Susceptibility of Aluminum Alloy 7075 T6 to Stress Corrosion Cracking

One of the most important aluminum (Al) alloys among the 7XXX series is 7075 in the T6 temper condition. However, 7075 T6 alloy is proven to be susceptible to stress corrosion cracking (SCC) and has caused many service failures of airplanes. In this research, the susceptibility of 7075 T6 alloys to SCC is approached according to many variables of stress, sodium chloride (NaCl) concentration, pH variation, and aeration. The testing method selected was the three-point bending under complete immersion for a period of 40 days. The results indicate that the threshold for SCC in 7075 T6 alloy lies between 220 and 340 MPa in environments containing as low as 0.5wt% NaCl concentrations in both neutral and acid solutions. The cracking direction found was different from the expected using other techniques such as tensile or notched specimens, which opens a new gate for testing and monitoring SCC in the 7XXX series. In addition to SCC, pitting was highly noticed and was found related to stress intensity, the higher the stress the more intense the pitting. However, in a pH of 2 and pH of 4 acid solutions, uniform corrosion was more predominant than pitting and contributed to more uniform surface damage.

A very remarkable effect for the conjugation of pitting and SCC can be seen in samples stressed to Ultimate Tensile Strength (UTS) levels and UTS under aerated solution, SCC cross-connected horizontal pits together creating semi-loose particles of the alloy, a result that was not observed in previous literature of related researches.