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Corrosion and inhibition studies of copper in aqueous solutions of formic acid and acetic acid
Corrosion Science, Vol. 37, No. 9, pp. 1399-1410, 1995 Copyright 0 1995 Elsevier Science Ltd
Pergamon
Printed in Great Britain. All rights reserved OOll&938X/95 $9.50 + 0.00
0010-938X(95)00042-9
CORROSION AND INHIBITION STUDIES OF COPPER IN AQUEOUS SOLUTIONS OF FORMIC ACID AND ACETIC ACID V.B. SINGH Department
of Chemistry,
Banaras
and Hindu
R.N.
SINGH
University,
Varanasi
- 221005, India
Abstract-Corrosion behaviour of copper has been investigated in different compositions of formic acid and acetic acid at 30°C by a potentiostatic method. The maximum corrosion rate was found in 2&40mol/o formic acid and in 20 mol/o acetic acid in aqueous solution mixtures. The corrosion rate depended on the concentration of either acid. Formic acid is observed to be more corrosive than acetic acid. The metal exhibited active-passive behaviour in the concentration range of 30_70mol/o of HCOOH acid in the solution mixture. A short passivity range of potential with a high passivity current density was observed for the metal in the solution mixtures of HCOOH acid while in solution mixtures of acetic acid the metal exhibited only active dissolution. Some organometallic compounds, viz. BuzSnCIz, PhSnC13, Ph2SnC12, Ph3SnCI have been subjected to inhibition studies in the aqueous solution mixture (20 mol/o) of either acid. Among the inhibitors used Ph,SnCI functions as a better inhibitor in both acids. A strong interaction between the inhibitor and corroding surface of copper is speculated due to adsorption of the inhibitor.
INTRODUCTION Organic acids constitute a group of the most important chemicals used in industry. The acids are produced more as precursors for other chemicals than for end use as organic acids. Acetic acid is the best known member of the group and is produced in the largest volume, but other organic acids are also important for the preparation of compounds used in routine compounds from aspirin to plastics and fibers. Corrosion by organic acids is complicated not only because there are numerous acids to be considered but also because the acids typically are not handled alone but as a process mixture. They are even sometimes used as solvents for other chemical reactions. Though organic acids are weakly acidic they provide sufficient protons to act as true acids towards most metals, because most organic acids are neither oxidizing nor reducing to metals, such as copper, which does not displace hydrogen from acids. A high corrosion rate will be encountered if acid is free of air and other oxidants. In the absence of oxygen or other oxidants, copper is probably the most widely used material for handling HCOOH and CH,COOH acid. The electrochemical study of metals in organic solvent containing different amounts of water indicates that water plays a significant role’ on the corrosion behaviour in formic acid solution. Substantial corrosion studies for different metals and alloys in these acids have been and for handling reported in the literature.2’3 Despite the wide use in industry purposes of these acids the corrosion studies of copper are scarcely known on the entire composition range of formic and acetic acid. In view of the above, the corrosion
Manuscript
received
14 June 1993; in amended
forms 14 July 1993 and 14 February 1399
1995.
V.B. Singh and R.N.
1400
Singh
and inhibition behaviour of Cu in formic and acetic acid of different compositions was studied. The effect of de-aeration and electrolyte addition in the acid solution on the corrosion behaviour of the metal has been investigated. A new class of inhibitor, i.e. organometallic compounds of tin, has been tested for this system.
EXPERIMENTAL
METHOD
Aqueous organic acid mixtures of different compositions (20-70 moljo formic and acetic acid) were prepared using double-distilled water. Before experiments the solution mixture was kept at rest for about 24 h so that structuralization of the mixture may reach completion. For electrochemical polarization studies the working electrode (copper) of area 2 cm* was employed. First the electrode was properly polished with I/o to 4/o emery paper. The electrode was then dipped in soap solution which emulsified the particulate material sticking to the electrode surface, and subsequently washed in flowing water. The electrode was then degreased in acetone. The working electrode was pickled in 5% H2S04 solution for a few seconds. Finally it was washed several times with double-distilled water, acetone and dried by softly pressing between warm filter papers. The solution was de-aerated with purified nitrogen for 3 h and the corrosion studies were carried out potentiostatically (WENKING POS 73). The experimental arrangement and working procedures are the same as described elsewhere.4 The polarization studies were carried out when the open circuit potential was stabilized. The potential was carried in step of 40mV and steady state current was noted. A saturated calomel electrode was used as a reference electrode. The experiments were carried out in unstirred solution mixtures at 30* 1 “C. Additions of 0.5 M HCOONa and 0.5 M CH$OONa were made to solution mixtures of formic and acetic acid solutions, respectively, to examine the influence on corrosion behaviour of the metal. Some organometallic compounds, viz. Bu?SnCl,, PhSnCla, Ph2SnClz, PhaSnCl, were added in different quantities (20,40, 100 ppm) separately to the only selected composition of solution mixture (20 mol/o formic and acetic acid) to see the inhibition effect. The inhibition efficiency for each concentration of inhibitors was calculated according to the equation, tit, = (I -i) x 100. where II, is the percentage of inhibition for each concentration of inhibitor and i and i. are the corrosion current densities (mA/cm2) with and without inhibitor, respectively.
EXPERIMENTAL
RESULTS
AND
DISCUSSION
The illustrations and discussion in text are related to de-aerated solutions in the presence of respective electrolyte unless specifically mentioned. Open circuit potential (OCP) vs composition of either acid was plotted and is shown in Fig. 1. It was found that the OCP tended towards active potential (less noble direction) up to 40 mol/o of formic acid and above 40 mol/o it shifted towards the noble direction. A decrease in OCP indicates a greater corrosion tendency in 40 mol/o of HCOOH acid. In the case of acetic acid, the OCP shifted to the most noble potential values with increasing acid concentration, which shows the decreasing corrosion tendency of the metal. Polarization curves of Cu in HCOOH acid are shown in Fig. 2. It is observed that the cathodic current decreases as the concentration of HCOOH acid increases from 20 to 70 mol/o at all potentials, except in 40 mol/o of HCOOH acid. The cathodic curves are of a similar nature, which shows that the cathodic reactions are the same. The nature of the curve obtained in 40 mol/o formic acid indicates a different type of cathodic reaction at higher cathodic potentials. Similarly the current density decreases as the concentration of acetic acid increases (Fig. 3), however, the current density is comparatively lower in this acid than that in HCOOH acid for the respective compositions. The cathodic Tafel slope was found to lie between 100 and 140 mV/decade I for the solution mixtures of either acid. These values suggest that the
Corrosion
and inhibition
Concentration
1401
studies of copper
(molio)
CIIJ COOH
IIO-
9n -
g
7050 -
% 30 -
InI 20
in
I 30
Concentration Open circuit potential
Fig. 1.
Deaerated
2
12lln
Aerated
I 50
I 40
I 6n
(molio)
vs composition
I 40
I 70
HCOOH
of either acid (HCOOH,
CH$OOH).
(mol1o HCOOH)
(molto
HCOOH)
cA c > s ;;j .,_ 9 ‘3 a
8nn
-1no
0
I
I
I
I
In-’
Io”
II)’
I n?
Current
Fig. 2.
Polarization
dewily
curves of Cu in different
(mA cm’)
compositions
of HCOOH
at 30°C.
1402
V.B. Singh and R.N. Singh
10-l
I0" Current
Fig. 3.
Polarization
density
curves of Cu in different
(mA:cm’)
compositions
of CH$OOH
at 30°C.
cathodic reaction is hydrogen evolution. Sekine’ also considered hydrogen evolution as a cathodic reaction on iron in formic acid. With the addition of electrolyte to either acid solution, the cathodic current is increased. The cathodic curves remained almost unchanged when the solution was de-aerated. Anodic polarization curves of Cu in different compositions of formic acid at 30°C reveal active-passive behaviour in all compositions of the acid except in 20 mol/o of formic acid where it remained only active (Fig. 2). Electrochemical corrosion parameters, derived from the polarization curves, are given in Table 1. The corrosion potential of copper in different compositions of the HCOOH acid differed significantly. The metal possesses a negative value of corrosion potential in 2&40mol/o of HCOOH acid and shows positive potential in other compositions of the acid. Corrosion current (iorr ) and critical current density for passivity (icr) decrease with increasing concentration of formic acid except for 40 mol/o. The critical potential for the onset of passivity and passivity current density also vary with formic acid concentration. The curves showed that the dissolution tendency is more in the anodic region up to 400 mV, because current increases rapidly with small change in the potential. A transpassive region is not observed in any solution mixture. The passivity current density is generally high (40-150 mA) which shows considerable dissolution of metal in the passive region. The anodic polarization curves of copper in different compositions of acetic acid are shown in Fig. 3, copper exhibits active behaviour in all the compositions of acetic acid. The corrosion potential (I?,,,,) of copper progressively attained nobler values with increasing acetic acid concentration in the solution (20-70 mol/o) while corrosion current and critical current density decreased (Table 2). The values of i,,,,, i,, are found to be lower for copper in acetic acid than those obtained in formic acid in all the solution mixtures, though copper is active in all the compositions of acetic acid solution. In (2040 mol/o) acetic acid the curves show a high dissolution tendency in comparison to higher concentrations of acetic acid as the current increases with an
-100 + 100 + 100 + 100 +120
+ 160
70
E cDrr (mV)
x x x x x
lop2 10-3 10-3 10-I 10-3
1.2 x 10-3
1.8 1.5 1.2 1.0 1.0
i,,,, (mA/cm2)
mixtures
60
50
solution
40 60 40 40 60
60 80 80 90 100 100
0.01
ba (mV/decade
0.32 0.018 0.045 0.025 0.06
‘cr (mA/cm’)
I)
I)
-300 +80 +80 100 200 180
140
(?q)
100 110 110 120 136
bc (mV/decade I)
+60
120
(24
80 30 0.08 0.01 0.80 0.02
x x x x
1.8 1.6 1.5 1.1
1O-3 lop3 10-j 10-3 1.0x 10-3 1.2 x 10-3
(mkfLm2)
In the presence
(m.$m2)
10-2
40 60 80 80 90 100
b. (mV/deLade
80 100 100 120 120 130
(mV/dkade
80 80
40 40
b, (mV/decade 60 110 40 120
I)
I)
40 40 20 40
b, (mV/decade
of 0.5 M HCOONa
25 40
2.6x 85
90 10’
150 50 65
2.7x IO2 2.1 x 102 2.8 x 10’
10-2 1O-2 10-2 lO-2 10-2
1.5 x 1.6x 2.0 x I.8 xx 1.2 1.5 x
‘P
of 0.5 M HCOONa
(m,$~m2)
In the presence
of formic acid at 30°C
(m,$m2)
mixtures
-20 -25 -40 +80 +40
I)
solution
100 140 100 140 120
(mV/d:ade
of Cu in different
of acetic acid at 30°C
(mV/dkade 150 48 42 45
lP
In the absence of electrolyte
of Cu in different
1.2 x 10-2
parameters
20 30 40 50 60
(mol/o)
ofCH,COOH
Concentration
Corrosion
+80
70
Table 2.
1.1x102 1.8 x 10’ 1.9x IO2 90 1.6x lo*
1.2x 1.3 x 1.4x 1.3 x 1.2 x
-20 -25 -40 +60 +40
20 30 40 50 60
75
lW (mA/cm’)
i,,,,
(mA/cm2)
E,,,,
(mV)
(moI/o)
10-l 10-2 10-2 10-2 10-z
parameters
In the absence of electrolyte
Corrosion
ofHCOOH
Concentration
Table I.
I)
I)
L 8
3-I
0, s
0 E1 ;: 8. S S e 5’
V.B. Singh and R.N. Singh
1404
increase of potential in these compositions. However, at higher concentrations little increase in current is observed with successive increases of the potential. Beyond a certain potential the current remained almost constant and in few cases increased only a little. It can be considered that either the metal is tending to attain passivity, the corrosion product is not taken away promptly from the double layer, or the product is not further soluble. A salt-like film may be considered to be formed on the surface of the electrode through which the metal dissolution occurs sluggishly. This film would show a poor protective character. Despite of the fact that the passivity was obtained in formic acid in most of the solution mixtures the current density is high in comparison to acetic acid in the active region and the corrosion rate of copper in formic acid is found to be higher than that in acetic acid. Similar results were reported’-’ for stainless steel which corroded more easily in formic acid than acetic acid. A lower corrosion rate in the higher concentrations of organic acids (HCOOH, CHsCOOH) was obtained by these investigators. The present results can be explained on the basis of the conductivity of the solution mixture. The maximum conductivity of the solution was reported73’0”i for 2&40 mol/o of either acid composition which may be considered responsible for a higher corrosion rate of metal in these compositions. Comparing the values of the anodic Tafel slope in different compositions of acid solution (Table 2), the lowest value of the Tafel slope was obtained in the case of 40 mol/o HCOOH and 20 mol/o of CHsCOOH acid solution. This also indicates a higher metal dissolution rate in the said acidic solution mixture. It is seen that the corrosion current and critical current density (&,.) (Tables I and 2; Figs 4 and 5) increased in each composition of the solution due to the addition of respective electrolyte (HCOONa, CHsCOONa) in formic acid or acetic acid, for example the corrosion current and critical current density increased from 1.4 x 1OV2 to 2 x 1O-2 mA/cm2 and from 190 to 280 mA/cm2, respectively, in 40 mol/o of HCOOH acid solution when the electrolyte was added to the solution. Similarly the
Aerated 2000
-
A Deaerated
1600 -
% _
1200
-
(ml/o
HCOOH
+O..sM HCOONa)
50
(ml/o l
20
x
40
0
50
0
70
HCOOH
+O.SM
HCOOSa)
cj x00 -
Current Fig. 4.
Polarization
density
(mAicm’)
curves of Cu in HCOOH
+ 0.5 M sodium forrnate
Corrosion
and inhibition
studies of copper
1405
Aerated (molto
CH$WOH
+ O.SMCH$XXIN~) A
. ; i
SO
Deacrated
:
x00 10.’
I
I
1 I)”
10-I
Current Fig. 5.
Polarization
I
I I
density
0”
10’
101
(mAicm2)
curves of Cu in CH,COOH
+ 0.5 M sodium acetate
corrosion current and the critical current density increased from 5.0 x 10K3 to 2.8 x lop2 mA/cm* and from 0.32 to 80 mA/cm2, respectively, in the case of 20 mol/ o of CH$OOH in the presence of CHsCOONa. After the addition of the electrolyte (CH&OONa), feeble passivity was observed in 40 and 70 mol/o of acetic acid and in remaining compositions of acetic acid solution the nature of the curves was found to be almost similar to those obtained in the absence of the electrolyte, but the curves shifted to some extent towards higher current region (Figs 4 and 5). The passivity current density in each solution mixture of the HCOOH acid with or without electrolyte is found to be high for the metal, and the passivity range of potential is large. The passivity current density was found to increase when electrolyte was added to the solution. The corrosion current is observed to increase in the presence of electrolyte probably due to the increase in conductivity of the solution mixture in the presence of added electrolyte. The critical potential for the onset of passivity becomes more noble in the presence of electrolyte in either acid. Polarization studies were performed in non-de-aerated solution of only one selected composition of formic and acetic acid (50 mol/o). The corrosion potential was found to shift in the more active direction in each case (Figs 4 and 5). The polarization curves for de-aerated solutions and non-de-aerated solutions are almost similar in nature. The corrosion current did not vary significantly upon de-aeration of the solutions. The passivity current density was found to be higher in the aerated solution mixture in comparison to the de-aerated one. In the case of acetic acid the value of the dissolution current was found to be higher in aerated solution mixture in comparison to the de-aerated one, showing a higher rate of dissolution/reaction in the former case. The lower corrosion rate in deaerated solution can be ascribed to the virtual absence of oxygen in de-aerated observed that the corrosion rate of copper is solutions. Syrett and MacDonald’2,13 reduced as the content of oxygen is reduced in solution.
V.B. Singh and R.N. Singh
1406
Li
2000
-
I600
-
I200
-
x Without A IOOppm q IOOppm l IOOppm o IOOppm
inhibitor Bu2 SKI: Ph SnCI, Ph, SnCl Ph3 SnCl
‘d
m ‘2 > E
x00 -
Ann-
-z ‘Z o-
B =, P
-4on -
-xnn
I
I
10-l
Id’
I
Current Fig. 6.
Polarization
curves
I
dcnslty
I
I n2
In’
IO
(mA;cm’)
of Cu in 20 mol/o HCOOH + 0.5 M sodium presence of different inhibitors.
formate
in the
Corrosion data based on the polarization studies (Figs 6 and 7) of Cu in solution mixtures of 20 mol/o formic acid and 20 mol/o acetic acid, respectively, at 30°C in the PhSnCls, PhZSnClz, PhsSnCl), are presence of different inhibitors (BuzSnClz, summarized in Table 3. In both the acid solutions it is observed that the corrosion potential shifted in a noble direction. The magnitude of the shift depends upon the
x Without I OOppm o IOOppm l IOOppm o IOOppm
A
inhlbitor Bu: SnCI? Ph SnCll Ph: SnCl Phj SnCl
i i i
I 100 ‘4 x
!lnn
z J
-
J x’
JO0
v=z---~
-xnn
I Io-2
in-l
IO0
Current Fig. 7.
Polarization
J
I
I
10’
10’
density
IO’
(mA/cm’)
curves of Cu m 20 mol/o CH$ZOOH presence of different inhibitors.
+ 0.5 M sodium
acetate
in the
Corrosion Table 3.
(a) The effect of different
Concentration Inhibitor Nil Bu$SnCI~ PhSnCI, Ph,SnCl;? Ph$nCl
(wm)
100 100 100 100
(b) The effect of different
Concentration
and inhibition
studies of copper
inhibitors on corrosion behaviour M HCOONa solution at 30°C
of Cu in 20 mol/o formic acid + 0.5
$6
(mA/cm’)
ba (mV/decade I)
-20 -15 -10 +30 +40
1.5x 10-l 0.6 x lop2 0.5 x 10-2 0.4 x 10-2 0.3 X 10-2
40 40 60 80 90
inhibitiors
I?,,,,
‘corr
1407
bc (mV/decade I) 60 90 100 120 130
Percentage inhibition efficiency
60.0 66.6 73.3 80.0
on corrosion behaviour of Cu in 20 mol/o acetic acid + 0.5 M CH$OONa solution at 30°C
Inhibitor
@pm)
(mV)
i,,,, (mA/cm’)
Nil Bu,SnCIZ PhSnC& Ph2SnClz Ph$nCl
100 100 100 100
-100 -80 -70 -50 -50
1.8 0.6 0.5 0.4 0.3
x x x x x
1O-2 1O-2 10-2 10-2 lo-’
b, (mV/decade 40 60 70 75 80
I)
b, (mV/decade 80 120 130 140 140
I)
Percentage inhibition efficiency
66.67 72.2 77.7 80.5
type and concentration of inhibitor used. The corrosion current and current density at a particular potential was lower in the presence of these inhibitors; a high value of the cathodic Tafel slope is obtained in the presence of the inhibitors which verifies the formation of a film with a physical barrier effect. This view seems to support an inhibition mechanism based on surface film formation by initial de-adsorption followed by reduction polymerization reactions14 of inhibitor. Such a mechanism was emphasized by Growcock et a1.‘4’7 for 1-octyn-3-01 and trans-cinnamaldehyde when used as inhibitors in acidic solutions. The anodic Tafel slopes also increased in the presence of these inhibitors. According to Donahue et al. I8such increases in Tafel slope suggest a mode of inhibition involving an interposition of organic into the charge transfer process for the anodic reaction. It is observed that the corrosion current and current densities in the cathodic and anodic region decreased when inhibitors were present in the solution. This suggests that the inhibitors BuzSnCIZ, PhSnC13, PhZSnClz, PhSnC13 suppress the anodic and cathodic reactions involved in the corrosion process by being adsorbed on the metal surface where they act as a mixed inhibitor. The results show (Tables 3(a) and 3(b)) that among the inhibitors used Ph$nCl exhibits an inhibition efficiency superior to those of the aromatic organometallic compounds used in present studies. Percentage inhibition efficiencies of the organometallic compounds increases in the following order in both the acid solutions: BuzSnClz < PhSnC13 < PhZSnClz < PhSnC13. Such a trend in the variation of inhibition efficiency can be explained in terms of the molecular size of the inhibitors. In aqueous solutions the inhibition efficiency of a series of related organic compounds increased with an increase in the molecular size of organic compounds. A positive shift of the corrosion potential due to addition of BuZSnClz, PhSnC13, PhzSnC12, Ph$nCl, individually, is seen (Tables 3 and 4) which
1408
V.B. Singh and R.N. Singh Table 4. (a) The effect of concentration of Ph,SnCl on inhibition efficiency in 20 mol/o HCOOH + 0.5 M HCOONa at 30°C
Concentration
Lx, (mA/cm’)
@pm) 0
1.5 6.0 4.5 3.0
20 40 100 (b) The effect of concentration in 20 mol/o CH$ZOOH
Concentration (wm) 0
20 40 100
x x x x
10-2 10-j lo-’ lo-’
Percentage inhibition efficiency
60.0 70.0 80.0
of Ph,SnCl on inhibition efficiency + 0.5 M CH&OONa at 30°C
k”,, (mAjcm2) 1.8 0.7 0.5 0.35
x x x x
10~ z 10-2 lop2 10-z
Percentage inhibition efficiency
67.8 7 1.4 80.6
indicated that these compounds are effective suppressors of the anodic dissolution reaction. The functional group and structure of the inhibitor molecules play significant roles in the adsorption process. Adsorption of the organometallic compounds at the metal surfaces takes place by electron transfer through the loosely bound electrons of the 71bond or the aromatic rings. The results can be visualized on the basis that in Ph$nCl the de-activating effect of the phenyl group is predominant over the aggressive effect of chloride and Ph$nCl acts as best inhibitor. It appears that Ph$nCl is adsorbed on the copper surface through the de-activated phenyl group. Different concentrations of Ph$nCl (20, 40, 100 ppm) were added to 20mol/o HCOOH and also to 20 mol/o CHsCOOH. The lowest values of the corrosion current and current density in the entire potential range were observed when the experimental solution contained 100 ppm of the above inhibitor (Figs 8 and 9). The cathodic and anodic Tafel slopes are also found to be higher in such cases. The inhibition efficiency is found to be at a maximum at a concentration of 100 ppm of Ph$nCl in the aqueous acidic solution of either acid (Tables 4(a) and 4(b)). It appears that the efficiency of inhibition is in general directly proportional to the amount of inhibitor adsorbed or the surface coverage. This may be the reason for variation of efficiency with content of Ph$nCl.
CONCLUSIONS The corrosion of Cu in formic acid is more than in acetic acid in different solution mixtures, which can be attributed to their relative acid strengths. The corrosion behaviour of the metal in concentrated solutions of acid is different from that in lower concentrations of acidic solution. The corrosion rate exhibits a non-linear dependence upon HCOOH and CH,COOH concentration. Maximum corrosion is observed in
Corrosion
and inhibition
1409
studies of copper
c
x Without inhibitor o 20ppm Ph$nCI l 40ppm Ph$nCl . IOOppm PhJSnCI
l-
I-
I-
I-
I-
I
I
I ?
Current Fig. 8.
Polarization
2000
1bO0
I
IO-
density (mAi’cm’)
curves of Cu in 20 mol/o HCOOH + 0.5 M sodium presence of different concentrations of Ph3SnC1.
formate
in the
x Without inhibitor 0 20ppm PhGnCl l 4Oppm PhsSnCl l
10.’
I
IO’
n
10”
IO“
1 O‘-
1OOppm Ph$nCl
IO-':
IO“
IO”
IO’
If9
Current density (mAicm’)
Fig. 9.
Polarization
curves of Cu in 20 mol/o CH,COOH + 0.5 M sodium presence of different concentrations of Ph3SnC1.
acetate
in the
V.B. Singh and R.N. Singh
1410
40 mol/o HCOOH acid and 20 mol/o CHsCOOH acid. Addition of sodium formate or sodium acetate increases the rate of copper corrosion in respective acids. Among the inhibitors used 100 ppm of Ph$nCl was found to be the most suitable for inhibition of copper corrosion, although other inhibitors used are also suitable. Acknovledgemenrs-The Chemistry, for providing
authors necessary
wish to thank facilities.
Prof.
P.K.
Srivastava,
head
of the
Department
of
REFERENCES 1. E. Heitz, Advances in Corrosion Science and Technology (ed. M.G. Fontana and R.W. Staehle), Vol. 4, Ch. 3. Plenum Press, New York (1974). 2. 1. Sekine, Corros. Sci. 27, 275 (1978). 3. I. Sekine and K. Momi, Corrosion 44, 136 (1988). 4. N.N. Rao and V.B. Singh, Corros. Sci. 44, 136 (1985). 5. I. Sekine, Corros. Sci. 22, 1113 (1982). 6. I. Sekine and A. Chinda, Boshoku Gijutsu 31, 313 (1982). 7. I. Sekine and K. Senoo, Corros. Sci. 24, 439 (1984). 8. I. Sekine and A. Chinda, Corrosion 40, 95 (1984). 9. I. Sekine and M. Koeda, Boshoku Gijustu 33, 500 (1984). 10. I. Sekine Shvichi, Nrctrochim. Actu 32, 915 (1987). 11. J.J. Demo, Corrosion 24, 139 (1968). 12. B.C. Syrett, Corrosion 32, 242 (1976). 13. D.D. MacDonald, B.C. Syrett and S.S. Wing, Corrosion 349, 1289 (1978). 14. F.B. Growcock, W.W. Frenier and V.R. Lopp, Proc. 6th Europ. Symp. on Corrosion Inhibitors, Ann. Univ. Ferrara, N.S. Sez. V. Suppl. No. 8, 167 (1985). 15. W.W. Frenier, F.B. Growcock and V.R. Lopp, Proc. 6th Europ. Symp. on Corrosion Inhibitors. Ann. Univ., Ferrara, N.S. Sez, V. Suppl. No. 8, 183 (1985). 16. F.B. Growcock and V.R. Lopp, Corrosion 44, 248 (1988). 17. W.W. Frenier and F.B. Growcock. Proc. 7th Europ. Symp. on Corrosion Inhibitors. Ann. Univ. Ferrara, N.S. Sez, V. Suppl. 661 (1990). 18. F.M. Donahue, A. Akiyama and K. Nobe. J. Elecrrochem. Sot. 114, 1006 (1967).
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0010-938X(95)00042-9
CORROSION AND INHIBITION STUDIES OF COPPER IN AQUEOUS SOLUTIONS OF FORMIC ACID AND ACETIC ACID V.B. SINGH Department
of Chemistry,
Banaras
and Hindu
R.N.
SINGH
University,
Varanasi
- 221005, India
Abstract-Corrosion behaviour of copper has been investigated in different compositions of formic acid and acetic acid at 30°C by a potentiostatic method. The maximum corrosion rate was found in 2&40mol/o formic acid and in 20 mol/o acetic acid in aqueous solution mixtures. The corrosion rate depended on the concentration of either acid. Formic acid is observed to be more corrosive than acetic acid. The metal exhibited active-passive behaviour in the concentration range of 30_70mol/o of HCOOH acid in the solution mixture. A short passivity range of potential with a high passivity current density was observed for the metal in the solution mixtures of HCOOH acid while in solution mixtures of acetic acid the metal exhibited only active dissolution. Some organometallic compounds, viz. BuzSnCIz, PhSnC13, Ph2SnC12, Ph3SnCI have been subjected to inhibition studies in the aqueous solution mixture (20 mol/o) of either acid. Among the inhibitors used Ph,SnCI functions as a better inhibitor in both acids. A strong interaction between the inhibitor and corroding surface of copper is speculated due to adsorption of the inhibitor.
INTRODUCTION Organic acids constitute a group of the most important chemicals used in industry. The acids are produced more as precursors for other chemicals than for end use as organic acids. Acetic acid is the best known member of the group and is produced in the largest volume, but other organic acids are also important for the preparation of compounds used in routine compounds from aspirin to plastics and fibers. Corrosion by organic acids is complicated not only because there are numerous acids to be considered but also because the acids typically are not handled alone but as a process mixture. They are even sometimes used as solvents for other chemical reactions. Though organic acids are weakly acidic they provide sufficient protons to act as true acids towards most metals, because most organic acids are neither oxidizing nor reducing to metals, such as copper, which does not displace hydrogen from acids. A high corrosion rate will be encountered if acid is free of air and other oxidants. In the absence of oxygen or other oxidants, copper is probably the most widely used material for handling HCOOH and CH,COOH acid. The electrochemical study of metals in organic solvent containing different amounts of water indicates that water plays a significant role’ on the corrosion behaviour in formic acid solution. Substantial corrosion studies for different metals and alloys in these acids have been and for handling reported in the literature.2’3 Despite the wide use in industry purposes of these acids the corrosion studies of copper are scarcely known on the entire composition range of formic and acetic acid. In view of the above, the corrosion
Manuscript
received
14 June 1993; in amended
forms 14 July 1993 and 14 February 1399
1995.
V.B. Singh and R.N.
1400
Singh
and inhibition behaviour of Cu in formic and acetic acid of different compositions was studied. The effect of de-aeration and electrolyte addition in the acid solution on the corrosion behaviour of the metal has been investigated. A new class of inhibitor, i.e. organometallic compounds of tin, has been tested for this system.
EXPERIMENTAL
METHOD
Aqueous organic acid mixtures of different compositions (20-70 moljo formic and acetic acid) were prepared using double-distilled water. Before experiments the solution mixture was kept at rest for about 24 h so that structuralization of the mixture may reach completion. For electrochemical polarization studies the working electrode (copper) of area 2 cm* was employed. First the electrode was properly polished with I/o to 4/o emery paper. The electrode was then dipped in soap solution which emulsified the particulate material sticking to the electrode surface, and subsequently washed in flowing water. The electrode was then degreased in acetone. The working electrode was pickled in 5% H2S04 solution for a few seconds. Finally it was washed several times with double-distilled water, acetone and dried by softly pressing between warm filter papers. The solution was de-aerated with purified nitrogen for 3 h and the corrosion studies were carried out potentiostatically (WENKING POS 73). The experimental arrangement and working procedures are the same as described elsewhere.4 The polarization studies were carried out when the open circuit potential was stabilized. The potential was carried in step of 40mV and steady state current was noted. A saturated calomel electrode was used as a reference electrode. The experiments were carried out in unstirred solution mixtures at 30* 1 “C. Additions of 0.5 M HCOONa and 0.5 M CH$OONa were made to solution mixtures of formic and acetic acid solutions, respectively, to examine the influence on corrosion behaviour of the metal. Some organometallic compounds, viz. Bu?SnCl,, PhSnCla, Ph2SnClz, PhaSnCl, were added in different quantities (20,40, 100 ppm) separately to the only selected composition of solution mixture (20 mol/o formic and acetic acid) to see the inhibition effect. The inhibition efficiency for each concentration of inhibitors was calculated according to the equation, tit, = (I -i) x 100. where II, is the percentage of inhibition for each concentration of inhibitor and i and i. are the corrosion current densities (mA/cm2) with and without inhibitor, respectively.
EXPERIMENTAL
RESULTS
AND
DISCUSSION
The illustrations and discussion in text are related to de-aerated solutions in the presence of respective electrolyte unless specifically mentioned. Open circuit potential (OCP) vs composition of either acid was plotted and is shown in Fig. 1. It was found that the OCP tended towards active potential (less noble direction) up to 40 mol/o of formic acid and above 40 mol/o it shifted towards the noble direction. A decrease in OCP indicates a greater corrosion tendency in 40 mol/o of HCOOH acid. In the case of acetic acid, the OCP shifted to the most noble potential values with increasing acid concentration, which shows the decreasing corrosion tendency of the metal. Polarization curves of Cu in HCOOH acid are shown in Fig. 2. It is observed that the cathodic current decreases as the concentration of HCOOH acid increases from 20 to 70 mol/o at all potentials, except in 40 mol/o of HCOOH acid. The cathodic curves are of a similar nature, which shows that the cathodic reactions are the same. The nature of the curve obtained in 40 mol/o formic acid indicates a different type of cathodic reaction at higher cathodic potentials. Similarly the current density decreases as the concentration of acetic acid increases (Fig. 3), however, the current density is comparatively lower in this acid than that in HCOOH acid for the respective compositions. The cathodic Tafel slope was found to lie between 100 and 140 mV/decade I for the solution mixtures of either acid. These values suggest that the
Corrosion
and inhibition
Concentration
1401
studies of copper
(molio)
CIIJ COOH
IIO-
9n -
g
7050 -
% 30 -
InI 20
in
I 30
Concentration Open circuit potential
Fig. 1.
Deaerated
2
12lln
Aerated
I 50
I 40
I 6n
(molio)
vs composition
I 40
I 70
HCOOH
of either acid (HCOOH,
CH$OOH).
(mol1o HCOOH)
(molto
HCOOH)
cA c > s ;;j .,_ 9 ‘3 a
8nn
-1no
0
I
I
I
I
In-’
Io”
II)’
I n?
Current
Fig. 2.
Polarization
dewily
curves of Cu in different
(mA cm’)
compositions
of HCOOH
at 30°C.
1402
V.B. Singh and R.N. Singh
10-l
I0" Current
Fig. 3.
Polarization
density
curves of Cu in different
(mA:cm’)
compositions
of CH$OOH
at 30°C.
cathodic reaction is hydrogen evolution. Sekine’ also considered hydrogen evolution as a cathodic reaction on iron in formic acid. With the addition of electrolyte to either acid solution, the cathodic current is increased. The cathodic curves remained almost unchanged when the solution was de-aerated. Anodic polarization curves of Cu in different compositions of formic acid at 30°C reveal active-passive behaviour in all compositions of the acid except in 20 mol/o of formic acid where it remained only active (Fig. 2). Electrochemical corrosion parameters, derived from the polarization curves, are given in Table 1. The corrosion potential of copper in different compositions of the HCOOH acid differed significantly. The metal possesses a negative value of corrosion potential in 2&40mol/o of HCOOH acid and shows positive potential in other compositions of the acid. Corrosion current (iorr ) and critical current density for passivity (icr) decrease with increasing concentration of formic acid except for 40 mol/o. The critical potential for the onset of passivity and passivity current density also vary with formic acid concentration. The curves showed that the dissolution tendency is more in the anodic region up to 400 mV, because current increases rapidly with small change in the potential. A transpassive region is not observed in any solution mixture. The passivity current density is generally high (40-150 mA) which shows considerable dissolution of metal in the passive region. The anodic polarization curves of copper in different compositions of acetic acid are shown in Fig. 3, copper exhibits active behaviour in all the compositions of acetic acid. The corrosion potential (I?,,,,) of copper progressively attained nobler values with increasing acetic acid concentration in the solution (20-70 mol/o) while corrosion current and critical current density decreased (Table 2). The values of i,,,,, i,, are found to be lower for copper in acetic acid than those obtained in formic acid in all the solution mixtures, though copper is active in all the compositions of acetic acid solution. In (2040 mol/o) acetic acid the curves show a high dissolution tendency in comparison to higher concentrations of acetic acid as the current increases with an
-100 + 100 + 100 + 100 +120
+ 160
70
E cDrr (mV)
x x x x x
lop2 10-3 10-3 10-I 10-3
1.2 x 10-3
1.8 1.5 1.2 1.0 1.0
i,,,, (mA/cm2)
mixtures
60
50
solution
40 60 40 40 60
60 80 80 90 100 100
0.01
ba (mV/decade
0.32 0.018 0.045 0.025 0.06
‘cr (mA/cm’)
I)
I)
-300 +80 +80 100 200 180
140
(?q)
100 110 110 120 136
bc (mV/decade I)
+60
120
(24
80 30 0.08 0.01 0.80 0.02
x x x x
1.8 1.6 1.5 1.1
1O-3 lop3 10-j 10-3 1.0x 10-3 1.2 x 10-3
(mkfLm2)
In the presence
(m.$m2)
10-2
40 60 80 80 90 100
b. (mV/deLade
80 100 100 120 120 130
(mV/dkade
80 80
40 40
b, (mV/decade 60 110 40 120
I)
I)
40 40 20 40
b, (mV/decade
of 0.5 M HCOONa
25 40
2.6x 85
90 10’
150 50 65
2.7x IO2 2.1 x 102 2.8 x 10’
10-2 1O-2 10-2 lO-2 10-2
1.5 x 1.6x 2.0 x I.8 xx 1.2 1.5 x
‘P
of 0.5 M HCOONa
(m,$~m2)
In the presence
of formic acid at 30°C
(m,$m2)
mixtures
-20 -25 -40 +80 +40
I)
solution
100 140 100 140 120
(mV/d:ade
of Cu in different
of acetic acid at 30°C
(mV/dkade 150 48 42 45
lP
In the absence of electrolyte
of Cu in different
1.2 x 10-2
parameters
20 30 40 50 60
(mol/o)
ofCH,COOH
Concentration
Corrosion
+80
70
Table 2.
1.1x102 1.8 x 10’ 1.9x IO2 90 1.6x lo*
1.2x 1.3 x 1.4x 1.3 x 1.2 x
-20 -25 -40 +60 +40
20 30 40 50 60
75
lW (mA/cm’)
i,,,,
(mA/cm2)
E,,,,
(mV)
(moI/o)
10-l 10-2 10-2 10-2 10-z
parameters
In the absence of electrolyte
Corrosion
ofHCOOH
Concentration
Table I.
I)
I)
L 8
3-I
0, s
0 E1 ;: 8. S S e 5’
V.B. Singh and R.N. Singh
1404
increase of potential in these compositions. However, at higher concentrations little increase in current is observed with successive increases of the potential. Beyond a certain potential the current remained almost constant and in few cases increased only a little. It can be considered that either the metal is tending to attain passivity, the corrosion product is not taken away promptly from the double layer, or the product is not further soluble. A salt-like film may be considered to be formed on the surface of the electrode through which the metal dissolution occurs sluggishly. This film would show a poor protective character. Despite of the fact that the passivity was obtained in formic acid in most of the solution mixtures the current density is high in comparison to acetic acid in the active region and the corrosion rate of copper in formic acid is found to be higher than that in acetic acid. Similar results were reported’-’ for stainless steel which corroded more easily in formic acid than acetic acid. A lower corrosion rate in the higher concentrations of organic acids (HCOOH, CHsCOOH) was obtained by these investigators. The present results can be explained on the basis of the conductivity of the solution mixture. The maximum conductivity of the solution was reported73’0”i for 2&40 mol/o of either acid composition which may be considered responsible for a higher corrosion rate of metal in these compositions. Comparing the values of the anodic Tafel slope in different compositions of acid solution (Table 2), the lowest value of the Tafel slope was obtained in the case of 40 mol/o HCOOH and 20 mol/o of CHsCOOH acid solution. This also indicates a higher metal dissolution rate in the said acidic solution mixture. It is seen that the corrosion current and critical current density (&,.) (Tables I and 2; Figs 4 and 5) increased in each composition of the solution due to the addition of respective electrolyte (HCOONa, CHsCOONa) in formic acid or acetic acid, for example the corrosion current and critical current density increased from 1.4 x 1OV2 to 2 x 1O-2 mA/cm2 and from 190 to 280 mA/cm2, respectively, in 40 mol/o of HCOOH acid solution when the electrolyte was added to the solution. Similarly the
Aerated 2000
-
A Deaerated
1600 -
% _
1200
-
(ml/o
HCOOH
+O..sM HCOONa)
50
(ml/o l
20
x
40
0
50
0
70
HCOOH
+O.SM
HCOOSa)
cj x00 -
Current Fig. 4.
Polarization
density
(mAicm’)
curves of Cu in HCOOH
+ 0.5 M sodium forrnate
Corrosion
and inhibition
studies of copper
1405
Aerated (molto
CH$WOH
+ O.SMCH$XXIN~) A
. ; i
SO
Deacrated
:
x00 10.’
I
I
1 I)”
10-I
Current Fig. 5.
Polarization
I
I I
density
0”
10’
101
(mAicm2)
curves of Cu in CH,COOH
+ 0.5 M sodium acetate
corrosion current and the critical current density increased from 5.0 x 10K3 to 2.8 x lop2 mA/cm* and from 0.32 to 80 mA/cm2, respectively, in the case of 20 mol/ o of CH$OOH in the presence of CHsCOONa. After the addition of the electrolyte (CH&OONa), feeble passivity was observed in 40 and 70 mol/o of acetic acid and in remaining compositions of acetic acid solution the nature of the curves was found to be almost similar to those obtained in the absence of the electrolyte, but the curves shifted to some extent towards higher current region (Figs 4 and 5). The passivity current density in each solution mixture of the HCOOH acid with or without electrolyte is found to be high for the metal, and the passivity range of potential is large. The passivity current density was found to increase when electrolyte was added to the solution. The corrosion current is observed to increase in the presence of electrolyte probably due to the increase in conductivity of the solution mixture in the presence of added electrolyte. The critical potential for the onset of passivity becomes more noble in the presence of electrolyte in either acid. Polarization studies were performed in non-de-aerated solution of only one selected composition of formic and acetic acid (50 mol/o). The corrosion potential was found to shift in the more active direction in each case (Figs 4 and 5). The polarization curves for de-aerated solutions and non-de-aerated solutions are almost similar in nature. The corrosion current did not vary significantly upon de-aeration of the solutions. The passivity current density was found to be higher in the aerated solution mixture in comparison to the de-aerated one. In the case of acetic acid the value of the dissolution current was found to be higher in aerated solution mixture in comparison to the de-aerated one, showing a higher rate of dissolution/reaction in the former case. The lower corrosion rate in deaerated solution can be ascribed to the virtual absence of oxygen in de-aerated observed that the corrosion rate of copper is solutions. Syrett and MacDonald’2,13 reduced as the content of oxygen is reduced in solution.
V.B. Singh and R.N. Singh
1406
Li
2000
-
I600
-
I200
-
x Without A IOOppm q IOOppm l IOOppm o IOOppm
inhibitor Bu2 SKI: Ph SnCI, Ph, SnCl Ph3 SnCl
‘d
m ‘2 > E
x00 -
Ann-
-z ‘Z o-
B =, P
-4on -
-xnn
I
I
10-l
Id’
I
Current Fig. 6.
Polarization
curves
I
dcnslty
I
I n2
In’
IO
(mA;cm’)
of Cu in 20 mol/o HCOOH + 0.5 M sodium presence of different inhibitors.
formate
in the
Corrosion data based on the polarization studies (Figs 6 and 7) of Cu in solution mixtures of 20 mol/o formic acid and 20 mol/o acetic acid, respectively, at 30°C in the PhSnCls, PhZSnClz, PhsSnCl), are presence of different inhibitors (BuzSnClz, summarized in Table 3. In both the acid solutions it is observed that the corrosion potential shifted in a noble direction. The magnitude of the shift depends upon the
x Without I OOppm o IOOppm l IOOppm o IOOppm
A
inhlbitor Bu: SnCI? Ph SnCll Ph: SnCl Phj SnCl
i i i
I 100 ‘4 x
!lnn
z J
-
J x’
JO0
v=z---~
-xnn
I Io-2
in-l
IO0
Current Fig. 7.
Polarization
J
I
I
10’
10’
density
IO’
(mA/cm’)
curves of Cu m 20 mol/o CH$ZOOH presence of different inhibitors.
+ 0.5 M sodium
acetate
in the
Corrosion Table 3.
(a) The effect of different
Concentration Inhibitor Nil Bu$SnCI~ PhSnCI, Ph,SnCl;? Ph$nCl
(wm)
100 100 100 100
(b) The effect of different
Concentration
and inhibition
studies of copper
inhibitors on corrosion behaviour M HCOONa solution at 30°C
of Cu in 20 mol/o formic acid + 0.5
$6
(mA/cm’)
ba (mV/decade I)
-20 -15 -10 +30 +40
1.5x 10-l 0.6 x lop2 0.5 x 10-2 0.4 x 10-2 0.3 X 10-2
40 40 60 80 90
inhibitiors
I?,,,,
‘corr
1407
bc (mV/decade I) 60 90 100 120 130
Percentage inhibition efficiency
60.0 66.6 73.3 80.0
on corrosion behaviour of Cu in 20 mol/o acetic acid + 0.5 M CH$OONa solution at 30°C
Inhibitor
@pm)
(mV)
i,,,, (mA/cm’)
Nil Bu,SnCIZ PhSnC& Ph2SnClz Ph$nCl
100 100 100 100
-100 -80 -70 -50 -50
1.8 0.6 0.5 0.4 0.3
x x x x x
1O-2 1O-2 10-2 10-2 lo-’
b, (mV/decade 40 60 70 75 80
I)
b, (mV/decade 80 120 130 140 140
I)
Percentage inhibition efficiency
66.67 72.2 77.7 80.5
type and concentration of inhibitor used. The corrosion current and current density at a particular potential was lower in the presence of these inhibitors; a high value of the cathodic Tafel slope is obtained in the presence of the inhibitors which verifies the formation of a film with a physical barrier effect. This view seems to support an inhibition mechanism based on surface film formation by initial de-adsorption followed by reduction polymerization reactions14 of inhibitor. Such a mechanism was emphasized by Growcock et a1.‘4’7 for 1-octyn-3-01 and trans-cinnamaldehyde when used as inhibitors in acidic solutions. The anodic Tafel slopes also increased in the presence of these inhibitors. According to Donahue et al. I8such increases in Tafel slope suggest a mode of inhibition involving an interposition of organic into the charge transfer process for the anodic reaction. It is observed that the corrosion current and current densities in the cathodic and anodic region decreased when inhibitors were present in the solution. This suggests that the inhibitors BuzSnCIZ, PhSnC13, PhZSnClz, PhSnC13 suppress the anodic and cathodic reactions involved in the corrosion process by being adsorbed on the metal surface where they act as a mixed inhibitor. The results show (Tables 3(a) and 3(b)) that among the inhibitors used Ph$nCl exhibits an inhibition efficiency superior to those of the aromatic organometallic compounds used in present studies. Percentage inhibition efficiencies of the organometallic compounds increases in the following order in both the acid solutions: BuzSnClz < PhSnC13 < PhZSnClz < PhSnC13. Such a trend in the variation of inhibition efficiency can be explained in terms of the molecular size of the inhibitors. In aqueous solutions the inhibition efficiency of a series of related organic compounds increased with an increase in the molecular size of organic compounds. A positive shift of the corrosion potential due to addition of BuZSnClz, PhSnC13, PhzSnC12, Ph$nCl, individually, is seen (Tables 3 and 4) which
1408
V.B. Singh and R.N. Singh Table 4. (a) The effect of concentration of Ph,SnCl on inhibition efficiency in 20 mol/o HCOOH + 0.5 M HCOONa at 30°C
Concentration
Lx, (mA/cm’)
@pm) 0
1.5 6.0 4.5 3.0
20 40 100 (b) The effect of concentration in 20 mol/o CH$ZOOH
Concentration (wm) 0
20 40 100
x x x x
10-2 10-j lo-’ lo-’
Percentage inhibition efficiency
60.0 70.0 80.0
of Ph,SnCl on inhibition efficiency + 0.5 M CH&OONa at 30°C
k”,, (mAjcm2) 1.8 0.7 0.5 0.35
x x x x
10~ z 10-2 lop2 10-z
Percentage inhibition efficiency
67.8 7 1.4 80.6
indicated that these compounds are effective suppressors of the anodic dissolution reaction. The functional group and structure of the inhibitor molecules play significant roles in the adsorption process. Adsorption of the organometallic compounds at the metal surfaces takes place by electron transfer through the loosely bound electrons of the 71bond or the aromatic rings. The results can be visualized on the basis that in Ph$nCl the de-activating effect of the phenyl group is predominant over the aggressive effect of chloride and Ph$nCl acts as best inhibitor. It appears that Ph$nCl is adsorbed on the copper surface through the de-activated phenyl group. Different concentrations of Ph$nCl (20, 40, 100 ppm) were added to 20mol/o HCOOH and also to 20 mol/o CHsCOOH. The lowest values of the corrosion current and current density in the entire potential range were observed when the experimental solution contained 100 ppm of the above inhibitor (Figs 8 and 9). The cathodic and anodic Tafel slopes are also found to be higher in such cases. The inhibition efficiency is found to be at a maximum at a concentration of 100 ppm of Ph$nCl in the aqueous acidic solution of either acid (Tables 4(a) and 4(b)). It appears that the efficiency of inhibition is in general directly proportional to the amount of inhibitor adsorbed or the surface coverage. This may be the reason for variation of efficiency with content of Ph$nCl.
CONCLUSIONS The corrosion of Cu in formic acid is more than in acetic acid in different solution mixtures, which can be attributed to their relative acid strengths. The corrosion behaviour of the metal in concentrated solutions of acid is different from that in lower concentrations of acidic solution. The corrosion rate exhibits a non-linear dependence upon HCOOH and CH,COOH concentration. Maximum corrosion is observed in
Corrosion
and inhibition
1409
studies of copper
c
x Without inhibitor o 20ppm Ph$nCI l 40ppm Ph$nCl . IOOppm PhJSnCI
l-
I-
I-
I-
I-
I
I
I ?
Current Fig. 8.
Polarization
2000
1bO0
I
IO-
density (mAi’cm’)
curves of Cu in 20 mol/o HCOOH + 0.5 M sodium presence of different concentrations of Ph3SnC1.
formate
in the
x Without inhibitor 0 20ppm PhGnCl l 4Oppm PhsSnCl l
10.’
I
IO’
n
10”
IO“
1 O‘-
1OOppm Ph$nCl
IO-':
IO“
IO”
IO’
If9
Current density (mAicm’)
Fig. 9.
Polarization
curves of Cu in 20 mol/o CH,COOH + 0.5 M sodium presence of different concentrations of Ph3SnC1.
acetate
in the
V.B. Singh and R.N. Singh
1410
40 mol/o HCOOH acid and 20 mol/o CHsCOOH acid. Addition of sodium formate or sodium acetate increases the rate of copper corrosion in respective acids. Among the inhibitors used 100 ppm of Ph$nCl was found to be the most suitable for inhibition of copper corrosion, although other inhibitors used are also suitable. Acknovledgemenrs-The Chemistry, for providing
authors necessary
wish to thank facilities.
Prof.
P.K.
Srivastava,
head
of the
Department
of
REFERENCES 1. E. Heitz, Advances in Corrosion Science and Technology (ed. M.G. Fontana and R.W. Staehle), Vol. 4, Ch. 3. Plenum Press, New York (1974). 2. 1. Sekine, Corros. Sci. 27, 275 (1978). 3. I. Sekine and K. Momi, Corrosion 44, 136 (1988). 4. N.N. Rao and V.B. Singh, Corros. Sci. 44, 136 (1985). 5. I. Sekine, Corros. Sci. 22, 1113 (1982). 6. I. Sekine and A. Chinda, Boshoku Gijutsu 31, 313 (1982). 7. I. Sekine and K. Senoo, Corros. Sci. 24, 439 (1984). 8. I. Sekine and A. Chinda, Corrosion 40, 95 (1984). 9. I. Sekine and M. Koeda, Boshoku Gijustu 33, 500 (1984). 10. I. Sekine Shvichi, Nrctrochim. Actu 32, 915 (1987). 11. J.J. Demo, Corrosion 24, 139 (1968). 12. B.C. Syrett, Corrosion 32, 242 (1976). 13. D.D. MacDonald, B.C. Syrett and S.S. Wing, Corrosion 349, 1289 (1978). 14. F.B. Growcock, W.W. Frenier and V.R. Lopp, Proc. 6th Europ. Symp. on Corrosion Inhibitors, Ann. Univ. Ferrara, N.S. Sez. V. Suppl. No. 8, 167 (1985). 15. W.W. Frenier, F.B. Growcock and V.R. Lopp, Proc. 6th Europ. Symp. on Corrosion Inhibitors. Ann. Univ., Ferrara, N.S. Sez, V. Suppl. No. 8, 183 (1985). 16. F.B. Growcock and V.R. Lopp, Corrosion 44, 248 (1988). 17. W.W. Frenier and F.B. Growcock. Proc. 7th Europ. Symp. on Corrosion Inhibitors. Ann. Univ. Ferrara, N.S. Sez, V. Suppl. 661 (1990). 18. F.M. Donahue, A. Akiyama and K. Nobe. J. Elecrrochem. Sot. 114, 1006 (1967).