During this initial period, the alloy exhibits relatively slow degradation rates similar to normal oxidation. The protective oxide scales (typically Cr₂O₃ or Al₂O₃) continue to provide some protection. The duration of this stage depends on several factors including alloy composition, salt chemistry, and temperature. This stage ends when the protective scale is breached or compromised.

Once the protective oxide barrier is compromised, rapid and catastrophic degradation begins. Molten salts directly contact the metal substrate, resulting in accelerated corrosion rates that can be 10-100 times faster than normal oxidation. This stage is characterized by extensive internal attack, formation of non-protective corrosion products, and significant metal loss.

Unlike uniform oxidation, hot corrosion often produces localized attack patterns such as pitting, intergranular penetration, or selective grain boundary degradation. This non-uniformity reflects the electrochemical nature of the process and the influence of local microstructural features.

A hallmark of hot corrosion is the formation of metal sulfides beneath the corroded surface. These sulfides appear as discrete particles or continuous networks, often along grain boundaries. The presence of internal sulfides distinguishes hot corrosion from simple oxidation and indicates sulfur transport through the corrosion products.

Hot corrosion typically produces complex, multi-layered corrosion products rather than the simple, adherent scales seen in oxidation. These layers may include:

• An outer porous oxide layer
• An intermediate mixed oxide/sulfate zone
• An internal sulfidation zone
• A zone of alloy depletion (especially of chromium or aluminum)

Residual salt deposits are often found within or on top of the corrosion products. These may appear as crystalline structures during post-exposure examination, though they were molten during the corrosion process.

The weight change or metal loss curve typically shows an initial period of slow change (incubation) followed by a sudden acceleration (breakaway). This transition marks the failure of the protective oxide and the onset of rapid degradation.

Unlike the parabolic or sub-parabolic kinetics of protective oxidation, hot corrosion in the propagation stage often follows linear or even accelerating rate laws. This reflects the continuous dissolution of protective oxides and the absence of a diffusion-limiting barrier.

800-950°C, with maximum severity typically observed around 850-900°C. At these temperatures, sodium sulfate is molten, enabling rapid ionic transport and accelerated corrosion reactions.

• Uniform, broad-front attack with significant penetration into the alloy
• Distinctive layered structure with an outer porous oxide scale
• Internal sulfidation zone containing discrete metal sulfide particles
• Depletion zones where chromium or aluminum has been selectively removed from the alloy
• Minimal pitting compared to Type II hot corrosion

Type I hot corrosion primarily involves basic fluxing of the protective oxide scales. The molten Na₂SO₄ develops high basicity (high oxygen ion activity) near the metal-salt interface due to the removal of sulfur into the alloy as sulfides. This basic salt dissolves the normally protective oxides (Al₂O₃, Cr₂O₃) through reactions such as:

Cr₂O₃ + O²⁻ + 3/2 O₂ → 2CrO₄²⁻

Al₂O₃ + O²⁻ → 2AlO₂⁻

Type I hot corrosion is most commonly observed in:

• Hot sections of industrial gas turbines
• Aircraft engines operating at high power settings
• High-temperature heat exchangers in petrochemical plants
• Components in coal-fired power plants operating at high temperatures

600-750°C, with maximum severity typically observed around 670-730°C. At these temperatures, pure Na₂SO₄ would be solid, but it forms molten eutectic mixtures with transition metal sulfates (particularly NiSO₄ or CoSO₄).

• Localized, pitting attack rather than uniform degradation
• Characteristic "sulfate spike" morphology with deep, localized penetration
• Minimal internal sulfidation compared to Type I
• Layered corrosion products within the pits
• Unattacked regions between pits that retain their protective scales

Type II hot corrosion involves acidic fluxing of oxide scales, driven by the formation of transition metal sulfates and high SO₃ partial pressures. The process typically follows these steps:

1. Gas phase SO₂ oxidizes to SO₃
2. SO₃ reacts with metal oxides to form metal sulfates
3. These sulfates form low-melting eutectics with Na₂SO₄
4. The resulting acidic molten salt mixture dissolves protective oxides

A key reaction is:

NiO + SO₃ → NiSO₄

The NiSO₄-Na₂SO₄ eutectic melts at approximately 671°C, enabling liquid phase attack at temperatures below the melting point of pure Na₂SO₄.

Type II hot corrosion is most commonly observed in:

• Gas turbines operating at part-load or idle conditions
• Marine gas turbines exposed to sea salt
• Industrial gas turbines in areas with high atmospheric pollution
• Heat exchangers in waste incineration plants
• Components in the cooler sections of boilers and furnaces

The basicity of the molten salt increases near the metal surface due to several processes:

• Removal of sulfur from the salt by reaction with the alloy to form sulfides
• Oxygen consumption at the metal-salt interface
• Dissolution of oxides into the salt

This increased basicity enables the dissolution of normally protective amphoteric oxides such as Al₂O₃ and Cr₂O₃ through reactions like:

Al₂O₃ + O²⁻ → 2AlO₂⁻

Cr₂O₃ + 2O²⁻ + 3/2O₂ → 2CrO₄²⁻

Once the protective oxides dissolve at the high-basicity metal-salt interface, the resulting metal-containing ions (e.g., AlO₂⁻, CrO₄²⁻) diffuse outward through the salt to regions of lower basicity. There, they precipitate as non-protective, porous oxide particles:

2AlO₂⁻ → Al₂O₃ + O²⁻

This continuous cycle of dissolution and precipitation prevents the formation of a protective oxide barrier, leading to accelerated attack.

Basic fluxing can become self-sustaining because:

• The precipitation of oxides in the outer regions releases oxide ions that can diffuse back to the interface
• Continued sulfide formation at the metal-salt interface maintains high basicity
• The non-protective nature of the precipitated oxides allows continued salt access to the metal

Acidic fluxing can be driven by two primary sources:

1. Gas phase acidity: High SO₃ partial pressure in the gas phase increases the acidity of the salt through the reaction:
   SO₃ + O²⁻ → SO₄²⁻
2. Alloy-induced acidity: Oxides of certain alloy elements (Mo, W, V) dissolve in the salt and increase its acidity

Under acidic conditions, basic oxides like NiO and CoO readily dissolve:

NiO + SO₃ → NiSO₄

CoO + SO₃ → CoSO₄

The metal sulfates formed through acidic fluxing create low-melting eutectic mixtures with Na₂SO₄:

• Na₂SO₄-NiSO₄ eutectic: melts at ~671°C
• Na₂SO₄-CoSO₄ eutectic: melts at ~630°C
• Na₂SO₄-FeSO₄ eutectic: melts at ~650°C

These low-melting eutectics enable liquid-phase attack at temperatures well below the melting point of pure Na₂SO₄ (884°C), which is why Type II hot corrosion can occur at relatively low temperatures.

Under acidic conditions, even normally stable oxides like Cr₂O₃ can be dissolved:

Cr₂O₃ + 3SO₃ → Cr₂(SO₄)₃

The chromium sulfate formed is highly soluble in the molten salt, leading to loss of the protective barrier. This process is particularly damaging because chromium is the primary protective element in many high-temperature alloys.

Acidic fluxing often leads to localized attack because:

• Eutectic formation may be non-uniform across the surface
• Local variations in SO₃ concentration create acidic "hot spots"
• Once a pit initiates, it can become self-propagating as the geometry concentrates aggressive species
• The electrochemical nature of the process can create anodic and cathodic regions

Sulfur from the molten salt penetrates into the alloy through several mechanisms:

• Grain boundary diffusion: Rapid transport along high-diffusivity paths
• Lattice diffusion: Slower but significant transport through the crystal structure
• Transport through oxide defects: Movement through cracks or pores in the corrosion products

The driving force for sulfur penetration is the chemical potential gradient between the sulfur-rich salt and the sulfur-poor alloy interior.

Sulfur reacts preferentially with the most sulfidation-prone elements in the alloy, typically in the following order of preference:

1. Chromium: Forms Cr₂S₃ or CrS
2. Iron: Forms FeS
3. Nickel: Forms NiS or Ni₃S₂
4. Cobalt: Forms CoS

The formation of chromium sulfides is particularly damaging because it depletes the alloy of its primary protective element.

Internal sulfidation has several detrimental effects:

• Depletion of protective elements: Chromium and aluminum are consumed in sulfide formation, reducing the alloy's ability to form protective oxides
• Volume expansion: Sulfides typically have larger molar volumes than the parent metals, creating internal stresses
• Mechanical degradation: Sulfide particles can act as stress concentrators and crack initiation sites
• Continued sulfur transport: Many sulfides are porous or have high defect concentrations, allowing continued inward sulfur diffusion

Sulfidation and fluxing are interconnected processes:

• Sulfide formation removes sulfur from the salt, potentially increasing its basicity (promoting basic fluxing)
• Depletion of protective elements through sulfidation reduces the alloy's resistance to fluxing attack
• The porous nature of sulfides provides pathways for salt penetration deeper into the alloy

Examples of synergistic interactions include:

• Sulfidation-enhanced fluxing: Depletion of protective elements through sulfidation makes the remaining alloy more susceptible to fluxing attack
• Fluxing-enhanced sulfidation: Dissolution of protective oxides provides easier access for sulfur penetration
• Electrochemical coupling: Different regions of the surface can act as anodes and cathodes, driving localized attack
• Stress-enhanced transport: Volume changes from sulfide formation create stresses that enhance diffusion rates

Hot corrosion can become autocatalytic, meaning that the products of the corrosion process accelerate further corrosion:

• Porous corrosion products provide pathways for deeper salt penetration
• Depletion zones become preferential sites for continued attack
• Mechanical damage from volume changes creates new reactive surfaces
• Local chemistry changes can create more aggressive conditions

• Sodium sulfate (Na₂SO₄): The most common hot corrosion salt, formed from the reaction of NaCl with sulfur-containing gases
• Potassium sulfate (K₂SO₄): Similar behavior to Na₂SO₄ but with different melting characteristics
• Mixed alkali sulfates: (Na,K)₂SO₄ solid solutions with intermediate properties

Small amounts of impurities can dramatically alter salt behavior:

• Vanadium compounds (V₂O₅): Form extremely aggressive, low-melting vanadates; even 1-2% can cause catastrophic attack
• Chlorides (NaCl, KCl): Lower melting points, increase ionic conductivity, and can form volatile metal chlorides
• Lead compounds: Form low-melting eutectics and can cause lead-induced hot corrosion
• Phosphates: Can moderate the aggressiveness of sulfate salts in some cases
• Transition metal oxides: MoO₃, WO₃ can increase salt acidity and aggressiveness

The amount of salt deposit (salt flux) affects corrosion behavior:

• Low salt flux: May lead to salt starvation and reduced attack rates
• Moderate salt flux: Typically produces maximum attack rates
• High salt flux: Can sometimes reduce attack by diluting aggressive species or by forming thick, partially protective deposits

Oxygen partial pressure affects hot corrosion through several mechanisms:

• High pO₂: Favors formation of higher-valent metal oxides and can stabilize protective scales in some cases
• Low pO₂: Promotes sulfidation and can increase salt basicity by reducing sulfate to sulfide
• Intermediate pO₂: Often produces the most severe hot corrosion by providing optimal conditions for both oxidation and sulfidation

The ratio of different sulfur species is critical:

• SO₂/SO₃ ratio: High SO₃ levels promote Type II hot corrosion through acidic fluxing
• H₂S presence: Can lead to direct sulfidation and alter salt chemistry
• Total sulfur content: Higher sulfur levels generally increase hot corrosion severity

The equilibrium: SO₂ + ½O₂ ⇌ SO₃ is temperature and pressure dependent, affecting the SO₃ availability for Type II hot corrosion.

Water vapor can influence hot corrosion through:

• Affecting the stability of different oxide phases
• Influencing the SO₂/SO₃ equilibrium
• Modifying salt melting behavior and ionic conductivity
• Participating in hydrolysis reactions that can alter salt chemistry

CO and CO₂ can affect hot corrosion by:

• Altering the local oxygen activity
• Participating in reactions that affect salt chemistry
• Influencing the stability of carbonate phases in some salt systems

Hot corrosion typically exhibits maximum severity within specific temperature ranges:

• Type I window: 800-950°C, corresponding to molten Na₂SO₄ conditions
• Type II window: 650-750°C, corresponding to eutectic melting conditions
• Intermediate temperatures: 750-800°C may show mixed behavior or reduced attack

Temperature affects the rates of various processes:

• Diffusion rates: Higher temperatures increase ionic diffusion in salts and atomic diffusion in alloys
• Reaction rates: Chemical reaction rates generally increase exponentially with temperature
• Phase transformations: Temperature affects the stability of different oxide and sulfide phases

Temperature affects salt properties that influence corrosion:

• Viscosity: Lower viscosity at higher temperatures enhances mass transport
• Ionic conductivity: Generally increases with temperature, affecting electrochemical processes
• Solubility: Temperature affects the solubility of various species in the molten salt

• Scale spallation: Differential thermal expansion between the alloy and corrosion products leads to scale cracking and spallation
• Fresh surface exposure: Spallation exposes fresh, unprotected metal surface to attack
• Crack formation: Thermal stresses can create cracks that provide pathways for salt penetration
• Substrate cracking: Repeated thermal cycling can cause cracking in the alloy substrate, creating new reactive surfaces

• Salt redistribution: Melting and solidification cycles can redistribute salt deposits and concentrate aggressive species
• Phase changes: Cycling through different temperature regimes can alternate between Type I and Type II mechanisms
• Recrystallization: Thermal cycling can cause recrystallization of corrosion products, potentially changing their protective properties
• Renewed incubation: Fresh surfaces exposed by spallation must re-establish protective scales, extending the vulnerable incubation period

The severity of thermal cycling effects depends on several factors:

• Temperature range: Larger temperature excursions create greater thermal stresses
• Heating/cooling rates: Rapid temperature changes increase thermal shock effects
• Hold times: Longer holds at temperature allow more chemical attack between mechanical damage events
• Number of cycles: Cumulative damage increases with the number of thermal cycles

• Chromium: Generally improves hot corrosion resistance, but effectiveness depends on temperature and salt chemistry. Minimum levels of ~15-20% typically required.
• Aluminum: Provides excellent protection when present in sufficient quantities (>4-5%) to form continuous Al₂O₃ scales
• Silicon: Can improve resistance in some environments but may be detrimental in others

• Nickel-base alloys: Generally show good hot corrosion resistance, especially with adequate Cr or Al content
• Cobalt-base alloys: Often more susceptible to hot corrosion due to the formation of low-melting CoSO₄ in Type II conditions
• Iron-base alloys: Variable behavior depending on composition; generally require higher Cr content for protection

• Molybdenum and Tungsten: Can form volatile oxides and acidic species that accelerate hot corrosion
• Vanadium: Extremely detrimental, forming aggressive vanadates even in small quantities
• Sulfur: Alloy sulfur content can influence internal sulfidation behavior

• Grain size: Fine-grained structures may provide more rapid diffusion paths but also more nucleation sites for protective oxides
• Phase distribution: The distribution of different phases affects local composition and corrosion behavior
• Precipitates: Carbides, intermetallics, and other precipitates can create galvanic couples or act as preferential attack sites
• Surface condition: Surface roughness, residual stresses, and prior oxidation all affect initial hot corrosion behavior

Small additions of reactive elements can significantly improve hot corrosion resistance:

• Yttrium: Improves scale adherence and reduces growth rates
• Hafnium, Zirconium: Similar beneficial effects to yttrium
• Rare earth elements (La, Ce): Can improve sulfidation resistance and scale properties

These elements typically work by improving scale adherence, modifying transport properties, and gettering harmful impurities like sulfur.