Localized Corrosion Resistance of Nickel Alloy 718 in Chloride-Containing Environments

Among many variations resulting from the development project of Alloy 625 (UNS N06625⁽¹⁾), the nickel-based Alloy 718 (UNS N07718) was considered the most successful.¹  Alloy 718 combines good toughness and tensile strength, making it suitable for cryogenic, ambient, and moderate temperature services.²  Strengthening of Alloy 718 is achieved by the precipitation of secondary phases (e.g., γ’ and γ’’) into the austenitic matrix during age hardening. The fine precipitation of γ’ and γ’’ imparts uniform mechanical through-thickness properties, overcoming the limitations of strengthening thick and complicated sections by cold working, which may result in high residual stresses.³  Given its excellent low-cycle fatigue strength and high creep resistance,⁴  Alloy 718 has outstanding performance in high-temperature service, e.g., in the power industry and jet engine applications. From a corrosion resistance standpoint, Alloy 718 has good general and pitting corrosion resistance, making it the preferred high-strength corrosion-resistant alloy (CRA) for reducing downhole oil and gas environments. Furthermore, Alloy 718 has excellent resistance against sulfide stress cracking and stress corrosion cracking (SCC).^5-6 

Because of its outstanding mechanical properties, good corrosion resistance, and stock availability from the so-called Alloy 718 aerospace grade, Alloy 718 became widely used for marine, utility, and oil and gas applications, initially without modifications to the alloy processing and heat treatment.⁵  The first uses of Alloy 718 in the oil and gas industry started in the 1970s and included parts like fasteners, valve stems, shafts, and drill tools, where minimal corrosion was expected, and failure consequences were limited.^3-5  In addition, Alloy 718 found applications as CRA in, e.g., measurement-while-drilling systems.⁷  Alloy 718 has performed well for check valves in the hydrocracker for refinery applications since the 1970s.⁸  The unique combination of technological properties and the demand for more acidic and aggressive environments increased the request for the alloy and widened its application in the oil and gas industry.⁵ 

Reported failures of the high strength, i.e., 140 ksi (965 MPa) specified minimum yield strength (SMYS), Alloy 718, attributed to hydrogen embrittlement,⁵  led to modifications to the Alloy 718 specifications for oil and gas applications, which are now captured in the API 6ACRA standard. Changes included the adoption of chemical composition limits and the modification of the age-hardening heat treatment steps to reduce the formation of deleterious phases.^9-10  The main differences in the chemical composition include a stricter maximum carbon content of 0.045 wt% to control the formation of continuous grain boundary carbides¹⁰  compared with a limit of 0.080 wt% for the aerospace grade,¹¹  as carbide precipitation plays an important role for high-temperature properties.^5,12-13  The other main change was the limit of the niobium content, which provides the creep rupture resistance of the high-temperature Alloy 718 variant.¹³  For the oil and gas grade, niobium has been restricted to a maximum of 5.2 wt% to minimize δ (delta) phase precipitates, compared with 5.5 wt% in the aerospace grade. Phosphorus was also restricted to less than 0.01 wt% to improve toughness.¹² 

Despite its excellent performance in reducing oil and gas conditions, Alloy 718 can suffer localized corrosion in oxidizing halide-containing environments, e.g., seawater, given its low chromium and molybdenum content, i.e., Alloy 718 has a low pitting resistance equivalent (PRE) (PRE = %Cr + 3.3 [Mo% + 0.5 %W] + 16 %N). International materials selection standards, such as ISO 21457¹⁴  and NORSOK M-001,¹⁵  impose stringent restrictions on the service boundaries of engineering alloys for seawater applications, which are ranked based on their PRE, with a PRE > 40 as the minimum value to define seawater resistance.^14-16  In this regard, Alloy 718 has a nominal PRE < 32, with comparable localized corrosion resistance to that of low-PRE stainless steels, such as UNS S31603 (SS316L),¹⁷  excluding it from seawater service. As a result, other alloys, e.g., UNS N07725 and UNS N07716, have been developed to provide seawater corrosion resistance, maintaining an SMYS of 120 ksi (827 MPa).¹⁸  Table 1 shows the nominal chemical composition of the aerospace (ASTM B670) and oil and gas (API 6ACRA) Alloy 718 grades. The columns shaded in gray in Table 1 highlight the main elements with different limits between the two specifications.

Age-hardening steps and the precipitation of deleterious phases impact not only the material’s mechanical properties but also its corrosion resistance.⁷  For instance, lower corrosion resistance was attributed to the precipitation of carbides and other deleterious phases, such as δ-phase,¹⁹  which deplete the particle/matrix interface of Cr and Mo during aging.⁷  Chen, et al., suggested that the thermally formed γ’/γ’’ strengthening precipitates were cathodic to the austenite matrix, resulting in the formation of microgalvanic couples that caused pit nucleation.²⁰  Recently, Marya²¹  showed that Alloy 718 exhibited no crevice corrosion after immersion for 60 d in 3.5% NaCl at 25°C to 35°C and suggested that the alloy had a pitting potential (E_Pit) and a passive potential range comparable to the super duplex stainless steel UNS S322507, i.e., an alloy with a PRE > 40. Xu, et al.,²²  investigated the effect of three different aging processes on the pitting corrosion resistance of the API 6ACRA Alloy 718 grade tested in high chloride solutions (i.e., 80,000 ppm Cl⁻ at 23°C and 50°C). The results of polarization testing showed similar corrosion performance up to 50°C. At lower temperatures (below 50°C), the alloy remained passive and was unaffected by heat treatment variations. The limited and contradictory evidence in the literature regarding Alloy 718’s localized corrosion resistance required an extensive investigation to quantify the performance of the alloy under different metallurgical conditions.

This work studied the effect of variations in the chemistry, aging steps, and microstructure on the localized corrosion performance of Alloy 718. We compared the seawater resistance of commercial batches of Alloy 718 manufactured to both the ASTM B670 aerospace and API 6ACRA oil and gas standards. This study is the first in a series of articles on seawater resistance of alloys with a PRE < 40.

The samples used for testing were prepared from an Alloy 718 sheet produced to the AMS 5996/ASTM B670¹¹  aerospace specification and a bar of the API 6ACRA 120K grade.²³  The API 6ACRA 120K grade also complied with the oil and gas NACE/ISO 15156-3 sour service requirements.^24-25  The actual chemical compositions of both alloys and their PRE values were listed in Table 2.

Table 3 details the heat treatment history of each alloy grade. As can be seen, the aerospace condition involved a solution treatment for only 1 h at 954°C compared with the API 6ACRA case, which was solution annealed at 1,030°C for 1.5 h to improve the fracture toughness of the API 6ACRA grade.⁵  The aging treatment was also different. A two-step aging was performed for the aerospace grade, while a one-step aging sequence was followed for the Alloy 718 API 6ACRA condition. Consequently, the API 6ACRA samples had lower yield strength and hardness values to meet the sour service requirements outlined in NACE/ISO 15156-3.^5,7 

Samples of the two Alloy 718 conditions were prepared for metallographic analysis by etching in Kalling’s Etchant (Option A, as per API 6ACRA, Methanol 200 mL, HCl 200 mL, CuCl₂ 10 g) for 30 s to 60 s. A light optical microscope (LOM, Nikon ECLIPSE LV150N^†), a field emission-scanning electron microscope (SEM, MIRA3 Tescan^†), energy-dispersive x-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD) were used to characterize the microstructure of the alloys. EBSD data acquisition was performed at 25 kV and a step size of 0.5 μm and was processed in the HKL Channel 5^† software.

The PD-GS-PD tests were conducted in deaerated natural seawater at different temperatures, i.e., 10°C, 20°C, and 50°C. The natural seawater was obtained from Hillarys Harbour, Western Australia, Australia. Chemical analysis of the seawater was detailed in Table 4. Deaeration was achieved by purging high-purity (99.99%) nitrogen gas in the solution for 1 h before testing and continuously throughout the test duration. A chiller connected with a glass coil was used to reach the lower testing temperatures at 10°C and 20°C. A hot plate was used to control the higher temperature.

The electrochemical tests were conducted in a conventional three-electrode configuration, where a platinum-coated mesh was used as the counter electrode, and a commercial Ag/AgCl/KCl (3 M) (Orion) electrode was used as a reference with +205 mV with respect to the standard hydrogen electrode. A GAMRY Interface 1000B^† potentiostat was used for the electrochemical tests. Before testing, the open-circuit potential (OCP) was measured for 15 min, followed by applying a cathodic current of 5 μA for 5 min as a cathodic cleaning step.²⁸ 

Anodic PD polarization testing was used to quantify the electrochemical behavior of the Alloy 718 API 6ACRA grade in an environment that simulates an active pit or crevice at room temperature (22±2°C). To simulate the expected acidification inside a pit or crevice, which can lower the pH to zero or below zero,^29-31  it was decided to fix the solution pH to zero, this was achieved by adding HCl to acidify the solution to pH = 0. The chloride concentration with a minimum of 4 M Cl was suggested to sustain pitting or crevice corrosion,³¹  based on that for this set of experiments the chloride concentration was varied from 1 M to 7 M. NaCl was used to prepare solutions containing [Cl⁻] = 1 M, 3 M, and 5 M, and LiCl for the [Cl⁻] = 7 M condition. As explained elsewhere, LiCl was needed given its higher solubility.²⁹  All of the solutions were prepared with analytical-grade chemicals and ultrapure water.

The main parameters compared were the Flade potential, E_f; the critical current density, i_crit; and the breakdown potential, E_b. E_F, sometimes referred to as primary passivation potential, and the associated i_crit are two parameters indicating the onset of passive film formation. In this regard, a stable passive film develops when a sample is polarized above these two values.³⁵  E_b (sometimes referred to as pitting potential, E_Pit) is defined as the potential below which pits do not nucleate and above which stable pits can survive and grow.^20,30,35  The three critical parameters were compared as a function of chloride concentration as summarized in Table 7. The samples in the 1 M HCl and 3 M NaCl (pH = 0) solutions showed an active-to-passive behavior with a comparable passive range; however, the sample tested in the 1 M HCl easily passivated with a lower critical current density, which was one order of magnitude lower than that of the 3 M [Cl⁻] solution. The polarization curve in 5 M [Cl⁻] exhibited a narrower passive zone. The effect of the higher chloride concertation can be seen with the critical current density required to reach passivity, which was higher by one and two orders of magnitude compared with the 1 M HCl and the 3 M NaCl, respectively. At 5 M [Cl⁻] the E-i curve exhibited a primary passivation zone with a transition region before the onset of passivity. The breakdown potentials for the three different concentrations were close to each other, with an average value of +0.89 V_SSC. The polarization curve in 7 M [Cl⁻] showed no passive behavior. The high current density indicated a metal salt film precipitation in this case.²⁹ 

The two metallurgical conditions of Alloy 718 exhibited differences in chemical composition and microstructure. The aerospace-grade samples had higher carbon content and more carbide precipitates than the API 6ACRA grade, the increase in carbon content has been reported to result in more carbide precipitates.³⁶  Researchers have reported a detrimental influence of carbides in the localized corrosion resistance of Alloy 718. Some authors have suggested that carbides and carbonitrides act as preferential sites for pit initiation, reducing the alloy’s localized corrosion resistance.^37-38  Others concluded that carbides act as preferential cathodic sites, forming galvanic cells with the surrounding matrix and facilitating the anodic dissolution of the matrix at the sites near the carbides, similarly reducing pitting corrosion resistance.²⁰  In our work, the effects of carbide content were evidenced by longer crevice initiation times at 10°C (Figure 5), higher crevice repassivation potentials (Figure 8), shallower crevice attacks (Table 6), and fewer pits (Figure 8) associated with the API 6ACRA grade (which has the lowest carbide concentration) compared with the aerospace grade alloy batch.

In addition, the difference in the grain size between the two tested grades of Alloy 718 may play a role in the corrosion resistance. Limited work was published in this regard, the work completed by Mo, et al., did show that the coarse grain size of ASTM No. 5 has the best corrosion resistance (among tested different grain sizes varied between ASTM No. 9 and No. 3.5 grades), this has been attributed to the faster formation of passive film and its compact, homogeneous structure during constant immersion testing.³⁹  Other work reported no significant influence of the grain size on the pitting corrosion performance of alloy.⁴⁰  This point will need further work to confirm the influence of the grain size on the corrosion resistance of the alloy.

Temperature influenced the two Alloy 718 grades. In this regard, the crevice corrosion resistance decreased with increasing temperature,⁴¹  as seen by the decrease in the breakdown and repassivation potentials shown in Figure 8. Temperature facilitated localized corrosion initiation on both materials⁴²  and accelerated the localized corrosion rate.³⁰  Previous work reported critical crevice temperature (CCT) values for Alloy 718 to vary between <10°C (testing in 6% FeCl₃+ 1% HCl) and ≤25°C (tested in 4% NaCl+ 0.1% Fe₂(SO₄)₃+0.01 mol/L HCl);^43-45  the variation was due to the different testing solutions and method. Other work⁴⁶  reported a CCT of 30°C for Alloy 718 when tested in deaerated neutral 3.5% NaCl. The CCT of the aerospace grade alloy in natural deaerated seawater determined in this work was 10°C, in line with the literature and its expected performance, considering its low PRE. In contrast, the API 6ACRA condition, with its lower carbide precipitation, showed only transpassive behavior with no measurable corrosion at 10°C. At 10°C, the microstructure strongly influenced the localized corrosion resistance since, as with pitting, crevice corrosion initiation occurs at susceptible locations, i.e., carbides.^37,41  At 20°C, the aerospace-grade alloy had a breakdown potential in the forward scan, while crevice corrosion initiated in the GS step for the API 6ACRA condition, suggesting a somewhat better crevice corrosion resistance. At 50°C, both grades had similar E_b values, lower than at 20°C, and a narrow passive range. In addition, for both alloys, the extent of corrosion under the crevice formers increased with temperature.

The increase in chloride concentration in the pit/crevice-like solutions was investigated for the Alloy 718 API 6ACRA condition. In 1 M and 3 M [Cl⁻], the alloy did show active-passive behavior; however, the critical current density was higher in 3 M [Cl⁻] by almost one order of magnitude, indicating that the onset of passivation occurred readily in 1 M [Cl⁻] compared to the 3 M case. A narrower passive range was found in 5 M [Cl⁻] than in the 1 M and 3 M cases, associated with a higher current density. No active-passive behavior was observed in 7 M [Cl⁻], and the precipitation of a salt film took place, essential to maintain active dissolution during localized corrosion propagation.^29,47  The chloride concentration inside a stable pit is believed to reach 6 M with a pH close to zero.³⁰  In agreement with our work, some authors suggested that approximately 7 M [Cl⁻] should be around the minimum required chloride ion concentration to stabilize localized corrosion of stainless steels with comparable chromium and molybdenum content.³⁰ 

In this work, the localized corrosion of Alloy 718 was quantified using the PD-GS-PD method and anodic polarizations in simulated pit/crevice-like environments. The work represents the first thorough characterization of the crevice corrosion resistance of Alloy 718 in oxidizing seawater environments relevant to oil and gas and seawater treatment exposure conditions. The following conclusions were drawn based on the evidence presented above:

• The aerospace-grade Alloy 718 microstructure had an extensive amount of carbides, more than five times greater than the API 6ACRA heat treatment condition.

• The aerospace-grade Alloy 718 suffered crevice corrosion at 10°C, while transpassive dissolution was observed in the API 6ACRA grade. The better crevice corrosion resistance of the API 6ACRA grade Alloy 718 was attributed to the higher carbon content of the aerospace grade and the resultant microstructure with more carbide precipitates for this grade. Differences in grain size may play a role, but this will need further work to investigate the influence of the grain size on the alloy corrosion resistance.

• Both alloys experienced crevice corrosion at 20°C; however, the API 6ACRA samples performed better, as indicated by PD-GS-PD curves. In this regard, crevice corrosion started during the GS step for the API 6ACRA grade. In contrast, crevice corrosion started readily in the forward scan for the aerospace Alloy 718 grade. The number of affected sites under the crevice former and profilometry confirmed the more extensive attack on the aerospace-grade samples.

• Both metallurgical conditions showed the same extent of crevice corrosion attack at 50°C, indicating that the influence of the microstructure decreases with increasing exposure temperature. In this regard, the number of affected sites under the crevice former decreased with increasing temperature. In both cases, the depth of the corroded areas increased with increasing temperature.

• Anodic PD polarization testing of Alloy 718 in the API 6ACRA grade in simulated pit/crevice-like solutions suggested that the critical chloride concentration to sustain an active pit/crevice is 7 M [Cl⁻].

The authors acknowledge the financial support of Chevron (Australia Business Unit) and Woodside Ltd. Furthermore, we acknowledge the support of the John de Laeter Centre at Curtin University, where the characterization took place. Finally, the authors thank Dr Mobin Salasi (Curtin University) for his feedback on the interpretation of the results. Data will be made available upon request.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.