Melamine–isa in is Schiffbase as an efficien
co osion inhibi o o mild s eel in 0.5 mola
hyd ochlo ic acid solu ion: weigh loss,
elec ochemical and su ace s udies†
I zan A shad, *
a
Khiza Qu eshi,
a
Awais Siddique Saleemi,
b
Ali Abdullah,
b
Aboud Ahmed Awadh Bahajjaj,
c
Sha aqa Ali*
de
and Awais Bokha i
In he cu en s udy, 3,3
′
,3
′′
-((1,3,5- iazine-2,4,6- iyl) is(azaneylylidene)) is(indolin-2-one) (MISB), which is he
condensa ion p oduc o melamine ( iazine) and isa in, was in es iga ed as a mild s eel co osion inhibi o in
0.5 M HCl. The abili y o he syn hesized is-Schiffbase o supp ess co osion was e alua ed u ilizing weigh
loss measu emen s, elec ochemical echniques and heo e ical compu a ion. The maximum inhibi ion
efficiency o 92.07%, 91.51% and 91.60% was achie ed using 34.20 ×10
−3
mM o MISB in weigh loss
measu emen s, pola iza ion, and EIS es s, espec i ely. I was e ealed ha an inc ease in empe a u e
dec eased he inhibi ion pe o mance o MISB, whe eas an inc ease in he concen a ion o MISB inc eased
i . The analysis demons a ed ha he syn hesized is-Schiffbase inhibi o ollowed he Langmui adso p ion
iso he m and was an effec i e mixed- ype inhibi o , bu i exhibi ed dominan ca hodic beha io . Acco ding
o he elec ochemical impedance measu emen s, he R
c
aluesinc easedwi haninc easein heinhibi o
concen a ion. The weigh loss and elec ochemical assessmen s we e also suppo ed by quan um
calcula ions and su ace cha ac e iza ion analysis, and he SEM images showed a smoo h su ace mo phology.
In oduc ion
Mild s eel is e y cheap and i possesses excellen mechanical
s eng h, making i a popula choice in he pe oleum and
na u al gas indus ies. In his case, s ong acids, such as
hyd ochlo ic acid, a e u ilized o acid descaling, acid pickling,
and oil well acidica ion o elimina e he undesi able sal
deposi s and scales o imp o e oil eco e y.
1
Howe e , s ong
acids may co ode he su ace o mild s eel, ul ima ely esul ing
in expensi e epai s and sys em main enance and nancial,
physical, and en i onmen al losses.
1,2
O ganic co osion inhibi o s a e he mos common ype o
co osion inhibi o s,
3
which a e ca ego ized based on hei
chemical s uc u e, mechanism o ac ion, and o he p ope -
ies.
4,5
Thei affo dabili y, ease o applica ion, and high deg ee
o p o ec ion all con ibu e o hei inc easing popula i y. They
adso b on he su ace o me als and p o ec hem om co o-
sion,
6,7
and hei effec i eness in a ange o acidic solu ions is
also due o he he e oa oms hey con ain, such as ni ogen,
oxygen, phospho us, sul u and halogens.
8–10
The adso p ion o
o ganic molecules on he su ace o me al p e en s di ec
con ac be ween he me al and co osi e en i onmen .
11,12
Howe e , many o ganic co osion inhibi o s a e cos ly and
ha m ul o human heal h and he en i onmen . In an acidic
en i onmen , o ganic molecules con aining he e ocyclic and
a oma ic he e ocyclic ings exhibi g ea e co osion
inhibi ion.
5,13–18
The adso p ion o o ganic molecules is inu-
enced by bo h chemical and physical bonding. The effec i eness
o o ganic inhibi o s can be a ibu ed o hei low elec oneg-
a i i y and g ea pola izabili y, which allow hem o co e huge
me al su aces and quickly ans e elec ons o acan a omic
o bi als.
19
The iazine ing-con aining compound known as
melamine has h ee ni ogen a oms, and he e o e i is
a ni ogen- ich molecule.
20,21
These ni ogen a oms a e eadily
p o ona ed, hus inc easing he solubili y o melamine in pola
sol en s. Recen ly, subs an ial p og ess has been made in he
applica ion o melamine de i a i es o a ange o objec i es,
including he p e en ion o co osion. The ema kable inhibi-
ion efficiency o melamine de i a i es is a ibu ed o he
a
Depa men o Chemis y, Uni e si y o Managemen and Technology, Sialko ,
Pakis an. E-mail: mi [email protected]; a shadm[email p o ec ed]
b
Nano & Mic o FT Labo a o y, P og am o Physics, Uni e si y o Managemen and
Technology, Sialko Campus, Sialko , 51310, Pakis an
c
Depa men o Chemis y, College o Science, King Saud Uni e si y, Riyadh 11451,
Saudi A abia
d
Depa men o En i onmen al Science, Go e nmen College Uni e si y Faisalabad,
Faisalabad 38000, Punjab, Pakis an
e
Depa men o Biological Science and Technology, China Medical Uni e si y,
Taichung, 40402, Taiwan
Sus ainable P ocess In eg a ion Labo a o y, Facul y o Mechanical Enginee ing, B no
Uni e si y o Technology, 2896/2, 61600, B no, Czech Republic
†Elec onic supplemen a y in o ma ion (ESI) a ailable. See DOI:
h ps://doi.o g/10.1039/d3 a00357d
Ci e his: RSC Ad .,2023,13,19301
Recei ed 17 h Janua y 2023
Accep ed 1s June 2023
DOI: 10.1039/d3 a00357d
sc.li/ sc-ad ances
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adso p ion o hei p o ona ed si es and sha ing o hei elec-
ons and lone pai elec ons wi h he i on a om.
20,21
Also,
p esence o ni ogen and oxygen a oms makes isa in de i a i es
e y efficien co osion inhibi o s among he he e o-a om-
con aining compounds, and hus se e al isa in de i a i es
ha e been discussed in dep h as p omising co osion inhibi o s
o me als.
22–24
Recen esea ch showed a emendous inc ease
in he use o Schiffbase de i a i es as co osion inhibi o s o
me als including s eel, aluminum, and coppe in e y acidic
en i onmen s.
25–29
Schiffbases can be easily p epa ed using
ex emely cheap s a ing ma e ials and ha e li le oxici y;
he e o e, hey a e becoming inc easingly popula as co osion
inhibi o s.
30
The a o emen ioned ac o s encou aged us o syn hesize
a melamine–isa in Schiffbase and o e alua e he he mody-
namic ac o s affec ing i s adso p ion, namely, 3,3
′
,3
′′
-((1,3,5-
iazine-2,4,6- iyl) is(azaneylylidene)) is(indolin-2-one)
(MISB), on he su ace o mild s eel in an acidic en i onmen
using weigh loss measu emen s, elec ochemical echniques,
su ace mo phology and compu a ional s udies.
Expe imen al
Ma e ials and sample p epa a ion
Chemicals and sol en s o analy ical g ade we e used o
syn hesize he is-Schiffbase o melamine wi h isa in. All he
sol en s, including melamine and isa in, we e pu chased om
Sigma-Ald ich and used wi hou u he pu ica ion. 0.5 M
HCl solu ion was p epa ed in deionized wa e using analy ical-
g ade hyd ochlo ic acid (37%). The mild s eel specimens used
in his wo k mainly con ained he elemen al composi ion o C
(0.17%), Mn (1.6%), P (0.040%), Si (0.59%), and Fe ( emaining
po ion). Fo he weigh loss measu emen s, a specimen wi h
he dimensions o 2 cm ×2cm×0.3 cm was used. P io o he
s udies, he s eel specimen was polished wi h 600–1200 g ade
eme y pape unde a unning ap. Finally, i was cleaned wi h
deionized wa e , and hen degassed wi h ace one and alcohol,
ollowed by d ying wi h cold ai . Be o e he es s, he polished
mild s eel specimens we e kep in a acuum desicca o .
Syn hesis and cha ac e iza ion da a o MISB
The Schiffbase 3,3
′
,3
′′
-((1,3,5- iazine-2,4,6- iyl) is(azaneylylidene))
is(indolin-2-one) (MISB), as shown in Fig. 1, was syn hesized
ia he eac ion be ween melamine wi h 3 mol o isa in in
e hanol as he sol en unde eux, yielding 87% MISB, and i s
pu i y was de e mined by TLC.
3,3
′
,3
′′
-((1,3,5-T iazine-2,4,6- iyl) is(azaneylylidene))
is(indolin-2-one) (MISB). Yield: 87%, elemen al analysis: calc.:
C, 63.16; H, 2.94; N, 24.55; ound: C, 63.10; H, 2.89; N, 24.41%;
cha ac e is ics IR peaks (ATR): nN–H=3502, nC–H (a oma ic)
=3053, nC]O=1631, nC]N=1592 cm
−1
;
1
H NMR (400 MHz,
DMSO, d, ppm) d=10.02 (s, 3H, NH), 9.21 (ddd, 3H, A –H), 7.69
(ddd, 3H, A –H), 7.45 (ddd, 3H, A –H), 7.29 (ddd, 3H, A –H);
13
C
NMR (100 MHz, DMSO, d, ppm) d=168.7 (C]O), 162.5 (C]N,
iazine), 151.1 (C]N, imine), 148.4 (C–NH), 122.5, 131.9, 128.4,
128.2, 110.0.
Weigh loss measu emen s
Because o i s ou s anding accu acy, simplici y, and excellen
epea abili y, he weigh loss me hod was used o examine he
co osion p o ec ion beha io ini ially. The weighed me allic
specimens wi h dimensions o 2.0 ×2.0 ×0.3 cm we e allowed
o co ode in 50 mL o 0.5 M HCl wi hou and wi h a ying
concen a ions o MISB o animme sion imeo 4hin
a he mos a ically con olled wa e ba h. These specimens
we e aken ou ae a pa icula du a ion, and he co osion
p oduc s on hem we e gen ly wiped offwi h wa e and
ace one. To ensu e he ep oducibili y o he measu emen ,
heweigh lossexpe imen wasca iedou in iplica e o
each in es iga ed concen a ion o MISB.The ollowingequa-
ions we e used o de e mine he a e o co osion and inhi-
bi ion efficiency (IE%).
1,31
CRmg cm2h1¼Dw
s (1)
IE% ¼ðCRÞaðCRÞp
ðCRÞa
100 (2)
q¼IE%
100 (3)
whe e IE%, C
R
(mg cm
−2
h
−1
), and qdeno e he inhibi ion
efficiency (%), a e o co osion, and su ace co e age, espec-
i ely. The a es o co osion o he uninhibi ed and inhibi ed
condi ions a e (C
R
)
a
and (C
R
)
p
, espec i ely. sis he su ace a ea
in cm
2
and is he exposu e du a ion (4 h). Dws ands o he
Fig. 1 Syn he ic scheme o MISB inhibi o molecule.
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diffe ence be ween he ini ial and nal weigh o he specimens
a diffe en concen a ions unde s udy.
Elec ochemical measu emen s
A CHI660D elec ochemical wo ks a ion was employed o he
elec ochemical (EIS and PDP) s udies. The na u e o he elec-
odes and me hod used o p epa e hem we e simila o ha
epo ed in ou p e ious pape .
1
Ae 30 min o imme sion, he
elec ochemical expe imen s we e pe o med. The es ablish-
men o an open ci cui po en ial (OCP) necessi a ed almos
30 min imme sion. EIS da a we e eco ded by employing
a 10 mV signal in he equency ange o 0.01 Hz o 100 kHz. By
 ing he Nyquis cu es o bo h he me allic specimens ha
we e inhibi ed and uninhibi ed, he alues o pola iza ion
esis ance (R
p
) we e ob ained, and hen he pe cen age inhibi-
ion (IE%) was calcula ed by using he ollowing o mula.
1,7
IE% ¼1Ra
P
Rp
P100 (4)
whe e R
a
P
and R
p
P
a e he pola iza ion esis ance in he absence
and p esence o he syn hesized inhibi o , espec i ely. The
wo king elec ode po en ials agains he co osion po en ial
(E
co
) o he po en iodynamic pola iza ion s udy we e a ied in
he ange o −250 mV o +250 mV. To de e mine he alues o
co osion cu en densi y (i
co
), he linea Ta el cu e sec ions
we e ex apola ed. The pe cen age inhibi ion (IE%) by using
hese alues was calcula ed using eqn (5).
1,6
IEð%Þ¼ i0
co ico
i0
co
100 (5)
whe e i
co
and i
0
co
a e he co osion cu en densi ies in he
absence and p esence o he inhibi o , espec i ely.
Compu a ional calcula ions
The densi y unc ional heo y (DFT) me hod was employed o
calcula e he quan um pa ame e s o ou in es iga ed inhib-
i o MISB. Op imiza ion o he geome ical s uc u e o he
p epa ed inhibi o was done using Becks h ee-pa ame e
exchange unc ional B3 wi h he Lee–Yang–Pa (LYP) non-
local co ela ion unc ional and 6-321 + G basis se . The
molecules we e c ea ed using Gauss View 6.0 execu ed in he
Gaussian 09 p og am package.
32,33
Key pa ame e s such as he
ene gy o he highes occupied molecula o bi al (E
HOMO
)and
he ene gy o he lowes unoccupied molecula o bi al (E
LUMO
),
ene gy gap (DE)be ween heE
LUMO
and E
HOMO
, elec onega-
i i y (c), ha dness (h), and soness (s) ac ion o ans e ed
elec ons (DN) we e also calcula ed using he ollowing
o mulas:
34,35
Ioniza ion po en ial (I)=−E
HOMO
(6)
Elec on a ini y (A)=−E
LUMO
(7)
Elec onega i i y (c)=(I+A)/2 (8)
Elec onic ha dness (h)=(I−A)/2 (9)
Chemical so ness (s)=1/h(10)
Dn¼cFe cinhibi o
ðhFe þhinhibi o Þ(11)
Su ace s udies
To analyze he su ace, cleaned and d ied specimens we e
imme sed in 100 mL o 0.5 M HCl o 4 h, bo h wi h and wi hou
he addi ion o MISB a he op imum concen a ion. Subse-
quen ly, he samples we e cleaned, d ied, and subjec ed o SEM
analysis o su ace mo phological s udies. A model X Flash
De ec o 5010 BRUKER Nano scanning elec on mic oscope was
used o eco d he SEM images o he polished, inhibi ed, and
uninhibi ed specimens.
Resul s and discussion
Weigh loss measu emen s
Effec o concen a ion. Table 1 shows he a ia ion in IE%
alues wi h MISB molecule concen a ion in he acidic disso-
lu ion o mild s eel. The esul s show ha abili y o MISB o
p o ec om co osion inc eased as i s concen a ion inc eased,
a aining he highes efficiency a 42.80 ×10
−3
mM concen-
a ion. Ca e ul examina ion o Table 1 indica ed ha i s
p o ec i e abili y signican ly imp o ed as i s concen a ion
inc eased om 8.56 ×10
−3
o 34.20 ×10
−3
mM. Howe e , he e
was only a sligh inc ease in IE% when i s concen a ion
inc eased om 34.20 ×10
−3
o 42.80 ×10
−3
mM. These nd-
ings indica e ha he op imum concen a ion o MISB is 34.20
×10
−3
mM. The syn hesized compound could p e en co o-
sion because i con ains enough ee elec ons, including a lone
pai on ni ogen and elec on pai on ca bonyl, and hese
elec ons a e igh ly bound o he posi i ely cha ged me al
su ace.
1,12,36
Effec o empe a u e. Table 2 shows he effec o empe a-
u e on he capaci y o MISB o p e en he acidic dissolu ion o
mild s eel. The ndings show ha he inhibi ion efficiency o
he syn he ic inhibi o molecules de e io a ed and a co e-
sponding inc ease in he co osion a e alues wi h an inc ease
in empe a u e. Nume ous high empe a u e- ela ed p ocesses,
including molecula agmen a ion, acid-ca alyzed molecula
ea angemen , deso p ion and molecula e ching o adso bed
MISB inhibi o molecules, a e hough o be esponsible o he
dec ease in he abili y o MISB o p o ec agains co osion.
1,37
The inc ease in kine ic ene gy a highe empe a u es causes
Table 1 Weigh loss measu emen pa ame e s wi hou and wi h
a ying concen a ions o MISB inhibi o molecules
Inhibi o C(mM) C
R
(mg cm
−2
h
−1
) IE% q
Blank 5.83
MISB 8.56 ×10
−3
2.41 59.08 0.58
17.10 ×10
−3
1.22 78.80 0.77
25.70 ×10
−3
0.92 84.07 0.85
34.20 ×10
−3
0.51 91.48 0.92
42.80 ×10
−3
0.07 92.07 0.91
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he deso p ion o he adso bed inhibi o molecules om he
su ace o he me al, and consequen ly he o ce cons an
be ween he inhibi o molecules and he me al su ace is
educed.
38
The mos popula me hod o desc ibing he impac
o empe a u e on he in e ac ions o he inhibi o wi h me al is
he A henius equa ion, which can be w i en as ollows:
1
LogðCRÞ¼ Ea
2:303RT þlog A(12)
whe e E
a
,A,R, and T ep esen he ac i a ion ene gy, A henius
p e-exponen ial ac o , uni e sal gas cons an , and absolu e
empe a u e, espec i ely. The slopes o he A henius plo s
(Fig. 2) we e used o calcula e he E
a
alues. The E
a
was calcu-
la ed o be 65.70 kJ mol
−1
, whe eas i was jus 30.50 kJ mol
−1
in
he absence o MISB molecules. The adso p ion and o ma ion
o a de ensi e ba icade by he inhibi o molecules is he cause
o he ele a ed ac i a ion ene gy unde he inhibi ed condi-
ions. By adhe ing o he su ace, he inhibi o molecules
p oduced a coa ing, which imp o ed he ene gy ba ie o he
co osion p ocess.
Adso p ion iso he ms
The adso p ion iso he m model se es as he mos accu a e
ep esen a ion o he MISB in e ac ions wi h me allic
su aces.
1,12
To illus a e he adso p ion beha io o he
syn hesized MISB inhibi o on he su ace, some common
iso he ms we e es ed. Fig. 3 displays he Langmui adso p ion
iso he ms. By using he Langmui adso p ion iso he m equa-
ion p esen ed below, he alues o he adso p ion cons an s a
he op imal concen a ion o MISB a a ious empe a u es we e
e alua ed.
KadsC¼q
1q(13)
whe e Cs ands o he mola concen a ion o MISB molecules
and qs ands o he deg ee o su ace co e age. Typically, a high
K
ads
alue eec s a s ong abso p ion abili y. Table 2 displays
he calcula ed K
ads
alues o he a ious examined empe a-
u es. The G
ads
was de e mined o each empe a u e using
eqn (14).
DG
ads ¼RT lnð55:5KadsÞ(14)
whe e he nume al 55.5 s ands o he wa e concen a ion in
acidic solu ion and he o he symbols ha e hei con en ional
meanings. Table 2 also illus a es he calcula ed K
ads
alues. As
can be obse ed, MISB has a e y high nega i e G
ads
alue,
which indica es ha i has a signican capaci y o adso p-
ion,
1
and also he signican endency o adso p ion on he
me allic su ace was indica ed by he high alues o K
ads
o
MISB.
Table 2 The alues o a e o co osion (C
R
), inhibi ion efficiency (IE%), adso p ion cons an (K
ads
), and Gibb's ee ene gy (DG
ads
) o mild s eel in
0.5 M HCl a diffe en empe a u es
Tempe a u e
(°C)
Blank MISB
C
R
(mg cm
−2
h
−1
)DG
ads
(kJ mol
−1
)K
ads
×10
4
(L mol
−1
)C
R
(mg cm
−2
h
−1
) IE%
35 5.86 −34.52 1.3 0.50 91.47
45 8.20 −34.21 0.75 1.13 86.19
55 12.40 −34.53 0.56 2.17 82.52
65 16.60 −33.53 0.27 5.07 69.47
Fig. 2 A henius plo s o he co osion o mild s eel in he p esence
and absence o MISB.
Fig. 3 Langmui adso p ion iso he m plo ed o he adso p ion o
MISB on mild s eel su ace in 0.5 M HCl.
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Open ci cui po en ial es
To suppo he weigh loss expe imen , elec ochemical
measu emen s we e conduc ed. The open ci cui po en ial is
he diffe ence be ween he po en ials o he wo king elec ode
and he s anda d o e e ence elec ode when no ex e nal
cu en is applied. Fig. 4 displays he open ci cui po en ial
e sus ime cu es o 25 minu e cu es ollowing 30 min
imme sion. As can be obse ed, he OCP s. ime cu es o he
inhibi ed and uninhibi ed condi ions depic s aigh lines,
indica ing ha a s eady-s a e po en ial de eloped in bo h cases.
The s aigh lines also show ha he Fe
2
O
3
and Fe
3
O
4
oxide
laye s we e en i ely elimina ed, and ha a p o ec i e o inhibi-
i e lm by he MISB inhibi o was o med on he me al su ace.
Addi ionally, i can be obse ed ha he open ci cui po en ial
s. ime cu es shied wi h MISB in he ca hodic o nega i e
di ec ion. This esul indica es ha al hough he p esence o
MISB affec s bo h he ca hodic and anodic p ocesses, i s effec-
i eness owa ds ca hodic eac ions is compa a i ely highe due
o he p ecipi a ion o he inhibi o s molecule on he ca hodic
si es o he me al su ace.
Po en iodynamic pola iza ion s udies
Fo ho ough insigh in o he inhibi o beha io owa d he
anodic and ca hodic p ocesses, po en iodynamic pola iza ion
s udies we e conduc ed. Fig. 5 depic s he ca hodic and anodic
pola iza ion cu es o me allic dissolu ion in 0.5 M HCl wi h
and wi hou MISB, and Table 3 lis s he pola iza ion indices o
he inhibi o . Ob iously, a he a ious concen a ions es ed,
MISB had an impac on bo h he ca hodic and anodic eac ions
and p ocesses, causing a signican d op in he co osion
cu en densi y (i
co
) wi hou al e ing he ypical Ta el cu e
appea ance. This obse a ion e eals ha he MISB molecules
hinde he p ocess o co osion by blocking he su ace-ac i e
si es h ough adso p ion.
39
This esul implies ha he
examined inhibi o p ecipi a ed o e he ca hodic egion, and
subsequen ly beha ed as mos ly a ca hodic- ype inhibi o gi en
ha he co osion po en ial (E
co
) in he Ta el cu e inhibi ed
by MISB shied owa ds he nega i e si e.
40
The shiin he
alue o he co osion po en ial o he inhibi ed Ta el cu e
ela i e o he uninhibi ed Ta el cu e can be used o cha ac-
e ize he ca hodic and/o anodic na u e o he syn hesized
inhibi o .
Elec ochemical impedance spec oscopy
Fig. 6 and 7 show he Nyquis and Bode plo s o he co osion
o mild s eel in acidic condi ions wi h and wi hou MISB
molecules, espec i ely. A all concen a ions unde in es iga-
ion, he Nyquis plo s appea as a single semici cle, which
sugges s a single cha ge ans e eac ion. I is e iden ha he
diame e s o he semici cles in he Nyquis plo s a e la ge o
he inhibi ed me allic specimens han o he uninhibi ed
me allic specimens (blank). Addi ionally, a highe concen a-
ions o MISB, he diame e o he semici cle inc eased mo e
no iceably. The Nyquis plo s o he me allic samples unde he
inhibi ed and uninhibi ed condi ions we e  ed in he sui able
equi alen ci cui o de i e a ious impedance cha ac e is ics.
In he desc ibed ci cui , a cons an phase elemen (CPE) was
used in place o a pu e capaci o gi en ha i gi es a g ea e
unde s anding o he in e ac ions be ween he me al and elec-
oly e a in e aces. Subsequen ly, we p esen he impedance o
he cons an phase elemen , which is ypically indica ed by
Z
CPE
.
Z
CPE
=Y
0
−1
((iu)
n
)
−1
(15)
whe e u,Y
0
,n, and i ep esen he angula equency, CPE
cons an , phase shi, and an imagina y numbe , espec i ely.
Gi en ha a high alue o nco ela es wi h a high le el o
su ace smoo hness and ice e sa, i can also be used as
Fig. 4 Open ci cui po en ial s. ime cu e o co osion o mild s eel
in 0.5 M HCl in he absence and p esence o a ious concen a ions o
MISB molecules.
Fig. 5 Pola iza ion cu e o mild s eel in 0.5 M HCl wi h and wi hou
MISB molecules.
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a measu e o su ace oughness o smoo hness.
41
Table 4 lis s
he calcula ed impedance pa ame e s, % inhibi ion efficiencies,
and su ace co e age. The esul s show ha , wi h a ew excep-
ions, he n alues a e highe in he inhibi ed solu ion han in
he uninhibi ed condi ion. This disco e y shows ha he
me allic su ace was subs an ially smoo he in he p esence o
MISB, especially a a g ea e concen a ion, han in i s absence.
Addi ionally, he n alues o bo h he inhibi ed and uninhib-
i ed condi ions a e close o uni y, showing ha he CPE unc-
ioned as a pseudo-capaci o du ing he in es iga ion. O ganic
inhibi o s a e hough o be able o supp ess co osion in
agg essi e acidic media by adso p ion on me al–elec oly e
in e aces, which leads o he o ma ion o an elec ic double
laye .
1,42
The ndings in Table 4 demons a e ha he R
c
alues
o he inhibi ed case a e much g ea e han ha o he non-
inhibi ed condi ion. This esul sugges s ha he difficul y o
cha ge ans e in he p esence o MISB is due o i s adhesion o
he me al–elec oly e in e aces. Acco ding o addi ional nd-
ings, he inc ease in R
c
alue is much mo e no iceable a highe
inhibi o concen a ions.
Compu a ional s udies
Fig. 8 depic s he on ie molecula o bi als (op imized,
HOMO, LUMO and MEP) o MISB, while Table 5 lis s he
Table 3 Pola iza ion pa ame e s o he co osion o mild s eel in 0.5 M HCl wi hou and wi h a ious concen a ions o MISB molecules
Inhibi o C(mM) E
co
(mV pe SCE) b
a
(mAcm
−2
)b
c
(mV dec
−1
)i
co
(mAcm
−2
) IE% q
Blank −445 70.6 114.8 1130
MISB 8.56 ×10
−3
−522 69.2 107.8 462.0 59.21 0.60
17.10 ×10
−3
−545 164.7 146.3 229.0 79.83 0.80
25.70 ×10
−3
−525 72.3 76.4 173.0 84.78 0.83
34.20 ×10
−3
−541 132.6 152.1 97.0 91.51 0.92
Fig. 6 Nyquis plo s o co osion o mild s eel in 0.5 M HCl wi hou
and wi h a ious concen a ions o MISB molecules.
Fig. 7 Bode plo s o co osion o mild s eel in 0.5 M HCl wi hou and wi h a ious concen a ions o MISB molecules.
Table 4 EIS pa ame e s o he co osion o mild s eel in 0.5 M HCl
wi hou and wi h a ious concen a ions MISB inhibi o molecules
Inhibi o C(mM) R
s
(Ucm
2
)R
p
(Ucm
2
)nIE% q
Blank 1.13 10.71 0.828
MISB 8.56 ×10
−3
0.727 26.80 0.812 60.06 0.61
17.10 ×10
−3
0.520 51.19 0.816 79.10 0.80
25.70 ×10
−3
0.693 78.01 0.843 82.29 0.83
34.20 ×10
−3
0.569 127.21 0.856 91.60 0.93
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calcula ed DFT indices. The DFT pa ame e s in he able a e in
good ag eemen wi h he expe imen al da a. Gene ally, he
in e ac ions o co osion inhibi o s on a me allic su ace a e
comp ised o dono –accep o bonding, whe e in hese in e ac-
ions, E
HOMO
is associa ed wi h he capaci y o he inhibi o o
ansmi elec ons (cha ge), whe e E
LUMO
is associa ed wi h i s
abili y o accep elec ons. Hence, s ong me al–inhibi o
binding and good p o ec i e abili y a e ela ed wi h high alues
o E
HOMO
and low alues o E
LUMO
, espec i ely.
43–47
The highes
occupied-molecula -o bi al (E
HOMO
) ene gy shows he capabili y
o he es ed inhibi o o dona e elec ons. The capabili y o he
molecules o accep elec ons om he back dona ion o i on,
and o hus enhance he binding ene gy be ween he me al and
he inhibi o is shown by a lowe E
LUMO
alue. The g ea e he
E
HOMO
and he lowe he E
LUMO
, he be e he capaci y o he
es ed inhibi o o a ach o he me al su ace.
3–5,30
This sugges s
ha MISB is an efficien co osion inhibi o , which is in good
ag eemen wi h he expe imen al ndings.
48
The chemical
esponse was de e mined by he ene gy gap (E
gap
) alues. In
e ms o eac i i y, he mo e eac i e he molecule owa ds he
subs a e su ace, he highe i s inhibi o y efficiency, and he
smalle he DEgap, he mo e s able i is. Consequen ly, he
combina ion o MISB wi h he Fe subs a e was s able. The
dipole momen (m) is caused by he nonuni o m su ace cha ge
dis ibu ion o he a oms in he molecules. A low dipole
momen alue suppo s he capaci y o a molecule as an
Fig. 8 Op imized chemical s uc u e (a), highes occupied molecula o bi al (b), lowes unoccupied molecula o bi al (c) and molecula elec-
os a ic po en ial (d) o he es ed inhibi o MISB.
Table 5 DFT pa ame e s o neu al and p o ona ed o m o MISB inhibi o molecule
MISB E
HOMO
(eV) E
LUMO
(eV) DE(eV) h(eV) s(eV) c(eV) DN(eV)
m
(debye)
Neu al −0.21 −0.09 0.15 0.06 16.66 0.12 0.207 6.32
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inhibi o . Acco ding o he ha d–soacid–base concep , a so
molecule in has a lowe E
gap
alue and g ea e basici y, whe eas
he opposi e is obse ed o ha d molecules.
49
Consequen ly,
asomolecule has g ea e adso p ion abili y due o i s easie
elec on ans e and i is a s onge co osion inhibi o han
a ha d molecule.
Acco ding o Luko i 's esea ch, when he numbe o elec-
ons ansmi ed (DN) is less han 3.6, he inhibi ion pe o -
mance imp o es as a unc ion o he elec on ans e .
49
The
la ge he ac ion o elec on anspo (DN), he be e he
co osion inhibi o . Thus, he high alue o DN o MISB sugges s
ha i is a good co osion inhibi o and elec on dono .
The global elec onega i i y alue o MISB was also
compu ed. A high global elec onega i i y (c) alue sugges s
ha he in es iga ed compound is less po en o dona e/ ans e
i s elec on o he app op ia e accep o molecule, e.g., d-o bi al
o he su ace Fe a oms in he p esen case.
MISB shows an elec onega i i y alue o 0.12. The low alue
o c o MISB indica es ha i has high elec on ans e abili y,
and he eby ac s as good co osion inhibi o . Based on he alues
o E
LUMO
and E
HOMO
, he global ha dness (h) and soness (s)
alues we e also de i ed o he MISB molecule. A high alue o s
is ela ed wi h high eac i i y; elec on dona ing abili y, adso p-
ion endency and inhibi ion efficiency, while he opposi e is
obse ed o h.
49,50
The esul s show ha MISB has high s alue
(16.66 eV) and low h alue (0.66 eV), which sugges ha MISB is
a highly eac i e and po en co osion inhibi o . Based on he
abo e discussion, i can be concluded ha he DFT s udy
p o ided good suppo o he expe imen al esul s and ndings.
Sugges ed co osion-inhibi ion mechanism
The inhibi o y ine ia o o ganic molecules is caused by he
o ma ion o a p o ec i e laye ha is adso bed on o he i on
su ace. G a ime ic measu emen s e ealed ha he es ed
inhibi o g ea ly educed he co osion o mild-s eel. Mo eo e ,
he adso p ion iso he m s udies e ealed ha he in es iga ed
inhibi o molecules adhe ed o he mild-s eel su ace and
ollow he Langmui adso p ion model. Fu he mo e, he
adso p ion beha io o he gene a ed p o ec i e lm is p inci-
pally de e mined by he ollowing: (1) elec os a ic in e ac ions
Fig. 9 Sugges ed co osion-inhibi ion mechanism o mild s eel in
0.5 M HCl wi h he addi ion o he examined inhibi o (MISB).
Fig. 10 SEM image o mild s eel specimen o 4 h imme sion ime in 0.5 M HCl (a) be o e imme sion, (b) imme sed in 0.5 M HCl wi hou inhibi o
and (c) imme sed in 0.5 M HCl wi h MISB inhibi o molecules.
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h ough p o ona ed he e oa oms and (2) diffe en linkages
be ween inhibi o molecules and he mild-s eel su ace.
51
Fig. 9 illus a es in u he de ail how he es ed inhibi o
compounds in e ac wi h he mild-s eel su ace. The mos
common ou e o adso p ion be ween he inhibi o molecule
and he mild-s eel su ace in ol es he in e ac ion o bo h he
pi-elec ons o he a oma ic ings and he acan d-o bi al o he
me al a oms. The second me hod in ol es in e ac ions be ween
he lone elec on pai s in he he e oa oms and he unoccupied
d-o bi als on he su ace o i on (mild s eel) a oms. The d-
o bi als o he Fe a om will sha e hese ac i e elec ons.
Mo phological in es iga ion
Fig. 10 displays he SEM images o he mild s eel specimen ha
was co oded o 3 h. As can be seen om he ex emely ough
su ace and isible pi s and ssu es, he unp o ec ed me allic
specimens in he SEM pho os a e se e ely co oded and
damaged. Howe e , as can be seen om he SEM image, he
me allic su ace g ea ly imp o ed in he p esence o he
syn hesized MISB inhibi o . I can be pos ula ed ha MISB
gene a ed su ace lms, which shield he me allic su aces om
co osion, based on he be e su ace mo phologies o he
shielded me allic specimens. This esul also con med he
pa e n o inhibi o y effec i eness a ained h ough he weigh
loss and elec ochemical echniques.
Conclusion
In conclusion, we showed he syn hesis o a melamine–isa in
hyb id is-Schiffbase (MISB) and in es iga ed i s po en ial o
unc ion as a mixed- ype inhibi o . The syn he ic inhibi o
demons a ed a high mild s eel inhibi ion pe o mance in 0.5 M
HCl. Acco ding o he e alua ion o weigh loss, he efficacy o
inhibi ion inc eased as he inhibi o concen a ion inc eased
and i was educed as he empe a u e inc eased, and he
maximum efficiency o 92.06% was de i ed o MISB. Acco ding
o he elec ochemical impedance measu emen s, he inc eased
inhibi ion efficacy o he inhibi o solu ion was caused by i s
high cha ge ans e esis ance. The Langmui - ype iso he m
was iden ied h ough he analysis o he adso p ion iso he ms
and he modynamic pa ame e s, indica ing physiso p ion. The
pola iza ion analysis e ealed ha MISB unc ions p ima ily as
a ca hodic- ype inhibi o . Fu he e idence o he abili y o he
syn he ic is-Schiffbase o supp ess co osion was p o ided by
SEM images, which showed he de elopmen o a p o ec i e
laye on he su ace o mild s eel.
Conflic s o in e es
The e a e no conic s o decla e.
Acknowledgemen s
The wo k was unded by he Resea che s Suppo ing P ojec
Numbe (RSPD2023R763) King Saud Uni e si y, Riyadh, Saudi
A abia. We a e also highly g a e ul o he Highe Educa ion
Commission (HEC), Go . o Pakis an o nancial suppo . The
nancial suppo by Uni e si y o Sialko and UMT, Sialko is
also highly acknowledged.
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