Evaluation of Corrosion Resistance of Alloy 625 Obtained by Laser Powder Bed Fusion

Abstract

The aim of this work is to compare the corrosion resistance of nickel-base Alloy 625 (UNS N06625) produced by laser powder bed fusion with that obtained via conventional casting and hot working. Cyclic potentiodynamic polarization and potentiostatic tests were performed in order to evaluate the corrosion resistance of the differently manufactured alloys according to ASTM G5, and in NaCl 0.6 M solution at pH 7 and pH 3, at 40°C. The electrochemical characterization was carried out on the as-produced alloy and after annealing at 980°C for 32 minutes (according to ASTM B446). This heat treatment was also performed on the commercial hot worked alloy. Two surface conditions, namely as-built and polished surfaces, were investigated on the additive manufactured specimens. The alloy produced by laser powder bed fusion was not susceptible to pitting in the considered environments and had a good localized corrosion resistance, slightly higher than that of traditional wrought material. However, as predicted, the corrosion resistance of the as-built surfaces increased after mechanical polishing. The correlation between the corrosion performance and microstructure is also discussed.

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 .…”

“…Several other examples of microstructural features and processing defects that have been investigated are; nanoscale oxides/inclusions 7,20,23,24 , solute segregation (i.e. cell boundaries, melt pool boundaries) 7,13,[22][23][24][25][26][27][28][29] , texture and grain character 29 , residual stresses 3,30,31 , surface roughness 18,20,[32][33][34][35] , and porosity 7,9,[18][19][20][21][22] .…”

“…Another result of the powder-based AM processes is high surface roughness, a major concern for AM part reliability and a characteristic that has received minimal attention with regards to its impact on corrosion. Currently, most of the aforementioned corrosion studies investigate samples in a ground surface condition (planar grinding) with very few addressing how the as-printed AM surfaces impact corrosion behavior 18,20,[32][33][34][35] . The work by Cabrini et al showed a reduction in corrosion resistance and crevice potential for the as-printed surface of the 625 Ni alloy when compared to a polished surface in 0.6 M NaCl solution 18 .…”

“…Currently, most of the aforementioned corrosion studies investigate samples in a ground surface condition (planar grinding) with very few addressing how the as-printed AM surfaces impact corrosion behavior 18,20,[32][33][34][35] . The work by Cabrini et al showed a reduction in corrosion resistance and crevice potential for the as-printed surface of the 625 Ni alloy when compared to a polished surface in 0.6 M NaCl solution 18 . The polished AM parts improved corrosion resistance was stated to stem from a decrease in surface porosity, which served as sites for crevice corrosion, and from the polishing imparting a less defective passive oxide film.…”

“…on the rougher surface 20,36 . For an AM parts surface the large roughness, high aspect ratio, and tortuous features will exacerbate this pit initiation frequency effect as compared to a ground or machined surface 18,40 . Also the development of aggressive solution chemistries at the bottom of the crevice-like/nodular features will increase the likelihood a pit will stabilize and propagate when compared to a smooth surface without these asperities 9,41 .…”