Corrosion Science 50 (2008) 1998–2006

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Corrosion Science
journal homepage: www.elsevier.com/locate/corsci

Inhibition of mild steel corrosion in acidic medium using synthetic

and naturally occurring polymers and synergistic halide additives
S.A. Umoren a,*, O. Ogbobe b, I.O. Igwe b, E.E. Ebenso c
a
Department of Chemistry, Faculty of Science, University of Uyo, P.M.B 1017 Uyo, Nigeria
b
Department of Polymer and Textile Engineering, School of Engineering and Engineering Technology, Federal University of Technology, P.M.B. 1526 Owerri, Nigeria
c
Department of Chemistry and Chemical Technology, National University of Lesotho, P. O. Roma180, Lesotho, South Africa

a r t i c l e i n f o a b s t r a c t

Article history: The corrosion inhibition of mild steel in H2SO4 in the presence of gum arabic (GA) (naturally occurring
Received 25 January 2008 polymer) and polyethylene glycol (PEG) (synthetic polymer) was studied using weight loss, hydrogen
Accepted 23 April 2008 evolution and thermometric methods at 30–60 °C. PEG was found to be a better inhibitor for mild steel
Available online 29 April 2008
corrosion in acidic medium than GA. The effect of addition of halides (KCl, KBr and KI) was also studied.
Results obtained showed that inhibition efficiency (I%) increased with increase in GA and PEG concentra-
Keywords: tion, addition of halides and with increase in temperature. Increase in inhibition efficiency (I%) and
A. Mild steel
degree of surface coverage (h) was found to follow the trend Cl < Br < I which indicates that the radii
A. Polymers
Corrosion inhibition
and electronegativity of the halide ions play a significant role in the adsorption process. GA and PEG alone
Synergism and in combination with halides were found to obey Temkin adsorption isotherm. Phenomenon of chem-
Halides ical adsorption is proposed from the trend of inhibition efficiency with temperature and values DG0ads
obtained. The synergism parameter, SI evaluated is found to be greater than unity indicating that the
enhanced inhibition efficiency caused by the addition of halides is only due to synergism.
Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction and protecting the metals from corrosive agents present in solution
[10]. Some research groups have investigated the use of polymers
Mineral acids are widely used in various industries for pickling as corrosion inhibitors of metals in aggressive media. Polymers
of steel at elevated temperatures up to 60 °C. This technique be- such as polyvinyl alcohol, polyethylene glycol, polyvinyl pyridine,
sides being used to remove corrosion scales from steel surface polyvinylbipyridine, polyvinylpyrrolidine, polyvinylpyrrolidone
without causing acid attack of the bulk metal is also effectively ap- (PVP), polyethylenimine, polyacrylic acid, polyaniline, polyacryl-
plied in cleaning of industrial equipment and acidisations of oil amide and polyvinylimidazoles have been reported [11–24].
wells [1]. Corrosion of oil equipment is of significant cost, which Naturally occurring substances as inhibitors of acid cleaning
is additional to mechanical and other operational problems that re- process has continued to receive attention as replacement for syn-
quire work-overs and repairs. It is therefore imperative to add cor- thesized organic inhibitors which are considered to be very toxic,
rosion inhibitors to the solution during pickling in other to reduce expensive and environmentally unfriendly. The greatly expanded
the degree of metal attack and the rate of acid consumption [2–6]. interest on naturally occurring substances, otherwise tagged ‘green
Most of the efficient acid inhibitors are organic compounds which inhibitors’ is attributed to the fact that they are cheap, ecologically
contain nitrogen, sulphur and/or oxygen atoms in their molecules friendly and posses no threat to the environment. In addition, they
[7–9]. The mechanism of inhibition is by adsorption with the polar are readily available and renewable source of materials. Available
groups acting as the active centers for the adsorption process and literature has shown that naturally occurring materials such as
the resulting adsorption layer function as a barrier, isolating the natural honey [25,26], henna [27], opuntia extract [28], guar gum
metal from the corroding medium. [29], jojoba oil [30], artemisia oil [31], Telferia occidentalis extract
The use of polymers as corrosion inhibitors have drawn consid- [32] to mention but a few have been found to be very efficient cor-
erable attention recently due to their inherent stability and cost rosion inhibitors for iron and steel in contact with acidic media.
effectiveness. More so, through their functional groups, they form Most acid inhibitors are known for their specificity of action.
complexes with metal ions and on the metal surface, these com- However, the combination of inhibitors is more likely to provide
plexes occupy a large surface area thereby blanketing the surface multiple effects required for effective corrosion inhibition. Inter-
estingly, addition of halide salts to acid solutions containing any
* Corresponding author. Tel.: +234 802 3144 384.
organic compound had been reported to result in synergistic effect
E-mail address: saviourumoren@yahoo.com (S.A. Umoren). thereby inhibiting iron corrosion. Corrosion inhibition synergism

0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.corsci.2008.04.015
 S.A. Umoren et al. / Corrosion Science 50 (2008) 1998–2006 1999

results from increased surface coverage as a result of ion–pair centrations of the acid, inhibitors (GA and PEG), halides and inhib-
interactions between the organic cation and the anions. Synergistic itors-halide mixtures have been determined for a 7 day (168 h)
effect of halide ions on the corrosion inhibition of metals using var- immersion period from the weight loss measurements using the
ious substances have been reported by some research groups to expression:
mention a few [33–37]. Reports on the influence of halide ions in DW
1
combination with synthetic polymers as corrosion inhibitors are Corrosion rateðmg cm2 h Þ ¼ ð2Þ
At
very scanty [38,39] while there is no report yet to our knowledge
on the effect of halide ions on naturally occurring polymer as cor- where DW is the weight loss (mg) (obtained as difference between
rosion inhibitor. A series of reports have been highlighted in our initial weight and weight at a given time), A is the area of the spec-
laboratory on the synergistic effect of halide ions and polymers imen (cm2) and T is the exposure time (h).
on the corrosion inhibition of mild steel/aluminium in acidic med-
ium [40–42]. 2.3. Hydrogen evolution measurements
In our continuous quest to explore more polymeric materials as
corrosion inhibitors, the present study investigates and compares The procedure followed in this method was as previously de-
the inhibiting effect of PEG (a synthetic polymer) and GA (a natu- scribed [40–42]. The dimension of the mild steel coupons used
rally occurring polymer) on mild steel corrosion in strong acidic was 3 cm  3 cm  0.046 cm. The test solution was kept at
solution as well as influence of halide ions on the inhibitory action 100 ml. The volume of hydrogen gas evolved when mild steel cou-
of the polymers using weight loss, hydrogen evolution and thermo- pons were dropped into the test solutions was monitored by the
metric methods at 30–60 °C. depression in the level of paraffin oil. The depression in paraffin
oil level was monitored at fixed time interval. The experiment
2. Experimental was performed for different concentrations of H2SO4 (blank), GA
and PEG acting as inhibitors, halides and inhibitors-halides combi-
2.1. Material preparation nation but with the corrodent concentration of 1 M H2SO4.
The inhibition efficiency (I%) was calculated using the equation:
!
Mild steel sheets of composition (Mn 0.6, C 0.15, P 0.36, Si 0.03 V 1Ht
and balance Fe) and 0.046 cm thickness were used in the study. I% ¼ 1  100 ð3Þ
V 0Ht
The sheets were mechanically press cut into 5 cm  4 cm coupons.
These coupons were used as cut without further polishing. How- where V iHt is the volume of H2 evolved at time ‘t’ for inhibited solu-
ever, they were degreased in absolute ethanol, dried in acetone tion and V oHt for uninhibited solution.
and stored in a desiccator free of moisture prior to their use in cor-
rosion studies. 2.4. Thermometric method
Gum Arabic (GA) and polyethylene glycol (PEG) used as inhibi-
tor were obtained from BDH laboratory supplies, England and was The reaction vessel and procedure for determining the corrosion
used as sourced without further purification. The concentrations of behaviour by this method has been described elsewhere [43–45].
GA and PEG (inhibitors) prepared and used for the study range The dimension of the mild steel coupons was the same as in hydro-
from 0.1 g/l to 0.5 g/l and 104 M–103 M respectively. The con- gen evolution technique. The test solution was kept at 50 ml. The
centrations of H2SO4 (BDH Supplies Chemical, England) prepared initial temperature in all experiments was kept at 30 °C. The tem-
and used in the study range from 0.002 M to 1.0 M. The halides perature was measured to ±0.05 °C in a calibrated thermometer
used (KCl, KBr, and KI) were all BDH laboratory supplies chemicals, (0–100 °C). This method allowed for the evaluation of the reaction
England and the concentrations prepared for the study are 0.01– number (RN). The reaction number is defined as
0.1 M. 0.05 M KCl, KBr and KI were used for the synergistic studies.
1 Tm  Ti
The studies were carried out at 30–60 °C. RN ð C min Þ ¼ ð4Þ
t
2.2. Weight loss measurements where Tm is the maximum temperature, Ti is the initial temperature
of the systems and ‘‘t” is time (min) taken to reach the maximum
In the weight loss experiments, the pre-cleaned mild steel cou- temperature. The inhibition efficiency (I%) was calculated from
pons were suspended in 250 ml beakers containing 200 ml of test the percentage reduction in the reaction number namely:
solutions maintained at 30–60 °C in a thermostated bath with the
RN aq  RN wi
aid of glass rods and hooks. The weight loss was determined by I% ¼  100 ð5Þ
RNaq
retrieving the coupons at 24 h interval progressively for 168 h (7
days), washed in 20% NaOH solution containing 200 g l1 of zinc where RNaq is the reaction number in the absence of inhibitors (GA
dust with bristle brush, rinsed in deionised water, cleaned, dried and PEG) and/or halides and RNwi is the reaction number of the
in acetone and reweighed. The weight loss was taken to be the dif- aqueous 2 M H2SO4 in the presence of the inhibitors.
ference between the weight at a given time and the original weight
of the coupons. The measurements were carried out for the unin-
hibited solution (blank), halides, inhibitors and inhibitors-halide 3. Results and discussion
mixtures.
The inhibition efficiency (I%) of GA and PEG, halides and GA/ 3.1. Weight loss measurements
PEG-halide mixtures was evaluated using the following equation:
  The corrosion of mild steel in 0.1 M H2SO4 in the absence and
W1 presence of GA and PEG as inhibitors, halides and halides in com-
I% ¼ 1  100 ð1Þ
W2 bination with GA and PEG was studied using the weight loss tech-
nique at temperature range of 30–60 °C. Fig. 1 shows the variation
where W1 and W2 are weight losses for mild steel in the presence of weight loss with time (days) for mild steel dissolution in 0.1 M
and absence, respectively, of the inhibitor in H2SO4 solution at the H2SO4 devoid of and in the presence of halides (0.05 M KCl,
same temperature. The corrosion rate of mild steel in different con- 0.05 M KBr and 0.05 M KI), GA (0.5 g/l) and GA in combination with
2000 S.A. Umoren et al. / Corrosion Science 50 (2008) 1998–2006

0.7 0.7

0.6 0.6

0.5 0.5
Weight loss (g/dm2)

Weight loss (g/dm2)

0.4 0.4

0.3 0.3

0.2 Blank 0.05MKCl 0.2 Blank 0.05MKCl

0.05MKBr 0.05MKI 0.05MKBr 0.05MKI
0.1 GA (0.5g/l) GA + 0.05MKCl PEG (1x10 M) PEG + 0.05MKCl
0.1
GA + 0.05MKBr GA + 0.05MKI PEG + 0.05MKBr PEG + 0.05MKI
0
0
1 2 3 4 5 6 7
1 2 3 4 5 6 7
Time (days)
Time (days)
1.4
1.4

1.2
1.2

1
Weight loss (g/dm2)

1
Weight loss (g/dm2)

0.8
0.8
0.6

0.6
0.4
Blank 0.05MKCl
0.05MKBr 0.05MKI 0.4
0.2 GA GA + 0.05MKCl
GA + 0.05MKBr GA + 0.05MKI
Blank 0.05MKCl
0.05MKBr 0.05MKI
0 0.2 -3
PEG (1x10 M) PEG + 0.05MKCl
1 2 3 4 5 6 7
PEG + 0.05MKBr PEG + 0.05MKI
Time (days)
0
1 2 3 4 5 6 7
Fig. 1. Variation of weight loss with time (days) for the corrosion of mild steel in
0.1 M H2SO4 in the absence and presence of halides, GA and GA-halide mixtures at Time (days)
(a) 30 °C and (b) 60 °C.
Fig. 2. Variation of weight loss with time (days) for the corrosion of mild steel in
0.1 M H2SO4 in the absence and presence of halides, PEG and PEG-halide mixtures at
(a) 30 °C and (b) 60 °C.
the halides at the lowest (a = 30 °C) and the highest (b = 60 °C)
temperatures studied. Similar plots are depicted in Fig. 2 for mild
steel corrosion in the same acid concentration containing PEG Table 1
(1  103 M), halides and PEG-halide mixtures at (a) 30 and (b) Calculated values of corrosion rate and inhibition efficiency (I%) for mild steel
corrosion in 0.1 M H2SO4 for different systems at different temperatures from weight
60 °C, respectively. Inspection of the figures revealed that weight loss measurements
loss of mild steel decreases upon addition of halides and the inhib-
itors to the acid solution compared to the blank. For the halides, Systems/ Corrosion rate (mg cm2 h1) Inhibition efficiency (I%)
concentration
the greatest reduction in weight loss was recorded on addition of 30 °C 40 °C 50 °C 60 °C 30 °C 40 °C 50 °C 60 °C
KI. The figures also show that a further reduction in weight loss Blank 0.174 0.277 0.319 0.359 – – – –
was obtained on addition of halides to both GA and PEG with a 0.05 M KCl 0.168 0.217 0.236 0.259 4.27 22.3 25.0 27.7
remarkable weight loss reduction obtained with GA/PEG-KI mix- 0.05 M KBr 0.166 0.216 0.229 0.244 5.13 22.0 27.1 32.2
0.05 M KI 0.143 0.154 0.161 0.174 17.9 44.3 48.9 53.6
tures at 30 °C. Similar trend was observed at 60 °C but with rather
GA (0.5 g/l) 0.136 0.205 0.214 0.223 21.9 25.9 32.9 37.9
higher values of weight loss. GA + 0.05 M KCl 0.136 0.179 0.201 0.223 21.4 35.6 37.0 38.7
Table 1 shows the calculated values of corrosion rates and inhi- GA + 0.05 M KBr 0.119 0.166 0.177 0.190 31.6 40.9 44.6 47.1
bition efficiency for the different systems studied at different tem- GA + 0.05 M KI 0.099 0.123 0.136 0.147 39.3 55.6 57.3 59.1
PEG 0.145 0.188 0.201 0.215 16.6 32.3 36.3 40.2
peratures from the weight loss measurements. From the table, it is
(1  103 M)
clearly seen that corrosion rate was reduced in the presence of the PEG + 0.05 M KCl 0.136 0.174 0.193 0.213 22.1 37.4 39.0 46.4
halides and inhibitors (GA and PEG) compared to the free acid solu- PEG + 0.05 M KBr 0.131 0.167 0.186 0.208 24.6 39.9 40.9 52.5
tion. Also the corrosion rate increased with rise in temperature. PEG + 0.05 M KI 0.103 0.125 0.127 0.182 40.7 54.3 59.3 64.3
Further reduction in corrosion rate was observed on addition of ha-
lides to the inhibitors. The plot of inhibition efficiency as a function
of the inhibitors concentration for (a) GA and (b) PEG at 30–60 °C
reveals that inhibition efficiency increases with increase in concen- to evaluate the effects of the halides. The figures also indicate that
tration of the inhibitors to attain a maximum value at 0.5 g/l for GA the inhibition efficiency increases with increase in temperature
and 1  103 M for PEG. These concentrations were therefore used (Fig. 5).
 S.A. Umoren et al. / Corrosion Science 50 (2008) 1998–2006 2001

60 35
Blank
0.05MKCl
50 34
0.05MKBr
Volume of H2 evolved (cm3)

0.05MKI
GA
33
40
GA + 0.05MKCl

Temperature (°C)
GA + 0.05MKBr 32
GA + 0.05MKI
30
31

20
30

10 Blank 0.05MKCl
29
0.05MKBr 0.05MKI
GA GA + 0.05MKCl
0 28
GA + 0.05MKBr GA + 0.05MKI
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340
Time (s) 27
5 10 15 20 25 30 35 40 45 50 55 60
60 Time (mins.)
Blank
0.05MKCl 35
50 0.05MKBr
0.05MKI 34
Volume of H2 evolved (cm3)

PEG (1x10-3 M)

40 PEG + 0.05MKCl
33
PEG + 0.05MKBr
PEG + 0.05MKI
Temperature (°C)

32
30
31

20 30

Blank 0.05MKCl
29
10 0.05MKBr 0.05MKI
28 PEG (1x10-3M) PEG + 0.05MKCll
PEG + 0.05MKBr PEG + 0.05MKI
0 27
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340
Time (s) 26
5 10 15 20 25 30 35 40 45 50 55 60
Fig. 3. Variation of volume of H2 evolved with time for the corrosion of mild steel in
Time (mins.)
1 M H2SO4 in the absence and presence (a) GA and GA-halide mixtures and (b) PEG
and PEG-halide mixtures at 60 °C.
Fig. 4. Temperature–time curves for the corrosion of mild steel in 2 M H2SO4 in the
absence and presence (a) GA and GA-halide mixtures and (b) PEG and PEG-halide
mixtures.
The inhibition of mild steel corrosion in the presence of these
polymeric materials (GA and PEG) could be attributed to the 4-O-methyl-b-D-glucuronopyranosyl [46]. It also contains gluco-
adsorption of these compounds onto mild steel surfaces, which proteins. The adsorption of these compounds on the mild steel sur-
blocks the metal and thus do not permit the corrosion process to face makes a barrier for mass and charge transfers. Consequently,
take place. Polyethylene glycol contains oxygen atoms in its molec- the metal is protected from the aggressive anions of the acid. An-
ular structure having lone pair of electrons. It could be adsorbed by other possibility may be due to the formation of positively charged
the interaction between the lone pair of the electrons of the oxygen protonated GA (since it contains glucoproteins) species in acidic
atoms and mild steel surface. This process may be facilitated by the solution, which facilitates adsorption of the compound on the me-
presence of d-vacant orbital of iron making the steel, as observed tal surface through a coordinate type of linkage between the organ-
in the d-group metals or transition metals. In addition to the ic molecules and the metal surface. The degree of protection
molecular form, PEG can be present in protonated species in acid increases with increase in GA concentration due to higher degree
solution. The formation of positively charged protonated species of surface coverage resulting from enhanced inhibitor adsorption.
facilitates adsorption of the compound on the metal surface Further investigation using surface analytical techniques will en-
through electrostatic interaction between the organic molecules able the characterization of the active materials in the adsorbed
and the metal surface. Owing to the complex chemical composition layer and assist in identifying the most active ingredients.
of GA, it is quite difficult to assign the inhibitive effect to a partic- Results presented in Table 1 also points to the inhibition of mild
ular constituent. GA is a branched, neutral or slightly acidic, steel corrosion by the halide ions. The inhibition of mild steel cor-
complex polysaccharide obtained as a mixed calcium, magnesium rosion by halide ions has been reported to be caused by the adsorp-
and potassium salt. The backbone consists of 1,3-linked b-D-galac- tion on the metal surface and by the formation of surface
topyranosyl units. The side chains are composed of two to five compounds which are insoluble in the corrosive media [47,48].
1,3-linked b-D-galactopyranosyl units joined to the main chain by The adsorption ability on the metal surface and hence inhibition
1,6-linkages. Both the side and main chains contain units of a-L- potential of the halides has been estimated in the order
arabinofuranosyl, a-L-rhamnosyl, b-D-glucuronopyranosyl and I > Br > Cl[49,50]. Generally, the adsorbability of anions is re-
2002 S.A. Umoren et al. / Corrosion Science 50 (2008) 1998–2006

lated to the degree of hydration, the less hydrated ion is preferen- Hence mild steel can form compounds with halide ions, thereby
tially adsorbed on the metal surface. The ease of adsorption (great- inhibits corrosion of mild steel in the polymeric materials studied.
er inhibition efficiency) shown in case of iodide ions may be due to
its less degree of hydration. The inhibitive effect of halide ions is 3.2. Hydrogen evolution measurements
found to be in the same order as that of adsorption ability [51].
The table also shows that the inhibition efficiency of the inhib- Experiments were also undertaken using gas – volumetric tech-
itors synergistically increased on addition of halides. The synergis- nique. This technique apart from its experimental rapidity ensures
tic effect increases with the addition of 0.05 M of the halides to GA a more sensitive monitoring in situ of any perturbation by an
and PEG solutions in the order KI > KBr > KCl. Further inspection of inhibitor vis-à-vis gas evolution on the metal – corrodent surface.
the table reveals that the synergistic effect on the addition of the This assertion has been firmly established in earlier reports [54–
halides was more in the presence of PEG compared to GA. This 57]. Fig. 3 shows the variation of volume of hydrogen gas evolved
observation is in agreement with earlier published reports. Gomma with time for 1 M H2SO4 in the absence and presence of halides, (a)
[35,52] and Ebenso [33,34] have reported that addition of halide GA and GA-halide mixtures and (b) PEG and PEG in combination
salt to sulphuric acid solution containing any organic compound with halides at 60 °C. Similar plots were obtained at other temper-
causes a synergistic or cooperative effect which inhibit iron corro- atures studied. Inspection of the figures reveal that the volume of
sion. Halide ions have been shown to inhibit the corrosion of some H2 evolved varies linearly with time. The hydrogen evolution rates
metals in strong acids and this effect depend on the ionic size and decreased in the presence of the halides and inhibitors compared
charge, the electrostatic field set up by the negative charge of the to the blank solution. Further reduction in the hydrogen evolution
anion on the adsorption site and the nature and concentration of rates was observed on addition of halides to the inhibitors with the
the halide ions. Stabilization of the adsorbed halide ions by means most remarkable reduction obtained for both GA and PEG-KI
of interaction with PEG and GA leads to greater surface coverage combination.
(h) and thereby greater inhibition efficiency (I%) (see Tables 1–3). The corrosion rate of mild steel in the absence and presence of
From the observed trend of increase in the inhibitory action in the additives were assessed from the linear portion of the hydro-
the order I > Br > Cl, it is likely that the radii and the electroneg- gen evolution plots and the corresponding values for the different
ativity of the halide ions has a profound influence on the adsorp- systems studied are given in Table 2. The corrosion rates were seen
tion process. Electronegativity increases from I to Cl (I = 2.5, to increase with increasing temperature and were reduced in the
Br = 2.8, Cl = 3.0) while atomic radius decreases from I to Cl presence of the halide ions and inhibitors compared to the blank
(I = 135pm, Br = 114pm, Cl = 90pm) [53]. The iodide ion is more solution. Table 2 also shows the calculated values of inhibition effi-
predisposed to adsorption than the bromide ion and chloride ion. ciency (I%) for the different test solutions determined using the
gasometric technique. The data presented in the table follows the
same trend observed for weight loss method. Inhibition efficiency
Table 2 (I%) increased with rise in temperature and synergistically in-
Calculated values of corrosion rate (cm3 s1) and inhibition efficiency (I%) for mild
creased on addition of halide ions to both GA and PEG. The increase
steel corrosion in 1 M H2SO4 for different systems at different temperatures from
hydrogen evolution measurements
in inhibition efficiency with increase in temperature suggests that
these inhibitors were chemically adsorbed onto the mild steel sur-
Systems/ Corrosion rate (cm3 s1) Inhibition efficiency (I%)
face. The mechanism of inhibition action could possibly be attrib-
concentration
30 °C 40 °C 50 °C 60 °C 30 °C 40 °C 50 °C 60 °C uted to interaction between the corroding steel surface and the
Blank 0.0027 0.0045 0.0059 0.0081 – – – – lone pair of the electrons on the oxygen atom in case of PEG and
0.05 M KCl 0.0024 0.0035 0.0050 0.0055 14.0 15.1 17.2 25.1 some phytochemical constituents present in GA, resulting in their
0.05 M KBr 0.0023 0.0034 0.0044 0.0048 17.2 23.1 25.1 35.1 being adsorbed at cathodic sites and hinder the hydrogen evolu-
0.05 M KI 0.0022 0.0028 0.0037 0.0044 21.1 37.0 38.2 40.0
tion reaction. The synergistic effect was observed to be greater in
GA (0.5 g/l) 0.0017 0.0025 0.0032 0.0035 40.1 43.2 46.0 52.1
GA + 0.05 M KCl 0.0014 0.0022 0.0025 0.0031 48.2 50.3 57.1 59.1 PEG than GA and could possibly be attributed to specific adsorp-
GA + 0.05 M KBr 0.0013 0.0021 0.0023 0.0028 53.2 55.2 61.0 62.2 tion of PEG molecules through the oxygen atom compared to com-
GA + 0.05 M KI 0.0012 0.0019 0.0022 0.0025 54.2 57.1 62.1 66.0 plex and varied composition of GA molecules.
PEG (1  103 M) 0.0015 0.0020 0.0029 0.0033 44.8 47.3 49.8 54.7
PEG + 0.05 M KCl 0.0013 0.0017 0.0027 0.0030 49.3 50.3 51.4 57.1
PEG + 0.05 M KBr 0.0012 0.0015 0.0024 0.0029 53.9 55.6 57.3 60.1
3.3. Thermometric method
PEG + 0.05 M KI 0.0009 0.0012 0.0021 0.0025 60.2 62.0 63.9 64.7
Thermometric method has proved to be of considerable value
and help in studying the corrosion behaviour of a number of metals
and alloys in various corroding environments. The technique is also
useful in evaluating the inhibitor efficiency of a number of active
Table 3
Calculated values of reaction number and percent reduction in reaction number agents. Results obtained using thermometric method was con-
(inhibition efficiency) for mild steel corrosion in 2 M H2SO4 containing GA, PEG and firmed by other well established methods such as weight loss,
GA/PEG- halides mixtures potentiostatic and polarization measurements [43]. Fig. 4 repre-
Systems/concentration Reaction number (°C/min) Inhibition efficiency (I%) sents temperature–time curves for the corrosion reaction of mild
steel in 2 M H2SO4 devoid of and containing (a) GA, halides and
Blank 0.090 –
0.05 M KCl 0.060 33.3 GA–halides mixtures and (b) PEG, halides and PEG in combination
0.05 M KBr 0.050 44.4 with halides. An interesting behaviour was observed in the figure
0.05 M KI 0.040 55.6 where the temperature of the corroding systems increases gradu-
GA (0.5 g/l) 0.048 43.7 ally to reach a maximum value before it starts to decrease again.
GA + 0.05 M KCl 0.040 66.7
GA + 0.05 M KBr 0.030 70.0
Inspection of the figure reveals that the maximum temperature
GA + 0.05 M KI 0.013 85.6 reached decreases and the time elapsed to reach the maximum
PEG (1  103 M) 0.040 55.6 temperature increases in the presence of the halides and GA/PEG
PEG + 0.05 M KCl 0.039 55.6 compared to the blank. Further decrease in the maximum temper-
PEG + 0.05 M KBr 0.030 66.7
ature attained and increase in time elapsed to attain the maximum
PEG + 0.05 M KI 0.019 77.8
temperature was also observed on addition of halides to GA and
 S.A. Umoren et al. / Corrosion Science 50 (2008) 1998–2006 2003

PEG. The most profound effect was noticed with GA/PEG-KI mix- PEG in combination with halides was elucidated from the degree
tures. This behaviour reflects the high inhibition efficiency ob- of surface coverage (h) values calculated from the weight loss data
tained for GA/PEG- halide mixtures toward mild steel dissolution (h = I%/100). The values of surface coverage, h for the inhibitors and
in acidic medium. The calculated values of reaction number and inhibitors + halides have been used to explain the best isotherm to
the percent reduction in reaction number (inhibition efficiency) determine the adsorption process. Attempts were made to fit h val-
for varying concentrations of the inhibitors in combination with ues to various adsorption isotherms namely Frumkin, Temkin. Flo-
0.05 M KCl, 0.05 M KBr and 0.05 M KI are given in Table 3. The table ry-Huggin, Langmuir and Freundlich. By far the best fits were
shows that reaction numbers of the inhibitors-halides mixtures are obtained with Temkin adsorption isotherms.
lower than that of different inhibitors alone. Table 3 also shows The characteristics of the Temkin adsorption isotherm is given
that the percentage inhibition efficiency for the inhibitors im- by
proved on the addition of halides and was found to be in the order:
expð2ahÞ ¼ KC ð6Þ
inhibitors + 0.05 M KI > inhibitors + 0.05 M KBr > inhibi-
tors + 0.05 M KCl. where ‘a’ is the lateral interaction parameter describing the molec-
ular interaction in the adsorption layer and heterogeneity of the
3.4. Adsorption considerations metal surface, h is the degree of surface coverage, ‘K’ is the equilib-
rium constant of adsorption process and ‘C’ is the inhibitor concen-
Two main types of interaction can describe the adsorption of or- tration. Fig. 6 shows the plot of surface coverage as a function of
ganic compounds namely: physical adsorption and chemical logarithm of inhibitor concentration for (a) GA and GA-halide mix-
adsorption. These are dependent on the electronic structure of tures and (b) PEG and PEG-halide mixtures at 60 °C. Similar plots
the metal, the nature of the electrolyte and the chemical structure involving the halides at the same temperature is given in the inset
of the inhibitor. Fig. 5 shows the plot of inhibition efficiency of Fig. 6b. Linear plots were obtained indicating that the experimen-
against inhibitor concentration at different temperatures for (a) tal results at all the temperatures studied for mild steel corrosion in
GA and (b) PEG. Inspection of the figure reveals that inhibition effi- the presence of GA and GA-halide mixtures as well as PEG and PEG
ciency increases with increase in inhibitors’ concentration and in combination with the halides obey the Temkin adsorption iso-
with increase in temperature. Increase in inhibition efficiency with
increasing inhibitor concentration and increased efficiency with
increase in temperature is suggestive of chemical adsorption 0.7
mechanism. The character of adsorption of GA and PEG and GA/
0.6

40 0.5

35 o
30 C 0.4
θ
Inhibition efficiency (%l)

o
30 40 C
o
50 C 0.3
25 o
60 C GA

20 0.2 GA+ KCl

GA+ KBr
15 0.1
GA+ KI
10
0
5 -1 -0.7 -0.
-0.52 -0.4 -0.3

0 Log C
0.1 0.2 0.3 0.4 0.5
0.7
0.
Inhibitor concentration (g/l)
0.6
0.
45

40 0.5
0.
35
Inhibition efficiency (%l)

0.4
0.
30
θ
25 o 0.3
0.
30o C KCll KBr KI

o
20 o
40 C
o 0.2
0. PEG
15 50o C
o
o
PEG+KCl
60 C
10 PEG+KBr
0.1
0.
PEG+KI
5
0
0 -4 -3.5 -3.3 -3.2
-3. -3
1 3 5 7 10 Log C
Inhibitor concentration x 10–4 M
Fig. 6. Temkin adsorption isotherm plot as h against log of concentration for (a) GA
Fig. 5. Plot of inhibition efficiency (I%) against inhibitor concentration for mild steel and GA-halide mixtures and (b) PEG and PEG-halide mixtures at 60 °C: inset shows
in 0.1 M H2SO4 containing (a) GA and (b) PEG different temperatures. Temkin isotherm plot for the halides at 60 °C.
2004 S.A. Umoren et al. / Corrosion Science 50 (2008) 1998–2006

Table 4 0
Some parameters of the linear regression from Temkin isotherm plot at 60 °C for mild Blank
steel in 0.1 M H2SO4 containing halides, inhibitors and inhibitors-halide mixtures 0.05MKCl

Systems/concentration Temkin isotherm -0.2 0.05MKBr

0.05MKI
2
a K R GA (0.5g/l)

0.05 M KCl 4.72 4.49  102 0.97 -0.4 GA+ 0.05MKCl

0.05 M KBr 3.62 1.17  102 0.75 GA+ 0.05MKBr

Log (CR)
0.05 M KI 6.78 1.13  1013 0.87 GA+ 0.05MKI

GA (0.5 g/l) 3.68 18.67 0.98 -0.6

GA + 0.05 M KCl 3.74 57.16 0.96
GA + 0.05 M KBr 4.18 2.45  104 0.97
GA + 0.05 M KI 9.45 8.64  1020 0.98 -0.8
PEG (1  103 M) 13.57 2.75  1036 0.88
PEG + 0.05 M KCl 13.57 2.38  1028 0.88
PEG + 0.05 M KBr 15.51 3.76  1027 0.92 -1
PEG + 0.05 M KI 19.74 4.37  1029 0.92

-1.2
therm. The values obtained from hydrogen evolution measure- 3 3.1 3.2 3.3
ments and thermometric methods also obey the Temkin adsorption 1/T x10-3 (K-1)
isotherm in this study although not shown here.
The adsorption parameters deduced form Temkin adsorption
0
isotherms are presented in Table 4 for mild steel corrosion in Blank
0.1 M H2SO4 in the presence of GA and PEG and GA/PEG-halide 0.05MKCl
mixtures at 60 °C. The molecular interaction parameter ‘a’ can have -0.2 0.05MKBr

both positive and negative values. Positive values of ‘a’ indicates 0.05MKI
PEG (1x10 M)
attraction forces between adsorbed molecules while negative val- -0.4 PEG + 0.05MKCll
ues indicate repulsive forces between the adsorbed molecules
Log (CR)

PEG + 0.05MKBr
[58]. It is seen in the table that the values of ‘a’ in all cases are neg- PEG + 0.05MKI
-0.6
ative indicating that repulsion exists in the adsorption layer [59]. It
is generally known that K denotes the strength between the adsor-
bate and adsorbent. Large values of K imply more efficient adsorp- -0.8
tion and hence better inhibition efficiency [60]. K values are seen to
increase with increase in temperature suggesting that the inhibi- -1
tors are chemically adsorbed onto the mild steel surface. Inspec-
tion of the tables also reveal that K values are in the order
-1.2
Inhibitors + halides > halides > inhibitors. 3 3.1 3.2 3.3
1/T x10-3 (K-1)
3.5. Effect of temperature
Fig. 7. Plot of log corrosion rate (CR) against 1/T for mild steel in 0.1 M H2SO4
Corrosion of mild steel in 0.1 M H2SO4 was studied in the tem- (blank), halides and (a) GA and GA-halide mixtures and (b) PEG and PEG-halide
mixtures.
perature range of 30–60 °C in the absence and presence of halides,
inhibitors (GA and PEG) and these inhibitors in combination with
the halides. The dependence of logarithm of corrosion rate (log CR)
Table 5
on the reciprocal of absolute temperature (I/T) for 0.1 M H2SO4 is
Calculated values of kinetic/thermodynamic parameters for mild steel corrosion in
presented in Fig. 7 for (a) blank, GA, halides and GA-halide mix- 0.1 M H2SO4 containing halides, inhibitors and inhibitors-halide mixtures
tures and (b) blank, halides, PEG and PEG-halide mixtures Linear
Systems/ Ea (kJ/ DH0ads ðkJ=molÞ DS0ads ðJ=mol=KÞ DG0ads ðkJ=molÞ
plots were obtained which indicates that it follows Arrhenius
concentration mol)
equation given by
Blank 19.72 16.47 54.56 
Ea 0.05 M KCl 10.91 8.81 58.39 116.42
log CR ¼ log A  ð7Þ 0.05 M KBr 10.34 7.08 59.07 202.32
2:303RT
0.05 M KI 4.98 3.06 62.51 272.64
where ‘CR’ is the corrosion rate, A is the Arrhenius constant, R is the GA (0.5 g/l) 13.02 10.34 59.07 176.99
molar gas constant and T is the absolute temperature. The Ea values GA + 0.05 M KCl 13.02 10.34 59.41 183.81
GA + 0.05 M KBr 12.06 10.15 60.60 199.57
obtained from the slope of the linear plot are presented in Table 5.
GA + 0.05 M KI 10.53 8.04 63.28 218.63
From the table it is seen that Ea decreases in the presence of the PEG (1  103 M) 9.76 11.49 37.82 220.77
additives compared to the blank which may be attributed to an PEG + 0.05 M KCl 12.44 13.59 37.24 256.57
appreciable increase in adsorption process of the inhibitors on the PEG + 0.05 M KBr 8.09 11.10 37.08 298.53
metal surface with rise in temperature (chemisorption). Enthalpy PEG + 0.05 M KI 5.74 9.38 41.26 378.53
of adsorption, DH and entropy of adsorption, DS for the corrosion
of mild steel corrosion in 0.1 M H2SO4 in the presence of halides,
inhibitors and inhibitors-halides mixtures was obtained by apply- of log (CR/T) against 1/T for blank, 0.05 M KCl, 0.05 M KBr, 0.05 M KI
ing transition state equation given by and (a) GA and GA in combination with halides and (b) PEG and PEG
    in combination with halides. Linear plots were obtained and from
RT DS DH
CR ¼ exp exp  ð8Þ the slope (DH/2.303RT) and intercept of the linear plots, enthalpy
Nh R RT
of adsorption and entropy of adsorption were obtained respectively.
where N is the Avogadro’s number, h is Planck’s constant, R is molar The calculated values are presented in Table 5 for the different sys-
gas constant and T is the absolute temperature. Fig. 8 shows the plot tems studied. Results shown in the table indicate that the enthalpy
 S.A. Umoren et al. / Corrosion Science 50 (2008) 1998–2006 2005

-2.6 2
Blank
-2.7 0.05MKCl 1.8
0.05MKBr
-2.8 0.05MKI 1.6
GA(0.5g/l)
-2.9 GA+ 0.05MKCl 1.4
GA+ 0.05MKBr
-3 1.2 2
Log (CR/T)

GA+ 0.05MKI

Log %I
-3.1 6
1

Log %I
1.2
-3.2 0.8
GA
0.8
-3.3 0.6 GA+ KCll
0.4 KCl KBr KI
-3.4 GA+ KBrll
0.4 0
GA+ KI -2 -1.5 -1.3 -1.2 -1
-3.5 0.2 Log [halides]

-3.6
3 3.1 3.2 3.3
0
-1 -0.7 -0.52 -0.4 -0.3
-3 -1
1/T x10 (K ) Log C
-1.5 1.85
Blank
-1.6 0.05MKCl 1.8
0.05MKBr 1.75
-1.7 0.05MKI
PEG (1x10-3M) 1.7
-1.8
PEG+ 0.05MKCll
1.65
Log (CR/T)

Log %I
-1.9 PEG + 0.05MKBr
PEG + 0.05MKI 1.6
-2
1.55
PEG
-2.1 1.5 PEG+ KCl
-2.2 1.45 PEG+ KBr
PEG+ KI
-2.3 1.4

-2.4 1.35
-5 -4.5 -4.3 -4.2 -4
-2.5 Log C
3 3.1 3.2 3.3
-3 -1 Fig. 9. Plot of log I% against log C for mild steel corrosion in 0.1 M H2SO4 for (a) GA
1/T x10 (K )
and GA-halide mixtures (b) PEG and PEG-halide mixtures at 60 °C inset shows plot
Fig. 8. Transition state plot of the corrosion rate for mild steel in 0.1 M H2SO4 of log I% vs log C for the halides at 60 °C.
(blank), halides and (a) GA and GA-halide mixtures and (b) PEG and PEG-halide
mixtures.
3.6. Synergism considerations

of adsorption decreases in the presence of the additives compared Corrosion inhibition synergism results from increased surface
to the free acid solution, which further supports the chemisorption coverage as a result of ion–pair interactions between organic cat-
mechanism proposed. In all cases, negative values of entropy of ions and the anions. This interaction of inhibitor molecules can
adsorption were obtained and ranged between 37.05 and be described by a parameter called synergism parameter (SI)
63.28 J mol1 K1 both in the absence and presence of the addi- [38,63] which is defined as
tives. The negative values are indicative of increase in the systems I  I1þ2
order [28]. S1 ¼ ð10Þ
I  I01þ2
Free energy of adsorption, DG0ads was obtained from the inter-
cept of plot of log inhibition efficiency (log I%) against log inhibitor where I1+2 = I1 + I2; I1 is inhibition efficiency of the halides, I2 is the
concentration (log C) (Fig. 9) and evaluated using the following inhibition efficiency of inhibitor (GA) and I01þ2 is measured inhibi-
equation [61]: tion efficiency for the inhibitor in combination with the halides.
  S1 approaches 1 when no interaction between the inhibitor mole-
h
log C ¼ log  log B ð9Þ cules exists, while S1 > 1 indicates a synergistic effect. In the case
1h
of S1 < 1, antagonistic behaviour prevails which may be attributed
where log B = 1.74  (DG0ads =2:303RT) and C is the concentration of to competitive adsorption. The calculated values for the halides
the system studied. The calculated values of DG0ads at 60 °C for the from the three methods employed in this study are presented in Ta-
various systems studied are presented in Table 5. The DG0ads values ble 6. The values of S1 as presented in Table 6 are greater than unity
obtained are negative and indicate the spontaneous adsorption of which clearly shows that the enhanced inhibition efficiency brought
the GA/PEG and GA/PEG in combination with halides. Generally, about by GA/PEG and in combination with halides is only due to
values of DG0ads more negative than 20 KJ mol1 (as obtained in synergistic effect [64]. Similar observation have been reported in
the present study) involve charge sharing or transfer from the our earlier publication [65] and also by Larabi and Harek [39] and
inhibitor molecules to the metal surface to form a coordinate type Larabi et al. [38] in their study of addition of iodide ions to
of bond indicating chemical adsorption [33,61,62]. poly(4-vinyl pyridine) (P-4VP) in 0.5 M H2SO4 and 1 M HCl, respec-
2006 S.A. Umoren et al. / Corrosion Science 50 (2008) 1998–2006

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1085.
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