Corrosion Science 53 (2011) 448–457

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Corrosion Science

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Acoustic emission during the electrochemical corrosion of 304 stainless steelin H2SO4 solutions

Jian Xu, Xinqiang Wu ⇑, En-Hou HanState Key Laboratory for Corrosion and Protection, Liaoning Key Laboratory for Nuclear Material and Safety Assessment, Institute of Metal Research,Chinese Academy of Sciences, 62 Wencui Road, 110016 Shenyang, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 May 2010Accepted 24 September 2010Available online 1 October 2010

Keywords:A. Acid solutionsA. Stainless steelB. PolarizationC. Acid corrosionC. Transpassivity

0010-938X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.corsci.2010.09.056

⇑ Corresponding author. Tel.: +86 24 23841883; faxE-mail address: xqwu@imr.ac.cn (X. Wu).

Acoustic emission (AE) behaviour during the electrochemical corrosion of 304 stainless steel (304SS) inH2SO4 solutions was studied. AE signals which related to transpassive dissolution are detected in solu-tions with low pH, and are very slightly influenced by current density and pre-strain. During hydrogenbubble evolution, a weak correlation exists between the AE signal amplitude and the hydrogen bubblediameter. The concept of potential – pH – AE diagram is proposed and such a diagram is drawn basedon AE activity and b-values. The main mechanisms of AE sources which are transpassive dissolutionand hydrogen bubble evolution, are also discussed.

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1. Introduction

The acoustic emission (AE) phenomenon is the result oftransient elastic wave propagation generated by a rapid releaseof energy within a material due to changes in local stress and strainfields. Due to its advantages of continuous monitoring and littledisturbance to the object, AE technique has been used to monitorcorrosion behaviour of material since the 1970s [1,2]. Much ofthe work has been concerned with stress corrosion crackingprocesses [2–12] while only a small part has focused on purelyelectrochemical corrosion [13–22]. In the latter case, one aim ofthe research is to correlate AE activity with corrosion rate, whichcan be estimated in terms of hydrogen evolution rate [1], weightloss [13–16], surface damage [16], or applied current density[17]. Another aim is to find characteristic parameters of AE to iden-tify certain corrosion behaviour such as pitting [18–20] and crevicecorrosion [16]. Fregonese et al. [18,19] studied pitting corrosion of316L stainless steel intensively and used rise time and countsnumber to discriminate pitting initiation and propagation steps.In other work they obtained similar conclusions when monitoringcrevice corrosion of 304L stainless steel. Recently Jirarungsatianand Prateepasen [20] used duration to distinguish rupture of pas-sive film and pit growth, and got satisfying results. Modal AE tech-nique has been used [21] to analyze signals produced bypolarization corrosion of LY12CZ aluminium alloy. The results

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showed that there were two-type waveforms for both anodic andcathodic polarization.

The mechanism of AE sources during electrochemical corrosionis a matter of utmost concern for the reason that the success of AEwhen used as a tool for non-destructive testing depends largely onhow well the AE sources detected during experimental work can beinterpreted. Usually, the generation of acoustic signals is attributedto the formation and evolution of hydrogen bubbles [1]. More con-cretely, Fregonese et al. [19] assigned AE signals to the friction ofthe hydrogen bubbles along walls of the pits or the impacts onthe cap of the occluded cells, while according to Idrissi et al. [23]the release of hydrogen bubbles gave rise to AE signals. However,recently Jirarungsatian and Prateepasen [20] claimed the AE signalwas from bubble break-up and they also calculated the break-upfrequency which agreed well with experimental results. Neverthe-less, in another study [24] concerning stainless steel, Magaino et al.postulated that AE was associated with instantaneous stresschanges in the case when no bubbles were observed. In additionto the above mechanisms, rupture of a cap covering the pit [25]has also been proposed as the AE sources.

As mentioned above, a lot of work has been done to determinethe range in which the AE technique is suitable for detecting pureelectrochemical processes, but little of it has focused on transpas-sive dissolution though it is generally regarded as a type of corro-sion damage. One aim of the present work is to study the AEbehaviour during the transpassive dissolution process. Also, theAE behaviour recorded during hydrogen bubble evolution has beenstudied because some doubts still exist even though it is the mostcommonly accepted AE source.

Fig. 1. Specimen geometry.

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2. Experimental method

2.1. Material and specimen preparation

AISI 304SS plate, the chemical composition of which is given inTable 1, was used for this study. The material was solution an-nealed for 0.5 h at 1050 �C and quenched in water. Some speci-mens were pre-strained up to 30% and 50% respectively throughtensile tests. The specimens (Fig. 1) were cut from the plate andmachined in order to be fixed in a waveguide with a bolt throughthe hole. The waveguide was also made of austenitic stainless steeland the attenuation of AE signals caused by the waveguide wasabout 5 dB measured by the lead break test. The standard Hsu Niel-sen source was used (ASTM E976) and the Nielsen Shoe on HsuPencil Source was produced by PAC. First the lead was broken onthe surface of a specimen then on the waveguide, the differenceof the amplitude was defined as the attenuation. For each test,the attenuation measurement was done 3 times at least and themean value taken. The threshold was set at 25 dB. The specimenwas wet-ground to 1500 grit finish by using silicon carbide paper,rinsed with de-ionised water and ethanol then dried in a stream ofcool air.

2.2. Experimental apparatus

A schematic diagram of the experimental apparatus is shown inFig. 2. The specimen was partly immersed in the electrolyte andthe immersed area was 6 cm2. Polarization curves were conductedby an EG&G 273A Potentiostat controlled by PowerSuite software.AE instrumentation consisted of a wideband transducer from Phys-ical Acoustic Corp, a preamplifier and an acquisition device (PCI-2from PAC). A long-distance microscope (Questar QM-100) wasused to observe in situ the surface of specimen.

Fig. 2. Experimental apparatus.

Table 2Testing solutions used in the present work.

Number H2SO4 (mol/L) Na2SO4 (mol/L)

Solution 1 0.5 0.25Solution 2 0.05 0.25Solution 3 0.005 0.25Solution 4 5 � 10�5 0.25Solution 5 – 0.25

2.3. Experimental solutions

All the studies reported here were conducted at room tempera-ture in 0.25 mol/L Na2SO4 solutions with various H2SO4 concentra-tions (Table 2). The electrolytes were prepared from analyticalgrade chemicals and de-ionised purified water. For polarizationtests, a Pt counter electrode and a saturated calomel reference elec-trode (SCE) were employed. When the specimen (work electrode)is anodically polarized, the counter electrode is cathodically polar-ized. Hydrogen bubbles can generate on the counter electrodewhich can influence the specimen if two electrodes were put inone corrosion cell. Two corrosion cells connected with a salt bridgewere used.

2.4. Experimental procedure

Anodic and cathodic polarization curves were measured sepa-rately from open current potential (OCP) after 10 min immersionin the electrolytes with a scan rate 0.4 mV/s. The specimens werealso anodically polarized at 1.05 VSCE, 1.15 VSCE, and 1.25 VSCE (forpre-strained specimens only at 1.15 VSCE) respectively in solution1 to study the AE behaviour during the transpassive dissolutionprocess.

Table 1Compositions of 304SS investigated presently, wt.%.

C Si Mn Cr Ni

0.035 61.00 2.00 18.50–20.00 9.00

2.5. Hydrogen bubble experiment

A special experiment was designed to investigate the hydrogenbubble evolution process. The experimental apparatus was similarto that mentioned above except that the specimen was larger thanthe previous one to connect the transducer directly on it and wassealed by wax mixed with rosin. The exposed area in the solutionwas 2 mm � 1 mm approximately which suited the observationscope of the long-distance microscope, i.e. the long-distance micro-scope could image the whole exposed area. This enabled thehydrogen bubbles to be studied in detail and hence built a directlink between hydrogen bubble evolution and AE signals. The spec-imen was cathodically polarized from open current potential (OCP)

Cu Co B Fe

–10.00 1.00 60.06 0.0018 Bal.

Fig. 4. Anodic potentiodynamic polarization curves of annealed 304SS in varioussolutions.

Table 3Summary of some important potentials in Fig. 4.

Potential (VSCE) Solution 1 Solution 2 Solution 3 Solution 4 Solution 5

Firsttranspassivation

0.91 0.74 0.68 0.58 0.44

Secondpassivation

– 1.03 0.87 0.80 0.79

Secondtranspassivation

– 1.30 1.25 1.08 1.09

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to �0.8 VSCE after 10 min immersion in 0.05 mol/L H2SO4 solutionand the AE threshold was set at 30 dB.

3. Results

3.1. Microstructure observation

Fig. 3 shows the microstructures of annealed and pre-strained304SS. The annealed steel shows the typical equiaxed structure(Fig. 3a), while martensite appears in the pre-strained steels.Increasing the degree of pre-strain increases the proportion ofmartensite (Fig. 3b and c). According to Xu et al. [26], the amountof martensite under 30% and 50% pre-strain were (in weight per-cent) 5 and 7.5 respectively.

3.2. Anodic polarization

3.2.1. Polarization curvesThe anodic potentiodynamic polarization curves for annealed

304SS in various solutions are plotted in Fig. 4. Two passive regionsand two transpassive regions can be distinguished clearly from allthe curves except for the one in solution 1. Both of the passive cur-rents increase monotonically with the H2SO4 concentration. Someimportant potentials are summarized in Table 3. The mechanismsassociated with each region have been studied by Song et al.[27,28] thoroughly. It should also be noticed that the curves insolutions of lower H2SO4 concentrations (solutions 2–5) overlapto some extent in the high potential range (E P 1.45 VSCE approxi-mately), in which the oxygen evolution process was clearly ob-served by the long-distance microscope. In the case of solution 1,when the potential reaches a higher potential (1.55 VSCE approxi-mately), oxygen bubbles also emerge in agreement with the resultsof Song et al. [27,28].

Fig. 5 shows the anodic potentiodynamic polarization curves ofdifferent pre-strained 304SS in solution 1. The pre-strain seems tohave no obvious influence on corrosion resistance. This might berelated to the stability of the passive film in this solution [29].

Fig. 6 shows the current–time (I–t) curves of different potentio-static polarization processes in solution 1. At different appliedpotentials, the current density decreases at beginning and then

Fig. 3. Optical images of (a) solution-annealed (b) 3

tends to become constant. The small influence of pre-strain on cor-rosion resistance is confirmed again by the I–t curves.

3.2.2. AE resultsFig. 7 shows the evolution of AE data of annealed 304SS in var-

ious solutions in anodic potentiodynamic polarization processes.There are two stages in the AE hit-time response for the tests insolutions 2–5 (Fig. 7b–e). The potentials corresponding to the tran-sition times are similar for all cases above which the polarizationcurves are superposed as mentioned before. In this potential range,

0% pre-strained and (c) 50% pre-strained 304SS.

Fig. 5. Anodic potentiodynamic polarization curves of different pre-strained 304SSin solution 1.

Fig. 6. The potentiostatic polarization current densities of annealed 304SS atdifferent potentials applied.

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no obvious localized corrosion occurs and many oxygen bubblesemerge which are responsible for the AE signals. However, in solu-tion 1(Fig. 7a), three stages are observed and the first transitionpoint appears much earlier than that in the other four solutions.At the second stage (the corresponding potentials are 1 VSCE –1.55 VSCE approximately), there are no obvious oxygen bubbleson the surface and only slight corrosion is observed initially. Withincreasing potential, the localized corrosion becomes more andmore serious and finally a reticular corrosion zone appears. Atthe third stage, oxygen bubbles appear too. Similar results havebeen obtained for the pre-strained specimens.

Fig. 8 is the AE signals evolution during various potentiostaticpolarization processes in solution 1. The applied potentials arethe same as those in Fig. 6 and are also in the range of the secondstage shown in Fig. 7a. The amplitude and activity of the AE signalsdecrease as time increased in all cases and both the activity andamplitude of the AE signals seem to be independent of the currentdensity and the degree of pre-strain. This is different from the re-sults obtained from hydrogen bubble evolution [30] because theAE sources in the two studies are different.

3.3. Cathodic polarization

3.3.1. Polarization curvesFig. 9 shows the cathodic potentiodynamic polarization curves

for annealed 304SS in various solutions. Both in solution 1 and 2,

the cathodic currents increase linearly with the polarized potential.However, in the other three solutions, when the polarized potentialreaches a certain value, the currents become constant because theyare limited by the diffusion controlled process.

3.3.2. AE resultsThe AE results are shown in Fig. 10. Few AE signals are detected

in the polarization processes in solutions 4 and 5 and no obvioushydrogen bubbles are observed (Fig. 10d and e). In the other threesolutions when current densities reach certain values (in the mag-nitude of 10�4–10�3 A/cm2), AE signals increase significantly.Fig. 11 shows the hydrogen bubbles formed on the surface of thespecimen.

3.3.3. Hydrogen bubble experimentFig. 12 is the AE signal evolution during the potentiodynamic

polarization process in the hydrogen bubble experiment. Theexperimental conditions have been mentioned above. By usinglong-distance microscopy it is found that almost every AE signalis accompanied by the release of one or more hydrogen bubbles.Table 4 summarizes the characteristics of the first 17 AE signals,including the diameter of the released bubbles, and the amplitudeand peak frequency of the signals.

It is noted that the diameter of a high percentage of the bubblesis larger than 0.3 mm, which seems to be a necessary condition forgenerating a detectable AE signal. However, not all the bubbleshaving a diameter larger than 0.3 mm could produce a detectableAE signal. It seems that larger bubbles corresponded to lower fre-quency generally, but there are no direct links between the bubblediameter and AE signal amplitude, indicating that the characteris-tics of the AE signal may be controlled by more complicatedfactors.

4. Discussion

4.1. Characterization of the AE signals during electrochemicalcorrosion processes

4.1.1. AE activityThe results show that the AE activity vary greatly between the

different ranges (Figs. 7 and 10). Oxygen bubbles and hydrogenbubbles evolution are the main two AE sources. For transpassivedissolution AE signals can be detected only in solution 1 and theactivity is lower than oxygen/hydrogen bubbles. In other solutionsalthough transpassive dissolution occurs no corresponding AE sig-nals are present. This indicates that the ability of AE technique todetect transpassive dissolution is limited to low pH value.

4.1.2. Analysis of amplitude distributionBesides AE activity, the characteristic of the AE signals associ-

ated with each processes is also given by analysis of amplitude dis-tribution. By the way, the amplitude distribution is related to thesource mechanism by which AE signals are generated, so the anal-ysis of amplitude distribution is a useful tool for distinguishing dif-ferent mechanisms [31].

One of the most widely used model for characterizing ampli-tude distribution is the power law model, which has its origin inseismology [31]. The amplitude distribution of acoustic signals of-ten approximate to a power law distribution of the form

nðaÞ ¼ ða=a0Þ�b ð1Þ

where n(a) is a function which can be defined as the fraction of theemission population whose peak amplitude exceeds the value a; a0

is the lowest detectable amplitude, and b is the exponent whichcharacterizes the amplitude distribution.

Fig. 7. Evolution of AE data for annealed 304SS in various solutions in anodic potentiodynamic polarization processes. (a) Solution 1. (b) Solution 2. (c) Solution 3. (d) Solution4. (e) Solution 5.

Fig. 8. The amplitude and the AE activity during various potentiostatic polarization processes. (a) Different potentials for annealed 304SS in solution 1. (b) Different pre-strained 304SS at 1.15 VSCE in solution 1.

452 J. Xu et al. / Corrosion Science 53 (2011) 448–457

Fig. 9. Cathodic potentiodynamic polarization curves of annealed 304SS in varioussolutions.

Fig. 10. Evolution of AE data and polarization curves for annealed 304SS in various solut2. (c) Solution 3. (d) Solution 4. (e) Solution 5.

J. Xu et al. / Corrosion Science 53 (2011) 448–457 453

Fig. 13 shows that the b-values are similar for hydrogen bubblesevolution and oxygen bubbles evolution indicating that the sourcemechanism is the same for both processes. While the b-value cor-responding to transpassive dissolution is much higher indicatingthat there is a decrease in the proportion of high amplitude AE sig-nals. The detailed analysis of the mechanisms of AE sources is gi-ven in the latter part (see Sections 4.2 and 4.3).

4.1.3. Potential (E) – pH – AE diagramThe ultimate goal for our research is to use AE technique to

monitor the corrosion on line. As we all know, potential (E) andpH are the main factors that affect the corrosion condition of amaterial. The present work has investigated wide potential(�1 VSCE – 1.6 VSCE) and pH (0–7 approximately) ranges and an E– pH – AE diagram (Fig. 14) is given on the basis of our results.The method to draw an E – pH – AE diagram is to divide the E– pH diagram by characters of AE signal, e.g. AE activity and b-va-lue in present work. The diagram would be helpful in utility of AE

ions in cathodic potentiodynamic polarization processes. (a) Solution 1. (b) Solution

Fig. 11. Hydrogen bubbles formed on the surface of specimen during cathodicpotentiodynamic polarization process.

Fig. 12. AE evolution of the hydrogen bubbles experiment.

Table 4Characters of the first 17 AE signals during hydrogen bubble experiment.

Number ofsignal

Time (s) Bubblediameter (mm)

Amplitude(dB)

Peakfrequency (kHz)

1 397 0.38 31 2782 571 0.38 36 2343 626 0.38 30 2144 723 0.5 34 1955 733 0.25 35 2296 770 * 31 2347 772 – 30 1668 787 0.5+**0.44 30 1669 793 0.44 35 234

10 795 – 33 10711 820 0.313 36 9212 829 0.5 + 0.44 30 19513 844 – 35 14614 865 0.16 + 0.16 37 22915 901 – 30 14116 916 0.44 34 19517 929 0.44 + 0.25 + 0.44 32 146

* Dash means no released bubble was observed.** Plus means several bubbles released simultaneously.

Fig. 13. b-value of different electrochemical processes.

Fig. 14. Experimental potential – pH – AE diagram of 304SS in H2SO4 solutions withNa2SO4 and the dash line presented the traditional experimental potential – pHdiagram.

454 J. Xu et al. / Corrosion Science 53 (2011) 448–457

technique for monitoring corrosion process via linking the AE re-sults to corrosion condition more obviously. This enables the re-gions in which AE is suitable for monitoring corrosion processesto be identified easily, and by combining the diagram with theexperimental results, enables the corrosion condition of the mate-rial to be assessed.

According to the above E– pH – AE diagram, transpassive disso-lution and oxygen/hydrogen bubble evolution are the main AE pro-cesses and the mechanisms of the AE sources are discussed asfollows.

4.2. The mechanism of AE source during transpassive dissolution

A number of models have been proposed to account for thetranspassive dissolution, of which many models are from the view-point of electrochemical reactions [27,28,32,33] and few concernsthe mechanical behaviour of the passive films [34]. The main reac-tion during transpassive dissolution is [35]

CrðOHÞ3ðfilmÞ þ 5OH�ðfilmÞ ! CrO2�4 ðfilmÞ þ 4H2Oþ 3e� ð2Þ

According to Figs. 6 and 8a, the reaction currents vary largelywhile the AE activities are similar, so the reaction is not consideredas the AE source.

Because seldom work has been done on transpassive dissolu-tion by AE, the mechanisms proposed for pitting corrosion is usedfor reference considering occurrence of both of the corrosion arecaused by breakdown of passive films [34,36]. For pitting corrosionthe AE source can be divided into two categories. Fregonese et al.[18,19] and Darowicki et al. [37] proposed hydrogen bubbles insidethe pitting caused AE signals while other authors related thesource to surface changes during pitting, e.g. instantaneous stresschanges proposed by Magaino et al. [24], rupture of salt cap by

J. Xu et al. / Corrosion Science 53 (2011) 448–457 455

Jones et al. [9] and passive film breakdown by Jirarungsatian andPrateepasen [20]. In our case, no gas bubbles or salt cap have beenobserved so two potential processes which may cause stresschange on the surface are taken into account.

(1) Release of the stress concentration caused by the dissolutionof pileup of dislocations.

(2) Breakdown of passive films in the form of mechanical defor-mation and or crack formation.

It should be stressed that the most obvious and significant AEsource is oxygen bubble when the potential is high enough, buthere only the process without oxygen bubble is discussed.

The first hypothesis is deduced from the AE signal produced bydeformation of metals [38]. For the solution-annealed specimens, itis believed that the density of dislocations is too low to result inobvious AE activity. For the pre-strained specimens, the deforma-tion caused the formation of martensite (Fig. 3b and c). However,there was no direct relationship between the amount of martensiteand AE activity (Fig. 8b), suggesting that the above first hypothesisis not reasonable.

For the second hypothesis, although it has been proposed byJirarungsatian and Prateepasen [20], they did not give detailedexplanations. Here a further analysis is given according to Sato’stheory [34]. Sato has pointed out that the film can be broken downin three forms: brittle crack, plastic slip and plastic flow. In the caseof plastic flow, the anodic oxidation induces enough film pressurefor film deformation, and then film breakdown proceeds and pro-duces pores on top of the film, which eventually align in aclosed-packed pattern because of the most stable form for stressrelief. This agreed well with the present optical observation andthe similar results were also reported by Ye et al. [39]. Due tothe low activity of AE signal (Fig. 8), it is believed that AE signalsare generated through the formation of large pores and/or somesmall pores simultaneously produced by film breakdown. This pro-cess would mainly occur at the beginning of transpassive dissolu-tion which results in the higher amplitude of AE signals at thebeginning (Fig. 8). The similar results are obtained by Jirarungsa-tian and Prateepasen [20]. They detected the AE signals during pit-ting corrosion of 304SS by potentiostatic polarization and foundthat at the stable pit growth stage, current density became con-stant, and the number of hits from both AE sources (passive filmbreakdown and pit growth) decreased. These results may indicatethat breakdown of the passive films mainly occurs at the beginningof transpassive dissolution/pitting while when the current densityis constant, the breakdown may also occurs to some extent. How-ever, further detailed work, however, needs to be done to providedirect evidence.

In the present work, it is found that the current density and pre-strain have slight influence on AE activity (Fig. 8), which can alsobe explained according to Sato’s theory [34].Sato stated that thefilm pressure, p, acting vertically to the film is determined by thefollowing equation.

p� p0 ¼eðe� 1ÞE2

8p� c

Lð3Þ

where p0 is the atmospheric pressure, e is the dielectric constant ofthe film, E is the electric field in the film, c is the surface tension ofthe film and L is the film thickness. When p reaches a critical value,the film will break down.

For the annealed specimens, it is assumed that e, c and L areconstants and p � p0 only varies with E which only changedslightly, resulting in the small difference in AE activity (Fig. 8a).For the pre-strained specimens, the tests were conducted at thesame potential (E = potential/L), e, c are assumed to be constant, L

is supposed to be main variable. Due to the little influence ofpre-strain on corrosion resistance (Figs. 5 and 6), E and L keep al-most unchanged. As a result, no obvious difference in AE activityis observed in the present work (Fig. 8b).

4.3. The AE source mechanism of hydrogen bubble evaluation

According to the b-values, the mechanism of hydrogen bubblesand oxygen bubbles evolution should be the same so only hydro-gen bubbles evolution process is discussed. In the present work,the release of hydrogen bubbles should be the AE source as pro-posed by many other researchers [1,15,23,24,30,40]. But the exactmechanism producing AE signals is still unknown. Here a model isproposed to try to clarify the mechanism.

It is well known that hydrogen is generated by the followingreaction during electrochemical corrosion.

2Hþ þ 2e� ! H2: ð4Þ

For hydrogen evolution the possible reaction steps are [41]

Hþ þ e� ! H ðAtomic hydrogenÞ ð4:1Þ

HþH ! H2 ðHydrogen moleculeÞ ð4:2Þ

H2 þH2 ! 2H2 ðGas bubbleÞ ð4:3Þ

Once the hydrogen forms, it should firstly dissolve into the solu-tion although its solubility is very low. Then a new phase, hydro-gen bubble, will form with continuous generation of hydrogen.

For decreasing the critical free energy to form a stable nucleus,hydrogen bubble nucleation occurs on the surface of the specimenas shown in Fig. 11 and in other reports [20,23]. After stable nuclei(gas bubbles) have been formed in the solution, these nuclei growlarger by overcoming the exterior pressure, namely,

pin > pex ¼ pat þ qghþ 2rLG

rð5Þ

where pin is the interior pressure of the bubble, pex is the exteriorpressure, pat is the atmosphere pressure, rLG is the surface free en-ergy between liquid and gas and r is the radius of gas bubble. A sim-ilar process of hydrogen bubble evolution was also addressed byVolkov et al. [42] during hydrogen evolution inside corrosion pitin aluminium.

Fig. 15 shows the forces to which the bubble subjects, wherenotations are the same as Eq. (3). In addition, SF is the support forceby the specimen and the subscript V and H mean the vertical andhorizon component.

When the bubble is in stable state, in the vertical direction, G ismuch smaller than pexV which should be balanced by rLGV. How-ever, in the horizon component, the magnitudes of rLGH and pexH

are unknown, so the magnitude of SF is uncertain, resulting inthe indeterminacy of the stress condition of the specimen. Whenthe bubble releases, the specimen only subjects to the hydrostaticpressure. It is believed that this change of stress condition on thespecimen is the mechanism of AE source.

According to the above mechanism of AE source, the amplitudeof the AE signal is related to the stress condition of the specimen,the real contact area of the specimen and bubble, and the contactangle between solution and bubble. All of these parameters arehard to be measured. Moreover, only the stable state is simply con-sidered here. If the bubble evolution is very fast, the situation ismore complex. This is why there is no certain relationship betweendiameter of hydrogen bubble and amplitude of AE signal in thepresent work (Table 4).

Although some previous works have obtained good correlationbetween hydrogen bubbles and AE signals [1,30], there still exist

Fig. 15. Schematic illustration showing the forces to which the bubble subjects. Thegap between bubble and surface is enlarged for identifying the force easily.

456 J. Xu et al. / Corrosion Science 53 (2011) 448–457

some contradictions. Mazille et al. [17] demonstrated that thereexisted a perfect correlation between the pit growth and acousticactivity, but Darowicki et al. [37] pointed out there was no correla-tion between potentials of pitting corrosion and AE signals. Itshould be noted that thousands of signals were produced in Maz-ille’s experiment, while the average number of AE events was 4.6 inDarowicki’s work. Assuming all the AE signals were generated byhydrogen bubbles which had been generally accepted by manyresearchers [18,19,37], the different results obtained by Mazilleand Darowicki may be due to the different number of AE signalsthey analyzed. In this context, a good correlation between hydro-gen bubbles and AE signals could only be obtained in term of sta-tistic analysis of a large number of signals rather than every singlesignal.

One of the necessary conditions for bubble to be released is thatthe sound wave must propagate through whole bubble circumfer-ence [43]. Supposing the velocity of sound wave is 340 m/s, the ra-dius of contact surface (not but smaller than the presentobservation) is 0.3 mm, then the time for a bubble to be completelyreleased is 5.5 ls. Theoretically, the frequency range can thereforebe extended up to 180 kHz, which is in agreement with the presentresults (Table 4).

It should be also note that some puzzles still exist. Table 4shows that AE signals whose signals are of similar amplitude andfrequency than the signals associated with hydrogen bubbles, weredetected even when no hydrogen bubble was observed. Somedeductive reasons instead of a determinate conclusion will be gi-ven as follows. First, our specimen was sealed by wax mixed withrosin and some sites (not in our visual field) may not be sealed verywell, resulting in generating some hydrogen bubbles. Second, thebreak-up of hydrogen bubbles on the surface of solution may alsogenerate AE signals, especially breaking up simultaneously. Inaddition, noise might also account for some signals.

Further experiments are also needed to analyze the waveformof AE signals based on the modal AE theory, which can give a directlink between the mechanism and AE signal. Some new experimen-tal data will be presented in a later paper.

5. Conclusions

(1) Three electrochemical processes, i.e. oxygen bubbles, hydro-gen bubbles evolution and transpassive dissolution, are themain AE sources. The AE signals associated with each pro-

cess are characterized by their AE activity and b-value.Transpassive dissolution has lower AE activity and higherb-value than oxygen/hydrogen bubbles evolution.

(2) In our experimental configuration, AE activity correspondingto transpassive dissolution is detected in low pH solution,and current density and pre-strain have little influence onthe AE activity. For hydrogen bubbles evolution, the lowerlimit current density above which AE signals can be detectedis in the magnitude of 10�4–10�3 A/cm2. There exists a weakcorrelation between the AE signal amplitude and the hydro-gen gas bubble diameter. Small bubble tends to producehigh frequency AE signal.

(3) A concept of E– pH – AE diagram has been proposed and anexperimental E– pH – AE diagram plotted based on AE activ-ity and b-value. This enables the regions in which AE is suit-able for monitoring corrosion processes to be identifiedeasily, and by combining the diagram with the experimentalresults, enables the corrosion condition of the material to beassessed.

(4) The breakdown of passive films is believed to be the AEsource mechanism during transpassive dissolution and itseems that AE signals are produced by larger pores and/orsome small pores simultaneously. During hydrogen bubbleevolution, the change of stress condition of the specimenduring release of bubbles could be the mechanism of AEsource.

Acknowledgements

This study was jointly supported by the Science and TechnologyFoundation of China (50871113), the Major Scientific and Techno-logical Special Project (2010ZX06004-009), the Special Funds forthe Major State Basic Research Projects (2006CB605005) and theInnovation Fund of Institute of Metal Research (IMR), ChineseAcademy of Sciences (CAS).

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