Electronic Theoretical Study of the Influence of Cr on Corrosion Resistance of Fe-Cr Alloy
Yao WANG1,Chunfu LI1,Yuanhua LIN1,2()
1 School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China 2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,
Cite this article:
Yao WANG,Chunfu LI,Yuanhua LIN. Electronic Theoretical Study of the Influence of Cr on Corrosion Resistance of Fe-Cr Alloy. Acta Metall Sin, 2017, 53(5): 622-630.
Based on the empirical electron theory (EET) of solids and molecules, the valence electron structure caculation results of Fe-Cr alloy containing (0~30%)Cr were analyzed semi-quantitatively. The electron density differences of interface (Δρ) between Fe-Cr alloy and Cr2O3, Fe2O3 passivation films were calculated. According to the results, adding Cr to α-Fe matrix can strengthen the matrix by improving the number of hybid atomic orbitals σn, the number of the strongest bond covalent electron pairs nA and the strongest covalent bond energy EA of Fe-Cr alloy. Once the content of Cr rises up to 12.52% and 24.3%, the corrosion resistance of Fe-Cr alloy is improved because of Cr being changed to a higher hybrid level, where Cr becomes more unstable and easily reacts with environment to form a complete passivation layer of Cr2O3. Moreover, among the electronic density differences of 24 low-index faces between Fe-Cr and Cr2O3, Fe2O3, only the Δρ of Fe-Cr(112)/ Cr2O3(0001), Fe-Cr(112)/Cr2O3 (101?0)Cr,Fe-Cr(112)/Fe2O3(112?0) are lower than 10%. For the matrix with same content of Cr, the Δρ between Fe-Cr(112) and Cr2O3(101?0)Cr is the lowest, but the number of hybid atomic orbitals σ satisfied Δρ<10% is the largest. Δρ (σ) of Fe-Cr(112)/Cr2O3(0001) and Fe-Cr(112)/Fe2O3(112?0) is decreased (increased) with the increase of Cr, therefore the interface bonding strength between Cr2O3, Fe2O3 and matrix will be enhanced, it has been found that the corrosion resistance of Fe-24.3%Cr is better. The calculation results of variation of Fe-Cr corrosion resistance with Cr content are in better agreement with Tammann's law.
Fund: Supported by National High Technology Research and Development Program of China (No.2006AA06A105) and Fund of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (No.PLN0609)
Fig.1 Unit cell structure of M (M is considered to be an average atom of Fe and Cr; A, B are the
non-negligible covalent bonds)
Fig.2 Atomic arrangement of X2O3 (X=Cr, Fe) in different crystallographic faces(a) (0001) (b) (101?0)O (c) (101?0)X (d) (112?0)
Crystal face
Bond
Ia
na
S / cm2
ρ / nm-2
(110)
M-M
4
0.3875
0.1162
14.8559
M-M
2
0.0882
(100)
M-M
4
0.0882
0.0822
4.2942
(112)
M-M
27
0.3875
0.0503
2.2004
Table 1 Interfacial electron densities of Fe-5%Cr (atomic fraction) alloy on the different crystal faces
Crystal face
Bond
Ia
na
S / cm2
ρ / nm-2
Cr2O3
Fe2O3
Formula
Cr2O3
Fe2O3
Cr2O3
Fe2O3
(0001)
O1-O3
6
0.0261
0.0340
32a02
0.2123
0.2187
1.0930
1.2756
O1-O2
12
0.0063
0.0102
(101?0)Cr
O1-O2
6
0.0063
0.0102
a0c
0.6736
0.6921
0.0560
0.0884
(101?0)X
X1-X2
2
0.4076
0.2186
a0c
0.6736
0.6921
1.9754
1.0957
X1-X3
2
0.2327
0.1456
X1-X4
2
0.0201
0.0111
X1-X5
2
0.0049
0.0039
(112?0)
X-O2
4
0.9528
1.1290
3a0c
1.1667
1.1988
6.2157
6.2597
X-O1
4
0.8024
0.6689
X-O3
4
0.0032
0.0033
O1-O3
6
0.0261
0.0340
O1-O4
6
0.0103
0.0160
Table 2 Interfacial electron densities (ρ) of Cr2O3 and Fe2O3
Atomic fraction
Atomic state
σn
nA
EA
of Cr / %
Hybridization
ncFe
Hybridization
ncCr
kJmol-1
level of Fe
level of Cr
0
8
3.5955
-
-
3
0.3835
59.8761
1
8
3.5955
1
4
54
0.3840
60.0479
2
8
3.5955
1
4
54
0.3844
60.2197
3
8
3.5955
1
4
54
0.3848
60.3926
4
8
3.5955
3
3.9515
54
0.3851
60.6105
5
8
3.5955
5
3.8779
54
0.3851
60.7721
6
8
3.5955
5
3.8779
54
0.3854
60.8624
7
8
3.5955
5
3.8779
54
0.3857
61.0289
8
7
3.5560
1
4
61
0.3831
60.1916
9
7
3.5560
1
4
65
0.3836
60.3758
10
7
3.5560
2
3.9851
65
0.3839
60.6539
11
7
3.5560
3
3.9515
67
0.3840
60.8699
12
7
3.5560
4
3.9360
67
0.3842
61.0611
12.50
7
3.5560
4
3.9360
68
0.3844
61.1598
12.52
9
3.7743
13
3.1290
68
0.3940
65.3692
13
9
3.7743
13
3.1290
68
0.3937
65.3777
14
7
3.5560
5
3.8779
65
0.3841
61.3981
15
7
3.5560
5
3.8779
64
0.3845
61.6811
16
7
3.5560
5
3.8779
64
0.3848
61.7875
17
9
3.7743
11
3.2932
60
0.3939
65.9435
18
9
3.7743
11
3.2932
58
0.3934
65.9905
19
9
3.7743
10
3.3538
61
0.3941
66.2459
20
9
3.7743
10
3.3538
63
0.3937
66.3043
21
9
3.7743
10
3.3538
64
0.3932
66.3630
22
9
3.7743
9
3.3972
65
0.3938
66.5806
23
9
3.7743
9
3.3972
69
0.3934
66.5781
24
9
3.7743
9
3.3972
71
0.3930
66.7128
24.3
10
3.9723
13
3.1290
71
0.4019
70.2113
25
10
3.9723
13
3.1290
73
0.4013
70.1827
26
10
3.9723
12
3.1635
72
0.4013
70.3102
27
10
3.9723
12
3.1635
70
0.4004
70.2761
28
8
3.5955
6
3.6807
75
0.3861
64.0870
29
8
3.5955
6
3.6807
76
0.3862
64.2539
30
8
3.5955
6
3.6807
78
0.3863
64.6460
Table 3 Vanlance electron structures of Fe-Cr alloy
Fig.3 Number of hybrid atomic orbital σn and EA of different Fe-Cr alloys
Fig.4 Hybrid levels of Fe (a) and Cr (b) in different Fe-Cr alloys
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