Effects of solution pH and Cl− on electrochemical behaviour of an Aermet100 ultra-high strength steel in acidic environments

In this work, the effects of solution pH and Cl− on the electrochemical behaviour of an Aermet100 ultra-high strength steel in 0.5 M (Na2SO4 + H2SO4) solution were studied by polarization curve and electrochemical impedance spectroscopy (EIS) measurements, combined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) characterization. The results show that, when solution pH is below 4, the steel is in the active dissolution state, and corrosion current decreases with the increase of pH. There exists a critical pH value, above which the steel is passivated. Moreover, the oxides and hydroxides of Fe, Co, Ni and Cr are the primary components of the passive film. With addition of Cl−, pits are initiated on the steel electrode.

► At pH greater than 4.2 passive films will be formed on the surface of Aermet100.
► Several kinds of metal-oxide/hydroxide species were found in the passive film.
► In acidic solutions below pH 4.2 without chloride ions, Aermet100 was uniformly corroded.
► Pits became apparent when chloride ions are added to the solution.