Dr Shagufta Khan AMCO Integrity Pty Ltd Australia, Muhammad Hussain, University of Wollongong, Australia.
SUMMARY: Austenitic stainless steels (SSs) have been used as structural materials for various components in the aqueous reprocessing plants. These alloys are widely used in nuclear spent fuel reprocessing and waste management plants and the process fluid is nitric acid at temperature up to boiling point. Incorporation of oxidizing ions in nitric acid stream make the environment highly corrosive to stainless steels. Several types of nitric acid grade (NAG) alloys having compositions similar to Types 304L, 310L, and several new proprietary alloys have been developed worldwide. NAG alloys show lower corrosion rates compared to conventional Type 304L in boiling HNO3 (Huey). However, several reports of failure have been reported in components made of NAG grade austenitic SS also. In this study it has been shown that the potential attained in process solution determines the corrosion behaviour of SS. Potentials were applied to Types 304 L NAG, and 310L in boiling 6 M nitric acid for a period of 48 h and corrosion behaviour was observed. Influence of Cr6+ concentration present in nitric acid solution on end grain corrosion rate was also studied. Some important observations of this investigation were: (1) Rate of IGC increased exponentially with the increase in the applied potential. (2) No significant difference in corrosion rate of Type 304L NAG1 having step and dual microstructures was observed (3) Increase in Cr6+ concentration present in nitric acid solution increase end grain corrosion rate of Type 304L NAG2.
Keywords: Steel, intergranular corrosion, transpassive potential, heat treatment, Microstructure, Nitric acid.
1. Introduction
Austenitic stainless steels (SSs) with low carbon content such as AISI types 304L, 310L, 347, 321, several other proprietary alloy steels having modified chemical composition and microstructures have been used as structural materials for various components in the aqueous reprocessing plants [1-4] Several types of nitric acid grade (NAG) alloys having compositions similar to Types 304L, 310L, and several new proprietary alloys have been developed worldwide [5]. NAG alloys show lower corrosion rates compared to conventional Type 304L in boiling HNO3 (Huey) test [6-9].This is achieved by (i) controlled chemical composition, (ii) modified microstructures leading to elimination of weaker sites for passive film break down and dissolution, and (iii) enhanced strength against transpassive dissolution. Increasing chromium content increases the stability and strength of passive film. However, in practice chromium levels below 30 % are used. The reason for this is that increase in chromium contents beyond this level affects the stability of the austenite phase. Nickel is another element which improves the strength of passive films through stabilizing austenite. Nickel is kept within 15% as increases in nickel content increases the cost of the alloy. Molybdenum is completely removed from the alloy due to two important reasons namely, (i) increasing ferrite formation, and (ii) submicroscopic sigma precipitation during multipass welding of components thicker than 6 mm. Sigma phase can preferentially dissolves in hot oxidizing HNO3 leading to excessive corrosion rates. Manganese, silicon, copper and aluminium are normally present as impurities in the steel making. Mn content less than 1 % is recommended to avoid inclusions. These inclusions are harmful as they are preferentially attacked in HNO3 service. Copper and aluminium are also reduced to low levels as they enhance selective corrosion attack in HNO3 service. Silicon has dual role with respect to corrosion of Type 304L in HNO3. Good corrosion resistance is observed when Si content is less than 0.2%, and also when Si content beyond 1.6 % [5]. However, between 0.4 and 1% Si content, excessive IGC has been observed [5]. Hence, two grades of NAG alloys have evolved, one with Si less than 0.2%, and the other with 4% Si. The impurity elements like S, P and B significantly affect the corrosion resistance of Type 304L in HNO3 [5]. Sulphur forms MnS inclusions which degrade the alloy with selective dissolution. It is major cause of end grain corrosion or tunnel corrosion in Type 304L tubular and bar product [10]. Phosphorous is controlled within 200 ppm in NAG grade alloys and both S and P together within 250 ppm to ensure good corrosion resistance. Boron forms dichromium boride (∆Cr2B) along grain boundaries during quenching after solution annealing. This results in decrease in corrosion resistance of the conventional Type 304L and hence for NAG alloys, boron is not exceeded 10 ppm. In a typical Type 304, the carbon content range is 0.06–0.08%. During heat treatment in temperature regime 723–1073 K, carbon enhances the formation of chromium rich M23C6 carbide at grain boundaries [11-12].This leads to selective depletion of Cr in a narrow region adjacent to the grain boundary. M23C6 carbides, thus promotes excessive corrosion along the grain boundaries. Now a days ultra low carbon alloys with carbon levels less than 0.015% are being developed for HNO3. In this paper we have discussed the role of composition and microstructure (“step” and “dual”) on corrosion behaviour of NAG SS in near boiling 6 M HNO3. Potentials were applied to NAG SS specimens in boiling 6M HNO3 for a period of 48 h. The corrosion rates measured in such experiments were plotted as a function of applied potential. Threshold potential, above which IGC was observed, was established with the help of these plots. Below this potential, uniform and low rate of corrosion occurred. Corrosion behaviour of Type 304L NAG1 and Type 310L NAG (all heat treated at 675 °C for 1h) were investigated and compared by Practice A, A262, ASTM, DL- EPR, tafel polarization, potentiostatic experiments and morphological study. Type 304L NAG1 specimens were subjected to various heat treatment in order to produce change in microstructure. The effect of microstructure on corrosion behaviour of Type 304L NAG1 was investigated. How presence of oxidising ion in nitric acid influence end grain corrosion was also investigated on tubular Type 304L NAG2.