Applied Surface Science 255 (2009) 9193–9199

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Applied Surface Science

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In situ characterization of localized corrosion of stainless steel by scanningelectrochemical microscope

Yuehua Yin a, Lin Niu a,*, Min Lu a, Weikuan Guo a, Shenhao Chen a,b

a School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, Chinab Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

A R T I C L E I N F O

Article history:

Received 30 December 2008

Received in revised form 30 June 2009

Accepted 1 July 2009

Available online 8 July 2009

Keywords:

Localized corrosion

304 stainless steel

Characterization

In situ

Scanning electrochemical microscope

(SECM)

A B S T R A C T

Scanning electrochemical microscopy (SECM) area scan measurements have been performed to

investigate the localized corrosion of type 304 stainless steel in neutral chloride solution. Variations in

the Faradaic current measured at selected tip potential values can be related to changes in the local

concentration and electrochemical activities of electroactive species involved in corrosion reactions

occurring at the substrate as a function of immersion times of the substrate and polarized currents or

potentials applied on the substrate. To further verify the results acquired from cyclic voltammetric

experiments, SECM measurements were employed to in situ study the compositions and

electrochemical activity distribution profile of the pitting corrosion products of stainless steel. It has

been demonstrated that the combination of feedback current mode with generation-collection (G-C)

mode of SECM is suitable to elucidate the possible reaction mechanisms and paths involved in the

localize corrosion of stainless steel in neutral chloride solution.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The study of localized electrochemical reactions occurred onmetal surfaces is of great significance to corrosion science.Generally, conventional electrochemical techniques such assteady-state polarization curves and electrochemical impedancespectrum (EIS) [1–5] are capable of providing insight into corrosioncurrent and/or corrosion rate at surface, but this information isactually the average electrochemical behaviour of the total surface.

In more recent years, research has focused on understandingthe problem from a surface science perspective to determine thelocalized chemistry involved in localized corrosion processes. Toachieve the goal, a series of local electrochemical scanningtechniques have been developed and applied in electrochemistryand corrosion studies, including scanning reference electrodetechnique (SRET) [6–8], scanning vibrating electrode technique(SVET) [6,9–11], local electrochemical impedance spectrum (LEIS)[6,12,13], scanning Kelvin probe (SKP) [6,14,15], electrochemicalmicrocell technique [6,16,17] and scanning electrochemicalmicroscope (SECM), etc. Among these techniques, the SECM hasbecome a very powerful technique for probing a variety ofelectrochemical reactions in corrosion process owing to its highspatial resolution and electrochemical sensitivity to characterizethe topography and redox activities of the metal/solution interface.

* Corresponding author. Tel.: +86 531 88361385; fax: +86 531 88564464.

E-mail addresses: lniu@sdu.edu.cn, niulin80@yahoo.com.cn (L. Niu).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.07.003

Its capability for the direct identification of chemical species (i.e.

corrosion products) in localized corrosion processes with highlateral resolution would be greatly advantageous. For example, ithas been used to characterize electroactive defect sites in passiveoxide films on aluminium [18] and titanium [19], local breakdownof passive layer and pitting of iron [20–25], concentration profilesof redox-active species of iron dissolution [26,27], role of sulfurspecies inclusion dissolution on the initiation of pitting corrosionof stainless steels [28–31], metastable pit nucleation [28,32,33],dynamic nature and product distribution of localized corrosion onstainless steel [34–36], degradation of organic coating propertieson steels [37–39], and galvanic corrosion of a Zn/steel couple[10,40].

In the present study, the SECM area scan measurements wereperformed to investigate the effects of factors, including immer-sion times of stainless steel substrate in NaCl solution and thepolarized currents or potentials applied for the substrate, on thesize and shape of current peaks in SECM scan area images,associating with the proceedings of initiation and growth oflocalized corrosion at sites of passivated films with inhomogeneityand/or dislocation. In addition to the cyclic voltammetric method,the SECM images were used to directly study the compositions andelectrochemical activity distribution profile of the pitting corrosionproducts of stainless steel. It was indicated that the combination offeedback current mode with generation-collection (G-C) mode ofSECM was capable of elucidating the possible reaction mechanismsand paths involved in the localize corrosion of stainless steel inneutral chloride solution.

Fig. 1. SECM area scan images of 304 stainless steel in 0.1 M NaCl solution at

different immersion times. (a) 2 h, (b) 5 h and (c) 15 h. Is = �80 mA, Et = 0.56 V.

Y. Yin et al. / Applied Surface Science 255 (2009) 9193–91999194

2. Experimental

2.1. Chemicals and materials

The electrolytes considered were 0.1 M NaCl and 0.1 MNaCl + 10 mM KI solutions. They were prepared from analytical-grade chemicals and twice-distilled water. All measurements wereperformed at ambient temperature in the naturally aeratedsolutions. The specimen of type 304 stainless steel (1 mm2) wasmounted into an epoxy resin sleeve, such that only the circular endsurface formed the testing steel face (substrate electrode). Beforeeach experiment the specimen was polished mechanically withsilicon carbide paper to 2000# and 4000# grit finish, and cleanedthoroughly with twice distilled water.

2.2. Apparatus and methods

The SECM area scan and cyclic voltammetric measurements wereperformed on scanning electrochemical microscope (SECM 270,UNISCAN Instruments Ltd., UK). The tips for the SECM measure-ments were glass insulated, disk-shaped Pt microelectrode whichwas fabricated from 25 mm diameter Pt wires. The tip was polishedwith 0.05 mm alumina polish, then cleaned with twice distilledwater prior to use. A saturated calomel electrode (SCE) was used as areference electrode and a looped Pt wire surrounding the specimenas a counter electrode. All the potentials in the experiments werereferred to SCE. The electrochemical cell with the specimen pressfitted at its bottom hole was mounted on the three-axis translationstage. Specimen was mounted horizontally facing upwards. A videomicroscope (CCD) was employed to aid in accurately positioning thetip over the specimen (substrate). The SECM measurements wereperformed with the tip at a height of 30 mm over the specimensurface, unless special tip-specimen distances were selected. Thepotentials of the tip and specimen were controlled by a bipotentio-stat (PG 580, USA). Potentiostatic or galvanostatic polarization wasapplied for the specimen to induced localized corrosion, while thetip was set at appropriate potentials for the electroactive speciesresulted from corrosion to undergo a redox reaction.

3. Results and discussion

3.1. Localized corrosion as a function of immersion and polarization

3.1.1. SECM images—effect of immersion times

The SECM area scan images of 304 stainless steel in 0.1 M NaClsolution with different immersion times were measured, whilegalvanostatically polarizing the stainless steel sample with anodicoxidation currents of�80 mA and setting the SECM Pt tip potentialat +0.56 V, as shown in Fig. 1. It can be seen that the contour plotclearly showed a localized production of Fe2+ ions above the defect.Moreover, the shape and size of current peak varied distinctly withthe increase of the immersion times. In the initial stage ofimmersion, a single sharp current peak appeared on the samplesurface, implying the sites of passivated films with inhomogeneityand/or dislocation may be readily attacked by chloride ions.Subsequently, pitting corrosion was induced by localized anodicdissolution occurred on the stainless steel sample surface,resulting in a high ratio of depth to width of the pit (Fig. 1(a)).With the extension of the immersion time, the anodic dissolutionrate at the depth direction of the pit slowed down, while the anodicdissolution developed along the side wall of the pit, exhibiting acurrent peak with progressively shortened ratio of depth to widthand at some sites even displaying multiple current peaks or asshoulders of the larger peak (Fig. 1(b) and (c)). In addition to theremarkable variations of size, shape and number of current peakswith extending immersion times, an increase in the background

current was also observable, which was likely due to accumulationof electroactive ionic species near the steel surface which diffusedvery slowly into the bulk. Generally, the background currentdetected away from the defect area is considerably smaller thanthe anodic tip current over a defect [27].

Y. Yin et al. / Applied Surface Science 255 (2009) 9193–9199 9195

3.1.2. SECM images—effect of polarization currents

Fig. 2 depicted the SECM area scan images of 304 stainless steelin 0.1 M NaCl solution while galvanostatically polarizing thestainless steel sample with anodic currents of�50 mA,�80 mA and

Fig. 2. SECM area scan images of 304 stainless steel in 0.1 M NaCl solution with

different anodic polarizing currents on the steel substrate. (a) �50 mA, (b) �80 mA

and (c) �120 mA. Et = 0.56 V.

�120 mA, respectively, and setting the Pt tip potential at +0.56 V. Itwas found that the current peak grew gradually as the polarizationcurrents passing through the stainless steel substrate increased,indicating the amounts of the Fe2+ ions emanating from the steelsubstrate dissolution and subsequently reaching the Pt tip bydiffusion process increased noticeably, which could be ascribed tothe promoting effect of increasing oxidation current on the growthof pitting corrosion. When the polarization current applied onstainless steel substrate reached the highest level of �120 mA, alarger peak accompanying multiple smaller peaks appeared on theSECM images, demonstrating the fact that higher oxidationcurrents not only promoted the growth rates of pitting corrosionbut also readily induced more defects like pits, that is, theprobability of pitting corrosion initiation rose remarkably withincreasing polarization currents. Lister and Pinhero [31] proposeda method for the first time for imaging localized sulfurconcentrations dissolved from sulfide inclusions in type 304stainless steels and found that while galvanostatically inducingcorrosion a current inversion at the microelectrode was observedat higher sample current densities in close proximity to the areawhere sulfur was detected at lower sample current.

3.1.3. SECM images—effect of polarization potentials

The SECM images are capable of exhibiting activity distributionprofile of corrosion products of substrate more directly comparedto the traditional electrochemical methods such as cyclic voltam-metric measurement [30]. Usually, concentration profiles of theredox species participating in the anodic and cathodic microcellsdeveloped on the metal surface can be measured from theappropriate option of the tip potential. In other word, the imageswere generated by setting the probe tip potential to detectelectroactive species from the localized corrosion sites andmonitor the anodic current there from [27]. Moreover, SECMevidently detected those sites that were most vulnerable and thosepits that would initiate earliest, while the visibility of the peaksdepended largely on the potential applied to the metal substrate(Es).

Fig. 3 displayed the SECM images with the 304 stainless steelelectrode held at fixed potential of 0.05 V, 0.1 V and 0.15 V,respectively, in 0.1 M NaCl solution while setting the SECM Pt tippotential at +0.56 V. As the lowest polarization potential (0.05 V)was applied to the substrate electrode, the peak currents appearingin the area scan images of Fig. 3(a) corresponded most likely to thecurrent fluctuation of metastable pitting corrosion events on thestainless steel substrate [31]. In this case, the process of pittingcorrosion initiation yielded small amounts of Fe2+ ions whichdiffused to Pt tip and were oxidized thereby and eventually led tothe enhancement of tip currents. On the whole, however, thegrowth and decline of the metastable pitting corrosion in generalgave rise to low electrochemical activities. As the potentialsapplied to the substrate shifted gradually to more positive values,not only the number of the pitting corrosion but simultaneouslyalso the electrochemical activities corresponding to the pittingcorrosion grew substantially, which likely matched the formationof stable pitting corrosion (in Fig. 3(b) and (c)). In short, by applyingappropriate polarization potentials for substrate, SECM area scanwas able to detect the pitting corrosion events at different stages,from metastable to stable states [28]. Accompanying the positiveshift of polarization potentials applied to the substrate, the electricfield intensity within passivated film would thereby rise, allowingthe Cl� ions to easily permeate the passivated film and as a resultinducing pitting corrosion. Generally, the higher the polarizationpotentials applied on the substrate, the more corrosion productsresulting from the active dissolution and the higher the Faradaiccurrents on the tip, that is, the higher the correspondingelectrochemical activities.

Fig. 3. SECM area scan images of 304 stainless steel in 0.1 M NaCl solution with

different anodic polarizing potentials on the steel substrate. (a) 0.05 V, (b) 0.1 V and

(c) 0.15 V. Et = 0.56 V.

Fig. 4. Cyclic voltammograms of tip in 0.1 M NaCl + 10 mM KI solution in the

absence of stainless steel substrate. Sweep rate:10 mV/s.

Y. Yin et al. / Applied Surface Science 255 (2009) 9193–91999196

3.2. Concentration and electrochemical activity distribution

profile of localized corrosion products

3.2.1. Cyclic voltammograms of tip—effect of substrate and

tip-substrate distances

Taking advantage of the characteristic of SECM over traditionalelectrochemical methods to study localized corrosion of metals, in

this section, we employed both SECM and cyclic voltammetricmeasurements to study compositions and the concentrationdistribution profile of pitting corrosion products as well as therelated electrochemical activities. The combination of feedbackcurrent mode with generation-collection (G-C) mode of SECM willbe adopted to elucidate the possible reaction mechanisms andpaths involved in the localize corrosion of stainless steel in neutralchloride solution.

In the absence of stainless steel substrate, the cyclic voltam-mograms of Pt tip in 0.1 M NaCl + 10 mM KI solution at sweep rateof 10 mV/s was shown in Fig. 4. It was to be noted that an oxidationcurrent peak appeared at around 0.5 V in the forward sweep.Thereafter, the tip currents dropped sharply to a lowest point, roseagain and eventually reached a current plateau after 0.55 V, themagnitude of the plateau current (i.e. steady-state diffusioncurrent) was about �1.7 � 10�8 A. According to the potentialcorresponding to the oxidation peak and the character of thecurrent plateau, it could be deduced that the reactions taking placeon the tip would be oxidation of I� ions as follows:

2I� �2e� ! I2 (i)

I2þ I� ! I3� (ii)

or expressed with a single reaction (iii) by combination of reactions(i) and (ii):

3I� �2e� ! I3� (iii)

The sharp drop of the tip currents after the peak current may berelated to the low solubility of the oxidation product (I2) and itsweak adsorption on the surface of tip [31]. The plateau currentcorresponded to the steady-state limiting diffusion currentsoriginating from the oxidation reaction (iii). The ionic couple I�/I3� in this case played the role as a redox mediator in the feedback

current and, additionally, the I�/I3� was involved in the reactions

with corrosion products in association with substrate generation-tip collection (SG-TC) mode of SECM.

To study the compositions and electrochemical activitydistribution profile of the products of pitting corrosion on stainlesssteel, SECM tip could be placed over an individual active site (e.g. apit) and the tip current was collected. Images of the surfacefollowing corrosion indicated the existence of pits on the surface,suggesting that the events observed would be due to corrosionprocesses exposing metallic sites for electron transfer to occur [34].In the present study, the stainless steel substrate was firstly

Fig. 5. Cyclic voltammograms of tip in 0.1 M NaCl + 10 mM KI aqueous solution in

the presence of stainless steel substrate at different distances between tip and pit.

(a) 30 mm, (b) 100 mm and (c) 600 mm. Sweep rate: 10 mV/s.

Y. Yin et al. / Applied Surface Science 255 (2009) 9193–9199 9197

potentiostatically polarized at 0.15 V in 0.1 M NaCl + 10 mM KIsolution to induce pits as a role of active sites. Then, the cyclicvoltammetric measurement of tip in 0.1 M NaCl + 10 mM KIaqueous solution was performed at different heights (30 mm,100 mm and 600 mm, respectively) above the pit resulted frompotentiostatic polarization as mentioned above. The CV resultswere shown in Fig. 5. It was interesting to note that theconcentration profile of Fe2+ ions was highly dependent on thedistance from the steel sample to tip. With increasing the distancebetween tip and the pit, on one hand, the tip current peakdecreased distinctly and the potentials corresponding to thecurrent peak slightly shifted to the positive values; on the otherhand, the peak width enlarged and the plateau currents (i.e. steady-state diffusion currents) reduced progressively in the meanwhile.As a matter of fact, the ratio between the intensity of the currentpeak and the corresponding plateau current greatly increased asthe tip height over the sample became smaller.

The steady-state limiting diffusion currents at the tip can beexpressed as follows [10,30]:

It;1 ¼ 4nFDCa (1)

where n is equal to 1, F is the Faraday constant, D is the diffusioncoefficient, C is the local ferrous concentration for the reaction anda the Pt disk electrode radius. Clearly, the limiting diffusioncurrents (It,1) would be proportional to the concentration ofelectroactive species (C) if other parameters in Eq. (1) keptconstant. It was to be expected that with the increase of thedistance between tip and pit, the amount of the electroactivespecies that diffused to the tip diminished and consequently led tothe progressive fell of the steady-state diffusion currents. Althoughthere was no proportional relation between the It,1 and tip-substrate distance, as the tip gradually approached the substrate,the shorter and shorter diffusion path from substrate to tip wouldpromote the corrosion product (Fe2+) to be diffused to the tip morequickly and effectively in comparison with that in the case of alonger tip-substrate distance. Therefore, the peak current in Fig. 5was an inverse proportion to the tip-substrate distance.

In addition to concern the qualitative correlation of limitingdiffusion currents (It,1) with tip-substrate distances, it would bebeneficial to quantitatively elucidate the relation by consideringthe reactions probably occurred on the tip. As shown in Fig. 5, whenthe distance between tip and pit was 600 mm, the steady-statediffusion current on the tip was around �1.7 � 10�8 A, inagreement with the tip steady-state diffusion current shown in

the CV plot which was measured in the same solution while in theabsence of stainless steel substrate (Fig. 4). This fact, therefore,indicated that at the tip-substrate distance of 600 mm or aboveonly oxidation reaction represented by oxidation reaction (iii)most likely occurred on the tip, while no products of pittingcorrosion diffused to the tip and being oxidized therein. If thedistance between tip and pit was reduced gradually to 100 mm and30 mm, the corresponding diffusion currents on the tip increased toa level of�6 � 10�8 A and�1.5 � 10�7 A, respectively. Apparently,both of them were higher than the diffusion current on the tip(�1.7 � 10�8 A) resulting only from the oxidation reaction (iii) inthe absence of stainless steel substrate (Fig. 4). In the case of shortdistance between tip and pit, therefore, it can be proposed that inaddition to the oxidation reaction (iii) the other oxidation reactionshown by (iv) as follows may take place simultaneously on the tipand, consequently, the currents on the tip would be the sum ofboth oxidation reactions of (iii) and (iv).

Fe2þ � e� ! Fe3þ (iv)

Next, SECM experiments will provide convincing evidences tosupport the point of views in this section.

3.2.2. SECM images—effect of redox mediator I�/I3�

To verify the possibility that, in addition to the oxidation of I�

ions in reaction (iii), the oxidation of Fe2+ ions in reaction (iv)would lead likely to the additional increase of tip diffusion currentin the presence of stainless steel substrate (Fig. 5) and the tip wasvery near to the pit (e.g. below 100 mm), SECM area scanexperiments were performed to directly display images whichmay be associated with the reactions taking place on both stainlesssteel substrate and tip, by combining current feedback mode withsubstrate generation-tip collection (SG-TC) mode.

Fig. 6 showed the SECM images of 304 stainless steel in 0.1 MNaCl and 0.1 M NaCl + 10 mM KI solutions, respectively, whilepolarizing the stainless steel sample potentiostatically at 0.15 V,setting the Pt tip potential at +0.56 V and controlling the tip-substrate distance at 30 mm. It was observed that the peak currenton tip in 0.1 M NaCl solution as shown in Fig. 6(a) was around�1.1 � 10�8 A, and the background current located nearly at zero.The reaction paths involved in Figs 4 and 5Figs. 5 and 4(a) may besuggested reasonably as follows:

In comparison, the situation in the NaCl solution containing I�

ions was much different from that in the NaCl solution, similar tothe cyclic voltammogram responses of a SECM tip in a 0.1 MNaCl + 10 mM solution and in a blank solution containing onlysupporting electrolyte (0.1 M NaCl) [30]. As shown in Fig. 6(b), thepeak current on tip in 0.1 M NaCl + 10 mM KI solution was around�1.6 � 10�7 A, while the background current was about�2 � 10�8 A which was close to the tip steady-state diffusioncurrents (�1.8 � 10�8 A), originating from the oxidation of I� ionsin reaction (iii), displayed in the CV plot of tip in the same solutionwhile in the absence of stainless steel substrate (Fig. 4). Obviously,the I� ions added to the solution had played an important role inthe electrode reactions on both tip and the stainless steel substrate.It would be worthwhile to clarify the role of the I� ions [30,34].

By comparing the peak current intensity in Fig. 6(a) and (b), itwas observed that the peak current (��1.4 � 10�7 A) afterdeducting the background current in Fig. 6(b) was much higherthan the peak current on tip (�1.1 � 10�8 A) originating only fromthe oxidation of Fe2+ in reaction (iv) (Fig. 6(a)). It is of interest to

Fig. 6. SECM area scan images of 304 stainless steel in (a) 0.1 M NaCl and (b) 0.1 M

NaCl + 10 mM KI solutions. Es = 0.15 V, Et = 0.56 V.

Fig. 7. Schematic diagram of the SECM detection of Fe2+ ions above pit in 304

stainless steel.

Y. Yin et al. / Applied Surface Science 255 (2009) 9193–91999198

probe the possible local chemistry involved with such high peakcurrent as �1.4 � 10�7 A on the tip. By comprehensively con-sidering the possible reactions taken place and thus contributed tothe characteristic of both peak current and background current inFigs. 5 and 4(b), the following reaction schemes could be proposed:

Dissolutionreaction on substrate : Fe � 2e� ! Fe2þ (v)

It was worth noting that, in addition to the reactions (iv) and (v)occurred through substrate generation-tip collection (SG-TC)mode which associated with the occurrence of pitting corrosion,the I3

� yielding on the tip expressed by reaction (iii) was reducedby Fe2+ to I� ions as shown by reaction (vi) which diffused and fedback to the tip and were oxidized to I3

� again and, as a result, thecurrent on the tip was enhanced further. As a matter of fact, thereactions (iii) and (vi) together constituted the feedback currentcircuit and consequently enhanced the tip current. The reactionschemes as mentioned above can be represented by the schematicdiagram in Fig. 7. It can be concluded that, therefore, the

compositions and the electrochemical activity distribution profileof pitting corrosion can be studied by the combination of substrategeneration-tip collection (SG-TC) mode with feedback currentmode of SECM. As reported previously, by using redox mediator I�/I3� in the feedback current mode, Paik et al. studied the pitting

corrosion behavior of stainless steel in NaCl solution [30]. Theyproposed that the sulfur species (HS� or S2O3

2�) are slowlygenerated above previously identified MnS inclusions, however,they did not pay attention to the possible reduction action of Fe2+

presented in the pitting corrosion products.

4. Conclusions

(i) T

he size and shape of current peaks in SECM scan area imagesvaried distinctly with the increase of the immersion times ofstainless steel substrate in NaCl solution, indicating the courseof initiation and growth of localized corrosion at defect sites ofpassivated films.

(ii) T

he peak current in SECM scan area images grew gradually asthe palorization currents passing through the stainless steelsubstrate increased, demonstrating the amounts of the Fe2+

ions emanating from the substrate dissolution and subse-quently reaching the tip by diffusion process increasedremarkably.

(iii) B

y applying different polarization potentials for substrate andsetting appropriate tip potential, SECM area scan was able todetect the pitting corrosion events at different stages frommetastable to stable states. Generally speaking, the higher thepolarization potentials applied on the substrate, the morecorrosion products resulting from the active dissolution andthe higher the Faradaic currents on the tip and, consequently,the higher the electrochemical activities.

(iv) C

ompared to cyclic voltammetric method, the SECM area scanmeasurements possessed the advantages to in situ study thecompositions and electrochemical activity distribution profileof the products of pitting corrosion on stainless steel. Thecombination of feedback current mode with substrategeneration-tip collection (SG-TC) mode of SECM was suitable

Y. Yin et al. / Applied Surface Science 255 (2009) 9193–9199 9199

to elucidate the possible reaction mechanisms and pathsinvolved in the localize corrosion of stainless steel in neutralchloride solution.

Acknowledgements

This work was supported by the Special Funds for the MajorState Basic Research Projects (973 Projects) (Grant no.2006CB605004) and the Natural Science Foundation of ShandongProvince, China (Grant no. Y2006B16).

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