The Corrosion Behavior of Pure Iron under Solid Na2SO4 Deposit in Wet Oxygen Flow at 500 °C

Abstract

The corrosion behavior of pure Fe under a Na2SO4 deposit in an atmosphere of O2 + H2O was investigated at 500 °C by thermo gravimetric, and electrochemical measurements, viz. potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and surface characterization methods viz. X-ray diffraction (XRD), and scanning electron microscope (SEM)/energy dispersive spectroscopy(EDS). The results showed that a synergistic effect occurred between Na2SO4 and O2 + H2O, which significantly accelerated the corrosion rate of the pure Fe. Briefly, NaFeO2 was formed in addition to the customary Fe oxides; at the same time, H2SO4 gas was produced by introduction of water vapor. Subsequently, an electrochemical corrosion reaction occurred due to the existence of Na2SO4, NaFeO2, and H2O. When this coupled to the chemical corrosion reaction, the progress of the chemical corrosion reaction was promoted and eventually resulted in the acceleration of the corrosion of the pure Fe.

Corrosion Behavior of Pure Ti under a Solid NaCl Deposit in a Wet Oxygen Flow at 600 °C

“…The electrochemical corrosion was weak but accelerated the corrosion rate greatly by interacting synergistically with the chemical corrosion [6,7,9]. For pure Ti, the measured I g of the Ti-Pt electrodes under NWO600, which is 1.7ˆ10´4 A/cm 2 as shown in Figure 8c, is significantly larger than that of the Pt-Pt electrodes, which is near to zero as shown in Figure 8a.…”

“…The three electrodes were in quartz tubes, which acted as insulators. All the gaps were sealed by a high temperature inorganic glue [6,7,9]. To decrease the interference of the furnace electric current, the connection wire was covered with a metal shielding layer.…”

“…When pure Cr and pure Fe were exposed in a marine condition at 600˝C, the electrochemical reactions, observed by Liu et al [5] and Tang et al [6,7,9], were involved in the whole corrosion process; thus, the entire corrosion included electrochemical corrosion and chemical corrosion. The electrochemical corrosion was weak but accelerated the corrosion rate greatly by interacting synergistically with the chemical corrosion [6,7,9].…”

“…This reaction destroys the protective oxide scale formed on the metal surface, and the gathered Cl 2 or HCl reacts with substrate metal cyclically, resulting in a fast corrosion. Additionally, electrochemical reactions have been observed during the corrosion process and were involved in the whole corrosion process [5][6][7]9]. The interaction of chemical reactions with electrochemical reactions accelerates the corrosion rate significantly.…”

“…To better understand the corrosion mechanism of these alloys, further studies have been conducted to study the corrosion behavior of pure Fe [5][6][7] and pure Cr [5,8,9], which are the major alloy elements of the blade materials. The results demonstrate that the corrosion rate of pure Fe and pure Cr is sensitive to the presence of a solid NaCl deposit layer because the solid NaCl deposit could react with their oxide at 500-700˝C.…”