Effects of ultrasonic impact treatment on the corrosion resistance of laser-cladded CrMnFeCoNi high-entropy alloy coatings

Ultrasonic impact treatment (UIT) was performed on the surface of the laser-clad CrMnFeCoNi high-entropy alloy coatings. The fluences of the UIT on the microstructure, microhardness, nanoindentation metrics and corrosion resistance of the coatings were explored. Furthermore, the enhancing mechanisms of the corrosion resistance of these coatings by the UIT were clarified. The results showed that the UIT markedly enhanced the corrosion resistance of the laser-clad CrMnFeCoNi high-entropy alloy coatings, and the enhancing effect continuously improved with the increasing of the UIT time. The most significant enhancement of corrosion resistance occurred under the action of UIT with a current of 1.6 A for 20 min. Compared to the untreated group, corrosion potential rose by 65.68 %, corrosion current density fell by 74.95 %, and equivalent resistance increased by 340.37 %. Corrosion morphology analysis showed that the primary forms of corrosion before the UIT were stress corrosion and pitting, which vanished after the UIT with a significant improvement in pitting. Analysis of corrosion products revealed an increased Cr content and a decreased FeOOH and O content on the coating surface after the UIT, leading to enhanced corrosion resistance. Additionally, microstructural analysis showed that large columnar crystals in the laser-clad CrMnFeCoNi high-entropy alloy coatings transformed into finer columns, drastically reducing the grain size of the surface. Concurrently, surface microhardness and nanoindentation mechanical properties markedly improved after the UIT, with the effects growing stronger over time. After the UIT with the current of 1.6 A for 20 min, the increasement of surface microhardness reached 109.01 %. A comprehensive analysis of the fluences on the corrosion resistance of laser-clad CrMnFeCoNi highentropy alloy coatings by the UIT indicated that the refinement of grain size, improvement of the stress state of the coating, increasement of Cr element and its compounds, and decreasement of FeOOH content are the main reasons of enhanced corrosion resistance.