Issue 50

Z.-y. Han et alii, Frattura ed Integrità Strutturale, 50 (2019) 21-28; DOI: 10.3221/IGF-ESIS.50.03

1 m p RT    + + + m m s m

1



( M m

)

G

(3 ）

=

=

=

i

i

exp(

)

i

exp(

)

k i

p 0

0

p 0

(1 ) +

RT

where ΔG is the change of Gibbs free energy, R is universal gas constant, T is absolute temperature value, i 0 is anodic dissolution current density in the condition of no applied stress, M and ρ are the molar mass and the density of the electrode material, p and m are the material constants using the Ramberg-Osgood power model, σ s is the yielding strength of the electrode material, and k p is the current concentration factor.

b

a

Figure 9 : Stress concentration at the corrosion pit and microcrack: (a) corrosion pit, (b) microcrack.

From Fig. 9, the stresses at the bottom of the corrosion pit and the tip of crack are far bigger than the applied tensile stress. According to Eqn. (3), the anodic dissolution current density i p increases drastically, which significantly promote the anodic dissolution of the metal at the pits and microcracks, resulting in the accelerating loss in cross-sectional area and fracture process of the sample. Fig. 10 depicts the variation of current concentration factor k p with the concentrated stress, and the measured current densities of X80 steel from Ref. [19] are quoted and were depicted in Fig. 10 for comparison. Although the current concentration factors of the measured currents under different stresses have certain differences with the theoretical k p , the fact that high stress increases the anode dissolution current can be affirmative. From the above analysis, it can be seen that even under lower applied stress, the stress concentration can cause local high stress at the corrosion pit and crack tip. The interaction of high stress and electrochemical reaction promotes the anodic dissolution and the growth of corrosion pits and cracks, thereby accelerates the process of stress corrosion cracking of the X80 steel in the seawater.

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

7

( m

)

m

m

+

1

+

1

    (

   

M m 1 +



+ 1

 

)

m

+

   

s

M m



+



test data [19]

k

=

exp

s

  

k

=

p exp

m

(

) 

6

 m p RT

1 )

+

p

(

m p RT

1

+

3 56g/mol, =7.84g/cm 206GPa, =695MPa 4.37, 293K  = p 3 56g/mol, =7.84g/cm 206GPa, =695MPa 4.37, 293K  = p = = = M E = = m T = m T M E

5

4

k p

R=8.31J/mol.K

R=8.31J/mol.K

k

k

3

2

2



 

 M



  

M

k

k =  exp

exp = 



e

2

e

2  E RT

2

 E RT 





1

0

0

200

400 400

600 600

800 800

0

200

0

200

Stress /MPa 400

600

800

Figure 10 : The effect of applied stress on active anodic dissolution stre s/MPa

stress/MPa

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