Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution

316L austenitic stainless steel is a kind of ultra-low carbon stainless steel with carbon content less than 0.03%. Its material properties are equivalent to 00Cr17Ni14Mo2 steel in China [1,2]. 316L stainless steel, as the main material of PWR primary pipe and core components [3], has excellent mechanical properties and corrosion resistance, and is widely used in nuclear power field. 316L stainless steel flange is prone to stress corrosion cracking under the condition of stress and corrosive medium at the same time. This kind of corrosion is mainly caused by the destruction of the passivation film on the surface of 316L stainless steel flange, which leads to intergranular corrosion. Boric acid is also an important factor leading to this kind of corrosion in the operating environment of nuclear power equipment, which brings greater risk to the safe operation of nuclear power equipment [4,5] Therefore, it is of great value to master the corrosion behavior of 316L stainless steel flange in boric acid solution for improving the service life and safety of nuclear power equipment.


At present, the research on the corrosion mechanism of 316L stainless steel flange mainly focuses on its electrochemical corrosion or stress corrosion in different media. In terms of electrochemical corrosion behavior, Shi Yanhua et al. [6] studied the corrosion behavior of 316L stainless steel in different Cl concentration. The results showed that when Cl concentration was 3% (mass fraction), the pitting corrosion degree of 316L stainless steel flange was the most serious. Han Yajun et al. [7] studied the electrochemical corrosion behavior of 316L stainless steel in different conductivity seawater environment. The experimental results showed that the conductivity The stability of the passivation film of 316L stainless steel flange is affected by changing the activity of the corrosion ion in the corrosion medium by affecting the solution resistance. In the aspect of stress corrosion under different conditions, the influence of pH value on stress corrosion cracking of 316L stainless steel in high temperature and high pressure water environment was studied by means of slow strain rate tensile test. It was found that the corrosion cracking sensitivity of 316L stainless steel flange in acid and alkaline solution was higher, and the corrosion cracking sensitivity increased with the increase of acid and alkaline solution The results show that the crack growth rate of 316L stainless steel increases with the increase of dissolved oxygen content, and the change trend is very significant when the oxygen content is less than 0.2 mg / L, and the change trend becomes slow when the oxygen content is more than 0.2 mg / L. In addition, dikici et al. [10] studied the effect of post heat treatment on the corrosion resistance of 316L cold spray coating. The results showed that the hardness and porosity of cold spray 316L stainless steel flange coating decreased with the increase of heat treatment temperature (250500750 and 1000 ℃); tanhaei et al. [11] studied the different cold rolling levels (0-50%) The results show that with the increase of cold rolling percentage, the ratio of hardness to yield strength increases, that is, the work hardening energy decreases.
Up to now, most of the researches on the effect of external potential on stress corrosion of metal are focused on pipeline steel and alloy steel, but few on 316L stainless steel flange. Liu Zhiyong et al. [12] studied the stress corrosion behavior of X80 Pipeline Steel in Yingtan soil simulation solution by electrochemical potentiodynamic polarization, slow strain rate tensile (SSRT) experiment and SEM [12], and found that the SCC sensitivity of X80 pipeline steel increased with the decrease of external negative potential in acid soil environment. In this paper, the SCC sensitivity difference and corrosion mechanism of 316L stainless steel flange in different range of potentiodynamic polarization were studied through the slow strain rate tensile test of 316L stainless steel flange in the near neutral boric acid solution with different applied potential, which provided some reference for the corrosion mechanism and protection technology research of 316L stainless steel flange in nuclear power environment.

Experimental method

The experimental material is 316L austenitic stainless steel flange for nuclear power. Its chemical composition (mass fraction,%) is: C 0.003, Si 0.65, Mn 1.62, p0.03, s 0.03, Ni 12.00, Cr 17.63, Mo 2.32, Fe margin.
Before the experiment, 316L stainless steel flange was processed into 100 mm × 45 mm × 3 mm sample, which was used as the working electrode for electrochemical test. After being polished to 1000 μ w with abrasive paper, passivation was carried out in 25% (mass fraction) HNO3 solution. The passivation temperature was 45 ℃, and the passivation time was 30 min. After passivation, clean it with deionized water and anhydrous ethanol, and dry it in a desiccator for standby. The solution used in the experiment is boric acid with pH value of 6.9 ± 0.5, and the temperature of the solution is maintained at 25 ℃.
The three electrode system used in the electrochemical experiment and the slow stretching experiment (SSRT) is: 316L stainless steel flange as the working electrode, Ag / agcl3 electrode (SCE) as the reference electrode, Pt as the auxiliary electrode. All the potentials in this paper are relative to the SCE potential. During the experiment, nitrogen was introduced to remove O2.
The size of the electrochemical sample is 100 mm × 45 mm × 3 mm. The potentiodynamic polarization curve test was carried out on Shanghai Chenhua chi1660e electrochemical workstation. The scanning rate is 50, 10 and 0.5 MV / s. The electrolytic cell is a flat electrolytic cell. Before the experiment, grind the sample to 2000 × with sandpaper. It has been pointed out in literature that the SCC sensitivity of pipeline steel in a specific solution can be determined by using the potentiodynamic polarization curves with different scanning rates. Among them, fast scanning can characterize the electrochemical characteristics of crack tip, mainly because fast scanning shields the film forming process, making the electrode always exposed to the fresh metal surface, while slow scanning focuses on the electrochemical characteristics of non crack tip and metal surface.
SSRT test was carried out on the stress corrosion testing machine produced by Xi’an Lichuang Material Testing Technology Co., Ltd. with a strain rate of 3.0 × 10-6 mm / min. The sample is prepared in accordance with ASME B16.48 figure-8 blink 150# RF, with the size shown in Figure 1, thickness of 3 mm. Before the experiment, grind the sample to 1000 × with sandpaper step by step. The grinding direction is consistent with the stretching direction. Clean the sample with acetone and alcohol in the ultrasonic cleaner to remove the oil and impurities on the sample surface. On the basis of SSRT, the potentiostat was applied to 1200 MV (over passivation area), 300 MV (passivation area), 200 mV (passivation area) and – 600 MV (cathode area) respectively in the solution. 200 mV was used as the parallel experiment of passivation area to ensure the reliability of the experiment. After the experiment, su8010 field emission scanning electron microscope (SEM) was used to observe and analyze the tensile fracture morphology.
20200105081844 50319 - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Fig.1 Dimensional drawing of 316L stainless steel flange SSRT specimen

Experimental results

Electrochemical experiment

Figure 2 shows the potentiodynamic polarization curves of 316L stainless steel flange in neutral boric acid solution at different scanning rates (50, 10 and 0.5 MV / s).
20200105083039 53813 - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Fig.2 Dynamic potential polarization curves of 316L stainless steel flange at different scan rates
According to the polarization curve fitting, the corresponding self-corrosion potential from slow scanning to fast scanning is: – 0.338, – 0.378, – 0.282v, respectively. With the increase of scanning rate, the self-corrosion potential of 316L stainless steel flange increases. It can be seen from Figure 2 that the passivation range decreases with the increase of scanning rate, and there is a significant difference between fast scanning and slow scanning, and the difference becomes more obvious with the increase of scanning rate. When the potential is above the passivation region, the current density of the fast scanning decreases rapidly with the increase of the potential, while the slow scanning has an increasing trend, indicating that the corrosion rate of the crack tip occurs rapidly, while that of the non crack tip decreases. For the cathode polarization region, the self-corrosion current density of fast scanning is higher than that of slow scanning, indicating that the non crack tip has better corrosion resistance under the effect of cathodic protection.

Slow tensile test

The results of SSRT experiment are shown in Figure 3, and the data fitting results are shown in Table 1.
20200105083157 81467 - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Fig.3 Stress-strain curves of 316L stainless steel flange at differ-ent applied potentials
Table 1  Experimental results of 316L stainless steel flange under strain rate in 3.0×10-6 s-1 and 16.7 MPa

Condition

Absorbedenerge

Elongation

Reductionof
area /

Rupturetime
/ h

In air

437

70.6

77.5

58.5

200 mV

411

69.7

77.3

64.3

300 mV

417

67.0

76.8

58.1

EOCP

399

65.6

73.1

57.9

-600 mV

423

63.2

73.7

56.4

1200 mV

404

57.0

70.4

53.4

According to the potentiodynamic polarization curve of 316L stainless steel flange, it can be seen from Figure 3 that 316L stainless steel flange has certain SCC sensitivity in near neutral boric acid solution compared with the tensile test in air environment, and at the same time, the SCC sensitivity of 316L stainless steel flange under different applied potentials is different and meets certain laws. It can be seen from table 1 that the elongation, reduction of area and fracture time of the sample after stretching are reduced to a certain extent compared with those of the sample in the non potentiated solution under the applied potential of – 600 and 1200 MV, indicating that the corrosion of 316L stainless steel flange is promoted when the applied potential is in the cathode area and the over passivation area of the potentiodynamic polarization curve. According to table 1, the SCC sensitivity index of 316L stainless steel flange under the applied potential of 200 and 300 MV is slightly better than that under the non applied potential solution environment, indicating that the SCC sensitivity of 316L stainless steel flange under the applied passivation potential is relatively small.
Figure 4 shows the change rule of the elongation and fracture time of 316L stainless steel flange under different applied potentials. It can be seen from the figure that the elongation and fracture time of 316L are higher under the applied potential of 200mV. With the increase and decrease of the potential, the elongation and fracture time of 316L decrease to different degrees, indicating that the SCC sensitivity of the material is lower at this time. When the potential is 1200 MV, the elongation and fracture time of 316L are smaller, which indicates that the corrosion degree of 316L stainless steel flange is more serious and SCC sensitivity is greater.
20200105083826 28251 - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Fig.4 Elongation and fracture time of 316L stainless steel flange at different potentials

Fracture morphology analysis

The macroscopic fracture morphology of the specimen shows that the fracture of 316L stainless steel flange in air shows obvious necking phenomenon, while the macroscopic fracture in the solution without potential has slight necking phenomenon, while the macroscopic fracture in the external cathodic protection zone with potential of – 600 MV and the overpassivation zone with potential of 1200 MV does not show necking phenomenon.
Figure 5 shows the fracture morphology of 316L stainless steel flange after SSRT under different conditions. Fig. 5A is the fracture surface of the tensile specimen in air. The fracture surface is in the shape of dimple with uniform dimple size, which is a typical ductile fracture. Fig. 5B shows the fracture morphology when the external potential is located in the passivation zone (300 MV). It also shows the ductile fracture characteristics. The fracture morphology is close to that in the air, but there are some differences in the size, number and depth of dimples. Fig. 5C shows the fracture morphology when no potential is applied to the solution. At this time, the dimple is obviously deep, and the dimple size is different, which belongs to ductile fracture.
20200105084155 51549 - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Fig.5 SEM appearance of SSRT fracture in 316L stainless steel flange under different conditions: (a) in air, (b) 300 mV, (c) in solution, (d) -600 mV, (e) 1200 mV
The fracture surface of the external cathodic protection zone potential of – 600 MV and the overpassivation zone potential of 1200 MV tends to be flat as a whole (Fig. 5D, e). The fracture surface is obviously different from that in the air, showing quasi cleavage characteristics, with a small amount of micropores, showing brittle fracture characteristics. When the potential is in the over passivation zone, there are large cleavage zones, and there are a lot of deep holes in the cleavage zone, and there are corrosion products in the local holes. It can be concluded that the fracture characteristics of SSRT of 316L stainless steel flange tend to change from ductile cracking to brittle cracking when the cathodic potential and overpassivation potential are applied.
Fig. 6 shows the SEM appearance of side cracks after SSRT under different conditions. It can be seen from the figure that there is no microcrack on the surface of the sample when the potential is 300 MV. When the potential of – 600 MV and 1200 MV are applied, microcracks appear on the surface near the fracture, but at – 600 MV, the microcracks on the side show the characteristics of multiple cracks, while at 1200 MV, the number of microcracks is small and relatively straight, and the tensile direction of the crack surface and the specimen is roughly vertical. Combined with the fracture morphology, it can be inferred that what happens to 316L stainless steel flange in neutral boric acid solution is Stress corrosion transgranular fracture.
20200105084416 30625 - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Fig.6 Side of SEM appearance of SSRT fracture in 316L stainless steel flange under different conditions: (a) in air, (b) 300 mV, (c) in solution, (d) -600 mV, (e) 1200 mV

Analysis and discussion

The simulated solution used in this experiment is neutral boric acid solution. In this experiment, the main reactions between anode and cathode are as follows:
Cathode reaction:
20200105084708 65137 - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Anodic reaction:
20200105084814 93850 - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
When the cathode potential is applied, both the crack tip and the non crack tip are controlled by the cathode process. At this time, the crack tip and the non crack tip produce cathodic protection to a certain extent, which increases the tensile strength of 316L stainless steel flange (Fig. 3). Due to the presence of H in the system, H + is easier to pass through the corrosion product film to promote the hydrogen evolution reaction [15], which makes h diffuse to the crack tip to accelerate the crack growth, resulting in more serious hydrogen embrittlement of the material and higher SCC sensitivity (Table 1), that is, at this time, the SCC mechanism is hydrogen embrittlement mechanism. It has been pointed out that for SCC controlled by the anodic dissolution mechanism of continuous active channel composed of hydrogen embrittlement and strengthening phase, the strength rise caused by hydrogen embrittlement will increase the SCC sensitivity of steel [16]. The experimental results in Fig. 3 show that the tensile strength and SCC sensitivity of the material are relatively high when – 600 MV potential is applied, which is consistent with the research conclusion.
When anode potential is applied, both crack tip and non crack tip are controlled by anode process. At the same time, the reaction of Fe3 +, Cr3 + and solution on the metal surface is more sufficient. Xu Haisong found in his research on the re passivation behavior of 316L stainless steel flange and the stability of passivation film that the cations on the exposed metal surface constantly contact with the solution during the anodizing process, which is easy to form a stable double-layer passivation film structure [17]. Therefore, the corrosion resistance is good, the passivation range is relatively large, and the main components of the passivation film are Fe2O3 and Cr2O3. During the rapid scanning, the system is in a transient state, the crack tip is always exposed due to the potentiodynamic polarization, the reaction speed of metal ions is generally slow and the newly formed passivation film is unstable, which makes it difficult to form a complete and compact passivation film on the metal surface, so the self-corrosion potential is higher and the passivation range is smaller during the rapid scanning. Combined with the above analysis, the corrosion of the material is mainly caused by the anodic dissolution at the crack tip when the potential of 200 and 300 MV is applied. Under this condition, the fracture morphology is close to that in the air.
When the potential of passivation zone is applied, the difference of anode current between crack tip and non crack tip becomes larger, and the crack is more likely to produce and expand. As the reaction proceeds, the passivation film begins to crack seriously, which accelerates the corrosion. At this time, the main anodic reaction is the anodic dissolution of Fe. Therefore, the corrosion of the material is more serious when the potential of 1200 MV is applied, which shows higher SCC sensitivity. The SCC mechanism is anodic dissolution mechanism.

Conclusion

  • (1) The SCC sensitivity of 316L stainless steel flange in the near neutral boric acid solution environment was affected by the applied potential. The SCC sensitivity of 316L stainless steel flange was different under different polarization range potentials, among which, the SCC sensitivity was lower at 200 and 300 MV, and higher at the applied cathodic area potential and the applied overpassivation area potential.
  • (2) The fracture characteristics of SSRT of 316L stainless steel flange changed from ductile cracking to brittle cracking when the cathodic potential and overpassivation potential were applied.
  • (3) When the applied potential is – 600 MV, the SCC mechanism is mainly hydrogen embrittlement, which is more serious for the sample; when the applied potential is 1200 MV, the SCC mechanism is anodic dissolution, which is mainly the anodic dissolution of Fe.

Source: China Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.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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Reference:

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Summary
alloy 20 spectacle blind flange - Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Article Name
Effect of external potential on stress corrosion behavior of 316L stainless steel flange in boric acid solution
Description
The stress corrosion cracking (SCC) sensitivity and corrosion mechanism of 316L stainless steel flange in boric acid solution by different applied potentials were studied by means of slow strain rate tensile (SSRT) test, potentiodynamic polarization measurement and SEM morphology characterization.
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