Corrosion Mechanism of Nickel-Containing Stainless Steels in Concentrated Aqueous Solutions of Sulfuric Acid

The present investigation was undertaken to interpret the spontaneous oscillating activation-passivation corrosion behavior of nickel-containing stainless steel (SS) in concentrated sulfuric acid (H2SO4) solutions using a galvanic passivation model. This was attempted by modeling the oscillating sequence of UNS S30403 in 93.5 wt% H2SO4 at 60°C using a nickel sulfide/nickel (NiS/Ni)-UNS S43000 (a nickel-free SS with similar chromium content to UNS S30403) galvanic couple. Electrochemical measurements showed that galvanic passivation of UNS S43000 is possible when connected to NiS. The behavior is due to the ability of NiS to depolarize the reduction of undissociated H2SO4 molecules to a level that stabilizes the passive film. The model system was found to reproduce the major process involved in the oscillating sequence, namely passivation induction, passivation, and activation. Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) analyses on the surface of UNS S30403 provided evidence to support the model, namely the presence of a chromium-rich oxide-hydroxide film and NiS after spontaneous passivation. The major processes involved in the oscillating sequence are considered to relate to the manner in which NiS grows and subsequently dissolves during corrosion. The findings have meaningful implications regarding the development of more corrosion-resistant SS.