Influence mechanism of underwater hyperbaric environment on the corrosion behavior of high nitrogen steel fabricated by underwater laser direct metal deposition

Abstract

The underwater laser direct metal deposition (UDMD) technique presents a potential new approach for on-site repair in the field of underwater on-site repair. However, the influence mechanism of the underwater hyperbaric environment on the corrosion behavior of UDMD-fabricated samples remains unclear. In the current research, the corrosion behavior of high nitrogen steel (HNS) samples fabricated by UDMD (sample U1) at the ambient pressure of 0.3 MPa was evaluated. The sample fabricated by DMD in the ground environment with the same process parameters served as a comparison (sample A1). Results manifested that the elevated ambient pressure induced sample U1 to possess a higher nitrogen content than the raw HNS powder, unlike sample A1 where nitrogen outgassing occurred. The increased nitrogen content induced the formation of a denser passive film, enhanced re-passivation ability, and promoted synergistic effect between nitrogen and molybdenum. Furthermore, the water-quenching effect, on the one hand, inhibited elemental partitioning and resulted in a lower ∆|PRENγ-PRENα|, thereby leading to a slower galvanic corrosion rate. On the other hand, the water-quenching effect refined grain size and promoted the formation of LAGBs, further leading to the formation of a more stable and robust passive film. Based on the aforementioned factors, the corrosion resistance of sample U1 was substantially improved.