Cyclic Polarization Study of Thick Welded Joints of Lean Duplex Stainless Steel for Application in Biodiesel Industry

Thick welded joints of UNS S32304 lean duplex stainless steel (LDSS) has been studied in acidified glycerin, a byproduct of biodiesel plants, using cyclic polarization technique. LDSS S32304 thick plates were welded by different processes commonly employed in the fabrication process: shielded metal arc welding, gas metal arc welding and flux cored arc welding. The electrochemical behavior of different weldment zones was characterized. Transmission electron microscopy and scanning electron microscopy (SEM) were used for sample characterization. SEM images showed that the heat affected zone was the most critical zone for pitting corrosion for all samples. The results were associated to the microstructural features of the materials. Generally, the breakdown potential of the top regions of welds was higher in acidified glycerin than the weld root for welds fabricated with two different filler metals. Basically, all the welded samples exhibited similar or higher corrosion resistance, compared with the corresponding base metal.


Introduction
Duplex stainless steels (DSSs) are materials that exhibit a combination of high yield strength, toughness and localized corrosion resistance, and they are being widely used in chemical, petrochemical, marine, nuclear and paper industry 1 .The best properties are obtained with approximately equal amount of austenite (γ) and ferrite (α) phases 2 .
Volatility of the Ni price has led to development of low alloy duplex steels 3 .The nickel content is partly replaced by manganese and nitrogen and also the molybdenum content is reduced in the so-called lean DSS (LDSS) 4 .
The localized corrosion of stainless steels is related to alloying elements as Cr, Mo and N, whereas Ni mainly acts to stabilize the austenite phase [13][14][15] .A great number of investigations have studied the effect of alloying elements on corrosion resistance of stainless steels, especially N, which is most effective 6,8,13,16,17 .According to Hsieh et al. 18 , a higher content of nitrogen and nickel favors the reformation of austenite.However, the effect of the nitrogen on the reformation of austenite is much greater than that of nickel.Hsieh et al. 18 also showed that the effect of manganese on the formation of austenite can be negligible.Bhatt et al. 19 found that the presence of 5 and 10% nitrogen in the shielding gas, decreases the ferrite contents to 35 and 29%, respectively, resulting in an improved pitting corrosion resistance of the weldments obtained by gas tungsten arc.According to Mesquita et al. 15 the general corrosion resistance of Mo-containing DSS grades is increased.Mo has a stronger beneficial effect on the corrosion resistance of ferrite compared to austenite.
The pitting corrosion resistance of 2304 DSS welded by autogenous plasma-arc was evaluated by Tan et al. 20 .In this study, the as-welded samples showed impaired corrosion resistance and pitting preferentially occurred at ferrite grains in the heat affected zone (HAZ) near the fusion line.In another study in welded joints of 2304 DSS, Chen et al. 8 found pits occurring in the ferrite phase for all simulated HAZ specimens and the corrosion resistance of welded joints is influenced by Cr 2 N and inclusions.According to their results, the corrosion resistance of the HAZ in 2304 DSS decreased with the cooling rate.
Based on that, this study aims to evaluate the corrosion resistance of UNS S32304 LDSS thick plates welded by three different welding processes in acidified glycerin, a byproduct of biodiesel industry.

Materials
Three different welding processes were used: shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and flux cored arc welding (FCAW).For each welding process, two filler metals with different chemical compositions, (23%Cr 7%Ni) and (22%Cr 9%Ni 3%Mo) were evaluated.The shielding gas for GMAW process was a ternary gas of 95%Ar, 3%CO 2 and 2%N 2 , while for FCAW process a shielding gas with 75% Ar and 25%CO 2 was used.The same shielding gas flow rate of 18 L.min -1 was utilized for all welding processes.
The electrochemical behavior of different weldment zones (fusion zone (FZ), base metal (BM) and heat affected zone (HAZ)) was characterized in the top and root of the specimens.The chemical composition of the LDSS is shown in Table 1.
This work is a part of a global project to study the weldability of the UNS S32304 alloys 21 .The filler metals compositions, the welding parameters and the sample preparation for electrochemical tests were described by the authors in a previous work 22 .

Microstructural characterization
Microstructural examination of specimens was conducted using an optical microscope (Olympus PME 3, LECO).After polishing, the specimens were then electrolytically etched in 10% oxalic acid electrolyte at 6 V for 60 s.A Tecnai G2-20 -SuperTwin FEI -200kV transmission electron microscope (TEM) was used for identification of the precipitates in LDSS.After the polarization tests, microstructural observations were carried out using scanning electron microscopy (SEM, Quanta 200F, FEI).

Electrochemical measurements
The tests were performed at a temperature of 65±1°C and under constant stirring to simulate the operational conditions in the pipes of biodiesel plants.A conventional three-electrode cell was used, the working electrodes being prepared from the LDSS welded samples with an exposed surface of approximately 1 cm 2 .The reference was a saturated calomel electrode (SCE) and a Pt mesh was used as the counter electrode.
Cyclic potentiodynamic polarization tests were performed in an industrial acidified glycerin provided by a biodiesel plant with 49.5 wt.% glycerin, 29 wt.% methanol, 17 wt.%water, 3 wt.%NaCl, and 1.5 wt.% organic materials, pH=6.2 and conductivity of 4.9 mS/cm.An Ivium compactstat potentiostat was used to measure the open circuit potential (OCP) for 1 h and the cyclic potentiodynamic polarization curves were then measured at a scan rate 0.167 mV/s from the open circuit potential (OCP) until the current density reached 5 mA.cm -2 at which point the scanning direction was then reversed.Three replicate tests of each measurement were performed.

Microstructure
Similarly to previous investigations 22 , the fusion zone (FZ) structure consists of austenite grains in the form of Widmanstätten plate precipitates and secondary austenite (γ 2 ) within a matrix of ferrite, and also, FZ region showed higher proportions of austenite than HAZ.The presence of secondary austenite (γ 2 ) can be verified in FZ, as detailed in Figure 1.
It is noteworthy that this type of austenite formed in the temperature range 800-1000°C and it is derived by dissolving Cr 2 N precipitates, which provides nitrogen to the formation of γ 2 23 , and its nucleation originates in inclusions and dislocations 23 .The HAZ exhibited chromium nitrides mainly in the interior of the ferrite grains.Nitride precipitation is more likely to occur in the ferrite phase because the solubility of nitrogen in ferrite drops rapidly with the decrease in temperature.
Figure 2 shows the weld profile of the sample (BM, HAZ and FZ) and the nitride colonies within the ferrite grains in the HAZ of UNS S32304.
Transmission electron microscopy indicated the presence of precipitates in the welded specimens as shown in Figure 3.Comparison with the results of other studies indicates that the precipitates are Cr 2 N 24 .
Figure 3a and 3b shows the results of the TEM analysis of the Cr 2 N formed in the HAZ of as-welded sample.Figure 3a indicates that all precipitates observed in the HAZ are rod-like Cr 2 N precipitated intragranularly.In addition to the intragranular rod-like nitrides, some Cr 2 N particles are also observed along the ferrite/austenite interfaces (Figure 3b).

Electrochemical behavior
Figure 4(a-c) shows typical cyclic potentiodynamic polarization curves obtained for samples welded using SMAW, GMAW and FCAW processes, respectively.The OCP values and breakdown potentials (E b ) from polarization tests are summarized in Figure 5.
The welded joints exhibited higher values of corrosion potential than the lean duplex steel in the as received condition, except for the root region of joint welded using SMAW process and 2307 consumable, which showed the lowest corrosion and breakdown potentials among the studied samples, which can be related with the lowest heat input used in the welding process in the case of SMAW 2307 root region, as showed in a previous work 22 .The highest breakdown potential was identified for the top region of the lean duplex joint welded using SMAW process and 2209 consumable, which can be related with the highest heat input used for this sample.This difference in the heat input was described by Cardoso Junior et al. 21as being an experimental deviation occurred during the manual SMAW welding process.In general, top regions showed higher values of E b than root samples, due to higher filler metal dilution for root regions.The filler metals used (2307 and 2209) have higher Ni content than the base metal and presence of Mo in the case of 2209.Nickel stabilizes austenite, while molybdenum stabilizes ferrite and contributes to increase PREN of ferrite.Tan et al. 25 reported that the pitting corrosion resistance of a super duplex stainless steel was determined by the pitting resistance equivalent number of weaker phase.If the PREN of the weaker phase increases, the global corrosion resistance of steel increases.The content of Ni and Mo extends the passive region and strengthens the passive layers of the samples 26 .Considering the standard deviation, no remarkable difference between the welding processes was observed (Figure 5).
As the potential increased, a sharp rise in current density was seen, suggesting the occurrence of pitting corrosion.Wide passive regions were observed for all samples tested and the pitting potentials were all 200-400 mV higher than the corresponding open circuit potentials.These observations indicate that the samples have excellent resistance to the initiation of pitting corrosion in this chloride environment.In general,  no remarkable decrease in pitting potential was found after the welding.The repassivation potentials during the reverse scans were also similar.The backward polarization (Figure 4) showed a positive hysteresis, a large hysteresis loop and low repassivation potential, similar in value to the OCPs, which indicate that pitting corrosion might occur over long periods of open circuit exposure to the acidified glycerin solution, which contained 3% NaCl.According to Marcus et al. 27 , after depassivation, the repassivation can be hindered by chloride adsorbed on the metal surface resulting in formation of nanopits.
For a more detailed understanding of the pitting corrosion of the joints, the samples were observed using SEM analysis after the cyclic potentiodynamic testing.An example is shown in Figure 6.
As shown by the authors in a previous work 22 , the FZ was more resistant than the HAZ and the pits, in all of welded joints, occurred mostly in the HAZ (Figure 6).The sites for nucleation of pits were probably chromium nitride particles, as suggested by Tan et al. 20 .All of the HAZ regions exhibited more pits than the other regions and the HAZ was always the most susceptible zone.Some pits were found to initiate in the vicinity of inclusions in either the δ or γ phase.Others were formed at the δ/γ interface or within the δ phase.
The higher susceptibility of the HAZ to pitting corrosion was found for all welded joints, and it resulted from two aspects of the microstructure evolution during welding.The Cr and Mo contents are diluted by an increase in the δ volume fraction, so Cr and Mo content of ferrite phase in the HAZ is lower than in the parent metal and, also, Cr 2 N precipitated in the ferrite phase due to the lower solubility of nitrogen in ferrite 6,8,9,14,20,23, 28-32 .According to Hsieh et al. 18 the reformation of austenite in HAZ is limited with nitrogen and nickel contents lower than 0.15 and 5.5% (as the studied UNS S32304), respectively, even in the cases of increased cooling time 18 .According to Hsieh et al. 18 the austenite content in the HAZ cannot fall below 25% in order to avoid precipitation of Cr 2 N and guarantee superior pitting corrosion resistance at the HAZ.The pits showed in Figure 6 were believed to be related to the Cr 2 N precipitates.These precipitates resulted in depletion in Cr in the adjacent matrix region.The chromium-depleted zones provide a favorable site for pitting corrosion nucleation, as reported by several authors 6,8,9,14,20,23,[28][29][30][31][32] .
According to Marcus et al. 27 , the presence of microstructural defects on the samples such as metal grain boundaries or interfaces between the matrix and inclusions or second phase particles can promote local breakdown.In this work the breakdown has, preferentially, occurred at surface defect sites such as non-metallic inclusions or precipitates.

Conclusions
The corrosion behavior of joints of lean duplex stainless steel, UNS S32304, welded by three different processes, was studied.The following was observed: • All welded samples exhibited similar or higher corrosion resistance, compared with the corresponding base metal.

•
The pitting corrosion resistance of the whole joint has improved compared to as received samples, as a consequence of alloying elements enhanced in ferrite and austenite.• Considerable precipitation of chromium nitrides was observed at grain boundaries or ferrite grains on the as-welded specimens.• Some pits were found to initiate in the vicinity of inclusions and others were formed at the interface δ/γ or within δ phase.

•
The pits in HAZ are believed to be related to the Cr 2 N precipitates.

Figure 1 :Figure 2 :Figure 3 :
Figure 1: OM micrograph of secondary austenite in the FZ (FCAW root of weld joint with 2209 as filler metal)

Figure 5 :
Figure 5: Experimental data for cyclic polarization tests.T -top of weld, R -root of weld