Zhuojin Pipe Fitting Co., LtdOxide Film Characteristics of Low Alloy Steel and Stainless Steel Overlay Welds in High-Temperature Water with Varying Oxygen Content
Literature Overview
This study by Xiong Qi and colleagues from Shanghai University investigates the corrosion and oxide film formation behavior of A508 Grade III low alloy steel overlaid with 309L and 308L stainless steel in high-temperature water environments with different dissolved oxygen levels. The research is directly relevant to pressurized water reactor (PWR) nuclear power plant applications, where overlay welds serve as corrosion-resistant barriers on reactor internals. The work was supported by the National Natural Science Foundation of China and published in the journal "Corrosion and Protection" in 2018.
Core Technical Findings
The study systematically compared oxide film morphology and composition under two conditions: oxygenated high-temperature water and deaerated high-temperature water. The key findings reveal significant differences in corrosion product phases depending on the oxygen environment.
| Condition | A508 III Base Steel Corrosion Product | 309L/308L Overlay Corrosion Product |
|---|---|---|
| Oxygenated high-temperature water | Primarily γ-Fe₂O₃ | Smaller, less dense spinel-type oxide particles |
| Deaerated high-temperature water | Primarily Fe₃O₄ | Larger, denser spinel-type oxide particles |
The transition from γ-Fe₂O₃ to Fe₃O₄ in deaerated conditions indicates that reduced oxygen availability shifts the thermodynamic equilibrium toward magnetite formation, which is consistent with established nuclear corrosion literature. The spinel-type oxide particles on the stainless steel overlay surface were notably larger and more densely packed in deaerated water, suggesting that lower oxygen content promotes the formation of more protective spinel structures.
Interpretation of Technical Points
Oxide Film Evolution Mechanism
The formation of γ-Fe₂O₃ in oxygenated water reflects the oxidative dissolution mechanism where dissolved oxygen acts as a cathodic depolarizer, accelerating iron ion release and subsequent precipitation as hematite. In deaerated water, the absence of oxygen reduces the cathodic reaction rate, allowing Fe²⁺ ions to accumulate and react with Fe³⁺ to form magnetite (Fe₃O₄). This has direct implications for reactor coolant chemistry management, where dissolved oxygen control is a critical parameter for managing corrosion rates.
309L versus 308L Performance Comparison
A particularly important finding is that 309L overlay welds exhibited pitting corrosion in both oxygenated and deaerated high-temperature water, whereas 308L overlay welds showed no obvious pitting under either condition. The authors attribute this to the preferential dissolution of inclusions in 309L surface in high-temperature water. This is a critical insight for nuclear engineering applications where overlay weld selection is made based on base material compatibility rather than corrosion resistance alone.
| Parameter | 309L Overlay | 308L Overlay |
|---|---|---|
| Spinels particle size | Larger | Smaller |
| Spinels particle density | Higher | Lower |
| Pitting susceptibility in oxygenated water | Present | Absent |
| Pitting susceptibility in deaerated water | Present | Absent |
| Root cause of pitting | Preferential inclusion dissolution | — |
Engineering Implications for PWR Applications
The finding that 309L is more susceptible to pitting than 308L challenges the conventional practice of using 309L as a transition layer between ferritic base metals and austenitic cladding in nuclear applications. While 309L is typically selected for its wider compositional compatibility with dissimilar metals, this study demonstrates that its microstructural features—particularly inclusion content—can become detrimental under long-term high-temperature water exposure. Engineers should consider whether 308L could serve as a more corrosion-resistant alternative in certain overlay weld configurations, provided that the weld metal composition remains compatible with the base material.
Key Questions and Reflections
The preferential dissolution of inclusions in 309L raises important questions about the role of non-metallic inclusions in overlay weld microstructures. Inclusions such as MnS, Al₂O₃, and TiN can act as initiation sites for localized corrosion, particularly in high-temperature aqueous environments where the protective oxide film is continuously formed and dissolved. The study does not provide detailed characterization of the specific inclusion types responsible for pitting initiation, which represents a gap in the understanding of the mechanism.
From a practical standpoint, this research underscores the importance of controlling inclusion content in overlay welding consumables used for nuclear applications. Welding process parameters that minimize inclusion pickup—such as proper shielding gas purity, consumable dryness control, and appropriate heat input—should be emphasized in nuclear-grade overlay welding procedures.
Study Insights and Implications
This work provides valuable data for corrosion management strategies in PWR nuclear power plants. The demonstration that deaerated water promotes the formation of denser and larger spinel oxide particles on stainless steel overlays is encouraging, as denser oxide films generally offer better barrier protection against further corrosion. However, the pitting susceptibility of 309L remains a concern that cannot be resolved simply by controlling coolant oxygen levels. The study reinforces the principle that overlay weld material selection must consider not only mechanical compatibility but also long-term corrosion behavior in the specific service environment, and that microstructural factors such as inclusion distribution can dominate the corrosion performance of stainless steel overlay welds.