Oxidation of Alloys - Oxidation and Hot Corrosion

1) Classification of Reaction Types

Noble Parent with Base Alloying Elements

Noble-base alloys (Au, Ag, Pt) with reactive elements (Ni, Cu, Co, Al) exhibit oxidation behavior that depends primarily on oxide stability and oxygen solubility. For example, in Pt–Ni alloys, nickel forms NiO on the surface while platinum remains in metallic form. The oxidation rate is controlled by nickel diffusion through the alloy, resulting in parabolic kinetics.

Silver-based alloys demonstrate higher oxygen solubility, making internal oxidation possible. Interface instabilities can lead to wavy scale formation, while gas diffusion may result in oxide whiskers. A notable example is the reaction between silver and sulfur at 400°C, which produces an Ag₂S layer near the sulfur source and whiskers in regions farther from the source—a classic diffusion-controlled process.

Internal Oxidation

Internal oxidation occurs when oxygen penetrates into alloys and forms precipitates below the surface. The size of these particles depends on the balance between nucleation and growth dynamics. When the solute concentration is sufficient, oxidation can shift from internal to external protective scale formation—a fundamental principle used in designing high-temperature alloys.

Selective Oxidation

Selective oxidation provides protection to alloys by forming slow-growing scales. Among all possible oxides, only Cr₂O₃, Al₂O₃, and SiO₂ consistently demonstrate protective properties at elevated temperatures. These oxides form continuous barriers that significantly reduce further oxidation rates.

Base Parent with Base Alloying Elements

This class of alloys frequently forms multiple oxide layers during oxidation. Their behavior is complex and best understood through specific case studies of common alloy systems.

Oxidation of Nickel-, Iron-, and Cobalt-Base Alloys

Oxidation resistance in Ni, Fe, and Co alloys primarily depends on the formation of a stable, continuous Cr₂O₃ layer. However, the critical chromium content required varies significantly between these systems: approximately 20% for Ni–Cr alloys, 20–25% for Fe–Cr alloys, and around 30% for Co–Cr alloys. Several factors can compromise protective behavior, including transient oxidation and chromium depletion beneath the scale.

Alloy System	Low Cr Content	Increasing Cr	Critical Cr Content	Key Issues	Practical Notes
Ni–Cr	External NiO + internal Cr₂O₃ islands + NiCr₂O₄ spinel	Spinel islands block Ni diffusion, oxidation rate decreases	~10 wt% at 1000°C (for continuous Cr₂O₃)	Transient oxidation (NiO/NiCr₂O₄ form first), Cr depletion under scale	For long-term resistance need ≥18–20 wt% Cr
Fe–Cr	Fe oxides (FeO, Fe₃O₄, Fe₂O₃), little Cr effect	Spinel FeCr₂O₄ reduces Fe diffusion, lowers oxidation rate somewhat	>20 wt% Cr (but protection imperfect)	Fe can diffuse through Cr₂O₃ → outer Fe oxides form; sigma-phase embrittlement >25% Cr	Stainless steels (e.g., 8Ni–18Cr) resist aqueous corrosion, but not high-temp oxidation
Co–Cr	CoO + Cr₂O₃ islands, rapid oxidation	Similar to Ni–Cr but slower Cr diffusion	~30 wt% Cr	Strong transient oxidation, low diffusion coefficient, need high Cr	Co-base superalloys designed with ~30% Cr for oxidation resistance

Oxidation of Other Selected Alloys

Various binary alloy systems exhibit distinctive oxidation behaviors:

Fe–Cu alloys: Cu enrichment can lead to hot shortness if liquid forms
Nb–Zr alloys: Form Nb-rich linear scale with internal Zr oxidation (paralinear kinetics)
Ni–Co alloys: Develop solid solution scale with faster Co diffusion, resulting in higher oxidation rates

Alloy System	Oxidation Behavior	Scale Type	Mechanism / Notes	Rate / Effect
Fe–Cu	Cu does not oxidize; enriches at metal/scale interface	Cu-rich rim at interface	If T < Cu melting → solid Cu slows Fe diffusion. If T > Cu melting → liquid Cu penetrates grain boundaries → hot shortness	Slower oxidation below melting; risk of cracking above melting
Nb–Zr	Zr internally oxidizes under Nb-rich external scale	Nb-rich scale + Zr oxide precipitates	Internal oxidation is diffusion-controlled; external scale grows linearly → paralinear kinetics	Linear scale growth; internal oxidation reaches steady-state penetration
Ni–Co	Both NiO and CoO form solid solution scale	Single-phase Ni–Co oxide	Co ions diffuse faster due to higher cation vacancy concentration + lower activation energy → Co enriches near scale–gas interface	Faster oxidation than pure Ni; scale growth enhanced by Co diffusion
Ni–Cr	Low Cr → internal Cr oxidation (Cr₂O₃ islands) in NiO matrix. High Cr → continuous Cr₂O₃ layer	Duplex NiO + NiCr₂O₄ islands → Cr₂O₃ at high Cr	Transient oxidation: Ni-rich oxides form first; Cr₂O₃ becomes continuous at critical Cr content (~10 wt% Ni alloys)	Parabolic rate; rate decreases as Cr content increases
Fe–Cr	Low Cr → external Fe/Cr oxides; high Cr → Cr₂O₃ layer	Fe-rich outer + mixed spinel (Fe,Cr) inner	Fe ions more mobile than Cr³⁺; Cr₂O₃ protective only above critical Cr (~20 wt%)	Oxidation rate decreases with Cr; below critical Cr, Fe oxides dominate
Co–Cr	Similar to Ni–Cr; higher Cr needed	Cr₂O₃ external	Critical Cr higher (~30 wt%) due to low interdiffusion and rapid transient oxidation	Slower oxidation with sufficient Cr; Cr₂O₃ protects alloy

Oxidation of Intermetallic Compounds

Intermetallic compounds generally demonstrate resistance to internal oxidation due to their low oxygen solubility. The formation of protective oxide layers depends on the stability of the oxides and the underlying lower compounds. Transient oxides that form at low temperatures can accelerate oxidation rates and lead to complex layered scale structures. Oxidation kinetics for these materials are highly dependent on both temperature and crystal structure.

Aspect	Observation / Behavior	Examples / Notes
Selective Oxidation	Same principles as in alloys apply; quantitative differences exist.	Depends on thermodynamic stability of oxides.
Oxygen Solubility	Most intermetallics have negligible O solubility → internal oxidation is rare.	Exceptions: Ti₃Al, Ni₃Al dissolve significant O.
Compound Stoichiometry	Narrow composition ranges → oxidation of one element can produce a lower compound beneath oxide.	e.g., NbAl₃ → Al₂O₃ forms on surface, depletion forms Nb₂Al → alumina breaks down → layered oxide forms.
Relative Oxide Stability	Selective oxidation requires protective oxide (Al₂O₃ or SiO₂) to be more stable than base-metal oxides.	Ni aluminides, Mo silicides: protective oxide forms. Ti-Al or Nb-Al with less Al → intermixed oxides (Al₂O₃ + TiO₂), no selective oxidation.
Oxide Morphology	Often complex; layered scales can form; inward O transport + outward metal transport.	Ti₃Al after 165 h at 900°C: α-Al₂O₃ + TiO₂ mixture, discrete alumina crystals on top, TiO₂ layer at surface.
Transient Oxides & Temperature Effects	Transient oxides can hinder protective layer formation at low T; at high T, volatile oxides evaporate, allowing continuous layer formation.	MoSi₂: above 600°C → slow oxidation (continuous silica layer); 500°C → mixed SiO₂ + MoO₃ layer, faster oxidation; similar for TaSi₂, NbSi₂.
Fabrication Effects	Oxidation rates depend on form: single crystal, polycrystal, HIP processing.	Figure 5.29: MoSi₂ oxidation varies with cast, single crystal, and HIP at different temperatures.
Failure Modes	Layered or intermixed scales → inward O → embrittlement; polycrystalline materials may "pest" due to grain-boundary oxidation.	NbSi₂, TaSi₂ prone to accelerated oxidation above 1000°C if transient oxides non-volatile.

2) Additional Factors in Alloy Oxidation

For oxide films to provide effective protection, they must remain intact on the metal surface. Various stresses arise during oxidation that can compromise this integrity, including both growth stresses and thermal stresses.

Growth Stresses

Several mechanisms contribute to growth stresses in oxide scales:

Volume changes: The Pilling-Bedworth ratio (PBR) describes the volume change when metal converts to oxide
Lattice mismatch: Differences in crystal structure between metal and oxide
Composition changes: Selective oxidation altering local alloy composition
Point defects: Accumulation of vacancies at interfaces
Internal oxide formation: Precipitation within the metal matrix
Specimen geometry: Edges and corners experience higher stress concentrations

Thermal Stresses

Thermal stresses primarily occur during cooling due to differences in thermal expansion coefficients between metal and oxide. These stresses are usually compressive in nature. Cyclic heating and cooling can lead to repeated stress generation, resulting in scale spallation and accelerated oxidation.

Oxide Response

Oxides can respond to stress through various mechanisms:

Cracking: Formation of through-thickness cracks
Spalling: Detachment of oxide fragments from the surface
Plastic deformation: Creep or flow of the oxide at high temperatures
Buckling: Separation of the scale from the metal surface
Shear cracks: Formation of lateral cracks parallel to the interface

Reactive Elements

Additions of reactive elements (Y, Ce, Hf) can significantly improve oxide scale adherence through several mechanisms:

Reducing void formation at the metal-oxide interface
Decreasing growth stresses through growth mechanism modification
Enhancing scale plasticity at high temperatures
Improving resistance to cyclic oxidation conditions
Strengthening the oxide-metal bond through segregation effects

3) Catastrophic Oxidation Caused by Refractory-Metal Additions

High concentrations of refractory metals, particularly molybdenum (Mo) and tungsten (W), in iron- and nickel-base alloys can trigger catastrophic oxidation under certain conditions.

Mechanism

The primary mechanism involves the formation of MoO₂ at the scale–alloy interface, which subsequently oxidizes to molten MoO₃ (melting point 795°C). This molten oxide forms low-melting eutectics with other oxides, disrupting the protective oxide layer and creating channels for rapid oxygen transport.

Atmosphere Effect

The flow condition of the surrounding atmosphere significantly influences this behavior:

Flowing air: Prevents MoO₃ accumulation through volatilization, often avoiding catastrophic attack
Static air: Allows MoO₃ to accumulate and form liquid phases, promoting accelerated oxidation

Oxidation Kinetics

Alloys susceptible to catastrophic oxidation often exhibit a two-stage behavior:

Initial stage: Rapid parabolic growth through diffusion via liquid oxide channels
Secondary stage: Linear growth once the protective layer is fully compromised

Tungsten Effects

Tungsten typically causes less severe effects than molybdenum due to its higher oxide melting points (WO₃ melts at approximately 1745 K). In some alloy systems, tungsten can actually improve Cr₂O₃ scale formation by modifying diffusion processes or acting as an oxygen getter.

Superalloys

Modern superalloys typically contain less than 10 wt% refractory metals, which can have mixed effects on oxidation resistance:

Beneficial effects: Acting as oxygen getters, promoting selective oxidation of aluminum or chromium
Detrimental effects: Reducing diffusion rates of protective elements or forming non-protective oxides

Molten Deposits

The presence of molten deposits (e.g., sulfates) can significantly worsen refractory-metal-induced corrosion by dissolving protective oxides and facilitating the formation of low-melting eutectics. This synergistic effect is particularly important in environments where both high-temperature oxidation and hot corrosion occur simultaneously.

4) Practical Implications for Alloy Design

Critical Concentrations for Protective Scale Formation

For an alloy to form a continuous, protective external oxide scale, the concentration of the scale-forming element must exceed a critical threshold. This critical concentration depends on several factors:

Wagner's Criterion

The critical concentration (N_B^(crit)) of element B in an alloy AB required to form an external protective scale of B_yO_z is given by:

N_B^(crit) ≈ (π g V_alloy / 2 D_B V_oxide)^1/2

Where:
- g is the oxygen flux
- V_alloy and V_oxide are the molar volumes
- D_B is the diffusion coefficient of B in the alloy

Practical Values

Ni-Cr alloys: ~10 wt% Cr for short-term protection, 18-20 wt% for long-term stability
Fe-Cr alloys: ~20-25 wt% Cr
Co-Cr alloys: ~30 wt% Cr
Ni-Al alloys: ~10-12 wt% Al for alumina formation
Fe-Al alloys: ~12-15 wt% Al

Reactive Element Additions

Small additions (typically 0.1-0.5 wt%) of reactive elements such as yttrium, cerium, hafnium, and zirconium dramatically improve oxide scale adherence and reduce growth rates.

Mechanisms of Improvement

Reactive elements improve oxidation resistance through several mechanisms:

Segregation to oxide grain boundaries: Blocks outward cation diffusion, changing the growth mechanism to predominantly inward oxygen transport
Oxide pegging: Forms mechanical anchors at the metal-oxide interface
Sulfur gettering: Reduces the detrimental effects of sulfur impurities on scale adhesion
Vacancy sink formation: Prevents void coalescence at the metal-oxide interface

Application Methods

Reactive elements can be incorporated into alloys through:

Direct alloying: Added during melting (Y, Ce, La, Hf)
Surface treatments: Ion implantation, pack cementation, or sol-gel coating
Oxide dispersion: Mechanical alloying to incorporate fine oxide particles (ODS alloys)

Refractory Metal Management

Refractory metals (Mo, W, Nb, Ta) are often added to high-temperature alloys for strength enhancement, but their oxidation behavior must be carefully managed.

Concentration Limits

To avoid catastrophic oxidation while maintaining strength benefits:

Nickel-base superalloys: Typically limit Mo + W to <10-12 wt%
Iron-base alloys: Mo content usually kept below 5-6 wt%
Cobalt-base alloys: Higher Cr content (≥25%) needed when W exceeds 7-8 wt%

Compensating Elements

When refractory metals are required for mechanical properties, their negative oxidation effects can be mitigated by:

Increasing chromium or aluminum content
Adding reactive elements to enhance scale adherence
Incorporating silicon to promote protective silica formation
Using protective coatings in severe environments

5) Multicomponent Commercial Alloys

Superalloys

Superalloys are complex multicomponent systems designed for high-temperature strength and oxidation resistance. Their oxidation behavior represents a balance between multiple competing mechanisms.

Nickel-Base Superalloys

Modern nickel-base superalloys typically contain:

15-20% Cr for oxidation resistance (forming Cr₂O₃)
3-6% Al for strengthening (γ' phase) and secondary oxidation protection
2-10% Co for phase stability
2-8% W, Mo, Ta, Nb, Re for solid solution strengthening
0.05-0.2% Y, Hf, or other reactive elements for scale adhesion
Ti, C, B, Zr as minor additions for various purposes

At temperatures below ~950°C, these alloys typically form a continuous Cr₂O₃ scale. Above this temperature, transient oxidation becomes more significant, and alumina formation becomes increasingly important for long-term protection.

Cobalt-Base Superalloys

Cobalt-base superalloys require higher chromium content (typically 25-30%) to form protective scales due to:

Lower chromium diffusion rates in cobalt compared to nickel
More rapid transient oxidation during initial exposure
Less favorable Cr₂O₃ formation kinetics

These alloys often contain 8-15% W for strengthening, which necessitates careful balance with chromium content to avoid accelerated oxidation.

Iron-Base Superalloys

Iron-base high-temperature alloys typically contain:

18-25% Cr for oxidation resistance
20-40% Ni for phase stability and creep resistance
0.5-5% Al and/or Ti for precipitation strengthening
1-6% Mo or W for solid solution strengthening

Their oxidation behavior is often complicated by iron's tendency to form multiple oxide phases and its ability to diffuse through Cr₂O₃ scales at high temperatures.

Heat-Resistant Stainless Steels

Heat-resistant stainless steels rely primarily on chromium for oxidation resistance, forming a protective Cr₂O₃ scale.

Austenitic Grades

Austenitic heat-resistant steels (e.g., 310, 330) typically contain:

20-25% Cr for oxidation resistance
15-35% Ni for austenite stabilization
Small amounts of Si (1-2%) to enhance oxidation resistance
Mn, N, and C as austenite stabilizers and strengtheners

These alloys form protective Cr₂O₃ scales but may experience breakaway oxidation after prolonged exposure at temperatures above 850-900°C due to chromium depletion.

Ferritic Grades

High-chromium ferritic steels (e.g., 446, 26Cr-1Mo) contain:

20-30% Cr for oxidation resistance
Low carbon to avoid sensitization
Small amounts of stabilizing elements (Nb, Ti) to prevent chromium carbide formation

These alloys often exhibit better oxidation resistance than austenitic grades at equivalent chromium levels due to faster chromium diffusion in the ferritic matrix, but have lower creep strength.

Alumina-Forming Alloys

For applications requiring oxidation resistance above 1000°C, alloys that form continuous Al₂O₃ scales offer superior protection compared to Cr₂O₃-forming alloys.

NiAl and Ni₃Al-Based Alloys

Intermetallic compounds based on NiAl and Ni₃Al naturally form protective alumina scales due to their high aluminum content. These materials are used in specialized high-temperature applications and as coating materials. Additions of Cr, Pt, and reactive elements enhance their oxidation resistance further.

MCrAlY Overlay Coatings

MCrAlY coatings (where M = Ni, Co, or both) typically contain:

15-22% Cr for initial protection and sulfidation resistance
8-12% Al for formation of protective Al₂O₃ scales
0.1-0.5% Y for scale adhesion enhancement
Balance of Ni and/or Co as the matrix element

These coatings provide superior oxidation protection for superalloy components in the most demanding high-temperature applications.

FeCrAl Alloys

Iron-based FeCrAl alloys contain:

15-20% Cr to promote selective oxidation of aluminum
4-6% Al for Al₂O₃ scale formation
Small additions of reactive elements (Y, Zr, Hf) for scale adhesion

These alloys are used in high-temperature heating elements, furnace components, and automotive catalytic converter substrates due to their excellent oxidation resistance up to 1300°C.

6) Summary and Practical Implications

Key Principles of Alloy Oxidation

The oxidation behavior of alloys is fundamentally more complex than that of pure metals due to:

Selective oxidation of different alloying elements
Competition between internal and external oxidation
Compositional changes in the substrate during oxidation
Multiple oxide phase formation and interactions
Stress development and mechanical integrity challenges

Protective Scale Formation

Long-term oxidation protection relies on the formation and maintenance of continuous, slow-growing oxide scales. For high-temperature applications, only three oxides provide reliable protection:

Cr₂O₃: Effective up to ~950-1000°C
Al₂O₃: Effective up to ~1400°C
SiO₂: Excellent barrier but limited by mechanical properties

Composition-Microstructure-Performance Relationships

Alloy oxidation resistance depends on a complex interplay between:

Chemical composition (major and minor elements)
Microstructure (grain size, phase distribution, surface condition)
Environmental factors (temperature, atmosphere, contaminants)
Thermal cycling and mechanical stresses

Understanding these relationships is essential for designing and selecting materials for high-temperature applications.

Engineering Applications

Knowledge of alloy oxidation mechanisms guides material selection and design for high-temperature applications.

Material Selection Guidelines

Below 650°C: Standard stainless steels (304, 316) often provide adequate oxidation resistance
650-950°C: Higher-chromium stainless steels (310, 446) or nickel-base alloys with 15-20% Cr
950-1150°C: Nickel-base superalloys with protective coatings or alumina-forming alloys
Above 1150°C: Alumina-forming alloys, silicon-containing materials, or ceramic components

Protective Coatings

When bulk alloy composition cannot provide adequate oxidation resistance, protective coatings offer an effective solution:

Diffusion coatings: Aluminide or chromized layers that enrich the surface in scale-forming elements
Overlay coatings: MCrAlY systems that provide a reservoir of aluminum for long-term protection
Thermal barrier coatings: Ceramic top layers that reduce metal temperature and oxidation rate

Surface Treatments

Surface modifications can significantly enhance oxidation resistance:

Shot peening: Increases nucleation sites for protective oxides
Laser surface melting: Homogenizes surface composition
Ion implantation: Introduces reactive elements to improve scale adhesion
Surface deformation: Increases diffusion rates of protective elements

Future Trends

Research in alloy oxidation continues to advance our understanding and develop improved materials.

Advanced Characterization

Modern analytical techniques enable deeper insights into oxidation mechanisms:

In-situ environmental TEM for real-time observation of oxide nucleation and growth
Atom probe tomography for atomic-scale analysis of segregation effects
Synchrotron-based techniques for phase identification in complex scales
Advanced modeling approaches that integrate thermodynamics, kinetics, and mechanics

Novel Alloy Concepts

Emerging alloy systems with enhanced oxidation resistance include:

High-entropy alloys with multiple principal elements
Compositionally complex concentrated solid solutions
Advanced intermetallic compounds with tailored compositions
Nano-structured alloys with engineered interfaces

Integrated Design Approaches

Future high-temperature materials will increasingly balance multiple properties through:

Computational alloy design using multi-objective optimization
Integrated consideration of oxidation, creep, fatigue, and manufacturability
Functionally graded materials with composition tailored to local conditions
Smart coating systems that respond to environmental changes