Study Note on High-Temperature Oxidation Characteristics of Iron-Based Alloy Composite Cladding Layer

Literature Overview

This 2025 paper by Duan Moran from Datang International Power Generation Zhangjiakou Branch, published in Materials Reports, investigates the high-temperature oxidation behavior of iron-based alloy composite cladding layers. The power generation industry faces a persistent challenge in protecting boiler tubes, superheater tubes, and other heat transfer surfaces from high-temperature oxidation and corrosion. Iron-based alloy cladding layers offer a cost-effective alternative to nickel-based or cobalt-based alloys, but their oxidation performance at elevated temperatures requires careful evaluation. This study provides the most current understanding of the oxidation mechanisms and protective scale formation in iron-based cladding alloys, with direct implications for the design of cladding specifications in coal-fired power plants.

Cladding Material System and Composition

The iron-based alloy composite cladding layer studied in this paper is a multi-component system designed to balance oxidation resistance, thermal stability, and cost-effectiveness. The typical composition includes:

The composite nature of the cladding layer means that it is deposited in multiple passes with different compositions, creating a gradient in alloy content from the base metal interface to the outer surface. The inner layer (closest to the base metal) typically has a lower alloy content to ensure good bonding, while the outer layer has a higher alloy content to maximize oxidation resistance.

Scale Formation Mechanism

The high-temperature oxidation of iron-based alloys is governed by the selective oxidation of the most reactive alloying elements. The sequence of oxidation is:

1. Aluminum: Forms Al2O3 (alpha-alumina), which is the most protective oxide due to its low diffusion coefficient and high melting point (2050°C).
2. Chromium: Forms Cr2O3 (chromia), which is protective but less stable than alumina at temperatures above 1000°C.
3. Silicon: Forms SiO2 (silica), which can be protective but is often incorporated into the mixed oxide scale.
4. Iron: Forms Fe2O3, Fe3O4, and FeO, which are non-protective and lead to spalling and rapid material loss.

The key to achieving long-term oxidation resistance is to form a continuous, adherent, and self-healing protective scale. The study found that the presence of both Al and Cr in the cladding layer was essential for forming a duplex scale consisting of an outer Al2O3 layer and an inner Cr2O3 layer. This duplex scale provides superior protection compared to a single Cr2O3 scale, as the alumina layer acts as a diffusion barrier for both oxygen inward diffusion and metal outward diffusion.

Experimental Results and Performance

The oxidation behavior was evaluated by exposing specimens to air at temperatures of 800°C, 900°C, 1000°C, and 1100°C for durations of up to 500 hours. The weight gain was measured at regular intervals, and the scale morphology was examined using SEM and XRD.

Weight Gain Data

The results clearly show that the cladding layer maintains protective oxidation behavior up to 900°C, with a parabolic growth rate indicating the formation of a stable, adherent scale. At 1000°C, the transition from parabolic to logarithmic kinetics indicates the onset of scale breakdown, likely due to the formation of internal oxidation and the disruption of the protective alumina layer. At 1100°C, the linear growth rate indicates rapid, non-protective oxidation with extensive spalling of the oxide scale.

Scale Morphology

At 800–900°C, the scale consisted of a well-adhered, continuous outer layer of alpha-Al2O3 (approximately 5–10 μm thick) with a thin inner layer of Cr2O3 (1–3 μm). The alumina layer was dense and free of cracks, providing an effective diffusion barrier. The presence of small amounts of SiO2 was detected in the inner region of the scale, likely formed by the oxidation of silicon dissolved in the metal matrix.

At 1000°C, the scale morphology changed significantly. The alumina layer became discontinuous, with cracks and voids forming at the scale-metal interface. Internal oxidation was observed, with oxide particles (primarily Al2O3 and Cr2O3) forming within the metal matrix to a depth of 50–100 μm. This internal oxidation weakens the metal substrate and accelerates the overall degradation rate.

At 1100°C, the scale was largely spalled, with only isolated patches of oxide remaining on the surface. The metal substrate showed extensive internal oxidation, with a heavily degraded zone extending 200–300 μm into the cladding layer. The rapid oxidation at this temperature is attributed to the formation of a mixed Fe-Cr-Al oxide scale that lacks the protective properties of a pure alumina layer.

Comparison with Conventional Materials

The iron-based cladding layer outperforms 310 stainless steel and 2.25Cr-1Mo steel at all temperatures, demonstrating the effectiveness of the multi-element alloy design. However, it is outperformed by Inconel 625, which is expected given the superior oxidation resistance of nickel-based superalloys. The key advantage of the iron-based cladding is its significantly lower cost, making it a practical choice for applications where the operating temperature is below 1000°C.

Engineering Implications and Design Recommendations

Based on the oxidation data and scale analysis, the following design recommendations are proposed for the application of iron-based composite cladding layers in power generation equipment:

1. Temperature limit: The maximum recommended operating temperature for the iron-based cladding layer is 950°C. Above this temperature, the protective scale becomes unreliable, and the oxidation rate increases rapidly.
2. Aluminum content: The aluminum content should be maintained at a minimum of 5% to ensure the formation of a continuous alumina scale. Lower aluminum contents lead to internal oxidation and reduced protection.
3. Chromium content: A chromium content of 20–25% is recommended to provide a sufficient Cr2O3 layer beneath the alumina scale. This dual-layer scale provides redundancy in protection.
4. Cooling rate: The cooling rate after welding should be controlled to avoid excessive grain growth in the cladding layer, which can promote internal oxidation during service.
5. Inspection interval: Components clad with iron-based alloys should be inspected at 500-hour intervals for signs of scale spalling or internal oxidation, particularly in regions subject to thermal cycling.

Key Reflections and Study Insights

The most valuable contribution of this paper is the clear delineation of the temperature regimes in which the iron-based composite cladding layer is effective. The finding that the cladding layer maintains protective behavior up to 900°C, with a significant performance drop at 1000°C and above, provides engineers with a clear basis for material selection. For applications operating below 950°C, the iron-based cladding offers an excellent balance of oxidation resistance and cost-effectiveness.

The study also highlights the importance of the duplex scale (Al2O3 + Cr2O3) in providing long-term protection. The presence of both aluminum and chromium in the cladding composition is not redundant but synergistic, as each element contributes to a different aspect of the protective scale. This insight has direct implications for the formulation of future iron-based cladding alloys, where the optimization of the Al/Cr ratio may further improve oxidation performance.

The practical significance of this research for the power generation industry cannot be overstated. Boiler tubes, superheater tubes, and other heat transfer surfaces are routinely exposed to temperatures in the 800–1000°C range, and the oxidation resistance of the cladding layer directly affects the service life and reliability of these critical components. The data presented here should be incorporated into the design specifications and material selection criteria for cladding applications in coal-fired power plants.