Surface Reactive Peptides and Polymers - ACS Publications

Chapter 21

Corrosion Inhibition by Thermal Polyaspartate Downloaded by UNIV MASSACHUSETTS AMHERST on August 1, 2012 | http://pubs.acs.org Publication Date: November 26, 1991 | doi: 10.1021/bk-1991-0444.ch021

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Brenda J.Little and C. Steven Sikes Naval Oceanographic and Atmospheric Research Laboratory, Stennis Space Center, MS 39529-5004 Mineralization Center, University of South Alabama, Mobile, AL 36688 1

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Polyanhydroaspartic acid, synthesized by thermal condensation of L-aspartic acid at 190 °C for 24 to 96 hours and converted to polyaspartate by mild alkaline hydrolysis, was evaluated as an inhibitor of corrosion. Inhibitor efficiency was judged on mild steel coupons exposed to salt water by visual inspection and by electrochemical testing. Electrochemical potential, polarization resistance and impedance spectra were evaluated. The mild steel surfaces were also examined using scanning electron microscopy/energy-dispersive x-ray spectrometry (SEM/EDAX) before and after exposure. The results indicated that thermal polyaspartate binds to surfaces of mild steel and moderately suppresses both anodic and cathodic corrosion reactions. The use of polymers as additives to water to control corrosion and mineral scaling has become widespread (1-3). Total organic and total polymer formulations are replacing traditional corrosion-control additives like zinc and chromate, principally due to concern about toxic effects (4, 5). To date, polyacrylate, polyacrylateacrylamide copolymers, and their derivatives have been most widely studied and used (6,7). Toxicity and biodegradability are still areas of concern, however, in that polyacrylate-acrylamides are not biodegradable and have toxic effects at doses used in process water (8,9). The use of biodegradable and non-toxic polypeptides as water treatment additives presents a promising alternative to hydrocarbonbased polymers (10-14). These polypeptides, like polyacrylate polymers, are polyanionic. They have a variety of interesting surface-reactive properties, including inhibition of crystallization (15), dispersant activity (16,17), and antifouling activity (10). Based on studies of the mechanism of polyanionic peptides as inhibitors of crystallization (18, 19), it is likely that the peptides will bind to metal surfaces. The purpose of the present study is to demonstrate that the simplest of the 0097-6156/91/0444-0263$06.00/0 © 1991 American Chemical Society In Surface Reactive Peptides and Polymers; Sikes, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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SURFACE REACTIVE PEPTIDES AND POLYMERS

polyanionic peptides, polyaspartate, has corrosion inhibiting activity when bound to surfaces of mild steel. In addition, a simple thermal polymerization to produce polyaspartic acid is decribed. The method is based on the early work by Fox et al. (20,21) on the abiotic origin of polyamino acids and proteins in the primitive atmosphere and ocean.

Downloaded by UNIV MASSACHUSETTS AMHERST on August 1, 2012 | http://pubs.acs.org Publication Date: November 26, 1991 | doi: 10.1021/bk-1991-0444.ch021

Preparation of Thermal Polyaspartate L-aspartic acid (500 g, Sigma Chemical Co.) was placed in a two-liter, roundbottom reaction vessel, originally designed as the evaporator vessel in a rotary evaporator apparatus. The reaction vessel was partially immersed in cooking oil in a deep-fryer set at 190°C ± 2°C. The reaction vessel was coupled by a ground glass fitting to a condenser vessel attached to a rotator shaft driven by a rheostated electric motor. The fittings were sealed with tape and fastened with hose clamps. A stream of nitrogen was continuously purged into the condenser vessel to eliminate O2 and lessen the possibility of charring. The water of dehydration during peptide bond formation collects in the condenser vessel and is a visible indicator of the progress of the reaction. The product converts to a tawny color during polymerization. The reaction was allowed to proceed for up to 96 hours. At the end, the bulk product was weighed, then dialyzed against 2 1 distilled H 2 O for two hours at least 4 times to remove residual unreacted aspartic acid and small reaction products. Dialysis tubing (Spectrapor) with a pore size to retain molecules larger than 1000 daltons was used. The dialysate was lyophilized and the purified product weighed. The yield after 24 hours of polymerization was about 30% peptide and about 50% by 72 hours with little change with further reaction. The sample used in this study was polymerized for 48 hours with a yield of about 40% peptide. The purified thermal product was in the form of the polyimide, polyanhydroaspartic acid (22) in which a 5 membered imide ring forms by a condensation reaction between the free carboxyl group of the aspartic acid R-group and the secondary amine of the peptide backbone. Prior to experimentation, the polyanhydroaspartic acid was converted to polyaspartate by opening the imide rings by mild aqueous alkaline hydrolysis: pH 10, 60 °C, 1 h. This treatment yields a mixed L,D polymer of alpha and beta residues (23).

Asp£>

-hCH-CO \ CH -CO 2

OH H0 2

-+NH—CH—COI CH

-fNH—CH-CH —CO 2

I COOH

2

COOH or- linkage

β - linkage

In Surface Reactive Peptides and Polymers; Sikes, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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LITTLE AND SIKES

265

Thermal Polyaspartate


Polyaspartate was characterized by gel permeation chromatography (Toya Soda PW 3000 XL), amino acid analysis (Waters, Picotag), and automated amino acid sequencing (Applied Biosystems model 477A) with on-line identification of phenylthiohydantoin derivatives (ABI, model 120). The purified thermal polyaspartate used in the corrosion measurements had an apparent molecular weight of 5000 daltons based on standard of polyaspartate molecules made by solid-phase methods (12). The amino acid analysis and sequence analysis confirmed that the product consisted of a pure polyaspartate. Corrosion Testing Inhibitor Solution. Mild steel coupons were exposed to a 0.01% solution of polyaspartate (PASP) in a synthetic salt solution (Instant Ocean) made up to a salinity of 35 ppt, pH 8.0. Results were compared to control samples which were exposed under identical conditions to blank synthetic salt solutions and solutions containing 10 mM potassium dichromate (Κ2&2θ7), a known anodic corrosion inhibitor. Corrosion Samples. Mild steel corrosion coupons were prepared by cleaning in 20% HCI, polishing with 400 emery paper and degreasing in acetone and hexane. They were then either totally immersed in the test solution or placed into an electrochemical cell. One cm coupons of mild steel were used for polarization resistance measurements, while 20 cm coupons were used for impedance measurements. 2

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Electrochemical Testing. Corrosion potentials (E rr) were measured using a saturated calomel reference electrode (SCE), a high impedance volt meter and a recording device. The impact of inhibitor on E r r was evaluated by allowing the mild steel electrode to equilibrate 16 to 20 hours before polymer was added. Polarization resistance (R ) was measured as the slope of a curve of potential (E) vs. current density (i) at E r r , where i = 0 using an EG&G Pare, Model 173, potentiostat/galvanostat with a three electrode system - a saturated calomel reference electrode, graphite rod counter electrodes and a mild steel working electrode (24). The corrosion current density (icorr) was calculated from R by: CO

C O

p

CO

p

icorr = B / R

(1)

P

with Β = b b /2.303(ba + b ) a

c

c

where b , b are the Tafel slopes. Impedance spectra were obtained at E r r with a Solartron model 1250 frequency response analyzer (FRA) and a model 1286 potentiostat. The data were collected and analyzed with software developed at the Corrosion and Environmental Effects Laboratory (CEEL) at the University of Southern California. a

c

C O



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Surface Analysis. Surface topography and chemistry were documented using a Kevex 7000 energy-dispersive x-ray spectrometer (ED AX) coupled to an AMRay 1000A scanning electron microscope (SEM). Electrodes were removed from the electrolytes (salt water or salt water + polymer), air dried and examined.


Results Visual examinations showed severe corrosion in the blank salt solutions, mild corrosion in the salt solutions containing 0.01% polyaspartate (PASP) and corrosion inhibition in the dichromate-containing solutions (Figure 1). In the absence of polymer, the carbon steel typically had a corrosion potential of 684 mv ± 20 mV. Addition of 0.01% PASP caused the corrosion potential to move 20 to 50 mV in the positive direction (Figure 2). The anodic and cathodic polarization behavior of the electrode was essentially unchanged by the addition of the polymer (Figure 3). The cathodic reaction became diffusion limited at a cd. of 50 /iAcm" in the absence of polymer and 20 μΑαη" in the presence of the polymer. Anodic and cathodic Tafel slopes remained essentially unchanged in the presence of the polymer. Addition of the polymer reduced the corrosion rate from 0.17-0.5 mm/y to 0.05-.13 mm/y (Table I). 2

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Table I. Typical Corrosion Rate Data for Mild Steel 35 ppt artificial seawater

35 ppt artificial seawater +0.01% PASP

-686 15.5 0.18

-667 4.9 0.06

Ε (i = 0) (mV) i-corr (μA/cm ) Corr Rate (mm/y) 2

Table II shows the values of E rr (vs. SCE), R and corrosion rates from the analyses of the impedance data obtained after two hours. As before, an estimate of the corrosion rate is given by the polarization resistance, R , which is inversely proportional to the corrosion rate. A value of R = 1000 ohm-cm corresponds to a corrosion rate of about 0.2 mm/y, assuming Β = 20 mV (eq 1). The corrosion rates for the blank, 0.01% PASP, and 10 mM dichromate solutions were 0.17, 0.16 and 0.08 mm/y, respectively. The impedance data are presented in Figure 4 as complex plane plots where Z a i is the real part of the impedance (Z) and Zimag is the imaginary part ( Z - Z real + Z imag). In a first approximation, R is given by the diameter of the semicircles. The impedance data in Figure 4 reflect mild corrosion inhibition in the presence of 0.01% PASP. Figures 5, 6 and 7 are S E M micrographs and E D A X spectra of the mild steel surfaces after exposure to salt water, salt water plus PASP and salt water CO

p

p

2

p

re

2

2

p


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21.

Figure 1. Photographs of mild steel exposed to (A) blank synthetic salt solution, (B) 0.01% PASP, (Q 10 mM K C r 0 . 2

2

7


268



200-

E

corr

VS TIME

oH

-200

H

-400

H

1

ADDITION OF POLYMER

-600

-800-

H

Τ TIME (hr) EFFECT OF POLYMER ADDITION ON CORROSION POTENTIAL

Figure 2.

E

C O

rr

86

vs. time for mild steel electrode before and after PASP addition.


LITTLE AND SIKES



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270


plus dichromate, respectively. In the absence of polymer the corroded mild steel surface was crystalline and chlorine dominated the spectrum. In the presence of the polymer, some areas of the surface were covered with an organic film, and sulfur and calcium have accumulated on the surface. Surface bound chlorine decreased in the presence of the polymer. In the presence of the dichromate inhibitor, the polished surface had not reacted with the constituents in the salt water. Only peaks for iron were found on the surface.


Table II. Typical EIS Data for Mild Steel (2 hr/Instant Ocean) Electrolyte

E rr CO

(mV)

2

R (ohm- cm )

Corrosion Rate (mm/y)

P

Blank synthetic salt solution

-666

1172

0.17

0.01% PASP in synthetic salt solution

-648

1268

0.16

10 mM dichromate in synthetic salt solution

-676

2660

0.08

The corrosion of mild steel in aerated seawater involves an anodic process whereby metal ions from the surface are oxidized and passed into solution and a cathodic process in which oxygen is reduced. Complete inhibition of corrosion of mild steel in seawater is difficult, if not economically impossible. Most systems make use of inhibitors to give marked reductions in corrosion rates. Furthermore, the majority of inhibitors are specific in their actions in metal/electrolyte environments. An inhibitor may decrease the rate of the anodic process, the cathodic process or both processes. Adsorbed inhibitors can retard corrosion reactions by forming a surface film which acts as a physical barrier to restrict diffusion of ions or molecules to or from the metal surface. These types of inhibitors are usually large molecules, e.g. proteins, such as gelatine; polysaccharides, such as dextrin; or compounds containing long hydrocarbon chains. Surface films of these types of inhibitors give rise to resistance polarization affecting both anodic and cathodic reactions (25). Inhibition in neutral solutions like aerated seawater can be due to compounds which form or stabilize protective surface films. An inhibitor may form a surface film of an insoluble salt by precipitation or reaction. Inhibitors forming films of this type include: (a) salts of metals such as zinc, magnesium, manganese and nickel form insoluble hydroxides at cathodic areas which are more alkaline due to the hydroxyl ions produced by reduction of oxygen; (b) soluble calcium salts can precipitate as calcium carbonate at cathodic areas; (c) polyphosphates in the presence of zinc or calcium produce a thin amorphous salt film. Salt


LITTLE AND SIKES


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80


1 - Blank 2 -PASP

^

3 - Dichromate

CO

Ε ο c

ce Ε

z

real (

z

m

o

h

m

s

)

Figure 4. Complex plane plots for mild steel.


271

272



CI

Figure 5. Micrograph and E D A X spectrum of mild steel after exposure to salt solution.


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Figure 6. Micrograph and E D A X spectrum of mild steel after exposure to 0.01% PASP.


274



Fe

Figure 7. Micrograph and E D A X spectrum of mild steel after exposure to 10 mM K C r 0 . 2

2

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films are usually thick and visible, restricting diffusion, particularly of dissolved oxygen to the metal surface (26). Since salt films are poor electronic conductors restricting oxygen reduction at the film surface, they are referred to as cathodic inhibitors. Another type of inhibitor that is effective in near-neutral solutions stabilizes oxide films on metals to form thin protective passivating films. These inhibitors are the anions of weak acids such as chromate, nitrite, benzoate, silicate, phosphate, and borate. Passivating oxidefilmson metals inhibit metal dissolution by preventing diffusion of metal ions. Because the anodic reaction is inhibited, these anions are referred to as anodic inhibitors. Anodic inhibitors are more frequently used than cathodic inhibitors to inhibit the corrosion of iron, zinc, aluminum, copper and their alloys in seawater (26). It is obvious from the visual inspection of the mild steel electrodes that the 0.01% solution of PASP reduces the corrosion of mild steel in a synthetic salt solution. The S E M / E D A X data confirm that the polymer is surface active, adsorbing to the surface and decreasing the amount of surface bound chlorine. The effects of adsorbed inhibitors on individual electrode processes of corrosion may be determined from electrochemical data. The measured potential of a corroding metal is a mixed potential resulting from the superposition of the anodic and cathodic reactions and is known as the corrosion potential ΟΓ -Ε/corr· The change in the corrosion potential due to addition of the inhibitor is an indication of which process is retarded. Displacement of the corrosion potential in the positive direction indicates retardation of the anodic process (anodic control), whereas displacement in the negative direction indicates mainly retardation of the cathodic process (cathodic control). Little change in the corrosion potential suggests mixed control whereby both anodic and cathodic processes are retarded (26). The observed shift of Ecorr in the positive direction upon addition of PASP to the synthetic salt solution suggests that the polymer mainly retards the anodic dissolution reaction. While measurements of E rr provide clues as to electrochemical behavior, the corrosion potential alone provides no information on the corrosion rate. Skold and Larson (27) found in studies of the corrosion of steel and cast iron in natural seawater that a linear relationship existed between potential and the applied anodic and cathodic current densities in the region of E rr (± 20 mV). Stern and his co-workers (28, 29) used the term "linear polarization" to describe the linearity. The slope of this linear curve, ΔΕ-Δΐ, is termed the polarization resistance, R , and has units of ohms*cm . On the basis of a detailed analysis of the polarization curves of the anodic and cathodic reactions involved in the corrosion of a metal, the following expression was derived: CO

CO

p

2

Icorr


276


where R is the polarization resistance determined at E rr, and b , b are the Tafel slopes. This equation shows that the corrosion rate is inversely proportional to R . It is evident from equations 1 and 2 that the evaluation of icorr from R requires a knowledge of Tafel slopes b and b . Figure 8 is a graphic representation of potential vs. log current density curves showing the relationship between E rr, icorr, b , b and the limiting diffusion current density (ii). The Tafel slopes must be derived from complete E-i curves ( >60 mV in each direction) for each system studied. In the experiments described Tafel slopes were determined independently for the anodic and cathodic reactions. As stated previously, the Tafel constants were not altered by the addition of polymer. Using polarization resistance data to calculate corrosion rates for 0.01% PASP in synthetic salt solution, a decrease of the corrosion rate from 0.17-0.50 mm/y to 0.05-0.13 mm/ y was observed. Thorough reviews of polarization resistance (R ) techniques for the measurement of corrosion currents have been reported elsewhere (30). The use of polarization resistance for determining corrosion rates has a number of significant advantages. It provides a method for rapidly monitoring instantaneous corrosion rates, requiring small changes in potential that do not significantly disturb the system. Electrochemical impedance spectroscopy (EIS) is a relatively new technique in corrosion research which has found many successful applications. A recent summary of the background of EIS and applications in many areas of electrochemistry can be found in the proceedings of the First International Symposium on EIS which was held in 1989 (31). Mansfeld (32, 33) has discussed some basic concepts concerning the recording and analysis of impedance data, and with Lorenz (34) has compared the results obtained with ac and dc techniques for iron in the presence of corrosion inhibitors. In the EIS technique the impedance data are recorded as a function of the frequency of the applied signal at a fixed working point (E,i) of a polarization curve. This working point often is the corrosion potential (E = E rr, i= 0). Usually a very large frequency range must be investigated to obtain the complete impedance spectrum, ranging from 65 kHz to 10 or lower mHz. The impedance data are usually determined with a three-electrode system. A potentiostat is used to apply the potential at which the data are to be collected. A frequency response analyzer (FRA) is programmed to apply a series of sine waves of varying frequency and constant amplitude small enough to remain in the linear potential range. Impedance data are determined by the F R A at each frequency and stored in memory. Properties of the corrosion system at a fixed potential (or current) can be determined through the analysis of the frequency dependence of the impedance. One of the advantages of EIS is that only very small signals are applied. The corrosion rates resulting from the EIS technique were similar to those obtained using linear polarization. Mild corrosion inhibition was observed with addition of the polymer to the synthetic salt solution. p

a

CO

p

p

a


CO

a

c

c

p

CO


c


21.

LITTLE AND SIKES

I 10

1

ι 10

ι 2

277


10

ι

Limiting diffusion current density, 'L

ι

3

Log Current density, /iAcrrr

2

Figure 8. Ε vs. log icorr curve.


278



Conclusion PASP is a surface active polymer that adsorbs to mild steel after brief exposure. The polymer appears to increase the concentration of calcium and sulfur on the surface of the metal and to form a diffusion barrier that inhibits chloride ions from reacting with the surface. The shift of the corrosion potential in the positive direction upon addition of polymer indicates decreased anodic dissolution processes, suggesting that PASP is an anodic inhibitor. Corrosion rate measurements by linear polarization and EIS confirm that PASP is a mild inhibitor of corrosion on mild steel exposed to aerated seawater. It did not inhibit corrosion to the extent that the anodic inhibitor, potassium dichromate did. In this regard, it will be interesting to evaluate derivatives of polyaspartate for improved performance. That is, it is clear that the nature and distribution of both anionic and nonionic residues are critical to the activity of polyanionic polymers (3538). Phosphorylated, phosphonated, and sulfonated residues, as well as neutral and hydrophobic residues may impart increased activity to the molecules as regulators of crystallization (12,36). Therefore, it is likely that polyamino acids with improved performance as inhibitors of corrosion will be forthcoming. Acknowledgments This work was supported by the Office of Naval Research and the Naval Océanographie and Atmospheric Research Laboratory (NOARL), Program Element 61153N, Ms. H. E. Morris, program director, NOARL Contribution SP 005:333:90. Additional funding was provided by the Mississippi-Alabama Sea Grant Consortium. Approved for public release; distribution is unlimited. Literature Cited 1. Smyk, E.B.; Hoots, J.E.; Fivizzani, K.P.; Fulks, K.E. Corrosion 88, 1988, paper 14, 20 p. 2. Lipinski, R.J.; Chang, K.Y. Corrosion 88, 1988, paper 12, 7p. 3. Fivizzani, K.P.; Dubin, L.; Fair, B.E.; Hoots, J.E. Corrosion 89, 1989, paper 433, 15p. 4. Environmental Protection Agency. Federal Register 1985, 15, 111, Notice ADL-FRL 2784-4. 5. Environmental Protection Agency. Federal Register 1985, 50, 134, Notice OW-FRL-2815-5. 6. Rohm and Haas Company. Polymer additions for aqueous systems; 1985, 21 p. 7. Nalco Chemical Company. Prism AAA-LH06; advance technical data sheet. 8. Cekolin, C.S. Echinoderm Conference abstract, Dauphin Island Sea Lab, 1989. 9. Cekolin, C.S. M.S. thesis, University of South Alabama, Mobile, 1990.



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10. Sikes, C.S.; Wheeler, A.P. U.S. Patent 4 534 881, 1985. 11. Sikes, C.S.; Wheeler, A.P. In Chemical Aspects of Regulation of Mineralization: Sikes, C.S.; Wheeler, A.P., Eds.; Univ. S. Alabama Publ. Services: Mobile, 1988; pp 15-20. 12. Sikes, C.S.; Yeung, M.L.; Wheeler, A.P. In Surface Reactive Peptides and Polymers: Discovery and Commercialization; Sikes, C.S.; Wheeler, A.P., Eds; American Chemical Society; Washington, D.C., 1990. 13. Fujimoto, Y.; Teranishi, M . West German Patent 2 253 190, 1973. 14. Boehmke, G. West German Patent 3 626 672, 1988. 15. Wheeler, A.P.; Sikes, C.S. In Biomineralization: Chemical and Biochemical Perspectives; Mann, S., Webb, J.; Williams, R.J.P., Eds.; V C H Publishers; Weinheim, FRG, 1989; 95-132. 16. Barbucci, R.; Marabini, A.M.; Barbaro, M . ; Nocentini, M . ; Corezzi, S. Reagents Miner. Ind. 1984, 245-250. 17. Garris, J.; Sikes, C.S. Ocean's'89abstract, 1989. 18. Low, K.C. M.S. Thesis, Univ. S. Ala., Mobile, 1990. 19. Wheeler, A.P.; Low, K.C.; Sikes, C.S. In Surface Reactive Peptides and Polymers: Discovery and Commercialization; Sikes, C.S.; Wheeler, A.P., Eds.; American Chemical Society, Washington, DC, 1990. 20. Fox, S.W.; Harada, K. J. Am. Chem. Soc. 1960, 82, 3745-81. 21. Fox, S.W.; Harada, K.; Rohlfing, D. In Polyamino Acids, Polypeptides, and Proteins; Stahman, M.A., Ed.; Univ. Wisconsin Press: Madison, 1962; pp 47-53. 22. Kokufuta, E.; Suzuki, S.; Harada, K. Biosystems 1977, 9, 211-214. 23. Saudek, V.; Pivkova, H.; Drobnik, J. Biopolymers 1981, 20, 1615-1623. 24. Kendig, M.; Mansfeld, F.; Tsai, S. Corr. Sci. 1983, 23, 317. 25. Manchu, W. First European Symposium on Corrosion Inhibitors, 1961, ρ 111. 26. Shrier, L. L. Corrosion; Newnes-Butterworths: Boston, M A , 1979; Chapter 16.3, 18.3. 27. Skold, R. V.; Larson, T. E. Corrosion 1957, 13, 1392. 28. Stern, M.; Geary, A. L. J. Electrochem. Soc. 1957, 104, 56. 29. Stern, M . Corrosion 1958, 14, 440. 30. Mansfeld, F. In Advances of Corrosion Science and Technology; Plenum Press: New York, 1976, Vol. 6, ρ 163. 31. Proc. 1st Intern. Symp. on EIS, 1989. 32. Mansfeld, F. Corrosion 1981, 37, 301. 33. Mansfeld, F.; Kendig, M . W.; Tsai, S. Corrosion 1982, 38, 570. 34. Lorenz, W. J.; Mansfeld, F. Corr. Sci. 1981, 21, 647. 35. Fong, D.W. U.S. Patent 4 703 092, 1987. 36. Fong, D.W.; Kowalski, D.J. U.S. Patent 4 678 840, 1987. 37. Fong, D.W.; Kowalski, D.J. U.S. Patent 4 762 894, 1988. 38. Sikes, C.S. U.S. Patent Appl. 07/339 672, 1989. RECEIVED August 27, 1990


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