High-Temperature Oxidation Behavior of Iron-Based Alloy Composite Cladding Layers

Literature Overview

This 2025 publication by Duan Moran from Datang International Power Generation Co., Ltd. Zhangjiakou Branch, published in Materials Reports, presents a systematic investigation of the high-temperature oxidation characteristics of iron-based alloy composite cladding layers applied to power plant components. This research is particularly timely given the ongoing challenges faced by coal-fired power plants in China regarding component degradation in high-temperature, high-sulfur combustion environments.

The study addresses a critical operational problem: boiler tubes, superheater elements, and air preheater components in coal-fired power plants experience severe oxidation and hot corrosion when exposed to flue gas temperatures of 550-950°C containing SO2, SO3, HCl, and ash particles. Conventional protective coatings often fail within months under these aggressive conditions, leading to unplanned outages and significant economic losses.

Experimental Methodology and Results

The researchers investigated several iron-based alloy compositions with varying chromium, aluminum, and silicon contents, deposited using plasma transferred arc (PTA) cladding and laser cladding techniques. The alloys were exposed to simulated flue gas environments at temperatures ranging from 600°C to 900°C for durations up to 500 hours.

Oxidation Mechanism Analysis

The study identifies three distinct oxidation regimes based on temperature and alloy composition:

1. Parabolic regime (600-750°C): Formation of protective Cr2O3 scale with parabolic rate constant kp < 10⁻¹⁴ g²/cm⁴·s. This regime represents acceptable service life for most power plant applications.
2. Catastrophic regime (750-850°C): Breakdown of protective scale due to thermal stress, sulfur attack, or phase instability. The rate constant increases by 2-3 orders of magnitude, leading to rapid material loss.
3. Steady-state regime (850-900°C): After initial spallation, a new protective scale forms at higher growth rate. Long-term performance depends on the alloy's ability to continuously regenerate protective oxide layers.

The addition of titanium (Ti) as a micro-alloying element proves particularly effective at preventing scale spallation. Ti promotes the formation of TiO2 inclusions within the oxide scale, which act as pinning points that reduce scale thickness and improve adhesion. The Ti-stabilized alloy (Fe-22Cr-5Al-1Ti) demonstrated superior performance compared to the Ti-free equivalent, with oxidation rates 30-40% lower at all test temperatures.

Microstructural Evolution and Scale Adhesion

A critical finding from this research is the relationship between cladding microstructure and oxide scale adhesion. The PTA-clad layers exhibited columnar dendritic structures with grain boundaries aligned perpendicular to the surface, while laser-clad layers showed equiaxed grain structures with finer grain size (5-15 μm versus 20-50 μm for PTA).

The equiaxed grain structure produced by laser cladding provided superior scale adhesion because:

• Grain boundaries were more randomly oriented, preventing crack propagation along a single direction
• Finer grain size reduced thermal expansion mismatch stress at the scale-metal interface
• Higher cooling rates produced more solid solution strengthening, improving substrate resistance to oxidation

However, the PTA process offered superior economics for large-scale power plant component repair, with deposition rates 3-5 times higher than laser cladding. The researchers recommend PTA for initial cladding applications and laser cladding for critical repair scenarios where maximum oxidation resistance is required.

Engineering Recommendations

Based on this research, the following recommendations are provided for power plant engineers:

1. Alloy selection: For superheater and reheater tubes operating at 650-750°C, the Fe-22Cr-5Al-1Ti composition provides optimal cost-performance balance with expected service life exceeding 50,000 hours.
2. Deposition process: PTA cladding is recommended for new fabrication and major repair, with a minimum overlay thickness of 1.5 mm to ensure complete coverage of surface irregularities.
3. Surface finish: Post-cladding machining to Ra 0.8-1.6 μm is essential for optimal oxide scale adhesion. Rough surfaces promote early scale initiation at stress concentration points.
4. Inspection protocol: In-service inspection should include periodic thickness measurements using ultrasonic testing (UT) and visual examination of scale adhesion. Removal of loose scale and re-cladding should be performed before the scale thickness exceeds 0.5 mm.
5. Service life prediction: Based on the parabolic oxidation kinetics observed, the service life of the recommended alloy at 800°C can be estimated as t = (Δx)²/kp, where Δx is the allowable thickness loss and kp is the parabolic rate constant determined from accelerated testing.

Study Insights and Future Directions

This 2025 study represents a significant contribution to the understanding of high-temperature oxidation in iron-based cladding alloys, particularly relevant to China's large coal-fired power generation fleet. The finding that titanium micro-alloying substantially improves scale adhesion without significantly increasing cost opens new possibilities for extending component life in existing plants through targeted repair programs.

The research also highlights the importance of considering the full lifecycle of cladding performance — not just initial oxidation resistance but also scale adhesion, spallation resistance, and regenerative capacity. Future work should focus on developing self-healing cladding compositions that can autonomously repair damaged oxide scales during service, potentially incorporating rare earth elements (Y, La, Ce) known to improve oxide scale adhesion through grain boundary segregation effects.