Effect of microstructure composition on pitting initiation and propagation of 2002 duplex stainless steel
The 2002 duplex stainless steel samples with different proportion and element content were obtained by solution treatment. The effects of microstructure composition on the initiation and propagation law and mechanism of pitting corrosion were studied and discussed by potentiostatic polarization, potentiodynamic scanning and surface morphology analysis. The results show that with the increase of solution temperature, the main alloy elements tend to γ Gather together, α The phase content increases, but the corrosion resistance decreases, and pitting corrosion tends to be in weak phase α The overall pitting resistance of 2002 duplex stainless steel decreased due to phase initiation. When pitting occurs, it has the shape of lace, and the pitting pit under the lace cover has the characteristics of shallow and wide dish. The better the pitting resistance of 2002 duplex stainless steel, the easier the pitting pit expands along the width direction, and has little effect on the depth direction.
Duplex stainless steel has excellent mechanical properties and corrosion resistance, and has been widely used in many fields such as petroleum, chemical industry, natural gas and marine engineering [1-3]. Pitting corrosion is one of the main corrosion failure forms of duplex stainless steel in high corrosive environment. Duplex stainless steel is composed of ferrite and austenite. The proportion of the two phases and corrosion resistance will affect the initiation and development of pitting corrosion. Research  shows that with the increase of solution treatment temperature, the proportion of ferrite phase increases and the proportion of austenite phase decreases. The solution treatment temperature range with good corrosion resistance is 1050 ~ 1075 ℃. Han Dong  studied the effect of solution treatment on the two-phase corrosion resistance of 2304 duplex stainless steel. The results showed that when the solution temperature was below 1050 ℃, the overall pitting corrosion resistance of the sample was limited by weak phase austenite phase and above 1050 ℃, it was limited by weak phase ferrite phase. It can be seen that heat treatment affects the overall pitting resistance by changing the proportion and corrosion resistance of ferrite and austenite in duplex stainless steel.
Economical duplex stainless steel is one of the future development trends of duplex stainless steel . Similar to ordinary duplex stainless steel, heat treatment has a significant effect on the pitting corrosion resistance of economical duplex stainless steel. Zhang Lihua  studied the two-phase corrosion resistance of 2101 duplex stainless steel. It shows that the pitting corrosion resistance of the sample is limited by the weak ferrite phase in the solid solution temperature range of 1000 ~ 1300 ℃. Considering the large difference of two-phase ratio at 1000 ℃, 1050 ℃ is selected as the best solid solution treatment temperature. Guo et al.  studied the effects of different cooling rates on the microstructure evolution and pitting corrosion resistance of 2002 duplex stainless steel in the simulated heat affected zone. In the range of 800 ~ 1350 ℃, with the cooling rate decreasing from 100 ℃/s to 10 ℃/s, CPT and pitting potential increased, that is, pitting corrosion resistance increased.
At present, the research on the influence of constituent phases on pitting corrosion in economical duplex stainless steel mostly focuses on the overall pitting resistance after microstructure distribution, such as the variation law of pitting potential and critical pitting temperature, or the mechanism of PITTING INITIATION caused by the second phase such as inclusions and precipitation. However, there are few studies on the influence of composition on the law and mechanism of pitting initiation and propagation. Therefore, in this paper, 2002 duplex stainless steel samples containing different constituent phases with different proportions and element contents were obtained by solution treatment. The effects of microstructure composition on pitting initiation and propagation law and mechanism were studied and discussed by potentiostatic polarization, potentiodynamic scanning and surface morphology analysis, which is helpful to deeply understand the PITTING INITIATION and propagation mechanism of economical duplex stainless steel, Provide basic data support for the development and application of economical duplex stainless steel.
The experimental material is the 2002 duplex stainless steel hot rolled plate provided by Baowu group. Its chemical composition (mass fraction,%) is: C 0.019, Si 0.48, Mn 4.52, P 0.024, s 0.009, Cu 0.19, Cr 20.21, Mo 0.01, Ni 2.04 and Fe allowance. The three samples are the original hot rolled sample (1# sample) of 2002 duplex stainless steel and the water quenched sample after solution treatment at 1100 ℃ (2# sample) and 1200 ℃ (3# sample) for 30 min.
The sample is cut into ϕ 11.3 in the form of a disc, the copper conductor shall be welded on the non working surface of the sample, and cold inlaid and encapsulated with epoxy resin, leaving a working surface of 1 cm2. Grind with 180 ~ 2000# sandpaper and then 2.5 μ M diamond grinding paste is polished to the mirror surface, and then placed in alcohol solution for ultrasonic oscillation cleaning and cold air drying.
The electrochemical experiment was tested by cs310 electrochemical workstation, a three electrode system, in which 2002 duplex stainless steel was used as the working electrode, Pt sheet was used as the counter electrode, and saturated calomel electrode (SCE) was used as the reference electrode. In the experiment, the solution temperature was controlled at (30 ± 1) ℃ by water bath.
Pitting potential shall be measured according to GB/T 17899-1999. High purity N2 was introduced into 3.5% (mass fraction) NaCl solution for 30 min to remove dissolved oxygen, and then the working electrode was stabilized in NaCl solution for 30 min before anodic scanning. The polarization range was – 0.5 ~ + 1.5 V (unless otherwise specified, the potential in the paper was relative to SCE). The scanning rate was 0.33 MV/s. when the current density reached 100 μ The corresponding potential at a/cm2 is pitting potential epit.
In the potentiostatic polarization experiment, the working electrode was polarized at – 0.9 V for 3 min to remove the passive film on the electrode surface, and then stabilized in NaCl solution for 30 min. For the pitting density statistical experiment, the polarization parameter is set to polarization at 0.8 V for 15 s to produce pitting pits with small size and large number. For the observation experiment of pitting micro morphology, the polarization parameter is set to polarize at 1 V for 5 s to produce pitting pits with large size and small number.
The metallographic structure of 2002 duplex stainless steel sample after etching (30% NaOH solution, 3 V potentiostatic polarization for 3 s) was observed by using super depth of field three-dimensional microscope system (vhx-5000). The density and size of pitting of 2002 duplex stainless steel after potentiostatic polarization were statistically analyzed. The ratio of ferrite and austenite was statistically analyzed by using statistical software image pro Plus6.0. The content of alloying elements in ferrite and austenite of 2002 duplex stainless steel was analyzed by scanning electron microscope (Fei Nova 400 nano, SEM) and energy spectrum analysis attachment (EDS).
Metallographic structure observation and element distribution
Fig.1 shows the metallographic structure of the original hot rolled sample (1# sample) and water quenched sample of 2002 duplex stainless steel after solution treatment at 1100 ℃ (2# sample) and 1200 ℃ (3# sample) for 30 min respectively. 2002 duplex stainless steel is made of ferrite（ α Phase) and austenite（ γ Phase) two phase composition, with the increase of solid solution temperature, α The phase content increases gradually, γ The phase content decreases gradually.
Fig.1 Metallographic observation of 1# (a), 2# (b) and 3# (c) 2002 duplex stainless steel samples
Table 1 shows three 2002 duplex stainless steel samples α and γ Proportion of two phases and distribution of main elements. It can be seen that compared with the 1# original sample, the 2# and 3# samples after solution treatment, α The phase proportion increased from 53.3% to 57.4% and 68.4%, while γ The phase proportion decreased from 46.7% to 42.6% and 31.6%. At the same time, α The contents of Cr and Mo in the phase decreased, while γ The contents of Cr, Mo and N in the phase increased.
Table.1 three 2002 duplex stainless steel samples α and γ Proportion of two phases and distribution of main elements
Pitting potential test
Figure 2 shows the anodic polarization curves of three 2002 duplex stainless steel samples. It can be seen that the 1# sample has the highest pitting potential of 0.317 v. The pitting potential of 2# and 3# samples after solution treatment decreased to 0.283 and -0.028 v. From the local enlarged view of the polarization potential range of – 0.15 to 0.1 V (Fig.2b), it can be seen that the polarization current of 1# and 2# samples is relatively stable, and obvious metastable pitting current peaks can be observed. The polarization current of 1# sample is basically stable at 1 μ A/cm2, while 2# the polarization current of the sample increases slowly. Although the 3# sample also has the process of current rise and recovery, the current peak is not obvious because the polarization current rise is very significant.
Fig.2 Anodic polarization curves (a) and local enlarged drawings (b) of three 2002 duplex stainless steel samples
Pitting density statistics
Fig.3 shows the statistical results of pitting density of three 2002 duplex stainless steel samples after potentiostatic polarization. As you can see, in the 1# sample α and γ The pitting densities on the two phases were 12.33 and 8.87/mm2, respectively. After solution treatment, 2# and 3# samples α The pitting density on the phase increased to 14.68 and 19.21/mm2, while γ The pitting density on the phase decreased to 6.79 and 4.97/mm2.
Fig.3 Pits morphologies and densities for 2002 duplex stainless steel samples after potentiostatic polarization: pits morph-ologies for samples 1# (a), 2# (b) and 3# (c); pits density calculated from Fig.3a (d), 3b (e) and 3c (f)
Pitting size statistics
Figure 4 shows the statistical results of pitting size of three 2002 duplex stainless steel samples after potentiostatic polarization. It can be seen that the average diameter of 1# sample pitting pit is 46.52 μ m. The average diameter of pitting pits of 2# and 3# samples after solution treatment gradually decreased to 40.13 and 36.04 μ m. The average pitting depth of the three samples is 10 μ M, and the depth diameter ratios of pitting pits are 0.22, 0.24 and 0.28 respectively.
Fig.4 Morphologies and three-dimensional size of pits for 2002 duplex stainless steel samples after potentiostatic polariza-tion: pits morphologies for samples 1#(a), 2#(b) and 3#(c); enlargement of Fig.4a (d), 4b (e) and 4c (f); average three-dimensional size measured from Fig.4a (g), 4b (h) and 4c (i)
Effect of microstructure composition on pitting initiation of 2002 duplex stainless steel
According to the experimental results in Table 1 and Fig.3, the structure of 2002 duplex stainless steel is divided after solution treatment, resulting in two obvious changes: first, compared with the original 1# samples, 2# and 3# samples α The phase ratio increases, γ The phase ratio decreases; The second is α The content of Cr and Mo (mass fraction) in the phase decreases, and the content of N remains unchanged (at α Saturated in phase), γ The contents of Cr, Mo and N in the phase increased. The effect of element content change on the corrosion resistance of stainless steel can be measured by pitting equivalent (prEN), and its value can be calculated by the following formula :
Where WCR, WMO and wn are the mass fractions of Cr, Mo and N respectively. Calculated results in different samples α and γ The prEN value of the phase is shown in Figure 5. In 1# sample α and γ The prEN value of phase is only 0.55 (2.3%), which can be considered as approximately equal, and the proportion of two phases is close (1 ∶ 1.14); In 2# and 3# samples α The phase ratio increases, but its prEN value decreases, γ The phase ratio decreased, but its prEN value increased. The difference of prEN values between the two phases was 1.86 (7.8%) and 4.41 (18.9%) respectively. This shows that the solution treatment makes the alloy elements redistribute in the two phases, resulting in α The phase ratio increases, but the corrosion resistance decreases. This is also confirmed by the statistical results of pitting density of three 2002 duplex stainless steel samples after potentiostatic polarization (Fig.3). along with α With the increase of phase ratio, the pitting density increased from 12.33/mm2 to 14.68 and 19.21/mm2; At the same time, with γ With the decrease of phase ratio, the pitting density decreased from 8.87 to 6.79 and 4.97/mm2.
Fig.5 PREN values of two phases in three 2002 duplex stainless steel samples
It is due to the weak phase in 2002 duplex stainless steel after solution treatment（ α Phase) becomes weaker, making pitting easier α It sprouts in phase. α The increase of phase ratio decreases the pitting corrosion resistance of duplex stainless steel. Therefore, the pitting potential of the original 1# sample decreased from 0.317 V to 0.283 and -0.028 V of the solution treated 2# and 3# samples.
Effect of microstructure composition on pitting expansion of 2002 duplex stainless steel
After potentiostatic polarization, the characteristic corrosion morphology of lace cover appeared on the surface of 2002 duplex stainless steel (Fig.6). There have been many related studies on this phenomenon [10-14]. It is generally believed that after pitting initiation, the concentration C0 of metal ions dissolved in the pit decreases from the pit bottom to the pit mouth, and decreases sharply near the pit mouth. The metal ion concentration on the pit surface is lower than the minimum solubility c * (critical metal ion concentration) required for active dissolution. At this time, the pit wall is in a passive state. The metal ion concentration increases rapidly towards the depth of the pit, and FeCl2 precipitation is generated after reaching the saturated solubility CSAT (Fig.7a), which slows down the expansion speed of the pitting pit in the depth direction. Since the metal ion concentration forms a concentration gradient from the pit bottom to the pit mouth, connecting the places where the metal ion concentration in the solution reaches c * can obtain a c * line (Fig.7a). In the area higher than the c * line, the metal ion concentration C0 < C *, and the pit wall is in a passive state; In the area below the c * line, the metal ion concentration C * < C0 < CSAT; The pit wall is in active dissolution state. However, the formation of FeCl2 precipitation hinders the development of the pit to the depth. Only the side wall of the pit can dissolve rapidly, so the pit develops rapidly to the width. When the dissolution of the side wall of the pit causes a new pit near the original pit (Fig.7b), the high concentration metal ions diffuse outside the pit, the pit wall near the pit mouth is passivated, and the pit develops again in the width direction. This process continues to cycle, resulting in a wide and shallow disc-shaped pitting pit with lace cover on the surface (Fig.7C).
Fig.6 Surface morphologies of 1# (a) and 3# (b) 2002 duplex stainless steel samples after polarization
Fig.7 Schematic of lace cover formation: (a) precipitation formation, (b) lace covers formation, (c) lace covers development
According to the statistics of pitting size (Fig.4), after potentiostatic polarization, the average pitting depth of 1# original sample and 2# and 3# sample after solution treatment is not different, but the average diameter gradually decreases. Some studies have found that the expansion of pit depth direction is linear with the square root of time by establishing the model of time and pit depth change . The quantitative relationship can be expressed by the following formula:
Where XD is the depth of corrosion pit; T is the time; CSAT is the saturated concentration of metal ions in the solution; Csolid is the concentration of metal atoms (metal density divided by average molar mass); D0 is the diffusion coefficient of Fe2 + at 20 ℃; T0 is 293.15 (k); T is the solution temperature (k).
After solution treatment of 2002 duplex stainless steel, only α and γ The proportion of the two phases and the distribution of alloy elements in the two phases do not change the saturation concentration CSAT of metal ions and the concentration csolid of metal atoms in the solution. Therefore, it can be considered that each parameter in formula (2) has not changed for 1#, 2# and 3# samples. Therefore, after the same time of potentiostatic polarization, the pitting pit depth of the three samples is close to 10 μ M (Fig.4).
The development rate of pit width is related to the concentration distribution of metal ions in the pit. When the corrosion resistance of the sample is poor (3# sample), more metal ions will be dissolved in the pit under the same polarization conditions, making the c * line move to the pit mouth (Fig.8), resulting in new pits near the original pit after the pit expands in the width direction; For the samples with good corrosion resistance (1# samples), the c * line is closer to the pit bottom, and new pits tend to appear near the original pits; After several cycles of activation passivation process in the pit, 1# the average diameter of pitting pit is the largest, which is 46.52 μ M, and the 3# sample has the smallest average diameter of pitting pits, which is 36.04 μ M or so.
Fig.8 Schematic of horizontal expansion of pit in samples with different anti-corrosion property
From the above discussion, it can be seen that the development of pit depth is mainly related to CSAT, csolid, d0 and T, while the development of pit width depends on the distribution of metal ions in pit solution. Solution treatment does not change CSAT, csolid, d0 and T, but significantly changes the corrosion resistance of the sample, resulting in the change of metal ion concentration distribution in the solution. Therefore, the depth of pitting pits of the three samples is basically the same, and the development of pitting pits in the width direction slows down with the decline of corrosion resistance.
- (1) Solution treatment changed the composition of 2002 duplex stainless steel α Phase harmony γ The proportion of phase and the distribution of alloying elements in the two phases. As the solution temperature increases, α With the increase of phase content, the main alloying elements move to γ Aggregation in phase, resulting in α The pitting potential and pitting resistance of 2002 duplex stainless steel decreased with the decrease of phase prEN value.
- (2) As the solution temperature increases, α The pitting density increases, while γ The pitting density decreases on the phase, and the pitting tends to be in the weak phase α It sprouts in phase.
- (3) The pitting corrosion of 2002 duplex stainless steel shows the corrosion morphology of lace cover. The pitting pit under lace cover has the characteristics of shallow and wide dish. The better the corrosion resistance of the sample, the easier the pitting pit is to develop in the width direction, and has little effect on the development in the depth direction.
Authors: LEI Zheyuan, WANG Yicong, HU Qian, HUANG Feng, LIU Jing
Source: China Duplex Stainless Steel Flanges 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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