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

Abstract

Additively manufactured austenitic stainless steels exhibit numerous microstructural and morphological differences compared to their wrought counterparts that will influence the metals corrosion resistance. The characteristic as-printed surface roughness of powder bed fusion (PBF) stainless steel parts is one of these morphological differences that increases the parts susceptibility to localized corrosion. This study experimentally determines the average surface roughness and breakdown potential (E b) for PBF 316L in 6 surface finished states: as-printed, ground with SiC paper, tumble polished in abrasive media, electro-polished, chemically passivated, and the application of a contour/re-melt scan strategy. In general, a smaller average surface roughness led to a larger E b. The smoothest surface treatments, ground and electro-polished conditions, led to E b near the materials limit (~+1.0 V Ag/AgCl) while all other surface treatments exhibited significantly lower E b (~+0.3 V Ag/AgCl) The build angle was also shown to impact surface roughness, where surfaces at high angles from the build direction resulted in larger roughness values, hence lower E b .

Corrosion of Additively Manufactured Stainless Steels—Process, Structure, Performance: A Review

“…[112][113] The few corrosion studies that have directly examined asbuilt AM surfaces have found severely diminished resistance relative to conventionally processed material and attributed this to the crevice-like geometry of surface porosity. 18,[114][115][116] A recent study by Melia, et al, found that the average breakdown potentials of SLM 316L as-built surfaces were at least 400 mV lower than a polished surface of the same material in 0.6 M NaCl. 116 Furthermore, E b varied widely with surface inclination angle with the rough downskin surfaces averaging 550 mV lower than the best performing and smooth upskin surfaces, Figure 14.…”

“…18,[114][115][116] A recent study by Melia, et al, found that the average breakdown potentials of SLM 316L as-built surfaces were at least 400 mV lower than a polished surface of the same material in 0.6 M NaCl. 116 Furthermore, E b varied widely with surface inclination angle with the rough downskin surfaces averaging 550 mV lower than the best performing and smooth upskin surfaces, Figure 14. Inclination angle dependence was attributed to the relative amounts and nature of the crevice-like porosity, as exemplified in Figure 13.…”

“…Distribution of corrosion studies in terms of stainless steel alloy, process, and environment. [2][3][4][5][6][7]10,[18][19]27,29,37,[47][48][49][50][51][53][54]57,[68][69]76,[91][92][101][102][103][104][105]109,116,129,134,137,144,[146][147][148][149]153,178,[185][186][187][188][189][190][191][192][193][194]…”

“…178 A systematic study of several types of surface treatments by Melia, et al, found that the lower the average surface roughness (S a ) imparted by a finishing technique, the higher the breakdown potential, Figure 23. 116 Exceptions to this trend came with finishing techniques that deformed the tortuous "cling-on" features, which maintained the sharp crevice-like roughness but showed a reduction in S a value, suggesting the S a roughness metric will not capture all of the features that will be detrimental to corrosion resistance.…”

“…The top edge length of the prism shown in center is 15 mm. Additional material details are given in Melia, et al116…”