EFFECT OF HYDROSTATIC PRESSURE AND PRE-STRESS ON CORROSION BEHAVIOR OF A NEW TYPE Ni-Cr-Mo-V HIGH STRENGTH STEEL
Lin FAN,Kangkang DING,Weimin GUO,Penghui ZHANG,Likun XU()
State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266101, China
Cite this article:
Lin FAN,Kangkang DING,Weimin GUO,Penghui ZHANG,Likun XU. EFFECT OF HYDROSTATIC PRESSURE AND PRE-STRESS ON CORROSION BEHAVIOR OF A NEW TYPE Ni-Cr-Mo-V HIGH STRENGTH STEEL. Acta Metall Sin, 2016, 52(6): 679-688.
The efforts on deep sea exploration and development have posed many challenges on the corrosion resistance and safety use of high strength steel in recent years. This has attracted a lot of attentions on the corrosion behavior of high strength steel in deep sea environment. Hydrostatic pressure has been identified as one of the most significant factors that affect pitting corrosion of materials or steel structures used in deep sea. However, pre-stress introduced by the actual service conditions is probably another critical factor of deep sea corrosion. The purpose of this work is to investigate the corrosion behavior of a new type Ni-Cr-Mo-V high strength steel under the combined stresses of hydrostatic pressure and preloaded tensile stress in simulated deep-sea environment. Corrosion rate measurement, SEM observation, statistical analysis of pitting geometry and finite element (FE) analysis were used in this work. The results indicated that corrosion rate of Ni-Cr-Mo-V high strength steel increased with the increase of hydrostatic pressure and pre-stress. The deterioration of corrosion resistance of the steel mainly reflected in pit initiation, pit growth and pit coalescence. Rather than pre-stress, hydrostatic pressure exhibited obvious effect on promoting pit initiation. Pits initiated in the form of corrosion pin-holes, which randomly distributed at the corroded surface. Both hydrostatic pressure and pre-stress facilitated pit growth, and there was an interaction between them, which was more remarkable at higher pre-stress. Hydrostatic pressure was mainly responsible for pit growth parallel to the steel surface, while pre-stress was essential to pit depth increase. Adjacent pits were inclined to coalesce in the direction perpendicular to pre-stress. With the increase of hydrostatic pressure and pre-stress, the aspect ratio of pits increased, which can lead to the formation of uniform corrosion.
Fig.1 Schematics of experimental setup (a) and tensile stress preloading device with specimen (b) (1—solution tank, 2—piston pump, 3—valve, 4—cooling equipment, 5—temperature/pressure monitor, 6—thermocouple, 7—pre-stress loading device, 8—pressure sensor)
Fig.2
Relationships between corrosion rate (D1~D4) of Ni-Cr-Mo-V high strength steel and pre-stress (σ) after 168 h immersion in simulated deep-sea environment (5 ℃, 3.5%NaCl) at different hydrostatic pressures
Fig.3 Relationships between measured (a) and calculated (b) corrosion rates of Ni-Cr-Mo-V high strength steel and hydrostatic pressure and pre-stress
Fig.4 Plot show corrosion rate of Ni-Cr-Mo-V high strength steel at different hydrostatic pressures and pre-stresses
Fig.5 Pitting morphologies of Ni-Cr-Mo-V high strength steel after 168 h immersion in simulated deep-sea environment (5 ℃, 3.5%NaCl) at different hydrostatic pressures and pre-stresses (The pre-stress in the horizontal direction of the plane) (a) macro-pits at 8 MPa and 30%σ0.2 (b) micro-pits at 8 MPa and 60%σ0.2 (c) macro-pits at 8 MPa and 90%σ0.2 (d) micro-pits at 12 MPa and 60%σ0.2 (e) macro-pits at 20 MPa and 90%σ0.2 (f) micro-pits at 28 MPa and 60%σ0.2 (g) morphology at 28 MPa and 90% σ0.2 (h) micro-pits at 28 MPa and 90%σ0.2
Fig.6 Pit sizes at different hydrostatic pressures and pre-stresses(a) 12 MPa and 10%σ0.2(b) 28 MPa and 10%σ0.2(c) 28 MPa and 60%σ0.2
Fig.7 Cumulative probabilities (PC) of pit sizes of Ni-Cr-Mo-V high strength steel at different hydrostatic pressures and pre-stresses(a) 12 MPa and 10%σ0.2(b) 28 MPa and 10%σ0.2(c) 28 MPa and 60%σ0.2
Fig.8 Stress distributions in and around pits at various hydrostatic pressures and pre-stresses (The pre-stress direction is along x axis)
(a) pit at 28 MPa (b) quarter pit at 28 MPa
(c) pit at 28 MPa and 60% s0.2 (d) quarter pit at 28 MPa and 60% s0.2
(e) pit at 28 MPa and 10% s0.2 (f) quarter pit at 28 MPa and 10% s0.2
(g) pit at 12 MPa and 10% s0.2 (h) quarter pit at 12 MPa and 10% s0.2
Fig.9 Stress distributions around adjacent pits aligned along the pre-stress direction (a) and vertical to pre-stress direction (b) (The pre-stress direction is along x axis)
Fig.10 Pitting geometry of Ni-Cr-Mo-V high strength steel at different hydrostatic pressures and pre-stresses (W—diameter of pit, h—depth of pit)
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