Assessment of Galvanic Corrosion in Galvanic Couples of Sensitized and Nonsensitized AISI Type 304 Stainless Steel in Nitric Acid

Waste vault tanks made of 304 stainless steel (SS) meant for the storage of high level liquid waste of 6M HNO3 showed precipitation of M23C6 in heat affect zone (HAZ) in the weldments and high corrosion rates in boiling nitric acid test (ASTM A262 Practice C test). In this connection a study of possible galvanic corrosion effect at the junction of sensitized HAZ and the adjacent base metal in 304 SS in sulphuric as well as nitric acid, was undertaken. The galvanic couples were prepared using sensitized 304SS as the anodes, and the base metal as the cathodes, electrically connected in different anode to cathode ratios. Potentiodynamic anodic polarization studies were conducted in deaerated 0.5M sulphuric acid whereas electrochemical noise (ECN) experiments were performed in 3M deaerated nitric acid at room temperature as well as 60°C. The data analysis of current and potential signals was conducted using statistical methods; the data was analysed in the frequency domain using Maximum Entropy Method (MEM). The corrosion rates observed in nitric acid medium at room temperature did not show enhanced corrosion rate due to galvanic coupling. However, the corrosion rates were higher at 60°C in the same medium. Based on these results, a drastic rise in corrosion rate and the subsequent failure of the waste vault tank was not expected.