SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 28, 119–122 (1999)

EIS and XPS Study of the Corrosion of CarbonSteel in Inhibited Natural Seawater

J. H. Qiu* and P. H. ChuaDivision of Materials Engineering, School of Applied Science, Nanyang Technological University, Nanyang Avenue, Singapore639798

Electrochemical and surface analytical techniques were used to study the corrosion behaviour of a carbonsteel in inhibited seawater. Linear polarization resistance and Tafel polarization measurements showed theeffect of inhibitor injection on the corrosion rate of carbon steel in natural seawater. A 90% reduction in thecorrosion rate was observed for the steel under investigation.In situ electrochemical impedance spectroscopy(EIS) measurements indicated that the inhibiting film formed in seawater was stable and no pitting corrosionwas observed. Surface analysis of steel samples by XPS revealed conclusively that the inhibiting film wasthree-dimensional in nature because the steel surface was essentially buried under it. Copyright 1999John Wiley & Sons, Ltd.

KEYWORDS: carbon steel; inhibitor; seawater; XPS; impedance; corrosion

INTRODUCTION

Corrosion is an interfacial reaction at the metal/environ-ment interface and the reaction processes are most oftenelectrochemical in nature. It is thus desirable to understandnot only the electrochemical characteristics of a corro-sion process but also the surface and interface propertiesthat affect corrosion. With recent advances in vacuumtechnology and electrochemical instrumentation, corro-sion researchers can now turn to surface-sensitive tech-niques such as XPS and AES for characterization of thesurface and interface properties.1 – 7 The traditional elec-trochemical techniques (d.c. polarization, a.c. impedance)and surface analytical techniques complement each otherin painting a complete picture of corrosion.6,7 This isparticularly true in the case of corrosion inhibitor stud-ies, where the surface and interface properties of themetal/environment system are important.

Commercial formulations of corrosion inhibitors arecomplex mixtures of organic and inorganic compoundsthat, when added to a metal/environment system, cansignificantly reduce metal losses. The performance of cor-rosion inhibitors is often evaluated using electrochemicaltechniques that measure the corrosion rate of a metal in acorrosive environment.2,3 It is, however, also very impor-tant to study the interaction of inhibitor molecules with ametal surface under realistic conditions of service. This iswhere surface analytical techniques can be used to provideunique surface-sensitive information about the protectedmetal, because most surface analytical techniques have aninformation depth of only a few nanometres. Consideringa metal coupon exposed to an inhibited solution, the sur-face state of the protected metal will be changed due tothe interaction of inhibitor molecules and the metal. Using

* Correspondence to: J. H. Qiu, Division of Materials Engineer-ing, School of Applied Science, Nanyang Technological University,Nanyang Avenue, Singapore 639798.E-mail: asjqiu@ntu.edu.sg

surface-sensitive techniques such as XPS, it is possible todetect the adsorbed or surface active components of theinhibitor formulation. These components and the nature oftheir interaction with the metal surface often play a vitalrole in reducing metal loss.

This work aims to investigate the corrosion behaviourof a carbon steel in inhibited and uninhibited naturalseawater through the combined use of electrochemical andsurface analytical techniques.

EXPERIMENTAL

Materials and test solution

Commercial carbon steel specimens (composition: 0.5%C, 0.5% Mn, 0.25% Si) were mounted in cold-settingAcrylic resin (exposed area: 1.3 cm3) and polished downto a 1 µm finish. The polished specimens were washedwith distilled water and dried in laboratory air.

Electrolyte used was natural seawater taken from Tuasin Jurong (Singapore). The inhibitor used in this work wasan amine-based formulation. Specimens were immersedin electrolyte for 15 min before each test, to stabilizethe open-circuit corrosion potential. To minimize cross-contamination, fresh seawater and newly polished speci-mens were used for each test.

Electrochemical measurements

Direct current techniques. The open-circuit potentialEcorr

vs. exposure time for steel specimens immersed in nat-ural seawaters with and without inhibitor injection wasrecorded. Linear polarization resistance (LPR) was usedto determine the corrosion rates of steel coupons immersedin natural seawater with and without inhibitor injection.The inhibiting efficiency was then calculated from thepolarization resistance measurements. Tafel polarization

CCC 0142–2421/99/130119–04 $17.50 Received 30 November 1998Copyright 1999 John Wiley & Sons, Ltd. Accepted 19 January 1999

120 J. H. QIU AND P. H. CHUA

was also conducted to investigate the corrosion behaviourof the carbon steel in the inhibited seawater. From Tafelpolarization measurements, an anodic potential at whichthe inhibited and uninhibited seawater showed greatestdifference in the corrosion rate was determined, and poten-tiostatic measurements at that potential were carried out.All potentials reported are relative to the saturated calomelelectrode (SCE).

Alternating current impedance measurement. Alternatingcurrent impedance measurements were conducted atopen-circuit potential over a frequency range of10 mHz–10 kHz and at an amplitude of 10 mV.

Surface analysis by XPS.Surface analyses of specimensbefore and after immersion in the inhibited and uninhib-ited seawater were carried out.

RESULTS AND DISCUSSION

Corrosion potential vs. time

The changes in open-circuit potential for steel specimensin inhibited and uninhibited seawater were monitored. Forthe uninhibited system, the open-circuit potential drifts inthe negative direction from an initial value of�580 mVto �700 mV over the first 15 min of immersion. Thisrapid change in open-circuit potential is caused by therapid loss of air-formed oxide in seawater. Thereafter, theopen-circuit potential stabilizes in the range of�700 to�750 mV, which is in close agreement with a previouslyreported value.4 At the end of the 1 h immersion test,the steel surface was seen to be covered partially byyellow-green deposits.5 In contrast to this, the open-circuitpotential of the steel specimen in the inhibited seawaterremains very stable at a value of�415 mV over the entireduration of the test. There were no visible rust depositson the steel surface at the end of the 1 h immersion test.This remarkable stability in the open-circuit potential iscertainly attributed to the interaction of steel with theinhibiting compounds of the corrosion inhibitor.

Quantitatively, the effectiveness of the inhibitor inreducing the corrosion rate can be determined by d.c.polarization or a.c. impedance measurements, whereas themechanisms of the interfacial reactions at the metal/elec-trolyte interface can be revealed byin situ a.c. impedancemeasurements andex situsurface analysis of steel samplesafter immersion in inhibited seawater.

Polarization measurements

The polarization resistance for steel immersed in blankand inhibited seawater was measured to be 721.0 and12040�Ðcm2, respectively. As the polarization resistanceis inversely proportional to the corrosion rate, the inhibit-ing efficiency of the inhibitor can thus be calculated from

� D(

1� Rp

Rp,i

)ð 100%

where� is the inhibiting efficiency andRp and Rp,i arethe polarization resistances of steel in blank and inhibited

seawater, respectively. It is seen that a 94% reduction inthe corrosion rate of carbon steel was achieved in inhibitedseawater.

Tafel polarization

The effect of inhibitor injection on the corrosion behaviourof carbon steel can be revealed to some extent by Tafelpolarization measurements. Figure 1 shows a marked dif-ference between the blank and inhibited seawater systems.The open-circuit potential was shifted by>200 mV inthe positive direction by the inhibiting compounds andthe anodic current density was reduced significantly atpotentials up to�100 mV. Thereafter, the effect grad-ually diminished and at 0 mV the anodic behaviour ofthe inhibited system became very close to the uninhibitedsystem.

Potentiostatic test

It was seen in Fig. 1 that at�220 mV the reductionin anodic current was most significant. A potentiostatictest was performed at this potential and the current decaywas monitored. Over the 1 h exposure period, the currentdensities for the inhibited and uninhibited systems werequite stable but with a great difference. At the end ofthe 1 h test, the current density for the inhibited systemwas 0.4 mA cm�2 and for the uninhibited system was24.5 mA cm�2.

Alternating current impedance measurements

Electrochemical impedance spectroscopy (EIS) is a power-ful in situ technique for the characterization of reac-tions at the metal/electrolyte interface, particularly forinhibitor studies. The adsorption and desorption of inhibit-ing species at the metal surface can be revealed by thechanges in the impedance characteristics. The impedanceand phase shift for steel samples upon immersion in blankand inhibited seawater were measured. Immediately uponimmersion, the inhibited system showed a more capacitive

Figure 1. Tafel polarization.

Surf. Interface Anal. 28, 119–122 (1999) Copyright 1999 John Wiley & Sons, Ltd.

CORROSION OF CARBON STEEL IN INHIBITED SEAWATER 121

Figure 2. Impedance and phase shift after 1 h of immersion.

behaviour with a much greater impedance value than thatof the uninhibited system. A similar trend is observed inFig. 2 for samples after 1 h of immersion. This seems tosuggest that the inhibiting compound formed at the metalsurface is the primary cause of corrosion inhibition, andthat the compound is stable during the 1 h test period.

Surface analysis by XPS

The surface of the metal after reaction with an elec-trolyte carries the fingerprint of the adsorbed or surfaceactive components. These components and the nature oftheir interaction with the surface of the metal often deter-mine the effectiveness of a corrosion inhibitor. Thus, it isimportant to analyse or probe the surface state of metalsamples after exposure to an inhibited electrolyte. X-rayphotoelectron spectroscopy is a powerful surface-sensitivetechnique that has an information depth of only a fewnanometres, so in studies of exposed metal samples onlythe adsorbed or surface active components of the formu-lation are detected.

A Survey scan of XPS spectra for the control sample(after polish and wash) and the samples after 1 h ofimmersion in blank and inhibited seawater is shown inFig. 3. For the control sample, the spectra consist ofcarbon, oxygen and iron photoelectron and Auger peaks.In the spectra for the sample immersed in blank seawater,extra peaks for magnesium, sulphur, chlorine and sodiumwere observed owing to the nature of blank seawater. Forthe sample after 1 h of immersion in inhibited seawater,the XPS spectra were drastically altered as comparedwith the control sample and the sample immersed inblank seawater. One of the most significant changes is theabsence of iron peaks. This is conclusive evidence thatthe inhibiting film with a thickness of>3 nm was formeduniformly on the metal surface such that the substrateiron is totally buried under the inhibiting compounds.In view of the surface sensitivity of XPS, iron signalswould have been detected for a thinner or patchy coverageof inhibitor. Another significant change is that nitrogenfrom amine is detected and the surface is seen to belargely carbonaceous, as indicated by the intensity of thecarbon peaks.

A narrow or high-resolution scan of XPS spectra canbe used to derive the chemical state information of the

Figure 3. The XPS spectra for steel: SP1 D polish C wash;SP2 D SP1C seawater; SP3 D SP2C inhibitor.

Figure 4. The XPS spectra of iron for steel.

surface. Figure 4 shows the high-resolution spectra ofiron for the control sample and the samples immersedin blank and inhibited seawater. Two peaks are observedfor the control sample at 706 and 710 eV (with referenceto C 1s at 284.8 eV binding energy), corresponding to themetallic and oxidized (Fe2C) states of iron. In this case,the air-formed oxide film after polish and wash was so

Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 28, 119–122 (1999)

122 J. H. QIU AND P. H. CHUA

Figure 5. The XPS spectra of nitrogen for steel.

thin (¾10 A) that the substrate metallic iron signal wasclearly detected. After 1 h of immersion in blank sea-water, however, the metallic shoulder at 706 eV in theiron spectra was gone, suggesting that the surface of steel

was totally covered by oxide. A drastic change in the ironspectra was observed for the steel sample immersed ininhibited seawater for 1 h. The weak or noisy iron sig-nal at 710 eV can only indicate that the steel substratewas buried underneath by the film-forming inhibitor com-pounds. Further analysis of the XPS spectra for carbon,oxygen and nitrogen identified the presence of carbonbonding to quaternary and tertiary amines (Fig. 5), as wellas hydrocarbon. It is these film-forming compounds at themetal surface that are responsible for the effectiveness ofthe inhibiting action. In fact, microscopic examination ofthe steel coupon after 1 h of immersion in inhibited sea-water showed no visible changes when compared with thecontrol sample.

CONCLUSION

The corrosion behaviour of carbon steel in inhibited nat-ural seawater was characterized usingin situ electro-chemical andex situ surface analytical techniques. Theimportance of analysis of the exposed metal surfacewas demonstrated. Results obtained from d.c. polarizationresistance and Tafel polarization agreed well with electro-chemical impedance measurements. The high inhibitingefficiency of the system was due to the presence of astrongly adsorbed inhibiting species on the metal surface.Surface analysis of the exposed samples by XPS identifiedthe chemical state of the surface active compounds andrevealed conclusively the three-dimensional nature of theinhibiting film.

REFERENCES

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2. I. L. Rosenfeld, Corrosion 37, 371 (1981).3. G. Trabanelli, Corrosion 47, 410 (1991).4. J. C. Rowlands, in Corrosion, 2nd Edn, Vol. 1, ed. by

L. L. Shreir, p. 54. Newnes Butterworths, Boston (1976).

5. U. R. Evans, in An Introduction to Metallic Corrosion, 3rd Edn,p. 32, Edward Arnold, London (1981).

6. J. E. Castle and J. H. Qiu, Corros. Sci. 29, 591 (1989).7. J. H. Qiu and J. E. Castle, J. Electrochem. Soc. 138, 1908

(1991).

Surf. Interface Anal. 28, 119 122 (1999) Copyright 1999 John Wiley & Sons, Ltd.