Influence of Clay size on corrosion protection by Clay nanocomposite thin films

Progress in Organic Coatings 140 (2020) 105489

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Influence of Clay size on corrosion protection by Clay nanocomposite thin films

T

Michael A. Meliaa, Stephen J. Percivala, Shuang Qinb, Erin Barrickc, Erik Spoerkea, Jaime Grunlanb, Eric J. Schindelholza,* a

Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210 Department of Materials Science and Engineering, Texas A&M University, College Station, Texas, 77843, USA c Department of Materials Science & Engineering, Lehigh University, Bethlehem, Pennsylvania, 18015, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Coating Anti-corrosion Electrochemical impedance Polymer Layer-by-layer assembly Permeability

The influence of clay platelet size on the corrosion barrier performance of highly-aligned polymer clay nanocomposite (PCN) thin films was examined. Layer-by-layer (LbL) deposition of alternating branched polyethylenimine (PEI) and either laponite (LAP), montromorillonite (MMT) or vermiculite (VMT) clay platelets were assembled on mild steel plates to obtain 20 bilayer (BL) films and cross-linked using glutaraldehyde after deposition. The clay platelets were chosen based on their aspect ratio, approximately 30:1, 400:1, and 2000:1, respectively. Electrochemical impedance spectroscopy of the coated steel plates during immersion showed corrosion rates and coating permeability followed LAP > MMT > VMT for up to 7 days of exposure in 0.6 M NaCl. The PEI/VMT films, ∼250 nm thick, slowed corrosion by a factor of > 1000 compared to bare steel. The results support the premise that high aspect ratio clay platelets can improve the corrosion barrier efficacy of LbL PCN films by decreasing film permeability and provide exceptional protection to steel in saline environments compared to other thin multilayer coatings and pretreatments.

1. Introduction Coatings are the most common means of limiting corrosion in metals. Despite their ubiquity, there is a perpetual demand for higher performance coatings that balance the demands of functionality, weight, cost and environmental compatibility. In recent years, polymernanoparticle composites have been of great interest in this regard [1–6]. The bulk addition of dispersed nanoparticles, such as exfoliated clay, to conventional polymer coatings has been demonstrated as a costeffective means of increasing mechanical durability, thermal stability, and corrosion barrier properties [6–12]. Several studies have demonstrated that polymer clay nanocomposites (PCN), containing as little as 1 wt% exfoliated clay can considerably increase corrosion barrier performance [2,13–16]. For example, Yeh and coworkers demonstrated a hundred-fold decrease in corrosion rate of mild steel in salt water coated with polymethylmethacrylate (PMMA) loaded with 10 wt% exfoliated montmorillonite (MMT) clay relative to coating with PMMA alone [2,16]. The enhanced corrosion protection imparted by clay and other nanoparticle additives is largely attributed to the creation of tortuous diffusion pathways in the coating, serving to decrease permeability ⁎

[2,3,17,18]. The relative permeability of nanocomposite coatings is a strong function of particle loading and dispersion, aspect ratio, and orientation, and can be generally modeled for gases as [19]:

Ps = Pp 1+

1 − ϕs L ϕ 2W s

( )(S + ) 2 3

1 2

(1)

Where Ps/Pp represents the relative permeability, Ps is the permeability of the nanocomposite coating, and Pp is the permeability of the polymer. From Eq. (1), relative permeability decreases with increasing particle loading (ϕs), particle aspect ratio (L/W), and particle order, S, ranging from -1/2 (long direction is perpendicular to substrate surface) to 1 (long direction is parallel to substrate surface). Alexandre et al. further demonstrated that although these relationships generally hold for water permeability under immersion conditions, several other factors concerning the polymer must be considered, including degree of crystallinity, water solubility, and resistance to water-induced plasticization [20]. Taken together, one could expect that a water resistant polymer with a high loading of high aspect ratio particles, aligned parallel to the substrate, would maximize barrier effectiveness. Although some factors, such as polymer choice, can be optimized in bulk

Corresponding author. E-mail address: [email protected] (E.J. Schindelholz).

https://doi.org/10.1016/j.porgcoat.2019.105489 Received 31 July 2019; Received in revised form 27 November 2019; Accepted 8 December 2019 0300-9440/ © 2019 Published by Elsevier B.V.

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M.A. Melia, et al.

2.2. Electrochemical analysis

nanocomposite coatings, particle orientation (S) is difficult to control and loading is generally limited to 10 wt% [19,21–23]. Layer-by-layer (LbL) assembly has been demonstrated in recent years as a means of constructing highly impermeable nanocomposite coatings via alternating layers of polymer and highly aligned (S ≈ 1) nanoparticles at loadings exceeding 90 wt% [8,11]. LbL films are constructed by alternating deposition of complimentary polymer and nanoparticle components by simple and scalable aqueous dip-coating or spraying processes [7,24]. The components sequentially bind together with attractive forces (e.g., electrostatic interactions or hydrogen bonding). Adjustments to component solution chemistry and process parameters provide precise control over film composition and architecture [7–11,25]. The resulting compositional versatility, conformal nature, impermeability, and thinness of LbL films makes them attractive candidates as anticorrosion coatings [26–28]. Recent work demonstrated that corrosion on steel immersed in salt water was significantly inhibited by chemically crosslinked LbL polyethyleneimine/montmorillonite (PEI/MMT) thin films, improved by increasing the number of bilayers and crosslinking of the PEI [29]. This and similar studies have demonstrated PCN multilayer films considerably improved the corrosion resistance of steel and other alloys in immersed and atmospheric conditions [2,4,6,29–34]. However, a deeper understanding of the relationship between coating architecture and corrosion barrier performance, requisite for informed materials design, is lacking [33–35]. The present study builds on these results, examining the influence of clay platelet size on the corrosion barrier performance of LbL PCN thin films. Specifically, bilayer films comprising PEI and either laponite (LAP), montmorillonite (MMT), or vermiculite (VMT) clay were assembled on mild steel, with all films crosslinked after assembly. These clay platelets provided a range of particle sizes spanning two orders of magnitude, ∼30–2,000 nm. The coated steel samples were then immersed in 0.6 M NaCl and corrosion barrier performance assessed using electrochemical impedance spectroscopy (EIS) and post-mortem electron microscopy. The results show that corrosion barrier performance can be considerably enhanced by increasing clay platelet size, supporting the premise that permeability is a primary factor in corrosion protection for this class of coatings.

Corrosion experiments were performed on coated specimens using a Biologic VMP-300 potentio/galvanostat (Seyssinet-Pariset, France) in a standard three cell electrode setup with the coated steel samples as the working electrode, Pt mesh as the counter electrode, and a Accumet Ag/AgCl reference electrode (Fisher Scientific, USA). Specimens were immersed in quiescent 0.6 M NaCl (Sigma-Aldrich, USA) aqueous solution for 168 h. This environment and length of time (7 days) was chosen to match common conditions used in the literature for assessing the viability of thin coatings on steel [36–39]. During immersion, electrochemical impedance spectroscopy (EIS) measurements were taken at the start of the experiment, every hour for the first 24 h, and every 6 h for the next 6 days. The EIS measurements used an excitation voltage of VRMS = 10 mV versus open circuit and a frequency range from 10−1 to 105 Hz. The EIS data was analyzed assuming the equivalent circuit shown in Fig. 2with the circuit elements as solution resistance (Rs), coating impedance (Zc), charge transfer resistance (Rct), and the double layer capacitance (CPEDL) overlaid on a schematic cross-section of the specimen. The same approach developed by Percival et al. is used in this work and described in brief here [29]. At high frequencies (≥ 105) in our experiments the impedance is dominated by the Rs. The Zc dominates from high to mid frequencies (105–102). From mid to low frequencies (102 –10−1) the Rct and CPEDL dominate [40–42]. First, the equivalent circuit model was fit to the low frequency regime of the impedance to determine the CPEDL, Rct, and to isolate the Zc from the total impedance. The Voigt measurement model, adapted from Chen et al., was used to fit the Zc to a number (n) of resistances (Rn) and capacitances (Cn) in parallel to estimate the resistivity (ρ) distribution through the PCN film [43]. A detailed description of the analysis procedure is given in the Supplemental Data. The ρ values qualitatively correspond to the extent of solution ingress through the film, lower ρ suggests higher solution content. This modeling approach to determine Zc is unreliable when the coating and charge transfer contributions become indistinguishable from one another, typically correlating to the time when corrosion of the steel substrate occurs. Similar approaches to tracking coating degradation have been utilized by others [29,43,44].

2. Materials and methods 2.3. Electron microscopy 2.1. Materials Imaging of the coated steel plates was performed using a Supra 55VP Field Emission (Zeiss, Germany) scanning electron microscopy (SEM) at an accelerating voltage of 5 kV. An FEI Scios DualBeam (ThermoFisher Scientific, USA) focused ion beam- scanning electron microscope (FIB-SEM), with a Ga ion source operating at 30 kV, was used to prepare thin foil samples of the as-deposited PCN films for scanning transmission electron microscopy (STEM) analysis. STEM medium angle annular dark field (MAADF) imaging was performed using an aberration-corrected JEM-ARM200CF (JEOL, Japan) operating at 200 kV. Thickness measurements of the cross-linked films deposited on the steel plates were performed during FIB-SEM and STEM imaging. Cross-sectional imaging of the corroded samples after 7-day immersions was performed using the FIB-SEM. Before milling, Pt was deposited onto the surface to prevent damaging the films during milling. A trench was milled into the sample using the FIB with a Ga ion source operating at 30 kV and 52° tilt to enable observation of the cross-section of the corrosion scale, PCN film, and underlying substrate. The cross-section was milled with successively decreasing currents to produce a polished face. Imaging of the cross-section was performed using the SEM operating at 5 kV and 52° tilt in backscattered electron (BSE) imaging mode.

Low carbon steel (UNS G10180) plates were polished to a final finish with SiC 1200 grit paper. After polishing, they were soaked in a passivation solution of 1 M NaOH and 100 mM sodium nitrate for 12 h, followed by rinsing with deionized (DI) water and drying with compressed nitrogen. The passivation step was necessary to prevent corrosion during the coating process [29]. The polymer solution used for coating was 0.1 wt% branched PEI, with an average molecular weight of 25,000 g/mol (Sigma-Aldrich, USA). The clay solutions were comprised of 1 wt% of one of three clay platelets in DI water: laponite (BYK Additives Inc., Germany), montmorillonite (BYK Additives Inc., USA), or vermiculite (963++, Specialty Vermiculite Corp., USA). Coatings were deposited onto the passivated steel plates using an automated dipcoater, dipping the steel substrate sequentially in the PEI and clay solutions for 1 min to create a single bilayer (BL), schematically shown in Fig. 1. In between each dipping step, samples were rinsed with DI water and then dried with compressed air using a set of air knives. Crosslinking of PEI was performed after depositing the desired number of BLs, 20 for this study, by immersing the coated plates in an aqueous solution of 25 wt% glutaraldehyde (Sigma-Aldrich, USA) in water for 12 h followed by rinsing with DI water and drying with N2. A crosssection schematic of these PEI/clay thin films is shown in Fig. 1 for 4 BLs. All aqueous solutions were made using 18.2 MΩ·cm DI water.

2

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Fig. 1. A schematic of the PCN film LbL deposition process which is cycled until the desired number of BLs (20 in this case) are deposited.

thinner and more difficult to resolve [45,46]. Previous work by the authors on LbL PEI/LAP films use a similar deposition process and showed individual LAP platelet layers in TEM cross-section images, suggesting they are similarly present and aligned in this studies films [8]. A variation in extent of exfoliation is seen for the MMT and VMT platelets with most platelets on the order of a few nm thick, while several are 10–20 nm thick. These less exfoliated platelets will contribute substantially to the films overall cross-sectional thickness, as measured by SEM and TEM and presented in Table 1. Average clay diameter and expected clay loading determined by the authors in previous studies is also given in Table 1 [8,9,11].

3.2. Barrier characteristics and performance during immersion From the plots of EIS measurements in Figs. 4 and 5, the PEI/VMT film consistently exhibits the largest impedance values across all frequencies and times during immersion of the coated plates in 0.6 M NaCl solution. The modulus of impedance plots in Fig. 4 show the initial separation of sample impedance at early times, with all films at an order of magnitude larger impedance or more than the bare, passivated steel substrate. The impedance of all coated steel decreased over time. This reduction is caused by the ingress of solution through the PCN film and eventual penetration to the substrate which will cause intense corrosion of the substrate and debonding of the film [47–49]. After 168 h all coatings, except for the PEI/VMT, were substantially compromised, with impedance values approaching the steel substrate. The Nyquist plots in Fig. 5 reveal the coating impedance Zc at high to mid frequencies for the PEI/VMT film in (a) is significantly larger than the other films at upwards of 20,000 Ω. cm2. PEI/LAP and PEI/MMT specimens exhibit small (< 1000 Ω.cm2) Zc values at short immersion times, shown in the inlaid plot in Fig. 5 (a). This leads to indistinct inflection points signifying where Zc and the charge transfer and double layer frequency range overlap making modelling and isolating the Zc difficult. The estimated charge transfer resistance, Rct, values determined by simulating the PCN coated steel specimen’s impedance spectra were generally VMT > MMT > LAP, with higher values indicative of lower corrosion rate and higher barrier integrity. The extracted Rct values, which are inversely proportional to corrosion rate, are plotted versus immersion time in Fig. 6 for each film type and passivated steel [50]. The PEI/VMT specimens large Rct values never degraded to below 10,000 Ω-cm2 for the entire 168 h experiments. In contrast, the PEI/ MMT and PEI/LAP films started with large Rct values, 300,000 and 40,000 Ω-cm2 respectively, but quickly reduced to below 20,000 Ω-cm2 after 6 h, plateauing at values similar to the bare steel after 72 h. Besides PEI/VMT, all other films approached the Rct value for the steel substrate after 168 h. These results show a strong correlation between increasing platelet size/aspect ratio and corrosion barrier performance of the PCN films. The estimated resistivity, ρ, distributions through the PCN films

Fig. 2. A schematic of the substrate, film, solution cross-section with the equivalent circuit used to fit the EIS data and extract the coating impedance, Zc.

3. Results and discussion 3.1. Characterization of PCN film structure In all cases, the films deposited on steel were uniform and contiguous with each clay type imparting unique surface characteristics, shown by the PCN film surface images in Fig. 3 (a–c). The PEI/LAP film Fig. 3 (a) is the smoothest and exhibited several dark lines randomly distributed across the surface. These dark lines originate from topological features on the steel substrate (verified by a FIB cross-section not shown here), possibly created after the passivation process. Similar dark lines were present on the PEI/MMT films as well as a rougher surface Fig. 3 (b). The PEI/VMT coating was also rough and lacked any apparent dark lines, likely from its larger thickness compared to the other films Fig. 3 (c). The larger VMT clay platelets, some folded, are visible in certain areas, such as in the upper right corner of Fig. 3 (c). The roughness of the PEI/MMT and PEI/VMT may be due to platelet or polymer agglomeration. The cross-sectional images in Fig. 3 (d–f), except for PEI/LAP, reveal layers of exfoliated clay platelets intercalated with compact polymer layers. The light grey contrast in Fig. 3 (e) and (f) correlate to the distinct MMT and VMT platelet layers, confirming their parallel orientation to the substrate surface (S ∼ 1 for Eq. 1). No distinct layers of LAP platelets are shown for PEI/LAP films, Fig. 3 (d), possibly because the LAP platelets were more exfoliated (separation of individual clay platelet molecular layers) than the MMT and VMT platelets, hence 3

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Fig. 3. Upper row are secondary electron images of the surface for as-deposited 20 BL (a) PEI/LAP, (b) PEI/MMT, and (c) PEI/VMT. Bottom row are MAADF STEM images of the same films in cross-section, showing the layered structures for (d) PEI/LAP, (e) PEI/MMT, and (f) PEI/VMT. The light grey striations in the crosssections, especially apparent in (e) and (f), are the clay layers, while the associated dark bands are the polymer.

thickness as a function time reveal all films overall ρ magnitude to reduce over the 7 day experiments, suggesting gradual water ingress through the films with the slowest rate of ingress for the PEI/VMT specimen, shown in Fig. 7 (a). The PEI/MMT films exhibit a ρ distribution at short immersion times in Fig. 7 (b) which degrade to a two ρ values after ∼168 h while the PEI/LAP films exhibit single ρ values for the entire experiment duration (not shown). These trends in ρ are a convenient means for tracking relative solution ingress through the films. However, the general approach lacks temporal sensitivity to acknowledge when corrosion begins on the steel substrate below the film or when debonding of the film occurs which were observed for all films after 7 days per characterization in the next section. The PEI/VMT and PEI/MMT comparison in Fig. 7 corroborates the initial hypothesis which suggests that VMT, with the largest platelet size and ρ, served to most effectively impede solution transport. These trends follow the expected relative permeability (Ps/Pp) of the coatings according to Eq. (1). From Table 1 and assuming the same clay platelet orientation and overlap, the calculated Ps/Pp of PEI/VMT is roughly 5 times lower than PEI/MMT and 10 times lower than PEI/LAP. Although the orientation and overlap assumptions seem reasonable for the MMT and the VMT coatings, Fig. 3, the applicability to the LAP coating is uncertain given the lack of confirmed platelet orientation. Nonetheless, water vapor and oxygen gas permeability of these same coatings deposited onto polymer substrates in previous studies also follow the calculated trends, suggesting clay size and aspect ratio are the dominant differentiating factor [7–11,25].

3.3. Film characteristics after immersion Debonding of the films from the steel substrate, a common mechanism for coating breakdown caused by ingress of solution and corrosion product buildup at the substrate/film interface, is the primary coating failure mode for all films, regardless of clay type [49,51]. The PEI/LAP and PEI/MMT coatings developed blisters, as exemplified in Fig. 8 (a), which covered a majority (> 50% area) of the exposed surfaces after 7 days immersion. These blisters were also present in the PEI/VMT coatings after exposure, as shown in Fig. 8 (b), but were far less prevalent (< 30% area). Furthermore, FIB cross-sectioning of areas of the PEI/LAP and PEI/MMT coatings in Fig. 8 (c) where no blistering or film rupture was apparent revealed rust had formed both under and over the coatings with complete debonding of the film from the substrate. On the left side of the PEI/LAP cross-section image in Fig. 8 (c), the thin black region indicative of the film (ii) is absent, suggesting the film had dissolved or fully detached and fell off from the substrate, another potential factor contributing to coating breakdown. By contrast, similar examination of the PEI/VMT coatings in areas without film rupture,shown in Fig. 8 (d), revealed no signs of debonding along with negligible change to its thickness. These results further corroborate the superior barrier performance of PEI/VMT over the other films in this study. Given debonding resistance is largely dictated by the bond at the metal/film interface, which is the steel oxide/PEI for all films in this study, it is reasonable to surmise that all films examined had similar debonding characteristics [49,51,52]. Consequently, the debonding resistance likely played a minor role in the differences observed in barrier performance between the three PCN films. The debonding

Table 1 As-deposited 20 bilayer coating characteristics. Coating Type

Thickness (nm)

Average clay diameter (nm)

Clay loadinga(wt%)

PEI/laponite (LAP) PEI/montmorillonite (MMT) PEI/vermiculite (VMT)

89 ± 13 155 ± 57 235 ± 27

30 400 2000

83 83 92

a

Clay loading was previously determined for these multilayer films8,

11

. 4

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Fig. 4. Bode magnitude EIS plots for all coated and uncoated steel plates during immersion in 0.6 M NaCl after (a) 2 min, (b) 24 h, and (c) 168 h.

failure mode results from migration of corrosion promoting solution species such as H2O and Cl− through the coating which lead to the generation of corrosion products, all of which can serve to weaken or stress adhesion at the metal/film interface [47–49,53–56]. Therefore, the controlling factor in time to coating breakdown of the PCN films is their ability to slow the permeation of solution species through the film, which is supported by the evidence in this study. The demonstration of these LbL PCN films as exceptional gas barriers in previous studies is further evidence of this [9–11].

3.4. Performance relative to similar coatings The PEI/VMT specimen in this study outperformed other thicker multilayered coatings along with several pretreatments on mild steel under similar test conditions, Fig. 9. One exception to this is a spincoated epoxy and graphene oxide (Epoxy/GO) coating recently reported by Zhang et al [39]. The investigators reported similar starting Rct values to the PEI/VMT in this study. However, the Rct of their coating after 7 days immersion was an order of magnitude larger than

Fig. 6. Charge transfer resistance, Rct, as a function of immersion time in 0.6 M NaCl for coated and uncoated steel plates. The error bars represent min and max values of Rct for each time of immersion.

Fig. 5. Nyquist EIS plots for all coated and uncoated steel plates during immersion in 0.6 M NaCl after (a) 2 min and (b) 168 h. The Zc for PEI/VMT is discernable in (a) as the large semi-circle nearest to the origin, a region that degrades in magnitude with time. 5

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Fig. 7. Distribution of ρ through (a) a PEI/VMT film and (b) a PEI/MMT film over the course of immersion in 0.6 M NaCl.

the PEI/VMT in this study. It is important to point out that the Epoxy/ GO was nearly 100 times thicker than the LbL PCN films in this study (∼10 μm versus ∼100 nm) and the greater thickness may have served to decrease relative permeability. Furthermore, each of the individual GO layers in the Epoxy/GO coatings were the same thickness as the total thickness of the 20 BL coatings in this study. More comparable are the results of Faure et al., who developed and tested a 10 BL 3,4-dihydroxyphenylalanine and [2-(methacryloxy)ethyl] trimethylammonium chloride (DOPA-DMAEMA+)/LAP film assembled using LbL [57]. Despite the use of a polymer that is known to be a strong corrosion inhibitor, Rct values were similar to PEI/LAP in this study. This could be partly due to the fewer number of layers used in that study (10 BL versus 20 BL in this study), but our results suggest that the lower performance may have been due to use of small aspect ratio platelets. The other coatings presented in Fig. 9 are more conventional for steel as pretreatments prior to application of much thicker industrial coating systems [36,38,58–62]. All of these have considerably smaller Rct values compared to PEI/VMT.

Fig. 9. Coating performance as a function of film thickness for the PEI/clay films in this study compared to other reported coatings on mild steel in 0.6 M NaCl solution. Solid symbols represent Rct at < 2 h immersion, open symbols represent Rct after 7 days immersion.

4. Conclusions This study has demonstrated that increasing platelet diameter can critically improve the corrosion barrier performance of LbL PCN films.

Fig. 8. Surface and FIB cross-section secondary electron images of the (a,c) PEI/LAP and (b,d) PEI/VMT films after 168 h of immersion in 0.6 M NaCl. The label (i) indicates corrosion product above the film, (ii) is the PCN film, and (iii) is corrosion product under the film. The steel substrate and the Pt deposit used for FIB cross-sectioning are labelled.

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The larger clay platelets (vermiculite) likely served to decrease coating permeability, supporting the premise that coating permeability is a primary factor in corrosion barrier effectiveness for these films. The 250 nm thick crosslinked coating with the largest diameter clay platelets (PEI/VMT) was able to slow corrosion by a factor of > 1000 compared to bare mild steel (Rct = 106 Ω-cm2 vs. Rct = 103 Ω-cm2) in 0.6 M NaCl solution over 7 days. This performance exceeded most other, thicker multilayered coatings and several conventional pretreatments on mild steel under similar test conditions that have been reported to date. Film debonding with the steel was the common failure mode during exposure and replacement of PEI with a more debondingresistant polymer may be a pathway to further improving LbL PCN coatings. This work demonstrates low cost and extremely thin PCN films (< 200 nm) fabricated by a scalable LbL process can provide exceptional protection to steel in saline environments.

[6]

[7]

[8]

[9]

[10]

[11]

Declaration of competing Interests [12]

None. [13]

CRediT authorship contribution statement Michael A. Melia: Methodology, Investigation, Writing - original draft, Visualization. Stephen J. Percival: Writing - review & editing. Shuang Qin: Methodology, Investigation. Erin Barrick: Methodology, Investigation. Erik Spoerke: Conceptualization, Supervision. Jaime Grunlan: Conceptualization, Supervision. Eric J. Schindelholz: Conceptualization, Supervision, Writing - review & editing.

[14]

[15]

Acknowledgements

[16]

The authors thank Sara Dickens for recording SEM micrographs and Christina Profazi for cutting and polishing the steel coupons. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. Sandia National Laboratories is a multi-mission 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. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Data availability statement: The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

[17] [18]

[19]

[20]

[21]

[22]

[23]

Appendix A. Supplementary data [24]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2019. 105489.

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