Effect of Surface Pretreatment on Quality and Electrochemical Corrosion Properties of Manganese Phosphate on S355J2 HSLA Steel

Article Effect of Surface Pretreatment on Quality and Electrochemical Corrosion Properties of Manganese Phosphate on S355J2 HSLA Steel

Filip Pastorek 1,*, Kamil Borko 2, Stanislava Fintová 3,4, Daniel Kajánek 1,2 and Branislav Hadzima 1,2

1 Research Centre, University of Zilina, Zilina 010 08, Slovakia; [email protected] (D.K.); [email protected] (B.H.) 2 Department of Materials Engineering, Faculty of Mechanical Engineering, University of Zilina, Zilina 010 08, Slovakia; [email protected] 3 Institute of Physics of Materials, Academy of Science of the Czech Republic, Brno 616 62, Czech Republic; ﬁ[email protected] 4 CEITEC IPM, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno 616 62, Czech Republic * Correspondence: ﬁ[email protected]; Tel.: +421-41-513-7622

Academic Editor: Robert B. Heimann Received: 5 September 2016; Accepted: 11 October 2016; Published: 15 October 2016

Abstract: High strength low alloy (HSLA) steels exhibit many outstanding properties for industrial applications but suffer from unsatisfactory corrosion resistance in the presence of aggressive chlorides. Phosphate coatings are widely used on the surface of steels to improve their corrosion properties. This paper evaluates the effect of a manganese phosphate coating prepared after various mechanical surface treatments on the electrochemical corrosion characteristics of S355J2 steel in 0.1 M NaCl electrolyte simulating aggressive sea atmosphere. The manganese phosphate coating was created in a solution containing H3PO4, MnO2, dissolved low carbon steel wool, and demineralised H2O. Scanning electron microscopy (SEM) was used for surface morphology observation supported by energy-dispersive X-ray analysis (EDX). The electrochemical corrosion characteristics were assessed by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PD) measurements in the solution of 0.1 M NaCl. Method of equivalent circuits and Tafel-extrapolation were used for the analysis of the obtained results. Performed experiments and analysis showed that the morphological and corrosion properties of the surface with manganese phosphate are negatively inﬂuenced by sandblasting surface pretreatment.

Keywords: corrosion; steel; sandblasting; manganese phosphate

1. Introduction The prime motivation of developing high strength low alloy (HSLA) steels was the replacement of low-carbon steels for the automotive industry in order to improve their strength-to-weight ratio and meet the need for higher-strength construction grade materials. HSLA steels have excellent strength, good weldability and ductility, and also outstanding low temperature impact toughness superior to that of high yield strength (HY) steels [1]. However, proper surface treatment is necessary for many applications to increase their resistance to aggressive environment. One of the most commonly used method for surface preparation is sandblasting, used for various purposes of surface inﬂuence including: modiﬁcation [2], strengthening [3], cleaning and rust removal [4]. Sandblasting abrasive particles are smaller compared with particles in shot peening technology and lower pressure of compressed air is used. Therefore, a smoother surface can be

Coatings 2016, 6, 46; doi:10.3390/coatings6040046 www.mdpi.com/journal/coatings Coatings 2016, 6, 46 2 of 9 obtained by sandblasting in general. During the sandblasting process, the surface of samples is blasted repeatedly by sand particles or other hard particles with high speed, which leads to the removal of oxide scale and generation of local plastic deformation in the surface layer. However, Wang et al. [5] observed decrease of corrosion resistance of steel after sandblasting resulting from the existence of high-density lattice defect: dislocation, that limits the application possibilities of this technique [6,7]. Phosphate coatings are the preferred surface treatment of several alloys since they exhibit a good adhesion, good corrosion resistance, improved abrasive resistance of the structures and a low cost in comparison with other coating systems [8]. The main types of phosphate coatings are manganese, iron, and zinc based [9,10]. Manganese phosphate coatings exhibit outstanding lubricant retention properties and mechanical resistance. Excellent corrosion resistance and anti-galling properties are the results of uniform crystal size distributions and high coating weight [11]. As revealed by accelerated corrosion tests, manganese phosphate demonstrates the best corrosion resistance compared to zinc and iron phosphates [12]. Manganese phosphate coating can be used on high strength steels for the oil and gas extraction industry [11]. Sufﬁcient quality of phosphate coatings is reached by proper control of process and material parameters including: composition of the metal, structure of the metal surface, surface preparation, pH, bath chemical composition, time, temperature, etc. [8,13] There are many studies focused on corrosion properties of various phosphate coatings on various steels [8,10–12,14,15] but no deeper research is performed on how sandblasting inﬂuences the electrochemical properties of manganese phosphate on HSLA steels. For these reasons, the aim of this study is to investigate this phenomenon.

2. Materials and Methods S355J2 steel was used for the experimental investigation. The chemical composition of this steel can be found in Table1. The microstructure was observed by the light metallographic microscope CARL ZEISS AXIO Imager.A1m (ZEISS, Oberkochen, Germany), equipment ﬁnanced by the project ITMS 26220220121). All the samples were prepared by standard metallographic methods. 3% Nital was used as an etchant for visualization of the S355J2 steel microstructure.

Table 1. Composition of S355J2 steel.

Component C Mn P S Si Cu Fe wt % 0.200 1.600 0.025 0.025 0.550 0.550 balance

S355J2 steel samples were ground (p500 grit SiC paper) to reach the homogenous surface roughness. Some samples were further sandblasted. Sandblasting was performed by sand particles on a mobile pressure sandblasting machine with a nozzle diameter 6 mm and pressure of 0.6 MPa. The final operation was chemical treatment of both surfaces by manganese phosphating (MnP). Composition of phosphating bath was 300 mL of demineralised water, 4.49 g of H3PO4, the same amount of MnO2 and 3 g of ultrafine low carbon steel wool of the grade 0000 (mean width of fibres 15.24–25.4 µm). After the initial dissolving of the steel wool in phosphate solution at 90 to 95 ◦C the subsequent phosphating of the samples was performed for 75 min at the same temperature of 90 to 95 ◦C. Demineralised water and ethanol were used for cleaning of the samples after final surface treatment, followed by dried air streaming to remove chemical and mechanical residues. The phosphating mechanism was initially described by Ghali and Potvin [16]. The phosphating process consists of four stages: (1) initial substrate dissolution caused by medium acidity. This dissolution leads to an increase of the bath pH value by the hydrogen evolution reaction (HER); (2) massive precipitation of very fine phosphate particles; (3) phosphate crystallization and growth; and (4) reorganization of the crystals, i.e., high rate dissolution and re-precipitation of the phosphate in the coating that reduce the exposed area and coating porosity. This mechanism was originally Coatings 2016, 6, 46 3 of 9 observed in the zinc phosphating process, but can be theoretically adopted for manganese phosphating also [11,16]. Dissolution of ultrafine low carbon steel wool helps to add Fe2+ ions to the phosphate solution,Coatings 2016 reduces, 6, 46 the first stage of phosphating causing the dissolution of substrate surface3 and of 9 supports the second phosphating stage. The manganese manganese phosphate phosphate morphology morphology was was observed observed by scanning by scanning electron electron microscopy microscopy (SEM) (SEM)LYRA3 LYRA3 TESCAN TESCAN and chemical and chemical analysis of analysis coatings of was coatings realized was by realizedenergy dispersive by energy X‐ dispersiveray (EDX) X-raytechnique. (EDX) Cross technique. section images Cross section were taken images by were light takenmetallographic by light metallographic microscopy (LM). microscopy Surface (LM).roughness Surface was roughness measured by was surface measured roughness by surface tester roughness Mitutoyo SJ tester‐210 according Mitutoyo to SJ-210 ISO 4287–1997 according tostandard ISO 4287–1997 focusing standard on maximum focusing roughness on maximum (Rz) roughnessand arithmetic (Rz) andaverage arithmetic (Ra) averagevalues [17]. (Ra) valuesThermodynamic [17]. Thermodynamic corrosion electrochemical corrosion electrochemical characteristics characteristics (corrosion potential) (corrosion together potential) with together kinetic withcorrosion kinetic electrochemical corrosion electrochemical characteristics characteristics (corrosion (corrosion current density current densityand corrosion and corrosion rate) rate)and andelectrochemical electrochemical impedance impedance spectroscopy spectroscopy corrosion corrosion characteristics characteristics (polarization (polarization resistance) resistance) were assessed byby corrosion corrosion tests tests in ain solution a solution simulating simulating aggressive aggressive sea environment sea environment (0.1 M NaCl)(0.1 M at NaCl) ambient at temperature.ambient temperature. A cell composed A cell composed of three electrodes of three electrodes (counter electrode,(counter electrode, working electrodeworking electrode and reference and electrode)reference electrode) was used forwas electrochemical used for electrochemical tests. The potentiodynamic tests. The potentiodynamic polarization (PD) polarization measurements (PD) andmeasurements electrochemical and electrochemical impedance spectroscopy impedance (EIS) spectroscopy measurements (EIS) were measurements realized by were potentiostat realized VSP by frompotentiostat BioLogic VSP SAS from France BioLogic (equipment SAS France financed (equipment by the project financed ITMS by 26220220048). the project ITMS EIS measurements 26220220048). wereEIS measurements running under were potential running controlunder potential with scanning control with frequency scanning range frequency 100 kHz–10 range 100 mHz kHz–10 [18]. ThemHz perturbation [18]. The perturbation amplitude amplitude was 10 mV was and 10 stabilization mV and stabilization time in NaCl time solution in NaCl was solution set to 5 was min. set to 5 min.The PD curves were obtained at ambient temperature using an applied potential from −250The mV PD to curves +250 mV were vs. obtained open circuit at ambient potential temperature with a constantusing an voltageapplied potential step 1 mV from·s−1 −as250 used mV byto +250Taghavikish mV vs. open et al. circuit [19]. Measured potential potentiodynamic with a constant voltage curves werestep 1 analyzed mV∙s−1 as using used Tafel by Taghavikish fit by EC-Lab et softwareal. [19]. (softwareMeasured financed potentiodynamic by the project curves ITMS were 26220220183). analyzed Theusing obtained Tafel diagramsfit by EC were‐Lab expressedsoftware in(software semi-logarithmic financed by scale the project in order ITMS to 26220220183). reach better interpretation The obtained diagrams of the measured were expressed data. Thein semi PD‐ measurementslogarithmic scale were in order performed to reach at better least threeinterpretation times, so of that the reproducibilitymeasured data. ofThe the PD test measurements results was ensuredwere performed [20]. at least three times, so that reproducibility of the test results was ensured [20].

3. Results Results and and Discussion Discussion Figure1 1 showsshows the the microstructure microstructure of of tested tested S355J2 S355J2 steel. steel. The microstructure The microstructure revealed revealed ferrite‐ ferrite-pearlitepearlite matrix matrixwith a withlow pearlite a low pearlite content content (local pearlite (local pearlite occurrence) occurrence) and with and average with average grain size grain of size10 μm. of 10 µm.

Figure 1. Microstructure of etched S355J2 steel. Figure 1. Microstructure of etched S355J2 steel.

The coatings produced on steel from manganese phosphate solutions consist almost entirely of The coatings produced on steel from manganese phosphate solutions consist almost entirely of hurealite (Mn,Fe)5H2(PO4)4∙4H2O. They may also contain variable amounts of secondary and tertiary hurealiteferrous phosphates. (Mn,Fe)5H2 Iron(PO4 and)4·4H manganese2O. They may in hurealite also contain are mutually variable amounts replaceable of secondary [21]. The andevidence tertiary of ferrousproper hurealite phosphates. formation Iron and on manganese the surface inof hurealiteS355J2 steel are was mutually confirmed replaceable by performed [21]. The EDX evidence analysis of proper hurealite formation on the surface of S355J2 steel was conﬁrmed by performed EDX analysis (Figure 2). The major form of hurealite was Mn5H2(PO4)4∙4H2O as obvious from the almost doubled concentration of Mn in the phosphate surface layer compared to Fe. The surface morphologies of phosphate coating formed on ground and sandblasted surfaces are shown in Figure 3 and their cross section images are shown in Figure 4. Moderate grinding of the steel surface prior to phosphating leads to the formation of thin (4 to 9 μm) but very continuous and homogenous phosphate coating with a crystal size from 1 to 5 μm. Crystal organization is very compact and reveals a minimum of defect areas. On the other hand, the phosphate coating formed

Coatings 2016, 6, 46 4 of 9

on a sandblasted surface is thicker on average (thickness ranges from 3 up to 12 μm) compared to a ground surface but there is an obvious inhomogeneity and frequent occurrence of coating defects and pores of different size. Crystals are of irregular shape, more elongated compared to crystals formed on a ground surface with a size ranging from 1 to 7 μm. Sandblasted surface has significantly higher roughness as seen from Table 2. Moreover, sandblasting causes more intensive surface Coatingsdeformation2016, 6, that 46 is responsible for the increase of dislocations and defect numbers, and for higher4 of 9 thermodynamic differences between local surface micro areas in comparison to ground surface. This inhomogeneity is then reflected in non‐uniform coating formation. Crystal nucleation and further (Figuregrowth2 ).is Thepreferentially major form present of hurealite in the was micro Mn 5 Hareas2(PO with4)4·4H increased2O as obvious inner from energy the almost(small doubledresidual concentrationstresses after sandblasting). of Mn in the phosphate surface layer compared to Fe.

Coatings 2016, 6, 46 4 of 9 on a sandblasted surface is thicker on average (thickness ranges from 3 up to 12 μm) compared to a ground surface but there is an obvious inhomogeneity and frequent occurrence of coating defects and pores of different size. Crystals are of irregular shape, more elongated compared to crystals formed on a ground surface with a size ranging from 1 to 7 μm. Sandblasted surface has significantly higher roughness as seen from Table 2. Moreover, sandblasting causes more intensive surface deformation that is responsible for the increase of dislocations and defect numbers, and for higher thermodynamic differences between local surface micro areas in comparison to ground surface. This inhomogeneity is then reflected in non‐uniform coating formation. Crystal nucleation and further growth is preferentially presentFigure in 2. EDXthe microanalysis areasof created with phosphate increased layer. inner energy (small residual stresses after sandblasting). The surface morphologies of phosphate coating formed on ground and sandblasted surfaces are shown in Figure3 and their cross section images are shown in Figure4. Moderate grinding of the steel surface prior to phosphating leads to the formation of thin (4 to 9 µm) but very continuous and homogenous phosphate coating with a crystal size from 1 to 5 µm. Crystal organization is very compact and reveals a minimum of defect areas. On the other hand, the phosphate coating formed on a sandblasted surface is thicker on average (thickness ranges from 3 up to 12 µm) compared to a ground surface but there is an obvious inhomogeneity and frequent occurrence of coating defects and pores of different size. Crystals are of irregular shape, more elongated compared to crystals formed on a ground surface with a size ranging from 1 to 7 µm. Sandblasted surface has signiﬁcantly higher roughness as seen from Table2. Moreover, sandblasting causes more intensive surface deformation that is responsible for the increase of dislocations and defect numbers, and for higher thermodynamic (a) (b) differences between local surface micro areas in comparison to ground surface. This inhomogeneity is thenFigure reﬂected 3. SEM in non-uniform images of phosphate coating surface formation. morphology Crystal on: nucleation (a) ground and surface; further (b) growth sandblasted is preferentially surface. present in the micro areasFigure with increased 2. EDX analysis inner energy of created (small phosphate residual layer. stresses after sandblasting).

(a) (b)

Figure 4. Cross‐section images of MnP phosphate surface on: (a) ground surface; (b) sandblasted surface. (a) (b)

FigureFigure 3. SEM 3. SEMimages images of phosphate of phosphate surface surface morphology morphology on: on: ( a()a ground) ground surface; surface; (b) sandblasted(b) sandblasted surface. surface.

(a) (b)

Figure 4. Cross‐section images of MnP phosphate surface on: (a) ground surface; (b) sandblasted surface.

Figure 2. EDX analysis of created phosphate layer.

(a) (b) Coatings 2016, 6, 46 5 of 9 Figure 3. SEM images of phosphate surface morphology on: (a) ground surface; (b) sandblasted surface.

(a) (b)

Figure 4. 4. CrossCross-section‐section images images of MnP of phosphate MnP phosphate surface on: surface (a) ground on: surface; (a) ground (b) sandblasted surface; (surface.b) sandblasted surface.

Table 2. Measured surface roughness parameters of the samples with different surface treatment.

Surface Treatment Ground (p500) Sandblasted Ground + MnP Sandblasted + MnP Roughness Parameters

Ra (µm) 0.28 1.67 0.67 1.29 Rz (µm) 1.99 12.70 5.15 9.40

Figure5 shows the measured potentiodynamic curves of S355J2 steel samples after various surface treatments in 0.1 M NaCl corrosive solution. Measured kinetic and thermodynamic corrosion characteristics together with Tafel constants (βa and βc) values are listed in Table3. Mechanical and chemical surface treatments caused changes in both thermodynamic and kinetic characteristics of the base material. Thermodynamic stability of the surface is represented by the values of corrosion potentials Ecorr in our case. More positive value of this electrochemical characteristic means that the surface is nobler and thermodynamically more stable. The kinetics of the corrosion process are expressed by the corrosion current density icorr reflecting the intensity of running corrosion processes in an electrolyte. This electrochemical characteristic has a direct relation to the corrosion rate rcorr [22]. Sandblasting caused a more intensive attack of the steel surface compared to grinding. A destroyed surface with a higher concentration of structure defects and with larger area of an active surface resulted in lower electrochemical stability, which is reflected in the 51 mV decrease of Ecorr value in comparison with a simply ground surface. A part of the kinetic energy of the sandblasting particles transformed to added internal energy of the base material, which caused its absolute value to increase. As the result of higher surface activity, an increase is observed of the corrosion current density icorr and the directly related value of corrosion rate rcorr on sandblasted surface compared to the ground surface. However, this increase is not very significant and represents just 1.3 µA·cm−2 (29 µm·y−1). It is a beneficial finding that the corrosion resistance decrease caused by intensive sandblasting during surface cleaning is acceptable. The fact that sandblasting deteriorates corrosion properties was observed on stainless steel [7] as well as pure titanium [23]. Manganese phosphating significantly improved both thermodynamic and kinetic characteristics of the steel surface after both mechanical pretreatment operations (grinding and sandblasting). However, the negative influence of sandblasting on surface corrosion properties persisted. By forming a stable layer several microns thick, manganese phosphate forms a mechanical barrier against corrosion. The higher stability of manganese phosphate in 0.1 M NaCl solution compared to the base metal resulted in considerably more positive values of Ecorr. The positive shift was 252 mV on ground surface and 250 mV on sandblasted surface, a negligible difference. The most positive and noble value of Ecorr was reached with ground and subsequently phosphated surface (−399 mV). Thanks to the mechanical barrier action of manganese phosphate, Coatings 2016, 6, 46 6 of 9 the diffusion of oxygen to base metal active surface is more difficult and concentrated mainly in more porous areas. This complication of the kinetics of the corrosion process results in a decrease of iCoatingscorr and 2016rcorr, 6, 46values of both phosphated surfaces. Creation of uniform phosphate surface coating6 of on 9 ground surface led to almost 17-fold reduction of the corrosion rate and almost 13-fold reduction on sandblastedground and surface.subsequently The lowest phosphated kinetic corrosionsurfaces. characteristicsThis value was were 34 times again higher reached compared with ground to only and subsequentlyground surface phosphated and almost surface. twice as Phosphate high as on treatment sandblasted chiefly and influences further phosphated the anodic surface, reaction which as seen is fromevidence almost of uniform double values manganese of βa phosphatereached after coating phosphating. formation.

-1 ]

-2 -2

-3

-4 log i [mA.cm i log -5 Ground p500 Sandblasted -6 Ground + MnP Sanbl. + MnP -7 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

E [mV]

Figure 5. Potentiodynamic curves of S355J2 steel after various surface treatments in 0.1 M NaCl.

Table 3. Measured electrochemical corrosion characteristics.

Electrochemical Characteristics Ecorr icorr rcorr βa βc Electrochemical Characteristics E i −2 r −1 β −1 β −1 Surface Treatment (mVcorrSCE) (μAcorr∙cm ) (μcorrm∙y ) (mV∙deca ) (mV∙decc ) Surface Treatment (mV ) (µA·cm−2) (µm·y−1) (mV·dec−1) (mV·dec−1) Ground (p500) −651SCE ± 10 8.7 ± 0.2 202 ± 4 84 ± 5 350 ± 7 GroundSandblasted (p500) −−651702± ± 1510 8.710.0± ± 0.20.2 202231± ± 45 8482± ± 65 283 350 ± 77 GroundSandblasted + MnP phosphated −−702399± ± 2215 10.00.5 ± 0.10.2 23112± ± 15 152 82 ± ± 116 268 283 ±± 127 SandblastedGround + MnP + MnP phosphated phosphated −−399452± ± 2522 0.50.8± ± 0.1 1218± ± 11 152159± ± 1311 250 268 ± 1012 Sandblasted + MnP phosphated −452 ± 25 0.8 ± 0.1 18 ± 1 159 ± 13 250 ± 10

1000 25000 Ground (p500) Ground (p500) The ﬁndings from potentiodynamicSandbl. polarization tests were supported by non-destructiveSandbl. Ground + MnP 800 Ground + MnP electrochemical20000 impedance spectroscopy (EIS) measurements. Figure6 shows the Nyquist diagrams of

] Sanbl. + MnP Sanbl. + MnP ] 2 S355J2 steel samples after various surface treatment. Nyquist2 600 plots are frequently selected tool for EIS

.cm 15000 .cm interpretation, allowing precise determination of equivalent circuit components [22,24,25]. Figure7  [ [

im 10000 400 shows the equivalent circuit best describing the electrochemicalim processes at the sample-electrolyte Z Z interface. Polarization resistance Rp is the most important electrochemical characteristic. The value of 5000 200 Rp expresses how resistant the mono- or multi- surface layers are against corrosion [26–28]. Component CPE represents0 a constant phase element. Its function in the expression of EIS data is deﬁned 0 elsewhere [250,29]. 10000 The values 20000 of equivalent 30000 40000 circuit elements0 were 200 obtained 400 600 from 800 the 1000 analysis 1200 1400 of 2 a selected equivalent circuitZre [ using.cm ] a software EC-Lab (Bio-Logic Science InstrumentsZre [.cm2] SAS, Claix, France, version 11.01). These(a) values are listed in Table4. The highest value of(b)R p was reached with ground and subsequently phosphated surfaces. This value was 34 times higher compared to only Figure 6. Measured Nyquist diagrams of S355J2 steel samples with different surface treatment (0.1 M ground surface and almost twice as high as on sandblasted and further phosphated surface, which is NaCl solution): (a) overall view; (b) detail. evidence of uniform manganese phosphate coating formation.

Figure 7. Equivalent circuit for Nyquist plots analyses.

CoatingsCoatings 2016 2016, 6, 466, 46 6 of6 of9 9 groundground and and subsequently subsequently phosphated phosphated surfaces. surfaces. This This value value was was 34 34 times times higher higher compared compared to to only only groundground surface surface and and almost almost twice twice as as high high as as on on sandblasted sandblasted and and further further phosphated phosphated surface, surface, which which is is evidenceevidence of of uniform uniform manganese manganese phosphate phosphate coating coating formation. formation.

1 1

0 0

-1 -1 ] ] -2 -2 -2 -2

-3 -3

-4 -4 log i [mA.cm i log log i [mA.cm i log -5 -5 GroundGround p500 p500 SandblastedSandblasted -6 -6 GroundGround + MnP + MnP Sanbl.Sanbl. + MnP + MnP -7 -7 -1.0-1.0 -0.8 -0.8 -0.6 -0.6 -0.4 -0.4 -0.2 -0.2 0.0 0.0 E [mV] E [mV] FigureFigure 5. 5.Potentiodynamic Potentiodynamic curves curves of ofS355J2 S355J2 steel steel after after various various surface surface treatments treatments in in0.1 0.1 M M NaCl. NaCl.

TableTable 3. 3.Measured Measured electrochemical electrochemical corrosion corrosion characteristics. characteristics.

ElectrochemicalElectrochemical Characteristics Characteristics EcorrEcorr icorricorr rcorrrcorr βa βa βc βc −2 −1 −1 −1 SurfaceSurface Treatment Treatment (mV(mVSCESCE) ) (μ(Aμ∙Acm∙cm−2) ) (μ(mμ∙my−∙1y) ) (mV(mV∙dec∙dec−1) ) (mV(mV∙dec∙dec−1) ) GroundGround (p500) (p500) − −651651 ± 10± 10 8.78.7 ± 0.2± 0.2 202202 ± 4± 4 8484 ± 5± 5 350350 ± 7± 7 SandblastedSandblasted − −702702 ± 15± 15 10.010.0 ± 0.2± 0.2 231231 ± 5± 5 8282 ± 6± 6 283283 ± 7± 7 CoatingsGround2016Ground, 6 ,+ 46 MnP+ MnP phosphated phosphated − −399399 ± 22± 22 0.50.5 ± 0.1± 0.1 1212 ± 1± 1 152152 ± 11± 11 268268 ± 12± 12 7 of 9 SandblastedSandblasted + MnP+ MnP phosphated phosphated − −452452 ± 25± 25 0.80.8 ± 0.1± 0.1 1818 ± 1± 1 159159 ± 13± 13 250250 ± 10± 10

10001000 2500025000 GroundGround (p500) (p500) GroundGround (p500) (p500) Sandbl.Sandbl. Sandbl.Sandbl. 800 GroundGround + MnP+ MnP 800 GroundGround + MnP + MnP 2000020000

] Sanbl. + MnP ] Sanbl. + MnP Sanbl. + MnP

] Sanbl. + MnP ] 2 2 2 2 600600

.cm 15000

.cm 15000 .cm .cm     [ [ [ [

im 400 im 10000 10000 im 400 im Z Z Z Z

50005000 200200

0 0 0 0 00 10000 10000 20000 20000 30000 30000 40000 40000 00 200 200 400 400 600 600 800 800 1000 1000 1200 1200 1400 1400 2 2 2 ZreZre [ .cm[.cm] ] ZreZre [ .cm[.cm2] ] (a)( a) (b()b )

FigureFigure 6. 6.Measured Measured Nyquist Nyquist diagrams diagrams of ofS355J2 S355J2 steel steel samples samples with with different different surface surface treatment treatment (0.1 (0.1 M M Figure 6. Measured Nyquist diagrams of S355J2 steel samples with different surface treatment NaClNaCl solution): solution): (a )( aoverall) overall view; view; (b ()b detail.) detail. (0.1 M NaCl solution): (a) overall view; (b) detail.

Figure 7. Equivalent circuit for Nyquist plots analyses. FigureFigure 7. 7.Equivalent Equivalent circuit circuit for for Nyquist Nyquist plots plots analyses. analyses. Table 4. Electrochemical characteristics of S355J2 steel after various surface treatments.

Electrochemical Characteristics R (Ω·cm2) R (Ω·cm2) CPE (10−6·F·sn − 1) n Surface Treatment Ω p Ground (p500) 97 ± 3 1220 ± 53 538 ± 24 0.81 ± 0.02 Sandblasted 98 ± 4 770 ± 42 953 ± 33 0.79 ± 0.01 Ground + MnP phosphated 94 ± 4 41880 ± 627 1652 ± 34 0.69 ± 0.02 Sandblasted + MnP phosphated 95 ± 4 23600 ± 491 1924 ± 46 0.71 ± 0.03

Combination of complex surface morphology observations with complete electrochemical corrosion evaluation of tested surface treatments led to integrated findings. The sandblasting pretreatment generally helps to remove surface contaminants and corrosion products from the surface and forms rougher surfaces suitable for further painting operations. On the one hand, the quality and morphology of the sandblasted surface are responsible for rougher, less uniform, and more porous manganese phosphate coating in a comparison with manganese phosphate created on ground surfaces. On the other hand, increased surface roughness and porosity of the manganese coating are beneficial if additional painting or coating is applied in the case of improving integrity of whole coating system with the base metal. When the manganese phosphating represents the final surface treatment operation whereby higher quality and maximal corrosion protection are required, a more appropriate pretreatment process is a simple grinding. Coatings 2016, 6, 46 8 of 9

4. Conclusions The present work describes the inﬂuence of various surface treatments on morphological and corrosion properties of manganese phosphate on S355J2 steel. The most relevant conclusion is as follows:

• Using the manganese phosphating process created a continuous coating on the steel surface after both mechanical surface pretreatment techniques. However, the higher uniformity of the coating and crystal size together with lower porosity and defect occurrence was obtained on ground surfaces; • Sandblasting caused increased surface roughness of the substrate compared to a ground surface. This inﬂuence was reduced after subsequent manganese phosphate formation; • Sandblasting had a negative effect on the corrosion resistance of S355J2 steel compared to ground surfaces. Worsening was observed of the relevant electrochemical corrosion characteristics including Ecorr, icorr, rcorr, and Rp after sandblasting; • The phosphate treatment reduces the corrosion rate as ascertained both in polarization curves and impedance tests by about the same extent on ground and sandblasted samples; • Manganese phosphating results in an increase of the polarization resistance of the steel surface by more than 30 times.

Acknowledgments: The research has been supported by Science Grant Agency of the Slovak Republic through project No. 1/0720/14. Authors are grateful works to the Slovak Research and Development Agency for support in experimentalby the projects No. APVV-14-0284, No. APVV-14-0772 and No. APVV-14-0096. This research was carried out under the project CEITEC 2020 (LQ1601) with financial support from the Ministry of Education, Youth and Sports of the Czech Republic under the National Sustainability Programme II. Author Contributions: Branislav Hadzima and Stanislava Fintová conceived and designed the experiments; Kamil Borko performed the experiments; Daniel Kajánek analysed the data; Filip Pastorek contributed reagents/materials/analysis tools; Filip Pastorek wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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