Corrosion Of 304 Stainless Steel Exposed To Nitric Acid - Chloride Environments

In an effort to examine the combined effect of HNO₃, NaC1, and temperature on the general corrosion behavior of 304 stainless steel (SS), electrochemical studies were performed. It was found that the corrosion response of 304 SS was bifurcated: materials were either continuously passive following immersion or spontaneously passivated following a period of active dissolution. Active dissolution was demonstrated to be autocatalytic, with the corrosion rate increasing exponentially with time and potential. The period of active corrosion terminated following spontaneous passivation, resulting in a corrosion rate decrease of up to five orders of magnitude. The length of the active corrosion period was found to be strongly dependent on the solution volume to surface area ratio. This finding, coupled with other results, suggested that spontaneous passivation arises solely from solution chemistry as opposed to changes in surface oxide composition. Increasing NaCl concentrations promoted pitting, active dissolution upon initial immersion, a smaller potentird range for passivity, longer active corrosion periods, larger active anodic charge densities preceding spontaneous passivation, and larger corrosion current and peak current densities. In contrast, intermediate HNO₃ concentrations were found to promote active dissolution, with continuous passivity noted at HNO₃ concentration extremes. During active corrosion, increasing HNO₃ concentration increased anodic charge density, corrosion current density, and peak current density. The time required for spontaneous passivation was found to be maximized at intermediate HNO₃ concentrations, Susceptibility to pitting was found to be highest at intermediate HNO₃ concentrations. The pit initiation and repassivation potentials decreased with increasing HNO₃ until the HNO₃ concentration exceeded a critical concentration where pitting susceptibility was entirely eliminated. No differences between 304 SS, 304 L SS, and sensitized 304 SS were apparent in solutions yielding active dissolution, passive, dissolution or pitting. Increasing solution temperatures increased the susceptibility to both pitting and active dissolution.

The processing of actinide salts in nitric acid (HNO₃) process streams plays an important role in high level radioactive waste reduction and long term storage of radioactive material. Container materials for HNO₃ process streams, such as pipes and holding vessels, are typically comprised of AISI 304 stainless steel (304 SS). However, the corrosion resistance of 304 SS in HNO₃ / halide environments has been questioned.

In general, the corrosion behavior of 304 SS has been very well documented^1,2,3. The recent literature alone is replete with studies of general and localized corrosion of 304 SS exposed to HNO₃ solutions ^4,5,6,7,8,9 and to sodium chloride (NaC1) solutions ^{10,11,12,13,14,15} individually. However, little work incorporating the corrosion of 304 SS exposed to solutions containing both HNO₃ and NaC1 has been published.

No single reported work to date has comprehensively examined the general corrosion behavior of 304 SS in the 0.01 M to 10 M HNO₃ range. A variety of researchers have performed weight loss measurements in only one or a few different HNO₃ / HF^16,17 or HNO₃ / C1 ^18,19,20,21 environments. However, publication of potentiodynamic polarization curves in HNO₃ / NaC1 environments has been scant.