Indian Journ al of Engineerin g & Matcrial s Sc iences
Vo l. 9, April 2002, pp. 141 - 146
Inhibition of corrosion of mild steel by nitrite, hydrogen phosphate and
molybdate ions in aqueous solution of sodium chloride
Rita Mehra & Ad iti Soni
Departill ent of Pu re & A pp li ed Chemi stry, Maharshi Dayanand Saraswa ti Uni ve rsity. Aj mer 305 009, India
Received 1 March 200 1: accepled 30 Jal/uary 2002
Inh ibiti on of Illild stce l in aqucous sodium chl oride by sodium nitrite. di sodiulll hydrogen phosphatc and sodi um molybdate with respec t to th e inhi bitor co nce ntrat ion and temperature is reponed here. Na2M o04 shows hi ghest effic iency
(90.00%) foll owed by NaN0 1 (87.56%) and Na2HP04 (85.36%). A s such, th e order of inhibition observed is
Nal M o0 4> aN0 2> Nal H P04 . Therm odyna mi c as well as ki neti c param eters including acti vation energy (Ell )' ch:.lIlge in free
cnergy of ad sorptio n (LlC), entropy of adsorpti on (LIS), corros ion current (I,,,,,.), open circuit potential (OCP). Tafel slopes
(pa, {3d and threshold limi tin g value (TL V) for prot ec ti ve film form ati on have also been calcul ated from we ight loss and
potenti osta ti c polari sati on Illeth ods.
Sod ium chl oride is present in large co ncentrations in
all types of water, potable as we ll as marine water.
Sodium chloride conce ntrati on vari es from 150 ppm
to 2000 ppm in various types of water; for drinking
water allowable below 1000 ppm I, and 250 ppm for
aes thetic va lue as per WHO guidelines. But, the pres2
ence of large co ncentration of sodium chloride is
harmful for water pipeline materi al because of aggressive behaviour of chloride ions towards corrosion and
increase in conducti vity of the solution by strong acid
and strong base formed after hydrolysis in spite of a
sli ght variation in pH of aqueous solution of sodium
chloride.
Various studi es have been carri ed out to mitigate
the corrosion of meta ls due to sodium chloride by or37
ganic as well as inorgani c inhibitors - . Th e addition
of these inhibitors in large amounts in potabl e water
may change the quality of potable water resulting in
various health hazard s.
Sodium nitrite, di sodium hydrogen phosphate, sodiu m molybdate are among th e sui tab le inorgani c corrosion inhibiting ions, which ca n co ntrol the corros ive
behaviour of sod ium chloride without affecting the
quality of water to a great ex tent. B efore controlling
the corrosion in water pipelines, it is important to
identify the factors which influence co rrosion reacti ons as well as the mutual effect of ions on one anoth er, particul arl y while in vesti gatin g aqueou s solutions co ntaining mixture of ions 8 -'J .
Electrochemical and weight loss investi gations for
mild steel in aqueous solution of NaCI in presence of
sodium nitrite, disodium hyd rogen phosphate, and
sodium molybda te as corros ion inhibitors are reported
here.
Experimental
Weight loss studi es were per formed on heat-treated
2
mild steel square specimens (2x 2 cm of 0.03 cm
thickness) . The chemical composition of mil d teel
was determined by known chemi cal methods10- 12 and
found to be: C 0.22%, S 0.018%, Ni 0.25 %, Mn
0.62%, P 0.022%, M o 0.15 %, Si 0.28 %, Cr. 0.095 %,
Cu 0.20%, besides iron . Th e specimens were degreased in benzene, washed with 50% acetone, dried
and weighed. Cleaned specimens were suspended by
a glass hook in th e aqueous solution of sodium chloride (concentrations ranging from 200 to 3000 ppm)
(pH-7) for 24 h. Corroded coupons were then cleaned
in a solution of hydrochloric aci d, 50 giL stannous
chloride and 20 giL antimony (1I1) chloride'J·lo and
weighed. Temperature of corrosive medium was co ntroll ed from 293 K to 353 K for 24 h immersion peri od. Experimental set up for polarisat ion measurements consisted of potentiostatlgalvanostat model CL95 of Eli co with a sweep current generator and three
electrode cell. All potenti al values were recorded on
the saturated calomel (SCE) scale.
Results and Discussion
Th e ef fect of sodium chloride co nce ntration on co r2
rosion rate ( mg/dm ) and open circuit potential recorded immed iately after the immersion of spec imen
in co rrosi ve media against reference ca lomel electrode, is shown in Fig. I . The Figure shows that co r-
INDIAN 1. ENG . MATER . SC I , APR IL 2002
142
4 0r-----------------------------------~
35
;
E
(a)
30
"0
en
.§.
25
-o-r.NV;No-;or-.;t--.;t - ON - r-\O
N\clnN\o In - OO\
C v; \0 00 N C"'") <'I r- N
o'OOr-:o'o':xi"<iMM
r '10t NNNNM('f"'"I ( ""'I
1000
500
1500
2000
2500
C"'")\o\oOC"'") C"'") oo -
.450
_ ·550
>
.
·600
Q..
·700
=:
1500
2000
2
a
0\ 0
0 ("' I 0\ N
r----01t.r,N['-- ........
'\
·500
E
1000
500
(b)
~ ·650
;;
O
c-ri~0N M .r)('f'"; r-...: r-:
Sodiurn Chloride ConcentraUon (ppm)
-
"<t N
--
N
V)~
r-:o'oNooo'~ ~ "<i0
-
-
N
("1 -
_ .. N
N
C""t 01
8~:g~~~8gg
o'Mr-:v-iOv-iOMO
r-oooor--ooooooooQ'\
·750
·800
Sodium Chloride Co ncen trati on (ppm)
Fig. I- Effec t of sodium chloride co nce ntrati on on (a) we ight loss
and (b) po tenti al of mi ld steel in aq ueous solution of sodi um chl oride
o o O N - "<t v ; O - \ O
C"'")O\O\OV;OOOC"'")-.;t
o'"<ioo0 ~ v-io"<io
r-oooor-ooooooooO\
rosion rate increases with th e increase in concentration of salt and OCP shi fts toward more negative
va lue with th e increase in sodium chl oride co ncentration . Increase in cOITOsion rate and decrease in OCP
indicates the in verse relati onship of rate and open circui t potenti al.
The corrosion rate and inhibition effi ciency (lE%)
(calcul ated on percentage bas is ' 3) of mild steel In
aq ueous solution of NaC I in presence and absence of
inhibi tors at di ffe rent temperatures and at varying inhi bi tor concentrati on is show n in Table I along with
£", L1G, L1S and Q va lues.
IE% =
W
II
~.:;b~~;;;8~~;;;
~ ~ 0.: r-: C"i r-: ~ v-) -:
&
E
~
v;
r-oooor-oox>ooooo..
===
00
('f')
~N
(3
E
....J:'2
gE
-w' x IOO
W
II
00
o
C"'")
where W is the weight loss of the uninhibited aqueous medium and Wi is the we ight loss of metal in inhibited medium . The inhibition effi ciency increases
with the increase In co ncentrati on of inhibitor and
decreases wi th the temperature.
£ " has been calcul ated from the slope of plots of
Fig. 2 [log weight loss vs . IIT 103 (T being absolute
temperature)] and with the help of the integrated fo rm
of the Arrhenius equ ation :
til",
(3 E
....J-O
' bQ
~ E
II
u
c:
o
... u
C"'") v;O C"'")V;OC"'")v; O
00 - 00- 0 0 -
00 00 0 0 0 0 0
9
:E
:.2
,.5 ...
E
o:s
Z
oo
::E
';;'
z
MEHRA & SON!: INHIBITION OF CORROSION OF MILD STEEL BY N ITR ITE
1.8
N
(a)
E 1.6
+---------+----.
-
:g,1,4
E 1.2
~
--+--Blank
....
-0--0 .03%
1
.2 0.8
~ 0.6
~ 0,4
g' 0.2
-tr-0 .05%
-0- 0 .10%
o
2.9
3
3.1
11T10
3
3.2
3.3
(b) ~
1.6
E
"C
II)
II)
--+-Blank
o
:: 0.8
-0--0.03%
~
.21 0.6
~
t3J
temperature)
Heat of adsorption (Q) was co mputed using l4:
1,4
.sen 1.2
III
= [{ log (811 - 8 ) - log
where log fJ
= - 1.74 - (CoA I2 .303 RT) and 8 and
( 1- 8) are the surface coverage and open surface at any
1.8 , . - - - - - - - - - - - - ,
N
th e corrosion of mild steel in 600 ppm NaCI solution
is - 17.79 kJ morl.
In so lu tions containing various inhibitors, E" va lues
are found to be sli ghtly hi gher, i.e. , 19.20, 20 .50 ,
l
1
22.22 kJmo l- for NaN0 2; 18. 19, 19. 12,2 1.11 kJmor
for di sod ium hydrogen phos phate ; and 2 1.1 4, 24 .52 ,
l
26. 12 kJmor for sodium mo lybdate (at 0.03 , 0.05
and 0.1 0% inhibitor co ncentration, respec ti ve ly).
Heat of adso rption (Q) and free energy of adsorpti on .1C were calc ul ated by usi ng l3:
Log C
K
143
Q = 2.303 R [{ log (8 2/ 1- 82) } - {log (8ll - 8 1 )}]
(TI x T2 ) I (T2 - T I )
-tr-0.05%
0.4
-0-0.10%
en
.2 0.2
o
~---------~
2.9
3
3.1
3.2
3.3
3
11T 10 K
where 8 1 and 8 2 are the fraction s of the metal surface
covered by the inhibitors at temperatures TI and T2
respectively and (1-8 1) and ( 1-8 2) are the open surfaces which react with the stud ied aggressive ions.
Surface coverage (8) is calcu lated b/ 5 :
1.8 , . - - - - - - - - - - - - ,
N~
1.6
.5
1.4
(C) ~
~ 1.2
-;
1
.2
0.8
--+-Blank
-0--0.03%
1: 0.6
en
-tr-0.05%
en 0.2
o
0 ~------------------~
2.9
3
3.1
32
3.3
-0-0.10%
.~ 0.4
11T 10 3 K
Fig. 2- A rrhenius plots for corrosion of mild steel in aqueous
so luti on of 600 ppm sod ium ch loride at varyi ng inhibitor concent!"ali on [(a) NaNO] . (b) Na] HPO,,, (c) Na] M o0 4 ]
where WI & W2 are the corresponding weig ht loss at
temperature TI and T2 (K), respectively. The Figure
also depicts the Anhenius plots for weight loss at different temperature and concentration of various i nhibitors. The hi gher slopes in presence of inhibitors
show high act ivation energy value. The E" values
given in Tabl e I show that the energy of activation for
where, Wu is the weight loss in uninhibited corrosive
medium and Wi is the weight loss in inhibited conosive medium.
The Q values are very less at low inhibitor concentration , increase as the inhibitor concentration increases ranging from 3.3 to 6.6 kJ mol- I for NaN0 2,
l
2.0 to 5.3 kJmor for Na2HP04, and 3.8 to 7.0
kJ mor l for Na2Mo04. The average .1C value for all
the cases are negative. The Na2Mo04 inhibitor gives
more negative free energy of adsorption than NaN0 2
and Na2HP04, supporting the order of reported inhibition efficiency for these inhibitors. The comparison
with Mo0 4- has not been investi gated I6-17 .
Thus, with the increase in concentration of inhibitors, .1C becomes less negative depictin g decrease in
corrosion rate. Similarly, with temperature, .1C becomes more negative indicating the increase in corrosion rate with temperature which supports the Arrhenious relationship of temperature and reaction rate.
.1S (as given in Table I) has been calculated by using
the Gibbs-Helmholtz eq uation:
INDIAN 1. ENG . MATER. SCI .. APR IL 2002
144
200
1600
r - - - - - -' - - - - - - - - .
(c)
(a)
12 00
BOO
800
400
4 00
0
'"
til
:;:;
:;;
c:
..
C
·400
III
~Blank
0
a.
....0ClI
· 4 00
-<>--0.03%
G-
-l:r- 0 .05%
·SOO
·BOO
--0-- 0 .10%
·1 200
· 1200
·1 GOO
· 1600
· 2000
·2000
·2 40 0
0
log curre nt de ns ity
log current density
1600
1600
(d)
(c)
1200
1200
800
800
400
400
0
..,'"
.!!!
'E
C
Q)
·. 00
2l
-<>-0.03%
0
!l-
-(r-0.05%
·800
-0-0.10%
-400
-<>-- 0.03%
0
!l-
-ls-O.05%
-800
-0-
0.10%
-1200
.1200
-1600
·1600
-2000
· 2000
-2400
2
0
·2400
0
log curr" nt density
log current density
Fig. 3- Pote nti ostati c po lari sa ti o n c urves for co rros io n of mild stee l in aqueou s soluti on o f 600 ppm sodium c hl o ride at varyin g inhibito r
co nce ntrati o n I(a) Bla n ' , (b) NaN0 2, (c) Na2HP04' (d) Na2MoO. l
l
L1C
= Q - TL1S
The values of L1S are hi gher for Na2Mo04 than NaN02
and Na2HP04 which range from 100.3 to 106.00
l
JK- Imor l for NaN0 2, 98.47 to 104. 14 JK-1mor for
Na2HP04 and 117.629 to 124.7 JK-1mor for
Na2Mo04 (for 0.03-0. 10% concentration).
A comparison of IE%, L1C , L1S, E", Q rev eals th at
negative value of L1C , small Q value and hi gh E" values are associated with high inhibition efficiency.
MEHRA & SON I: INH IBITION OF CORRO SION OF MILD STEEL BY NITRITE
Tabl e 2 -
Tafel paramcters and inhi bi ti on effi ciency of various inhibitors for mild stee l in 600 ppm sodium chloride at 28
Inhibitor
ame
Blank
NaN0 2
Na2l-1P0 4
Na2M o04
145
OCP
Conc.
- mY
0.03
0.05
0.1 0
0.03
0.05
0.10
0.03
0.05
0. 10
586
387
3 12
260
4 10
387
380
360
278
260
Tafel slopes
A nodi c (f3a)
+ mY/decade
Ca thod ic ((3c)
- mY/decade
72
120
240
244
250
230
238
240
250
260
274
202
202
200
200
200
200
204
204
208
Polarisation behaviour
Anodic and cathodic polari zat ion curves for mild
steel in 600 ppm NaCI so luti o n (in absence and presence of inhibito r) are g ive n in Fig. 3 at 3S± 1°C. With
the increase in concentration of inhibitor, curves beco me anodically po lari sed indi cating film fo rm ati o n
at th e anodi c sites ' 8. ' 9 The values of various kin etic
parameters of Tafel pl o ts are g iv e n in Tab le 2. Th e
OCP values shi ft towards less negative with the increase in concentration of inhibito r. Th e values of log
2
IC{)/T and l eo!! ()..lA/cm ) also dec rease in the presence of
2
inhibitor. For blank solution, log 1m!! is 1.05 )..lA/cm
which dec reases in presence of NaN0 2 by 0.32, 0. 16,
2
2
0.06 )..lA/cm for NaN0 2, 0.42, 0. 14, 0. 13 )..lA/cm for
2
Na2HP04 and 0.27, 0.09, 0.08 )..lA/cm for Na2Mo04
(at 0.03, 0.05 and 0.10% inhibitor concentration respectively). Inhibition efficiencies calculated from
corrosion current obtained by ex trapolation of Tafe l
slopes to OCP are also given in Table 2. In almost al l
cases, the Tafel plot efficiencies agree wel l with the
values obtained from weight loss data.
± I °C
Log
/COI.,..
Corrosion
currelll
~tA/cm 2(a)
Inhibiti on
effi ciency
(lE%) from(a)
Inhi bi ti on
effic iency
(lE%) fro m WI.
Loss
1.09
0.36
0.20
0. 10
0.46
0.33
0. 17
0. 3 1
0.13
0.09
12.30
2.29 1
1.585
1.259
2.884
2. 138
1.479
2.042
1.349
0.9 12
81.37
87 .11
89 .76
76.55
82.62
87.97
83.40
89.03
92.59
80.44
85 .77
89.77
78 .22
82.66
87.55
82.22
87. 11
92.44
Na
Na
"() ~
o~
./ ) )
/
Mo
~o
. METAL
Na
/
/0
I'
o t/'"
~O- H
MF.TAL
Fig. 4-Adsorpti on model of Nal Mo04, NaN0 2 and Na2HP04 on
metal surface
Mechanism
The inhibitin g behaviour of these inhibitors is due
to the formation of thin films on the surface of
metal ' 8. '9 . Th e metal s urface in contact with aqueous
so lutions becomes electron deficient, i.e. positively
charged and higher electron density centres in anions
of inhibitors get adsorbed on the metal surface to form
a protective film. In case of NaN0 2, nitrogen as well
as oxygen atoms have high e lectron density , hen ce
both act as reaction centres. Similarly, in Na2HP04
phosphorou s as well as oxygen atoms act as reaction
centres and in Na2Mo04 oxygen atom acts as reaction
center.
The adsorption mode ls of NaN 0 2 , Na2Mo04 and
a2HP04 are show n in Fig. 4, accord ing to which
Na2Mo04 has greatest inhibition efficiency due to
strong adsorption on mi ld steel fo ll owed by NaN0 2
and Na2HP04.
The TLV values (the concentration of inhibito rs
required to inhibit corrosion) were determined by
varying inhibitor concentrations up to 2.0%. It was
found that at 308.15 K, Na2Mo04 gives nearly 100%
inhibition efficiency at 0.9%, NaN0 2 at 1.2% and
Na2HP04 at 2.0%. At these concentrations, the weight
loss beco mes zero for 24 h immersion period and
protection efficiency becomes nearly hundred percent,
TLV values for Na2Mo04, NaN0 2 and Na2HP04 bein g 0.9%, 1.2% and 2.0% at 308. 15 K.
IND IAN J. ENG. MAT ER. SC I.. APRIL 2002
146
6
Conclusions
Th e co rrosion rate of mild steel has direc t relati onship to the co ncentration of NaC I in aqueous so luti on
and has an in verse relationship w ith inhibitor co ncentrati on. The effici ency or inhibitors dec reases in th e
order Na2M oOej>NaN02> Na} HPOej . Th e interaction
between inhibitor and mild steel surface is accomplished by negative free energy, low Q va lues hi gh .15
va lues , high E" va lues and hi gh va lue of inhibiti on
effici ency w i th in crease in co ncentrati on of inhibitor.
I
2
3
Th ird Edi ti on (Gov l.
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Denny A & Jone D A, Prillciples alld prel'elllioll (!l CO''I'OSiO Il , Second Editio n (Prenti ce Hall Upper Sadd le Ri ve r.
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Thoma s 1 G N & Davies J D, Br Corms J , 12 ( 1977) 108.
Mallllal
()JI
\l'me r sllpplv alld
/1'1'(/11111'111,
4
W ebe rJ ,Br Co rm.l' J , 14( 1979)69.
5
Gda ll as h A G & Tamous H M, Bill/ EleC/rochell/. II ( 1995)
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7
Abdu ll ah M & Elhalke m S M, Bill/ E/eC/mchell/. 12 ( 1996)
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Tandel PB&Oza l3 .TmllsSAEST: 33( 1998).I-l.
Cha udha ry 13 P, Jha G S Prasad D. Sanyal S. J elee/roehelll
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10
II
12
13
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