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MATERIALS AND INTERFACES Investigation of Low-Toxic Organic Corrosion Inhibitors for CO2 Separation Process Using Aqueous MEA Solvent Amornvadee Veawab* and Paitoon Tontiwachwuthikul Process Systems Laboratory, Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada S4S 0A2
Amit Chakma Office of VP Acadamic and Provost, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
This work investigated the possibility of using low-toxic corrosion inhibitors, instead of heavymetal inhibitors, for CO2 separation using aqueous solutions of monoethanolamine (MEA). The performances of eight low-toxic organic inhibitors (amines, carboxylic acid, and sulfoxide) were evaluated by conducting electrochemical corrosion experiments with carbon steel-1020 specimens immersed in 3.0 kmol/m3 MEA solutions under CO2 saturation at 40 and 80 °C. The experimental results showed that carboxylic acid had the best inhibition performance (as high as 92%), followed by sulfoxide and long-chain aliphatic amine. Their performances depended on inhibitor concentration and temperature. 1. Introduction The removal of carbon dioxide (CO2) from industrial gas streams using amine treating units has never been considered a problem-free technology despite the fact that it has been used in industry for over 50 years. The process is constantly subjected to a number of operational difficulties, the most severe of which is corrosion of the process equipment.1 Because of the tremendous impact of corrosion, suppression of excessive corrosion to an acceptable level becomes necessary for amine treating units. Generally, the corrosion problems can be reduced by a number of approaches including (i) the use of proper equipment design, (ii) the use of corrosionresistant materials instead of carbon steel, (iii) the removal of solid contaminants from liquid solution, and (iv) the use of corrosion inhibitors.2-4 Of these alternatives, the use of corrosion inhibitors is considered the most economical technique for corrosion control.5-8 Various corrosion inhibitors have been developed, patented, and commercialized by many major chemical companies for uses in amine treating units. The patented organic inhibitors include thiourea and salicyclic acid, while the inorganic inhibitors are vanadium, antimony, copper, cobalt, tin, and sulfur compounds. Inorganic inhibitors are more favored, in practice, than the organic compounds because of their superior inhibition performance. Vanadium compounds, particularly sodium metavanadate (NaVO3), are the most extensively and successfully used in amine treating plants. The corrosion rate in a monoethanolamine (MEA) plant was reduced to less than 1 mpy (0.0254 mm/year) when 1300 ppm (mg/kg) of the vanadate compound was used.5 * To whom correspondence should be addressed. Tel: (306) 585-5665. Fax: (306) 585-4855. E-mail:
[email protected].
Despite the successful use of the inorganic corrosion inhibitors, concerns about the impacts of their toxicity on human health and environment have recently increased. As shown in Table 1, the toxicity index (lethal dose LD50) of inorganic corrosion inhibitors (vanadium compounds) is much lower than that of the absorption solvents. This indicates that the corrosion inhibitors exhibit relatively greater toxicity than the absorption solvents themselves. According to the U.S. Environmental Protection Agency (EPA), corrosion inhibitors containing vanadium, antimony, copper, and thiocyanate compounds are considered to be hazardous substances and priority/toxic pollutants under the Clean Water Act (CWA), whereas those containing thiocyanate and cobalt compounds are categorized as hazardous air pollutants under the Clean Air Act (CAA). Many countries and regions around the world are introducing a series of controlling regulations to provide guidelines on the use and discharge of the toxic substances, especially inorganic salts and salts of heavy metals. In Europe, for example, the European Economic Community (EEC) has charged the Paris Commission (PARCOM) with providing a framework for legislation.6,10 Other highlighted regulations include the Emergency Planning and Community Right to Know Act of 1986, regulations adopted by the U.S. Occupational Safety and Health Administration (OSHA) in 1993, and adoption of the Chemical Hazard Assessment and Risk Management (CHARM) model in the U.K. and other European countries.11 In Canada, uses of toxic substances including several inorganic heavy metals are controlled under the Canadian Environmental Protection Act (CEPA). Because of the adopted regulations, the use of toxic corrosion inhibitors makes the disposal of the industrial wastes difficult and costly.1,11 To respond to the envi-
10.1021/ie010248c CCC: $20.00 © 2001 American Chemical Society Published on Web 10/10/2001
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Table 1. Information on Toxicity of Absorption Solvents, Conventional Inhibitors, and Test Inhibitors (Acros Chemicals) LD50-orala (mg/kg) chemical absorption solvents monoethanolamine (MEA) diethanolamine (DEA) methyl diethanolamine (MDEA) diisopropanolamine (DIPA)
rabbit
mouse
1000 2200 4780 4765
700 3300
conventional inhibitors vanadium pentaoxide sodium metavanadate ammonium metavanadate organic inhibitors investigated A imidazole (aromatic amine with two N atoms) B piperazine (aromatic amine with two N atoms) C hexamethyleneimine (aromatic amine with one N atom) D cycloxylamine (aromatic with one N atom) E 2,4-lutidine (aromatic with one N atom) Fb (long-chain aliphatic amine) Gb (carboxylic acid) Hb (sulfoxide)
rat 1720 710
23 74.6 25
10 98 58.1
880 600
200 1900 410 156 200 2500 1960
224 5500
1600 750
a
LD50 (lethal dose) is the dose large enough to kill 50% of a sample of animals under test and is normally quoted in units of mg/kg, where kg refers to the body weight of the animal concerned.9 b Exact names proprietry.
ronmental concern and reduce the cost of waste disposal, as well as prepare for more stringent regulations for chemical uses, low-toxic corrosion inhibitors have to be developed. The objective of this work was to explore the possibility of using low-toxic chemical substances instead of the heavy-metal corrosion inhibitors that are commonly used in amine treating units. The following characteristics must all be thoroughly examined if an ideal inhibitor is to be found: (i) low toxicity in comparison with that of current corrosion inhibitors; (ii) ability to suppress excessive corrosion rates (inhibition performance); (iii) compatibility with all chemical species in the system; (iv) no undesirable environmental impacts; (v) no undesirable side effects on plant operation, such as foaming and formation of emulsions, sludges, and precipitates; and (vi) ability to tolerate traces of oxygen and other acid gases such as H2S, NOx, and SOx when used for the removal of CO2 from flue gas. However, this work focuses only on a preliminary evaluation of inhibition performance of selected potential low-toxic substances. The results of the investigation will pioneer the extensive tests for effective low-toxic corrosion inhibitors that can lead to more environmentally friendly plant operation. 2. Chosen Corrosion Inhibitors Organic corrosion inhibitors are generally more environmentally friendly than inorganic ones. Eight organic inhibitors (Table 1) were chosen on the basis of toxicity level for performance evaluation. According to their lethal doses (LD50), the chosen organic inhibitors exhibit lower toxicities than the current inhibitors (vanadium compounds), although some are more toxic than the absorption solvents. Inhibitors A-F are amines that have nitrogen functional groups with different molecular structures, i.e., aromatic and long-chain aliphatic. Inhibitor G is a carboxylic acid with oxygen and nitrogen reaction centers, and inhibitor H is a sulfoxide containing a sulfur functional group. In addition, the test inhibitors also include sodium metavanadate (VND), which is a
Table 2. Test Conditions for Performance Evaluation Experimentsa temperature (°C)
corrosion inhibitor
inhibitor concentrations (ppm)
80
VND A B C D E F G H
1000 1000, 5000 1000, 5000 1000 1000, 5000 1000, 5000 1000, 5000 100, 500, 1000, 5000 1000
40
VND F G H
1000 1000 1000 1000
a Samples immersed in 3.0 kmol/m3 MEA solution under CO 2 saturation.
conventional inhibitor commonly used in the CO2 removal process. 3. Experiments The performance of chosen corrosion inhibitors was evaluated by conducting the electrochemical corrosion experiments under the test conditions summarized in Table 2. The experiments were carried out with carbon steel-1020 specimens immersed in uninhibited and inhibited 3.0 kmol/m3 aqueous solutions of monoethanolamine (MEA) under CO2 saturation. Results from the experiments were generally interpreted in terms of percent inhibition (PI) indicating the reduction in corrosion rate resulting from the addition of inhibitor. This was determined by the equation12
[
PI ) 1 -
]
CRinhibited × 100% CRuninhibited
(1)
where CRinhibited is the corrosion rate in the inhibited system and CRuninhibited is the corrosion rate in the uninhibited system. It should be noted that the corrosion rates used in this paper are the average values with an uncertainty of (5% of the replicated data.
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4773 Table 3. Chemical Compositions of Test Specimens chemical composition
Figure 1. Schematic diagram of experimental setup using electrochemical techniques.
3.1. Experimental Apparatus. Figure 1 is a schematic diagram of the corrosion experimental setup using the electrochemical techniques. The setup consists of a number of components: corrosion cell, potentiostat, data acquisition system, water bath equipped with temperature controller, gas supply set, condenser, and pH meter. The corrosion cell is a standardized three-electrode cell approved by the ASTM [model K47 from EG&G Instruments Corporation (PerkinElmer Instruments), Oak Ridge, TN]. A model 273A potentiostat (EG&G), providing an accuracy of (0.2% of potential and current readings, was used in this work. The data acquisition system, model 352 SoftCorr III (EG&G), was installed on a Pentium IBM-compatible computer for control of the potentiostat and also for recording and analysis of the produced corrosion data. A water bath with a temperature controller was used to maintain a constant water temperature, to within (0.1 °C, during the experiment. To minimize heat loss to (or gain from) the surroundings, the water surface was covered with hollow balls. The gas supply set was the source of CO2 introduced into the corrosion cell to simulate the test environment through a series of gas flow meters that can measure the gas flow with (2% accuracy. An Allihn condenser (jacket length of 300 mm and height of 445 mm) was connected to the corrosion cell to prevent any change in solution concentration due to evaporation. The pH meter, model P-59003-10 (Cole-Parmer, Vernon Hills, VA) was used to measure the pH value of the test solution with an accuracy of (0.01. 3.2. Specimens and Solutions. Carbon steel-1020 was chosen for corrosion inhibitor tests because it is the most commonly used material for the construction of amine treating plants. The carbon steel specimens were manufactured as cylindrical elements with dimensions of 12.7-mm length and 9.50-mm diameter. To accommodate the specimen holder for mounting purposes, the specimens were drilled to a depth of 6.40 mm and tapped to accept a 3-48 UNC-2A 8.00-mm minimum thread depth. The specimens were prepared by wet grinding with 600 grit silicon carbide papers using deionized water in accordance with the ASTM standard E3-80.13 After grinding, the dimensions of each specimen were measured by a vernier caliper with an accuracy of (0.03 mm. The specimens were then degreased with high-purity methanol and dried with hot air. The prepared specimens were kept in a desiccator
type of specimen
component
amount (%)
carbon steel-1020
C Mn P S Si Fe
0.20 0.51 0.013 0.039 0.17 balance
until use in the experiments. Their chemical compositions are given in Table 3. The CO2 absorption solvent used in this work was MEA purchased from Sigma-Aldrich, Oakville, Ontario, Canada. The pure MEA (99 wt %) was diluted with deionized water to the desired concentration (3.0 kmol/ m3). The exact solution concentration was determined by titration with a 1.0 kmol/m3 standard hydrochloric acid (HCl) solution using methyl orange as the indicator. CO2-saturated solution was prepared by purging CO2 gas into the fresh solution for approximately 8 h. The CO2 content in solution was determined by a standard method given by the Association of Official Analytical Chemists (AOAC).14 This method involved acidification of a precisely measured quantity of the sample through the addition of excess HCl solution. The CO2 gas released was then collected in a precision gas buret. The amount of released CO2 was later used to calculate the CO2 loading of the amine solution. 3.3. Experimental Procedure. The experiments were carried out in accordance with ASTM test standard G5.15 Each test began with immersion of the corrosion cell containing a test solution and specimen in a water bath, whose temperature was set and controlled to within an accuracy of (0.1 °C. A stream of CO2 gas was introduced into the corrosion cell at a flow rate of 150 mL/min to maintain the prepared CO2 content in the test solution. The potential of the specimen was measured relative to a calomel reference electrode and recorded as a function of time. Once the potential remained constant for at least 5 min, the potentiodynamic polarization was initiated with a scanning rate of 0.60 V/h. This basically caused the corrosion cell to produce electrical currents, which were recorded in a computer file together with the applied potentials. During the test, both potentials and currents were also plotted as a polarization curve to allow for the observation of corrosion behavior. Finally, the corrosion rate was determined from the plot by using the Tafel extrapolation technique, which involves extrapolation of currents in anodic and cathodic Tafel regions to the corrosion potential and corrosion current. The obtained corrosion current then can be converted to corrosion rate using the equation
CR )
0.129icorra nD
(2)
where CR is the corrosion rate in mpy (mils per year), icorr is the corrosion current density in µA/cm2, a is the atomic weight, n is the number of electrons, and D is the density of the specimen in g/cm3. Details of the Tafel extrapolation technique can be found in Baboian (1995).16 4. Results and Discussion The experimental results are primarily presented to show how the inhibition performance is affected by
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parameters such as inhibitor type, inhibitor concentration, and system temperature. However, some general observations on system characteristics that were made during the experiments should be discussed first. 4.1. General Observations. The characteristics of both inhibited solutions and test specimens were observed in this work. The solution observations, providing us with valuable information on how the system would respond to the presence of test inhibitors, were made when the inhibited solutions were prepared. It was found that, at a concentration of 1000 ppm, most of the test inhibitors dissolved very quickly and completely in the aqueous amine solutions. Only one inhibitor (sulfoxide, inhibitor H) had difficulty dissolving at room temperature. The application of a certain amount of heat was required to facilitate the dissolution of this inhibitor. Complete dissolution of inhibitor H could not be achieved at a concentration of 5000 ppm, where the other inhibitors still dissolved readily. The appearance of the solutions was also observed. The inhibited solutions remained colorless (clear), with no formation of emulsions, foams, or precipitates. The appearance of most solutions generally remained unchanged during the tests. This could be an indication of the compatibility of the test inhibitors with aqueous amine-CO2 environments. However, a solution containing inhibitor C did show an indication of chemical reactivity, i.e., the solution turned orange during the test. Visual observations were also made on the test specimens. Generally, the specimens before the test were clean and shiny. However, after the test, the specimens in most cases were dull and covered by gray and orange products, indicating that a certain amount of corrosion had taken place. This change was not found in the case of solutions containing sodium metavanadate (VND). The VND specimens recovered after the test remained shiny and clean, with no visible corrosion product covering the specimen surface. 4.2. Inhibition Performance. (i) Inhibitor Type. The inhibition performance of eight organic inhibitors (AH) was experimentally evaluated and compared with the performance of the conventional sodium metavanadate (VND). Initially, the evaluation was carried out at 80 °C using MEA solutions containing 1000 ppm of each inhibitor. This inhibitor concentration was chosen on the basis of the range of concentrations recommended in patents and the literature. Figure 2 shows that the conventional inhibitor VND reduced corrosion by as much as 97%, the best result. The organic inhibitors gave a wide range of inhibition performances, varying from 21 to 92% (F-H were better than 75%). The rest of the test inhibitors (A-E) gave relatively low corrosion inhibitions (less than 60%). The inhibition performance could vary with inhibitor concentration, so additional evaluations were conducted at 5000 ppm. Inhibitors C and H were not included because of their limited solubilities. Figure 2 shows that the increase in inhibitor concentration to 5000 ppm had no significant effect on the performance of inhibitors A, B, and D-F, as also shown in the polarization curves (Figure 3). This suggests that maximum inhibition already occurred at a concentration of 1000 ppm. For inhibitor G, the increase in concentration from 1000 to 5000 ppm enhanced inhibition from 92 to 98%, which was comparable to inhibition by the vanadium compound. Judging from these results, inhibitor G is
Figure 2. Comparison of inhibition performance (3.0 kmol/m3 MEA solution, 80 °C, and CO2 saturation of 0.565 mol CO2/mol MEA). The corrosion rate of the uninhibited MEA solution under these conditions is 136 ( 7 mpy.
Figure 3. Polarization curves for systems containing 1000 and 5000 ppm of corrosion inhibitors (3.0 kmol/m3 MEA solution, 80 °C, and CO2 saturation).
considered to be the most promising organic inhibitor, followed by inhibitors F and H. The high inhibition performances of inhibitors F-H can be explained by considering the polarization curves of the inhibited and uninhibited systems (Figure 4). From Figure 4a, the polarization curve of the inhibited system G appears in the passive region where the corrosion potential is much greater than the potential of the uninhibited system. Addition of inhibitor G caused the specimen potential to shift in the noble (positive)
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Figure 6. Schematic diagram of inhibiting function of inhibitors F and H.
Figure 7. Schematic representation of inhibition mechanism by adsorption.18
Figure 4. Polarization curves for 3.0 kmol/m3 MEA solutions in the presence of 1000 ppm of corrosion inhibitors (80 °C and CO2 saturation).
Figure 5. Schematic diagram of inhibiting function of inhibitors G and sodium metavanadate (VND).
direction, and the surface of the test specimen was covered by a passive film. As illustrated in Figure 5, a passive film basically functions as a barrier, inhibiting the oxidation reaction (iron dissolution) at the anodic sites. This inhibition mechanism, commonly known as metal passivation, was also found in the inhibited system VND (Figure 4a). In the case of inhibitors F and H (Figure 4b), the corrosion potential appears in the active region where the polarization curve of the uninhibited system is also located. Generally, no potential shift toward the passive state and no passive film formation resulting from the addition of the inhibitor occurred. Instead, these inhibitors enhanced the corrosion resistance by suppressing
the reduction of active agents (such as HCO3-) at the cathodic sites. As illustrated in Figure 6, when the inhibitor is introduced into the system, it is adsorbed onto the metal surface and prevents the oxidizing agents from undergoing reduction (accepting e-) on the cathodic sites. This phenomenon can be observed from the deviation of the current density at the cathodic polarization curves shown in Figure 4b. The inhibited systems yielded a lower current density than the uninhibited systems. The fact that inhibitor F provided a superior inhibition performance in comparison with the other amine inhibitors A-E (Figure 2) can be explained as follows. Because amine inhibitors function as adsorbates, their inhibition performance primarily relies on the adsorption bond between the metal atoms and the inhibitor molecules.17 The strength of the bond depends on the molecular structure of the inhibitors. When compared with other amines, aliphatic inhibitor F, having a relatively high molecular weight and greater electron density at the nitrogen functional groups, tends to establish stronger bonds with the metal surface (via nitrogen atoms) in a flat configuration. In addition, moieties of the amine molecules that are not on the metal surface might also retard the mass transfer of the active species to the metal-solution interface (Figure 7). This basically inhibits the reduction reaction at the cathodic sites and results in a relatively low cathodic current density in the system containing inhibitor F, as shown in Figure 8. (ii) Inhibitor Concentration. The inhibitor concentration was found to have a significant impact on the inhibition performance, as illustrated in Figure 9. Raising the concentration of the inhibitor G, for example, from 100 to 5000 ppm improved inhibition performance from 23 to 98% in the case of a 3.0 kmol/ m3 MEA solution saturated with CO2 at 80 °C. The increase in inhibition performance can be explained by the anodic and cathodic polarization behavior shown in Figure 10. At the low concentration of 100
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Figure 8. Polarization curves for 3.0 kmol/m3 MEA solutions in the presence of 1000 ppm of amine-group inhibitors (80 °C and CO2 saturation).
Figure 9. Effect of inhibitor concentration on inhibition performance (3.0 kmol/m3 MEA solution in the presence of inhibitor G, 80 °C, and CO2 saturation).
Figure 11. Effect of solution temperature on inhibition performance (3.0 kmol/m3 MEA solutions in the presence of 1000 ppm of corrosion inhibitors under CO2 saturation). The corrosion rate of the uninhibited MEA solution at 40 °C and under CO2 saturation (0.656 mol CO2/mol MEA) is 83 ( 4 mpy.
increased to 500 ppm, the test specimen was covered by a passive film, resulting in a significant change in the corrosion potential, from -0.562 to -0.181 V vs SHE. For the higher concentrations of 1000 and 5000 ppm, the system remained in the passive region, with little change in the corrosion potential. However, the percent inhibition was significantly improved because of the substantial reduction in the current density with the concentrated systems. (iii) Temperature. Figure 11 shows the effect of temperature on the inhibition performance of systems containing conventional VND and the test organic inhibitors. The experimental results show that the temperature had no significant effect on the protection efficiency of the conventional VND. That is, the inhibition performance remained at a high level regardless of the temperature. In contrast, a change in the temperature had a significant effect on the performance of systems containing inhibitors F-H. Increasing the system temperature considerably improved the protection efficiency. Raising the temperature from 40 to 80 °C can double the inhibition performance of the system containing inhibitor G. This might be because higher temperature leads to greater diffusivity of the inhibitor molecules and also lowers the viscosity and surface tension of the solution. As a result, the inhibitors can reach the surface of the metal more effectively. 5. Conclusions
Figure 10. Effect of inhibitor concentration on polarization behavior (3.0 kmol/m3 MEA solution in the presence of inhibitor G, 80 °C, and CO2 saturation).
ppm, no passive film developed on the specimen surface to block the metal dissolution at the anodic sites; therefore, the polarization curve appears in the active region. Generally, the polarization curves of both inhibited and uninhibited systems at this point are very similar. Only a small difference can be noticed in the cathodic polarization. Thus, this indicates a small degree of corrosion inhibition. When the inhibitor concentration
This work responds to the environmental concern over the impacts of using toxic heavy-metal corrosion inhibitors by evaluating the performance of low-toxic organic corrosion inhibitors. The principle conclusions are the following: Organic compounds (amine, carboxylic acid, and sulfoxide) are promising low-toxic corrosion inhibitors for the aqueous amine-CO2 system. They can reduce corrosion by as much as 75-92%. Carboxylic acid gave the best performance, followed by sulfoxide and longchain aliphatic amine. The performance of the organic inhibitors varies with inhibitor concentration. Increasing the inhibitor concentration improves the inhibition performance. Solution temperature has a significant effect on the performance of organic inhibitors. An increase in the
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solution temperature from 40 to 80 °C leads to a greater inhibition performance. In contrast, the performance of the currently used but toxic inhibitor sodium metavanadate remains at a high level, regardless of the temperature. Acknowledgment The Natural Sciences and Engineering Research Council of Canada (NSERC) and Saskferco Products Inc. are gratefully acknowledged for financial support and analytical equipment. Literature Cited (1) Kohl, A. L.; Nielsen, R. B. Gas Purification, 5th ed.; Gulf Publishing Company: Houston, TX, 1997. (2) Smith, R. F.; Younger, A. H. Tips on DEA Treating. Hydrocarbon Process. 1972, July, 98-100. (3) Smith, R. F.; Younger, A. H. Operating Experiences of Canadian Diethanolamine Plants. In Proceedings of the 22nd Annual Gas Conditioning Conference; University of Oklahoma, Norman, OK, 1972; pp E1-E17. (4) Kohl A. L.; Riesenfeld F. C. Gas Purification, 4th ed.; Gulf Publishing Company: Houston, TX, 1985. (5) Williams, E.; Leckie, H. P. Corrosion and Its Prevention in a Monoethanolamine Gas Treating Plant. Mater. Prot. 1968, 7 (7). (6) Haslegrave, J. A.; Hedges, W. M.; Montgomerie, H. T. R.; O’Brien, T. M. The Development of Corrosion Inhibitors with LowEnvironmental Toxicity. In Proceedings of 67th SPE Conference; SPE: Richardson, TX, 1992; Paper SPE 24846. (7) Webster, S.; Harrop, D.; McMahon, A. J.; Partridge, G. J. Corrosion Inhibitor Selection for Oilfield Pipelines. In Proceedings of Corrosion 93; NACE International: Houston, TX, 1993; Paper 109. (8) Kinsella, B.; Tan Y. J.; Bailey, S. In Proceedings of Corrosion & Prevention 95; Australasian Corrosion Association Inc.: Perth, Australia, 1995; Paper 54.
(9) Porteous, A. Dictionary of Environmental Science and Technology; Open University Press: Buckingham, U.K., 1991. (10) McMahon, A. J.; Harrop, D. Green Corrosion Inhibitors: An Oil Company Perspective. In Proceedings of Corrosion 95; NACE International: Houston, TX, 1995; Paper 32. (11) Singh, W. P.; Bockris, J. O’M. Toxicity Issues of Organic Corrosion Inhibitors: Applications of QSAR Model. In Proceedings of Corrosion 96; NACE International: Houston, TX, 1996; Paper 225. (12) Al-Hashem, A.; Carew, J.; AI-Muhanna, K. In Proceedings of 13th International Corrosion Congress; Australasian Corrosion Association Inc.: Perth, Australia, 1996; Paper 346. (13) ASTM Standard E3-80. Standard Methods of Preparation of Metallographic Specimens. In Annual Book of ASTM Standards; The American Society for Testing and Materials: Philadelphia, PA, 1989; Vol. 03.01. (14) Horowitz, W. Official Methods of Analysis of AOAC International, 12th ed.; George Banta Co., Inc.: Nanasha, WI, 1975. (15) ASTM Standard G5-94. Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements. In Annual Book of ASTM Standards; The American Society for Testing and Materials: Philadelphia, PA, 1994; Vol. 03.02. (16) Scully, J. R. Electrochemical. In Corrosion Tests and Standards; Baboian, R., Ed.; The American Society for Testing and Materials: Philadelphia, PA, 1995. (17) Sastri, V. S. Corrosion Inhibitors: Principles and Applications; John Wiley & Sons: New York, 1998; pp 33-51. (18) Harrop, D. Chemical Inhibitors for Corrosion Control. In Chemical Inhibitors for Corrosion Control; Clubley, B. G., Ed.; The Royal Society of Chemistry: Cambridge, U.K. 1988.
Received for review March 20, 2001 Revised manuscript received July 30, 2001 Accepted August 15, 2001 IE010248C
