1. 1 Crevice corrosion resistance of super-austenitic and super-duplex stainless steels in chloride solutions Pablo A. Martínez, Edgar C. Hornus, Martín A. Rodríguez and Ricardo M. Carranza Comisión Nacional de Energía Atómica, ARGENTINA Raúl B. Rebak GE Global Research, NY, USA C2015-5740

2. 2Outline Introduction Super-austenitic and super-duplex stainless steels / Crevice corrosion / PRE N / Crevice corrosion testing Objective Experimental Method Tested alloys / Testing conditions / Experimental procedure Results & Discussion Crevice corrosion repassivation potentials by PD-GS-PD & PD-PS-PD methods Conclusions

3. 3Introduction Spontaneous development and build-up of a Cr-rich passive film on stainless steels avoid the formation of rust and provide low corrosion rates. Stainless steels are usually classified according to their metallurgical structure: Austenitic Ferritic Martensitic Duplex Stainless steels may suffer localized corrosion, in the forms of pitting and crevice corrosion, and stress corrosion cracking when exposed to chloride-containing solutions.

4. 4444Introduction The Pitting Resistance Equivalent (PRE N ) is commonly used as an indicative measure of resistance to localized corrosion of stainless steels PRE N = %Cr + 3.3%Mo + 16%N PRE N ≥ 40  highly resistant to localized corrosion Localized corrosion resistance of stainless steels depends also on the metallurgical conditions: Delta ferrite Chromium carbides Sigma, Chi and Laves phases Sulfide inclusions, etc.

5. Introduction E R,CREV is commonly used for quantifying the crevice corrosion susceptibility of an alloy in a given environment. Reliable testing methods and crevicing devices are key to obtain conservative and reproducible values for E R,CREV. In crevice corrosion testing, PTFE-wrapped ceramic are more demanding crevice formers than solid PTFE. There are a few works reported in literature regarding to crevice corrosion of stainless steels S32750, S31254 and S32654. Polymeric-crevice formers were used in all of them.

6. 6Objective Study the crevice corrosion resistance of super-duplex S32750, and super-austenitic S31254 and S32654 stainless steels using experimental techniques and devices developed for the more corrosion-resistant Ni-Cr-Mo alloys

7. 7 Alloy 654SMO (UNS S32654) Fe-24Cr-22Ni-7.3Mo PRE N = 55 Alloy 254SMO (UNS S31254) Fe-20Cr-18Ni-6.3Mo PRE N = 44 Alloy 2507 (UNS S32750) Fe-25Cr-7Ni-4Mo PRE N = 43 Experimental Methods Testing solutions [Cl - ] = 10,000 and 100,000 ppm (0.282 and 2.82 mol/L NaCl) PCA specimen Torque 5 N.m Area 14 cm 2 Surface finished at #600

8. 8 Repassivation Potential Potentiodynamic-Galvanostatic-Potentiodynamic method PD-GS-PD 1.Potentiodynamic polarization (@ 0.167 mV/s) in the anodic direction up to reaching +300 µA 2.Application of a constant anodic current for 2 hours I GS = 300 µA (i GS ~ 25 µA/cm 2 ) 3.Potentiodynamic polarization (@ 0.167 mV/s) in the cathodic direction from the previous potential up to reaching alloy repassivation. E R,CREV = E CO (PD-GS-PD)

9. 9 Repassivation Potential Potentiodynamic-Potentiostatic-Potentiodynamic method PD-PS-PD 1.Potentiodynamic polarization (@ 0.167 mV/s) in the anodic direction up to reaching a pre-defined potential 2.Application of this pre-defined potential (E PS ) for 20 hours 3.Potentiodynamic polarization (@ 0.167 mV/s) in the cathodic direction from the previous potential up to reaching alloy repassivation. E R,CREV = E CO (PD-PS-PD)

10. 10Results [Cl - ] = 10,000 ppm [Cl - ] = 100,000 ppm Polarization curves at 30ºC Wide passive range Small anodic peak at 0.5-0.7 V SCE Secondary passivity Passivity breakdown at 0.9-1.0 V SCE O 2 evolution likely E > 0.57 V SCE Gas bubbles not observed Peak

11. 11Results Super-duplex S32750 [Cl - ] = 10,000 ppm [Cl - ] = 100,000 ppm PD-GS-PD tests at different temperatures in chloride solutions [Cl - ] = 10,000 ppm 60ºC Peak

12. 12Results [Cl - ] = 10,000 ppm [Cl - ] = 100,000 ppm PD-GS-PD tests at different temperatures in chloride solutions Super-austenitic S31254 [Cl - ] = 10,000 ppm 60ºC Peak

13. 13Results [Cl - ] = 10,000 ppm [Cl - ] = 100,000 ppm PD-GS-PD tests at different temperatures in chloride solutions Super-austenitic S32654 [Cl - ] = 10,000 ppm 60ºC Peak

14. 14Results Repassivation potentials and peak potentials in PD-GS-PD tests Crevice corrosion current density peaked at 0.2-0.3 V above the corresponding E CO.

15. 15Results Repassivation potential from PD-GS-PD tests as a function of temperature At 30ºC, alloy S32654 showed a significantly higher E CO than alloys S32750 and S31254 However, at 60ºC and 90ºC, E CO was similar for the three tested alloys

16. 16Results Super-duplex S32750 PD-PS-PD tests in [Cl - ] = 100,000 ppm solutions, at 30ºC Potentiodynamic scans (PD)Potentiostatic stage (PS) Peaks

17. 17Results Repassivation potential and current density after 20 hours of polarization as a function of the applied potential in the PS stage Repassivation PotentialFinal Current Density PD-PS-PD may be better than PD-GS-PD when crevice corrosion is difficult to initiate. However, PD-PS-PD tests with E PS > 0.4 V SCE may also lead to transpassive dissolution of the alloys.

18. 18Results Another set of PD-PS-PD tests was performed for the tested alloys in [Cl - ] = 10,000 ppm solutions varying E PS in the vicinity of the previously determined E CO (PD-GS-PD). The selected values of E PS included potentials higher and lower than the corresponding E CO (PD-GS-PD). Consequently, crevice corrosion was not observed in all the tested conditions. E CO (PD-PS-PD) was obtained only from those tests which led to crevice corrosion.

19. 19Results Repassivation potential from PD-PS-PD tests as a function of temperature E CO (PD-PS-PD) values were qualitatively in agreement with E CO (PD-GS- PD) values in [Cl - ] = 10,000 ppm solutions.

20. 20Results Repassivation potential from PD-PS-PD tests as a function of temperature Zoom E PROT was defined from PD-PS-PD tests between the maximum E PS at which crevice corrosion was not observed and the minimum E PS at which crevice corrosion occurred for each alloy In general, E CO was more conservative than E PROT

21. 21Discussion Temperature showed a more pronounced effect than [Cl - ] in the tested conditions. Alloy S32654 did not suffer crevice corrosion at 30ºC, but only at 60 and 90ºC. For alloys S31254 and S32654, E CO generally decreased with increasing temperatures and [Cl - ] For alloy S32750, E CO did not show a clear dependence neither with temperature nor with [Cl - ] Crevice corrosion resistance ranking at 30ºC: S32750 < S31254 < S32654 However, at 60 and 90ºC the three tested alloys showed comparable E CO within experimental error

22. 22Conclusions Alloys S32750 and S31254 suffered crevice corrosion at 30, 60 and 90ºC while alloy S32654 only suffered crevice corrosion at 60 and 90ºC. In general, the crevice corrosion repassivation potential of alloys S31254 and S32654 decreased with increasing temperatures and chloride concentrations. The repassivation potential of alloy S32750 did not show a clear dependence neither with temperature nor with chloride concentration, in the tested conditions. Crevice corrosion initiation was significantly affected by temperature. Localized corrosion initiated at high potentials in PD-GS-PD tests for the low testing temperatures and at low potentials for the higher temperatures.

23. 23Conclusions Repassivation was less affected by temperature. At 30ºC, alloy S32654 clearly outperformed alloys S32750 and S31254 as indicated by the PRE N. However, at 60 and 90ºC the repassivation potentials of the alloys were similar within experimental error. The crevice corrosion current density of the tested alloys showed a maximum value in a potential region above the repassivation potential and below the secondary passivity. The lower crevice corrosion current densities observed at the secondary passivity may be due to the inhibiting effect of oxyanions such as molybdates and chromates which are released in these conditions.

24. 24Acknowledgements Financial support from the Agencia Nacional de Promoción Científica y Tecnológica of the Ministerio de Ciencia, Tecnología e Innovación Tecnológica and from the Universidad Nacional de San Martín from Argentina is acknowledged.