Mechanism of pitting corrosion induced by inclusions in Al-Ti-Mg deoxidized high strength pipeline steel

Highlights

• In deoxidized Al steel, the Ti-Mg composite deoxidization treatment can obtain finer and more dispersed non-metallic inclusions, promoting ferrite nucleation and significantly improving the performance of the welding heat-affected zone. At the same time, Mg treatment can modify alumina and sulfide inclusions and reduce their hazards. It is the first time to study the galvanic corrosion of Al-Ti-Mg killed steel induced by inclusions.

• Researchers concluded that the composite inclusions have lower surface potentials than the matrix by traditional experiment measurements, which induces pitting corrosion. This opinion cannot explain why in the same composite oxide inclusions, one part did not dissolve, while the other part dissolved prior to the matrix in the pitting corrosion process. It is difficult to determine by traditional experimental method which inclusions in composite inclusions play a greater role in the process of pitting corrosion. In this work, we determined which inclusions in composite inclusions play a greater role in the process of galvanic corrosion, and explained why in the same composite oxide inclusions, one part did not dissolve, while the other part dissolved prior to the matrix in the pitting corrosion process.

Abstract

In order to reveal the mechanism of galvanic pitting corrosion initiation induced by typical complex inclusions in Al-Ti-Mg deoxidized high strength pipeline steel, first-principles calculations, combining with immersion tests, scanning electron microscopy was used to study the correlation between electronic work function and galvanic corrosion of Al-Ti-Mg killed steel. The results show that MnS inclusions act as anodes in the electrochemical environment, preferentially corroding and dissolving; MgAl₂O₄ and Al₂O₃ inclusions act as cathodes, leading to the corrosion of Fe matrix; different end planes of MgTiO₃ and MgTi₂O₄ act as both anodes and cathodes, but they have little effect on pitting corrosion. Microcrevices on the MgTiO₃ and MgTi₂O₄ inclusions boundary are the main cause of matrix dissolution.

Introduction

Marine pipeline transportation is the most safest, economical and effective way to transport marine oil and gas resources. In corrosive environments, pitting corrosion is the main cause of failure of steels because of the heterogeneous structure between inclusions and the steel matrix (Cao et al., 2019; Baik et al., 2018; Idczak et al., 2016). In addition to purify the molten steel (Sun et al., 2020), scholars also tried to refine inclusions. In Al deoxidized pipeline steel, the Ti-Mg complex deoxidization treatment can obtain finer and more dispersed non-metallic inclusions, promoting ferrite nucleation and significantly improving the performance of the welding heat-affected zone. At the same time, alumina and sulfide inclusions are modified by Mg treatment. Mg treatment has become a research hotspot in inclusion control (Hou et al., 2016; Ono and Nakajima, 2015; Ono et al., 2009; Ono and Ibuta, 2011), but there are few studies on the pitting corrosion induced by inclusions in Al-Ti-Mg killed steels.

When inclusions exist on a metal surface, they may be interacting with the matrix to form a micro-battery circuits under the corrosive environment medium (Cl⁻, SO₄^2-, H⁺), which can cause pitting corrosion at inclusion sites. In situ measurement techniques, such as Kelvin probe force microscope (SKPFM), were used in Ref. (Rahimi and Rafsanjani-Abbasi (2018); Esfahani and Rahimi, 2018; Jia and Yu, 2019; Zheng and Li, 2013) to study pitting corrosion, and it was revealed that the local corrosion of alloys was closely related to the intrinsic potential difference between the inclusions and the matrix. Zheng et al. (Zheng and Li, 2013) studied the effect of Mg-Al-Ca complex oxide inclusions on pitting corrosion in 316L stainless steel. According to the results of SKPFM measurements, the complex inclusions have lower surface potentials than the matrix, which induces pitting corrosion. But this experimental results cannot explain why in the same Mg-Al-Ca complex oxide inclusions, part I (Mg, Al-rich and Ca-poor) did not dissolve, while part II (Ca-rich and Mg, Al-poor) dissolved prior to the matrix during the pitting corrosion process. The reason is that a lot of inclusions in commercial steel are complex inclusions, and each inclusion of the same component has different effect on the pitting corrosion, which limited the experimental work to identify which non-metallic inclusions are effective in the formation of the galvanic corrosion. Furthermore, considering that galvanic corrosion occurs on the surface of alloys, and it is mainly due to the potential differences between different inclusions crystal surface and the matrix, the intrinsic potential difference between the inclusions and the matrix obtained by using SKPFM can not accurately reflect the mechanism of galvanic corrosion.

Therefore, in this work, by combining with the immersion and scanning electron microscopy tests, density functional theory was used to reveal the mechanism of galvanic pitting corrosion induced by typical complex inclusions in Al-Ti-Mg deoxidized X80 pipeline steel (taking single phase as examples, including MnS, MgAl₂O₄, Al₂O₃, MgTi₂O₄, and MgTiO₃) from the point of view of electronic structure.