Effect of Cerium on Mechanisms of Pitting Corrosion Induced by Inclusions in a 304 Stainless Steel

Abstract

Laboratory experiments were performed to study the pitting process induced by different kinds of inclusions in stainless steels with and without cerium. When the content of total cerium in the stainless steel increased from 0 to 430 ppm, the evolution path of inclusions in the stainless steel was Al₂O₃–SiO₂–MnO(–CaO)–MnS → Ce–Al–Si–Ca–O–S → homogeneous Ce–O–S → heterogeneous Ce–O–S. Immersion test was performed and morphologies of pits induced by different types of inclusions were observed. The dissolution sequence was Ce–O phase in heterogeneous Ce–O–S and MnS → homogeneous Ce–O–S, Ce–S phase in heterogeneous Ce–O–S and steel matrix → Ce–Al–Si–Ca–O–S → Al₂O₃–SiO₂–MnO(–CaO). A corrosion index was defined to compare the corrosion resistance of the steel matrix around inclusions. Besides, first-principles calculations of related inclusions were performed to clarify the mechanism of pitting corrosion induced by different kinds of inclusions in the stainless steel.

1. Introduction

In recent years, rare earth elements have attached more and more attentions of steelmakers due to their strong affinity to oxygen and sulfur.¹⁾ Many works have been conducted to investigate the effect of rare earth elements in steels and it has been proved that rare earth elements in the steel can purify the molten steel,^2,3,4,5,6) modify inclusions,^{7,8,9,10,11,12,13)} and improve the structure and properties of steels.^{14,15,16,17,18,19,20,21,22)} Inclusions in the steel are inevitable during the smelting process^{23,24,25,26,27,28)} and many studies have indicated that inclusions have a negative impact on the pitting corrosion resistance of steels.^{25,26,29,30,31)} Avci et al.³²⁾ and Wei et al.³³⁾ reported that MnS inclusions provided initiation sites for the pitting corrosion in carbon steels. Zheng et al.³⁴⁾ reported that Ca–Al–Mg–O inclusions had a negative impact on the corrosion resistance of stainless steels. Suter et al.³⁵⁾ proposed that pitting originated from inclusions smaller than 1 μm would hardly transform into steady pits. Rare earth elements in the steel can modify sulfide inclusions and oxide inclusions into rare earth-containing inclusions and different types of inclusions such as REAlO₃, RE₂O₂S, RE₂O₃ and RES can be formed in the steel after the addition of rare earth elements.^36,37,38) Zang et al.³⁹⁾ reported that the steel was purified and the fraction of ferrite structure was obviously increased after adding 200 ppm yttrium into martensitic stainless steels, leading to the improvement of the corrosion resistance. Cai et al.⁴⁰⁾ studied the effect of Ce on the corrosion resistance of 204Cu stainless steels and reported that adding 180 ppm Ce into the steel could decrease the number and size of inclusions, resulting in obvious improvement of the corrosion resistance. Liu et al.⁴¹⁾ reported that rare earth elements could improve the corrosion resistance of low-carbon steels and weathering steels. Tang et al.⁴²⁾ studied the role of rare earth-containing inclusions in the initial marine corrosion process and reported that inclusions containing sulfur tended to dissolve preferentially while oxides were not easy to dissolve, and the dissolution sequence in the micro-alloyed steel was La₂S₃ → (La₂O₂S ≈ Fe ≈ La₂O₃) → LaAlO₃ → La₂Zr₂O₇. Hou et al.^43,44) studied the effect of La and Y on the corrosion resistance of pipeline steels in NaCl solution and reported that pitting was initially caused by micro-crevices when the inclusion was modified into LaAlO₃ and Y–O, while La₂O₂S had lower corrosion resistance and Mg–Y–S had higher corrosion resistance than the steel matrix. Liu et al.⁴⁵⁾ studied the effect of inclusions modified by rare earth elements on the localized marine corrosion in Q460H steels and reported that the steel matrix had a higher corrosion resistance than RE₂O₂S–RE_xS_y and REAlO₃–RE₂O₂S–RE_xS_y and pitting was initiated by the dissolution of RE₂O₂S–RE_xS_y. Shi et al.⁴⁶⁾ studied the correlation between inclusions and the pitting corrosion in stainless steels with yttrium addition by observing the corrosion morphology and proposed that MnS, composite oxide inclusions and YN deteriorated the pitting corrosion resistance, while regular Y₂O₃ showed the best pitting corrosion resistance.

It could be concluded that many previous works mainly focused on investigating the pitting corrosion resistance of steels with rare earth elements addition by comparing the pitting potential, corrosion morphologies before and after the corrosion. However, the detailed evolution of the corrosion morphology during the corrosion process was hardly recorded, and further analysis on the corrosion morphology was rarely performed.

In the current study, laboratory experiments were performed to study the effect of cerium on mechanisms of pitting corrosion induced by inclusions in the 304 stainless steel (SS). Pitting process induced by different kinds of inclusions in the SS was investigated by in situ analysis of the corrosion morphology evolution.

2. Laboratory Experiments and Analysis

The raw material used in this study was a commercial 304 SS and the chemical composition is shown in Table 1. Approximately 700 g steel sample was melted in a MgO crucible with a 52 mm diameter and a 120 mm depth using an electrical resistance furnace with MoSi₂ heating bars under a pure argon atmosphere at a flow rate of 2 L/min. The experimental device is illustrated in Fig. 1. The steel sample was heated to 1873 K, after the steel was melted for 5 min, a certain amount of CeFe alloy was added into the molten steel and was kept for 30 min before furnace cooling, and steel specimens with the total cerium content of 0 ppm, 76 ppm, 300 ppm and 430 ppm were obtained, the schematic of experimental process is shown in Fig. 2. The specimen was not heat treated in any other way after cooling in the furnace. The diagram of sample processing is shown in Fig. 3. Steel samples with the size of φ 6 mm × 5 mm were mechanically ground using 2000 grit silicon carbide paper and polished with 0.5 μm diamond paste to eliminate the effect of surface roughness on the pitting corrosion, then specimens were rinsed with deionized water, degreased in alcohol, and dried immediately.

Table 1. Chemical composition of the steel (wt%).

[C]	[Si]	[Mn]	[Cr]	[Ni]	[Al]	T.Ca	T.O	T.S
0.07	0.37	0.88	18.23	8.12	0.0017	0.0009	0.0042	0.0015

Fig. 1.

Schematic of the experimental device. (Online version in color.)

Fig. 2.

Schematic of experimental process.

Fig. 3.

Diagram of sample processing.

A series of immersion tests were conducted to clarify the relationship between different kinds of inclusions in the SS and the pitting corrosion. The test solution was comprised of 350 mL deionized water with 69.9 g FeCl₃·6H₂O and 20 mL hydrochloric acid. Samples were first fully immersed in the solution for different time intervals to observe the corrosion morphology around several typical inclusions. According to different corrosion morphologies, ten immersion periods including 0, 0.5 min, 1 min, 2 min, 3 min, 5 min, 8 min, 13 min, 20 min and 30 min were designed and several inclusions in each sample were chosen to be observed in situ before and after immersion for different times. After every step of immersion, the sample was slightly washed with alcohol and dried for surface observation. Original morphologies and compositions of different kinds of inclusions were detected using an automatic scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS, FEI, USA) inclusion analysis system (Aspex-Explorer) operated at 15 kV. After every step of immersion, the corrosion morphology was observed using SEM to reveal the dissolution of inclusions and the steel matrix around these inclusions, and the composition of the residual inclusion was confirmed using EDS mapping as well. Area measurements of inclusions and the dissolved steel matrix before and after the immersion were performed using ImageJ. The region of the remain inclusion and the pit was distinguished by the gray level of the picture. Schematic of the picture processing is shown in Fig. 4. Meanwhile, the three-dimensional morphology of the observed inclusion was recorded by confocal laser scanning microscopy (CLSM), and the scanning step of the scanning was set as 0.1 μm.

Fig. 4.

Schematic of the picture processing. (Online version in color.)

3. Inclusions in the Steel with Different Cerium Contents

Elemental mappings of typical inclusions in the steel with different cerium contents are shown in Fig. 5. In the steel without cerium addition, inclusions were mainly Al₂O₃–SiO₂–MnO(–CaO) with MnS wrapped outside the oxide, the subscript of (CaO) implies that the inclusion contained a small amount of CaO. In the steel with 76 ppm Ce, inclusions were mainly Ce–Al–Si–Ca–O–S in gray phase. In the steel with 300 ppm Ce, inclusions were mainly homogeneous Ce–O–S in white phase with uniform distribution of elements. While in the steel with 430 ppm Ce, inclusions were mainly heterogeneous Ce–O–S inclusions consisted of Ce–O phase and Ce–S phase.

Fig. 5.

Elemental mappings of typical inclusions in the steel with different cerium contents: (a) Without Ce; (b) With 76 ppm Ce; (c) With 300 ppm Ce; (d) With 430 ppm Ce. (Online version in color.)

Thermodynamic calculations were performed using FactSage 7.1 with databases of FactPS, FToxid, FSstel and MYRE. Due to the unavailable parameter of Ce₂O₂S in the software, the database MYRE was self-developed according to the result reported by Vahed et al.¹⁾ and was added in FactSage 7.1 to consider the formation of Ce₂O₂S inclusions. The transformation of inclusions in the steel with different contents of cerium was analyzed, as shown in Fig. 6. In the steel without cerium, inclusions were composed of Al₂O₃, SiO₂, MnO, CaO and MnS after solidification, which was in consistence with Fig. 5(a). Since the precipitation temperature of MnS was lower than that of oxides, MnS usually wrapped around the oxide. In the steel with 76 ppm Ce, inclusions were modified to Ce₂O₃, Al₂O₃, SiO₂, MnO, CaO and MnS, which agreed well with the elemental mapping of typical inclusions in the steel with 76 ppm Ce, as shown in Fig. 5(b). In the steel with 300 ppm Ce and 430 ppm Ce, when the solidification process was completed, inclusions in the steel were composed of Ce₂O₃, Ce₂O₂S and a small amount of CaO, which was consistent with the result shown in Figs. 5(c) and 5(d).

Fig. 6.

Transformation of inclusions in the steel with (a) 0 Ce, (b) 76 ppm Ce and (c) 300 ppm Ce, (d) 430 ppm Ce during the solidification process. (Online version in color.)

4. Evolution of Pitting Morphology around Inclusions

The evolution of pitting morphologies around Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusions is shown in Fig. 7. Morphologies recorded using scanning electron microscope under backscattered electron detecting mode (BSED) and secondary electron mode (SEM) were placed in the middle column and the right column, respectively. Manganese sulfide wrapped around the oxide began to dissolve after corrosion for 0.5 min, and MnS was completely dissolved 2 min after corrosion, which could be identified through elemental mappings of the inclusion, as shown in Fig. 8. Then, the steel matrix gradually dissolved and the pit propagated to the steel matrix. Thirty minutes after corrosion, the pit became larger, however, the Al₂O₃–SiO₂–MnO(–CaO) inclusion didn’t dissolve significantly.

Fig. 7.

Evolution of pitting morphologies around Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusions. (Online version in color.)

Fig. 8.

Elemental mappings of Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusions after corrosion for 2 min. (Online version in color.)

The evolution of pitting morphologies around Ce–Al–Si–Ca–O–S inclusions is shown in Fig. 9. After corrosion for 0.5 min, pitting corrosion occurred at the interface between the inclusion and the steel matrix, and discontinuous pits gradually formed at the interface. Five minutes after corrosion, obvious dissolution of the steel matrix was observed and discontinuous pits were connected to form an annular pit. Then, the annular pit expanded with the gradual dissolution of the steel matrix, and the inclusion fell off from the annular pit, 13 min after corrosion, forming a circular pit. The circular pit became larger with the continuous dissolution of the steel matrix.

Fig. 9.

Evolution of pitting morphologies around Ce–Al–Si–Ca–O–S inclusions. (Online version in color.)

The evolution of pitting morphologies around homogeneous Ce–O–S inclusions is shown in Fig. 10. After corrosion for 0.5 min, pitting corrosion occurred at the interface between the inclusion and the steel matrix. Discontinuous pits were gradually formed and were connected to form an annular pit after corrosion for 5 min. After corrosion for 13 min, the steel matrix was further dissolved, accompanied by the dissolution of the homogeneous Ce–O–S inclusion. Thirty minutes after corrosion, the homogeneous Ce–O–S inclusion completely dissolved or fell off from the steel matrix, forming a circular pit with larger size.

Fig. 10.

Evolution of pitting morphologies around homogeneous Ce–O–S inclusions. (Online version in color.)

The evolution of pitting morphologies around heterogeneous Ce–O–S inclusions is shown in Fig. 11. After corrosion for 0.5 min, the Ce–O phase dissolved obviously. Elemental mappings of the heterogeneous Ce–O–S inclusion after corrosion for 3 min is shown in Fig. 12, the Ce–O phase region in white color disappeared while the Ce–S phase region remained, indicating that the Ce–O phase completely dissolved 3 min after corrosion, and an irregular pit at the inclusion was formed. Then, the pit became larger with the simultaneous dissolution of both the Ce–S phase and the steel matrix. After corrosion for 30 min, the Ce–S phase completely dissolved and a larger irregular pit was formed.

Fig. 11.

Evolution of pitting morphologies around heterogeneous Ce–O–S inclusions. (Online version in color.)

Fig. 12.

Elemental mappings of heterogeneous Ce–O–S inclusions after corrosion for 3 min. (Online version in color.)

Three-dimensional morphologies variations of pits induced by different kinds of inclusions at different times after corrosion are shown in Figs. 13, 14, 15 and 16. For the Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusion, micro-crevice with a maximum depth of 0.86 μm was observed at the interface between the inclusion and the steel matrix before corrosion. The steel matrix around the inclusion dissolved gradually and the surface of the Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusion became obviously higher than the matrix. Thirty minutes after the corrosion, the Al₂O₃–SiO₂–MnO(–CaO) inclusion protruded from the steel matrix, the maximum depth of the gap between the inclusion and the steel matrix was 1.44 μm.

Fig. 13.

Three-dimensional morphologies variation of Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusions: (a) before corrosion; (b) 1 min after corrosion; (c) 2 min after corrosion; (d) 5 min after corrosion; (e) 13 min after corrosion; (f) 30 min after corrosion. (Online version in color.)

Fig. 14.

Three-dimensional morphologies variation of Ce–Al–Si–Ca–O–S inclusions: (a) before corrosion; (b) 1 min after corrosion; (c) 2 min after corrosion; (d) 5 min after corrosion; (e) 13 min after corrosion; (f) 30 min after corrosion. (Online version in color.)

Fig. 15.

Three-dimensional morphologies variation of homogeneous Ce–O–S inclusions: (a) before corrosion; (b) 1 min after corrosion; (c) 2 min after corrosion; (d) 5 min after corrosion; (e) 13 min after corrosion; (f) 30 min after corrosion. (Online version in color.)

Fig. 16.

Three-dimensional morphologies variation of heterogeneous Ce–O–S inclusions: (a) before corrosion; (b) 1 min after corrosion; (c) 2 min after corrosion; (d) 5 min after corrosion; (e) 13 min after corrosion; (f) 30 min after corrosion. (Online version in color.)

For the Ce–Al–Si–Ca–O–S inclusion, the maximum depth of the micro-crevice at the interface between the inclusion and the steel matrix was 0.86 μm before corrosion. An annular groove around the inclusions gradually formed during the corrosion process, and the inclusion feel off from the matrix 13 min after corrosion, forming a circular pit with the maximum depth of 4.56 μm. Then, the steel matrix gradually dissolved with time, the size of the circular pit became larger while the depth became smaller, and the maximum depth of the pit was 3.66 μm after corrosion for 30 min.

For the homogeneous Ce–O–S inclusion, the maximum depth of the micro-crevice at the interface between the inclusion and the steel matrix was 0.64 μm before corrosion. The dissolution of the inclusion and the steel matrix occurred simultaneously. Meanwhile, an annular groove around the inclusions also gradually formed during the corrosion process, and after corrosion for 30 min, the homogeneous Ce–O–S inclusion completely dissolved or fell off from the steel matrix, forming a circular pit with larger size with a maximum depth of 9.04 μm.

For the heterogeneous Ce–O–S inclusion, the Ce–O phase obviously dissolved and a pit with a maximum depth of 2.91 μm formed at the initial Ce–O phase region after corrosion for 1 min. Then, the Ce–S phase and the steel matrix gradually dissolved with time, enlarging the size and reducing the maximum depth of the pit. After corrosion for 30 min, the maximum depth of pit was 1.79 μm.

In order to analyze the depth of pits induced by different types of inclusions during the corrosion process, the average depth of pits was calculated using the method reported by Liu et al.⁴⁷⁾ Average depth variation of pits induced by different types of inclusions with corrosion time is shown in Fig. 17. When corrosion time was zero, the average depth of micro-crevices before corrosion was also calculated. The average depth of pits induced by Al₂O₃–SiO₂–MnO(–CaO)–MnS fluctuated around 0.2 μm. For pits induced by Ce–Al–Si–Ca–O–S and homogeneous Ce–O–S, the average depth of pits increased from 0.13 μm and 0.07 μm to 2.46 μm and 3.98 μm, respectively. The average depth of pits induced by heterogeneous Ce–O–S first increased from 0.08 μm to 0.57 μm after corrosion for 5 min, the showed a decreasing tendency with corrosion time.

Fig. 17.

Average depth variation of pits induced by different types of inclusions with corrosion time.

5. Corrosion Index of the Steel around Different Kinds of Inclusions

To further analyze the pitting morphology, areas statistics of inclusions and the dissolved steel matrix before and after the corrosion were performed. The schematic of several definitions of the pitting morphology around the inclusion is shown in Fig. 18. The number of measured inclusions was five for Al₂O₃–SiO₂–MnO(–CaO)–MnS, four for Ce–Al–Si–Ca–O–S, and three for homogeneous Ce–O–S type inclusions and homogeneous Ce–O–S type inclusions. The evolution of pitting morphologies around the other four inclusions is shown in Fig. 19, and the dissolution behavior of the four types of inclusions was similar with to that displayed in Section 4.

Fig. 18.

Schematic of the definition of the pit around inclusions. (Online version in color.)

Fig. 19.

Evolution of pitting morphologies around inclusions: (a) initial Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusion; (b) pitting morphologies around Al₂O₃–SiO₂–MnO(–CaO)–MnS; (c) initial Ce–Al–Si–Ca–O–S inclusion; (d) pitting morphologies around Ce–Al–Si–Ca–O–S inclusion; (e) initial homogeneous Ce–O–S inclusion; (f) pitting morphologies around homogeneous Ce–O–S inclusion; (g) initial heterogeneous Ce–O–S inclusion; (h) pitting morphologies around heterogeneous Ce–O–S inclusion. (Online version in color.)

Pitting morphologies of several inclusions for every kind of inclusions mentioned above were analyzed. To better clarify the corrosion resistance of the steel with different Ce contents, the corrosion index (η) of the steel matrix was defined to eliminate the effect of the initial inclusion size on the area of the dissolved steel matrix, as shown in Eq. (1):   

$$\eta =\frac{{\mathrm{S}}_s}{{\mathrm{S}}_i}$$(1)

where S_s is the area of the dissolved steel matrix, S_i is the initial area of the inclusion.

The variation of average corrosion index of the steel matrix around different kinds of inclusions during the pitting process is shown in Fig. 20. In the early corrosion stage, the corrosion index of the steel matrix around Ce–Al–Si–Ca–O–S inclusions was slightly lower than that around Al₂O₃–SiO₂–MnO(–CaO)–MnS ones. In the later corrosion stage, the corrosion index of the steel matrix around Ce–Al–Si–Ca–O–S inclusions was obviously higher than that around Al₂O₃–SiO₂–MnO(–CaO)–MnS ones. The corrosion index of the steel matrix around homogeneous Ce–O–S inclusions and heterogeneous Ce–O–S inclusions was higher than that around Al₂O₃–SiO₂–MnO(–CaO)–MnS ones during the whole corrosion process.

Fig. 20.

Variation of average corrosion index of the steel matrix around different kinds of inclusions during the pitting process: (a) early stage; (b) middle stage; (c) later stage.

6. First-Principles Calculations

To theoretically explain the pitting process induced by different kinds of inclusions, the Vienna ab-initio simulation package (VASP) was employed to calculate the band structure and work function of different kinds of inclusions. The projector augmented wave (PAW) method was used to describe the interaction between valence electrons and ions core. The Perdew-Burke-Ernzerhof (PBE) in generalized gradient approximation (GGA) was chosen as the exchange-correlation functional. For the geometry optimization of the bulk phase model of inclusions, the cut-off energy of the plane wave basis was taken as 600 eV. The constraint of the force and energy were 0.01 eV/Å and 1.0 × 10⁻⁵ eV/atom, respectively. The default method was adopted for cleaving surfaces of inclusions, and a vacuum of 20 Å was added to prevent the interaction between the atom layers.

Considering the complexity to achieve the convergence during the characteristic calculation of different kinds of inclusions, compounds Al₂Si₃Mn₃O₁₂, MnS, CeAlO₃, Ce₂O₃, Ce₂O₂S and CeS were chosen to represent different kinds of inclusions studied in the current work. The conductivity of Al₂Si₃Mn₃O₁₂, MnS, CeAlO₃, Ce₂O₃, Ce₂O₂S and CeS was calculated from their energy band structure, as shown in Fig. 21. Usually, materials with the value of bandgap smaller than 5 eV were considered to be semiconductors. The bandgap of Al₂Si₃Mn₃O₁₂ was 3.5196 eV, and those of MnS, CeAlO₃, Ce₂O₃, Ce₂O₂S and CeS were all approximately zero, indicating that inclusions investigated in the current study were all conductive. Generally, galvanic corrosion occurs due to the direct contact of different kinds of conductive materials with different electron work functions, resulting in local corrosion at the contact site. Electron work functions of different kinds of inclusions were also calculated using the VASP, as shown in Fig. 22. Calculated values of the bandgap and the electron work function in the current study was similar with the result that had been reported previously, as shown in Table 2.^48,49,50,51) Guo et al. reported that the work function of the austenite phase of a duplex stainless steel was 4.57 eV.⁵²⁾ The studied steel in the current paper was a typical austenite stainless steel, thus, the work function of the stainless steel matrix was roughly thought to be 4.57 eV. It’s obvious that the work function of MnS and Ce₂O₃ was smaller than that of Fe, and the work function of Ce₂O₂S was slightly higher than that of Fe, however, the work function of CeAlO₃, CeS and Al₂Si₃Mn₃O₁₂ was significantly higher than that of Fe. When the inclusion was exposed in the corrosion environment, MnS and Ce₂O₃ with lower work functions would dissolve prior to the steel matrix, however, Al₂Si₃Mn₃O₁₂, CeAlO₃, Ce₂O₂S and CeS were more stable than the steel matrix and the dissolution of these inclusions may be caused by the chemical dissolution in the acidic environment.

Fig. 21.

Energy band structure of (a) Al₂Si₃Mn₃O₁₂, (b) MnS, (c) CeAlO₃, (d) Ce₂O₃, (e) Ce₂O₂S and (f) CeS.

Fig. 22.

Electron work functions of different kinds of inclusions.

Table 2. Comparison of values of bandgap and electron work function between current results and reported results.

Compounds	Bandgap (eV)		Electron work function (eV)
	Current results	Reported results	Current results	Reported results
Al2Si3Mn3O12	3.520	3.556	5.504	–
MnS	0.0931	0	4.103	2.197–4.419
CeAlO3	0.001	0	5.406	4.581–6.875
Ce2O3	0.0002	0	4.407	–
Ce2O2S	0.0394	0	4.725	2.829–5.221
CeS	0.0004	0	5.063	–

7. Mechanisms of Pitting Corrosion Induced by Inclusions

The mechanism of pitting corrosion induced by different kinds of inclusions in the SS is summarized in Fig. 23. For the Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusion, the work function of the MnS phase was the lowest, thus MnS would act as the anode phase and dissolved first in the corrosion environment. When the dissolution of MnS was completed, the steel matrix would dissolve because of the lower work function of Fe compared with that of Al₂Si₃Mn₃O₁₂. For the Ce–Al–Si–Ca–O–S inclusion, the compound CeAlO₃ was chosen to analyze the dissolution process due to the low content of Si, Ca and S in the inclusion. The steel matrix would act as the anode phase due to the lower work function compared with that of CeAlO₃, when the dissolution of the steel matrix reached a certain extent, the Ce–Al–Si–Ca–O–S inclusion fell off from the matrix. For the homogeneous Ce–O–S inclusion, the work function of the steel matrix was slightly lower than that of Ce₂O₂S, and the dissolution of the steel matrix firstly occurred. The Ce₂O₂S inclusion began to dissolve after corrosion for 13 min which might be caused by the chemical dissolution in the acidic environment. For the heterogeneous Ce–O–S inclusion, the Ce–O phase possessed the lowest work function and acted as anode to dissolve first, after the dissolution of the Ce–O phase, the steel matrix began to dissolve. Then, the CeS phase dissolved in the acidic environment with the corrosion process, enlarging the area of the pit.

Fig. 23.

Mechanisms of pitting corrosion induced by different kinds of inclusions in SS. (Online version in color.)

8. Conclusions

In the current study, laboratory experiments were performed to study the effect of cerium content on the pitting corrosion resistance of a 304 SS. Pitting process induced by different kinds of inclusions were studied by in situ observation of the corrosion morphology around different types of inclusions. The following conclusions were obtained:

(1) When the content of total cerium in the SS increased from 0 to 430 ppm, evolution path of inclusions in the SS was Al₂O₃–SiO₂–MnO(–CaO)–MnS → Ce–Al–Si–Ca–O–S → homogeneous Ce–O–S → heterogeneous Ce–O–S.

(2) The dissolution sequence during the corrosion process could be summarized as: Ce–O phase in heterogeneous Ce–O–S and MnS → homogeneous Ce–O–S, Ce–S phase in heterogeneous Ce–O–S and steel matrix → Ce–Al–Si–Ca–O–S → Al₂O₃–SiO₂–MnO(–CaO).

(3) The corrosion index of the steel matrix around Al₂O₃–SiO₂–MnO(–CaO)–MnS inclusion was lower than that around homogeneous and heterogeneous Ce–O–S ones, and was higher than that around Ce–Al–Si–Ca–O–S ones only in the early corrosion stage.

(4) Inclusions Al₂Si₃Mn₃O₁₂, MnS, CeAlO₃, Ce₂O₃, Ce₂O₂S and CeS were all conductive. Compared with the work function of Fe, those of MnS and Ce₂O₃ was smaller, those of CeAlO₃, Ce₂O₂S, CeS and Al₂Si₃Mn₃O₁₂ was higher. MnS and Ce₂O₃ would dissolve prior to the steel matrix, CeAlO₃, Ce₂O₂S, CeS and Al₂Si₃Mn₃O₁₂ was more stable than the steel matrix.

Acknowledgement

The authors are grateful for the support from S&T Program of Hebei (Grant No. 20311004D, 20311005D), the High Steel Center (HSC) at Yanshan University, Hebei Innovation Center of the Development and Application of High Quality Steel Materials, Hebei International Research Center of Advanced and Intelligent Manufacturing of High Quality Steel Materials, Baotou Research Institute of Rare Earths, State Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization, Baotou Inner Mongolia 014030, China, and the High Steel Center (HSC) at North China University of Technology, Yanshan University and University of Science and Technology Beijing, China.

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