High-Temperature Oxidation Kinetics of Iron-Based Multicomponent Alloy Weld Overlay Layers

Literature Overview

This study investigates the high-temperature oxidation behavior of iron-based multicomponent alloy weld overlay deposits, which are widely employed in components operating in aggressive oxidizing environments such as furnace tubes, boiler headers, heat exchanger surfaces, and petrochemical reactor internals. Understanding the oxidation kinetics of these overlay layers is essential for predicting service life, establishing inspection intervals, and selecting appropriate overlay compositions for specific thermal environments. The research employs classical oxidation kinetics models to characterize mass gain behavior and identifies the controlling mechanisms at different temperature regimes.

Core Technical Points

The oxidation behavior of iron-based multicomponent alloys is governed by the formation and stability of oxide scales, which depends critically on the alloy composition, microstructure, and the oxygen partial pressure of the environment. The study identifies three distinct oxidation regimes:

Parabolic regime (low temperature, long duration): Oxidation rate is controlled by diffusion of metal cations through the oxide scale. The mass gain follows the equation Δm² = kp·t, where kp is the parabolic rate constant and t is time.
Linear regime (high temperature, short duration): Oxidation rate is controlled by the reaction rate at the metal-oxide interface or the oxide-gas interface. Mass gain follows Δm = kl·t.
Transient regime (intermediate conditions): Both diffusion and reaction kinetics contribute, often resulting in a power-law relationship Δm = ktⁿ where 0.5 < n < 1.

The multicomponent nature of these alloys—typically containing combinations of Cr, Al, Si, Mo, and W—creates complex oxide scale compositions including Cr2O3, Al2O3, Fe2O3, and spinel phases (FeCr2O4, Fe3O4). The protective quality of the scale depends on whether a continuous, adherent, and slow-growing oxide layer can form, which is strongly influenced by the Cr and Al content of the overlay.

Oxidation Kinetics Analysis

Overlay Composition	Test Temperature	Oxidation Regime	Rate Constant (kp, mg²/cm⁴·h)	Protective Scale
Fe-15Cr-5Al	800°C	Parabolic	2.3 × 10⁻⁴	Cr2O3 + Al2O3
Fe-25Cr-5Mo	900°C	Parabolic	8.7 × 10⁻⁴	Cr2O3
Fe-10Cr-2Si	700°C	Transient	1.1 × 10⁻³	Mixed Fe-Cr-Si oxides
Fe-18Cr-8Ni-3Mo	1000°C	Linear	5.2 × 10⁻²	Fe2O3 (non-protective)
Fe-20Cr-5Al-2Ti	900°C	Parabolic	1.5 × 10⁻⁴	Al2O3 + Cr2O3

The study reveals that the addition of aluminum to chromium-bearing overlays significantly improves oxidation resistance by promoting the formation of a continuous Al2O3 layer, which has a lower cation diffusion rate than Cr2O3. However, this improvement comes at the cost of reduced ductility and weldability of the overlay deposit. The titanium addition acts as a reactive element, enhancing the adhesion of the Cr2O3 scale and preventing spallation during thermal cycling.

Microstructural Evolution During Oxidation

The microstructure of the overlay layer undergoes significant changes during prolonged high-temperature exposure. Key observations include:

Carbide precipitation: M23C6 and M6C carbides precipitate at grain boundaries during long-duration exposure at 700-900°C, creating chromium-depleted zones susceptible to preferential oxidation.
Phase transformations: BCC ferrite transforms to FCC austenite in Ni-bearing alloys above 800°C, altering diffusion pathways and oxidation kinetics.
Scale spallation: Repeated thermal cycling causes differential thermal expansion between the oxide scale and the metal substrate, leading to cracking and spallation, particularly in alloys forming non-protective Fe2O3 scales.

Engineering Practice Implications

For pressure vessel and heat exchanger applications where overlay layers are subjected to cyclic thermal loading, the following design considerations emerge from this literature:

Composition selection: For temperatures above 800°C, overlays containing at least 20% Cr with 3-5% Al are recommended to ensure formation of protective oxide scales. For temperatures below 700°C, 12-15% Cr with 1-2% Si provides adequate protection at lower cost.
Overlay thickness: Minimum overlay thickness should account for expected oxidation allowance over the design life. A typical allowance of 0.1-0.3 mm per decade of service at 800-1000°C is recommended, depending on composition.
Thermal cycling resistance: For applications involving frequent start-stop cycles, overlays with higher Cr content (≥25%) and lower Al content are preferred to avoid brittle Al2O3 scale formation that spalls under cyclic conditions.
Inspection strategy: Ultrasonic thickness measurement should be performed at intervals determined by the oxidation rate constant, with particular attention to areas of overlay discontinuity or thinning.

Key Questions and Reflections

A critical question raised by this study is the extrapolation validity of oxidation kinetics data. Laboratory measurements typically span hundreds to thousands of hours, while industrial service may extend over decades. The transition from parabolic to linear kinetics observed in some compositions at extended durations suggests that long-term oxidation resistance cannot be reliably predicted from short-term testing alone. This has direct implications for pressure vessel design codes, where overlay layer thickness allowances are often based on limited oxidation data. The study also highlights the importance of considering the overlay microstructure—particularly grain size and phase distribution—in oxidation resistance predictions, as these factors significantly influence scale growth kinetics and spallation resistance. Engineers should request long-duration oxidation testing data (minimum 1000 hours) when specifying overlay compositions for high-temperature applications, and should account for microstructural coarsening that occurs during prolonged thermal exposure.