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

Abstract

The corrosion behavior of selective laser melted (SLM) 304L was investigated and compared to conventional wrought 304L in aqueous chloride and acidic solutions. Through immersed electrochemical testing and exposure in acidic solutions, the SLM 304L exhibited superior pitting resistance in the polished state compared to wrought 304L. However, the surface condition of the SLM material had a great impact on its corrosion resistance, with the grit-blasted condition exhibiting severely diminished pitting resistance. Local scale, capillary micro-electrochemical and scanning electrochemical microscopy investigations, identified porosity as a contributing factor to decreased corrosion resistance. Preferential corrosion attack was not observed to be related to the characteristic underlying cellular microstructure produced through SLM processing. This study highlights the effects of SLM microstructural features on corrosion resistance, specifically the substantial influence of surface finish on SLM corrosion behavior and the need for development and optimization of processing techniques to improve surface finish. Powder bed selective laser melting (SLM) has become a desirable and widely used technique for the additive manufacturing (AM) of metal parts. While a significant amount of research has been carried out on the mechanical properties of SLM materials, little is known regarding their corrosion behavior. Of the few corrosion studies that do exist, the primary focus has been on evaluating general corrosion resistance.1,2 A few recent investigations have examined the role of processing and microstructure on the material corrosion behavior in more detail.3,4 However, there is still a need for further investigation in order to develop a full understanding of the microstructural and morphological characteristics inherent to AM materials due to the unique processing conditions and their relative contributions to the materials' corrosion behavior. Ideally, a ranking of the deleterious or advantageous properties formed in AM materials with respect to corrosion should be established. This is necessary for a variety of alloy systems in engineering relevant solutions and environments to help inform and guide future processing parameters, build designs, and materials selection.The powder bed process along with extremely high cooling rates and temperature gradients during SLM processing create microstructures and part surfaces that differ greatly from their conventional thermo-mechanically processed counterparts. Several characteristics are expected or have known effects on corrosion behavior including surface finish, porosity, inclusions (MnS, oxides, etc.), and microstructures formed through non-equilibrium cooling conditions. For example, a strong relationship between surface roughness and corrosion susceptibility has been observed in which pit initiation decreases with decreasing surface roughness in chloride solutions.5-7 Porosity, formed by entrapped gas or lack of fusion of powder particles in the SLM build, can also reduce...

Corrosion of metallic materials fabricated by selective laser melting

“…10b, c, which was also confirmed by other reports. 50,[188][189][190] The oxide inclusions of this scale were not deleterious to the pitting corrosion resistance. 191,192 However, no MnS inclusions or large (Al, Ca)-oxide precipitates have yet been observed in the SLMed 316L stainless steel, which should be attributed to the extremely high solidification rate during the SLM process, however, there was enough time for the element diffusion (such as Mn and S) to form those inclusions during the traditional casting process.…”

Effect of Manufacturing Parameters on the Mechanical and Corrosion Behavior of Selective Laser‐Melted 15‐5PH Stainless Steel

“…When laser power was 150 W and scanning speed was 1400 mm s −1 , there was obvious corrosion product on the surface of the sample. Figure f shows that there was obvious oxygen enrichment near the manufacturing defect, and the corrosion product was more obviously detected by energy‐dispersive spectrometry due to its crevice‐like geometry …”

“…Manufacturing defects also have complex effects on pitting corrosion resistance and passivation film stability. Schaller et al showed that manufacturing defects in the selective laser‐melted 304 L stainless steel could result in a decrease in corrosion resistance in the solution with 5% HCl + 100 ppm FeCl 3 , which was attributed to the crevice‐like geometry of manufacturing defects . Manufacturing defects of the crevice‐like structure can significantly reduce pitting corrosion resistance of the sample.…”

“…In addition to the mechanical properties, corrosion resistance is also increasingly important for metallic materials. Some studies have proposed that manufacturing defects in selective laser‐melted metals resulted in decreased corrosion resistance and increased pitting sensitivity due to their crevice‐like geometry. This research mainly focused on the manufacturing of low‐strength metals, but it was very important to explore the effect of manufacturing defects on the mechanical and corrosion behavior for ultra‐high‐strength stainless steel to choose appropriate process parameters by selective laser melting.…”

How build angle and post-processing impact roughness and corrosion of additively manufactured 316L stainless steel

“…A recent review by Sander et al summarizes how some of these features can cause higher susceptibility toward the initiation of local corrosion than others and in relation to a wrought counterpart 17 . For instance internal part porosity, a common processing defect, has received significant attention and was shown to serve as preferential site for corrosion initiation in chloride solutions by several authors 7,9,[18][19][20][21][22] . Schaller et al and Melia et al empirically showed corrosion to preferentially occur at lack of fusion pores for powder-based AM austenitic stainless steels, leading to a reduction in breakdown potential (E b ) compared to areas excluding this porosity 7,20 .…”