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DOI: 10.1039/C5RA11082C (Paper) RSC Adv., 2015, 5, 57254-57262

Study on the efficiency of some amine derivatives as corrosion and scale inhibitors in cooling water systems†

M. A. Migahed*a, A. A. Attiab and R. E. Habibb
aEgyptian Petroleum Research Institute, Nasr City, Cairo, 11727, Egypt. E-mail: mohamedatiyya707@hotmail.com
bChemistry Department, Faculty of Science, Zagazig University, Egypt

Received 10th June 2015 , Accepted 17th June 2015

First published on 17th June 2015

Abstract

Corrosion and scaling are the main problems in cooling water systems. To address these problems, this paper aimed to study the inhibition efficiency of some amine derivatives for carbon steel in sea water using electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The selected compounds, with different structures and sizes, were found to be adsorbed onto the carbon steel surface and had good performance as corrosion and scale inhibitors. Potentiodynamic polarization studies indicate that the amine derivatives act as mixed type corrosion inhibitors, while the data obtained from EIS revealed that the value of the charge transfer resistance (Rt) increased when increasing the inhibitor concentration leading to an increase in the inhibition efficiency. Also, the obtained results from this study show that the selected amines can decrease scale build-up growth under the experimental conditions. The surface morphology of the carbon steel was investigated in the absence and presence of the selected inhibitors through scanning electron microscopy (SEM) and energy dispersive analysis of X-rays.

1. Introduction

Two of the main problems in cooling water systems are corrosion and scale phenomena. These problems have a great economic impact, since the first involves deterioration of the metallic surface, whereas the second causes loss of capacity for thermal exchange. These phenomena occur simultaneously in many industrial applications like cooling systems, and hence a study of their development and inhibition under real conditions becomes an issue of great interest. Significant scientific and technological efforts have been made to control these phenomena, yet they are still controlled through the addition of chemicals that inhibit their development. Many inhibitors have been used in cooling water systems in order to solve these problems.1–4 Corrosion and scale occur because of the electrochemical oxidation reduction reaction and the metal salt sediments on the metal surface.5

Water is the most commonly used cooling fluid in industrial systems, such as heat exchangers, cooling towers, and related equipment. Depending on the quality and availability of a fresh water supply, the recirculating cooling water systems contain varying amounts of solids, suspended or dissolved, but most likely both.6 Dissolved solids usually consist of mineral complexes in the form of crystals, which, in a process described as nucleation, continue to grow. They are characterized by the extent of their solubility and especially by their potential insolubility at particular temperatures.

Carbon steel has many industrial applications because of its easy availability, low cost and uncomplicated fabrication, and it is used extensively in water pipe lines,7,8 cooling water systems,9 boilers, process industries, oil and gas, refining and extraction, etc.

Corrosion occurs when an electric current flows from one part of the metal (anode) through the water (electrolyte) to another part of the metal (cathode). Corrosion takes place at the anode only. This process degrades the metal, reduces its strength and thickness, and in some extreme cases, creates pits and then holes in the material. Corrosion, in general, and pitting corrosion, in particular, must be guarded against in order to ensure the long term integrity of a cooling system.10

Corrosion inhibitors are substances which, when added in small concentrations to a corrosive media, decrease or prevent the reaction of the metal with the media.11 Most of the effective inhibitors are molecules that contain heteroatoms such as O, N, and S and multiple bonds through which they are adsorbed onto the metal surface.12–21 It has been observed that adsorption depends mainly on certain physicochemical properties of the inhibitor group, such as the functional groups, electron density at the donor atoms, p-orbital character, and electronic structure of the molecule.22–47 Amines are well known as corrosion inhibitors of iron and its alloys. The relatively high water solubility of low molecular weight amines is an advantage for their use as corrosion inhibitors.48–53 The efficiency of an organic compound as an inhibitor is mainly dependent on its ability to be adsorbed onto the metal surface, which involves the replacement of water molecules at the corroding interface.

Scale is formed from minerals, formerly dissolved in water, that were deposited from water onto heat transfer surfaces or in-flow water lines. As water is evaporated in a cooling tower, the concentration of dissolved solids becomes greater until the solubility of a particular scale-causing mineral salt is exceeded. The most common scaling minerals are calcium carbonate (CaCO3), calcium phosphate Ca3(PO4)2, calcium sulfate (CaSO4), and silica. Formation of magnesium silicate scale is also possible under certain conditions. Silica will form in areas with the lowest water temperature, such as in the cooling tower fill.54 The critical parameters for cooling water are: conductivity, total dissolved solids (TDS), hardness, pH, alkalinity and saturation index.55 A number of techniques, such as the controlled acidification of sea water,56 use of anti-scale agents57,58 and utilization of sponge–ball cleaning, are employed to control scaling.

Some amine derivatives have been evaluated as corrosion inhibitors against several industrial media,59 and they showed inhibition efficiencies in the range of 50–80%. Since scale formation and metal corrosion problems appear in cooling water systems, this work aimed to examine the inhibition efficiency of some amine derivatives as corrosion and scale inhibitors for cooling water systems. The added value of this work arises from the point that the prepared compounds act as both corrosion and scale inhibitors which provides double action protective properties. Furthermore, using a double action chemical to replace the injection of two different chemicals into cooling water systems provides an economic advantage. Moreover, it eliminates the potential interference in the case of using two different chemical injections for corrosion and scale inhibition in a system. This work is the onset of a series of works currently under investigation in our labs.

2. Experimental

2.1. Materials

2.1.1. Specimen preparation. The principal material used in the experiments was cylindrical carbon steel of grade X-65 with 1 cm2 surface area. The electrodes were degreased with acetone in an ultrasonic water bath for about 10 min, air-dried, embedded in a two-component epoxy resin, and mounted in a PVC holder. A copper wire was soldered to the rear side of the electrode as an electrical connection. The exposed surface of the electrode (of area 1 cm2) was wet polished with silicon carbide abrasive paper up to 800 grits, rinsed with ethanol, placed in an ultrasonic acetone bath for about 5 min to remove any possible residue from polishing, and air dried. This was used as the working electrode during the electrochemical tests.
2.1.1.1. Chemical composition of the tested carbon steel. The average range of the chemical components of the tested carbon steel are shown in Table 1.
Table 1 Chemical composition of carbon steel
Element C Si Mn P S Ni Cr Mo V Cu Al Fe
Content (wt%) 0.09 0.22 1.52 0.01 0.05 0.04 0.02 0.004 0.002 0.02 0.04 Rest

2.1.2. Chemical composition of the studied cooling water. Most of the elements that can be found on earth are present in sea water, at least in trace amounts. However, 11 constituents account for 99.95 percent of the total solutes, as indicated in Table 2, with chloride ions being by far the largest constituent. The concentration of dissolved materials in the sea varies greatly with location and time because of rivers diluting the sea water, rain, or melting ice and sea water can be concentrated by evaporation.
Table 2 Average concentrations of the 11 most abundant ions and molecules in clean sea water (35.0% salinity, density of 1.023 g cm−3 at 25 °C)
Species Concentration
mmol−1 kg−1 g kg−1
Na+ 468.5 10.77
K+ 10.21 0.399
Mg2+ 53.08 1.290
Ca2+ 10.28 0.4121
Sr2+ 0.090 0.0079
Cl− 545.9 19.354
Br− 0.842 0.0673
F− 0.068 0.0013
HCO3− 2.30 0.140
SO42− 28.23 2.712
B(OH)3 0.416 0.0257

The most important properties of sea water are:

(i) Remarkably constant ratio of the concentrations of the major constituents worldwide.

(ii) High salt concentration, mainly sodium chloride.

(iii) High electrical conductivity.

(iv) Relatively high and constant pH.

(v) Buffering capacity.

(vi) Solubility of gases, of which oxygen and carbon dioxide in particular are of importance in the context of corrosion.

(vii) The presence of a myriad of organic compounds.

(viii) The existence of biological life, to be further distinguished as microfouling (e.g., bacteria, slime) and macrofouling (e.g., seaweed, mussels, barnacles, and many kinds of animals or fish).

Some of these factors are interrelated and depend on physical, chemical, and biological variables, such as depth, temperature, intensity of light, and the availability of nutrients. The main numerical specification of sea water is its salinity.

2.2. Inhibitors

Amines and their derivatives are good selected compounds in this work which inhibit the corrosion and scale deposition of carbon steel in cooling water systems. The chemical structure, 3D structure, nomenclature and molecular weight of the inhibitors used in this work are listed in Table 3.
Table 3 The chemical structures, 3D structures, nomenclature and molecular weights of the inhibitors used
No. Inhibitor name Chemical and 3D structures M. wt
I Diethylenetriamine image file: c5ra11082c-u1.tif 103
II Triethylenetetramine image file: c5ra11082c-u2.tif 146
III Pentaethylenehexamine image file: c5ra11082c-u3.tif 232

2.3. Techniques

2.3.1. Potentiodynamic polarization measurements. Electrochemical measurements were carried out using a Volta lab80 (Tacussel-radiometer PGZ402) controlled by a Tacussel corrosion analysis software model (Volta master 4). A platinum electrode was used as the auxiliary electrode. All potentials were measured against a saturated calomel electrode (SCE) as the reference electrode. All measurements were carried out in air saturated solutions and at ambient temperature (298 K). The Tafel extrapolation method used an extrapolation interval of 250 mV with respect to Ecorr.
2.3.2. Electrochemical impedance spectroscopy (EIS). Electrochemical impedance measurements were carried out using a Volta lab 80 potentiostat (Tacussel-radiometer PGZ402) controlled by a Tacussel corrosion analysis software model (Volta master 4). Impedance spectra were obtained in the frequency range between 100 kHz and 50 mHz using 20 steps per frequency decade at an open circuit potential after a 1 hour immersion time. An AC signal with 20 mV amplitude peak to peak was used to perturb the system. The EIS diagrams are given as both Nyquist and Bode representations.
2.3.3. Evaluation of the selected amine derivatives as scale inhibitors of calcium sulfate deposition. The experimental procedure involved dissolving 9.11 g of CaCl2 per L of double distilled water (brine A). On the other hand, the sulfate solution (brine B) was prepared by dissolving 7.3 g of Na2SO4 per L of double distilled water. The test protocol consisted of mixing 50 mL of calcium solution with 50 mL of sulfate solution. The solutions were then incubated in a water thermostat at 90 °C for 24 hours. At the end of the test duration, the solution was filtered through 0.22 micron filter paper and the calcium concentration was analyzed by titration using 0.01 M EDTA and Murexide as the indicator. The experiments were repeated for different doses (25–125 ppm) of the selected amine derivatives for the same test duration.60
2.3.4. Scanning electron microscopy (SEM). Immersion corrosion analysis of the carbon steel samples in sea water with and without the optimal concentration of the inhibitor (III) was performed using SEM. In order to study the surface morphology, the samples were subjected to SEM immediately after the corrosion tests using a JSM-6510LA analytical scanning electron microscope.
2.3.5. Energy dispersive analysis of X-rays (EDX). An EDX system attached to a JSM-6510LA analytical scanning electron microscope was used for the elemental analysis or chemical characterization of the film formed on the carbon steel surface before and after applying compound III.

3. Results and discussion

3.1. Potentiodynamic polarization measurements

Dynamic potential polarization is a common method that evaluates the inhibition efficiency of corrosion inhibitors.61

The electrochemical measurements were carried out in a cell using a three electrode mode; a platinum electrode and saturated calomel electrode (SCE) were used as the counter and reference electrodes respectively. A 1 cm2 area of the carbon steel sample as the working electrode (WE) was abraded, washed, and finally immersed in the cooling water. The working electrode was first immersed into the test solution for 1 h to establish a steady state open circuit potential. After determining the open circuit potential, potentiodynamic polarization curves were obtained with a scan rate of 1 mV s−1 in both cathodic and anodic potentials to investigate the polarization behavior. Fig. 1 shows the cathodic and anodic polarization curves of carbon steel immersed in sea water in the absence and presence of different concentrations of compound III as a representative sample.


image file: c5ra11082c-f1.tif
Fig. 1 Potentiodynamic polarization curves (E–log[thin space (1/6-em)]I relationship) of carbon steel in sea water in the absence and presence of different concentrations of compound III.

Electrochemical parameters such as the corrosion potential (Ecorr), corrosion current density (Icorr), and cathodic and anodic Tafel slopes (βc and βa) were calculated62 and listed in Table 4. The inhibition efficiency (IE%) values were evaluated from the measured Icorr values using the following equation:

 
IE% = [(Icorr − Icorr(inh)/Icorr)] × 100 (1)
where Icorr and Icorr(inh) are the corrosion current values without and with the addition of various concentrations of inhibitor.

Table 4 Data obtained from potentiodynamic polarization measurements of carbon steel in sea water solution in the absence and presence of various concentrations of compounds I, II, and III at 298 K
Inhibitor Conc., ppm −Ecorr, mV vs. SCE Icorr, μA cm−2 βa, mV dec−1 βc, mV dec−1 IE%
Blank 0 760 25.62 (±1.39) 119.1 301.4 —
(I) 50 776.1 14.14 (±0.65) 115.1 229.6 44.78 (±1.99)
100 738 12.28 (±0.65) 112.3 149.7 51.89 (±4.92)
150 807.6 10.97 (±0.65) 133.2 206.8 57.18 (±0.34)
200 737.1 10.51 (±0.86) 129.6 162.8 59.01 (±1.69)
250 734.4 9.15 (±0.74) 125.2 138.4 64.31 (±1.54)
300 699.8 6.44 (±0.83) 127.7 157.2 74.93 (±2.00)
(II) 50 788.6 13.02 (±1.16) 125.2 211.7 49.23 (±2.68)
100 735.7 11.96 (±0.60) 120.9 190.8 53.30 (±1.13)
150 796.4 10.76 (±0.95) 135.4 216.7 57.98 (±3.44)
200 714.1 9.20 (±0.60) 135.2 195.3 64.09 (±1.43)
250 695.5 7.99 (±0.63) 130.7 188.1 68.80 (±2.03)
300 651.7 5.68 (±0.85) 131.6 144.5 77.83 (±2.76)
(III) 50 689.1 9.01 (±0.72) 247.8 168.7 64.86 (±1.76)
100 682.9 7.59 (±0.48) 206.5 173.2 70.30 (±2.67)
150 682.1 6.70 (±0.53) 199.4 171.3 73.87 (±0.88)
200 679.9 5.04 (±0.66) 187.5 171.2 80.34 (±2.21)
250 672.6 3.85 (±0.58) 195.1 160.7 84.99 (±1.89)
300 610.8 2.02 (±0.19) 192.6 166.1 92.13 (±0.34)

From Fig. 1, it is clear that both the anodic metal dissolution and cathodic reduction reactions were inhibited when compound III was added to the sea water and this inhibition was more pronounced with increasing inhibitor concentration. The Tafel lines were shifted from more negative to more positive potentials with respect to the blank curve by increasing the concentration of the inhibitor. The results show that an increase in the inhibitor concentration leads to a decrease in the corrosion current density (Icorr), but the βc and βa are approximately variable indicating that the retardation of the two reactions (cathodic oxygen reduction and anodic metal dissolution) was affected without changing the dissolution mechanism. The obtained results indicate that the percentage inhibition efficiency (IE%) of compound III is greater than that of compounds II and I.

3.2. Electrochemical impedance spectroscopy (EIS)

In order to understand the corrosion behavior of carbon steel in sea water in the absence and presence of various concentrations of compounds I, II and III, using compound III as a representative sample, electrochemical impedance spectroscopy (EIS) measurements were carried out. Nyquist plots for compound III are shown in Fig. 2 and Bode plots are also shown in Fig. 3.
image file: c5ra11082c-f2.tif
Fig. 2 Nyquist plots for carbon steel in sea water in the absence and presence of different concentrations of compound III.

image file: c5ra11082c-f3.tif
Fig. 3 Bode plots for carbon steel in sea water in the absence and presence of different concentrations of compound III: (a) log frequency vs. log[thin space (1/6-em)]Z and (b) log frequency vs. phase angle.

The previous plots show that the impedance response of carbon steel in sea water was significantly changed after the addition of the inhibitor molecules. Various parameters such as the charge transfer resistance (Rt), double layer capacitance (Cdl) and percentage inhibition efficiency (IE%) were calculated and listed in Table 5.

Table 5 Data obtained from electrochemical impedance spectroscopy (EIS) measurements of carbon steel in sea water solution in the absence and presence of various concentrations of compounds I, II and III
Inhibitor Conc., ppm Coefficient Rs (Ω cm2) Cs (μF cm2) Rt (Ω cm2) Cdl (μF cm2) IE%
Blank 0 0.99 — — 358.13 (±3.5) 1302 —
(I) 50 0.99 230 48.1 642.00 (±8.0) 1217 44.21 (±0.63)
100 0.99 270 43.4 746.33 (±10.0) 1104 52.01 (±1.06)
150 0.99 320 37.2 798.00 (±8.0) 989.3 55.12 (±0.78)
200 0.99 350 32.5 870.33 (±5.0) 930.1 58.85 (±0.58)
250 0.99 380 29.3 1001.00 (±10.0) 902.5 64.22 (±0.61)
300 0.99 390 28.7 1401.00 (±17.7) 887.4 74.43 (±0.51)
(II) 50 0.99 250 46.2 703.00 (±11.8) 1159 49.05 (±0.86)
100 0.99 290 42.4 761.45 (±11.7) 1090 52.96 (±0.72)
150 0.99 350 36.7 838.33 (±8.4) 970.6 57.28 (±0.03)
200 0.99 370 31.1 996.00 (±3.0) 911.8 64.04 (±0.42)
250 0.99 410 28.8 1135.33 (±7.1) 873.1 68.46 (±0.12)
300 0.99 420 27.6 1592.00 (±20.4) 855.7 77.50 (±0.20)
(III) 50 0.99 280 43.2 1015.33 (±10.2) 1004 64.73 (±0.37)
100 0.99 330 40.5 1203.00 (±12.0) 978.1 70.23 (±0.52)
150 0.99 390 35.2 1356.15 (±12.2) 914.4 73.59 (±0.50)
200 0.99 420 29.1 1799.33 (±6.7) 877.5 80.10 (±0.27)
250 0.99 430 27.9 2364.33 (±8.3) 822.3 84.85 (±0.15)
300 0.99 440 26.4 3987.33 (±9.0) 786.9 91.02 (±0.08)

The percentage inhibition efficiency IE% was calculated from the values of Rt using the following equation:63

 
IE% = [1 − (Rt/Rt(inh))] × 100 (2)
where Rt and Rt(inh) are the charge transfer resistance values in the absence and presence of inhibitor, respectively. The increasing value of the charge transfer resistance (Rt) and decreasing value of the double layer capacitance (Cdl) observed with increasing inhibitor concentration indicate that the inhibitor molecules inhibit the corrosion rate of carbon steel in sea water by an adsorption mechanism.64 For the analysis of the obtained impedance spectra, an equivalent circuit (EC) was obtained using the Boukamp program, as shown in Fig. 4, where Rs is the solution resistance, Rt is the charge transfer resistance, Cdl is the electrochemical double layer capacitance, Rf is the film resistance and Cf is the film capacitance. From the EIS data it was found that the percentage inhibition efficiency of inhibitor III is greater than that of inhibitors I and II, thereby agreeing with the aforementioned results from the potentiodynamic polarization measurements.


image file: c5ra11082c-f4.tif
Fig. 4 The equivalent circuit used to model the impedance data of the carbon steel in sea water.

3.3. Evaluation of the selected amine derivatives as scale inhibitors

The present work was extended to establish the effectiveness of the selected amine derivatives as scale inhibitors of calcium sulfate deposition in sea water. The laboratory procedures were carried out as described in the Experimental part. The percentage inhibition efficiency was calculated as follow:
 
Scale inhibition efficiency% = [Cai − Cab/Cac − Cab] × 100 (3)
where, Cai = calcium ion concentration for the sample treated with the inhibitor after precipitation. Cab = calcium ion concentration in the blank solution after precipitation. Cac = calcium ion concentration in the blank solution before precipitation.

The obtained results are listed in Table 6 and graphically shown in Fig. 5. It is clear that the percentage inhibition efficiency increases with increasing inhibitor concentration, reaching 86.7% at 125 ppm.

Table 6 Efficiency of the selected amine derivatives (I, II and III) as calcium sulfate scale inhibitors at various concentrations as calculated from ASTM G 3-89 Re-approved 1994
Conc., ppm Percentage inhibition efficiency (IE%) of inhibitor I Percentage inhibition efficiency (IE%) of inhibitor II Percentage inhibition efficiency (IE%) of inhibitor III
0 — — —
25 30.71 (±0.55) 38.43 (±0.31) 52.03 (±0.51)
50 45.32 (±0.79) 51.50 (±0.52) 63.40 (±1.10)
75 56.30 (±0.78) 62.60 (±1.08) 74.80 (±1.30)
100 63.91 (±1.12) 69.33 (±0.67) 79.30 (±0.46)
125 65.10 (±1.13) 75.20 (±0.46) 86.70 (±0.96)


image file: c5ra11082c-f5.tif
Fig. 5 Variation in the efficiency of compounds I, II and III as calcium sulfate scale inhibitors with concentration as determined from ASTM G3-89-re-approved 1994.

3.4. Scanning electron microscopy (SEM)

Fig. 6a shows an SEM image of the polished carbon steel surface. The micrograph shows a characteristic inclusion, which was probably an oxide inclusion.65 Fig. 6b shows an SEM image of the surface of the carbon steel specimen after immersion in sea water for 90 days in the absence of inhibitor, while Fig. 6c shows an SEM image of the surface of another carbon steel specimen after immersion in sea water for the same time interval in the presence of 300 ppm of compound III. The resulting scanning electron micrographs reveal that the surface was strongly damaged in the absence of the inhibitor, but in the presence of 300 ppm of compound III, there was less damage to the surface. This confirms the observed high inhibition efficiency of compound III at this concentration.
image file: c5ra11082c-f6.tif
Fig. 6 SEM images of the carbon steel surface: (a) polished sample, (b) after immersion in the sea water and (c) after immersion in the sea water in the presence of 300 ppm of compound III.

3.5. Energy dispersive analysis of X-rays (EDX)

The EDX spectrum in Fig. 7a shows the characteristic peaks of some of the elements constituting the polished carbon steel surface. The spectra of the polished carbon steel surface after immersion in sea water in the absence and presence of compound III for 90 days are shown in Fig. 7b and c, respectively. The spectrum in Fig. 7c shows that the Fe peak is considerably decreased relative to the samples shown in Fig. 7a and b. This decrease in the Fe band indicates that a strongly adherent protective film of compound III formed on the polished carbon steel surface, which leads to a high degree of inhibition efficiency.66 EDX and SEM examinations of the carbon steel surface support the results obtained from chemical and electrochemical methods that indicate the compounds are good inhibitors for carbon steel in sea water.
image file: c5ra11082c-f7.tif
Fig. 7 EDX spectra of the carbon steel surface: (a) polished sample, (b) after immersion in sea water and (c) after immersion in sea water in the presence of 300 ppm of compound III.

The results from both the SEM and EDX techniques confirm the formation of a good protective layer on the surface of the carbon steel in the presence of 300 ppm of inhibitor III.

3.6. Inhibition mechanism of the selected amine derivatives

3.6.1. Corrosion inhibition mechanism. The adsorption of the amine derivative molecules onto the carbon steel surface can take place by chemisorption. The adsorption process arises from donor acceptor interactions between the p-electrons of the nitrogen atoms and the vacant d-orbitals of iron.67,68 The orientation of the molecules may depend on the pH and/or the electrode potential.69 In this case, the molecules can be adsorbed onto the surface of the carbon steel through the chemisorption mechanism that occurs via the displacement of adsorbed water molecules from the metal surface and then the sharing of electrons between the heteroatom and iron. A schematic illustration of the different modes of adsorption at the metal/solution interface is shown in Fig. 8.70
image file: c5ra11082c-f8.tif
Fig. 8 The interaction between pentaethylenehexamine and the carbon steel surface through chemical adsorption processes.

The corrosion inhibition efficiency for inhibitors I–III was tested using potentiodynamic polarization and the EIS technique and follows the sequence:

III > II > I

From the obtained results, one can conclude that, as the number of nitrogen atoms in the amine derivatives increases, their inhibition efficiency as a corrosion inhibitor is increased.

3.6.2. Scale inhibition mechanism. Scale inhibition is a technique in which a certain type of chemical additive is used to inhibit the growth rate of scale crystals formed in aqueous media. This process delays, reduces or prevents scale formation upon adding small amounts of inhibitor to normally scaling water. The use of a chelating agent such as an amine derivative can break up acid resistant scale by isolating and locking up the scale metallic ions within a closed ring-like structure as shown in Fig. 9.71
image file: c5ra11082c-f9.tif
Fig. 9 Chelation of diethylenetriamine with a calcium ion to prevent the formation of calcium sulphate scale.

4. Conclusions

The results showed that all compounds used in this work act as effective corrosion and scale inhibitors for carbon steel in sea water. The percentage inhibition efficiency (IE%) of the inhibitors increases with the increasing number of ethylene amine units in the inhibitor molecule. EIS data indicates that the value of the charge transfer resistance (Rt) increased with increasing inhibitor concentration, while the value of electrochemical capacitance (Cdl) decreased. The potentiodynamic polarization curves indicated that the inhibitor molecules inhibit both anodic metal dissolution and also cathodic oxygen reduction, so the inhibitors used in this work are classified as mixed type. The inhibition mechanism is attributed to the strong adsorption ability of the selected amine derivatives onto the carbon steel surface, forming a good protective layer, which isolates the surface from its aggressive environment, as confirmed by SEM and EDX techniques.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11082c
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