Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

304L austenitic stainless steel has been widely used in many fields such as aerospace, transportation, petrochemical industry and marine buildings due to its excellent mechanical properties, corrosion resistance and welding properties. Usually, the surface of austenitic stainless steel will form a dense passivation film in atmospheric environment, which will isolate the matrix from the environment, thus ensuring that the matrix has excellent corrosion resistance [3,4]. However, in harsher atmospheric environment (such as ocean atmosphere), wind carries sea water containing chloride suspended particles to the steel surface, chloride emits Cl-, Cl-adsorbed on the surface of austenitic stainless steel through tidal interpretation, which easily causes the rupture of passivation film on the local area of stainless steel surface and causes more serious local corrosion [5,6, 7].

  1. Microstructure of Austenitic Stainless Steel Welded Joints
  2. Corrosion resistance of austenitic stainless steel welded joints
  3. Chemical passivation of austenitic stainless steel welded joints
  4. XPS Analysis of Welded Joints

During the use of austenitic stainless steel pipe fittings, arc welding is often used to connect. The passive film on the surface of the welded joints is damaged, and the structure of the welded joints is changed due to the difference of heat input. Therefore, the surface rust of the welded joints is very easy to occur, which affects the aesthetics and safety of the tubes [8,9]. Therefore, it is particularly important to explore the corrosion causes of austenitic stainless steel welded joints in marine atmospheric environment, and to study the surface protection methods and processes of welded joints, which plays a very important role in the safe use and life evaluation of stainless steel in marine engineering. Traditional chromate, phosphate and 45%~55%(volume fraction) high concentration nitric acid are often used as passivating agents for stainless steel surface, which can promote the rapid film formation of austenitic stainless steel surface. However, the release of Cr6+ from chromate passivation solution into the environment is highly carcinogenic to human body and seriously endangers human health [10,11]. Waste water discharged after phosphate treatment will cause eutrophication of water body, which will affect the stability of water environment [12,13]. The amount of nitric acid used in high concentration nitric acid passivation solution is large and the waste is serious. With the enhancement of people’s awareness of environmental protection, new green and environmentally friendly surface treatment technology has gradually been favored by researchers [14]. Therefore, the study of new citric acid passivation treatment to improve the corrosion resistance of austenitic stainless steel welded joints has important engineering value.

The microstructures of 304L austenitic stainless steel welded joints by manual arc welding and argon arc welding were characterized by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The difference of corrosion resistance in different regions of welded joints was studied by electrochemical experiments and salt spray experiments. The welded joints were butted with citric acid passivator of different proportions. The head was passivated chemically, and the corrosion resistance of different regions of passivated welded joints was characterized by polarization curves. Finally, the composition of passivation film on welded joints treated with different passivation liquid ratios was analyzed by X-ray photoelectron spectroscopy (XPS), which provided guidance for environmental protection passivation of austenitic stainless steel welded joints.

1 Experimental method

The experimental welding base metal is 304L austenitic stainless steel, and the welding material is 316L austenitic stainless steel with better corrosion resistance. The main components of the two stainless steels are listed in Table 1. Among them, the C content of 316L stainless steel is low, about 0.009%, and high content of Mo, Ni and other corrosion resistant alloy elements are added. With 316L stainless steel as filler, manual arc welding (HD) and argon arc welding (HWS) are used for welding operation. It is required that the two steel plates welded together must be cut on the same plate as the previous base plate to ensure the consistency of the test. The welded 304L steel plate is 300 mm in length, 150 mm in width and 8 mm in thickness. The groove welding structure is adopted with four bead layers and E316L-16 electrode is used as welding material. The welding parameters are shown in Table 2 and the schematic diagram of the welding joint is shown in Figure 1.
The microstructures of 304L austenitic stainless steel welded joints by manual arc welding and argon arc welding were characterized by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The difference of corrosion resistance in different regions of welded joints was studied by electrochemical experiments and salt spray experiments. The welded joints were butted with citric acid passivator of different proportions. The head was passivated chemically, and the corrosion resistance of different regions of passivated welded joints was characterized by polarization curves. Finally, the composition of passivation film on welded joints treated with different passivation liquid ratios was analyzed by X-ray photoelectron spectroscopy (XPS), which provided guidance for environmental protection passivation of austenitic stainless steel welded joints.

Table 1  Chemical compositions of 304L and 316L stainless steels (mass fraction / %)

Steel C Cr Mo Ni Si Mn P S Fe
304L 0.023 18.6 8.1 0.58 1.67 0.026 0.005 Bal.
316L 0.009 19.25 2.48 12.50 0.36 1.57 0.018 0.005 Bal.

Table 2  Manual arc welding parameters of 304L stainless steel

Layer No. Weld method Welding material Size Current A Interpass temp. / ℃
1 HD E316L-16 Φ3.2 72 17
2 HD E316L-16 Φ3.2 103 67
3 HD E316L-16 Φ3.2 103 65
4 HD E316L-16 Φ3.2 100 80
347e2390 e57c 4a17 b38b 2a02c21e99c8 F001 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.1   Welded joint schematic of 304L stainless steel

Sampling along the welded joint, mechanical polishing, electrolytic erosion of 16 seconds with 20 V voltage in 80% (mass fraction) perchloric acid + 20% alcohol solution, and finally washing and drying with alcohol. The microstructure and morphology of welded joint matrix (BM), heat affected zone (HAZ) and weld zone (WM) were observed by means of EBSD (AZtecHKLH EBSD). The phase distribution of welded joint was analyzed by XRD (Smart Lab). Salt fog experiment was carried out in salt fog test box according to GB/T 10125-1997 neutral salt fog test standard. The experimental solution was 5% (mass fraction) NaCl and the experimental temperature was 35 C.
With green passivation solution, citric acid is 3% (mass fraction), hydrogen peroxide content is 10% and 20%, and anhydrous ethanol is twice as much as hydrogen peroxide. Two kinds of welded joints were passivated in 10% and 20% passivation solution for different time (15, 30 and 45 minutes respectively). Modulab XM electrochemical workstation is used for electrochemical test, which adopts traditional three-electrode system, saturated calomel electrode as reference electrode, Pt sheet as cathode and welding joint sample as working electrode; test solution is 3.5% (mass fraction) NaCl solution, room temperature; after 30 minutes of open circuit system stabilization, test polarization curve, scanning rate is 0.166. 7 mV/s. By means of XPS (PHI Quantera SXM), the composition of passivation film for welded joints passivated by different concentration of environmentally friendly passivating fluids was studied. XPS uses monochrome AlKalpha radiation source and hemispherical electronic analyzer with operating energy of 55 eV. Standard peaks (C1s, 285.0 eV) are used to correct all elemental peaks. Finally, the experimental results are fitted by XPS PEAK software.

2 Results and discussion

2.1 Microstructure of Austenitic Stainless Steel Welded Joints
Figure 2 shows the EBSD results of 304L stainless steel manual arc welding and argon arc welding. It can be seen that due to the large heat input, the grain growth is vertical to the weld and grows rapidly. The grain size in the weld interface area is columnar, and the grain size in the heat affected zone is larger than that in the manual welding heat affected zone [15]. The average grain size of HAZ in manual arc welding is 173.515 micron, and that of HAZ in argon arc welding is 234.042 micron. There is little difference in grain size between manual welding matrix and weld zone, and the grain size of manual arc welding is more uniform, and the overall grain size of manual arc welding is smaller than that of argon arc welding.

347e2390 e57c 4a17 b38b 2a02c21e99c8 F002 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.2   EBSD images of welded joints of 304L stainless steel after welding by 304L-HD (a) and 304L-HWS (b)

Figure 3 shows the XRD spectrum of welded joints. It can be seen that the weld zone of the two welding processes is composed of gamma-Fe phase and delta-Fe phase, and no harmful phase is formed. The peak value of the delta-Fe phase in the structure of TIG welding is more obvious because of the large heat input.

347e2390 e57c 4a17 b38b 2a02c21e99c8 F003 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.3   XRD spectra of welded joints of 304L stainless steel after two different welding processes

2.2 Corrosion resistance of austenitic stainless steel welded joints
Figure 4 shows the potentiodynamic polarization curve of 304L stainless steel welded joint in 3.5% NaCl solution. It can be seen that with the increase of electrode potential, the anode current density increases, which indicates that the anode activation dissolution reaction takes place on the surface of stainless steel welded joints. When the electrode potential reaches a certain value, the corresponding anode current density changes slightly. Each area on the surface of the welded joint begins to enter the passivation area, and the passivation process takes place [16,17]. During the passivation process, a dense passivation film was formed on the surface of the sample, and the current density of the anode did not change with the increase of the electrode potential.

347e2390 e57c 4a17 b38b 2a02c21e99c8 F004 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.4   Potentiodynamic polarization curves of welded joints of 304L stainless steel after welding by 304L-HD (a) and 304L-HWS (b)

From FIG. 4A (304L-HD), it can be seen that the weld metal is 316L welding material with high Cr and Ni, and its surface is easy to form passive film and is in passive state, so it can prevent further corrosion. The pitting corrosion potential in the weld zone is relatively high, and there is no obvious breakdown [18,19]; the base metal is less affected by heat and the grain size is small in the welding process. The pitting potential is higher than that in heat affected zone, and the corrosion resistance is relatively good. In the heat affected zone, grain growth or mixed crystal structure appears, which has lower corrosion potential, pitting potential and larger dimension passive current density, and the corrosion resistance is the worst. It can be concluded that the corrosion resistance of 304L-HD welded joint from strong to weak is as follows: weld > base metal > heat affected zone [20,21].
It can be seen from Fig. 4B that the heat input of 304L austenitic stainless steel after argon arc welding is larger and the cooling rate is slower, so the phase stability of the weld zone is poor, the mixed crystal structure appears in the weld zone and the grain size is relatively uneven, so the pitting potential of the weld zone is lower than that of manual arc welding; the columnar crystal group appears in the heat affected zone. Weaving, grain size is relatively large, corrosion resistance is poor.
Fig. 5 shows the corrosion morphology of manual arc welding and argon arc welding joints polished by steel wire brush and placed in salt spray box for 1 day. It can be seen that the corrosion of manual arc welding is slight, mainly in scratches, the corrosion of heat affected zone is more serious; after argon arc welding, the corrosion is more serious, the scratches of grinding appear obvious corrosion phenomenon, the corrosion of matrix and heat affected zone is more serious, and there are weak corrosion pits locally.

347e2390 e57c 4a17 b38b 2a02c21e99c8 F005 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.5   Corrosion morphologies of welded joints of 304L stainless steel after salt spray test for 1 d: (a) BM (HD), (b) WZ (HD), (c) HAZ (HD), (d) BM (HWS), (e) WZ (HWS), (f) HAZ (HWS)

2.3 Chemical passivation of austenitic stainless steel welded joints
Fig. 6, 7, 8 and 9 are the results of potentiodynamic polarization curves of passivated welded joints. It can be seen that the pitting corrosion potential of the welded joint is obviously increased under the passivation of citric acid as a whole. The corrosion resistance of stainless steel has been improved after citric acid passivation, because citric acid has stronger activity to iron than chromium, which makes the oxides of iron and iron dissolve preferentially. When passivation, the surface of stainless steel tends to be evenly balanced, which enriches Cr on the surface of stainless steel, thus improving the corrosion resistance of stainless steel.

347e2390 e57c 4a17 b38b 2a02c21e99c8 F006 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.6   Potentiodynamic polarization curves of the matrix (a), weld zone (b) and heat affected zone (c) of 304L stainless steel after manual arc welding and then chemical passivation

347e2390 e57c 4a17 b38b 2a02c21e99c8 F011 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.7   Pit potentials of the matrix (BM), weld zone (WZ) and heat affected zone (HAZ) of HD welded joint of 304L stainless steel after chemical passivation

347e2390 e57c 4a17 b38b 2a02c21e99c8 F007 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.8   Potentiodynamic polarization curves of the matrix (a), weld zone (b) and heat affected zone (c) of 304L stainless steel after HWS welding and then chemical passivation

347e2390 e57c 4a17 b38b 2a02c21e99c8 F010 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.9   Pit potentials of the matrix, weld zone and heat affected zone of HWS welded joint 304L stainless steel after chemical passivation

According to the theory of metal passivation, hydrogen peroxide can adjust the polarization potential of metal surface in the process of metal passivation, and control the polarization point of metal surface in the stable passivation region, so as to realize metal passivation. On the contrary, if the hydrogen peroxide content is too high, the potential will continue to move, and the polarization potential of the metal will cross the pitting corrosion potential. Then the metal surface will pass through the passivation state and enter the passivation state. At this time, the metal will generate high valence ions, which will lead to the destruction of the metal passivation film [22, 23, 24]. For 304L stainless steel welded joints, after passivation with 10% hydrogen peroxide, the pitting corrosion potential is the highest. A dense passivation film is formed on the surface of welded joints, which hinders the dissolution of metals and covers the electrode surface. The contact area between solution and metal is greatly reduced, and the corrosion rate is greatly reduced. Increasing the concentration of oxidant can reduce the concentration of metal ions in the solution near the metal surface, weaken the electrochemical polarization of the anode to a certain extent, and make the passivated metal re-active. Therefore, the passivation effect in 10% hydrogen peroxide water is relatively good.
In addition, it can be seen from figs. 6, 7, 8 and 9 that in 10% hydrogen peroxide passivation solution, pitting potential decreases with the increase of passivation time, and the passivation effect is the best in 15 minutes, and decreases gradually after more than 15 minutes. The quality of passivation film is closely related to the passivation time. If the passivation time is too short, the compactness of passivation film can not be guaranteed, but if the passivation film is too long, the passivation film will be destroyed. So when the concentration of hydrogen peroxide is 10% and the passivation time is 15 minutes, the pitting corrosion potential of the welded joint is the highest and the corrosion resistance is the best.
2.4 XPS Analysis of Welded Joints
In order to investigate the effect of hydrogen peroxide ratio in passivation solution on passivation film composition of welded joints, XPS was used to analyze the composition of passivation film on the surface of welded joints treated with passivation solution of different proportions. Figs 10 and 11 are the results of full and narrow scan spectra of Cr, Fe and O in the passive film passivated by 10% and 20% hydrogen peroxide respectively. Corresponding to the peak of bonding energies 576.88 and 579.18 eV, Cr2O 3 and Cr(OH)3, Cr2O3 and Cr(OH)3 can form stable oxide passivation films and improve the corrosion resistance of stainless steel. Corresponding to the peak of 574.11 eV, it should be the atomic form of Cr, which indicates that the surface of the sample is not completely oxidized to form a film. For the Fe2p peak, the corresponding binding energy is 716.55 eV, which should be FeOOH; the corresponding binding energy is 711.07 eV, which should be Fe2O3; and the corresponding binding energy is 706.98 eV, which should be the atomic form of Fe. The peak at 711.07 eV is the strongest. It can be seen that the main form of Fe in passive film is Fe2O3. Two fitting peaks were obtained from the analysis of O1s peak, of which the corresponding binding energy of 532.00 eV should be M-OH. The corresponding compounds are Cr(OH)3, CrOH and FeOH. The corresponding binding energy of 530.51 eV should be M-O, and the corresponding compounds are Cr2O3, Fe3O4 and Fe2O3. The peak of O at 532.00 eV is the strongest in the passive film on the sample surface, which indicates that the main form of O in the passive film is M-OH [25,26]. After passivation with 20% hydrogen peroxide, the peak value of M-OH decreases and the corrosion resistance is poor.

347e2390 e57c 4a17 b38b 2a02c21e99c8 F008 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.10   XPS full spectrum and fine spectra of Cr 2p3/2 (a), Fe 2p3/2 (b) and O 2p (c) for the surface film form-ed on 304L stainless steel after chemical passivati-on in the solution containing 10%H2O2

347e2390 e57c 4a17 b38b 2a02c21e99c8 F009 - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel

Fig.11   XPS full spectrum and fine spectra of Cr 2p3/2 (a), Fe 2p3/2 (b) and O 2p (c) for the surface film formed on 304L stainless steel after chemical passivation in the solution containing 20%H2O2

According to the full spectrum of XPS and the fine spectrum of Cr, Fe and O after passivation, the main passivation films on the surface of samples are CrOOH, Cr (OH) 3, Cr2O3, FeOH, Fe3O4 and Fe2O3, among which Fe is mainly in the form of Fe2O3. The passivation film composed of oxides and hydroxides of Fe and Cr effectively improves the corrosion resistance of the sample surface. In addition, there are a few atoms of Cr and Fe, which constitute the local defects of passive film.

3 Conclusion

  • (1) The grain size of manual arc welding joint is relatively small, the corrosion is slight in simulated marine atmosphere environment, and the corrosion resistance is better. For the same welding joint, the alloy element content is higher and the corrosion resistance is better when the weld material is 316L stainless steel. Because the grain size of HAZ increases due to heat input, the order of corrosion resistance is: weld > base metal > HAZ.
  • (2) When the content of citric acid is 3%, the content of hydrogen peroxide is 10%, and the passivation time is 15 minutes, the pitting corrosion potential of different regions of welded joint is higher, the corrosion resistance of passivation film is better, and the passivation effect is more obvious.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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[27] SUN Xiaoguang, HAN Xiaohui, ZHANG Xingshuang, ZHANG Zhiyi, LI Gangqing, DONG Chaofang. Corrosion Resistance and Environmentally-friendly Chemical Passivation of Welded Joints for Ultra-low Carbon Austenitic Stainless Steel. Journal of Chinese Society for Corrosion and Protection[J], 2019, 39(4): 345-352 doi:10.11902/1005.4537.2019.054

Summary
corrosion resistance and environmental protection chemical passivation of welded joints of ultra low carbon austenitic stainless steel - Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel
Article Name
Corrosion Resistance and Environmental Protection Chemical Passivation of Welded Joints of Ultra-Low Carbon Austenitic Stainless Steel
Description
The passive film on the surface of the welded joints is damaged, and the structure of the welded joints is changed due to the difference of heat input.
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