CLADDING TECHNOLOGY SHANXI CO., LTDOxidation Scale Characteristics of Low-Alloy Steel and Stainless Steel Cladding Layers in High-Temperature Water with Varying Oxygen Content
Research Background and Technical Significance
The performance of cladding layers on pressure vessels and heat exchangers in high-temperature water environments is critically dependent on the stability and protective characteristics of the oxide scales that form on the metal surface. This study investigates the oxidation behavior of low-alloy steel and stainless steel weld overlay cladding layers in high-temperature water at varying dissolved oxygen concentrations, a topic of considerable importance in nuclear power, fossil fuel power generation, and chemical processing industries where water-cooled or water-reactant systems operate at elevated temperatures and pressures.
The dissolved oxygen content in high-temperature water is a key variable that significantly influences the type, structure, and protective properties of the oxide scale formed on metal surfaces. At high temperatures, oxygen solubility decreases dramatically, but the available oxygen still plays a decisive role in determining whether the oxide scale is magnetite (Fe3O4), hematite (Fe2O3), or a mixed oxide, and whether the scale is protective or detrimental to the underlying metal.
Experimental Conditions and Materials
The study examines multiple cladding materials, including low-alloy steels such as 15CrMo and 12Cr1MoV, and stainless steels such as 304, 316, and 347, applied as weld overlay cladding on carbon steel substrates. The experimental conditions involve immersion in high-temperature water at temperatures ranging from 250 to 400 degrees Celsius, with dissolved oxygen concentrations varied from zero (deoxidized) to approximately 100 ppb, simulating the range of oxygen levels encountered in industrial water systems.
The cladding layers are applied using submerged arc welding (SAW) or gas metal arc welding (GMAW) with appropriate filler metals. The overlay thickness is typically 3 to 6 mm, and the number of passes is optimized to ensure uniform composition throughout the layer thickness. The bond line quality is verified by metallographic examination and mechanical testing to ensure that the oxidation behavior is not compromised by interface defects.
| Material | Cr Content (wt%) | Mo Content (wt%) | Ni Content (wt%) | Typical Application |
|---|---|---|---|---|
| 15CrMo | 1.0-1.5 | 0.4-0.6 | - | High-pressure piping |
| 12Cr1MoV | 1.0-1.4 | 0.4-0.6 | - | Boiler tubes |
| 304 | 18.0-20.0 | - | 8.0-10.5 | General corrosion resistance |
| 316 | 16.0-18.0 | 2.0-3.0 | 10.0-14.0 | Chloride environments |
| 347 | 17.0-19.0 | - | 9.0-13.0 | High-temperature service |
Oxidation Scale Formation and Characteristics
The oxidation scale formed on the cladding surfaces is characterized by its thickness, phase composition, morphology, and adhesion. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) are employed to identify the oxide phases and analyze their distribution. The results reveal that the oxide scale composition and structure are strongly dependent on both the alloy composition and the dissolved oxygen content in the water.
For low-alloy steels, the oxide scale typically consists of a multi-layer structure with an outer layer of hematite (Fe2O3), an intermediate layer of magnetite (Fe3O4), and an inner layer of wustite (FeO) adjacent to the metal. The thickness of the magnetite layer increases with dissolved oxygen content and exposure time, following a parabolic growth law at lower temperatures and transitioning to a linear growth law at higher temperatures. The presence of chromium and molybdenum in the low-alloy steel promotes the formation of a thin, continuous chromium oxide layer at the metal-oxide interface, which acts as a diffusion barrier and significantly slows the oxidation rate.
For stainless steels, the oxide scale is predominantly composed of chromium oxide (Cr2O3) with varying amounts of iron oxide depending on the alloy composition and oxygen content. The 316 stainless steel, with its molybdenum addition, forms a more stable and adherent oxide scale compared to 304, particularly at higher oxygen concentrations. The molybdenum enriches the oxide scale, forming MoO3 or molybdenum-containing spinel phases that improve the scale's resistance to dissolution and spallation.
Effect of Oxygen Content on Scale Properties
The dissolved oxygen content exerts a profound influence on the oxidation scale characteristics. At very low oxygen levels (below 10 ppb), the oxide scale is thin, primarily composed of magnetite, and relatively adherent. As the oxygen content increases to 50 to 100 ppb, the scale thickness increases significantly, and the outer hematite layer becomes thicker and more porous. This thicker scale can be more protective in terms of diffusion barrier properties but is more susceptible to mechanical spallation due to thermal stress and hydrodynamic forces.
The study also examines the effect of temperature on the oxygen-dependent oxidation behavior. At 250 degrees Celsius, the oxide scales are relatively thin and well-adhered regardless of oxygen content. At 350 to 400 degrees Celsius, the scales become thicker and more susceptible to spallation, particularly at higher oxygen concentrations. The critical temperature above which scale spallation becomes a significant concern is approximately 320 degrees Celsius for low-alloy steels and 380 degrees Celsius for stainless steels.
Engineering Implications and Quality Control
The findings of this study have direct implications for the design and maintenance of cladded pressure vessels and heat exchangers operating in high-temperature water environments. The following engineering recommendations are derived from the study:
- Control dissolved oxygen levels in water systems to minimize excessive oxide scale growth, particularly for low-alloy steel cladding layers.
- Select stainless steel cladding for applications where high oxygen levels cannot be avoided, as the chromium oxide scale provides superior protection.
- Monitor oxide scale thickness and adhesion periodically during service to detect early signs of scale degradation.
- Consider the effect of thermal cycling on scale adhesion, as repeated heating and cooling can induce spallation even at moderate oxygen levels.
- Ensure adequate cladding thickness to accommodate expected oxide scale growth without compromising the structural integrity of the component.
Key Reflections and Study Insights
The most important insight from this research is the recognition that the dissolved oxygen content in high-temperature water is not merely a parameter to be minimized but a variable that must be carefully managed in conjunction with the alloy selection and operating temperature. The study demonstrates that there is an optimal oxygen level for each alloy system, below which the oxide scale may be too thin to provide adequate protection, and above which the scale becomes excessively thick and prone to spallation. This nuanced understanding is critical for developing effective water chemistry control strategies in industrial systems.
The study also highlights the importance of considering the long-term evolution of the oxide scale during service. The initial oxide scale formed during the first few hours of exposure may differ significantly from the steady-state scale formed after extended exposure. Engineers must account for this time-dependent behavior when predicting component life and scheduling maintenance activities.
In summary, this literature provides essential guidance for the selection and qualification of cladding materials for high-temperature water service, emphasizing the critical role of dissolved oxygen in determining oxidation scale behavior and offering practical recommendations for optimizing both material selection and water chemistry control.