Corrosion behavior of additively manufactured 316L ... · Due to the presence of non-homogeneous (chromium depleted re- gions) passive oxide ﬁlm and instability of iron oxide ﬁlm,

ARTICLE IN PRESS JID: MTLA [m5GeSdc; July 25, 2018;9:15 ]

Materialia 000 (2018) 1–11

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Materialia

journal homepage: www.elsevier.com/locate/mtla

Full Length Article

Corrosion behavior of additively manufactured 316L stainless steel in

acidic media

M.J.K. Lodhi a , K.M. Deen b , Waseem Haider a , c , ∗

a School of Engineering and Technology, Central Michigan University, Mt. Pleasant, MI, USA b Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada c Science of Advanced Materials, Central Michigan University, Mt. Pleasant, MI, USA

a r t i c l e i n f o

Keywords:

Additive manufactured 316L

Microstructure

Passive film

Acidic environment

Corrosion

a b s t r a c t

Recently, additive manufacturing has got tremendous attention due to ease in the production of complex metal-

lic parts for different applications i.e. aerospace, petrochemical etc. However, there is a scarcity of literature,

addressing the corrosion behavior of additive manufactured (AM) alloys. This study presents, the chemical com-

position and corrosion response of the passive oxide film formed on the AM 316L stainless steel in acidic regime

(pH ≤ 3) and its comparison to wrought counterpart, by applying X-ray photoelectron spectroscopy (XPS) and

electrochemical analysis, respectively. Microstructural characterization of AM specimen revealed the presence of

nanometer-ranged ripples type sub-granular structure confined within the macro grains. XPS analysis indicated

the formation of mono layered and bi layered passive oxide film in pH 1 and 3 electrolytes, respectively. In-

terestingly, higher charge transfer resistance (50 times) and significantly decreased corrosion current density (2

order of magnitude) in aggressively acidic solution (pH 1) has been observed by AM specimens compared to con-

ventional wrought 316L stainless steel. The higher corrosion resistance has been attributed to the development

of fine sub-granular structure, which most likely regulates the stability of the passive oxide film and the raid

solidification rate (approx. 10 7 K/s) involved in the additive manufacturing process rationalizing the reduction

of MnS inclusions. In comparison, a significantly higher corrosion resistance by the AM 316L stainless steel in

highly acidic environment (pH ≤ 3) has been recoded, surpassing the conventional wrought material.

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. Introduction

The improvement in corrosion resistance and mechanical strength

f stainless steel are always desirable for the components in many ap-

lications including but not limited to chemical processing, pressurized

ater reactors and petrochemical industries [1–4] . The corrosion resis-

ant property of stainless steel is attributed to the stability of passive

lm formed on its surface. The structure and nature of the passive film

as been studied rigorously using different in-situ and ex-situ techniques

nd there is a consensus on the formation of duplex structure, constitut-

ng oxides and hydroxides of chromium and iron [5–12] . The outer layer

f passive film is reported as to be of iron oxide and the inner layer is

omposed of chromium oxide due to lower mobility, resulting from re-

uced diffusion property of chromium compared to iron [13,14] . The

ature and growth of the passive film formed on stainless steel is con-

rolled by the chemical composition of an alloy and its stability depends

n the pH of the aqueous environment [15,16] . It was found that with

n increase in pH, thick and resistive passive film is formed on the Fe –Cr

lloy, containing outer iron oxide layer which is more stable in the alka-

∗ Corresponding author.

E-mail addresses: [email protected] , [email protected] (W. Haider).

d

ttps://doi.org/10.1016/j.mtla.2018.06.015

eceived 16 May 2018; Received in revised form 27 June 2018; Accepted 28 June 20

vailable online xxx

589-1529/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Please cite this article as: M.J.K. Lodhi et al., Materialia (2018), https://doi

ine solutions. On the other hand, the stability of chromium oxide film is

uch higher than iron oxide in acidic electrolytes [17,18] . Furthermore,

he structure of passive film formed on stainless steel has heterogeneities

nd defective structure that facilitates localized corrosion attack under

hese conditions [19–21] . Second phase particles e.g., sulfide inclusions

resent in the stainless steel could induce heterogeneities in the passive

lm and the chromium depleted regions in the vicinity of these inclu-

ions are considered as the triggering points for localized dissolution

22–24] .

Due to the presence of non-homogeneous (chromium depleted re-

ions) passive oxide film and instability of iron oxide film, resulting in

oor corrosion resistant properties, the application of stainless steel is

imited in highly acidic conditions. Various reports in the literature sug-

ests that by laser surface melting and producing nano structured grain

an significantly enhance the corrosion resistance of stainless steel due

o the formation of compact chromium enriched passive film [25–29] .

here are many processes reported in the literature for grain refine-

ent of stainless steel [30–33] . The grain refinement can enhance the

rowth of passive film which is possibly attributed to the decrease in

iffusion length for the chromium towards the formation of chromium

18

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.org/10.1016/j.mtla.2018.06.015

https://doi.org/10.1016/j.mtla.2018.06.015http://www.ScienceDirect.comhttp://www.elsevier.com/locate/mtlamailto:[email protected]:[email protected]://doi.org/10.1016/j.mtla.2018.06.015https://doi.org/10.1016/j.mtla.2018.06.015

M.J.K. Lodhi et al. Materialia 000 (2018) 1–11


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Table 1

Chemical composition (in wt%) of wrought and AM 316L stainless steel.

Elements C Mn Mo S P O Ni Cr Fe

Wrought 0.03 2.3 2.1 0.03 0.06 0.04 11.5 17.1 Bal

AM 0.03 2.2 2.3 0.03 0.05 0.05 11.2 17.5 Bal

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nriched surface film [34,35] . Fine microstructure due to rapid inter-

ace boundary diffusion process leads to the formation of more compact

nd uniform passive film [36] . Carboni et al. showed the enhanced re-

istance to localized corrosion by solutionizing the MnS inclusions by

aser surface melting and rapid quenching process [29] . The formation

f homogeneous and more tenacious passive film on 316L stainless steel

ould enhance its electrochemical properties in the aggressive environ-

ent [37] .

Additive manufacturing (hereafter abbreviated as AM) is a net shape

roduction method to fabricate 3-dimensional objects by digitally con-

rolling the deposition of layers to produce the final product. The prop-

rties associated with AM objects are dependent on the microstructure,

hich consequently depends on the process parameters [38,39] . The

apid cooling (approx. 10 7 K/s) of AM alloys could avoid the crystal

rowth [40] . This higher cooling rate associated to AM process decreases

ime for the nucleation of MnS inclusions in stainless steel, confirming to

he homogeneous distribution of alloying elements in the metal matrix,

voiding the formation of chromium depleted regions [41] . The similar

eason for the development of fine grain structure in the AM 316L stain-

ess steel in comparison to the commercially available wrought stainless

teel has also been discussed in the literature [42] . The dual triumph of

voiding the favorable conditions for the nucleation of MnS inclusions

nd development of refined sub-granular structure in AM 316L stain-

ess steel can hypothesize its enhanced resistance to corrosion in acidic

nvironment.

There have been a limited number of studies in the literature, report-

ng the corrosion behavior of AM 316L stainless steel [41,43,44] . This

tudy is elucidating the preliminary results about the corrosion response

f AM 316L stainless steel in chloride containing acidic solutions for the

ery first time. A comparison was also made to validate the results with

espect to commercially available wrought 316L stainless steel under

ame conditions.

. Experimental

The austenitic 316L stainless steel specimens prepared from addi-

ive manufacturing route was used in this study and its comparison was

ade with the commercially available wrought counterpart. For the AM

pecimens, the alloy powder was obtained from REINSHAW, US having

particle size in the range of 15–50 μm. AM–250 unit was used to man-

facture the specimens. A 200-W continuous ytterbium beam operating

t a wavelength of 1060 nm with a hatch distance of 100 μm was used.

igh power beam fused the powder granules and formed layer over the

ther with a thickness of 30 μm. For comparison, the commercially avail-

ble wrought 316L stainless steel was obtained from OnlineMetals®.

he circular disk shape specimens (both wrought and AM) of 1.5 cm

iameter and 0.5 cm thickness were cut and used for analysis. The spec-

mens were prepared by grinding them sequentially using silicon carbide

apers of 180–1200 grit size under running water and cleaned in ace-

one via ultra-sonication for 15 min. These specimens were washed in

I water and dried for further use.

To evaluate the compositional variations, the EDX analysis was car-

ied out by using Hitachi S-3400-II scanning electron microscope. X-

ay diffraction (XRD) patterns were obtained to identify the constituent

hases by using Cu K 𝛼 radiation ( 𝜆 = 0.154 nm) source in the RigakuiniFlexII diffractometer within 2 𝜃 ranging from 30

0 to 90

0 .

Specimens for microstructural analysis were ground sequentially

rom 180 to 1200 grit size by using SiC grinding papers followed by

olishing on the cloth impregnated with 1 μm diamond suspension to

roduce mirror like surface finish. To reveal the microstructural fea-

ures, the polished surfaces were etched for 40 s in 15 ml HCl + 10 mlNO 3 + 15 ml Acetic acid solution containing 5 drops of glycerols given in ASTM-E407. The microstructural analysis of the AM and

rought specimens was carried out in scanning electron microscope (Hi-

achi S-3400-II SEM).

2

X-ray photoelectron spectroscopy (XPS) was used to analyze the sur-

ace composition and to estimate the thickness of the passive film which

as intentionally formed on the wrought and AM specimens by immers-

ng in DI water of pH 1 and 3 for 24 hours. The XPS analysis was carried

ut by Kratos Axis ultra-photoelectron spectroscope using a monochro-

atic aluminum excitation source with 10 mA and 15 kV to produce

-rays in a chamber of 1 × 10 − 9 torr. XPS spectra were calibrated bytandardizing C1s peak at 284.8 eV. To analyze the chemical composi-

ion of the passive oxide film formed on the surface, XPS depth profil-

ng was done for 1200 s etching time. The XPS signals were processed

n CASAXPS software to evaluate the constituent species in the surface

assive film.

Electrochemical response of the specimens was measured by using

amry Reference 1000E potentiostat. The three electrodes cell config-

ration was employed to investigate the electrochemical behavior of

rought and AM specimens. The polished specimens (both wrought and

M) with exposed surface area of 1.27 cm 2 served as a working elec-

rode in this cell. The saturated calomel electrode (SCE) ( + 0.242 V vs.HE) and platinum coil were used as reference and counter electrodes,

espectively. The electrochemical testing was carried out in deionized

ater with and without containing Cl − ions (400 ppm) at different pH

1, 2 and 3). The pH of the electrolyte was adjusted within the tolerance

f ± 0.1 by the addition of sulfuric acid and the desired Cl – concentra-ion was maintained by adding calculated amount of sodium chloride.

he test solutions were degassed by bubbling N 2 for 30 minutes prior to

ach test and the specimens were immersed in the electrolyte immedi-

tely after preparation.

The open circuit potential (OCP) of each specimen was measured

or 5 h to achieve the stability (0.1 mV/min). The electrochemical

mpedance spectra were obtained after OCP stabilization and 5 mV AC

otential perturbation was applied within 100 kHz to 10 mHz frequency

ange. To investigate the corrosion behavior and barrier properties of

he passive film, the cyclic polarization (CP) test was also conducted.

P scans were executed at a rate of 1 mV/s from − 0.5 to 0.9 V vs. OCP.he scan was reversed after 0.9 V vs. OCP to evaluate the pitting behav-

or of the specimens. All the electrochemical testing was performed at

oom temperature.

. Results and discussion

.1. Microstructural analysis

To verify the chemical composition of the AM and wrought speci-

ens, the energy dispersive X-ray spectroscopy (EDX) and X-ray diffrac-

ion (XRD) analyses were carried out. In order to determine the ele-

ental composition, the EDX results on the polished surface of each

pecimen are given in Table 1 . The EDX results verified that chemical

omposition of the wrought and AM specimens was comparable with

light variations. However, overall compositional analysis emulated to

he ASTM 316L grade.

Fig. 1 shows the XRD patterns of wrought and AM 316L stainless

teel specimens. The similar diffraction peaks at 43.8 0 , 50.9 0 and 74.8 0

ere evident in both specimens, originating from the (111), (200) and

220) planes, respectively. In comparison, the intensities of the diffrac-

ion peaks associated with the (111) and (200) of AM specimen were

ecreased and slightly broadened, indicating the formation of fine grain

tructure. These characteristic peaks also confirmed the formation of

ure austenitic phase according to standard reference pattern JCPDS



Fig. 1. X-ray diffractogram of AM and wrought 316L stainless steel.

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1-0619 [42] . No additional peaks were observed in the XRD patterns

uggesting the purity of the stainless steel specimens.

Microstructural characterization of wrought and AM 316L stain-

ess steel using scanning electron microscopy (SEM) is illustrated in

ig. 2 . In case of wrought specimen ( Fig. 2 (a)), typical faceted morphol-

gy was revealed. The average grain size within the microstructure of

rought specimen was found to be ∼35 μm. The twin bands within thequiaxed grains structure indicated the mechanically deformed struc-

ure as shown in Fig. 2 (a), which is most likely associated with the prior

anufacturing history.

Compared to wrought, the AM specimen represented a very hetero-

eneous grain structure as illustrated in Fig. 2 (b). A very broad range of

ub-micron to nano meter ranged sub-granular structures was found to

e distributed in AM specimen. The interconnected polygonal shape fine

ub-grain structure (as shown in the inset of Fig. 2 (b)) was distributed

n small colonies without exhibiting any sharp boundaries. At the inter-

ection of two melt pools, the relatively coarser sub-grains structure was

lso evident, indicating the re-melting of the previously solidified ma-

erial in the precedent overlay cycle. The effect of directional heat flow

uring re-melting process was also revealed as delineated grain struc-

ure across the boundaries. In additive manufacturing, the laser beam

s targeted at the small segment to selectively melt the powder speci-

en. The intense temperature of the laser beam and large heat input

uring multiple overlays greatly modified the microstructure, which is

irectly related with the mechanical and electrochemical performance

f the final object. The non-equilibrium conditions prevail during laser

e-melting process, restricted the grain growth [45] . In the successive

elt pools and due to their instantaneous cooling, fine dendritic and

olumnar complex structure may also form.

Different orientation (cellular and columnar) of sub-grains for AM

pecimen observed in the SEM image was due to the preferred grain

rowth in the direction of heat flow. Fig. 2 (b) (inset) presents the higher

agnification image of the structure reveled in the AM specimen. The

ne structure revealed in the AM specimen was associated with the uni-

orm mixing of the molten pool and subsequent rapid cooling rate. The

orphologically similar microstructure of 316L stainless steel produced

ia additive manufacturing route has also been reported in the literature

42,46] .

.2. Surface analysis of the oxide film

X-ray photoelectron spectroscopy (XPS) analysis was carried out to

xamine the chemical composition of the passive oxide film developed

n AM and wrought 316L stainless steel after immersion in the acidic

lectrolytes of pH 1 and 3. Survey spectra of the AM and wrought spec-

mens after 24 h of immersion are shown in Fig. 3 .

3

The presence of Cr, Fe, Mn, Ni, O and Mo on the surface is evident

rom the Fig. 3 . Qualitatively, no change in the surface chemistry of the

pecimens was observed from the survey spectra.

The high resolution spectra depicting the chemical state of the pri-

ary constituents (oxygen, chromium and iron) in the passive film are

resented in Fig. 4 . After background subtraction by Shirley method, the

pectra were de-convoluted by using Gaussian function to differentiate

he chemical species. The de-convolution of the O 1s peak indicated the

resence of O 2– and OH – species originating at 530.2 eV and 531.9 eV,

espectively [47] . Compared to the peak for O 2– species, the board peak

ssociated with the OH − species can be used to predict the dominancy of

etallic hydroxide layer on the surface for both wrought and AM spec-

mens. XPS analysis being performed after immersion in water based

lectrolytes clearly justifies the existence of hydroxide layer on the sur-

ace of these specimens. However, after exposure to electrolyte of pH 3,

he intensity of collar peak associated with O 2– species was increased,

hich suggested the higher probability of oxide formation, under these

onditions. Similarly, the chromium spectra dissociated into two peaks

t 574.2 eV and 577.1 eV attributing to the presence of Cr 0 and Cr 3 +

pecies, respectively in the passive film [47,48] . The presence of Cr 3 +

eak along with small shoulder peak affiliated with the Cr 0 was evident

n all specimens except wrought 316L at pH 1. This could be related

ith the sole contribution of the Cr 3 + –OH – species in the development

f passive film on the wrought 316L stainless steel in the highly acidic

onditions. In electrolyte (pH 3), the presence of both Cr 0 and Cr 3 +

eaks corresponded to the existence of both Cr 2 O 3 and Cr(OH) 3 phases

n the passive oxide film. The deconvolution of the Fe 2p 3/2 high res-

lution spectra revealed the presence of Fe 0 and Fe + 3 species centered

t 706.5 eV and 710.5–711.5 eV, respectively, on the binding energy

cale. However, as reported in literature, it is difficult to identify the na-

ure of oxides (Fe 2 O 3 and/or Fe 3 O 4 ) from the Fe 3 + signals, which may

ary from 710.5 to 711.5 eV [14,49] . Observing the passive film formed

n the wrought specimen at pH 1, there was no indication of metallic

ron, which could be attributed to the dominancy of Fe 3 + species either

n the form of oxides or hydroxides in the passive film. However, the

ominancy for the OH – signals suggested the presence of hydrated iron

xides in the surface film.

The XPS depth profile results of both wrought and AM specimens

xposed to electrolyte of pH 1 and 3 are also shown in Fig. 5 . It was evi-

ent that the relatively high concentration of oxygen in the surface film

as mainly associated with the oxidation of alloying elements, e.g. iron,

hromium and nickel, when exposed to aqueous environment indepen-

ent to the microstructural features. Seo and Sato [50] demonstrated

hat the extrapolation of oxygen curve parallel to the sputtering time

abscissa) gives an estimation of the film substrate interface. As shown

n Fig. 5 (a) and (b), the oxygen (O) contents in the surface film formed

t pH 1, decreased rapidly and became almost constant after 600 s etch

ime. On the other hand, the “O ” concentration in the surface film on

oth wrought and AM specimens at pH 3 decreased gradually (up to

725 s etch time) before reaching to constant value. From these re-

ults, it can be estimated that the oxide film formed on both specimens

n pH 3 electrolyte was a little bit thicker than the film formed in pH

electrolyte. However, the distribution of other alloying elements (e.g.

r, Fe and Ni) on the surface of both wrought and AM specimens was

lmost similar independent to the nature of the electrolytes.

Moreover, to differentiate the influence of electrolyte pH on the com-

osition of the cationic species (i.e. Cr, Fe and Ni), the fraction of each

lement as a function of etch time is shown in Fig. 6 . It was evaluated

hat the specimens immersed in the highly acidic electrolyte (pH 1),

he top surface was enriched with chromium. The chromium fraction

ropped form ∼0.6 to ∼0.2 in almost 100 s in contrast to Fe fraction,hich increased from ∼0.3 to 0.65. However, the composition of the Nias small and remained almost constant within the surface film. On the

ther hand, the specimens (both the wrought and AM) exposed to pH 3

lectrolyte revealed the high concentration of Fe in the surface film ( Fig.

(c)–(d)). From these results, it is predicted that the pH has significant



Fig. 2. SEM images of chemically etched surface of 316L stainless steel (a) wrought, (b) AM.

Fig. 3. XPS survey spectra of AM and wrought 316L stainless steel after 24 h immersion in the electrolyte of pH 1 and 3.

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nfluence on the structure of oxide film. The oxide film formed on either

rought or AM stainless steel when exposed to pH 3 electrolyte was a

ixture of both Cr and Fe oxides/hydroxides. In highly acidic electrolyte

pH 1) the top surface of the film was mainly composed of Cr enriched

xides/hydroxide phases, which was found to be independent to the

rain structure. The literature well supported the findings and also sug-

ests the formation of duplex structure oxide film on the 316L stainless

teel when exposed to high pH aqueous media [13,14] . In contrast to

he relatively more stability of the Fe oxide at high pH, the oxide film

s mainly composed of Cr oxide/hydroxide at low pH. The diffusion of

xygen within the surface and migration of metal atom toward the outer

urface is always limited and depends on the environmental conditions

51] . Due to the limited stability of iron and its oxides at low pH ≤ 2,

mono layer of Cr enriched oxide/hydroxide film is expected. The XPS

nalysis of both specimens at pH 1 and pH 3 confirmed the existence of

hromium enriched and duplexed (Fe and Cr oxides) nature of surface

lms, respectively [52,53] . To summarize, at low pH, the surface film

as mainly composed of chromium enriched outer layer compared to

he iron oxide phase which may co-exist at relatively high pH. Zheng et

l. [32] also reported that the chemistry of the passive film formed on

ustenitic stainless steel with refined grain structure remains similar to

he wrought specimen when exposed to 0.5 M H 2 SO 4 .

.3. Electrochemical characterization; effect of pH and Cl −

The electrochemical impedance spectra (EIS) of both wrought and

M specimens were obtained in the electrolytes (DI (pH = 1, 2, 3))ithout and with the presence of 400 ppm Cl − concentration as shown

n Fig. 7 (a)–(d). The chloride ions were added to the electrolyte to

4

bserve the behavior of both stainless steel specimens in highly acidic

onditions and in the presence of Cl – (400 ppm). Supplementary figure

S1) showed the variation in the R ct of the specimen as a function of Cl –

oncentration and pH. An overall decreasing trend was observed with

he addition of Cl – (up to 400 ppm). In this study, we only selected the

00 ppm Cl – concentration to investigate the stability of the passive ox-

de film in highly acidic conditions and the results are further discussed

n detail. The electrochemical behavior of both wrought and AM speci-

ens and interfacial characteristics of the passive film with electrolyte

ere quantitatively estimated by simulating and fitting the experimen-

al impedance spectra with the equivalent electrical circuit (EEC) model

s shown in Fig. 7 (e).

Both specimens in different electrolytes showed identical feature for

ll Nyquist plots ( Fig. 7 (a)–(d)), i.e, the presence of one depressed semi-

ircle, over the whole frequency range. The larger diameter is indica-

ive of higher impedance response and that represents higher resistance

o corrosion phenomenon. The EEC model represented the electrolyte

esistance ( R s ) coupled in series with the single time constant (corre-

ponded to the parallel combination of double layer capacitance) and

harge transfer resistance ( R ct ). The capacitance is presented as con-

tant phase element ( Y dl ) as evaluated from the depressed semi-circle

Nyquist plot) and phase shift < 90° (in the Bode plot, not shown here)

nd can be quantitatively predicted from the phase coefficient n < 1.

his effect may be associated with the non-uniform surficial distribu-

ion of charge within the electrical double layer and was attributed to

he non-faradaic response of the surface. On the other hand, the charge

ispersed in the electrical double layer was associated with the dielectric

haracteristics of the oxide film. The extent of faradaic reactions (dis-

olution tendency) can be estimated from the R ct element in the EEC,



Fig. 4. High resolution spectra of O 1s, Cr 2p 3/2 and Fe 2p 3/2 species for AM and wrought 316L stainless steel after immersion in electrolytes at pH 1 and 3 (a) AM

(pH = 1), (b) wrought (pH = 1), (c) AM (pH = 3), (d) wrought (pH = 3).

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hich was inversely related with the kinetic response of the surface. The

uantitative information of the EEC parameters is given in Table 2 .

As given in the Table 2 , the R ct of wrought specimen significantly

mproved from 6.4 to 178 k Ω cm 2 when the pH of an electrolyte in-

reased from 1 to 3 in the absence of Cl − . At pH 3, the top iron oxide

ayer formed on the surface of wrought specimen as validated from the

PS analysis, would become stable and may provide additional barrier

o the electrochemical reactions. Furthermore, the R ct of AM specimen

lso increased from 398.2 to 577.0 k Ω cm 2 with an increase in the pH of

n electrolyte from 1 to 3, further suggesting the higher stability of top

5

ron oxide passive film on the specimen surface at pH 3. Moreover, im-

ortant to notice here is the notably higher R ct (398.2 k Ω cm 2 ) offered

y the AM compared to the wrought specimens (6.40 k Ω cm 2 ) even at

ery low pH level (pH 1), suggesting a significantly enhanced resistance

o charge transfer phenomenon by the AM specimen. This higher resis-

ive behavior could be directly related with the fine grain structure of

he AM specimen and resulting into the formation of more compact and

niform passive film on the surface. The refined grain structure of AM

pecimens could effectively reduce the diffusion path for metal atoms to

igrate at the metal/electrolyte interface and may facilitate the rapid



Fig. 5. The elemental composition (atomic %) of the passive film formed on the AM and wrought 316L stainless steel specimens after immersion in the electrolytes

of pH 1 and 3 (a) AM (pH = 1), (b) wrought (pH = 1), (c) AM (pH = 3), (d) wrought (pH = 3).

Table 2

Kinetic parameters of wrought and AM stainless steel obtained from the fitting of experimental

impedance spectra and by simulating with the EEC model; effect of electrolyte pH and Cl − ions.

Material Measured parameters 0 ppm Cl − 400 ppm Cl −

pH = 1 pH = 2 pH = 3 pH = 1 pH = 2 pH = 3

Wrought R s ( Ω cm 2 ) 10.65 48.22 89.04 9.90 20.03 27.34

R ct (k Ω cm 2 ) 6.40 10.65 178.0 4.81 6.42 109.1

Y dl (μSs n /cm 2 ) 790.2 515.4 59.3 584.5 545.8 122.4

n 0.94 0.85 0.91 0.92 0.91 0.91

AM R s ( Ω cm 2 ) 8.68 34.31 89.39 9.11 17.62 25.01

R ct (k Ω cm 2 ) 398.2 436.7 577.0 298.0 299.7 603.5

Y dl (μSs n /cm 2 ) 61.7 58.1 51.1 86.37 53.5 59.1

n 0.91 0.91 0.93 0.87 0.92 0.90

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2

i

c

A

H

R

(

t

l

ormation of compact and uniform oxide film at the surface [36] . As rep-

esented by Table 2 , the R ct of the AM specimens was much higher than

he wrought specimens measured in the similar electrolytes, suggesting

he pronounced hindrance in the charge transfer processes by the AM

pecimens.

Furthermore, the Y dl values which may be used to estimate the di-

lectric characteristics of the passive film were found to be much lower

or AM specimens compared to wrought specimens. This also justified

he existence of more uniform and less defective oxide film on the sur-

ace of AM specimens in the aggressive acidic conditions [54] . The sig-

ificantly lower R ct and larger Y dl of wrought specimens at low pH 1

nd 2 indicated the dissolution of iron oxide and formation of larger

6

oncentration of the defect sites (MnS inclusions) within the oxide film,

espectively.

The effect of 400 ppm Cl – addition in the same solution of pH 1,

and 3 was also determined as shown in Fig. 7 (b) and (d). It was ev-

dent that the addition of Cl – can significantly influence the electro-

hemical stability of oxide film. The R ct values of both the wrought and

M specimens decreased with the addition of Cl – in the electrolyte.

owever, the AM specimen still represented almost 50 times higher

ct (298.0 k Ω cm 2 at pH 1; 400 ppm Cl –) than the wrought specimen

4.81 k Ω cm 2 at pH 1; 400 ppm Cl –) under these conditions. Compared

o commercially available wrought 316L stainless steel, the AM stain-

ess steel can withstand very aggressive environmental conditions (pH



Fig. 6. Cationic fraction of the constituents for AM and wrought 316L stainless steel after immersion in electrolytes of pH 1 and 3 (a) AM (pH = 1), (b) wrought (pH = 1), (c) AM (pH = 3), (d) wrought (pH = 3). (Note: The concentration of the most dominant O 1s species is neglected to present the distribution of cationic species).

1

W

e

2

o

o

T

f

t

i

t

c

o

c

a

i

a

w

a

o

c

m

a

(

m

p

t

A

v

p

fi

i

t

b

w

d

t

p

c

s

o

o

p

c

o

s

c

; 400 ppm Cl –) and can be suitable for many demanding applications.

ith increase in pH from 1 to 3, the R ct also increased even in the pres-

nce of Cl – ions. However, the R ct values of AM specimens at pH 1 and

were almost 1.5 times less in the same solutions with the addition

f 400 ppm Cl – ions. This behavior is attributed to the adverse effects

f Cl – ions on the electrochemical stability of Cr enriched oxide film.

his behavior can be explained as the adsorption of Cl – ions on the sur-

ace and by replacing the oxygen atoms in the oxide film. Ultimately,

hese adsorbed anions can hydrolyzed the cationic species (Fe and Cr)

n the oxide film and may initiate the localized dissolution [54,55] of

he surface. Interestingly, the R ct of AM specimen at pH 3 slightly in-

reased from 577.0 to 603.5 k Ω cm 2 , possibly due to the formation

f relatively thick oxide film. The relatively low Y dl (51.1 μSs n /cm 2 )

alculated for specimen exposed to pH 3 with 0 ppm Cl – electrolyte

lso validated the more compact and homogeneous structure of the ox-

de film. The significant decrease in Y dl from 584.5 to 122.4 μSs n /cm 2

t pH 1 and 3, respectively, by the wrought specimen was affiliated

ith the improved stability of the Fe-oxide as confirmed from the XPS

nalysis.

The electrochemical dissolution of the 316L stainless steel depends

n the stability of the passive oxide film. The physical structure and

hemical composition of this oxide film would have decisive role in the

echanical integrity of the final component.

The barrier characteristic of the oxide film and its stability in the

ggressive environment was also evaluated by using cyclic polarization

7

CP) analysis. The CP curves obtained for both wrought and AM speci-

ens in varying electrolytes are presented in Fig. 8 .

At lower pH (pH 1 and 2), the relatively more negative corrosion

otential, E corr (–323 and –452 mV vs. SCE, respectively) registered by

he wrought specimens suggested their higher corrosion tendency than

M specimens. The E corr shifted to noble (positive) direction (–64 mV

s. SCE), when the wrought specimen was tested in the electrolyte of

H 3. This behavior indicated the increase in the stability of the passive

lm, moving towards higher pH. The addition of Cl – in the electrolyte

nduced a minor effect on the E corr , which is most likely associated with

he low concentration of these species in the diffuse layer. However,

oth the cathodic and anodic polarization curves for wrought specimen

ere significantly influenced by the change in pH. The large current

rawn by the wrought specimens during cathodic polarization indicated

he instability of the pre-existing passive film formed at an open circuit

otential. During anodic polarization, in the active region, the iron and

hromium would dissolve as Fe 2 + and Cr 2 + . The later species can be

ubsequently oxidized to Cr + 3 and may hydrolyzed to form a hydrated

xide film possibly, Cr(OH) 3 which may restrict the further dissolution

f the surface. In other words, the hydrolysis of Fe 2 + is restricted at

H 1 and 2 due to its thermodynamic instability under highly acidic

onditions. It is therefore, Cr enriched oxide film is expected to form

n the surface [53,56] . The large polarization observed during anodic

cans represented the formation of passive film mainly composed of

hromium oxide. Furthermore, the ever increasing current during an-



Fig. 7. Nyquist plots of (a) wrought (0 ppm Cl − ), (b) wrought (400 ppm Cl − ), (c) AM (0 ppm Cl − ), (D) AM (400 ppm Cl − ), (e) Equivalent electrical circuit (EEC)

used to extract quantitative information about the elements.

o

T

y

w

t

e

r

f

w

s

h

d

s

p

g

p

C

d

o

p

dic polarization also indicated the formation of defective oxide film.

hese results further comprehend the conclusion made from EIS anal-

sis about the highly defective nature of the oxide film formed on the

rought specimens. At pH 3, the relatively smaller anodic current (in

he passive region) indicated the formation of duplex oxide film. The

xistence of Fe-oxides at pH 3 as evaluated from the XPS analysis cor-

oborated to its increased thermodynamic stability. This behavior of-

ered by wrought stainless steel at varying values of pH was in line

ith the conclusions made by other researchers. This can be demon-

trated as the pronounced stability of Fe 2 + species and hydrolysis at

8

igher pH [57] . The large anodic current in the trans-passive region in-

icated the complete breakdown of passive film leading to uniform dis-

olution of the surface. Upon reverse anodic scan, the relatively larger

ositive loop was observed at pH 3 (in the presence of Cl –). This sug-

ested the more hindrance in the re-passivation and appreciably high

itting tendency of the wrought specimen. The deleterious effects of

l – ions can also be estimated from the increase in corrosion current

ensity I corr at pH 1 and 2 ( Table 3 ). The current fluctuations during an-

dic scans (in the passive region) revealed the formation of metastable

its on the surface [58] .



Fig. 8. Cyclic polarization curves of wrought and AM 316L stainless specimens (a) wrought (in 0 ppm cl ˉ), (b) wrought (in 400 ppm Cl ˉ), (c) AM (in 0 ppm Cl ˉ), (d)

AM (in 400 ppm Cl ˉ).

Table 3

Quantitative information obtained from the cyclic polarization curves.

Material Measured parameters 0 ppm Cl – 400 ppm Cl –

pH = 1 pH = 2 pH = 3 pH = 1 pH = 2 pH = 3

Wrought E corr (mV) –323 –452 –64 –335 –484 –156

E bd (mV) – – 329 – – 470

I cor (μA cm 2 ) 5.88 2.63 0.20 6.48 4.02 0.23

AM E corr (mV) –76 –126 –134 –10.1 –69.5 –139

E bd (mV) 815 823 836 783 792 870

I cor (μA cm 2 ) 0.056 0.051 0.049 0.114 0.106 0.078

(

t

s

l

t

v

d

g

c

h

(

b

M

t

o

a

a

n

d

d

4

s

t

On the other hand, the E corr of the AM specimens was much higher

noble) than the wrought specimen which is most likely associated with

he presence of more stable oxide film on the surface. Notably, irre-

pective to the pH variations, the relatively smaller anodic current and

arger passive region of the AM specimen ( Fig. 8 (c) and (d)) suggested

he better barrier characteristics of the oxide film. These results further

alidated the pronounced tendency of the AM 316L stainless steel to

evelop stable passive film due to its intrinsic finer polygonal shape

rain structure and the less possibility for the presence of MnS in-

lusions. The breakdown potential of AM specimens was significantly

igher ( > 800 mV vs. SCE) at low pH than wrought 316L stainless steel

∼ 400 mV vs. SCE at pH = 3). This higher breakdown potential offered

9

y the AM specimens rationalize the reduction or complete absence of

nS inclusions and consequently the absence of Cr depleted region in

he vicinity of sulphide inclusions, due to rapid solidification [41] . Two

rder of magnitude lower I corr and more positive E corr of AM specimens

lso demonstrated the improved capability of the former to withstand

ggressive acidic conditions as given in Table 2 . The presence of Cl –

egligibly influenced the breakdown potential at pH 3. However, un-

er highly acidic conditions (pH 1 and 2), the breakdown potential was

ecreased to 783 and 792 mV vs. SCE, respectively, in the presence of

00 ppm Cl –. Zheng et al. [32] also reported the compact nature of pas-

ive film formed on the stainless steel and its improved corrosion resis-

ance resulting from the refined microstructure. The fine grain structure



c

g

a

o

i

w

t

r

s

w

t

o

h

n

d

i

4

c

S

t

R

[

[

[

[

[

[

[

[

[

[

[

[

[

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[

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[

[

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[

ould accelerate the growth rate of passive film. The increase in the

rain boundary area could reduce the diffusion path for the chromium

toms to migrate to the surface and to form the uniform and compact

xide film [28] . Additionally, under the applied conditions, the AM spec-

mens showed negative hysteresis loop on reverse anodic polarization,

hich also represented their higher resistance to localized attack under

hese testing conditions.

It is apparent from these results that stability of passive film and

e-passivation tendency of the commercially available wrought 316L

tainless steel is significantly inferior to the AM stainless steel. Also, the

rought 316L stainless steel is found to be more sensitive to any varia-

ion in pH and Cl –. On the other hand, the relatively fine microstructure

f AM stainless steel exhibited higher resistance to dissolution in the

ighly aggressive acidic conditions. Also, the addition of 400 ppm Cl –

egligibly influenced the stability of the passive oxide film, which ad-

resses the suitability of this material for many applications, i.e. chem-

cal, biomedical, pharmaceutical, textile, nuclear and food industries.

. Conclusions

Based on the above reported research work, following conclusions

an be drawn.

• A non-conventional microstructure compared to wrought 316L stain-less steel has been observed in the AM specimens, possessing a very

fine web shaped cellular structure composed of ∼0.5 μm fine polyg-onal sub-grains confined within the large grains.

• XPS analysis revealed the influence by the pH of an aqueous solutionon the chemical composition of the passive oxide film formed on

the surface of an alloy. The formation of mono layered chromium

oxide/hydroxide passive film on the surface of both wrought and AM

specimens on exposure to pH 1 electrolyte has been noted. However,

in pH 3 electrolyte a duplex structured oxide passive film is formed

with iron oxide/hydroxide enriched outer layer.

• Electrochemical impedance spectroscopy suggested the dependencefor the occurrence of faradic and non-faradic reactions on the na-

ture of the passive oxide film. The high resistance to charge transfer

processes has been observed by the films formed on exposure to an

electrolyte with increasing level of the pH. Interesting to observe is

the significantly higher charge transfer resistance and much lower

dielectric characteristic in chlorinated extreme acidic regime offered

by the AM specimens in lieu to its wrought counterpart. This pro-

poses the exceptionally higher barrier properties being offered by

the passive film formed on the surface of AM specimens.

• Cyclic polarization results revealed the decrease in i corr of wroughtstainless steel specimen with increase in pH and in the absence of Cl –.

However, the relatively large i corr indicated by wrought stainless at

pH 1 and 2 in the presence of Cl – was attributed to the instability

of the passive film. The dissolution of oxide film (at pH 1, 2) during

cathodic polarization and relatively large anodic current of wrought

specimen compared to AM stainless steel also corresponded to the

poor stability of the passive film formed on the wrought stainless

steel. AM specimens also presented an order of magnitude lower cur-

rent density and significantly higher breakdown potential ( > 800 mV

vs. SCE) compared to wrought specimens in extremely acidic and Cl –

containing solutions. In support with the impedance spectroscopy re-

sults, the cyclic polarization results also validated the better barrier

characteristics of the passive film formed on the AM stainless steel

which is related with its fine grain structure compared to wrought

stainless steel.

upplementary materials

Supplementary material associated with this article can be found, in

he online version, at doi:10.1016/j.mtla.2018.06.015 .

10

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Corrosion behavior of additively manufactured 316L stainless steel in acidic media1 Introduction2 Experimental3 Results and discussion3.1 Microstructural analysis3.2 Surface analysis of the oxide film3.3 Electrochemical characterization; effect of pH and Cl−

4 Conclusions Supplementary materials References

Documents

Corrosion behavior of additively manufactured 316L ... · Due to the presence of non-homogeneous (chromium depleted re- gions) passive oxide ﬁlm and instability of iron oxide ﬁlm,