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Dastpak, Arman; Hannula, Pyry Mikko; Lundström, Mari; Wilson, Benjamin P.A sustainable two-layer lignin-anodized composite coating for the corrosion protection of high-strength low-alloy steel

Published in:PROGRESS IN ORGANIC COATINGS

DOI:10.1016/j.porgcoat.2020.105866

Published: 01/11/2020

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Please cite the original version:Dastpak, A., Hannula, P. M., Lundström, M., & Wilson, B. P. (2020). A sustainable two-layer lignin-anodizedcomposite coating for the corrosion protection of high-strength low-alloy steel. PROGRESS IN ORGANICCOATINGS, 148, [105866]. https://doi.org/10.1016/j.porgcoat.2020.105866

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Progress in Organic Coatings

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

A sustainable two-layer lignin-anodized composite coating for the corrosionprotection of high-strength low-alloy steel

Arman Dastpak, Pyry-Mikko Hannula, Mari Lundström, Benjamin P. Wilson*Hydrometallurgy and Corrosion, Department of Chemical and Metallurgical Engineering (CMET), Aalto University, P.O. Box 16200, FI-00076. Aalto, Espoo, Finland

A R T I C L E I N F O

Keywords:SteelOxide coatingBiopolymerOrganic coatingCorrosion protection

A B S T R A C T

In this study, plasticized kraft lignin was used to prepare anti-corrosive coatings on steel, which was galva-nostatically anodized (80 °C, 25 wt.% NaOH) by utilizing three different current density values (24, 50 and120mA/cm2). It was shown that the anodization affects not only the properties of anodic coatings but also theirinteraction with lignin coatings. A combination of pull-off adhesion measurements with linear sweep voltam-metry (LSV) demonstrated an enhanced adhesion strength of plasticized lignin coatings (4.32MPa on anodizedsurface vs. 2.38MPa on bare steel) and decreased corrosion current density from 15 μA/cm2 (bare steel) up to ca.0.5 μA/cm2 for anodized and lignin coated steel.

1. Introduction

Steel is an integral part of everyday life and is the most commonlyused metal for an extensive selection of objects, equipment and open-airstructures [1]. Among the different grades of steels available, high-strength low-alloy (HSLA) steels, also known as micro-alloyed steel, area unique category of steel as they are designed based on desired me-chanical properties (rather than a pre-defined chemical composition)and usually contain low carbon content (0.05 – 0.25 wt.%) and lessthan 2wt.% of other alloying elements [2]. The enhanced mechanicalproperties of HSLA steels make it an ideal candidate material for a widevariety of applications like off-road vehicles, construction and farmmachinery, building panels and off-shore structures, where the com-ponents are exposed to outdoor conditions [2]. With operation in suchenvironments, the corrosion of steel - and naturally its protection -becomes an important issue as factors such as atmospheric pollutionand proximity to maritime conditions can accelerate this phenomenon[1].

Numerous methods already exist to minimize the corrosion of steelsurfaces and these are primarily based on the use of a barrier layerbetween the surface and the surrounding environment [3]. Examples ofthese technologies are sacrificial coatings, barrier oxides and organiccoatings, with the latter being the most widely applied method used incorrosion protection applications [4]. Although organic coatings havebeen proven to minimize the effects of corrosion, their dependence ondepleting non-renewable hydrocarbon resources has motivated the re-search community to explore renewable and sustainable feedstocks for

the preparation of such coatings [5]. Amongst the potential alter-natives, lignin offers significant potential for utilization as a componentof sustainable anticorrosion coatings due to its antioxidant properties[6], corrosion inhibition capabilities [7,8] and economic competitive-ness. Lignin, as a key component of plant biomass, is one of the mostabundant biopolymers on Earth [9] and constitutes a large portion ofthe by-product from pulp, paper, and lignocellulosic-based biorefiningindustries [10]. Although lignin and its derivatives have been pre-viously studied as functional additives for anticorrosive coatings ondifferent metals [11–15], the performance of unmodified lignin as themain constituent of protective coatings has not been comprehensivelystudied to date. In our previous studies [16,17], the anticorrosioncapability of different types of lignin as a surface coating for differentgrades of steel (stainless steel, iron-phosphated steel) and in differenttest mediums (physiological solutions and 5wt.% NaCl) has been de-monstrated. Our previous findings suggest that lignin coatings decreasethe corrosion rate of steel by 1–2 orders of magnitude, and the pro-tection performances are dependent on the source of lignin [16]. Fur-thermore, it has also been shown that lignin coatings have a thickness-dependent susceptibility to develop cracks during and after drying [17].

The effectiveness of organic coatings for corrosion protection isfundamentally affected by the adhesion performance at the coating-metal interface [18,19]. Often, the surface chemistry of a metal sub-strate can be tailored by a pre-treatment step in order to enhance themetal-coating interfacial bonding capability. For example, the anodi-zation of a metallic surface to form an oxide layer increases microscopicroughness that gives rise to a higher surface area for the coating

https://doi.org/10.1016/j.porgcoat.2020.105866Received 22 April 2020; Received in revised form 29 June 2020; Accepted 1 July 2020

⁎ Corresponding author.E-mail address: ben.wilson@aalto.fi (B.P. Wilson).

Progress in Organic Coatings 148 (2020) 105866

0300-9440/ © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

T

adhesion as well as enhanced surface wettability characteristics [20].Furthermore, the presence of this oxide layer also provides an extrabarrier layer that further reduces the electrochemical activity of theinnermost surface of the steel [21]. Anodization is an electrochemicaloxidation process, in which the surface of metal, immersed in a specifictype of solution, is polarized by the application of an external current,resulting in oxide film formation. This method is commonly utilized inindustry for the modification of aluminium (Al) surfaces, although itcan also be applied to other metallic materials like titanium (Ti) [22],zinc (Zn) [23], magnesium (Mg) [24], as well as different grades of steelunder controlled alkaline (caustic) conditions [21,25,26]. Although,previous studies have demonstrated the possibility to produce differentcompositions of iron(II)/iron(III) oxides on steel by adjustment of theanodization parameters [21,25,26], to the best of the authors knowl-edge the addition of lignin coatings to such pre-oxidized steel surfacesand characterization of the subsequent effects on corrosion, for ex-ample, has not been reported.

This novel work aims to investigate the anticorrosion capability oflignin coatings, containing triethyl citrate (TEC) as a plasticizer, for ahigh-strength low-alloy steel (S355 MC), and to explore the effect of asurface pre-treatment step, i.e. anodization, on the adhesion and elec-trochemical properties of lignin coatings.

2. Materials and methods

2.1. Sample preparations

A standard grade of high-strength low-alloy S355 steel (SSAB,Sweden) was used as the substrate for preparing oxide films and ap-plication of lignin coatings. The chemical composition of the substrateis outlined in Table 1. A set of specimens with dimensions of25×25×3mm3 and a bigger set with 140×70×3mm3 dimensionswere sequentially sanded with SiC papers (Struers, USA) from 80 to 500grit. The samples were then cleaned by sonication in ethanol (94.2 %,Altia, Finland) before being thoroughly rinsed with DI water prior tofurther surface modification or coating.

2.1.1. Preparation of lignin coatingsA commercial grade softwood kraft lignin (KL, BioPiva™ 190, UPM,

4 g) was dissolved in an organic solvent mixture (10mL) comprised of a1:3 ratio of diethylene glycol monobutyl ether:1-methoxy-2-propanol(Sigma-Aldrich) and magnetically stirred (300 rpm, 24 h) to produce a400 g/l organic dissolution. A thorough analysis of the utilized kraftlignin and its performance in the organic solvents can be found in ourprevious study [17]. After the lignin-solvent mixture was obtained, thesolution was centrifuged at 5000 rpm for 600 s (ThermoFisher ScientificHeraes Megafuge, Germany) in order to separate any insoluble residues,and the weight of insoluble fraction was measured after two weeksdrying in a closed fume hood environment to obtain the solubility of thelignin in the utilized solvent mixture. The average solubility for 400 g/ldissolutions and from triplicate measurements were 98.08 % (±0.18%). Following centrifugation, 5 wt.% —vs. the initial weight of lignin—of triethyl citrate (≥ 98 %, Sigma-Aldrich), was added as a plasticizer,before the solution was magnetically stirred for 1 h to ensure a throughmixing of the components. The resultant mixture is subsequently re-ferred to as plasticized lignin (PL) in this study. As a comparison, adissolution with the same concentration of lignin without addition oftriethyl citrate was also prepared and is referred to as reference lignin

(RL). The PL and RL dissolutions (200 μL) were separately applied oncleaned steel or anodized surfaces by spin-coating (2000 rpm, 300 s)onto the small substrate samples (25mm×25mm×3mm). For theapplication of coatings onto larger substrate configuration(140mm×70mm×3mm), spray coating with a conventional cupgun equipment (Satajet 20 B, Q-lab, USA) was used. The PL and RLdissolutions (10mL) were sprayed separately onto steel or anodizedsurfaces at the same spray angle (∼90°), distance (∼20 cm) and airpressure (∼106 Pa) relative to the surface of plates. The spray coatingapplication was conducted twice for each surface at an interval of 48 h,in order to ensure complete surface coverage by the coating. After ap-plication of the lignin coating, samples were placed in a pre-heatedoven (Mermet 400, Germany) at 50 °C for 48 h to cure the coatings andaccelerate the evaporation of solvents from the coatings. All of thesamples with or without lignin coatings were stored in a desiccator.

2.1.2. Anodization of surfacesThe anodization of S355 steel was based on a modification of the

potentiostatic protocol described previously by Burleigh et al. [21,25]and three different current density values were selected as outlined inTable 2. Initially, samples were immersed in a pre-heated sodium hy-droxide solution (NaOH, 25 wt. %, 80 °C, 250mL) before being ano-dized galvanostatically using a conventional three-electrode setup:S355 as the working electrode (WE), stainless steel as the counterelectrode (CE) and Ag/AgCl/KClSaturated as the reference electrode (RE).Throughout the anodization process, the solution was under a con-tinuous magnetic stirring (150 rpm) and in order not to alter the com-position of NaOH medium, each solution was used to anodize a max-imum of three separate surfaces. Following the completion of theanodization step, all surfaces were thoroughly rinsed with DI water andthen stored in a desiccator prior to further use.

2.2. Characterization of lignin

2.2.1. Thermal propertiesThe effect of the triethyl citrate plasticizer on the glass transition

temperature (Tg) of lignin was studied by differential scanning calori-metry (DSC, TA Instruments, MT-DSC Q2000). In brief, multiple dropsof reference lignin (RL) and plasticized lignin (PL) were placed on se-parate glass petri dishes, and alongside the raw kraft lignin placed in apre-heated oven (50 °C, 48 h). Dried powder was placed in an alumi-nium pan (∼7mg) and heated from 20 °C to 190 °C (10 °C/min), thenheld at 190 °C for 5min to remove any previous thermal history, beforebeing subsequently cooled to 0 °C and reheated (10 °C/min) to 190 °C.The Tg values were obtained from the last heating cycle and from themidpoint of the temperature range where there was a demonstrablechange in the heat capacity [27].

The contribution of residual solvents on the lignin’s thermal beha-viour was further studied by thermogravimetric analysis (TGA, TAInstrument, TGA Q500). Dried lignin powders (50 °C, 48 h) were placedin a platinum pan (10−11mg) and were heated from 25 °C to 500 °C(10 °C/min). Both thermal characterizations (DSC and TGA) wereconducted under constant nitrogen flow (50mL/min).

2.2.2. ATR-FTIRThe chemical characteristics of lignin powders before and after

plasticization was studied by ATR-FTIR (Platinum-ATR, Bruker). FTIRmeasurements comprised of 24 scans over spectral range of 4000 to

Table 1Elemental composition of the utilized S355 steel substrate (wt. %), as specified by the manufacturer [W1].

Element C Si Mn P S Al Nb V Ti Fe

Wt. % < 0.10 < 0.03 < 1.50 < 0.02 < 0.01 > 0.01 < 0.09 < 0.20 < 0.15 B

B: balance.

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500 cm−1 with a reflection diamond ATR-D cell and the measurementof each sample was conducted in triplicate. The lignin and plasticizedlignin powders for the measurements were obtained from the samedrying procedure described in section 2.2.1 (50 °C, 48 h).

2.2.3. Optical microscopyThe general condition of dried lignin coatings, as well as their

thickness values were investigated by optical microscopy (OM, MoticBA310Met-t). Prior to thickness measurements by the cross-sections,samples were mounted in an epoxy resin and after drying they weresequentially polished with SiC paper (#80 to #1200 grit) and poly-crystalline diamond (3 μm and 1 μm, Struers, USA) to obtain a mirrorfinish and with a subsequent cleaning of samples by sonication in DIwater.

2.3. Characterization of anodized surfaces

2.3.1. Scanning electron microscopyThe microstructure and morphological features of the anodized

layers were examined by SEM (Zeiss Sigma VP, Germany), using an in-lens detector with an acceleration voltage of 1 kV and fixed specimendistance (∼4mm). Additional cross-sectional micrographs were ob-tained from each oxide layer using similar parameters, in order to de-termine the thicknesses of the different oxide layers. Prior to the cross-sectional imaging, one side of the anodized plates were sequentiallypolished using different SiC paper grades (#80 to #1200 grit) andpolycrystalline diamond (3 μm and 1 μm, Struers, USA) to obtain amirror finish and with a subsequent cleaning of samples by sonicationin ethanol (94.2 %, Altia, Finland). The pore sizes and thickness valuesfor the oxide layers were determined from the SEM micrographs usingfreely available ImageJ software (National Institutes of Health (NIH)Bethesda, MD, USA) [W2].

2.3.2. Goniometry measurementsThe wetting properties of the anodized surfaces were investigated

by contact angle (CA) measurements (KSV CAM200, Finland). A drop ofwater (4 μL) was applied separately on each surface and the watercontact angle was measured for 60 s. Measurements for each type sur-face was conducted in triplicate.

2.3.3. Raman spectroscopyThe chemical composition of anodized surfaces was determined by

Raman spectroscopy using a single stage dispersive Raman microscope(LabRAM HR UV-NIR, Japan). Measurements were conducted using a633 nm laser source, with 4 accumulations during 20 s of integrationtime and spectra were obtained from 4 random spots on each surface.

2.3.4. NanoindentationThe nanomechanical properties of oxide surfaces were determined

by Nanoindentation experiments (Hysitron Triboindenter Ti 950,Bruker) using a Berkovich diamond tip geometry. The Oliver-Pharrmethod was used to deduce the mechanical properties from the loading-unloading curves [28]. Samples were tested using an identical proce-dure and at room temperature. In total, 20 indentations on each samplewere made using a standard load control quasi-static method withcorresponding loading, holding and unloading times of 5, 1 and 5 s. Themaximum load applied during the measurements was 500 μN.

2.4. Electrochemical characterization

The corrosion protection performance of the obtained surfaces wasstudied by linear sweep voltammetry (LSV) and Tafel extrapolation. Allthe electrochemical measurements were performed in 3.5 wt.% sodiumchloride (NaCl), in order to simulate the salinity of seawater, using athree-electrode setup and an exposed sample surface area of 0.785 cm2.All experiments only commenced after 1 -h of immersion, once a stableopen circuit potential (OCP) was obtained. An Ag/AgCl/KClsat was usedas the reference electrode (RE), platinum (Pt) sheet as the counterelectrode (CE), and anodized or lignin coated steel surfaces as theworking electrode (WE). For LSV measurements, the potential sweeprange was between -0.4 V to 0.4 V vs. OCP with a sweep rate of 0.5mV/s. The corrosion current density was evaluated from the resulting LSVcurve from the intersection of anodic and cathodic curves at the cor-rosion potential. As corrosion current density is directly proportional tocorrosion rate, lower values indicate improved corrosion resistance.

2.5. Pull-off adhesion measurements

The adhesion strength of lignin coatings on the oxide or bare steelsurfaces was investigated by using pull-off adhesion tests. Pull-off ad-hesion strength was determined following the standard method ASTMD4541−17 [29]. The results from this measurement quantify the re-quired force to detach a dolly glued to the surface of a coating. Briefly,three aluminium dollies were glued to the surface of plasticized lignincoatings, using a two-component epoxy adhesive (EP11HT 1:1, Re-sinlab, Wisconsin, USA) and left to cure at room temperature for 24 h.After curing, the dollies were detached with the pull-off test device(PosiTest AT-M, DeFelsko) and quantitative values for the adhesionstrength of each surface was obtained. In addition, the appearance ofthe surfaces after the measurements were visually examined to ascer-tain the mode of coating failure, e.g. adhesion or cohesion, and the areapercentage of adhesion/cohesion failure was determined from digitalimages of the surfaces by using ImageJ software.

3. Results and discussion

3.1. Effect of plasticizer on lignin coatings

Differential scanning calorimetry (DSC) measurements were con-ducted to investigate the effect of a plasticizer, i.e. triethyl citrate(TEC), on glass transition temperature (Tg) of lignin. As illustrated inFig. 1, sample containing 5 wt.% of the plasticizer (PL) demonstrates aslight decrease in Tg values (78 °C cf. 86 °C for the non-plasticized re-ference lignin). The relatively small drop in Tg value of the PL indicatesthat 5 wt.% of TEC results in the plasticization of the kraft lignin, tosome extent [30]. Furthermore, the Tg value obtained for the referencelignin (RL) is also smaller than the value measured for the unmodifiedkraft lignin (KL, 151 °C), which is in the same range as previously re-ported values for the same type of lignin [31,32]. This is attributed tothe solvent-induced plasticization effect that occurs due to presence ofresidual solvents in the polymer’s structure [17].

The presence of residual solvents in the reference lignin (RL) asopposed to the unmodified kraft lignin (KL) is also evident in thermo-gravimetric analysis (TGA) and the corresponding derivative curves ofRL sample (Figure S1, supporting information), with observation of

Table 2Parameters used for the anodization of S355 steel surfaces.

Sample code Utilized current density (mA/cm2) Solution temperature (°C) Anodization duration (s) Electrode spacing (mm)

O-24 24 80 300 45O-50 50 80 300 45O-120 120 80 300 45

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decomposition peaks centered at ∼ 145 °C (related to 1-methoxy-2-propanol solvent with boiling temperature of ∼ 120 °C) and ∼ 240 °C(related to diethylene glycol monobutyl ether solvent with boilingtemperature of ∼ 230 °C).

As can be seen from Fig. 2, the optical microscopy (OM) imagesfurther illustrates the plasticization effect of TEC. For the RL coatings(Fig. 2a), it was observed that after drying (48 h, 50 °C) the coatings(average thickness of 19.2 ± 0.8 μm) possess a cracked texture, unlikethe PL coatings (Fig. 2b). Notwithstanding, a few unpropagated localcracks - shown by the circled region in Fig. 2b – can be observed on thesurface of PL coatings (average thickness of 15.7 ± 0.8 μm), whichsuggest that the degree of plasticization is relatively small, and that

these coatings may be susceptible to crack formation with e.g. increasedlevels of thickness or lignin concentration in the coatings. Nevertheless,this result is in line with previously reported results demonstrating thecapability of TEC as a plasticizer of lignin [27,33,34].

It is worth noting that the non-plasticized reference lignin coatingswere always cracked, irrespective of whether the coatings were pre-pared on bare steel or anodized surfaces (Figure S2, supporting in-formation). Consequently, only plasticized lignin coatings were selectedfor further investigation in this study.

3.2. Chemical characteristics of reference and plasticized lignin

Fig. 3 displays the infrared spectra of the reference lignin (RL) andthe plasticized lignin (PL). The samples show similar spectral char-acteristics, with the exception of the bands centered around∼3400 cm−1 and ∼1700 cm−1 that represent the OeH stretching inhydroxyl and C]O stretching of unconjugated lignin carbonyl groups,respectively [35,36]. A comparison of the RL and PL spectra highlightsthat the OeH stretching (3384 cm−1) and C]O stretching (1703 cm−1)bands for lignin demonstrate a shift to higher wavenumbers in thepresence of TEC (3396 cm−1 and 1724 cm−1), which may result fromweaker intermolecular/intramolecular hydrogen bonds within theplasticized lignin sample as noted by previous studies [35,37]. Thishypothesis is in line with the assumption that TEC acts as a plasticizerthat increase the structural spacing in lignin through insertion betweenthe polymeric chains of the lignin structure [30]. Moreover, strongerhydrogen bonds in the lignin structure results in a higher glass transi-tion temperatures (Tg) [35] as was observed for RL when compared toPL (Fig. 1). The assignment of the IR bands for RL and PL coatings arelisted in Table 3.

3.3. Production of anodized surfaces

Anodized surfaces were produced by the electrochemical galvano-static treatment of steel substrate surfaces in 25 wt.% NaOH at 80 °C atthree different current densities of 24, 50 and 120mA/cm2 using athree-electrode setup [21,25]. Typical time-potential curves for theanodization procedure are shown in Fig. 4, along with the visual ap-pearance of the resultant oxide surfaces. In general, the anodizationprocedure on S355 steel closely followed the previously reported ano-dization results [25], with the formation of a uniform blue anodic oxideat the potential of ca. 1.9 V vs. Ag/AgCl/KClsat at constant currentdensity of 24mA/cm2 (O-24). An increase in the current density to50mA/cm2 (ca. 2.0 V vs. Ag/AgCl/KClsat) produced a multicolour di-chroic oxide layer (O-50), whilst at 120mA/cm2 (ca. 2.3 V vs. Ag/

Fig. 1. DSC curves of the reference and plasticized lignin (+5wt.% triethylcitrate). The thermograms represent the second heating cycle from the mea-surement of each sample.

Fig. 2. Optical microscopy images of a) reference lignin, RL and b) plasticizedlignin (PL) coatings after 72 h drying at room temperature. The circled region in(b) highlights the presence of a crack within the PL coating (scale bar: 100 μm).

Fig. 3. ATR-FTIR Transmittance spectra of a) reference lignin (RL) and b)plasticized lignin (PL).

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AgCl/KClsat), the oxide layer produced featured a similar multicolourappearance of a darker shade (O-120). The difference in the observedoptical characteristics between samples follows the same trend as foundpreviously [21]. The anodic oxide surfaces of O-24 and O-50 were re-sistant to discolouring or oxide removal upon rubbing with a cottonswab, whereas the O-120 sample became discoloured when wiped andtraces of oxide removal, i.e. scratches, could be observed on the surface.

Additionally, during the anodization process evolution of oxygen atthe steel surface (WE) and the formation of a violet colour in proximityto the surface undergoing anodization was noticed in an otherwise clearsolution. On completion of the anodization, the solution once morebecame clear and the appearance of the violet colour during anodiza-tion was most noticeable at the highest applied current density, whenthe anodization reaction was most vigorous. This effect is attributed tothe formation of ferrate anions (FeO4

−2) due to the dissolution of thesteel surface [21,25]. This was subsequently confirmed by atomic ab-sorption spectroscopy (AAS) analysis of the NaOH solution followingthe anodization procedure (Table S1, supporting information), whichrevealed the presence of dissolved Fe and Mn from the S355 steel.

3.4. Morphological features of the anodic coatings

SEM micrographs at two different magnifications (25000x and75000x) from the surface of the different anodic coatings are shown inFig. 5. Unlike the sanded steel surface which has no obvious oxide filmand a relatively smooth morphology (Fig. 5a), the anodized surfacespossess a nanostructured oxide film of varying porosity and roughness,the morphology of which can be drastically affected by the anodizingcurrent density. For example, in the case of the O-24 sample (Fig. 5b),the surface structure appears heterogeneous; the oxide is mostly

covered with pores while some areas exhibit less pores and a more solidappearance, which indicates that the anodizing procedure is not com-pletely homogeneous along the sample surface. As the anodizationcurrent density is increased (O-50), the oxide layer appears more 3D-like, showing a more porous network (Fig. 5c) and at the highest ap-plied anodization current density (O-120), the appearance of a roughand highly porous network is most obvious (Fig. 5d). Analysis of themicrographs obtained from various anodized surfaces showed thatthere was little discernible difference in the individual pore size be-tween the samples – the average measured pore size was 26.2 nm(±10.9 nm). However, the highly anodized surfaces clearly containeda higher pore density and distribution. These observations are in ac-cordance with previous study [26] that demonstrated higher levels ofanodization resulting in an increased level of oxide roughness andporosity.

Table 4 lists the oxide layer thickness values measured from thecross-sectional micrographs obtained from the anodized samples(Figure S3, supporting information). From Table 4, it can be clearlyseen that an increase in the anodization current density for a constantanodization time results in the formation of thicker oxide layers.Naturally, a higher current density results in a higher driving force foroxidation and gives rise to an increased thickness of anodized layers asnoted previously by Chen et al. [26] during potentiostatic anodizationof steel.

3.5. Wetting properties of the anodized surfaces

Goniometry measurements were performed to provide a comparisonof the wetting properties of the anodic oxide coatings, i.e. hydro-phobicity/hydrophilicity, and related surface energy of the oxides thatcan affect their interaction with the subsequently applied lignin-basedcoating (Fig. 6). As can be seen, the sanded surface of S355 steel pos-sesses a hydrophilic character (contact angle, CA ∼47°), the level ofwhich is seen to increase in line with the extent of surface anodization.An increase in current density during the surface treatment clearly re-sults in enhanced hydrophilicity, with the greatest hydrophilic beha-viour – i.e. lowest CA value – obtained for O-120 samples (CA ∼14°).The enhanced hydrophilicity of the surfaces as a function of anodiza-tion current density confirms the noted increase in roughness of thesurfaces from the SEM observations as it is known that increasing sur-face roughness enhances the hydrophilicity of an inherently hydrophilicsurface [40].

This increased hydrophilicity of the anodized surfaces is in ac-cordance with the previous findings of Burleigh et al. [21], in which theformation of anodic coatings on steel resulted in enhanced hydro-philicity. Nevertheless, Chen et al. [26], have also shown that increasedlevels of anodization can also decrease the hydrophilicity of the

Table 3. Infrared (IR) bands and their assignments for reference lignin (RL) and TEC-plasticized lignin (PL) samples [36–39].

Peak No. Reference Lignin Wavenumber (cm−1) Plasticized lignin Wavenumber (cm−1) Assignment

1 3384 3396 OeH stretching2 2934 S CeH stretching3 2867 S CeH stretching4 1703 1724 C]O stretching (unconjugated)5 1593 S aromatic skeletal vibration+C]O stretching6 1510 S aromatic skeletal vibration7 1455 S asymmetric bending deformation of methyl and methylene groups8 1424 S CeH in-plane deformation with aromatic ring stretching9 1366 S OeH in-plane deformation vibration of phenolic OH10 1264 S CeO of guaiacyl ring11 1210 S CeC+C-O stretching12 1124 S CeH in-plane deformation vibration of guaiacyl ring13 1078 S CeO deformations of secondary alcohols and aliphatic ethers14 1028 S aromatic CeH in-plane deformation+CeO deformation in primary alcohols

S: Similar band position.

Fig. 4. Time-potential graphs for the galvanostatic anodization of S355 surfacesat various current densities and images of the resultant oxide surfaces (platedimensions of 25mm×25mm).

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resultant oxide surfaces even though an increase in the roughness andporosity was observed, although in their case the applied degree ofanodization was much higher than that for the samples investigated inthis study.

3.6. Compositional characteristics of the anodic coatings

Composition of the anodized surface coatings was examined byRaman spectroscopy shown in Fig. 7. The results demonstrate that theoxide surface obtained from the O-24 surface is comprised solely ofmagnetite (Fe3O4) as indicated by the characteristic strong peak at670 cm−1 and a shoulder at 331 cm-1 [41]. When the anodizationcurrent density is increased to 50mA/cm2 (O-50), new peaks - at225 cm-1 and 291 cm-1 - related to hematite (α-Fe2O3) can be detectedwithin the surface matrix [42], although not in all cases (Figure S4,supporting information). At the highest anodization current density of120mA/cm2 (O-120), the hematite-related peaks indicate a clear in-crease in the relative amount of α-Fe2O3 present and an additional peak- indicative of the presence of maghemite (γ-Fe2O3) – can be seen at∼500 cm-1 [42]. These results are in agreement with findings by Chenet al. [26], who reported the formation of pure magnetite at potentialsless than 2.0 V and the appearance of hematite and maghemite on the

Fig. 5. SEM micrographs of samples a) sanded steel, b) O-24, c) O-50 and d) O-120 oxides produced at 24mA/cm2, 50mA/cm2 and 120mA/cm2, respectively.

Table 4Average thickness values determined from the cross-sectional micrographs ofthe anodized samples using ImageJ software.

Sample Thickness (μm) Standard Deviation (μm)

O-24 1.56 0.17O-50 2.55 0.19O-120 6.20 0.90

Fig. 6. Average water contact angle (CA) values obtained after 60 s for sandedand differently anodized steel surfaces.

Fig. 7. Compositional characteristics of steel and anodized surfaces of O-24, O-50 and O-120 as measured by Raman spectroscopy.

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surface of magnetite at anodization potentials above 2.0 V in 50wt.%NaOH at 30 ℃. However, it must be noted that the Raman measure-ments conducted on both the O-50 and O-120 samples demonstratedvariation in the intensity of recorded iron (II) oxides at different loca-tions across the sample surfaces, while the magnetite peak intensity didnot vary considerably (Figure S4, supporting information). The pre-sence of compositional differences within these highly anodized layerscould occur due to differences in current density at the surface of theworking electrode (S355) as a result of inhomogeneities of the steelsurface, thus leading to different driving force at different parts of thesample surfaces (Figure S4, Supporting information).

3.7. Nanomechanical properties of the anodized surfaces

The nanomechanical properties of the prepared samples were ex-amined via nanoindentation measurements and the results are outlinedin Table 5. The non-anodized steel shows the highest hardness and ef-fective modulus. As can be seen, an increase the degree of anodizationleads to a decrease in the measured values of both hardness and ef-fective modulus of the coatings produced. Although it has been pre-viously reported that hematite (α-Fe2O3) - which is present in both theO-50 and O-120 samples - has higher hardness values when comparedto magnetite Fe3O4 [43]. The decrease in hardness and effective mod-ulus from O-24 to O-120 in this study is likely because of increasedsurface oxide porosities rather than the inherent mechanical char-acteristics of the iron oxides present.

3.8. Electrochemical characterization of surfaces

Corrosion performance of samples with and without oxides or lignincoatings was estimated with the Tafel method in 3.5 wt.% NaCl solu-tion. Tafel plots are shown in Fig. 8, and the measured corrosion po-tentials and current densities are summarized in Table 6. The untreatedS355 steel exhibits the highest susceptibility to corrosion with a cor-rosion current density of 15 μA/cm2 and corrosion potential of−700mV vs. Ag/AgCl/KClsat. In contrast, when the surface was coatedwith a plasticized lignin (PL) layer, the corrosion current density de-creased by an order of magnitude to 1.6 μA/cm2, while exhibiting aslightly more positive corrosion potential.

Fig. 8a shows that the corrosion current density (and thereforecorrosion rate) for the anodized surfaces is lower (9.5 – 3.5 μA/cm2)when compared to bare S355, and that there is a direct correlationbetween the level of applied anodization current density and corrosionprotection performance. This observed increase in the level of corrosionprotection with higher anodization current density likely results fromthe increase in surface oxide thickness (section 3.4), which extends thediffusion pathways of the corrosive moieties. Also, a similar trend forincreased corrosion protection as a function of oxide thickness has beenreported previously for oxidized carbon steel [44]. Nevertheless, al-though steel samples with an anodized oxide layer and no coating (O-24, O-50, O-120) all display a reduction in the corrosion rate whencompared to bare steel, they are not as effective as the lignin coating onthe same substrate as shown in Table 6. This is attributed to the inertnature of lignin, whose inhibition properties limits the electrochemicalreactions at both anodic and cathodic branches (Fig. 8) [15,16], andfurther isolates the underlying steel from the corrosive electrolyte.

The application of lignin coatings onto all three anodized surfaces(O-24 + PL, O-50 + PL, O-120 + PL) results in a marked decrease inthe corrosion current density for all these samples (Fig. 8b and Table 6).The values for all three anodized surfaces in combination with the PLcoating show highly similar levels of improved corrosion protection cf.either the untreated steel or untreated steel with the PL coating(Table 6). Another noticeable difference in Fig. 8 is the corrosion po-tential of O-120 + PL coating, which demonstrates a drastic shift to-wards more positive potentials and is a sign of increased nobility of theO-120 + PL coating combination. Ahlström et al. [45] have producedrelatively pore-free hematite and magnetite oxides on the steel surfaceand conducted potentiodynamic polarization (PDP) in a chloride

Table 5Nanomechanical properties of sanded S355 steel and the anodized layers.

Sample Hardness (GPa) Er Effective (reduced)modulus (GPa)

Max displacement (nm,under 0.5 mN load)

S355 6.99 ± 1.04 183 ± 24 35O-24 4.42 ± 1.07 157 ± 20 67O-50 1.49 ± 0.52 84 ± 14 117O-120 1.34 ± 0.50 48 ± 13 170

Fig. 8. Linear sweep voltammetry spectra for the bare steel and anodized sur-faces a) before and b) after the addition of plasticized lignin coatings.

Table 6Measured corrosion potentials and calculated current densities from the Tafelplots of bare steel and the different coatings, after 1 -h immersion in 3.5 wt.%NaCl.

Sample Corrosion potential (mV) vs.Ag/AgCl/KClsat

Corrosion current density(μA/cm2)

Bare S355 −700 ± 19 14.10 ± 1.10O-24 −604 ± 7 8.60 ± 0.50O-50 −614 ± 5 5.20 ± 0.60O-120 −600 ± 4 3.40 ± 0.30Bare S355+PL −642 ± 10 1.63 ± 0.15O-24 + PL −625 ± 23 0.66 ± 0.15O-50 + PL −610 ± 81 0.49 ± 0.16O-120 +PL −289 ± 12 0.70 ± 0.07

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containing solution, illustrating that hematite exhibits higher nobility—by several hundreds of mV— when compared to an oxide composedof magnetite. Therefore, the observed nobility of O-120 + PL coatingcould be related to the compositional characteristics of the underlyingporous oxide layer (which contains the most of noble oxide, hematite).Thus, when the porous oxide is covered by PL coating, the contributionof oxide’s composition on the measured corrosion potential becomesmore pronounced. This can also be seen in the LSV spectra of the oxidesurfaces without lignin (Fig. 8a), which demonstrate only slightlyhigher corrosion potential values for all the three surfaces when com-pared with the pure steel, likely due to the porous nature of all theoxides that allow the permeation of electrolyte through the oxide layertowards the interface with the underlying steel.

3.9. Adhesion strength of plasticized lignin to S355 steel and anodizedsurfaces

The adhesion strength of plasticized lignin on the bare and anodizedsteel surfaces was investigated by pull-off adhesion measurements tofurther probe the functionality of the produced coatings [15,29]. Fig. 9,emphasizes that the adhesion strength of spray-coated PL coatings(average coating thickness of 27.3± 1.3 μm) on anodized surfaces issignificantly higher than adhesion on bare steel, with the highestaverage adhesion strength of 4.32MPa for O-24 + PL sample cf.2.38MPa for the S355+PL coating sample. Furthermore, the adhesionstrength of PL coatings on the thicker oxides follow a decreasing trend(relative to O-24), with average values of 3.86MPa for O-50 + PL and3.22MPa for O-120 + PL. Nevertheless, the adhesion strength ofplasticized lignin on the oxide surfaces was considerabley higher thanthat of the bare steel. Consideration of the coating failure mode afterthe pull-off measurements, showed that all the surfaces demonstrate amixture of cohesion failure in the plasticized lignin and adhesion failurebetween the plasticized lignin and underlying surface. Furthermore, theratio of adhesion:cohesion failure was found to slightly increase from25:75 for O-24 + PL to 30:70 for O-50 + PL and 35:65 for O-120 + PL(Table S2 and Figure S5, supporting information).

The effect of interface roughness on adhesion strength of organiccoatings on metallic substrates is well-stablished [46–48], and it isgenerally accepted that for a surface an optimum roughness results inthe highest adhesion strength [49]. Based on the results from the sur-faces investigated, it can be determined that anodization with 24mA/cm2 (O-24) provides the most favourable interfacial profile for theadhesion of plasticized lignin to the S355 steel substrate.

4. Conclusions

This study demonstrates the enhanced anticorrosion and adhesion

capabilities of sustainable triethyl citrate-plasticized lignin coatings onhigh-strength low-alloy steel (S355) when combined with an anodizingpre-treatment in heated 25 wt.% NaOH. Three different anodizationpre-treatment conditions were investigated in order to produce oxidelayers with different morphologies, surface wettability and composi-tions. It was found that all anodized surfaces considerably increased theadhesion strength and the corrosion protection of the lignin coating onthe steel surface. Of the three anodization conditions explored, O-24(anodized with 24mA/cm2) provided the best interfacial properties forthe adhesion of plasticized lignin coatings. In all three cases, the ap-plication of plasticized lignin coating on the oxide surfaces resulted inimproved corrosion protection with the observed corrosion currentdensity decreased by an order of magnitude (down to 0.49 ± 0.16 μA/cm2 for O-50 + plasticized lignin compared with 1.63 ± 0.15 μA/cm2

for bare steel+ plasticized lignin and 14.10 ± 1.10 μA/cm2 for baresteel without any coating). This enhancement in performance wasfound to result from the synergistic of corrosion protection provided bythe combination both the anodized oxide surface and lignin coatings,coupled with the improved interfacial adhesion between the layers.

CRediT authorship contribution statement

Arman Dastpak:Methodology, Formal analysis, Investigation, Datacuration, Writing - original draft, Writing - review & editing,Visualization. Pyry-Mikko Hannula: Methodology, Formal analysis,Investigation, Data curation, Writing - original draft, Writing - review &editing.Mari Lundström: Conceptualization, Methodology, Validation,Resources, Writing - review & editing, Supervision, Project adminis-tration, Funding acquisition. Benjamin P. Wilson: Conceptualization,Methodology, Validation, Resources, Writing - review & editing,Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgements

This research was funded by Technology Industries of Finland/Janeand Aatos Erkko Foundations, Academy of Finland “Future Makers:Biorefinery Side Stream Materials for Advanced Biopolymer Materials(BioPolyMet)” and Academy of Finland (NoWASTE, No. 297962).Furthermore, this work utilized facilities provided by the RawMatTERSFinland Infrastructure (RAMI), supported by the Academy of Finland, aswell as OtaNano Infrastructure at Aalto University. We also are gratefulfor the support by the Finn CERES Materials Bioeconomy Ecosystem.Additionally, special thanks to Esa Virolainen (SSAB Europe Oy) forproviding steel sheets, and Michal Trebala for conducting nano-mechanical analysis of the coatings of this study.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in theonline version, at doi:https://doi.org/10.1016/j.porgcoat.2020.105866.

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