Stress corrosion cracking of sensitized Type 304 stainless steel in thiosulphate solution. II. Dynamics of fracture

Corrosion Science 48 (2006) 1608–1622 www.elsevier.com/locate/corsci

Stress corrosion cracking of sensitized Type 304 stainless steel in thiosulphate solution. II. Dynamics of fracture Monika Gomez-Duran, Digby D. Macdonald

*

Centre for Electrochemical Science and Technology, Department of Materials Science and Engineering, The Pennsylvania State University, 201 Steidle Building, University Park, PA 16802, United States Received 2 February 2004; accepted 26 June 2005 Available online 24 August 2005

Abstract This work focuses on the study of noise (EN) in the coupling current that is generated during stress corrosion cracking (SCC) of sensitized Type 304 stainless steel (304SS) in thiosulphate solution. The noise was acquired under open circuit conditions using a zero-resistance ammeter to monitor the coupling current that flows from the crack in an insulated compact tension (CT) specimen to external cathodes. The time record is transformed to the frequency domain using a fast Fourier transform algorithm and wavelet analysis; the wavelet analysis proved to be of greater facility in determining the frequency values at which the fracture events occur. The mechanism proposed to explain the behaviour of the noise is hydrogen-induced fracture (HIF), in which the entry of hydrogen into the matrix ahead of the crack tip is catalyzed by adsorbed sulfur. Additionally, it was found that a 0.5 M sodium thiosulphate solution is capable of initiating and propagating the localized corrosion process under unloaded conditions and that the effect of the load is, simply, to increase the rate with which the process occurs.  2005 Elsevier Ltd. All rights reserved. Keywords: Stress corrosion cracking; Type 304SS; Thiosulphate; Electrochemical noise

*

Corresponding author. Tel.: +1 814 863 7772; fax: +1 814 863 4718. E-mail address: [email protected] (D.D. Macdonald).

0010-938X/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.06.010

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1. Introduction Studies published on stress corrosion cracking (SCC) in sensitized Type 304 stainless steel (304SS) in thiosulphate solutions have focused on the effect of thiosulphate ion concentration on the crack growth rate [1–3], possible mechanisms for the phenomenon [2,4,5], initiation and propagation of cracks [1,5,6], and the electrochemistry of the system [2,4,6]. Part I of the present study addressed the fate of the coupling current generated within the crack and it was found that a measurable (but unknown) fraction of the current is consumed on the external surfaces [7]. That work (Part I) showed that the coupling current contains considerable noise and transformation of the current into the frequency domain indicated the existence of events of preferred frequency. Part II of the study addresses the origin of the noise and discusses the implications for the mechanism of crack advance. EN is a general term given to fluctuations in the potential and current generated spontaneously by uniform or localized corrosion processes. The corrosion and corrosion-associated processes and mechanisms identified as sources of EN include, among others, the nucleation and propagation of stress corrosion cracks [4,8–10]; hydrogen bubble nucleation, growth, and detachment [11–13]; passive film formation and growth [14–16]; crevice corrosion [17–19]; nucleation, growth, and propagation of pits [20–23]; abrasion [14]; resistance change and diffusion in solution [24]; high-temperature corrosion [25–27]; microbial corrosion [28–30]; and uniform corrosion [31,32]. EN is now recognized as a powerful tool in the study of corrosion processes, since it produces information that can be used to define corrosion mechanisms without the need to perturb the system [33]. Thus, EN measurements at open circuit, as in the present study, register the current and potential while the natural corrosion process is occurring in the system. This method has the advantages of reproducing ‘‘real’’ corrosion conditions, and hence allows its use for ‘‘real time’’ corrosion monitoring [34,35], and also for reducing the cost of instrumentation, when compared with other electrochemical techniques [36]. The most commonly used method for analyzing EN in the frequency domain is to obtain spectra calculated from time-domain records. The spectra can be represented as power spectral density (PSD) plots showing log[PSD(A2/Hz or V2/Hz)] vs. log[frequency (Hz)], or as amplitude plots showing log[amplitude (A or V)] vs. log[frequency (Hz)]. These spectral plots are produced by transforming the information collected in the time domain to the frequency domain using standard mathematical algorithms, such as the fast Fourier transform (FFT) [37] or the maximum entropy method (MEM) [38]. Another way of analyzing EN is to use the wavelet analysis (WA) [39] method. This method is similar to FFT, since it is also a mathematical transformation of the information collected in time domain, but WA presents information from the frequency domain while also retaining information from the time domain. For WA, the representation of the data in the frequency domain comprises an energy density plot (EDP) showing the fraction of total energy (%) contained in various ‘‘crystal’’ (bins), each covering a specific range of frequency. The crystals are arrangements of the

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‘‘detail coefficients’’, obtained through the mathematical transformation, which contain information about the features in the time domain when transformed to the frequency domain.

2. Experimental 2.1. Experimental systems Information regarding specimen configuration, cathodes, solution and experimental equipment has been presented in detail elsewhere [7]. 2.2. Time domain data acquisition and transformation EN data for this corrosion system were acquired under open circuit conditions by recording the current fluctuations in the coupling current flowing from the crack to external Type 304SS cathodes located on the side surfaces of insulated (PTFEcoated), sensitized, and pre-cracked Type 304SS compact tension [C(T)] specimens. The coupling current was measured using a zero-resistance ammeter (Keithley 485 Autoranging Picoammeter in the ZRA mode), which allowed the current to be measured without incurring a voltage drop across the measuring device. The current acquisition frequency was 2 Hz for all the experiments and 28,925, 27,460 and 23,606 points were measured for the 4-, 14- and 24-h sensitized specimens, respectively. The experimental set-up used to acquire the EN data is shown schematically in Fig. 1.

σ

Load Cell

Software

Zero Resistance Ammeter

σ

Fig. 1. Schematic representation of the experimental set-up.

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The EN information was analyzed in the frequency domain by generating plots of log[amplitude (A)] vs. log[frequency (Hz)] using a Fast Fourier Transform (FFT) algorithm, of tested reliability, applied through Lab-View software. The reliability test was performed by combining several sine waves of known amplitude, frequency, and phase using an Excel program to form an artificial noise signal, which was then processed using the FFT algorithm to obtain the original combination of sinusoidal components of preselected amplitudes and frequencies. The EN information was also analyzed using the WA method, and the results are presented as energy density plots (EDP) that show the fraction of total energy (in %) contained within crystals covering specified frequency ranges. The transformation of the time domain information was accomplished using the wavelet function Daubechies DAUB4 [40]. A more detailed explanation of the mathematics of the transformation process used in the present work is given elsewhere [41].

3. Results The experimental results, previously reported in Part I [7], were collected in the time domain, in which the current displays transients that overlap, thereby rendering delineation of the contributing events difficult, if not impossible. Accordingly, the time record in the coupling current was transformed into the frequency domain for further analysis. Current amplitude spectra plotting log[amplitude (A)] vs. log[frequency (Hz)] were obtained via FFT, as noted above. Energy density plots (EDP) plotting fraction of total energy (%) vs. crystals were also obtained by feeding the time domain data to the wavelet function Daubechies DAUB4, as described above. Comparison of the current amplitude spectra obtained for each condition of sensitization and loading was performed for both types of EN analysis. Comparison of current amplitude spectra obtained for each condition of sensitization and loading is the principal thrust of this analysis. It is necessary to note that the experimental equipment was limited in its ability to sustain the applied load, causing a gradual decrease in average current with time, as explained in Part I [7]. It is also worth noting that the drift in the background current, due to this limitation, was not eliminated before frequency domain analysis, so an error is incurred, primarily in the amplitude. We judge this not to be a serious issue, because we are primarily interested in the range of frequency over which distinct events occur in the current source. The behaviour of the material for different sensitization conditions, but under the same loading state analyzed by FFT, can be seen in Figs. 2–4. The diagrams show that the current amplitude decreases with increasing frequency. At frequencies higher than about 0.05 Hz, the noise is dominated by instrument noise and hence contains no probative information. At lower frequencies, the noise spectrum displays peaks corresponding to preferred event frequencies. While these frequencies clearly depend upon the loading and sensitization conditions, they can be attributed to dominant events yielding current within the crack, such as the periodic fracture of the matrix at the crack tip. Of particular interest is the low frequency region (<0.01 Hz) at

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Fig. 2. Coupling current amplitude spectra before application of the load.

pffiffiffiffi Fig. 3. Coupling current amplitude spectra for the loaded state ðK I ¼ 21.5 MPa mÞ.

which the dominant events occur, signifying that crack advance occurs by infrequent, but presumably large, micro fracture events. The WA transformed current data for different sensitization times, but for the same loading state, can be seen in Figs. 5–7. The main advantage of using WA in

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4h 14h

D17

D16

D15

D14

D13

D12

D11

D10

D9

D8

D7

D6

D5

24h

D4

0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

D3

Fraction of Total Energy (%)

Fig. 4. Coupling current amplitude spectra after unloading the specimen.

Crystal

Fig. 5. Coupling current energy density plot before application of the load.

this work arises from its greater ability to differentiate with respect to the frequencies at which the fracture events occur, as shown in the Figs. 5–7, with the frequency ranges for the 17 crystals being given in Table 1. It is necessary to clarify here that the D1 and D2 values are not included in the Figures or the Table, because they contain aliased data [41]. Once more, the behaviour of the 4-h sensitized specimen is different from those of the 14- and 24-h sensitized specimens, which perform similarly. In this case, the fraction of the total energy contained within any given crystal is larger for the 4-h specimen than for the other two, regardless of the state of loading. In all three cases, crystals with significant energy content (crystal 5–11) range in frequency from 0.0625 to 0.001953 Hz, indicating events that cover a wide dynamic range. Interestingly,

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Fraction of Total Energy (%)

0.12

14h

0.1

24h

0.08 0.06 0.04 0.02

D17

D16

D15

D14

D13

D12

D11

D9

D10

D8

D7

D6

D5

D4

D3

0

Crystal

pffiffiffiffi Fig. 6. Coupling current energy density plot for the loaded state ðK I ¼ 21.5 MPa mÞ.

0.3

Fraction of Total Energy (%)

4h

0.25

14h 24h

0.2 0.15 0.1

D17

D16

D15

D14

D13

D12

D11

D9

D10

D8

D7

D6

D5

D4

0

D3

0.05

Crystal

Fig. 7. Coupling current energy density plot after unloading the specimen.

events exist before loading and after the load has been removed, indicating that a high tensile stress is not required to cause the events to occur. Presumably, residual stress present at the crack tip from fatigue pre-cracking (before loading case) and from prior constant loading (after loading case) provide the driving force for event occurrence. Also, in the unloaded condition, sensitization has relatively little impact on the populations of the crystals ranging from crystals 6–10, but under active loading (Fig. 6), the energy contents of these crystals for the 4-h sensitized specimen is considerably greater than those for the 14- and 24-h sensitized specimens. The fact that the amplitudes in the FFT spectra and the energy content of crystals 6–10 are finite in the absence of active mechanical loading suggests that crack advance is best described as ‘‘stress-assisted fracture’’, rather than as ‘‘stress corrosion cracking’’, in which stress is an essential element in crack advance. Clearly, the existence of an active load causes differentiation of the specimens with respect to the impact of sensitization and is generally consistent with the impact of sensitization time on the degree of sensitization of Type 304SS having a carbon content of 0.08%. Thus, prolonged heating of the steel at the sensitization temperature of 650 C is expected to

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Table 1 Frequency ranges for D crystals Crystal

Minimum frequency (lHz)

Maximum frequency (lHz)

D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15 D16 D17

250,000 125,000 62,500 31,250 15,625 7,813 3,906 1,953 977 488 244 122 61 30.5 15.3

500,000 250,000 125,000 62,500 31,250 15,625 7813 3906 1953 977 488 244 122 61 30.5

Fig. 8. Coupling current amplitude spectra for specimens that had been sensitized at 650 C for 4 h pffiffiffiffi (loading was to K I ¼ 21.5 MPa m).

reduce the degree of sensitization by back diffusion of chromium into the chromium depleted zones at the grain boundaries, although this expected behaviour was not evident from the DOS given by the double loop electrochemical reactivation method (DLERM) employed in Part I [7] to characterize the specimens. A possible explanation of this apparent discrepancy is that the energy content of the noise depends more upon the depth of chromium depletion, whereas the DLERM measures the extent of depletion.

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Fig. 9. Coupling current amplitude spectra for specimens that had been sensitized at 650 C for 14 h pffiffiffiffi (loading was to K I ¼ 21.5 MPa m).

Fig. 10. Coupling current amplitude spectra for specimens that had been sensitized at 650 C for 24 h pffiffiffiffi (loading was to K I ¼ 21.5 MPa m).

The WA data are generally consistent with the behaviour observed from FFT transformation, where the spectra for the 14- and 24-h sensitized specimens at low frequencies are similar. After unloading, the spectrum for the 24-h sensitized specimen is practically indistinguishable from that for the 14-h sensitized specimen (Fig. 4). In the WA (Fig. 7), however, the energy content of the 14-h sensitized specimen is about half of that for the 24-h sensitized case, suggesting a somewhat complex relationship between energy content and DOS, although the observed discrepancy may reflect differences in the residual stress state at the crack tip.

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Before load First load Second load Third load

D17

D16

D15

D14

D13

D12

D11

D9

D10

D8

D7

D6

D5

D4

Fourth load

D3

Fraction of Total Energy (%)

0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

1617

Crystal

Fig. 11. Coupling current energy density plot for specimens that had been sensitized at 650 C for 4 h pffiffiffiffi (loading was to K I ¼ 21.5 MPa m).

Fraction of Total Energy (%)

0.3 Before load First load Second load Third load Fourth load

0.25 0.2 0.15 0.1 0.05

D17

D16

D15

D14

D13

D12

D11

D10

D9

D8

D7

D6

D5

D4

D3

0

Crystal

Fig. 12. Coupling current energy density plot for specimens that had been sensitized at 650 C for 14 h pffiffiffiffi (loading was to K I ¼ 21.5 MPa m).

0.35 Before load

Fraction of Total Energy (%)

0.3

First load

0.25

Second load Third load

0.2

Fourth load

0.15 0.1

D17

D16

D15

D14

D13

D12

D11

D10

D9

D8

D7

D6

D5

D4

0

D3

0.05

Crystal

Fig. 13. Coupling current energy density plot for specimens that had been sensitized at 650 C for 24 h pffiffiffiffi (loading was to K I ¼ 21.5 MPa m).

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The behaviour of each sensitized specimen under different loading states analyzed by FFT is shown in Figs. 8–10. Comparison of the spectra for different loading conditions for each sensitization state shows that before load application current activity already exists, as evidenced by the presence of current amplitude peaks at specific frequencies, and this remains true for all the sensitization states. Then, when load is applied for the first time, the current amplitude rises, but the subsequent increases in loading level does not appear to modify the current behaviour significantly, as was already observed in the time domain behaviour [7]. In the case of the 4-h sensitized specimen, when the load is applied, the current amplitude rises by about an order of magnitude, at frequencies below 0.02 Hz, but above this frequency it becomes indistinguishable from the unloaded condition. The probable explanation for this phenomenon is, again, that, for this specific specimen, the noise produced in the current is very low, so it is barely greater than the external or instrumentation noise, which prevails at low amplitudes and high frequencies. On the other hand, the current amplitude for the 24-h sensitized specimens increases by one to two orders of magnitude when the load is applied and the amplitude difference remains practically constant for all the frequencies. The behaviour of the material under different loading states as a function of sensitization time, as indicated by WA, can be seen in Figs. 11–13. Again for all sensitization conditions current activity arising from discrete events occur before the load is applied, in the form of fraction of total energy peaks in specific crystals. In every case, once the load is applied for the first time the fraction of total energy in the crystals decreases considerably, but the frequency ranges over which the events appear remain unchanged. Thereafter, subsequent increases in the stress intensity do not produce a significant change in the current behaviour, although a slight reduction in the fraction of total energy values in crystals 6–10 occurs with each load increase.

4. Discussion The current amplitude spectra show that the magnitude of the current increases with increasing load for a constant degree of sensitization and increases with the degree of sensitization for a given load, as was previously found from the time domain coupling current data [7]. Increasing the sensitization time from 4 to 14 h results in an order of magnitude increase in the current amplitude, whereas an increase in the sensitization time at 650 C to 24 h produces no further increase in the current amplitude. Thus, fourteen h of heating at 650 C is sufficient to produce a fully sensitized microstructure in this heat of steel. The current amplitude/fraction of total energy peaks at definite frequencies/crystals found in Figs. 2–4 and 6–13 can be explained through the combination of the mechanism explained below and the loading condition. When the unloaded stainless steel specimen and the coupled external cathodes are introduced in the thiosulphatecontaining environment, the thiosulphate ion immediately attacks the steel within the crack and begins the corrosion process, together with the disproportionation reaction [42]

M. Gomez-Duran, D.D. Macdonald / Corrosion Science 48 (2006) 1608–1622 2 S2 O2 3 ! S þ SO3

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ð1Þ

to produce adsorbed sulfur [42]. It is postulated that elemental sulfur adsorbed at the crack tip catalyzes the entry of hydrogen produced by the reduction of H+ and/or water into the matrix ahead of the crack, in addition to catalyzing the dissolution of the chromium-depleted grain boundaries (note that the crack morphology is intergranular in nature). Thus, the postulated fracture mechanism is one that is dominated by hydrogen-induced, with brittle micro fracture events occurring across the crack front. During the unloaded period, combination of all these processes produces slow, microscopic brittle intergranular penetration, registered in the current amplitude spectra as peaks at definite frequencies. In the absence of a load, it is envisioned that this occurs via recombination of hydrogen atoms at carbide/matrix interfaces, which results in void nucleation, followed by void pressurization until the local stress exceeds that necessary to rupture the matrix between the void and the crack tip, marking a brittle micro fracture event. When the load is applied, micro voids are envisioned to form initially as the result of the mechanically imposed tensile stress, but the stress is augmented by recombination of hydrogen in the voids. Thus, the load simply enhances an already on-going process of void nucleation, void pressurization, and eventual fracture of the matrix. In Part I of the present study [7] the fracture events were assumed to be semicircular in form and the micro fracture dimension was expressed as: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Bn  dL=dt r¼ ð2Þ 2f where Bn is the specimen thickness at the groove, dL/dt is the crack growth rate and, and f is the frequency at which the fracture events occur. The values employed in Part I [7] to calculate the micro fracture dimension were dL/dt = 1.7 · 106 cm/s, taken from the literature [42], Bn = 1.416 cm and f  0.1–0.01 s1, from the FFT spectra reported in that work. The use of WA to further analyze the EN in the present paper has produced a more precise frequency range, 0.001953–0.0625 s1, with an average value of 0.03 s1, for the micro fracture event occurrence. Employing the crack growth rate reported in the literature [42] the calculated micro fracture dimension is 60 lm, comparable to the micro fracture event dimensions reported in other systems [43,44], and with the grain size of the present material of 50– 60 lm. On the other hand using the crack growth rate for 14-h sensitization case, dL/ dt = 4.4 · 105 cm/s, experimentally obtained in the present work, the calculated micro fracture dimension is 300 lm, which corresponds to fracture events of multigrain dimension. In both cases, the calculated micro fracture dimension is considerably larger than the dimension involved in the slip dissolution mechanism, which is of the order of 102 to 101 lm [45]. As previously exposed in Part I [7], the calculated micro fracture dimension is consistent with a hydrogen embrittlement mechanism. In the FTT spectra, for all cases, the high frequency noise, above 0.02 Hz, appears to be less well-defined than that for the low frequency noise and it becomes indistinguishable for the 14- and 24-h sensitized specimens under the same loading conditions

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and for the same specimens under different loading conditions. On the other hand, WA provides for a clear delineation between these cases for these frequencies above 0.02 Hz, since the analysis shows the fraction of total energy in the crystal covering the frequency range between 0.001953 and 0.0625 Hz, and also display the subtle differences between the behaviour of each specimen under each loading condition. This allows us to more precisely determine the frequency values at which the micro fracture events occur for the particular phenomenon being studied in the present work. 5. Summary and conclusions The findings of this work may be summarized as follows: 1. The mechanism proposed to explain the behaviour of the coupling current data and the current amplitude spectra is essentially one of hydrogen-induced fracture, in which the entry of hydrogen into the steel matrix at the crack tip is catalyzed by adsorbed elemental sulfur on the steel surface as the result of decomposition or reduction of thiosulphate ion. Elemental sulfur may also catalyze dissolution of the chromium-depleted grain boundaries, thereby leading to a stress corrosion cracking component and a source of electrons for hydrogen evolution at the crack tip, in addition to oxygen reduction on surfaces external to the crack. 2. The fracture dimension (60–300 lm) ranges between the order of the grain size and multigrain dimensions. These findings are consistent with the proposed hydrogen-induced fracture mechanism, for which fracture events extending over macroscopic dimensions. 3. The presence of similar current amplitude (FFT)/fraction of total energy (WA) peaks at approximately the same frequencies for all load states also supports the proposed hydrogen embrittlement failure mechanism, in that the fracture dimension is primarily a materials property, possibly related to the spacing of voids on the grain boundaries ahead of the crack tip. 4. The use of wavelet analysis to analyze the experimental data in the frequency domain, as compared with fast Fourier transformation, allows for a better definition of the frequency range over which the micro fracture events occur, and thus for a more precise calculation of the micro fracture dimension. 5. Analysis of the current noise from experiments in the absence of a load shows that thiosulphate ion induces events within the crack, suggesting that the principal role of mechanical loading is to accelerate the penetration process along grain boundaries that is already occurring in the unloaded condition.

Acknowledgements The authors gratefully acknowledge the support of this work by COLCIENCIAS of Colombia and Fulbright Commission through a scholarship awarded to M.G.-D. and by the US Department of Energy/Environmental Management Science Program

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through Grant No. DE-FG07-97ER62515. The authors also gratefully acknowledge the help of Dr. Sue Liu in setting up the experimental apparatus, and the help of Morgan Smith in applying the wavelet transformation program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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