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Diallyl-1,12-diaminododecane-based cyclopolymers and their use as inhibitors for mild steel corrosion.

INTRODUCTION

An inhibitor molecule's ability to arrest corrosion of metals in a hostile environment involves its strong adsorption (physical or chemical) to the metal surface and subsequent interference with either cathodic or anodic reactions (or both) occurring at the adsorption sites (1), (2). Polymers with multiple adsorption sites have been shown to undergo much stronger adsorption and exhibit greater inhibition efficiency (IE) than their corresponding monomer analogs (3), (4). In the recent past, the corrosion protection of intrinsically conducting polymers has been extensively investigated (5-7). An organic molecule with donor atoms may undergo chemisorptions through the interactions of its lone pair of electrons with the metal ions on the surface (8), (9). Adsorption may also be aided by electrostatic attractions among the charges on the metal surface and the inhibitor molecule. The presence of hydrophobic alkyl chain helps to displace water from the metal surface thereby forming a physical barrier to the electrolytic process and thus reduces the corrosion owing to physisorption. Adsorption process becomes entropy driven since a single polymer chain can displace many water molecules from the metal surface, and its multiple adsorption sites lock it strongly on the metal surface thereby making the desorption of the inhibitor chain a slower process.

With the above discussion in mind, we would like to synthesize water-soluble polymers with donor atoms as well as hydrophobic moieties that are likely to impart greater efficiency in arresting mild steel corrosion. Butler's pioneering work (10) involving cyclopolymerization of a variety of diallyl quaternary ammonium salts [[([CH.sub.2]=CH--[CH.sub.2]).sub.2][N.sup.+][R.sup.1][R.sup.2]][Cl.sup.-] yielded linear water-soluble cationic polyelectrolytes that have found extensive industrial and commercial applications [11, 121. Polydial-lyldimethylammonium chloride ([R.sup.1] = [R.sup.2] =Me) alone accounts for over 1000 patents and publications. The polymer-architecture, having the five-membered cyclic units embedded in the backbone, has been recognized as the eighth major structural type of synthetic polymers. Over 33 million pounds of poly(diallyldirnethylammo-nium chloride) alone are sold annually for water treatment and another 2 million pounds are used for personal care formulation (11). The 1,6-hexanediamine-based cyclopol-ymers (13) prepared via Butler's cyclopolymerization as well as hydrophobically associating polymers (14) derived from [[([CH.sub.2]=CH--[CH.sub.2]).sub.2][N.sup.+][R.sup.1][R.sup.2]][Cl.sup.-], have been tested for their inhibition efficiencies against mild steel corrosion in 1 M HCI. The polymers were found to be very good inhibitors (~90% IE%), but still there is room for further improvement in the IE. With this in mind we report, herein, the synthesis of several new monomers based on 1,12-dodecanediamine (1) and their polymers via Butler's cyclopolymerization process (Scheme 1) as well as their inhibiting effects on the corrosion of mild steel in acidic and saline media. It is anticipated that inhibitors derived from 1 with a 12-carbon spacers between the nitrogens would impart greater IE against mild steel corrosion.

EXPERIMENTAL

Materials

1,12-Diaminododecane, ethyl formate, benzyl chloride, p-methoxybenzyl chloride, ally! chloride, t-butylhydroper-oxide (70 wt% solution in water), and silica gel 100 obtained from Fluka Chemie AG, were used as received. Azo-bisisobutyronitrile (AIBN) from Fluka was purified by recrystallization from a chlorofom-ethanol mixture. Ammonium persulfate (APS) from BDH Chemical Co. (Poole, UK), was used as received. For dialysis, Spectra/Por membrane with MWCO of 6-8000 was purchased from Spectrum Laboratories Inc. Dimethylsulfoxide (DMSO) was dried over calcium hydride overnight and then distilled under reduced pressure at a b.p. of 64[degrees]C-65[degrees]C (4 mm Hg). All solvents were of hplc grade. All glassware was cleaned using deionized water. All the reactions were carried out under a positive atmosphere of [N.sub.2].

Physical Methods

Melting points are recorded in a calibrated Electrother-mal-IA9100- Digital Melting Point Apparatus. IR spectra were recorded on a Perkin Elmer 16F PC FTIR spectrometer. [.sup.1]H and [.sup.13]C NMR spectra were measured in CD[Cl.sub.3] and [CD.sub.3]OD using TMS or [D.sub.2]0 using dioxane as internal standard on a JEOL LA 500 MHz NMR spectrometer operating at 500.00 and 125.65 MHz, respectively. Elemental analysis was carried out on a Carlo-Erba Elemental Analyzer Model 1106.

Viscosity measurements were made by an Ubbelohde viscometer (having viscometer constant of 0.005718 cSt/s at all temperatures). Kinetic energy corrections were made to determine the viscosity data using polymer solution in the concentration range 1.5-0.25 g/dL at 30[degrees]C in 0.1 N NaCl.

Corrosion Inhibition Tests

Corrosion inhibition tests by gravimetric measurements were performed using coupons prepared from mild steel having the composition: 0.089% (C), 0.34% (Mn), 0.037 (Cr), 0.022 (Ni), 0.007 (Mo), 0.005 (Cu), 0.005 (V), 0.010 (P), 99.47% (Fe). Mild steel containing 0.184% (C), 0.070% (Si), 0.29% (Mn), 0.097 (Cr), 0.071 (Ni), 0.021 (Mo), 0.065 (Cu), 0.014 (V), 0.012 (P), 0.029% (S), 99.15% (Fe) were used for electrochemical measurements.

Solutions of 1 M, 4 M and 7.7 M HC1 and 0.5 M [H.sub.2][SO.sub.4] were prepared from reagent A.C.S. concentrated HCI and [H.sub.2][SO.sub.4] (Fisher Scientific Company) using distilled and deionized water. Inhibitor efficiency was determined at 60[degrees]C for 6 h by hanging the steel coupon measuring 2.5 X 2.0 X 0.1 [cm.sup.3] into 1 M HCl (180 [cm.sup.3]) containing various amounts (from 0-400 ppm) of the synthesized inhibitors. (However, in 4 M and 7.7 M HC1 a volume of 500 [cm.sup.3] and in 0.5 M [H.sub.2][SO.sub.4] a volume of 250 [cm.sup.3] was used). The details of the gravimetric measurements are described elsewhere (15-17). Percent inhibition efficiency (%IE) was determined using the Eq. I:

%IE = Weight loss (blank) - Weight loss (inhibitor) / Weight loss (blank) x 100 (1)

where weight loss (blank) and weight loss (inhibitor) represent weight loss in absence and presence of inhibitor, respectively. Triplicate determinations were made with each of the inhibitors and with solutions containing no inhibitor. The average percent losses were found to have a standard deviation of 0.3%-3%.

For potentiodynamic polarization studies, carbon steel coupons embedded in araldite (affixing material) with an exposed area of 2.0 [cm.sup.2] were used. The details of the electrochemical measurements are described elsewhere (15-17). The %IE by Tafel extrapolation method was determined using Eq. 2:

%IE = [CR.sub.blank] - [CR.sub.inhibitor] / [CR.sub.blank] x 100 (2)

where [CR.sub.blank] and [CR.sub.inhibitor] represent the corrosion rate in mmpy (mm per year) in the absence and presence of inhibitor, respectively.

Synthesis

N,N-Diallyl-N'-formyl-1,12-diaminododecane (5). To a stirring liquefied diamine 1 (100 g, 0.5 mmol) (68[degrees]C-70[degrees]C) in a 1 L round bottom flask fitted with a reflux condenser was added ethyl formate (37 g, 0.5 mol) drop-wise over a period of 20 min maintaining the temperature around 60[degrees]C. The reaction mixture was then kept over-night at room temperature. NMR spectrum revealed the presence of the starting diamine 1, the monoformate 2, and diformate 3 in an approximate ratio of 1:2:1, respectively (Scheme Allyl chloride (0.25 mol) was added dropwise (~20 min) to the reaction mixture in methanol (100 [cm.sup.3]), keeping the temperature around 50[degrees]C. Next, allyl chloride (0.25 mol) and solid NaOH (0.25 mol) were added over a period of 20 min. The reaction mixture was then heated at 60[degrees]C for 1.5 h. Then another portion of solid NaOH (0.25 mol) was added followed by dropwise addition of allyl chloride (0.5 mol) (~20 min) at 50[degrees]C. After heating the reaction mixture at 70[degrees]C for 2 h, a solution of [K.sub.2][CO.sub.3] (0.5 mol) in water (100 [cm.sup.3]) was added and heated at 70[degrees]C for an additional 2 h. At the end, the reaction mixture was diluted with water (100 [cm.sup.3]) and extracted with ether (3 x 200 [cm.sup.3]). The organic layer was dried ([Na.sub.2][SO.sub.4]) and concentrated to give a thick liquid which on silica gel chromatography using hexane as the eluant gave tetraallyl compound 4 as a colourless liquid (21 g, 47% based on the amount of diamine I present after reaction with ethyl formate). Further elution with hexane/ether mixture afforded diallyl compound 5 as a low melting solid (50 g, 65% based on the amount of monoformate 2). The compound belonging to the lower [R.sub.f], values were not analyzed and was expected to contain mostly diformate 3.

N,N-Diallyl-AP-formyl-1,12-diaminododecane (5): (Found: C, 73.76; H, 11.62; N, 8.95. [C.sub.19][H.sub.36][N.sub.2]O requires C, 73.97; H, 11.76; N, 9.08%). [v.sub.max].(neat) 3285, 3069, 2919, 2846, 2796, 1699-1660 (broad), 1535, 1461, 1384, 1249, 1156, 1007 and 924 [cm.sup.-1]; [[delta].sub.H] (CD[C1.sub.3]) 1.26 (16 H, m), 1.44 (2 H, m), 1.52 (2 H, m), 2.60 (2 H, app t, J 7.3 Hz), 3.08 (4 H, d, J 6.4 Hz), 3.28 (2 H, q, J 7.0 Hz), 5.13 (4 H, m), 5.85 (2 H, m), 8.15 (1 H, s); [[delta].sub.c] (CD[C1.sub.3]) 26.86, 26.89, 27.48, 29.13, 29.23 (3 C), 29.55 (3 C), 38.18, 53.40, 56.83 (2 C), 117.24 (2 C), 135.81 (2 C), 161.27.

N,N-Diallyl-N-benzyl-(12-M-formylamino)-1-dodecylam-monium chloride (6a) A mixture of compound 5 (10.0 g, 32.5 mmol), acetone (10 [cm.sup.3]) and benzyl chloride (5.33 g, 42.1 mmol) was heated under [N.sub.2] in a closed vessel at 70[degrees]C for 48 h. After removal of the solvent the syrupy liquid was triturated with ether several times to remove any unreacted amine and excess benzyl chloride. The quaternary salt (6a) was obtained as a colourless thick liquid and found to be soluble in acetone and chloroform. (Found: C, 70.95; H, 10.3; N, 6.25. [C.sub.26][H.sub.43]Cl[N.sub.2]O requires C, 71.77; H, 9.96; N, 6.44%); [v.sub.max]. (neat) 3200, 3025, 2923, 2835, 1672, 1536, 1468, 1380, 1240, 1213, 999, 930, 852, 732 and 709 [cm.sup.-1]; [[delta].sub.H] (CD[C1.sub.3]) 1.22-1.31 (16 H, m), 1.53 (2 H, m), 1.89 (2 H, m), 3.20 (2 H, app. q, J 7.0 Hz), 3.27(2 H, app. t, J 7.0 Hz), 4.17 (4 H, m), 4.89 (2 H, s), 5.72 (4 H, m), 6.10 (2 H, m), 7.64 (2 H, d, J H, m), 7.47 (3 H, 7.2 Hz), 8.18 (1 H, s). [[delta].sub.c] (CD[Cl.sub.3]) 22.57, 26.35, 26.84, 28.88, 29.09, 29.15 (3 C), 29.26, 29.37, 38.01, 58.85, 61.74 (2 C), 63.49, 124.84 (2 C), 127.30, 128.46 (2 C), 129.37 (2 C), 130.84, 132.75 (2 C), 161.93.

N,N-Diallyl-N-(p-methoxybenzy1)-(12-N'-formylamino)-dodecylammonium chloride (6b) The above reaction was repeated using p-methoxybenzyl chloride at 52[degrees]C for 24 h to obtain the quaternary salt 6b as a solid (97% yield). [M.sub.p] (closed capillary) 56-58[degrees]C; (Found: C, 69.0; H, 10.15; N, 5.85. [C.sub.27][H.sub.45]Cl[N.sub.2][O.sub.2] requires C, 69.72; H, 9.75; N, 6.02%); [v.sub.max]. (neat) 3202, 3030, 2922, 2837, 1666, 1612, 1516, 1468, 1382, 1308, 1260, 1184, 1118, 1030, 948, 846, 770 and 716 [cm.sup.-1]; [[delta].sub.H] (CD[C1.sub.3]) 1.23-1.31 (16 H, m), 1.54 (2 H, m), 1.88 (2 H, m), 3.22 (4 H, m), 3.84 (3 H, s), 4.13 (4 H, m), 4.81 (2 H, s), 5.71 (4 H, m), 6.08 (2 H, m), 6.94 (2 H, d, .1 8.6 Hz), 7.54 (2 H, d, J 8.6 Hz), 8.00 (1 H, broad s), 8.18 (1 H, s); [[delta].sub.c] (CD[Cl.sub.3], TMS) 22.59, 26.44, 26.86, 28.92, 29.10, 29.18 (2 C), 29.25, 29.28, 29.41, 38.07, 55.47, 58.57, 61.53 (2 C), 63.34, 114.76 (2 C), 118.94, 124.93 (2 C), 128.39 (2 C), 134.23 (2 C), 161.37, 161.96.

N,N-Diallyl-N-benzyl-1,12-dodecyldiammonium dichloride (7a) A solution of the quaternary salt 6a (4.35 g) in 10% HCI (10 [cm.sup.3]) was heated at 50[degrees]C for 48 h. After removal of the solvent, the residual thick liquid was dissolved in methanol and precipitated in ether. The resultant salt was dried under vacuum to give 7a.HCl as a faint yellow semisolid (4.3 g, 97%). (Found: C, 64.7; H, 10.15; N, 5.8. [C.sub.25][H.sub.44][Cl.sub.2][N.sub.2][H.sub.2]O requires C, 65.06; H, 10.05; N, 6.07%); [v.sub.max]. (KBr) 3384, 2918, 2857, 1607, 1457, 1390, 1209, 994, 946, 852, 737 and 704 [cm.sup.-1]; [[delta].sub.H] ([CD.sub.3]OD) 1.33 (16 H, m), 1.68 (2 H, quint, J 7.6 Hz), 1.90 (2 H, m), 2.93 (2 H, app. t, J 7.7 Hz), 3.21 (2 H, app. t, J 8.2 Hz), 3.98 (4 H, app. s), 4.63 (2 H, s), 5.73 (4 H, m), 6.22 (2 H, m), 7.54 (3 H, m), 7.62 (2 H, d, J 7.9 Hz); [[delta].sub.c] ([D.sub.2]O, dioxane:67.4 ppm) 22.44, 26.48, 26.70, 27.58, 29.14, 29.36, 29.63, 29.73 (2 C), 29.82, 40.27, 58.88, 61.91 (2 C), 63.04, 125.12 (2 C), 128.06, 129.26 (2 C), 130.12 (2 C), 131.52, 133.42 (2 C).

N,N-Diallyl-N-(p-methoxybenzyl)-1,12-dodecyldiammonium dichloride 7b Using procedure as above the quaternary salt 6b (3.0 g, 6.45 mmol) upon acidic hydrolysis gave 7b.HC1 as a pale highly hygroscopic yellow semisolid (96% yield).

(Found: C, 63.12; H, 10.05; N, 5.63. [C.sub.26][H.sub.46][Cl.sub.2][N.sub.2]O [H.sub.2]O requires C, 63.53; H, 9.84; N, 5.70%); [v.sub.max (neat) 3406, 2926, 2854, 1610, 1516, 1468, 1308, 1260, 1184, 1026, 952, 846, 770 [cm.sup.-1]; [[delta].sub.H] ([D.sub.2]O) 1.15 (16 H, m), 1.52 (2 H, quint, J 7.2 Hz), 1.68 (2 H, m), 2.85 (2 H, app. t, J 7.5 Hz), 2.93 (2 H, app. t, J 8.3 Hz), 3.72 (7 H, app. s), 4.29 (2 H, s), 5.59 (4 H, m), 5.98 (2 H, m), 6.93 (2 H, d, J 8.6 Hz), 7.34 (2 H, d, J 8.6 Hz); be (D20, dioxane:67.4 ppm) 22.31, 26.39, 26.57, 27.61, 29.00, 29.22, 29.45, 29.57 (2 C), 29.65, 40.38, 56.44, 58.73 61.62 (2 C), 62.11, 115.51 (2 C), 120.19, 125.15 (2 C), 129.08 (2 C), 134.97 (2 C), 161.50.

Polymer Synthesis

Homopolymer 8a. To a solution of the monomer in 6a (5.15 g) in deionized water (2.77 g) was added t-butylhy-droperoxide (120 mg) and heated at 50[degrees]C for 24 h and at 80[degrees]C for 40 h. The mixture was dialyzed against distilled water to remove unreacted monomer. The polymer solution was then freeze-dried to obtain polymer 8a as a faint yellow powder (51% yield). [M.sub.p] 130[degrees]C-140[degrees]C; the melted liquid did not decompose up to 400[degrees]C; [v.sub.max] (KBr) 3433, 3229, 3033, 2927, 2861, 1670, 1534, 1458, 1382, 1240, 1030, 892, 736 and 704 [cm.sup.-1].

Homopolymer 8b. Using the polymerization process and isolation procedure as in the case of 6a, monomer 6b was polymerized to give polymer 8b as a pale yellow powder in 53% yield. [M.sub.p]. 95[degrees]C-100[degrees]C with no apparent decomposition up to 400[degrees]C; vmax (KBr) 3417, 3046, 2926, 2850, 1667, 1513, 1455, 1375, 1253, 1177, 1030, 847 and 712 [cm.sup.-1].

Acidic Hydrolysis of Homopolymer 8a to 9a. A solution of polymer 8a (0.70 g) in a mixture of methanol (20 [cm.sup.3]) and conc HCl (6 [cm.sup.3]) was heated at 50[degrees]C for 48 h. The mixture was then dialyzed against distilled water to remove HCl. Polymer solution was then freeze-dried to obtain the polymer 9a as a white powder (0.65 g, 91%). Mp 175[degrees]C-190[degrees]C with no apparent decomposition up to 400[degrees]C. [v.sub.max] (KBr) 3425, 2927, 2849, 1620, 1460, 1390, 1206, 1030, 888, 736 and 706 [cm.sup.-1].

Acidic Hydrolysis of Homopolymer 8b to 9b. As in the case of 8a, a solution of the polymer 8b was hydrolyzed to give 9b (88% yield) as a white powder. Mp turned brownish at 190[degrees]C with no apparent decomposition up to 400[degrees]C; [v.sub.max] (KBr) 3441, 2927, 2847, 1614, 1515, 1459, 1257, 1187, 1025, and 842 [cm.sup.-1].

Copolymer 10a. In a typical experiment, [SO.sub.2] (0.660 g, 10.3 mmol) was absorbed in a solution of the monomer 6a (4.50 g, 5.15 mmol) in DMSO (4.0 g). The initiator AIBN (180 mg) was then added under N2 and the contents in the closed flask was stirred using a magnetic stir bar at 60[degrees]C for 48 h. The reaction mixture was dialyzed against deionized water for 48 h. Polymer 10a, which remained insoluble in water, was collected and then dried under vacuum at 50'C to a constant weight (3.5 g, 68%). The onset of melting point/thermal decomposition (Closed capillary) 135[degrees]C-l40[degrees]C (melted to a liquid) (Found: C, 61.7; H, 8.9; N, 5.4; S,6.2. [C.sub.26][H.sub.43]Cl[N.sub.2][O.sub.3]S requires C, 62.56; H, 8.68; N, 5.61; S, 6.42%); [v.sub.max] (Kar) 3427, 2926, 2853, 1666, 1532, 1461, 1385, 1309, 1129, 1035, 893, 738 and 704 [cm.sup.-1].

Copolymer 10b. The above polymerization was repeated except that monomer 6b instead of 6a was used. The copolymer 10b, which remained insoluble in water, was collected after dialysis and then dried under vacuum at 50[degrees]C to a constant weight (64% yield). The onset of melting point/thermal decomposition (Closed capillary) 115C-122'C (melted to a liquid) (Found: C, 60.5; H, 8.7; N, 5.1; S, 5.8. [C.sub.27][H.sub.45]Cl[N.sub.2][O.sub.4]S requires C, 61.28; H, 8.57; N, 5.29; S, 6.06 %); [v.sub.max] (KBr) 3428, 2926, 2852, 1667, 1612, 1517, 1464, 1385, 1257, 1183, 1128, 1030, 843, and 769 [cm.sup.-1].

Acidic Hydrolysis of the Copolymer 10a. A solution of the copolymer 10a (2.7 g, 5.4 mmol) in methanol (30 [cm.sup.3]) and concentrated HCl (12 [cm.sup.3]) was stirred in a closed flask at 50[degrees]C for 48 h. The reaction mixture was then dialyzed against deionized water for 48 h. The resulting solution was freeze dried and subsequently dried under vacuum at 50'C to a constant weight (2.5 g, 91.2%) of polymer 11 a. The onset of thermal decomposition (Closed capillary) 180[degrees]C-185[degrees]C; (Found: C, 58.7; H, 8.8; N, 5.3; S, 6.1. [C.sub.25][H.sub.44][Cl.sub.2][N.sub.2][O.sub.2]S requires C, 59.16; H, 8.74; N, 5.52; S, 6.32%); [v.sub.max] (KBr) 3448, 2925, 1636, 1460, 1308, 1130, 1035, 873 and 704 [cm.sup.-1]; intrinsic viscosity [[eta]] in 0.1 N NaCl at 30[degrees]C: 0.0169 dL/g.

Acidic Hydrolysis of the Copolymer 10b. The above acidic hydrolysis was repeated except that polymer 10b instead of 10a was used. The polymer 1 lb was obtained in 90% yield. The onset of thermal decomposition (closed capillary) 171[degrees]C-176[degrees]C; (Found: C, 57.5; H, 8.7; N, 5.1; S. 5.8. [C.sub.26][H.sub.46][Cl.sub.2][N.sub.2][O.sub.3]S requires C, 58.09; H, 8.62; N, 5.21; S, 5.96%); [v.sub.max] (KBr) 3447, 2926, 2854, 1613, 1516, 1464, 1307, 1258, 1183, 1130, 1030 and 844 [cm.sup.-1]; intrinsic viscosity [[eta]] in 0.1 N NaCl at 30[degrees]C: 0.00770 dL/g.

RESULTS AND DISCUSSION

Synthesis of the Monomers

The reaction of 0.5 mole each of 1,12-dodecanediamine 1 and ethyl formate afforded a non separable mixture of the monoformate 2, diforrnate 3 and unreacted starting material 1. One nitrogen terminal in 2 is thus protected as an amide while the other terminal containing trivalent nitrogen can be elaborated as desired. Thus the mixture of the products upon treatment with 1.0 mole of allyl chloride afforded a separable mixture of tetraallyl derivative 4 and desired diallyl compound 5 (Scheme 1). The selective protection of one end of a diamine is indeed a challenging problem, and we are gratified to obtain compound 5 (with a formyl protected nitrogen terminal) in a reasonable yield. The structure of the synthesized compounds was established by NMR, IR and elemental analyses.

The diallyl monoformate 5 on alkylation with alkyl halides RC[H.sub.2]Cl afforded quaternary ammonium monomers 6a and 6b in excellent yields (Scheme 1). The quaternary salts on acidic hydrolysis removed the formyl group and led to the formation of diammonium HCl salts 7a and 7b.

Synthesis, Solubility, and Spectroscopy of Homopolymers

Attempted polymerization of aqueous solution of monomer 6a (70 wt%) using ammonium persulfate (1 wt%) at 100[degrees]C for 3 h failed to give any polymeric product; the monomer remained unreacted. However, it underwent cyclopolymerization in water using t-butylhydroperoxide as the initiator to give cyclopolymer 8a which on acidic hydrolysis afforded defomylated polymer 9a (Scheme 1). The monomer 6b, likewise, underwent cyclo-polymerization to give cyclopolymer 8b, which on acidic hydrolysis afforded deformylated polymer 9b.

While polymers 8a and 8b were found to be soluble in methanol but insoluble in water, the corresponding deformylated amine salts 9a and 9b were soluble in both the solvents. This is expected since the less polar NCHO group is changed to ionic [[NH.sub.3].sup.+][Cl.sup.-]] functionality thus imparting water-solubility. The low intrinsic viscosity [[eta]] values of 0.0386 and 0.0376 dL/g for 9a and 9b in 0.1 N NaCl at 30[degrees]C ascertained the low molecular weight of the polymers. The five-membered cyclic structure of the polymers is based on precedent literature [18-201 on cyclopolymerization reactions involving various diallyl quaternary ammonium salts. The [.sup.]1H NMR spectra of polymers 8a and 9a along with their corresponding monomers 6a and 7a are shown in Fig. 1. The hydrolysis of formyl group was ascertained by the absence of NCHO proton signal at 7.9 ppm in the NMR spectra of 7a and 9a. The drastic decrease in the intensity of the alkene proton signals at [delta] ~ 5.7 ppm demonstrates the participation of the double bonds in the cyclopolymerization reaction. In the overwhelming majority of cases in literature, the polymer spectra do not show the presence of any residual alkene. However, even after extensive dialysis (see experimental), the current polymers did reveal the presence of residual alkene in the 11-1 NMR spectra (Fig. 1C and D). Butler's cyclopolymerization happens via inter- and intramolecular propagation, and any termination immediately after intermolecular propagation step is expected to leave an alkene moiety at the end of the polymer chain. The low molar mass of the polymers as suggested by the low [[eta]] values makes it possible to see the alkene end group in the NMR spectra. The homopolymers 8 and 9 have melting points below 200[degrees]C and the polymers did not decompose up to 400[degrees]C.

Synthesis, Solubility, and Spectroscopy of [SO.sub.2]-copolymers

The monomer 6a underwent cyclocopolymerization with sulfur dioxide in DMSO using AIBN as the initiator at 60[degrees]C to yield the copolymer 10a, which on acidic hydrolysis was converted into polymer 1 la (Scheme 1). Likewise, monomer 6b afforded copolymers 10b and I lb. Like the homopolymers, the copolymers 10a and 10b were also found to be insoluble in water but soluble in methanol, while the corresponding amine salts I la and 1 lb were found to be water-soluble. The [.sup.1]H NMR spectra of the copolymers 10b and 1lb and their corresponding monomers 6b and 7b are shown in Fig. 2. The ultra-low intrinsic viscosity [[eta]] values of 0.0169 and 0.00770 dL/g for 1 la and 1 lb in 0.1 N NaCl at 30[degrees]C ascertained the low molecular weight of the polymers. The copolymers 10 and 11 were found to be stable up to 240[degrees]C and thereafter decompose as result of elimination of [SO.sub.2].

TABLE 1. Inhibition efficiency (%IE) for different concentrations
of inhibitors for the inhibition of corrosion of mild steel in 1 M
HCI (6 h), 4.0 M
HC1 (3 h), 7.7 M HCI (2 h), and 0.5 M
[H.sub.2][SO.sub.4] (6 h) exposed at 60[degrees]C


                        1.0 M                 4.0 M  7.7 M  0.5
                          HCl                   HCl    HCl    M

Inhibitor    5  10  25     50  100  200  400    400    400  100

1            -  35  40     47   50   57   62      -      -  6.0

5           40  48  57     65   75   81   85      -      -    -

6a           -  39  47     57   66   72   77     95     92   55

6b         9.0  20  43     72   95   97   97     96     90   52

7a           -   -   -     80   92   95   96     97     88   57

7b         8.7  11  31     79   91   98   99     97     90   63

9a (a)      58  73  79     83   86   93   98     98     65   75
           (b)                                         (a)

9b          85  86  92     94   96   97   99     99     68   76
           (c)                                         (a)

11a (a)     49  60  71     86   87   94   97     97     44   71
           (d)                                  (a)    (a)

11b         48  63  73     85   88   95   98     99     32   69
           (e)                                  (a)    (a)

            [H.sub.2]
           [SO.sub.4]

Inhibitor         400

1                  13

5                   -

6a                 81

6b                 79

7a                 74

7b                 66

9a (a)             85

9b                 87

11a (a)            87

11b                91

(a.) Cloudy.

(b.) 2.5 ppm, IE% 43; 1.25 ppm, IE% 27; 0.625 ppm, IE% 15.

(c.) 2.5 ppm, IE% 67; 1.25 ppm, IE% 40; 0.625 ppm, IE% 17.

(d.) 2.5 ppm, IE% 32.

(e.) 2.5 ppm, IE% 41; 1.25 ppm, IE% 27.

TABLE 2. Corrosion inhibition efficiencies using gravimetric
and electrochemical methods: Results of Tafel Plots of mild
steel sample in various solutions containing 200 ppm
inhibitors in 1 M HC1 at 60[degrees]C for 6 h.

Compound   [E.sub.corr]  [[beta].sub.a]  [[beta].sub.c]
            vs SCE (mV)        (mV/dec)        (mV/dec)

Blank (a)          -452            59.9             131

1                  -484            50.5             186

5                  -490            47.9             137

6a                 -485            76.7             146

6b                 -504            87.8             147

7a                 -477            65.0             128

7b                 -487            79.4             137

9a                 -490            54.5             126

9b                 -469            63.9             119

11a                -465            55.5             118

11b                -490            56.3             120

Compound         [I.sub.corr]  Corr rate  Tafel  Gravimetric
           ([mu]A/[cm.sup.2])  (mm/year)     IE    IE (6 hr)

Blank (a)                4140      48.23      0         0.00

1                        1300       15.1     69           57

5                         219       2.55     95           81

6a                        134       1.56     97           72

6b                       56.0      0.651     99           97

7a                       78.3      0.911     98           95

7b                        112       1.31     97           98

9a                        122       1.42     97           93

9b                        197       2.30     95           97

11a                       216       2.52     97           94

11b                       100       1.16     98           95
(a.) The blank was a 1 M HCI solution.


The elemental analysis of polymers 10 and 11 indicates the presence of monomer and [SO.sub.2] in a 1:1 ratio, hence suggesting the formation of alternating copolymer. The IR spectra of the copolymers indicate the presence of [SO.sub.2] into the polymeric backbone. The two strong bands around 1308 and 1130 [cm.sup.-1] were assigned to the asymmetric and symmetric vibrations of SO2 unit.

Corrosion Inhibition Study

Gravimetric Method. The synthesized monomers and their corresponding homo- and copolymers as listed in Scheme 1 have been investigated for their inhibition efficiencies (IEs) in arresting mild steel corrosion in several corroding environments such as 1 M, 4 M, 7.7 M HCI, 0.5 M [H.sub.2][SO.sub.4], and 3.5% NaCl at 60[degrees]C for 6 h in the presence of 0-400 ppm of the inhibitors. The results are included in the Tables 1-3. In order to obtain a better window for adsorption isotherms (i.e. %IE in the range 20-80), some of the inhibitors were also tested in the presence of less than 5 ppm of the inhibitors. The results are included as footnotes in Table 1. The results of the inhibition tests by gravimetric method are corroborated by the results of the electrochemical method (Table 2). All the compounds used in this study are highly surface-active and showed excellent inhibition of corrosion in 1 M HO (Table 1); the inhibitor molecules at 400 ppm achieved inhibition efficiencies (IE%) in the range 62%99%. Even though the parent diamine 1 is found to be an ineffective inhibitor, its derivative monomers 6a,b and 7a,b as well as homopolymers 9a,b and copolymers 11 a,b were found to be excellent inhibitors (Table 1). The polymers are adsorbed strongly on the metal surface owing to multiple adsorption centers which make the desorption process less favorable. p-Methoxybenzyl derivative 6b performed slightly better than 6a presumably as a result of the aromatic ring of the former having increased [pi]-electron density via electron donation by the methoxy group. Better IE of the polymers (in compare to the monomer) was amply demonstrated in the lower concentration range. As evident from Table 1, the IE% of 9b is much higher in the lower concentration range than its corresponding monomer 7b. It is interesting to note that copolymer 9b at a concentration of a meager 5 ppm gave a protection efficiency of 85%, (Table 1). Further increase in the polymer concentration is not reciprocated by proportionate increase in the IE. It is possible that after a certain concentration (5 ppm) the available free space scattered on the metal surface does not have the empty domains of appropriate areas to accommodate the semicoil dimension of a larger polymer chain.

TABLE 3. Corrosion inhibition efficiencies using gravimeiric and
clectrochemical methods: Results of Tafel Plots of mild steel
sample in various solutions containing 200 ppm of the inhibitors
in 3.5% NaCI al 60[degrees]C.

Compound  [E.sub.corr]  [[beta].sub.a]  [[beta].sub.c]
           vs SCE (mV)        (mV/dec)        (mV/dec)

Blank             -659            72.7             376
NaCl (a)

6a                -696            42.5             122

9a                -700            36.0            71.6

9b                -692             287             189

11a               -690            36.3             111

11b               -703            34.5             211

Compound    [I.sub.corr]  Corr rate  Tafel   Gravimetric
              ([micro]A/  (mm/year)     IE  IE, (100 hr)
             [cm.sup.2])

Blank             174.8       2.04       0             0
NaCl (a)

6a                 53.6       0.63      69            75

9a                 25.1       0.29      86            90

9b                 25.6       0.30      85             -

11a                25.7       0.30      85            88

11b                32.6       0.38      81             -

(a.) The blank was a 3.5% NaCl solution.


Some of the inhibitor molecules (at a concentration of 400 ppm) exhibited IE in the ranges 95%-99% in 4 M HCl, 13%-91% in 7.7 M HCl, 13%-91% in 0.5 M [H.sub.2][SO.sub.4] and 75%-90% in 3.5% NaCl. There are not many compounds in the literature, which reports the excellent corrosion inhibition activity both in acidic and saline media. The current polymers performed very well in HCl, [H.sub.2][SO.sub.4] (Table 1) as well as in saline media (Table 3). The inhibitor molecules even withstood the hostile environment of 4 M HCl and gave excellent performance. However at 7.7 M, the polymers performed poorly as a result of poor solubility.

Electrochemical Method: Polarization Curves. The inhibitors were subjected to electrochemical study for the purpose of comparison with the gravimetric methods. Anodic and cathodic polarization curves for mild steel in 1 M HCl at 60[degrees]C in the absence and presence of 200 ppm of some of the inhibitors are shown in Fig. 3. The figure and results, given in Table 2, shows that in general all chemicals have reduced the corrosion rates; the [I.sub.corr] values decrease considerably in the presence of the inhibitors thereby confirming their inhibitive nature. Tafel plots were analyzed to estimate corrosion current ([I.sub.corr]), corrosion potential ([E.sub.corr]), anodic and cathodic Tafel slopes ([[beta].sub.a] and [[beta].sub.c]). The corrosion current was used to calculate corrosion rate in mm per year (mmpy) and inhibition efficiency [14-161. Likewise, the inhibitors which are soluble in 3.5% NaCl were tested for their IEs at 60[degrees]C by polarization Curves. The results are given in Table 3. Gravimetric method is indeed the most simple and reliable method for the determination of IEs, nonetheless the results from the electrochemical method using Tafel plots corroborated the results obtained by the gravimetric method. It is evident from the Tafel plots (Fig. 3 and Tables 2 and 3) that the inhibitor adsorption shifted the corrosion potential ([E.sub.corr]) in the negative direction with reference to the blank in 1 M HCl or 3.5% NaCl signifying that suppression of the cathodic reaction is the main effect of these corrosion inhibitors.

Adsorption Isotherms

Surface coverage ([theta], i.e. fractional inhibition efficiency: %IE/100) values for the inhibitor molecules as determined by the weight loss measurements for various concentrations of the inhibitors in 1 M HCl are reported in the Table 1. The [theta] values and C (the concentration in ppm i.e., mg/L, was changed mol/L) were used to find the best adsorption isotherm among those more frequently used, i.e. Temkin ([K.sub.ads]C = [e.sup.f[theta]]), Langmuir ([theta] /(1 - [theta]) = [K.sub.ads]C), Frumkin [21] ([K.sub.ads]C = [theta] /(1 - [theta]) [e.sup.-2a[theta]]).

The correlation coefficient indicated the best fit for the Frumkin's isotherm for some inhibitors, while others followed the Temkins isotherm (Table 4 and Fig. 4) where [K.sub.ads] is the equilibrium constant of the adsorption process, 'a' is the attraction constant. The linear fitting slope for the Frumkin's isotherms gave the values of '2a' for the compounds. For the Temkins isotherm the linear fitting slope gave the values of I/f, (Table 4) where f is a molecular interaction parameter (22) related to the molecular interactions in the adsorption layer as well as energetic inhomogeneity of the surface (23).

TABLE 4. Square of coefficient of correlation ([R.sup.2]) and
values of the constants (f and a) in the adsorption isotherms in
1 M HCl of Temkin, Langrnuir, and Frumkin. Adsorption equi
ibrium constani and free energy parameter of the mild steel
dissolution in the presence of inhibitors at 60[degrees]C in 1
M HCl.

             Frumkin      Temkin   Langmuir
Inhibitor  [R.sup.2],  [R.sup.2],  [R.sup.2]  [K.sub.ads]
                    a           f

1             0.9865,  0.9934, 13     0.9173       1.85 x
                 -4.6                          [10.sub.6]

5             0,9781,     0.9917,    (1.9830       2.68 x
                 -2.3         9.3              [10.sub.6]

6a            0.9921,     0.9824,     0.9218       6.47 x
                 +1.0         3.1              [10.sub.3]

6b            0.9802,     0.9660,     0.9686       8.17 x
                +0.72         3.7              [10.sub.3]

7b            0.9937,     0.9411,     0.9799       3.97 x
                 +1.4         2.7              [10.sub.3]

9a            0.9044,     0.9982,     0.9992       1.36 x
                -0.12         4.7              [10.sub.6]

9b            0.9955.     0.9939,     0.9918       1.29 x
                -0.88         3.0              [10.sub.6]

11a           0.6446,     0.9855,     0.9765       1.55 x
                -0.56         5.9              [10.sub.6]

11b           0.9020,     0.9923,     0.9904       2.73 x
                -0.94         6.5              [10.sub.6]

               [DELTA]
               [G.sub.
            ads.sup.o]  Isotherm
              (kJ/mol)      used
Inhibitor
1                -51.1    Temkin

5                -52.1    Temkin

6a               -35.4   Frumkin

6b               -36.1   Frumkin

7b               -34.1   Frumkin

9a               -50.2    Temkin

9b               -50.1    Temkin

11a              -50.6    Temkin

11b              -52.2    Temkin


The equilibrium constant of the adsorption process Kads is related to the free energy of adsorption ([DELTA][G.sub.ads]), by Eq. 3:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

where 55.5 is the molar concentration of water in solution. Adsorption equilibrium constants [K.sub.ads] and [DELTA][G.sub.ads.sup.[degrees]] as well as interaction parameters 'a' and 'f' that were obtained from the adsorption isotherms are summarized in Table 4. The negative values of [DELTA][G.sub.ads.sup.[degrees]] ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the steel surface. Generally, values of [DELTA][G.sub.ads.sup.[degrees]] up to 20 kJ/mol are consistent with the electrostatic interaction between the charged molecules and the charged metal (physisorption) while those between 80 and 400 kJ/mol are associated with chemisorption as a result of sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond. The calculated [DELTA][G.sub.ads.sup.[degrees]] values of around 50 kJ/mol in most cases indicate, therefore, that the adsorption mechanism of the inhibitors on steel in 1 M HCl solution was both electrostatic adsorption and chernisorption (24), (25).

The monomers or each repeating unit in the polymers have two polar heads in nitrogens and a non-polar spacer of ([C[H.sub.2])12.sub.12] The effect of energetic inhomogeneity of the surface--on the process of the inhibitors' adsorption--is found to be greater than that of the attraction in the adsorbed layer (i.e. f> a). The positive values of 'a' provide evidence for attraction (26), (27) between adsorbed organic molecules owing to van der Waals attractions between the hydrophobic spacers, while negative values indicates repulsion as a result of coulombic repulsion among the positive nitrogens. The overall effect will depend on the magnitudes of the two effects. Note that Frumkin's adsorption isotherm for polymer 9b (Fig. 4C) at the lower concentration range reveals the prevalence of the attractive forces (a > 0), while repulsive effects (a < 0) predominate at the higher concentration regime. At higher surface coverage too many positive nitrogens lead to repulsion. Molecular interaction constant f is dependent on the charge at the hydrophilic nitrogen ([N.sup.+]) as well as steric factors of the hydrophobic chain (28). A higher f value signifies stronger forces of repulsion between the adsorbed and adsorbing molecules as in the case of parent diamine 1. The lower values of f (Table 4) for the monomers and polymers in 1 M HCl indicate that the sterically crowded hydrophilic [N.sup.+] groups in the quaternary salts are unable to approach close enough for forces of repulsion to have significant effect on f.

CONCLUSIONS

A group of new monomers of N,N-diallyl quaternary ammonium salts were synthesized from 1,12-diaminodo-decane. Polymers of quaternary ammonium salts of low molecular weights with a hydrophobic cover of pendent [(CH.sub.2]).sub.12] spacer group have been prepared for the first time. The main objective of the study was to synthesize inhibitor molecules that would provide effective protection of corrosion of mild steel in acidic and saline media. The polymers performed better than their corresponding monomers, especially in the lower concentration range. The inhibitors performed excellent in HCl but very good in [H.sub.2][SO.sub.4] and saline media. The polymers even withstood the harsh corroding environment of 4 M HCl. The results are indeed very promising and pave the way to exploit the polymers' excellent ability to inhibit corrosion of mild steel. Further study has to be carried out to investigate the effectiveness of the inhibitor molecules exposed in acidic media for longer duration and with flowing systems using additional techniques of corrosion inhibition study. The [DEITA][G.sub.ads.sup.o] point towards physisorption as the major and chemisorption as the minor contributor for the adsorption of the inhibitors on the metal surface.

ACKNOWLEDGMENTS

Facilities provided by King Fahd University of Petroleum and Minerals, and University of Dammam and financial assistance by King Abdulaziz City of Science and Technology (KACST) (under the grant: AR-20-72) are gratefully acknowledged.

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Shaikh A. Ali, (1) M.T. Saeed, (1) Asma M.Z. EI-Sharif (2)

(1.) Department of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

(2.) Department of Chemistry, University of Dammam, Dammam 31113, Saudi Arabia

Correspondence to: S.A. Ali; e-mail: shaikh@kfupm.edu.sa

Contract grant sponsor: King Abdulaziz City of Science and Technology (KACST); contract grant number: AR-20-72.

DOI 10.1002/pen.23224

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2012 Society of Plastics Engineers

DOI 10.1002/pen.23224
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Date:Dec 1, 2012
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