Novel class of bisquaternary ammonium salts in inhibition of mild steel corrosion in HCl and H 2 SO 4

Abstract

A series of new bisquaternary ammonium inhibitor molecules p-xylylenebis[trialkylammonium chloride] were prepared in excellent yields by reacting α,α′-dichloro-p-xylene with a group of tertiary amines such as N,N-diallyl-N′-formyl-1,12-diaminododecane, N,N-diallyl-, N,N-diethyldodecylamine and N-dodecylpyrrolidine. Inhibition efficiencies (IEs) for different concentrations of bisquaternary salts for the inhibition of corrosion of mild steel in 1, 4 and 7·7M HCl as well as 0·5M H₂SO₄ exposed for 6 h at 60°C were determined gravimetrically. While in the presence of 400 ppm all the inhibitor molecules exhibited excellent %IEs of ⩾99 in 1M HCl, p-xylylenebis[N,N-diallyl, N-dodecylammonium chloride] provided very good %IE of 97 and 94 respectively in 4 and 7·7M HCl. The inhibitors imparted good to very good IE in arresting mild steel corrosion in 0·5M H₂SO₄; p-xylylenebis[N,N-diallyl, N-dodecylammonium chloride] performed the best, giving %IE of 94 at 400 ppm. The interactions of the π electrons in the diallyl moiety with the low lying vacant d orbitals of iron help the inhibitor molecules to undergo adsorption and interfere with the anodic dissolution process. Comparable results for the IEs were obtained by the electrochemical method using Tafel plots of the synthesised compounds. The adsorption of these compounds on the mild steel surface was found to obey Frumkin and Temkin adsorption isotherms in 1M HCl and 0·5M H₂SO₄ respectively.

Keywords

Corrosion inhibitors Mild steel Bisquaternary ammonium salts Acid solutions Acid corrosion Polarisation Weight loss

Introduction

Organic compounds containing oxygen, sulphur and nitrogen are widely used in industrial acid cleaning, acid descaling, acid pickling and oil well acidising in order to restrain the corrosion attack on metallic materials.1 ^– 5 Since chloride ions impart effective synergism in the inhibition of corrosion by organic cationic inhibitors, they are highly efficient in arresting corrosion of mild steel in HCl.6 ^– 8 However, these compounds are not good inhibitors in plain sulphuric acid media.9 Since the surface charge of iron in both HCl and H₂SO₄ is positive, the organic cations are reluctant to be adsorbed on the metal surface. This is mostly true in H₂SO₄ solutions but not so in HCl as a result of the poor adsorbability of sulphate ions.10,11 The specific adsorption of Cl⁻ having a smaller degree of hydration leads to the species , which is expected to shift the position of the corrosion potential E _corr with respect to the potential for zero charge E _q = 0 to more positive values than E _corr, thus allowing the physical adsorption of organic cation.12,13 The formation of a close packed layer of (FeCl⁻…Inh⁺)_ad would inhibit iron ions to enter the bulk of the solution, thereby arresting the anodic dissolution of iron in HCl solutions.14,15 It is well known that the protective properties of a cation type of inhibitor in H₂SO₄ medium can be improved with the addition of halide ions.16 The cathodic hydrogen evolution, on the other hand, is the competitive adsorption cationic inhibitor molecule In⁺ to form (Fe…In⁺)_ads instead of (Fe…H⁺)_ads and thus inhibit corrosion by hydrogen evolution reaction.

There are not many organic inhibitors that are equally effective against corrosion attack on mild steel in HCl as well as H₂SO₄ media. In recent years, organic compounds containing isoxazolidine10,17 ^– 20 and diallylammonium moieties21 have been reported to be quite effective in the inhibition of mild steel corrosion in both HCl and H₂SO₄ media. The organic inhibitors may undergo physi- or chemisorption on the metal surface by displacing water molecules on the surface to form a compact barrier film22 or by the formation of a coordinate covalent bond via interaction of the non-bonded (lone pair) and π electrons in the inhibitor molecules with the vacant d orbitals of the metal.23

It has been shown24 that because of multiple adsorption sites, the polymers as well as oligomers are better corrosion inhibitors than their corresponding monomers25,26 as a result of an entropically favourable displacement of many water molecules from the metal surface by a single large oligo/polymer molecule. The presence of multiple bonding sites makes the desorption of the inhibitor molecule a slower process. Incorporation of multiple functional groups within a molecule is expected to help the inhibitor lock in on the metal surface strongly. With this in mind, we report herein the synthesis of a novel class of bisquaternary ammonium salts (Fig. 1) and their inhibiting effect on the corrosion of mild steel in 1M HCl and 0·5M H₂SO₄ solutions using gravimetric measurements and potentiodynamic polarisation curves. There are few reports that dealt with bisquaternary ammonium salts as the corrosion inhibitor for mild steel.27 ^– 29 Bisquaternary ammonium salts in general are widely used as surfactant that always have antimicrobial properties.30

Figure 1

New bisquaternary ammonium salts

Experimental

Materials

1,12-Diaminododecane, pyrrolidine, ethyl formate, diethylamine, 1-bromododecane, allyl bromide and allyl chloride, α,α′-dichloro-p-xylene, obtained from Fluka Chemie AG, were used as received. All solvents were of high pressure liquid chromatography grade. All glassware was cleaned using deionised water.

Physical methods

Melting points are recorded in a calibrated Electrothermal IA9100 Digital Melting Point Apparatus. 1H spectra were measured in CDCl₃ using TMS as internal standard on a JEOL LA 500 MHz nuclear magnetic resonance spectrometer. All the reactions were carried out under a positive atmosphere of N₂. Elemental analysis was carried out on a Carlo-Erba Elemental Analyzer Model 1106. The new compounds synthesised below gave satisfactory elemental analyses. Infrared and nuclear magnetic resonance analysis confirmed that the required molecular structure had been correctly synthesised.

Synthesis of inhibitor molecules

Preparation of amines 1

Amine 1a was prepared from the reaction of 1,12-diaminododecane with ethyl formate followed by treatment with two equivalents allyl chloride. Amine 1c was prepared as described in the literature.31

A mixture of 1-bromodecane (7·5 g, 30 mmol) and diethylamine (11 g, 150 mmol) was heated at 60°C in a closed vessel for 72 h. At the end, the reaction mixture was cooled and stirred with a solution of NaOH (0·6 g, 0·03 mol) in water (50 cm³) and extracted with ether (2×25 cm). The organic layer was dried (Na₂SO₄), concentrated and distilled to obtain the diethyl derivative 1d as a liquid (5·42 g, 80%). The compound 1e (85%) was similarly prepared using pyrrolidine instead of diethyl amine, as described above.

p-Xylylenebis[N,N-diallyl-N-(12-formylamino-1-dodecyl)ammonium chloride] (3a)

A solution of amine 1a (6·00 g, 19·4 mmol) and α,α′-dichloro-p-xylene (2) (1·40 g, 8·0 mmol) in acetone (6 cm³) was stirred under N₂ in a closed vessel at 75°C for 72 h. After removing acetone, the reaction mixture was washed with ether to obtain bisquaternary salt 3a (89%). Mp (closed capillary) was 119-122°C. The compound gave satisfactory spectral (1H, ¹³C), IR and elemental analyses.

p-Xylylenebis[N,N-diallyl-N-(12-amino-1-dodecyl)ammonium chloride hydrochloride] (3b)

A solution of diquaternary salt 3a (2·00 g, 2·525 mmol) in 10%HCl (6 cm³) was heated at 40°C for 48 h. After the removal of the solvent, compound 3b was obtained (91·5%). Mp (closed capillary) was 79-81°C. The compound gave satisfactory spectral (1H, ¹³C), IR and elemental analyses.

p-Xylylenebis[N,N-diallyl-N-(1-dodecyl)ammonium chloride] (3c)

A solution of diallylamine 1c (2·24 g, 8·4 mmol) and α,α′-dichloro-p-xylene (2) (0·538 g, 3·07 mmol) in dry acetonitrile (4 Å Linde molecular sieve) (6 cm³) was stirred under N₂ in a closed vessel at 70°C for 4 days. Work-up as described in the preparation of 3a gave quaternary salt 3c (85%). Mp (closed capillary) was 137-139°C. The compound gave satisfactory spectral (1H, ¹³C), IR and elemental analyses.

p-Xylylenebis[N,N-diethyl-N-(1-dodecyl)ammonium chloride] (3d)

A solution of N,N-diethyl-1-dodecylamine 1d (3·62 g, 15 mmol) and α,α′-dichloro-p-xylene (2) (1·05 g, 6·0 mmol) in acetone (6 cm³) was stirred under N₂ in a closed vessel at 78°C for 40 h. Similar workup as described in the preparation of 3a afforded the salt 3d (3·20 g, 81%). Mp (closed capillary) was 206-208°C. The compound gave satisfactory spectral (1H, ¹³C), IR and elemental analyses.

p-Xylylenebis[N-(1-dodecyl)pyrrolidinum chloride] (3e)

A solution of N-decylpyrrolidine 1e (3·6 g, 15 mmol) and α,α′-dichloro-p-xylene (2) (1·05 g, 6·0 mmol) in acetone (6 cm³) was stirred under N₂ in a closed vessel at 78°C for 40 h. Work-up as described in the preparation of 3a gave bisquaternary salt 3e (95%). The salt was found to be soluble in water as well as in chloroform. Mp (closed capillary) was 207-210°C. The compound gave satisfactory spectral (1H, ¹³C), IR and elemental analyses.

Specimens

Corrosion inhibition tests by gravimetric and electrochemical measurements in 1, 4 and 7·7M HCl and 0·5M H₂SO₄ were performed using coupons prepared from mild steel having the composition: 0·089C, 0·34Mn, 0·037Cr, 0·022Ni, 0·007Mo, 0·005Cu, 0·005V, 0·010P and 99·47Fe. The test specimens for the electrochemical measurements were machined in a flag shape from a mild steel sheet of 1 mm thickness. The stem of the flag, which measured ∼3 cm, was insulated by an insulating paint araldite. The remaining area was ∼1 cm² and provided 2 cm² of exposed area. The mild steel specimens were abraded with increasing grades of emery papers (100, 400, 600 and 1500 grit size), then degreased with acetone and washed with deionised water. The specimens were dried and kept in a desiccator.

Solutions

Solutions of 1, 4 and 7·7M HCl and 0·5M H₂SO₄ were prepared from reagent A.C.S. concentrated HCl and H₂SO₄ (Fisher Scientific Company) using distilled and deionised water. The concentration range of the inhibitors employed was 0-400 ppm (ppm refers to parts per million by weight). All corrosion experiments were carried out with electrolyte solutions in equilibrium with the atmosphere (i.e. aerated solutions) while no additional aeration was performed.

Gravimetric measurements

Inhibitor efficiency was determined at 60°C for 6 h by hanging the steel coupon measuring 2·5×2·0×0·1 cm into 250 cm³ 1 or 4M HCl or 0·5M H₂SO₄ containing various amounts (from 0-400 ppm) of the synthesised inhibitors. (However, in 7·7M HCl, a volume of 500 cm³ and the steel coupon measuring 2·5×2·0×0·15 cm were used.) After the elapsed time, the coupons were cleaned through a procedure that consisted of wiping the coupons with a paper tissue, washing with distilled water and acetone, followed by oven drying at 110°C.

The %IE was determined using the following equation Weight loss (blank) and weight loss (inhibitor) represent weight loss in the absence and presence of inhibitor respectively. Triplicate determinations were made with each of the inhibitors and with solutions containing no inhibitor. In some cases where the deviations are larger, the weight loss measurements were carried out for the 4th or even 5th time in order to get the desired standard deviation. In most of the cases where the coupons were not of identical size and mass, the relative weight losses of the coupons were used to calculate the per cent inhibition efficiency (%IE) as described.19,21 The average %IE reported is found to have a standard deviation of 0·3-3·5%. In 1M HCl, the deviation in the higher concentration range of the inhibitor molecules was within 0·5%.

Electrochemical measurements: Tafel extrapolation method

For potentiodynamic polarisation study, mild steel coupons with an exposed area of 2·0 cm² were used, and experiments were carried out in 250 cm³ of solution of 1M HCl or 0·5M H₂SO₄ containing 0 and 200 ppm (or 400 ppm) inhibitors at 60°C with exposure time of 30 min [or until a steady state open circuit potential (OCP) was obtained]. The electrochemical cell was assembled in a 500 mL round bottomed flask consisting of mild steel coupon as working electrode; a graphite rod of ∼5 mm diameter worked as a counter electrode, and a saturated calomel electrode (SCE) was used as reference electrode. All three electrodes were connected to a potentiostat (Model 283, EG&G PARC). Potential range of ±250 mV with respect to the OCP and a scan rate of 1·6 mV s⁻¹ were applied.

Results

Synthesis of inhibitor molecules

The new bisquaternary ammonium inhibitor molecules 3a–e are prepared in excellent yields from tertiary amine 1 and inexpensive α,α′-dichloro-p-xylene (2), as shown in Scheme 1. Concise experimental procedures are incorporated in the experiment section to describe the synthesis of precursor amines 1a–e. Assuming that the aromatic ring has an equivalent length of four carbons, the bisquaternary salt 3c, for instance, has an equivalent chain length of ∼32 carbons, which would indeed provide a huge coverage of the metal surface from both ends of the quaternary nitrogens. For the compounds 3a and 3b, while the former has amide (NHCHO) terminals, the later has four positive nitrogens on the molecule. In acidic solution, the compound 3a is expected to be hydrolysed to 3b, and as such, the inhibiting action of 3a alone may not be accurately assessed. All the compounds have a flat aromatic ring in the centre and are expected to help anchor the positive nitrogens firmly on the metal surface.

Gravimetric measurements

The results of weight loss measurements for various concentrations of inhibitor molecules at 60°C after 6 h of immersion in 1M HCl, 3 h immersion in 4·0M HCl and 2 h immersion in 7·7M HCl are reported in Table 1. At higher concentrations of HCl, the %IEs of the compounds were determined only at the concentration of 400 ppm. For the sake of comparison, the %IEs at 200 ppm in 1M HCl by gravimetry are also included in Table 2, which reports the results of electrochemical results by Tafel extrapolations. The %IEs in the presence of various concentrations of inhibitor molecules in 0·5M H₂SO₄ are reported in Table 3. The weight loss measurements were carried out for the 4th or even 5th time in order to get the desired standard deviation of 0·3-3·5% in the %IE values in the lower concentration range and within 0·5% in the higher concentration range.

Table 1

Inhibition efficiency for different concentrations of inhibitors for inhibition of corrosion of mild steel in 1M HCl (6 h), 4·0M HCl (3 h) and 7·7M HCl (2 h) exposed at 60°C

Compound	Inhibition efficiencies (%) at concentration (ppm) of compounds and HCl
3	5	10	25	50	100	200	400	400	400
3	1·0M HCl							4·0M HCl	7·7M HCl
a	…	…	…	92	95	99	99·2	94	91
b	68*	90	90	94	96	98	99·1	90	89
c	17†	51	82	94	96	99	99·3	97	94
d	95	95	97	97	98	99	99·0	87	77
e	93	96	97	97	98	98	99·2	86	75

*2·5 ppm, IE% 31; 7·5 ppm, IE% 83.

†7·5 ppm, IE% 33.

Table 2

Corrosion IEs using gravimetric and electrochemical methods: results of Tafel plots of mild steel sample in various solutions containing 200 ppm inhibitors in 1M HCl at 60°C for 6 h

Compound 3	E _corr/mV (SCE)	β _a/mV/decade	β _c/mV/decade	I _corr/μA cm⁻²	Corrosion rate/mm/year	Tafel IE	Gravimetric IE, 6 h
Blank*	−452	59·9	131	4140	48·23	0	0·00
a	−491	71·4	145	211	2·45	95	99
b	−479	66·8	129	110	1·29	97	98
c	−512	90·6	190	120	1·40	97	99
d	−523	80·4	157	194	2·26	95	99
e	−489	67·0	130	281	3·22	93	98

*Blank was a 1M HCl solution.

Table 3

Inhibition efficiency for different concentrations of inhibitors for inhibition of corrosion of mild steel in 1M H₂SO₄ exposed for 6 h at 60°C

Compound 3	Inhibition efficiencies (%) at concentration (ppm) of compounds
Compound 3	10	25	50	100	200	400
a	23	38	44	50	63	75
b	24	38	42	53	62	73
c	65	73	78	83	87	94
d	35	46	55	57	64	77
e	20	32	43	52	62	78

Electrochemical measurements: Polarisation curves

Each pair of Tafel plots was analysed to obtain the corrosion current density and the corrosion potential.19 The collected data were analysed by 352 SoftCorrII software, which utilises an advanced numerical fit to the Butler–Volmer equation by taking into consideration that many systems do not provide sufficient linear region to permit accurate extrapolation. The curve fitting procedures have the advantage that it does not require a fully developed linear portion of the curve.32,33

The results of the Tafel plots for mild steel in 1M HCl (blank) and 1M HCl solution containing 200 ppm of the inhibitors at 60°C are summarised in Table 2, whereas the results in 0·5M H₂SO₄ are presented in Table 4.

Table 4

Results of Tafel plots of mild steel sample in various solutions containing 400 ppm inhibitors 3b and 3c in 0·5M H₂SO₄ at 60°C

Inhibitor	Tafel plots
Inhibitor	E _corr/mV (SCE)	β _a/mV/decade	β _c/mV/decade	I _corr/μA cm⁻²	%IE
Blank*	−503	91·8	191	4625	…
3b	−487	55·0	160	829	82
3c	−464	37·0	157	324	93

*Blank was a 0·5M H₂SO₄ solution.

Adsorption isotherms

Surface coverage (θ, fractional IE) values, as determined by the weight loss measurements for various concentrations C (ppm in mg L⁻¹, was changed to mol L⁻¹) of the inhibitors, were used to find the best adsorption isotherm between those more frequently used, i.e. Temkin (K _adsC = e ^fθ), Langmuir [θ/(1−θ) = K _adsC] and Frumkin34 [K _adsC = θ/(1−θ)e ^−2aθ]. The correlation coefficient indicated the best fit for the Frumkin adsorption isotherm for inhibitors in 1M HCl (Tables 1 and 5 and Fig. 2) and the Temkin isotherm in 0·5M H₂SO₄ (Tables 3 and 5 and Fig. 3), where K _ads is the equilibrium constant of the adsorption process, and ‘a’ is the attraction constant. Frumkin adsorption isotherms for some of the inhibitors in 0·5M H₂SO₄ are displayed in Fig. 4. The linear fitting slope for the Frumkin isotherms gave the values of ‘2a’ for the compounds. Some of the molecules also gave good fit for the other adsorption isotherms (Table 5). For the Temkin isotherm, the linear fitting slope gave the values of 1/f (Table 5), where f is a molecular interaction parameter35 related to the molecular interactions in the adsorption layer as well as the energetic inhomogeneity of the surface.36

Figure 2

Frumkin adsorption isotherms of 3b and 3c in 1M HCl at 60°C

Figure 3

Temkin adsorption isotherms of 3a–e in 0·5M H₂SO₄ at 60°C

Figure 4

Frumkin adsorption isotherms of 3a, 3c and 3e in 0·5M H₂SO₄ at 60°C

Table 5

Square of coefficient of correlation (R ²) and values of constants in adsorption isotherms in 1M HCl and 0·5M H₂SO₄ of Temkin, Langmuir and Frumkin

Compound	Frumkin	Temkin	Langmuir
Compound	R ², a	R ², f	R ²
1M HCl
3b	0·9910, 1·3	0·9774, 2·3	0·9712
3c	0·9869, 1·2	0·9685, 2·9	0·9785
0·5M H₂SO₄
3a	0·9843, −1·3	0·9855, 7·5	0·9938
3b	0·9675, −1·7	0·9939, 7·7	0·9888
3c	0·9943, −3·9	0·9944, 14	0·9637
3d	0·8978, −3·0	0·9668, 9·4	0·9692
3e	0·9840, −1·3	0·9909, 6·5	0·9908

The equilibrium constant of the adsorption process K _ads is related to the free energy of adsorption () by where 55·5 is the molar concentration of water in solution. The adsorption equilibrium constants K _ads and that were obtained from the Frumkin adsorption isotherms are summarised in Table 6.

Table 6

Adsorption equilibrium constant and free energy parameter of mild steel dissolution in presence of inhibitors at 60°C in 1M HCl and 0·5M H₂SO₄

Compound	K _ads	/kJ mol⁻¹	Isotherm used
1M HCl
3b	61 698	−41·7	Frumkin
3c	19 555	−38·5	Frumkin
0·5M H₂SO₄
3a	402 727	−46·9	Temkin
3b	424 127	−47·0	Temkin
3c	531 403 729	−66·7	Temkin
3d	1 838 885	−51·1	Temkin
3e	216 318	−45·1	Temkin

Discussion

The bisquaternary ammonium salts 3a–e were synthesised for the first time in excellent yields (Fig. 1). The objective of this work is to investigate the corrosion inhibition effect of the newly synthesised compounds on carbon steel in acidic medium.

All the salts used in this study were surface active and exhibited excellent IEs (Table 1). At an inhibitor concentration of 400 ppm, all the compounds exhibited IE of ∼99% in 1M HCl. Note that compounds 3d and 3e imparted IE of ∼94% even at a low concentration of 5 ppm. The molar masses of inhibitor molecules 3a–e are 792, 809, 706, 658 and 654 respectively; thus, a 100 ppm concentration of 3a–e translates into their respective molar concentrations of 1·26×10⁻⁴, 1·24×10⁻⁴, 1·42×10⁻⁴, 1·52×10⁻⁴ and 1·53×10⁻⁴M. Since the molar masses of the compounds do not vary widely, in fact they are very similar, the efficiency of the inhibitor molecules can be compared using concentration values either in ppm or molarity.

The compounds 3a–e were tested further for their inhibition activity in 4 and 7·7M HCl at 60°C and was found to be very good inhibitors at this higher acid strength. The lower IE at the higher end of the acid strengths (Table 1) may be attributed to the higher rate of desorption of the inhibitor molecules from the metal surface with the increase in acid concentration.37

Even though the gravimetric method is the more simple and reliable method for the determination of IE, the inhibitors were also subjected to electrochemical study for the purpose of comparison. The results presented in Table 2 revealed that the IE in the presence of 200 ppm, determined by the electrochemical method using Tafel plots, corroborated the results from gravimetric method. Table 2 and Fig. 5 showed that E _corr values in the presence of all the inhibitors shifted significantly in the more negative direction by 26-70 mV. However, such moderate shifts may not permit to classify the inhibition of corrosion solely under cathodic control. A displacement of OCP by at least 85 mV in relation to that measured for the blank solution is suggested to classify a compound as cathodic or anodic type inhibitor.38 Cathodic current densities in the presence of inhibitor are found to be significantly lower than in its absence, indicating that the cathodic hydrogen evolution reaction is mostly retarded. A moderate increase in the values of the Tafel slopes in most cases indicated that the inhibitors did affect the corrosion reactions to some extent. The most significant effect is assumed to be the formation of a barrier film that hindered the diffusion of ions to or from the metal surface. The above results along with the OCP shifts suggest that the studied compounds in HCl solution act as mixed type inhibitors under the predominance of cathodic control.

Figure 5

Potentiodynamic polarisation curves for mild steel in 1M HCl (blank) and 1M HCl containing 200 ppm of 3a–e at 60°C

Next, we focused our attention to study corrosion inhibition of 3b and 3c in 0·5M H₂SO₄ using gravimetric and Tafel extrapolation methods (Fig. 6 and Table 4). All the compounds showed good to very good IEs in the range 73-94%, as obtained from gravimetric measurements and Tafel extrapolations. There are not many compounds in the literature, which reports the excellent corrosion inhibition activity in both HCl and H₂SO₄ media. Diammonium salt 3c containing diallyl groups performed the best, indicating the beneficial role of the π electron rich allyl groups whose π electrons may interact with the d orbitals of iron or Fe²⁺ to provide a protective film.

Figure 6

Potentiodynamic polarisation curves for mild steel in 0·5M H₂SO₄ (blank) and 0·5M H₂SO₄ containing 400 ppm of 3b and 3c

The Tafel plots (Fig. 6 and Tables 3 and 4) in 0·5M H₂SO₄ revealed that the corrosion potentials (E _corr) have been shifted in the noble (less negative) direction with reference to the blank. The modest anodic shifts are indicative of the suppression of the anodic reaction as the main effect. However, lower values of the slopes of both branches indicate that corrosion reactions are not adversely affected in the presence of the inhibitors. The diffusion of ions to or from the metal surface is hindered owing to the formation of a barrier film. While the corrosion inhibition by these molecules was predominantly under cathodic control in 1M HCl, the inhibition in 0·5M H₂SO₄ was found to be under anodic control.

The adsorption of inhibitor molecules 3b and 3c in 1M HCl gave better fit in Frumkin isotherm than Temkin or Langmuir. However, the inhibitor molecules 3a–e followed the Temkin isotherm in 0·5M H₂SO₄, even though most of them also gave a good linear fit in Frumkin and Langmuir isotherms. A higher f value in 0·5M H₂SO₄ than in 1M HCl signifies stronger forces of repulsion between the positive charges on the nitrogens of the adsorbed and adsorbing molecules in H₂SO₄ medium. The rationale behind these findings could be attributed to the stronger adsorption of Cl⁻ on the metal surface, whereby the positive charges on the nitrogens could be shielded by the adsorbed Cl⁻, as shown by the formation of the species (FeCl⁻…Inhibitor⁺)_ad.14,15 The poor adsorbability of sulphate ions10,11 may not be able to screen the positive charges on the nitrogens in the same fashion, thereby leading to repulsion between the N⁺, as indicated by higher f values.

The positive value of the attraction constant ‘a’ for the inhibitor molecules in 1M HCl provides evidence of the mutual attraction between adsorbed organic molecules, presumably as a result of the van der Waals forces of attraction between the hydrophobic spacers linking the quaternary nitrogens.39,40 The negative values of ‘a’ in H₂SO₄, on the other hand, indicate mutual repulsion presumably among the positive nitrogens.

The spontaneity of the adsorption process is assured by the negative values of (Table 6) in acidic media. The values, as obtained by analysing the adsorption isotherms, were found to be ˜−40 kJ mol⁻¹ in HCl and −45 to −67 kJ mol⁻¹ in H₂SO₄. Since the values up to −20 kJ mol⁻¹ are consistent with the physisorption, while those between −80 and −400 kJ mol⁻¹ are associated with chemisorption, the experimental values, therefore, indicate that the adsorption mechanism of the inhibitor molecules on steel in 1M HCl and 0·5M H₂SO₄ solution was both electrostatic adsorption and chemisorption.41,42 The highest value of −67 kJ mol⁻¹ for the adsorption of 3c points towards the ability of π electrons of the diallyl moiety to facilitate the adsorption on anodic sites through the interaction of the electron pairs with the low lying vacant d orbitals of iron or Fe²⁺ to provide a protective film.

Conclusions

Bisquaternary ammonium salts 3a–e are prepared for the first time. The results are indeed very promising and pave the way to exploit the bisquaternary ammonium salts’ 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. points towards physisorption as the major contributor and chemisorption as the minor contributor for the adsorption of inhibitors on the metal surface. Among the compounds studied, 3c seems to be the most promising inhibitor molecule in 1-7·7M HCl as well as in 0·5M H₂SO₄. The presence of π electrons attached to the nitrogens helps adsorption on the anodic sites to provide a protective film.

Footnotes

Acknowledgements

Facilities provided by the King Fahd University of Petroleum and Minerals and the University of Dammam and financial assistance by King Abdulaziz City of Science and Technology (KACST) (grant no. AR-20-72) are gratefully acknowledged.

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