Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Abstract:

Effect of composition and size of inclusions on the pitting initiation of 2205 duplex stainless steel was studied by means of potentiostatic pulse technique, capillary microelectrode technique and scanning electron microscope. Results showed that the inclusions in 2205 duplex stainless steel were all oxides, and the pitting of duplex stainless steel is mainly triggered by inclusions. The oxide inclusion influences the initiation of pitting corrosion by affecting the pitting corrosion potential of the surrounding matrix, while pitting extends along the inclusions rather than the metal matrix. Not only the chemical composition but also the size of oxide inclusions will affect the pitting corrosion initiation. Pitting initiated preferentially on the inclusions of Al2O3-CaO-SiO2 and Al2O3-MgO-CaO-SiO2 but the inclusions of Al2O3-MgO, of the former the greater the size is the easier the pitting initiation is.

Key words2205 duplex stainless steel ; inclusion ; pitting corrosion ; potentiostatic pulse technique ; capillary microelectrode technique

Cite this article:
ZHANG YaohuaLIU JingHUANG FengHU QianLU SaiwenEffect of Composition and Size of Oxide Inclusions on Pitting Initiation of 2205 Duplex Stainless SteelCorrosion Science and Protection Technology[J], 2018, 30(2): 105-112 doi:10.11903/1002.6495.2018.011

Duplex stainless steel (DSS) has excellent corrosion resistance and high mechanical processing performance. Its physical properties are between austenitic stainless steel and ferritic stainless steel. It gradually replaces traditional Austenitic Stainless Steel to become petroleum, chemical, and Indispensable structural materials in important areas such as papermaking and marine exploration have made tremendous contributions to the development of human society [1,2,3,4]. In many corrosion types of duplex stainless steel, pitting corrosion is one of the most important forms of corrosion failure. It usually occurs at the interface between inclusions and the steel substrate and is affected by the nature of the inclusions [5, 6, 7, 8].
Ninety percent of the inclusions in the duplex stainless steel are oxide inclusions [9], and the composition of the oxide inclusions varies and varies in form. There is limited research on how the size, shape, composition, and distribution of these inclusions affect pitting nucleation and growth. In addition, there has been controversy over the effect of oxide inclusions on pit initiation. Szldarska-Smialowska et al. [10] believe that pitting will only initiate in the presence of sulfide inclusions or inclusions containing sulphides, but not in inclusions consisting solely of oxides. In contrast, Jeon et al [11,12,13] studied the effect of different elements on the pitting resistance of super duplex stainless steels and found that with the increase of oxygen content and copper content in the steel, oxide inclusions in the super duplex stainless steel The quantity and area fraction of the material greatly increased. The anti-pitting ability of the super duplex stainless steel gradually decreased, and oxide inclusions were the main sites for initiation of pitting corrosion. Many recent studies [14,15,16] have shown that there are significant differences in the ability of different oxide inclusions to initiate pitting corrosion due to the nature of oxide inclusions. This shows that the shape, size, distribution and composition of the oxide inclusions have an important influence on the initiation and development of pitting corrosion of duplex stainless steels. At present, there is no detailed report on the influence of the size and composition of oxide inclusions in duplex stainless steel on pit initiation.
In this paper, the 2205 duplex stainless steel widely used in marine engineering, using electrochemical testing methods and capillary microelectrode technology, combined with microscopic analysis technology, systematically analyzed the size and composition of inclusions before and after pitting initiation of 2205 duplex stainless steel. Intrinsic correlations of inclusion size and composition with pit initiation.
1 Experimental method
The experimental material is a 2205 duplex stainless steel whose chemical composition is (mass fraction, %): C 0.018, Cr 22.14, Si 0.66, Mn 1.47, P 0.04, S 0.004, Cu 0.46, Mo 3.09, Ni 5.31, N 0.17, Fe margin. Prior to the experiment, the studied materials were not heat treated. The diameter of the sample used in the experiment was 10 mm. During the electrochemical experiment, the surface of the sample was coated with silica gel, leaving only an area of 1 cm2 for electrochemical testing. All specimens were sanded with 2000 sandpaper and polished with a diamond paste to avoid surface roughness effects on pitting. It was then ultrasonically washed in ethanol and deoiled with acetone.
The potentiodynamic polarization curve measurement is based on the national standard GB/T 17899-1999. Nitrogen was deoxygenated for 30 min before the experiment and continued to deoxygenate during the experiment. During the experiment, the solution was kept at a constant temperature of 30 °C and the test solution was 3.5% (mass fraction) of NaCl. The sample was first polarized at -1.3 V for 3 min to remove the surface passivation film, and then the working electrode was stabilized under OCP for 10 min. The anode scan was performed at a scan rate of 0.33 mV/s until the current density reached 500 mA/cm2. The corresponding potential at the specified current density of 100 μA/cm2 was the pitting potential. The experiment was repeated three times to ensure accuracy. The final pitting potential of the 2205 duplex stainless steel was 1.05 V.
The pitting was initiated on a 2205 duplex stainless steel substrate using a constant potential pulse method [17]. The experimental solution was a 3.5% NaCl solution. The experimental temperature was 30 °C. Before the experiment, the solution was purged with nitrogen for 30 min and the oxygen was continuously removed during the experiment. The sample was subjected to potentiostatic polarization at −1.3 V for 3 min to remove the surface oxide film, and a constant potential pulse test was performed after the OCP was stabilized for 15 min. Since the pitting potential of the 2205 duplex stainless steel is 1.05 V, the high potential of the constant potential pulse is 1.3 V for 1 s and the low potential is 0.8 V for 3 s. 3000 and 5000 pulse experiments are performed respectively.
Capillary microelectrode technique was used to study the ability of pitting corrosion caused by inclusions of different compositions. Figure 1 shows the capillary electromicro-electrode technique experimental apparatus diagram made by the laboratory. Capillary microelectrode experiments The working electrode diameter is 50 to 100 μm to ensure that only one inclusion is included in a single working area. The potentiostatic polarization curve was measured using 3.5% NaCl. The electrode was first polarized at -1.3 V for 3 minutes, and then the potentiostatic scan was performed after the OCP was stabilized for 15 minutes. The potentiodynamic sweep sweeps from -1 V to 1.5 V to stop scanning at a sweep rate of 1 mV/s.
The macroscopic distribution of inclusions in duplex stainless steel was observed using an Axioplan2 type microscope. The morphology and composition of the inclusions in the duplex stainless steel were analyzed by backscatter diffraction (BSED, FEI Nova 400 Nano) and energy diffraction spectroscopy (EDS, INCA IE 350 PentaFET-3). Scanning electron microscopy (SEM, FEI Nova 400 Nano) was used to observe the pitting morphology of duplex stainless steel. Image-Pro-Plus 6.0 software was used for statistical analysis of inclusions in 2205 duplex stainless steel.

img 1 3 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.1 Device diagram of experimental capillary

2 Results and Discussion
2.1 Analysis of Inclusions in 2205 Duplex Stainless Steel
Figure 2 is a photomicrograph of the macroscopic distribution of inclusions in a 2205 duplex stainless steel. It can be seen that the inclusions are small in size and diffusely distributed.

img 2 3 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.2 OM image of the inclusions in 2205 duplex stainless steel

Figures 3 and 4 are BSED images of typical inclusions in 2205 duplex stainless steel and their EDS diagrams, respectively. There are three types of inclusions in 2205 duplex stainless steel (designated as A, B and C in the following). Type A is mainly composed of Al2O3-MgO, and its shape is a regular polygon; Type B is mainly composed of Al2O3-CaO-SiO2. Is ellipsoidal; C type is A type and B type compound inclusions, mainly composed of Al2O3-MgO-CaO-SiO2, spherical in shape. BSED was used to randomly select 50 fields of view of the 2205 duplex stainless steel sample for observation of inclusions and take statistics. The corresponding number, size, and type of inclusions are shown in Figure 5. Among the 328 inclusions, there are 136 inclusions less than 1 μm in size, accounting for 41.5% of the total; 170 inclusions between 1 and 4 μm, accounting for 51.8% of the total; more than 4 μm Inclusions were 22, accounting for only 6.7% of the total. A type of inclusions were 33, accounting for 10.1% of the total; B type inclusions were 49, accounting for 14.9% of the total; C type inclusions were 246, accounting for 75% of the total. Therefore, inclusions in the 2205 duplex stainless steel are all oxide inclusions, most of which are less than 4 μm inclusions, and mainly spherical Al2O3-MgO-CaO-SiO2 inclusions.

img 3 3 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.3 BSED images of the inclusions in 2205 duplex stainless steel: (a) tyle A; (b) type B; (c) type C

img 4 3 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.4 EDS spectras of inclusions in Fig.3a (a), Fig.3 b (b),Fig.3 c spectum 1 (c) and Fig.3 c spectum 2 (d)

img 5 3 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.5 Size (a) and type (b) distribution of inclusions in 2205 duplex stainless steel

2.2 Relationship between Inclusion Composition and Pitting Initiation
Figure 6 shows the surface morphology of 2205 duplex stainless steel after 5000 constant potential pulse experiments. It can be seen from Fig. 6 that pitting occurred in the 2205 duplex stainless steel after the constant potential pulse experiment. Inclusions were found in some pits, and no inclusions were found in some pits. The SEM-EDS was used to statistically analyze pitting pits on 2205 duplex stainless steel constant potential pulse specimens. About 68% of pitting pits were found to contain oxide inclusions, and about 32% of pitting pits were found to contain no oxide inclusions. The compositional elements indicate that pitting corrosion on the 2205 duplex stainless steel is mainly caused by oxide inclusions under the constant potential pulse method.
img 6 1 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.6 Corrosion macroscopic morphology of 2205 duplex stainless steel

In order to facilitate the study of the influence of inclusion components on the pitting corrosion of 2205 duplex stainless steel, the inclusions larger than 1 μm in size were selected for analysis. Fig. 7 shows the pitting appearance of different types of inclusions. Table 1 shows the chemical composition of each region in Fig. 7. As can be seen from Figure 7a, Type A inclusions did not cause pitting in duplex stainless steels; in Figure 7b, Type B inclusions partially dissolved; and Figure 7c shows that Type C inclusions selectively dissolved. From the results of Fig. 7c and Table 1, it can be seen that part I is intact and part II is dissolved. Part II occurs preferentially in the presence of chloride ions, a phenomenon also found in super duplex stainless steels [18]. Table 2 shows the results of statistical analysis of inclusion types in pitting pits by SEM-EDS. Table 2 shows that the probability of pitting corrosion caused by type A inclusions is 0. B-type inclusions and C-type inclusions cause points. The probability of eclipses is similar, which is about 60%. Based on this, it can be considered that Al2O3-MgO-CaO-SiO2 inclusions and Al2O3-CaO-SiO2 inclusions easily induce pitting, while Al2O3-MgO inclusions do not easily cause pitting, which may be related to the physical and chemical properties of the inclusions themselves. . Al2O3-MgO type inclusions have high hardness (HV2100-2400 kg/mm2) and are difficult to be deformed in the process of mechanical grinding, and do not produce loose and dense phenomenon. Al2O3-CaO-SiO2 type inclusions are often lower than Stainless steel substrate surface, its surface is loose, porous, easy to deform [9]. This is related to the fact that the voltaic potential at the Al2O3-CaO-SiO2 inclusions is lower than that of the stainless steel matrix [14], and the electrochemical activity is higher, and pitting corrosion is easily caused.

img 7 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.7 Corrosion morphologies of 2205 duplex stainless steel: (a) tyle A; (b) type B; (c) type C

20180511164155 71335 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Table 1 Element percentage of four positions in Fig.6 obtained by EDS (atomic fraction / %)

2.3 Relationship between Inclusion Size and Pitting Initiation
Figure 8 shows different sizes of inclusions in pits, including less than 1 μm inclusions (Figure 8a), 1–4 μm inclusions (Figure 8b), and more than 4 μm inclusions (Figure 8c). It shows that different sizes of inclusions can cause pitting of 2205 duplex stainless steel. In order to further study the influence of the size of oxide inclusions on pit initiation, the size of inclusions and the size of inclusions in pits were measured and statistically analyzed by using SEM-EDS before and after the constant potential pulse test of 2205 duplex stainless steel. The results are shown in the table. 3 shows. The results showed that there were 178 pits containing inclusions, of which 11 were less than 1 μm inclusions, accounting for 8.1% of the total inclusions less than 1 μm; 147 inclusions of 1~4 μm, accounting for 1-4 86.5% of μm inclusions; 20 inclusions larger than 4 μm, accounting for 90.9% of total inclusions larger than 4 μm. It can be seen that inclusions larger than 4 μm are more likely to cause pitting corrosion, followed by 1 to 4 μm inclusions, and less than 1 μm inclusions are not likely to cause pitting, indicating that the larger the size of inclusions, the more likely to cause pitting corrosion. This is because the large-sized inclusions have a large contact area with the metal substrate, and pitting corrosion generally occurs at the interface between the inclusions and the metal substrate, and thus the surface area in the activated state is also large, and pitting corrosion is more likely [19, 20].
img 8 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.8 Corrosion morphologies of 2205 duplex stainless steel: (a) <1 μm inclusions cause pitting, (b) 1 μm-4 μm inclusions cause pitting, (c) >4 μm inclusions cause pitting

2.4 Analysis of initiation mechanism of pitting corrosion of 2205 duplex stainless steel caused by oxide inclusions
In order to further clarify the mechanism of the influence of inclusion chemical constituents on pit initiation, the 2205 duplex stainless steel substrate containing different types of inclusions was subjected to micro-zone potentiodynamic polarization, and the shape of inclusions before and after potentional polarization was observed and analyzed. Appearance and chemical composition. Figure 9 shows the potentiodynamic polarization curves of duplex stainless steels with different compositional inclusions for capillary microelectrode testing. The pitting potential of duplex stainless steels varied from 0.943 V to 1.162 V. Figure 10 shows the composition analysis of the inclusions and their topography before and after the potentiometric scanning. It can be seen from Fig. 9 that the pitting potential of the 2205 duplex stainless steel containing MgO-Al2O3 inclusions is higher than that of the inclusion 2205 duplex stainless steel base, and according to Fig. 10g, the MgO-Al2O3 inclusions are after the electrochemical test. The dissolution did not occur and the 2205 duplex stainless steel matrix dissolved, indicating that MgO-Al2O3 inclusions are less likely to cause pitting than the matrix. The pitting potentials of the 2205 duplex stainless steel substrate containing Al2O3-SiO2-CaO inclusions and the 2205 duplex stainless steel inclusions containing MgO-Al2O3-SiO2-CaO inclusions are not much different, and they are all smaller than the 2205 duplex phases containing no inclusions. The pitting potential of the stainless steel substrate. Comparing Fig. 10h and i, it can be seen that the Al2O3-SiO2-CaO and MgO-Al2O3-SiO2-CaO inclusions partially dissolved during the anodic polarization and caused pitting corrosion. This is consistent with the conclusion that Ha et al. [20] found that different compositional inclusions cause different pitting corrosion of ferritic stainless steels through capillary technology.

20180511164248 30710 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Table 2 Statistics of three type inclusions cause 2205 duplex stainless steel pitting corrosion

20180511164323 18878 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Table 3 Statistics of three type size inclusions cause 2205 duplex stainless steel pitting corrosion

img 9 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.9 Polarization curves of 2205 duplex stainless steel matrix containing different inclusions by capillary test

Figures 11a and b are 3000 and 5000 inclusions SEM images of 2205 duplex stainless steel circulating in a constant potential pulse test. From Fig. 11a, it can be seen that in 3000 constant potential pulse experiments, pitting initiated at the interface between the inclusion and the metal matrix, consistent with the results of Szummer et al. [21]. This is because the local impedance at the interface between the inclusions and the metal matrix is significantly reduced, the electrochemical activity is high, and pitting corrosion is prone to occur [9]. From Fig. 11b, it can be seen that with the increase of the number of constant potential pulses, pitting initiated at the interface between the inclusion and the metal substrate expands outward. In combination with Fig. 7 and Table 2, the Al2O3-CaO-SiO2 inclusions preferentially dissolved to form pitting pits during 5000 constant potential pulse experiments, demonstrating that pitting corrosion propagates along inclusions rather than metal matrix.

img 10 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.10 Inclusions of different components before and after the capillary experiment: (a, d, g) type A inclusion; (b, e, h) type B inclusion; (c, f, i) type C inclusion

img 11 - Effect of Composition and Size of Oxide Inclusions in 2205 Duplex Stainless Steel on Pitting Initiation

Fig.11 SEM images of the inclusions in the alloys after the potentiostatic pulse technique measurement:(a) 3000 cycles pulse experiment, (b) 5000 cycles pulse experiment

3 Conclusion
(1) The inclusions in the 2205 duplex stainless steel are mixed oxide inclusions and are classified into three types according to their morphology and composition: regular polygonal Al2O3-MgO, ellipsoidal Al2O3-CaO-SiO2, spherical Al2O3-MgO-CaO -SiO2. The size of inclusions is small. Among them, 41.5% are inclusions less than 1 μm in size, 51.8% are contained in 1-4 μm, and only 6.7% are larger than 4 μm.
(2) Oxide inclusion in 2205 duplex stainless steel is the main cause of pitting corrosion. Al2O3-CaO-SiO2 type and Al2O3-MgO-CaO-SiO2 type inclusions can easily cause pitting in duplex stainless steels. Al2O3-MgO type Inclusions are less likely to cause pitting, and the larger the size, the easier it is to cause pitting.
(3) Pitting initiates at the interface between the inclusion and the metal matrix and preferentially propagates along the inclusion. The preferential dissolution of Al2O3-CaO-SiO2 and Al2O3-MgO-CaO-SiO2 inclusions is the main cause of pitting corrosion.

The authors have declared that no competing interests exist.

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(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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