The dissolution of deposited magnetite layers on AISI(1) 304 stainless steel (SS) surfaces has been studied in acidic (pH = 2.6 to 3.0) ethylenediaminetetraacetic acid (EDTA) and EDTA/oxalate solutions at 80 to 90 C. Oxide layers were grown on SS coupons in 1.0 mol·L−1 NaOH at 275 C in SS-lined titanium autoclaves for 14 days. These layers consisted of a base layer of composition Ni01CrFe0.9O4 and a deposited upper layer of crystals of NixFe3−xO4 (with Ni <10% by weight). Dissolution experiments were performed in thermostatted glass cells under potential-controlled and freely dissolving conditions. The concentrations of dissolved iron, chromium, and nickel were analyzed by inductively coupled plasma spectrometry. Cyclic voltammetric and anodic polarization experiments were performed on clean SS disks to determine the corrosion characteristics in EDTA/oxalate solutions.
These experiments showed that dissolution of deposited magnetite films occurred via a reductive dissolution process providing the electrode potential was <0 mV (vs SCE at 25 C). Under freely dissolving conditions, dissolution appeared to involve electron transfer from dissolved FeII species to FeIII sites in the oxide via an EDTA or oxalate anion bridge. The oxalate anion is a more effective bridge and accelerates the overall oxide dissolution process. A decrease in pH from 3 to 2.6 accelerates oxide dissolution.
An active-to-passive transition was observed in solutions with a concentration ratio of oxalate to EDTA ≳ 4. A pitting potential of ~+ 600 mV was obtained in acidic EDTA solutions. This value is over 700 mV more positive than corrosion potentials observed during oxide dissolution.
