Mechanism Understanding of the Role of Rare Earth Inclusions in the Initial Marine Corrosion Process of Microalloyed Steels

Abstract

In this study, the corrosion behavior of rare earth (RE) microalloyed steels was first evaluated through potentiodynamic polarization tests and corrosion weight loss experiments, and then the corrosion morphologies were observed by scanning electron microscope (SEM). After immersion in a NaCl solution, the sulfides (or oxygen sulfides) dissolved preferentially, followed by corrosion at the boundary between the Fe matrix and oxides. Afterwards, the inclusions fell off as a whole, which promoted pitting nucleation. The first principle modeling demonstrated that the work functions of various kinds of inclusions descended in the following order: La2Zr2O7 > LaAlO3 > (La2O3 ≈ Fe ≈ La2O2S) > La2S3, which provided a theoretical explanation to the dissolution behaviors of inclusions. That is, inclusions containing sulfur tend to dissolve preferentially, whereas the oxides do not dissolve easily. Subsequently, the surface current distributions were detected by the scanning vibrating electrode technique (SVET), which provided more microscopic insight into the role of inclusions in the corrosion propagation. Results showed that the active sites of pitting nucleation accelerated the transverse propagation of corrosion. Finally, local corrosion spread to the whole surface as uniform corrosion.

Local Corrosion Behaviors in the Coarse-Grained Heat-Affected Zone in a Newly Developed Zr–Ti–Al–RE Deoxidized High-Strength Low-Alloy Steel

“…The scan rate was 0.333 mVs −1 . All electrochemical measurements were performed more than three times for greater reproducibility [ 31 , 45 ]. Electrochemical impedance spectroscopy (EIS) tests were performed at a frequency ranging from 10 5 Hz to 0.01 Hz with amplitude of 10 mV.…”

“…After adding rare earth, the lattice distortion of inclusions decreased dramatically and the dissolution of the (RE) 2 O 2 S-(RE) x S y caused pitting corrosion [ 30 ]. Using the first-principles theory, Tang et al calculated the work function of rare earth oxysulfide and confirmed that rare earth sulfide is easier to dissolve, from an atomic scale perspective, than rare earth oxide [ 31 ]. The mechanism of inclusion-induced local corrosion is still controversial.…”

A critical review of the recent advances in inclusion-triggered localized corrosion in steel

“…Nonetheless, another innovative work studied by Li and Moelans 23 provides the third explanation for the above situation when they paid attention to the effects of rareearth elements modified inclusions (Fig. 13a, c, e) on initialization of localized corrosion of pipeline steels in NaCl solution, whilst reports on modifying the corrosion resistance of steel by adding rare-earth elements have also been introduced elsewhere [94][95][96][97][98][99] . In this investigation, the bandgap values (E g ) of LaAl 2 O 3 obtained by experiment and calculation is around 5.47 eV and 5.60 eV, respectively (Fig.…”

Effects of Ti and Cu Addition on Inclusion Modification and Corrosion Behavior in Simulated Coarse-Grained Heat-Affected Zone of Low-Alloy Steels

“…Additionally, the fractions of coarse inclusions (2-3 μm) in Ti-and Cu-bearing steels were 29% and 24%, respectively, which significantly increased in comparison with the base steel (11%) ( Figure 3a). The ability of inclusions to induce localized corrosion was not only limited by the chemical composition but also influenced by the dimension and density of inclusions [25,27,33,34]. Hence, the number density and size distribution of inclusions in three types of low-alloy steels were statistically analyzed via an SEM/EDS (Figure 3a,b).…”

“…With the addition of alloying elements, such as S and Al, the number density and average size of MnS, MnO, and Al 2 O 3 obviously increased, which would lead to the decrease of corrosion resistance [ 20 , 22 , 23 ]. On the contrary, the addition of RE elements can improve the corrosion resistance of the steel, owing to the much smaller average size of the modified inclusions than that of normal inclusions [ 24 , 25 , 26 , 27 ].…”