- Email: [email protected]
Use of vapour phase corrosion inhibitors in packages for protecting mild steel against corrosion
Corrosion Science 51 (2009) 921–925
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Use of vapour phase corrosion inhibitors in packages for protecting mild steel against corrosion U. Rammelt *, S. Koehler, G. Reinhard Excor Korrosionsforschung GmbH Dresden, Magdeburger Str. 58, D-01067 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 21 November 2008 Accepted 21 January 2009 Available online 4 February 2009 Keywords: A. Mild steel A. VCI B. EIS B. OCP C. Passivation
a b s t r a c t Some aspects of the passivation of mild steel in the presence of selected vapour phase corrosion inhibitors (VCIs) were considered. In particular their ability to vapourize was evaluated by sublimation tests and their role in the inhibition mechanism of mild steel was investigated by electrochemical methods such as open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements. In the presence of some carboxylates, amines and azoles alone and as mixtures a protective layer can be formed on mild steel in neutral and alkaline solution. It was shown that the passivation mechanism strongly depends on the pH of the solution. In addition the influence of contaminants from industrial alkaline cleaning baths on the protective properties was analyzed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In general, polymer based packaging materials are used in order to protect metallic equipment during transport and storage against atmospheric corrosion. This temporary corrosion protection is achieved by incorporation of vapour phase corrosion inhibitors (VCIs, volatile corrosion inhibitors) in the polymer film, whereby the packaging material functions as a VCI source. Protection is necessary especially during the time of wetness, where the metal surface can corrode due to the formation of a thin electrolyte layer on the surface. Therefore, the efficiency of a compound used as VCI mainly depends on two parameters [1,2]: 1. its vapour pressure (more exactly: its tendency to sublime) under atmospheric conditions is high enough allowing significant vapour phase transport of the inhibitor within an enclosed space to the metal surface; 2. the VCI adsorbed directly on the metal surface or dissolved in a condensed water film on the metal surface inhibits the metal corrosion during storage and transport by interaction with the surface. Under ambient conditions VCIs have to possess moderately high vapour pressure, which should be in the range of 10–10 4 Pa [3]. If the vapour pressure is too low, another volatile substance, such as water vapour or urea can be used as carrier of the VCIs. The vola* Corresponding author. Tel.: +49 351 88857324; fax: +49 351 88857331. E-mail address: [email protected] (U. Rammelt). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.01.015
tility can also be enhanced by use of host–guest systems where the VCIs are incorporated to a porous substrate [4]. Then the VCIs are transferred to the metal surface by diffusion through the gas room and are adsorbed directly on the metal or dissolved in a surface moisture layer and reduce the corrosion rate markedly. As reported in the literature the inhibition mechanism proposed for VCIs, is mainly based on its adsorption on the metal surface whereby a thin monomolecular barrier film should be formed [1,5–7]. Especially the efficiency of amine-based inhibitors should be due, along with their basicity, to their ability to form a hydrophobic adsorption layer to shield the metal surface from corrosive contaminants [2,8,9]. Only in a few papers it was considered that the surface of unnoble metals like iron is covered with a thin oxide layer, which is formed rapidly on contact of the metal with air [10–13]. Usually this inherent natural oxide layer provides only limited protection so that the main purpose of VCIs is to maintain or reinforce this defect layer. It is well known that some salts of weak carboxylic acids are able to suppress the corrosion process by forming of insoluble deposits at the defect sites in the natural oxide layer. However, this passivation process is only possible under near neutral conditions in the presence of oxygen. In this case, oxygen cannot be considered as a corrosion accelerator [7], but it is itself the passivator supported by carboxylate [14–16]. Therefore, the protection mechanism of VCIs may involve a combination of passivation and adsorption. The protective properties of VCIs can be tested directly by metal loss measurements [1,10,17,18] or by electrochemical data of the corrosion process, such as corrosion current density icorr, corrosion potential Ecorr, polarization resistance Rpol and EIS data [5,10,18–20].
922
U. Rammelt et al. / Corrosion Science 51 (2009) 921–925
In this study, it was tested the ability of some typical VCIs to sublime in a dry nitrogen atmosphere. As VCIs some carboxylic acids and their salts alone or in combination with selected azoles or amines were chosen. The purpose of this work was to study the possibility to improve the protection of mild steel by combining carboxylates with azoles or amines. The inhibition mechanism of the inhibitors and their mixtures on mild steel was investigated by open circuit potential – time and EIS measurements in aqueous solution with different pH values. The according aqueous solutions should simulate the corrosion conditions which are given inside the package, if water is condensed on the metallic equipment.
2. Experimental 2.1. Materials Carboxylic acids (benzoic-, octanoic- and nonanoic acid) and some of their salts (sodium octanoate, sodium benzoate, ethanolammonium benzoate, cyclohexyl-ammonium benzoate) as well as some amines (ethanolamine, cyclohexylamine) and azoles (benzotriazole, benzimidazole) were used as inhibitors. Ethanolammonium and cyclohexyl-ammonium benzoate were synthesized via the stoichometric acid–base reaction of the amine with benzoic acid according to a procedure described by Estevao et al. [4] for dicyclohexyl-ammonium p-nitrobenzoate. All other compounds are commercial products and were used without further purification. The carboxylic acids as well as the amines were used alone and as a mixture with benzotriazole or benzimidazole. All electrochemical experiments were performed at room temperature in aerated 0.01 M inhibitor solutions with a mild steel electrode (A = 0.5 cm2) as working electrode. The surface was polished with 600 grit emery paper, rinsed with distilled water and then immersed immediately in the test solution. 2.2. Sublimation The sublimation tendency of the VCI compounds was investigated by sublimation of 1 g VCI substance at 40 °C in dry nitrogen atmosphere for 2 h. The sublimation was performed using a BÜCHI glass oven B-580. The VCI substance was placed in a glass boat and then introduced in a drying tube. The sublimation accessory was inserted into the drying tube and sublimation was started by heating the glass oven at 40 °C. Under nitrogen flow of about 50 cm3/min the sublimate was transferred to a washing bottle filled with 50 ml methanol. After 2 h the eluate was injected in a GC-2010 gas chromatographer (SHIMADZU) with a Varian CP-Sil 8 CB silica column or in a HPLC liquid chromatographer (MACHEREY + NAGEL) with a Dyonex RP18 column, respectively, for quantitative evalua-
tion. A calibration curve had previously been plotted using adequate standards. 2.3. Electrochemical set-up The electrochemical measurements (electrochemical impedance spectroscopy (EIS) and open circuit potential measurements) were made at room temperature in a three-electrode arrangement, consisting of the mild steel working electrode, a large area Pt gauze as counter electrode and a saturated calomel electrode (SCE) as reference. All potentials are referred to the SCE. The EIS measurements were made using the impedance measurement system IM6 of ZAHNER-elektrik. An ac amplitude of 10 mV was applied and data were collected in the frequency range 50 kHz–0.05 Hz using five points per decade. The EIS data were fitted with the Thales software of ZAHNER-elektrik. 3. Results and discussion 3.1. Sublimation As can be seen from Table 1 the investigated carboxylic acids and amines are volatile and can be detected by gas chromatography. However, whereas the ammonium salts of carboxylic acids are also sublimable, it is not possible to detect the sodium salts of these acids. The ability to sublime correlate rather well with vapour pressures reported in the literature. Although the vapour pressure values present some uncertainties and strongly depend on the experimental conditions they can be used to evaluate the tendency of a solid substance to undergo sublimation and of a liquid substance to vaporize, respectively, under normal conditions. The volatility of the sodium salts of carboxylic acids is too small (vapour pressure is approximately 10 7 Pa at ambient conditions) considering their use as VCI materials. Contrary to the sodium salts of carboxylic acids the vapour pressure and the vapourization rate of the investigated liquid amines ethanolamine and cyclohexylamine is too high for optimal protection because their effectiveness should be limited to a short time, as their consumption rate will be high. On the other hand, if only amines are in the vapour, the pH of the surface moisture film on the metal to be protected should be too high (pH > 10) for a multi-metal protection. Whereas iron or mild steel are in the passive state other unnoble metals like Al, Mg or Zn are attacked in this pH region. In order to avoid this it is usually to use amines as solid salts. The influence of the pH value on the protection of mild steel is investigated in the next section. The sublimation tendency of benzotriazole and benzimidazole is much smaller than this of the investigated amines and carboxylic acids. Whereas 20 lg/g weighed benzotriazole was found benzimidazole could not be quantified. The detection limit of 500 lg/
Table 1 Results of volatilization test. VCI
Sublimation mg/1 g weighed substance
Vapour pressure/Pa (from literature)
Benzoic acid Octanoic acid Nonanoic acid Sodium benzoate Sodium octanoate Ethanolammonium benzoate Cyclohexyl-ammonium benzoate Ethanolamine Cyclohexylamine Benzotriazole Benzimidazole
0.18 0.41* 0.26* – – 0.8/0.2 0.2/0.2 15.1* 440.5* 0.02
0.09 0.49 0.21 4.9 10 7 6.5 10 7 1.62 0.04 53 1.3 103 0.09 0.01
* **
Vaporisation of the liquid substance. Could not be proved.
**
U. Rammelt et al. / Corrosion Science 51 (2009) 921–925
g weighed substance is too high for GC analysis of benzimidazole. Sample enrichment or stronger sublimation conditions (higher temperature, extended sublimation duration) are necessary to reach the detection limit. However, also benzimidazole should be volatile enough for use as VCI since its vapour pressure exceeds 10 4 Pa. 3.2. Electrochemical measurements In 0.01 M aqueous solution of the carboxylic acid salts (the pH values are between 6.5 and 7.5) the steel surface can be protected. It is well known that at neutral conditions carboxylic acid salts support the passivation of mild steel by dissolved oxygen in aerated solution [15]. In previous studies [16,21] it was shown that the protection mechanism is due to the sealing of defects in the natural oxide layer, which already has been formed on the mild steel surface after short contact with air. High resolution autoradiography of benzoate labelled with 14C [22,23] and more recently XPS measurements [24] revealed that the passive oxide layer composed of an iron oxide/hydroxide mixture contained the carboxylate in the form of insoluble carboxylate particles plugging the defects. In order to characterise the passive layer formed in the different solutions EIS measurements were performed at the open circuit potential after 1 h of immersion. As an example, the impedance data are displayed in Figs. 1 and 2, respectively. It is evident, that in all cases the EIS data can be interpreted with two RC time constants
90 75
10k
1
60
2
45
3
30
1k
|Phase|(deg)
|Z|(Ohm)
100k
15 100
100m
1
10
100
1k
10k
0
f (Hz) Fig. 1. Bode plots of mild steel in aerated 0.01 M benzoate solution after 1 h of immersion at different pH: (1) ethanolammonium benzoate + ethanolamine (pH 10.5), (2) ethanolammonium benzoate (pH 6.6), (3) benzoic acid (pH 3.1).
90 75
1
60
2
10k
45
3
30
1k
|Phase|(deg)
|Z|(Ohm)
100k
15 100
100m
1
10
100
1k
10k
0
f (Hz) Fig. 2. Bode plots showing the influence of salt on the efficiency of ethanolamine (pH 10.5) after 1 h of immersion: (1) without salt, (2) with KNO3 and sodium benzoate, (3) with KNO3.
923
in parallel. The high frequency time constant (about 10 kHz– 10 Hz) can be explained with the adsorption of the inhibitor, while the low frequency time constant (about 10–0.05 Hz) is related to the oxide layer [5,16]. The resistance Rel reflects the electrolyte resistance which is measured at frequencies >10 kHz. The corresponding equivalent circuit is included in Fig. 1. With only one time constant as it was used for example by Subramanian et al. [19] and Zang et al. [10] the behaviour of mild steel in VCI containing solutions cannot be described sufficiently. It should be mentioned that both capacitances C of the time constants are replaced by a constant phase element CPE [ZCPE = ACPE(jx) n] [25], considering the nonideal behaviour of the layers, which is due to inhomogeneity in the conductance or dielectric constant inside the layers [26]. The exponent n of the CPE as a measure of the inhomogeneity was between 0.82 and 0.96 for all experiments. Only in acid solutions the exponent n is lower (0.73. . .0.85). ACPE is a constant with the dimension X cm2 s n. If n = 1 then 1/ACPE = C with the dimension F cm 2. The parameters associated to the EIS data are given in Table 2– 4. 3.2.1. Effect of pH on the protection mechanism The different volatility of the VCI components strongly affects the pH of the surface moisture layer on the metal to be protected. If the volatility of both components (amine and carboxylic acid) is nearly the same, the pH is in the neutral range. As an example, three different salts of benzoic acid are presented in Table 2. Independent of the cation the open circuit potential is in the passive range of mild steel, the resistance of the oxide layer Rox is about 50 kX cm2 and the surface of the electrode remains shiny after 1 h of immersion. The adsorption resistance ranged between 0.1 kX cm2 for sodium benzoate and 0.23 and 0.31 kX cm2 for the amine benzoates indicating a better adsorption ability of ethanolamine and cyclohexylamine on the oxide covered surface than benzoate. However, amine alone cannot passivate mild steel at this pH. In 0.01 M solution of ethanolamine and cyclohexylamine, respectively, the open circuit potential given in Table 2 remains in the active range of iron dissolution and the low resistance of about 0.5 kX cm2 is a typical value of a corroding iron electrode. The corrosion starts at weak points in the natural oxide layer and after 1 h of immersion the surface is covered with rusty males. It is clearly to see that the passivation of mild steel in neutral aerated solution is due to the presence of benzoate which forms insoluble ferric benzoate complexes in the defects of the natural oxide layer. As can be seen from the sublimation results of ethanolammonium benzoate (Table 1) the volatility of ethanolamine is about four times higher than the volatility of benzoic acid in this case. Therefore, the pH of the moisture layer on the metal surface increases and the inhibition mechanism may be based on the alkalization of the solution. The pH value of an aqueous solution containing 0.04 M ethanolamine and 0.01 M benzoic acid is about 10.5. The mild steel electrode is in the passive state and the resistance of the passive oxide layer in the alkaline solution is two times higher than that into neutral solution (see Table 3). If the volatile acid is in excess in the vapour the pH value of the surface moisture layer decreases and this may accelerate the corrosion process. As expected there is no passivation in aerated benzoic acid solution at pH 3.1 and after 1 h of immersion the metal surface is covered with rust. The same result was received in 0.01 M octanoic and nonanoic acid, respectively. In acid solution uniform corrosion is observed and both resistances are attributed to the corrosion process; adsorption of carboxylate cannot be detected. Therefore, the resistance and capacitance of benzoic acid in the high frequency range were parenthesized (Table 3). The influence of the pH on the passivation of mild steel is summarized in Table 3 and represented in Fig. 1.
924
U. Rammelt et al. / Corrosion Science 51 (2009) 921–925
Table 2 Fit results of EIS measurements in aerated 0.01 M solutions at neutral pH after 1 h of immersion. VCI
pH
Sodium benzoate Ethanolammonium benzoate Cyclohexyl-ammonium benzoate Ethanolamine Cyclohexylamine
7.1 6.6 6.9 7.3* 7.4*
*
E(VSCE) 0.105 0.126 0.092 0.413 0.341
Rox (kX cm2)
Cox (lF cm
54 51 58 0.7 0.9
25.0 21.8 26.4 42.8 41.0
2
)
Rad (kX cm2)
Cad (lF cm
0.09 0.23 0.31 0.30 0.39
19.4 19.6 24.2 38.8 32.2
2
)
Adjusted with nitric acid.
Table 3 Influence of pH on the passivation of mild steel in aerated 0.01 M solutions of ethanolammonium benzoate (etbzo), etbzo + ethanolamine (et) and benzoic acid after 1 h of immersion. VCI
pH
Etbzo Etbzo + et Benzoic acid
6.6 10.5 3.1
E(VSCE) 0.126 0.165 0.550
Rox (kX cm2)
Cox (lF cm
51 109 0.8
21.8 23.8 16.4
2
)
Rad (kX cm2)
Cad (lF cm
0.23 0.26 (0.18)
19.6 20.8 (5.6)
2
)
Remarks Passivation Passivation Corrosion
Table 4 EIS data in aerated 0.01 M ethanolamine solution (pH 10.5) in the presence of different salts (c = 0.01 mol/l) after 1 h of immersion (Nabzo = sodium benzoate).
Ethanolamine (et) Et + Nabzo Et + Nabzo + KNO3 Et + KNO3 Et + NaClO4 Et + Na2SO4
E(VSCE) 0.134 0.127 0.393 0.413 0.445 0.562
Rox (kX cm2)
Cox (lF cm
114 103 5.3 4.7 2.6 1.4
23.8 21.6 31.4 32.8 36.2 44.6
3.2.2. The influence of contaminants Before the metallic equipment is wrapped up into the VCI containing package one of the necessary manufacturing stages is the cleaning process. During the repeated use of industrial alkaline cleaning baths contaminants can be enriched in the bath and remain on the metal surface after the washing procedure. Therefore, the influence of some salts (nitrate, perchlorate, sulphate) on the passivation of mild steel in alkaline solution was investigated. Typical Bode plots of mild steel in 0.01 M ethanolamine solution (pH 10.5) with and without 0.01 M potassium nitrate are given in Fig. 2. There is no passivation of mild steel in alkaline solution in the presence of nitrate. The open circuit potential is in the active range of iron dissolution and the resistance of the oxide layer is clearly decreased (Fig. 2, curve 2 and 3). After 1 h of immersion the surface of the electrode is shiny but with some rust points. As can be seen in Table 4 in the presence of all investigated salts local corrosion is observed. That implies that in all cases the corrosive attack starts at local defects of the natural oxide layer independent of the aggressivity of the salt. The more aggressive the salt the larger are the rust points and the lower is the resistance of the oxide layer. An interesting point is the behaviour of mild steel in the presence of sodium benzoate and potassium nitrate in neutral and alkaline solution presented in Fig. 3. Whereas in neutral benzoate solution with and without nitrate passivation is reached after a few minutes in alkaline solution it takes more time to build up the passive oxide layer. However, in alkaline solution passivation cannot take place in the presence of nitrate and the potential drops down to 0.4 VSCE after 60 min of immersion independent of the presence of benzoate. The result leads to the conclusion that in more alkaline solution the inhibitive effect arises from the ability of the system to precipitate ferric hydroxide at weak points in the natural oxide layer whereas the plugging of defect sites with insoluble ferric benzoate decreases steadily with increasing pH [22]. It was also shown by Turgoose [27] that with increasing pH a subse-
2
)
Rad (kX cm2)
Cad (lF cm
1.0 0.31 0.44 0.17 0.28 0.21
15.8 19.6 30.4 31.6 32.0 40.8
2
)
Remarks Passivation Passivation Local corrosion Local corrosion Local corrosion Local corrosion
-0.1 sodium benzoate (Nabzo) Nabzo+KNO3
-0.2 Ecorr(V/SCE)
VCI
ethanolamine (et) et+Nabzo et+KNO3
-0.3
et+Nabzo+KNO3
-0.4 -0.5 0
10
20
30
40
50
60
t (min) Fig. 3. Open circuit potential vs. time for mild steel in neutral benzoate solution and alkaline ethanolamine solution with and without KNO3 and sodium benzoate.
quent conversion of ferric benzoate to ferric hydroxide occurs. Obviously, in the presence of nitrate, perchlorate or sulphate the susceptibility of iron to local corrosion increases with respect to the forming of very soluble ferrous salts. 3.2.3. VCI mixtures In practice mixtures of VCIs are widely used in metal packaging in order to provide for multi-metal protection. While azoles like benzotriazole and benzimidazole are effective for Cu, Al, Zn and their alloys [2,5,28,29] a large variety of amines and amine carboxylates are used for ferrous metals [2,11,17,29]. However, azoles are not only protective for non-ferrous metals they also can show synergistic action with other inhibitors in some cases [15,16,30–32]. We have investigated the influence of benzotriazole and benzimidazole, respectively, on the corrosion protection of mild steel dependent on the pH of the solution. In 0.01 M benzoic acid (pH 3.1) both azoles cannot prevent corrosion. After 1 h of immersion the open circuit potential is about
U. Rammelt et al. / Corrosion Science 51 (2009) 921–925
0.5 V(SCE) and the resistance of the corroding oxide layer Rox = 1 kX cm2. With and without azoles uniform corrosion is observed. In neutral solution, (0.01 M sodium benzoate, pH 7.1) the effectiveness of benzoate is increased in the presence of azoles. In both solutions, benzotriazole/benzoate and benzimidazole/benzoate, the passive oxide layer can be reinforced and the resistance is two times higher than in benzoate solution alone. The effect of the investigated azoles is probably due to a more favourable adsorption of the azoles on the oxide covered surface compared with benzoate [16]. In contrast to the mixtures azoles alone cannot protect mild steel under neutral conditions. After 1 h of immersion in 0.01 M benzotriazole E = 0.314 V(SCE) and the resistance of the oxide layer Rox = 6 kX cm2; in 0.01 M benzimidazole E = 0.310 V(SCE) and Rox = 7 kX cm2. The surface is covered with rusty males indicating that corrosion at weak sites of the natural oxide layer cannot be prevented by azoles. In alkaline solution, (0.01 M ethanolamine, pH 10.5) a synergistic effect as observed in neutral solution of azole/carboxylate cannot be measured. With and without the azoles the mild steel surface is passivated, the resistance of the passive oxide layer Rox is about 100 kX cm2 and the open circuit potential is between 0.12 and 0.17 V(SCE) in all cases. It seems that the influence of the azoles on the adsorption behaviour of amines in alkaline solution is not significant. 4. Conclusion It was shown that the inhibition by VCI compounds mainly depends on their ability to vaporize to a sufficient extent and subsequently to protect the metallic surface. The mechanism of the protection is strongly influenced by the pH value of the moisture layer on the metallic surface whereby this value is largely determined by the volatility of the VCI components. The passivation of mild steel in aerated solution was investigated in the presence of selected VCIs at different pH values. In acid solution, (the volatile acid is in excess) uniform corrosion was observed also in the presence of amines and/or azoles. In neutral solution, (the volatility of both VCI components, amine and carboxylic acid, is nearly the same) a passive oxide layer can be formed on the mild steel surface due to the precipitation of insoluble ferric carboxylate complexes in the defects of the natural air-formed oxide layer. The passive oxide layer is also stable in the presence of some contaminants e.g. nitrate or sulphate remained on the metal surface after the washing procedure. In the presence of azoles such as benzotriazole or benzimidazole the passive layer can be reinforced by strong adsorption of azole on the oxide covered surface. In alkaline solution at pH > 10 (the volatile amine is in excess or the surface pH is alkaline after the washing procedure) a passive oxide layer can be formed as well. However, it takes more time to build up the passive oxide layer in alkaline solution than in neutral carboxylate solution. This could be the reason that the passive layer formed in alkaline solution is more sensitive to contaminants. Independent of the presence of carboxylate local corrosion starting at defect sites of the natural oxide layer was observed. It can be concluded that in alkaline solution ferric hydroxide is precipitated at weak points which is more sensitive to corrosive attack than the ferric carboxylate complexes plugging the weak points in neutral solution. The results show that also for ferrous metals protection by VCIs in packages is more effective under neutral to slightly alkaline conditions than with strong alkaline conditions sometimes caused by the alkaline cleaning procedure.
925
References [1] W. Skinner, A new method for quantitative evaluation of volatile corrosion inhibitors, Corr. Sci. 35 (1993) 1491–1494. [2] D.M. Bastidas, E. Cano, E.M. Mora, Volatile corrosion inhibitors: a review, AntiCorr. Methods Mater. 52 (2) (2005) 71–77. [3] N.N. Andreev, Y.I. Kuznestov, Prediction of vapor pressure of volatile corrosion inhibitors, Prot. Met. 32 (1996) 148–154. [4] L.R.M. Estevao, R.S.V. Nascimento, Modifications in the volatilization rate of volatile corrosion inhibitors by means of host–guest systems, Corr. Sci. 43 (2001) 1133–1153. [5] G. Gao, C.H. Liang, 1,3-Bis-diethylamino-propan-2-ol as volatile corrosion inhibitor for brass, Corr. Sci. 49 (2007) 3479–3493. [6] A. Furman, Test methods for vapor corrosion inhibitors, Prog. 9th Europ. Symp. on Corrosion Inhibitors, Ferrara, 2000, pp. 465–479. [7] B. Boyle, A look at developments in vapor phase corrosion inhibitors, Met. Finish. 102 (2004) 37–41. [8] N.N. Andreev, Y.I. Kuznestov, Protective properties of solutions of volatile corrosion inhibitors, Prog. 9th Europ. Symp. on Corrosion Inhibitors, Ferrara, 2000, pp. 453–464. [9] S.B. Valdez, R.K. Zlatev, W.M. Schorr, G.N. Rosas, Ts. Dobrev, M. Monev, I. Krastev, Rapid method for corrosion protection determination of VCI films, Anti-Corr. Methods Mater. 53 (6) (2006) 362–366. [10] D. Zang, Z. An, Q. Pan, L. Gao, G. Zhou, Volatile corrosion inhibitor film formation on carbon steel surface and its inhibition effect on the atmospheric corrosion of carbon steel, Appl. Surf. Sci. 253 (2006) 1343–1348. [11] N.N. Andreev, N.P. Andreeva, R.S. Vartapetyan, Y.I. Kuznetsov, T.V. Fedotova, Volatile corrosion inhibitors based ethanolamines, Prot. Met. 33 (1997) 470– 473. [12] Y.I. Kuznetsov, Organic Inhibitors of Corrosion of Metals, Plenum Press, New York, 1996. [13] G. Reinhard, M. Radtke, U. Rammelt, On the role of the salts of weak acids in the chemical passivation of iron and steel in aqueous solutions, Corr. Sci. 33 (1992) 307–313. [14] D. Eurof Davies, Q.J.M. Slaiman, Mechanism of the corrosion inhibition of Fe by sodium benzoate – I. The influence of concentration and pH in air saturated solutions of sodium benzoate, Corr. Sci. 11 (1971) 671–682. [15] A.D. Mercer, Types of inhibitors used in neutral aqueous solutions, Prog. 7th Europ. Symp. on Corrosion Inhibitors, Ferrara, 1990, 449–469. [16] U. Rammelt, S. Koehler, G. Reinhard, Synergistic effect of benzoate and benzotriazole on passivation of mild steel, Corr. Sci. 50 (2008) 1659–1663. [17] E. Vuorinen, W. Skinner, Amine carboxylates as vapour phase corrosion inhibitors, Br. Corros. J. 37 (2002) 159–160. [18] M.A. Quraishi, J. Rawat, Investigations on some hydrazide based vapour phase corrosion inhibitors for mild steel, Prog. 9th Europ. Symp. on Corrosion Inhibitors, Ferrara, 2000, pp. 493–485. [19] A. Subramanian, A.R.S. Priya, T. Vasudevan, Diethylamine phosphate as VPI for steel components, Mater. Chem. Phys. 100 (2006) 193–197. [20] E. Cano, D.M. Bastidas, J. Simancas, J.M. Bastidas, Dicyclohexylamine nitrite as volatile corrosion inhibitor for steel in polluted environments, Corrosion 61 (2005) 473–479. [21] U. Rammelt, S. Koehler, G. Reinhard, EIS characterization of the inhibition of mild steel corrosion with carboxylates in neutral aqueous solution, Electrochim. Acta 53 (2008) 6968–6972. [22] J.E.O. Mayne, C.L. Page, Inhibition of the corrosion of iron by benzoate and acetate ions, Br. Corros. J. 9 (1974) 223–226. [23] J.E.O. Mayne, Some problems which have been resolved, Br. Corros. J. 28 (1993) 175–176. [24] N. Srisuwan, N. Ochoa, N. Pebere, B. Tribollet, Variation of the carbon steel corrosion rate with flow conditions in the presence of an inhibitive formulation, Corr. Sci. 50 (2008) 1245–1250. [25] K. Jüttner, Electrochemical impedance spectroscopy (EIS) of corrosion processes on inhomogeneous surfaces, Electrochim. Acta 35 (1990) 1501– 1508. [26] H. Göhr, Über Beiträge einzelner Elektrodenprozesse zur Impedanz, Ber. Bunsenges. Phys. Chem. 85 (1981) 274. [27] S. Turgoose, The relevance of the stability of ´basic ferric benzoate´to the inhibition by benzoate of the corrosion of iron, Prog. 6th Europ. Symp. on Corrosion Inhibitors, Ferrara, 1985, pp. 1041–1050. [28] R. Subramanian, V. Lakshminarayanan, Effect of adsorption of some azoles on copper passivation in alkaline medium, Corr. Sci. 44 (2002) 535–554. [29] K.L. Vasanth, An overview of structure, mechanism and applications of volatile corrosion inhibitors, J. Electrochem. Soc. India 91 (2002) 39–49. [30] C. Georges, E. Rocca, P. Steinmetz, Synergistic effect of tolutriazol and sodium carboxylates on zinc corrosion in atmospheric conditions, Electrochim. Acta 53 (2008) 4839–4845. [31] M.A. Barbosa, The use of benzotriazole in the corrosion inhibition of grey cast iron, Proc. 6th Europ. Symp. on Corrosion Inhibitors, Ferrara, 1985, pp. 613– 626. [32] R.C. Pemberton, A.D. Mercer, E.J. Wright, J.G.N. Thomas, Sodium sebacate– benzotriazole mixtures as inhibitors in neutral solutions, Proc. 6th Europ. Symp. on Corrosion Inhibitors, Ferrara, 1985, pp. 1241–1254.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Use of vapour phase corrosion inhibitors in packages for protecting mild steel against corrosion U. Rammelt *, S. Koehler, G. Reinhard Excor Korrosionsforschung GmbH Dresden, Magdeburger Str. 58, D-01067 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 21 November 2008 Accepted 21 January 2009 Available online 4 February 2009 Keywords: A. Mild steel A. VCI B. EIS B. OCP C. Passivation
a b s t r a c t Some aspects of the passivation of mild steel in the presence of selected vapour phase corrosion inhibitors (VCIs) were considered. In particular their ability to vapourize was evaluated by sublimation tests and their role in the inhibition mechanism of mild steel was investigated by electrochemical methods such as open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements. In the presence of some carboxylates, amines and azoles alone and as mixtures a protective layer can be formed on mild steel in neutral and alkaline solution. It was shown that the passivation mechanism strongly depends on the pH of the solution. In addition the influence of contaminants from industrial alkaline cleaning baths on the protective properties was analyzed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In general, polymer based packaging materials are used in order to protect metallic equipment during transport and storage against atmospheric corrosion. This temporary corrosion protection is achieved by incorporation of vapour phase corrosion inhibitors (VCIs, volatile corrosion inhibitors) in the polymer film, whereby the packaging material functions as a VCI source. Protection is necessary especially during the time of wetness, where the metal surface can corrode due to the formation of a thin electrolyte layer on the surface. Therefore, the efficiency of a compound used as VCI mainly depends on two parameters [1,2]: 1. its vapour pressure (more exactly: its tendency to sublime) under atmospheric conditions is high enough allowing significant vapour phase transport of the inhibitor within an enclosed space to the metal surface; 2. the VCI adsorbed directly on the metal surface or dissolved in a condensed water film on the metal surface inhibits the metal corrosion during storage and transport by interaction with the surface. Under ambient conditions VCIs have to possess moderately high vapour pressure, which should be in the range of 10–10 4 Pa [3]. If the vapour pressure is too low, another volatile substance, such as water vapour or urea can be used as carrier of the VCIs. The vola* Corresponding author. Tel.: +49 351 88857324; fax: +49 351 88857331. E-mail address: [email protected] (U. Rammelt). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.01.015
tility can also be enhanced by use of host–guest systems where the VCIs are incorporated to a porous substrate [4]. Then the VCIs are transferred to the metal surface by diffusion through the gas room and are adsorbed directly on the metal or dissolved in a surface moisture layer and reduce the corrosion rate markedly. As reported in the literature the inhibition mechanism proposed for VCIs, is mainly based on its adsorption on the metal surface whereby a thin monomolecular barrier film should be formed [1,5–7]. Especially the efficiency of amine-based inhibitors should be due, along with their basicity, to their ability to form a hydrophobic adsorption layer to shield the metal surface from corrosive contaminants [2,8,9]. Only in a few papers it was considered that the surface of unnoble metals like iron is covered with a thin oxide layer, which is formed rapidly on contact of the metal with air [10–13]. Usually this inherent natural oxide layer provides only limited protection so that the main purpose of VCIs is to maintain or reinforce this defect layer. It is well known that some salts of weak carboxylic acids are able to suppress the corrosion process by forming of insoluble deposits at the defect sites in the natural oxide layer. However, this passivation process is only possible under near neutral conditions in the presence of oxygen. In this case, oxygen cannot be considered as a corrosion accelerator [7], but it is itself the passivator supported by carboxylate [14–16]. Therefore, the protection mechanism of VCIs may involve a combination of passivation and adsorption. The protective properties of VCIs can be tested directly by metal loss measurements [1,10,17,18] or by electrochemical data of the corrosion process, such as corrosion current density icorr, corrosion potential Ecorr, polarization resistance Rpol and EIS data [5,10,18–20].
922
U. Rammelt et al. / Corrosion Science 51 (2009) 921–925
In this study, it was tested the ability of some typical VCIs to sublime in a dry nitrogen atmosphere. As VCIs some carboxylic acids and their salts alone or in combination with selected azoles or amines were chosen. The purpose of this work was to study the possibility to improve the protection of mild steel by combining carboxylates with azoles or amines. The inhibition mechanism of the inhibitors and their mixtures on mild steel was investigated by open circuit potential – time and EIS measurements in aqueous solution with different pH values. The according aqueous solutions should simulate the corrosion conditions which are given inside the package, if water is condensed on the metallic equipment.
2. Experimental 2.1. Materials Carboxylic acids (benzoic-, octanoic- and nonanoic acid) and some of their salts (sodium octanoate, sodium benzoate, ethanolammonium benzoate, cyclohexyl-ammonium benzoate) as well as some amines (ethanolamine, cyclohexylamine) and azoles (benzotriazole, benzimidazole) were used as inhibitors. Ethanolammonium and cyclohexyl-ammonium benzoate were synthesized via the stoichometric acid–base reaction of the amine with benzoic acid according to a procedure described by Estevao et al. [4] for dicyclohexyl-ammonium p-nitrobenzoate. All other compounds are commercial products and were used without further purification. The carboxylic acids as well as the amines were used alone and as a mixture with benzotriazole or benzimidazole. All electrochemical experiments were performed at room temperature in aerated 0.01 M inhibitor solutions with a mild steel electrode (A = 0.5 cm2) as working electrode. The surface was polished with 600 grit emery paper, rinsed with distilled water and then immersed immediately in the test solution. 2.2. Sublimation The sublimation tendency of the VCI compounds was investigated by sublimation of 1 g VCI substance at 40 °C in dry nitrogen atmosphere for 2 h. The sublimation was performed using a BÜCHI glass oven B-580. The VCI substance was placed in a glass boat and then introduced in a drying tube. The sublimation accessory was inserted into the drying tube and sublimation was started by heating the glass oven at 40 °C. Under nitrogen flow of about 50 cm3/min the sublimate was transferred to a washing bottle filled with 50 ml methanol. After 2 h the eluate was injected in a GC-2010 gas chromatographer (SHIMADZU) with a Varian CP-Sil 8 CB silica column or in a HPLC liquid chromatographer (MACHEREY + NAGEL) with a Dyonex RP18 column, respectively, for quantitative evalua-
tion. A calibration curve had previously been plotted using adequate standards. 2.3. Electrochemical set-up The electrochemical measurements (electrochemical impedance spectroscopy (EIS) and open circuit potential measurements) were made at room temperature in a three-electrode arrangement, consisting of the mild steel working electrode, a large area Pt gauze as counter electrode and a saturated calomel electrode (SCE) as reference. All potentials are referred to the SCE. The EIS measurements were made using the impedance measurement system IM6 of ZAHNER-elektrik. An ac amplitude of 10 mV was applied and data were collected in the frequency range 50 kHz–0.05 Hz using five points per decade. The EIS data were fitted with the Thales software of ZAHNER-elektrik. 3. Results and discussion 3.1. Sublimation As can be seen from Table 1 the investigated carboxylic acids and amines are volatile and can be detected by gas chromatography. However, whereas the ammonium salts of carboxylic acids are also sublimable, it is not possible to detect the sodium salts of these acids. The ability to sublime correlate rather well with vapour pressures reported in the literature. Although the vapour pressure values present some uncertainties and strongly depend on the experimental conditions they can be used to evaluate the tendency of a solid substance to undergo sublimation and of a liquid substance to vaporize, respectively, under normal conditions. The volatility of the sodium salts of carboxylic acids is too small (vapour pressure is approximately 10 7 Pa at ambient conditions) considering their use as VCI materials. Contrary to the sodium salts of carboxylic acids the vapour pressure and the vapourization rate of the investigated liquid amines ethanolamine and cyclohexylamine is too high for optimal protection because their effectiveness should be limited to a short time, as their consumption rate will be high. On the other hand, if only amines are in the vapour, the pH of the surface moisture film on the metal to be protected should be too high (pH > 10) for a multi-metal protection. Whereas iron or mild steel are in the passive state other unnoble metals like Al, Mg or Zn are attacked in this pH region. In order to avoid this it is usually to use amines as solid salts. The influence of the pH value on the protection of mild steel is investigated in the next section. The sublimation tendency of benzotriazole and benzimidazole is much smaller than this of the investigated amines and carboxylic acids. Whereas 20 lg/g weighed benzotriazole was found benzimidazole could not be quantified. The detection limit of 500 lg/
Table 1 Results of volatilization test. VCI
Sublimation mg/1 g weighed substance
Vapour pressure/Pa (from literature)
Benzoic acid Octanoic acid Nonanoic acid Sodium benzoate Sodium octanoate Ethanolammonium benzoate Cyclohexyl-ammonium benzoate Ethanolamine Cyclohexylamine Benzotriazole Benzimidazole
0.18 0.41* 0.26* – – 0.8/0.2 0.2/0.2 15.1* 440.5* 0.02
0.09 0.49 0.21 4.9 10 7 6.5 10 7 1.62 0.04 53 1.3 103 0.09 0.01
* **
Vaporisation of the liquid substance. Could not be proved.
**
U. Rammelt et al. / Corrosion Science 51 (2009) 921–925
g weighed substance is too high for GC analysis of benzimidazole. Sample enrichment or stronger sublimation conditions (higher temperature, extended sublimation duration) are necessary to reach the detection limit. However, also benzimidazole should be volatile enough for use as VCI since its vapour pressure exceeds 10 4 Pa. 3.2. Electrochemical measurements In 0.01 M aqueous solution of the carboxylic acid salts (the pH values are between 6.5 and 7.5) the steel surface can be protected. It is well known that at neutral conditions carboxylic acid salts support the passivation of mild steel by dissolved oxygen in aerated solution [15]. In previous studies [16,21] it was shown that the protection mechanism is due to the sealing of defects in the natural oxide layer, which already has been formed on the mild steel surface after short contact with air. High resolution autoradiography of benzoate labelled with 14C [22,23] and more recently XPS measurements [24] revealed that the passive oxide layer composed of an iron oxide/hydroxide mixture contained the carboxylate in the form of insoluble carboxylate particles plugging the defects. In order to characterise the passive layer formed in the different solutions EIS measurements were performed at the open circuit potential after 1 h of immersion. As an example, the impedance data are displayed in Figs. 1 and 2, respectively. It is evident, that in all cases the EIS data can be interpreted with two RC time constants
90 75
10k
1
60
2
45
3
30
1k
|Phase|(deg)
|Z|(Ohm)
100k
15 100
100m
1
10
100
1k
10k
0
f (Hz) Fig. 1. Bode plots of mild steel in aerated 0.01 M benzoate solution after 1 h of immersion at different pH: (1) ethanolammonium benzoate + ethanolamine (pH 10.5), (2) ethanolammonium benzoate (pH 6.6), (3) benzoic acid (pH 3.1).
90 75
1
60
2
10k
45
3
30
1k
|Phase|(deg)
|Z|(Ohm)
100k
15 100
100m
1
10
100
1k
10k
0
f (Hz) Fig. 2. Bode plots showing the influence of salt on the efficiency of ethanolamine (pH 10.5) after 1 h of immersion: (1) without salt, (2) with KNO3 and sodium benzoate, (3) with KNO3.
923
in parallel. The high frequency time constant (about 10 kHz– 10 Hz) can be explained with the adsorption of the inhibitor, while the low frequency time constant (about 10–0.05 Hz) is related to the oxide layer [5,16]. The resistance Rel reflects the electrolyte resistance which is measured at frequencies >10 kHz. The corresponding equivalent circuit is included in Fig. 1. With only one time constant as it was used for example by Subramanian et al. [19] and Zang et al. [10] the behaviour of mild steel in VCI containing solutions cannot be described sufficiently. It should be mentioned that both capacitances C of the time constants are replaced by a constant phase element CPE [ZCPE = ACPE(jx) n] [25], considering the nonideal behaviour of the layers, which is due to inhomogeneity in the conductance or dielectric constant inside the layers [26]. The exponent n of the CPE as a measure of the inhomogeneity was between 0.82 and 0.96 for all experiments. Only in acid solutions the exponent n is lower (0.73. . .0.85). ACPE is a constant with the dimension X cm2 s n. If n = 1 then 1/ACPE = C with the dimension F cm 2. The parameters associated to the EIS data are given in Table 2– 4. 3.2.1. Effect of pH on the protection mechanism The different volatility of the VCI components strongly affects the pH of the surface moisture layer on the metal to be protected. If the volatility of both components (amine and carboxylic acid) is nearly the same, the pH is in the neutral range. As an example, three different salts of benzoic acid are presented in Table 2. Independent of the cation the open circuit potential is in the passive range of mild steel, the resistance of the oxide layer Rox is about 50 kX cm2 and the surface of the electrode remains shiny after 1 h of immersion. The adsorption resistance ranged between 0.1 kX cm2 for sodium benzoate and 0.23 and 0.31 kX cm2 for the amine benzoates indicating a better adsorption ability of ethanolamine and cyclohexylamine on the oxide covered surface than benzoate. However, amine alone cannot passivate mild steel at this pH. In 0.01 M solution of ethanolamine and cyclohexylamine, respectively, the open circuit potential given in Table 2 remains in the active range of iron dissolution and the low resistance of about 0.5 kX cm2 is a typical value of a corroding iron electrode. The corrosion starts at weak points in the natural oxide layer and after 1 h of immersion the surface is covered with rusty males. It is clearly to see that the passivation of mild steel in neutral aerated solution is due to the presence of benzoate which forms insoluble ferric benzoate complexes in the defects of the natural oxide layer. As can be seen from the sublimation results of ethanolammonium benzoate (Table 1) the volatility of ethanolamine is about four times higher than the volatility of benzoic acid in this case. Therefore, the pH of the moisture layer on the metal surface increases and the inhibition mechanism may be based on the alkalization of the solution. The pH value of an aqueous solution containing 0.04 M ethanolamine and 0.01 M benzoic acid is about 10.5. The mild steel electrode is in the passive state and the resistance of the passive oxide layer in the alkaline solution is two times higher than that into neutral solution (see Table 3). If the volatile acid is in excess in the vapour the pH value of the surface moisture layer decreases and this may accelerate the corrosion process. As expected there is no passivation in aerated benzoic acid solution at pH 3.1 and after 1 h of immersion the metal surface is covered with rust. The same result was received in 0.01 M octanoic and nonanoic acid, respectively. In acid solution uniform corrosion is observed and both resistances are attributed to the corrosion process; adsorption of carboxylate cannot be detected. Therefore, the resistance and capacitance of benzoic acid in the high frequency range were parenthesized (Table 3). The influence of the pH on the passivation of mild steel is summarized in Table 3 and represented in Fig. 1.
924
U. Rammelt et al. / Corrosion Science 51 (2009) 921–925
Table 2 Fit results of EIS measurements in aerated 0.01 M solutions at neutral pH after 1 h of immersion. VCI
pH
Sodium benzoate Ethanolammonium benzoate Cyclohexyl-ammonium benzoate Ethanolamine Cyclohexylamine
7.1 6.6 6.9 7.3* 7.4*
*
E(VSCE) 0.105 0.126 0.092 0.413 0.341
Rox (kX cm2)
Cox (lF cm
54 51 58 0.7 0.9
25.0 21.8 26.4 42.8 41.0
2
)
Rad (kX cm2)
Cad (lF cm
0.09 0.23 0.31 0.30 0.39
19.4 19.6 24.2 38.8 32.2
2
)
Adjusted with nitric acid.
Table 3 Influence of pH on the passivation of mild steel in aerated 0.01 M solutions of ethanolammonium benzoate (etbzo), etbzo + ethanolamine (et) and benzoic acid after 1 h of immersion. VCI
pH
Etbzo Etbzo + et Benzoic acid
6.6 10.5 3.1
E(VSCE) 0.126 0.165 0.550
Rox (kX cm2)
Cox (lF cm
51 109 0.8
21.8 23.8 16.4
2
)
Rad (kX cm2)
Cad (lF cm
0.23 0.26 (0.18)
19.6 20.8 (5.6)
2
)
Remarks Passivation Passivation Corrosion
Table 4 EIS data in aerated 0.01 M ethanolamine solution (pH 10.5) in the presence of different salts (c = 0.01 mol/l) after 1 h of immersion (Nabzo = sodium benzoate).
Ethanolamine (et) Et + Nabzo Et + Nabzo + KNO3 Et + KNO3 Et + NaClO4 Et + Na2SO4
E(VSCE) 0.134 0.127 0.393 0.413 0.445 0.562
Rox (kX cm2)
Cox (lF cm
114 103 5.3 4.7 2.6 1.4
23.8 21.6 31.4 32.8 36.2 44.6
3.2.2. The influence of contaminants Before the metallic equipment is wrapped up into the VCI containing package one of the necessary manufacturing stages is the cleaning process. During the repeated use of industrial alkaline cleaning baths contaminants can be enriched in the bath and remain on the metal surface after the washing procedure. Therefore, the influence of some salts (nitrate, perchlorate, sulphate) on the passivation of mild steel in alkaline solution was investigated. Typical Bode plots of mild steel in 0.01 M ethanolamine solution (pH 10.5) with and without 0.01 M potassium nitrate are given in Fig. 2. There is no passivation of mild steel in alkaline solution in the presence of nitrate. The open circuit potential is in the active range of iron dissolution and the resistance of the oxide layer is clearly decreased (Fig. 2, curve 2 and 3). After 1 h of immersion the surface of the electrode is shiny but with some rust points. As can be seen in Table 4 in the presence of all investigated salts local corrosion is observed. That implies that in all cases the corrosive attack starts at local defects of the natural oxide layer independent of the aggressivity of the salt. The more aggressive the salt the larger are the rust points and the lower is the resistance of the oxide layer. An interesting point is the behaviour of mild steel in the presence of sodium benzoate and potassium nitrate in neutral and alkaline solution presented in Fig. 3. Whereas in neutral benzoate solution with and without nitrate passivation is reached after a few minutes in alkaline solution it takes more time to build up the passive oxide layer. However, in alkaline solution passivation cannot take place in the presence of nitrate and the potential drops down to 0.4 VSCE after 60 min of immersion independent of the presence of benzoate. The result leads to the conclusion that in more alkaline solution the inhibitive effect arises from the ability of the system to precipitate ferric hydroxide at weak points in the natural oxide layer whereas the plugging of defect sites with insoluble ferric benzoate decreases steadily with increasing pH [22]. It was also shown by Turgoose [27] that with increasing pH a subse-
2
)
Rad (kX cm2)
Cad (lF cm
1.0 0.31 0.44 0.17 0.28 0.21
15.8 19.6 30.4 31.6 32.0 40.8
2
)
Remarks Passivation Passivation Local corrosion Local corrosion Local corrosion Local corrosion
-0.1 sodium benzoate (Nabzo) Nabzo+KNO3
-0.2 Ecorr(V/SCE)
VCI
ethanolamine (et) et+Nabzo et+KNO3
-0.3
et+Nabzo+KNO3
-0.4 -0.5 0
10
20
30
40
50
60
t (min) Fig. 3. Open circuit potential vs. time for mild steel in neutral benzoate solution and alkaline ethanolamine solution with and without KNO3 and sodium benzoate.
quent conversion of ferric benzoate to ferric hydroxide occurs. Obviously, in the presence of nitrate, perchlorate or sulphate the susceptibility of iron to local corrosion increases with respect to the forming of very soluble ferrous salts. 3.2.3. VCI mixtures In practice mixtures of VCIs are widely used in metal packaging in order to provide for multi-metal protection. While azoles like benzotriazole and benzimidazole are effective for Cu, Al, Zn and their alloys [2,5,28,29] a large variety of amines and amine carboxylates are used for ferrous metals [2,11,17,29]. However, azoles are not only protective for non-ferrous metals they also can show synergistic action with other inhibitors in some cases [15,16,30–32]. We have investigated the influence of benzotriazole and benzimidazole, respectively, on the corrosion protection of mild steel dependent on the pH of the solution. In 0.01 M benzoic acid (pH 3.1) both azoles cannot prevent corrosion. After 1 h of immersion the open circuit potential is about
U. Rammelt et al. / Corrosion Science 51 (2009) 921–925
0.5 V(SCE) and the resistance of the corroding oxide layer Rox = 1 kX cm2. With and without azoles uniform corrosion is observed. In neutral solution, (0.01 M sodium benzoate, pH 7.1) the effectiveness of benzoate is increased in the presence of azoles. In both solutions, benzotriazole/benzoate and benzimidazole/benzoate, the passive oxide layer can be reinforced and the resistance is two times higher than in benzoate solution alone. The effect of the investigated azoles is probably due to a more favourable adsorption of the azoles on the oxide covered surface compared with benzoate [16]. In contrast to the mixtures azoles alone cannot protect mild steel under neutral conditions. After 1 h of immersion in 0.01 M benzotriazole E = 0.314 V(SCE) and the resistance of the oxide layer Rox = 6 kX cm2; in 0.01 M benzimidazole E = 0.310 V(SCE) and Rox = 7 kX cm2. The surface is covered with rusty males indicating that corrosion at weak sites of the natural oxide layer cannot be prevented by azoles. In alkaline solution, (0.01 M ethanolamine, pH 10.5) a synergistic effect as observed in neutral solution of azole/carboxylate cannot be measured. With and without the azoles the mild steel surface is passivated, the resistance of the passive oxide layer Rox is about 100 kX cm2 and the open circuit potential is between 0.12 and 0.17 V(SCE) in all cases. It seems that the influence of the azoles on the adsorption behaviour of amines in alkaline solution is not significant. 4. Conclusion It was shown that the inhibition by VCI compounds mainly depends on their ability to vaporize to a sufficient extent and subsequently to protect the metallic surface. The mechanism of the protection is strongly influenced by the pH value of the moisture layer on the metallic surface whereby this value is largely determined by the volatility of the VCI components. The passivation of mild steel in aerated solution was investigated in the presence of selected VCIs at different pH values. In acid solution, (the volatile acid is in excess) uniform corrosion was observed also in the presence of amines and/or azoles. In neutral solution, (the volatility of both VCI components, amine and carboxylic acid, is nearly the same) a passive oxide layer can be formed on the mild steel surface due to the precipitation of insoluble ferric carboxylate complexes in the defects of the natural air-formed oxide layer. The passive oxide layer is also stable in the presence of some contaminants e.g. nitrate or sulphate remained on the metal surface after the washing procedure. In the presence of azoles such as benzotriazole or benzimidazole the passive layer can be reinforced by strong adsorption of azole on the oxide covered surface. In alkaline solution at pH > 10 (the volatile amine is in excess or the surface pH is alkaline after the washing procedure) a passive oxide layer can be formed as well. However, it takes more time to build up the passive oxide layer in alkaline solution than in neutral carboxylate solution. This could be the reason that the passive layer formed in alkaline solution is more sensitive to contaminants. Independent of the presence of carboxylate local corrosion starting at defect sites of the natural oxide layer was observed. It can be concluded that in alkaline solution ferric hydroxide is precipitated at weak points which is more sensitive to corrosive attack than the ferric carboxylate complexes plugging the weak points in neutral solution. The results show that also for ferrous metals protection by VCIs in packages is more effective under neutral to slightly alkaline conditions than with strong alkaline conditions sometimes caused by the alkaline cleaning procedure.
925
References [1] W. Skinner, A new method for quantitative evaluation of volatile corrosion inhibitors, Corr. Sci. 35 (1993) 1491–1494. [2] D.M. Bastidas, E. Cano, E.M. Mora, Volatile corrosion inhibitors: a review, AntiCorr. Methods Mater. 52 (2) (2005) 71–77. [3] N.N. Andreev, Y.I. Kuznestov, Prediction of vapor pressure of volatile corrosion inhibitors, Prot. Met. 32 (1996) 148–154. [4] L.R.M. Estevao, R.S.V. Nascimento, Modifications in the volatilization rate of volatile corrosion inhibitors by means of host–guest systems, Corr. Sci. 43 (2001) 1133–1153. [5] G. Gao, C.H. Liang, 1,3-Bis-diethylamino-propan-2-ol as volatile corrosion inhibitor for brass, Corr. Sci. 49 (2007) 3479–3493. [6] A. Furman, Test methods for vapor corrosion inhibitors, Prog. 9th Europ. Symp. on Corrosion Inhibitors, Ferrara, 2000, pp. 465–479. [7] B. Boyle, A look at developments in vapor phase corrosion inhibitors, Met. Finish. 102 (2004) 37–41. [8] N.N. Andreev, Y.I. Kuznestov, Protective properties of solutions of volatile corrosion inhibitors, Prog. 9th Europ. Symp. on Corrosion Inhibitors, Ferrara, 2000, pp. 453–464. [9] S.B. Valdez, R.K. Zlatev, W.M. Schorr, G.N. Rosas, Ts. Dobrev, M. Monev, I. Krastev, Rapid method for corrosion protection determination of VCI films, Anti-Corr. Methods Mater. 53 (6) (2006) 362–366. [10] D. Zang, Z. An, Q. Pan, L. Gao, G. Zhou, Volatile corrosion inhibitor film formation on carbon steel surface and its inhibition effect on the atmospheric corrosion of carbon steel, Appl. Surf. Sci. 253 (2006) 1343–1348. [11] N.N. Andreev, N.P. Andreeva, R.S. Vartapetyan, Y.I. Kuznetsov, T.V. Fedotova, Volatile corrosion inhibitors based ethanolamines, Prot. Met. 33 (1997) 470– 473. [12] Y.I. Kuznetsov, Organic Inhibitors of Corrosion of Metals, Plenum Press, New York, 1996. [13] G. Reinhard, M. Radtke, U. Rammelt, On the role of the salts of weak acids in the chemical passivation of iron and steel in aqueous solutions, Corr. Sci. 33 (1992) 307–313. [14] D. Eurof Davies, Q.J.M. Slaiman, Mechanism of the corrosion inhibition of Fe by sodium benzoate – I. The influence of concentration and pH in air saturated solutions of sodium benzoate, Corr. Sci. 11 (1971) 671–682. [15] A.D. Mercer, Types of inhibitors used in neutral aqueous solutions, Prog. 7th Europ. Symp. on Corrosion Inhibitors, Ferrara, 1990, 449–469. [16] U. Rammelt, S. Koehler, G. Reinhard, Synergistic effect of benzoate and benzotriazole on passivation of mild steel, Corr. Sci. 50 (2008) 1659–1663. [17] E. Vuorinen, W. Skinner, Amine carboxylates as vapour phase corrosion inhibitors, Br. Corros. J. 37 (2002) 159–160. [18] M.A. Quraishi, J. Rawat, Investigations on some hydrazide based vapour phase corrosion inhibitors for mild steel, Prog. 9th Europ. Symp. on Corrosion Inhibitors, Ferrara, 2000, pp. 493–485. [19] A. Subramanian, A.R.S. Priya, T. Vasudevan, Diethylamine phosphate as VPI for steel components, Mater. Chem. Phys. 100 (2006) 193–197. [20] E. Cano, D.M. Bastidas, J. Simancas, J.M. Bastidas, Dicyclohexylamine nitrite as volatile corrosion inhibitor for steel in polluted environments, Corrosion 61 (2005) 473–479. [21] U. Rammelt, S. Koehler, G. Reinhard, EIS characterization of the inhibition of mild steel corrosion with carboxylates in neutral aqueous solution, Electrochim. Acta 53 (2008) 6968–6972. [22] J.E.O. Mayne, C.L. Page, Inhibition of the corrosion of iron by benzoate and acetate ions, Br. Corros. J. 9 (1974) 223–226. [23] J.E.O. Mayne, Some problems which have been resolved, Br. Corros. J. 28 (1993) 175–176. [24] N. Srisuwan, N. Ochoa, N. Pebere, B. Tribollet, Variation of the carbon steel corrosion rate with flow conditions in the presence of an inhibitive formulation, Corr. Sci. 50 (2008) 1245–1250. [25] K. Jüttner, Electrochemical impedance spectroscopy (EIS) of corrosion processes on inhomogeneous surfaces, Electrochim. Acta 35 (1990) 1501– 1508. [26] H. Göhr, Über Beiträge einzelner Elektrodenprozesse zur Impedanz, Ber. Bunsenges. Phys. Chem. 85 (1981) 274. [27] S. Turgoose, The relevance of the stability of ´basic ferric benzoate´to the inhibition by benzoate of the corrosion of iron, Prog. 6th Europ. Symp. on Corrosion Inhibitors, Ferrara, 1985, pp. 1041–1050. [28] R. Subramanian, V. Lakshminarayanan, Effect of adsorption of some azoles on copper passivation in alkaline medium, Corr. Sci. 44 (2002) 535–554. [29] K.L. Vasanth, An overview of structure, mechanism and applications of volatile corrosion inhibitors, J. Electrochem. Soc. India 91 (2002) 39–49. [30] C. Georges, E. Rocca, P. Steinmetz, Synergistic effect of tolutriazol and sodium carboxylates on zinc corrosion in atmospheric conditions, Electrochim. Acta 53 (2008) 4839–4845. [31] M.A. Barbosa, The use of benzotriazole in the corrosion inhibition of grey cast iron, Proc. 6th Europ. Symp. on Corrosion Inhibitors, Ferrara, 1985, pp. 613– 626. [32] R.C. Pemberton, A.D. Mercer, E.J. Wright, J.G.N. Thomas, Sodium sebacate– benzotriazole mixtures as inhibitors in neutral solutions, Proc. 6th Europ. Symp. on Corrosion Inhibitors, Ferrara, 1985, pp. 1241–1254.