Additively manufactured (AM) stainless steels (SSs) exhibit numerous microstructural differences compared to their wrought counterparts, such as Cr-enriched dislocation cell structures. The influence these unique features have on a SSs corrosion resistance are still under investigation with most current works limited to laboratory experiments. The work herein shows the first documented study of AM 304L and 316L exposed to a severe marine environment on the eastern coast of Florida with comparisons made to wrought counterparts. Coupons were exposed for 21 months and resulted in significant pitting corrosion to initiate after 1 month of exposure for all conditions. At all times, the AM coupons exhibited lower average and maximum pit depths than their wrought counterparts. After 21 months, pits on average were 4 μm deep for AM 316L specimen and 8 μm deep for wrought specimen. Pits on the wrought samples tended to be nearly hemispherical and polished with some pits showing crystallographic attack while pits on AM coupons exhibited preferential attack at melt pool boundaries and the cellular microstructure.

The natural passivity of stainless steels (SSs) makes them great candidates for material applications when exposure to marine environments is expected.1  However, the propensity of SS to localized corrosion and pitting can be of concern. The susceptibility of metals like SS have to the breakdown of passivity is largely dictated by exposure to aggressive halide-containing environments and microstructural features such as nonmetallic inclusions or other chemical heterogeneities.2-5  For this reason, recent studies have been performed investigating how the nonequilibrium microstructures that form in additively manufactured (AM) metals influence corrosion behavior.6-13  Determining these microstructure/corrosion relationships are a critical step in the assessment of AM metals that will ultimately lead to qualifying AM parts for use in an industrial setting.

The corrosion behavior of powder-based AM austenitic SS have received the most attention with several publications showing microstructure-processing-corrosion relationships under full immersion environments.6,14-19  However, a limited amount of work has focused on the atmospheric corrosion behavior of AM metals, in particular SSs, with all work to date using simulated salt fog environments.8,20-24  These salt fog studies, such as that Barile, et al.,21  investigate the corrosion response when exposed to NaCl spray solutions which can help predict potential susceptibility, but are insufficient at accurately simulating real marine environments.25 

For full immersion studies using chloride containing solutions, AM SS made with powder bed fusion (PBF) generally exhibit less susceptibility to local corrosion initiation than their wrought counterparts, determined via electrochemical and postmortem measurements.6,17,19,26-27  Work by Sander, et al., suggests the enhanced corrosion resistance of PBF SS is caused by the reduction in size of nonmetallic inclusions, such as MnS, compared to their wrought counterpart.6  In addition, the microchemical segregation associated with the nonequilibrium microstructures of PBF SS, such as Cr segregation to cell boundaries, play a minor role in determining the susceptibility to corrosion which has often been attributed to process defects, such as porosity and surface roughness.6,26,28-33  Viable mitigation strategies to enhance the predictability of local corrosion behavior for PBF SS are being investigated to reduce surface roughness and porosity through polishing or fine control of processing parameters/powder feedstock, respectively.6,30 

The conclusions from full immersion studies bode well for future applications where PBF SS parts are exposed to marine-like service environments, assuming the processing defects are addressed. The work presented here looks to correlate this enhanced corrosion resistance of PBF SS compared to a wrought counterpart from full immersion studies to a severe marine corrosive atmospheric environment. Two PBF and wrought austenitic SSs, 304L and 316L, were exposed to a coastal environment for 21 months with postmortem analysis showing typical pit morphology and pit depth with respect to time.

Materials

The wrought 304L and 316L plate were purchased from McMaster-Carr and cut into approximately 1.5 in square samples. AM 304L and 316L samples were printed with a PBF technique on a Renishaw AM 250 system under an Argon cover gas with a 1,070 nm wavelength Yb fiber laser with a spot size of 70 μm. The scan hatch distance was set to 85 μm (measured to be 82±15 μm) with a layer thickness of 50 μm (measured to be 40±10 μm) and scans were patterned in the “stripes” build mode. A laser pulse of 75 μs at a power of 200 W with 60 μm movements was used with a dwell time of 1 µs to 3 μs between pulses to enable repositioning of the beam with the mirror.

304L or 316L powders, both within the ASTM specification A240, were used for the build.34  The powder particles had diameters of 15 μm to 45 μm, with a median diameter of 29 μm. Samples were printed to dimensions of 38 mm × 38 mm × 6.8 mm, with the build direction along a 38 mm axis. Post print, all samples were removed from the build plate with wire electrical discharge machining (EDM) and abrasively blasted to remove remaining metal powder with silicon oxide particles.

The compositions of the austenitic SS coupons (304L and 316L), both PBF and wrought, examined in this study are provided in Table 1. The compositions were determined by a LECO combustion furnace for C, S, O, and N, and x-ray fluorescence (XRF) for Co, Cr, Cu, Mn, Mo, Nb, Ni, P, Si, and Sn, while Fe was determined by the remaining balance.

Table 1.

Stainless Steel Composition (wt%)(A)

Stainless Steel Composition (wt%)(A)
Stainless Steel Composition (wt%)(A)

Material Characterization

In preparation for metallographic analysis, coupons were polished to a mirror polish surface finish with diamond suspension. The samples were then rinsed, dried with air, then etched using an applied voltage of 10 V for 10 s in a bath of 60/40 vol% nitric acid/water solution.

Samples were imaged with optical and electron microscopy techniques in the etched condition, pre-exposed, postatmospheric exposure, and in the postatmospheric exposure with corrosion product removed conditions. Optical imaging was performed with a Keyence VHX 5000 while scanning electron microscopy (SEM) was performed with a Zeiss Supra 55-VP field emission SEM equipped with an Oxford Instruments X-Max SDD energy dispersive spectroscopy (EDS) detector. SEM was performed at a working distance of 9 mm to 12 mm at 20 kV. EDS was applied under the same operating conditions to compare pre- and postexposure coupons as well as to identify atmospheric aerosol contaminants.

Atmospherically exposed samples were imaged both with corrosion product and postcorrosion product removal. Corrosion product removal was performed by immersing samples in 1 M nitric acid for 30 min of sonication, rinsing with DI water, and drying with nitrogen. Measurements of the pit depth was performed with white light interferometry (Zygo, NexView, USA). The coherence scanning interferometry (CSI) mode was used with a combination of 5×, 20×, or 50× objective lenses (1× zoom lens), based on the size of the pits being investigated. Pit depth was determined by line profile measurements from the 3D heat map.

In a few select cases, Xe plasma-focused ion beam tool (P-FIB) and a Ga-sourced focused ion beam (FIB) tool were used to cross section a representative pit from each atmospheric exposure sample after 21 months. The exposed sample was placed in the Xe P-FIB (HyperFIB upgrade to an FEI DB235 Ga dual beam FIB, Applied Beams, LLC, Beaverton, Oregon, USA) and material containing the target pit was cut free from the surrounding material. This was accomplished with an accelerating voltage of 25 kV and a beam current of 1 μA. The sample was then transferred to a Ga sourced dual beam FIB (G2 Helios, FEI, Hillsboro, Oregon, USA) for final sample preparation. The material milled free in the Xe PFIB was lifted out using standard methods that involved attaching the sample to the micromanipulator tip (Omniprobe 200, Oxford Instruments, Ma, USA). Once attached, the sample was transferred to a standard lift out sample grid using standard lift out methods. The analysis surface was then prepared for SEM by careful milling of the sample surface using a 30 kV Ga ion beam at a variety of currents followed by a series of lower voltage (5 kV followed by 2 kV) polishes.

Atmospheric Exposure

PBF coupons were atmospherically exposed in either the abrasively (with silica grit) blasted state or ground to 120 grit with SiC grinding paper. Wrought coupons were exposed only with the 120 ground finish.

Samples were atmospherically exposed at Florida Atlantic University—SeaTech near Dania Beach, FL (Figure 1[a]) for 1 month to 21 months, with pull times of 1 month, 3 months, 9 months, and 21 months. Two samples from each condition were pulled at each time interval. The site of exposure was classified as “severe marine” with a location 115 m inland from the ocean (Figure 1[b]).35-36  Exposure racks were designed such that samples were exposed at a 30° angle, south facing, and a minimum of 30 cm above the ground according to ASTM G50-20.37  The conditions of the exposure site were characterized by wet candle as well as NOAA weather data information. Wet candle data were collected for a period of 4 y, once per month, to determine chloride deposition rates.38-39  Temperature and relative humidity were determined from the nearest NOAA weather station 72202212803 at the Boca Raton Airport.

FIGURE 1.

(a) Location and exposure racks (inset) and (b) temperature and RH (NOAA Weather Data, Station number: 72202212803) during the exposure period.

FIGURE 1.

(a) Location and exposure racks (inset) and (b) temperature and RH (NOAA Weather Data, Station number: 72202212803) during the exposure period.

Close modal

Chloride deposition from wet candle site measurements showed an average of 26±18 μg/cm2/d, with max: 80.92 and min: 4.31 μg/cm2/d. Diurnal cycle variations for temperature range from 64°F nighttime temperature to 76°F midday in December and 80°F to 92°F, respectively, in June. For RH, the diurnal cycle values range from 87% nighttime RH to 53% midday in December and 87% to 64% RH, respectively, in June.

Electrochemical Analysis

Electrochemical tests were performed using a standard three electrode cell with a Pt mesh counter electrode and a saturated calomel reference electrode (SCE, +0.24 VSHE [saturated hydrogen electrode]). Anodic potentiodynamic polarization measurements were performed to determine each sample’s breakdown potential (Eb) on a Biologic VMP300 multichannel potentio/galvanostat. Prior to the potentiodynamic measurement samples were immersed at open-circuit potential (OCP) for 1 h in a quiescent 0.6 M NaCl solution (pH ∼ 6) at 21±1°C. This solution was chosen based on the [Na]/[Cl] concentration in seawater. The potentiodynamic measurements started 20 mV below the samples’ OCP and scanned at a rate of 0.167 mV/s in the anodic direction to +0.6 VSCE or the experiment was stopped when a current density of 10−2 A/cm2 was reached.

Microstructural Analysis

Optical micrographs of the etched microstructure for wrought and PBF 304L and 316L samples are shown in Figure 2. The wrought microstructures in (a) and (b) show annealed, equiaxed grains with the dark scattered lines throughout the microstructure indicating nonmetallic inclusion stringers, likely oxide or oxide/sulfide inclusions. The etched microstructure for the PBF 304L, a face perpendicular to the build direction in Figure 2(c), shows certain areas to be more etched than others (dark bandings) which correlate to differing solidification modes. The more heavily etched (dark) regions solidify quickly as primary austenite while the less etched regions solidify somewhat slower as primary ferrite. The entire microstructure is still austenitic (FCC). The PBF 316L microstructure in Figure 2(d) is heavily etched because it maintains the primary austenite solidification pathway throughout the PBF process. Common processing defects in lack of fusion porosity can be seen on both etched PBF 304L and 316L samples. Lack of fusion porosity was more abundant in the PBF 304L material. The grit-blasted surfaces of the PBF 304L and 316L material were characterized with SEM and EDS, shown in Figure 3. The silica grit is shown to be lodged in the surface of the PBF material, a common occurrence for grit-blasted surfaces.

FIGURE 2.

Optical images of the etched microstructure of unexposed wrought (a) 304L and (b) 316L, and PBF (c) 304L and (d) 316L coupons.

FIGURE 2.

Optical images of the etched microstructure of unexposed wrought (a) 304L and (b) 316L, and PBF (c) 304L and (d) 316L coupons.

Close modal
FIGURE 3.

Example SEM BSE micrograph of the (a) grit-blasted unexposed surface of PBF 316L and (b) corresponding EDS maps displaying a grit blast particle composed of primarily oxygen and silicon.

FIGURE 3.

Example SEM BSE micrograph of the (a) grit-blasted unexposed surface of PBF 316L and (b) corresponding EDS maps displaying a grit blast particle composed of primarily oxygen and silicon.

Close modal

Electrochemical Measurements

OCP and anodic potentiodynamic measurements performed in quiescent 0.6 M NaCl on wrought and PBF SSs are shown in Figure 4 for all conditions. The OCPs for the grit-blasted AM samples show small fluctuations in potential, in comparison to the generally stable OCP response for the ground surfaces. The lessened stability of the grit-blasted surfaces’ OCP could be from increased roughness/tortuosity, shown in Figure 3, compared to the smoother ground surfaces and possibly more defective coverage of the passive oxide film. Regardless, the OCP for all samples remained in the range of −0.25 VSCE to −0.1 VSCE for the 304L samples and a range of −0.2 VSCE to −0.05 VSCE for the 316L samples at the end of the 1 h immersion.

FIGURE 4.

One hour open-circuit potential (OCP) (a) 304L and (b) 316L followed by anodic potentiodynamic polarization (c) 304L and (d) 316L in quiescent 0.6 M NaCl.

FIGURE 4.

One hour open-circuit potential (OCP) (a) 304L and (b) 316L followed by anodic potentiodynamic polarization (c) 304L and (d) 316L in quiescent 0.6 M NaCl.

Close modal

The potentiodynamic measurements in Figures 4(c) and (d) exhibited similar responses for the 304L and 316L materials. The grit-blasted PBF samples (in red) showed no passive regime, for both 304L and 316L, unlike the PBF and wrought ground surfaces. The ground wrought 304L and 316L samples show a passive window, both yielding stable pitting to start at 0.42 VSCE and 0.37 VSCE, respectively, and showing numerous instances of metastable pitting events throughout the passive window. These metastable pitting events are suspected to be caused by the nucleation of pits at the larger nonmetallic inclusions present in the microstructure, seen as the “stringers” in the etched microstructure from Figures 2(a) and (b).40-42  For the ground PBF SS samples, the breakdown potential was similar to wrought, PBF 304L (0.29 VSCE) and PBF 316L (0.36 VSCE), but the lack of metastable pitting events can be attributed to the smaller size of the non-metallic inclusions.6  At least two measurements were taken for each sample type.

Atmospheric Exposure

The exposure of wrought and PBF SS coupons 115 m from the coastline of Florida resulted in significant localized corrosion for all conditions. The sea salt deposited on the surface of a ground and grit-blasted 304L sample is shown in Figure 5 after the 1 month long exposures, prior to corrosion product removal. Salt deposits on the ground surface, Figures 5(a) and (c), showing three distinct NaCl crystals, on the order of 40 μm in diameter. Other salts deposited on the surface contain sulfur/oxygen particles with smaller sizes. For the grit-blasted surface, there is more coverage of the salts on the surface, shown in Figure 5(b), with less distinct salt particles but the salts show a similar composition to that observed on the ground surfaces, shown in Figure 5(d). The difference in distribution and possibly in total salt captured on the ground vs. the grit-blasted surfaces could lead to a difference in the area of surface attacked by corrosion. The salt distribution on the ground surface in Figures 5(a) and (c) would likely support the formation of individual droplets whereas the grit-blasted surface could result in a thin film/wider spreading of the electrolyte and could result in a larger cathode area causing a higher density of pits.

FIGURE 5.

SEM backscatter electron micrographs of the exposed surfaces of PBF 304L coupons post 1 month exposure of (a) ground- and (b) grit-blasted coupons. EDS maps of surface contamination and corrosion product of PBF 304L (c) ground- and (d) grit-blasted coupons.

FIGURE 5.

SEM backscatter electron micrographs of the exposed surfaces of PBF 304L coupons post 1 month exposure of (a) ground- and (b) grit-blasted coupons. EDS maps of surface contamination and corrosion product of PBF 304L (c) ground- and (d) grit-blasted coupons.

Close modal

Optical images were taken after 0, 1 month, 3 month, 9 month, and 21 month exposure times for the 304L (Figure 6) and 316L (Figure 7) coupons. The wrought 304L and 316L coupons show general discoloration of the entire surface after 1 month of exposure, while the PBF ground surfaces show less of this general discoloration. The wrought sample also shows rust showing up at the edge of the samples, starting after 1 month and increasing in intensity at longer exposure times. Corrosion at the edges was also seen on the ground PBF coupons, likely due to either enhanced salt/electrolyte build up or potential crevicing at points of contact with the exposure setup. The grit-blasted coupons of both PBF 304L and 316L experience similar discoloration to the wrought coupons after 1 month in addition to darker brown spots indicating intense localized corrosion. At longer exposure times the brown discoloration becomes more intense.

FIGURE 6.

Optical images of 304L wrought and PBF (ground- and grit-blasted) coupons unexposed and exposed on the coast of Florida vs. time.

FIGURE 6.

Optical images of 304L wrought and PBF (ground- and grit-blasted) coupons unexposed and exposed on the coast of Florida vs. time.

Close modal
FIGURE 7.

Optical images of 316L wrought and PBF (ground- and grit-blasted) coupons unexposed and exposed on the coast of Florida vs. time.

FIGURE 7.

Optical images of 316L wrought and PBF (ground- and grit-blasted) coupons unexposed and exposed on the coast of Florida vs. time.

Close modal

Some of the discoloration may be attributed to the variety of salts that can be deposited on the surface of these parts during the exposure, which may be more easily captured on the grit-blasted surface, as seen in Figure 5. Color-based analysis of corroded steel parts have proven valuable in assessing the severity of corrosion that has occurred, largely based on the brown to red hue of rust indicating the severity.43  Similar dark spots begin to show up on the ground wrought coupons after 1 month and after 3 months these spots show up on the PBF coupons. The darker spots typically coincide with local pitting corrosion.

To quantify the local corrosion damage which occurred on the four ground sample conditions, the salts and corrosion product were removed from these samples. The grit-blasted samples were not extensively investigated in this way because the surface roughness made it difficult to observe distinct pits reliably. White light interferometry was used to measure pit depth, shown in Figure 8, across these samples with the number of pits (n) > 2 for the 316L coupons and n > 10 for 304L coupons across all exposure times. The roughness from the 120 grit surface finish made distinguishing pits from grinding marks difficult at early times, mostly for 316L coupons which did not experience substantial local corrosion until longer exposure times, > 1 months.

FIGURE 8.

(a) Average pit depth and (b) maximum pit depth with respect to time for the four ground coupon conditions. Measured with white light interferometry, error bars in (a) are one standard deviation.

FIGURE 8.

(a) Average pit depth and (b) maximum pit depth with respect to time for the four ground coupon conditions. Measured with white light interferometry, error bars in (a) are one standard deviation.

Close modal

At all times the PBF coupons exhibited lower average and maximum pit depths than their wrought counterparts, shown in Figures 8(a) and (b), respectively. At early exposure times, 9 months and earlier, the PBF 304L coupon pit depths were statistically smaller (two tailed t-test p value < 0.03) than their wrought counterpart. This changed after 21 months showing near identical average pit depths for the PBF and wrought 304L coupons. The 316L coupons showed the opposite trend, with statistically similar (two-tailed t-test p-value = 0.25) average pit depths after 1 month, likely owing to the limited number of detected pits nucleated on these samples. At longer times the average pit depths on the PBF 316L coupon were statistically smaller (p value < 0.005) than their wrought counterpart.

The distribution of pits across a single coupon was generally scattered for all coupons from 9 months and before, never measuring more than 15 pitted locations on a single coupon, typically spaced several hundred micrometers from one another. This trend of individual pits held for the 316L coupons at 21 months, rarely seeing clusters of pits. For the 304L coupons, clustering of pits was more common after 21 months of exposure, in particular for the PBF 304L coupon. The PBF 304L coupon had numerous clusters (10 or more pits) of mostly shallow (<5 μm deep) pits after the 21 months of exposure. The wrought 304L coupon only had a few instances where several pits were in close (∼50 μm) proximity to one another, but never to the extent of that observed on the PBF 304L coupon. The abundance of pits on the PBF 304L coupon suggests a large increase in susceptibility to pit nucleation even though its average and maximum pit depths are similar to wrought. This was unexpected given the noticeable optical difference, less apparent corrosion on the PBF 304L, from Figure 6.5 

The typical pit morphology found on each coupon, shown in Figure 9, gives evidence of the microstructure controlling pit propagation pathways with some form of preferential etching occurring in all pits. Pits on the wrought samples tended to be ellipsoidal and faceted with some pits, like those shown in Figures 9(a) and (b), to have selective attack in and surrounding the polished pitted region. This type of crystallographic attack has been observed by others during the atmospheric corrosion of austenitic SSs, suggested to be attack along slip bands and dependent on relative humidity.44 

FIGURE 9.

SEM secondary electron micrographs of the exposed surfaces of ground coupons post 21 month exposure after corrosion product removal of the wrought (a) 304L and (b) 316L compared to the PBF (c) 304L and (d) 316L coupons. Higher magnification images of the (e) PBF 304L and (f) PBF 316L surfaces are also shown.

FIGURE 9.

SEM secondary electron micrographs of the exposed surfaces of ground coupons post 21 month exposure after corrosion product removal of the wrought (a) 304L and (b) 316L compared to the PBF (c) 304L and (d) 316L coupons. Higher magnification images of the (e) PBF 304L and (f) PBF 316L surfaces are also shown.

Close modal

Crystallographic attack was not observed on the pits formed on PBF 304L and 316L coupons, shown in Figures 9(c) through (f). The pits on PBF 304L coupons were generally hemispherical in shape but not polished, showing some roughness in Figure 9(e). This roughness did not appear to correlate to any particular microstructural feature from the PBF 304L material. Pits on the PBF 316L coupons were not hemispherical, typically irregularly shaped like the pit in Figure 9(d). At higher magnification preferential etching of the cellular structure common in PBF 316L material is shown in Figure 9(f), along with some preferential etching at what are suspected to be melt pool boundaries. This melt pool boundary attack was also shown to occur in the largest pit in Figure 9(c) for the PBF 304L coupon. These boundaries were observed at the bottom of multiple pits on the PBF 304L and 316L samples, identified by the dark contrast in the images, suggesting preferential etching, and the curvature of the etched lines which is typical for these boundaries, in addition to solute (Mo) depletion as shown by Godec, et al.45  Melt pool boundary attack has been shown by others, such as the work by Macatangay, et al., however the solutions typically used to elicit this response are highly aggressive and oxidizing.46  The preferential attack of melt pool boundaries in a severe marine environment suggests mitigation strategies may need to be taken (e.g., heat treatments) to prevent unexpected degradation at these boundaries.

Cross sectioning of a pit on the wrought and PBF 316L coupons revealed the jagged pitted morphology resulting from crystallographic attack on the wrought pits and surface undercutting along with etching of the cellular structure for the PBF pits, shown in Figure 10. The preferential etching of the cellular structure, clearly shown in Figures 10(c) and (e), for the PBF 316L pit was 1 µm to 2 μm deep across most the pits surface. This preferential etching could be a result of two effects: the enrichment of the cellular boundaries with Cr/Mo (common for 316L) resulting in a more protective passive layer than the inner cell structure itself or the Cr/Mo enrichment causing a microgalvanic couple between the cell boundaries (cathode) and the cell (anode).29  Figure 11 shows a comparison between the electrolytically etched PBF 316L microstructure where the cell boundaries were preferentially attacked, in comparison to a pit from the PBF 316L sample after the 21 month exposure, showing the inverse attack, the cell body was preferentially attacked with many of the boundaries still intact. The cells for the etched sample in Figure 11(a) were approximately 500 nm in diameter, similar to the cell boundary spacing in the pit from Figure 11(b). Additionally, the propagation of the PBF 316L coupons pit to a volume below the surface of the coupon, shown on the right side of Figure 10(e), was not observed for any other cross-sectioned pits. The pit from the wrought coupon did not show this undercutting of the surface, in Figure 10(d).

FIGURE 10.

SEM secondary electron micrographs of the (a) wrought and (b) PBF 316L coupons post 21 month exposure after corrosion product removal showing a representative pit. Detailed SEM secondary electron micrograph of the (c) preferential etching of the cellular structure for the PBF 316L FIBed pit. After plasma FIB cross-sectioning, secondary electron micrographs of the (d) wrought and (e) AM cross-sectioned pits.

FIGURE 10.

SEM secondary electron micrographs of the (a) wrought and (b) PBF 316L coupons post 21 month exposure after corrosion product removal showing a representative pit. Detailed SEM secondary electron micrograph of the (c) preferential etching of the cellular structure for the PBF 316L FIBed pit. After plasma FIB cross-sectioning, secondary electron micrographs of the (d) wrought and (e) AM cross-sectioned pits.

Close modal
FIGURE 11.

SEM secondary electron micrographs of the (a) polished and etched PBF 316L microstructure, electrolytically in 60/40 vol% nitric acid/water solution and (b) a high-magnification image of a PBF 316L pit after the 21 month exposure.

FIGURE 11.

SEM secondary electron micrographs of the (a) polished and etched PBF 316L microstructure, electrolytically in 60/40 vol% nitric acid/water solution and (b) a high-magnification image of a PBF 316L pit after the 21 month exposure.

Close modal

Implications

From a broader implication’s perspective, surface finish (grit-blasted vs. ground-) was observed to play a role in deposition of aerosols (clustered vs. even distribution), thus may have influenced the subsequent corrosion response. The use of optical identification for localized corrosion may not be the best indication of pitting and/or localized corrosion as shown from profilometry results. Additionally, AM austenitic SSs exhibited significant differences in localized corrosion morphologies compared to wrought counterparts and may exhibit better than or equivalent resistance to localized corrosion. No crystallographic attack was observed in AM materials where cellular and melt pool boundary etching was observed along with undercutting in the pit. These geometrical differences could play significant roles in continued pit propagation and material susceptibility to other forms of corrosion damage such as stress corrosion cracking. However, as AM materials exhibited pits with smaller depths in general, they may be less susceptible overall to cracking.

  • In general, the PBF SS specimens were less susceptible to atmospheric pitting corrosion than the wrought SS specimens, showing shallower pits and less crystallographic attack in a severe marine environment on the eastern coast of Florida. Coupons were exposed for 21 months and resulted in significant pitting corrosion to initiate after 1 month of exposure for all conditions. The grit-blasted coupons of both PBF 304L and 316L experience the most discoloration, correlating to intense localized corrosion after just 1 month of exposure. At longer exposure times the brown discoloration becomes more intense.

  • At all times, the PBF coupons exhibited lower average and maximum pit depths than their wrought counterparts. The distribution of pits across a single coupon were generally scattered (>100 μm spacing) for all coupons exposed for 1 to 9 months. After 21 months the PBF 304L coupons showed numerous clusters (10 or more pits) of mostly shallow (<5 μm deep) pits. These clusters of pits were not found on any 316L coupons and only one such cluster was found on the wrought 304L coupon.

  • Pits on the wrought samples tended to be nearly hemispherical and polished with some pits showing crystallographic attack in and surrounding the polished pitted region. Pits on the PBF 316L coupons were not hemispherical and showed preferential etching of the cellular structure. Melt pool boundary attack was observed in the pits on both PBF 304L and 316L coupons. Cross sectioning of pits on the 316L coupons revealed jagged pitted morphology resulting from crystallographic attack of slip bands on the wrought pits while the PBF pits showed surface undercutting along with severe etching of the dislocation cell structure. The impact a difference in pit morphology may have on material integrity is an active area of investigation.

Trade name.

The authors wish to thank S. Dickens, B. McKenzie, L. J. Jauregui, D. L. Perry, and J. R. Michael for materials preparation and characterization. This study was supported by the Department of Defense and Department of Energy Joint Munitions Program along with DOE funding from the ACT program and the Aging and Lifetime program. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

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