Effects of rare earth modifying inclusions on the pitting corrosion of 13Cr4Ni martensitic stainless steel

doi:10.1016/j.jmst.2021.03.014

Abstract

Abstract:

In this study, the pitting corrosion behavior of 13Cr4Ni martensitic stainless steel (BASE) and that modified with rare earth (REM) in 0.1 mol/L NaCl solution were characterized. Techniques such as automatic secondary electron microscope (ASPEX PSEM detector), scanning electron microscope (SEM), transmission electron microscope (TEM), scanning Kelvin probe force microscope (SKP), potentiodynamic and potentiostatic polarizations were employed. The results obtained indicate that BASE steel contains Al₂O₃/MnS, Al₂O₃ and MnS inclusions, while REM steels contain (La, Ce, Cr, Fe)-O and (La, Ce, Cr, Fe)-O-S inclusions. Compared with BASE steel, REM steel is more susceptible to induce the metastable pitting nucleation and repassivation, whereas it restrains the transition from metastable pitting to stable pitting. Adding 0.021% rare earth element to BASE steel can reduce the number and area of inclusions, while that of 0.058% can increase the number and enlarged the size of inclusions, which is also the reason that pitting corrosion resistance of 58REM steel is slightly lower than that of 21REM steel. In the process of pitting corrosion induced by Al₂O₃/MnS inclusions, MnS is preferentially anodic dissolved, and also the matrix contacted with Al₂O₃ is subsequently anodic dissolved. For REM steels, anodic dissolution preferentially occurs at the boundary between inclusions and matrix, while (La, Ce, Cr, Fe)-O inclusions chemically dissolve in local acidic environment or are separated from steel matrix. The chemically dissolved substance (La³⁺ and Ce³⁺) of (La, Ce, Cr, Fe)-O inclusions are concentrated in pitting pits, which inhibits its continuous growth.

Key words: 13Cr4Ni martensitic stainless steel, Rare earth modifying inclusion, Al₂O₃/MnS inclusion, Metastable pitting corrosion, Stable pitting corrosion

Changgang Wang, Rongyao Ma, Yangtao Zhou, Yang Liu, Enobong Felix Daniel, Xiaofang Li, Pei Wang, Junhua Dong, Wei Ke. Effects of rare earth modifying inclusions on the pitting corrosion of 13Cr4Ni martensitic stainless steel[J]. J. Mater. Sci. Technol., 2021, 93: 232-243.

Figures/Tables 12

Table 1. Chemical compositions of the stainless steels (wt.%).

	C	Si	Mn	Cr	Ni	Mo	Al	S	O	RE (La+Ce)	Fe
BASE	0.035	0.31	0.68	12.41	3.98	0.49	0.017	0.003	0.0012	-	Bal.
0.021REM	0.037	0.32	0.70	12.37	3.96	0.51	0.015	0.003	0.0015	0.021	Bal.
0.058REM	0.044	0.32	0.71	12.34	3.99	0.50	0.019	0.003	0.0013	0.058	Bal.

Fig. 1. Morphology characterization of inclusions in BASE steel. (a) Al2O3/MnS-type inclusion. (b) Al2O3-type inclusion. (c) MnS-type inclusion. (d) EDS results of the points from (a)-(c). (e) TEM image of an Al2O3/MnS inclusion in BASE steel. (f) Line-scan analysis of elements by EDS corresponding to (e).

Fig. 2. Morphology characterization of inclusions in REM steel. SEM images of (La, Ce, Cr, Fe)-O inclusions in (a) 21REM steel and (b) 58REM steel. SEM images of (La, Ce, Cr, Fe)-O-S inclusions in (c) 21REM steel and (d) 58REM steel. (e) EDS results of the points from (a)-(d). (f) TEM image of a (La, Ce, Cr, Fe)-O inclusion in 21REM steel. (g) line-scan analysis of elements by EDS corresponding to (f).

Fig. 3. Statistical diagram of inclusion properties: Effects of rare earth metals addition on the area, number and area distributions of inclusion per frame area of the experimental steels: (a) the total area and number of inclusions, (b) the area distribution of inclusions. Area distribution of different types of inclusions in experimental steels: (c) BASE steel, (d) 21REM steel, (e) 58REM steel.

Table 2. Standard free energy of formation of relevant oxides and sulphides at 298.15 K [34].

Substance	Free energy of formation (J/mol)
Ce₂O₃	-1706.2
La₂O₃	-1705.8
Al₂O₃	-1582.3
CeS	-451.5
LaS	-451.5
MnS	-218.4

Fig. 4. Electrochemical characterization of pitting corrosion. (a) Potentiodynamic polarization tests of the experimental steels in 0.1 mol/L NaCl solution at 25 °C. (b) Cumulative probability results of pitting potential of the experimental steels obtained from 10 groups of potentiodynamic polarization tests. (c) Potentiostatic tests at an applied potential of -80 mV/SCE of the experimental alloys in 0.1 mol/L NaCl solution at 25 °C. (d) Amplification section of red box in Fig. 4(c). (e) Statistical distribution of current spike heights from Fig. 4(c). (f) Statistical distribution of transient lifetimes from Fig. 4(c).

Table 3. Numbers of peak for experimental steels obtained from Fig. 9(a).

Alloy	BASE	21REM	58REM
Numbers of peak	66	358	395

Fig. 5. SEM image and SKPFM analysis of an Al2O3\MnS type inclusion in BASE steel: (a1) SEM image; (a2) topographical map; (a3) line-scan section analysis result obtained from the topographical map; (a4) volta potential map; (a5) line-scan section analysis result obtained from the volta potential map. SEM image and SKPFM analysis of a (La, Ce, Cr, Fe)-O type inclusion in 58REM steel: (b1) SEM image; (b2) topographical map; (b3) line-scan section analysis result obtained from the topographical map; (b4) volta potential map; (b5) line-scan section analysis result obtained from the volta potential map.

Fig. 6. SEM-EDS analysis of the corrosion morphology evolution of Al2O3/MnS inclusions in BASE, (a) the inclusion morphology before applying -80 mV/SCE for 10 min; (b) the inclusion morphology after applying -80 mV/SCE for 10 min; (c) EDS analysis results corresponding to (a) and (b); (d) the inclusion morphology before applying 0 mV/SCE for 10 min; (e) the inclusion morphology after applying 0 mV/SCE for 10 min; (f) EDS analysis results corresponding to (d) and (e); (g) The morphology of the maximum stable pitting pit on the electrode surface; (h) EDS analysis results corresponding to (g).

Fig. 7. Schematic of the pit initiation and propagation induced by Al2O3/MnS in BASE steel.

Fig. 8. SEM-EDS analysis of the corrosion morphology evolution of inclusions in 58REM steel, (a) the inclusion morphology before applying -80 mV/SCE for 10 min; (b) the inclusion morphology after applying -80 mV/SCE for 10 min; (c) EDS analysis results corresponding to (a) and (b); (d) the inclusion morphology before applying 120 mV/SCE for 10 min; (e) the inclusion morphology after applying 120 mV/SCE for 10 min; (f) EDS analysis results corresponding to (d) and (e); (g) The morphology of the maximum stable pitting pit on the electrode surface; (h) EDS analysis results corresponding to (g).

Fig. 9. Schematic of the pit initiation and propagation induced by (La, Ce, Cr, Fe)-O type inclusion in REM steel.

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