The electrochemical behavior of UNS(1) N08031 was investigated as a function of electrode potential in Green-Death(2) solution at 40 °C. The UNS N08031 surface is in a stable passive state during cyclic potentiodynamic polarization without an initiation and/or propagation of localized corrosion. In potentiostatic polarization of UNS N08031 for 3600 s, passive current density increases with an increase in the passivation potential from 0.7 to 1.0 VSSE (silver/silver chloride reference electrode in saturated potassium chloride) Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) analysis showed that a more defective n-type semiconductive passive film forms as the potential increases. X-ray photoelectron spectroscopy (XPS) revealed that passive film consists of mainly chromium and minor iron and nickel oxides. The mechanism of the defective passive film formation is discussed. The increase of the applied potential is considered to be a reason for the change in passive film stability.
The passivity of iron-chromium nickel (FeCrNi) alloy depends on electrochemical and/or chemical stability of the passive film in an aggressive anion-containing environment. Passive film is a chemically quasi-inert oxide film with several nanometer scales formed on the metal surface. The chemical composition of the film consists of iron, chromium cations, oxygen and hydroxide anions. It has been reported that the outermost layer of passive film becomes enriched with Fe cations,1,2 while some studies have shown that the Cr is more concentrated the entire passive film than Fe.4-6 The film composition varies with passivation conditions such as solution temperature, pH, and passivation potential, which is related to the film’s electrochemical behavior.2, 5 6 It is necessary to investigate the composition of passive film formed on FeCrNi alloys.
UNS N08031 is a type of super austenitic stainless steel and has a superior resistance against localized corrosion, and stress-corrosion cracking compared to other austenitic stainless steel such as UNS N08926, which contains 5 wt.% less Cr than UNS N08031. UNS N08031 is particularly suitable for applications in the chemical and petrochemical industries.7 However, the electrochemical behavior of UNS N08031 surface has seldom been reported.8 The semiconductive property of passive film formed on UNS N08031 surface has been investigated in phosphoric acid solution.9-11 However, the chemical composition of the film formed on UNS N08031 has never been reported.