Enhanced Corrosion Resistance of Additively Manufactured 316L Stainless Steel After Heat Treatment

Abstract

The microstructures and passivation behavior of selective laser melted 316L stainless steel (SLM SS316L) after various heat treatments (500 °C, 950 °C, and 1100 °C) were investigated. The electrochemical results showed that the SLM SS316L sample that was heat treated at 950 °C exhibited the lowest passive current density. The microstructural characterization analysis indicated that the subgrain structures transformed from dislocation-rich subgrain boundaries into island-like cellular trace structures after heat treatment at 950 °C. This led to improved corrosion resistance due to the elimination of dislocations and the homogenization of the composition. Compositional analyses of the passive film indicated that there was no notable change in the passive film composition after heat treatment at 500 °C and 950 °C. However, heat treatment at 1100 °C promoted the formation of Cr(OH)3 in the passive film, resulting in a reduced corrosion resistance. Based on these results, heat treatment at 950 °C appears to be an adequate post-process for SLM SS316L to optimize the microstructure, while also improving corrosion resistance.

Effect of Thermal Treatment on Corrosion Behavior of AISI 316L Stainless Steel Manufactured by Laser Powder Bed Fusion

“…Kong et al reported that a thermal treatment at 650 °C for 30 min did not modify the passive behavior of as-built L-PBF AISI 316L stainless steel, while recrystallization annealing at 1050 °C for 30 min promoted the formation of a thicker and more protective oxide film due to the homogenization effect of the thermal treatment at a high temperature [ 24 , 25 ]. Similar results were reported by Zhou et al, which showed that a thermal treatment at 950 °C for 4 h produced a homogenization effect on the microstructure and distribution of alloying elements [ 37 ]. In contrast, thermal treatment at 1100 °C reduced the corrosion resistance due to the formation of a high amount of Cr(OH) 2 and to the segregation of alloying elements after complete recrystallization [ 37 ].…”

“…Similar results were reported by Zhou et al, which showed that a thermal treatment at 950 °C for 4 h produced a homogenization effect on the microstructure and distribution of alloying elements [ 37 ]. In contrast, thermal treatment at 1100 °C reduced the corrosion resistance due to the formation of a high amount of Cr(OH) 2 and to the segregation of alloying elements after complete recrystallization [ 37 ]. Moreover, it was hypothesized that nanoscale inclusions of silicon oxide observed after heat treatment at 1100 °C could promote pit initiation [ 37 ].…”

“…The enhanced corrosion resistance of L-PBF AISI 316L stainless steel was also associated with the formation of very fine cellular/columnar sub-grains, which are thought to promote the formation of a more protective oxide film compared to other additive-manufacturing techniques, producing more coarse microstructures or conventionally produced stainless steels [ 35 , 36 ]. Other authors attributed the marked passivity of additively manufactured stainless steel to the absence of deleterious MnS inclusions in L-PBF stainless steel and to the formation of nano-oxide inclusions which are too small to initiate a localized attack [ 18 , 37 ].…”

“…In contrast, thermal treatment at 1100 °C reduced the corrosion resistance due to the formation of a high amount of Cr(OH) 2 and to the segregation of alloying elements after complete recrystallization [ 37 ]. Moreover, it was hypothesized that nanoscale inclusions of silicon oxide observed after heat treatment at 1100 °C could promote pit initiation [ 37 ]. Vignal et al observed a decrease in corrosion resistance in L-PBF AISI 316L after thermal treatment at 1050 °C for 6 h, which led to the precipitation of carbides that were precursor sites for pitting [ 38 ].…”

Pitting Corrosion of Type 316L Stainless Steel Elaborated by the Selective Laser Melting Method: Influence of Microstructure

“…The positive influence of the ratio O 2À / OH À in the native passive film (section 3.2) is then counterbalanced by the negative role of carbides. The corrosion resistance of 316L elaborated by SLM can increase after certain heat treatment, while others may reduce it ( Ref 17,18,28,29). It depends on modifications of the microstructure and physicalchemical properties of the passive film formed after aging in aqueous solutions.…”