Quaternary aryl phosphonium salts as corrosion inhibitors for iron in HCl

Article abstract of DOI:10.1016/j.jallcom.2018.06.241

Cathodic and anodic Tafel extrapolation data for the corrosion inhibition of iron in deaerated 1 M HCl at 22 °C are obtained for five synthesized phosphonium salts, 4-tolyltriphenyl-phosphonium chloride (TTPPC), 4-phenoltriphenylphosphonium chloride (PTPP

Full text of DOI:10.1016/j.jallcom.2018.06.241

Accepted Manuscript
Quaternary aryl phosphonium salts as corrosion inhibitors for iron in HCl
A.H. Nahlé, T.J. Harvey, F.C. Walsh
PII:
S0925-8388(18)32362-4
10.1016/j.jallcom.2018.06.241
JALCOM 46577
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 30 January 2018
Revised Date: 13 June 2018
Accepted Date: 19 June 2018
Please cite this article as: A.H. Nahlé, T.J. Harvey, F.C. Walsh, Quaternary aryl phosphonium
salts as corrosion inhibitors for iron in HCl, Journal of Alloys and Compounds (2018), doi: 10.1016/
j.jallcom.2018.06.241.
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ACCEPTED MANUSCRIPT
Quaternary aryl phosphonium salts as corrosion inhibitors for iron in HCl
a,*
b
b,c
A.H. Nahlé , T.J. Harvey , F.C. Walsh
a
Department of Chemistry, College of Sciences, University of Sharjah, Sharjah, PO Box 27272,
United Arab Emirates
b
National Centre for Advanced Tribology at Southampton, Department of Mechanical
Engineering, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom
c
Electrochemical Engineering Laboratory and Materials Engineering Research Group,
Engineering Sciences, University of Southampton, Highfield, Southampton, SO17 1BJ, United
Kingdom
*
Corresponding author; Email: anahle@sharjah.ac.ae
Abstract
Cathodic and anodic Tafel extrapolation data for the corrosion inhibition of iron in deaerated 1
o
M HCl at 22 C are obtained for five synthesised phosphonium salts, 4-tolyltriphenyl-
phosphonium chloride (TTPPC), 4-phenoltriphenylphosphonium chloride (PTPPC), 4-
anilinetriphenylphosphonium bromide (ATPPB), 4-benzyl alcoholtriphenylphosphonium
chloride (BATPPC), 4-hlorophenyltriphenylphosphonium bromide (CPTPPB), and two
commercially available phosphonium salts, tetraphenylphosphonium bromide (TPPB) and
methyltriphenylphosphonium bromide (MTPPB). The inhibitor concentrations ranged from 1 ×
-7
-3
o
1
0 to 1 × 10 M. The inhibition of iron corrosion in 1 M HCl at 22 C was found to be in the
order TPPB > BATPPC > CPTPPB > PTPPC > MTPPB > TTPPC > ATPPB.
Keywords: polarisation, Tafel region, voltammetry.
(Approx. 8.500 words, 11 tables, 32 figures, 6 reactions/equations and 54 references).
1

ACCEPTED MANUSCRIPT
1
. Introduction
Corrosion of metals is a serious problem in many industries, including civil services such as
water and sewage supplies. Inhibitors are frequently used in order to prevent or minimize the
corrosion. Corrosion increases running costs and reduces plant efficiency, availability, and
product quality.
Most inhibitors are organic compounds containing polar groups by which the molecule can
strongly or specifically adsorbed on the metal surface constitute [1]. Such inhibitors, which
include the organic N, P, S, and OH groups, are known to be similar to catalytic poisons, since
they decrease the reaction rate at the metal/solution interface without, in general, being involved
in the reaction. It is generally accepted that most organic inhibitors act via adsorption at the
metal/solution interface [1]. The mechanism by which an inhibitor decreases the corrosion
current is achieved by interfering with some of the steps for the electrochemical process.
The inhibition of iron corrosion in aqueous acidic solutions has been widely investigated. In
many industries, hydrochloric acid solutions are used to remove scale and salts from steel
surfaces, cleaning tanks and pipelines. This treatment may be prerequisite for coating by
electroplating, galvanizing or painting techniques. The acid is treated with a soluble inhibitor to
prevent extensive dissolution of the underlying metal. Organic inhibitors are normally adsorbed
at the metal/solution interface by displacing water molecules and forming a compact barrier
film.
Quaternary phosphonium compounds are known to control the bacterial growth in several
systems [2]; they are environmentally friendly [3] and can offer relatively low toxicity [4].
2

ACCEPTED MANUSCRIPT
Many authors have synthesized and used various phosphorus‐containing compounds in their
corrosion inhibition investigations. These compounds have included imidazoline double
quaternary phosphonium salt [5-12], P,P'-bis (triphenylphosphonio) methyl benzophenone
dibromide [13], 4-vinylbenzyl triphenyl phosphonium chloride [14] aromatic phosphonium salts
[
15], (4-ethoxybenzyl)-triphenylphosphonium bromide [16], tetra butyl phosphonium hydroxide
and iodide [17], tetraphenyl phosphonium iodide [18], benzyl triphenyl phosphonium chloride
and halide ions [19], butyl triphenyl phosphonium bromide [20], (1,3-dioxolan-2-ylmethyl)-
triphenyl phosphonium bromide [21], allyl triphenyl phosphonium bromide [22], phosphonium
chloride derivatives [23], tetra-benzyl phosphonium chloride [24], phosphating , salts [25], d-
amino acid-derived ionic liquid [26], 1-(4-nitrophenyl)-5-amino-1H-tetrazole [27], and others
have used plant extracts as corrosion inhibitors in their investigation [28-32].
In such studies, the phosphorus atom(s) in the compounds were shown to be able to strongly
adsorb on the metal surface and form a protective layer which, in turn, increased the corrosion
inhibition at higher inhibitor concentration.
In this work, The synthesis, including the preparation of the tris(triphenylphosphine)nickel
(
Ni[P(C H ) ] ) catalyst and subsequent preparation of the quaternary aryl phosphonium salts, is
6 5 3 3
considered. In most cases, tetraphenylphosphonium chloride was used as a reference for
comparison purposes as its structure can be considered the parent of a family of phosphonium
salts. Electrochemical techniques are used to determine inhibitor properties of the synthesized
phosphonium salts and two commercially available phosphonium salts.
2
. Experimental details
3

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2
.1 Synthesis of quaternary aryl phosphonium salts
The synthesis of quaternary aryl phosphonium compounds involved the use of a nickel (0)
complex of triphenylphosphine, tris(triphenylphosphine) nickel, as shown in Figure 1.
As shown in reaction (1) [33], the nickel catalyst (2) undergoes oxidative addition in the
presence of aryl halide (1) to form the arylnickel complex (3). Oxidative addition has also been
reported for the synthesis of other arylnickel (0, II) complexes [34, 35].
P(C H )
6
5
3
R
X
+
Ni[P(C H ) ]
R
Ni
X
+
P(C H )
6 5 3
6
5
3
3
P(C H )
6
5
3
(1)
(1)
(2)
(3)
The arylnickel complex (3) formed in reaction (1) reacts with triphenylphosphine to give the
phosphonium salt (4) and regenerate the nickel catalyst (2) via reaction (2).
P(C H )
6
5
3
R
Ni
X
+
2P(C H )
P
RX
+ Ni[P(C H ) ]
6 5 3 3
6
5
3
P(C H )
6
5
3
(2)
(3)
(4)
The overall reaction can be summarized as:
Ni[P(C H ) ]
6
5 3 3
R
X
+
P(C H )
5 3
P
RX
6
(3)
4

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The preparation of tris(triphenylphosphine) nickel has been described in the literature [36] (yield
7
0%). It was reported to be extremely oxygen sensitive and tends to trap solvent when
precipitated from solution. It was also reported that the compound decomposes in solution over
several hours; even under rigorous oxygen-free conditions and on standing the deep-red solution
darkens and a black precipitate was formed. From the information above, it was decided that
isolation of the compound would be impractical and thus preparation of
tris(triphenylphosphine)nickel and its subsequent reaction with aryl halides was carried out in-
situ, under nitrogen inert gas blanket.
2
.2 Preparation of tris(triphenylphosphine)nickel catalysts
Anhydrous nickel chloride (0.0071 mol, 0.99 g) and triphenylphosphine (0.0304 mol, 8.0 g)
were placed into a 1000 mL quickfit conical flask and refluxed in acetonitrile (240 mL) for one
hour. The reaction mixture was allowed to cool to room temperature then zinc (0.0117 mol, 0.76
g) was added. The mixture was stirred under nitrogen overnight. A red-brown solid formed and
was subsequently used in-situ for the preparation of the phosphonium salt.
2
.3 Preparation of quaternary aryl phosphonium salts [37]
Ethanol (200 mL), triphenylphosphine (0.05 mol, 13.06 g) and p-aryl halides (0.05 mol) were
introduced to Ni[P(C H ) ] (0.005 mol, 4.23 g) whilst the apparatus was purged with nitrogen
6
5 3 3
gas. The boiling mixture was stirred for 24 hours. The solution was filtered and the filtrate
volume was reduced using a rotary evaporator (under reduced pressure). The remaining solids
were then dissolved in water and the solution was filtered to remove any residues. The resulting
solution was treated with diethyl ether to remove any non-ionic organics (mainly unreacted
triphenylphosphine). The aqueous solution was evaporated to dryness. The crude phosphonium
5

ACCEPTED MANUSCRIPT
salts were recrystallized from water (with the exception of 4-anilinetriphenylphosphonium,
o
which was recrystallized from methanol and dried in a vacuum oven at 70 C overnight). Some
of the synthesized inhibitors had restricted solubility in HCl. Table 1 lists the prepared and the
commercially available phosphonium salts which were evaluated by electrochemical techniques.
FAB-MS (fast atom bombardment-mass spectroscopy) were obtained on a Joel JMS-DX303
GC/Mass Spectrometer. Using fragmentation patterns, the structure of the organic molecule
could be elucidated (Figures 2-6). IR (infra-red) spectra of the synthesized compounds, using a
1
Mattson Instrument Research Series 1 FTIR spectrometer are shown in Figures (7-11). H NMR
spectra were obtained on a Joel GSX 270 MHz Fourier transform NMR spectrometer. Proton
NMR can reveal information about the protons and the environment they are in, by measurement
of the chemical shifts (δ) and coupling constants (J) of each synthesized compound (Figures 12-
1
6). Melting points were obtained for each compound and the yields are shown in Table 2.
2
.4 Working electrode preparation
Iron metal (purity 99.5%) used was obtained, in sheet form, from Goodfellow. The 4 mm
diameter working electrode material was punched from sheet material. The working electrode
surface was prepared initially with silicon carbide paper, grades 200 to 1200 and followed by
polishing with progressively finer grades of alumina slurry (grades 5 µm down to 1 µm) with
absolute alcohol to achieve a reproducible surface finish and rinsed with absolute alcohol before
use.
2
.5 Electrolyte preparation
6

ACCEPTED MANUSCRIPT
A 1M HCl stock solution was made up in 2 L volumes from concentrated HCl (Fisons).
Inhibited solutions were prepared by dissolution of the phosphonium salts (Table 1) to produce
-3
5
00 mL of 1×10 M solutions. Subsequent dilution by a factor of 10 allowed a range of
--7
-3
solutions from 1×10 to 1×10 M to be prepared. All studies was carried out at a temperature
o
of 22 C using solutions which had been deoxygenated with a fast stream of nitrogen gas for 25
minutes prior to experiments.
2
.6 Equipment
o
Tafel extrapolation data for the inhibition of iron in deaerated 1 M HCl at 22 C are considered
for the five synthesised and two commercially available phosphonium salts solutions at
--7
-3
concentrations ranging from 1×10 to 1×10 M.
All electrochemical measurements were performed using an EG & G potentiostat / galvanostat,
model 273A, interfaced (via the computer's RS232 port) by a National Instruments GPIB-PCII
interface board, running NI-488.2 software, to a Viglen PC, model Genie Executive 3SX25
(386SX with a clock speed of 25 MHz and a maths co-processor).
A three electrode cell was used, with a working electrode consisting of an iron disc of circular
2
configuration of 4 mm diameter (geometric surface area 0.1257 cm ); a saturated calomel
reference electrode (SCE); and a platinum foil counter electrode with a surface area of 3.4 cm2.
Tafel extrapolation measurements were performed over a polarisation range of ±200 mV each
-1
side of the corrosion potential, Ecor. at a linear potential sweep rate of 0.050 mV s and an initial
delay of 7 minutes preceeded the experiments. Experiments were usually performed in triplicate
to ensure reproducibility of the results.
7

ACCEPTED MANUSCRIPT
3
3
. Results and discussion
.1 Tafel extrapolation data for iron in HCl
o
Tafel extrapolation data for the inhibition of iron in deaerated 1 M HCl at 22 C were considered
for the five synthesized and two commercially available phosphonium salts solutions at
-7
-3
concentrations ranging from 1×10 to 1×10 M.
Each of the compounds tested by electrochemical techniques will be displayed as a table
showing the mean values for corrosion current density (j ), inhibitor efficiency (%τ) and
cor
corrosion potential (E ) versus the saturated calomel reference electrode (SCE) together with
cor.
anodic and cathodic Tafel slopes for each of the inhibitor concentrations tested. The percentage
inhibition efficiency (%τ) was calculated according to the following equation:
(
ꢁ
)
ꢉꢀ(ꢁ
)
ꢂꢃꢄ ꢅꢆꢇꢆꢈ.
ꢂꢃꢄ ꢇꢆꢈ.
%
τ =ꢀ
ꢀ× 100
(4)
(
ꢁ
)
ꢂꢃꢄ ꢅꢆꢇꢆꢈ.
where %
τ
= percentage inhibitor efficiency, (ꢊ
)
= corrosion current density in
ꢋꢌꢍ ꢎꢏꢐꢏꢑ.
uninhibited solution, and (ꢊ
)
= corrosion current density in inhibited solution.
ꢋꢌꢍꢍ ꢐꢏꢑ.
The effects of each compound are shown in a figure representing a typical Tafel experiment that
most closely depicts the mean of the triplicate experimental results. Each figure has an insert
showing the values of corrosion current density and inhibitor efficiency for each inhibitor
concentration tested.
8

ACCEPTED MANUSCRIPT
3
.2 4-Anilinetriphenylphosphonium bromide (ATPPB)
-3
High inhibition is achieved at the highest inhibitor concentration of 1×10 M with inhibition at
1.7% (Figure 17), with more extensive data in Table 3. Inhibition dropped rapidly as inhibitor
9
-5
concentration lowered while below 1×10 M, the corrosion rate is increased compared to the
uninhibited iron. The value of corrosion potential moves in a more positive direction as the
concentration of inhibitor increases. This is indicative of predominant anodic control. This
behaviour at high inhibitor concentrations was explained by Hoar and Holliday [37] and by
West [38] for a predominately blocking mode of inhibition. At high surface coverages (near
monolayer) any gaps due to irregular packing, will be small and because of the relative size of
the hydrated hydrogen and metal ions, anodic inhibition will always predominate. The cathodic
Tafel slopes remain near the value of those obtained for uninhibited conditions until the
-5
concentration drops below 1×10 M when an increase is noted. For the anodic Tafel slopes, the
values remain near 100 mV per decade with the exception of the highest concentration which is
somewhat higher at 136 mV per decade, possibly due to stacking of the adsorbed inhibitor at the
metal surface.
The first step in inhibition would be expected to be protonation of the aniline group to form an
anilinium ion which can be electrostatically adsorbed at the metal surface. This was considered
o
by Banerjee and Malhotra [39] who studied the corrosion of mild steel in HCl at 30 C inhibited
by aniline. This may help elucidate the reasons for corrosion acceleration at inhibitor
-5
concentrations of 1×10 M and below; the increase in cathodic Tafel slopes could indicate
cathodic depolarisation as the protonated anilinium ion may catalyse the cathodic reaction. This
is similar to the mechanism suggested by Davolio and Soragni [40] who reported corrosion
acceleration of carbon steel in 1 M sulphuric acid at 30 oC in the presence of
9

ACCEPTED MANUSCRIPT
phenylmethylsulphoxide. A catalytic effect has been observed for other nitrogen-based
inhibitors on the hydrogen evolution reaction for zinc and zinc alloy corrosion in HCl. Similar
behaviour has been observed for protonated quinolines [41] and amines [42]. Foroulis [43]
observed stimulation of copper corrosion for aniline and n-butylamine in 4.2 M sulphuric acid
o
and aniline and pyridine in 2.6 M perchloric acid at 50 C.
3
.3 4-Benzyl alcoholtriphenylphosphonium chloride (BATPPC)
-3
-4
At the highest concentrations of 1×10 and 1×10 M (Figure 18), this compound exhibits high
inhibition with values of 94.7% and 89.5% respectively as shown in Table 4 which summarises
the results for this inhibitor. However, inhibition drops rapidly as the inhibitor concentration
-6
-5
decreases. Between the concentrations of 1×10 and 1×10 M an inflection is observed. The
most likely explanation for this effect is a change in the mode or configuration (or both) of
adsorption. This has been observed previously by Hackerman et al. [44] who studied
o
homopiperazine adsorption on iron in 6 M HCl at 25 C. A change in configuration might
present itself as the adsorption of a single phenyl ring at high inhibitor concentrations where
packing is compacted but, at low inhibitor concentrations, the possibility of a second phenyl ring
adsorbing is maybe due to the low covering of inhibitor molecules at the metal surface. This
could be initiated through the aliphatic alcohol group via the lone-pair on the oxygen atom or
protonation of the alcohol group. The corrosion potential is polarised in the more positive
direction compared to uninhibited values. Inhibition is anodic at high inhibitor concentrations
indicating a predominately blocking mode of inhibition. The anodic Tafel slopes for iron
dissolution as ferrous ions remained close to those obtained for uninhibited conditions until the
-5
concentration drops below 1×10 M when deviations are noted. The values of cathodic Tafel
1
0

ACCEPTED MANUSCRIPT
slopes for hydrogen evolution remained near -145 mV per decade with the exception of the 1×
-6
1
0 M solution which gave somewhat higher values of -172 mV per decade.
3
.4 4-Chlorophenyltriphenylphosphonium bromide (CPTPPB)
The results of Tafel analysis are shown in Table 5 and Figure 19 for this inhibitor. At the highest
-3
-4
concentrations of 1×10 and 1×10 M, this compound exhibited a high inhibition with values of
5.6% and 92.7%, respectively being achieved. However, the degree of inhibition dropped
rapidly as the inhibitor concentration decreased. A promotion of corrosion was experienced at 1
9
-5
-7
×
10 M (%τ = -149.4%) and 1×10 M (%τ = -157.8%), as previously encountered at low
inhibitor concentrations [45-47]. The corrosion potential remained almost constant over the
entire range of inhibitor concentration near uninhibited acid values indicating mixed inhibition.
The absence of anodic inhibition at high inhibitor concentrations would suggest that blocking is
not the important inhibition mode. The cathodic Tafel slopes exhibited a decrease with inhibitor
concentration. This may indicate a change in cathodic mechanism with the increase of inhibitor
concentration. However, the anodic Tafel slopes showed the opposite effect, suggesting that the
anodic reaction became more difficult. Such complex behaviour is often experienced due to the
diversity of adsorption modes, which can involve electrostatic interaction of charged inhibitor
species with oppositely charged local surface sites, delocalised
π-electron in the inhibitor, d-
electrons in the outer metal surface shell, molecular polarizability, inductive and steric effects.
3
.5 Methyltriphenylphosphonium bromide (MTPPB)
The plot of inhibitor efficiency against inhibitor concentration (Figure 20 and Table 6) showed
-3
-4
-5
-6
two steps at 1×10 to 1×10 M and at 1×10 to 1×10 M. Acceleration of corrosion was
-7
achieved at inhibitor concentration of 1×10 M (signified by a negative inhibitor efficiency, %
τ
1
1

ACCEPTED MANUSCRIPT
=
-33.7%). The corrosion potential is polarised 20 mV more positive compared to uninhibited
values for the highest concentration and shifts to approximately 15 mV basic compared to
-4
uninhibited values at concentrations < 1×10 M. The anodic Tafel slopes remained near the
value of that obtained for uninhibited conditions with the exception of the 1×10-3
M
concentration which is 50 mV per decade higher. The cathodic Tafel slopes values appear to
oscillate around the value obtained for that in uninhibited conditions. The only close literature
comparison appears to be due to Allen et al [48], who reported an inhibitor efficiency of 98.9%
-3
o
at an inhibitor concentration of 1×10 M for carbon-steel in 5.7 M HCl at 80 C using weight
loss and solution analysis techniques.
3
.6 4-Phenoltriphenylphosphonium chloride (PTPPC)
Good inhibition was observed at the two highest concentrations with inhibitor efficiencies of
5.0% and 89.1% as illustrated in Table 7 and Figure 21. Below these concentrations, however,
9
inhibitor efficiency dropped close to zero. The two highest concentrations exhibited corrosion
potentials near those obtained for uninhibited solutions to within 1 mV. Concentrations that have
inhibitor efficiencies approximately zero are polarised at less than 10 mV more positive
compared to the uninhibited value. The anodic and cathodic slopes for all concentrations, except
-3
1
×10 M, showed large deviations compared to uninhibited values, the shift indicating strong
adsorption of inhibitor molecules at the metal surface.
Figure 22 shows the possible resonance structures of 4-phenoltriphenylphosphonium chloride
and indicates the initiation of electrostatic adsorption or the protonated phenol group, stabilized
by similar resonance structures to the above examples that can offer an alternative initiation step.
The resonance structures shown in Figure 22 could indicate the loss of inhibition at
1
2

ACCEPTED MANUSCRIPT
-5
concentrations of 1×10 M and below. Cathodic splitting, as explained by Horner et al. [49] in
reaction (5), could produce a phosphine containing the phenol ring, Ph Ph(OH)P, by attack of
2
the phosphorus-carbon bond. The subsequent phosphine might accelerate corrosion, nullifying
the effect of the phosphonium parent, probably through interaction with the corrosion
intermediate. The Tafel slopes have increased significantly above the values for inhibited
concentrations. An alternative reason for acceleration of corrosion might arise via an analogy to
the 4-aniline system. The phenol group can protonate similar to the aniline group, although
much less readily. The resulting protonated phenol ion may then catalyse the cathodic reaction
through cathodic depolarisation.
−
−
+
+
e
.
.
e , H
R PR'   → R PR'  →R P + R'    →R P + R' H
(5)
3
3
3
3
3
.7 Tetraphenylphosphonium bromide (TPPB)
Inhibition is achieved over the entire concentration range as shown in Table 8 and Figure 23.
The corrosion potential shows little variation throughout the concentration range but all values
are approximately 15-20 mV more positive than that in uninhibited solution. The cathodic slopes
show only minor variations but remain slightly smaller when compared to the uninhibited
solution. However, the anodic slope is 20 mV lower than uninhibited solutions at the lower
concentrations and rise rapidly to 201 mV per decade at the highest concentration. The only
literature comparison is attributable to Pagetti et al. [50] and Troquet and Pagetti [51] who
-3
reported an inhibitor efficiency of 75% at an inhibitor concentration of 1×10 M for Armco iron
o
in 1 M HCl at 20 C using Tafel analysis. The only secondary inhibitor for TPPB is
triphenylphosphine [51-53] due to cathodic splitting.
1
3

ACCEPTED MANUSCRIPT
3
.8 4-Tolyltriphenylphosphonium chloride (TTPPC)
From Table 9 and Figure 24, this compound exhibits a high degree of inhibition with a value of
6.4% being achieved at maximum concentration. However, the degree of inhibition drops
9
-6
rapidly as the inhibitor concentration decreases, until acceleration occurs at 1×10 M (%τ = -
-7
1
93.6%) and 1×10 M (%τ = -150.2%). The two concentrations that exhibited high inhibitions
have corrosion potentials near uninhibited ones but the lower concentrations have slightly more
positive values. The cathodic Tafel slopes for the three lower concentrations are similar to
uninhibited one and show a decrease at higher concentrations. The anodic Tafel slopes remain
similar to those for uninhibited conditions, with the exception of the highest concentration which
is 84 mV per decade higher. Adsorption of the para-substituted ring is likely to be enhanced due
to the hyperconjugation effect (Figure 25) previously proposed by Ayers and Hackerman [54]
who studied the effect of methyl substitution on the pyridine ring using 2-, 3-, and 4-picoline
o
inhibition of iron in 6.08 M HCl at 35 C.
A comparison was made among the seven inhibitors and the results are summarised in Table 10.
TPPB gives the best overall inhibition throughout the concentration range tested, as shown in
Figure 26 and Table 10, although TTPPC gives the highest individual inhibition of 96.4% at the
-3
1
×10 M level. BATPPC was the only other inhibitor to function over the entire concentration
range studied. The other five inhibitors accelerated the corrosion rate of iron in 1 M HCl at low
concentration levels. These five did give some degree of corrosion protection at the two highest
concentration levels (%τ = 75 - 97%).
1
4

ACCEPTED MANUSCRIPT
ATPPB produces a linear relationship between corrosion inhibition (Figure 26) and current
density (Figure 27) vs. inhibitor concentration despite this compound accelerating corrosion at
low inhibitor levels. TPPB also shows a similar relationship but in this case inhibition is
observed over the entire concentration range. MTPPB exhibits the lowest inhibition at the
highest concentrations but shows a lower rate of decrease in inhibition compared to the
synthesized phosphonium salts. This would seem to indicate that the 4-substituted phenyl
compounds exhibit greater structural effects than replacement of a benzene ring. The two
features noted above, namely very good inhibition at high inhibitor concentration and a rapid
loss of inhibition, in some cases leading to corrosion acceleration are most prominent for the
para-substituted compounds. This infers that surface coverage and molecular packing are
important in the nature of these inhibitors at the iron surface. For BATPPC, the percentage
-3
inhibition increased, fell and rose once more to give 94% inhibition at 1×10 M. This may be
due to this compound undergoing a change in molecular configuration as inhibitor concentration
varies. At high inhibitor concentrations, where packing is compacted, singularly chemisorbed
phenyl ring is the predominant configuration. At low inhibitor concentrations, the possibility of
a second phenyl ring adsorbing might due to the low covering of inhibitor molecules at the metal
surface. Acceleration might be achieved via a catalytic pathway involving the para-substituted
ring or a decomposition product that is repelled by the layer of phosphonium ions at high
inhibitor concentrations. ATPPB is the poorest inhibitor of the compounds tested and
-5
acceleration of corrosion is achieved at inhibitor concentrations of 1×10 M and below. The
order of inhibition is TPPB > BATPPC > CPTPPB > PTPPC > MTPPB > TTPPC > ATPPB for
o
iron in 1M HCl at 22 C, by Tafel extrapolation.
1
5

ACCEPTED MANUSCRIPT
Figure 28 shows a Hoar-Holliday plot [37] of corrosion potential against the logarithm of
-2
corrosion current density. At corrosion current densities < 150 µA cm , the inhibitors can be
divided into two groups. The first consists of CPTPPB, TTPPC and PTPPC where Ecor. is within
5
mV of the uninhibited value. The second group comprises ATPPB, BATPPC, TPPB and
MTPPB which are polarised approximately 20 mV more positive. ATPPB appears to shows a
nearly linear relationship between corrosion potential and corrosion current density. The shape
and features of the curve show a similarity for this compound with those shown in Figures 24
and 26. For ATPPB, the mode of inhibition and acceleration are due to changes in the electrical
double layer. From the inclusion of an aniline group to the phosphonium salt another positive
centre is created due to protonation from electrolyte; counter ions must be present to neutralise
this charge. At high concentrations, the adsorption of the positive phosphonium ions causes the
charge of the double layer to decreases, which will decrease corrosion. At lower concentrations,
the majority of the phosphonium ion species is more widely distributed about the metal surface
during adsorption. Adsorption of the bromide, counter ion, is more likely as these are smaller
and can exert greater influence over surface charge, thus corrosion increases. The linear
relationship between potential and corrosion current density indicates that adsorption is mainly
through electrostatic adsorption. Chemisorption of a phenyl ring in the phosphonium ion is not
indicated to a significant degree. BATPPC shows linearity in Figure 28, despite the reversal in
the direction of change in corrosion current density as the inhibitor concentration is change
-6
-5
between 1×10 and 1×10 M. MTPPB is only the compound that demonstrates significant
cathodic characteristics as the corrosion potential is more basic than in the uninhibited solution.
As general rule, the cathodic Tafel slopes for all the phosphonium salts tested are shown in
Figure 29. The dotted line indicates the Tafel slope of uninhibited acid. At high concentrations
1
6

ACCEPTED MANUSCRIPT
the majority of the data points are lower than the dotted line, then move to within 10 mV of the
dotted line. A similar trend is shown for the anodic Tafel slopes in Figure 30, with the exception
that the data points at the highest concentration are all larger than the dotted line. There are two
exceptions in both cases, PTTPC shows values that are significantly higher than any other at the
lower concentrations; TPPB shows the opposite effect.
3
.9 Adsorption isotherms
o
Table 11 shows the adsorption isotherm data for the Inhibition of iron in 1M HCl at 22 C by
varous phosphonium salts, determined by Tafel extrapolation.
The Langmuir adsorption isotherm can be described by the following equation:
ꢕ
ꢙ
ꢒꢓꢔ
= log ꢗ + log ꢘ −ꢀꢚ.ꢛꢜꢛꢝꢞꢀ
(6)
ꢖꢉꢕ
where c = inhibitor concentration and
θ
= surface coverage; q and a are constants characteristic
of each system.
To show a relationship for the tested Langmuir adsorption isotherm the correlation coefficient of
apparent linear fit (determined by Least squares analysis ), r, was calculated for each curve and
is displayed adjacent to the appropriate legend. A value of r =1.00 represents an ideal straight
line; values close to 1.00 show the degree of linearity. For general purposes, ≥0.99 indicates a
very good correlation and values of ≥0.98 can be considered to show fair agreement.
An invariable test for Langmuir isotherm adsorption is a rectilinear plot of log
θ /1-θ against log
c, as illustrated by Figure 31. The correlation is poor; TPPB shows the highest correlation
1
7

ACCEPTED MANUSCRIPT
coefficient, of 0.977. Inhibitor adsorption is complex and rarely shows a close fit to simple
adsorption isotherm models.
For electrochemical impedance spectroscopy (EIS) studies, a PC controlled Autolab
PGSTAT128N Modular Potentiostat (electrochemical workstation) (supplied by Metrohm)
capable of driving currents up to ± 800 mA with an output potential across the cell of up to ± 10
V was used. EIS scans were performed at a sinusoidal excitation AC voltage of +10 mV (rms)
and the response was determined in the frequency range of 100 kHz–10 mHz with 50 points per
decade. A Nyquist plot for iron in 1.0 M HCl in the presence of various concentrations of
o
methyltriphenylphosphonium bromide (MTPPB) at 22 C is presented in Figure 32.
4
. Comments on the techniques used and the mechanism of corrosion inhibition
While a wide range of techniques can be deployed to study the inhibition of corrosion, several
comments may be made:
a) Imaging techniques, such as optical microscopy, scanning electron microscopy and atomic
force microscopy are useful in examining surface morphology but. In the present case, merely
conform that uniform corrosion takes place over the metal surface, inhibitors giving rise to less
pronounced etching at a given time. No quantitative information is provided on the rate of
corrosion.
b) Surface spectroscopy techniques, including infra-red and Raman spectroscopy can provide
useful information on surface adsorbed species, leading to mechanistic clues but only qualitative
data tends to be produced.
c) Established, classical techniques provide trusted data yet no mechanistic or rate information.
1
8

ACCEPTED MANUSCRIPT
Weight loss requires multiple replicate samples and becomes insensitive at high levels of
corrosion inhibition, hence low degrees of weight loss. Solution analysis can provide a useful
means of analysing a wide range of general corrosion rate but gives no information on the
electrochemistry or mechanism.
d) Electrochemical techniques are well developed and can provide information on quantitative
corrosion rates via the corrosion current density, provided adequate precautions are made in cell
design/operation and data analysis. Steady state direct current (dc) polarisation methods are
particulary well accepted. Adequate attention must be paid to minimising uncompensated ohmic
drop, using well-defined flow conditions and known dissolved oxygen levels, as in the present
experiments. Such methods can be used to examine cathodic and anodic (metal dissolution)
kinetics and, via Tafel extrapolation, corrosion current. However, Tafel kinetics take place over
a limited range of potential, due to the effects of mass transfer control and ohmic drop. Small
perturbation techniques, such as linear polarisation resistance, can be convenient and fast but can
be subject to application of unchecked software analysis and sometimes lack a preliminary
characterisation of the electrode reactions using wider potential range polarisation via
voltammetric sweep). In the present studies, we have used both linear dc polarisation and Tefel
extrapolation to obtain quantitative data. Small perturbation alternating current (ac) techniques,
such as electrochemical impedance spectroscopy have become increasing available, can
maintain the electrode potential near its open-circuit value and can be powerful tools for organic,
e.g., paint films but are subject to less confidence than dc and classical techniques, due to
complex data analysis and the need for software to rationalise behaviour via equivalent circuit
models.
The mechanism of corrosion inhibition is complex due to the number of (electronic, structural
and steric) chemical interactions between the primary and secondary inhibitor and the surface,
1
9

ACCEPTED MANUSCRIPT
participation of anodic and cathodic reactions and the complex adsorption of organic inhibitor
species, which can involve different coverage (rather than a single monolayer, stacking can take
place), several molecular layers (rather than one) and all the above being concentration- and
time-dependent. Hence, application of a simple adsorption isotherm, such as a Langmuir one,
over a wide concentration range is expected to present challenges.
In the case of phosphonium inhibition of transition metals in aqueous acidic electrolytes, we aim
to elaborate on the factors considered in this section in a forthcoming review.
5
. Conclusions
o
The order of inhibition for iron in 1 M HCl at 22 C is TPPB > BATPPC > CPTPPB > PTPPC >
MTPPB > TTPPC > ATPPB. Another noticeable trend is that Tafel extrapolation measurements
show that the two highest concentrations impart high inhibition (%τ = 75 - 97%), which is
-5
followed by a rapid fall at 1×10 M, indicating a different inhibition mechanism or mode of
action between high and low inhibitor concentration. CPTPPB, TTPPC and PTPPC showed
mixed inhibition, while ATPPB, BATPPC, TPPB and MTPPB primarily exhibited anodic
-5
inhibition. ATPPB showed corrosion acceleration at concentrations below 1×10 M, whereas
-6
corrosion acceleration occurred at concentrations lower than 1×10 M for both CPTPPB and
TTPPC. Langmuir isotherm adsorption studies showed that TPPB exhibits the highest
correlation coefficient (0.977) to a simple adsorption isotherm, compared to the other inhibitors.
2
0

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List of symbols
Symbol
Meaning
Anodic Tafel slope
Units
ba
bc
c
mV per decade
mV per decade
Cathodic Tafel slope
-1
Molar concentration
M = mol L
Ecor
j
Corrosion potential
V
-
2
2
Current density
A cm
A cm
ohm
ohm
-
jcor
Z
Corrosion current density
Real component of impedance
Imaginary component of impedance
Z’
Greek
%
τ
Percentage Inhibitor efficiency
Surface coverage
Dimensionless
Dimensionless
θ
2
1

ACCEPTED MANUSCRIPT
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Figure captions
Figure 1. Structure of tris(triphenylphosphine)nickel, Ni[P(C H ) ].
6
5 3
Figure 2. FAB-MS of 4-anilinetriphenylphosphonium bromide (ATPPB).
Figure 3. FAB-MS of 4-benzyl alcoholtriphenylphosphonium chloride (BATPPC).
Figure 4. FAB-MS of 4-chlorophenyltriphenylphosphonium bromide (CPTPPB).
Figure 5. FAB-MS of 4-phenoltriphenylphosphonium chloride (PTPPC).
Figure 6. FAB-MS of 4-tolyltriphenylphosphonium chloride (TTPPC).
Figure 7. Infra-red spectra of 4-anilinetriphenylphosphonium bromide (ATPPB) (KCl Disc).
Figure 8. Infra-red spectra of 4-benzyl alcoholtriphenylphosphonium chloride (BATPPC)
(KCl Disc).
Figure 9. Infra-red spectra of 4-chlorophenyltriphenylphosphonium bromide (CPTPPB) (KCl
Disc).
Figure 10. Infra-red spectra of 4-phenoltriphenylphosphonium chloride (PTPPC) (KCl Disc).
Figure 11. Infra-red spectra of 4-tolyltriphenylphosphonium chloride (TTPPC) (KCl Disc).
Figure 12. 1H NMR of 4-anilinetriphenylphosphonium bromide (ATPPB), Expansion of 8.0-
6.8 ppm Range.
Figure 13. 1H NMR of 4-benzyl alcoholtriphenyl-phosphonium chloride (BATPPC),
Expansion of 8.0-7.5 ppm Range.
Figure 14. 1H NMR of 4-chlorophenyltriphenyl-phosphonium chloride (CPTPPB), Expansion
of 8.0-7.4 ppm Range.
Figure 15. 1H NMR of 4-phenoltriphenylphosphonium chloride (PTPPC), Expansion of 8.0-7.3
ppm Range.
Figure 16. 1H NMR of 4-tolyltriphenylphosphonium chloride (TTPPC), Expansion of 7.4-8.0
ppm Range.
Figure 17. Tafel polarisation curves for 4-anilinetriphenylphosphonium bromide (ATPPB)
o
inhibition of iron in 1 M HCl at 22 C.
Figure 18. Tafel polarisation curves for 4-benzylalcoholtriphenylphosphonium chloride
o
(
BATPPC) inhibition of iron in 1 M HCl at 22 C.
Figure 19. Tafel polarisation curves for 4-chlorophenyltriphenylphosphonium bromide
o
(
CPTPPB) inhibition of iron in 1 M HCl at 22 C.
2
7

ACCEPTED MANUSCRIPT
Figure 20. Tafel polarisation curves for methyltriphenylphosphonium bromide (MTPPB)
o
inhibition of iron in 1 M HCl at 22 C.
Figure 21. Tafel polarisation curves for 4-phenoltriphenylphosphonium chloride (PTPPC)
o
inhibition of iron in 1 M HCl at 22 C.
Figure 22. Resonance structures of 4-phenoltriphenylphosphonium chloride (PTPPC).
Figure 23. Tafel polarisation curves for tetraphenylphosphonium bromide (TPPB) inhibition of
o
iron in 1 M HCl at 22 C.
Figure 24. Tafel curves for 4-tolyltriphenylphosphonium chloride (TTPPC) inhibition of iron in
o
1
M HCl at 22 C.
Figure 25. Resonance structures of 4-tolyltriphenylphosphonium chloride (TTPPC).
Figure 26. Variation in inhibitor efficiency with inhibitor concentration determined by Tafel
o
extrapolation for iron in 1 M HCl at 22 C inhibited by various phosphonium salts.
Figure 27. Variation in corrosion current density with inhibitor concentration determined by
-3
o
Tafel extrapolation for iron in 1 mol dm HCl at 22 C inhibited by various
phosphonium salts.
Figure 28. Variation in corrosion potential with corrosion current determined by Tafel
o
extrapolation for iron in 1 M HCl at 22 C in the presence of various phosphonium
salts.
Figure 29. Variation in anodic Tafel slope with inhibitor concentration determined by Tafel
o
extrapolation for iron in 1 M HCl at 22 C inhibited by various phosphonium salts
(Uninhibited data are shown by dotted lines).
Figure 30. Variation in cathodic Tafel slope with inhibitor concentration determined by Tafel
o
extrapolation for iron in 1 M HCl at 22 C inhibited by various phosphonium salts.
(Uninhibited data are shown by dotted lines).
o
Figure 31. Langmuir isotherm plot for iron in 1 M HCl at 22 C inhibited by various
phosphonium salts, determined by Tafel extrapolation (the number indicates the
correlation coefficient of apparent linear fit).
Figure 32. A Nyquist plot for iron in 1 M HCl and in the presence of various concentrations of
o
methyltriphenylphosphonium bromide (MTPPB) at 22 C.
2
8

ACCEPTED MANUSCRIPT
Phosphonium salt
Structure
4
-anilinetriphenylphosphonium
bromide (ATPPB)
Ρ
NH , Br
2
(Synthesised)
4
-benzyl alcoholtriphenylphosphonium
chloride (BATPPC)
Ρ
CH
OH, Cl
2
(Synthesised)
4
-Chlorophenyltriphenylphosphonium
Ρ
Cl , Br
bromide (CPTPPB)
(Synthesised)
Methyltriphenylphosphonium bromide
MTPPB)
Commercially available)
(
P
CH
, Br
3
(
4
-phenoltriphenylphosphonium
chloride (PTPPC)
Ρ
OH , Cl
(Synthesised)
Tetraphenylphosphonium bromide
TPPB)
Commercially available)
(
P
Br
(
4
-tolyltriphenyl-phosphonium chloride
TTPPC)
Synthesised)
(
P
CH , Cl
3
(
Table 1. Synthesized and commercially available phosphonium salts evaluated by
electrochemical techniques in this paper.