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38
Процессы и аппараты химических и других производств. Химия
УДК 620.193
The Inhibition of Corrosion and Hydrogen Permeation of Steel in Media, Containing H2S and CO2 L.E. Tsygankova, V.I. Vigdorovitch, S.E. Siniutina
Department of non-organic and physical chemistry,
Tambov State University after G.R. Derzhavin
Represented by a member of Editorial Board Professor V.I. Konovalov
Key words and phrases: corrosion; diffusion; ethoxy amines; inhibitor; hydrogen permeation.
Abstract: The efficiency of ethoxy higher aliphatic amines as composite inhibitors of carbonic-acid and hydrogen-sulfide corrosion and hydrogen diffusion into carbonaceous steel in acid solutions were studied. The influence of the number of ethoxy groups (2, 5 and 14), the length of the hydrocarbon radical (R=C10-C13 and R=C17-C20), medium acidity (0.005 - 0.05 M HCl) and electrode potential was investigated.
Introduction
Development of composite inhibitors and, specifically, general, hydrogen sulfide and carbonic-acid corrosion and hydrogen permeation of steel will enable to reduce the expanded nomenclature of inhibitors to a great extent, heighten the economical efficiency of inhibiting protection and solve a wide range of ecological problems in both production and application. With that end in view, the multifunctional properties of homological ethoxy amines (EOAs) mixture and, in addition, higher aliphatic amines bottoms that are a mixture of primary and secondary aliphatic amines (emulgin) were studied. Practical insolubility in aqueous media in molecular modes, that is increased as a result of protonation, is common to both EOAs and emulgin.
Experimental
0.005 to 0.05 M solutions of HCl were used as background. H2S was injected through saturating the background solutions with gaseous hydrogen sulfide which concentration was checked by iodine-metric titration. In a number of cases, the working solutions were saturated with CO2 (1.7 g/l) under gravimeter checking.
Corrosion tests of the steel samples sized 20x15x3 mm and processed up to sur-face-finish rating 6 after acetone degreasing were conducted in the cells with the plugs ground in (6 to 72 hours). Polarization potentiostatic measurements were performed in the three-electrode cell (pyrex) with a divided anode and cathode space. The electrophysical properties of the steel surface were studied using a photo-electrical polarization
method (PEP) [1]. The impedance measurements were carried out on a full-impedance meter (VM 507).
The hard-phase diffusion rate of H (iH) was evaluated under Ecor, cathodic (AEc=Ecor-Ec) and anodic (AEa= Ea-Ecor) polarization of the working side of the membrane (MB) in the electrochemical hydrogen permeation cell (pyrex) which was similar to that used by Devanathan. The procedure is described in detail in [2].
The EOAs and emulgin (averaged M=320 g/mole) efficiency with the concentration being 25 to 200 mg/l (Table 1) was studied.
Table 1
Products investigated as inhibitors of corrosion and hydrogen permeation of the St.3 steel
EOAs Number Formula R n = x + y
I O 0 - 3 2
II / (CH2CH2O)xH R - N 1 C 0 - 1 1 3 5
III \ (CH2CH2O)yH C10 - C13 14
IV C17 - C20 5
V C17 - C20 14
Emulgin R - NH2 C10 - C15, C16 - C20 -
R-NH-R C10 - C15, C16 - C20 -
The protective action of the inhibitors was calculated according to equation
Z,% = (K0 -Kj)-100/K0,
where K0 and Ki are steel corrosion rates in uninhibited and inhibited solutions. The hydrogen diffusion depressing coefficient a was calculated according to equation
y = iH /iH
where i0H and iH are flows of hard-phase diffusion in solutions without and with inhibitors respectively.
Results
1. The influence of EOAs on steel corrosion. In 0.005 M solutions of HCl or in addition containing CO2, ZEOAs is not high (Table 2). Most efficient are the amines with R=C17-C20. The presence of H2S heightens Z. The growth of nEOAs (in the presence of CO2 and H2S) with R=C10-C13 decreases Z and, for the amines with R=C17-C20, the dependence is reverse. The simultaneous presence of H2S and CO2 heightens ZEOAs. Also, the growth of CHCL influences Z. The presence of CO2 (1.7 g/l) decreases Z.
Table 2
Steel corrosion rate (K, g/m2h) and protective action (Z, %) of inhibitors (100 mg/l) in 0,005 M (numerator) and 0,05 M HCl (denominator) solutions containing 80 mg/l H2S, 1.7 g/l CO2. 293 K, tests duration 18 hours
Additive
Inhnbitor - H2S CO2 H2S + CO2
K Z K Z K Z K Z
absent 0,28 1,27 - 0,41 3,38 - 0,09 0,09 - 0,36 1,30 -
I 0,17 39 0,07 83 0,06 33 0,07 81
0,12 91 0,06 98 0,07 22 0,06 95
II 0,20 29 0,14 66 0,06 33 0,06 83
0,10 92 0,08 98 0,08 11 0,05 96
III 0,14 50 0,13 68 0,06 33 0,05 86
0,13 90 0,10 97 0,07 22 0,06 95
IV 0,13 54 0,12 70 0,05 44 0,07 81
0,12 91 0,10 97 0,07 22 0,05 96
V 0,15 46 0,08 80 0,05 44 0,04 89
0,26 80 0,14 96 0,10 0 0,06 95
2. The influence of EOAs on the kinetics of partial electrode reactions (PERs).
According to the nature of influence on PERs in 0.005 M solutions of HCl, the studied EOAs can be divided into three groups (Fig. 1):
E,B
Fig. 1 Potentiostatic polarization curves of steel in 0.005 M HCl. Inhibitor (100 mg/l):
1 - absent, 2 - I, 3 - II, 4 - III, 5 - IV, 6 - V. 293 K. Stationary electrode
- those that equally change the kinetic characteristics of PERs (product II), Ecor"=Ecor°, in which the upper index is product number and 0 is its absence;
- those that only inhibit anodic reactions without substantial influence on the ca-thodic process in the kinetic area (product I, Ecor'>Ecor°);
- those that retard H3O+ reduction and stimulate the anodic process (products III, IV and V, Ecor<Ecor°) with the first effect prevailing. For them, this sequence holds true: III>IV>V. In general, substances with a lesser R retard the anodic reaction or in like measure influence on the kinetics of PERs. The greatest effect is achieved when n is small. An increase of R length stimulates the anodic ionization of steel and depresses the cathode reaction; the effect being intensified by growing n. The limiting cathodic current, when injecting additives, is reduced as in the sequence: II > I > V > III > IV.
In the presence of H2S and CO2, the situation changes drastically: I and II essentially retard metal ionization (Fig. 2) and stimulates the cathodic reaction. The quantities of ba and bc actually do not change. The first effect prevails as corrosion rate, according to the polarization measurements, decreases by an order.
In the 0,005 M solutions of HCl, the initial signal of negative-sign photoelectric polarization Epep falls off to zero during the first 5 minutes and in 5 minutes after polarity reversal practically reaches a constant value (Fig. 3). In the presence of H2S, signal sign inversion is also observed whereas its time Tinv reduces considerably (Fig. 3). In comparison with the background solution, the quantity of resistance Rp decreases and the capacity slightly increases.
The influence of CO2, in the absence of H2S, is qualitatively analogous and Tinv reduces to a greater extent. In the mutual presence of H2S and CO2, the quantities of Tinv ,
E, b
-0.40-
-0,30-
-0,20-
-| 0
Fig. 2 Potentiostatic polarization curves of steel in 0.005 M HCl in the presence of H2S (80 mg/l) and CO2 (1.7 g/l). Inhibitor (100 mg/l): 1 - absent, 2 - I, 3 - II
Rp and Epep acquire an intermediate value (Table 2, Fig. 3). In 0,005 M HCl containing H2S or H2S+CO2, the injection of inhibiting additives increases the stationary amplitude of the positive signal (Fig. 3). Moreover, to a greater extent, this effect is caused by homologies with a lesser R. At the same time, they decrease the resistance and, in a number of cases, increase the capacity (Table 2). The dependence Epep =F(CEOAs) goes through the maximum, which is shown in example of I (Fig. 3).
Fig. 3 Dependence of Epep of steel on the presence of hydrogen permeation stimulators (a), the nature of EOAs (b) and concentration of I (с) in 0.005 M HCl.
a - additives: 1 - absent, 2 - 1.7 g/l CO2, 3 - SO mg/l H2S, 4 - 1.7 g/l CO2+S0 mg/l H2S.
b - EOAs (50 mg/l): 1 - absent, 2 - I, 3 - III, 4 - V.
All in the presence 1.7 g/l CO2 and SO mg/l H2S. c - C (mg/l) of product I in the presence of 1.7 g/l CO2 and SO mg/l H2S:
1 - absent, 2 - 25, 3 - 50, 4 - 100, 5 - 200. 293 K
3. The influence of EOAs on the flow of hydrogen diffusion through a steel membrane. In 0,005 M HCl, the studied EOAs increase the flow of hydrogen diffusion through a membrane (MB) at Ecor (Table 3), and amines with R=C17-C20 - to the maximum; their effect is intensified with a growing number of ethoxy groups. An increase in n of compounds with R=Ci0-Ci3 causes a reverse effect. In the presence of CO2, the influence of I, III - V on iH is poorly pronounced (Table 3). II considerably suppress hydrogen permeation. In the 0,005 M HCl solutions containing hydrogen sulfide, amines decrease iH. II is the most effective. In the co-presence of H2S and CO2, the latter weakens the retardation of hydrogen permeation.
Table 3
Influence of EOAs (100 mg/l) on the rate of hydrogen diffusion through a steel membrane (iH,A/m2) and the y coefficient under Ecor in 0,005 M HCl solution.
Temperature 293K
Additive
Inhibitor - H2S CO2 H2S + CO2
i Y i Y i Y i Y
I 0,35 0,4 0,14 2,6 0,24 1,0 0,28 1,2
II 0,21 0,7 0,00 T* 0,006 4,0 0,06 5,7
III 0,43 0,4 0,15 2,5 0,32 0,8 0,31 1,1
IV 0,45 0,3 0,19 2,0 0,32 0,8 0,31 1,1
V 0,49 0,3 0,24 1,5 0,29 0,8 0,32 1,1
T* - total protection
Under MB anode polarization in 0,005 M HCl solution, the dependence of iH on AEa goes through the maximum (0.05 B, Fig. 4) and y close to 1 in the interval of
in, A/m2
Fig. 4 Dependence of iH on AEa in 0.05 M HCl containing EOAs (100 mg/l):
1 - absent, 2 - I, 3 - II, 4 - IV in the absence (a) and presence of 1.7 g/l CO2 (b, d) and 80 mg/l H2S (c, d). Duration 2 hours, 293 K
potentials AEa=0...0.2 B. A stimulating effect is observed then AEa>0.2 B. Dependence with the maximum is also characteristic of other studied media, which correlates with literature data and is, evidently, common to all. In the presence of CO2, the influence of
I, II and IV is qualitatively the same. It is only when injecting H2S in the whole area of the investigated anodic potentials that II and, especially, I decrease iH (Fig. 4). The presence of CO2 together with H2S weakens the effect and I only remains as an inhibitor of hydrogen permeation (HP).
Under cathodic polarization, the dependence iH=F(AEc), especially in the presence of amines when pH 2, 3... 3 (pH grows due to EOA protonation), is also with the maximum. In 0,005 M HCl, all EOAs under investigation stimulate HP and IV only remains stimulating when CO2 is injected. When CO2 is changed into H2S, amines retard HP. It is also characteristic of the co-presence of H2S and CO2.
4. The influence of emulgin on steel corrosion and PERs kinetics. In 0.005 M background solutions of HCl, emulgin creates a considerable protective effect (Z up to 62 %) already in the concentration 25 mg/l. Its concentration increase of 200 mg/l leads to Z increase of 80 %. In the presence of CO2, estimation becomes complicated. Firstly, CO2 itself exhibits inhibiting action in such media (Z up to 40 %). Secondly, Z can be calculated in relation to Ko in the solution either containing CO2 or not. In the fist case, the quantity Z is appreciably lower. The H2S injected together with CO2 essentially increases Z (up to 90 %) which was repeatedly observed for amines. In the presence of 200 mg/l H2S and with the simultaneous CHCi increase of 2 and 10 times, Z comes to 97 % but slightly decreases in the presence of CO2 (Table 4).
In 0,005 M solutions of HCl containing 200 mg/l H2S and CO2 (1.7 g/l), emulgin slows down the anode reaction with dlgia/dlgCem close to -0.6 (ia - steel anodic ionization rate) and accelerates the cathodic process. Steel dissolves anodically in inhibited solutions being active with the Tafel slope ba=55 mV. Tafel sections of the cathodic polarization curve are not long and have a slope less than 120 mV, which is stipulated by their closeness to the limiting cathodic current (ic,lim). The inhibitor rather retards the anodic reaction than accelerates the cathodic process.
Table 4
Influence of emulgin and HCl concentration on an inhibitor’s protective action with respect to St3 corrosion in the presence of H2S (200 mg/l ) and CO2 (1,7g /l)
Chcl Cnh-103 Corrosion stimulator
mole/l CO2 H2S CO2 + H2S
0,005 0,625 72 89 90
0,010 0,625 87 86 92
0,050 0,078 86 94c 9 О 0
0,156 75 95s 95s
0,312 87 97i 96i
0,625 81 97 96
Note. Pitting formation: c - considerable, s - small, i - isolated, absent in other cases.
When CHCl is increased by an order, the pattern qualitatively remains in the presence of H2S (dlgia/dlgCem=-0,4). ia retardation prevails again, although dlgic/dlgCem>0 and AEcoj>0.
5. Steel hydrogen permeation in the presence of emulgin. The iH essentially increases under Ecor potential when injecting CO2 and, especially, H2S, which correlates with other data [3 - 5]. The quantity n is used to define the fraction of the adsorbed hydrogen diffusing into the metal in its general discharge from the cathode including the gaseous phase (rate ip). The following equality is obvious:
igen _ ip + iH
The increase of the acidity of solution, containing 1.7 g/l CO2 or 200 mg/l H2S in it separately or the co-presence decrease p 1.8, 2.2, 3.6 and 2.9 times respectively. In the background solution and that containing CO2, emulgin stimulates steel hydrogen permeation, thus heightening both iH(y > 1) and its contribution to igen(Pinh/ Pn/inh > 1).
In the concentration 200 mg/l, emulgin practically arrests hydrogen permeation completely, even in 0.05 M solution of HCl that contains CO2 and H2S simultaneously. With an increase in temperature up to 353 K, the efficiency of hydrogen permeation depression increases essentially, especially in media containing hydrogen sulfide (fig. 5).
Table 5
Influence of emulgin concentration on steel hydrogen permeation (iH, A/m2) in HCl solutions (T = 293 K, CCo2 = 1,7 g/l, two-hour experiments)
Stimulator nature and concentration, mg-l CchIj mole/l iH, A/m2 1/y P -E V -*-^cor? v iH, 1/y P -Ecor*,V
25 mg/l emulgin 200 mg/l emulgin
1 2 3 4 5 6 7 8 9 10
Absent 0,005 0,21 3,72 0,86 0,29 0,12 2,12 ~1 0,28
<N О О 0,34 2,56 ~1 0,30 0,12 0,89 0,86 0,35
50 H2S 0,09 0,50 - 0,35 0,11 0,64 - 0,35
200 H2S 0,09 0,40 0,53 0,35 0,07 0,29 0,50 0,35
50 H2S+CO2 0,07 0,58 - 0,36 0 0 0 0,34
200 H2S+C02 0,09 0,50 0,43 0,36 0,06 0,37 0,32 0,34
Absent 0,050 - - - - 0,02 0,20 0,32 034
<N О С - - - - 0,06 0,20 0,10 0,29
50 H2S - - - - 0 0 0 0,32
100 H2S - - - - 0 0 0 0,29
My
Fig. 5 Influence of temperature in 0.05 M HCl solution on 1/y. Emulgin concentration 200 mg/l.
CO2: 1 and 3 - 0, 2 and 4 - 1.7 g/l. H2S, mg/l: 1 and 2 - 0, 3 and 4 - 200
Continued Table 5
1 2 3 4 5 6 7 8 9 10
150 H2S - - - - 0,01 0,02 - 0,30
200 H2S - - - - 0,02 0,04 0,10 0,30
200 H2S+C02 - - - - 0 0 0 0,32
Note. Ecor* - corrosion potential of the reaction side of a membrane.
Discussion
When injecting EOAs and emulgin, in an acidic medium proceeds the reaction
R;N + H3O+^ R;NH+ + H2O, (1)
in which R stands for all the fragments of corresponding molecules, apart from the atom of nitrogen. We failed to find out any basicity constants for RjN in other works. From general considerations, nevertheless, it may be presumed that the degree of protonation is high enough because their basicity grows being influenced by the induction effect of alkyl radicals and charge localization in the presence of substitutes is weakened by the interaction between solvent and protonated amine, thus lowering cation acidity and stabilizing it.
The discharge stage is delayed on iron in HCl solutions [6].
H3O+ + e ^ Hads + H2O, (2)
whereas Hads removal proceeds in the quick stage of chemical recombination
Hads + Hads ^ H2 (3)
and also at the expense of its absorption by metal
Hads ^ Habs , (4)
which stipulates steel hydrogen permeation. The acceleration of the cathode reaction at the potentials correspond to the Tafel sections of the polarization curves, is evidently connected with the promotion of organic cations’ reduction compared to (2).
R;NH+ + e ^ Hads + R;N . (5)
Possibly, amines adsorption on steel results in polymolecular film formation, through which H3O+ mass transfer determines ic,lim. This explains the kinetic regularities of the cathode process in 0,005 M HCl solutions containing H2S and CO2. The situation practically does not differ from the one observed in the background solution. The only difference is that under pH=2 - 3, the dissociation of the weak acids H2S and H2CO3 is depressed and together with (1) and (5), there can only proceed concurrently grand (7) thus increasing the summary rate of the cathode process.
H2S + e ^ HS- + Hads, (6)
H2CO3 + e ^ HCO3 + Hads, (7)
In view of photoelectric polarization (Fig. 3), we will examine the origin of anodic reaction retardation by EOAs. According to the theory of method [1], Cl- anions of the background solution interact with the lattice of surface oxide (SO) by the mechanism of oxygen substitution and exhibit electron donor properties. A decrease in xmv in the presence of H2S is evidently connected with a more active interaction of H2S and the prod-
ucts of its dissociation adsorption with the lattice of SO. Hydrogen sulfide acts as donor for electrons, owing to which Rp decreases. Anion HS- or S2- can substitute ion O2- in the lattice of SO. Additive states with a positive effective charge (negative charge deficiency) that arise under substitution act as centers for recombination of unbalanced electrons leading to EPEP sign inversion. The negative charge deficiency is compensated for, proceeding from the conditions of oxide electric neutrality, by the reaction
Fe3+ + e ^ Fe2+. The products of the reaction Fe2+ are ‘active centers’ of preferable adsorption and dissolving of metal covered non-stoichiometric oxide film of n-type. Their concentration growth leads to an increase in corrosion rate. After Epep inversion, some of the states arising under oxygen substitution participate in the exchange of electrons with a valent zone. Being below the Fermi level, they are able to catch electrons, thus increasing the concentration of holes and lowering resistance.
The influence of CO2 is analogous, but xmv is slightly decreased (Fig. 3). Carbon oxide (IV) is also a donor for electrons, however the mechanism of its interaction with iron oxide is evidently different, because its substitution or introduction into SO is improbable. Most likely CO2 forms a surface adsorption complex (SAC) and, being in its
composition and transferring electrons to the zone of conduction, forms CO+ . The arising cation field retards Fe2+ output into the solution, thus slightly inhibiting corrosion (Z=40 %). As a result, Rp becomes greater than in the solution with H2S, whereas the capacity decreases.
The simultaneous presence of H2S and CO2 leads to intermediate values of xmv, Epep and Rp. The injection of I, III and V decreases Rp and oinv under the general tendency of increasing the amplitude of EPEP (Fig. 5). The large size of protonated molecules of amines does not allow them to penetrate into or substitute in the SO lattice. Like in the case of CO2, a SAC is formed at the expense of the undivided electron pair of nitrogen atom in molecules from the layer of amines that is closest to the metal and d-orbital of iron atoms. A positively-charged coverage is formed that hinders the transfer of metal cations into the solution, which decreases corrosion rate.
Conclusion
EOAs and emulgin inhibit corrosion of carbonaceous steel in dilute hydrochloric acid solution containing H2S with their protective efficiency Z up to 95-99,9% and remove the local destruction of metal. Z increases with increasing H2S concentration. Under some conditions these inhibitors are capable to supress hydrogen diffusion into the steel almost completely what is promoted by the acidity increase, H2S presence and the temperature increase (emulgin). The hydrogen permeation inhibition is obviously caused by decrease of surface coverage by Hads because of decreasing hydrogen portion of total Hads diffusing into the metal.
References
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2. V. I. Vigdorovitch, Protection of Metal (Rus), 31, 541, (2000).
3. I. L. Rosenfeld. Inhibitors of Corrosion. Khimiya, Moscow (1977).
4. J. I. Bregman. Corrosion Inhibitors. Khimiya. Moscow (1966).
5. M. V. Loskutova, A. V. Boldyrev. Vestnik Tambovskogo Universiteta. 2, 41 (1997).
6. V. I. Vigdorovitch, T. P. Dyachkova, O. L. Pupkova, L. E. Tsygankova. Electrochemistry (Rus), 37, 1437 (2001).
Ингибирование коррозии и водородопроницаемости стали в средах, содержащих И28 и СО2
Цыганкова Л.Е., Вигдорович В.И., Синютина С.Е.
Ключевые слова и фразы: коррозия; диффузия; оксиэтилированные амины; ингибитор; водородопроницаемость.
Аннотация: Изучена эффективность оксиэтилированных высших алифатических аминов в качестве комплексных ингибиторов углекислотной коррозии и наводороживания углеродистой стали в кислых растворах. Исследовано влияние числа оксиэтильных групп (2, 5 и 14), длины углеводородного радикала (Я=С10-С13 и Я=С17-С20), кислотности среды (0.005 - 0.05 М НС1) и потенциала электрода.
Inhibierung der Korrosion und der Wasserstoffdurchdringungsfahigkeit des Stahls in den H2S und CO2 enthaltenen Mitteln
Zusammenfassung: Es ist die Effektivitat von ethoxy hochsten aliphatic Ami-nen als Komplexinhibitoren der Kohlensaurekorrosion in Sauerlosungen erlernt. Es ist der EinfluB der Zahl von Ethoxygruppen (2, 5 und 14), der Lange des Kohlenstoffradi-kals (R=Ci0-Ci3 und R=C17-C20), der Mediumsauerkeit (0.005 - 0.05 M HCl) und des Elektrodenpotentials untersucht.
Inhibitation de la corrosion et de l’etancheite de l’acier dans les milieux contenant H2S et CO2
Resume: On a etudie l’efficacite des amines oxyethylees primaires aliphatiques en qualite d’inhibiteurs complexes de la corrosion carbonique-acide et de l’hydrogenation de l’acier dans les solutions acides. On a etudie l’influence du nombre de groupes oxyethyles (2, 5 et 14), de la longueur du radical hydrocarbonique (R=Ci0-Ci3 et R=Ci7-C20), du milieu acide (0.005 - 0.05 M HCl) et du potentiel de l’electrode.
