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Multiphasic inhibition of mild steel corrosion in H2S gas environment
⁎Corresponding author at: Consultant Corrosion Science & Engineering Division, Central Electrochemical Research Institute, Karaikudi 630006, Tamil Nadu,India. Tel.: +91 4565 227550–559 (Office), +91 9442266269 (Personal); fax: +91 4565 227779, +91 4565 227713. mnatesan@rediffmail.com (M. Natesan)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Compounds like Octylpalmamide (OTP), Octylsteramide (OTS), Octylcaprylamide (OTC), Octylbenzamide (OTB) and one complex compound Dicyclohexylaminebenzotriazole (DCHAB) were synthesized and characterized by Fourier Transform infrared spectroscopy (FT-IR). These synthesized compounds were drawn as volatile corrosion inhibitor (VCI) in H2S gas environment on mild steel (MS) at 323 K. Surface morphology and elemental analysis have been examined by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) respectively. Various studies like weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used for evaluating corrosion rate, inhibitor behaviour and change in charge transfer resistance (Rct) value, respectively. All the above experiments proved that DCHAB was the most efficacious corrosion inhibitor. Adsorption behaviour of the inhibitor was evaluated and it obeys Langmuir adsorption isotherm.
Keywords
Mild steel
Volatile corrosion inhibitor
Hydrogen sulphide
EIS
SEM
EDX
1 Introduction
Metals are more prone to atmospheric corrosion due to the variation of temperature and humidity during transport and storage. Most of the industrial parts like vessels, pipeline, cooling tanks, boilers, etc. will easily get corroded. These materials are very sensitive to some corrosive like acid, base and corrosive gas. Many organic and inorganic plant extracts (Green inhibitors), dyes and drugs were used as inhibitors to make these materials passive in those media. Recently, VCIs are used for making the material more protective to corrosion. Inhibitor forms the primary adsorptive layer on the metal surface which separates the metal from the environment. Frequent condensation and deposition of vapours increase the thickness of the layer. The inhibition is transmitted as vapours and the vapour phase is controlled by the structure of the crystal lattice and the characteristic atomic bond present in the molecule. The protective vapours expand within the enclosed space until the equilibrium determined by the practical pressure of the vapour is reached (Subramanian et al., 2000). So many VCIs have been employed for protective efficiency in corrosion environment. These are used to protect the metallic instruments and auto mobile equipments (Sastri, 1998). VCIs are transported to the metal surface and then adsorbed; thereby it prevents corrosion (Furman et al., 2004). Bis-piperidiniummethyl-urea was used for the inhibition of corrosion on carbon steel in the atmosphere for temporary protection (Zhang et al., 2006). β-Naphthol and m-dinitrobenzene were used as VCIs in SO2 (Rajagopalan and Ramaseshan, 1960). Dicyclohexylamine and cyclohexylamine salts were used as volatile nature compounds for mild steel, copper and zinc in SO2. Dicyclohexylamine exhibited maximum inhibition on those metals (Rosenfeld, 1978). In some cases like alkaline medium of sodium sulphate, mild steel was protected using piperazine derivatives (Quraishi et al., 2009). On the other hand, more humid environment like NaCl which may cause corrosion on mild steel is greatly influenced by menthol as a VCI (Premkumar et al., 2009). Menthol is also used for the protection of copper in Hydrochloric acid environment which exhibits up to 78% efficiency (Premkumar et al., 2010). Thyme is also used as a VCI for mild steel which exhibits 77% efficiency in HCl (Premkumar et al., 2008). The extract of wood bark oil has been used as a VPI for NaCl and SO2 environment. The inhibitor efficiency of trychnos-nux-vomica was higher even at lower concentration on mild steel and copper (Poongothai et al., 2005). Benzoichydrazide salts were employed as VCI for different metals (Quraishi et al., 2005). The vapour phase inhibition effect of morpholine carbonate, borate and phosphate derivatives on mild steel has been examined (Subramanian et al., 1998). In SO2 and NaCl environment, a mixture of azole, amine, nitrate and benzoate was used as VCI for mild steel and copper which exhibits inhibition up to 90% (Poongothai et al., 2007). Using Octylsteramide (OTS) as volatile corrosion inhibitor for mild steel in Cl2 environment, the inhibition efficiency was up to 98% (Kannan et al., 2012). CHA and DCHA are found to be more effective for mild steel and zinc than for copper in a vigorous SO2 environment and protection efficiency was 76 and 85% respectively (Subramanian et al., 1999). Earlier, military equipments were protected by using camphor and ferrous metals (Singh and Banerjee, 1984). Recently, VCIs are being used in wrapper (Kraft paper) for protection of the metallic compounds. Many organic compounds containing hetero atoms like nitrogen, sulphur and oxygen will act as good corrosion inhibitors (Sudheer et al., 2012). Petroleum products from one place to other have been transported through huge pipelines. In this long process there will be a chance for corrosion inside the pipeline by effluent gases like sulphur, carbon dioxide etc. which are present in petroleum products. In this present work, VCI of OTP, OTS, OTC, OTB and DCHAB were exploited to prevent such type of corrosion on mild steel in hydrogen sulphide environment. In addition to this, some different immersion systems were also done and studied the protection efficiency.
2 Methods and materials
2.1 Gravimetric measurements
The Gravimetric test was conducted to evaluate the inhibition effect of VCI. Mild steel specimens were made with dimensions of 5 cm × 2.5 cm × 1 cm and a hole was drilled in each of the specimen. The final geometrical area of each specimen was 25 cm2. Prior to the tests, the metal surface was polished with 80–1200 grit emery sheets, degreased with trichloroethylene, and dried. The specimens were weighed on four digit weighing balance. After initial weighing, the test specimens were exposed in a 1000 ml of glass desiccators, which contained weighed solution of inhibitor in isopropyl alcohol. The specimens were hanged by hooks in the desiccators which contain 500 ml of electrolyte solution. This setup was kept in a thermostat water bath maintained at 323 K. This arrangement produced a continuous under saturation of water vapours at 100% relative humidity. One cycle included a 5 h exposure in the thermostat. The experiments were carried out in the absence and presence of inhibitor in the 1.2 wt% concentration. The observations were made at the end of the test period. Specimens were then cleaned using pickling solution as given in ASTM G1 specification. Specimens were washed, dried and reweighed. Mass loss was then found to determine the corrosion rate. The corrosion rate was calculated by Eq. (1) and inhibition efficiency (IE) by Eq. (2). The same procedure was done for the partially and fully immersed condition of metal in corrosive environment.
Rct(i) = Charge transfer resistance with inhibitors.
Rct = Charge transfer resistance without inhibitors.
RP(i) = Polarization resistance with inhibitors.
RP = Polarization resistance without inhibitors
2.2 Synthesis of inhibitors
The synthesis of OTP, OTS, OTC and OTB was done by adding stoichiometric amount of octylamine (Aldrich) with corresponding acids such as palmitic, stearic, caprylic, and benzoic acid (Merck) in an RB flask and refluxing with conc. HCl, pH was adjusted by adding ammonia solution. DCHAB was synthesized by complex formation of dicyclohexylamine with benzotriazole (Aldrich). Finally, synthesized inhibitors were characterized by FT-IR. The molecular structure and molecular mass of inhibitors are shown below (Table 1).
Sl.No
Structure
Molecular formula & weight
Vapour pressure (mmHg)
Name of VCIs
1
C15 H23 NO & 233.3492
2.4853 × 10−3
Octylbenzamide
2
C16 H33 & 255.4393
2.5653 × 10−3
Octylcaprylamide
3
C24 H49 NO & 367.652
1.9667 × 10−3
Octylpalmamide
4
C26 H53 NO & 395.7051
1.2740 × 10−3
Octylsteramide
5
C18 H29 N4 & 301.4498
1.253 × 10−3
Dicyclohexylaminebenzotriazole
2.3 Materials and medium
The electrolyte was prepared by intimation of ASTM G1 (0.08 M NaCl, 0.01 M acetic acid, 0.5 M Na2S). The combination of the above mixture gives H2S atmosphere (Andreev et al., 2000). The specimens were containing the following composition: Carbon – 0.07% Sulphur – nil, Phosphorous – 0.008%, Silicon – nil, Manganese – 0.34%, Fe – 99.582%.
2.4 Fabrication of volatile corrosion inhibitor monitor cell (VCIMC)
The VCIMC was constructed for mild steel. The schematic diagram of VCIMC is presented in Fig. 1. Each cell consists of two mild steel plates of size 2 cm × 0.2 cm. VCIMCs were prepared, according to the known report (Sudheer et al., 2012). They were insulated from one another by means of a very thin Mylar insulator. Then the cell set-up was embedded in epoxy and this forms the mono metal two-electrode type atmospheric corrosion monitor. Alternate metal plates were connected in a series by welding and these two end plates form the terminals for connection. The surface of the VCIMC was prepared by polishing first in a surface grinder and then using different grades of emery papers and finally degreased with trichloroethylene.
Schematic representation of VCIM.
2.5 Potentiodynamic measurement
The polarization measurement was taken with the help of two electrode system. Working and counter electrode modes were considered as one electrode and the saturated calomel electrode as the reference one. The potential values were measured with respect to SCE (Ag/AgCl2). The potentiodynamic measurements were started by changing step wise 60 mv/m on a PGP20IP potentiostat/galvanostatic.
2.6 Electrochemical impedance spectroscopy (EIS)
The cell configuration of EIS was done by the electrochemical measurement unit using solartron 1280b with the employed amplitude of ±20 mv, and frequency ranging from 0.5 Hz to 100 kHz.
2.7 FT-IR
The surface analysis of adsorbed film on the mild steel surface during the gravimetric tests in the presence of inhibitor was examined by FT-IR spectrum (Model No – TENSOR 27 Software – OPUS version 6.5).
2.8 SEM and EDX
The surface morphology was examined using SEM and elemental analysis of the inhibitor adsorbed on the metal surface has been determined via EDX. 1 cm2 surface area specimen samples were used for these studies. HITACHI S-3000H was used for scanning both blank and inhibitor coated samples.
3 Results and discussion
3.1 Weight loss method
The inhibition effect of volatile corrosion inhibitor on the carbon steel after 5 h exposure at 323 K was analysed by the weight loss method. The inhibition efficiency of these inhibitors was evaluated by ASTM G1-67. Three different environments like, without immersion, partial immersion and full immersion with electrolyte were studied and are shown in Fig. 2. The corrosion parameters of VCIs are shown in Tables 2–4 respectively. The concentration of the studied inhibitors is 1.2 wt% (Lavanya et al., 2012). In all three different conditions, DCHAB shows unrivalled inhibition effect on MS in the corresponding conditions. The corrosive H2S gas medium caused the metal dissolution which produces a film layer of corrosion product on the metal surface. By introducing the VCI a passive layer on the metal surface will be formed that will hinder further contact of corrosive gas with the material. Fig. 3(A) and (B) shows the corrosion effect in the absence and presence of inhibitor on mild steel without immersed condition. These figures clearly show the corrode product of FeS formed and delaminated from the surface for further corrosion.
Three different conditions for mild steel.
Sl. No
Inhibitors
Weight loss (g)
Corrosion rate (μm/y)
Inhibition efficiency (%)
1
Control
0.0464
4.1
–
2
OTP
0.0240
2.1
48.2
3
OTS
0.0110
0.98
76.2
4
OTC
0.0060
0.53
87
5
OTB
0.0030
0.26
93.5
6
DCHAB
0.0023
0.2
95
Sl. No
Inhibitors
Weight loss(g)
Corrosion rate (μm/y)
Inhibition efficiency (%)
1
Control
0.0260
2.3
–
2
OTP
0.0086
0.8
66.3
3
OTS
0.0059
0.5
77.3
4
OTC
0.0048
0.4
81.5
5
OTB
0.0033
0.3
87.3
6
DCHAB
0.0024
0.2
90.7
Sl. No
Inhibitors
Weight loss (g)
Corrosion rate (μm/y)
Inhibition efficiency (%)
1
Control
0.0131
1.168
–
2
OTP
0.0111
0.9896
15
3
OTS
0.0072
0.6419
45
4
OTC
0.0047
0.4190
63
5
OTB
0.0047
0.4190
64
6
DCHAB
0.0014
0.1248
89
(A) Blank, (B) specimen with VCI in without immersion condition.
Fig. 4(A) exhibits the image of a blank metal surface immersed totally in the corrosive medium and Fig. 4(B) shows the metal surface coated with DCHAB in the same corrosive medium. The total surface of the metal exposed in medium blank (A) is corroded more than the VCI present (B). Similarly in Fig. 5(A), the surfaces above and below the immersed area were severely attacked by the corrosive ion and gas but in (B) both areas were less corroded than the former.
(A) Blank, (B) specimen with VCI in fully immersed condition.
(A) Blank, (B) specimen with VCI in partially immersed condition.
Corrosion rate and inhibition efficiency are shown in Tables 2–4 in without immersed, partially immersed and fully immersed conditions respectively.
3.2 EIS studies
The Nyquist plot of the mild steel with these five inhibitors which is exposed to about 5 h in the H2S gas environment is shown in Fig. 6. It gives a clear idea about the corrosion behaviour of mild steel in the sense of resistance to the corrosive medium in the presence and absence of inhibitor. The VCI will not amend the mechanism of dissolution of metal. Hence, it will purely depend on the charge transfer process.
The impedance diagram for the blank and inhibitors after 5 h exposure at 323 K corrosive medium.
All experimental plots have approximately a semi-circular shape. The impedance measurement shows that the inhibition of VCIs was increased by increasing the diameter of the semicircle (optimized 1.2 wt% concentration was shown for all inhibitors). Rsol is the solution resistance between the reference and working electrode. When the aggressive medium contains sulphur and water, the ionic species permeate the protective film; reach the active site of the metal. Hence, corrosion becomes measurable and associated parameters like double layer capacitance Cdl and charge transfer resistance Rct can be estimated.
The Cdl and Rct values are shown in Table 5. It shows DCHAB having the Rct and Cdl value of 36,445 ohm cm2 and 6.4 × 10−5 μF/cm2 respectively. Inhibition efficiency can be calculated using the Eqs. (3) and (4).
Inhibitors
Rct (ohm cm2)
Cdl (μF cm−2)
Inhibition efficiency (%)
Control
8625
68
–
OTP
10091
63
14
OTS
10926
59.5
21
OTC
12445
59
30
OTB
16099
49
46
DCHAB
36445
6.4 × 10−5
73
3.3 Tafel polarization technique
Tafel polarization monitoring is an effective electrochemical method for measuring corrosion. Monitoring the relationship between the electrochemical potential and current generated between electrically charged electrodes, in a process stream allows the calculation of the corrosion rate. In the present study the current produced by the blank i.e. without inhibitor increases with the increase in volt. But it is interesting that the current produced by the best inhibitor (DCHAB) is very low, when compared to other inhibitors. With this account the most efficacious inhibitor is DCHAB in H2S gas environment. Tafel polarization curve and parameter values are given in Fig. 7 and Table 6 respectively, which explains clearly that all the inhibitors are mixed inhibitors. RP and IE were increased for all VCIs at constant wt% concentration as shown in Fig. 8. If it is anodic inhibitor the value is positively shifted to 185 mv with respect to the OCP. Likewise, if the potential shift is negatively shifted to 185 mv, it will be a cathodic inhibitor (Sanatkumar et al., 2012). RP is directly proportional to inhibition efficiency. Inhibition efficiency can be calculated from the Eq. (4).
Tafel polarization curve at 323 K.
Inhibitors
icor (μA cm−2)
Tafel slope (mv/decade)
Rp (K ohm cm2)
Inhibition efficiency (%)
ba
−bc
Control
28.9
172.8
173
2.59
–
OTP
4.5
362.3
61.1
6
56.8
OTS
2.4
302
66.8
12
78.4
OTC
3.1
245.7
98.6
14.47
82.1
OTB
1.5
352.8
58
22.68
88.5
DCHAB
0.75
167.2
156
31.52
91.7
Rp and IE of inhibitor of the H2S gas medium at 323 K.
3.4 Adsorption isotherm
The graph (θ/1 − θ) vs concentration (C) of VCI was plotted and gives a straight line. This study is based on Langmuir adsorption isotherm equation, given by (θ/1-θ) = KC.where K is the equilibrium adsorption constant, C is the concentration of VCI and θ is the surface coverage. The equilibrium adsorption constant K was calculated from the slopes of straight lines obtained for all inhibitors at different concentrations (shown in Fig. 9). This suggests that the inhibitor prevents the contact of the metal with corrosive environment by forming a barrier. Corrosion inhibitor added to the medium will be adsorbed to the metal surface and forms a compact barrier film (Nahle et al., 2012). Considering optimum concentration of the inhibitor, maximum inhibition efficiency was observed at 1.2 wt%. Surface area coverage increases with the increase in concentration of VCI and then decreases because of the leaching out of the protective film from the surface area of the materials. All these VCIs in the H2S environment obey Langmuir adsorption isotherm equation.
Adsorption isotherms of the inhibitor for mild at 323 K.
3.5 FT-IR
The synthesized VCI compounds and their inhibitive films formed on the surface of mild steel after 5 h exposure in the corrosive medium have been studied using the FT-IR spectral analysis. The mechanism for inhibition of corrosion on mild steel by VCI compounds is clear from Figs. 10–19. Spectral data obtained are shown in Table 7.
FT-IR spectrum of the DCHAB inhibitor.
FT-IR absorption spectrum for the DCHAB with metal in the environment.
FT-IR spectrum of the OTB inhibitor.
FT-IR absorption spectrum for the OTB with metal in the environment.
FT-IR spectrum of the OTC inhibitor.
FT-IR absorption spectrum for the OTC with metal in the environment.
FT-IR spectrum of the OTS inhibitor.
FT-IR absorption spectrum for the OTS with metal in the environment.
FT-IR spectrum of the OTP inhibitor.
FT-IR absorption spectrum for the OTP with metal in the environment.
Inhibitors
Bond
Frequency range (cm−1)
Measured band
Inhibitor (cm−1)
VCI deposited on MS (cm−1)
DCHAB
C—N
1250–1020
1109
1066
N—H
3500–3200
3394
3361
N⚌N
1640–1570
1638
1633
N—H
1600–1550
1551
1540
OTB
C⚌O
1800–1600
1602
1642
C—N
1360–1080
1165
1019
N—H
1600–1550
1552
1513
OTC
C⚌O
1800–1600
1632
1646
C—N
1360–1080
1310
968
N—H
1600–1550
1568
1513
OTS
C⚌O
1800–1600
1635
1645
C—N
1360–1080
1103
1000
N—H
1600–1550
1562
1539
OTP
C⚌O
1800–1600
1643
1700
C—N
1360–1080
1301
1012
N—H, C⚌O and C—N bonds of the OTB are shifted from 1551 to 1540 cm−1, 1602–1642 cm−1 and 1165–1019 cm−1 respectively. Similarly for the OTC N—H, C⚌O and C—N bonds are shifted from 1552 to 1513 cm−1, 1632 –1646 cm−1, and 1310 –968 cm−1. The N—H bonds of OTS and OTP inhibitors are shifted from the reign of 1568 cm−1 and 1562 cm−1 to 1513 cm−1 and 1539 cm−1 respectively. The C⚌O bonds of OTS and OTP at 1635 cm−1 and 1643 cm−1 are shifted to 1645 cm−1 and 1700 cm−1 respectively. The C—N bond values are shifted from 1103 cm−1 and 1301 cm−1 to 1000 cm−1 and 1012 cm−1 respectively.
In DCHAB C—N, N⚌N bond shifted from 1109 cm−1 and 1638 cm−1 to 1066 cm−1 and 1633 cm−1 respectively. The N—H bond in the inhibitors was shifted from 3394 cm−1to 3361 cm−1.
From the above FT-IR investigations, it shows that the amide compounds derived from octylamine such as OTB, OTC, OTP and OTS are having the lone pair electron containing atoms like oxygen and nitrogen. In amide compounds the nitrogen atom donates its lone pair electron to the metal to form the Fe-inhibitor coordinate bond. It is clear that the IR value of C—N is decreased and the C⚌O value is increased after exposure in the corrosive medium.
But in the case of the DCHAB complex compound, there are 3 nitrogen atoms and they will donate their lone pair electrons to the metal atom in order to make the coordinate bond. The inhibition efficiency is determined by the number of the electron donating atoms present in the compound.
FT-IR spectra of VCI compounds and iron specimens after 5 h exposure were recorded and the corresponding IR spectra are shown in Figs. 10–19.
3.6 SEM analysis
SEM images of the specimen revealed that they are containing corroded area in the blank specimen after exposure in the corrosive medium. Due to the mechanical polishing of the metal line, SEM images are clearly visible. The magnitude of the surface area affected by the corrosion is decreased by applying the inhibitor DCHAB on the MS. White particles in the image (A) reveal that the rust is formed by the metal dissolution.
Comparing the entire SEM image (Fig. 20), the image of blank specimen (A) has corroded surface whereas the inhibitor applied specimens have smooth surface. Especially DCHAB has smoother surface, even after exposure in corrosive gas atmosphere. Thus along with the weight loss method and electrochemical analysis, SEM images also scaffold the result that DCHAB shows exceptional inhibition efficiency compared with other inhibitors.
(A) Surfaces of the mild steel in the absence of inhibitor. (B–F) are inhibitors OTP, OTS, OTC, OTB and DCHAB respectively in the H2S environment.
3.7 EDX studies
The presence of sulphur in the EDX spectra for the blank specimen in the corrosive H2S medium indicates that the deposition of sulphide takes place on the metal surface after exposure. The presence of nitrogen and carbon in the EDX spectra for the MS coated with inhibitors exposed in the same atmosphere shows that the adsorption of inhibitor takes place on the metal surface. Figs. 21–26 exhibit the EDX results of the blank specimen and the inhibitor coated specimen respectively.
Blank specimen.
Specimen with DCHAB.
Specimen with OTP.
Specimen with OTS.
Specimen with OTC.
Specimen with OTB.
The strong peak of sulphur in the spectra of inhibitors revealed that some poly sulphide passive layer is formed on the metal surface which will give an additional inhibition of corrosion (Ma et al., 2000). Thus, from the EDX spectra it is clear that the mechanism of corrosion inhibition is by the Fe-inhibitor complex formation resulting in the adsorption of thin inhibitive layer on the metal surface.
4 Conclusions
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Five inhibitor compounds namely OTP, OTS, OTC, OTB and DCHAB were synthesized and characterized.
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In the weight loss method, DCHAB showed unrivalled inhibition efficiency when compared with other inhibitors in three different environments.
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EIS analysis showed that the adsorbed protective layers on the metal surface act as a hindrance for the metal dissolution in the H2S environment.
-
The surface analysis of the mild steel in the presence of inhibitor was investigated and identified by the FT-IR analysis.
-
SEM analysis revealed that the best inhibitor is DCHAB by showing the clear polished like surface after exposure in the corrosive medium.
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EDX spectra for DCHAB after 5 h exposure in the corrosive medium were recorded. The presence of N peak in the spectra clearly shows the adsorption of the VCI layer on the metal surface. It confirmed that the mechanism of inhibition is by the inhibitor-Fe complex formation.
Acknowledgement
The authors are very thankful to the Director of CECRI, Karaikudi for giving us support to publish this paper and constant encouragement.
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