Assessment of copper resistance to stress-corrosion cracking in nitrite solutions by means of joint analysis of acoustic emission measurements, deformation diagrams, qualitative and quantitative fractography, and non-linear fracture mechanics

A study of stress-corrosion cracking (SCC) of copper in 0.1M NaNO2 aqueous solution is presented. The fracture kinetics was monitored by measuring the acoustic emission (AE) signals. Macro- and micro-fractography analysis, using scanning electron microscopy (SEM), was employed to investigate the fracture mechanisms. Estimates of stress intensity factor, KI, and J-integral were derived in order to assess the resistance of copper to stress corrosion cracking. Two kinds of SCC tests under continuous circulation of the corrosive solution were employed in the present study: 1. Constant extension rate (2x10-6/s) tests on pre-cracked, middle tension (MT) panel specimens. 2. Tests on pre-cracked, compact tension (CT) specimens at a fixed (by a fixing bolt) opening of the crack walls (δ = 0.3 mm, Ki = 27 MPax√m). The time base for these tests was about two months. After the completion of the SCC test, the CT specimen was additionally tested, under a constant-rate (0.02 mm/s) off-center extension. In the both kinds of tests, the SCC fracture kinetics is found to exhibit two typical stages: Stage 1: SCC initiation stage (after a certain incubation period, Ti, measured to be Ti ≅ 3-4 hours for MT specimens under constant extension, the corresponding stress was σ ≅ 40-70 MPa, and Ti ≅ 200 hours for CT specimens under a fixed crack wall opening). Stage 2: Active fracture process (SCC macro-fracture) distinguished by strong AE pulses (which are registered after time T2 ≅ 8 hours for MT specimens and T2 ≅ 800 hours for CT specimens). Fractography analysis has shown that the zone of SCC fracture in MT specimens extends to approximately 1,500 μm. A 400-700 μm deep zone of brittle transgranular fracture, which included small areas showing characteristic SCC 'striations', was observed adjacent to the fatigue pre-crack area. At higher straining of MT specimens, the SCC crack front is found to shrink, due to crack tunneling between the shear lips extending from the specimen sides, and also to propagate by characteristic 'SCC lobes', which have a dark appearance and a rough surface morphology. At even higher loads on MT specimens, the SCC process becomes completely suppressed by the developed plastic deformation. The plastic crack opening proceeds by the usual mechanism of ductile fracture. The total crack advance in the CT specimen after 1,200 hours of testing (total time of SCC testing about 1,400 hours, minus the incubation period, 200 hours) was about 1.3 mm, which gives an estimate of the average crack growth rate of order of 3x10-7 mm/s, for the conditions of continuous pumping of the corrosive solution through the crack. The fracture resistance of copper was estimated to be Ki = 10.5 MPax√m, Ji = 114.6 kJ/m2 and Ki = 29.3 MPax√m, Ji = 578 kJ/m2 for the first and the second stage of SCC kinetics, respectively. For the off-center loaded CT specimen, pre-embrittled after the static SCC test, the J-integral estimates were Ji 98.3 kJ/m2 and Ji = 380.4 kJ/m2, for the first and the second kinetic stages of SCC fracture, respectively