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DOI: 10.1016/j.jmrt.2018.05.012
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Comparative study of the pitting corrosion resistance, passivation behavior and metastable pitting activity of NO7718, NO7208 and 439L super alloys in chloride/sulphate media
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Roland Tolulope Loto
Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria
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Table 1. Nominal composition (wt.%) of N07718, N07208 and SS439L.
Table 2. Potentiodynamic polarization data for NO7718, NO7208 and SS439L in 4M H2SO4/0–6% NaCl concentration.
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Abstract

Comparative study of the corrosion resistance and passivation characteristics of NO7718, NO7208 and SS439L super alloys in 4M H2SO4 media at 0–6% NaCl concentration was done by potentiodynamic polarization, open circuit potential measurement and optical microscopic analysis. The metal alloys showed relatively high resistance to corrosion as variation in Cl ion concentration had no significant influence on their corrosion rate results. SS439L showed the weakest resistance to pitting despite its relatively low corrosion rate. Passivation behavior disappeared after 2% NaCl concentration. NO7208 displayed exceptional pitting resistance characteristics, however, its metastable pitting and repassivation domain responds to changes in Cl ion concentration. The passivation domain of NO7718 significantly decreased with increase in Cl ion concentration due to delayed repassivation of the protective film after metastable pitting. NaCl concentration caused significant negative shift in open circuit potential (OCP) for NO7718 and SS439L alloys, compared to NO7208. The OCP of NO7208 at comparatively positive values contrasts those of NO7718 and SS439L alloys. Micro analytical studies shows a highly corrosion resistant morphology for NO7208 with numerous macro-pits present but their effect on the overall corrosion resistance of the nickel alloy is negligible. The corrosion pits on SS439L coupled with the appearance of grain boundaries are deleterious to the overall viability of the alloy. Enlarged corrosion pits are present on NO7718 due to the action of Cl ions.

Keywords:
Corrosion
Pitting
Passivation
Alloy
Full Text
1Introduction

Super alloys display excellent mechanical properties, surface stability, low corrosion susceptibility and high resistance to thermal deformation. This necessitates their extensive application as construction materials in nuclear industries, for piping and pumps in chemical processing and petrochemical industries, heat exchangers, in coal gasification and liquefaction systems, for the combustor and turbine sections of the aero engine and land-based industrial-gas turbines, marine applications and pulp and paper manufacturing [1,2]. Their corrosion resistance is due primarily to the presence of chromium within their metallurgical structure, and responsible for the formation and stability of an impenetrable passive oxide film on the alloy surface when exposed to oxidizing conditions. The passive film hinders the electrochemical action of corrosive anions prevalent in industrial conditions due to their adherent inert nature. However there are circumstances where the passive film breaks due to a variety of causes in very hostile environments where high pressure and temperature conditions and strong halides are encountered. Collapse of the passive film and the consequent localized corrosion in the presence of chlorides is the major underlying factor responsible for failure of these alloys [3,4]. The electrochemical behavior of super alloys with respect to the formation, durability and breakdown of their passive film is integral to understanding of the metastable and pitting corrosion resistance of these alloys. In understanding the nature of passive film stability, potentiodynamic polarization and open circuit potential measurement are employed while the resulting microstructure from electrochemical study are further analyzed. Previous research on pit initiation and growth on stainless steel alloys have contributed to their current understanding and have help improve the pitting corrosion resistance of the alloys especially in the area of material selection and appropriate operating conditions [5–10]. Yi et al. [11] studied the pitting corrosion behavior of AISI type 316 SS in NaCl solutions by potentiodynamic polarization and concluded that its critical pitting potential is significantly influenced by the scan rate. Hodge and Wilde [12] observed through anodic polarization that Ni–Cr alloys with minimum 10% chromium become passive and displayed sufficient resistance to pitting in sulphate/chloride media. Piron et al. [13] observed similar behavior with Ni 200 and Inconel 600 in sulphate media at 0–3.5wt.% NaCl. Breakdown of passivity on alloy 22 was observed by Friend [14] at a critical NaCl concentration by maintaining its potential inside the passive domain. The electrochemical behavior of alloy 600 and alloy 800 passive films was studied in various chemical environments [15,16]. Nickel chromium molybdenum alloys (NO7718 and NO7208) and ferritic stainless steel 439L exhibit excellent resistance to deterioration and failure in both oxidizing and reducing conditions. The ability of these alloys to withstand the effect corrosive anions is subject to the nature and constituent of their passive film, repassivation capacity, metastable pitting activity and pitting corrosion resistance. This research aims to study the passivation characteristics of the above-mentioned alloys in chloride/sulphate solutions and the resulting microstructure.

2Experimental methods

NO7718 and NO7208 nickel alloys (NO7718, NO7208), and 439L ferritic stainless steel (SS439L) obtained from PCC Airfoils, LLC Prototype foundry, OH USA, Haynes International Incorporation, Kokomo, USA and McMaster University, Ontario, Canada are the super alloys studied. Their nominal (wt.%) compositions are laid out in Table 1. NO7718, NO7208 and SS439L were segmented and machined to samples with exposed surface areas of 0.4cm3, 0.4cm3 and 0.5cm3. They were subsequently prepared metallographically with the aid of silicon carbide abrasive papers of 60, 120, 220, 320, 600, 800 and 1000 grits before polishing with diamond liquid suspension to 6μm, and then purified with distilled water and acetone for electrochemical tests. Recrystallized sodium chloride sourced from Titan Biotech, India was formulated in proportional concentrations of 0%, 1%, 2%, 3%, 4%, 5% and 6% in 200mL of 4M H2SO4 solution, prepared from standard grade of H2SO4 acid (98%) with distilled water.

Table 1.

Nominal composition (wt.%) of N07718, N07208 and SS439L.

Element symbol  Cu  Si  Ni  Cr  Mn  Co  Al  Ti  Mo  Fe 
% Composition (N07718)  0.3  0.35  51  18  0.35  0.04  0.05  0.08  0.8  0.3  1.1  –  24.72 
% Composition (N07208)  –  0.15  56  20  0.3  –  –  0.06  10  1.5  2.1  8.5  –  1.5 
% Composition (SS439L)  –  0.75  0.2  17.35  0.5  0.04  0.03  0.01  –    0.335  0.1  0.03  80.66 

Electrochemical tests were performed on the super alloys at 37°C ambient temperature with a three electrodes cell (platinum counter electrodes, Ag/AgCl reference electrodes and resin embedded NO7718, NO7208 and SS439L electrodes) (exposed surface area of 1cm2) containing 200mL of the acid media and connected to Digi-Ivy 2311 electrochemical workstation. Potentiodynamic polarization plots obtained at scan rate of 0.0015V/s from −1.25V and +1.5V set potentials. The corrosion current density, Cd (A/cm2) and corrosion potential, Cp (V) values were obtained from the polarization plots through Tafel extrapolation method. Corrosion rate, CR (mm/y) and inhibition efficiency, η (%) were determined from the following mathematical relationship:

D is the density in (g/cm3); Eq is the metal alloy equivalent weight (g). 0.00327 is the constant for corrosion rate. Open circuit potential measurement (OCP) was performed at 0.05V/s step potential for 1500s to study the thermodynamic stability of the alloys at rest potentials. Micro-analytical images of the super alloys surface configuration were studied before and after electrochemical degradation with Omax trinocular metallurgical microscope using ToupCam analytical software.

3Results and discussion3.1Passivation studies

The potentiodynamic polarization plots of NO7718, NO7208 and SS439L are shown from Fig. 1(a)–(c). Table 2 depicts the values of the polarization parameters obtained from the plots. The metal alloys showed relatively high resistance to corrosion. Variation in Cl ion concentration had no influence on their corrosion rate values, however, SS439L proves to be the most corrosion resistant followed by NO7208 from observation of corrosion rate values (Table 2). Passivity in metallic alloys emanates from the presence of hydrated Cr2O3 within the passive film. The passivation behavior of the metal alloys contrasts the corrosion rate values due to differential changes in their film composition to a more oxidized form in the presence of chlorides. The high reactivity of Cl ions is due to their small size, mobility, high dispersion solubility [17]. Under high potential difference Cl ions diffuse through the protective film displacing H2O and OH groups and inducing the localized breakdown of the passive layer. NO7208 maintained a stable passivation domain at all Cl ion concentration studied (1–6% NaCl). The potential at which pitting occurred on the alloy tends to be relatively stable signifying strong pitting corrosion resistance. Its passive film being highly resistive, pitting most probably do take place at sites where there are flaws, inclusions or defects. However, the metastable pitting and repassivation domain of NO7208 responds to changes in Cl concentration (Fig. 1(b)) resulting in extended anodic polarization plot due to short term dissolution, visible metastable pitting activity and decreased passivation domain. The passivation domain of NO7718 significantly decreased with increase in Cl ion concentration due to delayed repassivation of the protective film after metastable pitting. Shortening of the passive domain was accompanied by sufficient increase in corrosion current density. This is characteristic to dissolution of the passive film in addition to initiation of localized corrosion. Beyond 4% NaCl concentration passivation behavior was completely absent on NO7718 polarization plot due to the debilitating action of Cl ions. The increase in Cl ion concentration caused instability the passive films eventually resulting in breakdown as confirmed through micro analytical observations to be discussed later. The potential domain for passivation reduced, and formation of the protective stopped due to dominant electro-dissolution reaction processes. SS439L metal alloy showed the weakest resistance to pitting corrosion despite its relatively low corrosion rate. Passivation behavior disappeared after 2% NaCl concentration as a result of the inability of the protective film to reform after anodic polarization. The low Cl ion concentrations were not adequate to impair the passive film and initiate pitting on SS439L and NO7718 metal alloys. At higher Cl ion concentrations, the passive film breakdown exceeded film formation resulting in the formation of stable corrosion pits. The exceptional pitting resistance property of NO7208 is ascribed to the coaction of Cr and Mo [18,19]. Mo causes the passive film to be more stable against localized breakdown resulting from the Cl ions and promotes the repassivation characteristics.

Table 2.

Potentiodynamic polarization data for NO7718, NO7208 and SS439L in 4M H2SO4/0–6% NaCl concentration.

Sample  NaCl conc. (%)  Corrosion rate (mm/y)  Corrosion current (A)  Corrosion current density (A/cm2Corrosion potential (V)  Polarization resistance, Rp (Ω)  Cathodic Tafel slope, Bc (V/dec)  Anodic Tafel slope, Ba (V/dec) 
NO7718
1.70  6.65E−05  1.62E−04  −0.025  386.10  −9.130  6.489 
1.64  6.43E−05  1.57E−04  0.005  391.00  −10.780  26.670 
1.57  6.17E−05  1.50E−04  −0.035  416.60  −10.850  21.200 
1.59  6.22E−05  1.52E−04  −0.022  312.60  −12.430  21.270 
1.44  5.65E−05  1.38E−04  −0.045  454.70  −9.915  20.500 
1.39  5.45E−05  1.33E−04  −0.038  471.60  −11.600  22.700 
1.63  6.38E−05  1.56E−04  −0.057  360.20  −11.390  27.610 
NO7208
0.49  1.93E−05  4.82E−05  −0.024  1334.00  −8.292  −1.978 
0.48  1.85E−05  4.64E−05  −0.009  1985.00  −9.692  6.051 
0.55  2.15E−05  5.37E−05  0.004  1197.00  −10.570  9.464 
0.53  2.06E−05  5.14E−05  −0.006  1249.00  −10.800  8.225 
0.50  1.97E−05  4.92E−05  −0.002  1300.00  −10.700  9.936 
0.43  1.66E−05  4.15E−05  0.024  1549.00  −9.136  21.310 
0.47  1.83E−05  4.57E−05  −0.064  1352.00  −9.362  8.645 
SS439L
0.09  5.07E−06  8.59E−06  0.083  5472.00  −14.000  8.525 
0.07  4.12E−06  6.98E−06  −0.035  6242.00  −13.260  17.330 
0.11  5.83E−06  9.88E−06  −0.066  5007.00  −9.379  21.010 
0.07  3.76E−06  6.38E−06  −0.156  6829.00  −10.900  26.230 
0.07  4.12E−06  6.98E−06  −0.197  6240.00  −9.002  15.810 
0.09  5.19E−06  8.80E−06  −0.040  5171.00  −11.480  23.350 
0.11  5.88E−06  9.97E−06  −0.082  5093.00  −9.435  10.850 
Fig. 1.
(0.19MB).

Polarization plots of (a) NO7718 alloy (b) NO7208 alloy and (c) SS439L alloy in 4M H2SO4/0–6% NaCl concentration.

3.2Microanalytical studies

Fig. 2(a) and (b) shows the optical microscopic morphology of NO7718, NO7208 and SS439L metal alloys before corrosion test with visible metallurgical flaws present on NO7208 and SS439L morphology. The morphology of the alloys after corrosion in 4M H2SO4/0% NaCl (Fig. 3(a) and (b)) slightly contrast the images in Fig. 2(a) and (b). A dark superficial inert oxide is visible on NO7718 (Fig. 3(a)) coupled with an etched worn out morphology due to the electrochemical action of SO42− ions. Numerous macro-pits are present on NO7208 morphology, but their effect on the overall corrosion resistance of the nickel alloy is negligible. The protective film on the nickel alloy is basically Ni–Cr–Fe oxide. The high Cr content of the oxides accounts in large part for the high corrosion resistance of these alloys. Micro-pits are present on SS439L whose uncertain depths are most likely deleterious to the overall viability of the alloy with respect to results from polarization test. Increase in Cl ion concentration to 1% NaCl further deteriorates the morphology of the metal alloys (Fig. 4(a)–(c)) with visible formations of macro/micro-pits associated with pitting corrosion. The visible corrosion susceptibility is attributed to the dissolution of their passive film constituents. After corrosion in 4M H2SO4/6% NaCl (Fig. 5(a)–(c)), the corrosion pits on NO7718 (Fig. 5(a)) has enlarged due to increased action of Cl ions. Anodic dissolution of the alloy surfaces (Fig. 5(a)–(c)) are visible with grain boundaries gradually appearing on SS439L alloy.

Fig. 2.
(0.17MB).

Micro-analytical images of (a) NO7718, (b) NO7208 and (c) SS439L at mag. 100× before corrosion.

Fig. 3.
(0.21MB).

Micro-analytical images of (a) NO7718, (b) NO7208 and (c) SS439L at mag. 100× after corrosion in 4M H2SO4/0% NaCl.

Fig. 4.
(0.19MB).

Micro-analytical images of (a) NO7718, (b) NO7208 and (c) SS439L at mag. 100× after corrosion in 4M H2SO4/1% NaCl.

Fig. 5.
(0.2MB).

Micro-analytical images of (a) NO7718, (b) NO7208 and (c) SS439L at mag. 100× after corrosion in 4M H2SO4/6% NaCl.

3.3Open circuit potential measurement

Thermodynamic stability of passive oxide films constituents strongly influences the corrosion resistance of metal alloys and it gives suitable insight into the active passive behavior of the films without applied potential. The open circuit potential (OCP) measured against exposure time for NO7718, NO7208 and SS439L metal alloys in 4M H2SO4/0%, 1% and 6% NaCl concentration is shown in Fig. 6(a)–(c). The presence of NaCl in the acid media caused significant negative shift in OCP value for NO7718 and SS439L alloys, compared to NO7208 most especially at 6% NaCl signifying dominant cathodic corrosion reaction processes for both alloys with respect to Cl ion concentration. The positive increase in potential for SS439L at 0%, 1% and 6% NaCl signifies the time dependent gradual formation of a protective passive film at the alloy surface. Potential transients are visible on the curves for SS439L at 1% and 6% NaCl due to active-passive behavior of the alloy related to the breakdown and repassivation of the protective film. The decrease in potential for NO7718 (0% NaCl) to −0.012VAg/AgCl at 260.1s is due to cathodic polarization behavior related to interfacial degradation of the alloy. The alloy remained at this equilibrium till 700.01s at −0.013VAg/AgCl before potential increase to −0.005VAg/AgCl at 1075.36s due to passivation of the alloy. Slightly similar phenomenon occurred for NO7718 at 6% NaCl, however, the presence of Cl ions delayed the formation of the passive film on the alloy. The OCP of NO7208 (0%, 1% and 6% NaCl concentration) at values comparatively positive in contrast to NO7718 and SS439L alloys is due generally to the higher corrosion resistance displayed by NO7208 throughout the exposure hours. Increase in potential of NO7208 curves continued for the first 300s after which the OCP curve at 0% and 1% NaCl relatively stabilized due to the stability and strength of the protective film formed on NO7208 surface. However, a gradual but mild time dependent increase in potential was observed for NO7208 OCP curve at 6% NaCl before stabilizing at 0.021VAg/AgCl (1000.01s) after the first initial 300s due to crack/heal event during which sufficient hydrated chromium oxyhydroxide was formed and sustained.

Fig. 6.
(0.16MB).

OCP curves of corrosion potential versus exposure time for (a) NO77189, (c) NO7208 and SS439L in 4M H2SO4/0–6% NaCl.

4Conclusion

NO7208 proves to be highly resistant to pitting corrosion in acid chloride media due to the stability of its film at all chloride concentrations studied from optical microscopic images. However, it undergoes significant anodic dissolution resulting from metastable pitting and delayed repassivation of the protective film with respect to chloride concentration. The nickel alloy showed thermodynamic stability at positive potentials with minimal shift in OCP values. NO7208 and SS439L undergoes significant pitting corrosion with increase in chloride concentration resulting in loss of passivity at 4% and 2% NaCl. Significant shift in their OCP values occurred in the presence of chlorides at negative potentials due to cathodic polarization. Optical images confirmed the presence of corrosion pits on the metal alloys.

Conflicts of interest

The authors declare no conflicts of interest.

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Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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