Statistical study of the critical pitting temperature of 22-05 duplex stainless steel

Corrosion Science 60 (2012) 280–283

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Statistical study of the critical pitting temperature of 22-05 duplex stainless steel L. Peguet a,⇑, A. Gaugain a, C. Dussart a,b, B. Malki b, B. Baroux b a b

Aperam R&D, Stainless Europe Research Center, rue Roger Salengro BP15, 62330 Isbergues, France Institut National Polytechnique de Grenoble, CNRS/SIMAP, 1130, rue de la piscine BP75, 38402 Saint Martin d’Hères cedex, France

a r t i c l e

i n f o

Article history: Received 21 September 2011 Accepted 20 March 2012 Available online 4 April 2012 Keywords: A. Stainless steel C. Pitting corrosion

a b s t r a c t Using a multichannel electrochemical cell, critical potentials for passive film breakdown of a duplex stainless steel were measured at different temperatures in a 0.5 M NaCl aqueous solution. Statistical analysis shows that the transition between pitting and transpassivity does not occur at a well-defined ‘‘critical pitting temperature’’ (CPT) but takes place within a transition temperature interval (TTI), the width of which (10 to 1 °C) decreases as the size of the specimen increases. For practical purposes, the lower TTI bound appears to be the most reliable indicator of the CPT. Potentiostatic techniques were found to overestimate the CPT value compared to potentiodynamic estimations. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The pitting resistance of stainless steel is usually discussed in terms of pitting potential. This parameter is stochastic and extremely sensitive to environmental parameters. Therefore, to be reliable, studies must be based on statistical analysis of this criterion [1,2]. For the more resistant steels however, temperature has been found to be a more relevant parameter. A so called ‘‘critical pitting temperature’’ (CPT) can be defined, below which the pitting potential undergoes a sharp transition [3,4]. Surprisingly, this critical temperature is independent of potential, pH and chloride concentration [5]. Two experimental techniques based on pitting potential measurements exist to determine CPT, namely potentiodynamic and potentiostatic methods. These two techniques are reported to give very similar results [4,6] and for this reason, CPT is often considered to be a ‘‘quasi-deterministic’’ parameter for experiments performed under carefully controlled conditions. Therefore, for many years it was believed to be the most appropriate method for ranking highly stainless steel grades based on their chemical composition [7–10]. However, recent studies have shown CPT to be significantly influenced by electrolyte composition [11–16], surface preparation [17] and, to a greater or lesser extent; polarization effects [13–15]. For example, CPT has already been observed to be overestimated in MgCl2 when potentiostatic methods rather than potentiodynamic methods are used [18]. This effect has been attributed to an increase in passive film stability, which leads, in turn, to higher ⇑ Corresponding author. Present address: Constellium CRV, Voreppe Research Centre, 725 rue Aristide Bergès, BP 27, 38341 Voreppe cedex, France. Tel.: +33 04 76 57 83 64; fax: +33 04 76 57 80 99. E-mail address: [email protected] (L. Peguet). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.03.025

CPT. Nevertheless, this interpretation is not consistent with observations indicating that CPT transition is mainly related to pit propagation stage, rather than to passive film breakdown. This surprising observation was noted upon addition of inhibitor to the test solution [16,19]. From a practical viewpoint, the question also arises as to whether a unique CPT can be defined, or if there is a statistical dispersion, as is the case for the pitting potential. This paper addresses this question for a 22-05 duplex stainless steel using a multichannel type cell and offers some recommendations regarding best practices for CPT measurements. 2. Experimental 2.1. Material Samples for this study were 22-05 duplex stainless steel, the chemical composition of which has been previously described Table 1. Samples were cut from an industrial hot rolled, annealed and pickled 8 mm-thick plate. Prior to electrochemical testing, specimens were abraded (under water) with SiC paper up to a 1200 grit finish. They were then degreased in an ultrasonic acetone/ethanol bath, rinsed with distilled water, dried, and aged for 24 h in ambient air. 2.2. Electrochemical measurements Electrochemical tests were performed in a NaCl 0.5 M solution, made from distilled water and analytical grade sodium chloride, and deaerated with N2 (+H2). The reference electrode was a Saturated Calomel Electrode. The statistical distribution of both pitting potentials and pitting temperatures was investigated using

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L. Peguet et al. / Corrosion Science 60 (2012) 280–283 Table 1 Chemical composition of the studied 22-05 duplex stainless steel in weight%. %C

%N

% Cr

% Ni

% Mo

S (ppm)

% Mn

% Cu

0.016

0.16

22.8

5.5

2.8

<10 ppm

1.8

0.1

a multichannel cell allowing simultaneous polarization of 12 samples, as described previously [2]. Each specimen was exposed over an area of 0.785 cm2 using a PTFE encapsulated O-ring together with a silicone based grease in order to avoid the initiation of parasite crevice corrosion. The test solution was deaerated in a separate preparation vessel before transferring it to the electrochemical cell. Polarization was applied after 15 min at Open Circuit Potential (OCP). At the end of each period of polarisation, samples were observed under an optical microscope to check whether the exposed surface remained crevice free. Knowing that this aspect is crucial in this work, a special attention was given to the corrosion morphology identification. In order to validate a test result, pits had to be located within the exposed area and not at the edge of the O-ring. If not, the specimen was discarded. In potentiodynamic mode, potential was increased from OCP at a rate of 10 mV/min. The temperature of the electrolyte was thermostatically controlled at a fixed value between 50 and 80 °C. Pitting and/or transpassive potentials were defined as the potentials above which the anodic current remains above 10 lA, based on the polarization curves. CPT transition can then be simply determined by recording both pitting and transpassive potentials and plotting them against the temperature tested. In potentiostatic mode, CPT measurements were performed by raising the temperature by 0.5 °C/min from ambient, while maintaining a constant potential between 200 mV and 800 mV/SCE. Tests were interrupted when current reached 2 mA. Pitting was defined as the temperature at which the current continuously exceeded 10 lA. 3. Results and discussion 3.1. Potentiodynamic and potentiostatic findings Typical potentiodynamic curves obtained with a multichannel cell at 58 °C are shown in Fig. 1. Both pitting and transpassive potentials are clearly identified on these curves. A steep current increase in the 300–800 mV/SCE range indicates a pitting threshold, whereas transpassivity is reliably indicated by a large passive domain followed by a progressive increase of current for more anodic potentials in the range of Cr(III)/Cr(VI) transition and/or water

decomposition. In a second step, corrosion morphologies were definitively identified by observation of samples using an optical microscope. Pitting behaviour is characterised by the presence of localized attacks with a lacy metal overlay [20–23], whereas large shallow corroded areas with preferential dissolution of phase/grain boundaries are typical of a transpassive attack. CPT is generally described in the literature as a sharp transition between transpassive and pitting corrosion [5]. However, our statistical study using the multichannel cell technique appears to show that both pitting and transpassive corrosion may be observed at the same temperature. This makes it difficult to define a unique temperature. Indeed, pitting and transpassive potentials measured at various temperatures between 50 and 80 °C are shown in Fig. 2. As expected, pitting potentials decrease as temperatures rise: dropping from 550 to 150 mV when the temperature is raised from 61 to 81 °C. In the intermediate temperature range (between 51 and 61 °C), pitting and transpassive dissolution seem to occur at the same temperatures with a huge dispersion in breakdown potentials, up to 600 mV. This critical range will be referred to as the transition temperature interval (TTI), it is somewhat similar to the Critical Crevice Temperature (CCT) [24,25]. Whether or not the TTI should be considered as a special feature of duplex stainless steel has not been addressed so far but could deserve further work on a single phase austenitic grade. From this point of view, the present study is a complement to previous works mostly on AISI 904L grade. The idea that duplex stainless steel favours a broader TTI is a possibility we cannot rule out even if the complex relationships between crystallographic structure (austenitic, duplex, and ferritic) and CPT would require further investigations. One must notice that the annealing temperature of duplex SS including that of 2205 grade is adjusted in order to reach a good balance of ferrite and austenite phase ratio as well as a close PREN for each phase. This means that the lower and upper boundaries of the 10 °C TTI should not simply correspond to the CPT of austenite and ferrite in the material. On the other hand, due to their high content in alloying elements, duplex SS are prone to form chromium and/or molybdenum rich phases like sigma. At the vicinity of these precipitates, depletion in Cr and Mo can be observed leading to a low PREN in their surrounding area. These areas might be a possible source of CPT scattering for duplex SS. Temperatures below the TTI are regarded as low enough to prevent pitting; at these temperatures, SS always undergoes transpassive dissolution. In contrast, within the TTI range, it is tempting to define a pitting vs. transpassivity probability, obtained for each temperature by dividing the number of pitted samples n by the total number of samples tested N. A typical probability curve as a function of temperature is shown in Fig. 3(a). Based on this, a con-

900 transpassivity Potential (mV/SCE)

pitting 700

500

300

100 45

Fig. 1. Typical curves of potentiodynamic experiments performed in a multichannel cell at 58 °C in 0.5 M NaCl.

55

65 Temperature (°C)

75

85

Fig. 2. Pitting and transpassive potentials as a function of temperature from potentiodynamic experiments performed in a multichannel cell in 0.5 M NaCl.

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L. Peguet et al. / Corrosion Science 60 (2012) 280–283 1

1 ∞

76 ºC

S

Pitting vs. Transpassivity probability

S = 5 cm2 S = 0.785 cm2

67 ºC

P(n/N+1)

62 ºC

56 ºC 54 ºC

Experimental from multichannel cell ( S = 0.785 cm2 ) Estimated for S = 5 cm2 (Avesta type cell) Estimated for S → ∞

50 ºC 0

0 48

50

52

54

56 58 Temperature (ºC)

60

62

64

Fig. 3. Pitting vs. transpassivity probability n/(N + 1) as a function of the temperature given by the potentiodynamic method: (a) Experimental result using a multichannel cell with an exposed surface area of 0.785 cm2; (b) an estimation for a 5 cm2 surface area corresponding to an Avesta type cell and (c) an estimation for an infinite surface area.

900 potentiodynamic potentiostatic

Potential (mV/SCE)

58 ºC

700

500

300

100 45

55

65 Temperature (ºC)

75

85

Fig. 4. Pitting temperatures given by potentiostatic measurements performed in a multichannel cell in 0.5 M NaCl compared to pitting potentials from potentiodynamic ones.

servative CPT value can be defined as the lower bound of TTI, i.e. around 50 °C. In addition to this, current-temperature curves were determined using a potentiostatic method and the same multichannel cell. A comparative illustration of the resulting pitting temperatures is reported in Fig. 4. For lower potentials, few specimens undergo pitting before the maximum temperature reached in the experiment (100 °C). In the potential range 350–500 mV a wide scattering is observed. This is not surprising given the dependence of the pitting potential on temperature. On the other hand, this type of scattering is not expected at higher potentials. Pitting temperatures remain sensitive to potential, although the effect is less significant than at lower potentials. In addition, a significant proportion of pitting temperatures (for potentials between 500 and 800 mV/SCE) are shifted to higher values. Consequently, overestimation of the CPT is unavoidable. This was previously observed using an Avesta type cell (FlexcellTM – Gamry) [26]. 3.2. Statistical analysis of the CPT transition In line with the approach described by Shibata and Takeyama [1], a pitting probability can be calculated as a function of potential, expressed as n/(N + 1), where N is the total number of specimens examined, and n is the nth pitted specimen. This curve was plotted for different temperatures, and is shown in Fig. 5. Very high temperatures correspond to a narrower distribution of pitting

100

300

500 V (mV/SCE)

700

900

Fig. 5. Pitting probability at various temperatures showing the probability density transition between pitting and transpassivity.

potentials. The observed vertical slope at 50 °C can be attributed to transpassive behaviour. As a general rule, scattering of pitting potential increases as temperature drops, as previously observed in Fig. 2. Pitting probability unfolds in two distinct slopes, as shown in Fig. 5. For example, at 62 °C a first steep slope in the 350–450 mV/ SCE range is followed by a moderate slope up to 750 mV/SCE. Whatever the temperature, this moderate slope is always observed from 300 mV/SCE up to transpassivity. Moreover, since pitting is proportional to the probability density, the relative likelihood of pitting at a given potential is low. Consequently, for a temperature close to the CPT, if pitting does not occur before 400 mV/SCE, the potential will reach transpassivity leading to a sharp transition: below 300 mV/SCE and above 900 mV/SCE. 3.3. Surface area effect on CPT When calculating pitting probability the surface area effect must be taken into account. Following Baroux’s approach [2], an ‘‘elementary pitting probability’’ per unit area x can be defined. If a sample surface, S, is decomposed into small independent elementary parts, dS, the pitting probability, P(S), can then be written as:

PðSÞ ¼ 1  ð1  -:dSÞS=dS

ð1Þ

when dS?0:

PðSÞ ¼ 1  expð-:SÞ

ð2Þ

therefore:

-¼

lnð1  PðSÞÞ S

ð3Þ

In our study, TTI is most likely to be affected by the surface area of the sample investigated. To assess this effect, x was first evaluated from the ‘‘pitting vs. transpassivity’’ probability plotted in Fig. 3(a) for S = 0.785 cm2, using Eq. (3). Then, for a new surface area, an adjusted probability can be estimated from x using Eq. (2). This process can be iterated until the ideal case of an infinite surface area is attained, as illustrated in Fig. 3(c). This reveals that larger specimen surface area results in sharper transition. The CPT transition can be deduced with better accuracy (<1 °C) when the surface area of a test specimen is large enough. A conservative CPT can therefore be defined as the lower bound of the TTI near 50 °C. For example, from results obtained with an Avesta type cell, the pitting temperature for a 5 cm2 sample area is achieved with a precision of 7 °C (Fig. 3(b)), whereas 10 °C scattering is observed for the multi-channel cell (S = 0.785 cm2). As the most likely pitting temperature has a probability of around 0.5, a confident estimation

L. Peguet et al. / Corrosion Science 60 (2012) 280–283

of CPT would be between 52 and 54 °C. This value is comparable to the CPT previously measured by potentiodynamic method (54 °C) using the Avesta cell [26]. The results presented above show that the scatter in the CPT value tends to increase with the decrease in the surface area of the specimen which is linked to a higher mean CPT. This is directly related to the scatter increase in pitting potentials as well as they increase for lower specimen size as already mentioned [27]. The decreasing of the probability of finding pitting sites for generating stable pits when the surface area is decreased can explain why the probability of reaching transpassivity without experiencing pitting increases. 3.4. Influence of the polarization method on CPT Potentiostatic measurements above 400 mV/SCE result in higher CPT estimations than potentiodynamic methods (Fig. 4). A 400 mV/SCE threshold is given as the higher bound of metastable pitting activity for temperatures below the CPT [28,29] at the same time as an intense electrochemical dissolution of inclusions [30,31]. Therefore, potentiostatic polarization above 400 mV/SCE could have a significant ‘‘cleaning effect’’, dissolving most of the emerging inclusions on the surface, before reaching the pitting temperature. The consequence would be the need for a higher temperature to provoke pit development, thus resulting in a higher CPT. It could also be suggested that potentiostatic polarization above 400 mV/SCE at temperature below CPT is more efficient than a potentiodynamic sweep from lower potentials to reinforce the passive film [18]. However, such reinforcement is not necessarily linked with an increase of CPT as evidenced in case of inhibitor addition to the test solution [16,19]. In addition, CPT transition is often considered as not related to the passive film breakdown stage but more probably to pit propagation [28]. On the one hand, the presence of a lacy metal cover on propagating pits [32,33] is considered as a condition for pit stability and is likely to be affected by passive film stability. On the other hand, the repassivation ability of the pit embryo is also critical in allowing or hindering the subsequent pit development. Given that salt film could be a precursor of passivation [34], it may be easier under the thicker salt films formed at higher potentials [28,35]. This is a third possible explanation accounting for the higher CPT under potentiostatic solicitation. 4. Conclusions A statistical analysis has been applied to potentiodynamic and potentiostatic measurements of pitting potentials and temperatures for 22-05 duplex stainless steel in 0.5 M NaCl. The following conclusions can be drawn regarding the critical pitting temperature (CPT): (1) CPT is usually considered to be a sharp transition; however, critical potentials measured on small surface areas using a multichannel cell show that both pitting and transpassivity can be observed at the same temperature, close to the CPT; (2) In the case studied, a Temperature Transition Interval (TTI) ranging

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between 51 and 61 °C was defined, in which the probability of ‘‘pitting vs. transpassivity ’’ gradually increases from 0 to 1; (3) Using the ‘‘elementary pitting probability’’ concept to investigate the effect of the surface area of a specimen, large areas are found to favour sharp CPT transitions. For a sufficiently large area, the TTI width is found to be less than 1 °C; (4) A significant proportion of pitting temperatures measured using the potentiostatic method is shifted out of the pitting potential dispersion area measured with a potentiodynamic method. As a result, potentiostatic methods are likely to overestimate CPT; (5) For practical purposes, the most conservative CPT value is the lower bound of the TTI, as estimated based on potentiodynamic measurements; (6) Calculated pitting probability densities show that the likelihood of initiating and propagating a pit is lower between 400 mV/SCE and transpassive potential resulting in the CPT sharp transition. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]

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