Effect of Nitrate and Sulfate on Atmospheric Corrosion of 304L and 316L Stainless Steels

Abstract

The effects of nitrate and sulfate salts on the chloride-induced atmospheric pitting corrosion of 304L and 316L stainless steel was investigated through automated deposition of droplets of magnesium and calcium salts. Nitrate was found to inhibit pitting under magnesium salt droplets when the ratio between the deposition density of nitrate anions and chloride anions was above a critical value, which was the same for both 304L and 316L. This critical ratio was found to decrease with increasing humidity. Sulfate was also observed to inhibit pitting for MgCl 2 + MgSO 4 mixtures, but only at higher humidities. Sulfate did not show any inhibition for CaCl 2 + CaSO 4 mixtures, an effect attributed to the low solubility of CaSO 4 . At low relative humidities, precipitation of the inhibiting salt was observed, leading in some cases to crevice-like corrosion under salt crystals. The pitting behavior was explained in terms of the thermodynamic behavior of concentrated solutions. In the UK, stainless steel is used to package intermediate level radioactive waste, ILW, which is characterized by relatively large volumes and variable levels of radioactivity.1 Whatever the strategic approach to its management c , most ILW has been packaged in thinwalled containers (2.3-6 mm thick, typically grades 304L or 316L, UNS S304003 and UNS S316003, respectively) and will undergo long periods of exposure to atmospheric conditions, either in surface or underground facilities prior to permanent disposal.During these periods, waste containers will be exposed to regimes of varying temperature and relative humidity (RH), as well as to chloride-containing salts arising from aerosol deposition. As a result, it is important to identify suitable storage conditions to ensure durability of waste containers, in particular to avoid conditions associated with the development of pitting and, even more importantly, atmospherically-induced stress corrosion cracking (AISCC). 2,3Monitoring of ILW storage facilities and other indoor locations considered broadly representative of ILW stores suggests that temperature and relative humidity, which are key parameters in the development of atmospheric corrosion, are expected to vary between ∼0-30• C and ∼30-100% RH, respectively. 3 Ionic chemical species deposited on surfaces after relatively long periods of indoor storage found in swab tests in a variety of real storage facilities include calcium, magnesium, sodium, potassium, chloride, nitrate and sulfate ions. Inside the storage buildings surveyed, chloride deposition densities were found to be below ∼20 μg/cm 2 , with deposition rates of the order of 1 μg/cm 2 per year estimated. 3 With such a deposition rate, the chloride deposition density could increase to ∼100 μg/cm 2 over the next century.In atmospheric conditions relevant to this work, a number of tests have been carried out to evaluate the atmospheric pitting corrosion of stainless steel in the presence of chloride deposits (e.g., Refs. 2, 4-6) but none of these have been carried out in the presence of...

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