Intergranular Corrosion of Low Cr Ferritic Stainless Steel 429 Evaluated by the Optimized Double Loop Electrochemical Potentiokinetic Reactivation Test

Intergranular corrosion (IGC) of Nb-Ti stabilized ferritic stainless steel (FSS) 429 was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) test combined with the microstructure observation. The results indicated that the optimized DL-EPR test condition for FSS 429 was the solution of 0.5M H 2 SO 4 + 0.0001M KSCN with a scanning rate of 100mV/min at 30C. Based on this condition, the specimens aging at 400–700C for different duration were tested and a timetemperature-sensitization (TTS) curve for FSS 429 was obtained, which reveals the sensitization nose was located around 550C. The critical I r /I a value was determined to be about 3% above which IGC occurred. After aging treatment, Cr depletion zone was detected using energy dispersive spectroscopy (EDS), most possibly due to Cr segregation around intergranular TiC and NbC.


Introduction
Ferritic stainless steels (FSS) had been widely used in many elevated temperature applications (automotive exhaust systems, steam generators of power plants, and so on) due to its superior resistance to pitting corrosion, good machinability, and high thermal conductivity, as compared with conventional austenitic stainless steels [1][2][3][4][5].During application, they were inevitably exposed to the temperature range above recrystallization temperature; the thermal cycles would lead to the grain growth and sensitization, causing the intergranular corrosion (IGC) followed by the failure of the appliances [6].
According to the previous researches [7][8][9][10][11][12][13], IGC was mainly caused by the Cr depletion around the grain boundaries.During the welding or heat treatment process, Cr compounds such as Cr-carbides and Cr-nitrides precipitated at the grain boundaries, the Cr-depleted zone adjacent to the intergranular precipitates became the initiation sites, and then IGC was fully developed by the electrochemical potential difference between the Cr-depleted zone and matrix.
To prevent the precipitation of Cr compounds at the grain boundary, Bond and Lizlovs [7] proposed that the content of carbon and nitrogen should be lower than 0.01 wt% in 17 wt% Cr FSS.Various stabilizers such as Ti and Nb were added to react preferentially with carbon and nitrogen rather than Cr [14].Even if the contents of C and N and stabilizers in FSS meet the requirements mentioned above, failures due to the IGC still occurred in low Cr FSS materials during durability testing or actual application.Kim et al. suggested that the Crdepleted region was caused by the segregation of Cr around intergranular TiC in stabilized FSS 409L [15][16][17][18][19].However, researches on the IGC of low Cr (∼14 wt%) ferritic stainless steels were extremely lacking, especially for FSS 429.
In order to quantitatively detect the effect of fine precipitation and its consequence on the degree of sensitization (DOS) to IGC of stainless steels, Akashi et al. [20] developed a double loop electrochemical potentiokinetic reactivation (DL-EPR) method, which could be used to detect the DOS to IGC of stainless steels fast, nondestructively and quantitatively.To evaluate the DOS to IGC with DL-EPR test, a specific test condition (electrolyte composition, operating 2 Advances in Materials Science and Engineering  [22].In this paper, the DOS to IGC of type 429, which contains the interstitials of 0.017 wt.% C and 0.0048 wt.% N and the stabilizers of 0.12 wt.% Ti and 0.32 wt.% Nb, was investigated using DL-EPR test.The first objective of the present study was to determine the optimum conditions (including the electrolyte concentration, scan rate, and temperature) leading to a quantitative evaluation of the IGC susceptibility of type FSS 429 using the DL-EPR test; then the specimens with different heat treatment were tested in order to plot the timetemperature-sensitization (TTS) curve of type FSS 429.In our work, a generic control variate method was used to evaluate the DOS of IGC.

Material and Heat
Treatments.The material studied in present work was low Cr ferritic stainless steel 429.The specimens were machined into a form of 10 × 10 × 2 mm and the chemical composition was listed in Table 1.The specimens were solution-treated at 1200 ∘ C for 1 h (water quenched) and then aged at 400-700 ∘ C for 15 min-8 h (water quenched).All the heat treatments were operated in the atmosphere of N 2 .

Method and Criteria of DL-EPR Test.
The DL-EPR tests were conducted by using a potentiostat workstation CHI 660D comprised of three electrodes.A graphite rod and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively.The working electrode was constructed using the FSS samples embedded in epoxy resin with an exposure area of 1 cm 2 .Prior to each experiment, the specimens were successively grounded to 2000 grit using series of sandpaper, then cleaned with alcohol, and dried in hot air.
During the DL-EPR test, the working electrode was firstly cathodically polarized at 900 mV versus SCE for 2 min to improve reproducibility.Secondly, nearly steady-state open circuit potential ( ocp ) was reached (about 5 min).Thirdly, the potential was scanned from the open circuit corrosion potential to +600 mV (SCE) and then back to the initial corrosion potential.During the anodic scan, the specimen was anodically polarized from an active state to a passive state.In this process, the dissolution occurred and the passive film formed then.During the reverse scan, the passive film around the Cr-depleted region breaks while that on the matrix kept integrity.The current in the anodic and reverse scan was measured separately and the DOS to IGC was evaluated by   (the ratio of the reactivation (  ) and anodic (  ) current density) [23].For each specimen, at least three-time measurements were carried out.Due to the great difference in chemical composition and precipitation kinetics of test materials,   value was strongly dependent on the nature of the electrolyte and the test conditions, so it was very necessary to determine the optimal operation conditions for FSS 429 in the first place.According to the previous studies [20][21][22][23], an experimental design involving the change of the main factors controlling the DL-EPR test response was established as shown in Table 2.The operating conditions listed in Table 2 were discussed separately and the optimized condition was reached in the discussion section.

Microstructural Characterization.
After the DL-EPR measurement, the morphologies were observed by light optical microscopy (LOM) and scanning electronic microscopy (SEM Phillips XL30 FEG) with electron dispersive X-ray spectroscopy (EDS).R a (%) Figure 2 showed the   values and the microstructure of specimens after test, of all three types of specimens; the DOS increased with increasing KSCN concentration.The distinction of   values was much more significant when the concentration of KSCN was higher than 0.001 M.However, for KSCN above 0.001 M, the degree of sensitization was higher than 5% for the solution-treated specimen, and the microstructure of that indicated the occurrence of general corrosion.The specimens aged at 550 ∘ C for 15 min promoted severe sensitization after DL-EPR tests as shown in Figures 2(c) and 2(d), which implied that the concentration of KSCN has a positive effect during the procedure of IGC.Only when the concentration of KSCN was located at 0.0001 M, the nonsensitized specimens (solution-treated) retained the value of   /  < 2% and the sensitized specimens (aged for 15 min) presented a   value equal to 5%.Thus, it can be concluded that setting the concentration of KSCN as 0.0001 M was a fine balance between sufficiently aggressive attack and good selectivity.of 30, 60, 100, 150, and 300 mV/min and under conditions of 0.0001 M KSCN at a temperature of 30 ∘ C, as shown in Figure 4(a).From the figure, it was obvious that the   ratio decreased significantly with the increase of scanning rate.The increase of DOS at lower scanning rate was mainly attributed to an increase in the peak current during the reverse scan.When the scan rate was too slow, not only was more passive film on the Cr-depleted region dissolved, but also there was more general corrosion occurring on the entire specimen.On the contrary, at high scan rate, the passive film on the Cr-depleted region could not be dissolved completely, leading to the decrease of the sensitivity of DL-EPR test.

Optimization of DL-EPR Test Conditions
The above analysis was confirmed by the micrographs of the specimens after test, Figures 4(b)-4(f).Hence, the scan rate of 100 mV/min was suitable for an optimal test.

Effect of Heat Treatment on IGC.
To investigate the influence of the heat treatments on the IGC of FSS 429, the specimens aged at 400-700 ∘ C for various times are tested in the condition modified before (0.5 M H 2 SO 4 + 0.0001 M KSCN with a potential scanning rate of 100 mV/min at 30 ∘ C). such as stainless steel were capable of a process called healing [28] where Cr diffused into the depleted zones, raising the Cr level and curing the sensitized region.In this case, Cr in the intergranular precipitates would diffuse to the grain boundary for specimens aged for as long as 8 h and restore the chromium depleted region leading to the reduction of the sensitization to the IGC. Figure 6 compared IGC morphologies of the specimens aged at 550 ∘ C for various aging times after the DL-EPR tests.As the aging time increased, a deeper attack was evident along the grain boundary, as shown in Figures 6(a)-6(d).As the healing effect appeared, the morphology of the specimen aged for as long as 8 h became less ravined and the intergranular corrosion had been weakened, as shown in Figure 6(e).In a word, all the surface morphologies were closely related with DL-EPR results.

Effect of Aging Time on IGC.
Figure 7 showed the SEM-EDS characterization at the grain boundary of the solution-treated and aged specimen, in order to further examine the Cr-depletion caused by aging treatment.The Cr content (12.68 wt.%) along the grain )  boundary of the specimen aged at 550 ∘ C for 1 h was lower than that (14.25 wt.%) of the solution-treated specimen, which indicated that the Cr-depleted region developed with the increase of aging time.Similar results had been reported by other researchers as well [15,16,29].The previous studies had suggested that TiC redissolved and soluble C atoms were produced during the solution treatment at 1200 ∘ C. Suzuki et al. [30] suggested that Cr had a tendency to segregate at the TiC interface to relieve supersaturation in matrix at around 500 ∘ C. In this case, TiC/NbC reprecipitated along the grain boundary during the aging treatment, and then Cr segregated around TiC/NbC after TiC/NbC formation.Consequently, the Cr segregation developed Cr depletion zone that caused IGC adjacent to the grain boundary.the aging temperature continued increasing above 600 ∘ C, the   value decreased gradually and sensitization was relieved.The measured   value (2.18%) of the specimen aged at 700 ∘ C for 1 h was similar to that of solution-treated specimen.This was due to high diffusion rate of chromium at high temperature, and 1 h was long enough to heal the chromiumdepleted zones at 700 ∘ C.So the nose temperature of FSS 429 located at approximately 550 ∘ C.

Time-Temperature-Sensitization (TTS) Curve for FSS 429.
The TTS curve for FSS 429 was plotted based on   values (tabulated in Table 3) obtained in the optimized operating conditions and the microstructures after the DL-EPR tests, as shown in Figure 9.The curve showed that the IGC mainly occurred in the specimens aging at the temperature range of 450-600 ∘ C. Beyond this range (like 450 ∘ C, 650 ∘ C, and 700 ∘ C), the IGC was slight when the aging time was long enough.The sensitization nose was located around 550 ∘ C.

Conclusions
The Ti-Nb stabilized FSS 429 was investigated using DL-EPR test, LOM, and SEM-EDS.The results of this work suggested the following conclusions.
The optimum DL-EPR condition to evaluate DOS to IGC of FSS 429 was in a 0.5 M H 2 SO 4 + 0.0001 M KSCN solution with a potential scanning rate of 100 mV/min at 30 ∘ C.
The DOS to IGC of FSS 429 obtained by DL-EPR increased with the aging time in the range from 15 min to 2 h.This could be explained by the Cr segregation around TiC/NbC after the TiC/NbC reprecipitation, leading to the development of Cr depletion zone that caused IGC adjacent to the grain boundary.Further prolonging the aging time decreased the degree of sensitization, which resulted from the healing effect brought by the diffusion of Cr.The DOS to IGC of FSS 429 increased with the aging temperature between 450 and 550 ∘ C and decreased as the aging temperature increasing above 650 ∘ C. The nose temperature of FSS 429 was around 550 ∘ C.

Figure 5 :
Figure 5: DL-EPR curves and   values of FSS 429 aging at 550 ∘ C for different time.
Figure 8  illustrated the DL-EPR curves and the   values obtained for specimens aging at 400-700 ∘ C for 1 h.As aging temperature increased from 400 ∘ C to 550 ∘ C, Cr segregated around TiC/NbC at the grain boundaries and the corresponding sensitization became more obvious.Aging at 550 ∘ C and 600 ∘ C for 1 h produced the biggest degree of sensitization to IGC.While

Figure 7 :
Figure 7: SEM micrographs and EDS analysis results along the grain boundary in FSS 429.(a) Solution specimen, (b) EDS spectra of grain boundary in (a), (c) the specimen aged at 550 ∘ C for 1 h, and (d) EDS spectra of grain boundary in (d).

Table 1 :
[21]ical composition of FSS 429 (wt.%).In previous studies, Hong et al. used the DL-EPR test in the solution of 2 M H 2 SO 4 + 1.5 M HCl at 30 ∘ C and with a scanning rate of 1.5 mV/s to evaluate the DOS to IGC of UNS S32750 duplex stainless steel[21].Liet al. used the DL-EPR test in the solution of 0.5 M H 2 SO 4 + 0.005 M Na 2 S 4 O 6 at 40 ∘ C and with a scanning rate of 1.667 mV/s to evaluate the DOS to IGC of 316L austenitic stainless steel [12].Gong et al. used the DL-EPR test in the solution of 2 M H 2 SO 4 + 1 M HCl at 30 ∘ C and with a scanning rate of 1.66 mV/s to evaluate the DOS to IGC of UNS S31803 duplex stainless steel

Table 2 :
Design of experiments for the optimization of DL-EPR test conditions.

Table 3 :
values of FSS 429 for different aging treatments.