Corrosion Behavior of 316LN Welded Joints in High Temperature and High Pressure Water

Corrosion Behavior of 316LN Welded Joints in High Temperature and High Pressure Water

The primary circuit main pipeline, known as the “aorta” of nuclear power plants, is one of the seven key equipment in the nuclear island. It connects the reactor pressure vessel, steam generator and reactor coolant pump to form a closed loop. The main pipe consists of two loops, and there are 12 welded joints [1,2]. In the operation environment of nuclear power plant, these stainless steel welded joints will inevitably be corroded by high temperature and high pressure boron water in the primary circuit of nuclear power plant. The stability of the welded joints is related to the safety of the whole primary circuit. The analysis of pipeline rupture accident in nuclear power plant shows that pipelines with welds are more likely to occur accidents, which are mostly related to the corrosion resistance of welded joints [3,4,5].

Experimental method

Results and discussion

Conclusion

According to the literature, stainless steel is easy to form compact inner oxide film rich in Cr and loose outer oxide particles rich in Fe [6, 7, 8, 9, 10, 11, 12, 13] in high temperature and high pressure solution environment. The formation of this double-layer oxide film is closely related to the corrosion time. Many researchers believe that with the increase of corrosion time, the oxide film gradually changes from a single iron-rich layer to a double-layer structure of surface iron-rich layer and inner chromium-rich nickel [6,7,10]. At present, a large number of literatures have observed and studied the growth of oxidation film of stainless steel in high temperature and high pressure corrosion medium [8, 11, 12, 13], while the corrosion behavior of stainless steel welded joints in high temperature and high pressure environment is less studied. In addition, the difference of corrosion behavior between weld zone (WM) and base metal zone (BM) has not been reported. In order to ensure the safety of primary circuit main pipeline in nuclear power plant, it is necessary to study the corrosion behavior of stainless steel welded joints in high temperature and high pressure solution environment.
In this paper, the corrosion behavior of 316LN stainless steel welded joints in high temperature and high pressure boron-containing water is studied. The composition and structure of oxide film in different regions are observed and analyzed around the corrosion variation law and corresponding oxidation behavior, which can provide some reference and data for the safety service evaluation of stainless steel welded joints. It is supported.

1 Experimental method

1.1 Experimental Materials

In this paper, the experimental material is AP1000 Nuclear Power Plant Forging main pipe 316LN stainless steel, welding material is ER316L. The composition of base metal and welding material is shown in Table 1. The welding process adopts 25-26 V welding voltage, 80-100 A welding current, 24 kJ/cm heat input and 70-80 mm/min welding speed. The welding process is completed under the protection of argon gas. After welding, the samples were treated by post-weld heat treatment at 615 for 16 h.

Table 1 Chemical compositions of weld and base metal

Material	C	Cr	Ni	Mo	N	Mn	S	Si	Fe
316LN	0.020	17.6	10.8	2.02	0.11	1.16	0.004	0.41	Bal.
ER316L	0.017	18.6	10.7	2.74	0.05	1.30	0.007	0.42	Bal.

1.2 Microstructure Morphology Observation
Sampling was carried out on the cross section of the pipeline. Samples with size of 15 mm *15 mm *2 mm were intercepted from WM and BM respectively. All samples were grinded from 60, 120 The electrolyte is 10% (mass fraction) oxalic acid solution, the electrolytic voltage is 5V, and the electrolytic time is 30 s. Olympus metallographic microscope (OM) and Zeiss Auriga scanning electron microscopy (SEM) were used to observe and analyze the structure of the sample and the morphology of the oxide film. Rigaku DMAX-RB 12 kW rotating anode X-ray diffraction (XRD) and AXIS ULTRADLD X-ray photoelectron spectroscopy (XPS) were used to analyze the composition and structure of oxide films on WM and BM surfaces respectively.
1.3 Uniform Corrosion Test
Uniform corrosion test refers to JB/T 7901-2001 Standard of Uniform Corrosion Test Method in Metal Material Laboratory. The size of the corrosion specimen is 25 mm x 20 mm x 2 mm. The specimen is cut from the cross section of the welded joint. The specific sampling direction is shown in Figure 1. All the cut specimens were polished with SiC sandpaper, cleaned with deionized water and acetone, and dried for reserve. Before the beginning of the corrosion experiment, the uniform corrosion samples were weighed by an analytical balance with an accuracy of 0.01 mg. The equipment used in the high temperature and high pressure corrosion test is 5L static high temperature autoclave with the temperature set at 320 C and the corresponding saturated vapor pressure of 11 MPa. The experimental medium is simulated PWR primary loop working medium (aqueous solution), and the volume of solution is 2.6 L. Before the start of the experiment, the high temperature autoclave was deoxidized with 99.9999% nitrogen for 1 hour. The uniform corrosion test cycles were 50, 200, 360, 760, 1080 and 1440 h, respectively, with 3 parallel samples per cycle. After the completion of each cycle experiment, the samples were taken out of the autoclave, cleaned with acetone, alcohol and deionized water, and then dried and weighed. The weight of the samples under different corrosion cycles was recorded, and the corrosion weight gain and corrosion weight gain rate of the samples were obtained. The formula for calculating corrosion weight gain rate is as follows:

Fig.1 Cross-section overall appearance of 316LN stainless steel weld joint and sample cutting position

V= (W after−W before)/St
In the formula, V is the corrosion weight gain rate of the sample, ; W is the total mass of the sample before corrosion test, mg; W is the total mass of the sample after corrosion test, mg; S is the surface area of the sample, dm2; t is the experimental time, H.

2 Results and discussion

2.1 Microstructure Morphology Analysis
Fig. 2a and B are metallographic morphologies of 316LN welded joints BM and WM respectively. It can be seen from Fig. 2A that the structure of BM is equiaxed austenite grains with obvious twins. As can be seen from Fig. 2b, the structure of WM is austenite and vermicular or lath ferrite. Fig. 2C and D are local orientation difference diagrams of BM and WM of 316LN welded joints, respectively. It can be seen that WM has greater local orientation difference (residual strain) than BM.

Fig.2 Optical (a, b) and local orientation mismatch (c, d) images of base metal (a, c) and weld metal (b, d) in 316LN stainless steel weld joint, respectively

2.2 Corrosion Weight Increase and Corrosion Weight Increase Rate
Fig. 3a and B are uniform corrosion weight gain and corrosion weight gain rates of 316LN welded joints, respectively. It can be seen that in the early stage of the experiment (within 360 hours), the weight loss of the specimen is first and the corrosion weight gain rate is relatively high, which indicates that in the early stage of corrosion, the dissolution rate of metal in the welded joint area is higher than the formation rate of oxide film, which leads to the corrosion weight loss of the specimen; then, with the time prolonging, the corrosion weight loss decreases gradually until the end of corrosion. After 1080 h, the corrosion weight gain tends to be stable and the corrosion weight gain rate decreases slightly. The results show that with the increase of corrosion time, a dense and stable oxide film is formed on the surface of the welded joint specimen, which restrains the dissolution of metal and effectively protects the matrix material.

Fig.3 Mass gain (a) and mass gain rate (b) of 316LN stainles steel weld joint

2.3 Morphology and composition of oxide film
Fig. 4 shows the typical oxide film morphology on the sample surface at different corrosion time. It can be seen that in a short time of corrosion, fewer oxide particles were formed on the surface of welded joints, with an average size of about 200 nm. With the extension of corrosion time, on the one hand, the number of oxide particles gradually increased, and the oxide film became denser; on the other hand, the size of oxide particles gradually increased, among which, the size of oxide particles gradually increased. A few larger oxide particles (average size is 500 nm) appeared on the surface of the sample. These oxide particles have regular shape and straight edges.

Fig.4 SEM images of typical surface morphologies of the oxide films formed on the 316LN stainles steel weld joint after corrosion for 50 h (a), 200 h (b), 360 h (c), 760 h (d), 1080 h (e) and 1440 h (f)

In order to analyze the difference between WM and BM corrosion in 316LN welded joints, the surface oxide films of WM and BM after 360 and 1440 hours of corrosion were observed and analyzed respectively. The results are shown in Fig. 5. With the extension of corrosion time, the oxide particles on the surface of WM and BM distribute more uniformly and compactly, which can effectively slow down the dissolution rate of materials [9], thus ensuring that the welded joints have better uniform corrosion resistance.

Fig.5 Surface morphologies of the oxide films formed on base metal (a, c) and weld metal (b, d) after corrosion for 360 h (a, b) and 1440 h (b, d)

EDS analysis of oxide particles (points 1-4 in Fig. 5) at different corrosion time was carried out. The results are shown in Table 2. It can be seen that the oxide particles on the surface of the sample mainly consist of oxides of Fe and Cr. With the increase of the experimental period, the content of Cr and Ni in the oxide particles on the surface of the sample decreases relatively. Combining the results of Fig. 5 and Table 2 in this paper, it can be concluded that with the increase of corrosion time, more Fe-rich oxide particles are gradually formed on the surface of WM and BM oxide films, and there is no obvious difference in the composition and structure of the oxide films on the surface of WM and BM.

Table 2 Chemical compositions of oxide particles marked in Fig.5

Fig. 6 shows XRD spectra of oxide films on the surface of BM and WM of 316LN welded joints after 360 and 1440 hours of corrosion. It can be seen that the composition of oxide film on the surface of all samples is not different. Because the oxide film is thin and easy to be penetrated by X-ray, matrix diffraction peaks also appear in the spectrum. The oxide films on the surface of samples at different periods are mainly composed of FeOOH, FeCr2O4 and Fe3O4. At the same time, it can be seen that the FeOOH peaks appeared in both WM and BM after 360 hours of corrosion, indicating that at this time, ion dissolution will form more hydrates, so the corrosion sensitivity of welded joints is higher in short-term corrosion time. With the corrosion time prolonged to 1440 h, there was no obvious difference in the phase composition of the oxide film on the surface of WM and BM, mainly consisting of FeCr2O4 and Fe3O4.

Fig.6 XRD patterns of weld and base metals of 316LN stainless steel weld joint after corrosion for different time

2.4 Structure and phase composition of oxide film
Figures 7 and 8 are XPS spectra of oxide films on WM and BM surfaces after corrosion for 360 and 1440 h, respectively.

Fig.7 XPS fine spectra of Fe (a), Cr (b), Ni (c) and Mo (d) of the oxide films formed on weld and base metals of 316LN weld joint after corrosion for 360 h

It can be seen from the spectrum of Fe 2p 3/2 in Fig. 7a that the binding energy of Fe 2p 3/2 oxide film on the surface of samples is about 711 eV in different experimental periods. It can be seen from peak fitting that Fe exists mainly in the form of FeCr2O4 or Fe3O4 (binding energy 710.5 eV), FeOOH (binding energy 711.8 eV) and FeO (binding energy 709.5 eV). From the spectrum of Cr 2p 3/2 in Fig. 7b, it can be seen that the binding energy of the oxide film on the surface of the sample is about 577 eV. The main forms of Cr in the oxide film on the surface of the sample at different periods are Cr2O3 (binding energy 576 eV), Cr (OH) 3 (binding energy 577.4 eV) and CrO 3 (binding energy 579 eV) [8]. From the Ni 2p3/2 spectra of Fig. 7c, it can be seen that the binding energy of Ni 2p3/2 oxide film on the surface of the sample is about 856 eV, and Ni mainly exists in the form of Ni (OH) 2 (binding energy is 856 eV) in the oxide film on the surface of the sample at different periods [7,8]. From the Mo 3d5/2 spectrum of 7d, it can be seen that the binding energy of Mo 3d5/2 in the oxide film on the sample surface is about 233 eV. MoO 3 (binding energy 232.85 eV) mainly exists in the oxide film on the sample surface at different periods. In addition, it can be seen that there is no obvious difference in the composition of oxide film between WM and BM, which indicates that the corrosion mechanism of WM and BM is the same in high temperature and high pressure environment.
Figure 9 shows the XPS spectra of O in oxide films on WM and BM surfaces at different corrosion times. It can be seen that the binding energies of O 1s on the surface of the samples are all about 531 eV in different experimental periods. It can be seen from peak fitting that O exists mainly in the form of O 2 – (binding energy 531 eV) and OH – (binding energy 532.5 eV) in the oxide films on all samples. After 360 hours of corrosion, OH – is the main component of the oxide film on the surface of WM, which indicates that a large number of metal ions dissolve in WM and form a large number of hydrates in a short corrosion time. The above results show that there are more metal ions dissolved and deposited in WM, and WM exhibits higher corrosion sensitivity in a shorter corrosion time, which is closely related to the formation of larger residual strain (Fig. 2) and precipitates [2, 3, 4] in the welding process. At the same time, the existence of ferrite phase in WM (Fig. 2) leads to galvanic corrosion of WM and BM and galvanic corrosion of ferrite/austenite phase in WM [14], which further increases the corrosion sensitivity of WM. However, with the corrosion time prolonged to 1440 h, O in WM and BM was mainly O 2-. The XRD spectrum (Fig. 6) and EDS results (Table 2) show that there is no obvious difference between the oxide films on the surface of WM and BM after 1440 hours of corrosion. Both of them form stable oxide films, which make the 316LN welded joint area have better uniform corrosion resistance under high temperature and high pressure water environment.

Fig.8 XPS fine spectra of Fe (a), Cr (b), Ni (c) and Mo (d) of the oxide films formed on weld and base metals of 316LN weld joint after 1440 h immersion

Fig.9 XPS fine spectra of the oxide films formed on weld and base metals of 316LN weld joint after corrosion for 360 h (a) and 1440 h (b)

3 Conclusion

(1) In 316LN stainless steel welded joints, the weld zone has the same uniform corrosion resistance as the base metal zone.
(2) With the prolongation of corrosion time, corrosion weightlessness first occurs, and then decreases until a small weight gain occurs, and remains stable. At the initial stage of corrosion, a large number of ions dissolve to form hydrates in the weld zone, which shows higher corrosion sensitivity than that in the base metal zone.
(3) With the increase of corrosion time, the number and size of oxide particles increase gradually. The surface oxide film is mainly composed of FeOOH, FeCr2O4 and Fe3O4.

Source: China Welded Joints Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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