INTRODUCTION:
Several phosphonic acids have been used as corrosion inhibitors due to their hydrolytic stability, ability to form complex with metal ions and scale inhibiting property [1-15]. The present work is undertaken (i) to investigate the influence of Cl- ion on the inhibition efficiency of the ATMP – Zn2+ system. (ii) To analyse the protective film formed on metal surface by FTIR and UV – fluorescence spectra.
EXPERIMENTAL:
Preparation of carbon steel specimens:
The carbon steel specimens were selected from the same sheet of the following composition. Sulphur - 0. 026 percent, phosphorous - 0. 06 percent, manganese - 0. 4 percent, carbon - 0. 1 percent and the rest iron.
The carbon steel specimens of the dimensions 1. 0 x 4. 0 x 0. 2 cm were polished to mirror finish and then degreased with trichloroethylene. They were used in the weight–loss method and surface examination studies.
Carbon steel rod encapsulated in Teflon with an exposed cross section of 1 cm2 diameter was used as the working electrode in potentiostatic polarisation studies. The surface of the electrode was polished to mirror finish. Trichloroethylene was used to degrease the electrode.
Weight loss method:
Determination of surface area of the specimens were determined with specimens and the radius of the hole were determined with the help of of vernier calliper of high precision and the surface areas of the specimens were calculated.
Weighing the specimens before and after corrosion:
All the weighing of the carbon steel specimens before and after corrosion were carried out using a Mettle analytical balance –AE 240 dual range balance with readability of 0. 01 mg in 40g range and 0. 1 mg in 200g was supplied by Mettler instrument. AG, CH-8606 Greifensee, Switzerland
Determination of corrosion rate:
The weighed specimens in triplicate were suspended by means of glass hooks in 100 ml of various test solutions. After seven days of immersion, the specimens were taken out, washed in running water, dried and weighed. From the change in weight of the specimens, corrosion rates were calculated with the help of the following relationship.
Loss in weight (mg)
Corrosion rate = -----------------------------x Period of immersion (days)
Surface area of the specimen
The corrosion inhibition efficiency (I.E) was the calculated using the equation,
I.E = 100 [1-W2 / W1] %
Where, W1 = Corrosion rate in the absence of inhibitor
and W2= Corrosion rate in the presence of inhibitor
Potentiostatic polarisation study:
The measurement were carried out using corrosion measurements system (EG & G electrochemical Impedance Analyzer Model 6310). A three electrode cell assembly was used. The working electrode used was rectangular specimens of carbon steel, with one face of the electrode of 1 cm2 area exposed and the rest being shielded with red lacquer. A rectangular platinum foil was used as the counter electrode. The area of the counter electrode was much larger compared to the area of the working electrode. This can exert a uniform potential field on the working and minimize the polarization effect on the counter.
The reference electrode used was saturated calomel electrode (SEC). The reference electrode was placed close to the working electrode to minimize IR contribution. A time interval of about 5 to 10 min was given for the working electrode to attain a steady state open circuit potential. The results such as inhibition efficiency, Tafel slope, Ecorr and Icorr values are presented.
Surface analysis by FTIR spectroscopy:
After the immersion period of two days in various environments, the specimens were taken out the test solutions and dried. The film formed on the surface was scratched carefully and it was thoroughly mixed so was to make it uniform throughout. FTIR spectrum of the powder (KBr pellet) was recorded using Perkin –Elmer 1600 FTIR spectrophotometer with resolving power of 4 cm-1.
Surface analysis by UV – luminescence spectroscopy:
Luminescence spectra of the film formed on the metal surface were recorded using Hitachi 650-610S fluorescence spectrometer.
RESULTS AND DISCUSSION:
Weight loss study:
Corrosion rates of carbon steel immersed in various concentrations of Cl- (100 ppm and 150 ppm ) (Table-1 and 2) have been evaluated by weight- loss method in the absence and presence of Amino (trimethylene phosphonic acid ) ATMP and Zn2+. It is found that ATMP alone is corrosive at 100 ppm Cl- but at 150 ppm Cl- it shows some inhibition efficiency (IE). However interestingly, ATMP shows very good IE in the presence of Zn2+ at all concentrations of Cl-. A synergistic effect is noticed between ATMP and Zn2+. The protective film has been analysed by surface analysis techniques such as FTIR and fluorescence spectra.
Table 1 : Inhibition Efficiencies Offered to Carbon steel Immersed in Aqueous solutions containing 100 ppm of Cl- by the ATMP – Zn2+ system
|
ATMP ppm |
Zn2+ |
|||
|
0 |
25 |
50 |
75 |
|
|
25 |
-9 |
63 |
33 |
96 |
|
50 |
-13 |
88 |
63 |
97 |
|
75 |
-14 |
92 |
80 |
97 |
|
100 |
-15 |
35 |
80 |
98 |
|
125 |
-15 |
32 |
93 |
98 |
|
150 |
-17 |
30 |
98 |
98 |
Table II : Inhibition Efficiencies Offered to Carbon steel Immersed in Aqueous solutions containing 150 ppm of Cl- by the ATMP – Zn2+ system
|
ATMP ppm |
Zn2+ ppm |
|||
|
0 |
25 |
50 |
75 |
|
|
25 |
16 |
68 |
97 |
94 |
|
50 |
10 |
80 |
97 |
98 |
|
75 |
10 |
93 |
97 |
99 |
|
100 |
13 |
82 |
96 |
97 |
|
125 |
14 |
27 |
95 |
96 |
|
150 |
14 |
24 |
94 |
95 |
Polarization Study:
When carbon steel is immersed in 150 ppm Cl- solution, the corrosion potential is -370 m V vs. SCE. When 25 ppm Zn2+ is added, the potential is shifted to the cathodic side (-470 mV vs. SCE), suggesting that this system controls the cathodic reaction predominantly. The formulation consisting of 75 ppm ATMP shifts the corrosion potential to the anodic side (-340 m V vs. SCE). The formulation consisting of 75 ppm ATMP and 25 ppm Zn2+ has a corrosion potential of - 410 mV vs. SCE. This potential is between - 370 and - 470 mV vs. SCE suggesting that ATMP - Zn2+ system functions as mixed inhibitor (Table-3).
Table 3: Corrosion parameters derived from polarization curves of carbon steel in 150 ppm Cl- solution
|
ATMP Ppm |
Zn2+ ppm |
Ecorr mV vs. SCE |
Icorr A / Cm2 |
IE % |
|
0 |
0 |
- 370 |
250 |
- |
|
75 |
0 |
- 340 |
17. 5 |
93 |
|
0 |
25 |
- 470 |
25. 0 |
90 |
|
75 |
25 |
- 410 |
5. 0 |
98 |
FTIR spectra
The FTIR spectrum of ATMP is shown in Figure-1. The C-N stretching frequency appears at 1145 cm-1. The adsorption due to P-O stretch occurs at 1002 cm-1.
The FTIR spectrum (KBr) of the film formed on the surface of the metal specimen immersed in the solution containing 150 ppm Cl- 75 ppm ATMP, and 25 ppm Zn2+ is shown in the diagram. The C-N stretching frequency of ATMP decreases from 1145 cm-1 to 1161 cm-1. The P-O stretching frequency appears at 1019. 0 cm-1. This suggests the presence of ATMP on the metal surface as iron ATMP Complex. The band at 1383. 5 cm-1 is due to Zn(OH)2.
Fig. 1- FTIR Spectra
a) Pure ATMP
b) Film formed when carbon steel is immersed in the solution containing 150 ppm of Cl- 25 ppm of Zn2+ and 75 ppm of ATMP.
The UV-visible absorption spectra of solutions:
The uv-visible absorption spectrum of the solution containing 200 ppm ATMP is given in the figure and that of the solution containing 100 ppm Fe 2+ is given in figure-2. The spectrum of the solution containing 200 ppm ATMP and 100 ppm Fe2+ shows peak at 200 nm and 300 nm corresponding to Fe2+ - ATMP complex in solutions.
Figure 2 The UV-visible absorption spectra of solution.
a) 200 ppm of ATMP.
b) 100 ppm of Fe2+
c) 200 ppm of ATMP + 100 ppm of Fe2+
Florescence Spectrum:
The fluorescence spectrum of the film formed on the surface of the metal specimen immersed in the solution containing 150 ppm Cl -, 75 ppm ATMP and 25 ppm Zn2+ is shown in the figure-3. When the film is excited at 300 nm, peaks appear at 387, 415, 469, 536 and 561 nm. These peaks are due to Fe2+- ATMP complex entrailed in Zn(OH)2.
Figure3 UV-fluorescence spectrum of the film formed on carbon steel after immersion in the solution containing 150 ppm of Cl-, 75 ppm of ATMP and 25 ppm of Zn2+
Mechanism of corrosion inhibition by ATMP – Zn2+ system
1) When the solution containing 150 ppm Cl -, 75 ppm and 25 ppm Zn2+ is prepared, there is a formation of Zn2+ - ATMP complex in solution.
Zn2+ + ATMP Zn2+
-ATMP
2) When carbon steel specimen is immersed in this solution, the above complex diffuses towards the metal surface from the bulk.
3) On the metal surface, the zinc complex is converted to iron complex on the anodic sites. Zn2+ is released.
Zn2+ - ATMP +Fe2+ Fe2+-ATMP
+Zn 2+
4) The released Zn 2+ combines with OH- to form Zn(OH)2 on the cathodic sites
Zn2+ + 2 OH- Zn(OH)2
5) Thus the protective film consists of Fe2+ - ATMP complex and Zn(OH)2
CONCLUSIONS:
1) It is found that ATMP alone is corrosive at 100 ppm Cl- but at 1500 ppm Cl- it shows some inhibition efficiency (IE).
2) ATMP shows very good IE in the presence of Zn2+ at all concentrations of Cl-.
3) The protection film consists of Fe2+ - ATMP and Zn(OH)2.
4) The protective film is found to be UV fluorescent.
5) Polarization study reveals that ATMP – Zn2+ behaves like mixed inhibitor.
6) The ATMP – Zn2+ system can be used in cooling water system containing higher concentration of Cl- such a 150 ppm.
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Received on 15.10.2010 Modified on 27.10.2010
Accepted on 04.11.2010 © AJRC All right reserved
Asian J. Research Chem. 4(3): March 2011; Page 402-405