Passivity Degradation of Ni and Fe-Based Alloys in High-Temperature Acid Sulfate Solutions
Authors: Okoro Victor U , Daub Kevin , Carcea Anatolie , Newman Roger , Persaud Suraj Y 
Published: December 2024

During nuclear power plant (NPP) operation, materials are exposed to high-temperature water (280 °C to 340 °C) and various chemistries that can induce time-dependent aging phenomena and/or corrosion of materials. This encompasses routine and transient operations, leading to gradual changes in the behavior of materials when subjected to stress. Among the critical components within an NPP is the steam generator (SG), which facilitates heat transfer between the primary water side and the secondary steam side. The principal metallic constituents of a SG include tubing, tube support plates, and tube sheets. As nuclear steam generators age, ensuring continued excellent material performance is paramount. Degradation of SG alloys, such as 600, 690, and 800, can lead to forced outages. Thus, although performance has been relatively good, proactive research understanding the corrosion of SG alloys in off-chemistry conditions is important.

In SGs, ppb-levels of impurities in the water on the secondary side (outside the tubes) can concentrate at crevices between the tube, tube sheets, and tube support plates. These impurities, including sulfate, chloride, lead, silicates, and organic species, can reach ppm-level concentrations, although crevice chemistry is complex, and the quantity of dissolved and precipitated species tends to vary. Laboratory studies have shown that various forms of corrosion, including pitting and stress corrosion cracking, can occur in slightly acidic or alkaline environments outside normal operation specifications; in these environments, sulfate and lead tend to be most important for disturbing passivity, leading to degradation. Sulfate concentrations in the crevice measured in SG blowdown can reach levels of up to 10 ppb. Under SG operating conditions, the addition of hydrazine to remove oxygen, essential for maintaining the corrosion potential of SG tubes within a safe range to sustain stable passivity on the secondary side, may contribute to sulfate reduction, leading to the formation of sulfur species such as H2S, S0, HS-, and S2O3
2-.

The present research explores the passive film properties of various SG alloys in slightly acidic, sulfate-containing solutions. This includes the characterization of pitting corrosion from the micro-to-nanoscale and preliminary electrochemical measurements. Despite several studies, the mechanism of passivity degradation in Ni and Fe-based alloys exposed to acid sulfate environments remains unclear. Alloys 600 (Ni-9Fe-16Cr), 690 (Ni-10Fe-30Cr), and 800 (Fe-32Ni-21Cr) were exposed to an acid sulfate environment of 0.55M at a pH of 4.62 to evaluate corrosion, pitting, and stress corrosion cracking initiation, with a specific focus on the role of sulfur as an impurity element and its effect on loss of passivity in alloys with different Cr contents. Preliminary electrochemical measurements were also conducted on Ni and Fe-based alloys exposed to the same acid sulfate solutions. The temperature range for testing was 280°C to 305°C.

Results indicate that an acid sulfate environment can compromise the passivity and enhance the corrosion of alloys 600 and 800, which are Ni- and Fe-based alloys, respectively. Different corrosion morphologies were observed on the alloys. In alloy 600, the material exhibited shallow but aggressive intergranular corrosion. Lateral propagation of corrosion into adjacent grains led to the formation of pit-like features with time; alloy 690 formed wide and shallow pits, while alloy 800 formed elliptical pits. EDX elemental mapping using electron microscopy techniques from the micro-to-nanoscale revealed sulfur and chromium enrichment (Cr oxide) along the pit-metal interfaces. Sulfur was found to be present either incorporated in oxides or at the oxide-metal interface.

In alloys 600 and 690, sulfur was incorporated in oxides towards the center of the intergranular penetration and pit, overlapping with Cr-rich outer oxides, and low concentrations of Ni and Fe were detected in oxides formed in and around the intergranular penetration and pit. Conversely, in alloy 800, sulfur was enriched at the base of pits, possibly adsorbed at the metal-oxide interface. This contrasts with alloys 600 and 690, where sulfur incorporation in oxides or bonding with Ni, Ti, or Cr was observed. In all cases, the stable Cr-rich oxide is disturbed in Alloys 600 and 800, with alloy 690 exhibiting some resistance due to its high Cr content.

In summary, sulfur's incorporation in oxides or segregation at oxide-metal interfaces may be crucial in disturbing passivity in Ni- and Fe-based alloys. A nanoscale sulfur-rich layer forms at pit-metal interfaces with low oxygen concentrations. Alloys with high Cr content, such as alloy 690, may form a Cr-rich oxide film which is more resistant to degradation by sulfur species.

Figure 1