Effect of nitrogen on crevice corrosion and repassivation

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  • 1. Effect of Nitrogen on Crevice Corrosion and Repassivation Behavior of Austenitic Stainless Steel* Haruo Baba and Yasuyuki Katada National Institute for Materials Science, Tsukuba 305-0047, Japan Austenitic stainless steels were produced based on a Fe-23 mass%Cr-4 mass%Ni alloy with varying nitrogen (0.7–1 mass%) and molybdenum contents (0–1 mass%), through electro-slag remelting (ESR) under high nitrogen gas pressure. The effects of nitrogen on crevice corrosion behavior in an acidic chloride solution were investigated, and the passive film of the crevice corrosion area after corrosion tests was analyzed using X-ray photoelectron spectroscopy (XPS). At the same time, the effects of nitrogen on the passivation behaviors after scratching were also investigated. During crevice corrosion at a noble potential of 0.7 V (SCE), the nitrogen in solid solution in the steel dissolves into the solution as NO3 À , and its concentration increases with the nitrogen content in the steel. It was also established that the number of corrosion spots, the corrosion loss, and the maximum depth of corrosion all decrease with the increase in the nitrogen content present in the steel and the applied potential. Such results can be attributed to the presence of NO3 À dissolved into the aqueous solution. On the other hand, results from scratch tests show that the increase in the amount of added nitrogen decreases the peak value of passivation current as well as the amount of electricity during repassivation, suggesting that nitrogen stimulates the passivation process and suppresses the occurrence of crevice corrosion. XPS analysis shows the presence of nitrogen as nitrides and NH3 in the surface layer of crevice corrosion and the internal layer of passivation films. [doi:10.2320/matertrans.MRA2007273] (Received November 7, 2007; Accepted December 17, 2007; Published February 25, 2008) Keywords: stainless steel, nitrogen, crevice corrosion, X-ray photoelectron spectroscopy, polarization, scratch test, repassivation 1. Introduction It is well known from many studies that nitrogen has the effect of enhancing the resistance to crevice corrosion and pitting corrosion of austenitic stainless steel.1–6) Compared to other additives such as chromium or molybdenum, a minute nitrogen content is effective in improving resistance to localized corrosion. Moreover, nitrogen addition helps to refine the microstructure and increase the strength of the material, and it can be used instead of nickel as an austenite- forming element. Currently, the addition of nitrogen during fusion of austenitic stainless steel at ordinary pressure is limited by its solubility, and obtaining a stainless steel with a nitrogen content as high as 1% is extremely difficult. Because of this, the behaviors and localized corrosion control mechanisms of solid solution nitrogen are not yet understood.7) Austenitic stainless steel obtained through the nitrogen gas pressurized electroslag remelting (ESR) method increases the nitrogen solubility, and makes the use of manganese, which reduces the corrosion resistance of the material, unnecessary. The behavior of nitrogen on the surface of nitrogen-bearing austenitic stainless steel has been investigated using X-ray photoelectron spectroscopy (XPS) and Auger electron spec- troscopy (AES), and many research works have reported enhancement of the resistance to localized corrosion.8–13) On the other hand, formation of a chromium oxide film preserves the passivation of stainless steel, and alternate dissolution and regeneration of the passivation film in aqueous solutions keeps a constant thickness of this film. However, in an aqueous solution with a high concentration of chloride ions, the passivation film is locally destroyed and the corrosion advances at an accelerating rate. As the passivation film on stainless steels is difficult to remove by cathodic reduction, mechanical means such as scratching or polishing are usually employed to destroy the film and expose a new surface so that the corrosion resistance can be evaluated from the repassivation mechanism.14–17) In general, steels with high corrosion resistance are easily re-passivated, but it is more difficult to rebuild the passivation film, and easier to promote local corrosion on the steel loosing its anticorrosion characteristics. One available method to investigate the effect of nitrogen on local corrosion is to observe the repassivation behavior after mechanically destroying the passivation film. In this study, high nitrogen-bearing austenitic stainless steels manufactured through the nitrogen gas pressurized ESR method were used to investigate the effects of nitrogen on crevice corrosion characteristics in an acidic chloride solution, and the passive film on the crevice corrosion after corrosion tests was analyzed. The controlling mechanisms for localized corrosion resistance were elucidated using XPS. Also, the effect of nitrogen on corrosion resistance was investigated by observing the repassivation behavior after scratching the passivation film instantaneously. 2. Experimental 2.1 Sample preparation High nitrogen-bearing austenitic stainless steels with the compositions shown in Table 1 were used for the samples. Austenitic stainless steels were produced based on a Fe-23 mass%Cr-4 mass%Ni alloy with varying nitrogen (0.7–1 mass%) and molybdenum contents (0–1 mass%), through electro-slag remelting under high nitrogen gas pressure. After hot forging, hot rolling and cold rolling, the steels were solution treated at 1250 C for 30 minutes. It was confirmed that the steels consisted of a single-phase austenite structure and no grain boundary precipitation of Cr nitrides was detected. Samples with dimensions of 50 mm  50 mm  *This Paper was Originally Published in Japanese in J. Japan Inst. Metals 71 (2007) 570–577. Materials Transactions, Vol. 49, No. 3 (2008) pp. 579 to 586 #2008 The Japan Institute of Metals
  • 2. 3 mm were cut from the steel bars. After wet polishing with emery paper No. 600, the samples were washed in water, degreased with acetone, washed in alcohol, and dried. 2.2 Measurement of polarization Samples were immersed in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl, degassed in an argon atmosphere, at 35 C. After cathodic reduction by applying a voltage of À0:7 V (SCE) for 10 minutes, potentiodynamic polarization in the anodic direction was carried out at 20 mV minÀ1 . A saturated calomel electrode (SCE) was used as the reference electrode and a Pt plate as the counter electrode, and the potential value was expressed in the SCE standard. 2.3 Crevice corrosion test at constant potential and quantitative analysis of nitrogen products dissolved in aqueous media The constant potential multiple crevice corrosion test was carried out at a constant potential to assess crevice corrosion. The sample had a central hole of 10 mm in diameter. A multiple crevice device with 20 crevices was prepared from a polysulfone resin disc with a diameter of 25.4 mm according to the ASTM G78 standard. Samples were held from both sides by the crevice forming material, and a torque of 8.5 Nm was applied through a torque wrench. The multiple crevice device shown in Fig. 1 was immersed in the anodic solution (260 cm3 ) in a glass electrolytic cell separated into anodic and cathodic compartments by a glass filter. A calomel electrode (SCE) was used as reference electrode, using a Pt plate as counter electrode of cathodic side. Crevice corrosion tests under potentiostatic conditions were carried out to apply potentials of 0.2 V (SCE) and 0.7 V (SCE) for 72 hours to the solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl, pH 1 at 35 C, and the solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl + 0.02 kmol/m3 NaNO3, pH 1.2 at 35 C. Amounts of anodic current, and the weight loss caused by corrosion, maximum depth of corrosion and quantity of corrosion of the crevice corrosion produced were measured. The amounts of NH4 þ , NO2 À and NO3 À eluted into the anodic solution as a result of crevice corrosion were also calculated using absorption spectroscopy (ASTM D1426-93 and ATM D3867-90). Microscopic corrosion test equipment with a CCD laser displacement sensor (Nittetsu-ELEX) and an optical microscope were used to measure the maximum depth. 2.4 Surface analysis by X-ray photoelectron spectrosco- py (XPS) Chemical bonding conditions for each element in the passive film and in the surface film of the crevice corrosion area after the corrosion tests of the high nitrogen-bearing austenitic stainless steels were analyzed using XPS. The equipment employed is a Quantum 2000 made by Physical Electronics. Mono-Al K -rays were used as an X-ray excitation source at a take-off angle of 90 to the surface of film. Wave separation was carried out after smoothing and background adjustment of the obtained spectra. 2.5 Measurement of repassivation The passivation film of the sample immersed in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl was scratched with a diamond needle to observe if repassivation occurs. A schematic diagram of the equipment used for scratch tests is shown in Fig. 2. Inside an electrochemical cell composed of a SCE and Pt opposite electrode, a load of 100 g or 200 g was set on the tip of the diamond needle. The diamond needle was set on the surface of the sample, the stage was moved at a speed of 20 mm/s at a designated horizontal displacement, and the instantaneous scratch left the surface newly exposed. At the same time, the peak of current density was measured from the current decay curve at a constant potential, and the quantity of electricity measured was used to estimate the repassivation behavior of the sample. Table 1 Chemical compositions of steels (mass%). Sample No. C Si Mn P S Ni Cr Mo N Al(Total) O (1) 0.7N-1Mo 0.020 0.11 0.06 0.005 0.0002 4.15 22.55 1.02 0.73 0.14 0.0014 (2) 0.8N-0Mo 0.024 0.13 0.08 0.006 <0:0001 4.16 22.96 <0:01 0.81 — 0.0029 (3) 0.9N-1Mo 0.024 0.12 0.09 0.006 0.0004 4.23 22.44 1.04 0.93 0.13 0.0019 (4) 0.9N-0Mo 0.034 0.11 0.10 0.005 0.0020 4.53 23.30 0.02 0.96 0.018 0.0022 PotentiostatThermostat Glass filter Water bath Saturated calomel electrode Pt counter electrode Electrolyte Multiple crevice device Titanium (bolt,nut,washers) Specimen Glass cell Cathodic side Anodic side Fig. 1 Schematic illustration of electrochemical cell used for dissolved nitrogen compound analysis and crevice corrosion measurements. Weight Diamond bit Pt counter electrode Luggin probe Reference electrode Electrolyte Specimen Stage Electrochemical cell Fig. 2 Schematic of electrochemical equipment used for scratch test. 580 H. Baba and Y. Katada
  • 3. 3. Results and Discussion 3.1 Polarization curve Figure 3 shows the potentiodynamic polarization curves of 0.7N-1Mo, 0.8N-0Mo, 0.9N-1Mo, and 0.9N-0Mo steels in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl at 35 C. Regardless of the nitrogen content, the critical passive current density (icrit) shows a tendency to decrease with the increase of molybdenum content. In the case of no addition of Mo, the critical passive current density shows a peak in the vicinity of À0:4 V (SCE). For all the steels, a steady passive current density was observed in the range from À0:2 to +0.8 V (SCE). 3.2 Potentiostatic crevice corrosion characteristics Figures 4(a) and 4(b) show the electric current vs. time curves corresponding to a potentiostatic crevice corrosion test carried out at 0.2 V (SCE) and 0.7 V (SCE). A tendency for the current to decrease as the nitrogen and molybdenum contents increased was confirmed. Especially, the electric currents for the 0.9N-1Mo and 0.9N-0Mo steels were considerably lower at 0.7 V (SCE) than at 0.2 V (SCE). Figures 5(a) and 5(b) show the current vs. time curves for potentiostatic corrosion tests in a solution after adding 0.02 kmol/m3 NaNO3. It was confirmed that the presence of NO3 À in the solution causes a sharp decrease in the current at the high potential value of 0.7 V (SCE) value, inhibiting crevice corrosion. Figures 6 and 7 represent relationships between weight loss and number of crevice corrosion spots against the nitrogen and molybdenum contents in samples immersed in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl for an applied potential of 0.2 V (SCE) and 0.7 V (SCE). The number of crevice corrosion spots for a potential of 0.2 V (SCE) remained almost constant at 40/40 regardless of the nitrogen or molybdenum content. In contrast, at a high potential of 0.7 V (SCE), the number of spots showed a tendency to decrease as the nitrogen content increased. The phenomenon that the number of crevice corrosion spots show a sharp decrease at high potential was observed. The corrosion weight loss at potentials of both 0.2 V (SCE) and 0.7 V (SCE) showed a tendency to decrease as the nitrogen content increased, but this tendency was especially evident at the high potential of 0.7 V (SCE). This phenomenon has been reported for high nitrogen-bearing austenitic stainless steels immersed in solutions containing chloride ions, establishing the dependence of the number of crevice corrosion spots and corrosion weight loss on the potential value.18) -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1 kmol/m 3 NaCl + 0.1kmol/m 3 HCl 35 o C 0.7N-1Mo 0.8N-0Mo 0.9N-1Mo 0.9N-0Mo Currentdensity,I/Am -2 Electrode potential, E / V vs. SCE 102 10 1 10-1 10-2 10-3 Fig. 3 Potentiodynamic polarization curves of 0.7N-1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution. (1) (2) (4) (3) (1) (2) (4) (3) 0 10 20 30 40 50 60 70 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1 kmol/m3 NaCl+0.1 kmol/m3 HCl pH 1 35o C 0.2V(SCE) 72h (1) 0.7N-1Mo (2) 0.8N-0Mo (3) 0.9N-1Mo (4) 0.9N-0Mo Current,I/mA (a) 0 10 20 30 40 50 60 70 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1 kmol/m3 NaCl+0.1 kmol/m3 HCl pH 1 35o C 0.7V(SCE) 72h (1) 0.7N-1Mo (2) 0.8N-0Mo (3) 0.9N-1Mo (4) 0.9N-0Mo Current,I/mA Time, t / h (b) Fig. 4 Current-time curves for potentiostatic crevice corrosion of 0.7N- 1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution. (a): at 0.2 V (SCE), 72 h and (b): at 0.7 V (SCE), 72 h. (1) (2) (4) (3) (1) (2) (4)(3) (a) (b) 0 10 20 30 40 50 60 70 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1 kmol/m3 NaCl+0.1 kmol/m3 HCl +0.02 kmol/m3 NaNO3 pH 1.2 35o C 0.2V(SCE) 72h (1) 0.7N-1Mo (2) 0.8N-0Mo (3) 0.9N-1Mo (4) 0.9N-0Mo Current,I/mA 0 10 20 30 40 50 60 70 0.0 0.1 0.2 0.3 0.4 0.5 1 kmol/m3 NaCl+0.1 kmol/m3 HCl+0.02 kmol/m3 NaNO3 pH 1.2 35o C0.7V(SCE) 72h (1) 0.7N-1Mo (2) 0.8N-0Mo (3) 0.9N-1Mo (4) 0.9N-0Mo Current,I/mA Time, t / h Fig. 5 Current-time curves for potentiostatic crevice corrosion of 0.7N- 1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl + 0.02 kmol mÀ3 NaNO3 solution. (a): at 0.2 V (SCE), 72 h and (b): at 0.7 V (SCE), 72 h. Effect of Nitrogen on Crevice Corrosion and Repassivation Behavior of Austenitic Stainless Steel 581
  • 4. Figure 8 shows the relationship between maximum depth of crevice corrosion and the nitrogen content in crevice corrosion tests carried out in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl at potentials of 0.2 V (SCE) and 0.7 V (SCE). Maximum depth at both potentials decreased as the nitrogen content increased, especially at the high potential of 0.7 V (SCE), the depth tended to become shallow. When 0.02 kmol/m3 NaNO3 was added, the number of crevice corrosion spots, the corrosion weight loss and the maximum corrosion depth decreased further as the nitrogen content increased. In the same way, the depth became shallow at high potential of 0.7 V (SCE). 3.3 XPS surface analysis of passivation film and crevice corrosion spots Figures 9 and 10 show the XPS spectra of N 1s and Mo 3p3/2 corresponding to the surface film of the crevice corrosion area and to the passivation film after corrosion tests carried out on a 0.7N-1Mo steel in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl at 35 C under potentials of 0.2 V (SCE) and 0.7 V (SCE). As the bonding energy of the N 1s and Mo 3p3/2 spectra overlap, waveform separation was carried out. At both potentials, N 1s spectra for the surface of the crevice corrosion area and for the passivation film show peaks in the vicinity of 399.9 eV and 397 eV. The first peak corresponds to NH3, whereas the later one indicates the presence of nitride; Mo 3p3/2 spectrum indicated the presence of Mo0 and Mo6þ , whereas the surface film of the crevice corrosion area showed a NH4 þ peak in the vicinity of 401 eV.9,19) Peaks corresponding to oxides, hydroxides and metals were detected in the Cr and Fe spectra, it was confirmed that the passivation film is composed of chromium and iron oxides. Table 2 shows the quantitative value of nitrogen content calculated from the area of the N 1s spectrum after waveform separation. At 0.2 V (SCE) and 0.7 V (SCE), nitride and NH3 contents in the surface film of the crevice corrosion area were lower than those in the passivation film. This can be attributed to elution of nitrogen into the solution by effect of the crevice corrosion. Nitrogen content in the crevice Numberofcrevicecorrosion(n/40)Numberofcrevicecorrosion(n/40) 0.7N-1Mo 0.8N-0Mo 0.9N-0Mo 0.9N-1Mo 0 10 20 30 40 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35 o C 0.2V(SCE) 72h 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35 o C 0.7V(SCE) 72h 0 10 20 30 40 Fig. 6 Crevice corrosion number of spots caused at 0.2 V and 0.7 V for a 0.7N-1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution. Corrosionloss,/mgCorrosionloss,/mg 0.7N-1Mo 0.8N-0Mo 0.9N-0Mo 0.9N-1Mo 0 20 40 60 80 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35 o C 0.2V(SCE) 72h 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35 o C 0.7V(SCE) 72h 0 20 40 60 80 Fig. 7 Crevice corrosion weight loss at 0.2 V and 0.7 V for 0.7N-1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution. Maximumdepth,d/mmMaximumdepth,d/mm 0.7N-1Mo 0.8N-0Mo 0.9N-0Mo 0.9N-1Mo 0.00 0.05 0.10 0.15 0.20 0.25 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C 0.2V(SCE) 72h 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C 0.7V(SCE) 72h 0.00 0.05 0.10 0.15 0.20 0.25 Fig. 8 Maximum depth of crevice corrosion at 0.2 V and 0.7 V for 0.7N- 1Mo, 0.8N-0Mo, 0.9N-1Mo and 0.9N-0Mo steels in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution. 582 H. Baba and Y. Katada
  • 5. corrosion surface film was a little lower for 0.2 V (SCE) than for 0.7 V (SCE). 3.4 Electrochemistry in production of ammonium, ni- trite and nitrate salts Potentiostatic electrolyses of the samples (shown in Figure 1) were carried out after being immersed into the anodic solution in the electrolytic cell, followed by quanti- tative analyses of the nitrogen component eluted into the anodic solution. The calculated values for nitrogen content were used to produce the results shown in Fig. 11, which is the potential-pH equilibrium diagram20) in the NH3-H2O system at potentials of A (0.2 V, SCE) and B (0.7 V, SCE) for potentiostatic crevice corrosion tests. This figure shows that NH4 þ is relatively stable at low potential, whereas NO3 À is stable at high potential. Figure 12 shows the ratio of two values; the nitrogen content eluted into the solution obtained from calculating the 800 850 900 950 1000 1050 1100 1150 Intensity(Arb.unit) MoO 3 NH3 Nitride Met.Mo NH4 + 1100 1200 1300 1400 1600 1700 1500 404 402 400 398 396 394 392 Binding energy, E / eV Intensity(Arb.unit) NH3 MoO3 Nitride Met.Mo N 1s, Mo 3p3/2 0.7N-1Mo 0.2V(SCE) (a) (b) Fig. 9 N 1s and Mo 3p3/2 XPS spectra with wave identification for 0.7N- 1Mo steel after crevice corrosion at 0.2 V (SCE) in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution of pH 0.93 at 35 C. (a): surface film of the crevice corrosion area and (b): passivation film. 900 950 1000 1050 1100 1150 1200 Intensity(Arb.unit) NH3 MoO3 NitrideNH4 + 1100 1200 1400 1500 1600 404 402 400 398 396 394 392 Binding energy, E / eV Intensity(Arb.unit) Met.Mo MoO3 Nitride NH3 1300 N 1s, Mo 3p3/2 0.7N-1Mo 0.7V(SCE) (a) (b) Fig. 10 N 1s and Mo 3p3/2 XPS spectra with wave identification for 0.7N- 1Mo steel after crevice corrosion at 0.7 V (SCE) in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution of pH 0.93 at 35 C. (a): surface film of the crevice corrosion area and (b): passivation film. Table 2 Quantitative values for nitrogen content in a 0.7N-1Mo steel after crevice corrosion at 0.2 V (SCE) and 0.7 V (SCE) in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution of pH 0.93 at 35 C. (a): surface film of the crevice corrosion area and (b): passivation film. (at%) 0.2 V (SCE) 0.7 V (SCE) (a) (b) (a) (b) Nitride 0.2 0.5 0.4 0.7 NH3 0.3 1.8 0.7 1.3 NH4 þ 0.2 — 0.2 — 0 2 4 6 8 10 12 14 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 pH NH3 NH4 + NO3 _HNO2 25o C -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 NO2 A B _ O2 / H2O H+ / H2 Electrodepotential,E/Vvs.SHE Electrodepotential,E/Vvs.SCE Fig. 11 Potential-pH diagram for an ammonium salt, nitrite and nitrate system. A,B indicate potential of the crevice corrosion tests. Effect of Nitrogen on Crevice Corrosion and Repassivation Behavior of Austenitic Stainless Steel 583
  • 6. weight loss values after crevice corrosion tests of a 0.7N-1Mo steel and a 0.8N-0Mo steel in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl at 35 C under potentials of 0.2 V and 0.7 V (SCE) during 72 hours; and the nitrogen content in NH4 þ , NO2 À , and NO3 À measured by absorption spec- troscopy. Figure 13 shows similar results for 0.9N-0Mo steel and 0.9N-1Mo steel. For 0.7N-1Mo steel and 0.8N-0Mo steel, after potentiostatic crevice corrosion tests of 0.2 V (SCE), the ratio of the nitrogen content eluted into the solution calculated from weight loss measurements and the nitrogen content calculated from stoichiometric NH4 þ value is almost 1, indicating that almost all the nitrogen in the solution is present as NH4 þ . There are several reports regarding nitrogen compounds formed by nitrogen dissolved into the bulk solution after being solidified in the steel by the effect of localized corrosion. The results in the present work coincide with the report by Osozawa et al.2) regarding the presence of NH4 þ in the solution in the vicinity of natural potential. According to these results, the amount of NH4 þ formed from Hþ in the pitting area and N counterbalances the amount of NH4 þ as nitrogen eluted from the steel. Results also coincide in that the formation of NH4 þ promotes a repassivation effect as it raises the pH in the pitting area. After a potentiostatic crevice corrosion test in the high potential region of 0.7 V (SCE), small nitrogen content remained in existence as, apart from NH4 þ , NO3 À and NO2 À . For the 0.9N-0Mo steel with a high nitrogen content, after the potentiostatic crevice corrosion test at 0.2 V (SCE), the ratio of the nitrogen value into the solution measured from (a) (c) (d) 0 0.5 1 1.5 0 0.5 1 1.5 0 0 0 0 0 0.7N-1Mo 0.2V(SCE) 72h 0.7N-1Mo 0.7V(SCE) 72h Detectednitrogen/dissolvednitrogen NH4 + NO2 - NO3 - 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C(b) 0 0.5 1 1.5 0 0.5 1 1.5 0 0 0 0 0.8N-0Mo 0.2V(SCE) 72h 0.8N-0Mo 0.7V(SCE) 72h Detectednitrogen/dissolvednitrogen NH4 + NO2 - NO3 - (c) 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C Fig. 12 Ratio between the total amount of N from the steel and NH4 þ , NO2 À and NO3 À dissolved after the crevice corrosion tests, (a): 0.7N-1Mo steel, 0.2 V (SCE); (b): 0.7N-1Mo steel, 0.7 V (SCE); (c): 0.8N-0Mo steel, 0.2 V (SCE); (d): 0.8N-0Mo steel, 0.7 V (SCE). (a) (b) (c) (d) 0 0.5 1 1.5 0 0.5 1 1.5 0 0 00 0 0.9N-0Mo 0.2V(SCE) 72h 0.9N-0Mo 0.7V(SCE) 72h Detectednitrogen/dissolvednitrogen NH4 + NO2 - NO3 - 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C 0 0.5 1 1.5 0 0.5 1 1.5 2 00 0.9N-1Mo 0.2V(SCE) 72h 0.9N-1Mo 0.7V(SCE) 72h Detectednitrogen/dissolvednitrogen NH4 + NO2 - NO3 - 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 35o C Fig. 13 Ratio between the total amount of N and NH4 þ , NO2 À and NO3 À dissolved after the crevice corrosion tests, (a): 0.9N-0Mo steel, 0.2 V (SCE); (b): 0.9N-0Mo steel, 0.7 V (SCE); (c): 0.9N-1Mo steel, 0.2 V (SCE); (d): 0.9N-1Mo steel, 0.7 V (SCE). 584 H. Baba and Y. Katada
  • 7. weight loss to the stoichiometric nitrogen value calculated from NH4 þ in the solution was almost 1, confirming that almost all the nitrogen in the steel solid solution was present in the solution as NH4 þ after elution. In the case of the high potential of 0.7 V (SCE), for 0.9N-0Mo steel and 0.9N-1Mo steel, the nitrogen eluted forms besides NH4 þ , NO3 À and NO2 À . The amount of NO3 À shows a tendency to increase, as does the nitrogen content in the steel. As shown in Fig. 6 and 7, as the nitrogen content in solid solution in the steel increases, the number of crevice corrosion spots and the corrosion weight loss decrease, indicating that when NO3 À was present in the solution, the number of crevice corrosion spots in the high potential region was suppressed. 3.5 Repassivation characteristics after exposing new surface by scratch test Figure 14 shows the current density decay caused by the appearance of a new surface after scratching of 0.2 cm with the load of 200 g, which was made on a 0.7N-1Mo steel immersed in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl at a potential of 0.2 V (SCE). There was an instantaneous surge in current density when a new surface appeared after the passivation film was destroyed by scratching, and the value of current went back to 0 mA/cm2 when repassivation instantaneously occurred. This maximum value of current density was the peak. After 3.6 seconds, the total quantity of electricity corresponds to the sum of the current due to repassivation and the current caused by dissolution of the 0.7N-1Mo steel. It can be observed that the lower the peak of current density and total quantity of electricity, the easier repassivation occurs. Figure 15 (a) and (b) illustrate the relationship between potentials and peaks of current density for 0.7N-1Mo steel and 0.9N-1Mo steel immersed in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl, after using loads of 100 g and 200 g to make a 0.2 cm scratch respectively. For both steels, the peak of current density increases as the potential increases, and it also increase with the scratching load. On the other hand, the peak is lower for the steel with the higher nitrogen content, suggesting that the nitrogen in solid solution in the steel promotes the repassivation. Figure 16 illustrates the relationship between potential and total quantity of electricity for repassivation for 0.7N-1Mo steel and 0.9N-1Mo steel immersed in a solution of 1 kmol/m3 NaCl + 0.1 kmol/m3 HCl, after using a load of 200 g to make a 0.2 cm scratch. In the case of 0.7N-1Mo steel, there is a tendency for the total quantity of electricity to increase as the potential increases. In the case of 0.9N-1Mo steel, the electricity remains almost constant up to the potential region of 0.7 V (SCE), but in the transpassivation region there is a sudden increase in the value of total quantity of electricity the same as in the case of 0.7N-1Mo. The high nitrogen content in solid solution in the steel reduced the value of the total quantity of electricity repassivation. Scratching caused a sudden drop in the natural potential, but as the scratch was stopped, potential returned rapidly to Currentdensity,I/Am-2 0 0.01 0.02 0.03 0.04 0.05 1 kmol/m3 NaCl+0.1 kmol/m3 HCl 0.7N-1Mo 0.2V(SCE) 200g, scratch 0.2cm 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Time, t / s Fig. 14 Current density decay after exposing a new surface on a 0.7N-1Mo steel at 0.2 V (SCE) in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution. Scratch length: 0.2 cm; scratch load: 200 g. -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1 kmol/m 3 NaCl+0.1kmol/m 3 HCl 200g scratch 2cm 0.7N-1Mo 0.9N-1Mo Quantityofelectricity,Q/Cm -2 Potential, E / V vs. SCE 0 2 4 6 8 10 12 14 16 Fig. 16 Total amount of electricity for repassivation against applied potential for a 0.7N-1Mo steel and for a 0.9N-1Mo steel, in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution. Scratch length: 2 cm; scratch load: 200 g. -0.2 0.0 0.2 0.4 0.6 0.8 1.0 (a) 1 kmol/m 3 NaCl+0.1 kmol/m 3 HCl 100g scratch 0.2cm 0.7N-1Mo 0.9N-1Mo Peakcurrentdensity,I/Am -2 Potential, E / V vs. SCE -0.2 0.0 0.2 0.4 0.6 0.8 1.0 (b) 1 kmol/m 3 NaCl+0.1 kmol/m 3 HCl 200g scratch 0.2cm 0.7N-1Mo 0.9N-1Mo Peakcurrentdensity,I/Am -2 Potential, E / V vs. SCE 0 0.02 0.04 0.06 0.08 0.1 0 0.04 0.08 0.12 0.16 0.20 0.24 Fig. 15 Current density peak against potential for a 0.7N-1Mo steel and for a 0.9N-1Mo steel in a 1 kmol mÀ3 NaCl + 0.1 kmol mÀ3 HCl solution. Scratch length: 0.2 cm. (a) scratch load: 100 g, and (b) scratch load: 200 g. Effect of Nitrogen on Crevice Corrosion and Repassivation Behavior of Austenitic Stainless Steel 585
  • 8. its former value. From analyses of the XPS spectra with modified take-off angles for a stainless steel with a nitrogen content of approximately 1%, Sagara et al.13) have suggested the possibility that there is a high nitrogen concentration in the inner layer of the passivation film, and that this nitrogen concentration in the inner layer increases with the polar- ization potential. From these observations, it is established that the nitrogen concentration in the inner layer of the passivation film has the effect of promoting repassivation. On the other hand, it has been reported that in a NO3 À solution at stable high potentials, NO3 À tends to increase with the potential, and the number of crevice corrosion spots as well as weight loss due to corrosion are markedly suppressed.18) It was also reported that the presence of NO3 À in the solution increases the resistance to pitting corrosion,21) and that NO3 À has a controlling effect on pitting corrosion at the high potential region.22,23) In the present research, when NO3 À is present in an acidic chloride aqueous solution, a considerable decrease in electric current is observed at high electric potential, corroborating that the crevice corrosion is controlled. At relatively low potentials, solidified nitrogen dissolves with crevice corrosion to produce NH4 þ in the solution and control the acidification inside the pit.2) In the present research, increase of the nitrogen content in nitrogen-bearing austenitic steel results in (1) decreased of number of crevice corrosion spots and the corrosion weight loss, with a tendency to further decrease as the polarization potential reaches high values; (2) the amount of NO3 À eluted into the solution showed a tendency to increase, and at the same time, the eluted NO3 À was adsorbed onto the surface of the passivation film, which had a inhibitor effect suppressing the dissolution of the base metal; (3) the peak current density and the total quantity of electricity for the repassivation process decreased and indicated a high corrosion resistance which promotes repassivation. 4. Conclusions Austenitic stainless steels with a nitrogen content ranging from 0.7 to 1 mass% were produced by the electroslag remelting (ESR) method and the effect of nitrogen on the crevice corrosion in an acidic chloride aqueous solution as well as XPS analyses of the surface film after crevice corrosion were carried out. At the same time, the repassiva- tion process after scratching of the passivation film was observed, leading to the following conclusions. (1) After a crevice corrosion test in the high potential, the nitrogen in solid solution in the steel eluted into the solution and was present as NO3 À , The concentration of nitrogen in the solution showed a tendency to increase with the nitrogen content in the steel. The number of crevice corrosion spots, the corrosion weight loss, and the maximum depth of corrosion decreased with the increase in nitrogen content, and further decreased with the values of polarization potential. (2) The NO3 À eluted into the aqueous solution was adsorbed onto the surface of the passivation film, and acted as an inhibitor preventing the dissolution of the base material. (3) As the nitrogen content added to the steel and the polarization potential increased, the current density peak and the total quantity of electricity for repassiva- tion decreased, promoting the repassivation and sup- pressing the occurrence of crevice corrosion. (4) The XPS analyses confirmed the presence of nitrogen in the form of nitrides or as NH3 at the crevice corrosion surface film and the inner layer of passivation film. REFERENCES 1) S. J. Pawel, E. E. Stansbury and C. D. Lundin: Corrosion 45 (1989) 125–133. 2) K. Osozawa, N. Okato, Y. Fukase and K. Yokota: Boshoku-Gijyutsu (Corros. Eng.) 24 (1975) 1–7. 3) R. C. Newman and T. Shahrabi: Corros. Sci. 27 (1987) 827. 4) H. Baba, T. Kodama and Y. Katada: Corros. Sci. 44 (2002) 2393–2407. 5) H. Yashiro, D. Takahashi, N. Kumagai and K. 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