Corrosion behavior of ferritic stainless steel with 15wt ...

International Journal of Minerals, Metallurgy and Materials V olume 20 , Number 9 , September 2013 , P age 850 DOI: 10.1007/s12613-013-0806-4

Corrosion behavior of ferritic stainless steel with 15wt% chromium for the automobile exhaust system Hua-bing Li, Zhou-hua Jiang, Hao Feng, Hong-chun Zhu, Bin-han Sun, and Zhen Li School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China (Received: 16 December 2012; revised: 6 March 2013; accepted: 26 April 2013)

Abstract: The effect of chloride ion concentration, pH value, and grain size on the pitting corrosion resistance of a new ferritic stainless steel with 15wt% Cr was investigated using the anodic polarization method. The semiconducting properties of passive films with different chloride ion concentrations were performed using capacitance measurement and Mott-Schottky analysis methods. The aging precipitation and intergranular corrosion behavior were evaluated at 400900◦ C. It is found that the pitting potential decreases when the grain size increases. With the increase in chloride ion concentration, the doping density and the flat-bland potential increase but the thickness of the space charge layer decreases. The pitting corrosion resistance increases rapidly with the decrease in pH value. Precipitants is identified as Nb(C,N) and NbC, rather than Cr-carbide. The intergranular corrosion is attributed to the synergistic effects of Nb(C,N) and NbC precipitates and Cr segregation adjacent to the precipitates. Keywords: ferritic stainless steel; corrosion resistance; pitting; chlorides; intergranular corrosion; grain size

1. Introduction Ferritic stainless steels have been extensively used in automotive exhaust systems due to their low cost, high strength, and excellent high-temperature properties, low linear expansion coefficient, excellent corrosion resistance and cold forming properties [1-3]. A series of ferritic stainless steels such as T409L, SUS430JIL, SUS436JIL, SUS444, and R429EX are developed to join the hot and cold end parts in new automotive exhaust systems to meet stricter governmental regulations [4-7]. Water containing Cl− , Br− , 2− SO2− 4 , CO3 , and organic acids from exhaust gas condenses in the muffler and corrodes ferritic stainless steels. Cl− is a major contributor to corrosion in the muffler [1]. The weld joints of ferritic stainless steels are sensitive to the intergranular corrosion (IGC) due to the precipitation and segregation phenomena. The IGC mechanism of stainless steel is traditionally related to the depletion of chromium along grain boundaries due to the formation of Cr carbides or other Cr rich phases [8]. Cr-depletion due to the segregation of unreacted Cr atoms around carbides of stabilizer metals has been proposed to contribute to intergranular corrosion [9]. A new ferritic stainless steel with 15wt% Cr (FSSNEW) was developed for automotive exCorresponding author: Zhou-hua Jiang, Hua-bing Li

haust systems in the previous research [10]. Mo was used to improve the corrosion resistance and high-temperature oxidization resistance properties. Nb was added as a stabilizer to react preferentially with carbon and nitrogen. These additions prevent the precipitation of Cr at grain boundaries and improve the corrosion resistance. FSSNEW exhibits higher strength than T409L, SUS430JIL, and standard muffler metals. FSSNEW has good plasticity and deep drawing properties. The high-temperature tensile strength, yield strength, and high-temperature oxidization resistance are superior or equal to currently-used ferritic stainless steels. The effect of grain size, pH value, and chloride ion concentration on the pitting corrosion resistance was examined in this study. The IGC mechanism of Nb-stabilized FSSNEW was also evaluated.

2. Experimental The chemical composition (wt%) of FSSNEW manufactured in a vacuum induction furnace is C 0.012, Cr 14.3, Mo 1.28, Nb 0.48, S 0.004, N 0.0089, and P less than 0.03. The ingot was cold rolled into 2-mm plates with 60% cold deformation after forging, hot rolling, annealing, and picking. The specimens used for pitting corrosion and semi-

E-mail: [email protected], [email protected]

c University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2013

H.B. Li et al., Corrosion behavior of ferritic stainless steel with 15wt% chromium for ...

conductor property testing were annealed at 1100◦ C for 5 min followed by air cooling. The effect of grain size, chloride ion concentration, and pH value on pitting corrosion resistance was investigated using the anodic polarization method. The pH values of the solution were adjusted by adding HCl or NaOH to a 3.5wt% NaCl solution. The scan rate and test temperature in the anodic polarization tests were performed at 20 mV/s and 30◦ C, respectively. Semiconducting properties of passive films formed on FSSNEW were evaluated in solutions containing different chloride ion concentrations at 30◦ C. Capacitance measurements were performed using Mott-Schottky methods. The working face of the samples was wet ground and then polished to fine diamond grade (1 µm). The working electrode was kept at −0.5 V for 600 s to remove air-formed oxides on the surface. The sample was kept at 0.2 V vs. the saturated calomel electrode (SCE) for 1 h in different chloride ion concentrations to form the passive film. Capacitance measurements were performed at a fixed frequency of 1000 Hz with the amplitude of 5 mV. The potential range was varied from 0.2 to 0.8 V vs. SCE by 20 mV steps. The effect of sensitization temperature and time on the microstructure and susceptibility to intergranular corrosion were investigated using the double loop electrochemical potentiodynamic reactivation (DL-EPR) method. 1.5 M H2 SO4 plus 0.01 M KSCN was chosen as the DL-EPR solution. Specimens of 10.1 mm × 10.1 mm × 2 mm were machined from the cold rolled plate, solution annealed at 1200◦ C for 10 min, and water quenched. Sensi-

Fig. 1.

851

tization treatments were performed at 400 to 900◦ C for 2 h, followed by water quenching. Electrolytic etching with 10wt% oxalic acid was used to reveal the microstructure. Precipitation in the sensitized steels was identified using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). The volume fraction of precipitation was measured using Image-Pro Plus 5.0 software. All measurements were carried out using a three electrode PARSTAT 2273 potentiostat. Platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The solution was deaerated with high purity nitrogen for 0.5 h before testing and kept under a nitrogen atmosphere during testing.

3. Results and analysis 3.1. Pitting corrosion behavior (1) Effect of grain size on the pitting corrosion resistance The microstructures of FSSNEW manufactured using different annealing times at 1100◦ C were examined (Fig. 1). The grains extended into fibrous tissue along the rolling direction after 60% cold-rolling reduction (Fig. 1(a)). After annealing for 5 min, no deformation was seen, and the microstructure consisted of small equiaxial grains (Fig. 1(b)). Complete recrystallization of the steel with 60% cold-rolling reduction occurred after annealing for 5 min at 1100◦ C. Equiaxial grains were seen with the annealing time up to 10 min. Longer annealing times were associated with larger grain sizes (Fig. 1(d) and Fig. 1(e)).

Optical micrographs of FSSNEW with 60% cold-rolling reduction after different annealing times at 1100◦ C:

(a) 0 min; (b) 5 min; (c) 10 min; (d) 30 min; (e) 60 min.

852

Int. J. Miner. Metall. Mater., V ol. 20 , No. 9 , Sep. 2013

Fig. 2 shows the anodic polarization curves of FSSNEW made using different annealing times in a 3.5wt% NaCl solution. The effect of annealing time on the grain size and pitting corrosion were examined (Fig. 3). Longer annealing time was associated with larger grain size, increased corrosion current density, and a smaller passive region. The pitting corrosion resistance decreased with increasing the annealing time. Smaller grain size was associated with decreased Cl− adsorption on the steel surface, preventing the diffusion of Cl− into the passive film. Furthermore, smaller grain size was associated with the more uniform distribution of doped metals, which was associated with the formation of a more compact passive film [11]. Schino et al. [12] suggested that the pitting of coarsely grained steel started at larger and deeper pits. The large number of small pits in ultrafine-grained steel could lead to a decrease in anodic current density. The decreases in corrosion current density and pitting corrosion resistance were related to the compact passive film and smaller individual pits associated with smaller grains in the present research.

Fig. 2.

Anodic polarization curves of FSSNEW with

different annealing times in a 3.5wt% NaCl solution.

Fig. 3.

Effect of annealing time on the grain size and

pitting corrosion potential.

(2) Effect of chloride ion concentration on the pitting corrosion resistance and semiconducting properties of passive films The anodic polarization curves of FSSNEW annealed at 1100◦ C for 5 min followed by air cooling in 0.1, 0.6, and 3 M NaCl are shown in Fig. 4. The corrosion current density increases slightly as the chloride ion concentration increases. The passive region narrows significantly with the increase in chloride ion concentration. The pitting corrosion potential decreases significantly, especially at the chloride concentration of 3 M (Figs. 4 and 5). Chloride ions have a strong effect on the pitting corrosion resistance. The pitting corrosion resistance decreases significantly as the chloride ion concentration increases. The destructive effect of chloride ions on the stainless steel passive film has been well described [13-14]. Chloride adsorption leads to local film dissolution and weakening of oxide bonds, especially near cracks and fractures in the metal. Adsorption of corrosive anions, such as chloride on the surface of the passive film, is the first step in film dissolution. As the chloride concentration increases, pitting corrosion can occur, and pitting potential decreases.

Fig. 4.

Anodic polarization curves of FSSNEW in dif-

ferent concentrations of NaCl.

Fig. 5.

Plot of pitting corrosion potential versus NaCl

concentration.


The relationship between capacitance and applied potential was described using the Mott-Schottky (MS) equation [15-17]: 1 2 kT = − (n-type semiconductor) E − E FB C2 εε0 eND e (1) 2 kT 1 =− (p-type semiconductor) E − EFB − C2 εε0 eNA e (2) where ε is the dielectric constant of the passive film (15.6), ε0 the permittivity of free space (8.854 × 10−14 F/cm), e the electron charge (1.602 × 10−19 C), and ND and NA are the donor and acceptor densities, respectively. EFB is the flat-band potential, k the Boltzmann constant (1.38 × 10−23 J/K), and T the absolute temperature. ND and NA can be determined from the slope of experimentally derived 1/C 2 versus applied potential (E) curves. The flat-band potential EFB can be obtained from the extrapolation of 1/C 2 to 0. Fig. 6 shows the Mott-Schottky plots of passive films formed on FSSNEW using a film formation potential of 0.2 V vs. SCE for 1 h in solutions with 0.1, 0.6, or 3 M NaCl. One linear region is seen. The slope increases with increasing the chloride ion concentration due to different capacitance behaviors with applied potential. The positive slope indicates that the passive films formed on FSSNEW with different chloride ion concentrations behave as n-type semiconductors. The iron oxide portion of the space charge layer of the passive film is the enriched equivalent conductor, and the chromium oxide segment is the depleted portion. In the n-type semiconductor region, the decreasing capacitance found with applied potential is mainly attributed to the enlargement of the electron depleted layer and the reduction of charge carriers. The increase in capacitance is associated with increasing the concentration of holes in the valence band, due to the adsorption of anions [18]. The effect of chloride ion concentration on the donor

Fig. 6.

Mott-Schottky plots of passive films formed on

FSSNEW in different NaCl solutions.

853

density (ND ) and flat-band potential (EFB ) was evaluated using Mott-Schottky analysis (Fig. 7). The ND and EFB increase with the chloride ion concentration increasing. The chloride ion effect on ND was related to the point defect model (PDM) [19]. Chloride ions can be absorbed into the surface film and react with oxygen vacancies via a MottSchottky reaction to generate cation-oxygen vacancy pairs. Oxygen vacancies in turn react with additional chloride ions at the film-solution interface to generate more cation vacancies. The generation of cation-oxygen vacancies is autocatalytic. Excess vacancies move to the metal-film interface and condense. This leads to local detachment of the film from the underlying metal. Increases in chloride ion concentration promote this reaction. As oxygen vacancies and metal ion vacancies increase, the donor concentrations also increase. The passive film is more susceptible to damage and the pitting corrosion resistance of the steel decreases.

Fig. 7.

Relationship between ND , EFB , and Cl− con-

centration.

The relationship between the space-charge-layer thickness and E−EFB for FSSNEW with different chloride ion concentrations was examined. The thickness of the space charge layer (W ) in an n-type semiconductor can be calculated by the following equation [16]: 1/2 2εε0 kT E − EFB − W = (3) eND e The space-charge-layer thickness decreases with the chloride ion concentration increasing (Fig. 8). The space charge layer is known to be an effective barrier to the flow electrons and holes from semiconductor to electrolyte. The pitting corrosion resistance of steel has been shown to improve as the thickness of the space charge layer increases. Lee and Kim [16] suggested that increasing the Cr content in Fe-Cr alloys would result in a thicker space charge region, and the pitting corrosion resistance of the alloy would be improved. It has also been shown that the space charge layer thickens and the pitting corrosion resistance is improved as the nitrogen content of the steel increases [20].

854


The space-charge-layer thickness and the pitting corrosion resistance of alloys are related. Increasing the chloride ion concentration decreases the space-charge-layer thickness of the passive film and the pitting corrosion resistance of FSSNEW. These findings help define the relationship between the pitting corrosion and semiconducting properties of passive films formed on FSSNEW.

Fig. 10.

Relationship between pitting corrosion poten-

tial and pH value.

Fig. 8.

Relationship between W and E −EFB of FSS-

NEW at different Cl− concentrations.

(3) Effect of pH value on the pitting corrosion resistance The anodic polarization of FSSNEW in solutions with the pH value from 1 to 7 was examined (Fig. 9). The relationship between the pitting corrosion potential and the pH value was also evaluated (Fig. 10). At pH 7, the anodic current density is the smallest, the passive region is the widest, and the pitting potential is the largest. As the pH value is decreased to 5.61, the corrosion current density increases, and the passive region narrows. Further a decrease in pH value results in a much larger increase in corrosion density, a greater dissolution of FSSNEW, narrowing of the passive region, and a larger decrease in pitting potential. At pH 2, the pitting potential is almost 0 V. The pitting potential is −0.1 V vs. SCE at pH 1 and is associated with

Fig. 9.

Anodic polarization curves of FSSNEW at dif-

ferent pH values.

the minimal pitting corrosion resistance. The pitting corrosion resistance of FSSNEW at pH 5.61 to 7 is not changed. The pitting corrosion resistance of FSSNEW in 3.5wt% NaCl at pH 5.61 is superior to that of T409L, a metal used in automobile exhaust systems [10]. The effect of pH value on the pitting corrosion resistance is attributed to the low contents of Cr and Mo and the loss of the compact passive film [21].

3.2. Effect of sensitization temperature and time on the intergranular corrosion The volume fraction of precipitation in FSSNEW treated at sensitization temperatures from 400 to 900◦ C for 2 h followed by water quenching was evaluated (Fig. 11). There is no precipitation along grain boundaries at 400◦ C. The amount of precipitation along grain boundaries increases with the temperature increasing. The maximum precipitation occurs at 700◦ C along grain boundaries and in the matrix (Figs. 11 and 12). There is no precipitation in FSSNEW that was not sensitized. The amount of precipitates increases when the sensitization time prolongs (Fig. 13).

Fig. 11.

Relationship between sensitization tempera-

ture and the volume fraction of precipitation


Fig. 12.

855

Typical SEM morphology and EDS analysis results of precipitation in FSSNEW sensitized at 700◦ C for

2 h: (a) NbC along grain boundaries and in the matrix; (b) EDS spectra of NbC along grain boundaries; (c) EDS spectra of NbC in the matrix.

Fig. 13.

Relationship between sensitization time and

the volume fraction of precipitation in FSSNEW.

Precipitation in FSSNEW aged at 700◦ C for 2 h was analyzed using SEM and EDS (Fig. 12). Fine precipitates of NbC (EDS analysis, Fig. 12(b)) are commonly found along grain boundaries. The NbC precipitates are also found in the matrix (Figs. 12(a) and 12(c)). Small amounts of Nb(C,N) complexes are found along grain boundaries and in the matrix (Fig. 14). The Nb(C,N) is found to contain a core of Al2 O3 (Fig. 14(a)). A small amount of Cr is found in NbC and Nb(C,N) precipitates. The Cr-carbide, which has been hypothesized in the conventional theory of intergranular corrosion to be associated with Cr-depletion in the grain boundary, is not present. Our findings are similar to those by Kim et al. [22], who reported that TiN and TiC in Ti-stabilized 11wt% Cr ferritic stainless steel aged

Fig. 14. Typical Nb(C,N) SEM morphology and EDS analysis results of FSSNEW sensitized at 700◦ C for 2 h: (a) Nb(C,N) along grain boundaries; (b) EDS spectrum of Al2 O3 wrapped by Nb(C,N); (c) Nb(C,N) in the matrix; (d) EDS spectrum of Nb(C,N) along grain boundaries and in the matrix.

856


at 400 to 600◦ C were deposited along grain boundaries and in the matrix and were responsible for intergranular corrosion. The structure of precipitated material in FSSNEW aged at 700◦ C for 2 h was examined using TEM and selected area diffraction (SAD) patterns (Fig. 15). Precipitation along grain boundaries consists of Nb(C,N) with a face centered cubic structure exhibiting a lattice parameter of 0.476 nm. No Cr-carbide is detected by TEM or SAD analysis. NbC and Nb(C,N) are preferentially deposited at crystalline defects located along grain boundaries and inclusions during the aging process. The double loop electrochemical potentiokinetic reactivation (DL-EPR) was usually used to evaluate the effect of sensitizing temperature and time on the susceptibility to intergranular corrosion of FSSNEW. The electrolyte composition and DL-EPR scan rate are key factors in this analysis. 0.5 M H2 SO4 and 0.01 M KSCN were usually used during the evaluation of Mo-free stainless steels like 304SS, 18Cr-18Mn-0.9N [23-24]. The addition of Mo in stainless steel stabilizes the passive film and prevents IGC during the reactivation scan. 0.5 M H2 SO4 plus 0.01 M KSCN is not useful in evaluating chromium depletion zones in sensitized stainless steel and super alloys with the addition of Mo. NaCl, hydrochloric acid (HCl), and high concentra-

Fig. 15.

tions of H2 SO4 can react with chromium depletion zones and result in cracking of the passive film [25-26]. In the present research, the concentration of H2 SO4 was optimized and increased to 1.5 M for FSNEW with 1.28wt% Mo. Our DL-EPR findings confirmed previous experimental reports. The DL-EPR experiments of FSSNEW sensitized at different temperatures for 2 h and at 700◦ C for different times are performed, respectively. Some parameters are shown in Tables 1 and 2. The Ia values are about the same for all tests. Sensitizing temperature and time have obvious effect on Ir , Ir /Ia , and Qr /Qa . The critical value of Ir /Ia used to evaluate the susceptibility to intergranular corrosion increases and then decreases as the sensitizing temperature is increased from 400 to 900◦ C. A maximum Ir /Ia value is seen at 700◦ C (Fig. 16). The Ir /Ia value of annealed FSSNEW without sensitization treatment is about 0.0037 (Table 2). The Ir /Ia value increases as the sensitizing time increases (Fig. 17). The Ir /Ia value correlates well with the volume fraction of the precipitated material in FSSNEW. Fig. 18 shows the micrographs of FSSNEW aged for 2 h after DL-EPR at the sensitization temperatures ranging from 400 to 900◦ C. The susceptibility to intergranular

TEM image of precipitation in FSSNEW aged at 700◦ C for 2 h: (a) bright field image; (b) SAD pattern

of Nb(C,N).

Table 1.

DL-EPR experimental results of FSSNEW sensitized at different temperatures

Sensitization temperature / ◦ C 400 500 600 700 800 900

Ia / (mA·cm−2 ) 53.21 56.43 49.82 57.23 57.92 53.54

Ir / (mA·cm−2 ) 0.643 1.057 1.741 2.992 2.532 1.787

Qa / C 3.718 3.860 3.493 4.077 4.198 3.887

Qr / C 3.767 3.979 3.634 4.260 4.359 4.013

Ir / Ia 0.0121 0.0187 0.0349 0.0523 0.0437 0.0334

Qr / Qa 0.0132 0.0308 0.0404 0.0449 0.0384 0.0324

Note: Ia — peak activation current density; Ir — peak reactivation current density; Qa — activation electric charge; Qr — reactivation electric charge.

857

H.B. Li et al., Corrosion behavior of ferritic stainless steel with 15wt% chromium for ... Table 2.

DL-EPR experimental results of FSSNEW sensitized at 700◦ C for different times

Sensitization time / h 0 0.5 1 2 4

Fig. 16.

Ia / (mA·cm−2 ) 50.03 51.06 48.09 57.23 54.97

Ir / (mA·cm−2 ) 0.164 0.561 0.631 2.992 4.099

Relationship between sensitization tempera-

ture and I r /I a .

Fig. 17.

Relationship between sensitization time and

I r /I a .

corrosion is not obvious at 400◦ C (Fig. 18(a)). As the sensitization temperature is increased to 600◦ C, gain boundaries become clear and shallow ditches appear. These are the first changes found with intergranular corrosion. The ditches become wider and some pits are observed in the matrix at 700◦ C. This shows the next step in IGC occurs along grain boundaries and in the matrix. The ditches become shallow as the sensitization temperature increases from 800 to 900◦ C. No IGC is seen in annealed FSSNEW that did not undergo sensitization treatment because there is no precipitation in the steel (Fig. 19(a)). Ditches appear after sensitizing treatment at 700◦ C for 0.5 h and become deeper after 1 h (Figs. 19(b) and 19(c)). Wider ditches are seen along grain boundaries, and some pits appear in the matrix after

Qa / C 3.549 3.538 3.527 4.077 4.006

Qr / C 3.562 3.582 3.574 4.260 4.255

Ir / Ia 0.0033 0.0110 0.0131 0.0523 0.0746

Qr / Qa 0.0037 0.0124 0.0133 0.0449 0.0622

2 h. More advanced IGC takes place along grain boundaries and in the matrix after 4 h because of the presence of large amounts of Nb(C,N) and NbC precipitates along grain boundaries and in the matrix. Our analysis of the precipitates, DL-EPR, and micrographs of IGC suggests that IGC susceptibility is not related to Cr carbides located along grain boundaries and an adjacent Cr-depleted zone. The EDS spectra of NbC and Nb(C,N) precipitates confirm the presence of Cr in the precipitate. Therefore, it could be assumed that Cr segregated around fine Nb(C,N) and NbC, which result in the formation of a Cr depletion zone near the Cr segregation zone. To confirm this finding, the SEM morphology and a SEM-EDS line profile of Nb(C,N) along the grain boundary of FSSNEW sensitized at 700◦ C for 2 h were performed. Large (1 µm) Nb(C,N) precipitate is found along the grain boundary (Fig. 20(a)). C, N, and Nb are found to be enriched in the formation zone of Nb(C,N). Cr with lower than the matrix (14.3wt%) is found in and adjacent to Nb(C,N) (Fig. 20(b)). The amount of Nb adjacent to the formation zone of Nb(C,N) is larger than that found in the matrix. Some reports have shown that Cr can migrate to fine MC carbides, such as TiC and NbC, in order to relieve super saturation in the matrix [27-28]. The above results indicate that Cr, C, N, and Nb elements segregate near a position of crystal defects, such as along grain boundaries and inclusions. The formation of Nb(C) and Nb(C,N) was preferred over that of Cr-carbide during aging treatment. Cr could migrate toward fine Nb(C,N) and Nb precipitates, forming an adjacent Cr-depleted zone. The formation of fine Nb(C,N) and NbC and Cr precipitates found along grain boundaries and in the matrix could be responsible for IGC. IGC was induced by the formation of the Cr depletion zone in Ti-stabilized 11wt% Cr ferritic stainless steel due to the Cr segregation around intergranular TiC, rather than Cr-carbide and Cr-carbonitride [22].

4. Conclusions (1) The recrystallization behavior of FSSNEW steel after 60% cold-rolling reduction occurred within the annealing time of 5 min at 1100◦ C. A prolonged annealing time is associated with a larger grain size and a decreased pitting potential. (2) The pitting corrosion resistance of FSSNEW decreases with the chloride ion concentration increasing. The doping density and flat bland potential increase and the

858

Fig. 18.


Micrographs of FSSNEW at different sensitization temperatures: (a) 400◦ C; (b) 500◦ C; (c) 600◦ C; (d)

700◦ C; (e) 800◦ C; (f ) 900◦ C.

Fig. 19.

Micrographs of FSSNEW sensitized at 700◦ C for different sensitization times: (a) 0 h; (b) 0.5 h; (c)1 h;

(d) 2 h; (e) 4 h.

Fig. 20.

SEM morphology (a) and SEM-EDS line profile of Nb(C,N) (b) along the grain boundary of FSSNEW

sensitized at 700◦ C for 2 h.


thickness of the space charge layer decreases as the chloride ion concentration increases. The susceptibility of FSSNEW to pitting corrosion and the semiconducting properties of passive films formed on FSSNEW are closely interrelated. (3) The pitting corrosion resistance of FSSNEW in a 3.5% NaCl solution at pH 5.61 is superior to that of T409L used in automobile exhaust systems. The pH value has obvious effect on the pitting corrosion resistance from 5.61 to 1. The pitting corrosion resistance increases rapidly with the pH value decreasing. (4) Identified precipitants consist of Nb(C,N) and NbC, rather than Cr-carbide. As the sensitization temperature is increased from 400 to 700◦ C, the volume fraction of precipitants and intergranular corrosion susceptibility increase and then decrease. The intergranular corrosion is attributed to the synergistic effects of Nb(C,N) and NbC precipitates and Cr segregation adjacent to the precipitates.

Acknowledgements

859

resistant ferritic stainless steel with high formability for automotive exhaust manifolds: “JFE-MH1”, JFE Tech. Rep., 1(2004), No. 4, p. 61. [8] H.B. Li, Z.H. Jiang, Z.R. Zhang, Y. Cao, and Y. Yang, Intergranular corrosion behavior of high nitrogen austenitic stainless steel, Int. J. Miner. Metall. Mater., 16(2009), No. 6, p. 654. [9] J.K. Kim, Y.H. Kim, B.H. Lee, and K.Y. Kim, New findings on intergranular corrosion mechanism of stabilized stainless steels, Electrochim. Acta, 56(2011), No. 4, p. 1701. [10] H.B. Li, Z.H. Jiang, Z. Li, and Z.R. Zhang, Development of ferritic stainless steel with 15% chromium for automobile exhaust system, J. Northeast. Univ. Nat. Sci., 30(2009), No. 9, p. 1278. [11] L. Liu, Y. Li, and F.H. Wang, Influence of grain size on the corrosion behavior of a Ni-based superalloy nanocrystalline coating in NaCl acidic solution, Electrochim. Acta, 53(2008), No. 5, p. 2453. [12] A.D. Schino, M. Barteri, and J.M. Kenny, Grain size de-

This work was financially supported by the Program for Liaoning Innovative Research Team in University (No. LT20120008), the Fundamental Research Funds for the Central Universities (No. N100402015), and the General Scientific Research Project of the Department of Education of Liaoning Province, China (No. L2012077).

References

pendence of mechanical, corrosion and tribological properties of high nitrogen stainless steels, J. Mater. Sci., 38(2003), No. 15, p. 3257. [13] Y.A. Albrimi, A. Eddib, J. Douch, Y. Berghoute, M. Hamdani, and R.M. Souto, Electrochemical behaviour of AISI 316 austenitic stainless steel in acidic media containing chloride ions, Int. J. Electrochem. Sci., 6(2011), No. 10, p. 4614.

[1] E. Sato and T. Tanoue, Present and future trends of materials for automotive exhaust system, Nippon Steel Tech.

[14] S.U. Lee, J.C. Ahn, D.H. Kim, S.C. Hong, and K.S. Lee, Influence of chloride and bromide anions on localized corrosion of 15%Cr ferritic stainless steel, Mater. Sci. Eng.

Rep., 1(1995), No. 64, p. 13. [2] E. Ranjbarnodeh, S. Hanke, S. Weiss, and A. Fischer, Ef-

A, 434(2006), No. 1-2, p. 155. [15] S. Ningshe, U. Kamachi Mudali, V.K. Mittal, and H.S.

fect of welding parameters on the heat-affected zone of

Khatak, Semiconducting and passive film properties of

AISI409 ferritic stainless steel, Int. Mater., 19(2012), No. 10, p. 923.

Metall.

nitrogen-containing type 316LN stainless steels, Corros. Sci., 49(2007), No. 2, p. 481.

[3] J.X. Liu, Y.J. Zhang, and J.T. Han, Test research on stick-

[16] J.B. Lee and S.W. Kim, Semiconducting properties of passive films formed on Fe-Cr alloys using capacitance

J. Miner.

ing mechanism during hot rolling of SUS 430 ferritic stainless steel, Int. J. Miner. Metall. Mater., 17(2010), No. 5

measurements and cyclic voltammetry techniques, Mater.

p. 573. [4] N. Fujita, K. Ohmura, E. Sato, and A. Yamamoto, Devel-

Chem. Phys., 104(2007), No. 1, p. 98. [17] R. De Gryse, W.P. Gomes, F. Cardon, and J. Vennik, On

opment of ferritic stainless steel YUS 450 with high heat

the interpretation of Mott-Schottky plots determined at

resistance for automotive exhaust system components, Nippon Steel Tech. Rep., 2(1996), No. 71, p. 25.

semiconductor/electrolyte systems, J. Electrochem. Soc., 122(1975), No. 5, p. 711.

[5] H.T. Tsai, W.J. Sammon, and D.E. Hazelton, Characterization and countermeasures for sliver defects in cold rolled products, [in] Proceedings of the 73rd Steelmaking Confer-

[18] N.E. Hakiki, M.F. Montemor, M.G.S. Ferreira, and M. da Cunha Belo, Semiconducting properties of thermally grown oxide films on AISI 304 stainless steel, Corros. Sci.,

ence, Detroit, 1990, p. 49. [6] A. Miyazaki, M. Gunzi, and K. Yoshioka, High formabil-

42(2000), No. 4, p. 687. [19] J. Sikora, E. Sikora, and D.D. Macdonald, The electronic

ity R429EX and heat-resistant R444EX stainless steels for

structure of the passive film on tungsten, Electrochim.

automotive exhaust manifold, Kawasaki Steel Tech. Rep., 15(1994), No. 31, p. 21.

Acta, 45(2000), No. 12, p. 1875. [20] H.B. Li, Z.H. Jiang, Y. Yang, and Z.R. Zhang, Semicon-

[7] A. Miyazaki, J. Hirasawa, and O. Furukimi, Ferritic

ducting properties of passive films and pitting corrosion resistance of nickel free high nitrogen austenitic stainless

stainless steel for automotive exhaust systems-high heat-

860

Int. J. Miner. Metall. Mater., V ol. 20 , No. 9 , Sep. 2013 steels, [in] The Minerals, Metals & Materials Society 2009

[25] N. Lopez, M. Cid, M. Puiggali, I. Azkarate, and A. Pelayo,

Annual Meeting, San Francisco, 2009, p. 717. [21] F. Wong, The Effect of Alloy Composition on the Lo-

Application of double loop electrochemical potentiodynamic reactivation test to austenitic and duplex stainless

calized Corrosion Behavior of Ni-Cr-Mo Alloys [Disser-

steels, Mater. Sci. Eng. A, 229(1997), No. 1-2, p. 123.

tation], The Ohio State University, Columbus, 2009, p. 30. [22] J.K. Kim, Y.H. Kim, S.H. Uhm, J.S. Lee, and K.Y. Kim,

[26] Y. Cetre, P. Eichner, G. Sibaud, and J.M. Scarabello, Corrosion in chemical and parachemical industries, [in] Pro-

Intergranular corrosion of Ti-stabilized 11wt% Cr ferritic

ceedings of the 3rd European Conference on Corrosion,

stainless steel for automotive exhaust systems, Corros. Sci., 51(2009), No. 11, p. 2716.

Lyon, 1997, p. C4.1-C4.12. [27] M. Suzuki, S. Hamada, P.J. Maziasz, S. Jitsukawa, and A.

[23] R.F.A. Jargelius, S. Hertzman, E. Symniotis, H. H¨ anninen, and P. Aaltonen, Evaluation of the EPR technique for mea-

Hishinuma, Compositional behavior and stability of MCtype precipitates in JPCA austenitic stainless steel during

suring sensitization in type 304 stainless steel, Corrosion,

HFIR irradiation, J. Nucl. Mater., 191-194(1992), p. 1351.

47(1991), No. 6, p. 429. [24] H.B. Li, Metallurgical Fundamental and Properties of High

[28] V. Kuzucu, M. Aksoy, M.H. Korkut, and M.M. Yildirim, The effect of niobium on the microstructure of ferritic stain-

Nitrogen Austenitic Stainless Steels [Dissertation], North-

less steel, Mater. Sci. Eng. A, 230(1997), No. 1-2, p.

eastern University, Shenyang, 2008, p. 113.

75.