Corrosion assessment of nitric acid grade austenitic stainless steels

doi:10.1016/j.corsci.2008.09.038

Corrosion Science

Volume 51, Issue 2, February 2009, Pages 322-329

https://doi.org/10.1016/j.corsci.2008.09.038 Get rights and content

Abstract

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 (R_P) 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 R_P 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.

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], [2], [3], [4], [5], [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], [6], [7], [8], [9], [10], [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], [9], [10]. Close control of composition, 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], [13], [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], [12], [13], [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.

Section snippets

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

Open circuit potential (OCP) measurement

The measured OCP of different NAG SS obtained with 6 N and 11.5 N HNO₃ are shown in Fig. 2, Fig. 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).

Discussion

In Fig. 2, Fig. 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

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

Acknowledgement

The authors acknowledge M/s Mishra Dhatu Nigam Ltd. (MIDHANI), Hyderabad, India for supplying the NAG SS used in the present investigation.

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Corrosion assessment of nitric acid grade austenitic stainless steels

Abstract

Introduction

Section snippets

Materials and sample preparation

Open circuit potential (OCP) measurement

Discussion

Conclusions

Acknowledgement

Prog. Nucl. Energ.

Corros. Sci.

J. Nucl. Mater.

Mater. Chem.

Electrochim. Acta

Corros. Sci.

Corros. Sci.

Corros. Sci.

J. Nucl. Mater.

J. Nucl. Mater.

Mater. Eng. Perf.