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.
Highlights
Corrosion in steels normally originates from local corrosion, and propagates to the whole surface
It has been reported that, with the doping of rare earth (RE) elements, the main types of inclusions change into RE sulfides, RE oxides, and RE oxygen sulfides [14,24,34,35,36,37,38]
The role of rare earth inclusions in the initial marine corrosion process of microalloyed steels could be summarized as follows: (1) The corrosion of RE microalloyed steels originated from inclusions, which contained metal sulfides, metal oxides, and metal oxygen sulfides
Summary
Corrosion in steels normally originates from local corrosion, and propagates to the whole surface. The doping of RE elements can improve the comprehensive mechanical properties of microalloyed steels due to their effects on the purification of steel liquid, improvement of phase transformation, and modification of second phases, which has been investigated extensively [16,17,18,19,20,21,22,23,24,25,26]. The addition of RE elements has an effect on corrosion properties of microalloyed steels by promoting the formation of passive corrosion product films [22,27,28,29], and modifying inclusions [25,30].
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