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Journal of Chinese Society for Corrosion and protection  2026, Vol. 46 Issue (4): 1185-1196    DOI: 10.11902/1005.4537.2025.265
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Effect of Sodium Phosphate Concentration on Corrosion Resistance of 40Cr Steel
ZHOU Weilong1, GUO Yubing1, HU Mengru1, SHEN Kaijie2, YANG Zhenghuan2, XIE Linjun1()
1.School of Mechanical Engineering, Zhejiang University of Technology, Hangzhou 310014, China
2.China Nuclear Power Operation Management Co. Ltd., Haiyan 314300, China
Cite this article: 

ZHOU Weilong, GUO Yubing, HU Mengru, SHEN Kaijie, YANG Zhenghuan, XIE Linjun. Effect of Sodium Phosphate Concentration on Corrosion Resistance of 40Cr Steel. Journal of Chinese Society for Corrosion and protection, 2026, 46(4): 1185-1196.

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Abstract  

Nuclear power pump shaft, as a crucial load-bearing component experiences corrosion failure due to long-term exposure to cooling water, this corrosion failure is a critical factor limiting its service life. For such corrosion, sodium phosphate, as an important corrosion inhibitor, herein, of which the inhibition performance on the 40Cr steel was studied by using potentiodynamic polarization, electrochemical impedance spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometer. The results indicate that the corrosion rate decreases significantly with increasing sodium phosphate (Na3PO4) concentration. At lower concentrations (1 × 10-4 and 1 × 10-3 mol/L), corrosion still occurs, forming loose and porous corrosion products composed mainly of Fe3O4, Fe2O3, and γ-FeOOH, which provide poor corrosion protection. At a concentration of 1 × 10-3 mol/L, a more stable α-FeOOH phase is emerged, with volume fraction ratio of γ-FeOOH to α-FeOOH being 39.65% and 60.35%. The compact corrosion products of α-FeOOH and Fe3O4 effectively decelerate the corrosion process and improve the protective properties of the rust layer. When the concentration is increased to 1 × 10-2 mol/L, a dense and stable passive film forms on the 40Cr steel surface, primarily consisting of FePO4 and Fe2O3, this film effectively isolates the substrate from the corrosive medium, maintaining the steel in a passive state and thereby significantly inhibiting the corrosion process.
Key words:  Na3PO4      40Cr steel      electrochemical corrosion      corrosion inhibition performance      passive film     
Received:  25 August 2025      32134.14.1005.4537.2025.265
ZTFLH:  TG174  
Corresponding Authors:  XIE Linjun, E-mail: linjunx@zjut.edu.cn
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ZHOU Weilong
GUO Yubing
HU Mengru
SHEN Kaijie
YANG Zhenghuan
XIE Linjun

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https://www.jcscp.org/EN/10.11902/1005.4537.2025.265     OR     https://www.jcscp.org/EN/Y2026/V46/I4/1185

Fig.1  Polarization curves of 40Cr steel in Na3PO4 solution with different concentrations
C / mol·L-1Ecorr / VIcorr / A·cm-2
1 × 10-4-0.6176.690 × 10-6
5 × 10-4-0.6036.528 × 10-6
1 × 10-3-0.4566.500 × 10-6
5 × 10-3-0.4245.430 × 10-6
1 × 10-2-0.4215.197 × 10-6
Table 1  Ecorr and Icorr of 40Cr steel in Na3PO4 solution with different concentrations
Fig.2  Nyquist (a) and Bode (b) plots of 40Cr steel in Na3PO4 solution with different concentrations
Fig.3  Equivalent circuit diagrams for EIS of 40Cr steel in Na3PO4 solution at different concentrations: (a) 1 × 10-4 mol/L, 5 × 10-4 mol/L, 1 × 10-3 mol/L; (b) 5 × 10-3 mol/L, 1 × 10-2 mol/L
C / mol·L-1Rs / Ω·cm2Fitting error / %Rf / Ω·cm2Fitting error / %Rct / Ω·cm2Fitting error / %R / Ω·cm2
1 × 10-415850.4575688.74227171.8033285
5 × 10-411940.60921746.92035821.6625756
1 × 10-37220.6182.931 × 1042.2553.010 × 1044.0405.941 × 104
5 × 10-32780.831--8.341 × 1042.0368.341 × 104
1 × 10-21050.688--1.032 × 1052.4091.032 × 105
Table 2  Fitting results of EIS data of 40Cr steel in Na3PO4 solution at different concentrations
Fig.4  Polarization curves of 40Cr steel after immersion for different time in 1 × 10-4 mol/L (a), 1 × 10-3 mol/L (b), and 1 × 10-2 mol/L (c) Na3PO4 solutions
Fig.5  Nyquist (a, c, e) and Bode (b, d, f) plots of 40Cr steel after immersion for different times in 1 ×10-4 mol/L (a, b), 1 × 10-3 mol/L (c, d), and 1 × 10-2 mol/L (e, f) Na3PO4 solutions
Time / h1 × 10-4 mol·L-11 × 10-3 mol·L-11 × 10-2 mol·L-1
Ecorr / VIcorr / µA·cm-2Ecorr / VIcorr / µA·cm-2Ecorr / VIcorr / µA·cm-2
2-0.7207.533-0.6056.628-0.4195.064
4-0.7438.394-0.6516.935-0.4184.605
6-0.7559.288-0.6537.412-0.4124.445
8-0.76310.150-0.6587.943-0.4094.444
16-0.75912.140-0.70310.490-0.4074.421
32-0.76516.150-0.70211.920-0.4054.300
48-0.77818.750-0.73113.870-0.3963.984
72-0.79126.510-0.75420.130-0.3923.687
Table 3  Ecorr and Icorr of 40Cr steel after immersion for different time in Na3PO4 solutions
Fig.6  Surface SEM images (a-d) and EDS spectrum (e) of 40Cr steel after 72 h immersion in 1 × 10-4 mol/L Na3PO4 solution
Fig.7  Surface SEM images (a-d) and EDS spectrum (e) of 40Cr steel after 72 h immersion in 1 × 10-3 mol/L Na3PO4 solution
Fig.8  Surface SEM image (a-d) and EDS spectrum (e) of 40Cr steel after 72 h immersion in 1 × 10-2 mol/L Na3PO4 solution
Fig.9  XRD patterns of corrosion products of 40Cr steel after 72 h immersion in Na3PO4 solutions with different concentrations
Fig.10  XRD full spectrum fitting analysis results of corrosion products in 1 × 10-3 mol/L Na3PO4 concentration
Fig.11  XPS patterns of corrosion products of 40Cr steel after 72 h immersion in Na3PO4 solutions with different concentrations: (a, b) 1 × 10-4 mol/L, (c, d) 1 × 10-3 mol/L, (e-g) 1 × 10-2 mol/L
Fig.12  Schematic diagrams of corrosion of 40Cr steel in Na3PO4 solution with different concentrations: (a) 1 × 10-4 mol/L and 1 × 10-3 mol/L, (b) 1 × 10-2 mol/L
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