1 State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China 2 Shandong Provincial Key Lab of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China 3 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
Cite this article:
Suqiang ZHANG,Hongyun ZHAO,Fengyuan SHU,Guodong WANG,Wenxiong HE. Effect of Welding Thermal Cycle on Corrosion Behavior of Q315NS Steel in H2SO4 Solution. Acta Metall Sin, 2017, 53(7): 808-816.
As the main corrosion form of coal- or heavy oil-fired boilers, dew point corrosion occurs when corrosive gases (SO3, HCl, NO2, et al) are cooled and converted to condensed acids. The condensed acids (H2SO4, HCl and HNO3) are much corrosive to steel, causing corrosion damage to plant materials. The service temperature is designed lower and lower to improve energy efficiency recently, which makes dew point corrosion more and more serious. Q315NS steel produced by appropriate alloy design is much suitable for those parts vulnerable to dew point corrosion in power and petrochemical industry due to its excellent corrosion resistance in H2SO4 solution. As an efficient and low-cost process, welding is an essential process in the utilization of Q315NS. The corrosion mechanism of the heat affected zone is much complex due to the presence of microstructure gradients, which is largely determined by the welding thermal cycle. However, there is little research elucidating the effect of welding thermal cycle on corrosion behavior of Q315NS steel in H2SO4 solution. In this work, the microstructure evolution and corrosion behaviour in the 50%H2SO4 (mass fraction) solution of welding heat affected zones of Q315NS was investigated by comparison with base metal using welding thermal simulation technique, scanning electron microscope and electrochemical measurements. The results show that the microstructures of ferrite and pearlite are observed in base metal, fine-grained region and incomplete recrystallization region, while coarse-grained region consists of granular bainite. All the equivalent circuits of Q315NS with or without welding thermal cycle contain a resistor of corrosion product and a capacitor of electric double layer, and all specimens have passivation behavior. The base metal and the incomplete recrystallization region have the lowest corrosion current density and the largest charge-transfer resistance, which means the best corrosion resistance, while the coarse-grained region has the highest corrosion current density and the least charge-transfer resistance. Rod-like shaped corrosion product was formed by deposition on the surface of the coarse-grained region specimen while a porous-structured corrosion product was formed on the surface of other specimens.
Fig.1 SEM images of BM and HAZ of Q315NS steel (BM—base metal, HAZ—heat affected zone, CGHAZ—coarse-grained heat affected zone, FGHAZ—fine-grained heat affected zone, ICHAZ—incomplete recrystallization heat affected zone, GB—granular bainite, F—ferrite, P—pearlite)
(a) CGHAZ (b) FGHAZ (c) ICHAZ (d) BM
Fig.2 The microhardnesses and grain sizes of BM and HAZ
Fig.3 Tafel polarization curves for specimens immersed in 50% H2SO4 solution
Specimen
Ecorr
icorr
-βc
βa
Epp
Eb
ip
Corrosion rate
mV
μAcm-2
mV
mV
mV
mV
μAcm-2
mma-1
CGHAZ
-321.3
316.9
98.7
23.6
-159
403
8.71
7.390
FGHAZ
-333.9
148.4
91.7
25.3
-165
409
6.87
3.927
ICHAZ
-358.8
124.3
89.1
22.4
-162
420
6.60
3.419
BM
-371.9
121.2
91.6
22.2
-184
425
5.19
3.313
Table 1 Electrochemical parameters of Tafel polarization curves for specimens immersed in 50%H2SO4 solution
Fig.4 Nyquist plots for specimens immersed in 50%H2SO4 solution
Fig.5 Equivalent circuit of EIS plots (Rsol—solution/electrolyte resistance, Cdl—capacitance of the double electrode layer, Rct—charge transfer resistance)
Specimen
Rct / (Ωcm2)
Cdl / (μFcm-2)
CGHAZ
29.314
0.438
FGHAZ
61.228
0.554
ICHAZ
75.379
0.421
BM
76.421
0.579
Table 2 Electrochemical parameters of EIS for the specimens immersed in 50%H2SO4 solution
Fig.6 SEM images of corrosion products formed on the surface of electrodes immersed in 50%H2SO4 solution for 72 h
(a) BM (b) CGHAZ (c) FGHAZ (d) ICHAZ
Point
O
Si
S
Sb
Mn
Fe
Cu
I
28.72
10.48
14.92
1.30
-
39.09
5.50
II
25.74
5.80
15.10
1.28
0.67
43.70
7.72
Table 3 EDS analyses of the points I and II of the corrosion products formed on the surface in Fig.6(mass fraction / %)
Fig.7 SEM images of corrosion products formed on the surface of BM in 50%H2SO4 solution after different corrosion times of 1 h (a), 4 h (b), 12 h (c) and 72 h (d)
Fig.8 SEM images of corrosion products formed on the surface of CGHAZ in 50%H2SO4 solution after different corrosion times of 1 h (a), 4 h (b), 12 h (c) and 72 h (d)
Fig.9 Schematic of the corrosion mechanism of BM (a) and CGHAZ (b) in 50%H2SO4 solution (M/A—martensite-austenite)
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