Corrosion assessment of nitric acid grade austenitic stainless steels

Corrosion assessment of nitric acid grade austenitic stainless steels

Corrosion Science 51 (2009) 322–329 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci Co...

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Corrosion Science 51 (2009) 322–329

Contents lists available at ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Corrosion assessment of nitric acid grade austenitic stainless steels S. Ningshen, U. Kamachi Mudali *, G. Amarendra, Baldev Raj Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

a r t i c l e

i n f o

Article history: Received 13 July 2007 Accepted 1 September 2008 Available online 7 November 2008 Keywords: A. Stainless steel B. Polarization B. EIS B. AES C. Acid corrosion

a b s t r a c t The corrosion resistance of three indigenous nitric acid grade (NAG) type 304L stainless steel (SS), designated as 304L1, 304L2 and 304L3 and two commercial NAG SS designated as Uranus-16 similar to 304L composition and Uranus-65 similar to type 310L SS were carried out in nitric acid media. Electrochemical measurements and surface film analysis were performed to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO3 media. The results in 6 N HNO3 show that the indigenous NAG 304L SS and Uranus-65 alloy exhibited similar and higher corrosion resistance with lower passive current density compared to Uranus-16 alloy. In higher concentration of 11.5 N HNO3, transpassive potential of all the NAG SS shows a similar range, except for Uranus-16 alloy. Optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO3 with corrosion attacks at the grain boundaries. Frequency response of the AC impedance of all the NAG SS showed a single semicircle arc. Higher polarization resistance (RP) and lower capacitance value (CPE-T) revealing higher film stability for indigenous NAG type 304L SS and Uranus-65 alloy. Uranus-16 alloy exhibited the lowest RP value in both the nitric acid concentration. Auger electron spectroscopy (AES) study in 6 N and 11.5 N HNO3 revealed that the passive films were mainly composed of Cr2O3 and Fe2O3 for all the alloys. The corrosion resistance of different NAG SS to HNO3 corrosion and its relation to compositional variations of the NAG alloys are discussed in this paper. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The development of high corrosion resistant materials with adequate reliability is indispensable factor for ensuring the safety and economical operation of nuclear reprocessing plants. Austenitic SS with low carbon content are widely used in spent nuclear fuel reprocessing plant owing to their good corrosion resistance [1–6]. American Iron and Steel Institute (AISI) type 304L SS is extensively used for fabrication of vessels, tanks, piping and equipment in reprocessing plants where in the concentration of the acid is below 8 N and temperature of operation is below 80 °C [1,5,6]. Most SS, except certain high chromium types, exhibit an average corrosion rate of 0.13 mm per year in boiling 65% HNO3 [5,6]. However, several incidences of failures of components made of AISI type 304L SS have been reported in spent nuclear fuel reprocessing plants when they were used in HNO3 medium beyond 8 N concentration, and temperatures beyond 80 °C due to transpassive corrosion [5–11]. Nitric acid grade (NAG) SS are the alloys inherently developed with (i) controlled chemical composition of alloying elements, (ii) modified microstructures leading to elimination of weaker sites for passive film break down and dissolution, and (iii) enhanced strength against transpassive dissolution [2,5,8–10]. Close control of com* Corresponding author. Tel.: +91 44 27480121; fax: +91 44 27480301. E-mail address: [email protected] (U. Kamachi Mudali). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.09.038

position, good steel making practices and processing parameters are essential to achieve consistently good intergranular corrosion resistance in nitric acid. Several types of NAG SS having compositions similar to AISI types 304L, 310L and several new proprietary alloys with very low carbon have been developed worldwide [3,4,6,8]. AISI type 304L SS with extra low-carbon and restricted levels of C, Si, P, S, and Mo (AISI type 304ELC), with Nb (AISI type 347), Ti (AISI type 321), or AISI type 310L SS and its equivalent with Nb stabilization and high Cr content, have been used in several reprocessing plants in the world [1,5,6]. Proprietary alloys of 18Cr/15Ni and 25Cr/20Ni/ extra low carbon steels with silicon additions of about 4–5% have also been considered for many critical applications for reprocessing plants [1,5,6,9]. The beneficial effect of silicon addition in austenitic SS have been discussed by many authors [1,3,5,6,12–14]. The susceptibility of silicon-containing SS to corrosion resistance depends on the concentration of silicon and its distribution in the SS, as well as on the conditions and compositions of the corrosive environment [11–14]. However, the influence of Si and its mechanism in improving corrosion resistance in nitric acid still remains inconclusive. The objective of the present investigation is to study the corrosion resistance and passive film property of different NAG SS. The correlation between alloy composition and HNO3 concentration of different NAG SS are highlighted in this paper.

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2. Experimental work 2.1. Materials and sample preparation The chemical composition of different NAG SS used in the present investigation is shown in Table 1. Indigenously developed NAG SS are designated as 304L1, 304L2, 304L3, while proprietary SS were termed as Uranus-16 and Uranus-65. All the as-received alloys were solution annealed at 1050 °C for 30 min to homogenize the microstructure. Specimens of 10 mm  10 mm  2 mm were cut and then mechanically polished up to 1000 grit SiC emery paper on all sides. The surface of the specimen was then polished with diamond paste to mirror finish (1 lm finish). The specimens after polishing were ultrasonically cleaned in acetone and washed with double distilled water before the electrochemical experiments.

Fig. 1. The equivalent circuit used for the fitting of impedance data.

Open circuit potential, mV vs Ag/AgCl

1000

2.2. Electrochemical investigation for corrosion assessment 2.2.1. Open circuit potential (OCP) and polarization studies The variation in OCP was measured with time in aerated and non-stirred condition after immersing the specimen in 6 N and 11.5 N HNO3 for 30 min, and their OCP values were monitored with respect to time up to 3600 s. The potentiodynamic anodic polarization tests to evaluate the corrosion resistance of NAG SS were carried out in 6 N and 11.5 N HNO3 media. All the potentiodynamic polarization experiments were carried out at room temperature using the five neck ASTM electrochemical cell consisting of three electrodes system; reference electrode (Ag/AgCl-Sat), counter electrode (Pt) and working electrode. Solartron 1287 electrochemical interface was used for the polarization experiments. The potentiodynamic anodic polarization experiments were carried out at the scan rate of 10 mV/min and polarization was continued till the breakdown of transpassive potential occurred. During the potentiodynamic polarization, the potential at which there was a monotonic increase in the anodic current exceeding 80–100 lA was termed as the breakdown potential. Two to three sets of tests were conducted for each specimen and all the polarization plots were almost reproducible.

900 OCP - 6 N HNO3 304L 1 304L 2 304L 3 Ur an us -65 Ur an us -16

800

700

600

500 0

1000

2000

3000

4000

Time, sec Fig. 2. The OCP of different NAG stainless steels measured in 6 N HNO3.

the experimental impedance data using Zview Version 2.6 (Scribner Inc.) software. The impedance expression of CPE [15] is given by ZCPE = 1/[T(jx)n], where x is the angular frequency, T and n are frequency-independent fit parameters, j = (1)1/2 and x = 2pf, where f is the frequency Hz. CPE has been used in the present investigation to obtain better fit for experimental data and this will represent the capacitance of the passive oxide layer. EIS measurements under same condition are almost reproducible. 2.3. Surface analysis using AES

2.2.2. Electrochemical impedance spectroscopy studies Electrochemical impedance spectroscopy (EIS) measurements were carried out using Solartron 1255 frequency response analyzer (FRA) and Solartron 1287 electrochemical interface. The experiments were carried out in the frequency range of 0.01 Hz– 100 kHz by superimposing an AC voltage of 10 mV amplitude at open circuit potential in both 6 N and 11.5 N HNO3. The EIS results were interpreted using simple ‘‘equivalent circuit” shown in Fig. 1. The selection of this circuit was based on EIS results that revealed only one time constant and this model provides better and accurate fitting values of the experimental impedance data. The circuit description consist of the arrangement of (Rs (CPE || RP)) elements, where Rs is the solution resistance, CPE is the constant phase in parallel connection with RP which is the polarization resistance at the interface. The circuit elements values are obtained by fitting

In order to understand the composition of the passive films, Auger electron spectroscopy (AES) studies were carried out on passive films formed by potential sweep from OCP to passive region and by holding potentiostatically at +900 mV for 1 h. M/s OMICRON, Germany make, cylindrical mirror analyzer (CMA), having energy resolution of 0.4% was used for the analysis in a surface analysis chamber with a base vacuum of 5  1010 torr. AES analyses were performed with a primary electron beam voltage of 3 kV with 1 lA current. The AES system is computer controlled with provision for in situ Ar-ion sputtering. All the samples were sputtered with 500 eV Ar+ ions for 1 min, so as to remove the surface contamination due to air-handling. The major peaks chosen for the AES study were C (272 eV), O (503 eV), Cr (529 eV), Fe (701 eV), Ni (848 eV), N (379 eV), Si (96 eV) and Mo (186 eV).

Table 1 Chemical composition of the different NAG SS in wt%. Alloys

Cr

Ni

C

Si

Mn

P

S

Al

B

N

Mo

Ti

304L1 304L2 304L3 Uranus-16 Uranus-65

18.89 18.92 18.76 18.07 24.97

10 10 11.33 12.31 19.60

0.016 0.014 0.014 0.012 0.012

0.24 0.24 0.20 0.09 0.190

1.63 1.63 1.70 0.69 0.56

0.03 0.03 0.019 0.012 0.023

0.006 0.006 0.005 0.001 0.001

0.007 0.006 0.015 – –

0.001 0.001 0.001 – –

0.02 0.02 0.04 0.018 0.026

0.09 0.09 0.04 – –

0.01 0.02 0.05

Open circuit potential, mV vs Ag/AgCl

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S. Ningshen et al. / Corrosion Science 51 (2009) 322–329 Table 2 Polarization parameters of different NAG stainless steels in 6 N and 11.5 N HNO3 media (IPass were measured at 900 mV for 6 N HNO3 and at 1000 mV for 11.5 N HNO3).

1000

900

11.5 N HNO3 304L1 304L2 304L3 U ranus-65 U ranus-16

800

700

600

500

0

1000

2000

3000

4000

Alloy

ECorr (mV)

ICorr (lA/cm2)

IPass (lA/cm2)

ETP (mV)

6 N HNO3 304L1 304L2 304L3 Uranus-65 Uranus-16

726 646 691 744 600

0.255 0.975 0.845 0.212 0.119

11.20 9.59 8.46 3.42 33.78

1075 1077 1076 1070 1047

11.5 N HNO3 304L1 304L2 304L3 Uranus-65 Uranus-16

887 905 853 971 529

0.103 0.604 0.217 0.115 0.674

12.50 15.58 17.98 1.48 755

1090 1080 1085 1105 950

Time, sec Fig. 3. The OCP of different NAG stainless steels measured in 11.5 N HNO3.

3. Experimental results 3.1. Open circuit potential (OCP) measurement The measured OCP of different NAG SS obtained with 6 N and 11.5 N HNO3 are shown in Figs. 2 and 3. The data measured in 6 N HNO3 (Fig. 2) revealed that the OCP of 304L1 and 304L2 alloys (980 mV) showed marginally noble potential than the 304L3 (860 mV) alloy. Similarly, the OCP of Uranus-65 (950 mV) was nobler than Uranus-16 (570 mV). In Fig. 3, the OCP in 11.5 HNO3 showed that the OCP of Uranus-65 (960 mV), 304L2 (900 mV) are nobler than those of 304L1 (850 mV) and 304L3 (840 mV) and Uranus-16 (560 mV). The OCP of Uranus-16 SS is the lowest in both the nitric acid concentration. 3.2. Potentiodynamic anodic polarization studies The potentiodynamic polarization behavior of different NAG SS obtained in 6 N HNO3 are shown in Fig. 4 and measured polarization parameters such as corrosion potential (ECorr), corrosion current density (ICorr), and transpassive breakdown potential (ETP) are given in Table 2. It is evident from this plot (Fig. 4) that the transpassive ETP range was in similar range for all the NAG SS but Uranus-65 (3 lA/cm2) and NAG 304L3 (8 lA/cm2) SS showed lower passive current density (IPass) followed by NAG alloys of

1200

6N HN O3 304L 1 304L 2 304L 3 Uranus-65 Uranus-16

3.3. Microstructure and morphology of corrosion attack The microstructure of Uranus-16 and Uranus-65 SS are shown in Figs. 6 and 7, respectively. The electrochemically etched Uranus-16 in 10% oxalic acid showed step structure (Fig. 6a) revealing that the sample is free from sensitization and possess a homogenous microstructure. However, after polarization the microstructure changed, with clear indication of corrosion attack along the grain boundaries (Fig. 6b) at 6 N HNO3. In 11.5 N HNO3, the corrosion attack along the grain boundaries was more prominent (Fig. 6c). Similarly for Uranus-65 alloy (Fig. 7a) and 304L-1 (Fig. 8a) with step microstructure, severe corrosion attack along grain boundaries was observed in 11.5 N HNO3 (Fig. 7b) after the polarization test. Similar features of severe corrosion attack along grain boundaries was also observed in 11.5 N HNO3 after polarization for the other NAG alloys of 304L1 (Fig. 8b), 304L2 and 304L3 SS.

1500 Uranus-65

Uranus-65

900

P o tential, mV vs Ag/AgCl

Potential, mV vs Ag/AgCl

1500

304L3 (8.5 lA/cm2) and 304L1 (11.2 lA/cm2). Uranus-16 alloy showed the highest passive current density (33 lA/cm2). The typical anodic polarization curves of the alloys in 11.5 N HNO3 shown in Fig. 5 and the measured polarization parameters are given in Table 2. Similar to the pattern observed in 6 N HNO3 (Fig. 4), the obtained polarization parameters of ECorr, ICorr and ETP are in similar range for all the alloys except for Uranus-16 alloy, higher IPass and lower ETP was observed (Table 2).

Uranus -16

600

300

1200

900

600

300

0

0 10 -4

10 -3

10 -2

10 -1

10 0

101

102

103

104

105

106

107

Current density, μA/cm2 Fig. 4. Potentiodynamic polarization plots of different NAG stainless steels measured in 6 N HNO3.

1E-3

11. 5 N HNO3 304L1 304L2 304L3 U ranus-65 U ranus-16 0.01

0.1

Uranus-16

1

10

100

1000

10000 100000

Current density, μA/cm2 Fig. 5. Potentiodynamic polarization plots of different NAG stainless steels measured in 11.5 N HNO3.

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Fig. 6. Optical micrographs of Uranus-16 (a) etched in 10% oxalic acid; (b) after polarization in 6 N HNO3 and (c) after polarization in 11.5 N HNO3.

Fig. 7. Optical micrographs of Uranus-65: (a) etched in 10% oxalic acid and (b) after polarization in 11.5 N HNO3.

Fig. 8. Typical microstructure of NAG type 304L1: (a) electrochemically etched in 10% oxalic acid and (b) corrosion attacked observed after polarization in 11.5 N HNO3.

85000

As received samples 304L1 304L2 304L3 Uranus-16 Uranus-65

Intensity (arb. unit)

80000

75000

70000

Si

C

Ni

Cr Cr

65000

Fe O

60000

200

400

600

800

Electron energy (eV) Fig. 9. AES spectra of metallic surface of different NAG SS (as reference in air-formed).

1000

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S. Ningshen et al. / Corrosion Science 51 (2009) 322–329 85000

Passive film formed in 11.5N HNO3 304L1 304L2 304L3 Uranus-16 Uranus-65

Intensity (arb. unit)

80000

75000

70000

Ni C

Fe

Cr Cr

65000

60000

O 200

400

600

800

1000

Binding energy (eV) Fig. 10. AES spectra of passive film formed on different NAG SS in 11.5 N HNO3.

3.4. Passive film analysis by AES Table 3 AES elemental profiles for the passive film of five different NAG SS in as received and passive film formed in 11.5 N HNO3 solution at 900 mV for 1 h. Alloys

Si (96 eV)

Peak intensity 304L-1 304L-2 304L-3 Uranus-65 Uranus-16

(arbitrary unit) of as received samples 70407 77625 68320 70324 78566 68329 70764 78855 68756 70839 76050 67772 70418 77854 68538

O (503 eV)

Cr (529 eV)

Peak intensity 304L-1 304L-2 304L-3 Uranus-65 Uranus-16

(arbitrary unit) of passive film formed in 11.5 N HNO3 70700 79331 67800 69479 70745 78313 68735 69011 70931 79255 68315 69472 70700 78809 68449 69650 70893 80221 68741 69034

-30000

Ni (848 eV)

64123 64232 65463 65630 63818

70685 71151 71702 71275 72198

6N HN O 3 304L 1 304L 2 304L 3 Ur an us -6 5 Ur an us -1 6

-25000

-20000

-Z'' (Ωcm2)

Fe (709 eV)

71226 71027 71376 71533 72291

The measured AES spectra of as-received (taken as reference) and the potentiostatically passivated samples of different NAG SS are shown in Figs. 9 and 10, respectively. The analyzed peak intensity of each component of different NAG SS are shown in Table 3 to provide compositional differences observed between the alloys. In reference sample, peaks due to C, Cr, O, Fe and Ni were observed for all the NAG alloys (Fig. 9). The AES profiles of the passivated samples obtained in both 6 N and 11.5 N HNO3 showed similar features and only the AES spectra measured in 11.5 N HNO3 is presented (Fig. 10). With respect to passivated samples in 11.5 N HNO3, the main observation was the increase in Cr and O peak intensity and reduction in Fe peak (Fig. 10). Ni peak is relatively weak and the large signal of carbon (272 eV) was due to the contamination during exposure to the atmosphere during the sample transfer.

Uranus -16

Uranus -65

304L 3

-15000

304L 2 -10000

304L 1 -5000

Uranus -16 0 0

10000

20000

30000

Z'(Ωcm2) Fig. 11. Nyquist plots of different NAG stainless steels measured under OCP in 6 N HNO3.

40000

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11.5 N HNO3 304L1 304L2 304L3 Uranus-65 Uranus-16

-8000 -7000

304L 3

-Z'' (Ωcm2)

-6000 -5000 -4000

Uranus-65 -3000 -2000 -1000

Uranus-16 0 0

2000

4000

6000

8000

10000

12000

Z' (Ωcm2) Fig. 12. Nyquist plots of different NAG stainless steels measured under OCP in 11.5 N HNO3.

Small signal of Si (96 eV) was observed for reference samples (Fig. 9) but in passivated samples the Si peak was insignificant. 3.5. Passive film investigation by EIS The typical Nyquist plots measured in 6 N and 11.5 N HNO3 are shown in Figs. 11 and 12, respectively. As can be observed, all the samples revealed only one time constant of an unfinished semi-circle arc. In 6 N HNO3 (Fig. 11), distinct difference in the impedance spectra could be observed between the different alloys. The three NAG 304L SS and Uranus-65 alloy showed similar and higher semicircle arc radius compared to Uranus-16 alloy. The fitted impedance parameters shown in Table 4 revealed that Uranus-65 and 304L3 have lower CPE-T value and higher RP value while Uranus16 alloy has higher CPE-T value with lower RP value. The Nyquist plots in 11.5 N HNO3 are shown in Fig. 12 and the fitted impedance values are shown in Table 5. The impedance spectra of the different NAG SS showed a similar trend to the EIS measurements in 6 N HNO3. However, the semicircle arc radius (RP) is lower in higher concentration of 11.5 N HNO3 (Fig. 12).

Table 4 EIS fitted value of different NAG SS measured under OCP condition in 6 N HNO3. Alloys 304L1 304L2 304L3 Uranus-65 Uranus-16

RS (Ohm cm2) 0.143 0.117 0.265 0.175 0.534

CPE-T (F/cm2sn) 4

3.12  10 2.78  104 1.81  104 2.03  104 5.32  104

CPE-n

Rp (Ohm cm2)

0.785 0.759 0.943 0.865 0.916

29323 31643 33103 32854 1806

Table 5 EIS fitted value of different NAG SS measured under OCP condition in 11.5 N HNO3. Alloys 304L1 304L2 304L3 Uranus-65 Uranus-16

RS (Ohm cm2) 0.212 0.115 0.680 0.125 0.382

CPE-T(F/cm2sn) 4

3.28  10 4.36  104 2.77  104 2.91  104 6.07  104

CPE-n

Rp (Ohm cm2)

0.701 0.691 0.781 0.739 0.850

10744 8796 11058 11170 976

4. Discussion In Figs. 2 and 3, steady state OCP was observed for all the NAG SS just after immersion in both 6 N and 11.5 N HNO3 revealing the spontaneous passive film formation after immersion. The shift in OCP towards more noble values is related to faster growth of passive film and this depends on the nature of passive oxides layer formed [11,12]. In higher concentration of 11.5 N HNO3 (Fig. 3), more nobler OCP of Uranus-65 alloy revealed more corrosion resistance of the alloy compared to 304L2, 304L1 and 304L3 NAG alloys. Uranus-16 alloy showed the lowest OCP value in both the acid concentrations due to lower film stability compared to other NAG alloys. As shown in the results of anodic polarization in 6 N HNO3 (Fig. 4) and Table 2, Uranus-65 SS and the three NAG 304L SS possess higher corrosion resistance than Uranus-16 SS. The better performance of Uranus-65 alloy can be attributed to the alloy composition of higher Cr (24.97%) and Ni (19.60%) compared to Uranus-16 that has low Cr and Ni. The Si content of all NAG alloys are in the range of 0.09 wt% to 0.24 wt% Si and Uranus-16 alloy has the lowest Si content (0.09%). Despite insignificant Si peak in passivated samples (Fig. 10) and small Si signal observed in AES spectra of reference samples (Fig. 9), the important role of Si in nitric acid corrosion cannot be ignored. Silicon is one of the important element that significantly affects corrosion resistance of austenitic stainless steels [13,16–18]. Si additions are known to produce complex effects on both the corrosion and mode of attack. [13]. However, the exact role of Si in improving the corrosion resistance is not clearly understood and further studies will be required to clarify its role. In the polarization behaviors of different NAG SS shown in 11.5 N HNO3 (Fig. 5), higher IPass was observed prominently for Uranus-16 alloy (Table 2). Similarly, increased IPass observed in 11.5 N for all the alloys (except in Uranus-65) can be attributed to the highly aggressive and oxidizing nature of solution that leads to less protective nature of the passive film. Moreover, the decrease in corrosion resistance with increased nitric acid concentration could also be corroborated to the morphology and microstructure changes observed with the alloy. Particularly, the non-uniformity of the attack observed after the polarization test in the NAG SS (Fig. 6–8) can affect the passive film property and corrosion resistance. However, in both 6 N and 11.5 N HNO3, the correlation of the

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effects in the passive and transpassive regions in relation to the corrosion potential could not be observed. This can be due to complexity of the process of metal dissolution involved in HNO3 as different metals are known to exhibit different rates of dissolution in nitric acid under comparable experimental conditions [19,20]. Similarly, by comparing the AES elemental profile of as-received samples (Fig. 9) with passivated samples (Fig. 10), increase in Cr and O peak heights (Table 3) in passivated samples (Fig. 10), is due to the formation of predominantly Cr2O3 in the passive layer. The enrichment of chromium and oxygen in passivated samples with selective dissolution of iron in the passive film is attributed to lower mobility of chromium cations as compared to iron cations in the passive film [21]. The impedance spectra shown in Figs. 11 and 12 and the fitted circuit values given in Tables 4 and 5 are used to compare and evaluate the passive film properties of different NAG SS. The increase in semicircle is associated with increase in Rp and the RP values are strongly dependent on the passive film characteristic and are a measure of corrosion resistance of the materials [15,22,23]. In Table 4, higher RP value is observed for 304L3 and Uranus-65, followed by alloy 304L1, 304L2 and Uranus-16. The three indigenous NAG 304L SS and Uranus-65 alloy have almost similar range of semicircle arcs. Higher RP implies more protective and homogeneous passive film. Homogeneous passive films will have defect free uniform structure or similar composition throughout. Lower RP values observed in Uranus-16 may be related to increased ionic conductivity through the passive film or thinning of passive film resulting in non-protective passive film [22,23]. The relatively higher CPE-T value of Uranus-16 alloy could be attributed to the inhomogenous nature of the passive film. Similarly, lower value of CPE-T implies the formation of a thicker and more protective film on the metal surface. The RP values of all the NAG SS was much lower in 11.5 N HNO3 which can be due to generation of more reduction products of ‘‘electro-active” nitrous acid species which accumulate during the corrosion process [24,25]. Moreover, because of the autocatalytic metal dissolution mechanism, the dissolution will proceeds at higher rates in higher concentration [16,24]. In summary, these results are evidence of involvement of different reaction mechanisms of metal dissolution in HNO3. 4.1. Nitric acid corrosion mechanism of NAG SS Evans [26] described a mechanism that explains the autocatalytic corrosion of metals in nitric acid medium. The high rate of corrosion in HNO3 is attributed due to the main HNO2 ‘‘electroactive” species that is involved in the autocatalytic reactions [16,18,24,27–29]. Furthermore, the corrosion mechanism of stainless steel in nitric acid should consider: (i) concentration and (ii) boiling condition that promote transpassive corrosion by increasing the corrosion potential of SS [16,24,27,28]. The generation of the ‘‘electro-active” species of nitrous acid by autocatalytic reduction of HNO3 arises due to the production of the oxidant like NO2 [16,27,28]. But depending on the nitric acid concentration, temperature and the reducing agent involved, the end products can be different. At higher nitric acid concentrations (>8 N HNO3) or at high temperature, the main chemical reaction (heterogeneous) is the autocatalytic reduction of nitric acid to nitrous acid and nitrogen dioxide which accumulate in the liquid medium [27–30].

4HNO3 ! 2H2 O þ 4NO2 þ O2

ð1Þ

NO2 þ e ! NO2

ð2Þ

NO2

ð3Þ

þ

þ H ! HNO2

HNO2 þ HNO3 ! 2NO2 þ H2 O

ð4Þ

For lower concentrations or at low temperature, the reduction of nitrous acid is the main electrochemical reaction and the final product is NO and this set of reactions generate depolarizing cations [16,27–30]:

2HNO3 ! 2HNO2 þ O2 þ

þ

ð5Þ

HNO2 þ H ! NO þ H2 O

ð6Þ

NOþ þ e ! NO

ð7Þ

NO þ HNO3 ! HNO2 þ NO2

ð8Þ

For either mechanism, reduction of nitric acid autocatalytically generates oxidants (oxidizing agent or oxidizer) like NO, NO2 and HNO2 [27,29]. Similarly, the dissolution of metal will be autocatalytic in nitric acid because cations oxidized by solution would be available for reduction, thus increasing the metal dissolution. Based on the above points, the cathodic electrochemical reduction reaction (lower concentration) will be predominant in 6 N HNO3 and the reduction mechanism followed by regeneration of the ‘‘electro active” species, leading to autocatalytic metal dissolution will take place at higher nitric acid concentration (11.5 N HNO3) [27]. The details of nitric acid corrosion and its mechanisms involved in austenitic stainless steels have also been reported by others [27–30]. 5. Conclusions The electrochemical investigation and surface passive films analysis of different NAG SS alloys were carried out to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO3 media. The following conclusions can be drawn from the results of the present investigation: 1. The OCP of different NAG SS in both nitric acid concentrations (6 N and 11.5 N HNO3) revealed that NAG 304L SS and Uranus-65 alloy show nobler OCP. Uranus-16 SS showed the lowest OCP value in both nitric acid concentrations. 2. Similar transpassive potential range was observed for all the NAG alloys in the potentiodynamic anodic polarization results obtained in 6 N HNO3. However, Uranus-65 and NAG 304L3 SS showed lower passive current density followed by 304L1 and 304L2 NAG alloys. Uranus-16 alloy showed higher passive current density. 3. The potentiodynamic anodic polarization results in higher nitric acid concentration (11.5 N HNO3) are similar to measurement in 6 N HNO3. Except for Uranus-16 alloy, higher passive current density and lower transpassive potential was observed. 4. The optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO3 indicating corrosion attack along the grain boundaries depending on the alloy composition. 5. The AES analysis revealed that the composition of the passive films formed on all NAG SS alloys in both 6 N and 11.5 N HNO3 media are similar, and enrichment of Cr2O3 was observed. Small signal of Si was observed for reference samples but the peak was insignificant in passivated samples. 6. The frequency response of the AC impedance of all the NAG SS alloys in both 6 N and 11.5 N HNO3 showed a single semicircle arc. Higher polarization resistance and lower capacitance was observed in 6 N HNO3 for all the alloys compared to measurement in 11.5 N HNO3. Uranus-16 alloy showed the lowest polarization resistance value in both the nitric acid concentrations. 7. The present results have shown that the corrosion resistance of the three indigenously developed NAG 304L SS alloys is comparable to commercially available high Cr and Ni Uranus-65 alloy. Uranus-16 alloy possesses the lowest corrosion resistance in both 6 N and 11.5 N HNO3 media.

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