A new criterion to determine the critical pitting temperature (CPT) based on electrochemical noise measurement

Corrosion Science 58 (2012) 202–210

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A new criterion to determine the critical pitting temperature (CPT) based on electrochemical noise measurement Tao Zhang a,b,⇑, Danyang Wang a, Yawei Shao a,b, Guozhe Meng a,b, Fuhui Wang a,b a

Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology (Harbin Engineering University), Ministry of Education, Nantong ST 145, Harbin 150001, China b State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui RD 62, Shenyang 110016, China

a r t i c l e

i n f o

Article history: Received 2 October 2011 Accepted 26 January 2012 Available online 4 February 2012 Keywords: A. Stainless steel B. Electrochemical calculation B. Potentiostatic C. Pitting corrosion

a b s t r a c t Electrochemical noise (EN) was recorded during the temperature linear scanning period and analyzed using noise thermammetry method. The transition point, separating the Arrhenius plot into two regions, was observed. Below this point, the activation energy of material converse to negative and the corrosion events become spontaneous process. Therefore, this transition point can be as a criterion for the determination of CPT. The effect of inhibitor concentration, surface roughness and ageing time on the CPT were also investigated by means of EN method with comparison of potentiostatic results. These demonstrate the universality and measurement precision of this new criterion. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The concept of a critical pitting temperature (CPT) was introduced by Brigham and Tozer [1,2], and has been widely used as an important parameters to screen stainless steels. Below the CPT, the steel will not pit regardless of potential and exposure time, and the breakdown observed at high anodic potentials is caused by transpassive dissolution. However, once the temperature exceeded CPT, stable pitting can occur and breakdown potentials drop below those required for transpassivity. The CPT depends on inhibitor concentration [3–5], surface finish [6] and alloy elements [7], but hardly at all on potential [6] or chloride concentration within a lower concentration range [8]. The determination of CPT can be performed by immersion tests [9] and electrochemical methods [10–14]. The immersion method is described by ASTM G48 standard [9]. The specimen is immersed in 10% FeC13 solution at an initial temperature of interest and retained in the solution over a 72 h period. The specimen is then examined under an optical microscope to detect any pitting. By this way, the maximum temperature below which pitting does not occur is determined. The main limitation of this methodology is that it is either time-consuming or may not represent the actual corrosive environment.

⇑ Corresponding author at: Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology (Harbin Engineering University), Ministry of Education, Nantong ST 145, Harbin 150001, China. Tel./fax: +86 451 8251 9190. E-mail address: [email protected] (T. Zhang). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2012.01.026

The electrochemical methods can be grouped into two categories: direct-current (DC) measurement methods and electrochemical noise (EN) method. Both the potentiostatic and potentiodynamic CPT determination method are believed as DC method. The potentiostatic method was firstly introduced by Brigham involved potentiostatically controlling the sample potential in the chloride solution and increasing the temperature of the solution at a rate of 0.6 °C min1 [1,2]. The criterion for CPT is the temperature at which the current density increases sharply to a value of 10 lA cm2. But this criterion for CPT was not accepted by many scientists, they prefer to defined 100 lA cm2 as CPT criterion instead of 10 lA cm2 [3–8,10–14]. The potentiodynamic polarization method was favored by Qvarfort to determine the CPT of stainless steels in sodium chloride solutions [10]. The potentiodynamic polarization curves at different temperatures were measured and the breakdown potential as a function of temperature was plotted. The CPT was defined as the temperature where breakdown potential dropped from the transpassive range to the pitting potential range, which was typically a drop of several hundred millivolts. The criterion of breakdown potential used by Qvarfort was the potential where the current density exceeded 100 lA cm2. Furthermore, Li [11] and El-Meguid [12] developed the potentiostatic and potentiodynamic determination method and applied it to determine the critical protection temperature (CPrT), respectively. Salinas-Bravo and Newman [20] firstly proposed EN method to determine the CPT. They monitored the potential and current fluctuation of the specimen during the temperature scanning period. The criterion to determine the CPT of the specimens was the

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temperature at which the amplitude of current fluctuation increased to a value of 5 lA. Heyn [21] also applied EN technique on the determination of CPT. Recently, Iannizzi [22] presented a modified Salinas-Bravo’s approach based on EN to measure the CPT of stainless steel. Wong [23] also investigated the CPT of several kinds of steel by EN method. In summary, EN method has some distinguishing advantages, such as, EN is able to instantaneously monitor the corrosion rate [15,16]; EN is carried out without an artificial disturbance of the system [17]; EN can provide more information to research mechanism than conventional techniques [18,19]. Therefore, in the view of the point of corrosion monitoring, EN method is more promising than DC method. Especially, this method can determine the CPT without any external polarization and distinguish the CPT between stable and metastable pitting events. However, the disadvantage of the EN method is also clear: first, the criterion for the CPT determination (current fluctuation >5 lA) is empirical and lack of explicit physical meanings; second, this criterion was developed from 10% FeC13 [20] or FeC13 + HCl solution [21,23], which might not be suitable for the actual corrosive environments; third, the relationship between this CPT criterion and corrosion mechanism is still not clear. Thus, a more detailed investigation is required to refine the criterion for determining CPT. Mathematic analysis method is one of the absolute key on the application of EN in the corrosion research field. Many methods have been developed to analyze the EN data including statistical analysis [24,25], spectral analysis [26,27], stochastic and chaotic analysis [28,29] and chemometric analysis [30]. Regarding the statistical methods, the standard deviation of potential or current noise can be calculated at different times to monitor the intensity of a corrosion process. Another widely used parameter is the noise resistance, defined as the ratio of the standard deviations of the potential (rE) and current noise (rI), exhibited as following:

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn Pn 2

rE i¼1 ðEi  i¼1 Ei =nÞ =ðn  1Þ Rn ¼ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn Pn rI ðI  I =nÞ2 =ðn  1Þ i¼1

i

ð1Þ

i¼1 i

where Ei and Ii is a datum point of potential and current noise, respectively. Based on fundamental electrochemical theory (Butler–Volmer equation), the result of theoretical derivation indicated that the noise resistance is indeed equivalent to the polarization resistance if the anodic and cathodic reaction on the working electrode were totally activation controlled. In this case, the reciprocal of noise resistance, 1/Rn, which is proportional to the instantaneous corrosion rate [24]. However, noise resistance was also applied on the investigation of diffusion [31], passivation [32] and pitting system [33], which indicated that Rn has been confirmed to be roughly equivalent to polarisation resistance (Rp) experimentally. The aim of the present work is attempted to develop a new criterion for EN method, especially based on noise resistance method, to determine CPT. For this purpose, first, a new criterion of CPT with the explicit physical meaning is founded. Second, several typical experiments were carried out by means of EN method in order to provide the examples demonstrating the utility and validity of new criterion.

Table 1 The percentage composition of 304 SS and 2506 DSS. wt.%

C

Mn

Si

Cr

Ni

Mo

P

S

Fe

304 SS 2506 DSS

0.04 0.03

1.38 0.90

1.48 0.47

16.86 24.80

8.45 5.95

/ 2.13

0.03 0.03

0.03 0.001

Balance Balance

15  10  3 mm. In order to prevent the crevice corrosion, the specimens was specially treated: firstly, the specimen surface was subjected to passivation treatment at 50 °C for 1 h in the solution of 25 wt.% nitric acid. Secondly, the passivated specimens were mounted by epoxy powder coating whose adhesion force is more than 80 MPa. The epoxy powders were milled in high speed mixer and extruded by lab model twin screw extruder at 130 °C followed by cooling, grinding and sieving for obtaining the final powder for coating (120 mesh size). Electrostatic spray gun used for deposition of powder coating which was held at 100 kV potential with respect to the substrate (grounded). After obtaining uniform coverage of the powder, the coated specimens were placed in an oven for curing at 180 °C for 5 min. The powder coating thickness was in the range of 1–2 mm. Lastly, the treated specimens were embedded in the epoxy resin, exposing 10 mm  10 mm surface for testing. The design of electrode was illustrated in Fig. 1. For more accuracy, the interface between sample and mount resin was checked by microscope and no crevice was observed before and after experiment (Fig. 2). Prior to each experiment, the specimen was wet ground to a 1000-grit finish by abrasive paper, degreased with acetone, rinsed with distilled water and dried in a compressed hot air flow. The test solutions, 3.5 wt.% NaCl, were made up from analytical grade reagents and distilled water, which was kept being deaerated with pure nitrogen gas (N2) throughout the whole test. 2.2. Thermammetry technique The thermammetry technique is based on recording the potential or current flowing in the investigated system as a function of the system temperature. The temperature was dynamic controlled by the programmable temperature controller (Zjnbth No. THCD09). The relative error in establishing the real system temperature in relation to the applied temperature was equal to 0.1%. The temperature range is from 10 to 80 °C and its scanning rate in the heating direction was equal to 1 °C min1. 2.3. Electrochemical measurements Prior to determine the CPT, the working electrode was firstly cathodically polarized at 0.9 VSHE for 5 min. Then, the specimen was allowed to stabilize at open circuit potential for 30 min at 10 °C.

2. Experiment details 2.1. Materials The weight percentage compositions of 304 stainless steel (304 SS) and 2506 duplex stainless steel (2506 DSS) are listed in Table 1. These specimens were cut into coupons of dimensions

Fig. 1. Schematic diagram of the experimental design for working electrode.

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For better reproducibility, all electrochemical measurements were repeated more than five times using Zahner Zennium electrochemical workstation. 3. Results and discussion 3.1. Foundation of new criterion

Fig. 2. The interface between resin and stainless steel before (a) and after (b) CPT experiments.

Potentiostatic CPT determination experiments were performed in a three-electrode cell, using a platinum foil as counter electrode and an Ag/AgCl (saturated KCl) electrode as reference electrode. The anodic potential of 542 mVSHE (300 mVSCE) and 992 mVSHE (750 mVSCE) were applied on the 304 SS and 2506 DSS electrode, respectively, and then the solution temperature was increased at a rate of 1 °C min1. The current density was recorded simultaneously throughout the test (except for the first 5 s following the applied anodic potential). The CPT was defined as the temperature at which the current density reached the critical value of 100 lA cm2, which is commonly selected in the literatures to define CPT [3–7,10–14]. Electrochemical noise measurements were performed using a set-up in electrochemical noise module. Two identical specimens were used as the working electrode and an Ag/AgCl (saturated KCl) electrode as the reference electrode, respectively. The electrochemical current noise was measured as the galvanic coupling current between two identical working electrodes kept at the same potential. EN data was instantaneously recorded for 4800 s; meanwhile the solution temperature was increased at a rate of 1 °C min1. Each set of EN records is 100 s containing 2048 data points, recorded with a data-sampling interval of 0.049 s. Fortyeight time records were analyzed for each measurement. The DC trend removal was the key of electrochemical noise application. In this work, the direct current trend of the potential and current noise data was removed by using 5-order polynomial detrending method [34].

The current noise response of the 304 SS to the temperature scanning between 10 and 80 °C is shown in Fig. 3, where both current and temperature are plotted as separate simultaneous functions of time. The metal was initially cathodic polarized and then allowed to stabilize at OCP for 30 min in order to grow a new passive film, denoted as period ‘‘a’’ in Fig. 3. After this period of passivation, the solution was heated with the linear temperature ramp of 1 °C/min, which is denoted as period ‘‘b’’. The raw and trend-removal EN data of 304 SS are shown in Fig. 4. Several characteristic aspects of these EN plots can be outlined. At the beginning of the test, when the temperature is low, the amplitude of both potential and current transients is constant and low. As the temperature is further increased, the amplitude of both potential and current transients demonstrates the gradual changes. These changes correspond to the onset of stable pitting. The EN data was analyzed by the calculation of the reciprocal of noise resistance, 1/Rn (Fig. 5), which is roughly proportional to the instantaneous corrosion rate [24]. EN data were repeated more than three times. There was scatter in the 1/Rn data; however, the shifts in the 1/Rn data as a function of temperature exhibited a good reproducibility. As the mentioned in the Section 2.3, the time record of each EN set is 100 s, which revealed that the temperature interval in this test is 1.67 °C. Moreover, the CPT of 304 SS was determined by potentiostatic method, which indicated that the CPT of 304 SS was about 31.4 °C. 1/Rn result was also plotted in Fig. 5 for the convenience of the result comparison. From Fig. 5, it was noted that the tendency of 1/Rn was well consistent with that of potentiostatic method. During the heating period, once the temperature exceeded the threshold, the 1/Rn significantly increased and reached a higher value level. Recently, Burstein [35,36] introduced a new electrochemical technique, cyclic noise thermammetry (CNT), which combines the basis of cyclic thermammetry [36,37] and electrochemical noise measurement. Referencing the idea of CNT, the 1/Rn data in Fig. 5 are replotted as Arrhenius plot (Fig. 6). The 1/Rn and temperature followed an Arrhenius relationship, but with two distinct regions. The transition between the two regions occurs at about

Fig. 3. Effect of temperature scanning between 10 and 80 °C on the current noise response of 304 SS at OCP. The stabilization stage of OCP during the first 30 min at 10 °C (region ‘‘a’’), the temperature scanning stage (region ‘‘b’’).

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-3.8

Ea<0

2

log(1/Rn) (Ω .cm )

-4.0

-4.2

Ea>0 -4.4

33

-4.6

30

-4.8 0.0030

0.0031

0.0032

0.0033

0.0034

0.0035

0.0036

-1

1/T (K ) Fig. 6. Arrhenius plot of 1/Rn of 304 SS in NaCl solution.

Curve 1

Ea>0

ΔG < 0

Ea<0

Curve 2

Fig. 4. The raw (a) and DC-removal (b) electrochemical noise data of 304 SS at OCP during the temperature linear scanning period.

-4

1.0x10

Fig. 7. Schematic of the activation energy peaks for the nucleation, metastable pitting and stable pitting.

200

5000 4000 -2

2

-5

6.0x10

100

-5

4.0x10

50

-5

2.0x10

0 10

20

30

40

50

60

Temperature

3000

Arrhenius Slope

1/Rn (Ω .cm )

150

31.4

i (μA.cm )

-5

8.0x10

2000

31.7

1000 0 -1000 -2000 -3000 -4000 0.0031

Fig. 5. Comparison of 1/Rn and potentiostatic curve of 304 SS at 300 mVSCE during the temperature linear scanning period.

0.0032

0.0033

0.0034

0.0035

-1

1/T (K ) Fig. 8. Arrhenius slope plot of 1/Rn of 304 SS in NaCl solution.

30–33 °C. The Arrhenius plot showed a positive and negative slope in the higher (in left of transition point) and lower temperature range (in right of transition point), respectively. In the lower temperature range, the negative slope indicates the activation energy is positive value. The activation energy plays a key role on the corrosion process. If the activation energy was positive value, the metal atom must overcome the activation energy peak (Fig. 7 curve 1), which indicates that only a part of metal with higher energy can be corroded. In this temperature range, pitting can nucleate, and even propagate a very brief period before the repassivation ensues, which are called as metastable pitting. The lifetime of metastable pitting is only several seconds and is impossible to continuously

grow, suggesting a lower level of the corrosion reaction rate. As regard the higher temperature range, the positive slope of the Arrhenius plot (olog Rn/oT1) implies the activation energy is negative value. The metal atom can become ion without the overcome of energy peak and the corrosion process followed the curve 2. In this case, most of metal atom can continuously react and the reaction rate stayed at a higher level. Among all the corrosion type, the spontaneous corrosion process is usually with the feature of ‘‘acidation–selfcatalysis’’. Crevice corrosion follows the ‘‘acidation– selfcatalysis’’ mechanism. However, the specimens in this work were special treated, which suggested the probability of crevice

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T. Zhang et al. / Corrosion Science 58 (2012) 202–210

Fig. 9. Morphology of 304 SS after the immersion of 72 h in NaCl solution in a series of temperature around CPT, (a) 28 °C, (b) 30 °C, (c)32 °C and 34 °C.

-4.2

600 500 400 -2

i (μA.cm )

Ea<0

2

log(1/Rn) (Ω .cm )

-4.5

300

69.6 200 100

Ea>0 -4.8

-5.1

0

-5.4 10

20

30

40

50

60

70

80

0.0028

Fig. 10. The potentiostatic curve of 2506 DSS at 750 mVSCE in NaCl solution.

corrosion can be neglected. For the pitting growth process, the concentration of metal anion within pitting cavity increase, even reach saturation of the metal chloride. The hydrolysis of saturation salt induces the acidation of anolyte, which prevents the repassivation of pitting. Then, the pitting growth would succeed and obey the self-catalysis mechanism. This pitting stage is defined as stable pitting, which should be a kind of spontaneous process. In brief, once the temperature exceeded the transition point, the activation energy of corrosion events changed from positive value to negative value. The corrosion events would become spontaneous process and the stable pitting generate on the 304 SS surface. According to the definition of CPT, CPT is the lowest temperature at which the growth of stable pits is possible. Therefore, the transition point should be the criterion to determine the CPT and its corresponding temperature is CPT. However, due to the scattered

0.0029

0.0030

0.0031

0.0032

0.0033

-1

1/T (K )

Temperature

Fig. 11. Arrhenius plot of 1/Rn of 2506 DSS in NaCl solution.

1/Rn data, it is difficult to accurately determine the transition point from the Arrhenius plot. Therefore, the Arrhenius slope of each data point was calculated in order to the precise determination of the transition point. This method can be briefly explained as follows: consider the log(1/Rn)–T1 series consist of k data point ({log(1/Rn)}x = 1, 2, 3, ...i, i + 1, i + 2,.... k). Then, the adjacent data points of log(1/Rn)i, can be taken as an estimation of the Arrhenius slope and thus the Arrhenius slope of each data point (log(1/Rn)i) can be statistically calculate:

logð1=Rn Þx ¼ a þ b=Tx

x 2 ½i  p; i þ p

ð2Þ

where a is nodal increment, b is the Arrhenius slope of this point (log(1/Rn)i), and p is variation which can be 1, 2, 3 or more. For example, if we take p = 3, this means taking seven data points (three data points on the left and right of log (1/Rn)i, respectively) for the Arrhenius slope calculation.

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T. Zhang et al. / Corrosion Science 58 (2012) 202–210 3000

0

2000

-4000

69.5

Arrhenius Slope

1000

Arrhenius Slope

0.01 M

4000

0 -1000

0 -3000

0.0001 M 0

-3000

-3000

0.0029

0.0030

0.0031

0.0032

0.0033

69.5 0.0028

0.0029

0.0030

-1

1/T (K )

1 - 0.0 M

Na2MoO4

1

3

-4

2 - 10 M Na2MoO4

500

2

-3

4

3 - 10 M Na2MoO4

-2

i (μA.cm )

-2

4 - 10 M Na2MoO4

400 300

71 69

200

76

69

Table 2 Average and standard deviation of potentiostatic CPT and EN measurements of 2506 DSS in the NaCl solution containing various concentration of MoO2 4 . MoO2 4 concentration (M)

Method

Average

Standard deviation

Error between the two results (%)

0.0001

Potentiostatic EN Potentiostatic EN Potentiostatic EN

69.6 69.0 70.2 70.0 75.2 75.3

1.9 1.0 1.4 1.4 1.4 2.1

0.8

0.01

0 40

50

60

Temperature (

0.0032

Fig. 15. Arrhenius slope plot of 1/Rn of 2506 DSS in the NaCl solution containing various concentrations of Na2MoO4.

0.001

100

30

0.0031

-1

1/T (K )

Fig. 12. Arrhenius slope plot of 1/Rn of 2506 DSS in NaCl solution.

600

0.001 M

70.5

3000

-2000

0.0028

75.5

70

0.3 0.1

80

)

Fig. 13. The potentiostatic curves of 2506 DSS at 750 mVSCE in the NaCl solution containing various concentrations of Na2MoO4.

78

obtained by potentiostatic method obtained by EN method

76

0.01 M

74

)

-4.2

CPT (

-4.5

2

log(1/Rn) (Ω .cm )

-4.8 -5.1 -3.6

0.001 M

-3.9

72

70 68

-4.2

66

-4.2

0.0001 M

10

-4

10

-3

10

-2

2-

MoO4 concentration

-4.5 -4.8 -5.1 0.0028

0.0029

0.0030

0.0031

0.0032

Fig. 16. The relationship between the CPT of 2506 DSS in NaCl solution and the logarithm of Na2MoO4 concentration.

-1

1/T (K ) Fig. 14. Arrhenius plots of 1/Rn of 2506 DSS in the NaCl solution containing various concentrations of Na2MoO4.

Fig. 8 illustrates the Arrhenius slope as the function of T1. It is clear that the Arrhenius slope inversed from negative to positive value when the temperature reached 31.7 °C, which indicated that the transition point can be accurately determined and the CPT of 304 SS is 31.7 °C. This reasoning is also supported by a series of the corrosion morphology around CPT (Fig. 9). Below CPT, there was no any pitting on the 304 SS surface (Fig. 9a and b); once the temperature exceeded CPT, the large pitting cavity (diameter about 80–100 lm) instead of transpassivition was observed

(Fig. 9c and d). Furthermore, by comparing the result of Fig. 5 and Fig. 8, it suggests that this criterion has validity, and the CPT determined by this criterion show very good agreement with that obtained by potentiostatic method. 3.2. Examples for the precision and universality of new criterion 3.2.1. Determination to the CPT of 2506 DSS This new criterion is induced from the experimental results of 304 austenite stainless steel. In order to exhibit the validity of this new criterion, the CPT of 2506 DSS having two-phase of austenite– ferrite microstructure was carried out. Fig. 10 shows a typical i–t curve for 2506 DSS obtained by potentiostatic method. It is evident

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T. Zhang et al. / Corrosion Science 58 (2012) 202–210

2

1 - 60# 2 - 400# 3 - 1000#

500

1

400

-2

i (μA.cm )

3 300

66 200

Table 3 Average and standard deviation of potentiostatic CPT and EN measurements on 2506 DSS with different surface roughness in NaCl solution. Surface roughness

Method

Average

Standard deviation

Error between the two results (%)

60#

Potentiostatic EN Potentiostatic EN Potentiostatic EN

64.0 60.9 66.1 65.5 69.6 69.1

1.0 1.2 2.2 1.1 1.9 0.85

4.8

400#

69

64

1000# 100

0.8 0.7

0 30

40

50

60

Temperature (

70

)

1 - 0 min 2 - 10 min 3 - 60 min 4 - 300 min

600

Fig. 17. The potentiostatic curves of 2506 DSS at 750 mVSCE with different surface roughness in NaCl solution.

2

400

-2

i (μA.cm )

500

1

-4.4 1000# -4.6

2

200

43

69

57

100

-4.8

log(1/Rn) (Ω .cm )

3

4

300

65

-5.0

0

400# -4.4

30

35

40

45

50

55

60

Temperature (

65

70

75

)

-4.8 60#

-4.4

Fig. 20. The potentiostatic curves of 2506 DSS at 750 mVSCE with different ageing time in NaCl solution.

-4.6 -4.8

-4.5

-5.0 0.0028

0.0029

0.0030

0.0031

-4.8

-1

1000#

69.5

2

Fig. 18. Arrhenius plots of 1/Rn of 2506 DSS with different surface roughness in NaCl solution.

log(1/Rn) (Ω .cm )

1/T (K )

2000

-5.1 -4.2

60 min

-4.5 -4.8 -4.0 -4.4

0

-4.8 -2000

Arrhenius Slope

300 min

0.0032

10 min

-5.2 400#

67

4000

0.0029

0.0030

0.0031

0.0032

-1

1/T (K )

0 -4000 2000

60#

Fig. 21. Arrhenius plots of 1/Rn of 2506 DSS with different ageing time in NaCl solution.

0 -2000

60

-4000

0.0028

0.0029

0.0030

0.0031

0.0032

The transition point was determined and its corresponding temperature is about 69.5 °C (Fig. 12), which is well consistent with that obtained by potentiostatic method.

-1

1/T (K ) Fig. 19. Arrhenius slope plot of 1/Rn of 2506 DSS with different surface roughness in NaCl solution.

that the current density dramatically increases when the temperature exceeds a threshold. Considering 100 lA cm2 current density as a criterion for CPT evaluation, the CPT of 2506 DSS is about 69.6 °C. The EN data of 2506 DSS was recorded during the heating period. The 1/Rn of 2506 DSS was calculated, and then plotted as both Arrhenius plot and slope plot (Figs. 11 and 12), respectively.

3.2.2. Effect of inhibitor MoO2 4 on the CPT of 2506 DSS As an alloying element, Mo can be dissolved at local activation site reacting directly in the aggressive pitting environment. Therefore, the repassivation of metastable pitting is enhanced and the growth of stable pitting is hindered. Recently, some authors [3] also reported that molybdate has great inhibition influence on the CPT of duplex stainless steel. The potentiostatic curves of 2506 DSS in Cl containing solution with various molybdate concentrations are represented in Fig. 13. It can be seen that adding 0.0001, 0.001 and 0.01 M MoO2 4 increases CPT to about 0, 2 and

T. Zhang et al. / Corrosion Science 58 (2012) 202–210

3000

abraded to a 60 grit finish (about 4.8%). Furthermore, this experimental result is qualitatively coincident with the suggestion of published work [6,20]. The rough surface has bigger micro-crevices around surface inclusions and a longer diffusion length than that of smooth surface, which indicates that a pitting has higher probability to become stable pitting. This should be the reason why the CPT decreases with the increasing roughness.

300 min

Arrhenius Slope

0

45

-3000 2000

60 min

56

0 -2000 3000

62

0 -3000 -6000

10 min 0.0029

0.0030

0.0031

0.0032

-1

1/T (K ) Fig. 22. Arrhenius slope plot of 1/Rn of 2506 DSS with different ageing time in NaCl solution.

Table 4 Average and standard deviation of potentiostatic CPT and EN measurements on 2506 DSS with different ageing time in NaCl solution. Aged time (min)

Method

Average

Standard deviation

Error between the two results (%)

10

potentiostatic EN potentiostatic EN potentiostatic EN

65.0 62.3 56.5 56.3 43.0 45.0

1.6 2.5 1.7 2.1 2.8 2.1

4.2

60 300

209

0.4 4.7

7 °C, respectively. The EN data of 2506 DSS in those test solutions were recorded, and their Arrhenius plots and slope plots are shown in Figs. 14 and 15, respectively. The CPT-assessed values of 2506 DSS in the test solution with different inhibitor concentrations are listed in Table 2. The effect of MoO2 4 ion on CPT in NaCl solution revealed a negligible improvement effect at significantly low MoO2 concentrations (0.0001 M). At higher enough MoO2 con4 4 centration, such as more than 0.001 M, molybdate reveals clearly an inhibiting effect by increasing CPT values. Furthermore, unlike pitting potential, a nonlinear relationship could fit the CPT increase as a function of logarithm of MoO2 4 concentration (Fig. 16). These are qualitatively consistent with the result reported by Moayed [3].

3.2.3. Effect of surface roughness on the CPT of 2506 DSS The surface roughness of the specimen is another important factor that influences the CPT value [6]. Three nominally identical samples were tested for each of the following surface conditions: 60, 400 and 1000 grit. The CPT results for each of the different surface conditions determined by potentiostatic and EN method are shown in Fig. 17–19, respectively. Table 3 shows the mean and standard deviation of the data obtained by two methods. Clearly, the measured CPT increases as the surface roughness decreases. The lowest CPT, of about 64 °C, was measured for the specimens abraded to a 60 grit finish, whilst the highest CPT, of about 69 °C was obtained from specimens abraded to a 1000 grit finish. The results in Table 3 also show that the measurement reproducibility increased (i.e. the standard deviation decreased) as the surface roughness decreased. The standard deviation of three data points for the smoothest surface was all 0.85 °C (Table. 3). While, for the roughest surface tested, the standard deviation increased to 1.2 °C. These results assessed by different method showed good agreement each other, and the discrepancy between the two results is less than 1% except that obtained from the specimens

3.2.4. Effect of ageing time on the CPT of 2506 DSS Ageing has strong influence on the CPT of DSS [13,38]. During the temperature intervals between 550 and 1000 °C, r phase precipitates in the grain boundary, which is the major concern due to its detrimental influence on both mechanical properties and corrosion behaviour. Due to its high Cr and Mo content, the precipitation of r phase depletes the surrounding regions of Cr and Mo, leading to a decrease in corrosion resistance of duplex stainless steel [39]. Fig. 20 shows typical potentiostatic curves for different aged 2506 DSS specimens at 650 °C obtained from the CPT measurement test. The result of Fig. 20 suggest that the decrease in CPT is small after 10 min ageing because precipitates have not had time to form over this short time period. This was because Cr-rich precipitates could not completely precipitate in such a short time. As the ageing time increased further to 60 and 300 min, the CPT of aged specimens dropped to the 57 and 43 °C, respectively. It is clear that the decrease in CPT value for specimen with the increasing ageing time is mainly owing to the precipitation of relative large volume fraction of r phase [13,38]. The CPT results for each of the different ageing time determined by EN method are shown in Fig. 21 and 22, respectively. Although the standard deviation determined by EN method is slightly higher that of potentiostatic method, the discrepancy between the two results is less than 5% (Table 4), suggesting good precision for the EN method. This experimental result is also qualitatively agreement with the reports of published work [13,39]. 4. Conclusion In this work, a new criterion to determine CPT using EN method was presented: The 1/Rn was calculated based on EN data and replotted as Arrhenius plot and slope plot. Once the temperature exceeded the transition point, the activation energy of corrosion change to negative value and the corrosion events become spontaneous process. This transition point corresponds to CPT. In order demonstrate the validity of this new criterion, the CPT of 2506 DSS, effect of inhibitor concentration, surface roughness and ageing time on the CPT of 2506 DSS were investigated. The results show good agreement with that obtained by potentiostatic method and reported by the publishing literatures, which reveals that this new criterion has good universality and precision. Acknowledgements The authors wish to acknowledge the financial support of the program for New Century Excellent Talents in University of China (NCET-09-0052), the Fundamental Research Funds for the Central Universities (HEUCF201210001). References [1] R. Brigham, E. Tozer, Temperature as a pitting criterion, Corrosion 29 (1973) 33–35. [2] R. Brigham, E. Tozer, Effect of alloying additions on the pitting resistance of 18% Cr austenitic stainless steel, Corrosion 30 (1974) 161–166. [3] F. Eghbali, M. Moayed, A. Davoodi, N. Ebrahimi, Critical pitting temperature (CPT) assessment of 2205 duplex stainless steel in 0.1 M NaCl at various molybdate concentrations, Corros. Sci. 53 (2011) 513–522.

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