Influence of Ferrite Phase Content on the Electrochemical Properties of Duplex Stainless Steels

Abstract

Duplex stainless steels have a large number of industrial applications and may replace high cost materials, especially in chloride-containing environments like seawater in off-shore platforms due to their high mechanical properties and good corrosion resistance. The influence of the ferrite content on the performance of duplex stainless steels in these corrosive environments is not well known. For the present paper, new superduplex stainless steels with ferrite between 30 and 60% were developed and their microstructure and corrosion resistance were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in NaCl 3.5% (wt %) at 26°C and 78°C. The results obtained at 26°C showed that the pitting potential (E pitt ) is little affected by the ferrite content, but for the materials with higher ferrite it was possible to observe an increase in the repassivation potential with a decrease in the corrosion potential and passive currents due to the presence of more resistive passive films. Tests performed at 78°C indicated a high decrease in the E pitt for all the samples, independently of the ferrite percentage, although maintaining superiority in higher ferrite content. Alloys with a 55% ferrite phase content, i.e. less dependent of Ni element, present a superior performance of corrosion resistance.

Microstructure by Thermal Attack under Vacuum of a Superduplex Stainless Steels and Electrochemical Behavior in H2S/CO2-Saturated Synthetic Seawater

“…4,10 The chemical composition (wt.%), determined by optical emission spectrometry, the pitting resistance equivalent number, the percentage of magnetic phase (ferrite) using the ferritoscope magnetic and optical microscopy, for the SSS, are presented in Table 1.…”

“…[1][2][3][4] These stainless steels operating in a chloride environment, at high temperatures, exposed to H 2 S, CO 2 and other dissolved corrosive agents, have combined good localized mechanical properties and higher crevice corrosion and pitting attack resistance. 5,6 Duplex stainless steels are composed in two phases in comparable volume fractions of ferrite (δ) and austenite (γ), based on the Fe-Cr-Ni system, containing other chemical elements classified as either ferrite (Cr, Mo, Si) or austenite (Ni, N, C, Mn, Cu) stabilizers.…”

“…5,6 Duplex stainless steels are composed in two phases in comparable volume fractions of ferrite (δ) and austenite (γ), based on the Fe-Cr-Ni system, containing other chemical elements classified as either ferrite (Cr, Mo, Si) or austenite (Ni, N, C, Mn, Cu) stabilizers. 4 When the pitting resistance equivalent number (PREn = wt.% Cr + 3.3 wt.% Mo + 16 wt.% N) exceeds 40, they are called superduplex stainless steels (SSS). 7,8 The literature 9 reports the effects of the chemical composition of passive films formed in both phases of two types of duplex stainless steel (UNS S31803 and UNS S32304) by using Auger electron spectroscopy (AES) and quantitative relationships between electrochemical parameters in sodium chloride media.…”

“…Many publications 4,8,[10][11][12][13][14][15][16][17][18][19] have described the use of electrochemical techniques for studying the electrochemical corrosion phenomenon, such as polarization and electrochemical impedance spectroscopy (EIS), which can investigate passive film resistance, the corrosion rate, and the behavior of coatings and inhibitors. Ge et al 16 have discussed, through polarization curve analysis and EIS data, the influence of sulfide on 316 stainless steel passive films.…”

“…The analysis of the microstructure showed that the pitting number and size varied with the increase of the temperature of the tested medium and the Nyquist diagrams by EIS showed that the arc polarization resistance decreases as a function of the temperature, due to a catalytic degradation of the passive films. 10 In another work, Souza et al 4 investigated the influence of ferrite phase content on the electrochemical properties of new SSS, with ferrite between 30 and 60%, and their microstructure and corrosion resistance were evaluated by potentiodynamic polarization and EIS tests in NaCl 3.5% (wt.%) at room temperature and 78 °C. The results obtained at 25 °C showed that the pitting potential (E pit ) is not significantly affected by the ferrite content, but for the materials with higher ferrite it was possible to observe an increase in the repassivation potential, with a decrease in the corrosion potential and passive currents due to the presence of more resistive passive films.…”