu alloys with different techniques, namely, electron microprobe, 3 plasmon energy lOSS4 and Auger elecSHANTANU MAITRA, Staff Scientist, and G.C. ENGLISH, Technologist, are with Alloy Technology Division, Aluminum Company of America, Alcoa Technical Center, Alcoa Center, PA 15069. Manuscript submitted January 16, 1981. METALLURGICAL TRANSACTIONS A
tron spectroscopy. 5 However, in the T7351 temper, the difference between pitting potentials of these two regions was small enough so that preferential corrosion of the grain boundary region could not be sustained. This investigation deals with the effect of changing environment on the characteristic potentials of these two regions, and the consequences the environment has on the resistance to localized corrosion of 7075-T7351 alloy plate. Additions of nitrates to chloride solutions have been of interest because of the widely varying effects they have on the corrosion of aluminum
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