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The protection of Cu-Ni condenser tubes with high molecular weight water-soluble polymers
Corrosion
Science,
1969. Vol. 9, pp. 395 to 404. Pergamon
Press. Printed
in Great
Britain
THE PROTECTION OF Cu-Ni CONDENSER TUBES WITH HIGH MOLECULAR WEIGHT WATER-SOLUBLE POLYMERS* B. C.
EDWARDS
Department of Mining and Mineral Technology, Imperial College, London, S.W.4 Abstract-High molecular weight organic polymers, of the type used extensively in “solid-liquid separation”, function at very low concentrations as corrosion inhibitors of Cu alloy condenser tubes in cooling systems which use aggressive waters. Qualitative laboratory tests and plant observations have established that corrosion is principally due to “deposit attack”. Electrochemical studies and electron microscopy have led to a mechanism for inhibition in which adsorbed polymer molecules modify the growth of non-passivating oxide films so that they become passivating. R&m&-De hauts polymires du type couramment utilises en “stparation solide-liquide” peuvent g t&s faibles teneurs jouer le rBle d’inhibiteurs de corrosion d’alliage de cuivre dans le cas de tubes de condenseur de systbmes de refroidissement utilisant des eaux agressives. Des essais qualitatifs en laboratoire ainsi que des observations d’installations ont rCvtlC que la corrosion est principalement due g une attaque par dep8t. Des Etudes par electrochimie et microscopic Clectronique ont conduit g un mtihanisme d’inhibition dans lequel les mol&ules de polymere adsorbCes modifient la croissance de la pellicule d’oxyde non passivante au point de la rendre passivante. Zusammenfassung-Organische Polymere mit hohem Molekulargewicht, wie sie in groBem Umfang bei der Fest-Fliissig-Trennung benutzt werden, wirken bei sehr kleinen Konzentrationen als Korrosionsinhibitoren fiir Cu-Legierungs-Kondensatorrohre in Kiihlsystemen mit agressiven WLssern. Qualitative Laboratoriumsversuche und Beobachtungen in technischen Anlagen zeigen, dal3 die Korrosion grundsltzlich unter Ablagerungen einsetzt. Elektrochemische und elektronenmikroskopische Untersuchungen ergeben einen lnhibitionsmechanismus, der die Wirkung der Polymeren durch eine Umwandlung von nicht-passivierenden Oxidschichten in Passivschichten erkllrt. INTRODUCTION THIS research was initiated during a study of the interactions between high molecular
weight, water-soluble organic polymers and the clay minerals found in condenser cooling systemsin power generating stations located on the River Thames.l The polymers, generally referred to as flocculants, are widely used in the destabilization of colloidal suspensions, i.e. the separation of liquid and solid phases. Such polymers have found many industrial applications, e.g. improvement of rates of filtration and sedimentation, improvement of the degree of clarification of effluents and washings.z*3 The usefulnessof the polymers dependson their ability to aggregatethe solid phase. Aggregation of suspendedparticles is brought about by adsorption of the polymer at various points on the polymer chain on to suitable adsorption sites on the surface of the colloidal particles. Bridging of particles by polymer follows adsorption. Because of the high molecular weights (l-10 million) and the large number of adsorption sites on the chain, the adsorption energiesinvolved are very high. The actual length of the molecule in solution is large by molecular standards; for example, the non-ionic form of polyacrylamide with a molecular weight of 1 x lo6 has an end-to-end distance of *Manuscript received 25 October 1968. tPresent address: Department of Chemistry, Middle East Technical University, Ankara, Turkey. 39s
396
B. C. EDWARDS
about 1OOOA in water.4 A great variety of polymeric materials are used as flocculants, many of which are natural products (e.g. guar and gelatine) but the most effective materials are synthetic (e.g. polyacrylamide). The charge on the polymers can be neutral (non-iouic), negative (anionic) or positive (cationic) and can therefore be used according to the nature of the surface properties of the solid phase. In recent years, in an effort to eliminate corrosion, non-ionic polyacrylamide has been used to remove mud from condenser cooling systems using cooling water, containing appreciable quantities of suspended matter, on a once-through basis.jv6 A mechanism has now been proposed to explain this effect, again dependent on the high degree of adsorbability of the polymer molecules.’ A proprietary material (“Zimmite”, based on polyacrylamide) has been used with great success at West Thurrock Generating Station on the River Thames. It has been suggested that contact of the tubes with mud facilitates corrosion, which is largely eliminated by removal of the mud.6 However, treatment is limited to lOmin/d at concentrations as low as I-O~lppm. Evidence suggests that the polymer is an inhibitor in its own right, and not merely a means of eliminating corrosive conditions. Corrosion and protection of condenser tubes Cooling waters taken from the estuarine reaches of the River Thames generally contain clay minerals, silica, sewage decomposition products and various electrolytes. The water contains only a fraction (about 10%) of the usual level of oxygen but a high level of suspended solids (up to 500ppm). This is a particularly aggressive environment even for Cu/Ni tubes. Corrosion is eliminated however, if the water is either saturated with oxygen-this destroys oxygen depleting organic matter and maintains a protective film--or the suspended solids are removed by filtration prior to entry into the condenser tubes to minimise deposit attack. A tenuous explanation of the mechanism of corrosion prevention has been suggested and is based on the reduction of tube surface/mud interactions through an aggregation mechanism which reduces the adhesiveness of the mud to the tube surface.’ This explanation is now untenable, particularly as the polymer is used for such a short time on a once-through basis. In fact, the polymer is likely to enhance the degree of adhesion of very small particles bound to the surface of the tube by conventional physical forces of attraction.8 During the last few years the Taprogge system has been used in conjunction with polymer treatment at West Thurrock Generating Station. However, when the Taprogge system was maintained and the use of polymer discontinued, the tubes underwent pitting corrosion, particularly if the Taprogge balls became lodged in the tubes.g The presence of high molecular weight lyophilic colloids (e.g. gelatin or guar-gum) has some influence on electrochemical reactions in aqueous environments.‘O Campbell carried out some experiments concerning the presence of naturally occurring inhibitors in certain supply waters which prevent the corrosion of copper pipes.” Unfortunately, he did not identify the inhibitor, but it is possible that it could also be a high molecular weight polymer, i.e. a protein of the type found in biological decomposition products. Earlier reports have suggested that a high “free sulphide” content of the mud was responsible for condenser tube attack.6 However, analysis of the corrosion products
Protection of Cu/Ni condenser tubes with high molecular weight water-soluble polymers
397
taken from the tubes revealed an insignificant level of sulphide (O-~~/;W/W).Observations on the corrosion of the tubes in situ revealed a horizontal demarcation between corroded anodic areas (bottom half of the tube) and non-corroded cathodic areas (upper half of the tube). Analysis of “deposit attack” corrosion products of static tests showed that they contained Cu and Ni in the approximate ratio 3 : 1, appreciable amounts of CO, and very little Cl-. No sulphide was detectable when mud was used. The evidence strongly suggeststhat condensertube corrosion is due principally to galvanic attack resulting from differential aeration cells and not sulphide attack. EXPERIMENTAL
Potentiokinetic behaviour of Cu/Ni alloy in 0*2M NaCl Figure 1 shows the electrical assembly and the cell in circuit. Cu/Ni sheet containing 68Cu, 30Ni, 1Mn and 1Fe (wt.;/,) was used to prepare electrodes which were set in “Plasticene”. Unwanted areas of the electrodes were blocked with a quickdrying aerosol lacquer. The auxiliary electrode was Pt, and the reference electrode (SCE) was connected through a Luggin capillary. Potentiokinetic measurementswere performed by manually changing the potential by small regular increments at 30s intervals. The potential of the working electrode was measuredwith a high impedance valve voltmeter. All potentials reported are with reference to SCE. Potentiostatic measurements A stationary potential was applied to specimensunder test and the variation of current with time recorded automatically. A potential of - 150mV (SCE) was selected as this was within the range of the effectiveness of the inhibitor, determined by potentiokinetic measurements, and close to the corrosion potential of Cu/Ni in 0.2M NaCl. Solutions Cu/Ni specimenswere studied in 0*2M NaCl in the absenceand presenceof oxygen (10% saturation), and in the presenceand absenceof non-ionic polyacrylamide. The level of oxygen, i.e. 10% saturation was selected since it is similar to that which prevails in practice.s The polymers studied are listed in Table 2.
FIG. I. Corrosion counter electrode;
test cell and electrical assembly. (W)-Cu/Ni electrode; CC)--Pt (Lwlomel reference electrode; (V&-valve voltmeter; (P)potentiostat; (R)-chart recorder: (M)-ammeter.
B. C.
EDWARDS
RESULTS TABLE
1.
CHANGE
OF INHIBITOR CHARACTERISTICS sat.) CONTAINING NON-IONIC
(10x0,
1.5 2.9 5.1 7.5 10.2 13.1
TABLE
2.
EFECXOF
Polymer7 Zimmite* Separant NP20 Separant c-120 Separant AP30 Polyflok$ 209P WisproflokS P Floccotan$
VARIOUS
effective, -denotes
POLYMERS ONTHECORROSION lo%o, Sat., POTENTIAL -
Mol.wt.
x lo6 6 3
0.05-O. 2-3
10 0.05
10
Cu/Nit~ 0.2M NaCl (20ppm)
Effectiveness* + + T + -
PH
* fdenotes
WITH pH FOR POLYACRYLAMIDE
.
ineffective.
RATEOF Cu/Ni 15@-I-kV (SCE)
Charge and chemical type non-ionic polyacrylamide non-ionic polyacrylamide cationic polyethyleneimine anionic polyacrylamide non-ionic polyethylene oxide cationic starch derivative amphoteric tannin derivative
SPECIMENS
IN
0.2M NaCl,
Contact time (min) 35
o/oinhibition of initial corrosion rate 80
30
15
25
95
100
0
100
0
50
88
45
75
l Zimmite of Europe Ltd. tDow Chem. Co. *Yorkshire Dyware and Chemical Co. Ltd. §Forestal Industries Ltd. YConcentrations 20ppm.
E.
mV (S.C.E)
FIG. 2. Polarization curve for Cu/Ni in 0.2M NaCl (lO%O, saturation): o, in absence of polymer; 0, in presence of polymer (20ppm “Zimmite”).
Protection of Cu/Ni condenser tubes with high molecular weight water-soluble
E,
polymers
399
mV (S.C.E)
FIG. 3. Polarization curve for Cu/Ni in 0.2M NaCl (de-oxygenated): 7, in absence of polymer; 0, in presence of polymer (20ppm “Zimmite”).
0
I IO
I a
I 50 Time,
min
FIG.4. Typical graph of current variation with time for a Cu/Ni electrode in 0.2M NaCl at a fixed potential of - 150mV (SCE) - 10°~02 saturation: l , freely corroding specimen; G , polymer inhibited specimen (20ppm “Zimmite”). RESULTS
Potentiokinetic behaviour in @2M NaCl Figure 2 shows the anodic and cathodic polarization curves for Cu/Ni specimens in 10% oxygen-saturated 0*2M NaCl. In the anodic region of the polarization curve the presenceof polyacrylamide causesa shift to lower cds. In the absence of oxygen the polarization curves remain virtually unchanged
B. C. EDWARDS
400
25-
---. I.---
0
I IO
I 20
I
I
30
40
. 1 5(
min
Time.
FIG. 5. Graph of current variation with time for Cu/Ni in 0.2M NaCl at a fixed potential of - 150mV (SCE); lO%O? saturation: 0, in presence of polymer (20ppm “Zimmite”); l , transferred to polymer-free electrolyte solution.
__
I
Time, FIG.
-
min
6. Variation of current with time for Cu/Ni in 0.2M NaCl at fixed potential of IV (SCE) lO’~O, saturation: A, lppm “Zimmite”; n , IOppm “Zimmite”; o 20ppm “Zimmite”; v, 40ppm “Zimmite”; l , 80ppm “Zimmite”.
150m
except for slight difference in the c.d.s of the cathodic portion of the curve (Fig. 3). This is attributed to the presenceof reducible speciespresent asimpurities in the nonionic polyacrylamide, which is itself non-reducible. Potentiostatic behaviow Typical curves of polarization resistance vs. time are presented in Fig. 4 in the
Protection of Cu/Ni condenser tubes with high molecular weight water-soluble
30 FIG.
7. Variation
I
15
I
30
I
45
I
60
I
75 Time,
90
I
I
105
I
120
I
135
polymers
401
20
min
of capacitance and resistance with time for Cu/Ni in 0*2M NaCI10 %02 saturation : 0, capacitance in presence of polymer (20ppn-1); l , capacitance in absence of polymer; A, resistance in presence of polymer (20ppm); v, resistance in absence of polymer.
and absence of polymer, and it is evident that the polymer addition-results in a marked decreasein the corrosion current. Polarization resistance curves remain unchanged if partially inhibited specimens are transferred to polymer-free solutions of otherwise identical composition (Fig. 5). Figure 6 illustrates the dependence of the rate of inhibition on polymer concentration, and it was found that specimensallowed to corrode without restraint can subsequently be inhibited by the addition of polymer. The inhibition of the Cu/Ni specimensis pH dependent (Table 1). A number of high molecular weight polymers were investigated as inhibitors. The degreeof inhibition after time t is recorded in Table 2 for various solutions of polymer in contact with cupro-nickel in 0*2M NaCl (IO:/, oxygen saturation).
presence
Capacitance and resistancemeasurenzents The technique used to measure C capacitance and R resistancewas that developed by Isaacs and Leach.l2 Measurements were again performed in 0.2M NaCl containing 10% oxygen saturation, and specimenswere allowed to corrode freely in the presenceand absenceof polymer (Fig. 7). Electron microscopy Electron micrographs of inhibited and freely corroding cupro-nickel surfaceswere obtained. Specimenswere withdrawn from the polarization cell, washedwith acetone and “Formvar” replicas taken. The replicas were washed in loo;, H&SO, to remove adhering corrosion products (Fig. 8, (a) and (b)).
402
B. C.
EDWARDS
Other micrographs were prepared from specimenswhich were chemically cleaned and cathodically polarized and allowed to come into contact with oxygen-containing water with and without polymer (Fig. 8, (c) and (d)). It should be noted that the micrographs in Fig. 8 show active centres as either depressionsor projections depending on the mode of viewing; however, they are in fact interpreted from the shadowscast by the Au used in their preparation. DISCUSSION
Normally Cu/Ni resiststhe aggressivenature of saline waters. However, the alloy is not able to counteract the effects of differential aeration cells which occur in the condenser tubes. Thermodynamically, Cu is very reactive in oxygenated water,13 but the oxide film found at most metal-water interfaces imposes sufficient restraint, particularly if modified by the presenceof nickel oxide. There are numerous theories in the literature on the mechanismsof corrosion inhibition. Those involving an adsorption processat somestage require the establishment of an impervious film, either by oxidation of the metal surface to a passivating layer or the adsorption of the inhibitor by chemical or physical bonding. Little reference is made to materials which may change the structure of a corrosion product and bring about auto-passivation without themselvesbeing a major physical barrier to material transport. The mechanism of inhibition by colloidal material such as gelatine in the acid dissolution of steel has been attributed to the blocking of active sitesby chemical and physical adsorption and the exclusion of the corrosive environment.‘* The polarization curves in Fig. 2 are typical of Cu-based alloys. In the presence of polymer the curves are modified, the most marked effect being the decreasein the anodic c.d. between - 10 and - 300mV. The slight reduction in the anodic curve (Fig. 3) in the absenceof oxygen between - 20 and - 200mV, indicates a reduction in the rate of dissolution-a contribution to corrosion inhibition by physically adsorbed polymer cannot be ruled out. If the mechanism is solely dependent on a physically or chemically adsorbed barrier, then the polarization resistance curves would be expected to show rapid inhibition of the corroding specimens-the adsorption of polymer on mineral surfaces is reportedi to be 90% complete within 2s. However, it takes considerably longer (Fig. 4) to reduce the corrosion rate to lo?/, of the initial value. At low and high pH values, the inhibitor is ineffective. Although adsorption of polymer is modified at very high or low pH, it still adsorbs on to oxide surfaces. However, at high or low pH values oxide formation is impossiblebecausethe corrosion products are soluble,21but in the pH range 3.5-10.2 the oxide film is stable and the inhibitor is effective. No data are available on the zero point of charge of mixed hydrated Cu oxide and Ni oxide films. But data on the z.p.c. of the hydrated oxides suggesta value of pH 9-10 thus making it negative at neutral pH values.le This agrees with observations on anionic polymers which do not inhibit this system-their degree of adsorption on negatively charged mineral surfacesis small. Cationic polymers are particularly effective asinhibitors; oppositely charged polymers adsorb well on mineral surfaces. Molecules which do not adsorb strongly do not inhibit corrosion, e.g. polyethyleneoxide or sodium carboxymethyl cellulose.
FIG. 8. (a) freely corroding in 0.2M NaCI, 10%02 saturation, - 150mV (SCE), taken after 30min immersion.
FIG. 8. (b) as for (a) but in presence of “Zimmite”
(20ppm).
FIG.
8. (c) electrode prepared to cathodic reduction stage in 0.2M NaCI, 10%02 saturation, 30s immersion.
FIG. 8. (d) as for (c) but in presence of “Zimmite” (2Oppm). Note: Areas edged in white are projections. Electron micrographs of cupro-nickel specimens; magnification
x 40,000.
Protection of Cu/Ni condenser tubes with high molecular weight water-soluble
polymers
403
Figure 7 shows that in the initial stagesthe capacitance for the inhibited specimen is higher than the uninhibited; the resistance, however, is the same. This indicates a “physically adsorbed layer” at the metal solution interface. However, the resistance of the unhibited specimen increasesand the capacitance decreases’asa passivating oxide layer forms. The resistanceof the corroding specimenincreasesslightly as does the capacitance. The latter effect is presumably due to the hydration of the oxide film which is reported to increase capacitance.l’ Studies on metals such as Cu, Ni and Al have shown the existence of a duplex structure for the oxide film,13 i.e. a thin amorphous layer of oxide next to the metal covered by an outer porous layer of oxide which is subject to considerable mechanical stressbecauseof density differences between oxide and metal.‘* The presenceof polymer modifies the structure of the duplex film; it is well known that impurities affect crystal morphology, e.g. if gelatin is added to a CuSO, plating bath the grain size of the deposit is reduced from lmm to 0.2-l*Op, and such additions have a marked influence on hardness, mechanical strength and electrical resistance, all of which increase.lg Such impurities are not necessarily incorporated into the crystal lattice-they adsorb on high energy sites blocking growth and creating new nucleation sites, the rate of formation of which increases with increasing impurity level. This agreeswith the increased rate of inhibition on increasing the amount of polymer (Fig. 6). Electron micrographs of corroded and inhibited Cu/Ni specimensare significantly different. Figure 8(a) showsa freely corroding broken and fissuredsurface. Figure 8(b) is a film grown in the presenceof polymer; this surface is covered with projections which take the form of flat plateaux. The oxide growth (Fig. 8(a)) for the freely corroding specimenis constant and follows a rectilinear equation. The plateaux in Figure 8(b) have been formed by an equivalent number of coulombs. In this case growing active centres have been gradually suppressedby polymer adsorption. Minor plateaux can also be observed and could either be (a) nucleation sitessuppressedearly in growth or (b) fresh active centres suppressedafter a redistribution of corrosion centres from incipient active sites. Confirmation of the mechanism of inhibition, i.e. suppression of active sites, is observed when specimensare cleaned, cathodically reduced and allowed to come into contact with oxygen-containing water with and without inhibitor. Unrestricted growth and polymer inhibition of freshly cleaned surfaces have been arrested at a very early stage (within 30s) and examined. Micrographs (Fig. 8 (c) and (d)) show that uninhibited specimensform nucleation sites (which are suppressedby removal from solution before appreciable reaction takes place, but which would normally continue to grow outwards) and inhibited specimens show almost complete suppressionof active sitesbefore they have any opportunity to develop. Experimentally, it is difficult to assign a growth law to film build up and frequently the type of growth changes with film thickness.20However, the inhibited films exhibit an approximately logarithmic rate law, i.e. W = k, log (k,t + 1). An increasein polymer increasesthe rate of nucleation of the surface film, and passivity is soon established (Fig. 6). The inhibition of corroded surfacesis a slower process; presumably the diffusion of colloidal polymeric flocculant to the incipient active centres is restricted by the porosity of the existing corrosion products.
404
B. C. EDWARDS
Polymers adsorb strongly on to suitable surfacesand once adsorbed, their influence on crystal morphology persistseven in the absenceof bulk polymer solution (Fig. 5). There can be little doubt of the effectiveness of organic polymers as corrosion inhibitors in neutral aqueous environments. However, much work is required to obtain (1) a fuller understanding of the mechanism, and (2) suitable polymers for use under all conditions; for example, by including certain groups in the polymer moleculessuch as aryl amines, it might be possible to inhibit the corrosion of steel in acid or aqueous environments. Acknowledgements-The author wishes to express his gratitude to Dr. J. A. Kitchener for his helpful advice and criticism during this work. Financial support from the Central Electricity Generating Board and assistance from its staff are gratefully acknowledged. REFERENCES 1. B. C. EDWARDS, Ph.D. Thesis, University of London (1967). 2. S. K. KUZ’KIN and V. P. NEBERA, Synthetic Floccrrlants in Dewatering Processes, Translation, Nat. Lending Library, Boston, G.B. (1966). 3. J. B. POOLE and D. DOYLE, Solid-Liquid Separation, Ministry of Technology,
Moscow (1963) ; Her Majesty’s
StationeryOffice, London(1966). 4. K. SAKA~UCHI and F. NAG.&, Bull. them. Sot. Japan 39, 88 (1966). 5. W. E. ZIMMIE and R. F. W. BLOECHER.U.S. Patent 308.591.6 (19631. 6. A. SHERRYand E. R. GILL, Chemistry’and Industry 102’(19&$ ’ 7. W. E. ZIMMIE, paper presented to the Nat. Ass. Corros. Engrs, Nat. Meeting, St. Louis, Missouri, March (1965). 8. J. K. MARSHALL and J. A. KITCHENER, J. Colloid Sci. 22, 342 (1966). 9. E. R. GILL, personal communication (1967). 10. J. I. BREGMAN, Corrosion Inhibitors. Macmillan, New York (1963); I. N. PUTILOVA, S. A. BALEZIN and U. P. BARRANIK, Metallic Corrosion Inhibitors. Pergamon, London (1960). 11. H. S. CA~~PBELL, I. appl. Chem. 4, 633 (1954); H. S. CAMPBELL, Trans. Faraday Sot. 50, 1351 (1954); H. S. CAMPBELL. J. Inst. Metals. 77. 345 (1950). 12. H. S..&AACS and J. S. L. LEACH, J. electrochem. ioc. ilO, 680 (1963). 13. D. J. G. IVES and A. E. RAWSON, J. electrochem. Sot. 109,452 (1962). 14. W. MACHU and V. K. Go~A, Werkst. Korros. 13, 745 (1962). 15. G. PETERSONand T. K. KEWI, J. phys. Chem. 65, 1330 (1961). 16. G. A. PARKS, Chem. Rev. 65, 177 (1965). 17. W. J. BAYNARD and J. J. RANDALL, JR., J. electrochem. Sot. 108, 822 (1961). 18. N. B. PILLMG and R. E. BR)WORTH, J. Insf. Metals 29, 529 (1923). 19. R. H. DOREMUS, B. W. ROBERTS and D. TURNBULL (Eds.), Growrh and Perfection of Crystals. Wiley, New York (1963); S. W. SEARS,pp. 441-445; N. CABERA and D. A. VERMILYEA, pp. 392408.
K. HAUFFE, Oxidation of Metals. Plenum, London (1965). 21. M. POURBAIX, Atlas of Electrochemical Equilibria. Pergamon, London (1966). 20.
Science,
1969. Vol. 9, pp. 395 to 404. Pergamon
Press. Printed
in Great
Britain
THE PROTECTION OF Cu-Ni CONDENSER TUBES WITH HIGH MOLECULAR WEIGHT WATER-SOLUBLE POLYMERS* B. C.
EDWARDS
Department of Mining and Mineral Technology, Imperial College, London, S.W.4 Abstract-High molecular weight organic polymers, of the type used extensively in “solid-liquid separation”, function at very low concentrations as corrosion inhibitors of Cu alloy condenser tubes in cooling systems which use aggressive waters. Qualitative laboratory tests and plant observations have established that corrosion is principally due to “deposit attack”. Electrochemical studies and electron microscopy have led to a mechanism for inhibition in which adsorbed polymer molecules modify the growth of non-passivating oxide films so that they become passivating. R&m&-De hauts polymires du type couramment utilises en “stparation solide-liquide” peuvent g t&s faibles teneurs jouer le rBle d’inhibiteurs de corrosion d’alliage de cuivre dans le cas de tubes de condenseur de systbmes de refroidissement utilisant des eaux agressives. Des essais qualitatifs en laboratoire ainsi que des observations d’installations ont rCvtlC que la corrosion est principalement due g une attaque par dep8t. Des Etudes par electrochimie et microscopic Clectronique ont conduit g un mtihanisme d’inhibition dans lequel les mol&ules de polymere adsorbCes modifient la croissance de la pellicule d’oxyde non passivante au point de la rendre passivante. Zusammenfassung-Organische Polymere mit hohem Molekulargewicht, wie sie in groBem Umfang bei der Fest-Fliissig-Trennung benutzt werden, wirken bei sehr kleinen Konzentrationen als Korrosionsinhibitoren fiir Cu-Legierungs-Kondensatorrohre in Kiihlsystemen mit agressiven WLssern. Qualitative Laboratoriumsversuche und Beobachtungen in technischen Anlagen zeigen, dal3 die Korrosion grundsltzlich unter Ablagerungen einsetzt. Elektrochemische und elektronenmikroskopische Untersuchungen ergeben einen lnhibitionsmechanismus, der die Wirkung der Polymeren durch eine Umwandlung von nicht-passivierenden Oxidschichten in Passivschichten erkllrt. INTRODUCTION THIS research was initiated during a study of the interactions between high molecular
weight, water-soluble organic polymers and the clay minerals found in condenser cooling systemsin power generating stations located on the River Thames.l The polymers, generally referred to as flocculants, are widely used in the destabilization of colloidal suspensions, i.e. the separation of liquid and solid phases. Such polymers have found many industrial applications, e.g. improvement of rates of filtration and sedimentation, improvement of the degree of clarification of effluents and washings.z*3 The usefulnessof the polymers dependson their ability to aggregatethe solid phase. Aggregation of suspendedparticles is brought about by adsorption of the polymer at various points on the polymer chain on to suitable adsorption sites on the surface of the colloidal particles. Bridging of particles by polymer follows adsorption. Because of the high molecular weights (l-10 million) and the large number of adsorption sites on the chain, the adsorption energiesinvolved are very high. The actual length of the molecule in solution is large by molecular standards; for example, the non-ionic form of polyacrylamide with a molecular weight of 1 x lo6 has an end-to-end distance of *Manuscript received 25 October 1968. tPresent address: Department of Chemistry, Middle East Technical University, Ankara, Turkey. 39s
396
B. C. EDWARDS
about 1OOOA in water.4 A great variety of polymeric materials are used as flocculants, many of which are natural products (e.g. guar and gelatine) but the most effective materials are synthetic (e.g. polyacrylamide). The charge on the polymers can be neutral (non-iouic), negative (anionic) or positive (cationic) and can therefore be used according to the nature of the surface properties of the solid phase. In recent years, in an effort to eliminate corrosion, non-ionic polyacrylamide has been used to remove mud from condenser cooling systems using cooling water, containing appreciable quantities of suspended matter, on a once-through basis.jv6 A mechanism has now been proposed to explain this effect, again dependent on the high degree of adsorbability of the polymer molecules.’ A proprietary material (“Zimmite”, based on polyacrylamide) has been used with great success at West Thurrock Generating Station on the River Thames. It has been suggested that contact of the tubes with mud facilitates corrosion, which is largely eliminated by removal of the mud.6 However, treatment is limited to lOmin/d at concentrations as low as I-O~lppm. Evidence suggests that the polymer is an inhibitor in its own right, and not merely a means of eliminating corrosive conditions. Corrosion and protection of condenser tubes Cooling waters taken from the estuarine reaches of the River Thames generally contain clay minerals, silica, sewage decomposition products and various electrolytes. The water contains only a fraction (about 10%) of the usual level of oxygen but a high level of suspended solids (up to 500ppm). This is a particularly aggressive environment even for Cu/Ni tubes. Corrosion is eliminated however, if the water is either saturated with oxygen-this destroys oxygen depleting organic matter and maintains a protective film--or the suspended solids are removed by filtration prior to entry into the condenser tubes to minimise deposit attack. A tenuous explanation of the mechanism of corrosion prevention has been suggested and is based on the reduction of tube surface/mud interactions through an aggregation mechanism which reduces the adhesiveness of the mud to the tube surface.’ This explanation is now untenable, particularly as the polymer is used for such a short time on a once-through basis. In fact, the polymer is likely to enhance the degree of adhesion of very small particles bound to the surface of the tube by conventional physical forces of attraction.8 During the last few years the Taprogge system has been used in conjunction with polymer treatment at West Thurrock Generating Station. However, when the Taprogge system was maintained and the use of polymer discontinued, the tubes underwent pitting corrosion, particularly if the Taprogge balls became lodged in the tubes.g The presence of high molecular weight lyophilic colloids (e.g. gelatin or guar-gum) has some influence on electrochemical reactions in aqueous environments.‘O Campbell carried out some experiments concerning the presence of naturally occurring inhibitors in certain supply waters which prevent the corrosion of copper pipes.” Unfortunately, he did not identify the inhibitor, but it is possible that it could also be a high molecular weight polymer, i.e. a protein of the type found in biological decomposition products. Earlier reports have suggested that a high “free sulphide” content of the mud was responsible for condenser tube attack.6 However, analysis of the corrosion products
Protection of Cu/Ni condenser tubes with high molecular weight water-soluble polymers
397
taken from the tubes revealed an insignificant level of sulphide (O-~~/;W/W).Observations on the corrosion of the tubes in situ revealed a horizontal demarcation between corroded anodic areas (bottom half of the tube) and non-corroded cathodic areas (upper half of the tube). Analysis of “deposit attack” corrosion products of static tests showed that they contained Cu and Ni in the approximate ratio 3 : 1, appreciable amounts of CO, and very little Cl-. No sulphide was detectable when mud was used. The evidence strongly suggeststhat condensertube corrosion is due principally to galvanic attack resulting from differential aeration cells and not sulphide attack. EXPERIMENTAL
Potentiokinetic behaviour of Cu/Ni alloy in 0*2M NaCl Figure 1 shows the electrical assembly and the cell in circuit. Cu/Ni sheet containing 68Cu, 30Ni, 1Mn and 1Fe (wt.;/,) was used to prepare electrodes which were set in “Plasticene”. Unwanted areas of the electrodes were blocked with a quickdrying aerosol lacquer. The auxiliary electrode was Pt, and the reference electrode (SCE) was connected through a Luggin capillary. Potentiokinetic measurementswere performed by manually changing the potential by small regular increments at 30s intervals. The potential of the working electrode was measuredwith a high impedance valve voltmeter. All potentials reported are with reference to SCE. Potentiostatic measurements A stationary potential was applied to specimensunder test and the variation of current with time recorded automatically. A potential of - 150mV (SCE) was selected as this was within the range of the effectiveness of the inhibitor, determined by potentiokinetic measurements, and close to the corrosion potential of Cu/Ni in 0.2M NaCl. Solutions Cu/Ni specimenswere studied in 0*2M NaCl in the absenceand presenceof oxygen (10% saturation), and in the presenceand absenceof non-ionic polyacrylamide. The level of oxygen, i.e. 10% saturation was selected since it is similar to that which prevails in practice.s The polymers studied are listed in Table 2.
FIG. I. Corrosion counter electrode;
test cell and electrical assembly. (W)-Cu/Ni electrode; CC)--Pt (Lwlomel reference electrode; (V&-valve voltmeter; (P)potentiostat; (R)-chart recorder: (M)-ammeter.
B. C.
EDWARDS
RESULTS TABLE
1.
CHANGE
OF INHIBITOR CHARACTERISTICS sat.) CONTAINING NON-IONIC
(10x0,
1.5 2.9 5.1 7.5 10.2 13.1
TABLE
2.
EFECXOF
Polymer7 Zimmite* Separant NP20 Separant c-120 Separant AP30 Polyflok$ 209P WisproflokS P Floccotan$
VARIOUS
effective, -denotes
POLYMERS ONTHECORROSION lo%o, Sat., POTENTIAL -
Mol.wt.
x lo6 6 3
0.05-O. 2-3
10 0.05
10
Cu/Nit~ 0.2M NaCl (20ppm)
Effectiveness* + + T + -
PH
* fdenotes
WITH pH FOR POLYACRYLAMIDE
.
ineffective.
RATEOF Cu/Ni 15@-I-kV (SCE)
Charge and chemical type non-ionic polyacrylamide non-ionic polyacrylamide cationic polyethyleneimine anionic polyacrylamide non-ionic polyethylene oxide cationic starch derivative amphoteric tannin derivative
SPECIMENS
IN
0.2M NaCl,
Contact time (min) 35
o/oinhibition of initial corrosion rate 80
30
15
25
95
100
0
100
0
50
88
45
75
l Zimmite of Europe Ltd. tDow Chem. Co. *Yorkshire Dyware and Chemical Co. Ltd. §Forestal Industries Ltd. YConcentrations 20ppm.
E.
mV (S.C.E)
FIG. 2. Polarization curve for Cu/Ni in 0.2M NaCl (lO%O, saturation): o, in absence of polymer; 0, in presence of polymer (20ppm “Zimmite”).
Protection of Cu/Ni condenser tubes with high molecular weight water-soluble
E,
polymers
399
mV (S.C.E)
FIG. 3. Polarization curve for Cu/Ni in 0.2M NaCl (de-oxygenated): 7, in absence of polymer; 0, in presence of polymer (20ppm “Zimmite”).
0
I IO
I a
I 50 Time,
min
FIG.4. Typical graph of current variation with time for a Cu/Ni electrode in 0.2M NaCl at a fixed potential of - 150mV (SCE) - 10°~02 saturation: l , freely corroding specimen; G , polymer inhibited specimen (20ppm “Zimmite”). RESULTS
Potentiokinetic behaviour in @2M NaCl Figure 2 shows the anodic and cathodic polarization curves for Cu/Ni specimens in 10% oxygen-saturated 0*2M NaCl. In the anodic region of the polarization curve the presenceof polyacrylamide causesa shift to lower cds. In the absence of oxygen the polarization curves remain virtually unchanged
B. C. EDWARDS
400
25-
---. I.---
0
I IO
I 20
I
I
30
40
. 1 5(
min
Time.
FIG. 5. Graph of current variation with time for Cu/Ni in 0.2M NaCl at a fixed potential of - 150mV (SCE); lO%O? saturation: 0, in presence of polymer (20ppm “Zimmite”); l , transferred to polymer-free electrolyte solution.
__
I
Time, FIG.
-
min
6. Variation of current with time for Cu/Ni in 0.2M NaCl at fixed potential of IV (SCE) lO’~O, saturation: A, lppm “Zimmite”; n , IOppm “Zimmite”; o 20ppm “Zimmite”; v, 40ppm “Zimmite”; l , 80ppm “Zimmite”.
150m
except for slight difference in the c.d.s of the cathodic portion of the curve (Fig. 3). This is attributed to the presenceof reducible speciespresent asimpurities in the nonionic polyacrylamide, which is itself non-reducible. Potentiostatic behaviow Typical curves of polarization resistance vs. time are presented in Fig. 4 in the
Protection of Cu/Ni condenser tubes with high molecular weight water-soluble
30 FIG.
7. Variation
I
15
I
30
I
45
I
60
I
75 Time,
90
I
I
105
I
120
I
135
polymers
401
20
min
of capacitance and resistance with time for Cu/Ni in 0*2M NaCI10 %02 saturation : 0, capacitance in presence of polymer (20ppn-1); l , capacitance in absence of polymer; A, resistance in presence of polymer (20ppm); v, resistance in absence of polymer.
and absence of polymer, and it is evident that the polymer addition-results in a marked decreasein the corrosion current. Polarization resistance curves remain unchanged if partially inhibited specimens are transferred to polymer-free solutions of otherwise identical composition (Fig. 5). Figure 6 illustrates the dependence of the rate of inhibition on polymer concentration, and it was found that specimensallowed to corrode without restraint can subsequently be inhibited by the addition of polymer. The inhibition of the Cu/Ni specimensis pH dependent (Table 1). A number of high molecular weight polymers were investigated as inhibitors. The degreeof inhibition after time t is recorded in Table 2 for various solutions of polymer in contact with cupro-nickel in 0*2M NaCl (IO:/, oxygen saturation).
presence
Capacitance and resistancemeasurenzents The technique used to measure C capacitance and R resistancewas that developed by Isaacs and Leach.l2 Measurements were again performed in 0.2M NaCl containing 10% oxygen saturation, and specimenswere allowed to corrode freely in the presenceand absenceof polymer (Fig. 7). Electron microscopy Electron micrographs of inhibited and freely corroding cupro-nickel surfaceswere obtained. Specimenswere withdrawn from the polarization cell, washedwith acetone and “Formvar” replicas taken. The replicas were washed in loo;, H&SO, to remove adhering corrosion products (Fig. 8, (a) and (b)).
402
B. C.
EDWARDS
Other micrographs were prepared from specimenswhich were chemically cleaned and cathodically polarized and allowed to come into contact with oxygen-containing water with and without polymer (Fig. 8, (c) and (d)). It should be noted that the micrographs in Fig. 8 show active centres as either depressionsor projections depending on the mode of viewing; however, they are in fact interpreted from the shadowscast by the Au used in their preparation. DISCUSSION
Normally Cu/Ni resiststhe aggressivenature of saline waters. However, the alloy is not able to counteract the effects of differential aeration cells which occur in the condenser tubes. Thermodynamically, Cu is very reactive in oxygenated water,13 but the oxide film found at most metal-water interfaces imposes sufficient restraint, particularly if modified by the presenceof nickel oxide. There are numerous theories in the literature on the mechanismsof corrosion inhibition. Those involving an adsorption processat somestage require the establishment of an impervious film, either by oxidation of the metal surface to a passivating layer or the adsorption of the inhibitor by chemical or physical bonding. Little reference is made to materials which may change the structure of a corrosion product and bring about auto-passivation without themselvesbeing a major physical barrier to material transport. The mechanism of inhibition by colloidal material such as gelatine in the acid dissolution of steel has been attributed to the blocking of active sitesby chemical and physical adsorption and the exclusion of the corrosive environment.‘* The polarization curves in Fig. 2 are typical of Cu-based alloys. In the presence of polymer the curves are modified, the most marked effect being the decreasein the anodic c.d. between - 10 and - 300mV. The slight reduction in the anodic curve (Fig. 3) in the absenceof oxygen between - 20 and - 200mV, indicates a reduction in the rate of dissolution-a contribution to corrosion inhibition by physically adsorbed polymer cannot be ruled out. If the mechanism is solely dependent on a physically or chemically adsorbed barrier, then the polarization resistance curves would be expected to show rapid inhibition of the corroding specimens-the adsorption of polymer on mineral surfaces is reportedi to be 90% complete within 2s. However, it takes considerably longer (Fig. 4) to reduce the corrosion rate to lo?/, of the initial value. At low and high pH values, the inhibitor is ineffective. Although adsorption of polymer is modified at very high or low pH, it still adsorbs on to oxide surfaces. However, at high or low pH values oxide formation is impossiblebecausethe corrosion products are soluble,21but in the pH range 3.5-10.2 the oxide film is stable and the inhibitor is effective. No data are available on the zero point of charge of mixed hydrated Cu oxide and Ni oxide films. But data on the z.p.c. of the hydrated oxides suggesta value of pH 9-10 thus making it negative at neutral pH values.le This agrees with observations on anionic polymers which do not inhibit this system-their degree of adsorption on negatively charged mineral surfacesis small. Cationic polymers are particularly effective asinhibitors; oppositely charged polymers adsorb well on mineral surfaces. Molecules which do not adsorb strongly do not inhibit corrosion, e.g. polyethyleneoxide or sodium carboxymethyl cellulose.
FIG. 8. (a) freely corroding in 0.2M NaCI, 10%02 saturation, - 150mV (SCE), taken after 30min immersion.
FIG. 8. (b) as for (a) but in presence of “Zimmite”
(20ppm).
FIG.
8. (c) electrode prepared to cathodic reduction stage in 0.2M NaCI, 10%02 saturation, 30s immersion.
FIG. 8. (d) as for (c) but in presence of “Zimmite” (2Oppm). Note: Areas edged in white are projections. Electron micrographs of cupro-nickel specimens; magnification
x 40,000.
Protection of Cu/Ni condenser tubes with high molecular weight water-soluble
polymers
403
Figure 7 shows that in the initial stagesthe capacitance for the inhibited specimen is higher than the uninhibited; the resistance, however, is the same. This indicates a “physically adsorbed layer” at the metal solution interface. However, the resistance of the unhibited specimen increasesand the capacitance decreases’asa passivating oxide layer forms. The resistanceof the corroding specimenincreasesslightly as does the capacitance. The latter effect is presumably due to the hydration of the oxide film which is reported to increase capacitance.l’ Studies on metals such as Cu, Ni and Al have shown the existence of a duplex structure for the oxide film,13 i.e. a thin amorphous layer of oxide next to the metal covered by an outer porous layer of oxide which is subject to considerable mechanical stressbecauseof density differences between oxide and metal.‘* The presenceof polymer modifies the structure of the duplex film; it is well known that impurities affect crystal morphology, e.g. if gelatin is added to a CuSO, plating bath the grain size of the deposit is reduced from lmm to 0.2-l*Op, and such additions have a marked influence on hardness, mechanical strength and electrical resistance, all of which increase.lg Such impurities are not necessarily incorporated into the crystal lattice-they adsorb on high energy sites blocking growth and creating new nucleation sites, the rate of formation of which increases with increasing impurity level. This agreeswith the increased rate of inhibition on increasing the amount of polymer (Fig. 6). Electron micrographs of corroded and inhibited Cu/Ni specimensare significantly different. Figure 8(a) showsa freely corroding broken and fissuredsurface. Figure 8(b) is a film grown in the presenceof polymer; this surface is covered with projections which take the form of flat plateaux. The oxide growth (Fig. 8(a)) for the freely corroding specimenis constant and follows a rectilinear equation. The plateaux in Figure 8(b) have been formed by an equivalent number of coulombs. In this case growing active centres have been gradually suppressedby polymer adsorption. Minor plateaux can also be observed and could either be (a) nucleation sitessuppressedearly in growth or (b) fresh active centres suppressedafter a redistribution of corrosion centres from incipient active sites. Confirmation of the mechanism of inhibition, i.e. suppression of active sites, is observed when specimensare cleaned, cathodically reduced and allowed to come into contact with oxygen-containing water with and without inhibitor. Unrestricted growth and polymer inhibition of freshly cleaned surfaces have been arrested at a very early stage (within 30s) and examined. Micrographs (Fig. 8 (c) and (d)) show that uninhibited specimensform nucleation sites (which are suppressedby removal from solution before appreciable reaction takes place, but which would normally continue to grow outwards) and inhibited specimens show almost complete suppressionof active sitesbefore they have any opportunity to develop. Experimentally, it is difficult to assign a growth law to film build up and frequently the type of growth changes with film thickness.20However, the inhibited films exhibit an approximately logarithmic rate law, i.e. W = k, log (k,t + 1). An increasein polymer increasesthe rate of nucleation of the surface film, and passivity is soon established (Fig. 6). The inhibition of corroded surfacesis a slower process; presumably the diffusion of colloidal polymeric flocculant to the incipient active centres is restricted by the porosity of the existing corrosion products.
404
B. C. EDWARDS
Polymers adsorb strongly on to suitable surfacesand once adsorbed, their influence on crystal morphology persistseven in the absenceof bulk polymer solution (Fig. 5). There can be little doubt of the effectiveness of organic polymers as corrosion inhibitors in neutral aqueous environments. However, much work is required to obtain (1) a fuller understanding of the mechanism, and (2) suitable polymers for use under all conditions; for example, by including certain groups in the polymer moleculessuch as aryl amines, it might be possible to inhibit the corrosion of steel in acid or aqueous environments. Acknowledgements-The author wishes to express his gratitude to Dr. J. A. Kitchener for his helpful advice and criticism during this work. Financial support from the Central Electricity Generating Board and assistance from its staff are gratefully acknowledged. REFERENCES 1. B. C. EDWARDS, Ph.D. Thesis, University of London (1967). 2. S. K. KUZ’KIN and V. P. NEBERA, Synthetic Floccrrlants in Dewatering Processes, Translation, Nat. Lending Library, Boston, G.B. (1966). 3. J. B. POOLE and D. DOYLE, Solid-Liquid Separation, Ministry of Technology,
Moscow (1963) ; Her Majesty’s
StationeryOffice, London(1966). 4. K. SAKA~UCHI and F. NAG.&, Bull. them. Sot. Japan 39, 88 (1966). 5. W. E. ZIMMIE and R. F. W. BLOECHER.U.S. Patent 308.591.6 (19631. 6. A. SHERRYand E. R. GILL, Chemistry’and Industry 102’(19&$ ’ 7. W. E. ZIMMIE, paper presented to the Nat. Ass. Corros. Engrs, Nat. Meeting, St. Louis, Missouri, March (1965). 8. J. K. MARSHALL and J. A. KITCHENER, J. Colloid Sci. 22, 342 (1966). 9. E. R. GILL, personal communication (1967). 10. J. I. BREGMAN, Corrosion Inhibitors. Macmillan, New York (1963); I. N. PUTILOVA, S. A. BALEZIN and U. P. BARRANIK, Metallic Corrosion Inhibitors. Pergamon, London (1960). 11. H. S. CA~~PBELL, I. appl. Chem. 4, 633 (1954); H. S. CAMPBELL, Trans. Faraday Sot. 50, 1351 (1954); H. S. CAMPBELL. J. Inst. Metals. 77. 345 (1950). 12. H. S..&AACS and J. S. L. LEACH, J. electrochem. ioc. ilO, 680 (1963). 13. D. J. G. IVES and A. E. RAWSON, J. electrochem. Sot. 109,452 (1962). 14. W. MACHU and V. K. Go~A, Werkst. Korros. 13, 745 (1962). 15. G. PETERSONand T. K. KEWI, J. phys. Chem. 65, 1330 (1961). 16. G. A. PARKS, Chem. Rev. 65, 177 (1965). 17. W. J. BAYNARD and J. J. RANDALL, JR., J. electrochem. Sot. 108, 822 (1961). 18. N. B. PILLMG and R. E. BR)WORTH, J. Insf. Metals 29, 529 (1923). 19. R. H. DOREMUS, B. W. ROBERTS and D. TURNBULL (Eds.), Growrh and Perfection of Crystals. Wiley, New York (1963); S. W. SEARS,pp. 441-445; N. CABERA and D. A. VERMILYEA, pp. 392408.
K. HAUFFE, Oxidation of Metals. Plenum, London (1965). 21. M. POURBAIX, Atlas of Electrochemical Equilibria. Pergamon, London (1966). 20.