Inhibitive action of some dibenzofuran derivatives on corrosion of iron in sulphuric acid media

Materials Chemistry 7 (1982) 19- 34

INHIBITIVE ACTION OF SOME DIBENZOFURAN DERIVATIVES ON CORROSION OF IRON IN SULPHURIC ACID MEDIA*

A ZINGALES**, G QUARTARONE, A TASSAN Istttuto dl Chtmtca lndustrzale, Facolta dt Chlmwa lndustrtale, Untverstta degh Stud~ dt Venezta, D Duro 2137 - 30127 VENEZIA - l t a l y Recewed 7 July 1981, revised 19 October 1981 Summary - The inhibitive properlaes of Dlbenzofuran (D B F ) and some derxvatwes (3-NH2-, 2-NH2-, 2,3-NH2-, 3-NHCOCH3-, 2-SH-, 3-SH-, 3-NO 2) against the corrosion of iron m H2SO4 IN at 25°(2 and 50"12 have been mvestlgated Experimental data of electrochemical, colorlmetnc and weight loss measurements are reported The results show that these molecules act as qmte effective mhlbltors at 25*(2 They do show instead not a same effectiveness at 50*(2

INTRODUCTION It has been recogmzed 1, 2, 3 that most of polar orgamc molecules can be chemically adsorbed on a metalhc surface by electrodonatlon, leading to the formatron of a coordinate bond between the atom~ of the organic molecule and the metal 4 It was consldered also an electrostatic s'6 (Coulomb~c) lnteract~on, at electrode potenUal away from the p z c (potential of zero charge) * Presented at "XIV Congresso della Soclet~ Chlmlca Itahana" Catama (Itaha), September 21-25, 1981 Abstract book p 392 ** To whom correspondence should be addressed 0390-6035/82/010019-165200/0 Copyright © 1982by CENFORS R L All rights of reproducttonm any form reserved

20 Moreover In acidic solution some polar groups can act either as a neutral molecule or as protonated In some cases it has been suggested a ~r-bondmg interaction which can be related to a back-donation o f electrons m the metal to a vacant d-orbital o f the polar atom Due to these different interaction it ~s reasonable to conceive that aromatic molecules can adsorb on the iron surface by the formation o f a bond o f a different type a n d / o r strength than their derivatives with polar atoms The aim o f the present work is to relate the inhibition effectiveness of a compound (Dlbenzofuran), showing a planar aromatic structure, to the effectiveness of its substituted derivatives with polar groups, against the iron corrosion m 1N H2SO4 at 25 ° and 50"C

EXPERIMENTAL Corrosion inlu'bitors Orgamc molecules used as mhlbltors were Dlbenzofuran (D B F ) and its substituted derivatives 3-NO2- , 3-NH2- , 2,3-NH2- , 3 - S H - , 2-SH- and 3-NHCOCH 3Inlubltlve solution were prepared by dissolving the mlubltors in 1N H2SO4 (reagent grade). Owing low solubility, all these molecules were added to the acid and stirred for at least 15 hours, obtaining saturated solution The actual concentration m the solution resulted m every case below 10"4M, with flight differences between the various molecules Due to this reason It was not possible to make different runs at different concentration, to obtain more reformation on the inhibition efficlences o f these molecules Known amount (700 m l ) o f Filtered solution were Filled m corrosion cell and deaerated by prolonged bubbling o f very pure nitrogen The gas stream also provided the agitation o f the solution during the experunents

Metal Carbon steel disks o f 15 m m diameter were used in electrochenucal experl.

21 ments, The percentage composition was C = 0 06*/,, Mn = 0 15%, Nl = 0 02%, P = 0 016%, S = 0 017%, S n = 0 0 2 4 % , C u = 0 02% Each disk was soldered to a copper wxre and embedded m a metallographlc resin leaving only one face exposed Before the experiment the exposed surface of the electrodes was abraded m sequence with n 120-240-400-600 (grade) emery papers, washed with &stalled water, degreased with methanol and then kept m desiccator over sd~ca-gel for 24 hours Unembedded plates of the same carbon steel, treated as above measuring 40 x 30 x 2 mm, were used m colortmetnc and welgth-loss tests.

POTENTIODYNAMIC POLARIZATION MEASUREMENTS Tests were carried out m an ASTM cell 7 Platmum electrodes were used as counters, saturated calomel electrode (SCE) provided with a Luggm capdlary probe, as reference electrode Measurements were carried out using a mod 551 AMEL (Mdan-Italy) potentlostat The spectmens, prepared as above, were placed in the cell with the exposed face verhcally m contact with the solution The anodlc and cathodic polarization curves were recorded potentlodynamxcally on rephcated specimens Each measurement was started from the almost steady state corrosion potenhal, reached after 1 hour of free corrosion running, by scanning the potential at a rate of 3 mV/nun and 60 mV/mm respectively Potential against log 1 (current density) was recorded on a X-Y recorder, the cell was thermostated at 25 + 0 I*C respectwely Each test was repeated three ttmes to check reproduclbdlty, and an average curve plotted from the three mdlwdual curves

COLORIMETRIC ANALYSIS AND WEIGHT-LOSS MEASUREMENTS Specunens before each test, were placed m 1N sulphunc acid for 1 hour to homogemze the surface, then were washed, dried, weighed and plunged into the soluhon tested (800 ml) thermostated at 25 +--0 l'C, 50 --- 0 I*C respectwely Every 15 rmnutes a sample of the solution was withdrawn with a burette,

22

and the ~ron passed into solution was determined colonmetncally using o-phenanthrolme method The results were expressed m mg cm "2 h "1 After two hours the specmaen was removed, washed, rmsed and rewelghed to determine the overall wetgth-loss

DISCUSSION Fig 1 and Fig 2 show the effect of addltxon of the molecules studied, on

/

J /

/Y

J

46(

\ 550

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\ HAS04 .

.

.

.

. . . . . . .

~

98F

2 RH2 O B F 3111~0 B F

--~--~--2

3 NH2OB F

10000

Ftg 1 -Polartzatton curves at 250C, Vscan 3m V/lnm.

23

4~

,

/ •

.

.

.

H2SO4 .

3-NHCOCH~O B F

. . . . . .

- -

--

// /

3-SH-OD F

- -

2 SH-ODF

- - ~ - - . - -

3-H0~58

lb

F

16o

....

1dee

10600

Fig 2 - Polarzzatzon curves a 25°C, Vscan 3 m V / m m

the potentlodmarmc polarization curves m 1N H2SO4 at 25°C (scanning rate 3 mV/mm) It can be seen that free corrosion potential shafts towards more noble values when mlubitors are added Ttus ~ an evidence that these molecules affect mainly the anod~c process, raising the anod~c overpotentlal more than the cathodic one s The polarization curves when the mlubltors are added, are slufted towards lower current denmy w~th respect to the unmlub~ted one, retaining the same trend

24 The values of Tafel-slopes ba and bc (Table 1) are m agreement with those reported m hterature for iron m sulphunc acid and appear almost unaffected by added mhlbxtors s It can be argued that electrochermcal reaction mvolved in the overall corrosion process is the same both with and without the mhibitors investigated s Tlus is a-postenon conFLrrnatlon that the electrochemical method can be used to evaluate the lrdubltlon efficiency of these molecules with respect to the unmlublted sulphunc-acld "In Table 1 are reported the corrosion current density ('corr) values calculated from potentiodynarmc curves by extrapolating both the cathodic and anodlc Tafel-hnes to the common pomt (A) and, also by extrapolatmg the Tafel-cathochc line to the free corrosion potential (B) The inhibition percentage I P are calculated by the following equation l c ° r r H 2 SO4 - l c ° r r m h l b l t

100

I P = lcorr H2 SO 4

As can be seen m Table 1, D B F shows an I P of c a. 84% at 25"C, the amino-derivatives c a. 93"/,, while the SH-and NO2- derivatives show values a httie lower than the above mentioned ones The data obtained indicate that I.P are m some way affected by the polar effect of the substltuents, the most effective being the amino-derivatives s ' 9 " l ° These can increase the negative delocahsed charge of the aromatic compounds and/or can mteract through the polar atom with the metal It is known 10 that the prunary anunes can be adsorbed m chfferent ways a) by the electrondonation of N-atom to the positive charged surface, b) By formation of a hydrogen bond between the neutral polar group and the metalhc surface of a water molecule there adsorbed, as follow H I

R-N-H

Me

or

R- N

O-H

Me

or

R-N-H

O-H

I

I

I

I

I

H

H

H

H

H

Me

A thard way, considers the fact that m acidic solution the amines can exast m the protonated form It is possible that the ammonium ion interact with the surface of metal charged with opposite sagn Since molecules with substltuents that prowde polar atoms, hke attunes,

61

118

126

112

115

117

118

127

107

113

H2 SO4 1 N

DBF

2NH2-D B F

3NH2-D B F

3NO2-D B F

2SH-D B F

3SH-D B F

2,3NH2-D B F

3NHCOCH3-D B F 54

59

64

45

36

43

54

58

ba mV

bc mV

-499

-512

-502

-493

-494

-491

-502

-498

-543

85

82

191

113

86

85

87

178

1237

Ecorr lcorr mV/SCE /zA cm -2

Table 1 - Polanzahon data at 25°C, Vscan = 3 m V / m m

93

93

85

91

93

93

93

86

IP

A

01

01

02

01

01

01

01

02

13

74

70

269

138

115

79

69

162

1044

Rcorr lcol:r mg/cm2h /IA cm -2

93

93

74

87

89

92

93

84

IP%

01

01

03

02

01

01

01

02

11

Rcorr mg/cm 2 h

tO tab

59 75 62

113

101

90

H2SO 4 IN

DBF

3-NH2-D B F

ba

mV

bC

mV

-499

-501

-514

Ecorr mV/SCE

Table 2 - Polarization data at 25"C, Vscan = 60 m V / m m

106

143

1245

Icof1" uA cm -2

91

89

IP

A

O1

O1

13

127

207

1605

Rcorr 1COIr mg/cm2h /IA cm -2

92

87

IP

01

02

17

Rcorr mg/cm 2 h

t~

27 are more effective mhlbltors ~t can be argued that one or more o f these mechamsms enhance the interaction between the substituted orgamc compounds and the metal with respect the dlbenzofuran unsubstltuted The D B F m effect can interact by means o f electrostatic attraction between the n-delocallsed electrons and the metal or by the formation of ~r-complex It can be seen m Table 2 that the runs performed at 25°C w~th a scanning rate o f 60 m V / m m show shghtly different values o f electrochemical parameters

40C

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3 -

Hystereszs curves

3-NHCOCH30 3-NHCOCH3D0BF

~

~1~ i1~/

,'o

,~o

28 with respect at 3 mV/mm, wde the I P and the corrosion rates are consastent with the above mentlonated In Fig. 3 it can be seen a broadentng of the hysteresis cycle of the anodlc polarization curve caused by the molecules investigated It is know indeed that, also without mhlbltors, at the free corrosion potential m sulphunc acid the iron surface is almost completely covered by adsorbed hydrogen forming (Fell)ads By scannmg the potential toward nobler values, the coverage fraction decreases 11 Hence, scannmg the potential back-ward it results a gap between the "dtrect" and the "back" scanned curve In presence of mh~bltlng molecules, the current at a g~ven potential m the dtrect scanning is lower because the adsorbed orgamc compounds contribute to dnmnlsh the surface avadable for the reaction Tlus is done m two ways, firstly blocking by itself a number of sites onto metal surface, secondly by interaction of the molecule standing near the surface (by electrostatic interaction e g ), winch can act as a slueld between the adsorbed hydrogen and the solution In any case, once reached a potential sufficiently high to result m a coverage fraction of (Fell)ads and orgamc molecules almost zero, ff the potential is scanned backward, It can be seen a gap between the direct and back curve,broadenmg as the mhlb~tor ~s more effectwe In fact from our experimental data ~t can be seen that the ammo-denvatwes broaden the hysterests cycle more than the D B F itself, showing a more inhibiting effectiveness In Fig 4 are reported the potentlodynamlc polartzatlon curves obtamed at 50"C (scanmng rate 60 mV/mm) In Table 3 are reported the I P and Rcorr (corrosion rate) for the sulphurlc acid, D.B F., 3-NH2-, 2-NH2- and 3-NHCOCH3- derivatives at the same temperature Obwously, accordmg the Arrhenms law, the corrosaon rate increases with temperature. Nevertheless it has been clamaed 12 that m the case of very effectwe inlubltors, at 50"C the I P should be higher than at 25"C The explanation stands m the fact that the overall apparent act~vatlon energy of the inhibited reaction should be lower with powerful mhlbltors than uninhibited ones In other words, the rate of the adsorption reaction rises with the tempera-

29

/ 4o~

/,,1

/

/

/ t/

........

_----

j

~.

H2SO, OBF . . . . . . . .

3-N)'1208 F

Fig 4 - Polartzatton curves at 50°C, Vscan 6 0 m V / m t n

ture more than the rate of the corrosion reaction In this case the rate o f overall corrosion reaction m presence o f inhibitor rises w~th the temperature less than the reaction m absence o f mlub~tors In our case, instead, the I P (or the coverage fraction) decreases as the temperature increases Therefore it can be suggested that the rate o f the adsorption reaction rise w~th the temperature less than the corrosion reaction, leading to lower coverage fraction at 50"C than at 25"C

104

110

102

107

DBF

3-NH2-D B F

2-NH2-D B F

3-NHCOCH3 DBF

113

be mV

H2 SO4 1N

t

89

77

73 -510

- 507

- 506

-514

-533

70 78

Ecorr mV/SCE

ba mV

Table 3 - Polarization data at 50°C, Vscan = 60 mV/mm

2330

2056

1905

2951

5698

lcorr /2A/cm 2

59

64

67

48

IP

A

24

22

20

31

59

2577

2605

2376

3091

6763

Rcorr 1COl"r mg/cm 2 h /.tA/cm 2

62

61

65

54

IP

B

27

27

25

3'2

70

Rcorr mg/cm2h t.o Q

31 Table 4 - Colorunetrle and weight loss data at 25"C

Colonmetnc

Rc°rr mg/cm 2 h

IP

Weight loss

Rcorr mg/cm 2 h

IP

H2 SO4

235

DBF

0 84

64

096

57

3-NH2-D B F

0 73

69

080

64

221

2-NH2-D B F

0 47

80

050

78

3-NHCOCHa-D B F

0 59

75

069

69

2,3-NH2-D B F

0 66

72

065

71

2-SH-D B F

0 84

64

0 84

62

3-SH-D B F

0 82

65

082

63

3-NO2-D B F

0 92

61

089

59

Table 5 - Colormaetnc and weight loss data at 50"C Colonmetnc

Re°rr mg/cm 2 h

H2 SO4

IP

Weight loss

Re°rr mg/cm 2 h

IP

1042

11 78

DBF

6 82

42

6 75

39

3-NH2-D B F

4 91

58

4 79

56

2-NH2-D B F

4 93

58

5 01

54

3-NHCOCHa-D B F

4 26

64

4 35

60

32 Ttus is an evidence that the overall apparent activation energy of the inhabited reactxon is hagher that the unmtublted one Indicative calculatlons of Eac t from the experunental data, altough slightly s~gmficant because the investigated temperature are only two, support this argument It could be of a certain interest mvestlgate the influence of the mlubxtor con. centratlon over the I P and the Eac t , but it was not possible to make it due the scarce solubility of these compounds The colonmetnc data obtained are reported m Tables 4, 5 and m Figures 5, 6 It can be seen that, although the general trend of corrosion data IS the same as

91

m__.l em

• H2s% at ~ z r ~t ~NH2VBz r O 3DBzr

03Nt~DaZF i'

•

III

H~szr

s~ D B~ r

~a

4

IIO

rll

~o

I~1

I~10 t mln

F t g 5 - C o l o r t m e t r t c curves at 25"C

m the potentlodynamlc polanzatlon expertments, the absolute values are quite different A such behavlour is reported by other authors I 3 We can suppose that m potentlodynamlc polarization experunents, the actual surface changes during the measure For that the scanning rate of the potential must be chosen fast enough to avoid too large surface changes, and slow enough to reach quasi-steady state condmons

33

/

H~SQ~

gO-

AvBzr O~2vBz~ •

3 NH~ b Bz j-

•

3N~ICOP~3 D Bz T

¢ r~

Ib

l~tg

6 -

Colortmetrtc

I

$0

41~

aO

curves at 50°C

Whathever rate chosen is a compromise Moreover, m free corrosion tests, either c o l o n m e t n c and weight-loss, it is claamed the occurrence o f a "chemical" corrosion, superimposed to the electrochemical one In the electrochemical tests, obviously, the "chemical" corrosion is not detected

CONCLUSIONS We have found that D B F and its denvatwes 3-NO2, 3-NH 2, 2-NH2, 3-Stt, 3-NHt~OCH3 act as quite effechve mhlbltors for iron corrosion m the sulfuric acid, at 25"C They do not however show the same effectweness at 50°C These molecules are hkely to interact mamly with the anod~c reaction as shown from Ecorr displacement toward more nobles values, and by broadening of the hysteresis clcle of anod~c potentlodynamlc polarization curves The colonmetrlc and weight-loss data show a slmdar trend, confirming the electrochemical tests, although the absolute values of corrosxon rate are not coincident

34 REFERENCES

1 2 3 4 5

6 7 8 9 10 11. 12 13

G TRABANELLI, F ZUCCHI, G L ZUCCHINI - 3 rd European Sympostum on Corrosion Inhtbttors (Proceed), 121, 1970 V CARASSITI, F ZUCCHI, G TRABANELLI - 3 rd European Symposium on Corroston Inhtbttors (Proceed), 525, 1970 I N PUTILOVA - 4 th European Symposium on Corroston mhlbttors (Proceed), 425, 1975 K ARAMAKI - 5th European Sympostum on Corroszon Inhtbttors (Proceed), 267, 1980 J O'M BOCKRIS, A K N R E D D Y - Modern electrochemtstry, Plenum Press J G N THOMAS - 5 th European Symposium on Corroston Inhlbttors (Proceed ), 453, 1980 W H AILOR (Editor) - Handbook on Corroston Testtng and Evaluatton, Plenum Press, 177, 1971 H VAIDYANATHAN, N HACKERMAN - C o r r o s t o n Scwnce, II, 737, 1971 K KOBAYASHI, K ISHII - 5th European Symposium on Corroston lnhtbttors (Proceed), 489, 1980 M TEZUKA, Y TOMOE, S FUJII - 5 th European Sympostum on lnhtbttors (Proceed), 501, 1980 I EPELBOIN, P MOREL, H TAKENOUTI - 3 rd European Sympostum on Corrosion Inhtbztors (Proceed), 23, 1970 Q J M SLAIMAN, E M A KADHIMI - 5 th European Symposium on Corroszo~ Inhlbttors,(P~oceed ), 87, 1980 G TRABANELLI, F ZUCCHI, A FRIGN&NI, M ZUCCHINI, G BRUNORO, G ROCCHINI - 5th European Sympostum on Corroslon Inhtbttors (Proceed'), I39, 1980