Effect of Bending Deformation and Heat Treatment on Grain Boundary Character Distribution and Corrosion Behavior for Pure Nickel N6 Weld

The pure nickel N6 plasma arc weld was subjected to U-shaped bending deformation and heat treatment (grain boundary engineering, GBE). The grain boundary character distribution of non-GBE and GBE samples and its influence on corrosion behavior were studied. The results show that the proportion of low SCSL grain boundaries increase to 54.1% by U-shaped bending deformation and subsequent annealing at 900 degrees C for 10 min, and simultaneously the large-size highly-twinned grain-cluster microstructure is formed. The recrystallization nucleation sites increase and the grain size decreases from 468 mu m to 146 mu m during annealing. The corrosion resistance of GBE sample is higher than that of non-GBE sample, the self corrosion current density of GBE sample is less than that of non-GBE sample, and the capacitive arc radius, impedance modulus and phase angle of GBE sample are greater than those of non-GBE samples. Pitting corrosion occurs on the surface of two samples. The passivation film on the surface is destroyed by Cl- in the 3.5% NaCl solution and forms pitting pits at the initial stage, with the prolongation of immersion time, the concentration of Ni2+ near the anode metal reaches saturation or supersaturation, the bound water in the corrosion product film reacts with Ni2+ forming Ni(OH)(2), and then the Ni(OH)(2) are dehydrated and decomposed into NiO. Corrosion products cover the surface of the sample and form a dense corrosion product film, which are composed of NiO and Ni(OH)(2). The corrosion product film has a protective effect, which prevents Cl- from penetrating through the corrosion products to the sample surface and the pitting is controlled.