Corrosion Resistance of Anodic Layers Grown on 304L Stainless Steel at Diﬀerent Anodizing Times and Stirring Speeds

Article Corrosion Resistance of Anodic Layers Grown on 304L Stainless Steel at Diﬀerent Anodizing Times and Stirring Speeds

Laura Patricia Domínguez-Jaimes 1, María Ángeles Arenas Vara 2, Erika Iveth Cedillo-González 1 , Juan Jacobo Ruiz Valdés 1, Juan José De Damborenea 2 , Ana Conde Del Campo 2 , Francisco Javier Rodríguez-Varela 3 , Ivonne Liliana Alonso-Lemus 4 and Juan Manuel Hernández-López 1,*

1 Universidad Autónoma de Nuevo León, Facultad de Ciencias Químicas, Ciudad Universitaria, Av. Universidad s/n., Nuevo León C. P. 66455, Mexico; [email protected] (L.P.D.-J.); [email protected] (E.I.C.-G.); [email protected] (J.J.R.V.) 2 Department of Surface Engineering Corrosion and Durability, National Center for Metallurgical Research, CENIM-CSIC, Avda. Gregorio del Amo, 8, 28040 Madrid, Spain; [email protected] (M.Á.A.V.); [email protected] (J.J.D.D.); [email protected] (A.C.D.C.) 3 Sustentabilidad de los Recursos Naturales y Energía, Cinvestav Unidad Saltillo, Av. Industria Metalúrgica, 1062, Ramos Arizpe 25900, Coahuila, Mexico; [email protected] 4 Conacyt, Cinvestav Unidad Saltillo. Av. Industria Metalúrgica 1062, Ramos Arizpe 25900, Coahuila, Mexico; [email protected] * Correspondence: [email protected] or [email protected]; Tel.: +52-1-55-11941410

Received: 30 September 2019; Accepted: 25 October 2019; Published: 29 October 2019

Abstract: Different chemical and physical treatments have been used to improve the properties and functionalities of steels. Anodizing is one of the most promising treatments, due to its versatility and easy industrial implementation. It allows the growth of nanoestructured oxide films with interesting properties able to be employed in different industrial sectors. The present work studies the influence of the anodizing time (15, 30, 45 and 60 min), as well as the stirring speed (0, 200, 400, and 600 rpm), on the morphology and the corrosion resistance of the anodic layers grown in 304L stainless steel. The anodic layers were characterized morphologically, compositionally, and electrochemically, in order to determine the influence of the anodization parameters on their corrosion behavior in a 1 0.6 mol L− NaCl solution. The results show that at 45 and 60 min anodizing times, the formation of two microstructures is favored, associated with the collapse of the nanoporous structures at the metal-oxide interphace. However, both the stirring speed and the anodizing time have a negligeable effect on the corrosion behavior of the anodized 304L SS samples, since their electrochemical values are similar to those of the non-anodized ones.

Keywords: anodization; stainless steel; anodic layer; corrosion resistance

1. Introduction In recent years, surface engineering has developed physical, chemical, mechanical, and microstructural modiﬁcation treatments on steel to expand the range of applications. Literature is quite extensive about the surface treatments carried out to date [1–6]. Nevertheless, the anodizing process that was initially developed for aluminum alloys [7,8] and later for other metals, such as Mg [9], Ga [10], Co [11], W [12], Nb [13], Zr [14], Sn [15], and Ti [16,17], has emerged as a new alternative to surface functionalization of iron base alloys of particular interest in ﬁelds such as photocatalysis, sensors, corrosion, environmental remediation, and biomedical to name a few [18–30].

Coatings 2019, 9, 706; doi:10.3390/coatings9110706 www.mdpi.com/journal/coatings Coatings 2019, 9, 706 2 of 14

In photocatalysis applications, anodic layers have been considered an adequate way to achieve greater efficiency due to the increase in surface area. First studies about semiconducting properties and growth of nanostructured layers on ferrous materials were reported by Prakasam et al. [31] who achieved thicknesses of 300–600 nm and pore diameters from 50 to 250 nm on iron. Further, Zhang et al. have explored the photocatalytic decomposition of methylene blue [19] and the degradation of azo dye [20] by anodic iron oxides in different nanostructures. Likewise, the nanostructures growth on iron and its alloys is quite promising for water splitting reaction under solar or visible-light illumination [23,24]. Rangaraju et al. [25] studied different nanostructure configurations of photoanodes based on anodic iron oxide for photoelectrochemical 1 water oxidation. These authors obtained better photocatalytic properties in 1 mol L− KOH with Air mass 1.5 Global light illumination when the photoanode has a two-layered oxide structure comprised by a top layer with nano-dendrite morphology and a bottom layer of nanoporous morphology. Moreover, these authors state that layer close to the metal provide corrosion resistance compared with the top layer [26]. On the other hand, in terms of biomedical applications, Asoh et al. [29] reported the growth of anodic layers on type 304 stainless steel in an anodizing bath of hydrogen peroxide and sulfuric acid is suitable to prepare the surface prior to the deposit of synthetic bioactive hydroxyapatite in biomedical implant devices. However, even though corrosion resistance is an important issue in medical devices and applications [32,33], literature is scarce in the electrochemical characterization of anodic layers on stainless steel. The range of applications and properties, which can be tailored and improved by manipulating the nanostructure is wide. As the morphology and structural characteristics of the anodic layers are deeply correlated with the anodizing conditions, the control of the anodizing parameters such as potential applied, temperature, electrolyte composition and anodizing time is key to the formation of the nanoestructure of the anodic layers [18,34–36]. Various researchers have studied the growth of anodic layers on stainless steels, being 316L [25,26] and 304 [27–30] the most commonly used. The studies on 316L SS indicate that the addition of water to organic baths improves the diameter and thickness of the grown layers [37]. On the other hand, the use of fluoride-free solutions enables to obtain anodic layers composed of a mixture of hydroxides-oxides-sulfates [38]. Literature reporting the growth of anodic layers on stainless steel 304 preferentially use a mixture of glycerol or ethylene glycol and NH4F and different additions of H2O as organic anodizing bath. Theresearch performed in these media allowed concluiding three main aspects: (1) the anodic layers have similar Fe, Cr, Ni contents to the substrate [39], (2) the concentration of H2O concentration in the anodizing bath determines the morphological characteristics of the anodic layers [40] and (3) the voltage applied favored different growth mechanisms leading to the fabricationof anodic layers with different chemical compositions [29]. Nevertheless, most of the existing works so far are focused on the growth and chemical/ morphological characterization of the anodic layers on these ferrous materials [29,35,39–42], but not on their influence on the electrochemical response or corrosion resistance properties. For that reason, the objective of this work is to explore the corrosion properties in a NaCl solution of the anodic layers 1 grown on 304L SS at different stirring speeds and anodizing times inthylene glycol with 0.1 mol L− 1 NH4F and 0.1 mol L− H2O.

2. Materials and Methods Disks of diameter 22.2mm of 304L SS (18.3 wt.% Cr, 8.11 wt.% Ni, 1.52 wt.% Mn, 0.27 wt.% Si) supplied by Jay Steel Corporation, Mumbai, India, were ground using successive grades of SiC paper up to 2000 grade, degreased with detergent and rinsed with tap water followed by deionized water. Anodic layers were formed in a two-electrode cell by anodizing the 304L SS substrates at constant 1 1 voltage at 50 V in ethylene glycol electrolyte containing 0.1 mol L− NH4F and 0.1 mol L− H2O for 15, 30, 45 and 60 min at a constant temperature of 5 ◦C, under stirring conditions at 0, 200, 400 and 600 rpm. Finally, a platinum mesh was used as a cathode. Coatings 2019, 9, 706 3 of 14 Coatings 2018, 8, x FOR PEER REVIEW 3 of 14

Plan view morphology of the anodic oxide ﬁlmsfilms was examined by ﬁeldfield emission gun scanning electron microscopy (FEG–SEM) utilizing Hitachi 4800 instrument (Tokyo, Japan) equipped with Energy DispersiveDispersive X-rayX-ray Spectroscopy Spectroscopy (EDX) (EDX) facilities, facilities, operated operated at 15at keV15 keV for for EDX EDX analysis analysis and and 7 keV 7 forkeV secondary for secondary electron electron imaging. imaging. Each Each of the of areasthe areas and and local local EDX EDX analysis analysis results results are are quoted quoted as anas averagean average of 3of measurements. 3 measurements. The electrochemical measurements were done in triplicate in a a conventional conventional three-electrode three-electrode cell. cell. The working electrodes were were either either the the non-anodized non-anodized or or anodized anodized samples, samples, an an Ag/AgCl Ag/AgCl electrode electrode (3 −1 1 (3mol mol L L −KCl),KCl), was was used used as asreference reference electrode electrode and and the the counter counter electrode electrode was was a a platinum wire. The −11 solution to evaluateevaluate thethe corrosioncorrosion propertiesproperties waswas aa 0.60.6 molmol LL− NaCl at room temperature. Corrosion behavior was evaluated by potentiodynamicpotentiodynamic polarization using a Gamry Reference 600 potentiostat. The potentiodynamic curves curves were were conducted conducted at at a ascan scan rate rate of of0.16 0.16 mV/s. mV /Befores. Before starting starting the thescan, scan, the thesample sample remained remained in the in solution the solution for 15 formin 15 to minstabilize to stabilize the open the circuit open potential circuit potential (OCP). The (OCP). potential The potentialscan was scanstarted was in startedthe anodic in the direction anodic directionfrom a potential from a potentialvalue of 0.3 value V with of 0.3 respect V with to respectthe OCP to to the 1 OCPV with to respect 1 V with to respectthe Ag/AgCl to the electrode Ag/AgCl or electrode when the or sample when the reached sample a current reached density a current of density0.5 A/cm of2. 0.5Electrochemical A/cm2. Electrochemical Impedance Impedance Spectroscopy Spectroscopy (EIS) meas (EIS)urement measurement were performed were performed in a VSP-300 in a VSP-300Biologic Biologicpotentiostat, potentiostat, applying applying a sinusoidal a sinusoidal signal of 10 signal mV ofpeak 10 to mV peak peak of toamplitude peak of amplitude vs. OCP. The vs. frequency OCP. The frequencyrange was rangefrom 100 was kHz from to 100 kHzmHz toand 100 recording mHz and 10 recording points per 10 decade. points Measurements per decade. Measurements were carried wereout at carried 0 and 24 out h atof 0immersion. and 24 h of The immersion. experimental The experimental data were analyzed data were using analyzed the ZVIEW using software the ZVIEW (v. software3.1c). (v. 3.1c).

3. Results and Discussion

3.1. Influence Influence of Stirring Speed on the Growth of Anodic Layers To investigateinvestigate the eeffectffect ofof thethe stirringstirring speedspeed onon thethe growthgrowth ofof thethe anodicanodic layers,layers, thethe anodizinganodizing processes werewere carriedcarried out out at at four four di differentfferent stirring stirring speeds speeds (0, (0, 200, 200, 400, 400, and and 600 600 rpm). rpm). Figure Figure1 plots 1 plots the currentthe current density density versus versus time recordedtime recorded during during the anodizing the anodizing processes processes performed performed at different at different stirring speeds.stirring speeds. The shape The of shape the anodizing of the anodizing curve is curve characteristic is characteristic of the growthof the growth of of porous of of porous anodic anodic layers reportedlayers reported for diff erentfor different valve metals valve [ 19metals,43–45 [19,43] and–45] it comprises and it comprises three stages: three (I) anstages: abrupt (I) decreasean abrupt in thedecrease current in densitythe current during density the firstduring seconds the first of theseco anodizingnds of the processanodizing related process to the related growth to the of agrowth dense oxideof a dense film oxide on the film surface on the of surface the metal of the known metal as know barriern as layer, barrier (II) layer, a gradual (II) a increasegradual increase of the current of the densitycurrent density related torelated preferential to preferential dissolution dissolution zones randomly zones randomly distributed distributed on the surfaceon the surface of the barrier of the layer,barrier which layer, corresponds which corresponds to the nucleation to the nucleation of the pores of the and pores (III) and a step (III) of a stablestep of current stable current density density related torelated the coarsening to the coarsening of the anodic of the layeranodic [46 layer]. [46].

2.00 0 rpm 1.75 200 rpm 400 rpm 1.50 600 rpm ) 2 I II III 1.25 (mA/cm i 1.00

0.75

0.50 0 100 200 300 400 500 600 700 800 900 Anodization Time (s)

Figure 1. CurrentCurrent density–time density–time responses responses for foranodizing anodizing on 304L on 304LSS for SS 15 formin 15 in minethylene in ethylene glycol- NH4F-H2O electrolyte at different stirring speeds. glycol-NH4F-H2O electrolyte at diﬀerent stirring speeds.

Moreover, the charge density estimated by integrating the current density-time responses depends on the stirring speed. The charge density that passes through the system during the

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Coatings 2018, 8, x FOR PEER REVIEW 4 of 14 Moreover, the charge density estimated by integrating the current density-time responses depends onanodizing the stirring treatments speed. performed The charge for density15 min varies that passes from ~0.750, through ~0.864, the system ~1.027,during and ~1.061 the anodizingC/cm2, for 2 treatments0, 200, 400, performed and 600 rpm, for 15 respectively. min varies fromThis ~0.750,higher ~0.864,current ~1.027,density and (charge ~1.061 density), C/cm , forobserved 0, 200, 400,for andhigher 600 stirring rpm, respectively. rates (400, 600 This rpm) higher points current out density to an enhanced (charge density), transport observed of ionic for species higher from stirring the ratessolution (400, to 600 the rpm) metal/electrolyte points out to interface an enhanced and, transport therefore, of to ionic the species enhancement from the of solution the chemical to the metaldissolution/electrolyte mechanism interface as and,it has therefore, been reported to the in enhancement literature [47,48]. of the chemical dissolution mechanism as it hasThe beenelectrochemical reported in literatureresponse was [47,48 studied]. by polarization curves in 0.6 mol L−1 NaCl solution, 1 FigureThe 2. electrochemicalThe non-anodized response 304L SS was samples studied present by polarization a corrosion curves potential in 0.6 (𝐸 mol L) −of NaCl~−244.9 solution, mV, a −7 2 Figurepitting2 potential. The non-anodized (𝐸) of ~441.6 304L mV, SS samplesand a passive present current a corrosion density potential (𝑖) of ( Eaboutcorr) of 8.3 ~ × 244.910 A/cm mV, a. − 7 2 pitting potential (E ) of ~441.6 mV, and a passive current density (ipass) of about 8.3 10 A/cm . The anodic layers obtainedpit at different stirring speeds showed an electrochemical response× − similar Theto the anodic non-anodized layers obtained samples, at di characterizedﬀerent stirring by speeds 𝐸 showedvalues ranging an electrochemical about −312.1 response mV and similar 𝑖 toof the non-anodized−7 samples,2 characterized by Ecorr values𝐸 ranging about 312.1 mV and ipass of about about 4.10 × 10 A/cm . However, the pitting potential shifts toward− lower values, about ~329.5 4.10 10 7 A/cm2. However, the pitting potential E shifts toward lower values, about ~329.5 mV for mV for× the− anodic layers grown at 0, 200 and 400 rpm.pit While, for the oxide layer grown at 600 rpm theshowed anodic the layers lowest grown pitting at 0,potential, 200 and 400~140.5 rpm. mV, While, indi forcating the that oxide it layeris the grown most atprone 600 rpmto localized showed thecorrosion. lowest pitting potential, ~140.5 mV, indicating that it is the most prone to localized corrosion.

0.4 0 rpm 200 rpm 400 rpm 0.2 600 rpm ) Non-anodized 0.0 304L SS Ag/AgCl -0.2

-0.4 E (V vs. E vs. (V E

-0.6

-0.8 -10 -9 -8 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 10 10 10 2 i (A/cm ) Figure 2. 2. PotentiodynamicPotentiodynamic polarization polarization curves curves of the of sample the sampless anodized anodized for 15 formin, 15 at min,different at di stirringﬀerent stirringspeeds. speeds.

3.2. Influence Influence of Anodizing Time on the Growth of Anodic Layers Figure3 3 shows shows the the current current density-time density-time curves curves recorded recorded during during the anodizingthe anodizing treatments treatments at 0 rpm at 0 andrpm atand 600 at rpm 600 forrpm di ffforerent different anodizing anodizing times (15,times 30, (15, 45, 6030, min). 45, 60 The min). three The steps three described steps described in Figure in1 forFigure the anodizing1 for the anodizing process developed process developed for 15 min for at 15 diff minerent at stirringdifferent speeds stirring are speeds observed are but,observed the length but, ofthe each length one of varies each one significantly varies sign withificantly the stirring with the speed. stirring speed. As it shows in Figure3 3a,a, thethe firstfirst stagestage associatedassociated withwith thethe formationformation ofof thethe barrierbarrier layerlayer occursoccurs up to 500 s for the anodizing performedperformed atat 00 rpm,rpm, whilewhile at at 600 600 rpm rpm (Figure (Figure3b) 3b) it it is is shorter, shorter, about about 200 200 s. Thiss. This di ffdifferenceerence indicates indicates that that the the barrier barrier layer layer grows grows faster faster under under stirring stirring conditions conditions due todue the to mass the transfermass transfer is favored is favored under stirringunder stirring conditions. conditions Moreover,. Moreover, higher current higher densitiescurrent densities were also were observed also duringobserved the during anodization the anodization performed performed at 600 rpm. at 600 The rp currentm. The increasecurrent increase is also reflected is also reflected in the charge in the density,charge density, which is which about is 4 about times higher4 times at higher 600 rpm at 600 than rpm at 0 than rpm at for 0 arpm specific for a anodizingspecific anodizing time. It varies time. 2 2 fromIt varies ~0.710 from to ~0.710 ~3.626 to C /~3.626cm for C/cm 0 rpm2 for and 0 rpm from and ~1.631 from to ~1.631 ~4.438 to C /~4.438cm for C/cm 600 rpm,2 for 600 Figure rpm,3c. Figure 3c. From the charge density values, the theoretical thickness of the anodic layer can be estimated assumingFrom thatthe charge the entire density charge values, is used the for theoretical the formation thickness of FeOOH, of the acoordinganodic layer to Equationcan be estimated (1) This assuming that the entire charge is used for the formation of FeOOH, acoording to Equation (1) This compound has been previously reported as the main component of the anodic layers grown in ferrous materials [35,41,49].

𝐶 𝑀𝑤 𝑐𝑚 (1) 𝐴𝑛𝑜𝑑𝑖𝑐 𝑙𝑎𝑦𝑒𝑟 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 =𝑄 ∗ 𝐶 𝑐𝑚 𝑛𝐹𝜌

Coatings 2019, 9, 706 5 of 14 compound has been previously reported as the main component of the anodic layers grown in ferrous materials [35,41,49]. " # C Mw cm3 Anodic layer thickness = Q FeOOH (1) 2 Coatings 2018, 8, x FOR PEER REVIEW cm ∗ nFρ FeOOH C 5 of 14 where Q is the charge density of the current density–time plot, Mw is the molecular weight ρ is the ρ wheredensity, Q nisis the the charge number density of electrons of the current equivalent density–time and F is plot, the Faraday Mw is the constant. molecular weight is the density, n is the number of electrons equivalent and F is the Faraday constant. 2.00 2.00 1.3

1.2 1.75 1.75 1.1

1.0 1.50 I II III 0.9 1.50 100 200 300 400 ) ) I 2 2 1.25 I II II 1.25 III 1.00 (mA/cm (mA/cm i i 1.00 15 min 0.75 15 min 30 min 30 min 45 min 0.50 0.75 45 min 60 min 60 min 0.25 0.50 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 Anodization Time (s) Anodization Time (s) (a) (b)

5.0 0 rpm 4.5 600 rpm 4.0 )

2 3.5 3.0 2.5 Q (C/cm 2.0 1.5 1.0 0.5 15 30 45 60 Anodization Time (min)

(c)

FigureFigure 3. 3. CurrentCurrent density–time density–time responses responses for for anodiz anodizinging on on304L 304L SS in SS ethylene in ethylene glycol-NH glycol-NH4F-H42OF-H 2O electrolyteelectrolyte for for different diﬀerent anodizing anodizing time time at at(a)( a0 )rpm; 0 rpm; (b) (600b) 600 rpm rpm stirring stirring speeds. speeds. (c) Charge (c) Charge density density obteinedobteined of of current current density-time density-time plots plots at at0 and 0 and 600 600 rpm. rpm.

TheThe thicknesses thicknesses estimated estimated for for the the anodic anodic layers layers grown grown at at 00 rpm rpm and and different different anodizing anodizing times times are: are:581 581 nm nm (15 (15 min), min), 1.07 1.07µm µm (30 (30 min), min), 1.83 1.83µ µmm (45 (45 min) min) andand 2.60 µmµm (60 (60 min). min). For For the the layers layers grown grown at at600 600 rpm rpm the the thickness thickness obtained obtained for for di differentfferent anodizing anodizing times times are: are: 739739 nmnm (15(15 min),min), 1.431.43 µµmm (30(30 min), min),2.27 µ 2.27m (45 µm min) (45 andmin) 3.18 andµ 3.18m (60 µm min) (60 atmin) 600 at rpm. 600 Theserpm. These results results suggest suggest that the that higher the higher the anodizing the anodizingtime and stirringtime and speed, stirring the speed, thicker the the thicker anodic the layers; anodic however, layers; however, this simplistic this simplistic analysis doesanalysis not take doesinto accountnot take the into chemical account dissolution the chemical and dissolution dissolution and assisted dissolution by the assisted electric by field the processes electric field involved processes involved during the fabrication of the anodic layers. Both processes compete with the during the fabrication of the anodic layers. Both processes compete with the growth mechanism growth mechanism resulting an anodic oxide layers with lower thickness [42,50]. As can be seen in resulting an anodic oxide layers with lower thickness [42,50]. As can be seen in Figure4, where Figure 4, where the anodic layers generated for 15 and 60 min at 0 rpm show thicknesses around 500 the anodic layers generated for 15 and 60 min at 0 rpm show thicknesses around 500 and 1400 nm, and 1400 nm, respectively, these values being lower than those obtained theoretically. respectively, these values being lower than those obtained theoretically.

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508 nm 1.38 µm 1 µm 2 µm

(a) (b)

FigureFigure 4. 4. SEMSEM images images of anodic layerlayer thicknessthickness grown grown in in ethylene ethylene glycol-NH glycol-NH4F-H4F-H2O2O electrolyte electrolyte for for (a ) (15a) 15 min min and and (b )(b 60) 60 min min at 0at rpm. 0 rpm.

FigureFigure 5 showsshows the the morphology morphology of of the the anodic anodic layers layers grown grown during during different di fferent anodizing anodizing times times 15– 6015–60 min minat 0 rpm at 0 and rpm 60 and min 60 at min 600 atrpm. 600 The rpm. anodized The anodized layers grown layers for grown 15 min for show 15 min a surface show acovered surface withcovered a uniform with a uniform oxide layer oxide which layer whichreproduces reproduces the topography the topography of the of the surface surface finish. finish. At At higher higher magnificationsmagnifications this this surface surface shows shows nanoporous nanoporous morpholo morphologygy due due to to the the distribution distribution of of the the electric electric field field inin the the anodic anodic layer, layer, Figure Figure 55a,b.a,b. For 30 min of an anodizingodizing time the anodic oxide layer shows a similar appearance.appearance. At At higher higher magnifications magnifications the the nanoporous nanoporous mophology mophology of of the the anodic anodic layer layer is is observed, observed, FigureFigure 55c,d.c,d. The The samples anodized anodized for for 45 min shows a notably didifferentfferent surface surface appearance appearance whith whith twotwo zones zones clearly clearly distinguished. distinguished. A A lighter lighter Zone Zone 1 1 with with elongated elongated shape shape appearance appearance deposited deposited on on top top aa darker darker area area (Zone (Zone 2). 2). At At higher higher magnifications magnifications de de Zone Zone 2 2 still still keeps keeps the the nanoporous nanoporous morphology morphology observedobserved at at shorter shorter anodizing anodizing times times although although with with hi highergher distribution distribution in in pore pore diameter, diameter, Figure Figure 5e,f.5e,f. TheThe increase increase of of anodizing anodizing time time to to 60 60 min min enhances enhances the the features features observed observed in in the the previous previous images images FigureFigure 5g–j.5g–j. It It looks like there is a coarsening of the whiter areas (Zone 1), which become closer one toto others. others. As As results results the the top top view view of the of surface the surface shows shows a narrowing a narrowing of the ofdarker the darkerareas (Zone areas 2), (Zone Figure 2), 5g,h.Figure On5g,h. the other On the hand, other the hand, samples the samples grown for grown 60 min for at 60 600 min rpm, at 600 show rpm, a cleaner show a anodic cleaner surface, anodic thansurface, those than obtained those obtained at the same at the time same at time 0 rpm, at 0 rpm,Figure Figure 5i,j.5 i,j.This This is associated is associated with with a ahigher higher mass mass transfertransfer and and a a constant constant elimination elimination of of matter matter during during anodizing anodizing due due to to stirring stirring speed. speed. Despite Despite to to the the anodicanodic oxid oxid film film appears appears lo lo collapse collapse for for longer longer anodizing anodizing times, times, the the anodic anodic layer layer placed placed underneath underneath remainsremains adhered adhered to to the the substrate substrate and and keeps keeps an an ordered ordered nanoporous nanoporous morphology. TheseThese results revealreveal that that the the anodizing anodizing time time influences influences both theboth morphology the morphology and the and pore the diameter, pore diameter,obtaining obtaining values of values 42.56 of5.03 42.56 for ± 15 5.03 min for and 15 63.0min and11.0 63.0 nm ± for11.0 60 nm min for of 60 anodizing min of anodizing at 0 rpm at and 0 ± ± rpmvalues and of values 65.97 of10.53 65.97 nm ± 10.53 for 60 nm min for of 60 anodizing min of anodizing at 600 rpm. at 600 rpm. ± OnOn the the other other hand, hand, Table Table 1 shows the results of EnergyEnergy DispersiveDispersive X-ray Spectroscopy (EDX) analysisanalysis for for anodic anodic layer layer obtained obtained at at 0 0 rpm rpm for for diff differenterent time time of treatment. The The distribution distribution of of the the elementselements isis analyzedanalyzed in twoin two zones: zones: Zone 1Zone corresponds 1 corresponds to the outer to layerthe locatedouter layer at the oxidelocated/electrolyte at the oxide/electrolyteinterface; and Zone interface; 2 corresponds and Zone to 2 the corresponds nanoporous to layerthe nanoporous placed underneath layer placed that underneath is observed afterthat is45 observed min of treatment. after 45 min Both of zones treatment. show Both a similar zones trend show since a similar the content trend of since F and the O content elemental of increasesF and O elementalwhile the increases percentage while of other the percentage elements such of other as Cr, elements Fe, and such Ni tends as Cr, to Fe, decrease and Ni with tends the to anodizing decrease withtime. the The anodizing above is causedtime. The by aabove fluorine is andcaused oxygen by a incorporation fluorine and fromoxygen the incorporation electrolyte, which from likely the electrolyte,leads to the which formation likely of leads fluoride to the compounds, formation of mainly fluoride FeF compounds,2, FeF3, and mainly FeOOH FeF [402, ,FeF49].3, Nevertheless, and FeOOH [40,49].at 45 min Nevertheless, of treatment, at 45 F values min of decreasetreatment, abruptly F values from decrease 44.50 abruptly at.% to 16.41 from at.%44.50 and at.% O to values 16.41 at.% from and11.66 O at.%values to 5.64from at.% 11.66 between at.% to Zone5.64 at.% 1 and between Zone 2, Zone respectively. 1 and Zone 2, respectively.

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5 µm 500 nm (a) (b)

5 µm 500 nm (c) (d)

Zone 1

Zone 2 5 µm 500 nm

(e) (f)

Zone 2

Zone 1

5 µm 500 nm

(g) (h)

Zone 2

Zone 1

5 µm 500 nm

(i) (j)

FigureFigure 5. 5.SEM SEM images images of of anodic anodic layers layers grown grown inin ethyleneethylene glycol-NH44F-HF-H2O2O electrolyte electrolyte at at0 rpm 0 rpm for for (a,b(a),b 15) 15 min; min; (c ,(dc),d 30) 30 min; min; (e (,ef,)f 45) 45 min; min; ( g(g,h,h)) 6060 min;min; andand (i,j) 60 min min at at 600 600 rpm. rpm.

Coatings 2019 9 Coatings 2018,, 8,, x 706 FOR PEER REVIEW 8 8of of 14 14

Table 1. Energy Dispersive X-ray Spectroscopy (EDX) results for the anodic layers grown at different Table 1. Energy Dispersive X-ray Spectroscopy (EDX) results for the anodic layers grown at diﬀerent anodizing times at 0 rpm. anodizing times at 0 rpm. Element, Element, at. %C CO O F F Cr CrMn Mn Fe Fe Ni at. % 15 5.15 0.86 5.95 0.79 35.61 3.42 11.53 0.80 0.51 0.20 37.17 3.04 4.08 0.63 15 5.15 ± 0.86± 5.95± 0.79± 35.61 ± 3.42± 11.53 ± 0.80± 0.51 ± 0.20± 37.17 ±± 3.04 4.08 ± 0.63 30 4.88 0.97 9.33 1.85 41.42 7.60 9.92 1.61 0.56 0.23 30.80 6.37 3.08 0.72 30 4.88 ± 0.97± 9.33 ± 1.85± 41.42 ± 7.60± 9.92 ± 1.61± 0.56 ± 0.23± 30.80 ±± 6.37 3.08 ± 0.72 45 45 4.18 1.81 11.66 1.13 44.50 10.31 9.82 1.85 0.40 0.22 27.20 7.89 2.25 0.85 Zone4.18 1 ± 1.81± 11.66 ± 1.13± 44.50 ± 10.31± 9.82 ± 1.85± 0.40 ± 0.22± 27.20 ±± 7.89 2.25 ± 0.85 Zone 1 45 5.50 1.26 5.64 0.69 16.41 1.05 15.59 0.56 0.72 0.22 50.72 1.24 5.42 0.35 45 Zone 2 ± ± ± ± ± ± ±

(min) 5.50 ± 1.26 5.64 ± 0.69 16.41 ± 1.05 15.59 ± 0.56 0.72 ± 0.22 50.72 ± 1.24 5.42 ± 0.35 Zone 2 Anodization time 60 3.18 0.34 22.74 2.97 59.06 11.56 5.69 0.98 0.34 0.26 8.67 1.20 0.32 0.19 60 Zone 1 ± ± ± ± ± ± ± 3.18 ± 0.34 22.74 ± 2.97 59.06 ± 11.56 5.69 ± 0.98 0.34 ± 0.26 8.67 ± 1.20 0.32 ± 0.19 Zone 1 60 8.70 2.57 5.21 1.03 16.54 1.81 14.39 0.83 1.34 1.48 47.77 2.42 6.05 0.97 Zone 2 ± ± ± ± ± ± ±

Anodization time (min) Anodization time 60 8.70 ± 2.57 5.21 ±1.03 16.54 ± 1.81 14.39 ± 0.83 1.34 ± 1.48 47.77 ± 2.42 6.05 ± 0.97 Zone 2 The corrosion behavior of the anodic layers at 0 rpm is shown in Figure6. The polarization curve for theThe anodizing corrosion atbehavior 15 min showsof the anodic a passive layers region at 0 rpm similar is shown to the non-anodizedin Figure 6. The 304L polarization SS but broader curve forcompared the anodizing to the otherat 15 min treatments. shows a The passiveEcorr regionof the samplesimilar treatedto the non-anodized at 15 min is ~ 304L322.8 SS mV, butwhile broader for − comparedthe other anodizing to the other times treatments. this potential The 𝐸 shifts oftowards the sample nobler treated values at up 15 to min ~ 57.3is ~− mV.322.8 However, mV, while all for the 7 2 − theanodizing other anodizing treatments times show this a ipass potentialabout ~5.4shifts towards10− A/cm noblerand values a Epit of up ~281.3 to ~−57.3 mV indicatingmV. However, a similar all × −7 2 thecorrosion anodizing kinetics treatments than the show non-anodized a 𝑖 about samples ~5.4 × 10 but A/cm higher and susceptibility a 𝐸 of ~281.3 to localized mV indicating corrosion. a similarPerforming corrosion a thermal kinetics treatment than the could non-anodized be an interesting samples alternative but higher to improve susceptibility the electrochemical to localized corrosion.response ofPerforming the oxide layersa thermal since ittreatment allows the could obtaining be an more interesting stable oxidesalternative and the to eliminationimprove the of electrochemicalﬂuoride inside theresponse anodic of layers, the oxid as hase layers been since reported it allows of Titanium the obtaining and its more alloys stable [51]. oxides and the elimination of fluoride inside the anodic layers, as has been reported of Titanium and its alloys [51]. 0.6 15 min 0.4 30 min 45 min 60 min

) 0.2 Non-anodization 304L SS 0.0 Ag/AgCl

-0.2

E (V vs. E vs. (V E -0.4

-0.6

-0.8 -10 -9 -8 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 10 10 10 2 i (A/cm ) FigureFigure 6. 6. PotentiodynamicPotentiodynamic polarization polarization curves curves of of the the samples samples anodized anodized for for different different anodizing anodizing times times atat 0 0 rpm. rpm. 3.3. Electrochemical Spectroscopy Impedance 3.3. Electrochemical Spectroscopy Impedance Figure7 shows the electrochemical impedance spectra at di fferent immersion times 0 and 24 h, for Figure 7 shows the electrochemical impedance spectra at different immersion times 0 and 24 h, both non-anodized and anodized samples at different immersion times and stirring speeds. for both non-anodized and anodized samples at different immersion times and stirring speeds. Figure7a,b, shows the evolution with time of impedance corresponding to the non-anodized Figure 7a,b, shows the evolution with time of impedance corresponding to the non-anodized sample. The response is comprised by a single time constant at both immersion times. The /Z/ plot sample. The response is comprised by a single time constant at both immersion times. The /Z/ plot shows a linear stage in a wide frequency range with a slope of ~ 1 and a phase angle of about 80 shows a linear stage in a wide frequency range with a slope of ∼−−1 and a phase angle of about 80°◦ pointing out to a capacitive behavior or blocking electrode. This behavior is characteristic of passive pointing out to a capacitive behavior or blocking electrode. This behavior is characteristic of passive metals covered with a native oxide layer. The slight deviation observed at the Bode plots at low metals covered with a native oxide layer. The slight deviation observed at the Bode plots at low frequencies is related to the surface finish of the sample and the quality of the native oxide layer that frequencies is related to the surface finish of the sample and the quality of the native oxide layer that grows on it [52–54]. grows on it [52–54].

Coatings 2019, 9, 706 9 of 14

Coatings 2018, 8, x FOR PEER REVIEW 9 of 14 In the same way, the anodic layers grown at short and long anodizing time, 15 and 60 min at 0 rpm (FigureIn the same7c,d) way, show the a capacitiveanodic layers behavior grown asat short the anodizing and long anodizing time increases time, at15 theand same 60 min time at 0 of immersion,rpm (Figure being 7c,d) more show marked a capacitive in the spectrabehavior at 24as the h immersion. anodizing Thetime valueincreases of the at phasethe same angle time ranges of immersion, being more marked in the spectra at 24 h immersion. The value of the phase angle ranges about 85◦ from 100 Hz. This result reveals the high stability of the anodic layers grown at both 15 and about 85° from 100 Hz. This result reveals the high stability of the anodic layers grown at both 15 and 60 min of anodizing times. 60 min of anodizing times. The Nyquist diagrams of the anodic layers grown at 60 min at 600 rpm, Figure7e. There is not any The Nyquist diagrams of the anodic layers grown at 60 min at 600 rpm, Figure 7e. There is not signiﬁcant diﬀerence of the stirring speed on the anodizing time for 60 min. However, the impedance any significant difference of the stirring speed on the anodizing time for 60 min. However, the modulus in the Bode diagrams of the anodized samples at 600 rpm, Figure7f, seems to present two impedance modulus in the Bode diagrams of the anodized samples at 600 rpm, Figure 7f, seems to time-constantspresent two sincetime-constants there is a smallsince changethere is ina thesmall slopes change on thein the/Z/ response,slopes on whichthe /Z/ could response, be associated which to thecould properties be associated of the to porous the proper layer andties of the the barrier porous layer layer as reportedand the barrier in the literaturelayer as reported for anodic in layersthe grownliterature on other for anodic metals layers [54,55 gr]. own on other metals [54,55].

180 0 h -100 160 24 h 5

10 (deg) Angle Phase 140 -80 2

120 2 4

*cm 10 -60 Ω

100 *cm k

Ω 3 img 80 10 -40 -Z 60 /Z/ 2 -20 40 10 0 h 20 -0 1 24 h 0 10 -2 -1 0 1 2 3 4 5 6 0 20 40 60 80 100 120 140 10 10 10 10 10 10 10 10 10 2 Z kΩ*cm real Frequency (Hz) (a) (b)

5 80 10 -100 15 min - 0 h 70

15 min - 24 h (deg) Angle Phase 4 -80 60 60 min - 0 h 10 2

2 60 min - 24 h 50 -60 *cm *cm 3

Ω 10 Ω 40 k -40 img 30 /Z/

-Z 2 10 15 min - 0 h 20 15 min - 24 h -20 60 min - 0 h 10 1 10 60 min - 24 h -0 0 -2 -1 0 1 2 3 4 5 6 0 102030405060 10 10 10 10 10 10 10 10 10 2 Z kΩ*cm Frequency (Hz) real (c) (d)

80 5 -100 0 rpm 10 70 60 min - 0 h 60 min - 24 h -80 (deg) Angle Phase 60 4 2 600 rpm 2 10 50 60 min - 0 h -60 *cm

60 min - 24 h *cm Ω 3 k 40 10 Ω img 0 rpm -40

-Z 30

/Z/ 60 min - 0 h 2 20 10 60 min - 24 h -20 600 rpm 10 1 60 min - 0 h 10 -0 0 60 min - 24 h -2 -1 0 1 2 3 4 5 6 0 102030405060 10 10 10 10 10 10 10 10 10 2 Z kΩ*cm real Frequency (Hz) (e) (f)

FigureFigure 7. Electrochemical7. Electrochemical Impedance Impedance Spectroscopy Spectroscopy (EIS), (EIS), Nyquist Nyquist and and Bode Bode diagrams diagrams atat 00 andand 2424 hh of immersionof immersion times; times; (a,b) corresponding(a,b) corresponding to the to non-anodized the non-anodized specimen; specimen; (c,d )(c anodization,d) anodization treatments treatments at 15 andat 60 15 min and a60 0 min rpm; a (0e ,rpm;f) eﬀ ect(e,f) of effect stirring of stirring speeds speeds at 60min at 60 of min anodization. of anodization.

CoatingsCoatings2019Coatings 2018, 9, 706, 8 ,2018 x FOR, 8, PEERx FOR REVIEW PEER REVIEW 10 of 101410 of of 14 14 Coatings 2018, 8, x FOR PEER REVIEW 10 of 14 TableTable 2 summarizes 2 summarizes the proposed the proposed equivalent equivalent electr icalelectr circuitical circuit used toused simulate to simulate the behavior the behavior of of TabletheTable 2non-anodized thesummarizes2 summarizes non-anodized sample, the the proposedsample, for proposed which for equivalent which an equivalent equivalent an equivalent electr electrical Randlesical Randlescircuit circuit circuit usedcircuit was used to used. was simulate to simulateused.This equivalentThis the the behaviorequivalent behavior circuit of circuit of the thenon-anodized non-anodizedis suitableis suitable tosample, model sample, to model forthe which forcapacitive the which capacitive an equivalent anbehaviour equivalent behaviour ofRandles passive Randles of passive circuit real circuit systems real was systems was used. since used. This sinceit allows This equivalent it allows equivalent to model to circuit model the circuit the is suitableis suitabledeviations todeviations model to modelof the of thecapacitive idealthe capacitive idealbehavior. behaviourbehavior. behaviourWhere Where ofthe passive of CPthe passiveE CPvalues realE realvaluessystems obtained, systems obtained, since for since itboth forallows it bothimmersion allows to immersion model to modeltimes, the times, the α 2 α 2 deviationsdeviationscorrespond ofcorrespond ofthe the toideal idealthe to doublebehavior. the double layer Where Wherelayercapacitance capacitance the the CPE 30.70CP valuesE 30.70µSs values/cm obtained,µSs αatobtained,/cm 0 h2 atand for 0 h both28.91 andfor immersion28.91µSsboth/cm µSs immersion αat/cm times,242 h;at while24 correspond h;times, while the the Ω Ω2 2 Ω Ω2 2 correspondto thecharge double tocharge thetransfer layerdouble transfer resistance capacitance layer resistance capacitance -Rct- 30.70 presents-Rct- µpresents Ss30.70 αvalues/cm µSs 2values atαof/cm0 0.25 h2 ofat and M00.25 h 28.91 and cmM 28.91µ andcmSsα 0.61 /andµSscm 2 αM0.61/cmat 24 2cm Mat h; 24 at whilecm h;0 whileandatthe 0 24and the charge h 24 h immersionimmersion respectively, respectively, which which are typical are typical values values for bare for2Ω 304bare SS2 304 [56,57]. SS [56,57]. 2Ω 2 chargetransfer transfer resistance resistance -Rct -- presentsRct- presents values values of 0.25 of M0.25Ω cmM and cm 0.61 and M 0.61Ω cm M at cm 0 and at 240 and h immersion 24 h immersionrespectively, respectively, which are which typical are values typical for values bare for 304 bare SS [56 304,57 SS]. [56,57]. Table Table2. Values 2. Values of the ofelectrical the electrical parameters parameters value valueof the ofequivalent the equivalent circuit circuit used toused fit theto fitspectra the spectra of of the non-anodizedthe non-anodized specimen. specimen. TableTable 2. Values 2. Values of the of theelectrical electrical parameters parameters value value of ofth thee equivalent equivalent circuit circuit used used to to fit ﬁt the the spectra spectra of of the the non-anodizednon-anodized specimen. specimen. Re Re CPE dlCPE dl Circuit Parameter Circuit Parameter R ct R ct Re CPE dl

CircuitCircuitImmersion ParameterImmersion Parameter time (h)time (h) 0 0 R ct 24 24 e Ω 2Ω 2 R ( R cme ( ) cm ) 28.06 28.06 31.73 31.73 α 2 α 2 ImmersionImmersionCPE dltimeCPE (µSs time dl(h) /cm(µSs (h)) /cm ) 0 30.76 30.76 24 28.91 28.91 Ω 2α 2 0.91 0.92 Re (Re cm( Ω cm) ) α 28.06 0.91 31.73 31.73 0.92 2 Rct α(MΩαct 2cm2Ω) 2 0.25 0.61 CPECPEdl (µSsdl (µ/cmSsR / cm(M) ) cm ) 30.76 0.25 28.91 28.91 0.61 −3 Chi-sqrα αChi-sqr (10 ) (10−3) 0.911.71 1.71 0.92 1.37 1.37 2 Rct (MΩ cm ) 0.25 0.61 Rct (MΩ cm2) 0.25 0.61 Similarly,Similarly, to simulate toChi-sqr simulate the (10 behavior the3) behavior of the of anodic the1.71 anodic layers layers grown grown for 1.3715 for min 15 and min 60 and min 60 at min 0 rpm at 0 rpm Chi-sqr (10−3) − 1.71 1.37 the samethe sameequivalent equivalent circuit circuit has been has used,been used,Table Table 3. As 3.it Ascan it be can seen be theseen CPE the valueCPE value decreases decreases with with the immersionthe immersion time intime NaCl in NaClfrom 35.96from 35.96to 28.96 to 28.96µSsα/cm µSs2 αfor/cm the2 for anodic the anodic layers layers grown grown during during 15 min 15 min Similarly,Similarly, to simulate to simulate the thebehavior behavior of the of theanodic anodic layers layers grown grown for 15 for min 15 min and and60 min 60 min at 0 atrpm 0 rpm and fromand 40.79from 40.79to 36.23 to 36.23µSsα/cm µSs2 αfor/cm the2 for anodic the anodic layers layers grown grown for 60 for min 60 and min the and values the values of the of charge the charge the thesame same equivalent equivalent circuit circuit has hasbeen been used, used, Table Table 3. As3. Asit can it can be beseen seen the the CPE CPE value value decreases decreases with with transfertransfer resistance resistance slightly slightly increase increase with thewith imme the immeαrsion rsion2time. time.All, these All, thesevalues values are similar are similar to those to those the immersion time in NaCl from 35.96 to 28.96 µαSs 2/cm for the anodic layers grown during 15 min the immersionobtained time for the in NaClnon-anodized from 35.96 sample. to 28.96 These µSs results/cm suggest for the that anodic the barrierlayers layergrown of duringthe anodic 15 filmmin obtained for the non-anodizedα 2 sample. These results suggest that the barrier layer of the anodic film and from 40.79 to 36.23 µα Ss 2/cm for the anodic layers grown for 60 min and the values of the charge and fromshows 40.79shows similar to 36.23 similar electrochemical µSs electrochemical/cm for properties the anodic properties than layers thethan naturallygrown the naturally for grown 60 mingrown oxide and oxide layer the layervalueson 304 on SS.of 304 the SS. charge transfertransfer resistanceFinally, resistanceFinally, slightlythe slightlyanodic the increase anodic layers increase layers fabricatedwith with fabricated the theforimme 600 immersion forrsion rpm 600 were time.rpm time. weresimulated All, All,simulatedthese theseusing values using valuesa circuit are a circuit aresimilar equivalent similar equivalent to those with to those with obtainedobtainedtwo for time-constants, twothe for thenon-anodizedtime-constants, non-anodized the first sample.the associated first sample. associated These Thesewith results thewith results electrochemical suggestthe suggestelectrochemical that thatthe response barrier the response barrier of layer the of layerbody ofthe the bodyof anodic theanodic of anodic anodic layer film film layer showsshows similarand similar theand electrochemical second the electrochemical second with thewith bottomproperties the properties bottom of anodic than of anodic than films.the thenaturally films. naturally grown grown oxide oxide layer layer on 304 on 304 SS. SS. Finally, the anodic layers fabricated for 600 rpm were simulated using a circuit equivalent with TableTable 3. Values 3. Values of theof the electrical electrical parameter parameter of of thethe equivalentequivalent circuit circuit used used to to fit fitthe the spectra spectra of the of theanodic anodic two time-constants,Table the 3. first Values associated of the electrical with parameter the electrochemical of the equivale responsent circuit used of theto fit body the spectra of anodic of the anodiclayer filmsfilms for diforfilmsff differenterent for different anodizing anodizing anodizing times times and andtimes stirring stirring and stirringspeeds. speeds. speeds. and the second with the bottom of anodic films. Re Re CPE i CPE i Re Re CPE dlCPE dl Table 3. Values of the electrical parameter of the equivalent circuit used to fit the spectraR i of theR i anodicCPE dl CPE dl CircuitCircuit ParameterCircuit Parameter Parameter films for different anodizing times and stirring speeds.R ct R ct R ct R ct

Re i meansIiCPE Meansmeans anodic i Anodic anodic film Film film rpm Re CPE0 dl 600 rpmrpm 00 R i 600 600CPE dl CircuitImmersion ParameterImmersion time (h)time (h) 0 0 24 24 0 0 24 24 Immersion time (h) 0R ct 24 0R ct 24 AnodizedAnodized time (min)time (min) 15 15 60 60 15 15 60 60 60 60 60 60

Anodized timeΩ (min)2 15 60 15 60 60 60 Re ( Rcme (Ω) cm2) 22.29 22.2946.55 46.5527.02 27.029.13 9.13 21.44i means 21.44 anodic film 9.45 9.45 2 R (Ω cm ) α 2 22.29 46.55 27.02 9.13 21.44 9.45 rpmCPEe filmCPE (µSsfilm /cm(µSs)α /cm2) – – –0 – – – – – 16.83 16.83 600 22.45 22.45 α 2 ImmersionCPEfilm (µ timeSs /cmα (h) ) α – 0 – – –– – – 24 – – – –– 0 0.89 0.89 16.83 0.95 24 22.45 0.95 Ω 2Ω 2 AnodizedR timeanodicα R layer (min)anodic ( layer cm ( ) cm 15– ) – 60– –– 15– – –– 60 – –– 60 40.21 40.21 0.89 240.16 60 240.16 0.95 α 2 CPEdl2CPE (µSsdl2 /cm(µSs)α /cm2) 35.96 35.9640.79 40.7928.96 28.9636.23 36.23 23.01 23.01 1.37 1.37 RRanodice (Ω layer cm(Ω) cm ) 22.29– 46.55 –27.02 –9.13 –21.44 40.21 9.45 240.16 αα α 22 α 0.92 0.92 0.91 0.91 0.94 0.94 0.94 0.94 0.89 0.89 1.00 1.00 CPECPEfilm dl(µSs(µSs/cm/cm )) 35.96– – 40.79– 28.96– 36.23 16.83 23.01 22.45 1.37 Ω 2 Rct (MRct cm(MΩ) cm2) 0.11 0.11 0.68 0.68 2.10 2.10 2.14 2.14 1.09 1.09 0.93 0.93 α α 0.92 – – 0.91– 0.94– 0.94 0.89 0.89 0.95 1.00 Chi-sqrChi-sqr (10−3) (10−3) 1.52 1.52 2.10 2.10 0.15 0.15 0.65 0.65 2.63 2.63 0.38 0.38 anodic layer Ω 2 2 R Rct (M (Ω cmcm)) 0.11– – 0.68– 2.10– 2.14 40.21 1.09 240.16 0.93 α 2 CPEdl (µSs /cm3 ) 35.96 40.79 28.96 36.23α 2 α 2 23.01 1.37 Chi-sqrWith (10 WithCPE− ) valuesCPE values 1.52of the of anodic the anodic 2.10 layer layerof 16.83 0.15of 16.83µSs /cm µSs 0.65at/cm 0 hat which 0 h which reaches 2.63 reaches a value a value of 0.38 22.45 of 22.45 µSsαα/cmµSs 2 αat/cm 242 h at of 24 immersion h 0.92of immersion and0.91 a andload a transfer load0.94 transfer resistance0.94 resistance values values associated 0.89 associated with thewith body the 1.00 bodyof anodic of anodic Ω 2 Rct Finally, (M cm the) anodic layers0.11 fabricated0.68 for2.10 600 rpm2.14 were simulated 1.09 using a circuit equivalent 0.93 with −3 twoChi-sqr time-constants, (10 ) the first1.52 associated2.10 with0.15 the electrochemical0.65 response 2.63 of the body 0.38 of anodic layer and the second with the bottom of anodic films. With CPE values of the anodic layer of 16.83 µSsα/cm2 at 0 h which reaches a value of 22.45 µSsα/cm2 at 24 h of immersion and a load transfer resistance values associated with the body of anodic

Coatings 2019, 9, 706 11 of 14

With CPE values of the anodic layer of 16.83 µSsα/cm2 at 0 h which reaches a value of 22.45 µSsα/cm2 at 24 h of immersion and a load transfer resistance values associated with the body of anodic film of the order of 40.21 Ω cm2 at 0 h that reaches values of 240 Ω cm2 at 24 h which are values of the same order of magnitude to those reported by Lucas-Granados et al. [57] in anodic layers grown on iron, where they model the anodic layers they obtain with 3 time counters, where the first two time constants associate them with the morphological characteristics present along the iron nanotubes and the third time constant to the response of the barrier layer which is more compact and much less conductive than the nanostructures body. The results obtained from the electrochemical impedance are consistent with those found in the polarization curves. All anodic layers reveal similar corrosion kinetics and mechanism than the non-anodized 304 SS, which indicates that the anodizing process does not have detrimental effects on the electrochemical response of the 304 SS.

4. Conclusions

1 Nano-structured anodic layers are obtained on 304L SS using ethylene glycol + 0.1 mol L− H2O + 1 0.1 mol L− NH4F at different stirring speeds and anodizing times. Increasing the stirring speed results in a higher current density-time response and, therefore, in the charge that passes through the system. The increase of the anodizing time favors the growth of an oxide layers with defined nanoporous morphology with pore diameters ~63.0 nm and thicknesses about 1.4 µm. The chemical composition depends on the anodizing time since the mechanisms responsible for the growth of the anodic layer involve ion migration and chemical dissolution. For 45 and 60 min of anodizing a higher content of F and O is found in the outer zone of the anodic layer. The electrochemical tests confirmed that the responses to corrosion of the oxide layers are independent of the anodizing time and the stirring speed. Moreover, the electrochemical response of all anodic layers grown at different conditions exhibited a similar behavior and do not provide any improvement on the corrosion resistance of the non-anodized 304L SS.

Author Contributions: Conceptualization, M.Á.A.V.and J.M.H.-L.; Methodology, J.M.H.-L.; Investigation, L.P.D.-J. and J.M.H.-L.; Data Curation, L.P.D.-J. and J.M.H.-L.; Writing-Original Draft Preparation, L.P.D.-J. and J.M.H.-L.; Writing-Review & Editing, M.Á.A.V., E.I.C.-G., J.J.R.V., J.J.D.D., A.C.D.C., F.J.R.-V., I.L.A.-L. and J.M.H.-L.; Visualization, L.P.D.-J., E.I.C.-G., J.J.D.D., J.M.H.-L.; Resources, A.C.D.C., M.Á.A.V., J.J.R.V., F.J.R.-V., I.L.A.-L. Supervision, M.Á.A.V. and J.M.H.-L.; Project Administration, J.M.H.-L.; Funding Acquisition, M.Á.A.V. and J.M.H.-L. Funding: This work was funded by Faculty of Chemical Sciences of the UANL with 02-106939-PST-16-137 project and by the BACTERIA project (MAT2017-86163-C2-1-R), Ministry of Science, Innovation and Universities, SPAIN. Conﬂicts of Interest: The autors declare no conﬂict of interest.

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