Accepted Manuscript Title: Vegetable oil-based polybenzoxazine derivatives coatings on Zn-Mg-Al alloy coated steel Author: Matei Raicopol Brindusa B˘al˘anuc˘a Kirill Sliozberg Bert Schl¨uter Sorina Alexandra Gˆarea Nicoleta Chira Wolfgang Schuhmann Corina Andronescu PII: DOI: Reference:
S0010-938X(15)30050-0 http://dx.doi.org/doi:10.1016/j.corsci.2015.08.018 CS 6446
To appear in: Received date: Revised date: Accepted date:
28-4-2015 3-8-2015 5-8-2015
Please cite this article as: Matei Raicopol, Brindusa B˘al˘anuc˘a, Kirill Sliozberg, Bert Schl¨uter, Sorina Alexandra Gˆarea, Nicoleta Chira, Wolfgang Schuhmann, Corina Andronescu, Vegetable oil-based polybenzoxazine derivatives coatings on Zn-Mg-Al alloy coated steel, Corrosion Science http://dx.doi.org/10.1016/j.corsci.2015.08.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Vegetable oil-based polybenzoxazine derivatives coatings on Zn-Mg-Al alloy coated steel Matei Raicopola, Brindusa Bălănucăb, Kirill Sliozbergc, Bert Schlüterc, Sorina Alexandra Gâreab, Nicoleta Chiraa, Wolfgang Schuhmann*1,c, Corina Andronescu*2,b a
“Costin Nenitzescu” Department of Organic Chemistry , University “Politehnica” of Bucharest; 1-7, Gh. Polizu Street, Bucharest, Romania Advanced Polymer Materials Group; University “Politehnica” of Bucharest; 1-7, Gh. Polizu Street, Bucharest, Romania Analytical Chemistry – Center for Electrochemical Sciences (CES), Ruhr-Universität Bochum; Universitätsstr. 150, D-44780 Bochum, Germany
Highlights ► new benzoxazine derivatives were synthesized using precursors from renewable resources ► phenolated high oleic sunflower oil and aniline or 1,6-diaminohexane as amine components were used for the synthesis of benzoxazine derivatives ► heat curing of the benzoxazine precursors leads to crosslinked hydrophobic coatings on steel samples ► the open circuit potential and the break-through potential are shifted significantly to more anodic potentials ► using a scanning-droplet cell the film thickness dependence of the corrosion protection was evaluated ► ►
Based on environmentally friendly (bio-based) precursor materials a new class of benzoxazine derivatives was synthesized using phenolated high oleic sunflower oil as phenol component and either aniline or 1,6-diaminohexane as amine components. Hydrophobic and dense poly(benzoxazine) coatings on Zn-Mg-Al alloy coated steel were obtained after spin-coating or air-brush type spray coating by crosslinking during a heat treatment step. The poly(benzoxazine)-coated ZM-steel samples showed an anodic shift of the open circuit potential as well as the breakthrough potential. Using an automatic scanning droplet cell the impact of the polymer film thickness on corrosion protection was evaluated. 1.
Poly(benzoxazine) resins are a class of phenolic resins which shows good mechanical and thermal properties. The design flexibility of the monomers offers the possibility to tailor the properties of the resulting poly(benzoxazines) . Due to properties like low water absorption and near-zero shrinkage upon curing of the monomers , poly(benzoxazines) coatings were 1 2
Corresponding author: [email protected]
(W. Schuhmann) Corresponding author: [email protected]
investigated previously as anticorrosion layers. They were shown to inhibit corrosion of coated steel samples due to the formation of a stable network which diminished the permeability of corrosion-promoting agents to the metallic substrate [3-7]. In an attempt to providing sustainable solutions, research is increasingly oriented on the valorization of renewable resources. Simultaneously, the depletion of coal and petroleum resources and their continuously rising prices, at least over a longer time scale, are encouraging researchers to find a new feedstock in order to synthesize inexpensive and sustainably manufactured end products. In this context, plant oils are increasingly exploited as an alternative to petroleum resources in chemical synthesis. Besides their renewable character, vegetable oils provide other advantages like widespread availability, comparatively low price and they are inexhaustible if rationally used [8-10]. The presence of reactive sites, such as double bonds that enable multiple ways of functionalization, makes vegetable oils and their derivatives an attractive feedstock for the synthesis of polymeric materials. However, until now the production of related monomers and polymers is very low as compared to those based on petrochemical precursors . Only a few attempts of using vegetable oils to obtain poly(benzoxazine) based materials were reported, e.g. tryglycerides being modified with hydroxy-groups containing benzoxazine monomers by using isocyanates similar to urethane chemistry [11,12]. Polymer coatings are widely applied to achieve an at least temporary corrosion protection [13-15]. Zn-Al-Mg alloy (ZM) coated steel is progressively used in technological applications such as in automotive and building industry due to its intrinsically good corrosion protection caused by the formation of a protection layer upon alloy oxidation [16-18]. It is supposed that coating of ZM steel with a suitably designed polymer layer extends corrosion protection in time. Moreover, it can be hypothesized that if pinholes or scratches occur in the protecting polymer layer the underlying Zn-Al-Mg alloy coating acts as a second barrier against corrosion by formation of local oxidation product deposits. A few studies have demonstrated the principle applicability of poly(benzoxazine) derivatives as corrosion protection layer [3-6]. The synthesis of benzoxazine derivatives and their polymerization leading to the formation of the corresponding poly(benzoxazine) resins is well known and it is generally performed by thermally activated ring-opening polymerization [1,19]. A large variety of benzoxazine monomers with tailored properties is accessible by variation of the substituents at the amino and the phenol precursor. In this contribution, we report the synthesis of hydrophobic benzoxazine derivatives starting from high oleic sunflower oil (HO-SFO) and their exploitation as corrosion protection layer on ZM steel using a two-step strategy described recently for methyl oleate-based benzoxazine derivatives . First, a mixture of triglycerides bearing phenolic rings covalently attached to the fatty acid chains, i.e. phenolated oils (PHO-SFO), was obtained by alkylating phenol with
HO-SF in the presence of a superacid catalyst. In the second step, the prepared phenolated oil was further used as phenol component in the synthesis of new bio-based benzoxazine monomers. HO-SF was chosen as raw material for phenolation because it was demonstrated that monounsaturated fatty acids give the best yields in this type of reaction . Triglycerides with a high content of fatty acids with two double bonds (e.g. linoleic acid) and three double bonds (e.g. linolenic acid) tend to polymerize in the presence of superacids and are less reactive in the phenol alkylation reaction. Two new bio-based benzoxazine monomers were obtained by reacting the phenolated high oleic sunflower oil (PHO-SFO) with aniline (a) or 1,6-diaminohexane (ad6). The formation of benzoxazine monomers was investigated by 1H-NMR, FT-IR and DSC. Due to the hydrophobic flexible chains, the newly synthesized benzoxazine monomers were investigated as candidates for anti-corrosion coatings and were tested as such using a variety of electrochemical techniques. Especially, an electrochemical scanning droplet cell was applied to investigate the film-thickness dependence of spray-coated poly(benzoxazine) coatings on ZM-steel samples. 2.
High oleic sunflower oil (HO-SFO) was obtained from Cargill Podari, Dolj county, Romania, having a fatty acid profile of (mole ratio): 3.27% (C16:0), 1.60% (C18:0), 90.60% (C18:1), 4.01% (C18:2), 0.34% (C22:0), 0.18% (C20:1n11c); iodine value: 85 g I2/100 g oil; average molar weight: 883 g/mol; double bond equivalent: one double bond/fatty acid methyl esters (0.34 double bonds/100 g HO-SFO). o-Dichlorobenzene, trifluoromethanesulfonic acid, sodium carbonate, anhydrous magnesium sulfate were purchased from Merck. Phenol, aniline, para-formaldehyde, dioxane, 1,6-diaminohexane and diethyl ether were from Sigma-Aldrich. Aniline was distilled before use; all other chemicals were used as received. Araldite 35600 -6,6’(propane-2,2-diyl)bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine) (BA-a) was provided by Huntsman. Zn-Al-Mg alloy (ZM) coated steel was a gift from ThyssenKrupp Steel Europe. The specimens were produced at laboratory scale using a hot dip process simulator (Rhesca, Japan). The Zn-Al-Mg coating (ZM) contains 90.26 % Zn, 3.97 % Al and 5.78 % Mg by weight. The average coating thickness was 7 µm. 2.2.
Phenolation of the high oleic sun flower oil
0.6 mL trifluoromethanesulfonic acid were added as catalyst under argon to a mixture of 60 g (204 mmol double bond equivalent) high oleic sunflower oil and 38 g (404 mmol) phenol. The reaction mixture was stirred at 120 oC for 4.5 h. After cooling down to room temperature, 150 mL o-dichlorobenzene were added and the resulting mixture was carefully neutralized with solid NaHCO3 and then filtered. After washing three times with water, the solvent and excess
phenol were evaporated using a rotary evaporator (10 mm Hg, heating bath kept at 150 oC). The phenolated high oleic sunflower oil (PHO-SFO) was obtained as a brown viscous liquid. 1H NMR (300 MHz, CDCl3) δ (ppm) = 7.39-6.53 (m, 12H), 5.37 (d, J=37.0 Hz, 2H), 4.55 – 3.95 (m, 2H), 3.04 (d, J=35.3 Hz, 3H), 2.59 (s, 6H), 1.82-1.42 (m,6H), 1.27 (d, J=16.1 Hz, 78H), 0.95-0.62 (m, 9H); FT-IR ν (cm-1) = 1515 (ar C-C st.) (η = 84%). 2.3.
Synthesis of benzoxazine monomers 10 g of PHO-SF, 3.162 g (0.034 mol) aniline and 2.04 g (0.068 mol) para-formaldehyde
were heated for 0.5 h at 90 °C and 0.5 h at 130 °C. The resulting mixture was dissolved in diethyl ether and filtered to remove solid byproducts. The mixture was washed with water and dried over anhydrous magnesium sulfate. The ether was removed under vacuum and the resulting compound (PHO-SFO+a) was dried for 48 h at 10 mm Hg. 1H NMR (300 MHz, CDCl3) δ (ppm) = 7.34 – 6.39 (m, 24H), 5.42-5.30 (m, 6H), 5.19 (s, 1H), 4.67-4.47 (m, 6H), 4.19 (dd, J=48.1, 13.8 Hz, 2H), 3.08-2.79 (m, 3H), 2.22 (d, J=42.8 Hz, 6H), 1.75 – 1.37 (m, 6H), 1.23 (d, J=15.9 Hz, 78H), 0.80 (dd, J=40.4, 4.7 Hz, 9H); FT-IR ν (cm-1) = 1502 (ar C-C st.), 1228 (C-O-C st.), 949 (ar. C-H δ) (η = 72 %). 1.972 g (0.017 mol) 1,6-diaminohexane and 2.04 g (0.068 mol) para-formaldehyde were mixed for 0.5 h at room temperature in 20 ml dioxane. 10 g PHO-SF were added and the mixture was heated to 100 °C for 8 h. The resulting mixture was cooled down to room temperature and filtered to remove solid byproducts. Dioxane was removed under vacuum and the benzoxazine derivative (PHO-SFO-ad6) was obtained by precipitation in water. The product was dried under vacuum (48 h, 10 mm Hg).1H NMR (300 MHz, CDCl3) δ (ppm) = 7.10 – 6.51 (m, 6H), 4.77 (s, 4H), 4.69 (d, J=17.3 Hz, 1H), 4.12 (dd, J=25.2, 14.9 Hz, 2H), 3.92 (d, J=6.9 Hz, 4H), 2.94 (s, 2H), 2.64 (s, 4H), 2.32 – 2.09 (m, 4H), 1.67 – 1.36 (m, 4H), 1.29 (s, 4H), 1.19 (d, J=14.6 Hz, 56H), 0.82 (d, J=6.5 Hz, 6H); FT-IR ν (cm-1) = 1449 (ar C-C st.), 1230 (C-O-C st.), 935 (ar. C-H δ) (η = 61 %). 2.4.
Poly(benzoxazine) coatings on ZM steel sheets
The Zn-Al-Mg alloy (ZM) coated steel sheets were cut to the desired size (either 1.5 x 1.5 cm2 or 1.5 x 10 cm2) and were then sequentially cleaned in an ultrasonic bath for 10 min each in acetone, isopropanol (2 times) and ethanol. The samples were further immersed in 1.5 wt.% Ridoline solution at 60 °C. The benzoxazine monomers (BA-a, PHO-SH-a, or PHO-SFO-ad6) were deposited on the cleaned ZM-steel surfaces using either spin coating or air-brush spray-coating. For spin-coating, 50 µL of 0.25 g/mL solution of the respective benzoxazine monomer in toluene were coated for 2 min at 1000 rpm on the cleaned ZM substrates. Spray deposition was performed with a home-built air-brush type spray coater mounted on an x-,y-,z-positioner and
controlled by a specifically developed software. The benzoxazine solution (35 mg/50 ml) was sprayed on the ZM steel sheets which were heated to 70°C in order to attain immediate solvent evaporation. Two layers were sprayed. The speed of the spray nozzle was automatically adjusted to obtain a linear layer thickness gradient in the range from <1 µm to 10 µm along the length of the ZM sheet. The coated ZM steel sheets were cured in an oven either in air (spin-coated individual samples for initial corrosion tests) or in Ar atmosphere (samples for evaluating the impact of the coating thickness). A temperature program was applied - if not otherwise stated to 180°C (2 h); 200°C (2 h), 250°C (0.5 h) - in order to obtain the cured poly(benzoxazine) films. For BA-a the curing was performed for 2 h at 180 °C and 2 h at 200 °C (according to the manufacturer's data sheet). 2.5. 1
Characterization of the benzoxazine monomers and the poly(benzoxazine) coated ZM steel sheets
H-NMR spectra were registered on a Bruker Avance DRX 400 spectrometer. FT-IR spectra
were recorded on a Bruker Vertex 70 spectrometer in the wavelength range from 600 to 3800 cm-1 by attenuated transmission reflectance (ATR). Differential scanning calorimetry (DSC) was performed using a DSC 402 F1 equipment from Netzsch. Non-isothermal scans were registered from 20 to 350 °C with a 10 °C/min heating rate under nitrogen. Thermogravimetric analysis (TGA) was performed using a Q500 TA instrument from 30 to 600 °C with a heating rate of 10 °C/min under nitrogen. Static water contact angles were measured using a custom-built goniometer under ambient conditions. Droplets were formed by means of a syringe and carefully deposited on the sample surface under video control. Then the syringe was removed from the droplet, and the shape of the droplet was recorded with a CCD camera. The contact angle was determined by fitting the shape of the droplet according to Young’s equation. Electrochemical measurements were performed using a Palmsens potentiostat (open circuit potential (OCP) and linear-sweep voltammetry (LSV) of spin-coated samples). The setup consisted of an electrochemical cell with the coated sample (5 mm diameter) as working electrode, a Ag/AgCl/3 M KCl electrode as reference electrode and a platinum gauze as counter electrode. Corrosion tests were performed in aqueous 3.5 wt.% NaCl or in 0.1 M Na2SO4 solution. To avoid electromagnetic noise, the experiments were conducted in a Faraday cage. Initially the OCP was measured for 5 min. For the subsequent LSV measurements, the potential was scanned from 0.25 V cathodic to OCP to 0.25 V anodic to OCP with a scan rate of 1 mV/s. A scan direction from cathodic to anodic potentials was used to assure that the underlying ZM-coating was initially intact and not directly oxidized upon applying the start potential. LSV measurements along the spray-coated thickness gradients were performed with an in-house built scanning droplet cell (SDC) described previously [22,23]. The Teflon cell, which has an
aperture of 1 mm in diameter, was pressed onto the coated ZM steel sheet with a constant force to guarantee a constant and well-defined wetted surface area. 3.5 wt.% NaCl solution was used as electrolyte. After the cell was automatically positioned, the sample was polarized to -1.25 V vs SHE for equilibration and then the potential was swept from -1.25 V to -0.75 V at a scan rate of 1 mV/s. The chosen starting potential is close to the starting potential in the previous LSV measurements of the individual samples. Data are represented in a color-coded contour plot with log(abs(i)) vs. d vs. U, where i is the current, d the estimated layer thickness and U the applied potential vs. SHE. The violet/black areas correspond to the minimum of log(abs(i)) and indicate the breakthrough potentials. 3.
Results and discussion
The synthesis of several benzoxazine monomers was accomplished using precursor materials from renewable resources (Figure 1).
Figure 1: Synthesis of PHO-SFO-a and PHO-SFO-ad6 starting from high-oleic sunflower oil and the chemical formula of the commercial benzoxazine monomer Araldite 35600 (BA-a) First high oleic sunflower oil (HO-SFO) was reacted with phenol under acid catalysis to functionalize the fatty acid chains with phenolic rings. Phenolation of HO-SFO oil is a FriedelCrafts alkylation reaction in which the electrophile is generated from the double bonds of the unsaturated triglyceride in the presence of a superacid catalyst, trifluoromethanesulfonic acid in our case. Consequently, both sp2 carbon atoms forming the double bond can be protonated and there are two possibilities for the attachment of phenolic rings onto the fatty acid chain (Figure 1). Moreover, the electrophylic substitution can take place in the ortho- or para-position of the phenol molecule, where the electron density is high. The phenolated high oleic sunflower oil (PHO-SFO) was then reacted with either aniline or 1,6-diaminohexane as amine component and formaldehyde under formation of the benzoxazine monomers PHO-SFO-a and PHO-SFO-ad6, respectively. One of the main drawbacks of poly(benzoxazines) is the formation of a rigid polymer network during curing. We expect that the use of vegetable oils as precursor of such materials is a key to solving the flexibility problem due to the long fatty acid chains that confer a particular mobility .
H-NMR of a) high oleic sunflower oil (HO-SFO) and phenolated high oleic sunflower oil (PHO-SFO), and b) the benzoxazine monomers PHO-SFO-a and PHO-SFO-ad6
H-NMR was used to prove the successful synthesis of PHO-SFO (Figure 2a). The vinyl
protons from the unsaturated chains of HO-SFO (noted with "f") are no longer present in the PHO-SFO spectrum. Another proof for the successful alkylation reaction is the disappearance of the signal corresponding to the allylic protons (noted with "e" in the HO-SFO spectrum) and the appearance of a new signal at 2.98 ppm in the PHO-SFO spectrum which corresponds to the protons linked to the former sp2 C atom from the double bond which is linked now to the aromatic ring (noted with "f'" on PHO-SFO structure). In the PHO-SFO spectrum, protons from the aromatic ring give rise to new signals in the region from 6.5 - 7.5 ppm. The synthesis of the envisaged benzoxazine monomers starting from PHO-SFO is supported by the corresponding 1H-NMR spectra (Figure 2b). For PHO-SFO-a, the characteristic benzoxazine signals arise at 5.32 and 4.61 ppm, which are attributed to protons noted with x (NCH2-O) and y (N-CH2-Car). In the case of PHO-SFO-ad6, these signals appear at 4.77 and 3.92 ppm, respectively. Additional signals are assigned as shown in Figure 2b. Methanol traces can be seen in both spectra (3.37 ppm), in case of PHO-SFO-a due to the washing step, while for PHOSFO-ad6 methanol was added to increase the monomer solubility in CDCl3. The FT-IR spectra shown in Figure 3 additionally prove the formation of the benzoxazine monomers. The presence of the tri-substituted aromatic ring from the benzoxazine structure is supported by bands at 1502 cm-1 and 949 cm-1 for PHO-SF-a as well as 1499 cm-1and 935 cm-1 for PMO-SFO-ad6. The appearance of the C-O-C stretching characteristic for the benzoxazine ring at 1228 cm-1 and 1230 cm-1 for PMO-SFO-a and PMO-SFO-ad6, respectively, also confirms the structure of the monomers.
FT-IR spectra for PHO-SFO, PHO-SFO-a, and PHO-SFO-ad6
The thermal curing of both monomers was further investigated by means of DSC. The DSC curves (Figure 4a) show large exothermic peaks for both PHO-SFO-a (black) and PHO-SFO-ad6 (red). The polymerization starts at around 200 °C for PHO-SFO-a, and close to 150 °C for PHOSFO-ad6. The smaller exothermic peaks in the range between 50 and 100 °C may be attributed to side reactions caused by the presence of unreacted phenol structures from the PHO-SFO which can act as catalyst for the benzoxazine opening reaction. The temperature at which the polymerization process has the highest rate is 222 °C and 246 °C for PHO-SFO-ad6 and PHOSFO-a, respectively. Using the DSC curves as a qualitative guideline for assuring complete curing of the benzoxazine monomers, the thermal treatment of PHO-SFO-a and PHO-SFO-ad6 coatings deposited onto ZM substrates was performed for 2 h at 180 °C, 2 h at 200 °C and finally 0.5 h at 250 °C. The degradation of the poly(benzoxazine) derivatives obtained after the thermal polymerization was investigated by TGA (Figure 4b). It is obvious that the used curing temperature is substantially below the degradation temperature of the formed poly(benzoxazine) films.
Figure 4: a. DSC curves for PHO-SFO-a (1) and PHO-SFO-ad6 (2). b. TGA curves for poly(BA-a) (3), poly(PHO-SFO-a) (1), and poly(PHO-SFO-ad6) (2) Degradation was evaluated using the temperature at which the mass loss is 5% (Td5%). This value is attained at 330 °C for poly(PMO-SFO-ad6) as well as for the commercial poly(benzoxazine) (poly(BA-a)), while for poly(PMO-SFO-a) a temperature increase of 20 °C was observed. Poly(PMO-SFO-a) and poly(PMO-SFO-ad6) exhibit a lower degradation rate as compared with poly(BA-a). Above 400 °C poly(PMO-SFO-a) and poly(PMO-SFO-ad6) decompose faster leading to lower residual char at 600 °C (10.5 % for poly(PMO-SFO-a) and 8.97 % for poly(PMO-SFO-ad6)) than poly(BA-a) (32.5 %) due to the aliphatic chains from the oil structure which do not have a contribution to char formation. The properties of poly(BA-a), poly(PMO-SFO-a), and poly(PMO-SFO-ad6) as corrosion protection coatings on ZM-steel sheets were evaluated. For this, solutions of the respective monomers dissolved in toluene were spin-coated onto cleaned ZM-steel sheets in order to obtain highly uniform coatings (below 10 µm thickness). Following the results of the DSC measure-
ments (see Figure 4a), curing was performed for 2 h at 180 °C, 2 h at 200 °C and 0.5 h at 250 °C. As a matter of fact, an organic coating is blocking access of the electrolyte to the conducting underlying surface which depending on the film thickness may completely impede electrochemical measurements to be performed. We therefore tried to limit the film thickness below 10 µm, however, the poly(benzoxazine) coatings may contribute to the overall electrical circuit like a resistor in series. Attempts to investigate the surfaces using electrochemical impedance spectroscopy failed especially at high frequencies, which prevented the determination of the uncompensated resistance and the related iR drop. Thus, to obtain a first hint on the electrochemical properties, the open-circuit potential (OCP) was determined after immersion of the coated samples into 3.5 wt.% NaCl solution (Figure 5). OCP determination is considered to be not impeded by the resistance of the polymer film once electrical contact between electrode and electrolyte solution was obtained. The OCP of the only ZM-coated steel (not heat treated) was initially -1.01 V vs. SHE and dropped to -1.04 V vs. SHE before it increased with time to about -0.97 V vs. SHE. This can be attributed to the formation of the passive layer upon initial corrosion leading at first to a cathodic shift of the OCP due to the increased anodic partial reaction. The OCP value of the poly(BA-a)-coated ZM steel is about -1.0 V vs. SHE, that of poly(PHO-SFO-a)-coated ZM steel is initially at about -0.96 V vs. SHE and increases after about 30 h slowly to -0.94 V vs. SHE. Most importantly, the OCP of all poly(benzoxazine)-coated ZM-steel samples only show comparatively small changes over time suggesting that the Zn-Al-Mg alloy of the underlying passivation layer is not oxidized due to the protection by the organic film. The most substantial shift is seen for the OCP of the poly(PHO-SFO-ad6)-coated ZM steel which is initially -0.86 V vs. SHE and decreases to -0.88 V vs. SHE during 70 h. After about 5 h, the OCP of the poly(BA-a)-coated ZM steel falls below that of the uncoated ZM-steel due to the anodic shift of the OCP of the uncoated sample caused by the formation of the passive layer. The OCP values of the poly(PHO-SFO-a)-coated ZM steel and especially that of the poly(PHO-SFO-ad6)-coated ZM steel are at substantially less cathodic values. This is most likely due to a decrease of the anodic reaction caused by a hindered access of e.g. chloride ions to the steel surface through the dense and hydrophobic organic coating. From the molecular structure of poly(BA-a), poly(PHO-SFO-a), and poly(PHO-SFO-ad6) it is obvious that the crosslinking degree upon curing is high for poly(BA-a) leading to a more brittle film, while the degree of crosslinking is higher for poly(PHO-SFO-ad6) than for poly(PHOSFO-a) with both films being more smooth due to the long hydrophobic side chains. SEM images did not reveal differences in the film properties due to the used very small layer thicknesses (data not shown).
If an uncoated ZM-sample is subjected to the same thermal treatment as the coated samples, the value of the OCP is -1.23 V vs. SHE (results not shown). It is anticipated that this large cathodic shift of the OCP is due to an oxidic layer which is formed by oxidation of the passivation layer. Obviously, all poly(benzoxazine) coatings prevent the oxidation of the underlying passivation layer during heat curing as well as during immersion in an electrolyte solution.
Figure 5: OCP measurements of uncoated ZM steel (4), poly(BA-a)-coated ZM steel (3), poly(PHO-SFO-a)-coated ZM steel (1), and poly(PHO-SFO-ad6)-coated ZM steel (2) for 70 h (3.5% wt. NaCl solution). Figure 6 shows the LSV of the uncoated ZM steel (4), poly(BA-a)-coated ZM steel (3), poly(PHO-SFO-a)-coated ZM steel (1), and poly(PHO-SFO-ad6)-coated ZM steel (2) started after 5 min OCP measurement from -0.25 V vs. OCP to +0.25 V vs. OCP in 3.5 M NaCl solution (Figure 6a) as well as in 0.1 M Na2SO4 solution (Figure 6b).
Figure 6: LSV of uncoated ZM steel (4), poly(BA-a)-coated ZM steel (3), poly(PHO-SFO-a)coated ZM steel (1), and poly(PHO-SFO-ad6)-coated ZM steel (2). a) in 3.5% wt. NaCl solution, and b) in 0.1 M Na2SO4 solution. The uncoated ZM-steel shows the expected oxidation of the Zn-Mg-Al alloy concomitantly forming a passive layer with comparatively low anodic current densities at potentials above the break-through potential. In the cathodic branch of the Tafel representation current densities of 10-3 to -10-4 A cm-2 at -1.25 V vs SHE are observed, a break-through potential of -1.17 V vs. SHE and anodic current densities of about 10-5 A cm-2 up to a potential of -0.8 V vs. SHE at which the current density is sharply increasing. In 3.5% NaCl solution, the poly(BA-a)-coated ZM steel, poly(PHO-SFO-a)-coated ZM steel, and poly(PHO-SFO-ad6)-coated ZM steel show a similar behavior with a decreasing cathodic current density of below -10-5 A cm-2 (poly(BA-a)), below -10-6 A cm-2 (poly(PHO-SFO-a)), and below -10-7 A cm-2 (poly(PHO-SFO-ad6)) to the break-through potential of -0.78 V vs. SHE (poly(BA-a)), -0.75 V vs. SHE (poly(PHO-SFO-a)), and -0.76 V vs. SHE (poly(PHO-SFO-ad6)). At potentials anodic of the break-through potential the current density increases sharply for all samples, however, poly(PHO-SFO-a) and poly(PHOSFO-ad6) coated ZM-steel sheets show an overall smaller anodic corrosion current density. In Na2SO4 solution, the break-through potentials for the uncoated ZM-steel are unchanged while they are higher for the poly(BA-a)-coated sample (-0.81 V vs. SHE) and for the poly(PHO-SFOa)-coated sample (-0.77 V vs. SHE). The poly(PHO-SFO-ad6) coated ZM-steel sheets shows a lower break-through potential (-0.74 V vs. SHE) than in chloride-containing electrolyte solution. The bifunctionality of 1,6-diaminohexane is leading to a higher crosslinking probability during curing and hence obviously improves the corrosion protection for the poly(PHO-SFO-ad6) coated ZM-steel sheets. The LSV measurement suggest that in chloride-containing electrolyte the final breakthrough is very similar for all poly(benzoxazine) coatings. We assume that all polymer films prevent access of chloride as well as the oxidation of the Zn-Mg-Al alloy. In absence of chloride ions the individual properties of the poly(benzoxazine) films are additionally determining the breakthrough potential but as well the cathodic and anodic current densities. The
poly(BA-a)-coated ZM steel shows comparatively high cathodic current densities in both electrolyte solutions as well as the highest anodic current densities. The poly(PHO-SFO-a)coated sample exhibits substantially smaller cathodic and anodic current densities as compared with poly(BA-a)-coated ZM steel which we attribute to the smoother film and the decreased wettability due to the hydrophobic side chains. The poly(PHO-SFO-ad6) layers are showing an anodic shift in the breakthrough potential concomitantly with a substantially decreased anodic current density in chloride-free solution. To evaluate the impact of the curing procedure on the corrosion protection, poly(PHO-SFO-a)coated ZM steel and poly(PHO-SFO-ad6)-coated ZM steel samples were suspected to different curing procedures either up to a maximal temperature of 200°C, 220°C,or 250°C (Figure 7). In the case of the poly(PHO-SFO-a)-coated ZM steel the cathodic current density is lowest after curing at 220°C while in the case of the poly(PHO-SFO-ad6)-coated ZM steel it is lowest after curing at 250°C. Hence, all further experiments were performed with samples cured up to 250°C.
Figure 7: LSV uncoated ZM steel (4); a) poly(PHO-SFO-a)-coated ZM steel and b) poly(PHOSFO-ad6)-coated ZM steel. Curing for 2h at 180°C and 2h at 200°C (3), curing for 2h
at 180°C, 2h at 200°C and 2h at 220°C (1), curing for 2h at 180°C, 2h at 200°C and 0.5 h at 250°C (2). (in 3.5% wt. NaCl solution). Contact angle measurements were performed before and after the OCP/LSV experiments (Figure 8). The loss in hydrophobicity, which is most prominent for the poly(BA-a) coatings followed by the poly(PHO-SFO-a) and the poly(PHO-SFO-ad6) coatings, respectively, seems to correlate with the shift in the break-through potential. Especially the poly(PHO-SFO-ad6)-coated ZM steel retains largely its hydrophobic surface properties even after applying rather high anodic potentials during the LSV measurement. We hence suppose that the use of highly crosslinked poly(benzoxazine) derivatives with long hydrophobic tails are advantageous for improved corrosion protection.
Figure 8: Contact angle measurements for uncoated ZM steel (green), poly(BA-a)coated ZM steel (red), poly(PHO-SFO-a) coated ZM steel (blue) poly(PHO-SFO-ad6) coated ZM steel (violet) each before and after performing a LSV from -0.25V to +0.25 V with respect to the OCP in 3.5% wt. NaCl solution. In order to evaluate the correlation between the anticorrosive properties and the thickness of the poly(benzoxazine) coatings, ZM steel sheets were spray-coated with BA-a, PHO-SFO-a and PHO-SFO-ad6 using an air-brush type spray coater. Coating thickness gradients from about 1 µm to 10 µm were obtained after curing in Ar to assure the highest possible reproducibility. The anticorrosion performance was evaluated using an automatic scanning droplet cell (SDC). The heart of the SDC is shown in Figure 9. Using the in-built force sensor, the Teflon tip of the droplet cell is pressed with sufficient force to assure a tight sealing and a highly reproducible
wetted area. The droplet cell was complemented with a double-junction Ag/AgCl/3 M KCl reference electrode and a Pt wire counter electrode. The measuring head of the SDC was moved along the thickness gradient of the poly(benzoxazine) coating. After approaching the surface and sealing with a constant force a circular area of about 1 mm diameter was selected and a sequence of measurements consisting of the determination of the OCP followed by equilibration at -1.25 V vs. SHE and a LSV was performed. The LSV of the ZM steel samples covered with thickness gradients of poly(BA-a), poly(PHO-SFO-a) and poly(PHO-SFO-ad6) are shown in Figure 10 in a three-dimensional false-color representation.
Figure 9: Photograph of the automatic scanning droplet cell during evaluation of a thickness gradient of a poly(benzoxazine) coating on a ZM-steel.
Figure 10: Three-dimensional false-color representation of a series of LSV obtained using an automatic scanning-droplet well. a) poly(BA-a), b) poly(PHO-SFO-a), c) poly(PHOSFO-ad6). In the case of poly(BA-a) the cathodic currents are quite high at low film thickness (green color; about -10-4 A cm-2) decreasing slightly with increasing film thickness. Surprisingly, the breakthrough potential shifts from -730 to -750 mV vs. SHE with increasing film thickness which suggests a more pronounced limitation for cathodic reactions. In contrast, for the poly(PHOSFO-a) coated sample, the cathodic currents substantially decrease with increasing film thickness while the break-through potential shows only a small shift to higher values (from -740 mV vs. SHE to -730 mV vs. SHE). The poly(PHO-SFO-ad6) coated sample shows very little dependence on the film thickness. Already at the lowest film thickness the cathodic currents are low. Additionally, the break-through potential remains nearly unchanged. This is seen as an additional evidence that the crosslinked poly(benzoxazine) film exhibits besides highest hydrophobicity best adhesion and hence the most pronounced corrosion protection. Since no substantial anodic shift of the breakthrough potential with increasing film thickness was obtained in all cases, the uncompensated resistance caused by the polymer coating does not contribute significantly to the obtained results. In general, increasing poly(benzoxazine) film thickness is decreasing the cathodic current density while the breakthrough potential as well as the anodic branch of the Tafel representation remain mainly unchanged.
Benzoxazine derivatives were synthesized using phenolated high oleic sunflower oil as the phenol component and either aniline or 1,6-diaminohexane as the amine component. The corresponding poly(benzoxazine) films obtained by spin coating or spray coating of ZM steel sheets with the monomer solutions followed by a heat-curing step showed a significant impact on the corrosion protection of ZM-coated steel also in comparison with similar coatings obtained with the commercial poly(benzoxazine) resin BA-a. Especially, polybenzoxazine-coated steel samples based on phenolated sun flower oil and 1,6-diaminohexan showed a substantially increased OCP value and most importantly a significantly increased breakthrough potential at concomitantly decreased cathodic and anodic currents densities in LSV.
Acknowledgements The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132397. The authors are grateful to ThyssenKrupp Steel Europe AG for providing ZM coated steel sheets and for financial support of the Center for Electrochemical Sciences (CES) at Ruhr-University Bochum. Tim Bobrowski is acknowledged for his contribution during his in-depth practical, Kim Nolte for contact-angle measurements, Dr. Artjom Maljusch for valuable discussions (all Ruhr-University Bochum), and Dr. Anamaria Hanganu for NMR measurements (“C.D. Nenitzescu“ Institute of Organic Chemistry of the Romanian Academy).
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