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Acta Metall Sin  2018, Vol. 54 Issue (4): 527-536    DOI: 10.11900/0412.1961.2017.00149
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Investigation of Corrosion Behavior of Welded Joint of X70 Pipeline Steel for Deep Sea
Ge MA, Xiurong ZUO(), Liang HONG, Yinglun JI, Junyuan DONG, Huihui WANG
Key Laboratory of Material Physics, Ministry of Education, Zhengzhou University, Zhengzhou 450052, China
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X70 pipeline steel with thick specifications (40.5 mm) for 3500 m deep sea reached the international advanced level in the wall thickness and service depth. Due to the high heat input during the welding process, the corrosion resistance of inside welding and outside welding would vary depending on the microstructure differences. The corrosion resistance of the welded joints of X70 pipeline for deep sea was studied by the immersion test, the weight loss test, the electrochemical test in this work. The components of the passive film were analyzed by XRD and the microstructure was observed by SEM. The results show that the corrosion resistance of the weld metal is the best. The corrosion resistance of the heat affected zone follows. The corrosion resistance of the base metal is the worst. And for the same area, the corrosion resistance of the inside welding is better than that of the outside welding. The formation of dense Fe3O4 passivation film can effectively slow down the progress of the reaction, and the corrosion products of Fe2O3, FeOOH and Fe(OH)3 which are loose in the outer layer, have no protective effect on the matrix. The microstructure of the weld metal with the best corrosion resistance is mostly the intragranular nucleation ferrite and martensite-austenite (M-A) constituent is fine and uniform. The microstructure gradient of the heat affected zone is the largest, the M-A constituent is coarse and the corrosion resistance is inferior to the weld metal. The base metal consists of ferrite and bainite, the bainite is island-like distribution and the corrosion resistance is the worst. Microstructure of the inside welding is more refined, owing to the influence of outside welding thermal cycle, and the volume fraction of M-A constituent in inside welding is higher than that of the outside welding, so the corrosion resistance is better than that of the outside welding.

Key words:  X70 pipeline      pitting      inclusion      microstructure      M-A constituent     
Received:  25 April 2017     
ZTFLH:  TG172.6  

Cite this article: 

Ge MA, Xiurong ZUO, Liang HONG, Yinglun JI, Junyuan DONG, Huihui WANG. Investigation of Corrosion Behavior of Welded Joint of X70 Pipeline Steel for Deep Sea. Acta Metall Sin, 2018, 54(4): 527-536.

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Fig.1  Diagram (a) and macroscopic morphology (b) of X70 pipeline steel (BM—base metal, HAZ—heat affected zone, WM—weld metal, WMin—inside weld metal, HAZin—inside heat affected zone, BMin—inside base metal, WMout—outside weld metal, HAZout—outside heat affected zone, BMout—outside base metal)
Position C Mn P S Si Ni+Cr+Cu+Mo Nb+Ti Fe
Base metal 0.044 1.56 0.009 0.009 0.25 0.475 0.069 Bal.
Weld metal 0.074 1.54 0.011 0.002 0.28 0.691 0.043 Bal.
Table 1  Chemical compositions of base metal and weld metal in X70 pipeline steel (mass fraction / %)
Fig.2  Macro morphologies of welded joints after immersion 2 h in 3.5%NaCl solution

(a) untreated (b) polished

Fig.3  OM images of welded joints after immersion 2 h in 3.5%NaCl solution (d—diameter)

(a) WMin (b) HAZin (c) BMin (d) WMout (e) HAZout (f) BMout

Fig.4  Statistical results of pitting corrosion of welded joints after immersion 2 h in 3.5%NaCl solution

(a) number of pits per unit area (b) proportion of pits in a certain diameter range

Fig.5  Results of weight loss test of welded joints in 3.5%NaCl solution

(a) average mass change curve (b) average corrosion rate curve

Fig.6  XRD spectra of passive film of welded joints after immersion 96 h in 3.5%NaCl solution
Fig.7  Open circuit potentials of each region of welded joints
Fig.8  Potentiodynamic polarization curves in each region of welded joints (E—potential, i—current density)

(a) outside welding (b) inside welding

Position Ecorr / mV icorr / (mAcm-2)
BMout -691 1.340×10-4
HAZout -689 2.608×10-4
WMout -680 6.934×10-5
BMin -648 6.644×10-4
HAZin -609 6.996×10-4
WMin -583 4.919×10-4
Table 2  Self corrosion potential (Ecorr) and self corrosion current density (icorr) in different regions of welded joints
Fig.9  Different morphologies (a, b) and corresponding EDS analyses (c, d) of oxide inclusions of Ti (a, c) and composite oxide inclusions (b, d) in welded joints after immersion 96 h in 3.5%NaCl solution
Fig.10  SEM images of welded joints in different regions (AF—acicular ferrite, LB—lath bainite, GB—granular bainite, PF—polygonal ferrite, M-A—martensite-austenite)

(a) WM (b) coarse grain heat affected zone (CGHAZ) (c) fine grain HAZ (FGHAZ) (d) intercritical HAZ (ICHAZ) (e) BM

Fig.11  OM images of M-A constituents in each area of the outside welding

(a) WM (b) CGHAZ (c) FGHAZ (d) IGHAZ (e) BM

Fig.12  M-A constituent size distributions of per unit area of welded joints

(a) outside welding (b) inside welding

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