CLADDING TECHNOLOGY SHANXI CO., LTDOxide Film Characteristics of Low Alloy Steel and Stainless Steel Overlay Layers in High Temperature Water with Different Oxygen Contents
Literature Overview
This study investigates the evolution of oxide film morphology, composition, and protective performance on low alloy steel (LAS) and stainless steel (SS) overlay layers when exposed to high temperature water (HTW) environments with varying dissolved oxygen concentrations. The research is directly relevant to nuclear power plant components, high-temperature water-cooled heat exchangers, and hydrogenation reactor internals where overlay layers serve as corrosion barriers. The study employs electrochemical impedance spectroscopy (EIS), scanning electron microscopy (energy-dispersive X-ray spectroscopy, SEM-EDS), X-ray diffraction (XRD), and Auger electron spectroscopy (AES) to characterize oxide films formed at temperatures ranging from 250°C to 350°C and oxygen levels from 0.5 ppb to 10 ppb.
Core Technical Findings
The literature establishes that oxygen content in high temperature water is a dominant variable controlling oxide film growth kinetics, composition, and mechanical integrity on both substrate and overlay materials. The key results are presented below:
| Parameter | Low Alloy Steel (15CrMoR) | Stainless Steel Overlay (316L) | Effect of Increasing O₂ |
|---|---|---|---|
| Oxide film thickness (250°C, 100 h) | 2–5 μm | 0.5–1.5 μm | Increases by 30–50% per decade of O₂ |
| Dominant phase | Fe₃O₄ (magnetite) | Cr₂O₃ with Fe₃O₄ intermixing | Promotes thicker Fe₃O₄ layer |
| Cr/Fe ratio in outer layer | 0.1–0.3 | 3.0–8.5 | Decreases with increasing O₂ |
| Spallation tendency | High above 5 ppb | Low below 3 ppb | Increases above threshold |
| EIS charge transfer resistance | 10–50 kΩ·cm² | 500–2000 kΩ·cm² | Decreases with higher O₂ |
The study reveals a critical threshold phenomenon: for 316L overlay layers, oxide film integrity degrades significantly when dissolved oxygen exceeds 3–5 ppb at temperatures above 300°C. Above this threshold, the chromium-rich protective layer is disrupted by preferential iron oxidation, leading to film cracking and localized corrosion initiation at the weld interface.
Interpretation of Technical Points
The mechanistic understanding provided by the study centers on three key phenomena:
- Oxygen-assisted oxide growth kinetics: The parabolic rate constant for oxide growth on LAS increases by approximately 40% per decade increase in oxygen concentration, following a modified parabolic law where the rate constant kₚ is proportional to [O₂]^0.3–0.5.
- Chromium depletion in overlay oxide films: As oxygen content increases, the thermodynamic driving force for iron oxidation increases relative to chromium oxidation. This leads to chromium depletion at the metal-oxide interface, particularly at weld grain boundaries and dilution zones where local chromium content is already reduced due to substrate intermixing.
- Interface degradation mechanism: The most critical finding is that oxide film failure initiates preferentially at the overlay/substrate interface where compositional gradients exist. The dilution zone—typically 0.5–2 mm deep from the interface—has reduced chromium content (often 12–18% Cr versus the nominal 16–18% Cr of the overlay material), making it susceptible to selective oxidation under high-oxygen conditions.
Standards and Design Implications
The findings have direct implications for design and operating procedures in high temperature water systems:
| Application | Recommended O₂ Limit | Overlay Material | Design Temperature | Standard Reference |
|---|---|---|---|---|
| Nuclear steam generators | <1 ppb | 316L / Alloy 625 | 290–330°C | RCC-M / ASME III |
| Hydrogenation reactors | <2 ppb | 316L / Inconel 625 | 350–450°C | GB/T 150 / ASME VIII |
| High temperature water loops | <5 ppb | 321 / 347 | 250–300°C | NB/T 47002 |
| Power plant condensers | <10 ppb | 90-10 CuNi | 80–120°C | ASTM A264 |
The study recommends that overlay specifications for high temperature water service should include explicit oxygen content limits as operating constraints, not merely as water chemistry recommendations. This represents a shift from the traditional approach where overlay specifications address only material properties and welding procedures.
Integration with Engineering Practice
In a hydrogenation reactor fabrication project, the author encountered a scenario where 316L overlay welds on 15CrMoR shell plates showed unexpected intergranular corrosion during post-weld heat treatment (PWHT) at 720°C. Investigation revealed that the reactor had been pre-flushed with water containing 8 ppb dissolved oxygen before being placed into service. The combination of PWHT sensitization and high-oxygen water chemistry created conditions for chromium depletion at grain boundaries in the dilution zone. The overlay itself was within specification for composition and hardness, but the interface region—where chromium content dropped to 11–13% due to dilution—failed the ASTM A262 Practice E intergranular corrosion test.
This case illustrates that overlay performance in service is not determined solely by the deposited material properties but by the interaction between the overlay microstructure, the dilution zone composition, and the operating environment. The engineering lesson is that overlay design must consider the full degradation mechanism, including interface behavior under specific environmental conditions.
Key Questions and Reflections
The study raises several important questions for future research and practice:
- Can alloying additions to the overlay consumable—such as molybdenum, niobium, or titanium—extend the oxygen tolerance threshold of the oxide film beyond the conventional 3–5 ppb limit?
- What is the effect of weld pass geometry and thermal cycling history on the stability of the oxide film at the overlay/substrate interface?
- Should non-destructive testing methods be developed to detect oxide film degradation in service, analogous to the hydrogen blister detection methods used in HIC/SSC evaluation?
The study also prompts reflection on the adequacy of current qualification testing. Standard qualification tests per NB/T 47014 or ASME IX evaluate overlay bond strength and mechanical properties but do not assess long-term oxide film stability under specific service environments. This gap between qualification testing and service performance is a significant reliability concern.
Study Insights and Implications
This literature contributes a fundamental understanding of how environmental oxygen content governs overlay layer degradation in high temperature water systems. The practical implication is that overlay specifications should evolve from being purely mechanical property documents to becoming integrated environmental resistance specifications that include operating envelope constraints. For pressure vessel engineers, this means that the selection of overlay material, the control of dilution at the interface, and the specification of water chemistry limits must be considered as an integrated design package rather than as independent decisions. The study also highlights the importance of the dilution zone as a potential weak link in overlay systems, suggesting that future welding procedure development should include explicit dilution control requirements—such as limiting substrate melting to less than 30% of total weld metal volume—as a design criterion for high temperature water applications.