Polyester Powder Coatings in Aggressive Media

Article Experimental Evaluation of Polyester and Epoxy– Polyester Powder Coatings in Aggressive Media

Ivan Stojanovi´c 1,*, Vinko Šimunovi´c 1, Vesna Alar 1 and Frankica Kapor 2

1 Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Zagreb 10002, Croatia; [email protected] (V.Š.); [email protected] (V.A.) 2 Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Zagreb 10002, Croatia; [email protected] * Correspondence: [email protected]; Tel.: +385-1-6168-343

Received: 13 February 2018; Accepted: 7 March 2018; Published: 8 March 2018

Abstract: Protective coatings are the most widely used corrosion protection method for construction materials in different environmental conditions. They isolate metals from aggressive media, making the structure more durable. Today, alongside good anti-corrosive properties, coatings need to be safe for the environment and harmless to those who apply them. The high volatile organic compound (VOC) content in conventional solvent-borne coatings presents a huge ecological problem. A solution for indispensable solvent emission reduction is the application of powder coatings. This study evaluates the corrosion performance and surface morphology of polyester and epoxy–polyester powder coatings. Electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) measurement, salt spray chamber and humidity chamber testing followed by adhesion testing were used to investigate the protective properties of powder coatings. Scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX) was used to analyse the surface morphology and chemical composition, whereas the microstructure and coating uniformity were determined by optical microscope examination. The research revealed a negative inﬂuence of coating surface texture on coating thickness and consequently a lack of barrier and adhesion properties. The epoxy–polyester powder coating showed a better performance than the polyester coating. All tested coatings showed uniform structure.

Keywords: electrostatic spraying; powder coatings; corrosion performance; EIS study; SEM analysis

1. Introduction Electrostatic powder coating originated in the USA in the 1950s. For years, it has been extensively used for painting processes in the automotive industry. Compared to traditional coatings, it provides even, uniform and reproducible coatings with less waste [1]. The electrostatic coating process starts by electrostatically charging the powder. At the spray gun exit, charged particles repel each other, and spread evenly [2] on the object which is oppositely charged or grounded. The electrostatic attraction between the powder particles and the object allows the powder to adhere to the surface, forming an even coating [3,4]. The application of the electrostatic powder coating is followed by a high-temperature curing process, which provides a smoothly-coated surface. The powder is charged using either a corona or tribo-charging spray gun [3]. In the case of corona-charging, the powder particles pass through an ion-rich area and become charged based on their permittivity. Tribo-charging is based on the frictional charging of powder that is transported through a pipe of a specific material, such as polytetrafluoroethylene (PTFE), metal or other powder particles [1,5]. The powder particles are more extensively spread and have a higher transfer efficiency at higher charge [6]. Since tribo-charging generates less charge on powder particles, corona-charging is generally used for electrostatic powder

Coatings 2018, 8, 98; doi:10.3390/coatings8030098 www.mdpi.com/journal/coatings Coatings 2018, 8, 98 2 of 12 coating [1]. Today, thanks to the commercial success and wide application of powder coatings, extensive research is carried out in this field of study, in particular in the powder coating chemistry [5]. Compared to liquid paints, powder coatings feature several advantages including the absence of volatile organic content, the reduction in coating material loss (up to 68%), the reduction of dust formation (from 40% to 84%) [1], high utilization rates, fast curing, the minimal health risks involved, and the elimination of hazardous wastes [7]. However, a major disadvantage of electrostatic coating technology is that the curing of the powder coating requires a significant amount of heat energy. For this reason, the future of these technologies will include the development of low temperature curable (LTC) powder coatings, whose application will enable energy saving solutions and also the ability to coat heat-sensitive substrates [8]. The second disadvantage is the lack of coating uniformity in Faraday-cage areas. Aside from several challenges needing improvement, powder coating is an environmentally-friendly coating process with good performance properties. There are numerous methods to evaluate the corrosion performance of powder coatings. Mafi et al. [5] investigated the protective behaviour of powder coatings in 3.5% w/w NaCl solution using electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurement, as well as differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). Bhadu et al. [9] studied the properties of two polyester-based powder coatings using electrochemical impedance spectroscopy in 3.5% w/w NaCl solution, the salt spray test, and scanning electron microscope (SEM) analysis. Both papers [5,9] showed better protective properties of epoxy–polyester powder coating compared to polyester powder coating, which is attributed to the higher cross-linked density of the epoxy-based powder. In this paper, the protective properties of two epoxy–polyester and two polyester powder coatings with rough and smooth surface textures were evaluated in different aggressive media, that is, in 4% w/w NaOH solution using the EIS method, evaluated in a humid and warm atmosphere using humidity chamber testing and in a marine environment using the salt spray chamber testing. The structure, morphology, and uniformity of powder coatings were analysed by SEM and EDX analysis. The results were discussed regarding the chemical composition of the binder and coating thickness, which was strongly affected by surface textures. The coatings were provided by four different manufacturers, and are commonly used in switch-gear cabinet production.

2. Materials and Methods Mild steel plates with the dimensions 100 mm × 150 mm × 3 mm were chemically pre-treated using iron phosphating in a bath and a 0.5−2 µm thick Fe-phosphate coating was obtained. Subsequently, the epoxy–polyester and polyester powder coatings with different surface textures, used for indoor and outdoor exposure, were applied to the pre-treated samples using a PG1 electrostatic spray gun, with spraying parameters of 98 kV, 100 µA, and 17 kHz. The powder-coated samples were then cured in an oven for about 20 min at 185–200 ◦C. Table1 presents technical data for the tested powder coatings.

Table 1. Technical data for tested powder coatings.

Surface Particle Speciﬁc Weight Sample Chemical Type Purpose Gloss Appearance Size (µm) (kg dm−3) saturated carboxylated polyester A for indoor use semi-glossy textured (rough) < 100 1.6 resin and solid epoxy resin saturated carboxylated polyester river textured B resin cured with the for outdoor use glossy 30–40 1.7 (rough) β-hydroxyl-alkyl amide (HAA) saturated carboxylated polyester C resin cured with the for outdoor use glossy smooth < 100 1.4-1.7 β-hydroxyl-alkyl amide (HAA) saturated carboxylated polyester D for indoor use semi-glossy smooth 30–40 1.65 resin and solid epoxy resin Coatings 2018, 8, 98 3 of 12

The dry-ﬁlm thickness (DFT) of the cured powder coated samples was measured with a non-destructive magnetic induction method using an Elcometer 456 instrument (Elcometer Limited, Manchester, UK), according to ISO 2808 [10]. Measurements were performed on ten different locations per sample. The coating adhesion was determined using the Zehntner cross-cut device (Zehntner GmbH Testing Instruments, Sissach, Switzerland), according to ISO 2409 [11]. The 240-h salt spray and humidity chamber tests were used as accelerated laboratory tests to predict the corrosion performance of the powder coatings [5,12,13]. The salt chamber testing was conducted in 5% neutral NaCl solution in an Ascott cabinet, model S450 (Ascott Analytical Equipment Limited, Staffordshire, UK), according to ISO 9227 [14]. The humidity test was conducted according to ISO 6270-2 [15]. The samples were periodically examined according to ISO 4628 [16] in order to evaluate the degradation of coatings [17]. The accelerated testing was performed on two samples per each powder coating. The corrosion/stability tendency in the NaOH solution was assessed by measuring the open circuit potential (OCP) [5]. The open circuit potential of the polyester and epoxy–polyester powder-coated samples was measured against the saturated calomel electrode (SCE) as the reference electrode [5] in 4% w/w NaOH solution, pH = 12, at room temperature (23 ± 2) ◦C. The protective properties of the powder coatings were investigated by electro-chemical impedance spectroscopy [9,18], with a VersaSTAT 3 Potentiostat/Galvanostat (AMETEK Scientiﬁc 131 Instruments, Princeton applied research, Berwyn, PA, USA). The measurements were carried out after one hour and after 100 h of immersion in 4% w/w NaOH solution, pH = 12, at room temperature (23 ± 2) ◦C. The impedance spectra were performed at open circuit potential (OCP) with a 10 mV sinusoidal amplitude. The frequency range was from 100 kHz to 100 mHz. A three-electrode cell including a coated metal sample as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and graphite sticks as the auxiliary electrodes were used in the experiments [12]. The surface of the working electrode was 1 cm2. The EIS measurements were repeated three times for each sample. The Solartron Z-View 2.2 software was used to interpret data. The surface morphology and the microstructure of the powder coatings were observed by an Olympus GX51 (Olympus Corporation, Tokyo, Japan) inverted metallurgical microscope and a Tescan scanning electron microscope (SEM) (TESCAN Brno, Brno, Czech Republic) equipped with Oxford Instruments energy dispersive spectroscopy (EDX) (Oxford Instruments, Belfast, UK). The energy used for analysis was 20 keV.

3. Results and Discussion

3.1. Coating Thickness Measurement By measuring the coating thickness (ISO 2808), a large scatter of the results for the applied polyester coating with rough river surface texture (sample B) was found, with a minimum coating thickness of 40.4 µm. In other samples, greater variations in thickness were not determined. The average coating thickness was about 100 µm.

3.2. Chamber Testing Results The accelerated corrosion tests in the salt spray (ISO 9227) and the humidity (ISO 6270-2) chamber for a period of 10 days (240 h) did not show any blistering and rusting of the undercoated surface, according to ISO 4628 [16]. Corrosion appeared only on the hole through which the samples were suspended on a metal hook for grounding. At the hole, the metal hook physically disabled the access for coating application. This is certainly a drawback of electrostatic powder coating application because such places need to be repaired additionally with a brush or spray. The corrosion on the edges of the samples was not taken into consideration. After the corrosion test, samples BV 1 and BV 2, which were exposed to the humidity chamber, showed extremely poor adhesion—Gt = 5 (extreme flaking)—whereas the samples BS 1 and BS 2, Coatings 2018, 8, 98 4 of 12 after testing in the salt spray chamber, showed excellent adhesion—Gt = 0—according to ISO 2409 [11]. The samples tested in the salt spray chamber were protected with a greater coating thickness. The adhesion to the other tested samples in the salt spray chamber and the humidity chamber was classified as Gt = 0. The test revealed a strong negative influence of low coating thickness on the adhesion of powder coatings in a humid environment. Prior to the corrosion tests, all the samples had shown excellent adhesion Gt = 0 (no flaking). The chemical composition of the powder coating binder had no influence on the coating protection properties during 240 h of accelerated chamber testing. The results of powder coating dry-film thickness and corrosion testing after 240 h in the salt spray chamber and the humidity chamber, as well as the subsequent adhesion tests, are provided in Tables2 and3. Table 2. Assessment of coating protection properties after 240 h in the humidity chamber.

DFT DFT DFT Cross-Cut Testing Sample min max mean Blistering Rusting (µm) (µm) (µm) Adhesion AV 1 83.6 133 104.01 0 0 0 AV 2 100 152 124.1 0 0 0 BV 1 64.5 114 87.98 0 0 5 Humidity BV 2 40.4 130 81.28 0 0 5 chamber CV 1 96.7 116 107.28 0 0 0 CV 2 91.3 134 115.93 0 0 0 DV 1 80.8 106 94.74 0 0 0 DV 2 83.3 104 92.94 0 0 0

Table 3. Assessment of the coating protection properties after 240 h in the salt spray chamber.

DFT DFT DFT Cross-Cut Testing Sample min max mean Blistering Rusting (µm) (µm) (µm) Adhesion AS 1 95 115 103.61 0 0 0 AS 2 89.1 123 106.83 0 0 0 BS 1 76 150 107.73 0 0 0 Salt spray BS 2 73.5 154 117.71 0 0 0 chamber CS 1 97.7 119 107.95 0 0 0 CS 2 101 122 113 0 0 0 DS 1 82.7 103 91.81 0 0 0 DS 2 86.6 118 96.57 0 0 0

3.3. OCP and EIS Study

The open circuit potential (E/Ecorr) results after stabilization for 30 min in 4% w/w NaOH solution and the mean coating thickness (DFTmean) are provided in Table4.

Table 4. The results of the open circuit potential after stabilization in 4% w/w NaOH solution.

Sample A B C D 01 00

DFTmean (µm) 126.6 114.2 71.4 69.2 – – Ecorr vs SCE (V) −0.447 −0.240 −0.320 −0.300 −0.253 −0.270

A diagram of the open circuit potential in NaOH solution is shown in Figure1. The phosphate coating showed instability when exposed to the alkaline solution (sample 01), while the increase in the corrosion potential in time could be connected to the creation of a passive ﬁlm on the mild steel substrate (sample 00). The epoxy–polyester coating D showed very good protective properties because of its potential corrosion stability over time. Coatings 2018, 8, 98 5 of 12 Coatings 2018, 8, 98 5 of 12 Coatings 2018, 8, 98 5 of 12

Coatings 2018, 8, 98 5 of 12

Figure 1. TheFigure open 1. circuitThe open potential circuit potential of powder of powder coating coating as a as function a function of of time, time, in in 4% 4% w/w w/w NaOH NaOH solution. Figure 1. The open circuit potential of powder coating as a function of time, in 4% w/w NaOH solution. solution. The impedanceFigureThe impedance 1. dataThe open for data circuit the for testedpotential the tested powder of powder coatings coatingcoatings as at a at the function the beginning beginning of time, of inexposure 4% of w/w exposure to NaOH 4% w/w to 4% w/w NaOH solutionTheNaOHsolution impedance were solution. ﬁtted were data fitted by for the bythe theRandles tested Randles powder equivalent equivalent coatings electric electric at circuit the beginning for circuit a single for of electro exposure a single-chemical to electro-chemical 4%cell. w/w NaOHThe solution equivalent were ci rcuitsfitted consistby theed Randles of the solutionequivalent resistance, electric R circuits, the polarisation for a single resistance, electro-chemical Rp, and cell. cell. The equivalent circuits consisted of the solution resistance, Rs, the polarisation resistance, Rp, The equivalentdoubleThe -impedancelayer ci capacitance,rcuits data consist for Cdl ed,the [1 9 of]tested shownthe solutionpowder in Figure coatingsresistance, 2. For modellingat theRs, beginningthe the polarisation interface of exposure of resistance,a metal to surface4% Rw/wp, and and double-layer capacitance, C ,[19] shown in Figure2. For modelling the interface of a metal doubleNaOHin - contactlayer solution capacitance, with were an fitted electrolyte Cdl by, dl[ 1the9, ]as shownRandles well asin equivalent toFigure describe 2. electric For non modelling-porous circuit coatingfor the a single interface behaviour electro of, - chemicalthea metal Randles surface cell. surfacein contactTheequivalent equivalent with with circuit an anci rcuits electrolyte electrolyte,is often consist used, ased [ 20 as wellof]. wellthe as solution toas describeto describeresistance, non- non-porous porousRs, the polarisation coating coating behaviour resistance, behaviour,, the R p Randles, and the Randles equivalentequivalentdouble circuit-layer circuit isoften capacitance, is often used used C [20dl, [].1209]]. shown in Figure 2. For modelling the interface of a metal surface in contact with an electrolyte, as well as to describe non-porous coating behaviour, the Randles equivalent circuit is often used [20].

Figure 2. The Randles equivalent circuit [12,19].

After 100 h of exposureFigureFigure to 2.a 4%The2. The w/w Randles Randles NaOH equivalentesolution,quivalent the circuit circuitNyquist [12 [,1 12diagram9],.19 ]. for all tested powder coatings showed a two-time constant spectrum, thus the impedance data were fitted with the equivalent electric circuit forFigure porous 2. The coating. Randles The equivalent equivalent circuit circuit [12 ,1for9]. porous coating consisted of After 100After h of100 exposure h of exposure to a to 4% a 4% w/w w/w NaOH NaOH solution, the the Nyquist Nyquist diagram diagram for all for tested all testedpowder powder the solution resistance, Rs, the coating resistance, Rc, the charge transfer resistance, Rct, the coating coatingsAfter showed 100 h aof twoexposure-time to constant a 4% w/w spectrum NaOH solution,, thus the the Nyquistimpedance diagram data for were all tested fitted powder with the coatings showedcapacitance, a two-time Cc, and double constant-layer spectrum, capacitance, thus Cdl, [5, the21] impedanceshown in Figure data 3. were ﬁtted with the equivalent equivalent electric circuit for porous coating. The equivalent circuit for porous coating consisted of electric circuitcoatings for showed porous a coating. two-time The constant equivalent spectrum circuit, thus the for impedance porous coating data were consisted fitted with of the the solution theequivalent solution resistance, electric circuit Rs, thefor porouscoating coating. resistance, The equivalentRc, the charge circuit transfer for porous resistance, coating R consistct, the edcoating of resistance,capacitance,theRs ,solution the coatingCc ,resistance, and double resistance, Rs-, layerthe coating capacitance,Rc, the resistance, charge Cdl, [5,R transferc,21 the] shown charge resistance, in transfer Figure resistance, R3.ct , the coating Rct, the coating capacitance, Cc, and double-layercapacitance, capacitance, Cc, and doubleCdl-layer,[5, capacitance,21] shown C indl, [5, Figure21] shown3. in Figure 3.

Figure 3. The two-time constant electrical equivalent circuits for porous coating [5,12,21].

The EIS analysis results at the beginning and after 100 h of exposure of powder coatings to a 4%

w/w NaOH solution are given in Table 5. CPEdl is a constant phase element of the double layer FigureFigure 3. 3. The two-time constant electrical equivalent circuits for porous coating [5,12,21]. FigureThe two-time3. The two-time constant constant electrical electrical equivalentequivalent circuits circuits for porous for porous coating coating [5,12,21].[ 5 ,12,21]. The EIS analysis results at the beginning and after 100 h of exposure of powder coatings to a 4% The EIS analysisThe EIS analysis results results at the at the beginning beginning and and after after 100 100h of exposure h of exposure of powder of coatings powder to a coatings4% to a dl w/ww/w NaOH NaOH solution solution are are given given in in Table Table 5. 5. CPECPEdl is a constantconstant phase phase element element of of the the double double layer layer 4% w/w NaOH solution are given in Table5. CPE dl is a constant phase element of the double layer showing its capacitive properties, which depend on the empirical constant ndl. The constant n is in the range from 0 to 1. If n = 0, the CPE acts as a resistor, and if n = 1, the CPE acts as a capacitor [22]. CPEcoat is a constant phase element of the coating, and ncoat is its empirical constant. Coatings 2018, 8, 98 6 of 12 showing its capacitive properties, which depend on the empirical constant ndl. The constant n is in the range from 0 to 1. If n = 0, the CPE acts as a resistor, and if n = 1, the CPE acts as a capacitor [22]. CPEcoat is a constant phase element of the coating, and ncoat is its empirical constant. Coatings 2018, 8, 98 6 of 12 Table 5. The results of the EIS measurement at the begging (A, B, C, D) and after 100 h (A/100 h, B/100 Tableh, C/100 5. h,The D/100 results h) immersion of the EIS in measurement 4% w/w NaOH at the solution begging. (A, B, C, D) and after 100 h (A/100 h,

B/100 h,DFT C/100mean h, D/100 h) immersion in 4% w/w NaOH solution. Sample Rs (Ω) CPEcoat (F) ncoat Rcoat (Ω) CPEdl (F) ndl Rp/Rct (Ω) (µm) DFT SampleA mean 150.7R ( Ω) CPE– (F) – n R– (Ω)4.975 CPE × 10(F)−11 0.96016n 2.8963R /R ×(Ω 10) 6 126.6(µm) s coat coat coat dl dl p ct A/100 h 331.2 7.665 × 10−5 0.7334 1.323 × 104 2.584 × 10−4 1 1.527 × 104 A 150.7 – – – 4.975 × 10−11 0.96016 2.8963 × 106 126.6 −1 6 A/100B h113.5 331.2 7.665– × 10−5 – 0.7334 1.323– × 104 6.5492.584 ×× 1010−°4 0.944671 1.3721.527 × ×10 104 114.2 −5 2 −4 4 B/100B h 236.9113.5 7.705 × 10 – 0.4012 – 2.856 × 10 – 1.1006.549 × 1010−1◦ 0.69170.94467 2.8821.372 × ×10 106 114.2 −5 2 −4 4 B/100C h467.8 236.9 7.705– × 10 – 0.4012 2.856– × 10 6.4521.100 ×× 1010−11 0.961660.6917 1.75622.882 × ×10 106 C 71.4 467.8 – – – 6.452 × 10−11 0.96166 1.7562 × 106 C/100 h 71.4 121.6 1.064 × 10−4 0.5856 111.8 2.865 × 10−4 0.5569 4.252 × 103 C/100 h 121.6 1.064 × 10−4 0.5856 111.8 2.865 × 10−4 0.5569 4.252 × 103 −11 6 DD 672.3672.3 – –– –– – 7.3217.321 ×× 1010−11 0.940900.94090 1.8189× ×10 106 69.269.2 D/100D/100 h h100 100 2.7022.702 × 10× −910 −9 0.96200.9620 2.8092.809 × 10×410 4 3.4623.462 ×× 1010−6− 6 0.78970.7897 9.830× ×10 104 4

The results of the EIS analysis at the beginning of exposureexposure toto aa 4%4% w/ww/w NaOH NaOH solution solution show showeded that the polyester powder coating B B (for (for outdoor outdoor use) use) ha hadd the the weakest weakest protective protective properties, properties, that that is, is, the lowest coating resistance (Rp = 1.372 × 1066 Ω). The biggest coating resistance was determined for the lowest coating resistance (Rp = 1.372 × 10 Ω). The biggest coating resistance was determined for epoxyepoxy–polyester–polyester coating A. The coating resistance features the ability to resist electrolyte penetration and is often used to assess coating protective performance [99,,1919].]. After 100 h of immersion in in 4% 4% w/w w/w NaOH NaOH solution solution,, all all the the tested tested powder powder coatings coatings showed showed a decreasea decrease in in resistance, resistance, thus thus indicating indicating a aporous porous coating coating behaviour behaviour where where the the corrosive medium penetrated thethe metal–coatingmetal–coating interface interface [5 [].5] The. The lowest lowest coating coating resistance resistance after after long-term long-term immersion immersion was wasdetermined determined for polyester for polyester powder powder coating coating C, whereas C, whereasthe best protective the best protectiveproperties wereproperties established were establishedfor epoxy–polyester for epoxy powder–polyester coating powder D. coating Both powder D. Both coatings powder Ccoatings and D C were and appliedD were applied with similar with similarthickness thickness and had and a smooth had a smooth surface surface appearance. appearance. This suggests This suggests that the that chemical the chemical composition composition of the powderof the powder coating coating binder binder was responsible was responsible for the corrosion for the corrosion protection protection behaviour behaviour rather than rather the thickness. than the thickness.Polyester resins Polyester are not resins resistant are to not alkaline resistant hydrolysis to alkaline [23], which hydrolysis has led [ to23 the], which degradation has led and to lower the degradationresistance of theand polyester lower resistance powder of coatings the polyester B and C powder compared coatings to the hybridB and C epoxy—polyester compared to the powder hybrid epoxycoating-–polyester in the presence powder of NaOH. coating Alkaline in the presencehydrolysis of resulted NaOH. in Alkaline a cleavage hydrolysis reaction of result estered chains in a cleavageand a loss reaction of polyester of ester protective chains and properties. a loss of Thepolyester epoxy–polyester protective properties. powder coating The epoxy A was–polyester applied withpowder higher coating mean A was thickness applied but with showed higher slightly mean thickness lower resistance but showed than slightly the epoxy–polyester lower resistance powder than thecoating epoxy D,– duepolyester to an unevenpowder texturecoating ﬁlm. D, due to an uneven texture film. The impedance res results,ults, measured (Msd.) and fitted ﬁtted ( (Calc.),Calc.), were expressed by the NyquistNyquist and Bode diagramsdiagrams shown in Figure4 4..

Sample A.txt Model : R(Q(R(QR))) Wgt : Modulus Z , Msd. 6,000 Z , Calc. 5,500 158m 5,000 200m 4,500 251m 316m

4,000 398m )

m 3,500 501m h

o 631m

(

3,000

Z 1 2,500 - 1.26 794m 2,000 2 1.58 1,500 3.16 2.51 1,000 5.01 3.98 10 500 12.6 0 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 Z ' (ohm) (a) Figure 4. Cont.

Coatings 2018, 8, 98 7 of 12 Coatings 2018, 8, 98 7 of 12 Figure 4. Cont.

Sample A.txt Model : R(Q(R(QR))) Wgt : Modulus |Z|, Msd. 10,000 50 |Z|, Calc. Angle, Msd. 45 Angle, Calc.

n ) 30 g

l m

e h

1,000 ( (

25 d

e |

Z | 20 )

100 0 0.1 1 10 100 1,000 10,000 100,000 Frequency (Hz) (b)

Sample B.txt Model : R(Q(R(QR))) Wgt : Modulus Z , Msd. Z , Calc. 3,500 158m 3,000 200m 251m 2,500 316m

) 398m m

h 501m

o 2,000

(

631m

' '

794m

- 1,500 1

1.58 1,000 1.26 2.51 2 5.01 500 10 3.98 12.6 0 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000 Z ' (ohm) (c)

Sample B.txt Model : R(Q(R(QR))) Wgt : Modulus |Z|, Msd. 10,000 48 |Z|, Calc. Angle, Msd. 46 Angle, Calc. 44 42 40 1,000 38

n )

36 g

l m

e h

( (

32 e |

Z | 30 ) 100 28 26 24 22 20 18 10 0.1 1 10 100 1,000 10,000 100,000 Frequency (Hz) (d)

Figure 4. Cont. Figure 4. Cont.

Coatings 2018, 8, 98 8 of 12 Coatings 2018, 8, 98 8 of 12

Sample C.txt Model : R(Q(R(QR))) Wgt : Modulus Z , Msd. 1,200 Z , Calc.

1,100 100m

1,000 158m 126m 900 251m

800 398m 200m )

m 316m

700 631m

h o

( 501m

' 600 '

794m Z 1.58 - 500 1.26 400 2.51 2 3.98 300 3.16 7.94 200 5.01 20 10 100 31.6 0 0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200 2,400 Z ' (ohm) (e)

Sample C.txt Model : R(Q(R(QR))) Wgt : Modulus |Z|, Msd. 10,000 |Z|, Calc. 40 Angle, Msd. Angle, Calc.

1,000 30

n )

l m

e h

25 o

( (

e |

Z | 20 ) 100

10 0.1 1 10 100 1,000 10,000 100,000 Frequency (Hz) (f) Sample D.txt Model : R(Q(R(QR))) Wgt : Modulus Z , Msd. 3.20E+05 Z , Calc. 3.00E+05 2.80E+05 158m 2.60E+05 2.40E+05 200m 2.20E+05

) 2.00E+05 251m m

h 1.80E+05

(

316m

' 1.60E+05

Z 1.40E+05

398m - 1.20E+05 501m 1.00E+05 8.00E+04 794m 631m 6.00E+04 1.26 1 4.00E+04 2 1.58 2.00E+04 7.94 0.00E+00 0.00E+00 1.00E+05 2.00E+05 3.00E+05 4.00E+05 5.00E+05 Z ' (ohm) (g)

Figure 4. Cont. Figure 4. Cont.

Coatings 20182018, ,88, ,98 98 9 9of of 12 12 Coatings 2018, 8, 98 9 of 12

Sample D.txt Model : R(Q(R(QR)))Sample D.txt Wgt : Modulus |Z|, Msd. 1.00E+06 Model : R(Q(R(QR))) Wgt : Modulus 85 |Z|,|Z|, Calc.Msd. 1.00E+06 8085 Angle,|Z|, Calc. Msd. 7580 Angle, Calc.Msd. 7075 Angle, Calc. 1.00E+05 6570 1.00E+05 6065

5560 A

n )

55 l m

50 n

) h

( m 1.00E+04 50

45 e (

e |

1.00E+04 (

45 g

( 40 Z

)

e |

40 g

Z 35 |

) 3035 1.00E+03 2530 1.00E+03 2025 1520 1015 1.00E+02 510 1.00E+02 0.1 1 10 100 1,000 10,000 100,0005 0.1 1 10 Frequency100 (Hz) 1,000 10,000 100,000 Frequency (Hz) (h) (h)

FigureFigure 4 4.. TheThe Ny Nyquistquist diagram diagram (a,c (,ae,,cg,e) ,andg) and Bode Bode diagram diagram (b,d (,fb,h,d), fof,h )tested of tested powder powder coatings coatings after Figure 4. The Nyquist diagram (a,c,e,g) and Bode diagram (b,d,f,h) of tested powder coatings after 100after h 100immersion h immersion in 4% inw/w 4% NaOH w/w NaOH solution solution:: (a,b) Sample (a,b) Sample A; (c,d A;) Sample (c,d) Sample B; (e,f) B; Sample (e,f) Sample C; (g,h C;) 100 h immersion in 4% w/w NaOH solution: (a,b) Sample A; (c,d) Sample B; (e,f) Sample C; (g,h) Sample(g,h) Sample D. D. Sample D. 3.3.4.4. Morphology Morphology and and EDX EDX S Spectrapectra 3.4. Morphology and EDX Spectra TheThe crosscross-section-section of of the representative polyester and epoxyepoxy–polyester–polyester powder powder coating coatingss on on a The cross-section of the representative polyester and epoxy–polyester powder coatings on a phosphate steel surface are shown in Figure 55.. ItIt cancan bebe seenseen thatthat thethe coatingscoatings appliedapplied onon thethe samplessamples phosphate steel surface are shown in Figure 5. It can be seen that the coatings applied on the samples appearappear to to be adequately cured and uniform, with no defects and pores.pores. A s slightlylightly higher coating appear to be adequately cured and uniform, with no defects and pores. A slightly higher coating thicknthicknessess of of the the polyester coating C can be observed. The cracks and pores are not visible. thickness of the polyester coating C can be observed. The cracks and pores are not visible.

(a) (b) (a) (b) Figure 5. Cross-sectional photographs of powder coatings: (a) Sample A (epoxy-polyester), coating Figure 5.5. Cross-sectionalCross-sectional photographs of powder coatings: ( a) Sample A (epoxy-polyester),(epoxy-polyester), coating thickness 60–100 µm;µ (b) Sample C (polyester), coating thickness 80–120 µµm. thickness 60 60–100–100 µm;m; (b) Sample C (polyester), coatingcoating thickness 80–12080–120 µm.m. The SEM micrograph with an EDX spectrum is shown in Figure 6. The SEM micrograph The SEM SEM micrograph micrograph with with an an EDX EDX spectrum spectrum is shown is shown in Figure in 6Figure. The SEM 6. The micrograph SEM micrograph indicated indicated that the tested epoxy–polyester and polyester coating adhered firmly to the phosphate indicatethat thed tested that the epoxy–polyester tested epoxy–polyester and polyester and polyester coating adhered coating ﬁrmly adhered to thefirmly phosphate to the phosphate substrate substrate following the morphology of the substrate. The peak of the Ti pigment materials on the substratefollowing following the morphology the morphology of the substrate. of the Thesubstrate peak. of The the p Tieak pigment of the materialsTi pigment on materials the EDX spectraon the EDX spectra was predominant in both cases. EDXwas predominantspectra was predominant in both cases. in both cases.

Coatings 2018, 8, 98 10 of 12 Coatings 2018, 8, 98 10 of 12

(a) (b)

Figure 6. The SEM micrograph with EDX spectra of the powder coatings: (a) Sample A (epoxy- Figure 6. The SEM micrograph with EDX spectra of the powder coatings: (a) Sample A polyester), good penetration of coating into the substrate, no visible cracks, and pores, Ti pigment is (epoxy-polyester), good penetration of coating into the substrate, no visible cracks, and pores, dominant; (b) Sample C (polyester), no visible cracks and pores, finely micro-rough appearance of Ti pigment is dominant; (b) Sample C (polyester), no visible cracks and pores, ﬁnely micro-rough upper layer surface, Ti pigment is dominant. appearance of upper layer surface, Ti pigment is dominant.

4. Conclusions 4. Conclusions The tests performed in this study showed that the powder coatings exhibited very good The tests performed in this study showed that the powder coatings exhibited very good properties properties in humid and marine environments as well as in an alkali medium, but also displayed in humid and marine environments as well as in an alkali medium, but also displayed some failures. some failures. Extremely good protective properties were displayed in the epoxy–polyester coatings, Extremely good protective properties were displayed in the epoxy–polyester coatings, which are which are intended for indoor use, while the worst performance was delivered by polyester powder intended for indoor use, while the worst performance was delivered by polyester powder coating B, coating B, intended for outdoor use, which lost of adhesion after exposure to the humid and warm intended for outdoor use, which lost of adhesion after exposure to the humid and warm atmosphere atmosphere (ISO 6270-2). A possible cause of this adhesion loss was the distinct relief surface of (ISO 6270-2). A possible cause of this adhesion loss was the distinct relief surface of coating B, for which coating B, for which insufficient coating thickness was determined (DFTmin = 40.4 µm), which led to insufficient coating thickness was determined (DFT = 40.4 µm), which led to an early penetration an early penetration of water to the steel surface. Themin protective efficiency of an organic coating is of water to the steel surface. The protective efficiency of an organic coating is generally achieved generally achieved through the barrier mechanism, which confirms the strong influence of coating through the barrier mechanism, which confirms the strong influence of coating thickness on corrosion thickness on corrosion protection behaviour. Taking into account that the size of the particles in most protection behaviour. Taking into account that the size of the particles in most powders used for powders used for electrostatic spraying is 30–50 µm, the coating thickness should exceed 50 µm to electrostatic spraying is 30–50 µm, the coating thickness should exceed 50 µm to obtain a satisfactory obtain a satisfactory protective film. protective film. According to the EIS results, the coating resistance for all the tested coatings at the beginning of According to the EIS results, the coating resistance for all the tested coatings at the beginning of exposure showed an acceptable value (106 Ω·cm2) for protective coatings [19], but after 100 h in 4% exposure showed an acceptable value (106 Ω·cm2) for protective coatings [19], but after 100 h in 4% w/w NaOH, this decreased for all tested coatings. The worst coating resistance was established for w/w NaOH, this decreased for all tested coatings. The worst coating resistance was established polyester coating sample C. The established difference in corrosive durability between the epoxy– for polyester coating sample C. The established difference in corrosive durability between the polyester powder coating and the polyester powder coating determined by EIS can be related to the epoxy–polyester powder coating and the polyester powder coating determined by EIS can be related better corrosion properties and the higher cross-link density of the epoxy-based binder [5,9,13], as to the better corrosion properties and the higher cross-link density of the epoxy-based binder [5,9,13], well as the instability of polyester to alkaline hydrolysis [23]. The cross-sectional examination showed as well as the instability of polyester to alkaline hydrolysis [23]. The cross-sectional examination that all tested coatings were fully cured with no cracks and pores and with good adhesion to the substrate.

Coatings 2018, 8, 98 11 of 12 showed that all tested coatings were fully cured with no cracks and pores and with good adhesion to the substrate.

Author Contributions: Ivan Stojanovićand Vesna Alar conceived and designed the experiments; Ivan Stojanović and Vinko Šimunovićperformed the experiments and analysed the data; Frankica Kapor contributed with materials/analysis tools and to the experimental work; Ivan Stojanovićwrote the paper; Vesna Alar revised the paper. Conflicts of Interest: The authors declare no conflict of interest.

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