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Initiation and propagation steps in pitting corrosion of austenitic stainless steels: monitoring by acoustic emission
Corrosion Science 43 (2001) 627±641
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Initiation and propagation steps in pitting corrosion of austenitic stainless steels: monitoring by acoustic emission M. Fregonese a,*, H. Idrissi a, H. Mazille a,1, L. Renaud b, Y. Cetre b a
Lab. de Physicochimie Industrielle, Institut National des Sciences Appliqu ees de Lyon, Batiment 401, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex, France b Mat eriaux±Corrosion, Rhone Poulenc Industrialisation, 24 Avenue J. Jaur es, 69153 D ecines Charpieu, France Received 12 July 1999; accepted 15 June 2000
Abstract Acoustic emission (AE) technique was used to study the development of pitting corrosion on AISI 316L austenitic stainless steel, in a 3% NaCl solution acidi®ed to pH 2. The initiation and the propagation steps of the pits were separately studied owning to a speci®c polarization procedure. It appears that the initiation step of pitting corrosion is not signi®cantly emissive, whereas the propagation step is characterized by the emission of resonant signals. This kind of AE signals is representative of the development of the pits in the form of occluded cells, in which the evolution of hydrogen bubbles appears to be the emissive phenomenon. A subsequent change in the mode of corrosion, i.e. the transfer to uniform corrosion, can be detected by the AE technique. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Pitting corrosion; Acoustic emission; Austenitic stainless steel; Hydrogen evolution; Occluded cell
*
Corresponding author. Tel.: +33-4-72-43-62-19; fax: +33-4-72-43-87-15. E-mail addresses: [email protected] (M. Fregonese), [email protected] (H. Mazille). 1 Also corresponding author. Tel.: +33-4-72-43-82-81; fax: +33-4-72-43-87-15.
0010-938X/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 0 ) 0 0 0 9 9 - 8
628
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
1. Introduction The acoustic emission (AE) technique, based on the rapid release of energy within a material generating a transient elastic wave propagation, is widely used as a nondestructive technique (NDT) for testing vessels on-site. Many microscopic deformation or fracture processes have also been studied with this technique in laboratory experiments, but most of them concerned stress corrosion cracking investigations [1]. Some published papers also deal with abrasion or erosion corrosion studies, and only a few attempts have been made to study purely electrochemical corrosion types such as uniform corrosion [2±4] or pitting corrosion [4±11]. In the latter case, the studies mainly concern aluminium and austenitic stainless steels in the presence of chloride ions. In both cases, AE activity (number of events) has been correlated to the corrosion rate, which was estimated in terms of weight-loss, applied current density or hydrogen evolution rate. A direct quantitative correlation was even established between the number of AE events and the number of pits or the pitted area [11]. Most of the time, the generation of acoustic signals has been attributed to the evolution of hydrogen bubbles [4,6,7,11]. Yet, as no direct correlation was made between the formation and the release of bubbles and the generation of AE bursts, some other physico-chemical mechanisms were proposed, such as stress changes on metal surface [8], or the rupture of an oxide or salt cap covering the pits [10]. Moreover, a thorough investigation has been performed by Arora [9] in various wellcontrolled experimental conditions on aluminium alloys. As mentioned by the author, if AE has to be used for detecting and identifying an active corrosion process such as pitting, it is absolutely necessary to proceed to careful acoustic parameters analyses of recorded AE signals. In that sense, the authors recently reported that two kinds of AE signals were recorded during pitting corrosion investigations, which could be discriminated by their rise time and counts number, whether pits are initiated by potential or current application on specimens machined out from a bar or from a rolled sheet [12]. In that context, the aim of this work is to characterize more precisely by AE the pitting corrosion steps: initiation and propagation. Attention is paid to the morphological change of the pits during their development and its in¯uence on acoustic parameters of recorded AE signals.
2. Experimental method 2.1. Material and specimen preparation AISI 316L austenitic stainless steel, the chemical composition of which is given in Table 1, was used for this study. The specimens were cut out from a rolled sheet of 2 mm thick. The exposed surface (2.8 cm2 ) was wet ground up to 1200 grit silicon carbide paper. After a passivation treatment of 30 min in 20% HNO3 at 60°C, the specimens were rinsed with de-ionised water then acetone, dried in a stream of cool
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
629
Table 1 Composition of the studied materials Element
Fe
C
Si
Mn
Ni
Cr
Mo
S
P
Wt.%
Balance
0.02
0.45
1.78
11.76
17.20
2.40
0.006
0.027
air, and were stored overnight in a desiccator, which gave more reproducible results for the pitting behaviour. 2.2. Electrochemical environment All the studies reported here were conducted at room temperature in 3% NaCl solution with the initial pH adjusted to 2 with HCl addition. For polarization tests, the electrochemically applied current or potential was controlled with an EG&G 273A potentiostat, the sample being the working electrode, a platinum mesh as the counter-electrode and a saturated calomel electrode as a reference. In order to avoid acquisition of acoustic noise induced by hydrogen evolution from the counterelectrode during anodic polarization of the specimen, the platinum mesh had to be placed in a near-by annex cell connected to the corrosion cell via a salt bridge (Fig. 1). 2.3. Acoustic Emission monitoring AE instrumentation consisted of a transducer, a preampli®er and an acquisition device (MISTRAS from Physical Acoustic Corp.) (Fig. 1). The transducers were resonant R15D type from PAC (piezo-electric disks). They have been selected because of their high sensitivity in a bandwidth of 100±500 kHz. The acquisition
Fig. 1. Experimental device.
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M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
system was completely computer controlled. The waveforms, the events number and the characteristic acoustic parameters were stored on a hard disk as soon as detected, and were available for treatment under the form of ASCII ®les, as well as the electrochemical parameters. For each detected AE signal, following acoustic parameters are studied: (i) Events number: number of AE signals detected (discontinuous emission), (ii) amplitude: maximal amplitude of the considered AE event (peak), (iii) rise time: time between the ®rst overshoot of the de®ned threshold and the peak, (iv) counts number: number of times that the threshold is overshot for a given AE event, (v) duration: time between the ®rst and the last overshoot of the de®ne threshold. 2.4. Experimental procedure In order to record separately acoustic noise generated by the initiation of the pits on the one hand and by their propagation on the other hand, the following polarization procedure has been adopted: ®rst, the specimens were either anodically polarized in the passive domain (400 mV/SCE), or left at their open circuit (o.c.) potential in the solution for 15 min, in order to have dierent pit generation potentials (Egp ). Pits were then initiated by a cyclic polarization test always conducted above Egp , with a rate of 0.4 mV/s. When the current density has reached 200 lA/ cm2 , the potential was reversed in the negative sweep direction until the value of 450 mV/SCE, which was situated between the pit generation potential Egp and the repassivation potential Erp (200 mV/SCE for tested material) (Fig. 2). During the potential negative sweep, the current density still increased and stabilized at the value ireverse ranged between 1 and 10 mA/cm2 . The cyclic polarization step was then stopped and the potential was maintained at 450 mV/SCE for various pre®xed durations, while the current density was recorded. At this imposed potential, new pits did not initiate, whereas previously initiated pits did propagate. AE activity was recorded during all the steps of the test. 2.5. Pits characterization After each corrosion test, the aggressive solution was carefully removed with a pipette in order to keep intact eventual caps covering the pits. The pitted surface was observed by optical microscopy and SEM. Some specimens were transversally cut, in order to characterize the morphology and the size of the pits. 3. Experimental results Some experimental results are gathered in Table 2. The prepolarization of the specimens in the passive domain or their maintenance at their o.c. potential have
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
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Fig. 2. Evolution of the current density with the potential during the cyclic polarization step.
allowed to get an Egp range of [600, 950 mV/SCE] (the prepolarization in the passive region gives rise to higher Egp values compared with that in o.c.). This various values of Egp have a direct in¯uence on the values of the current density ireverse recorded during the reverse curve of potentiodynamic step (Fig. 2): the greater Egp , the greater ireverse . During the cyclic polarization step on the negative sweep, the nature of the AE signals recorded depends on the value of ireverse : if ireverse is less than 4 mA/cm2 , only short bursts are recorded (Fig. 3a); when ireverse is higher than 4 mA/cm2 , short bursts are ®rst recorded then resonant signals appear (Fig. 3b). These two kinds of AE signals are similar to those presented elsewhere [12]. For ireverse > 4 mA/cm2 , the presence of two populations of AE signals can be evidenced by 3D graphs on which representative acoustic parameters are distributed into classes (Fig. 4). As previously discussed [12], rise time and counts number are considered as discriminating parameters for the pitting phenomenon. These representations show that the short AE signals are characteristic of a ®rst population with rise time lower to 40 ls and counts number less than 7, whereas the resonant AE signals are representative of the second population with rise time higher than 40 ls and counts number ranged between 1 and 500. During that cyclic polarization step, it is further worth noting that the number of initiated pits increases with Egp (and ireverse ). Between the two polarization steps (cyclic then potentiostatic), a short transfer step occurs, due to the response time of the potentiostat. It actually corresponds to the time delay between the end of the cyclic polarization step (stopped at 450 mV/ SCE) and the potentiostatic step (at 450 mV/SCE). During this short time (1 s or 2 s), the sample supports a short o.c. potential step, during which the current density drops. Yet, it re-increases very quickly as soon as potential is applied again. It is assumed that pits do not have time to repassivate during this short time delay.
10 10
950 950
b
Short then resonant Short then resonant
Short
Short Short
Signals
Occluded pit: covered by a metallic cap (Fig. 6a and b). Open pit: the metallic cap is no more present (Fig. 6c).
1
600
a
4 1
ireverse (mA/cm2 )
790 730
Egp (mV/SCE)
0.4 mV/s ± cyclic polarization step
Table 2 Experiments carried out and results
± 3.0
4.3
4.0 3.6
Current density at the (re)emission of the resonant signals (mA/cm2 )
11 000, 3 h 30 min 76 000, 15 h 15 min
24 400, 18 h
1240, 1 h 15 min 4340, 7 h 45 min
Number of AE events (short resonant), duration of the test
450 mV/SCE ± potentiostatic step
3 Occluded pitsa 6 Open pitsb 1 with a cap that has just collapsed 7 Open pits uniform corrosion 15 Occluded pits 15 Open pits uniform corrosion
Observations
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M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
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Fig. 3. Waveforms of: (a) short signals and (b) resonant signals.
During the potentiostatic step at 450 mV/SCE, corresponding to pit propagation, the number of AE signals recorded increases with experiment duration (Table 2). For tests duration shorter than about 8 h at that applied potential, only pitting is observed. Two populations of AE signals are then recorded: only short signals at the beginning of the potentiostatic step, then both short and resonant signals when current density reaches 3 to 5 mA/cm2 (Table 2 and Fig. 5). Thus, a time delay is necessary for the (re)emission of resonant signals after the cyclic polarization step stopping, which is in good agreement with previously published data [12,13]. Moreover, the choice of dierent durations for the potentiostatic step allows to observe the change of the pits morphology. As evidenced in Fig. 6, pits develop as occluded cells during the potentiostatic step, with the formation of a metallic cap. When the duration of the test exceeds 8 h, this metallic cap collapses. The fallingdown of the cap is associated to the stopping of the development of the pits and to
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M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
Fig. 4. 3D representation of discriminating acoustic parameters for the cyclic polarization step (positive negative sweep) for which ireverse > 4 mA/cm2 : presence of both short and resonant signals.
Fig. 5. 3D representation of discriminating acoustic parameters for the potentiostatic step before the change in the corrosion mode, for test durations <8 h: presence of both short and resonant signals.
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
635
Fig. 6. Morphological evolution of the pits during the potentiostatic step.
a change in the mode of corrosion, which develops to more uniform type. As shown in Fig. 7, after a 15 h potentiostatic test, the whole surface of the specimen in contact with the aggressive solution is corroded on an average of 200 lm depth, whereas most of the pits developed before the change in the mode of corrosion are 450 lm deep. A signi®cant decrease of the resonant AE signals number is also associated with the change of corrosion mode, while cumulated AE activity still increases with time: thus, short signals become predominant during the development of uniform corrosion (Fig. 8). It is moreover worth noting that the collapsing of the pits cap does not induce any noticeable additional AE activity, at least during pitting corrosion emissivity recordings. Two complementary tests have been carried out in order to characterize the natural development of the pits when applied polarization is turned o. For the ®rst one, the cyclic polarization step is followed by 16 h at o.c. potential. During the cyclic polarization test, leading to a value of the reverse current density ireverse of 10 mA/cm2 , both short and resonant AE signals are recorded. As soon as the cyclic polarization test is completed and the polarization is turned o, AE activity sharply decreases: only 158 bursts are recorded in 16 h of test, with no more resonant signals (Fig. 9). The shape of the short signals is similar to those of signals
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M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
Fig. 7. Surface and section views of a specimen damaged by pitting then uniform corrosion after a 15 h potentiostatic step.
recorded during o.c. potential tests carried out on passivated and non-pitted samples. Consequently, it con®rms that pits initiated during the cyclic polarization step do not propagate but repassivate when no polarization is applied. This result is con®rmed by the observation of the pits at the end of the test (Fig. 10): pits are smaller, less developed and some are still surrounded by ``lace''. By comparison with the pits observed after a 1 h potentiostatic step (Fig. 6a), it can be assessed that the formation of the metallic cap above the pits occurs during the potentiostatic step, during which pits propagate rapidly as occluded cells, and that resonant signals are quite representative of this kind of propagation. For the second complementary test, a specimen is submitted to a 2 h potentiostatic test at 450 mV/SCE after the initial cyclic polarization stage, then followed by a 20 min test at o.c. potential, during which both AE activity and potential are recorded. The last two curves are compared to those drawn during the ®rst complementary test (Fig. 11). In both cases, the potential sharply drops to a more negative value witnessing the depassivation of the surface by pitting development (Fig. 11a). Then, the potential stabilizes and, when the experiment duration is suf®cient, it re-increases due to partial repassivation. The 20 mV gap between the two experiments is explained by the dierence in the pits number formed: 22 pits are
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
637
Fig. 8. 3D representation of discriminating acoustic parameters for the potentiostatic step after the change in the corrosion mode, for test durations ranged between 8 and 18 h after polarization application: disappearance of resonant signals.
detected for the ®rst complementary test (cyclic polarization o.c. potential) whereas only 10 pits developed during the second one (cyclic polarization potentiostatic o.c. potential), for a tested sample surface of 2.8 cm2 . Moreover, AE activity during the o.c. potential step depends on whether the propagation of the pits has been sustained by potentiostatic polarization or not
Fig. 9. 3D representation of discriminating acoustic parameters for the 16 h o.c. potential step following a cyclic polarization step: presence of short signals only.
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M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
Fig. 10. Pit surrounded by ``lace'' after a 16 h o.c. potential step following a cyclic polarization step.
(Fig. 11b). AE activity is much higher in the ®rst case (0.1 events/s compared to 7 10ÿ3 events per second for the ®rst one). In both cases, bursts are in majority short (rise time <40 ls), although some resonant signals are still recorded during the second test. Thus, more drastic conditions within the pits, induced by the potentiostatic step, lead to higher and more resonant residual AE activity.
4. Discussion As previously reported [11,12], our experimental work con®rms the occurrence of a time delay before recording any signi®cant AE activity when 316L stainless steel is tested in pitting conditions. Such a time-lag corresponds to the delay for resonant bursts emission. We observe also a current density threshold for both cyclic polarization and potentiostatic tests, i.e. the necessity of a minimal corrosion amount (or pit size) for the pit propagation to be emissive. This corrosion threshold can be linked to a minimum amount of metallic cations formed by anodic dissolution in the occluded cell. Indeed, the hydrolysis of the resulting corrosion products leads to acidi®cation within the pits, and then to hydrogen evolution. Thus, the current density threshold can correspond to a minimum amount of protons formed locally to release hydrogen bubbles, from the classical H reduction. Yet, it must be quoted that the value of this threshold depends on the number of pits that initiate and develop: it is lower for specimens more sensitive towards pitting in terms of number of initiation sites [12,13]. An interaction phenomenon between the pits must therefore be taken into account for acoustic activity considerations. From the results presented above, it appears moreover that the resonant AE signals are representative of the propagation step of the pits. Yet, the recording of
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
639
Fig. 11. (a) Evolution of potential, and (b) AE activity during the o.c. potential step whether it is preceded or not by a potentiostatic step.
resonant bursts is not only conditioned by the pitting rate, but also by the presence of occluded cells, as such signals are no more recorded when the pits are fully open. So, this type of bursts is linked to a mechanism speci®c to the propagation of the pits with the morphology of occluded cells. Among the phenomena that occur within the pits, the most energetic, i.e. susceptible to be the most emissive, is the formation and the evolution of hydrogen bubbles. This mechanism is the most frequently quoted in the literature as being responsible for the recorded acoustic noise [4,6,7,11]. Resonant hits can thus be representative of the evolution of hydrogen within the pits. More precisely, the friction of the hydrogen bubbles along the walls of the pits or the impacts on the cap of the occluded cells can confer the resonant characteristic on AE signals associated to pitting. The residual AE activity recorded after switching o anodic polarization, when severe acidic conditions have been locally established within the pits can thus be attributed to the evolution of some residual hydrogen
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M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
bubbles or to the formation of few new ones due to the high amount of H in a very occluded area. This phenomenon does not occur when H concentration is too low within the pits, i.e. when no potentiostatic step is imposed after the cyclic polarization stage: in the latter case, a sharp decrease of AE activity occurs, and resonant bursts are no more recorded. As already mentioned, a minimal amount of H ions within the pits is thus necessary to make it possible hydrogen evolution responsible for the emission of resonant signals, due to active corrosion inside the pit. On the other hand, the opening of the pits after a 8 h potentiostatic step, can be attributed to the eect of gravity. Actually, on our rolled samples, the pits develop preferentially laterally (Fig. 7), which makes it dicult to keep stable the cap above the pit. When the pits are widely open, not only the friction of the bubbles is sharply reduced because of the absence of metallic cap above the pits, but also H2 bubbles formation itself becomes dicult, indeed even impossible, as the occluded character of the pits is responsible for the particular pH and potential conditions leading to H2 evolution in the pits. In fact, at the applied potential considered here (450 mV/ SCE), H2 evolution is only possible if the bottom of the pits suers local acidi®cation and ohmic drop resulting from corrosion products accumulation. Moreover, the opening of the pits induces a release of acidi®ed solution outside the pit cell and a subsequent increase of the local pH inside the pit. However, the local presence of corrosion products limits ionic exchanges within the solution, and pH can remain relatively low in the vicinity of the specimen, with high level of Clÿ ions. With the conditions of potential and pH considered here, a very little pH decrease close to the surface of the specimen can lead to the global dissolution of the passive ®lm [14]. The progressive acidi®cation on the whole surface of the sample, associated to the opening of the pits, can thus explain the fact that the surface of the sample becomes entirely active, which leads to a change in the corrosion mode. As uniform corrosion of the specimen induces a very important dissolution of the metal (on a 200 lm depth for an exposed area of 2.8 cm2 ), and then leads to the formation of a great amount of corrosion products, the quite numerous but short AE signals recorded during the uniform corrosion stage could be associated to the formation and deposition of the corrosion products onto the sample. This hypothesis has to be con®rmed by complementary tests. Further experiments are also needed to determine the possible contribution of other phenomena occurring during pits development on the emission of resonant signals, for instance: (de)passivation, selective leaching, intergranular attack or formation and crystallization of corrosion products. A thorough study of the acoustic noise recorded during H2 bubbles formation and evolution would also be meaningful. Some new experimental data will be presented in a later paper. 5. Conclusions From this experimental work aimed to investigate separately the initiation and the propagation steps of pitting corrosion of austenitic stainless steels by the AE technique, the following conclusions can be drawn:
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
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1. During the initiation step of the pits, AE signals recorded are not very numerous. But AE activity becomes signi®cant when pits propagate. The increasing of AE activity is associated to the recording of resonant signals, which become predominant if these pits form occluded cells and develop at a sucient rate. 2. The emissive source within the pits appears to be the friction phenomenon of the evolving H2 bubbles inside the pits. Moreover, the evolution of H2 bubbles within the occluded pits confers the resonant characteristics of the signals recorded. 3. The opening of the pits induces, in our severe electrochemical conditions, the change of corrosion mode from pitting corrosion to uniform corrosion. This modi®cation is followed by AE technique, as representative acoustic parameters (rise time and counts number) are drastically aected. Detection by AE of the change of corrosion mode opens promising perspectives for industrial on-site monitoring.
References [1] A. Proust, P. Fleischmann, H. Mazille, R. Rothea, Corrosion Deformation Interaction, in: T. Magnin, J.M. Gras (Eds.), CDIÕ92, Les Editions de Physique, Les Ulis (F), 1993, p. 829. [2] K.H.W. Seah, K.B. Lim, C.H. Chew, S.H. Teoh, Corros. Sci. 34 (1993) 1707. [3] R.J. Grossman, IEEE Trans. Sonics and Ultrasonics SU-31 (1984) 25. [4] T.W. Retting, M.J. Felsen, Corrosion 32 (1976) 121. [5] D.L. Parry, 14th Conf. on Non-Destructive Evaluation, San Antonio, USA, 1983, p. 388. [6] F. Mansfeld, P.J. Stocker, Corrosion 35 (1979) 541. [7] S. Magaino, Boshoku Gijutsu 34 (1985) 657. [8] S. Magaino, A. Kawaguchi, A. Hirata, T. Osaka, J. Electrochem. Soc. 134 (1987) 2993. [9] A. Arora, Corrosion 40 (1984) 459. [10] R.H. Jones, M.A. Friesel, Corrosion 48 (1992) 751. [11] H. Mazille, R. Rothea, C. Tronel, Corros. Sci. 37 (1995) 1365. [12] M. Fregonese, H. Idrissi, H. Mazille, L. Renaud, Y. Cetre, J. Mat. Sci. 36 (2001). [13] M. Fregonese, H. Idrissi, H. Mazille, L. Renaud, Y. Cetre, in: D. Feron, B. Espelid (Eds.), Marine Corrosion of Stainless Steels, EFC Series, Institute of Materials, London, to be published. [14] M. Pourbaix, Atlas dÕequilibres electrochimiques a 25°C, Ed. Gauthier-Villars et Cie, Paris (F), 1963.
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Initiation and propagation steps in pitting corrosion of austenitic stainless steels: monitoring by acoustic emission M. Fregonese a,*, H. Idrissi a, H. Mazille a,1, L. Renaud b, Y. Cetre b a
Lab. de Physicochimie Industrielle, Institut National des Sciences Appliqu ees de Lyon, Batiment 401, 20 Avenue Albert Einstein, 69621 Villeurbanne Cedex, France b Mat eriaux±Corrosion, Rhone Poulenc Industrialisation, 24 Avenue J. Jaur es, 69153 D ecines Charpieu, France Received 12 July 1999; accepted 15 June 2000
Abstract Acoustic emission (AE) technique was used to study the development of pitting corrosion on AISI 316L austenitic stainless steel, in a 3% NaCl solution acidi®ed to pH 2. The initiation and the propagation steps of the pits were separately studied owning to a speci®c polarization procedure. It appears that the initiation step of pitting corrosion is not signi®cantly emissive, whereas the propagation step is characterized by the emission of resonant signals. This kind of AE signals is representative of the development of the pits in the form of occluded cells, in which the evolution of hydrogen bubbles appears to be the emissive phenomenon. A subsequent change in the mode of corrosion, i.e. the transfer to uniform corrosion, can be detected by the AE technique. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Pitting corrosion; Acoustic emission; Austenitic stainless steel; Hydrogen evolution; Occluded cell
*
Corresponding author. Tel.: +33-4-72-43-62-19; fax: +33-4-72-43-87-15. E-mail addresses: [email protected] (M. Fregonese), [email protected] (H. Mazille). 1 Also corresponding author. Tel.: +33-4-72-43-82-81; fax: +33-4-72-43-87-15.
0010-938X/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 0 ) 0 0 0 9 9 - 8
628
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
1. Introduction The acoustic emission (AE) technique, based on the rapid release of energy within a material generating a transient elastic wave propagation, is widely used as a nondestructive technique (NDT) for testing vessels on-site. Many microscopic deformation or fracture processes have also been studied with this technique in laboratory experiments, but most of them concerned stress corrosion cracking investigations [1]. Some published papers also deal with abrasion or erosion corrosion studies, and only a few attempts have been made to study purely electrochemical corrosion types such as uniform corrosion [2±4] or pitting corrosion [4±11]. In the latter case, the studies mainly concern aluminium and austenitic stainless steels in the presence of chloride ions. In both cases, AE activity (number of events) has been correlated to the corrosion rate, which was estimated in terms of weight-loss, applied current density or hydrogen evolution rate. A direct quantitative correlation was even established between the number of AE events and the number of pits or the pitted area [11]. Most of the time, the generation of acoustic signals has been attributed to the evolution of hydrogen bubbles [4,6,7,11]. Yet, as no direct correlation was made between the formation and the release of bubbles and the generation of AE bursts, some other physico-chemical mechanisms were proposed, such as stress changes on metal surface [8], or the rupture of an oxide or salt cap covering the pits [10]. Moreover, a thorough investigation has been performed by Arora [9] in various wellcontrolled experimental conditions on aluminium alloys. As mentioned by the author, if AE has to be used for detecting and identifying an active corrosion process such as pitting, it is absolutely necessary to proceed to careful acoustic parameters analyses of recorded AE signals. In that sense, the authors recently reported that two kinds of AE signals were recorded during pitting corrosion investigations, which could be discriminated by their rise time and counts number, whether pits are initiated by potential or current application on specimens machined out from a bar or from a rolled sheet [12]. In that context, the aim of this work is to characterize more precisely by AE the pitting corrosion steps: initiation and propagation. Attention is paid to the morphological change of the pits during their development and its in¯uence on acoustic parameters of recorded AE signals.
2. Experimental method 2.1. Material and specimen preparation AISI 316L austenitic stainless steel, the chemical composition of which is given in Table 1, was used for this study. The specimens were cut out from a rolled sheet of 2 mm thick. The exposed surface (2.8 cm2 ) was wet ground up to 1200 grit silicon carbide paper. After a passivation treatment of 30 min in 20% HNO3 at 60°C, the specimens were rinsed with de-ionised water then acetone, dried in a stream of cool
M. Fregonese et al. / Corrosion Science 43 (2001) 627±641
629
Table 1 Composition of the studied materials Element
Fe
C
Si
Mn
Ni
Cr
Mo
S
P
Wt.%
Balance
0.02
0.45
1.78
11.76
17.20
2.40
0.006
0.027
air, and were stored overnight in a desiccator, which gave more reproducible results for the pitting behaviour. 2.2. Electrochemical environment All the studies reported here were conducted at room temperature in 3% NaCl solution with the initial pH adjusted to 2 with HCl addition. For polarization tests, the electrochemically applied current or potential was controlled with an EG&G 273A potentiostat, the sample being the working electrode, a platinum mesh as the counter-electrode and a saturated calomel electrode as a reference. In order to avoid acquisition of acoustic noise induced by hydrogen evolution from the counterelectrode during anodic polarization of the specimen, the platinum mesh had to be placed in a near-by annex cell connected to the corrosion cell via a salt bridge (Fig. 1). 2.3. Acoustic Emission monitoring AE instrumentation consisted of a transducer, a preampli®er and an acquisition device (MISTRAS from Physical Acoustic Corp.) (Fig. 1). The transducers were resonant R15D type from PAC (piezo-electric disks). They have been selected because of their high sensitivity in a bandwidth of 100±500 kHz. The acquisition
Fig. 1. Experimental device.
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system was completely computer controlled. The waveforms, the events number and the characteristic acoustic parameters were stored on a hard disk as soon as detected, and were available for treatment under the form of ASCII ®les, as well as the electrochemical parameters. For each detected AE signal, following acoustic parameters are studied: (i) Events number: number of AE signals detected (discontinuous emission), (ii) amplitude: maximal amplitude of the considered AE event (peak), (iii) rise time: time between the ®rst overshoot of the de®ned threshold and the peak, (iv) counts number: number of times that the threshold is overshot for a given AE event, (v) duration: time between the ®rst and the last overshoot of the de®ne threshold. 2.4. Experimental procedure In order to record separately acoustic noise generated by the initiation of the pits on the one hand and by their propagation on the other hand, the following polarization procedure has been adopted: ®rst, the specimens were either anodically polarized in the passive domain (400 mV/SCE), or left at their open circuit (o.c.) potential in the solution for 15 min, in order to have dierent pit generation potentials (Egp ). Pits were then initiated by a cyclic polarization test always conducted above Egp , with a rate of 0.4 mV/s. When the current density has reached 200 lA/ cm2 , the potential was reversed in the negative sweep direction until the value of 450 mV/SCE, which was situated between the pit generation potential Egp and the repassivation potential Erp (200 mV/SCE for tested material) (Fig. 2). During the potential negative sweep, the current density still increased and stabilized at the value ireverse ranged between 1 and 10 mA/cm2 . The cyclic polarization step was then stopped and the potential was maintained at 450 mV/SCE for various pre®xed durations, while the current density was recorded. At this imposed potential, new pits did not initiate, whereas previously initiated pits did propagate. AE activity was recorded during all the steps of the test. 2.5. Pits characterization After each corrosion test, the aggressive solution was carefully removed with a pipette in order to keep intact eventual caps covering the pits. The pitted surface was observed by optical microscopy and SEM. Some specimens were transversally cut, in order to characterize the morphology and the size of the pits. 3. Experimental results Some experimental results are gathered in Table 2. The prepolarization of the specimens in the passive domain or their maintenance at their o.c. potential have
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631
Fig. 2. Evolution of the current density with the potential during the cyclic polarization step.
allowed to get an Egp range of [600, 950 mV/SCE] (the prepolarization in the passive region gives rise to higher Egp values compared with that in o.c.). This various values of Egp have a direct in¯uence on the values of the current density ireverse recorded during the reverse curve of potentiodynamic step (Fig. 2): the greater Egp , the greater ireverse . During the cyclic polarization step on the negative sweep, the nature of the AE signals recorded depends on the value of ireverse : if ireverse is less than 4 mA/cm2 , only short bursts are recorded (Fig. 3a); when ireverse is higher than 4 mA/cm2 , short bursts are ®rst recorded then resonant signals appear (Fig. 3b). These two kinds of AE signals are similar to those presented elsewhere [12]. For ireverse > 4 mA/cm2 , the presence of two populations of AE signals can be evidenced by 3D graphs on which representative acoustic parameters are distributed into classes (Fig. 4). As previously discussed [12], rise time and counts number are considered as discriminating parameters for the pitting phenomenon. These representations show that the short AE signals are characteristic of a ®rst population with rise time lower to 40 ls and counts number less than 7, whereas the resonant AE signals are representative of the second population with rise time higher than 40 ls and counts number ranged between 1 and 500. During that cyclic polarization step, it is further worth noting that the number of initiated pits increases with Egp (and ireverse ). Between the two polarization steps (cyclic then potentiostatic), a short transfer step occurs, due to the response time of the potentiostat. It actually corresponds to the time delay between the end of the cyclic polarization step (stopped at 450 mV/ SCE) and the potentiostatic step (at 450 mV/SCE). During this short time (1 s or 2 s), the sample supports a short o.c. potential step, during which the current density drops. Yet, it re-increases very quickly as soon as potential is applied again. It is assumed that pits do not have time to repassivate during this short time delay.
10 10
950 950
b
Short then resonant Short then resonant
Short
Short Short
Signals
Occluded pit: covered by a metallic cap (Fig. 6a and b). Open pit: the metallic cap is no more present (Fig. 6c).
1
600
a
4 1
ireverse (mA/cm2 )
790 730
Egp (mV/SCE)
0.4 mV/s ± cyclic polarization step
Table 2 Experiments carried out and results
± 3.0
4.3
4.0 3.6
Current density at the (re)emission of the resonant signals (mA/cm2 )
11 000, 3 h 30 min 76 000, 15 h 15 min
24 400, 18 h
1240, 1 h 15 min 4340, 7 h 45 min
Number of AE events (short resonant), duration of the test
450 mV/SCE ± potentiostatic step
3 Occluded pitsa 6 Open pitsb 1 with a cap that has just collapsed 7 Open pits uniform corrosion 15 Occluded pits 15 Open pits uniform corrosion
Observations
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633
Fig. 3. Waveforms of: (a) short signals and (b) resonant signals.
During the potentiostatic step at 450 mV/SCE, corresponding to pit propagation, the number of AE signals recorded increases with experiment duration (Table 2). For tests duration shorter than about 8 h at that applied potential, only pitting is observed. Two populations of AE signals are then recorded: only short signals at the beginning of the potentiostatic step, then both short and resonant signals when current density reaches 3 to 5 mA/cm2 (Table 2 and Fig. 5). Thus, a time delay is necessary for the (re)emission of resonant signals after the cyclic polarization step stopping, which is in good agreement with previously published data [12,13]. Moreover, the choice of dierent durations for the potentiostatic step allows to observe the change of the pits morphology. As evidenced in Fig. 6, pits develop as occluded cells during the potentiostatic step, with the formation of a metallic cap. When the duration of the test exceeds 8 h, this metallic cap collapses. The fallingdown of the cap is associated to the stopping of the development of the pits and to
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Fig. 4. 3D representation of discriminating acoustic parameters for the cyclic polarization step (positive negative sweep) for which ireverse > 4 mA/cm2 : presence of both short and resonant signals.
Fig. 5. 3D representation of discriminating acoustic parameters for the potentiostatic step before the change in the corrosion mode, for test durations <8 h: presence of both short and resonant signals.
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635
Fig. 6. Morphological evolution of the pits during the potentiostatic step.
a change in the mode of corrosion, which develops to more uniform type. As shown in Fig. 7, after a 15 h potentiostatic test, the whole surface of the specimen in contact with the aggressive solution is corroded on an average of 200 lm depth, whereas most of the pits developed before the change in the mode of corrosion are 450 lm deep. A signi®cant decrease of the resonant AE signals number is also associated with the change of corrosion mode, while cumulated AE activity still increases with time: thus, short signals become predominant during the development of uniform corrosion (Fig. 8). It is moreover worth noting that the collapsing of the pits cap does not induce any noticeable additional AE activity, at least during pitting corrosion emissivity recordings. Two complementary tests have been carried out in order to characterize the natural development of the pits when applied polarization is turned o. For the ®rst one, the cyclic polarization step is followed by 16 h at o.c. potential. During the cyclic polarization test, leading to a value of the reverse current density ireverse of 10 mA/cm2 , both short and resonant AE signals are recorded. As soon as the cyclic polarization test is completed and the polarization is turned o, AE activity sharply decreases: only 158 bursts are recorded in 16 h of test, with no more resonant signals (Fig. 9). The shape of the short signals is similar to those of signals
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Fig. 7. Surface and section views of a specimen damaged by pitting then uniform corrosion after a 15 h potentiostatic step.
recorded during o.c. potential tests carried out on passivated and non-pitted samples. Consequently, it con®rms that pits initiated during the cyclic polarization step do not propagate but repassivate when no polarization is applied. This result is con®rmed by the observation of the pits at the end of the test (Fig. 10): pits are smaller, less developed and some are still surrounded by ``lace''. By comparison with the pits observed after a 1 h potentiostatic step (Fig. 6a), it can be assessed that the formation of the metallic cap above the pits occurs during the potentiostatic step, during which pits propagate rapidly as occluded cells, and that resonant signals are quite representative of this kind of propagation. For the second complementary test, a specimen is submitted to a 2 h potentiostatic test at 450 mV/SCE after the initial cyclic polarization stage, then followed by a 20 min test at o.c. potential, during which both AE activity and potential are recorded. The last two curves are compared to those drawn during the ®rst complementary test (Fig. 11). In both cases, the potential sharply drops to a more negative value witnessing the depassivation of the surface by pitting development (Fig. 11a). Then, the potential stabilizes and, when the experiment duration is suf®cient, it re-increases due to partial repassivation. The 20 mV gap between the two experiments is explained by the dierence in the pits number formed: 22 pits are
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637
Fig. 8. 3D representation of discriminating acoustic parameters for the potentiostatic step after the change in the corrosion mode, for test durations ranged between 8 and 18 h after polarization application: disappearance of resonant signals.
detected for the ®rst complementary test (cyclic polarization o.c. potential) whereas only 10 pits developed during the second one (cyclic polarization potentiostatic o.c. potential), for a tested sample surface of 2.8 cm2 . Moreover, AE activity during the o.c. potential step depends on whether the propagation of the pits has been sustained by potentiostatic polarization or not
Fig. 9. 3D representation of discriminating acoustic parameters for the 16 h o.c. potential step following a cyclic polarization step: presence of short signals only.
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Fig. 10. Pit surrounded by ``lace'' after a 16 h o.c. potential step following a cyclic polarization step.
(Fig. 11b). AE activity is much higher in the ®rst case (0.1 events/s compared to 7 10ÿ3 events per second for the ®rst one). In both cases, bursts are in majority short (rise time <40 ls), although some resonant signals are still recorded during the second test. Thus, more drastic conditions within the pits, induced by the potentiostatic step, lead to higher and more resonant residual AE activity.
4. Discussion As previously reported [11,12], our experimental work con®rms the occurrence of a time delay before recording any signi®cant AE activity when 316L stainless steel is tested in pitting conditions. Such a time-lag corresponds to the delay for resonant bursts emission. We observe also a current density threshold for both cyclic polarization and potentiostatic tests, i.e. the necessity of a minimal corrosion amount (or pit size) for the pit propagation to be emissive. This corrosion threshold can be linked to a minimum amount of metallic cations formed by anodic dissolution in the occluded cell. Indeed, the hydrolysis of the resulting corrosion products leads to acidi®cation within the pits, and then to hydrogen evolution. Thus, the current density threshold can correspond to a minimum amount of protons formed locally to release hydrogen bubbles, from the classical H reduction. Yet, it must be quoted that the value of this threshold depends on the number of pits that initiate and develop: it is lower for specimens more sensitive towards pitting in terms of number of initiation sites [12,13]. An interaction phenomenon between the pits must therefore be taken into account for acoustic activity considerations. From the results presented above, it appears moreover that the resonant AE signals are representative of the propagation step of the pits. Yet, the recording of
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Fig. 11. (a) Evolution of potential, and (b) AE activity during the o.c. potential step whether it is preceded or not by a potentiostatic step.
resonant bursts is not only conditioned by the pitting rate, but also by the presence of occluded cells, as such signals are no more recorded when the pits are fully open. So, this type of bursts is linked to a mechanism speci®c to the propagation of the pits with the morphology of occluded cells. Among the phenomena that occur within the pits, the most energetic, i.e. susceptible to be the most emissive, is the formation and the evolution of hydrogen bubbles. This mechanism is the most frequently quoted in the literature as being responsible for the recorded acoustic noise [4,6,7,11]. Resonant hits can thus be representative of the evolution of hydrogen within the pits. More precisely, the friction of the hydrogen bubbles along the walls of the pits or the impacts on the cap of the occluded cells can confer the resonant characteristic on AE signals associated to pitting. The residual AE activity recorded after switching o anodic polarization, when severe acidic conditions have been locally established within the pits can thus be attributed to the evolution of some residual hydrogen
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bubbles or to the formation of few new ones due to the high amount of H in a very occluded area. This phenomenon does not occur when H concentration is too low within the pits, i.e. when no potentiostatic step is imposed after the cyclic polarization stage: in the latter case, a sharp decrease of AE activity occurs, and resonant bursts are no more recorded. As already mentioned, a minimal amount of H ions within the pits is thus necessary to make it possible hydrogen evolution responsible for the emission of resonant signals, due to active corrosion inside the pit. On the other hand, the opening of the pits after a 8 h potentiostatic step, can be attributed to the eect of gravity. Actually, on our rolled samples, the pits develop preferentially laterally (Fig. 7), which makes it dicult to keep stable the cap above the pit. When the pits are widely open, not only the friction of the bubbles is sharply reduced because of the absence of metallic cap above the pits, but also H2 bubbles formation itself becomes dicult, indeed even impossible, as the occluded character of the pits is responsible for the particular pH and potential conditions leading to H2 evolution in the pits. In fact, at the applied potential considered here (450 mV/ SCE), H2 evolution is only possible if the bottom of the pits suers local acidi®cation and ohmic drop resulting from corrosion products accumulation. Moreover, the opening of the pits induces a release of acidi®ed solution outside the pit cell and a subsequent increase of the local pH inside the pit. However, the local presence of corrosion products limits ionic exchanges within the solution, and pH can remain relatively low in the vicinity of the specimen, with high level of Clÿ ions. With the conditions of potential and pH considered here, a very little pH decrease close to the surface of the specimen can lead to the global dissolution of the passive ®lm [14]. The progressive acidi®cation on the whole surface of the sample, associated to the opening of the pits, can thus explain the fact that the surface of the sample becomes entirely active, which leads to a change in the corrosion mode. As uniform corrosion of the specimen induces a very important dissolution of the metal (on a 200 lm depth for an exposed area of 2.8 cm2 ), and then leads to the formation of a great amount of corrosion products, the quite numerous but short AE signals recorded during the uniform corrosion stage could be associated to the formation and deposition of the corrosion products onto the sample. This hypothesis has to be con®rmed by complementary tests. Further experiments are also needed to determine the possible contribution of other phenomena occurring during pits development on the emission of resonant signals, for instance: (de)passivation, selective leaching, intergranular attack or formation and crystallization of corrosion products. A thorough study of the acoustic noise recorded during H2 bubbles formation and evolution would also be meaningful. Some new experimental data will be presented in a later paper. 5. Conclusions From this experimental work aimed to investigate separately the initiation and the propagation steps of pitting corrosion of austenitic stainless steels by the AE technique, the following conclusions can be drawn:
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1. During the initiation step of the pits, AE signals recorded are not very numerous. But AE activity becomes signi®cant when pits propagate. The increasing of AE activity is associated to the recording of resonant signals, which become predominant if these pits form occluded cells and develop at a sucient rate. 2. The emissive source within the pits appears to be the friction phenomenon of the evolving H2 bubbles inside the pits. Moreover, the evolution of H2 bubbles within the occluded pits confers the resonant characteristics of the signals recorded. 3. The opening of the pits induces, in our severe electrochemical conditions, the change of corrosion mode from pitting corrosion to uniform corrosion. This modi®cation is followed by AE technique, as representative acoustic parameters (rise time and counts number) are drastically aected. Detection by AE of the change of corrosion mode opens promising perspectives for industrial on-site monitoring.
References [1] A. Proust, P. Fleischmann, H. Mazille, R. Rothea, Corrosion Deformation Interaction, in: T. Magnin, J.M. Gras (Eds.), CDIÕ92, Les Editions de Physique, Les Ulis (F), 1993, p. 829. [2] K.H.W. Seah, K.B. Lim, C.H. Chew, S.H. Teoh, Corros. Sci. 34 (1993) 1707. [3] R.J. Grossman, IEEE Trans. Sonics and Ultrasonics SU-31 (1984) 25. [4] T.W. Retting, M.J. Felsen, Corrosion 32 (1976) 121. [5] D.L. Parry, 14th Conf. on Non-Destructive Evaluation, San Antonio, USA, 1983, p. 388. [6] F. Mansfeld, P.J. Stocker, Corrosion 35 (1979) 541. [7] S. Magaino, Boshoku Gijutsu 34 (1985) 657. [8] S. Magaino, A. Kawaguchi, A. Hirata, T. Osaka, J. Electrochem. Soc. 134 (1987) 2993. [9] A. Arora, Corrosion 40 (1984) 459. [10] R.H. Jones, M.A. Friesel, Corrosion 48 (1992) 751. [11] H. Mazille, R. Rothea, C. Tronel, Corros. Sci. 37 (1995) 1365. [12] M. Fregonese, H. Idrissi, H. Mazille, L. Renaud, Y. Cetre, J. Mat. Sci. 36 (2001). [13] M. Fregonese, H. Idrissi, H. Mazille, L. Renaud, Y. Cetre, in: D. Feron, B. Espelid (Eds.), Marine Corrosion of Stainless Steels, EFC Series, Institute of Materials, London, to be published. [14] M. Pourbaix, Atlas dÕequilibres electrochimiques a 25°C, Ed. Gauthier-Villars et Cie, Paris (F), 1963.