On The Corrosion and Metastable Pitting Characteristics of 316L Stainless Steel Produced by Selective Laser Melting

Abstract

The austenitic stainless steel 316L was additively manufactured using Selective Laser Melting (SLM). The corrosion characteristics of the additively manufactured (3D printed) specimens were investigated by both potentiodynamic and potentiostatic techniques. The production parameters were deliberately varied during SLM, to produce 316L specimens fabricated by different laser scan speed (in the range of 860-1160 mm/s) and laser power (in the range of 165-285 W). The fabrication parameters were found to influence the porosity of the resulting specimens. The pitting potentials, metastable pitting rates and repassivation potentials of the 3D printed specimens are presented herein as a function of the laser scan speed and laser power, and also discussed in the context of specimen porosity. The corrosion characteristics of the 3D printed 316L were also compared with wrought 316L, revealing higher pitting potentials and lower rates of metastable pitting for most SLM 316L specimens, the related concepts of which are discussed herein. Stainless steels are a critically important class of alloy in several industries.1 The corrosion resistance of stainless steel (SS) is attributed to the presence of alloyed chromium (> ∼11 wt%), enabling the formation of a chromium oxide (Cr 2 O 3 ) based passive film upon the metal surface.1-3 The addition of elements such as nickel, nitrogen, molybdenum, carbon, aluminum, copper, sulfur and selenium can modify the corrosion resistance, strength, ductility, machinability, and the phases present (and their stability) in stainless steels.1-3 The types of stainless steel are most conveniently categorized according to their microstructure, classified as: austenitic, martensitic, ferritic and austenoferritic (duplex). Such structures are realized by specific alloying additions and metallurgical processing.1-4 Additive manufacturing has recently been explored as a means to produce SS components in net shape, 5-7 circumventing the requirement of traditional manufacturing methods such as casting, rolling, welding, machining, forging, etc. Selective Laser Melting (SLM) is one such additive manufacturing method, which can produce dense products by laser processing of metal powders. In SLM, metal powder layers are successively fused in a layer-by-layer manner into the requisite 3D structure, employing a fiber laser. [8][9][10][11] The metallic component in essence is therefore 3D printed into the requisite shape, by additive manufacturing processes like SLM. The process takes place in chamber of well-controlled inert atmosphere (either nitrogen or argon). The primary parameters that govern the microstructure of a 3D printed specimen are the laser power and the laser scan speed, as they influence the thermal gradients and growth rate of the metal at the solid-liquid interface (in the melt pool). 12 The porosity that may develop in 3D printed specimens depends upon the heating and cooling rates of the melt pool. 13 The laser scan speed relates to the duration for which the laser beam is in contact with ...

Corrosion behaviour of 316L stainless steel manufactured by selective laser melting

“…The corrosion behaviour has been systematically investigated as a function of the main laser parameters (laser power and scan speed), which mainly determine the amount and size of pores in the material. For materials containing pores with a size larger than 10 µm, it is reported that the porosity does not significantly affect the breakdown of passivity (E pit ) in potentiodynamic polarization curves, while it has a more relevant effect on metastable pitting and on the repassivation behaviour of SLM 316L stainless steel . Materials obtained with scan speeds in the range of 1,000–1,400 mm/s tend to display a higher density of pits, that were associated to the presence of pores with increasing size and number, although the pitting potential was only slightly affected by the variation of the scan speed …”

“…The size of the particles ranges between few nanometers up to 1 µm . MnS inclusions are typically not found in SLM 316L stainless steel . The formation of an intragranular cellular structure in combination with randomly distributed nanoinclusions improves the mechanical properties of SLM 316L stainless steel …”

“…Although there is already a significant body of literature on this topic, it appears that the effect of the microstructure of additively manufactured stainless steel on corrosion behaviour is not completely understood yet. As an example, it is commonly reported that selective laser melted 316L stainless steel displays a higher corrosion resistance than a traditional wrought material with comparable chemical composition . Nevertheless, it is also reported in some cases that as produced SLM stainless steel exhibits lower corrosion resistance than the wrought counterpart due to its complex microstructure or to the presence of defects related to the SLM process .…”

“…Porosity is reported to be a possible site for the initiation of localized corrosion in SLM 316L stainless steel . The corrosion behaviour has been systematically investigated as a function of the main laser parameters (laser power and scan speed), which mainly determine the amount and size of pores in the material.…”

The Role of Microstructure and Surface Finish on the Corrosion of Selective Laser Melted 304L

“…However, the SLM 316L also displayed increased pitting frequency and decreased repassivation potentials related to increasing specimen porosity. 20 The non-equilibrium nature of rapid solidification during the SLM process can lead to cellular-dendritic solidification substructures with locally heterogeneous compositions. [21][22][23] The comparatively Cr-lean cellular regions could have compromised passive oxide continuity relative to the surrounding cell boundaries and initiated preferential corrosion attack in chloride solutions.…”