QUANTITATIVE MEASUREMENTS OF THE DEGREE OF EXFOLIATION ON ALUMINUM-ALLOYS

The paper deals with quantitative measurements of the degree of exfoliation on three aluminium-lithium alloys (Al-Li-Cu, 8090 and 2091) in under-aged and over-aged conditions and on 7075-T7351, 7175-T6511 and 2024-T351. A special device was used in this work to measure the forces developed by the corrosion products during exfoliation in Exco-solution. Potentiokinetic curves were obtained on these alloys in Exco-solution and sodium chloride solutions (0.6, 1, 2 and 4 M Cl- ) at a sweep rate of 1000 mV min-1, to determine the pitting potential, E(p), of each alloy. The curve obtained by plotting the forces developed by the exfoliation process as a function of time could be divided into two stages; an incubation period, t(i), was followed by an increase in the forces to a level corresponding to a time of exfoliation equal to 240 h (F240h). The results obtained were compared to those observed in the conventional Exco-test (ASTM G34). Finally, stress-corrosion cracking tests using the slow strain-rate tensile technique were carried out on the aluminium-lithium alloys, either in Exco-solution or in the sodium chloride solutions containing various chloride concentrations. The results obtained show that, when the time of incubation, t(i), increases, the force of exfoliation, F240h, decreases and the resistance to stress-corrosion cracking, R(SCC), increases. This resistance is sensitive to the copper content, i.e. R(SCC) decreases also when the copper content increases. It is concluded that t(i) may be taken as an exfoliation criterion.