Synergistic inhibition of carbon steel corrosion by sodium tungstate ...

Synergistic inhibition of carbon steel corrosion by sodium tungstate and sodium silicate in neutral aqueous media V.S. Saji and S.M.A. Shibli

The authors V.S. Saji and S.M.A. Shibli are both based at the Department of Chemistry, University of Kerala, Thiruvananthapuram, Kerala, India. Keywords Corrosion inhibitors, Effectiveness, Steel Abstract Tungstate inhibitors are seldom used alone in open recirculating cooling water systems due to their low oxidising ability and high cost. The objective of the present work was to develop efficient synergistic inhibitor combinations comprising sodium silicate and very low concentration of sodium tungstate, keeping in view of their application in industrial cooling water system. It was demonstrated in the present study that all the combinations of the inhibitors exhibited synergistic benefit and higher inhibition efficiencies than did either of the individual inhibitors. It was also established that a 4:1 ratio of sodium silicate to sodium tungstate (total 1,000 ppm) was the best overall combination. The FTIR spectra also suggest that tungstate and silicate ions were incorporated in the passivating metal oxide layer formed on the surface of carbon steel in the inhibitor solutions. The effects of excess and depleted concentrations of the individual inhibitor components on overall inhibition behaviour are also discussed. Electronic access The current issue and full text archive of this journal is available at http://www.emeraldinsight.com/0003-5599.htm

Anti-Corrosion Methods and Materials Volume 49 · Number 6 · 2002 · pp. 433– 443 q MCB UP Limited · ISSN 0003-5599 DOI 10.1108/00035590210452789

Introduction For many years, chromates have been used as economical and efficient inhibitors for protection of many metals and alloys. The toxicity of chromates, however, has restricted their use in recent years and caused the search for new anodic inhibitors. Based on the similarity in chemical structure and periodicity between chromate and other Group VI ions, attention has been focused on molybdate, and also tungstate. Among these inhibitors, tungstate may be more effective than is molybdate, in terms of its inhibition efficiency and its applicability under wider experimental conditions (including a broader range of bath pH). The first published information on corrosion inhibition by tungstate appeared as a patent describing its use in organic antifreeze solution (U.S. Patent, 1939). Robertson (1951) demonstrated that tungstate lacked the oxidising properties of chromate and nitrite. Since then, its use has been studied extensively for the protection of iron, zinc and aluminium in neutral, acid and alkaline solutions. In order that Na2WO4 might act as an anodic inhibitor, the presence in solution of dissolved oxygen, or the presence of an appropriate oxidising agent, is essential (Abd el Kadher et al., 1998a; Pryor and Cohen, 1953; Sastri et al., 1989). This arises from the fact that the tungstate ion alone is not able to shift the potential of the metal substantially into the passive region. Abd el Kadher et al. (1998a), in their recent studies, showed that inhibition by tungstate requires the simultaneous presence of tungstate with oxygen in the solution, and the formation of an orderly arrangement of these two species on the metal surface. All successful formulations described in the literature combine tungstate with one or more co-inhibitors (Abd el Kader et al., 1998b; Sastri and Bednar, 1990; Sastri et al., 1991). Sodium silicate is an eco-friendly inhibitor and is very attractive in terms of cost and availability. Sodium silicate also has been reported to be a good synergistic co-inhibitor with molybdate in cooling waters (Chen et al., 1991; Oung and Wang, 1988; Oung et al., 1998). In spite of passivation by dissolved oxygen, Na2SiO3 also forms oxide layers of ferrous or ferric silicate, which act as efficient diffusion barriers, unlike the hydrous FeO. Dissolved oxygen may help to create the

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Inhibition of carbon steel corrosion

Anti-Corrosion Methods and Materials

V.S. Saji and S.M.A. Shibli

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required cathodic area, which can promote anodic passivation of the remaining surface at the prevailing rate of reduction of SiO22 or 3 WO22 (Uhlig, 1965). The lower surface and 4 interfacial tensions of silicates also enhance its ability to displace air from crevices, and assist it to penetrate deposits (James Veil, 1952). In all these respects, silicate can behave as an efficient co-inhibitor with tungstate, even at low concentrations. The objective of the present work was to improve the inhibition efficiency of sodium tungstate, especially at low concentrations, in neutral aqueous media, in view of its potential application in cooling water systems. This paper reports an evaluation of the synergistic inhibition efficiency of sodium tungstate with sodium silicate.

the addition of KSCN using the spectrophotometer at a wavelength of 480 nm. Concentration of the iron was determined from a calibration curve prepared using standard ferric solution. Electrochemical polarisation experiments were conducted potentiodynamically using Voltamaster 1 (Radiometer, Copenhagen) and the corrosion rates were determined by evaluating the corrosion currents from the Tafel plots. The working electrode was polished as mentioned earlier and embedded into a Teflon electrode holder. Area of the working electrode was 2 cm2. Saturated calomel electrode was used as the reference electrode while platinum was the counter electrode. Considering the characteristics of tungstate that immediately facilitate passivation, a quite high scan rate was preferred for polarisation experiments. In the present study, a scan rate of 500 mV/min was used. The working electrode was subjected to cathodic polarisation from the stable OCP value followed by anodic polarisation, i.e. 100 mV above and below the established Ecorr value. The Ecorr values were measured after giving a fixed interval of 10 min in all cases. They were found reproducible when the experiments were repeated. However, it slowly shifted with prolonged time. There was no perfect recognisable Tafel region for both the anodic and cathodic curves. However, it was plotted based on a software-based approximation that was followed in all the cases. Higher temperature experiments were carried out using a thermostatic water bath with mechanical stirrers. For the experiments where the effect of depletion was studied, the inhibitor solutions were diluted to the required extent by means of replacing the required volume of the original inhibitor solution by pure distilled water. A Perkin-Elmer spectrophotometer was used to record the FTIR spectra of samples of passive films that were scratched from the sample surfaces.

Experimental Commercial grade carbon steel specimens, with a composition of Fe, C-0.189, Mn-0.535, Si-0.284, P-0.043, S-0.031, Cr-2.75, Ni-0.148, Cu-0.043, Al-0.358, Ti-0.006, Sn-0.178 per cent, and measuring 30 £ 20 £ 1 mm; were used for weight loss tests and open circuit potential (OCP) measurements. The specimens were polished in standard sequence using different grade of emery papers down to metallographic grade of 000, degreased with trichloroethylene and washed with distilled water before immersion into the test solution. All inhibitor solutions were prepared with distilled water and analytical grade reagents. Different batches of specimens were immersed in 300 ml inhibitor solution contained in a 500 ml corrosion cell that was maintained at a temperature of 308C. The test solution was kept quiescent. The cell was tightly closed, but occasionally opened for OCP decay measurements. In certain cases where no appreciable corrosion was noticed, the dissolved iron present in the test solution was estimated by thiocyanate colorimetric method. In this procedure, the ferric ion content of the test solution was determined (Mustafa et al., 1997; Vogel, 1971), using a UV-Visible spectrophotometer manufactured by Shimadzu Co. Japan. A quantity of 10 ml of 20 per cent KSCN was added to a suitable portion of the test solution and the volume was adjusted to 50 ml with distilled water. Absorbance was measured immediately after

Results Comparison of individual inhibitors Table I shows the test results of evaluation of sodium tungstate and sodium silicate, individually, by polarisation and weight loss techniques. In certain cases, where no appreciable corrosion was evident, the weight

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Table I Test results of individual evaluation of sodium silicate and sodium tungstate at 308C, under stagnant conditions Inhibitor composition Sodium silicate Sodium tungstate (ppm) (ppm) 10 100 1,000 1,0000 0 0 0 0 0

0 0 0 0 10 100 1,000 10,000 0

OCP(V)

Ecorr (V)

Icorr (mA/cm2)

Weight loss (mpy)

Corrosion rate Colorimetry (mpy)

Polarisation (ipy)

2 0.510 2 0.470 2 0.390 2 0.220 2 0.400 2 0.355 2 0.265 2 0.195 –

2 0.451 2 0.434 2 0.429 2 0.351 2 0.310 2 0.295 2 0.289 2 0.250 –

0.2151 0.0982 0.0346 0.0095 0.1810 0.0730 0.0264 0.0062 –

1.747 1.422 – – 1.806 1.500 – – 2.7824

1.850 1.520 0.0850 0.0078 1.890 1.625 0.1012 0.0092 2.998

0.1986 0.0907 0.0320 0.0088 0.1672 0.0674 0.0244 0.0058 –

loss tests could not be completed, and the iron present in the solution was estimated colorimetrically in such cases. The corrosion rate determined based on weight loss tests, colorimetric estimation and polarisation experiments, with respect to logarithmic concentration of both sodium silicate and sodium tungstate, can be compared in the Table. Weight loss measurements were made after an immersion period of 1,000 h. The corrosion potential of carbon steel in sodium silicate decreased marginally in a cathodic direction with increase in the silicate concentration. A similar trend was observed for carbon steel immersed in sodium tungstate solutions, though these values were slightly more anodic than were the potentials of carbon steel samples in sodium silicate at each relative concentration. The corrosion rates calculated by both colorimetric and polarisation techniques were significantly lower when the individual inhibitor concentration was 1,000 ppm and above. However, severe localised attack was evident from visual examination of specimens that had been immersed in 1,000 ppm of the individual inhibitor solutions. Still higher concentrations, of the order of 10,000 ppm, may yield higher inhibition efficiency, but the cost factor makes such concentrations unattractive for commercial use. Since no appreciable inhibition efficiency was obtained with lower concentrations of sodium silicate or sodium tungstate applied individually, the present study proposed to determine the optimum synergistic combination. In this context, a minimum combined concentration of 1,000 ppm of both the inhibitors was selected for formulating synergistic combinations to be evaluated in the study, keeping in mind the necessity to

achieve excellent corrosion inhibition for very longer periods. The OCP values present in the table were noted prior to the commencement of each polarisation experiments. In parallel, the OCP of each specimen was monitored continuously in a separate set of experiments, some of which are included in Figure 1. The variation of OCP, as a function of duration of immersion, was recorded with each 1,000 ppm of sodium silicate and sodium tungstate, and is included in this figure. A gradual decrease in the OCP of the specimen immersed in 1,000 ppm of sodium silicate solution was observed after a period of 5 days, indicating the onset of corrosion attack, which was confirmed visual observation of localised attack. Similarly, the OCP corresponding to 1,000 ppm of sodium tungstate also shifted in a cathodic direction after 3 days, though initially it exhibited comparatively higher OCP values. Hence, higher concentrations of these individual inhibitors, or an optimum synergistic combination of these two inhibitors, were required for effective protection of the specimen. Evaluation of the synergistic combination In parallel to the evaluation of different synergistic combinations of sodium silicate and sodium tungstate by different techniques, the OCP values of such combinations were monitored continuously beyond an immersion period of 100 days. Based on the results of the preliminary experiments, different proportions of sodium silicate and sodium tungstate, with a total concentration of 1,000 ppm, were chosen for detailed evaluation. Variations of OCP values of the systems are compared in Figure 1. A similar

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Figure 1 (a) Variation of OCP as a function of duration of immersion, recorded for individual and synergistic combinations of sodium silicate (SS) and sodium tungstate (ST). A. (Blank), B. (1,000 ppm SS), C. (1,000 ppm ST), D. (200 ppm SS + 800 ppm ST), E. (400 ppm SS + 600 ppm ST), F. (600 ppm SS + 400 ppm ST), G. (800 ppm SS + 200 ppm ST), H. (1,600 ppm SS + 400 ppm ST), K. (400 ppm SS + 100 ppm ST); (b) A typical polarisation curve obtained with the synergistic combination

trend of OCP decay was observed for the four systems shown in the figure (curves D, E, F and G) throughout the entire period of immersion. The steady OCP reached after around 5 days of immersion was maintained throughout the experiment beyond 100 days. Moreover, the OCP was observed to increase with increase in concentration of tungstate after any particular period. Earlier observations suggest that this was attributable to the individual behaviour of sodium tungstate, in which steel exhibits a more anodic initial OCP compared to immersion in sodium silicate (curves B and C). For comparison, other higher and lower concentrations of the combinations (total 2,000 ppm and 500 ppm) with 4:1 ratio of sodium silicate to sodium tungstate are also presented in the same Figure (curve H and K). A combination of total 500 ppm did not maintain a steady OCP, even for 1 day. The other combination (curve H) of total 2,000 ppm also was not found superior to the corresponding combination of total

1,000 ppm (curve G). Hence, a 1:4 combination of sodium silicate and sodium tungstate, at a total concentration of 1,000 ppm, seemed to be optimal, based on these OCP decay experiments (curve D). Corrosion rates, as determined by polarisation experiments for individual and synergistic combinations of sodium silicate and sodium tungstate (total 1,000 ppm), are shown in Figure 2. Even though the corrosion rate corresponding to all synergistic combinations were lower than those of the individual inhibitors, a slight increase in the corrosion rate corresponding to the combination of 1:1 ratio also was noticed. Additionally, based on the results of the polarisation experiments, a 1:4 ratio of sodium silicate to sodium tungstate seemed to be the best synergistic combination. This could be confirmed if the 1:4 solution showed more effective inhibition based on long term tests such as weight loss measurements also. However, weight loss experiments gave negligible differences even after an immersion

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Figure 2 Variation of corrosion rate determined based on polarisation experiments with different ratios of SS and ST (total 1,000 ppm)

period of 6 months, and therefore the corrosion rates of the synergistic combinations were determined by colorimetric estimation of the iron present in the solution (Figure 3). The colorimetric estimations were carried out after different periods of immersion of the specimens. From these results, the rates determined after 25 days of immersion are compared in the Figure. Even though 1:4 Figure 3 Variation of corrosion rate determined based on colorimetric estimation experiments with different ratio of SS and ST total concentrations: A. (1,000 ppm), B. (2,000 ppm), C. (100 ppm)

combination of sodium silicate and sodium tungstate exhibited the highest anodic OCP (Figure 1) and the highest inhibition capability, as based on polarisation experiments (Figure 2), its inhibition efficiency determined by colorimetric estimation appeared to be the lowest among all the combinations studied. Here 4:1 ratio of sodium silicate and sodium tungstate exhibited the highest inhibition efficiency, irrespective of total inhibitor concentration. Higher concentrations of these combinations (total 2,000 ppm) indicated lower efficiency, although similar trends of change in corrosion rate were evident. When the total concentration of inhibitor was reduced to 100 ppm, rusting was evident in all the cases. For these combinations, the corrosion rates determined by colorimetric estimation after 5 days are also compared in Figure 3. A similar trend also was observed for the total 100 ppm combinations by actual weight loss data obtained after an immersion period of 25 days (not shown in the figure). Thus, in fact, a 4:1 ratio of sodium silicate and sodium tungstate was determined to be the best synergistic combination. Though all the inhibitor combinations perform well, the 4:1 ratio (800 ppm sodium silicate + 200 ppm sodium tungstate) was selected for further study, and was the most acceptable in terms of inhibition efficiency and industrial/commercial value. In situ addition of excess inhibitor The influence on the effect of passivation of further in situ additions of sodium silicate or sodium tungstate was evaluated by monitoring the OCP shift. Figure 4 shows the effects after 4 days of immersion of further addition of sodium silicate, which were added to the optimum inhibitor combination (800 ppm sodium silicate + 200 ppm sodium tungstate). For comparison purpose, the effect of addition of sodium silicate to 2,000 ppm of individual sodium tungstate solution is also included in the figure. The addition of excess sodium silicate to the optimum combination significantly improved the trend of OCP shift. The OCP of the optimal combination without addition of excess silicate was 2 0.120 V after 2 weeks (curve A), and rose to a value of 2 0.084 V when excess sodium silicate was added (curve A1). However, the addition of sodium silicate to the inhibitor solution of 2,000 ppm of sodium tungstate caused an immediate shift in

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Figure 4 Variation of OCP as a result of in situ addition of excess SS. Y – time of immersion, Y1 – time of excess addition, A. (800 ppm SS + 200 ppm ST), B. (2,000 ppm ST), A1, B1 – Addition of 1,000 ppm excess SS

OCP to a more negative value. Such initial depression of the curves due to the excess addition of sodium silicate may be attributable to the role of silicates, which establishes the OCP in the more negative region, compared to the OCP value typical of sodium tungstate solutions (Figure 1). The effect of individual additions of excess sodium tungstate to the optimum inhibitor combination is shown in Figure 5. For comparison, the effect of its addition to 2,000 ppm individual sodium silicate solution is also included in the figure. The addition of excess sodium tungstate did not bring any significant immediate shift in the OCP.

While there was a negligible effect due to the addition of excess sodium tungstate to the 2,000 ppm sodium silicate solution, its addition to the optimum inhibitor combination raised the OCP substantially. Effect of depletion of inhibitor solution The OCP’s of different batches of specimens were monitored continuously to evaluate the change due to the influence of depletion of the inhibitor solutions. Two cases of such OCP shifts when the inhibitor solution was depleted to 50 per cent of its original concentration are shown in Figure 6. As a period of minimum 5 days was observed

Figure 5 Variation of OCP as a result of in situ addition of excess ST. Y – time of immersion, Y1 – time of excess addition, A. (800 ppm SS + 200 ppm ST), B. (2,000 ppm SS), A1, B1 – Addition of 1,000 ppm excess ST

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Figure 6 Variation of OCP due to depletion of the inhibitor solution (50 per cent depletion). Y – time of immersion, Y1 – time of excess addition, A. (800 ppm SS + 200 ppm ST), B. (1,600 ppm SS + 400 ppm ST)

to be required for effective passivation, depletion of inhibitor solution was also done after 5 days of immersion of the specimen in its original inhibitor solution. It was found that the OCP was not retained after depletion of the optimum inhibitor combination, even for 1 day: viz. 800 ppm sodium silicate + 200 ppm sodium tungstate (curve A). However, the OCP remained essentially unchanged for more than 7 days when the concentration of the original inhibitor solution was high (1,600 ppm sodium silicate + 400 ppm sodium tungstate).

Variation of pH Figure 7 shows the variation of pH as a function of time for individual and mixtures of inhibitor solutions. All the curves begin from zero time, i.e. at the time of immersion, which is free from interference by any corrosion products. In the case of the test solution containing 1,000 ppm sodium tungstate alone, the pH was found to increase with increase in time up to 5 days, the period required for completion of passivation (curve D). Reduction of dissolved oxygen in the presence of tungstate caused an initial increase in pH, after which it attained an almost stable value. In contrast to tungstate, the pH for carbon steel in the 1,000 ppm sodium silicate solution showed a sudden

decrease to the extent of almost one pH value. In the case of three other combinations too, in all of which the ratio of silicate to tungstate was high, a steady decrease in pH corresponding to the effect of mixed sodium silicate and sodium tungstate was observed. This indicated the effective involvement of sodium silicate in all the synergistic combinations, including the optimum combination, viz. 800 ppm sodium silicate + 200 ppm sodium tungstate. Thus, the trend of overall change in pH for the inhibitor combinations was similar to the trend of variation of OCP vs time of the corresponding inhibitor combinations.

Spectral analysis The FTIR spectra recorded for the passive layer corresponding to three different inhibitor solutions, namely (A) 1,000 ppm sodium tungstate, (B) 1,000 ppm sodium silicate and (C) 800 ppm sodium silicate + 200 ppm sodium tungstate are compared in Figure 8. The spectrum recorded for 1,000 ppm sodium silicate predominantly exhibited four peaks at 3401, 1618, 1384 and 1018 cm2 1. While the peak at 3401 cm2 1 indicated the strong broad absorption due to -OH stretching of water, the peak at 1618 cm2 1 indicated -OH bending. The peak at 1384 cm2 1 confirmed the metal oxide in

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Figure 7 Variation of pH with time for individual and various combinations of SS and ST. A. (1,000 ppm SS), B. (1,600 ppm SS + 400 ppm ST), C. (800 ppm SS + 200 ppm ST), D. (1,000 ppm ST), E. (80 ppm SS + 20 ppm ST)

Figure 8 FTIR spectra of passive layers formed by different inhibitor solutions. A. (SS), B. (ST), C. (800 ppm SS + 200 ppm ST)

the passive layer while the peak at 1018 cm2 1 confirmed the existence of the orthosilicate in the passive layer. The spectrum obtained for the passive layer, corresponding to 1,000 ppm sodium tungstate alone, also showed two significant peaks at 1383 and 1018 cm2 1, in addition to two other peaks corresponding to -OH stretching and -OH bending of water at 3402 and 1636 cm2 1, respectively (Nyquist and Kagel, 1971). While the peak at 1383 cm2 1 indicated the metal oxide of the passive layer, the peak at 1018 cm2 1 confirmed the metal tungstate incorporation in the passive layer. The spectrum obtained for the optimum combination also did not reveal any significant peak, other than those obtained with individual sodium silicate or sodium tungstate. In this context, it was

proposed to analyse the passive layer formed initially by individual sodium silicate followed by addition of sodium tungstate after sufficient duration of immersion. The passive layer corresponding to the reverse addition i.e. sodium tungstate followed by addition of sodium silicate was also analysed by FTIR spectroscopy. Even though those two spectra resembled each other in terms of the peak position, the peak corresponding to metal oxide at 1384 cm2 1 became more pronounced (not shown in figure), when sodium silicate was added to sodium tungstate. This indicated the more significant role of sodium silicate over sodium tungstate in the synergistic combination. This finding was supported by other test results such as the OCP variations also. Thus, it may be concluded that it was the metal oxide layer, incorporated with tungstate and silicate, which caused effective passivation.

Evaluation at higher temperatures Figure 9 shows the variation of OCP values with time for steel specimens immersed in the optimum inhibitor combination (800 ppm sodium silicate + 200 ppm sodium tungstate) at different temperatures, with and without agitation. Variation of OCP of the specimens subjected to continuous heating at 608C and 758C are indicated by the curves A and E. In the case of continuous heating at 608C, a highly anodic OCP of the specimen was

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Figure 9 Variation of OCP of the mild steel specimens in the inhibitor solution containing 800 ppm SS + 200 ppm ST under different experimental conditions. A. with continuous heating at 608C, B. with intermittent heating between 308C and 758C, C. under stirring conditions at 308C, D. with intermittent heating and stirring, E. with continuous heating at 758C

observed. The specimen remained unattacked even after an immersion period of 1 month. When the temperature was raised by 158C, the performance of the inhibitor combination decreased and the OCP values shifted substantially to more cathodic values after about 10 days, indicating the onset of corrosion attack on the carbon steel samples. Thus, studies were repeated with intermittent heating conditions, between 758C and 308C (Here the specimen was subjected to continuous heating at 758C for 12 h, followed with cooling up to 308C for the next 12 h.). The curves D and B represent the OCP variations with time for the specimens in the inhibitor combination that were subject to intermittent heating, with and without continuous agitation, using a magnetic stirrer that rotated at a speed of 750 rpm. In both the cases, very effective passive films were formed and the specimens remained unattacked. A similar trend was obtained when the specimen was subjected to continuous agitation at 308C (curve C).

Discussion The performances of all the inhibitor combinations in the present study were observed to be considerably better than the individual inhibitors of the corresponding composition. The steady OCP reached after

around 5 days of immersion indicated effective passivation, which was maintained throughout the experiment beyond 100 days. Commencement of passivation commenced with the initial OCP, which increased still further, indicating increasingly effective passivation with longer sample immersion time. However, daily observations noted for the specimens immersed in individual inhibitor solutions of 1,000 ppm revealed severe localised (pitting) attack. Such localised attack commenced on the metal surfaces when they were in inhibitor solutions of less than the threshold concentration for complete protection. The precise concentration for complete protection may vary, depending on the metal/inhibitor system chosen (Abd el Khader et al., 1998a, b; Sastri et al., 1989, 1991). The localised attack evident in the cases of 1,000 ppm of the individual inhibitors may have been due to the presence of mild imperfections in the surfaces of the carbon steel specimens, or due to insufficient concentration of the inhibitors in solution. Although 1,000 ppm of either individual inhibitor showed more negative OCP values with time, they exhibited comparatively low corrosion rate values during both polarisation and colorimetric experiments. After arriving at the optimum synergistic combination, studies were also done to evaluate the effect of change in concentration of the inhibitor systems on the degree of effective passivation. The initial depression of the curves, due to the in situ addition of excess sodium silicate, may have been attributable to the role of the silicate, which established a more negative OCP than was the case with sodium tungstate. This indicated that there was an immediate interaction between the silicate ions with the already formed passive film. Although addition of excess sodium tungstate did not show any significant influence with the already-passivated specimen, its favourable influence in longer term cannot be ruled out. The depletion experiments were carried out in order to evaluate the extent of corrosion attack on metals if the inhibitor solution were to become depleted unintentionally in an industrial application. The optimum combination was observed to be highly effective at moderate temperature (608C), and under intermittent heating conditions between 758C and

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308C, with and without agitation. The comparatively lower performance of the inhibitor combination when subjected to continuous heating at high temperature (758C) may have been due to the decreased solubility of oxygen in water at high temperatures, and passivation was harder to achieve at higher temperatures. The simultaneous presence of both tungstate ions and oxygen in the solution was necessary for tungstate to act as an inhibitor, and inhibition was achieved by the formation of an orderly arrangement of the two species on the metal surface. As in the case of tungstate, silicate ions require the presence of oxygen for inhibition and oxygen acts as the main passivator. The behaviour of sodium silicate in aqueous solution has been of interest because of the wide variation in properties these solutions exhibit. Monomeric and dimeric silicate species polymerise to form cyclic poly-silicate anions, especially under alkaline conditions, and the distribution of the ionic species changes during ageing of the solution. Consequently, it might be expected that the silicate species, acting as ligands, would exhibit a range of reactivities towards any cations in solution, which would be evident from variation in the pH of these solutions with time. Any sudden decrease in pH of the silicate solution with time may be due to depletion in the concentration of silicate ions, due to their interaction with metal ions leading to partial complexation (Iler, 1979). A steady decrease in the pH, even after completion of passivation, indicates the mild and continuous consumption of the inhibitors due to complexing with the surface oxide film. The silicate ions facilitate adsorption of oxygen, which results in effective passivation. In addition to the formation of passive films of this kind, protection is supplemented by the development of diffusion barrier films of iron silicate also (Uhlig, 1965). The spectral evaluations of the passive films obtained in the present study also were in agreement with the earlier observations. Thus, it has been demonstrated that tungstate and silicate can be used to commercial advantage in a synergistic combination for the inhibition of corrosion on carbon steel in near-neutral and alkaline systems.

Conclusions The inhibition efficiency of individual sodium silicate, sodium tungstate and their synergistic combinations on carbon steel has been investigated by several independent techniques. All combinations of these compounds exhibited higher inhibition efficiencies than did either of the individual inhibitors. Even though 1:4 ratio of sodium silicate to sodium tungstate (total concentration 1,000 ppm) appeared to offer the best based on short-term performance, the reverse combination (800 ppm sodium silicate + 200 ppm sodium tungstate) was identified by weight loss data and overall performance criteria to be the optimal combination. The optimal combination also was quite effective at higher solution temperatures, both with and without intermittent heating, or stirring. The addition of excess individual inhibitors to the optimum combination enhanced significantly the trend in OCP shift. Depletion of the test solution resulted in damage to the passive layer after a period, the duration of which depended upon the concentration of the original inhibitor mixture. FTIR spectra confirmed that the metal oxide layer, incorporating the tungstate and silicate ions was the primary cause of effective inhibition.

References Abd el Kadher, J.M., El Warraky, A.A. and Abd el Aziz, A.M. (1998a), “Corrosion inhibition of mild steel by sodium tungstate in neutral solutions. Part-1: Behaviour in distilled water”, British Corrosion Journal, Vol. 33 No. 2, pp. 139-44. Abd el Kadher, J.M., El Warraky, A.A. and Abd el Aziz, A.M. (1998b), “Corrosion inhibition of mild steel by sodium tungstate in neutral solutions. Part-3: Co-inhibitors and synergism”, British Corrosion Journal, Vol. 33 No. 2, pp. 152-7. Chen, J.R., Chao, H.Y., Lin, Y.L., Yang, I.J., Oung, J.C. and Pan, F.M. (1991), “Studies on carbon steel corrosion in molybdate and silicate solutions as corrosion inhibitors”, Surface Science, Vol. 247 No. 2-3, pp. 352-9. Iler, R.K. (1979), The Chemistry of Silica, Wiley, New York, p. 667. James Veil (1952), Soluble Silicates, Reinhold, USA, Vol.1, p. 267. Mustafa, C.M., Shahinoor, S.M. and Islam, Dulal (1997), “Corrosion behaviour of mild steel in moderately alkaline to acidic simulated cooling water containing molybdate and nitrite”, British Corrosion Journal, Vol. 32 No. 2, pp. 133-7.

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Nyquist, A.R. and Kagel, O.R. (1971), Infrared Spectra of Inorganic Compounds, Academic Press, USA. Oung, J.C. and Wang, T.Y. (1988), “Study on molybdate based inhibitors in a synthetic cooling water system”, Corros. Australas, Vol. 13 No. 1, pp. 19-21. Oung, J.C., Chin, S.K. and Shih, H.C. (1998), “Mitigating steel corrosion in cooling water by molybdate based inhibitors”, Corrosion Prevention and Control, Vol. 45 No. 5, pp. 156-62. Pryor, M.J. and Cohen, M. (1953), “Inhibition of the corrosion of Fe by anodic inhibitors”, Journal of Electrochemical Society, Vol. 100, pp. 203-15. Robertson, W.D. (1951), “Molybdate and tungstate as corrosion inhibitors and the mechanism of inhibition”, Journal of Electrochemical Society, Vol. 98, pp. 94-100.

Sastri, V.S. and Bednar, J.S. (1990), “Corrosion inhibition in coal-water slurries”, Material Performance, Vol. 29 No. 5, pp. 44-6. Sastri, V.S., Tjan, C. and Roberge, P.R. (1991), “Corrosion inhibition by tungstate in aqueous solutions and its applications in coal water slurries”, British Corrosion Journal, Vol. 26 No. 4, pp. 251-4. Sastri, V.S., Packwood, R.H., Brown, J.R., Bendnar, J.S., Galbraith, L.E. and Moore, V.E. (1989), “Corrosion inhibition by some oxy anions in coal water slurries”, British Corrosion Journal, Vol. 24 No. 1, pp. 30-5. Uhlig, H.H. (1965), Corrosion and Corrosion Control, Wiley, p. 260. U.S. Patent (1939) No. 2, 147, 395. Vogel, A.I. (1971), A Text Book of Quantitative Analysis, Longman, London, p. 690.

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