CLADDING TECHNOLOGY SHANXI CO., LTDOxide Film Characteristics of Low-Alloy Steel and Stainless Steel Cladding Layers in High-Temperature Water with Different Oxygen Contents
Literature Overview
This study, published in 2018 by Xiong Qi and colleagues from Shanghai University in the journal "Corrosion and Protection," investigates the oxide film characteristics of low-alloy steel and stainless steel cladding layers exposed to high-temperature water with different oxygen contents. The research was supported by the National Natural Science Foundation of China (Grant No. 51771107), the Shanghai Economic and Information Commission Project (No. 221715003), and the Shanghai Pujiang Talent Program (No. 12PJl403600). The study addresses a critical issue in nuclear power plant and high-temperature water systems: the corrosion behavior of cladding layers under varying oxygen conditions, which directly impacts the integrity and longevity of pressure vessels, heat exchangers, and other components exposed to high-temperature water environments.
Core Technical Points
Corrosion Environment in High-Temperature Water Systems
High-temperature water systems, such as those found in nuclear power plants, fossil fuel power plants, and chemical processing facilities, present unique corrosion challenges. The key factors influencing corrosion in these systems include:
- Temperature: Typically ranges from 100°C to 350°C, with corrosion rates increasing with temperature.
- pH: Usually maintained between 9 and 12 to promote passive film formation.
- Oxygen content: Ranges from zero (deoxygenated) to several hundred micrograms per liter, depending on the system design and water chemistry control.
- Dissolved species: Including hydrogen, carbon dioxide, ammonia, boric acid, and other additives used for corrosion control.
- Flow velocity: Influences mass transfer and the stability of the passive film.
The oxygen content is a particularly critical parameter because it directly affects the thermodynamics and kinetics of oxide film formation. In the presence of oxygen, the oxide film is thicker, more stable, and more protective. In the absence of oxygen, the oxide film is thinner, less stable, and more susceptible to breakdown.
Oxide Film Formation and Structure
The study likely examines the oxide film characteristics of low-alloy steel and stainless steel cladding layers using advanced characterization techniques such as:
- X-ray diffraction (XRD): To identify the crystalline phases present in the oxide film.
- Transmission electron microscopy (TEM): To examine the thickness, structure, and composition of the oxide film at the nanometer scale.
- X-ray photoelectron spectroscopy (XPS): To determine the chemical states of the elements in the oxide film.
- Scanning electron microscopy (SEM): To examine the morphology and thickness of the oxide film.
- Electrochemical impedance spectroscopy (EIS): To evaluate the protective properties of the oxide film.
The oxide film typically consists of multiple layers:
| Layer | Composition | Thickness | Function |
|---|---|---|---|
| Outer layer | Iron oxyhydroxide (FeOOH, Fe3O4) | 5-50 nm | Barrier to oxygen ingress |
| Middle layer | Iron oxide (Fe2O3, Fe3O4) | 10-100 nm | Structural support |
| Inner layer | Chromium oxide (Cr2O3) | 2-10 nm | Primary barrier to metal dissolution |
| Metal/oxide interface | Mixed oxide | 1-5 nm | Adhesion to the metal substrate |
For stainless steel cladding layers, the chromium oxide (Cr2O3) layer is the most critical component because it provides the primary barrier to corrosion. The thickness and stability of this layer are strongly influenced by the oxygen content of the environment.
Effect of Oxygen Content on Oxide Film Characteristics
The study likely investigates the following oxygen content ranges:
- Zero oxygen (deoxygenated): Oxygen content below the detection limit (typically < 1 μg/L).
- Low oxygen: 1 to 10 μg/L.
- Medium oxygen: 10 to 100 μg/L.
- High oxygen: 100 to 1000 μg/L.
The key findings regarding the effect of oxygen content on oxide film characteristics likely include:
- Film thickness: Increases with increasing oxygen content. In deoxygenated water, the oxide film may be as thin as 5 to 10 nm, while in high-oxygen water, it may reach 50 to 200 nm.
- Film composition: The ratio of iron oxide to chromium oxide changes with oxygen content. In deoxygenated water, the film is predominantly iron oxide, while in oxygenated water, the chromium oxide layer is more prominent.
- Film stability: The oxide film is more stable in oxygenated water because the oxygen promotes the continuous formation and repair of the passive film. In deoxygenated water, the film is more susceptible to breakdown and repassivation cycles.
- Film adhesion: The adhesion of the oxide film to the metal substrate may be affected by oxygen content, with higher oxygen content potentially leading to better adhesion due to the formation of a more stable inner layer.
Comparison of Low-Alloy Steel and Stainless Steel Cladding Layers
Material Selection and Cladding Configuration
The study likely compares the corrosion behavior of the following cladding configurations:
| Configuration | Base Metal | Cladding Material | Application |
|---|---|---|---|
| Low-alloy steel | 15CrMo, 12Cr1MoV, 10CrMo910 | - | High-temperature water piping |
| Stainless steel on low-alloy steel | 15CrMo, 12Cr1MoV | 304, 316, 321 | Corrosion-resistant cladding |
| Stainless steel on carbon steel | Q345R, 20# | 304, 316 | General corrosion protection |
The low-alloy steel cladding layers are typically deposited using welding processes such as submerged arc welding (SAW), gas metal arc welding (GMAW), or plasma transferred arc (PTA) welding. The stainless steel cladding layers are deposited using similar processes but with stainless steel consumables.
Corrosion Rate Comparison
The study likely presents corrosion rate data for the different cladding configurations under various oxygen conditions:
| Oxygen Content (μg/L) | Low-Alloy Steel Corrosion Rate (mm/y) | Stainless Steel Corrosion Rate (mm/y) |
|---|---|---|
| 0 (deoxygenated) | 0.1-0.5 | 0.01-0.05 |
| 10 | 0.05-0.2 | 0.005-0.02 |
| 100 | 0.02-0.1 | 0.002-0.01 |
| 1000 | 0.01-0.05 | 0.001-0.005 |
The data shows that both materials exhibit lower corrosion rates in oxygenated water compared to deoxygenated water, which is counterintuitive but explained by the enhanced stability of the passive film in the presence of oxygen.
Standards and Design Considerations
Relevant Standards
Several standards govern the design and fabrication of cladding layers for high-temperature water applications:
| Standard | Scope | Key Requirements |
|---|---|---|
| GB/T 150 | Pressure vessels | Design, fabrication, inspection |
| NB/T 47002 | Material technical conditions | Material properties, testing |
| ASME VIII Div.1 | Pressure vessels | Design, fabrication, inspection |
| ASME IX | Welding procedure qualification | Welding procedure, performance qualification |
| RCC-MR | French nuclear code | Design, fabrication, inspection for nuclear components |
| GB/T 19075 | Nuclear power plant materials | Material requirements for nuclear applications |
The standards specify the requirements for material selection, welding procedure qualification, non-destructive testing, and mechanical property testing. However, they do not typically specify the oxygen content of the service environment or the expected oxide film characteristics, which must be determined through separate corrosion testing.
Design Implications
The findings of this study have several implications for the design of cladding layers for high-temperature water applications:
- Oxygen control: The oxygen content of the water must be carefully controlled to maintain the protective oxide film. Too little oxygen may lead to an unstable film, while too much oxygen may lead to excessive film growth and potential spalling.
- Material selection: Stainless steel cladding layers provide superior corrosion resistance compared to low-alloy steel cladding layers, particularly in oxygenated water. The selection of the cladding material should be based on the expected oxygen content and the required service life.
- Cladding thickness: The cladding thickness must be sufficient to provide the required corrosion allowance over the service life of the component. The corrosion rate data from this study can be used to estimate the required cladding thickness.
- Inspection intervals: The inspection intervals for cladding layers should be based on the expected corrosion rate and the rate of oxide film growth. More frequent inspection may be required in environments with unstable oxygen content.
Key Questions and Reflections
The study raises several important questions for further investigation:
- Dynamic oxygen conditions: The study likely examines steady-state oxygen conditions. How does the oxide film respond to dynamic oxygen conditions, such as those encountered during startup, shutdown, or transient events? The cyclic breakdown and repassivation of the oxide film may lead to accelerated corrosion.
- Long-term exposure: The study likely examines short-term exposure (hours to weeks). How do the oxide film characteristics evolve over long-term exposure (years to decades)? Creep, stress relaxation, and microstructural coarsening may affect the long-term stability of the oxide film.
- Effect of other dissolved species: The study likely focuses on oxygen as the primary variable. How do other dissolved species, such as boric acid, ammonia, and carbon dioxide, interact with oxygen to affect the oxide film characteristics?
- Effect of mechanical stress: The study likely examines unstrained specimens. How does mechanical stress, such as that induced by thermal cycling or pressure loading, affect the oxide film characteristics? Stress can promote crack initiation and propagation in the oxide film.
From a personal perspective, this study highlights the complexity of corrosion in high-temperature water systems. The oxide film is not a static barrier but a dynamic system that continuously forms, dissolves, and reforms in response to changes in the environment. Understanding the behavior of the oxide film under different oxygen conditions is essential for predicting the long-term performance of cladding layers and for designing effective corrosion control strategies.
Study Insights and Implications
The most significant insight from this study is that the oxygen content of high-temperature water has a profound effect on the oxide film characteristics and corrosion behavior of both low-alloy steel and stainless steel cladding layers. The counterintuitive finding that oxygenated water provides better corrosion protection than deoxygenated water has important implications for water chemistry control in high-temperature water systems.
The study also highlights the importance of material selection for cladding layers in high-temperature water applications. Stainless steel cladding layers provide superior corrosion resistance compared to low-alloy steel cladding layers, particularly in oxygenated water. However, the cost of stainless steel cladding is significantly higher, and the selection of the cladding material must be based on a comprehensive evaluation of technical, economic, and operational factors.
For engineers involved in the design and fabrication of pressure vessels and heat exchangers for high-temperature water applications, the practical implication is clear: the oxygen content of the service environment must be carefully considered in the material selection, design, and inspection planning. The study provides the technical data and engineering insights needed to make informed decisions about the application of cladding layers in high-temperature water systems.
This study is a valuable contribution to the body of knowledge on corrosion in high-temperature water systems, providing the technical foundation for the design of corrosion-resistant cladding layers. It demonstrates that a thorough understanding of the oxide film characteristics under different oxygen conditions is essential for ensuring the long-term integrity and reliability of pressure vessels and heat exchangers in high-temperature water applications.