Anodic Polarization at the Grain Boundary Region and Aging Embrittlement by Stress-Corrosion in Al-Zn-Mg Alloys in 3%NaCl Aqueous Solution

Nagaoka Kunio, Isano Toyojiro, Kawai Yasunori

doi:10.2320/jinstmet1952.45.1_13

Anodic polarization behaviors at the local region near a grain boundary groove of Al-Zn-Mg alloys in 3%NaCl aqueous solution containing K₂Cr₂O₇, and the relation between stress-corrosion (SC) and hydrogen were discussed. The break-down potential was shifted to the noble side by soluble oxygen in the electrolyte, and the potential in the grain boundary groove became more noble than that on the grain, and the break-down potential was shifted to the less noble side by the applied stress. Anodic polarization in the grain boundary perpendicular to the applied stress deteriorated in reproducibility with increasing stress. The tensile strength of the specimens subjected to the SC treatment (147.1 MPa, 3.6 ks, 3%NaCl+5 kg/m³ K₂Cr₂O₇ at 323 K) were measured as tensile strength at intervals of 0 s, 86.4 ks, 172.8 ks and 1.728 Ms. The tensile strength decreased up to 172.8 ks, but gradually recovered to the previous strength at 1.728 Ms. Thus, this will be inferred from absorption, diffusion and release of hydrogen gas in the specimen. Tensile fracture surfaces are observed by scanning electron microscopy. Specimens which were not treated with SC generated chemically reactive micro-cracks at the intergranular, but in the SC treatment, cracks initiated at the edges of three adjacent grains, and the former fracture surface showed intergranular cracking, but the latter showed a both intergranular cracking and transgranular cracking.

Anodic Polarization at the Grain Boundary Region and Aging Embrittlement by Stress-Corrosion in Al-Zn-Mg Alloys in 3%NaCl Aqueous Solution

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