Effect of Temperature and Chloride Concentration on the Crevice Corrosion Resistance of Austenitic Stainless Steels Edgar C. Hornus, Martín A. Rodríguez and Ricardo M. Carranza Comisión Nacional de Energía Atómica, ARGENTINA Raúl B. Rebak GE Global Research, NY, USA C2016-7170

Outline Introduction Objective Experimental Method Austenitic stainless steels / Localized corrosion / PREN / Crevice corrosion testing Objective Experimental Method Tested SSs / Testing conditions / Experimental procedure Results & Discussion Crevice corrosion repassivation potentials by PD-GS-PD method / Corrosion potential in the crevice-like solution / Localized acidification model. Conclusions This an outline of the presentation.

Introduction 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. Read the screen.

Introduction The Pitting Resistance Equivalent (PREN) is commonly used as an indicative measure of resistance to localized corrosion of stainless steels PREN = %Cr + 3.3%Mo + 16%N Localized corrosion resistance of stainless steels depends also on metallurgical variables (delta ferrite, Cr carbides, sigma, chi and Laves phases, sulfide inclusions, etc.) Austenitic stainless steels of the 300 series are the most popular corrosion-resistant alloys. In the nuclear industry, they are used to manufacture canisters for radioactive intermediate-level waste and they are also considered as metal waste forms for high-level waste. Read the screen. 4 4 4

Introduction ER,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 ER,CREV. In crevice corrosion testing, PTFE-wrapped ceramic are more demanding crevice formers than solid PTFE. Most of the research in the published literature on crevice corrosion of stainless steels used solid polymeric-crevice formers. In previous research, we tested super-duplex stainless steel S32750 (PREN = 43) and super-austenitic stainless steels S31254 (PREN = 44) and S32654 (PREN = 55). Read the screen.

Objective Determine the crevice corrosion resistance of austenitic stainless steels 304 (PREN = 18) and 316 (PREN = 26) using experimental techniques and devices previously developed for more corrosion-resistant materials, such as Ni-Cr-Mo alloys. The current assessment is necessary to obtain quantitative parameters on the same basis for a proper comparison of the crevice corrosion resistance of alloys. Read the screen.

Experimental Methods 304 SS (UNS S30400) Fe-18Cr-8Ni PREN = 18 316 SS (UNS S31600) Fe-18Cr-10Ni-2.3Mo PREN = 26 Full solubilization (no carbides) Testing solutions 0.1 and 1 mol/L NaCl, and 5 mol/L CaCl2 T = 0, 10, 30, 60 and 90ºC PCA specimen Torque 5 N.m Area ~14 cm2 Surface finished at #600 304 and 316 SSs specimens were prepared from wrought mill annealed plate stock. The specimens were heat treated for 15 minutes at 1050ºC and then water-quenched to obtain full solubilization of any precipitates. The nominal chemical compositions of the alloys in weight percent and the PREN are listed in the table. Prism Crevice Assemblies (PCA) specimens were used. The specimens were fabricated based on ASTM standards G 192 and G 48. The crevicing device contained 24 artificially creviced spots formed by two ceramic washers (crevice formers) wrapped with a 70 µm-thick PTFE tape. The applied torque was 5 N-m. The surface area exposed to the electrolyte solution was approximately 14 cm2. All specimens were surface finished using 600 grit abrasive paper and then degreased in ethanol and washed in distilled water. The surface preparation was performed 1 hour prior to testing. Testing solutions were 0.1 and 1 mol/L NaCl and 5 mol/L CaCl2. The temperature of the solution was controlled by immersing the cell in a water bath, which was kept at a constant temperature using a cryothermostat. The set point temperatures (T) were 0, 10, 30, 60 and 90ºC.

Repassivation Potential Potentiodynamic-Galvanostatic-Potentiodynamic method PD-GS-PD Potentiodynamic polarization (@ 0.167 mV/s) in the anodic direction up to reaching +300 µA Application of a constant anodic current for 2 hours IGS = 300 µA (iGS ~ 25 µA/cm2) Potentiodynamic polarization (@ 0.167 mV/s) in the cathodic direction from the previous potential up to reaching alloy repassivation. The crevice corrosion repassivation potential was determined by the Potentiodynamic-Galvanostatic-Potentiodynamic (PD-GS-PD) method. Before each PD-GS-PD test, the open circuit potential was measured for 15 minutes and afterwards a cathodic current of 50 µA was applied for 5 minutes (pre-treatment). The PD-GS-PD method consists of three stages: (1) a potentiodynamic polarization (at a scan rate of 0.167 mV/s) in the anodic direction until reaching an anodic current of 300 µA, (2) the application of a constant anodic current of IGS = 300 µA (approximately iGS = 22 µA/cm2) for 2 hours, and (3) a potentiodynamic polarization (at 0.167 mV/s) in the cathodic direction, from the previous potential until reaching alloy repassivation. Three or more tests were performed for each testing condition.

PD-GS-PD tests in 0.1 mol/L NaCl at 0ºC Results PD-GS-PD tests in 0.1 mol/L NaCl at 0ºC These figures show PD-GS-PD tests for 304 and 316 SS in 0.1 mol/L NaCl at 0ºC. The two tested SSs showed forward and reverse curves (Stages 1 and 3, respectively) with similar features. 316 SS showed an anodic shift in ECORR of ~100 mV compared to those of 304 SS (Figure on the right). Both SSs showed a large potential drop during the galvanostatic stage (Stage 2). Most of this potential drop in Stage 2 occurred within the first 200 seconds of galvanostatic polarization (Figure on the right). The repassivation potential was defined as the potential value in the reverse curve (Stage 3) at which the current density drops below 1 µA/cm2 without any further increase. The repassivation potential defined in this way is usually called ER1 and it is shown in the Figure on the left. This criterion for the repassivation potential was preferred over other criteria due to the poor reproducibility observed in the forward potential scan and the variability of the passive current density in the tested conditions. Stages 1, 2 & 3 Stage 2 ER,CREV  ER1: the potential value in the reverse curve at which the current density drops below 1 µA/cm2 without any further increase.

PD-GS-PD tests for 304 SS in chloride solutions at 30ºC Results 304 SS PD-GS-PD tests for 304 SS in chloride solutions at 30ºC This figure shows PD-GS-PD tests for 304 SS at 30ºC in solutions with different chloride concentrations ([Cl-]). ER1 decreased as [Cl-] increased as expected. The passive potential range became narrower as the chloride concentration increased. The test in 5 mol/L CaCl2 did not show a passive range.

PD-GS-PD tests for 316 SS in 5 mol/L CaCl2 Results 316 SS PD-GS-PD tests for 316 SS in 5 mol/L CaCl2 This figure shows PD-GS-PD tests for 316 SS in 5 mol/L CaCl2 at different temperatures. The potentials associated with passivity breakdown and repassivation decreased as the temperature increased. There was also an apparent increase of the passive current density with increasing temperatures. However, the test at 60ºC also showed an anodic branch starting at higher potentials and there was not a defined passive range.

Creviced stainless steels specimens after PD-GS-PD tests Results Creviced stainless steels specimens after PD-GS-PD tests These images correspond to specimens of 304 and 316 SSs after PD-GS-PD tests. The crevice corrosion damage for both materials in 0.1 and 1 mol/L NaCl occurred below the crevice formers borderline and the amount of corrosion products on the specimens surface increased with the increase of temperature. In the tests performed in 5 mol/L CaCl2, the specimens were apparently less corroded than in 0.1 and 1 mol/L NaCl. In 5 mol/L CaCl2 the attack spread out from the crevice formers borderline towards the specimen surface exposed to the bulk solution. The attacked area was larger in 10 mol/L chloride solutions compared with that in 0.1 and 1 mol/L chloride solutions, and as a consequence the attack in the latter solutions was visibly deeper. It must be considered that the PD-GS-PD technique applied a fixed charge of 2.16 Coulombs (300 µA * 7200 s) in Stage 2 (crevice corrosion propagation). This dramatic change in the mode of the crevice corrosion attack in very concentrated chloride solutions also occurs in Ni-Cr-Mo alloys. It may be argued that the localized attack always started below the crevice former borderline: in 0.1 and 1 mol/L chloride solutions the attack grew deeper at this location while in 10 mol/L solutions the attack spread out from the crevice towards the non-creviced surface.

Results SEM Images 304 SS 316 SS 304 SS 316 SS Tests in 0.1 mol/L NaCl at 0ºC 304 SS 316 SS These are SEM images comparing the crevice corroded 304 and 316 SSs tested in different experimental conditions. Corrosion pits were observed in the creviced corroded spots. The crevice corrosion damage for both SSs may be characterized as crystalline attack plus pitting corrosion. Crystalline type of attack in crevice corrosion is typical of Ni-Cr-Mo alloys, the different crystal planes corrode at different rates and consequently crystal planes are discernible. Tests in 1 mol/L NaCl at 30ºC

Results SEM Images 304 SS 316 SS 304 SS 316 SS Tests in 1 mol/L NaCl at 60ºC 304 SS 316 SS Pit density was lower in 10 mol/L chloride solutions compared to 0.1 and 1 mol/L chloride solutions. For 316 SS, pit density increased significantly from 30ºC to 60ºC. Pit coalescence was observed for both steels in certain conditions. The 2.5% Mo content of 316 SS was responsible for its significantly lower pitting density compared with that of 304 SS for all the tested conditions. Pit nucleation in the crevice corroded spots was apparently not associated with any metallurgical defects like inclusions or grain boundaries. Tests in 5 mol/L CaCl2 at 60ºC

Localized acidification model Results Open circuit potential (ECORR) of 304 and 316 SSs in HCl solutions Localized acidification model ER1 = ECORR* + η + Δϕ Crevice-like solution? Candidates 0.5 mol/L HCl 1 mol/L HCl The crevice-like solution can be inferred by comparison of the obtained ECORR with the lowest ER1 in the most aggressive tested conditions The open circuit or corrosion potential of 304 and 316 SSs was monitored for 2 hours in deaerated 0.5 and 1 mol/L HCl solutions at different temperatures. The values in the last 10 minutes were averaged obtaining a standard deviation lower than 2 mV. These average values are represented as a function of temperature. ECORR for 304 and 316 SSs showed a slight increase (< 20 mV) from 0ºC to 90ºC. Most of the ECORR increase occurred from 60ºC to 90ºC for 304 SS, and from 0ºC to 30ºC for 316 SS. ECORR of 304 SS was ~20 mV higher in 1 mol/L HCl than in 0.5 mol/L HCl. For 316 SS, ECORR in 1 mol/L HCl was ~60 mV higher than in 0.5 mol/L. ECORR of 316 SS exceeded that of 304 SS by 20 to 40 mV in 0.5 mol/L HCl, and by 60 to 80 mV in 1 mol/L HCl. The corrosion potential of the alloys in the crevice-like solution is an important parameter for the localized acidification model. The other terms of the equation are the polarization to sustain the critical chemistry (η) and an ohmic potential drop (ΔΦ). 0.5 and 1 mol/L HCl solutions were selected as candidate solutions based o previous research on more corrosion-resistant materials. The crevice-like solution can be inferred by comparison of the obtained ECORR with the lowest ER1 in the most aggressive tested conditions as we will see later. Tests in solutions with an acidity similar to the crevice-like solution

Results SEM Images 304 SS 316 SS 304 SS 316 SS 2-hour immersion at open circuit in 1 mol/L HCl at 30ºC 304 SS 316 SS These SEM images show non-creviced 304 and 316 SSs specimens after 2-hour immersions in 1 mol/L HCl at 30 and 60ºC. At 30ºC, both SSs showed crystalline type of attack and a few pits. 304 SS showed more pits than 316 SS. Pit density and pit diameter increased at 60ºC for both SSs. This type of attack resembles that observed in the crevice corroded spots of these materials. 2-hour immersion at open circuit in 1 mol/L HCl at 60ºC

Polarization curves of 304 and 316 SSs in 0.5 mol/L HCl Results Polarization curves of 304 and 316 SSs in 0.5 mol/L HCl These are anodic polarization curves of 304 and 316 SSs in 0.5 mol/L HCl, at different temperatures. 316 SS showed lower current densities than 304 SS in all the tested conditions reaching a passive or pseudo-passive state after an anodic peak. This anodic peak was probably associated with an active to passive transition though the quality of the passivity was rather poor since high currents were observed. The current density of the peak increased with temperature. 304 SS showed a small current drop at higher potentials. All the tested specimens were severely pitted after testing. The molybdenum addition of 316 SS with respect to 304 SS not only improved the corrosion resistance of the material in the active range (low potentials) but also allowed some degree of passivation at high potentials. 30ºC 60ºC 90ºC 316 SS showed lower current densities than 304 SS reaching a passive or pseudo-passive state after an anodic peak The Mo addition of 316 SS with respect to 304 SS not only improved the corrosion resistance of the material in the active range but also allowed some degree of passivation at high potentials

Discussion ER1 decreased linearly with the increase of temperature in the range from 0ºC (or 10ºC) to 60ºC for 304 SS, and in the entire temperature range for 316 SS. A linear equation was fitted to the data of both steels at the three tested chloride concentrations. The fit parameters are listed in the table. A and B are constants which depend on the tested stainless steel and chloride concentration. ER1 = A + BT Read the screen.

Discussion 304 SS ER1 in near neutral chloride solutions and ECORR in acidic chloride solutions as a function of temperature for 304 SS. ER1 for 304 SS decreased as temperature increased from 0ºC to 60ºC with slopes of ‑4.4 mV/K and ‑3.5 mV/K in [Cl-] = 0.1 mol/L and [Cl-] = 1 mol/L solutions, respectively. In [Cl-] = 10 mol/L solutions, the temperature dependence was significantly lower. ER1 showed a constant value in the temperature range from 60ºC to 90ºC for the three tested chloride concentrations. ECORR of 304 SS in 0.5 and 1 mol/L HCl solutions are also represented as a function temperature. 0.5 mol/L HCl is more representative as a crevice-like since the ECORR of 304 SS are lower and close to ER1 in the more concentrated and hot chloride solutions.

Discussion 316 SS ER1 in near neutral chloride solutions and ECORR in acidic chloride solutions as a function of temperature for 316 SS. ER1 for 316 SS decreased as temperature increased with slopes of ‑3.0 mV/K ([Cl-] = 0.1 mol/L), ‑2.5 mV/K ([Cl-] = 1 mol/L) and ‑0.7 mV/K ([Cl-] = 10 mol/L). The absolute value of the slopes decreased as the chloride solutions became more concentrated for the tested SS. This behavior is in agreement with that of Ni-Cr-Mo alloys in the same testing conditions. ECORR of 316 SS in 0.5 and 1 mol/L HCl solutions are also represented as a function temperature. As in the case of 304 SS, 0.5 mol/L HCl is more representative as a crevice-like since the ECORR of 316 SS are lower and close to ER1 in the more concentrated and hot chloride solutions.

Discussion Localized acidification model ER1 = ECORR* + η + Δϕ The crevice-like solutions of 304 and 316 SSs are more dilute and their ECORR* are hundreds of mV lower than those of Ni-Cr-Mo alloys. For sufficiently high temperatures and chloride concentrations the terms η + ΔΦ became nil. The crevice-like solutions of 304 and 316 SSs are more dilute and their ECORR* are hundreds of mV lower than those of Ni-Cr-Mo alloys. For sufficiently high temperatures and chloride concentrations the terms η + ΔΦ became nil and ER1 = ECORR* These conditions were attained for 304 SS in the tested chloride solution at 60ºC and higher temperatures and for 316 SS at 90ºC in 1 and 10 mol/L chloride solutions. ER1 = ECORR* + η + Δϕ For 304 SS  [Cl-] = 0.1, 1 and 10 mol/L at 60ºC and higher temperatures For 316 SS  [Cl-] = 1 and 10 mol/L at 90ºC

Discussion Localized acidification model Effect of 2.5% Mo addition in 316 SS compared to 304 SS ΔECORR* = ECORR* (316 SS) – ECORR* (304 SS) ~ 30 mV ΔER1 = ER1(316 SS) – ER1(304 SS) = f(T, [Cl-]) ΔECORR* = 30 mV was obtained as the difference of ECORR* for 316 SS and ECORR* for 304 SS. We also define ΔER1 as the difference of ER1 for 316 SS and ER1 for 304 SS, in identical experimental conditions, to quantify the effect of Mo on the crevice corrosion. ΔER1 is a function of temperature and chloride concentration. This figure shows ΔER1 and ΔECORR* as a function of the temperature. ΔER1 increased as [Cl-] decreased. In the context of the localized acidification model, this is a clear indication that ΔΦ increases with the Mo addition and this effect is more pronounced for dilute chloride solutions. For [Cl-] = 0.1 and 1 mol/L solutions, ΔER1 increased with temperature from 0ºC to 60ºC. Then it decreased from 60ºC to 90ºC due to the saturation of ER1 for 304 SS. We may infer that Mo is a more effective alloying element as temperature increases with regard to crevice corrosion resistance. In [Cl-] = 10 mol/L solutions, there was no other significant effect of Mo than the increase of ECORR*. At this chloride concentration, ΔER1 slightly decreased with increasing temperatures until reaching ΔER1 = ΔECORR* at 90ºC. This behavior indicates a marginal effect of the Mo addition on the term η. Present analyses indicate that the localized acidification model is able to rationalize the effect of Mo in two of the three terms of its fundamental equation.

Conclusions The repassivation potential of 304 SS decreased with increasing [Cl-] and T in the range from 0 to 60ºC, and it reached a constant value in the range from 60 to 90ºC regardless [Cl-]. This minimum and constant repassivation potential value was the corrosion potential in the crevice-like solution, ECORR* = ‑0.430 ± 0.015 VSCE. The repassivation potential of 316 SS showed a decrease with increasing [Cl-] and T in the entire tested temperature range. The repassivation potential of 316 SS is expected to saturate at ‑0.400 ± 0.017 VSCE reaching the corrosion potential in the crevice-like solution. Read the screen.

Conclusions The effect of the 2.5% Mo addition in 316 SS compared to 304 SS on the crevice corrosion resistance was analyzed in the context of the Galvele’s localized acidification model. The Mo addition led to a 30 mV increase in the corrosion potential in the crevice-like solution and an increase in the ohmic potential drop which was most significant for dilute chloride solutions. The polarization needed to sustain the critical chemistry in the crevice did not change significantly in the tested conditions. These findings highlights the fact that 40 years after the appearance of the seminal paper of the localized acidification model, it continues to give insights on localized corrosion and rationalizing new knowledge. Read the screen.

Acknowledgements 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.