Corrosion Behavior of Cu-7Ni-3Al-1Fe-1Mn Alloy Subjected to Different Processing Techniques

Copper–nickel alloys are critical materials for marine engineering, and their corrosion resistance is closely related to their microstructural states. However, the mechanism by which processing technologies regulate corrosion behavior through altering grain boundary character distribution remains unclear. In this study, the corrosion behavior of a Cu–7Ni–3Al–1Fe–1Mn alloy in 3.50 wt% NaCl solution was compared among three states: as-cast (AC), 60% deformation orthogonally cross-rolled (CR), and hot-extruded (HE) with an extrusion ratio of 12.30. The aim was to reveal the key role of grain boundary character distribution in corrosion behavior. The results showed that the corrosion resistance order was HE > CR > AC, with corrosion rates of 0.0062, 0.0068, and 0.015 mm/a, respectively. CR exhibited sufficient recrystallization (83.80%) and was dominated by high-angle grain boundaries, resulting in continuous intergranular corrosion. HE showed a low degree of recrystallization (25%), but possessed high absolute densities of CSL grain boundaries and triple junctions; its corrosion behavior was dominated by uniform dissolution and shallow pitting, with intergranular corrosion suppressed. The key factor determining the resistance to intergranular corrosion was the absolute number (density per unit area) of coincidence site lattice (CSL) grain boundaries rather than their relative proportion. Processing technologies regulate grain boundary character distribution and thereby alter corrosion paths: interconnected high-angle grain boundary networks lead to intergranular corrosion, whereas dense CSL grain boundary networks lead to uniform dissolution. Hot extrusion achieved both grain refinement and optimization of the CSL grain boundary network, transforming the corrosion mode from destructive localized corrosion to uniform and safe corrosion. These findings provide theoretical support for grain boundary engineering design of copper–nickel alloys for marine engineering.