Initial Corrosion Behavior and Local Corrosion Origin of 9%Cr Alloy Steel in Cl－Containing Environment

doi:10.11900/0412.1961.2021.00597

Acta Metall Sin

2023, Vol. 59

Issue (7): 926-938 DOI: 10.11900/0412.1961.2021.00597

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Initial Corrosion Behavior and Local Corrosion Origin of 9%Cr Alloy Steel in Cl^－Containing Environment

CHEN Runnong¹^,²^,³, LI Zhaodong¹(

), CAO Yanguang¹^,⁴, ZHANG Qifu², LI Xiaogang³

¹Department of Structural Steels, Central Iron and Steel Research Institute, Beijing 100081, China
²National Engineering Laboratory of Advanced Coating Technology for Metals, Central Iron and Steel Research Institute, Beijing 100081, China
³Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
⁴Maanshan Iron & Steel Co. Ltd., Maanshan 243003, China

Cite this article:

CHEN Runnong, LI Zhaodong, CAO Yanguang, ZHANG Qifu, LI Xiaogang. Initial Corrosion Behavior and Local Corrosion Origin of 9%Cr Alloy Steel in Cl^－Containing Environment. Acta Metall Sin, 2023, 59(7): 926-938.

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Abstract

The South China Sea is a marine atmosphere environment with high humidity, high salt content, and strong radiation. Traditional weathering steel and 3Ni advanced weathering steel cannot meet the service requirements in the South China Sea environment, necessitating the development of steel with improved corrosion resistance. Alloy steels with Cr of 2.5%-10% (mass fraction) provide a marginal gain in corrosion performance at a low cost and have great potential for marine atmospheric application. A 9%Cr alloy steel was designed to obtain higher corrosion resistance, and the relevant results can offer a reference for developing novel corrosion-resistant steels for the marine atmospheric environment. The initial corrosion behavior of 9%Cr alloy steel in a Cl^- containing environment was investigated using dry-wet cycle test, SEM, TEM, XRD, and electrochemical approaches, and the effects of composite inclusions (Mg, Si, Al)O-MnS and Cr-rich M₂₃C₆ on its local corrosion behavior were discussed. The findings demonstrate that the initial corrosion resistance of alloy steel was more than 12 times that of 09CuPCrNi, and local corrosion occurred during the 360-h dry-wet cycle. Pits' depth below the rust layer followed the lognormal distribution, and the pits' maximum depth (D_max) and average depth (D_ave) with time (t) were in line with the power functions D_max = 8.4844 × t^0.65717 and D_ave = 7.3181 × t^0.53866, respectively. The rust layer's compactness and the α / γ^* ratio increased over time, but the addition of high Cr delayed the corrosion. Thus, the rust layer did not entirely cover the surface and only provided limited protection, and an exponent value obtained by fitting the weight loss according to the power function was greater than 1. (Mg, Si, Al)O-MnS caused metastable pitting corrosion through a preferential dissolution of MnS or MgO regions, but its immersion in 2%NaCl solution for 300 min did not induce surrounding matrix's dissolution. The Cr consumption caused by Cr-rich M₂₃C₆'s precipitation was the primary reason for preferentially inducing local corrosion.

Key words: Cl^- containing environment local corrosion inclusion M₂₃C₆ 9%Cr alloy steel

Received: 31 December 2021

ZTFLH:

TG174.22

Fund: National Key Research and Development Program of China(2021YFB3701702);Central Iron and Steel Research Institute Foundation(20G61860A)

Corresponding Authors: LI Zhaodong, professor senior engineer, Tel: (010)62181284, E-mail: cisri_lizhaodong@126.com

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	CHEN Runnong
	LI Zhaodong
	CAO Yanguang
	ZHANG Qifu
	LI Xiaogang

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https://www.ams.org.cn/EN/10.11900/0412.1961.2021.00597 OR https://www.ams.org.cn/EN/Y2023/V59/I7/926

Fig.1 Schematic of manufacturing process of 9Cr steel

Table 1 Chemical compositions of 9Cr and 09CuPCrNi steels

Fig.2 Corrosion rates (a) and weight losses (b) of 9Cr and 09CuPCrNi steels as a function of time (W—weight loss, t—corrosion time)

Fig.3 Macro (a1-a4) and micro (b1-b4) surface corrosion morphologies of 9Cr steels corroded for different time and the high-resolution SEM images (c1-c4) corresponding to the square areas in Figs.3b1-b4
(a1-c1) 72 h (a2-c2) 168 h (a3-c3) 264 h (a4-c4) 360 h

Table 2 EDS results corresponding to the square areas in Figs.3c1-c4

Fig.4 Cross-sectional morphologies of rust layer (a-d) and corresponding main element distributions (e-h) of 9Cr steels after corrosion for 72 h (a, e), 168 h (b, f), 264 h (c, g), and 360 h (d, h)

Fig.5 Cross-sectional morphology (a) and EPMA elemental mappings (b-e) of the cross-sectional rust layer of 9Cr steel after corrosion for 360 h

Fig.6 Nyquist plots (a), phase angle plots (b), modulus plots (c), and equivalent circuit (d) of corroded 9Cr steels in 2%NaCl solution (Z_im－imaginary part of impedance, Z_re－real part of impedance, |Z|－impedance modulus, R_s－resistance of electrolyte, Q_dl－constant phase element of double layer, R_p－polarization resistance)

Table 3 EIS fitting data of corroded 9Cr steels in 2%NaCl solution via the equivalent circuit in Fig.6d

Fig.7 XRD spectra (a) and semi-quantitative results (b) of the scraped rust formed on 9Cr steel surface after corrosion for different time (α / γ^*－ratio of the content of α-FeOOH to the content of γ-FeOOH and Fe₃O₄)

Fig.8 Distributions of pit depth (a) and variation law of pit depth (b) of 9Cr steel surface after corrosion for different time (D_max－maximum value of the measured pit depth, D_ave－average value of the measured pit depth)

Fig.9 Distributions of diameter-depth ratio of pits on 9Cr steel surface after corrosion for 72 h (a), 168 h (b), 264 h (c), and 360 h (d)

Fig.10 Polarization curve of 9Cr steel in 2%NaCl solu-tion (a) and the magnified plot corresponding to the square region in Fig.10a (Inset shows the surface morphology of 9Cr sample after polariz-ation test) (b)

Fig.11 Representative metastable pitting morphologies (a-c) on the surface of 9Cr steel after polarization test

Fig.12 SEM images and corresponding EDS element maps of MnS-rich (a) and MnS-poor (b) composite inclusions

Fig.13 Corrosion morphologies of MnS-rich (a1-a3) and MnS-poor (b1-b3) composite inclusions immersed in 2%NaCl solution for 5 min (a1, b1), 50 min (a2, b2), and 300 min (a3, b3)

Fig.14 Local corrosion morphologies (a, b) of 9Cr steel immersed in 2%NaCl solution for 300 min and main element distributions (c, d) around a corrosion pit in Fig.14b (Inset in Fig.14a shows the corresponding high magnified image)

Fig.15 Distribution characteristics and composition analysis of M₂₃C₆ in 9Cr steel (Inset in Fig.15b shows the SAED pattern of zone 1)
(a) low magnified TEM image (b) high magnified TEM image
(c) EDS of zone 1 in Fig.15b (d) Cr poor phenomenon near M₂₃C₆

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