The effect of crack tip environment on crack growth behaviour of a low alloy steel at cathodic potentials in artificial seawater

doi:10.1016/j.jmst.2020.04.034

Abstract

Abstract:

The environment at crack tip and its effect on the crack growth behaviour of low alloy steel- E690 steel were studied at cathodic potentials in artificial seawater. The results showed that the micro environment at crack tip and crack growth behaviour were related to the electrochemical reactions at crack tip, which were affected by the stress state and applied potentials. The crack tip environment was acidified under cyclic loading, resulting from the crack tip anodic dissolution reaction and corresponding hydrolysis reaction. Because of the hydrogen evolution and the inhibited anodic dissolution inside the crack, the crack tip pH increases as the cathodic potential decreases. The effect of cathodic potentials on the electrochemical reactions caused the variation of the hydrogen content, which influenced the crack growth rate because the crack growth behaviour was controlled by hydrogen embrittlement mechanism. This resulted in a fact that with the negative decrease of potential, the crack growth rate first decreased and then increased, with the minimum rate at -0.75 V. And the crack growth path exhibited transgranular fracture.

Key words: Low alloy steel, Stress state, Cathodic potential, Crack tip environment, Crack growth behaviour

Yong Li, Zhiyong Liu, Endian Fan, Zhongyu Cui, Jinbin Zhao. The effect of crack tip environment on crack growth behaviour of a low alloy steel at cathodic potentials in artificial seawater[J]. J. Mater. Sci. Technol., 2020, 54: 119-131.

Figures/Tables 14

Fig. 1. Schematic diagram of the device: (a) cyclic loading experiment equipment, (b) dimensions of the CT specimen, (c) mini-electrode and (d) calibration graph for the Cl- concentration.

Fig. 2. CGR of E690 steel in air and artificial seawater: (a) crack length as a function of time and (b) da/dt as a function of ΔK.

Fig. 3. Fracture morphologies of E690 steel during the crack growth process in (a) air and (b) artificial seawater.

Fig. 4. Crack growth path of E690 steel in (a) air and (b) artificial seawater: (a1 and b1) IPF, (a2 and b2) IQ, (a3 and b3) KAM and (a4 and b4) KAM with grain boundaries (white arrows indicate LBBs and yellow circles indicate PAGBs).

Fig. 5. CGR of E690 steel under various potentials: (a) crack length as time elapses, (b) da/dt as a function of ΔK and (c) CGR as a function of the applied potential.

Fig. 6. Images of corrosion products on the fracture surface of E690 steel at various potentials: (a) OCP, (b) -0.75 V, (c) -0.85 V, and (d) -1.05 V. (e) EDS results and (f) in situ Raman spectra on the fracture surface of E690 steel.

Fig. 7. Fracture images of E690 steel under various potentials after derusting: (a) OCP, (b) -0.75 V, (c) -0.85 V, (d) -0.95 V, (e) -1.05 V and (f) -1.2 V.

Fig. 8. Crack growth path of E690 steel at (a1 and a2) -0.75 V and (b1 and b2) -1.05 V: (a1 and b1) IPF and (a2 and b2) KAM with grain boundaries.

Fig. 9. Dependence of the environment within the crack on the distance from the crack tip at OCP: (a) pH value and (b) Cl- concentration.

Fig. 10. Environment within the crack at various potentials: (a) pH value depending on the distance from the crack tip under cyclic loading, (b) crack tip pH value as a function of time with and without cyclic loading.

Fig. 11. Hydrogen content of E690 steel in 3.5 wt.% NaCl solution with various (a) pH and (b) cathodic potentials.

Fig. 12. Schematic illustrations of the electrochemical reactions at the crack tip under cathodic potential: (a) with and (b) without cyclic loading.

Fig. 13. (a) Polarisation curves of E690 steel in simulated solution at slow and fast scanning rates and (b) the contribution of crack growth factors to the CGR.

Fig. 14. Schematic diagrams of (a) the electrochemical reaction that occurred at the crack tip and (b) the relationship between CGR and the reaction at various potentials.

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