Corrosion behaviour of additively manufactured 316L and CoCrNi

Abstract

Alloys such as 316L and the medium‐entropy alloy CoCrNi are known for their excellent strength and corrosion properties. In the present study, bulk samples of 316L and CoCrNi (without and with 0.11 wt.% N) alloys fabricated using powder bed fusion laser beam (PBF‐LB) were tested in the as‐printed state for their corrosion behaviour in 0.5 M H2SO4 without and with added 3 wt.% NaCl. The tests were done using potentiodynamic measurements and the results were compared with those of the conventionally manufactured 316L. By means of angle‐resolved X‐ray photoelectron spectroscopy (ARXPS), the passive film characteristics were studied in terms of composition and film thickness. The 316L fabricated using PBF‐LB showed favourable passivation and corrosion behaviour as compared with its conventionally manufactured counterpart. It was observed that all the alloys fabricated using the PBF‐LB showed similar corrosion behaviour, but with CoCrNi and CoCrNi‐N showing better passivation behaviour than 316L alloys in the presence of NaCl. The ARXPS showed the presence of both hydroxide and oxides in all the alloys, with outer hydroxide layer and inner oxide layer. The ARXPS of both 316L alloys showed the expected presence of Cr–Fe oxide on the surface of as‐passivated samples, whereas the presence of sulphide was also depicted for the conventionally manufactured 316L, supposed to be detrimental to its corrosion behaviour. The CoCrNi‐based alloys showed the presence of only Cr2O3 layer in their passivated state, with Co and Ni acting as noble elements in the formation of the passive film. Upon micro‐alloying with the strong solid solution strengthener N, CoCrNi did not show any negative effect on either the corrosion behaviour or the passivation behaviour of the alloy.

Improved Ability to Resist Corrosion of Selective-Laser-Melted Stainless Steel Based on Microstructure and Passivation Film Characteristics

“…Moreover, the ratio of Cr/Fe in the facial film composition of stainless steel can reflect its passivation ability. Generally, the larger the ratio, the higher the ability to resist corrosion of the facial film [31]. The Cr/Fe ratio of the SLM 304 steel was 0.21, which was higher than that of the forging 304 steel by 0.18, indicating that the passivation ability of the SLM 304 was greater than that of the forging 304 steel.…”