Co esponding au ho : Olusegun Pe e s Akinyemi
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In es iga ion o he co osion inhibi ion po en ial o Axonopus comp essus (Ca pe
g ass) Ex ac on Mild s eel in 𝐶02co osi e media
Olusegun Pe e Akinyemi 1, *, Fa ou Oluwada asimi Emmanuel 1, Ogheneme ega Di ine Edoja 1 and Da id
Asuquo An e 2
1 Depa men o Chemical Enginee ing, Facul y o Enginee ing, Lagos S a e Uni e si y, Ojo, Nige ia.
2 Depa men o Chemis y, Louisiana S a e Uni e si y, USA.
Wo ld Jou nal o Ad anced Resea ch and Re iews, 2025, 27(02), 741-754
Publica ion his o y: Recei ed on 17 June 2025; e ised on 22 July 2025; accep ed on 25 July 2025
A icle DOI: h ps://doi.o g/10.30574/wja .2025.27.2.2640
Abs ac
Co osion emains a o midable challenge in p ocess plan s, necessi a ing eco- iendly mi iga ion s a egies. This s udy
explo es he po en ial o Axonopus comp essus (Ca pe g ass) ex ac as a co osion inhibi o o mild s eel in a 𝐶𝑂2
saline en i onmen .
The aim is o ad ance he unde s anding o en i onmen ally iendly co osion inhibi o s in a CO₂ saline en i onmen ,
con aining 3.5% NaCl. The p ojec 's objec i es encompass phy ochemical ex ac ion, cha ac e iza ion, and co osion
inhibi ion po en ial e alua ion. The ex ac ion o Axonopus comp essus was conduc ed h ough he sol en ex ac ion
me hod employing me hanol. Va ious concen a ions o he ex ac we e subjec ed o es ing wi hin a empe a u e o
30°C and 60°C, pH o 6.0, and o an imme sion pe iod o 163 hou s. The co osion inhibi ion assessmen u ilized he
g a ime ic (Weigh Loss) me hod.
The esul s o he expe imen s e ealed a ema kable co osion inhibi ion e iciency o 93% a a empe a u e o 30°C
and concen a ion o 2.8g in 100ml o solu ion o he Axonopus comp essus ex ac . A 60°C, he co osion a e was
ound o inc ease signi ican ly, and he inhibi ion e iciency dec eased sligh ly, pa icula ly a lowe concen a ions,
hough high concen a ions s ill main ained subs an ial p o ec i e e ec s. No ably, he inhibi ion e iciency exhibi ed a
posi i e co ela ion wi h he concen a ion o he ex ac . As he concen a ion inc eased, he inhibi i e e ec on
co osion demons a ed a co esponding enhancemen . The adso p ion iso he m o he adso p ion o Axonopus
comp essus ex ac on he s eel su ace was ound o ollow he Langmui adso p ion iso he m.
Keywo ds: G een co osion inhibi ion; Axonopus comp essus Ex ac ;𝑪𝑶𝟐 saline en i onmen ; Mild S eel; Langmui
Adso p ion Iso he m
1. In oduc ion
Co osion is a e m ha desc ibes he ans o ma ion o pu e me als and hei alloys o a mo e s able he modynamic
s a e as a esul o a eac ion wi h hei su oundings, and his s able s a e may include hei a ious sul ides,
hyd oxides, and oxides. The e ec o co osion on p ope ies is qui ea lo as hey may cause damage like leakages,
acili y down ime, and collapse o a sys em which can be inju ious and haza dous [1]. The undamen al elemen ha
d i es co osion is he po en ial ene gy di e ence be ween he po en ial ene gy o he co oding me al and he
co osion p oduc . In he ex ac ion o me als om hei o es, he e is a equi ed amoun o ene gy used o achie e his
and a e a while, his me al inds a way o e u ning o i s o iginal ene gy le el when i inally begins o in e ac wi h
i s en i onmen [2].
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𝐶02co osion,also e e ed o as swee co osion, is commonly associa ed wi h mild s eel and ep esen s a signi ican
challenge wi hin a ious p oduc ion sec o s, no ably in he oil and gas indus y, whe e he p esence o bo h wa e and
oil is p e alen [3].
The p esence o acidic gases such as 𝐶02 and 𝐻2𝑆 in p oduced luids agg a a es co osion endencies in p oduc ion
acili ies and can cause localized o pi ing co osion in s eels [4]. The eme gence o he use o 𝐶𝑂2 injec ion o
enhanced oil eco e y makes 𝐶𝑂2 co osi e media a common issue in he explo a ion o gas ese oi s [5]. 𝐶𝑂2 is
soluble in he o ma ion wa e ha comes o he su ace wi h p oduc ion luid and e en ually o ms a weak ca bonic
acid (𝐻2𝐶𝑂3) which is ex emely co osi e o ca bon s eel. 𝐹
𝑒𝐶𝑂3 is belie ed o be he main co osion p oduc o med
wi h ca bon s eel in a co osi e media and some ca hodic mechanisms ha e been p oposed o soo he he 𝐶𝑂2 co osion
beha io wi h 𝐶𝑂2 belie ed o accele a e he ca hodic eac ion [6].
1.1. 𝐂𝟎𝟐 Co osion Mechanism on Mild S eel
𝑪𝟎𝟐co osionis a signi ican conce n in indus ies like oil and gas, whe e mild s eel is commonly used due o i s cos -
e ec i eness and s eng h. The co osion p ocess in ol es se e al key s eps:
The p ocess begins wi h he dissolu ion o ca bon dioxide 𝑪𝟎𝟐in wa e (𝑯𝟐𝑶(𝒍)) ha accompanies p oduc ion luids.
𝑪𝟎𝟐, a gas, is highly soluble in wa e , leading o i s con e sion in o aqueous 𝑪𝟎𝟐 (𝒂𝒒) [7].
𝑪𝟎𝟐 (𝒈)⇌𝑪𝟎𝟐 (𝒂𝒒) ………………… (1)
𝐶02 (𝑎𝑞) eac s wi h wa e o o m ca bonic acid (𝐻2𝐶𝑂3). This eac ion occu s due o he in e ac ion be ween 𝑪𝟎𝟐
molecules and wa e molecules. Acco ding o a s udy by Nešić [8], as ci ed in he s udy by Fazal [6],only a mino po ion,
app oxima ely 0.2%o he dissol ed CO₂ eac s wi h wa e o o m ca bonic acid. The eac ion is desc ibed by (Eq 1)
and (Eq 2), espec i ely.
𝑪𝟎𝟐 (𝒂𝒒)+ 𝑯𝟐𝑶(𝒍)⇌𝑯𝟐𝑪𝑶𝟑……………………….. (2)
Ca bonic acid is a weak acid bu is co osi e o mild s eel because i ini ia es he co osion p ocess.
Ca bonic acid unde goes dissocia ion in o hyd ogen ions (𝐻+) and bica bona e ions (𝐻𝐶𝑂3−) in an equilib ium eac ion:
𝑯𝟐𝑪𝑶𝟑⇌𝑯++ 𝑯𝑪𝑶𝟑−…………………….. (3)
The p esence o hyd ogen ions is a c i ical ac o in he co osion p ocess.
Ca hodic Reac ion: Reduc ion o P o ons: One o he p ima y ca hodic eac ions in ol es he educ ion o p o ons 𝐻+
𝟐𝑯+ + 𝟐𝒆−⇌𝑯𝟐 …………………….. (4)
This educ ion eac ion p oduces hyd ogen gas (𝐻2) as a byp oduc .
While he educ ion o p o ons is commonly accep ed as he main ca hodic eac ion as in (Eq 5), a p oposed 'bu e
e ec mechanism' sugges s ha he equilib ium eac ion o ca bonic acid p ima ily se es o supply mo e hyd ogen
ions (𝐻+) a he han di ec ly pa icipa ing in he educ ion p ocess [6, 9].
𝑯𝟐𝑪𝑶𝟑 + 𝟐𝒆−⇌𝑯𝑪𝑶𝟑−+ 𝑯𝟐 …………………….. (5)
This mechanism posi s ha he p ima y educ ion occu s wi h p o ons (𝐻+).
Role o Tempe a u e: I 's impo an o no e ha he a e-con olling s ep o ca hodic eac ion kine ics is o en he
hyd a ion o ca bon dioxide (𝐶02 (𝑔)) o ca bonic acid (𝐻2𝐶𝑂3), especially a empe a u es below 50–60 °C [8].
Anodic Reac ion - I on Dissolu ion: To balance he ca hodic eac ions, mild s eel (i on) unde goes anodic dissolu ion:
𝑭𝒆(𝒔)→ 𝑭𝒆𝟐+ + 𝟐𝒆− …………………….. (6)
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The dissol ed i on ions en e he solu ion as pa o he co osion p ocess.
Adso p ion and Oxida ion o I on Ions: I on combines wi h wa e (𝐻2𝑂) o o m adso bed in e media e p oduc s:
𝑭𝒆(𝒔)+ 𝑶𝑯−→ 𝑭𝒆𝑶𝑯 + 𝒆− (adso p ion) …………………….. (7)
The adso bed in e media e p oduc s hen oxidize u he :
𝑭𝒆𝑶𝑯→ 𝑭𝒆𝑶𝑯𝟐+ + 𝒆− …………………….. (8)
These eac ions con ibu e o he o e all anodic dissolu ion o i on.
Fo he case o mild s eel one can w i e he o e all eac ion as;
𝑭𝒆(𝒔)+ 𝑪𝑶𝟐 + 𝑯𝟐O → 𝑭𝒆𝟐+ + 𝑪𝑶𝟑𝟐− + 𝑯𝟐 …………………….. (9)
In essence, 𝐶02 co osion on mild s eel is a complex elec ochemical p ocess in ol ing he dissolu ion o 𝐶02 (𝑔) in
wa e , he o ma ion o ca bonic acid, he educ ion o p o ons, and he anodic dissolu ion o i on [10]. The speci ic
eac ions can a y depending on ac o s such as empe a u e, pH, and he p esence o inhibi o s [11].
1.2. G een Co osion Inhibi o s
G een Co osion Inhibi o s can be na u al o syn he ic, as seen in igu e 1. The idea o he use o plan s has been
ecognized as a po en ial eplacemen o o ganic compounds and he e ha e been nume ous esea ch in he las decade
and mo e o be done in yea s o come, i is belie ed ha he use o na u ally occu ing compounds is o in e es because
o hei cos -e ec i eness, easy accessibili y o aw ma e ials and eco- iendliness. When selec ing an inhibi o , se e al
ac o s me i conside a ion, including he inhibi o 's cos , po en ial oxici y impac ing humans and o he li ing
o ganisms, i s a ailabili y, and i s en i onmen al iendliness [12]. Plan ex ac s se e as iable op ions o
en i onmen ally iendly al e na i es [13]. Acco ding o Sapunyo W.L. [14], he aqueous me hanol, and
dichlo ome hane: me hanol (1:1) ex ac s o Axonopus comp essus (ca pe g ass) possess phy ochemical compounds
wi h an icandidal ac i i y. The ex ac s ha e also been classi ied as non- oxic [14].
Figu e 1 Na u al and Syn he ic g een co osion inhibi o
Gi en he g owing en i onmen al and economic conce ns su ounding co osion, especially in CO₂- ich saline
en i onmen s such as oil and gas pipelines and p ocess plan s, he e is an inc easing need o eco- iendly co osion
mi iga ion s a egies. Con en ional inhibi o s a e o en syn he ic and oxic, posing en i onmen al and sa e y isks. As a
esul , a en ion has shi ed owa ds plan -based g een inhibi o s ha a e biodeg adable, cos -e ec i e, and sus ainable.
Wo ld Jou nal o Ad anced Resea ch and Re iews, 2025, 27(02), 741-754
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This s udy explo es Axonopus comp essus as a no el g een co osion inhibi o o mild s eel in a CO₂-sa u a ed 3.5%
NaCl medium. The aim is o ad ance he unde s anding o he inhibi ion mechanism o his plan ex ac by e alua ing
i s phy ochemical composi ion, adso p ion beha iou , and inhibi ion pe o mance unde di e en en i onmen al
condi ions.
2. Ma e ials and Me hods
2.1. P epa a ion o Ca pe g ass ex ac
Axonopus comp essus, sou ced om he icini y o Epe in Lagos, Nige ia, unde wen a se ies o p epa a ion s eps.
Ini ially, i was subjec ed o washing and subsequen ai -d ying. Following his, i was exposed o an o en se a 45°C
whe e i emained o 48 hou s. The esul ing d ied ca pe g ass was hen p ocessed in o a powde ed o m using a
mille . To achie e a ine consis ency, he powde was sie ed h ough a mesh wi h a size o 10mm. Finally, he ine
pa icles we e ca e ully s o ed in an ai igh con aine and placed inside a desica o o sa ekeeping.
Cold ex ac ion was used. A quan i y o one hund ed g ams (100g) o he milled ca pe g ass was measu ed in 100%
me hanol o 48 hou s, shaking spo adically e e y wo hou s. A e il e ing he ex ac h ough Wha man il e shee s
No. 42 (125 mm), he sol en was eco e ed using s eam dis illa ion wi h he wa e ba h se a 70𝑜𝐶 and he esul ing
we esidue was d ied in an elec ic o en se a 40𝑜𝐶. The c ude ex ac was s o ed in sealed plas ic con aine a 4 °C in
a e ige a o un il when needed. Using he ollowing o mula, he ex ac 's pe cen age yield was de e mined.
Ra io o Cold Mace a ion = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑟𝑝𝑒𝑡 𝑔𝑟𝑎𝑠𝑠
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑆𝑜𝑙𝑣𝑒𝑛𝑡 …………………….. (10)
Ra io o Me hanol Reco e ed = Volume o Reco e ed Me hanol
Volume o sol en used …………………….. (11)
Yield = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝐸𝑥𝑡𝑟𝑎𝑐𝑡
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑐𝑎𝑟𝑝𝑒𝑡 𝑔𝑟𝑎𝑠𝑠 ˟ 100 …………………….. (12)
2.2. P epa a ion o Me al Specimen
Mild s eel shee s o 1 mm hickness we e cu in o ele en iden ical coupons wi h dimensions o 3 cm × 3 cm × 0.1 cm. To
ensu e su ace cleanliness, each coupon was polished using sandpape o emo e su ace oxides and impu i ies, hen
insed wi h analy ical-g ade e hanol. Deg easing was subsequen ly ca ied ou using ace one o elimina e esidual oil
o g ease ha could in e e e wi h su ace in e ac ions du ing es ing. One o he coupons was selec ed and submi ed
o he Quali y Con ol Labo a o y o elemen al analysis. The emaining en coupons we e indi idually labeled and
ese ed o he co osion inhibi ion expe imen s. Each coupon was weighed using a p ecision balance (±0.01 g), and
he ini ial weigh s we e ound o be app oxima ely 6.43 g, con i ming uni o mi y in dimensions and ma e ial densi y.
Figu e2 The Ene gy Dispe si e X- ay Spec oscopy (EDS) shows he elemen al analysis o mild s eel be o e co osion
s udy. The EDS composi ional analysis esul s ob ained om he spec a show a weigh pe cen o Ca bon (3.0 w . %),
Oxygen (20.0 w . %), I on (54.0 w . %), Aluminum (2.00 w . %), Magnesium (4.00w %), Zinc (8.40), Calcium (1.30w %),
Po assium (2.00w %), Silicon (1.20w %) and Ti anium (4.00w %) o he mild s eel be o e co osion expe imen .
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Figu e 2 Ene gy Dispe si e X-Ray Spec oscopy Analysis o Mild S eel
A e d ying he specimens in an o en a 40 °C, hei ini ial weigh s we e e-measu ed. The olume o he es solu ion
was delibe a ely kep la ge enough o ensu e he de ec ion o any signi ican changes in i s co osi e p ope ies,
whe he due o he deple ion o ac i e co osi e species o he accumula ion o co osion byp oduc s. To p ese e he
p epa ed mild s eel in a dus - ee en i onmen , he specimens we e s o ed in desicca o s and used immedia ely o he
co osion expe imen s a e p epa a ion.
2.3. E alua ion o Co osion inhibi ion o mild s eel (Weigh Loss)
CO₂ gas was con inuously pu ged in o a sealed bo le con aining dis illed wa e o app oxima ely 15 minu es o ensu e
sa u a ion, as illus a ed in Figu e 3. Following his, a solu ion was p epa ed by combining 100 mL o he CO₂-sa u a ed
wa e wi h 3.5% NaCl, adjus ed o a pH o 6.0. This solu ion was hen e enly dis ibu ed in o en sepa a e beake s.
Subsequen ly, a ying concen a ions (10%, 20%, 30%, and 40%) o he me hanolic plan ex ac (as shown in Figu e
3) we e added indi idually o designa ed beake s. Each beake ecei ed a p e-weighed mild s eel coupon, a e which
all beake s we e placed in wa e ba hs se a 30 °C and 60 °C, espec i ely. The imme sion p ocess las ed o 163 hou s.
A e he exposu e pe iod, he s eel samples we e ca e ully emo ed, cleaned, d ied, and eweighed. The mass loss was
eco ded me iculously o u he analysis o co osion a es and inhibi ion e iciency. The weigh loss (Δw), co osion
a e (CR), inhibi ion e iciency (IE) and deg ee o su ace co e age we e calcula ed using s anda d equa ions
∆W = 𝑊𝑖 − 𝑊
𝑓 …………………….. (13)
𝐼𝐸% = 𝑊0−𝑊1
𝑊0 …………………….. (14)
𝜃 = 𝑊0 − 𝑊1
𝑊0 …………………….. (15)
CR (mmpy) = K x Weigh loss
D x A x (in hou s) …………………….. (16)
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Figu e 3 Pho og aphs showing p epa a ion o Axonopus comp essus Ex ac and solu ion.
Whe e 𝑊𝑖 and 𝑊
𝑓a e he ini ial and inal weigh o mild s eel samples espec i ely; 𝑊1 and 𝑊0 a e he weigh loss alues
in p esence and absence o inhibi o espec i ely. A is he a ea cm2, is he ime K = 8.76x104 (cons an )), D is densi y
in gm/cm3 (7.86).
3. Resul and discussion
3.1. Yield and Cha ac e iza ion o Me hanolic Ex ac
Table 1 Quali a i e Phy ochemical Analysis o Ex ac
Plan Phy ochemicals
Plan Me aboli e
Ca diac glycosides
+++
S e oid glycosides
++
Saponins
+ +
Tannins
++
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Alkaloids
+++
Phloba annins
+
Te penoids
+
Fla onoids
++
An h aquinones
+++
-= Absen , + = P esen , ++=Mode a e, +++ = Abundan
The me hanolic lea ex ac o Axonopus comp essus had a g eenish colo and yielded 7% w/w o he d y ma e . The
composi ion o he phy ochemical ex ac is summa ized in Table 1.
The phy ochemical sc eening o Axonopus comp essus indica ed he p esence o s e oids (s e oid glycoside), alkaloids,
saponins, annins, ca diac glycosides, la onoids, phloba annins, an h aquinones, and e penes. Quan i a i e analysis
e ealed high le els o la onoids, and alkaloids, along wi h signi ican amoun s o polyphenolsconsis en wi h indings
epo ed by A olabi [15].
3.2. Weigh Loss Measu emen
Table 2 – Table 3 show he a ious samples, hei espec i e concen a ions, weigh loss alues, inhibi ion e iciency (η
%) and su ace co e age (θ) o mild s eel a di e en concen a ions o Axonopus comp essus ex ac . These alues
we e calcula ed a e imme sion in CO₂ co osi e media. F om Table3, he analysis shows ha he minimum weigh loss
was obse ed when he mild s eel samples we e imme sed in he es solu ion con aining Axonopus comp essus ex ac
o 163 hou s, wi h concen a ion anges om 10% (w/w) o 40% (w/w) a 30°C. In pa icula , Sample C (40% w/w)
exhibi ed he leas weigh loss (0.01 g), ollowed by Sample B (30% w/w) wi h 0.04 g, indica ing e ec i e inhibi ion a
highe concen a ions.
Table 2 The labeling o all ex ac samples and he e co esponding concen a ion(w/ ).
Sample
Concen a ion (w/ )
Pe cen age (%w/w)
Sample A (Blank)
0
0
Sample B
2.1
30
Sample C
2.8
40
Sample D
0.7
20
Sample E
0.4
10
Table 3 The da a o a ying weigh o mild s eel wi h di e en concen a ion o Axonopus comp essus ex ac a he
wo empe a u es s udied.
Time (h .)
Sample A (g)
Sample B (g)
Sample C (g)
Sample D (g)
Sample E (g)
30 0C
60 0C
30 0C
60 0C
30 0C
60 0C
30 0C
60 0C
30 0C
60 0C
0
6.43
6.43
6.43
6.43
6.43
6.43
6.43
6.43
6.43
6.43
163
6.29
4.68
6.39
5.83
6.42
6.13
6.38
5.68
6.37
5.53
In con as , he blank sample (Sample A), which had no inhibi o , eco ded a weigh loss o 0.14 g a 30°C and 1.75 g a
60°C, con i ming he absence o any p o ec i e ba ie agains co osion.
Thus, compa ed o he esul o he blank, i.e., wi hou he p esence o inhibi o , i can be obse ed ha he Axonopus
comp essus ex ac (Inhibi o ) caused a educ ion in weigh loss and consequen ly educed he co osion a e a bo h
30°C and 60°
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F om he analysis abo e, i is e iden ha he ca pe g ass ex ac signi ican ly imp o ed co osion esis ance o mild
s eel by educing he weigh loss ac oss all concen a ions when compa ed o he blank. The highe he concen a ion
o he ex ac , he g ea e he inhibi ion e iciency, wi h op imal esul s seen a 40% and 30% w/w.
3.3. Co osion Ra e
The co osion a e o mild s eel in CO₂-sa u a ed co osi e media was e alua ed in he p esence and absence o
Axonopus comp essus (ca pe g ass) ex ac , and he esul s a e p esen ed in Figu e4 and Table 4. I was obse ed ha
he co osion a e dec eased wi h inc easing concen a ion o he ex ac , indica ing he e ec i eness o Axonopus
comp essus as a co osion inhibi o .
A 30°C, he blank sample (Sample A), which con ained no inhibi o , exhibi ed he highes co osion a e o 9.57
mm/yea , while he lowes co osion a e o 0.68 mm/yea was eco ded o Sample C (2.8 g/ml o 40% w/w ex ac
concen a ion). A simila end was obse ed a 60°C, whe e he co osion a e o he blank ose signi ican ly o 119.66
mm/yea , compa ed o 20.52 mm/yea o he same highes concen a ion o inhibi o . This educ ion in co osion a e
is a ibu ed o he adso p ion o phy ochemical cons i uen s o he ca pe g ass ex ac on o he su ace o he mild
s eel, o ming a p o ec i e ba ie ha hinde ed u he in e ac ion wi h he co osi e medium.
Table 4 The Co osion a e (CR), Inhibi ion e iciency (IE%) and su ace co e age (θ) wi h di e en concen a ion o
Axonopus comp essus ex ac a he wo empe a u es s udied.
Sample
Conc. (g/ml)
Weigh Loss (g)
CR (mmpy)
IE%
(θ)
30 0C
600C
300C
600C
300C
600C
300C
600C
Sample A
0
0.14
1.75
9.57
119.66
0
0
0
0
Sample E
0.4
0.06
0.9
4.1
61.54
57.14
48.57
0.57
0.49
Sample D
0.7
0.05
0.75
3.42
51.28
64.29
57.14
0.64
0.57
Sample B
2.1
0.04
0.6
2.73
41.02
71.43
65.71
0.71
0.66
Sample C
2.8
0.01
0.3
0.68
20.52
92.86
82.86
0.93
0.83
Figu e 4 The a ia ion o Co osion Ra e wi h ex ac concen a ion a he wo empe a u es s udied.
3.4. E ec s o Concen a ion on Inhibi ion E iciency
The inhibi ion e iciency o Axonopus comp essus ex ac inc eased p og essi ely wi h ising inhibi o concen a ion as
shown in Figu e 5. This end was e iden a bo h 30°C and 60°C, indica ing ha he p o ec i e ac ion o he ex ac on
mild s eel is s ongly dependen on i s concen a ion in he co osi e medium. A 30°C, inhibi ion e iciency inc eased
om 57.14% a 0.4 g/ml (Sample E) o 92.86% a 2.8 g/ml (Sample C), while a 60°C, a co esponding inc ease om
Wo ld Jou nal o Ad anced Resea ch and Re iews, 2025, 27(02), 741-754
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48.57% o 82.86% was obse ed. This s eady imp o emen sugges s ha mo e inhibi o molecules a e adso bed on o
he s eel su ace a highe concen a ions, he eby enhancing su ace co e age and educing access o he co osi e
species o he me al subs a e. The sligh educ ion in inhibi ion e iciency a ele a ed empe a u es may be a ibu ed
o inc eased deso p ion o he inhibi o molecules due o he mal agi a ion, which weakens he in eg i y o he adso bed
ilm. Despi e his, he ex ac s ill e ained conside able e iciency, e en a 60°C, con i ming i s po en ial as a iable g een
co osion inhibi o unde a ying he mal condi ions.
Figu e 5 The a ia ion o inhibi ion e iciency wi h ex ac concen a ion a he wo empe a u es s udied.
3.5 E ec s o Su ace Co e age (θ)
Su ace co e age (θ) is a dimensionless pa ame e ha ep esen s he ac ion o he me al su ace co e ed o p o ec ed
by he inhibi o molecules. I p o ides insigh in o he ex en o adso p ion o he inhibi o on o he me al su ace, whe e
a alue o 1 indica es comple e co e age and 0 implies no adso p ion. A highe su ace co e age alue sugges s mo e
e ec i e ba ie o ma ion agains co osion [16]. In his s udy, su ace co e age inc eased wi h he concen a ion o
Axonopus comp essus ex ac , consis en wi h he obse ed ends in co osion a e and inhibi ion e iciency. A 30°C, θ
inc eased om 0.57 a 0.4 g/ml (Sample E) o 0.93 a 2.8 g/ml (Sample C). A simila inc ease was eco ded a 60°C,
whe e θ ose om 0.49 o 0.83 ac oss he same concen a ion ange.
These esul s indica e ha he ex ac ’s ac i e componen s adso b mo e e ec i ely on o he mild s eel su ace a highe
concen a ions, leading o enhanced p o ec i e ilm o ma ion. Al hough su ace co e age alues we e sligh ly lowe a
60°C compa ed o 30°C, he ex ac s ill demons a ed signi ican adso p ion capabili ies unde ele a ed empe a u e
condi ions.
3.6 E ec o Tempe a u e
Tempe a u e plays a signi ican ole in unde s anding he mechanism o adso p ion and co osion inhibi ion e iciency.
As shown in Table 5, inc easing he empe a u e om 303 K o 333 K leads o a dec ease in he adso p ion equilib ium
cons an (K ads) and a less nega i e Gibbs ee ene gy o adso p ion (∇G0ads) sugges ing a educ ion in he adso p ion
s eng h a highe empe a u es. This end indica es ha he adso p ion o he Axonopus comp essus ex ac on he
mild s eel su ace is p edominan ly physiso p i e, as physical adso p ion ypically dec eases wi h inc easing
empe a u e due o he weakening o in e molecula o ces[17]. To u he in e p e he empe a u e e ec , he
A henius equa ion will be employed:
𝑙𝑜𝑔𝐶𝑅 = log 𝐴 − 𝐸𝑎
2.303𝑅𝑇 …………………… (17)
whe e CR is he co osion a e, A is he p e-exponen ial ac o , 𝐸𝑎is he ac i a ion ene gy, R is he gas cons an , and T is
he absolu e empe a u e. A plo o log (CR) s 1/T (wi h which mul iplica ion by 1000 is usually done o g aph plo ing
con enience) will p o ide he ac i a ion ene gy (𝐸𝑎) om he slope;
(𝐸𝑎= −𝑠𝑙𝑜𝑝𝑒 × 2.303𝑅) ………………… (18)
