CHAPTER VI EFFECT OF AZOMETHINE POLYAMIDES ON REBAR CORROSION. The influence of organic and inorganic corrosion inhibitors as investigated by

Transcription

1 CHAPTER VI EFFECT OF AZOMETHINE POLYAMIDES ON REBAR CORROSION 6.1 Preamble The influence of organic and inorganic corrosion inhibitors as investigated by several authors are discussed in the previous chapters. It would be apt to discuss the investigations of a few more authors in this chapter for a better understanding of corrosion inhibition by different kinds of inhibitors. In this connection a state of-the-art report was published on corrosion inhibitors for steel in concrete 1 which covers the entire spectrum of commonly used inhibitors such as amino alcohols, calcium nitrite and sodium monofluro phosphates. The related information on (a) basic mechanism study, which gives information about the mechanism of protection provided by inhibitors (b) effectiveness of inhibitors against corrosion in chloride contaminated and carbonated concrete, which deals with the preventive and curing effect of the inhibitors in different environments (c) penetrability of the inhibitor, which underlines some difficulties of penetration into concrete for migrating corrosion inhibitors (MCIs) (d) influence on fresh and hardened concrete properties, which compares fresh concrete properties, mechanical performance and durability with and without inhibitor (e) field trials, which gives the limited data on the long-term performance of the inhibitors in real structures were emphasized. Investigations on the setting time and strength properties on the addition of admixtures, such as calcium nitrite, tri-ethanolamine and tri-isopropanolamine at early stages were conducted by Aggoun et al. 2 Results showed that the calcium nitrite used alone acts as a setting accelerator and has little beneficial effect on the long term development of mechanical resistances. The alkanolamines, when used alone, regardless of the cement type, perform well as hardening accelerator at all stages. Moreover, the results of the combined additions 113

2 are very significant and promising with respect to setting. The hardening acceleration and continuous strength increase well with respect to time. In Chapter 5, the influence of azo polyesters on the corrosion of mild steel rebar in chloride contaminated concrete pore solution was discussed. However, the inhibition efficiencies have not exceeded beyond 80%. In search of inhibitors with still higher inhibition efficiencies, azomethine polyamides with the same dianiline precursors have been synthesised and investigated in this chapter. Polymer crystallinity, electrochemical characteristics and inhibition efficiencies have been evaluated for these polymers in simulated pore solution. 6.2 Experimental Polymeric inhibitors employed in the present study The names, structures and abbreviations of the polymers employed in the present study are summarized in Table 6.1. Table 6.1 Structures and Abbreviations of the Polymers employed Sl. No. Abbreviation Structure of the compounds 1 PAMA1 Polyazomethine amide 1 2. PAMA2 Polyazomethine amide 2 114

3 Sl. No. Abbreviation Structure of the compounds 3. PAMA3 Polyazomethine amide 3 4. PAMA4 Polyazomethine amide Synthesis of polymeric inhibitors Chemicals used All chemicals used were of AR grade 1. 4,4' - diamino diphenyl ether 2. 4,4' - diamino diphenyl methane 3. Ammonium thiocyanate amino benzoic acid 5. Terephthaldehyde 6. p-toluene sulphonic acid 7. Ethanol 8. Hydrochloric acid 9. Triphenyl phosphorite 10. Calcium chloride 115

4 11. Sodium carbonate 12. Pyridine 13. Chloroform 14. Potassium hydroxide Purification of Solvents Ethanol and Pyridine are purified according to standard procedure. a) Ethanol Ethanol was obtained by refluxing with quick lime with an efficient condenser and a guard tube for 6 hours and allowed to stand overnight. It was then distilled and the middle fraction boiling at 78 o C was collected and stored. b) Pyridine Pyridine was refluxed with potassium hydroxide in a round bottomed flask fitted with an efficient condenser and a guard tube and refluxed for 30 minutes and excess of potassium hydroxide was filtered off under reduced pressure. Pyridine was then distilled under the vacuum and stored in black container Synthesis of azomethine polyamides The four azomethine polyamides used in the present investigation, viz, PAMA1, PAMA2, PAMA3 and PAMA4 are synthesised by the method developed by Ravikumar et al 3. The method involves three stages. (i) (ii) (iii) Synthesis of dicarboxylic acid monomers having azomethine group. Synthesis of phenyl thiourea diamine monomer. Synthesis of polyamide by phosphorylation condensation of two monomers. 116

5 (i) Synthesis of dicarboxylic acid monomers having azomethine group A 50ml round bottomed flask was equipped with a reflux condenser, Dean-Stark trap and a magnetic stirrer. It was then charged with 0.2 mol of 4-aminobenzoic acid, 0.01 mol of terephthaldehyde and 30ml of absolute ethanol and catalytic quantities of p-toluene sulphonic acid. The reaction mixture was stirred at room temperature for 15minutes and heated in an oil bath at 110 o C. An azeotrope was formed and it was collected in the Dean- Stark trap. The contents were further refluxed for minutes and then about 10ml of ethanol was distilled off. The remaining slurry was again refluxed for 3 hours. At the end, a precipitate is formed which is cooled, filtered and washed several times with hot water. It is then finally washed with absolute ethanol and dried under vacuum. The product obtained is the dicarboxylic acid containing azomethine group. The synthetic route is shown in Fig 6.1. Fig. 6.1 Synthesis scheme for azomethine dicarboxylic acid monomer (ii) Synthesis of phenyl thiourea diamine monomer The procedure for the synthesis of phenyl thiourea diamine monomer required for the synthesis of two polymers PAMA1 and PAMA2 has already been described in this thesis as scheme I in chapter V (page #89). The scheme is shown in Fig

6 Fig. 6.2 Synthesis scheme for phenyl thiourea diamine monomer For the synthesis of PAMA3 and PAMA4, the precursor monomers viz., 4, 4 diamino diphenyl ether and 4, 4 - diamino diphenyl methane are used respectively, in stage 3. (iii) Synthesis of polymer by phosphorylation condensation The azomethine polyamides were synthesised by employing phosphorylation condensation method. A typical example of the synthesis of polyamide involving diamine diphenyl methane monomer is described as follows: A mixture of mol of the dicarboxylic acid monomer, 0.04 mol of diamino diphenyl methane monomer, 0.01 mol of triphenyl phosphorite, 1.2g of dry CaCl 2, 4ml of pyridine and 10ml of NMP was heated in a round bottom flask with stirring at 110 o C for 4-5 hours. A viscous liquid of the polymer was formed due to condensation. The polymer was thrown out in the form of a precipitate by pouring the viscous liquid into 300ml of absolute ethanol with stirring. It was filtered, washed several times with hot water, washed with dilute Na 2 CO 3 solution, dilute HCl and hot water, in succession. The polymer was finally washed with absolute ehanol and dried under vacuum. The polymerisation process and the structures of the polymers synthesised is summarized in Fig

7 Fig. 6.3 Synthesis scheme for azomethine polyamides Characterisation of polymers Physical characterisation of the synthesised polymers The colour and yield of the polymers, which are synthesised are presented in Table

8 Table 6.2 Physical characterisation of the polymers Sl. No. Structure of the compounds Colour Yield (%) 1. Polyazomethine amide 1 Dirty White 92 (PAMA1) 2. Polyazomethine amide 2 Dirty White 90 (PAMA2) 3. Polyazomethine amide 3 (PAMA3) White 95 4 Polyazomethine amide 4 Pale white 85 (PAMA4) 120

9 XRD Studies The XRD analysis of the four synthesised polymers namely PAMA1, PAMA2, PAMA3 and PAMA4 have been conducted and the resulting XRD spectra are presented in Figs. 6.4 to 6.7 respectively. The data derived from the XRD spectra such as angle 2θ, d value and relative intensities are presented in Tables 6.3 to 6.6 respectively. The percentage crystallinity (%X c ) is calculated from the intensities of XRD peaks by using the formula as discussed in Chapter 4 and the values are given in Table Simulation of pore solution The electrochemical evaluation of the synthesised polymers PAMA1, PAMA2, PAMA3 and PAMA4 towards inhibition efficiency for steel corrosion in alkaline medium has been carried out by using pore solution as electrolytes throughout the studies in the present chapter. The simulated pore solution is the pore solution for concrete which is contaminated with 1M Cl - ions (treated as blank). The ph of the chloride contaminated simulated pore solution is 10. This leads to the simulation of chloride contaminated concretes close to those in coastal zones Electrochemical characterisation In order to assess the inhibition efficiency and the corrosion rate of the system, well known techniques such as, potentiodynamic polarisation and impedance spectroscopy are employed Corrosion Rate Measurements The corrosion cell consisted of a reference electrode namely calomel electrode, a working electrode, with 1 cm 2 working area and a platinum counter electrode with working 121

10 area slightly more than the working electrode. The simulated chloride contaminated pore solution (blank) is used as the electrolyte. The luggin capillary is inserted near to the working electrode. The whole assembly is called as corrosion cell set up. The polymeric inhibitors are added in different concentrations to the electrolyte for evaluating their performance towards alkaline corrosion Polarisation studies The rebars having 1 cm 2 active area are tested for determining the corrosion current density, I corr, by using the linear polarisation measurement (LPRM) technique. The LPRM procedure is based on the Stern-Geary characterisation of the typical polarisation curve for the corroding metal. In this method, a linear relationship is described mathematically for a region on the polarisation curve in which slight change in the current applied to the corroding metal in an ionic solution causes corresponding change in the potential of the metal. In other words, if a large current is required to change the potential by a given amount, the corrosion rate is high and on the other hand, if only a small current is required, the corrosion rate is low. The rebar inside the corrosion cell is polarized by applying a small shift in potential to it ( E = 10 mv) and the resultant current ( I) between the working electrode and the counter electrode is measured. The linear polarisation resistance, R p, was determined from the slope of the plot of applied potential versus the measured current. The corrosion current density was then calculated by using Stern-Geary formula 4 I = E I = B R where, I corr is the corrosion current density (µa/cm 2 ) R p is the polarisation resistance (kω cm 2 ) 122

11 where, β a is the anodic Tafel coefficient β c is the cathodic Tafel coefficient B= 2.3(β.β ) (β +β ) The values of β a and β c are determined from the Tafel plot. In the present investigation, potentiodynamic polarisation measurements have been carried out to evaluate the inhibition efficiency of the inhibitors for steel samples in pore solutions. Before the polarisation test, the samples are immersed in pore solutions with inhibitor for 30 min. The polarisation graph obtained for the systems employed in the present investigation are shown in Figs. 6.8 to 6.12 and the corresponding data obtained from polarisation technique is presented in Tables 6.8 to Impedance Studies Among the various AC techniques, the method based on Faradaic impedance is widely used due to the main advantage that the double layer capacitance and charge transfer resistance can be determined 5. It deals with polarising the steel rebar with a small amplitude alternating current (10 mv) against a counter electrode, even as the steel rebar is impressed with a dc potential of E against a reference electrode. As discussed earlier in Chapter 3, the cell impedance consists of real and imaginary parts. For the determination of R t, a sinusoidal current (or potential) perturbation of very small amplitude (~ 10mV) is applied as a function of frequency. The impedance of the corroding system for various frequencies can be measured by using frequency response analyzer. From the plot Z versus Z (Nyquist plot), R ct value is determined. 123

12 To obtain the double layer capacitance (C dl ), the frequency at which the imaginary component of the impedance is maximum (-Z max) is found and C dl values are obtained from the following equation " F Z = 1 2πC R To investigate the overall corrosion behaviour and the interfacial structure of steelpore solution, electrochemical impedance spectroscopy (EIS) is performed for the steel samples exposed to the pore solutions with different types of polymeric inhibitor of varying concentrations. EIS is performed at the open-circuit potential (OCP), at a frequency range from 10 4 Hz down to 10-2 Hz. Z-view software is used for the analysis of EIS spectra. All the measurements are performed at room temperature and the consistency is checked at least three times. The impedance plot obtained for the systems employed in the present investigations are shown in Figs to 6.17 and the corresponding data obtained are presented in Tables 6.13 to Results and Discussion Characterisation of polymers with XRD The calculated % Crystallinity (%X c ) presented in Table 6.7 reveals that the PAMA1 polymer is 61% crystalline and 39% amorphous, PAMA2 polymer is 86.19% crystalline and 13.81% amorphous, PAMA3 polymer is 57.85% crystalline and 42.15% amorphous, and PAMA4 is 93.52% crystalline and 6.48% amorphous. The above calculated data indicates the dual nature of polymers, such as crystalline and amorphous state, which implies the complex characteristic behavior of the polymeric system in solid state. 124

13 6.3.2 Electrochemical behaviour of polymers a) Polarisation behaviour of polymeric inhibitors The electrochemical parameters including corrosion current (I corr ), corrosion rate (CR), Tafel-C (β c ), Tafel-A (β a ) and corrosion potential (E corr ), are computed from the potentiodynamic polarisation data for the system containing four polymeric inhibitors used in the present study. The polarisation graphs obtained from this study are shown in Figs. 6.9 to 6.12 and the data are presented in Tables 6.9 to 6.12, which indicate that all the four polymers, viz, PAMA1, PAMA2, PAMA3 and PAMA4 show excellent corrosion inhibition efficiency at 10 ppm itself. Beyond 10 ppm, the inhibitor efficiency follows the same trend upto 1000 ppm. b) Impedance behaviour of polymeric inhibitors The Nyquist plots obtained for the systems, investigated are shown in Figs to The analyses of these plots reveal that the steel electrolyte interface response contains only one loop as capacitive arcs. The impedance spectra for steel in contaminated pore solution in the presence and absence of azomethine polyamide inhibitors are similar in shape as shown in their corresponding figures. The radii of the semi-circle depend on the additive or inhibitor concentration. The charge transfer resistance (R ct ) and double layer capacitance (C dl ) are determined by analysis of complex plane impedance plots and their values are given in Tables 6.14 to The results show that the R ct value increases with increasing polymers concentration. Similar trend is noticed for all the four polymers, which indicates the formation of surface film with increasing concentration. Also, the increase in R ct and decrease in C dl with increase in polymer concentration for the four systems indicate higher surface coverage by the inhibitor. Further, the values of protection efficiencies indicate that there is an interaction of the compound with the active sites of the metal. 125

14 c) Inhibitor efficiency, Surface coverage and Adsorption isotherm The inhibitor efficiency and surface coverage of the polymeric inhibitor systems are calculated using the results obtained from the polarisation data and are presented in Tables 6.9 to The variation of inhibition efficiency as a function of inhibitor concentrations is plotted as shown in Figs to The adsorption isotherms are constructed by using surface coverage values and are presented in Figs to 6.25, respectively. Analysis of these plots reveals that, the azomethine polyamides in chloride contaminated pore solutions follow the Langmuir adsorption. As the adsorption is of Langmuir type, the organic molecule is attached to the metal surface as a monolayer by physisorption Characterisation of corrosion products using SEM Fig 6.26 to 6.28 show the scanning electron micrographs of steel rebar exposed to pore solution containing 1M Cl - ion and 1000 ppm of PAMA2 and PAMA3 inhibitors. Fig indicates the presence of corrosion products in chloride contaminated environment. Fig 6.27 and 6.28 show the existence of highly dense and uniform protective layer over the steel surface. This indicates that, the presence of inhibitor in the solution has an auto-repairing effect on the defective areas of the deposited layer, once the corrosion process starts at the bare metallic surface exposed on their base. It is also possible that the protective film might have been formed due to a complex formation between the corrosion products and the solution components such as chlorides and inhibitors. 126

15 6.3.4 Mechanism of inhibition by azomethine polyamides In the previous chapter, the mechanism of inhibition of rebar corrosion by azopolyesters was discussed at length. In this chapter, the polymers employed for inhibition studies are azomethine polyamides. Although the two groups of polymers used are different in structure, one of the monomers used in the polymer synthesis belong to the dianiline family. This means that inhibition mechanism of polyamides may not be so different from that adopted by the polyesters. Hence, in the case of the four polyamides used in this chapter, the mechanism is expected to involve the following steps: i. Displacement of Cl - ions and water molecules from the rebar steel surface by the polyamide molecules. ii. Formation of a gel like adduct between the displaced Cl - ions and polymer molecules. iii. Formation of coordination type of complex between Fe 2+ ions on metal surface and the gel leading to strong adsorptions, which may be both chemisorption and physisorption and thereby rendering metal surface hydrophobic. iv. Prevention of the ingress of both the aggressive Cl - ions as well as water molecules towards the metal surface. v. During the adsorption, the aromatic polyamide molecules orient themselves in near flat orientation with reference to the metal surface. The aromatic rings, azomethine groups CH=N-, -C=S and CONH, act as anchoring sites with their rich electron density. vi. The polymer molecules arrange themselves parallel to each other on the metal surface forming an organic monolayer that covers the entire metal surface and effectively block the ingress of Cl - ions. 127

16 given below. The general inhibition mechanism through adsorption is represented by Fig 6.29 as Aromatic Ring Fig 6.29 Mechanism of inhibition through adsorption of polymer on metal sites Further, the adsorption of individual polymer molecules PAMA1, PAMA2, PAMA3 and PAMA4 on steel surface can be depicted as shown in Figs 6.30 to Fig 6.30 Adsorption of PAMA1 polymer on steel surface Fig 6.31 Adsorption of PAMA2 polymer on steel surface 128

17 Fig 6.32 Adsorption of PAMA3 polymer on steel surface Fig 6.33 Adsorption of PAMA4 polymer on steel surface Comparative performance of azo polyesters (Chapter 5) and polyamides (Chapter 6) A comparison of the inhibition efficiency values obtained from polarisation studies of azo polyesters and azomethine polyamides (PAMA4 system is taken for comparative purpose) is presented in Table 6.18, which would reveal that the azomethine polyamides are far more superior to the azo polyesters. The higher inhibition efficiency of the azomethine polyamides compared to azopolyester can be attributed to the following reasons. In the synthesis of azo polyesters, a monomer derived from dianiline precursor is coupled with the diacid chloride containing only one aromatic ring (diacid chloride of terephthalic acid) as another monomer, which adds only one benzene ring per monomer to the polymer. On the other hand, in the case of the azomethine polyamides, a monomer 129

18 from dianiline precursor is condensed with another monomer derived from another dianiline precursor. Consequently during the synthesis of polyamides, a minimum of two aromatic rings are added to the polymer skeleton. This would mean essentially that the azomethine polyamides would contain larger number of aromatic rings than the polyesters per polymer molecule. This would mean an increased π electron density from the additional benzene ring in the case of poly amides as compared to polyesters. As a result, the degree of adsorption of polyamides will be more than that of polyesters, which ultimately lead to a higher inhibition efficiency of the polyamides than the polyesters. 6.4 Summary In this chapter, the study of corrosion inhibitive performance of four azomethine polyamides has been presented. All the four polyamides have been found to be equally efficient at all concentrations. A mechanism of adsorptive inhibition has been proposed. The polyamides are found to be more efficient than the polyesters discussed in Chapter V. 130

19 6.5 References 1. T.A.Soylev and M.G.Richardson, Constr Build Mater., 22 (2008) S.Aggoun, M.Cheikh-Zouaoui, N.Chikh and R.Duval, Constr Build Mater., 22(2008) A. Murugesan, L. Ravikumar, V.S. Bala, P. Senthil Kumar, T.Vidhya Devi, S. Dinesh Kirba, S.S. Kalaivani, S.Krithika and S. Sivanesan, Desaln. 271 (2011) M. Stern and A.L.Geary, J. of Electrochem Soc., 104(1) (1957) B.J.Vasanthi, L.Ravikumar and A.Selvaraj, Mater. corros., 59 (1) (2008) G. Achary, Y. A. Naik, S.Vijay kumar, T.V. Venkatesha, and B.S. Sherigara, Appl. Surf. Sci., 254 (2008)

20 X-RAY DIFFRACTION SPECTROSCOPY Table 6.3 XRD data of PAMA1 Index Angle (2θ) D value x 10-4 Rel. Intensity % % % % % % % % % % % % % % % % % % % % % % % % % % % % Table 6.4 XRD data of PAMA2 Index Angle (2θ) D value x 10-4 Rel. Intensity % % %

21 Table 6.5 XRD data of PAMA3 Index Angle (2θ) D value x 10-4 Rel. Intensity % % % % % % % % % % % % % % % % % % % % % % % % % % % % %

22 Table 6.6 XRD data of PAMA4 Index Angle (2θ) D value x 10-4 Rel. Intensity % % % % % Table 6.7 Crystallinity of polymers obtained from XRD data Compound Name Total intensity of stronger peaks (I c ) (counts) Total intensity of broader peaks (I a ) (counts) % Crystallinity I c %Xc = I + I c a x100 PAMA PAMA PAMA PAMA

23 POLARISATION TECHNIQUE Table 6.8 Polarisation data of pore solution with and without chlorides Inhibitor conc. (ppm) Simulated pore solution without Cl - E corr I corr B a B c Corrosion Rate (-)mv µa/cm 2 mv/dec mv/dec mm/year Simulated pore solution with Cl - Table 6.9 Polarisation data of PAMA1 Inhibitor conc. (ppm) Simulated pore solution with Cl - E corr (-)mv I corr µa/cm 2 B a mv/dec B c mv/dec Corrosion Rate mm/year IE (%) Degree of Surface coverage (θ)

24 Table 6.10 Polarisation data of PAMA2 polymer Inhibitor conc. (ppm) Simulated pore solution with Cl - E corr (-)mv I corr µa/cm 2 B a mv/dec B c mv/dec Corrosion Rate mm/year IE (%) Degree of Surface coverage (θ) Table 6.11 Polarisation data of PAMA3 Inhibitor conc. (ppm) Simulated pore solution with Cl - E corr (-)mv I corr µa/cm 2 B a mv/dec B c mv/dec Corrosion Rate mm/year IE (%) Degree of Surface coverage (θ)

25 Table 6.12 Polarisation data of PAMA4 Inhibitor conc. (ppm) Simulated pore solution with Cl - E corr (-)mv I corr µa/cm 2 B a mv/dec B c mv/dec Corrosion Rate mm/year IE (%) Degree of Surface coverage (θ)

26 IMPEDANCE TECHNIQUE Table 6.13 Impedance data of pore solution with and without chlorides Inhibitor conc. (ppm) R ct Ohms/cm 2 C dl µf/cm 2 Simulated pore solution without Cl - Simulated pore solution with Cl Table 6.14 Impedance data of PAMA1 Inhibitor conc. (ppm) Simulated pore solution with Cl - R ct Ohms/cm 2 C dl IE (%) µf/cm

27 Table 6.15 Impedance data of PAMA2 Inhibitor conc. (ppm) Simulated pore solution with Cl - R ct Ohms/cm 2 C dl IE (%) µf/cm Table 6.16 Impedance data of PAMA3 Inhibitor conc. (ppm) Simulated pore solution with Cl - R ct Ohms/cm 2 C dl µf/cm 2 IE(%)

28 Table 6.17 Impedance data of PAMA4 Inhibitor conc. (ppm) R ct Ohms/cm 2 C dl IE (%) µf/cm 2 Simulated pore solution with Cl Table 6.18 Comparison of % IE for azopolyester and azomethine polyamides Inhibitor conc. (ppm) PBTAE IE (%) PETAE IE (%) PMTAE IE (%) PAMA4 IE (%)

29 Fig 6.4 XRD spectra of PAMA1

30 Fig 6.5 XRD spectra of PAMA2

31 Fig 6.6 XRD spectra of PAMA3

32 Fig 6.7 XRD spectra of PAMA4

33 Fig 6.8 Polarisation plots of Pore solution with and without chloride -100 Pore solution without chlroride Pore solution with chlroride -200 Potential (mv) E-5 1E-4 1E Current (ma/cm²)

34 Fig 6.9 Polarisation plots of PAMA Blank 10 ppm 100 ppm 500 ppm 1000 ppm -100 Potential (mv) E-6 1E-5 1E-4 1E Current(mA/cm 2 )

35 Fig 6.10 Polarisation plots of PAMA2 000 blank $$$ - 10 ppm 100 ppm ppm

36 Fig 6.11 Polarisation plots of PAMA Blank 10 ppm 100 ppm 500 ppm 1000 ppm -200 Potential (mv) E-5 1E-4 1E Current (ma/cm2)

37 Fig 6.12 Polarisation plots of PAMA blank 10 ppm 100 ppm 500 ppm 750 ppm 1000 ppm -200 Potential(mV) E-6 1E-5 1E-4 1E current(ma/cm2)

38 Fig 6.13 Nyquist plots of Pore solution with and without chloride Pore solution without Chloride Pore solution with Chloride Z" (ohm.cm²) Z' (ohm.cm²)

39 Fig 6.14 Nyquist plots of PAMA1

40 Fig 6.15 Nyquist plots of PAMA blank $$$ - 10 ppm ppm ppm

41 Fig 6.16 Nyquist plots of PAMA3 Z''(ohm.cm 2 ) Z'(ohm.cm 2 ) blank 10 ppm 100 ppm 500 ppm 1000 ppm

42 Fig 6.17 Nyquist plots of PAMA z''(ohm.cm 2 ) blank 10 ppm 100 ppm 500ppm 750 ppm 1000 ppm Z'(ohm.cm 2 )

43 Fig 6.18 Log C vs % IE of PAMA Percentage IE Log c Fig 6.19 Log C vs % IE of PAMA Percentage IE Log c

44 Fig 6.20 Log C vs % IE of PAMA Percentage IE Log c Fig 6.21 Log C vs % IE of PAMA Percentage IE Log c

45 Fig 6.22 Adsorption Isotherm of PAMA Log(θ/(1-θ)) Log c Fig 6.23 Adsorption Isotherm of PAMA Log(θ/(1-θ)) Log c

46 Fig 6.24 Adsorption Isotherm of PAMA3 Log(θ/(1-θ)) Log c Fig 6.25 Adsorption Isotherm of PAMA Log(θ/(1-θ)) Log c

47 Fig 6.26 SEM micrograph for steel in Cl- ion contaminated pore solution Fig 6.27 SEM micrograph for steel in Cl- ion contaminated pore solution + inhibitor PAMA2

48 Fig 6.28 SEM micrograph for steel in Cl - ion contaminated pore solution + inhibitor PAMA3