coatings Article Electrodeposition of Nanocrystalline Fe-P Coatings: Influence of Bath Temperature and Glycine Concentration on Structure, Mechanical and Corrosion Behavior Natalia Kovalska 1,2, *, Natalia Tsyntsaru 3 , Henrikas Cesiulis 3 , Annet Gebert 4 , Jordina Fornell 5 , Eva Pellicer 5 , Jordi Sort 5,6 , Wolfgang Hansal 1 and Wolfgang Kautek 2, * 1 Hirtenberger Engineered Surfaces GmbH, Leobersdorferstrasse 31-33, A-2552 Hirtenberg, Austria; wolfgang.hansal@hirtenberger.com 2 Department of Physical Chemistry, University of Vienna, Währinger Strasse 42, A-1090 Vienna, Austria 3 Department of Physical Chemistry, Vilnius University, Naugarduko str. 24, LT-03225 Vilnius, Lithuania; ashra_nt@yahoo.com (N.T.); henrikas.cesiulis@chf.vu.lt (H.C.) 4 Leibniz Institute for Solid State and Materials Research Dresden (IFW), Helmholtzstrasse 20, D-01069 Dresden, Germany; a.gebert@ifw-dresden.de 5 Department de Física, Facultat de Ciències, Universitat Autònoma de Barcelona, Campus UAB, Building Cc, E-08193 Bellaterra, Spain; jordina.fornell@uab.cat (J.F.); eva.pellicer@uab.cat (E.P.); jordi.sort@uab.es (J.S.) 6 Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, E-08010 Barcelona, Spain * Correspondence: nataliyakovalskaya@ukr.net (N.K.); wolfgang.kautek@univie.ac.at (W.K.)  Received: 4 January 2019; Accepted: 11 March 2019; Published: 13 March 2019  Abstract: A detailed electrochemical study and investigation of a Fe-P glycine bath as a function of the temperature and glycine concentrations and current density, and their resulting corrosion and mechanical behavior is presented. A low addition of glycine to the electrolyte led to a drastic increase of the P content. At low Fe-P deposition rates, heterogeneous rough deposits with morphological bumps and pores were observed. By increasing the Fe-P deposition rate, the number of pores were reduced drastically, resulting in smooth coatings. Increasing the P content led to the formation of nanocrystalline grains from an “amorphous-like” state. Coatings with higher P contents exhibited better corrosion resistance and hardening, most likely attributed to grain boundary strengthening. Keywords: electrodeposition; iron-phosphorous alloys; temperature; glycine additive; corrosion; hardness; friction; wear 1. Introduction Amorphous alloys can exhibit a combination of striking characteristics, like magnetic [1–3] and mechanical [4] properties, as well as corrosion resistance [5,6], owing to their single-phase nature related with the lack of structural defects like grain boundaries and dislocations. Alloying transition metals with metalloids such as P, B, C, and Si can render amorphous phases. Fe-P coatings have recently raised scientific interest in respect to various advanced applications as temporary biodegradable bone replacement material [7] and alternative Li-ion storage anode [8,9]. Fe-P alloys may also serve as a substitute for widely banned hard chromium. Additionally, the co-deposition of phosphorous, e.g., with a Ni-carbon composite [10] or with Fe [11] can enhance the catalytic activity for the hydrogen evolution reaction. There are several methods for obtaining Fe-P materials, such as rolling of sheet alloys [1], vacuum induction melting [2,4,12], rapid quenching, and single roller melt-spinning technique [13]. Electrodeposition is a low-cost technique for producing transition metal-metalloid alloy coatings. Alloy Coatings 2019, 9, 189; doi:10.3390/coatings9030189 www.mdpi.com/journal/coatings Coatings 2019, 9, 189 2 of 12 electrodeposition is a complex process since various experimental parameters, such as current density, temperature, and concentration of active species in the electrolyte influence the overall electrochemical deposition reaction and, in turn, the alloy composition. Therefore, one has to define the experimental parameters and the electrolyte composition to obtain the targeted alloy composition and structure. The number of suitable electrolytes for Fe-P alloys is limited [6,14–17]. The most studied baths are based on iron sulfate and hypophosphite [14]. When deposition was performed at pH < 2.2, glycine addition led to amorphous-like alloys [15]. Under these conditions the electrolyte was stable, the alloy composition was reproducible, and the coatings were uniform, smooth, and bright [17,18]. An increase in pH, current density and glycine content in the electrolyte led to a decrease of the phosphorus content in the alloys. The addition of glycine acts as a complex-forming additive for nickel [19] and iron [20]. Even low concentrations show an influence on the quality of the iron deposits. The structural characterization of the deposited Fe-P films was performed by X-ray diffraction (XRD) measurements [15] and Mossbauer spectroscopy [3,13,21]. These investigations revealed that a P content over 14 at.% resulted in amorphous-like coatings. Property analysis of Fe-P coatings was mostly pursued in respect to magnetic applications [2,16,22]. Limited information is available about the corrosion behavior [12,23–25] and mechanical properties [26] of Fe-P coatings. In general, fundamental studies showed that high concentrations of P could have a beneficial effect on the corrosion resistance and the mechanical properties. The presence of P in bulk iron alloys can decelerate the corrosion of steels in acid media [12,27]. Fe-P alloys passivate more easily and are more corrosion resistant than P-free coatings [25,28,29]. The corrosion behavior of Fe-P coatings with low P content (up to 1 wt.%) was studied in H2 SO4 and HCI [12]. Phosphine generated by the hydrogen evolution during the corrosion process decelerated the acid corrosion. The mechanical properties of the Fe–P based alloys produced by hot powder forging showed high ductility for 0.35 wt.% P [26]. Alloys with 0.7 wt.% P showed poor ductility, but higher strength (∼500 MPa). In this work, a detailed electrochemical study of the Fe-P glycine bath at various temperatures and glycine concentrations is presented. Moreover, the corrosion resistance and the mechanical properties were investigated in relation to their phosphorous content and crystallinity. 2. Experimental Fe-P alloys were electroplated from an aqueous solution consisting of analytical grade 0.7 M FeSO4 ·7H2 O and 0.06 M NaH2 PO2 ·H2 O in deionized water. To this solution, glycine was added in different concentrations (0.11, 0.21 and 0.64 M). The pH was adjusted to 2.5 by adding drops of H2 SO4 and NaOH. The plating cell was a glass beaker (0.3 L) without stirring. The electrochemical measurements were performed in a conventional three-electrode set-up using a computer controlled potentiostat/galvanostat system (IPS PGU 200148, IPS Elektroniklabor GmbH & Co. KG, Münster, Germany). A brass plate with an active surface area of 2.25 cm2 was used as a working electrode for cathodic polarization curves measurements and for galvanostatic electrodeposition. A gold electrode, as more noble metal than brass, served as working electrode for cyclic voltammetry measurements in order to measure striping of the layer and prevent dissolution of the substrate. It was rinsed in 0.1 M H2 SO4 before deposition. Ag/AgCl/3 M KCl (U = +0.21 V vs. standard hydrogen electrode, SHE) [30] served as a reference electrode. All potential values are vs. the SHE in the context of this paper. The counter electrode was an 8 cm2 diameter steel plate positioned parallel to the working electrode in order to optimize the current distribution in the cell. The distance between the anode and the cathode was 10 cm. The temperature was set to 20, 40, or 60 ◦ C. Coatings were electrodeposited up to a thickness of 11 µm. This thickness value and current efficiencies, η, were calculated based on weighing the total coating mass and on an elemental weight analyses (EDX) of the electrodeposited alloys and Faraday’s law [31,32]. The surfaces were morphologically and compositionally characterized by a scanning electron microscope (SEM, Hitachi FEG-SEM S4800, Tokyo, Japan) equipped with energy dispersive X-ray Coatings 2019, 9, 189 3 of 12 spectroscopy (EDX) detector. The crystallographic structure of the obtained coatings is studied by X-ray diffractometry (Rigaku MiniFlex II, Cu Kα radiation, 30 kV, 30 mA, Tokyo, Japan). The mean crystallite size was estimated based on the peak width and position using Scherrer’s equation. Electrochemical polarization (corrosion) tests were also performed in a three-electrode configuration (potentiostat: Solartron SI 1287). The coated samples contacted with a copper tape were positioned as a working electrode towards an opening at the outer bottom of a Teflon beaker with ~50 mL electrolyte volume. A mercury-mercurous electrode (MSE, U = +0.64 V vs. SHE) served as a reference electrode. The counter electrode was a Pt sheet. After immersion of the sample under open circuit potential (OCP) conditions, a potential sweep was performed from −0.15 V vs. the individual OCP up to +1.14 V at a sweep rate of 0.5 mV·s−1 . Each measurement was repeated three times to ensure reproducibility. The mechanical test was performed on the cross-section surfaces of the as-plated coatings using a nanoindentation instrument (Anton-Paar NHT2 nanoindentation tester, Graz, Austria) equipped with a Berkovich pyramidal-shaped diamond tip under load control mode. A load of 10 mN was applied with a loading segments fixed to 30 s followed by a load holding segment of 5 s and an unloading segment of 30 s. The hardness and elastic modulus are reported as an average value of 15 indentations, performed in the middle of the layer cross-sections. Scratch tests were carried out applying an increasing normal load with the option for lateral force measurements. The normal load was linearly swept from 0 to 50 mN along the length of the scratch (600 µm) at a scratch velocity of 10 µm·s−1 . The tests were repeated four times for each sample. The initial profile at 10 µN at the location where the scratch was performed was carried out to assess the surface morphology. The actual penetration depth of the indenter under the sample surface was estimated by comparing the indenter displacement normal to the surface during scratching with the topography of the original surface at each position along the scratch length. The scratch segment, the roughness, and the slope of the surface were considered in the calculation of the indenter penetration. After the scratch, similarly a final profile was recorded to establish the residual scratch depth. 3. Results and Discussion 3.1. Potentiodynamic Investigations Cathodic polarization curves were measured on brass, in order to investigate Fe-P electrodeposition (Figure 1). The polarization curves were divided into two regions (by a dashed line). The reaction in the first region is mainly related to hydrogen evolution. In the second region, hydrogen evolution and Fe-P deposition coexisted. Figure 1a shows the influence of the glycine, Gly, content in the electrolyte at 20 ◦ C. The current density waves starting at ca. −0.5 V represent the proton reduction without Fe-P electrodeposition. An increase of the bath temperature caused a higher hydrogen evolution (Figure 1b). When the temperature of the Fe-P electrolyte (with Gly = 0.64 M) was increased from 20 up to 60 ◦ C, the reduction potential shifted to the anodic direction and the current density, j, for the metal reduction reactions increased due to a higher electron transfer rate and complex stability. The study of the Fe-P deposition potential with varying Gly concentration was performed by cyclic voltammetry (Figure 2). The potential transients were starting from the individual OCP to the negative direction with various reversal potentials. The first appearance of the anodic stripping peak correlates to the Fe-P deposition potential that is caused by the second current density wave. Cyclic voltammetry shows that the onset of the cathodic Fe-P deposition shifts to more negative potentials with increasing Gly concentration. It should be taken into account that the current density scale of Figure 2c contrasts to that of Figure 1a,b, when comparing the cathodic current waves. Table 1 shows the average potential of the second current density wave. The addition of Gly decreased the deposition rate caused by the competing hydrogen evolution. Therefore, the high concentration of adsorbed hydrogen species (H+ , compare [33]) decreased the active surface area for metal deposition. Coatings 2019, 9, 189 4 of 12 Coatings 2019, 9 FOR PEER REVIEW 4 Figure 1. 1. Cathodic Cathodicpolarisation polarisationcurves curves of the of the Fe-PFe-P alloys alloys electrodeposition electrodeposition depending depending on glycine on (a) the (a) the concentration glycine Gly at 20 concentration ◦ Cat Gly and 20 (b) the bath °C and temperature (b) the at 0.64 M bath temperature atGly. 0.64 M Gly. The study of the Fe-P deposition potential with varying Gly concentration was performed by cyclic voltammetry (Figure 2). The potential transients were starting from the individual OCP to the negative direction with various reversal potentials. The first appearance of the anodic stripping peak correlates to the Fe-P deposition potential that is caused by the second current density wave. Cyclic voltammetry shows that the onset of the cathodic Fe-P deposition shifts to more negative potentials with increasing Gly concentration. It should be taken into account that the current density scale of Figure 2c contrasts to that of Figure 1a,b, when comparing the cathodic current waves. Table 1 shows the average potential of the second current density wave. The addition of Gly decreased the deposition rate caused by the competing hydrogen evolution. Therefore, the high concentration of adsorbed hydrogen species (H+, compare [33]) decreased the active surface area for metal deposition. Figure 2. Cyclic voltammograms of the cathodic Fe-P deposition depending on the glycine concentration at 20 ◦ C. (a) Glycine free; (b) 0.11 M Gly; and (c) 0.64 M Gly. Table 1. Onset potential of the cathodic Fe-P deposition as a function of the glycine concentration Gly at 20 ◦ C. Concentration Gly/M Potential, U (vs. SHE)/V 0 −0.67 0.11 −0.71 Figure 2. Cyclic voltammograms 0.21 of the cathodic Fe-P deposition −0.75 depending on the glycine concentration at 20 °C. (a) Glycine free; (b) 0.11 M Gly; and (c) 0.64−M0.85 0.64 Gly. Table 1. Onset potential of the cathodic Fe-P deposition as a function of the glycine concentration Gly at 20 °C. Concentration Gly/M Potential, U (vs. SHE)/V Coatings 2019, 9, 189 0 −0.67 5 of 12 0.11 −0.71 0.21 −0.75 3.2. Morphological and 0.64 Structural Investigation −0.85 Galvanostatic 3.2. Morphologicalmodes dominate and Structural the technical praxis of electroplating processes. Therefore, Investigation the electrodeposition of the Fe-P coating was performed in galvanostatic mode. The deposition Galvanostatic modes dominate the technical praxis of electroplating processes. Therefore, the of theelectrodeposition coatings was performed of the Fe-Patcoating 30 mA wascm−performed 2 , as an optimal current density for a high deposition rate in galvanostatic mode. The deposition of the and acoatings homogeneous layer morphology. was performed at 30 mA cm−2,EDX as ananalysis of Fe-Pdensity optimal current coatingsfor showed a dependency a high deposition rate andof the phosphorous a homogeneous layer morphology. EDX analysis of Fe-P coatings showed a dependency ofofthe content in the layers on the glycine concentration (Figure 3a). A low addition glycine to thephosphorous electrolyte content alreadyinhad a pronounced the layers on the glycine impact on the P(Figure concentration content.3a). AThe lowdeposition addition of mechanism glycine to the changed electrolyte upon glycinealready had a [33]. adsorption pronounced Furtherimpact on the P increase concentration content. Theled deposition to a slight mechanism P increase, due to itschanged upon glycine complexation adsorption with iron. [33]. Further The formed complex concentration increase ledrate led to a deposition to a decreases slight P increase, due of Fe resulting to its complexation with iron. The formed complex led to a deposition rate decreases ◦ in a higher co-deposition rate of P. A temperature increase from 20 to 60 C resulted in ~50% more P. of Fe resulting in a higher co-deposition rate of P. A temperature increase from 20 to 60 °C resulted in ~50% more P. The coatings were smooth at 40 and 60 ◦ C, but became relatively rough at 20 ◦ C. The oxygen content The coatings were smooth at 40 and 60 °C, but became relatively rough at 20 °C. The oxygen content in the coatings was strongly reduced by an order of magnitude, from ca. 20 at.% to 5 at.% upon a in the coatings was strongly reduced by an order of magnitude, from ca. 20 at.% to 5 at.% upon a temperature rise rise from 40 to ◦ (Figure 3b). Oxygen can either derive from the deposition of oxygen temperature from 40 60 to 60C°C (Figure 3b). Oxygen can either derive from the deposition of oxygen species or may species arise or may from arise sample from sampleoxidation oxidationupon upon storage inthe storage in theair airafter after electrodeposition. electrodeposition. Figure Figure 3. The 3. The dependency dependency of of (a)(a)phosphorus phosphorusand and (b) (b) oxygen oxygencontents of of contents thethe Fe-P alloy Fe-P coatings alloy on the coatings on the Gly concentration in the electrolyte at various electrolyte temperatures; j = 30 mA cm −2. −2 Gly concentration in the electrolyte at various electrolyte temperatures; j = 30 mA cm . The partial The partial current current densitiesofofthe densities therespective respective elements elements“e”, “e”,i.e., FeFe i.e., and P, jP, and e, can provide further je , can provide further insights into the mechanism of the Fe-P deposition. je values were evaluated from the EDX data and insights into the mechanism of the Fe-P deposition. je values were evaluated from the EDX data and the weighed coating for the respective mass according to [34]: the weighed coating for the respective mass according to [34]: je = ne·ωe·F/me·t (1) je = ne ·ωe ·F/me ·t (1) ωe = ωte%/100 Δm/A (2) where je (mA cm−2) is the partial current ωe =density ωte %/100 ∆m/A for the corresponding element, ne the number of (2) electrons involved in the charge transfer reaction, ωe the mass of the element e calculated from where je (mA cm−2 ) is the partial current density for the corresponding element, ne the number weighing and EDX analyses (g∙cm−2), F the Faraday constant, me the atomic mass of the element e of electrons−1 involved in the charge transfer reaction, ωe the mass of the element e calculated from (g∙mol ), and t the electrodeposition time (s), ωte the weight content for the corresponding element weighing andwith analyzed EDX analyses EDX, (g·cm− A the surface 2 ), F the Faraday constant, m the atomic mass of the element e area of working electrode. e (g·mol−1 ),The and t the electrodeposition calculated je for the Fe and Ptime (s), ωte the weight electrodeposition and thecontent primaryfor sidethe corresponding reaction, element the hydrogen analyzed evolution EDX, A the with depended onsurface areaconcentration the glycine of working electrode. and the temperature (Table 2). For the sake of The calculated clarity, conditionsje for the Fethe yielding andgreatest P electrodeposition and thecontent difference in oxygen primary(cf. side reaction, Figure the used. 3) were hydrogen evolution depended on the glycine concentration and the temperature (Table 2). For the sake of clarity, conditions yielding the greatest difference in oxygen content (cf. Figure 3) were used. Therefore, coatings obtained at 20 and 60 ◦ C were compared. The partial current densities for Fe decreased with increasing the temperature, whereas that of P and the hydrogen evolution increased. An increase of Gly decreased the partial currents of Fe and P and increased the partial current for the proton discharge. The hydrogen evolution becomes the dominant reaction in the presence of a high glycine concentration at low current densities (j = 15 mA·cm−2 ). The occurrence of hydrogen evolution could also explain the higher amount of phosphorous in the alloy. The previous studies showed that the higher hydrogen evolution during the electrolysis process of Ni-P alloy supports the co-deposition of phosphorous [11]. decreased with increasing the temperature, whereas that of P and the hydrogen evolution increased. An increase of Gly decreased the partial currents of Fe and P and increased the partial current for the proton discharge. The hydrogen evolution becomes the dominant reaction in the presence of a high glycine concentration at low current densities (j = 15 mA∙cm−2). The occurrence of hydrogen evolution could also Coatings 2019,explain 9, 189 the higher amount of phosphorous in the alloy. The previous studies showed6 of that 12 the higher hydrogen evolution during the electrolysis process of Ni-P alloy supports the co- deposition of phosphorous [11]. Table 2. Influence of temperature and Gly concentration on the partial cathodic current densities, je , of the electrodeposition of Fe and P, and the hydrogen evolution (side reaction). Table 2. Influence of temperature and Gly concentration on the partial cathodic current densities, je, of the electrodeposition of Fe and P, and the hydrogen evolution (side− reaction). je /mA·cm−2 c[Gly]/M T/◦ C −jtotal /mA·cm−2 Fe P −2 −je/mA·cm H2 c[Gly]/M T/°C −jtotal/mA·cm−2 15 Fe11.1 P 0.4 H3.5 2 20 1530 11.1 0.4 0.9 3.52.6 26.5 0.11 20 3015 26.59.3 0.9 0.7 2.65.0 0.11 60 1530 9.318.9 0.7 1.5 5.09.5 60 30 18.9 1.5 9.5 15 8.8 0.9 5.4 20 1530 8.820.6 0.9 2.8 5.46.6 0.64 20 30 20.6 2.8 6.6 0.64 15 3.5 0.6 11.0 60 15 3.5 0.6 11.0 60 30 18.8 1.8 9.3 30 18.8 1.8 9.3 The results of The results of the the respective respectivecurrent currentefficiency,η, efficiency, η,ofofthethe Fe-P Fe-P alloy alloy deposition deposition together together withwith the the P layer content (Figure 4a) and the deposition rate (Figure 4b) are presented P layer content (Figure 4a) and the deposition rate (Figure 4b) are presented versus the applied versus the applied current ◦ The chosen temperature is 60 ◦ C, because it shows the best coating quality current density density at at 60 60°.. The chosen temperature is 60 °C, because it shows the best coating quality (roughness) (roughness)and andthethe lowest oxygen lowest content. oxygen Current content. efficiency Current and the deposition efficiency rate were determined and the deposition rate were based determined based on Faraday’s law [31]. The phosphorous content is practically unaffected the on Faraday’s law [31]. The phosphorous content is practically unaffected by j. However, by Pj. concentration However, theincreases drastically P concentration with the increases glycine concentration drastically with the glycineof theconcentration bath. The influence of the of the The bath. Gly concentration on the η, is significant at low j (glycine reduces η) and practically disappears influence of the Gly concentration on the η, is significant at low j (glycine reduces η) and practically at increased values, i.e., at at increased −2 . The deposition rate −2 30 mA cmvalues, j (Figure disappears i.e., at 30 mA cm is. Theincreasing withrate deposition 4b). is increasing with j (Figure 4b). FigureFigure (a) The 4. (a)4.The dependency dependency of the of the current current efficiency, efficiency, ηη (greysymbols), (grey symbols),and andthe the phosphorous phosphorous content (black(black symbols) symbols) in theinFe-P the alloy Fe-P coatings. alloy coatings. (b) Dependency (b) Dependency of the deposition of the deposition rate on rate on the cathodic the cathodic current current density, j. VariableGly parameter: Gly concentration. ◦ 60 C. density, j. Variable parameter: concentration. 60 °C. The The layer layer morphologies are very morphologies are much affected very much affected byby jj and and the the concentration concentration of of glycine glycine (Figure (Figure 5). 5). Both Both parameters parameters control η (Figure control η (Figure 4a), 4a), aa low low ηη indicates indicates aa high high proton proton reduction reduction rate rate and and aa strong strong hydrogen gas evolution. At low glycine content, η is relatively high and practically independent hydrogen gas evolution. At low glycine content, η is relatively high and practically independent of of j.j. Therefore, hydrogen bubbling should be moderate at all investigated current densities. Therefore, hydrogen bubbling should be moderate at all investigated current densities. The The competition between hydrogen competition betweenbubble evolution hydrogen and the bubble layer growth evolution and control the growth the layer morphology (Figure control the 5a,b). At low morphology j(Figure and low Fe-P deposition rate, inhomogeneous deposits with bumps leaving out pores 5a,b). At low j and low Fe-P deposition rate, inhomogeneous deposits with bumps leaving of ca. 0.3 µm in outdiameter pores ofwere observed ca. 0.3 (Figure 5a). μm in diameter were Hydrogen observedbubbles (Figuregenerated these bubbles 5a). Hydrogen bumps and pores. these generated With increased j and Fe-P deposition rate, the bumps cannot form again, and the number of pores is reduced drastically (Figure 5b). At high glycine content and low j, current efficiency showed <30%. The current efficiency caused vigorous hydrogen bubbling resulting in, bumps and many pores with ca. 0.1 µm in diameter (Figure 5c). An increase of j caused a drastic increase of η (Figure 4a) resulting in a minimum gas evolution and as such the deposit exhibited no pores and practically no bumps (Figure 5d). bumps bumps and and pores. pores. With With increased increased jj and and Fe-P Fe-P deposition deposition rate, rate, the the bumps bumps cannotcannot formform again, again, andand the the number of pores is reduced drastically number of pores is reduced drastically (Figure 5b). (Figure 5b). At At high high glycine glycine content content and and low low j,j, current current efficiency efficiency showed showed <30%.<30%. TheThe current current efficiency efficiency caused caused vigorous vigorous hydrogen hydrogen bubbling resulting in, bumps and many pores with ca. 0.1 bubbling resulting in, bumps and many pores with ca. 0.1 μm in diameter (Figureμm in diameter (Figure Coatings 2019, 9, 189 7 of 12 5c). 5c). An An increase increase of of jj caused caused aa drastic drastic increase increase of of η η (Figure (Figure 4a)4a) resulting resulting in in aa minimum minimum gas gas evolution evolution and and asas such such the the deposit deposit exhibited exhibited nono pores pores andand practically practically no no bumps bumps (Figure (Figure 5d). 5d). X-ray ◦ X-ray diffraction patterns of the Fe-P alloy layers grown at 60 °C showed aa transition diffraction diffraction patterns patterns of of the Fe-P alloy layers grown at 60 °CC showed transition from from aa nanocrystalline to an amorphous-like structure; identified by peak broadening nanocrystalline to an amorphous-like structure; identified by peak broadening (Figure 6). An increase (Figure 6). An increase increase of of the phosphorous the phosphorous phosphorouscontent content from from6 66at.% contentfrom at.% at.%toto to1616 at.% at.% 16 resulted resulted at.% resulted in in aa grain a grain in sizesize grain reduction reduction size reduction from from ~50 ~50 from to ~6 ~50 to tonm~6 ~6 nm (“amorphous-like”) (“amorphous-like”) basedbased on on the the analysis analysis of the of {110}the {110} reflection reflection at 44.9 ◦ , which at 44.9°, is which in good is in good agreement nm (“amorphous-like”) based on the analysis of the {110} reflection at 44.9°, which is in good agreement with with literature agreement with literature [15]. Pure Fe[15]. literature Pure Pure Fe Fe was was electrodeposited [15]. was electrodeposited from a similar bath electrodeposited from fromwithoutaa similar similar bath bath without hypophosphite without in hypophosphite order to compare hypophosphite in order inwith to to compare orderFe-P alloys. The compare with with Fe-P Fe-P alloys. alloys. The corresponding XRD The corresponding scan showed XRD corresponding scan narrower XRD showed scan peaks showed narrower indicating narrowera peaks indicating polycrystalline a polycrystalline body-centered cubicbody-centered Fe structure cubic [34]. peaks indicating a polycrystalline body-centered cubic Fe structure [34].Fe structure [34]. (a) (a) (b) (b) (c) (c) (d) (d) Figure Figure5. Figure 5.5.Morphology Morphology(SEM Morphology (SEMimages) (SEM images) of images) of Fe-P of coatings electrodeposited Fe-P coatings electrodeposited electrodeposited at at various at various Gly variousGly concentrations Glyconcentrations concentrations and and andcurrent current densities, current densities, j, densities, j, at at 60 at 60 ◦°C. 60 °C. (a) Fe-12 (a)Fe-12 C. (a) at.% Fe-12at.% P (j at.%PP(je(j= e = 15 mA cm e =1515mA mAcm − −2, c[Gly] = 0.11 M); (b) Fe-13 at.% P 2 cm , c[Gly] −2 c[Gly] == 0.11 0.11 M); M); (b) (b)Fe-13 Fe-13at.% at.%PP (j(jee =e= =30 (j 3030mA mAmAcm cm cm −2, c[Gly] = 0.11 M); (c) Fe-14 at.% P (je = 15 mA cm−2, c[Gly] = 0.64 M); and (d) Fe-16 at.% P −2,−c[Gly] 2 , c[Gly] = 0.11 M);M); = 0.11 (c) Fe-14 at.%at.% (c) Fe-14 P (jeP=(j15 =mA 15 cm mA cm −2 , c[Gly] −2, c[Gly] = 0.64=M); and 0.64 M); (d) andFe-16 (d) at.% Fe-16P e (j e = 30 mA cm−2 e = 30PmA (jat.% (j =cm −2, 30 mA c[Gly] , c[Gly] −= 2 0.64 cm = ,0.64 M). M).= 0.64 M). c[Gly] e Figure 6. Figure X-ray diffraction 6. X-ray X-ray diffraction patterns diffraction patterns of patterns of Fe-P of Fe-P alloys Fe-P alloys with with varying varying PP varying P contents. contents. The coatings contents. The The coatings were coatings were electrodeposited at 30 mA cm − −22and and 60 ◦ C from electrolytes containing various Gly concentration: 0 electrodeposited electrodeposited at 30 mA cm−2 and 60 °C from electrolytes containing various Gly concentration: 00 M at 30 mA cm 60 °C from electrolytes containing various Gly concentration: M (Fe-6 at.% P), 0.11 M (Fe-12 at.% P), 0.21 M (Fe-13 at.% P), and 0.64 M (Fe-16 (Fe-16 at.% at.% (Fe-6 at.% P), 0.11 M (Fe-12 at.% P), 0.21 M (Fe-13 at.% P), and 0.64 M (Fe-16 at.% P). P). P). 3.3. Corrosion Properties In order to simulate acidic rain in an industrial environment, the corrosion properties of electrodeposited Fe-P coatings with various alloy compositions were investigated in 0.05 M H2 SO4 3.3. Corrosion Properties In order to simulate acidic rain in an industrial environment, the corrosion properties of Coatings 2019,Fe-P electrodeposited 9, 189 coatings with various alloy compositions were investigated in 0.05 8 of 12M H2SO4 (pH 1.4). The average coating thickness was around 10–12 μm. The phosphorous content of the samples(pH were varied 1.4). from 6 coating The average at.% tothickness 16 at.% was (cf. Figures 5b,d µm. around 10–12 and The 6). The corrosion phosphorous of the ρcorr of a resistance content samples were varied from 6 at.% to 16 at.% (cf. Figure 5b,d and Figure pure Fe coating and bare brass was also evaluated (Figure 7) as reference samples. The 6). The corrosion resistance ρcorrcorrosion of a pure Fe coating and bare brass was also evaluated (Figure 7) as reference samples. The corrosion resistance is shown in Table 3 based on the corrosion potential Ucorr and the corrosion current density resistance is shown in Table 3 based on the corrosion potential Ucorr and the corrosion current density jcorr which jcorriswhich determined by the is determined byintersection the intersectionofofthe the cathodic and cathodic and anodic anodic TafelTafel line extrapolations. line extrapolations. Figure 7.Figure 7. Potentiodynamic Potentiodynamic polarization polarization curves of curves of Fe-P Fe-Pcoatings coatingswith various with phosphorous various content, content, phosphorous compared with Fe and brass (substrate). In 0.05 M H2 SO4 . Fe-6 at.% P (Gly 0 M), Fe-12 at.% P (Gly compared with Fe and brass (substrate). In 0.05 M H2SO4. Fe-6 at.% P (Gly 0 M), Fe-12 at.% P 0.11 M), and Fe-16 at.% P (Gly 0.64 M). (Gly 0.11 M), and Fe-16 at.% P (Gly 0.64 M). Table 3. Corrosion potential (Ucorr ), corrosion current density (jcorr ), corrosion resistance ρcorr of Fe-P and Fe samples. Table 3. Corrosion potential (Ucorr), corrosion current density (jcorr), corrosion resistance ρcorr of Fe-P samples. Content/at.% (±1) –U corr /V vs. SHE (±0.5) and FePhosphorous jcorr /mA cm−2 (±1) ρcorr /Ω·cm2 (±0.5) Fe 0.22 0.24 0.49 Phosphorous Content/at.% Fe-6 P (±1) –Ucorr/V vs. SHE (±0.5) 0.20 j0.30 corr/mA cm−2 (±1) ρcorr/Ω·cm2 (±0.5) 0.62 Fe-12 P 0.15 0.27 0.59 FeFe-16 P 0.120.22 0.16 0.24 0.67 0.49 Fe-6 P 0.20 0.30 0.62 Fe-12 P 0.15 0.27 0.59 Layers with a higher P content (12 at.% and 16 at.%) showed a more anodic Ucorr meaning that Fe-16 these alloys arePmore electropositive (noble)0.12 0.16pure Fe samples (Figure than the low alloyed and the 0.67 7). The layer with the highest P content of 16 at.% showed a moderate corrosion resistance: in a potential rangewith Layers of ca.a higher 0.4 V anodic of Ucorr(12 P content , theat.% current anddensity values 16 at.%) are almost showed a moreone anodic order of U magnitude corr meaning that lower than the values of all other samples. This is also expressed by a somewhat lower jcorr and these alloys are more electropositive (noble) than the low alloyed and the pure Fe samples (Figure 7). higher ρcorr (Table 3). Actually, all layers except the sample with the highest P content (16 at.%) were The layeralmost with completely the highest P content of 16 at.% showed a moderate corrosion resistance: in a potential dissolved after the corrosion test. The coatings with <16 at.% P were more or range ofless ca. inhomogeneous 0.4 V anodic of Ucorr, the exhibiting current pores density that allowed values contact aremore to the almost one orderbrass electropositive of magnitude electrode lower than the(Figure values5).ofHowever, all otherthesamples. thickness ofThis is also the 16 at.% Pexpressed sample wasby a somewhat moderately reducedlower jcorrtoand from ~10 higher ρcorr ~8 µm and showed a strong crackelation (Figure 8). All curves reach an anodic (Table 3). Actually, all layers except the sample with the highest P content (16 at.%) were current density plateau at almost − 2 relatively high values (>1 mA cm ). This may be interpreted as a diffusion-limited dissolution current completely dissolved after the corrosion test. The coatings with <16 at.% P were more or less or as a “pseudo-passive” behavior. inhomogeneous exhibiting pores that allowed contact to the more electropositive brass electrode Even though pure Fe is active in the studied pH region an inhibition process [35,36] involving an (Figure 5). However, adsorbed the film of thickness ofgenerated hypophosphite the 16 at.% P sample by the oxidationwas of P moderately reduced may play a major role: from ~10 to ~8 μm and showed a strong crackelation (Figure 8). All curves reach an anodic current density plateau at − + − relatively high values (>1 mA cm−2). This 2O → P + Hmay beHinterpreted 2 PO2 + 2H + ase a diffusion-limited dissolution (3) current or as a “pseudo-passive” The adsorbed Hbehavior.− 2 PO2 anion may be oxidized to H3 PO3 or H3 PO4 , providing a P-enriched Fe-P Even though surface. Thepure Fe with coatings is active incontent high P the studied pH (16 at.%) region were an inhibition measured process by EDX after [35,36]test the corrosion involving an adsorbed film of hypophosphite generated by the oxidation of P may play a major role: P (Table 4) to assess compositional consistency. The homogeneous 16 at.% P sample almost doubled its content. The O content increased by a factor of four probably due to the formation of the phosphite or phosphate layer. P + H2O → H2PO2− + 2H+ + e− (3) The adsorbed H2PO2− anion may be oxidized to H3PO3 or H3PO4, providing a P-enriched Fe-P surface. The coatings with high P content (16 at.%) were measured by EDX after the corrosion test (Table 4) to assess compositional consistency. The homogeneous 16 at.% P sample almost doubled its P content. The O content increased by a factor of four probably due to the formation of the phosphite Coatings 2019, 9, 189 9 of 12 Coatings 2019, 9 FOR PEER REVIEW 9 (a) (b) Figure 8. Surface Surface morphology morphology (SEM (SEM images) images) ofof the the Fe-16 Fe-16 at.% at.%PP alloy alloy before before (a) (a) and and after after (b) (b) the potentiodynamic corrosion test in 0.5 M H22SO44.. 4. PPand Table 4. Table andOOcontent of of content Fe-P alloys Fe-P before alloys and and before afterafter corrosion testing corrosion in 0.5 in testing MH 2 SO 0.5 for M4H 1 h (comp. 2SO4 for 1 h Figure 7). Determined by SEM-EDX. (comp. Figure 7). Determined by SEM-EDX. Elements Elements Content/at.% ±2) Content/at.% ((±2) Before Corrosion Test Before Corrosion Test After Corrosion After Test Corrosion Test 6 P-<10 6 P-<10 O O 16 P-74 O 16 P-74 O 12 P-<10 12 P-<10 O O 23 23 P-48 OO P-48 16 P-<10 16 P-<10 O O 24 24 P-41 OO P-41 3.4. 3.4. Mechanical Mechanical Properties Properties The The hardness, hardness, H, H, and and reduced reduced Young’s modulus, E Young’s modulus, Er,, values of the Fe-P coatings were extracted r values of the Fe-P coatings were extracted from from the unloading segment of the load-displacement curves using thethe the unloading segment of the load-displacement curves using method method of Oliver of Oliver and and ParrParr [37]. [37]. An increase An increase of H, of H, 8.04 from fromto8.04 9.01toGPa, 9.01wasGPa,observed was observed as the Pascontent the P content increases increases fromto from 6 at.% 6 at.% to 16 at.% 16 at.% (Table 5). The increase of H observed in the coatings from 6 at.% to 12 at.% (Table 5). The increase of H observed in the coatings from 6 at.% to 12 at.% P can be ascribed to grain P can be ascribed to grain boundary boundary strengthening strengthening due tosize due to grain grain size reduction reduction and toand to partial partial amorphization amorphization of theofalloy the alloy as a as a result of P addition. In turn, the further increase of H observed in the coating result of P addition. In turn, the further increase of H observed in the coating with 16 at.% P with 16 at.% P can can be be mainly attributed to the amorphous-like structure of the coating with substantial amounts ofP. mainly attributed to the amorphous-like structure of the coating with substantial amounts of P. Typically, amorphousmaterials Typically, amorphous materialsexhibit exhibitlarger larger HH values values compared compared to to their their crystalline crystalline counterparts counterparts as as the the conventional conventional deformation deformation mechanisms mechanisms of crystalline of crystalline materials materials (e.g., dislocation (e.g., dislocation motion,motion, stackingstacking faults, faults, grain boundary sliding) are no longer operative in amorphous alloys due to the lack of long-range long- grain boundary sliding) are no longer operative in amorphous alloys due to the lack of order. range order. from Er decreases Er decreases 167 to 142fromGPa167 to 142 when the GPa whenincreases P content the P content from 6increases at.% to 12from at.%.6 The at.%variation to 12 at.%. in The Er canvariation in Eto be related r can be related to the microstructural changes (i.e., amorphization) occurring upon the microstructural changes (i.e., amorphization) occurring upon P enrichment. P enrichment. Table 5. Hardness H (GPa) and Young’s modulus Er (GPa) of the Fe-P coatings with P (6 at.%; 12 at.% Table and 165.at.%). Hardness H (GPa) and Young’s modulus Er (GPa) of the Fe-P coatings with P (6 at.%; 12 at.% and 16 at.%). P Content/at.% H/GPa Er /GPa P Content/at.% H/GPa Er/GPa Fe-6Fe-6 P P 8.04 ±8.04 0.42± 0.42 167.0 ± 8.6167.0 ± 8.6 Fe-12 P 8.57 ± 0.27 142.3 ± 3.9 Fe-12 P 8.57 ± 0.27 142.3 ± 3.9 Fe-16 P 9.01 ± 0.16 146.6 ± 2.4 Fe-16 P 9.01 ± 0.16 146.6 ± 2.4 The wear behavior behavior of of the Fe-P coating coating was was studied studied for for the the different different investigated investigated samples. samples. performance is observed for the three investigated conditions, a smother friction Although similar performance force curve curveasasa function a function of the of scratch distance the scratch is observed distance for the Fe-16 is observed at.% for the P coating Fe-16 at.% Pwhen compared coating when with the curves of the Fe-6 at.% P and Fe-12 at.% P coatings. The sharp drops observed compared with the curves of the Fe-6 at.% P and Fe-12 at.% P coatings. The sharp drops observed in in the friction forcefriction the curvesforce and in the friction curves and incoefficient curves the friction (Figurecurves coefficient 9a,b) can be ascribed (Figure to the 9a,b) can be inhomogeneous ascribed to the inhomogeneous morphology observed by SEM (Figure 5). Nonetheless, no mechanical failure is observed for any of the investigated coatings in the measured range. Coatings 2019, 9, 189 10 of 12 morphology observed by SEM (Figure 5). Nonetheless, no mechanical failure is observed for any of the investigated Coatings 2019, 9 coatings in the measured range. FOR PEER REVIEW 10 Figure Figure 9. Friction 9. (a) (a) Friction coefficientand coefficient and(b) (b) Friction Friction force force of of measured measuredFe-P coatings Fe-P withwith coatings varying varying phosphorous phosphorous content. content. 4. Conclusions 4. Conclusions A detailed A detailed electrochemical electrochemical studyofofthe study theFe-P Fe-P glycine glycine bath bathatatvarious various temperatures temperatures and and glycine glycine concentrations together with corrosion and mechanical behavior investigation concentrations together with corrosion and mechanical behavior investigation was presented. was presented. A low addition of glycine to the electrolyte of 0.11 M led to a drastic increase of the P content A low addition of glycine to the electrolyte of 0.11 M led to a drastic increase of the P content from from ca. 4 at.% to 13 at.%. The deposition mechanism changed upon glycine adsorption. The increase ca. 4 at.% to 13 at.%. The deposition mechanism changed upon glycine adsorption. The increase of the of the Gly concentration decreased the partial currents of Fe and P, but increased the hydrogen Gly concentration evolution which decreased supportedthethe partial currents co-deposition of P.of Fe and P, but increased the hydrogen evolution which supported the co-deposition of P. The observed layer morphologies were controlled by the competition between the hydrogen The observed bubble evolutionlayer morphologies and the layer growth.were At a controlled by the competition low Fe-P deposition between rate, heterogeneous the hydrogen deposits with bubble evolution bumps andwere and pores the layer growth. observed. WithAt anaincreased low Fe-PFe-P deposition deposition rate, rate,heterogeneous deposits the number of pores was with reduced drastically, and smooth coatings resulted. An increase of the P content bumps and pores were observed. With an increased Fe-P deposition rate, the number of pores was from 6 at.% to 16 at.% resulted reduced in a grain drastically, andsize reduction smooth from ~50 coatings to ~6 nm, resulted. Annanocrystalline increase of the to P“amorphous-like”. content from 6 at.% to 16 at.% An increase of the bath temperature from 20 up to resulted in a grain size reduction from ~50 to ~6 nm, nanocrystalline to60 °C resulted in faster alloy deposition and “amorphous-like”. ~50% higher P content. The oxygen content in the coatings was sharply reduced by an order of An increase of the bath temperature from 20 up to 60 ◦ C resulted in faster alloy deposition and magnitude, from ca. 20 at.% to 5 at.%. ~50% higher P content. The oxygen content in the coatings was sharply reduced by an order of Coatings with higher P contents (12 at.% and 16 at.%) were more noble than the low alloyed and magnitude, the purefrom ca. 20 at.% Fe sample to 5 at.%. and showed potentially better corrosion resistance. All layers with <16 at.% P Coatings were morewith higher or less P contents (12 inhomogeneous at.% and exhibiting 16 at.%) pores were that led to more local noble element than the lowOnly corrosion. alloyed the and the pure Fe sample and showed potentially better corrosion resistance. All layers 16 at.% P sample exhibited a passive region. The measured composition after the corrosion test with <16 at.% P were moreshowed or less an inhomogeneous almost doubledexhibiting pores that P content probably dueledto to thelocal element formation of acorrosion. phosphite orOnly the 16 at.% phosphate layer.exhibited a passive region. The measured composition after the corrosion test showed an P sample The increase almost doubled of H hardness P content probablyobserved due to thein the coatings of formation from 6 at.% to 12orat.% a phosphite P can be layer. phosphate ascribed to The increase of H hardness observed in the coatings from 6 at.% to 12 at.% P can bethe grain boundary strengthening due to grain size reduction and to partial amorphization of alloy to ascribed as a result of P addition. A smoother friction force curve as a function of the scratch distance was grain boundary strengthening due to grain size reduction and to partial amorphization of the alloy observed for the Fe-16 at.% P coating. The abrupt drops observed in the friction force curves and in as a result of P addition. A smoother friction force curve as a function of the scratch distance was the friction coefficient curves can be ascribed to the inhomogeneous morphology. observed for the Fe-16 at.% P coating. The abrupt drops observed in the friction force curves and in Author Contributions: Conceptualization: N.K., W.H., and W.K.; Methodology: N.K. and W.K.; Validation: the friction coefficient curves can be ascribed to the inhomogeneous morphology. N.T., H.C., A.G., J.F., E.P., J.S., W.H., and W.K.; Formal Analysis: N.K. and W.K.; Investigation: N.K. and W.K.; Resources: W.H.; Writing—Original Draft Preparation: N.K. and W.K.; Writing—Review and Editing: N.K. and Author Contributions: Conceptualization: N.K., W.H., and W.K.; Methodology: N.K. and W.K.; Validation: W.K.; Visualization: N.K. and W.K.; Supervision: W.K.; Project Administration: W.H.; Funding Acquisition: N.T., H.C., A.G., J.F., E.P., J.S., W.H., and W.K.; Formal Analysis: N.K. and W.K.; Investigation: N.K. and W.K.; W.H. W.H.; Writing—Original Draft Preparation: N.K. and W.K.; Writing—Review and Editing: N.K. and Resources: W.K.; Funding: Visualization: N.K. and The authors W.K.; Supervision: acknowledge funding byW.K.; Project Administration: the HORIZON2020 W.H.;(No. SELECTA project Funding Acquisition: 642642), and partial W.H. financial Funding: The support authorsfrom the Generalitat acknowledge de Catalunya funding (2017-SGR-292 project). by the HORIZON2020 SELECTA J. Fornell and E. project Pellicer (No. are grateful 642642), and partial to MINECO financial supportfor the the from “Juan de la Cierva” Generalitat de (IJCI-2015-27030) and “Ramonproject). Catalunya (2017-SGR-292 y Cajal” (RYC-2012-10839) J. Fornell and E. contracts, Pellicer are respectively. grateful to MINECO for the “Juan de la Cierva” (IJCI-2015-27030) and “Ramon y Cajal” (RYC-2012-10839) contracts, respectively. Coatings 2019, 9, 189 11 of 12 Acknowledgments: Experimental assistance by M.B. Fernandez (IFW), M. Johne (IFW), V. Gman (Hirtenberger Engineered Surfaces GmbH), G. Quorri (Hirtenberger Engineered Surfaces GmbH), R. Mann (Hirtenberger Engineered Surfaces GmbH), and A. Nicolenco (Vilnius University) is gratefully acknowledged. Partial financial support from the MAT2017-86357-C3-1-R research project from the Spanish Ministerio de Economía y Competitividad (MINECO), cofinanced by the ‘Fondo Europeo de Desarrollo Regional (FEDER), is also acknowledged. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. References 1. Jafari, S.; Beitollahi, A.; Eftekhariyekta, B.; Kanada, K.; Ohkubo, T.; Herzer, G. Microstructural and magnetic properties study of Fe–P rolled sheet alloys. J. Magn. Magn. Mater. 2014, 358, 38–43. [CrossRef] 2. Manna, S.K.; Prabhu, D.B.; Gopalan, R.; Srinivas, V. 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Res. 2004, 19, 3–20. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).