MAGNESIUM THIODIALKANOATES: DUALLY-FUNCTIONAL

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Materials and Interfaces

MAGNESIUM THIODIALKANOATES: DUALLYFUNCTIONAL ADDITIVES TO ORGANIC COATINGS Lev Bromberg, Xiao Su, Katherine Reece Phillips, and T. Alan Hatton Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01997 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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Industrial & Engineering Chemistry Research

MAGNESIUM THIODIALKANOATES: DUALLY-FUNCTIONAL ADDITIVES TO ORGANIC COATINGS Lev Bromberg, Xiao Su, Katherine R. Phillips, T. Alan Hatton* Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, MA 02139 Abstract Magnesium salts can enhance corrosion-protective properties of organic coatings. Here, mirocrystalline magnesium thiodipropionate (MgTDPA) and amorphous magnesium acetate thiodipropionate (MgAcTDPA) are synthesized in water via metathesis and replacement reactions, respectively, from 3,3’-thiodipropionic acid (TDPA), a non-toxic, film-forming dicarboxylic acid. The ability of the uncovered magnesium thiodipropionates to accelerate ringopening polymerization of benzoxazines such as the aniline derivative of bisphenol and curing of the epoxy resin-amine hardener is confirmed using differential scanning calorimetry and rheological methods, respectively. The corrosion inhibition of galvanized steel by magnesium thiodipropionates was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy measurements. Adsorption of TDPA anions on the galvanized steel surface followed the Langmuir adsorption isotherm. Epoxy-amine powder compositions containing MgTDPA were designed and coated on galvanized steel by electrodeposition and performance of magnesium thiodipropionate as a delamination inhibitor was investigated by immersion and salt-spray cyclical tests. The widening of cuts in the coating (scribe creep) results demonstrated the dramatic protective action of magnesium thiodipropionate in the epoxy-amine powder coatings. Magnesium thiodipropionates appear to be economical and functional additives to organic coatings, aiding in curing and corrosion protection. E-mail: [email protected] Introduction Corrosion protection of metal surfaces afforded by organic coatings is the most cost-effective means of providing practical protection from corrosion in transportation and infrastructure,1 postponing or preventing mechanical instability, reducing replacement expense, and mitigating 1 ACS Paragon Plus Environment

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safety concerns of corroded components. The failure mechanism of these organic coatings is typically due to delamination. The present work reports on magnesium thiodialkanoate additives to organic coatings that can both accelerate the formation/curing of the coatings and augment their corrosion-inhibiting properties, in addition to inhibiting coating delamination. The corrosion-driven delamination of organic coatings adherent to steel substrates typically proceeds via a cathodic disbondment mechanism wherein the anodic metal dissolution by the electrolyte transported beneath the delaminated coating is interlinked with the cathodic oxygen reduction occurring at the site of the coating disbondment.2-7 Rare earth (Ce3+, La3+, Gd3+, etc)8,9 and alkaline earth (Mg2+, Ca2+, Sr2+, Ba2+, etc.)4 cations are capable of forming films of amphoteric oxide layers resulting from the cathodic oxygen reduction producing alkaline pH, thus inhibiting the corrosion-driven cathodic disbondment on bare and galvanized steel surfaces.10 Abundance, low cost, non-toxic nature, and very low solubility of the hydroxide corrosion product make magnesium Mg2+ our metal cation of choice.

Magnesium oxides and salts

exhibiting a wide range of acido–basic properties, which can be modified and tuned, possess active sites capable of catalyzing numerous organic11 and carbonation12 reactions. A wide band gap (low electron surface concentration) and high stability under alkaline conditions make magnesium hydroxide efficient in inhibiting cathodic delamination.13 For example, an ionexchange resin loaded with Mg2+ ions has been applied as a smart-release component of an organic coating capable of inhibiting cathodic disbondment.14 Herein, we applied a different approach toward a cation-releasing corrosion inhibitor via the design of a carboxylate salt capable of releasing magnesium cations upon dissociation. Sodium carboxylates such as salicylate, caprinate, cinnamate, decanoate or N-oleoylsarcosine, etc. are promising corrosion inhibitors in their own right in the presence of aqueous chloride ions as a standard corrosion medium.15-20 Carboxylates can heal the local defects in the passivating oxide layer on steel by forming weakly soluble Fe(III) compounds.17 Remarkably, however, a synergistic effect in combining both cathodic and anodic inhibition has previously been realized by applying salts of rare earths (RE) and carboxylate (HOOCR) moieties, prepared via a simple methathesis reaction (Fig. 1): 9

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Fig.1. Metathesis reaction between rare earth (RE) salt and carboxylic acid. At variance with the reported work on carboxylates and salts as corrosion inhibitors, we set out to explore functional additives to organic coatings, including powder coatings, which would preferably perform not only as delamination inhibitors but also aid in the curing of the organic coating. Hence, 3,3’-thiodipropionic acid (TDPA) attracted our attention, as it is a known catalyst of step growth ring-opening polyaddition of bis-benzoxazine monomers that result in thermosetting polybenzoxazine resins employed in coatings.21 Thus, it can accelerate the curing of corrosion-inhibitor coatings. TDPA is generally recognized as a safe, nonirritating cosmetic additive.22 It spontaneously self-assembles into monolayers upon adsorption onto metal surfaces from aqueous solutions.23 The acid strength of TDPA in the monolayers is higher than that of the molecules in solution due to the electrostatic and coordination interactions of the carboxylic groups with the metal surface. Corrosion inhibition properties of TDPA have been alluded to in the patent literature in the context of lubricant formulations,24 aids in passivating zinc-galvanized surfaces by chromium,25 and precursors for synthesis of polymeric chelating agents for zinc and galvanized steel,26-28 but TDPA as a corrosion inhibitor per se has not been examined. In order to combine the delamination-inhibiting effects of magnesium with the corrosion inhibition of TDPA, we synthesized magnesium thiodipropionates and investigated their effect as additives in organic coatings on steel. As described below, we discovered that magnesium thiodipropionates can act synergistically as corrosion inhibitors, which lower the temperature of the epoxy-amine gelation and benzoxazine curing as well as significantly improve an epoxyamine powder coating resistance to cathodic disbondment on galvanized steel in industrystandard cyclic tests. Experimental Section Materials



Metallic magnesium powder (≥99%, mesh 325), magnesium acetate tetrahydrate (≥99%), magnesium nitrate hexahydrate (99%), 3,3’-thiodipropionic acid (TDPA, 97%), dicyandiamide (99%), and montmorillonite K10 were all obtained from Sigma Aldrich Chemical Co. and used as received. Benzoxazine resin (product designation, XU35610), Araldite GT 6259 resin (bisphenol A based epoxy resin modified with an epoxy cresol novolac) and two-part curing system comprising Araldite® LY8601 resin and Aradur® 8602 hardener were supplied by Huntsman Advanced Materials, Inc. (Woodlands, TX). Araldite® LY8601 is composed of bisphenol A epoxy resin (60 – 100%), glycidylether of C12-C14 alcohols (7 – 13%), and butylphenyl glycidyl ether (3 – 7%), whereas Aradur® 8602 is composed principally of diethylenetriamine (13 – 30%), polyoxypropylene diamine (13 – 30%), 4-nonyl-phenol (7 – 13%), triethanolamine (3– 7%), and 2-(2-aminoethylamino)ethanol (0.1-1%). Carbon black (Emperor 1600) was obtained from Cabot Corp. (Billerica, MA). All other chemicals, solvents and buffers were obtained from commercial sources and were of the highest purity available. Syntheses A sample of Mg(OH)2 was prepared by precipitating it from an aqueous solution of magnesium nitrate by an ammonium hydroxide solution at pH 10-11. The precipitate was purified by centrifugation and resuspension cycles with deionized water, then dried at 100oC and stored as a powder. Magnesium Thiodipropionate (MgTDPA) was prepared by substitution reaction as follows. MgTDPA1:1. Metallic magnesium powder, 200 mg (8.23 mmol) was placed in an aqueous solution of 3,3’-thiodipropionic acid, 1.466 g (8.23 mmol) in 30 mL DI water (producing a solution with initial pH of 1.89) and the mixture was stirred at room temperature for 24 h, at which point all magnesium particles had completely dissolved leaving a clear solution, hydrogen evolution ceased, and the pH in the solution was measured to be 5.1. The entire solution was snap-frozen in liquid nitrogen and lyophilized for 7 days. The resulting compound appeared as transparent crystalline powder. Yield, 90%. 1H NMR (400 MHz, D2O): δ, ppm 2.67 (t, -S-CH2), 2.38 (t, -C(=O)CH2). Elemental analysis for C6H8MgS: calcd. C, 35.94; H, 4.02; Mg, 12.12; S, 15.99; found C, 34.04; H, 5.30; Mg, 12.04; S, 15.22.


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MgTDPA5:1. A salt of magnesium acetate and thiodipropionic acid (MgTDPA5:1) was prepared with 5-fold molar excess of magnesium relative to thiodipropionic acid as follows. Metallic magnesium powder (≥99%, mesh 325), 1.0 g (41.15 mol) was placed in an aqueous solution of 3,3’-thiodipropionic acid (97%), 1.466 g (8.23 mmol) in 50 mL DI water (producing a solution with initial pH of 1.89) and the mixture was stirred at room temperature for 48 h, at which point all hydrogen evolution seized and pH in the solution was measured to be 7.2. Magnesium hydroxide/TDPA particles were observed forming a stable suspension, which was snap-frozen in liquid nitrogen and lyophilized for 2 days until constant weight. The resulting compound appeared as white powder of irregularly shaped particles of ~5-50 µm. Yield, 92%. Elemental analysis for C6H14Mg5S: calcd. C, 18.03; H, 3.53; Mg, 30.40; S, 8.02; found C, 18.14; H, 3.64; Mg, 30.05; S, 8.01. Magnesium Acetate Thiodipropionate (MgAcTDPA) A mixed salt of magnesium acetate and thiodipropionate was prepared by methathesis reaction as follows. Magnesium acetate tetrahydrate (2.14 g, 1 mmol) and thiodipropionic acid (1.78 g, 1 mmol) were dissolved in deionized water (50 mL) and the solution was evaporated on air at 60oC for 16 h, resulting in a transparent glassy solid. The product was redissolved in 50 mL deionized water and the solution was snap-frozen in liquid nitrogen and lyophilized to dryness for 5 days at 10 mTorr vacuum resulting in glassy white particles of MgAcTDPA. Yield, 85%. 1H NMR (400 MHz, D2O): δ, ppm 2.68 (t, -S-CH2), 2.42 (t, -C(=O)CH2), 1.87 (s, -C(=O)CH3). Elemental analysis for C58H110Mg5O52S8: calcd. C, 34.53; H, 5.50; Mg, 6.02; S, 12.71; found: C, 34.33; H, 5.45; Mg, 6.01; S, 12.51. General Methods 1

H NMR spectra were taken at 400 MHz using a Bruker Avance 400 spectrometer. FTIR

measurements were conducted using a Nicolet 8700 FTIR spectrometer and a Specac Smart Golden Gate Attenuated Total Reflection Accessory (ATR). Spectra of the powderous materials were measured in KBr tablets at 1 cm-1 resolution with 64 scans, whereas electrode surfaces were subjected to ATR. A total of 128 spectra (2 cm-1 resolution) were acquired and averaged for every sample. XRD spectra were acquired at room temperature for 6 h each, using an X’Pert Panalytical Pro diffractometer equipped with an X’celerator high-speed detector coupled with a



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Ni β-filter. Source of X-rays was Cu K-α (wavelength 1.540598 Å). Programmable divergence slits were used to illuminate a constant length of the samples (4 mm), with 0.02 soller slits. The XRD analysis for was performed using the Treor indexing method.29 The maximum cell volume was set to 1500 A3, and both Pawley and mixed profile fits were used to narrow down the space group, and match the peak position and intensities. A manual range of peaks from 5 to 45o was used to perform the analysis, with the first 25 peaks used to carry out the fit. Thermogravimetric analysis (TGA) and simultaneous differential scanning calorimetry (DSC) were conducted using a Q600 TGA/DSC instrument (TA Instruments, Inc., New Castle, DE). Samples were subjected to heating scans (10 °C/min) under nitrogen atmosphere in a temperature ramp mode. Analysis of the magnesium thiodipropionate surface was performed with a Physical Electronics Versaproble II X-ray photoelectron spectrometer (XPS). The analysis was performed at ultrahigh vacuum (1 × 10–8 bar) with an argon-gun neutralizer. The survey scans were performed with 10 cycles from 1400 eV to 50 eV at 200 kV with a pass energy of 80 eV and a step size of 0.5 eV. The high-resolution scans were performed at 100 kV, a pass energy of 11 eV to 23.5 eV, and a step size of 0.05 eV, with 30 cycles for iron and 8 cycles for the remaining elements. The surface morphology of the materials was characterized by a FEG-XL-30 field-emission SEM at 20 kV using a beam size of 3 and high-vacuum conditions, and a ZEISS Merlin High-Resolution SEM at 15 kV under high vacuum. Rheological Testing of the Rate of Curing of Epoxy Resins by Gel Time Method Tested epoxy two-part curing system comprised Araldite® LY8601 resin and Aradur® 8602 hardener. The Araldite®LY8601/Aradur®8602 components taken at 4:1 w/w ratio result in epoxy curing infusion system (ECIS) with initial nominal viscosity of 175 cP.30 The rheological measurements were conducted using a controlled stress ARES-G2 Rheometer (TA Instruments) with a parallel-plate geometry system (steel plate, 25 mm diameter, equipped with a solvent trap), gap set at 1 mm. Time sweep oscillatory shear experiments were conducted at 1 Hz. Preliminary frequency and time sweeps were conducted with the control ECIS mixture to find optimum experimental, stress-strain linearity and temperature conditions. The empty chamber was preheated to the gel time test temperature set at various temperatures ≥ 60oC. Finely powdered

MgTDPA1:1

or

MgAcTDPA

particulates

were

blended

with

Araldite®LY8601/Aradur®8602 (4:1 w/w) to result in homogenous suspensions of fixed solids 6 ACS Paragon Plus Environment

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loading (5 wt%) upon mixing. After vigorous mixing, the suspension or the control ECIS resin was quickly deposited on a preheated plate, the upper plate was rapidly lowered to a set gap and the excess fluid was wiped up and removed from the plate edges. Typical gel time determination results are shown in S-1. Rate of benzoxazine curing using DSC As received XU35610 was dissolved in ethanol/methyl ethyl ketone (90:10 v/v) mixture and recrystallized after solvent evaporation under vacuum. A homogeneous blend of XU35610 resin and magnesium thiodipropionate (either MgTDPA1:1 or MgAcTDPA) was prepared as homogeneous solution in ethanol/MEK (90:10 v/v) followed by solvent evaporation under vacuum at 60oC. Concentrations of magnesium thiodipropionates in the blends were set at 10 mol% relative to the benzoxazine monomer (MW 463 Da).21 The resulting material was finely ground and placed in hermetically sealed aluminum pans. Differential scanning calorimetry (DSC) isotherms were measured under flowing nitrogen using a Discovery DSC (TA Instruments) with modulated temperature amplitude at heating ramp rates (β = dT/dt) varying in the 2-15 oC/min range. Curing peak maxima were determined using instrument software. Electrochemical Testing The electrochemical open-to-air cell (PTC1™ Paint Test Cell, Gamry Instruments, Warminster, PA) utilized for the potentiodynamic polarization (Tafel tests) and electrochemical impedance spectroscopy (EIS) experiments was composed of working electrode made of galvanized steel (zinc-galvanized sheets of type B carbon steel, ASTM A653, total thickness 0.305 mm, working area 2.2 or 15.2 cm2, McMaster Carr Supply Co., Robbinsville, NJ), reference Ag/AgCl electrode, and Pt wire auxiliary electrode (BASi® Corp., West Lafayette, IN). Prior to the immersion in the electrolyte (3.5% NaCl, pH adjusted to 5.25-5.3, 40 mL), the working electrodes were washed by acetone, ethanol, dried by nitrogen flow and polished with P1200 grit.31 The working electrode was immersed into electrolyte containing the tested compound at a given concentration with desired pH at 25oC for 60 min prior to each measurement. The measurements were conducted using a Parstat PMC-2000 multichannel potentiostat equipped with Versa Studio 2.50.3 (Princeton Applied Research, Oak Ridge, TN) and modeled with a ZSimpWinTM software for complex nonlinear least square analyses fitting the circuit models of



the electrode interface to the Nyquist (Zre vs Zim) impedance data. EIS measurements were carried out using AC signals of peak to peak amplitude of 10 mV at the open circuit potential in the frequency range of 100 kHz to 0.1 Hz.32 Parameters for the Tafel tests were as follows: initial potential, -1.25 V (vs reference); final potential, -0.45 V; scan properties: step height, 0.5 mV, step time, 2 s. In a series of measurements with the corrosion inhibiting additives, a range of the additive concentrations was studied and the scan range was set from 150 mV below the open circuit voltage (OCV) to 200 mV more positive than the OCV, with three measurements for each condition. The range in these measurements was limited in order to minimize the cathodic polarization causing local pH changes. 33 The corrosion current (icorr) and corrosion potential (Ecorr) were extracted from the Tafel plots as illustrated in S-2. The Ecorr value was initially identified by inspection and linear fits of the curve made between a small range of mV on either side of this Ecorr. In the vicinity of Ecorr both currents were present in the specimen, but experimentally it was possible to measure only the net current. The net current has a single measurable polarity, either positive or negative. At Ecorr the net measured current is exactly zero. The corrosion current ( ) was expressed as (for the slopes’ βa, βc value definitions, see S-2): =

∆ 2.3( + ) ∆

The corrosion rate (CR, units are mils per year, or mpy) is directly related to the corrosion current via the expression: () =

. (..)

,

where E.W. is the equivalent weight of the corroding species, g. In our case, the corroding species is Zn, so E.W.= 32.7 g. Icorr is the corrosion current density in µA/cm2, and d is the density of the corroding species. Density of zinc is 7.140 g/cm3. From the icorr values, inhibitor efficiencies (IE, %) were calculated as: !(%) =

# ($%&)'# (#$(#)#%) # ($%&)

× 100 (1)


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At the end of the potentiodynamic measurements, the electrochemical cell was disassembled and the galvanized steel electrode was gently air-dried. The electrode was then subjected to FTIR measurements in ATR mode. In a separate series of experiments, rinsing the air-dried electrode with deionized water removed the adsorbed additive, which was then undetectable by FTIR. Testing of Magnesium Thiodipropionate Performance in Coatings Powder compositions for electrodeposition on galvanized steel optimized for ease of compounding, temperature of epoxy-amine hardener curing, electrodeposition and adhesion to steel are given in Supporting Information (S-3, S-4). The final blend compositions are shown in Table 1. Table 1. Powder coatings compositions Epoxy resin Crosslinker

Carbon

Montmorillonite MgTDPA

(wt%)

(wt%)

black (wt%)

(wt%)

(wt%)

Control

75.7

15.2

8.9

0.2

0

Test coating

72.7

12.1

8.9

0.2

6.1

The release of magnesium from the powder compositions was tested as follows: a known amount of as-prepared powder (10 to 20 mg) containing 6.1 wt% MgTDPA was dispersed in 10 mM Tris buffer (3 mL) with pH adjusted in the 6-10 range. The dispersion was briefly sonicated and then gently shaken at room temperature overnight. The liquid was then separated from the solids by a syringe membrane filter (Millipore, dpore 0.45 µm) and was assayed for magnesium content in a commercial laboratory by ICP-AES spectrometry. The measurements were conducted in triplicate. Panels of galvanized steel (zinc-galvanized sheets of type B carbon steel, ASTM A653, 4" x 3" x 0.0150") were solvent-brushed according to ASTM D609-17 34 and kept at 400F immediately prior to the powder electrodeposition. The powders were deposited on preheated steel panels using an Eastwood Dual-Voltage HotCoat Powder-Coating Gun at 25 kV and baked in an electrical oven at 400F for 20 min and 375F for 15 min. The resulting coatings were 3.8-4.0 mil (97-102 µm) thick. Immersion Tests Prior to the commencement of the immersion test, the coated panels were scribed with a scalpel blade making 100 µm-wide and 3 cm-long cuts exposing the bare steel surfaces. The tests were 9 ACS Paragon Plus Environment


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conducted by complete immersion of the coated panels in 3.5 wt% aqueous NaCl solutions (pH 5.25) at 25oC. We conducted quantitative measurements of the scribe creep at the conclusion of the immersion tests after 60 days. Following the 60-day immersion, the panels were gently rinsed by deionized water, air-dried and any flaked or blistered coating along the edges of the scribe was removed using an adhesive tape according to ASTM D3359.35 Scribe creep measurement was performed by taking eight measurements along the length of the scraped scribe. The creep was measured edge-to-edge of the scribe using digital calipers and 3X magnifying glass, then averaged for each panel. Each type of panel (i.e. with and without the corrosion inhibitor) was measured in triplicate using three distinct panels, and the results were averaged among the panels of the same type. Cyclical Salt-Spray Tests The powder coatings prepared with and without the corrosion inhibitor, MgTDPA1:1, were subjected to six cyclical tests in a commercial laboratory according to ISO 11997.

36

Each cycle

consisted of consecutive wet (salt fog), dry, and humidity exposures. That is, each standard cycle corrosion test was set as a combination of neutral salt spray exposure according to ASTM B 11737 and ISO 922738 certified tests, 100% condensing humidity according to ISO 6270-2,39 drying and dwelling. One full corrosion cycle was a week in duration. The coated panels (4" x 3" x 0.0150") were scribed using a van Laar Model 426 scratching tool with spherical tungsten scratch needle (Erichsen GmbH & Co., Germany). Ferrous rust run-offs were not observed on the panels after 6 cycles. There was no blistering away from the scribe per ASTM D165440 in any of the tested panels. However, the appearance of white rust powdery deposits was observed on the surface of the scribes as well as in the wash-outs. Following the six cycles, the panels were gently rinsed by deionized water, air-dried and any flaked or blistered coating along the edges of the scribe was removed using an adhesive tape according to ASTM D3359.35 Scribe creep measurement was performed as described above for immersion tests. Results and Discussion Syntheses and characterization of magnesium thiodipropionates


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Salts of magnesium and thiodipropionic acid (TDPA) were prepared via “green” aqueous routes, both by a methathesis reaction from magnesium acetate and TDPA (Fig.1), and by a substitution reaction (Fig.2), reacting metallic magnesium and TDPA in water as described in Experimental.

Fig.2. Replacement reaction of metallic magnesium and 3,3’-thiodipropionic (TDPA) or 3,3’dithiodipropionic (DTDPA) acids in aqueous solution at room temperature and pH 5.1. The MgTDPA salts resulting from the substitution reaction were crystalline compounds as evidenced by powder XRD patterns (Fig.3). The MgTDPA1:1 species prepared from a 1:1 stoichiometric ratio of magnesium and TDPA exhibited a high degree of crystallinity, including the presence of low 2θ angle peaks, which point to large, ordered unit cells. Subtraction of the amorphous contribution from the background and refinement of the XRD spectrum fitting indicated a unique mixed phase structure that did not match any of the compounds listed in the Cambridge Structural Database (CSD), including specific searches for existing organic magnesium salts. The mixed-fit analysis yielded a triclinic SG1 space group, with the unit cell parameters being a(Å) =6.727, b(Å) =12.864, and c(Å)=14.645 Å, α(ο)=92.567, β(ο)=73.054, γ(ο)=111.428. For comparison, the orthorhombic crystal structure of TDPA grown from warm aqueous solutions that were left to evaporate at room temperature has been resolved using singlecrystal diffractometry: Pcan (Pbcn), a(Å) =5.063(1), b(Å) =8.648(1), c(Å)=18.073(2), U=791.3 Å3.41 The synthesis of MgTDPA via replacement reaction (Fig.2) in aqueous solution followed by lyophilization precluded formation of large multicrystalline MgTDPA powders, and hence, no single-crystal diffraction peaks were visible. Nevertheless, the powder XRD patterns confirm a unique MgTDPA1:1 structure that is different from the parent TDPA (Fig.3). Our attempts to grow MgTDPA crystals by evaporating warm (40-80oC) aqueous solutions of the Mg/TDPA reaction mixtures lead to TDPA crystallizing separately from magnesium salts, so that the powder XRD patterns of the products prepared in that fashion were indistinguishable from the original TDPA patterns. The powder XRD spectrum of MgTDPA5:1 species obtained from a 5:1 stoichiometric ratio of magnesium and TDPA starting materials (see Experimental) appeared to contain both broad and sharp peaks, indicating the presence of a significant fraction of 11 ACS Paragon Plus Environment


amorphous phase with a second, crystalline phase embedded into the former. Elemental analysis and solubility studies demonstrated that MgTDPA5:1 contained at least 40 wt% of water-insoluble magnesium hydroxide. The crystal structure of Mg(OH)2 (unit cell parameters, a(Å) =3.142(1), b(Å) =3.142(1), and c(Å)=4.766(2), α(ο)=90, β(ο)=90, γ(ο)=120)) has been resolved42 and somewhat broadened 2θ angle peaks at 18, 37, 50, 57, and 64o in the powder XRD of MgTDPA5:1 clearly point to the presence of [001], [[101], [102], [110] and [111] reflections, respectively, in the magnesium hydroxide crystals43 present in our MgTDPA5:1 material. In contrast to the MgTDPA materials, mixed magnesium acetate thiodipropionate salt (MgAcTDPA) appeared as an amorphous glassy solid devoid of any crystalline features in its powder XRD pattern (not shown).

Fig.3. Powder XRD pattern of parent magnesium (Mg), thiodipropionic acid (TDPA) and magnesium thiodopropionate prepared with magnesium: thiodipropionate stoichiometric ratio of 1:1 (MgTDPA5:1) and 5:1 (MgTDPA5:1). Thermogravimetric analysis of the magnesium thiodipropionate salts (Fig. 4) shows that the properties of the mixed MgAcTDPA salt resembled those of the parent MgAc tetrahydrate. with the multistep, over 30 wt% endothermic weight loss due to the dehydration above 110oC and decomposition over 200oC.


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Fig.4. Thermogravimetric and differential scanning calorimetry analyses (TGA-DSC) of magnesium thiodipropionate (MgTDPA1:1), magnesium acetate thiodipropionate (MgAcTDPA) and parent magnesium acetate tetrahydrate (MgAc) at 10oC/Min temperature ramp in nitrogen atmosphere. Solid and dashed lines show the weight and heat flow, respectively. The thermograms of MgAc were well-defined, demonstrating endothermic loss of water at around 83oC (heat of transition, 179 J/g) and multistep endothermic decomposition in the range of 315 to 400oC (total heat of transition, ~380 J/g), which correlated well with the data reported previously.44,45 As-received TDPA exhibited endotherms at 105 and 128 ◦C (departure from baseline, data not shown), corresponding to the morphology change between crystalline phases and crystal melting, respectively.46 Decomposition of TDPA occurred endothermically at T>220oC. The TDPA presence in MgTDPA and MgAcTDPA was evident in endothermic melting peaks with onsets at 108 and 112oC, respectively. Degradation of MgAcTDPA occurred via multiple steps above 200oC, whereas degradation of MgTDPA was sharper and occurred at temperatures above 340oC. Importantly, MgTDPA and MgAcTDPA lost approximately 20 and 24% at 170oC, respectively, indicating the presence of significant amounts of hydrate water in these compounds. Such water may enhance the Lewis acid/base properties and catalytic effect of magnesium thiodipropionates in reactive organic coatings. Overall, the thermal properties such as softening and dehydration at >150oC and decomposition at >300oC make these materials suitable as additives to powder coatings, most of which possess a melting temperature around 150 °C, and are cured at around 200 °C.47



The electronic environment of magnesium in the newly obtained magnesium thiodipropionates can be highlighted by X-ray photoelectron spectroscopy (Fig.5).

Fig.5. XPS spectra of the Mg 2p peak of magnesium hydroxide and MgTDPA1:1 . The binding energy (BE) of Mg 2p peaks of magnesium hydroxide and that of magnesium thiodipropionate (MgTDPA1:1) were observed at 49.2 and 48.5 eV, respectively (Fig. 5). The BE value for Mg(OH)2 corresponded well with the value reported previously.48 The shift of BE in magnesium thiodipropionate to less electronegative values indicates that the Mg2+ cation in MgTDPA is less positive than in Mg(OH)2. This is an interesting observation considering that the Mg2+ cation is less positive in magnesium hydroxide than in common magnesium salts such as magnesium chloride, nitrate or carbonate.48 Electron donating (Lewis base) groups such as OH- and, to a larger extent, thiodipropionate anions render Mg2+ cation less positive and shift the BE to lower values. The electron donating, radical trapping properties of thiodipropionic acid and its derivatives explain their reported utilization as food and polymer antioxidants, especially in synergistic mixtures with substituted phenols.49-51 Magnesium thiodipropionates as benzoxazine and epoxy curing accelerators TDPA is known to be an efficient catalyst of the ring-opening polymerization of benzoxazines, a promising class of organic coating constituents.52,53 We hypothesized that related magnesium thiodipropionates can also act as benzoxazine curing (crosslinking) catalysts, especially as they contain hydrate water that can participate in the auto-acceleration of curing via the reverse Mannich reaction and electrophilic aromatic substitution of the carbonium cation to a 14 ACS Paragon Plus Environment

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benzoxazine monomer or phenol.54 Salts exhibiting Lewis acid (metal cation) and Lewis base (organic anion) properties are known to be effective bifunctional catalysts of ring-opening polymerization of benzoxazines55,56 and of the curing of oxirane-functional resins.57,58 In the present work, we tested the hypothesis of the possible catalytic activity of magnesium thiodipropionates with two model systems, including bisphenol-A/aniline based benzoxazine thermoset resin XU 35610 and a two-part epoxy (Araldite® LY8601 resin) -amine (Aradur® 8602). Homopolymerization and crosslinking of the benzoxazine resin are depicted in S-5. Typical

DSC

thermograms

of

the

original

benzoxazine

and

magnesium

thiodipropionate/benzoxazine blends (Fig. 6) show the effect of the additives on the onset of the polymerization exotherm, which is significantly lower in the presence of magnesium thiodipropionates; the exotherm’s end point is barely affected. That is, the exotherms were significantly broadened in the presence of additives and the peaks moved to lower temperatures. Without attempting to arbitrarily deconvolve the peaks into various thermal events, we observed that the peak maxima were at 220, 204 and 178oC for exotherms without additives, with MgAcTDPA, and with MgTDPA as additives, respectively. Similar effects on benzoxazine polymerization were previously reported with TDPA, where the onset of the polymerization and the peak maxima moved to lower temperatures, whereas the end point of the exotherm did not move, leading to peak broadening.21 Such observation has been interpreted as TDPA having the greatest impact on the initial stages of the polymerization such as ring opening, with the subsequent processes associated with cross-linking being affected to a lesser degree. Using the variable heating rate method (“Ozawa corrected”),59-61 we estimated the Arrhenius activation energy of the benzoxazine monomer to be 88.6 kJ/mol (S-6), which is within the range of previously reported values ranging from 81.4 to 93.7 kJ/mol depending on the degree of monomer purification by crystallization.21



Fig. 6. DSC curing exotherms of benzoxazine XU 35610 on heating (10 oC/min) in nitrogen atmosphere without additives and blends of benzoxazine with 10 mol% additives MgTDPA1:1 and MgAcTDPA. At 10 mol% loading, MgTDPA and MgAcTDPA lowered the apparent activation energy of benzoxazine polymerization to 63.7 and 69.1 kJ/mol, respectively, in line with the notion of the salts acting as catalysts. Similar effects of additive presence have been observed previously with TDPA and 4,4′-thiodiphenol, both of which are recognized as benzoxazine polymerization accelerators.21,62,63 Next, we examined the effects of magnesium thiodipropionates as additives to epoxy-amine twopart curing systems. Amines and amine derivatives are the most diverse group of epoxy curing agents widely employed in the coating industry.64 Alkaline earth metal carboxylates have been found to be effective cure modifiers for epoxy resins providing latency and facile curing.65 We hypothesized that magnesium thiodipropionates may accelerate curing of the amino-epoxy blend. This hypothesis was tested with Araldite® LY8601 epoxy resin and Aradur® 8602 amine hardener. The two-component system is a relatively low-viscosity liquid that, upon rapid mixing, is capable of solubilizing magnesium thiodipropionates at ambient temperature without gelling, with or without the use of reactive diluents such as 1,4-butanediol diglycidyl ether and the like. The gel point (onset of gelation, tgel) of that liquid is very sensitive to temperature and additives,30 which makes it a convenient model for our purposes. The onset of gelation was measured using controlled stress rheometry at a given temperature (see Experimental and S-1) 16 ACS Paragon Plus Environment

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according to the Winter-Chambon criterion67 at the crossover of the storage and loss moduli (G’=G”; loss angle δ=45o). The results of the isothermal rheological tgel measurements in the 60100oC temperature range are shown in Fig. 7. The linearity of the plots in Fig.7 indicated that the Arrhenius model held very well. The magnesium thiodipropionates indeed lowered the tgel values throughout the temperature range studied. The apparent activation energy value for the fastcuring epoxy-amine system without additives was 39.2 kJ/mol, while at 5 wt% loading, MgTDPA and MgAcTDPA lowered the apparent activation energy of curing to 27.8 and 29.3 kJ/mol, respectively. Hence, we have demonstrated that magnesium thiodipropionates retain the useful feature of their parent TDPA, i.e., they act as curing accelerators of heterocyclic compounds, oxiranes and benzoxazines, widely employed as components of organic coatings.

Fig. 7. Plots of gel time (tgel) as a function of reciprocal temperature for the Araldite® LY8601 epoxy/Aradur® 8602 amine without additives and with 5 wt% of MgTDPA1:1 or MgAcTDPA added. Arrhenius model coordinates were used according to expression ln(tgel)=lnA+(∆Ea/RT), where tgel(s) is the gel time, A is the pre-exponential factor, Ea (kJ/mol) is the activation energy, and T (K) and R (J/K.mol) are temperature and gas constant, respectively. Magnesium Thiodipropionates as Corrosion Inhibitors in Aqueous Salt Media The effects of magnesium thiodipropionates as corrosion inhibitors on galvanized steel were first assessed using potentiodynamic polarization studies (Fig.8). We employed a standard 3.5% (0.6 M) NaCl electrolyte, equilibrated with open air at a slightly acidic pH 5.6, which is above the pKa of 4.11 of thiodipropionic acid (TDPA).68 TDPA is a dicarboxylic acid with pKa1 and pKa2 17 ACS Paragon Plus Environment


equal to 3.87 and 4.68, respectively.

69

Hence, at pH 5.6 most of the carboxylic groups are

dissociated.

Fig.8. Potentiodynamic polarization curve for a specimen of galvanized steel as the working electrode in 3.5% NaCl with MgAcTDPA (effective concentration, 2.5 mg/mL) added. pH=5.3, T=25oC. As is seen in Fig.8, the behavior of galvanized steel in the presence of TDPA anions can be described as active-passive.70 As the potential increases above a certain Ecorr, the current increases exponentially, as is expected for an actively dissolving metal: Zn→Zn2+ + 2e-. However, at a certain Flade potential,71 the trend reverses and the current decreases because of the formation of a passive oxide film. In the passivation region, the current density decreases with the potential and increases again at a higher potential due to the breakdown of the passive film by localized corrosion or by transpassive reactions such as oxygen evolution.72 The breakdown potential corresponding to the instability of the oxide film and pitting corrosion is increased by the presence of thiodipropionate salts. We observed that at 1-2 mM MgTDPA1:1 concentrations and above in the electrolyte, the breakdown potentials increased, presumably due to the thiodipropionate adsorption reinforcing the passive film by blocking the Cl- ions at the active site, thus inhibiting nucleation of pits. Formation of fluffy precipitates of white rust (approximate composition by elemental analysis, 3Zn(OH)2·ZnCO3·H2O) on the zinc-galvanized surface of the working electrode in the active 18 ACS Paragon Plus Environment

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Page 19 of 39

region was observed in 3.5% NaCl solution without any additives, but the presence of millimolar or higher concentrations of MgAcTDPA, MgTDPA or TDPA in the electrolyte dramatically reduced the formation of white rust, instead facilitating formation of layers of passivated material on the steel surface. It must be noted that the white rust, albeit superficial, can seriously degrade the quality and adhesion of the coating if the surfaces of the steel were to be overpainted. In contrast, formation of the passivating thiodipropionate layer can enhance the adhesion of the organic coating components. FTIR measured in the attenuated total reflection mode demonstrated the presence of the physisorbed thiodipropionate layers on the galvanized steel surface when MgTDPA1:1 was added to the 3.5% NaCl electrolyte (Fig.9). In particular, peaks at 1720 (C=O bonded asymmetric stretch) and 1440 cm−1 (symmetric COO− stretching) indicate the presence of the carboxylate anion on the zinc surface.19,20 FTIR spectra of the as-synthesized magnesium thiodopropionates and TDPA are collected in S-7.

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000

MgTDPA added

no additive

1500 1000 Wavenumbers (cm-1)

500

Fig.9. FTIR-ATR spectra of the galvanized steel electrode surface after potentiodynamic electrochemical experiments with 3.5% NaCl electrolyte without additives and with the same electrolyte containing 10 mg/mL MgTDPA1:1 as an additive. Vertical dotted lines indicate the following spectral assignments: 1720 cm-1, C=O stretching vibrations of TDPA-;73,74 1537, 1398 cm-1, stretching vibrations of the carbonate ion in hydrozincite [Zn5(CO3)2(OH)6] and simonkolleite [Zn5(OH)8Cl2·H2O];75,76 1440 cm-1, symmetric COO− stretching of the carboxylate anion20 and 671 cm-1, νs (C-S) stretching vibrations of TDPA-. 19 ACS Paragon Plus Environment


Typical results of the addition of MgTDPA, MgAcTDPA and control magnesium acetate (MgAc) and thiodipropionic acid (TDPA) on Tafel plots are shown in Fig.10. To compare the effect of the additives, each additive’s concentration in the 3.5% NaCl electrolyte (pH 5.6) was arbitrarily set at 10 mg/mL. It was observed that all the studied additives reduced the icorr, i.e., reduced the corrosion rate. All the anodic branches of the Tafel plots were somewhat similar prior to the onset of passivation. The anodic branches showed active zinc dissolution and a monotonically increasing current up to very steep slopes, which indicated a diffusion-controlled reaction rate. In the cathodic curves, representing the hydrogen evolution reaction, there exists a clear reduction in the values of the current density compared to the control (no additives), from the steel in 3.5% NaCl without additives, to sample immersed in the additive solutions. Varying concentration of the additive in the bulk electrolyte changed both the cathodic and anodic currents (not shown), which suggested that our magnesium thiodipropionate salts could be classified as mixed-type corrosion inhibitors.72 Tafel plots in Fig.10 clearly show that MgTDPA1:1 reduced the Icorr values the most compared to the other additive species. -1.0

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1.1

Control

-1.1 MgTDPA

-1.1

5:1

MgAcTDPA MgTDPA

-1.2

-2

TDPA 1:1

-1 0 1 2 2 log(Current density (µA/cm ))

3

Fig. 10. Typical Tafel plots illustrating the effect of additives on potentiodynamic polarization of galvanized steel in 3.5% aqueous NaCl (initial pH 5.6, 25oC), with either no additives, MgAcTDPA (brown curve), MgTDPA1:1 (black curve), MgTDPA5:1 (blue curve), TDPA (green curve). Additive concentrations: 10 mg/mL. EIS (Fig.11) was applied to further reveal the effects of the corrosion inhibitors on the electrolyte-galvanized steel electrode interface and to complement the potentiodynamic


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polarization measurements. The qualitative information was obtained using fit analysis embedded into the software used to model the spectra, employing electrical equivalent circuits (EECs). 0.3

-Z (kΩ.cm )

No additive

2

0.2

im

CPE 0.1

0.0

Rel 0

0.2

RCT

0.4 0.6 2 Z (kΩ.cm )

0.8

1

re

80

MgTDPA added

60 2

-Z (kΩ.cm )

CPE 40 20 0

CPEp

Rel

im

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RCT 0

50

Rp

100 150 2 Z (kΩ.cm )

200

re

Fig.11. Typical Nyquist plots for galvanized steel in aqueous 3.5% NaCl (pH 5.6, 25oC) with and without additive, MgTDPA1:1. Effective MgTDPA1:1 concentration when added was 10 mg/mL. Symbols are experimental datapoints and solid lines represent simulated (modeled) spectra. Electrical equivalent circuits (EEC) used to fit the electrical impedance spectra are shown on each corresponding spectrum. Designations on EEC are as follows: Rel is the ohmic resistance between the working and the reference electrode; RCT is the charge transfer resistance related to the corrosion reaction at open-circuit potential (OCP); CPE is the capacitance of the electric double layer at the electrode/electrolyte interface; CPEp is the pseudocapacitance and Rp is the pseudo-resistance of the surface-adsorbed additive (MgTDPA) layer. 77



Elements of the EECs are described in the legend to Fig.11. The corrosion resistance of the galvanized steel surface in the presence of the corrosion inhibitor represents the RCT + Rp sum. In the control measurement without the additive, the spectrum with one time constant EEC was used, wherein the electrode resistance is represented by RCT. An excellent agreement between the experimental data (symbols) and calculated model (lines) was obtained, justifying the use of the proposed EECs. Analogous EEC models have been employed previously to estimate the effects of corrosion inhibitors such as zinc benzoate, monomeric and oligomeric amines, natural products, aminoalkanoic acids, complex heterocycles, etc.77,79-81 The fitting (solid curves) in Fig.11 demonstrated a dramatic, 283-fold increase in the total resistance value Ri = RCT + Rp (kΩ.cm2) in the presence of MgTDPA1:1 versus the Ro=RCT value recorded in the absence of the corrosion inhibitor. EIS measurements enabled estimation of the corrosion inhibition efficiency: 77-81 η(%) =-1 −

/0 /1

2 × 100

(2)

The above expression (2) is equivalent to eq(1) where the corrosion inhibition efficiency is obtained via the ratio of the corrosion current values found via the potentiodynamic polarization measurements. Both EIS and Tafel measurements are widely utilized for the estimation of the corrosion inhibitor efficiency.82 Values obtained in Tafel and EIS measurements are collected in Table 2, which affords a comparison of the corrosion inhibition effects of the magnesium thiodipropionate salts with TDPA and magnesium acetate as controls. MgAc2 and especially calcium magnesium acetate (CMA, a blend of calcium and magnesium acetates resulting from the reaction of acetic acid and dolomitic limestone) are considered to be non-corrosive deicing agents and even marginally effective inhibitors of corrosion caused by chlorides. 83-84


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Table 2. Effect of magnesium thiodipropionate (MgTDPA1:1 and MgTDPA5:1), thiodipropionic acid (TDPA), magnesium acetate thiodipropionate (MgAcTDPA), and magnesium acetate (MgAc2) as additives on the corrosion potential (Ecorr), corrosion current density (Icorr), and corrosion rate (CR) of galvanized steel and resulting inhibitor efficiency from EIS (η, %) and from Tafel (IE, %) measurements in aqueous 3.5% NaCl solutions at 25oC at pH 5.6. Initial effective concentration of each additive in the electrolyte was 10 mg/mL. Additive

[TDPA]

[Mg]

(meq/L) None

a

[Ac]

b

-Ecorr

c

η(%) IE(%)

Icorr

CR

(meq/L) (meq/L) (V)

(µA/cm2)

(mpy)

0

0

0

1.099

25.8

15.4

0

0

MgTDPA1:1

49.9

49.9

0

1.064

1.79

1.07

99

93

MgTDPA5:1

25.0

125

0

1.068

7.82

4.65

72

69

MgAcTDPA 39.7

24.8

24.8

1.067

4.06

2.41

80

84

TDPA

56.1

0

0

1.076

4.15

2.47

81

84

MgAc2

0

46.6

93.3

1.059

13.5

8.03

41

48

a

Total magnesium concentration.

b c

Acetate concentration.

Measurements were performed in triplicate, and all calculated data reported as averages (n=3).

Maximum standard deviations for η and IE values were 12 and 6% of the mean values, respectively.

Data in Table 2 demonstrate that MgTDPA1:1 is a more effective corrosion inhibitor for galvanized steel than either TDPA of MgAc, due to the synergistic effect afforded by the presence of both water-soluble TDPA anions and magnesium cations when MgTDPA1:1 is added to the electrolyte. The presence of acetate in the MgAcTDPA mixed salt lowered the effective concentration of magnesium and TDPA ions compared to MgTDPA1:1 and hence, lowered the corrosion inhibition efficiency of MgAcTDPA. Likewise, MgTDPA5:1 was only fractionally soluble in the electrolyte and yielded a suspension rather than solution, which certainly lowered the concentration of the TDPA ions available for adsorption on the steel surface. Correlation



between η and IE(%) values was found satisfactory, with η having higher uncertainty of determination due to the data fitting procedure. In addition to the newly discovered MgTDPA1:1 inhibitor, there are other, related corrosion inhibitors such as metal carboxylates, the effects of which on steel have been studied previously.20,86,87 Analogously to the magnesium thiodipropionate described in the present work, the anodic inhibitory effect of carboxylates has been combined with cathodic inhibitory cations, such as cerium, lanthanum and other rare earth metals33,87-90 and zinc.91 Decanoic acid, considered to be an efficient corrosion inhibitor, enabled inhibition efficiency η=83.5% on galvanized steel at concentration 44.7 mM, in electrolyte containing 0.0165 wt% NaCl (pH not stated20). Since at that concentration of decanoic acid the η values plateaued,20 yet the total salt concentration was approximately 100-fold lower than in our experiments, we can conclude that decanoic acid was approximately as efficient a corrosion inhibitor as TDPA (compare with data in Table 1), but less efficient than MgTDPA1:1. On the other hand, a closer MgTDPA analogue, zinc decanoate, demonstrated inhibition efficiency η=65% on carbon steel, although direct comparison is imprecise due to the limited aqueous solubility of Zn decanoate of only 0.026 mM.88

Fig.12. Concentration dependencies of inhibition efficiency (IE,%) of MgTDPA1:1, MgAcTDPA and TDPA in protecting galvanized steel from corrosion in 3.5% NaCl at pH 5.6, 25oC. The data were derived from the potentiodynamic polarization measurements performed at varying


Page 24 of 39

Page 25 of 39

concentrations of additives. CTDPA (mM) is the effective concentration of thiodipropionate in electrolyte. Concentration dependencies of the inhibitor efficiency IE (%) for TDPA, MgAcTDPA and MgTDPA1:1 shown in Fig. 12 were of characteristic saturation type, with the IE values plateauing at CTDPA >30 mM. As we have seen, the inhibitor efficiency is related to the adsorption of TDPA on the steel surface and is reversible (physisorption), with the layers of the adsorbed thiodipropionate readily soluble in fresh electrolyte. Therefore, assuming that IE is proportional to the steel surface coverage upon TDPA adsorption,77,92 the experimental data in Fig.12 were plotted in the form of Langmuir adsorption isotherm: 34567 8

= 9 + ; , (3) :

where Ka is the equilibrium constant for the adsorption–desorption process and q=0.01.IE. 70 50

20

TDPA

/q (mM)

60

C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

MgAcTDPA

TDPA

30 MgTDPA

10 0

0

20 C TDPA

40 (mM)

60

Fig.13. Linearized form of Langmuir adsorption isotherms for adsorption of MgTDPA1:1, MgAcTDPA and TDPA on galvanized steel at 25 ˚C and pH 5.6. The data were obtained from Fig.12, utilizing the mean values of q=0.01.IE (n=3). In all cases, linear fits were excellent (R2>0.99). The agreement between the Langmuir isotherm interpretation (eqn(3)) and experimental data was excellent, with all fitted lines being linear (R2>0.995 in all cases) and the corresponding slopes were close to unity (1.03±0.06), as is predicted theoretically. The Y-axis intercept of the



Page 26 of 39

lines in Fig.13 yielded Ka in the range 150-190 M-1, which affords an estimate of the corresponding standard Gibbs free energy (∆? ) of the thiodipropionate adsorption from the expression: 77,93 −∆? = @AB(CD&EF$% G ) (4) where csolvent= 55.5 M is the water concentration, R= 8.314 J / mol. K is the gas constant and T=298 K is the absolute temperature. Our data yielded –∆? values of 22.4-23.0 kJ/mol, which is close to the range of -20 kJ/mol typically attributed to physisorption via electrostatic interaction between the charged carboxylates and the charged metal. 92,94,96 Spontaneity of the adsorption process is supported by the negative ∆? values indicating that the corrosion inhibitor species form stable layers on the steel surface. In summary, we have discovered that magnesium thiodipropionate salt is an efficient corrosion inhibitor for galvanized steel in 3.5% NaCl solutions of moderately acidic pH, acting via formation of thiodipropionate layers on the steel surface. The cathodic protection effect of magnesium ions can be seen in augmented inhibition efficiency of MgTDPA compared to that of thiodipropionic acid. Magnesium thiodipropionate performance in powder coatings As we have seen, magnesium thiodipropionates exhibit corrosion inhibitive properties in aqueous media, with the MgTDPA1:1 species being more efficient than the others we investigated. Given the suitable thermal properties of MgTDPA1:1 and its aqueous solubility and dispersibility in epoxy resins, we next examined its performance in powder coatings. Incorporation of corrosion inhibitors into a polymeric matrix for the purpose of limiting cathodic disbondment is a complex task, given the opposing requirements for the corrosion inhibitor accessibility for water in order to function versus the protective, barrier properties of the coating’s polymer matrix.64 In order to implement powder electrodeposition while still enabling water access to magnesium thiodipropionate, the salt was physically blended, without melting, with the epoxy resin and dicyandiamide (latent crosslinker with high melting point) as two major components

The

composition also contained specialty carbon black and montmorillonite clay that enhanced the blended powder dispersibility (see Experimental). We investigated the accessibility of the 26 ACS Paragon Plus Environment

Page 27 of 39

powder composition to water as a function of pH by quantifying the fraction of magnesium initially present in MgTDPA that is released from the powder in contact with water using ICP (Fig. 14).

100 Mg release (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0

5

6

7

8 pH

9

10

11

Fig. 14. Release of magnesium from MgTDPA-containing powder formulation into aqueous buffer as a function of pH. Dashed line shows the onset of Mg(OH)2 precipitation. Release (%)=100x(Amount of Mg in the buffer/Amount of Mg in the initial powder composition). At pH9.3, Mg2+ produced by dissociated MgTDPA formed insoluble Mg(OH)2, which was removed by filtration along with the powder material. These experiments indicate that when a corrosive cathodic O2 reduction occurs on the interface between metal and the coating (i.e., in the delamination front), the magnesium cations dissociating from the MgTDPA-containing coating will

exhibit an anticorrosion action by

stopping propagation of the corrosion front on the metal-coating interface because of the formation of the barrier Mg(OH)2 precipitate:14 O2 (g) + 2H2O + 4e- → 4OH- (aq) Mg2+ + 2H2O

Mg(OH)2↓ +2H+

Following the coating processes, the coated steel panels were scribed in a controlled fashion and subjected to the immersion and cyclical salt-spray tests (see Experimental). The scribed areas were visualized using SEM images (60-day immersion tests, see S-8, S-9), which showed that 27 ACS Paragon Plus Environment


the scribes were initially identical in width but became significantly (~30 to 60%) wider for the coatings devoid of the corrosion inhibitor. The scribe creep (widening) was used as a quantifiable parameter of the coating delamination in both immersion and industry-standard cyclic corrosion tests (Fig.15). The average scribe creep in the panels coated without the corrosion inhibitor was 2.4- to 2.6-fold larger than with an analogous coating containing the corrosion inhibitor magnesium thiodipropionate. These results provided an unequivocal proof of the protective action of magnesium thiodipropionate in the developed epoxy-amine powder coatings. 200

8

4 2

Immersion

6

150 Salt-spray

Scribe creep, (mm) (Salt Spray)

10

100 50

0

Scribe creep (µm) (Immersion)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

0 Without CI

With CI

Fig.15. Results of the scribe creep measurements after the six-cycle salt-spray test and after 60day immersion test of the panels powder-coated with and without the corrosion inhibitor (CI), magnesium thiodipropionate (MgTDPA1:1). Datapoints represent mean average scribe readings, whereas the error bars show average standard errors of the mean. The initial scribe widths were subtracted from the scribe creep reading. Concluding remarks Developments in the field of ‘‘green’’ corrosion inhibitors are directed toward inexpensive, effective molecules of minimal negative environmental impact.72 In that regard, the magnesium thiodipropionates uncovered here, which are prepared in water by either a simple replacement reaction of magnesium metal with thiodipropionic acid (TDPA) or metathesis reaction, represent promising entries in the green corrosion inhibitors arsenal. TDPA is a non-toxic, monolayerforming, non-irritating dicarboxylic acid that is catabolized by microbes,97 whereas magnesium 28 ACS Paragon Plus Environment

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is more abundant and certainly less expensive than rare earths, carboxylates of which are considered to be green corrosion inhibitors.9 Yet, magnesium thiodipropionates are quite effective corrosion inhibitors in aqueous media and, for the first time, have been shown here to lower delamination of the powder coatings. This, combined with the observation that magnesium thiodipropionates accelerate benzoxazine polymerization and curing of the epoxy resin with amine hardeners, represents a rarely found synergism of functionalities useful wherever coatings are applied. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. S-1. Oscillatory shear time sweep results of the gel time tests. S-2. Tafel plot resulting from a potentiodynamic polarization experiment. S-3. Powder compositions for electrodeposition. S-4. SEM microphotograph of the particulates comprising the test powder coating composition. S-5. Schematic of catalyzed homopolymerization and crosslinking of bisphenol-A based benzoxazine thermoset resin. S-6. Temperature of exothermic peak maxima versus heating rate. S-7. FTIR spectra of as-made MgTDPA, MgAcTDPA and TDPA. S-8 and S-9. SEM microphotographs of the scribes made on the powder-coated steel panels. Acknowledgments Funding from Metalsa, S.A. de C.V.(Mexico) is gratefully acknowledged. The authors are thankful to Marina Temchenko and Kai-Jher Tan for help in organizing and conducting experiments. References (1) Jones, F. N.; Nichols, M. E.; Pappas, S. P. Organic Coatings: Science and Technology 4th Ed., Wiley: NY, 2017, 512 pp. (2) Fürbeth, W.; Stratmann, M. Delamination of Polymeric Coatings from Electrogalvanized Steel - a Mechanistic Approach. Part 1: Delamination from a Defect with Intact Zinc Layer, Corros. Sci., 2001, 43, 207-227. (3) Dafydd, H.; Worsley, D.; McMurray, H. The Kinetics and Mechanism of Cathodic Oxygen Reduction on Zinc and Zinc–Aluminium Alloy Galvanized Coatings. Corros. Sci. 2005, 47, 3006-3018. 29 ACS Paragon Plus Environment


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MAGNESIUM THIODIALKANOATES: DUALLY-FUNCTIONAL

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