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Materials Chemistry and Physics 106 (2007) 387–393

The synergistic effect of polyacrylamide and iodide ions

on the corrosion inhibition of mild steel in H2SO4
S.A. Umoren a,∗ , E.E. Ebenso b
a
Department of Chemistry, Faculty of Science, University of Uyo, P.M.B. 1017, Uyo, Nigeria
b Physical Chemistry Unit, Department of Pure and Applied Chemistry, University of Calabar, P.M.B. 1115, Calabar, Nigeria
Received 16 December 2006; received in revised form 19 May 2007; accepted 13 June 2007

Abstract
The corrosion inhibition of mild steel in 1 M H2 SO4 using polyacrylamide (PA) in the presence of iodide ions was studied at 30–60 ◦ C using
weight loss and hydrogen evolution methods. Results obtained showed that inhibition efficiency increased with increase in concentration of PA
and decreased with increase in temperature. The inhibition efficiency of PA synergistically increased on addition of KI. The adsorption of PA alone
and in combination with iodide ions on the metal surface is found to obey Freundlich, Temkin and Flory–Huggins adsorption isotherms at all
◦
temperatures studied. Phenomenon of physical adsorption is proposed from the values of Ea and Gads obtained. Synergism parameter evaluated
is found to be greater than unity for all concentrations of PA indicating that the enhanced inhibition efficiency of PA caused by addition of iodide
ion is only due to synergism. Adsorption of PA and (PA + KI) on to mild steel surface is spontaneous.
© 2007 Elsevier B.V. All rights reserved.

Keywords: Polyacrylamide; Synergistic effect; Iodide ions; Adsorption isotherms; Mild steel; Corrosion

1. Introduction eter (S1 ) calculated from inhibition efficiency was found to be

greater than unity, suggesting that the phenomenon of syner-
The corrosion behaviour of mild steel has been extensively gism exists between P4VP and iodide ions. Wu et al. [19] and
studied in acid media. For H2 SO4 solution containing any Schweinsberg et al. [20] have also reported that synergistic
organic compounds, addition of halide salts results in syner- effect exists when benzotriazole and 1-[11,21 dicarboxyl ethyl]-
gistic effect thereby inhibiting iron corrosion [1,2]. In acid benzotriazole, respectively, were combined with iodide ions on
solution, halides are known both to stimulate and inhibit cor- the dissolution of copper in aerated H2 SO4 . Also reported is the
rosion. Previous reports [1–16] have shown that the inhibiting synergistic influence of aminopyrimidine derivatives [21] and
effect of halide ions in combination with organic compounds 2-mecarpto benzimidazole [22] and iodide ions in the control of
in acidic medium increases in the order I− > Br− > Cl− which corrosion of carbon steel in H2 SO4 , respectively.
seems to indicate that the radii (which increases in the order Literature available to date has revealed that the synergistic
Cl− < Br− < I− ) and the electronegativity (which increases effect of halide ions generally and iodide ions in particular using
in the order Cl− > Br− > I− ) of the halogens may have an polymers as corrosion inhibitors of metals in aggressive media is
important role to play in the adsorption hence inhibition pro- very scanty. In our earlier publications [3–5], we have reported
cess. the effects of halide ions on the corrosion inhibition of metals
Some authors have reported synergistic inhibition between in acidic medium using polymers. The present work is another
iodide ions and some compounds including polymers. For attempt, in furtherance aimed at investigating the synergistic
instance, Larabi and Harek [17] and Larabi et al. [18] have inhibition between polyacrylamide (PA) and iodide ions on the
reported that addition of iodide ions to poly(4-vinyl pyri- corrosion effect of mild steel in H2 SO4 and to propose a suitable
dine) (P4VP) in 0.5 M H2 SO4 and 1 M HCl, respectively, mechanism for the synergistic inhibitory action.
enhanced inhibition efficiency significantly. Synergistic param-
2. Experimental method

∗ Corresponding author. The mild steel of the following composition (wt.%), C (0.19), Si (0.26),
E-mail address: saviourumoren@yahoo.com (S.A. Umoren). Mn (0.64), S (0.05), P (0.06), Ni (0.09), Cr (0.08), Mo (0.02), Cu (0.27),

0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2007.06.018
388 S.A. Umoren, E.E. Ebenso / Materials Chemistry and Physics 106 (2007) 387–393

and the remainder iron (Fe) was used in the study. The metal was prepared obtained from Eq. (2):
as previously reported [3–5]. The corrodent concentration used was 1 M  
H2 SO4 (BDH Chemical Supplies Laboratory, England). Polyacrylamide (PA) Wi
surface coverage, θ = 1− (3)
(Hi-tek Polymers, Japan) [Mn = 50,000 g mol−1 ] was used as inhibitor in the W0
concentration range of 2 × 10−5 to 1 × 10−4 M. The halide salt, potassium
iodide (KI, BDH) used was in the concentration range 0.02–0.10 M. However, Results shown in the table revealed that corrosion rates
0.06 M KI was used for the synergistic study. increase with increase in temperature for all the systems stud-
The apparatus and procedure followed for weight loss and hydrogen evolu- ied. The highest corrosion rate (11.0 × 10−3 mpy) was obtained
tion methods were similar to that earlier reported [3–5,23]. In the weight loss
at 60 ◦ C. In the presence of PA and 0.06 M KI, the corro-
method, the progress of the corrosion reaction was monitored by determining
the weight loss of the coupons (obtained as the differences in the weight of the sion rate was observed to reduce significantly indicating that
coupons after immersion in different solutions of the system and the original PA actually inhibited the corrosion of mild steel in the acidic
weight of the coupons) and careful measurement of the volume of hydrogen gas environment. Further reduction in corrosion rate was observed
evolved for weight loss and hydrogen evolution methods, respectively, at fixed on addition of 0.06 M KI to PA. The reduction in corrosion
time intervals. In both techniques, the experiments were conducted at 30–60 ◦ C
rate of PA when combined with iodide ions was found to be
in a thermostated bath.
concentration dependent. Fig. 1 shows the plot of inhibition effi-
ciency against concentration for (a) PA and (b) KI at different
3. Results and discussion temperatures. Inspection of the figures revealed that inhibition
efficiency increase with increase in concentration of the inhibitor
3.1. Weight loss measurements and decrease with increase in temperature. Decrease in inhibi-
tion efficiency with increase in temperature may be attributed
The corrosion of mild steel in 1 M H2 SO4 in the absence to increase in the solubility of the protective films and of any
and presence of KI, PA and PA–KI mixtures was investigated reaction products precipitated on the surface of the metal that
at temperature range of 30–60 ◦ C using weight loss measure- may otherwise inhibit the reaction rate. From Table 1, it is
ments. The calculated values of corrosion rate (mpy), inhibition clearly seen that inhibition efficiency synergistically increase
efficiency (%I) and surface coverage (θ) for mild steel corrosion on addition of 0.06 M KI to PA at all the temperatures stud-
in 1 M H2 SO4 (blank) and in the presence of inhibitor (PA), ied with the highest inhibition efficiency (69.0%) obtained for
0.06 M KI and 0.06 M KI in combination with different concen- 0.06 M KI–10 × 10−5 M PA mixture at 30 ◦ C. Results obtained
trations of PA at 30–60 ◦ C from the weight loss measurement in this present study are in agreement with what has been earlier
are shown in Table 1. The corrosion rate, inhibition effi- reported by other authors [13–18]. One possible mechanism of
ciency and surface coverage were evaluated using the following inhibition action of inhibitors is the adsorption of the inhibitor
equations: onto the metal surface which blocks the metal surface and this
do not permit the corrosion process to take place. The adsorp-
534W
corrosion rate (mpy) = (1) tion process is made possible due to the presence of heteroatoms
ρAt such as oxygen, nitrogen and sulphur atoms, which are regarded
where W is the weight loss (g), ρ the density of the mild steel as centres of adsorption.
coupon (g cm−3 ), A the area of the coupon (cm2 ) and t is the Polyacrylamide contains nitrogen and oxygen atoms in its
exposure time (h). structure having lone pair and ␲-electrons. The compound could
be adsorbed by the interaction between the lone pair of the elec-
 
Wi trons of the oxygen and nitrogen atoms, respectively, on the PA
Inhibition efficiency I(%) = 1 − 100 (2) moiety and mild steel surface. This process may be facilitated
W0
by the presence of d-vacant orbital of iron making the steel,
where W0 and Wi are the weight losses of mild steel in inhibited as observed in d-group metals or transition metals. In addition
and uninhibited solution. The degree of surface coverage (θ) was to molecular form, PA can be present in protonated species in

Table 1
Calculated values of corrosion rate (mpy), inhibition efficiency (%I) and degree of surface coverage for mild steel corrosion in 1 M H2 SO4 for different systems at
30–60 ◦ C from weight loss measurements
Systems/concentration Corrosion rate (mpy) × 10−3 , inhibition efficiency (%I) and degree of surface coverage (θ)

30 ◦ C 40 ◦ C 50 ◦ C 60 ◦ C

Blank 8.6 – – 9.4 – – 10.0 – – 11.0 – –

0.06 M KI 3.3a 62.4b 0.62c 3.9a 58.0b 0.58c 4.9a 51.0b 0.51c 6.5a 39.0b 0.39c
PA (inhibitor) 3.5 58.1 0.58 5.3 44.1 0.44 5.7 43.8 0.44 6.7 37.2 0.37
2 × 10−5 M PA + 0.06 M KI 3.3 63.2 0.63 3.5 61.2 0.61 4.5 55.3 0.55 6.1 43.0 0.43
4 × 10−5 M PA + 0.06 M KI 3.2 65.0 0.65 3.3 62.4 0.62 4.3 57.0 0.57 6.0 44.1 0.44
6 × 10−5 M PA + 0.06 M KI 3.0 66.1 0.66 3.2 65.1 0.65 4.2 58.0 0.58 5.8 46.0 0.46
8 × 10−5 M PA + 0.06 M KI 2.9 68.4 0.68 3.0 65.4 0.65 4.0 61.0 0.61 5.5 49.0 0.49
1 × 10−4 M PA + 0.06 M KI 2.7 69.0 0.69 2.9 67.4 0.67 3.8 62.0 0.62 5.3 51.0 0.51

Notes: a = corrosion rate, b = inhibition efficiency, and c = degree of surface coverage.

 S.A. Umoren, E.E. Ebenso / Materials Chemistry and Physics 106 (2007) 387–393 389

of the halogens may have an important role to play in the

adsorption. The observed synergistic effect between PA and
KI may be attributed to the interaction between physically
adsorbed iodide ions and the polycation of PA, which leads to
greater degree of surface coverage and higher inhibition effi-
ciency.

3.2. Hydrogen evolution measurements

The corrosion of mild steel in 1 M H2 SO4 in the absence

and presence of PA, KI and PA–KI mixtures at different tem-
peratures was also investigated using the hydrogen evolution
measurements. The corrosion rates of mild steel in the absence
and presence of the additives were assessed by monitoring the
volume of hydrogen gas evolved. The calculated values of cor-
rosion rates are presented in Table 2. It is seen in the table that
corrosion rate increased with rise in temperature and decreased
in the presence of the additives (PA, KI and PA + KI).
Inhibition efficiency (%I) and the degree of surface coverage
(θ) from the hydrogen evolution measurements were obtained
using Eqs. (4) and (5), respectively:
 
VHt1
inhibition efficiency I(%) = 1 − 0 100 (4)
VHt
 
V1
surface coverage (θ) = 1 − Ht (5)
VHt
0

Fig. 1. Plot of inhibition efficiency against concentration for mild steel corrosion The results obtained are presented in Table 2. The data
in 1 M H2 SO4 in the presence of (a) PA and (b) KI at different temperatures. presented follows the same trend as observed for weight loss
technique. Inhibition efficiency decreased with rise in temper-
ature and increased with increasing concentration of PA in
an acidic solution. The formation of positively charged proto- combination with 0.06 M KI.
nated species facilitates adsorption of the compound on the metal In comparison with the weight loss measurements, the values
surface through electrostatic interaction between the organic of inhibition efficiency and surface coverage obtained from the
molecules and the metal surface [24,25]. Previous reports [1–16] hydrogen evolution method are lower. This can be attributed to
have shown that the inhibiting effect of halide ions in com- the difference in time required to form an adsorbed layer of the
bination with organic compounds in acidic medium increases inhibitor on the metal surface that can inhibit corrosion. More so,
in the order I− > Br− > Cl− which seems to indicate that the the weight loss method gives average corrosion rates whereas the
radii (which increases in the order Cl− < Br− < I− ) and the hydrogen evolution technique gives more or less instantaneous
electronegativity (which increases in the order Cl− > Br− > I− ) corrosion rates.

Table 2
Calculated values of corrosion rate (cm s−1 ), inhibition efficiency (%I) and degree of surface coverage for mild steel corrosion in 1 M H2 SO4 for different systems
at 30–60 ◦ C from hydrogen evolution measurements
Systems/concentration Corrosion rate (ml s−1 ) × 10−3 , inhibition efficiency (%I) and degree of surface coverage (θ)

30 ◦ C 40 ◦ C 50 ◦ C 60 ◦ C

Blank 4.3 – – 5.07 – – 6.60 – – 8.23 – –

0.06 M KI 2.07a 51.9b 0.52c 2.72a 46.4b 0.46c 3.80a 32.6b 0.33c 6.53a 20.80b 0.21c
PA (inhibitor) 2.15 50.0 0.50 2.97 42.8 0.43 4.45 42.4 0.42 5.67 31.3 0.31
2 × 10−5 M PA + 0.06 M KI 2.42 43.8 0.44 3.53 30.3 0.30 4.83 26.8 0.27 6.27 24.0 0.24
4 × 10−5 M PA + 0.06 M KI 2.18 49.2 0.49 2.98 41.1 0.41 4.03 38.9 0.39 5.92 28.3 0.28
6 × 10−5 M PA + 0.06 M KI 2.08 51.6 0.52 2.60 48.7 0.49 3.67 44.4 0.44 5.00 39.4 0.39
8 × 10−5 M PA + 0.06 M KI 1.78 58.5 0.59 2.25 55.6 0.56 3.62 45.2 0.45 4.90 40.6 0.41
1 × 10−4 M PA + 0.06 M KI 1.55 63.5 0.64 2.08 58.9 0.59 3.20 51.5 0.52 3.65 49.1 0.49

Notes: a = corrosion rate, b = inhibition efficiency, and c = degree of surface coverage.

390 S.A. Umoren, E.E. Ebenso / Materials Chemistry and Physics 106 (2007) 387–393

Fig. 3. Temkin adsorption isotherm plot for mild steel corrosion in 1 M H2 SO4
Fig. 2. Freundlich adsorption isotherm plot for mild steel corrosion in 1 M
in the presence of KI (inset), PA and PA + KI at 30 ◦ C.
H2 SO4 in the presence of KI (inset), PA and PA + KI at 30 ◦ C.

given by [14,26]:
3.3. Adsorption considerations  
θ
log = log K + x log(1 − θ) (7)
The trend in inhibition efficiency with temperature for PA C
alone and in combination with KI on the surface of mild
where θ is the degree of surface coverage, C the concentration
steel may be postulated that PA inhibits the corrosion of mild
of the systems studied, x the number of water molecule replaced
steel by physisorption mechanism (i.e. inhibition efficiency
by one inhibitor molecule and K is the equilibrium constant
decreases with increase in temperature). The veracity of the
for the adsorption process. The adsorption of an organic
above statement was ascertained by evaluating the degree of
adsorbate at a metal/solution interface can be represented as a
surface coverage (θ) of the adsorbed PA from the weight loss
substitution adsorption process between the organic molecules
measurement using Eq. (3). The values obtained for different
in the aqueous solution Org(sol) and the water molecule on the
concentrations of the systems studied at 30–60 ◦ C were applied
metallic surface H2 O(ads) [27,28]:
to determine the best isotherm for adsorption process. Fig. 2
shows the plot of log %I against log C for mild steel corrosion Org(sol) + xH2 O(ads) = Org(ads) + xH2 O
in 1 M H2 SO4 in the presence of PA and PA + KI at 30 ◦ C.
Inset of the figure shows the plot of log %I versus log C for where Org(sol) and H2 O(sol) are the organic molecules and water
KI at 30 ◦ C. Linear plots were obtained which indicates that molecules in the aqueous solution, respectively. x is the size
Freundlich adsorption isotherm was obeyed. Similar plots were ratio representing the number of water molecules replaced by
obtained at 40–60 ◦ C. one molecule of organic adsorbate. The calculated values of x
Fig. 3 shows the plot of surface coverage (θ) against loga- and equilibrium constant of adsorption process K obtained from
rithm of inhibitor concentration for mild steel in 1 M H2 SO4 Flory–Huggins isotherm are presented in Table 3. The values of
containing KI, PA and PA + KI at 30 ◦ C. Similar plots were x are less than unity showing that each molecule of the inhibitor
obtained at other temperatures (40–60 ◦ C) studied. Straight lines occupied less than one active site on the mild steel surface [14].
were obtained indicating that the adsorption of the additives onto The calculated values of molecular interaction parameters ‘a’
mild steel surface can be approximated using Temkin adsorption and the equilibrium constant of adsorption process K deduced
isotherm given as from Temkin adsorption isotherm plot (Fig. 3) are also shown
in the same table. It is seen that the values of ‘a’ are negative
exp(−2aθ) = KC (6) in all cases showing that repulsion exists in the adsorption layer
[29]. It is a known fact that K denotes the strength between
where ‘a’ is molecular interaction parameter, θ the degree of sur- adsorbate and adsorbent. Large values of K imply more efficient
face coverage, K the equilibrium constant of adsorption process adsorption and hence better inhibition efficiency [30]. K val-
and C is the concentration of the inhibitors. ues was found to be in the order (PA + KI) > KI > PA at all the
Flory–Huggins adsorption isotherm was also tested for its fit temperatures studied with the most significant value obtained
to the experimental data obtained from the weight loss measure- at 30 ◦ C. The values of the equilibrium constant for the adsorp-
ment. The plot of log(θ/C) against log(1 − θ) is shown in Fig. 4 tion process K obtained from both Temkin and Flory–Huggins
for (a) PA, (b) PA + KI and (c) KI at 30 ◦ C. Linear plots were adsorption isotherms decreases with temperature suggesting that
obtained and this clearly shows that Flory–Huggins adsorption the inhibitor is physically adsorbed on the metal surface and des-
isotherm is obeyed. The characteristic of Flory–Huggins is orption processes are enhanced by increase in temperature [14].
 S.A. Umoren, E.E. Ebenso / Materials Chemistry and Physics 106 (2007) 387–393 391

Fig. 5. Arrhenius plot for mild steel corrosion in 1 M H2 SO4 in the absence and
presence of KI, PA and PA + KI.

3.4. Kinetic/thermodynamic studies

In an acidic solution the corrosion rate is related to tempera-

ture by Arrhenius equation [31]:

Ea
log CR = log A − (8)
2.303RT
where ‘CR’ is the corrosion rate determined from the weight loss
measurement, Ea the apparent activation energy, A the Arrhenius
constant, R the molar gas constant and T is the absolute temper-
ature. The apparent activation energy was determined from the
slopes of log(CR) versus 1/T graph depicted in Fig. 5. Ea val-
ues obtained for the blank, KI, PA and PA + KI are presented in
Table 4. It is seen that Ea values were higher in the presence of
the additives compared to the blank. The higher value of Ea in
the presence of the additives compared to that in their absence
and the decrease in the %I with rise in temperature is interpreted
as an indication of physiosorption [32–34].
◦
Fig. 4. Plot of log(θ/C) against log(1 − θ) for mild steel corrosion in the presence Free energy of adsorption, Gads values were obtained
of (a) PA, (b) PA + KI and (c) KI at 30 ◦ C (Flory–Huggins adsorption isotherm). from the intercept of Fig. 2 and calculated using the following

Table 3
Some parameters of the linear regression from Temkin and Flory–Huggins adsorption isotherm
Inhibitor Temperature (◦ C) Temkin isotherm Flory–Huggins isotherm
◦
Gads (kJ mol−1 ) a K x K

30 −20.22 −9.44 1.1 × 1024 0.154 4.11

40 −20.60 −7.49 9.9 × 1016 0.147 3.97
KI
50 −20.96 −8.04 1.3 × 1016 0.146 3.61
60 −20.86 −12.77 8.9 × 1019 0.151 2.62
30 −18.64 −3.19 1.07 × 104 0.105 4315.19
40 −18.87 −4.93 2.8 × 105 0.113 3184.19
PA
50 −19.48 −3.62 4.64 × 102 0.077 3589.22
60 −17.01 −3.34 9.32 0.085 2798.98
30 −20.42 −14.47 1.4 × 1041 0.158 4246.19
40 −21.06 −14.47 4.6 × 1039 0.160 4036.45
PA + KI
50 −21.45 −12.06 3.6 × 1029 0.155 3810.66
60 −21.31 −10.34 1.5 × 1019 0.149 3184.19
392 S.A. Umoren, E.E. Ebenso / Materials Chemistry and Physics 106 (2007) 387–393

Table 4 Table 5
Activation energy for mild steel corrosion in 1 M H2 SO4 in the absence and Enthalpy and entropy of adsorption for mild steel corrosion in 1 M H2 SO4 in
presence of PA, KI and (PA + KI) the absence and presence of PA, KI and (PA + KI)
◦ ◦
Systems/concentrations Activation energy, Systems Hads (kJ mol−1 ) Sads (kJ mol−1 K−1 )
Ea (kJ mol−1 )
KI −20.09 −0.23
Blank 0.68 PA −19.57 −0.43
0.06 M KI 1.74 PA + KI −20.30 −0.31
PA (inhibitor) 1.91
2 × 10−5 M PA + 0.06 M KI 1.76
4 × 10−5 M PA + 0.06 M KI 1.76 ◦
6 × 10−5 M PA + 0.06 M KI 1.82
The figure also revealed that Gads decreases with increase in
8 × 10−5 M PA + 0.06 M KI 1.83 temperature which seems to indicate that adsorption of inhibitors
10 × 10−5 M PA + 0.06 M KI 2.64 was unfavourable at higher temperatures thus resulting in the
desorption of adsorbed inhibitor from the mild steel surface
with increase in temperature. From the figure, the values of
equation [35]: enthalpy and entropy of adsorption were obtained for KI, PA
 
θ and (PA + KI) and are presented in Table 5. The values of the
log C = log − log B (9) thermodynamic parameters are negative. The negative sign indi-
1−θ
cates that the adsorption process is spontaneous, exothermic and
◦
where log B = −1.74 − (Gads /2.303RT ) and C is the increases the system order [3,37].
concentration of the system studied. The calculated values of
◦
Gads at all the temperatures studied (30–60 ◦ C) are presented 3.5. Synergism consideration
in Table 4. The negative values obtained suggest that the
inhibitors molecules are strongly adsorbed on the mild steel The synergism parameter, S1 , was evaluated using the
surface. The values also indicate a spontaneous adsorption of relationship given by Aramaki and Hackerman and reported
the inhibitor molecules and usually characterize their strong elsewhere [2,4,11,14]:
◦
interaction with the metal surface. The value of Gads of
1 − I1+2
−40 kJ mol−1 is usually accepted as a threshold value between S1 = (11)
◦ 
1 − I1+2
chemisorption and physiosorption [36]. The values of Gads
obtained in this study is below −40 kJ mol−1 (between −17.01 where I1+2 = I1 + I2 ; I1 = inhibition efficiency of the iodide ions;
and −21.45 kJ mol−1 ). This is consistent with electrostatic I2 = inhibition efficiency of the PA and I = measured inhibition
interactions between the charged molecules and the charged efficiency for PA in combination with iodide ions. This parame-
metal, which are indicative of physical adsorption. ter was evaluated from the inhibition efficiency values obtained
From the basic thermodynamic Eq. (10), the entropy of from both the weight loss and hydrogen evolution measurements
adsorption could be obtained: using Eqs. (2) and (4), respectively. The results obtained are
◦ ◦ ◦
Gads = Hads − TSads (10) presented in Table 6. The S1 values for both methods employed
◦
as shown in Table 5 for different concentrations of PA are
Fig. 6 shows the plot of Gads against absolute temperature, greater than unity. This indicates that the improved inhibition
◦
T. The figure clearly shows a good dependence of Gads on T. efficiency caused by the addition of iodide ions to PA is only due
to synergistic effect. Similar results have been reported [17,18].
Strong chemisorption of iodide ions on the metal surface are
responsible the synergistic effect of iodide ions in combination
with the cation of the inhibitor (PA). The cation is then adsorbed
by columbic attraction on the metal surface where iodide ions
are already adsorbed by chemisorption. Stabilization of the
adsorbed iodide ions with cations leads to a greater surface
coverage and therefore greater inhibition. It could therefore

Table 6
Synergism parameter (S1 ) for different concentrations of PA

Concentration of PA (×10−5 M) Synergism parameter, S1

Weight loss Hydrogen evolution

2 1.53 1.69
4 1.54 1.58
6 1.75 1.66
8 1.76 1.68
Fig. 6. Plot of free energy of adsorption against temperature for KI, PA and 10 1.76 1.62
PA + KI.
 S.A. Umoren, E.E. Ebenso / Materials Chemistry and Physics 106 (2007) 387–393 393

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