Effect of microstructure on the erosion-corrosion behavior of 70/30 copper-nickel tubes

Grain size is a fundamental microstructural parameter governing the corrosion behavior of metallic materials, yet systematic comparative studies on its influence within conventional (micrometer-scale) 70/30 copper-nickel alloys remain limited. To address this gap, the erosion-corrosion behavior of five commercial 70/30 copper-nickel tubes with distinct grain sizes (ranging from 22.3 to 34.1 μm) and grain boundary character distributions was systematically investigated using a custom-built test rig in artificial seawater. The results demonstrate that the erosion-corrosion resistance of the alloy is predominantly governed by its average grain size and the fraction of low-ΣCSL (coincident site lattice) grain boundaries. Over the investigated range of 22.3 to 34.1 μm, the erosion-corrosion resistance of the alloy increased with its average grain size. Furthermore, a higher fraction of low-ΣCSL grain boundaries was found to enhance corrosion resistance. TEM and XPS analyses revealed that the corrosion product film possesses a stratified structure. This structure consists of a loose, porous, and Ni-enriched outer layer, along with a smooth, dense, and similarly Ni-rich inner layer. The outermost region of the film was Fe-rich and predominantly composed of FeO and γ-FeOOH. An optimal balance between mechanical properties and corrosion resistance is achieved at a grain size of 34.1 μm and a low-ΣCSL boundary fraction of 46.1%.