7.1 High-Temperature Oxidation

7.1 High-Temperature Oxidation

High-Temperature Oxidation is the non-aqueous, high-temperature chemical reaction of metallic iron and alloying constituents with oxygen or oxidizing gaseous species (such as carbon dioxide, water vapor/steam, and combustion flue gases). Documented under API RP 571 Section 3.48, oxidation is one of the most universal high-temperature degradation mechanisms encountered in petroleum refining and petrochemical processing.

Oxidation occurs whenever metal operating temperatures exceed thermodynamic and kinetic activation thresholds in the presence of oxygen-bearing process fluids or combustion environments. In carbon steels, oxidation becomes practically significant at temperatures above 1000 °F (538 °C). At or above this threshold, metal loss accelerates exponentially with increasing temperature unless sufficient alloying elements—predominantly chromium (Cr), assisted by silicon (Si) and aluminum (Al)—are present in the metallic matrix to synthesize a continuous, dense, and adherent protective oxide barrier.

                                [ High-Temperature Flue Gas / Steam / Air ]
                                              O2, H2O, CO2
                                                    |
                                                    v
                  +-------------------------------------------------------------+
                  | Outermost Scale:  Hematite (Fe2O3)  - Dense, ~1-2% of scale  |
                  |-------------------------------------------------------------|
                  | Intermediate:     Magnetite (Fe3O4) - Spinel, ~4-8% of scale|
                  |-------------------------------------------------------------|
                  | Innermost Scale:  Wüstite (FeO)     - Defect-rich, fast Fe2+|
                  | (Only > 1058 °F)                     diffusion, ~90-95%     |
                  +-------------------------------------------------------------+
                                                    ^
                                                    | Fe2+ Cation Diffusion Outward
                  +-------------------------------------------------------------+
                  | Metallic Base Metal: Carbon Steel / Low-Alloy Cr-Mo         |
                  +-------------------------------------------------------------+

Chemical Thermodynamics and Fundamental Reaction Chemistry

When iron and iron-base alloys are exposed to oxygen at elevated temperatures, iron atoms undergo oxidation, donating electrons to reduce gaseous oxygen to oxide anions ($\text{O}^{2-}$). The overall chemical reactions proceed according to the thermodynamic stability of the specific oxide phases formed:

1. Formation of Wüstite (Ferrous Oxide, FeO): $2\text{Fe} + \text{O}_2 \longrightarrow 2\text{FeO}$ Wüstite is a non-stoichiometric, metal-deficient compound ($\text{Fe}_{1-x}\text{O}$) characterized by a high concentration of cation vacancies. It is thermodynamically stable only at temperatures above 1058 °F (570 °C).

2. Formation of Magnetite (Ferrous-Ferric Oxide, Fe3O4): $3\text{Fe} + 2\text{O}_2 \longrightarrow \text{Fe}_3\text{O}_4$ Magnetite possesses an inverse spinel crystal structure containing both divalent ($\text{Fe}^{2+}$) and trivalent ($\text{Fe}^{3+}$) iron cations. It is stable from ambient temperatures up to its melting point.

3. Formation of Hematite (Ferric Oxide, Fe2O3): $4\text{Fe} + 3\text{O}_2 \longrightarrow 2\text{Fe}_2\text{O}_3$ Hematite exhibits a rhombohedral corundum crystal structure. It is the most oxygen-rich iron oxide phase and represents the outermost boundary of the scale adjacent to the oxidizing gas phase.

Steam Oxidation Chemistry

In fired heater steam coils, steam superheaters, and steam-methane reformer tubes, oxidation occurs in the absence of free molecular oxygen via direct reaction with water vapor:

$\text{Fe} + \text{H}_2\text{O} \longleftrightarrow \text{FeO} + \text{H}_2$ $3\text{Fe} + 4\text{H}_2\text{O} \longleftrightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$

Steam oxidation releases nascent atomic hydrogen ($\text{H}^0$) at the reacting oxide-metal interface. While a portion recombines into molecular $\text{H}_2$ gas and sweeps downstream, atomic hydrogen can also diffuse into the steel lattice, exacerbating creep void coalescence or high-temperature hydrogen degradation in susceptible microstructures.

Oxidation Growth Kinetics: Parabolic Law vs. Breakaway Oxidation

The rate at which oxide scales thicken is governed by mass transport—specifically the solid-state diffusion of iron cations ($\text{Fe}^{2+}, \text{Fe}^{3+}$) moving outward through the scale, and oxygen anions ($\text{O}^{2-}$) diffusing inward through the scale lattice.

1. Parabolic Oxidation Kinetics (Wagner's Theory)

When a continuous, defect-free oxide film forms on a metallic surface, the growing film acts as a physical diffusion barrier. The rate of scale growth is inversely proportional to the instantaneous scale thickness ($x$):

$\frac{dx}{dt} = \frac{k_p}{x} \quad \implies \quad x^2 = 2k_p t + C$

Where:

• $x$ is the oxide scale thickness (or metal mass gain per unit area).
• $t$ is the exposure time.
• $k_p$ is the parabolic rate constant, which follows an Arrhenius relationship with temperature: $k_p = k_0 \exp(-Q / RT)$.

Under pure parabolic growth, the oxidation rate diminishes continuously over time as the scale thickens, establishing a self-limiting protective behavior.

2. Linear and Accelerated Breakaway Oxidation

Parabolic kinetics assume that the oxide layer remains mechanically intact, adherent, and crack-free. However, as the oxide scale grows beyond a critical thickness, internal compressive growth stresses accumulate due to the Pilling-Bedworth Ratio (PBR), defined as the ratio of the molar volume of the oxide to the molar volume of the metal consumed:

$\text{PBR} = \frac{V_{\text{oxide}}}{V_{\text{metal}}} = \frac{M_{\text{oxide}} \cdot \rho_{\text{metal}}}{n \cdot M_{\text{metal}} \cdot \rho_{\text{oxide}}}$

When internal compressive stresses exceed the fracture toughness or cohesive bond strength of the oxide scale, microcracks, through-thickness fissures, and scale blisters develop. Oxidizing gases rapidly bypass the diffusion barrier through these fissures, directly reaching the underlying bare metal. The degradation mechanism transitions from protective parabolic kinetics into linear breakaway oxidation:

$x = k_l \cdot t$

Under breakaway conditions, metal thinning proceeds at a rapid, constant rate, leading to premature wall breach.

Crystallographic Stratification of Iron Oxide Scales

The metallurgical architecture of iron oxide scales on carbon and low-alloy steels is strictly dictated by thermodynamics and temperature. A fundamental principle of iron oxidation (useful background for Section 3.48) is the phase transition at 1058 °F (570 °C).

Scale Stratification Above 1058 °F (570 °C)

At temperatures above 1058 °F, iron and low-alloy steels develop a classic three-layered scale structure displaying distinct oxygen activity gradients from the outer gas interface to the inner metal substrate:

                                  GAS INTERFACE (High Oxygen Activity)
                                                |
                                                v
    =========================================================================
    [ Hematite: Fe2O3 ]  Rhombohedral | Dense | ~1-2% Scale Thickness
    -------------------------------------------------------------------------
    [ Magnetite: Fe3O4 ] Inverse Spinel | Intermediate | ~4-8% Scale Thickness
    -------------------------------------------------------------------------
    
    [ Wüstite: FeO ]     FCC Halite | Highly Defective Lattice (Fe1-xO)
                         Extremely Fast Fe2+ Cation Outward Diffusion
                         Constitutes ~90-95% of Total High-Temp Scale Thickness
                         ** ONLY STABLE ABOVE 1058 °F (570 °C) **
    
    =========================================================================
                                                ^
                                                |
                                  STEEL SUBSTRATE (Zero Oxygen Activity)

Scale Stratification Below 1058 °F (570 °C)

Below 1058 °F (570 °C), wüstite is thermodynamically unstable. It undergoes an internal eutectoid decomposition reaction into alpha-ferrite and magnetite:

$4\text{FeO} \longrightarrow \text{Fe}_3\text{O}_4 + \alpha\text{-Fe}$

Consequently, for carbon and low-alloy steels operating below 1058 °F, the scale consists strictly of two layers: an inner magnetite ($\text{Fe}_3\text{O}_4$) layer and an outer hematite ($\text{Fe}_2\text{O}_3$) layer. Because the highly defective, fast-diffusing wüstite phase cannot exist below 1058 °F, overall oxidation rates are dramatically lower.

Metallurgical Alloying Effects and Maximum Temperature Limits

To inhibit high-temperature oxidation, steels are alloyed with elements that possess a higher thermodynamic affinity for oxygen than iron, and which form dense, protective, cation-blocking scales.

1. The Critical Role of Chromium (Cr)

Chromium is the cornerstone alloying element for oxidation resistance. When chromium content exceeds 12 wt% to 18 wt%, the alloy forms a continuous surface layer of chromia (Cr2O3):

$4\text{Cr} + 3\text{O}_2 \longrightarrow 2\text{Cr}_2\text{O}_3$

Chromia is a dense, p-type semiconductor with an exceptionally low concentration of interstitial defects and vacancies. Cation and anion diffusion through $\text{Cr}_2\text{O}_3$ is several orders of magnitude slower than through iron oxides, effectively suppressing metal wastage up to the volatilization or spallation limit of the scale.

2. Synergistic Alloying Elements: Silicon (Si) and Aluminum (Al)

• Silicon (Si): When present at 0.5 wt% to 2.0 wt% (as in cast stainless steels and heat-resistant alloys), silicon oxidizes selectively at the metal-scale interface to form an amorphous or quartz-like silica (SiO2) interlayer. This vitreous subscale acts as an impermeable physical barrier against iron cation migration.
• Aluminum (Al): Forms alumina (Al2O3), the most thermodynamically stable and protective oxide scale known in high-temperature metallurgy. Alumina-forming nickel alloys (such as Alloy 601) maintain protective scaling resistance at temperatures exceeding 2100 °F (1149 °C).

3. Alloy Ranking and Temperature Guidance

API RP 571 makes three points that exam questions test directly:

• Carbon steel: oxidation becomes significant above about 1000 °F (538 °C), and rates increase with temperature.
• Chromium is the primary alloying element that improves oxidation resistance; silicon and aluminum are also effective but are limited by their effects on mechanical properties. Molybdenum does not improve oxidation resistance, so C-0.5Mo behaves like carbon steel.
• 300 series SS resist scaling up to about 1500 °F (816 °C).

Design temperature limits for heater tubes come from design standards and owner specifications (for example, the limiting design metal temperatures in API 530) and also reflect creep strength, so they are not the same thing as RP 571's oxidation guidance.

Thermal Cycling, CTE Mismatch, and Thermal Spalling Dynamics

Under perfectly isothermal, steady-state laboratory conditions, an alloy with moderate chromium can survive for thousands of hours because the growing oxide scale remains unbroken. In real refinery process heaters, however, operating conditions are rarely static.

The Mechanism of Thermal Spalling

The primary accelerator of high-temperature oxidation in industrial practice is thermal spalling, driven by the severe disparity in coefficient of thermal expansion (CTE) between the metallic substrate and the surface ceramic oxide scale:

• Austenitic Stainless Steels (300 Series): $\alpha \approx 16\text{ to }18 \times 10^{-6} /\text{°C}$
• Ferritic / Martensitic Steels (Cr-Mo): $\alpha \approx 11\text{ to }14 \times 10^{-6} /\text{°C}$
• Iron / Chromium Oxides (Scale): $\alpha \approx 7\text{ to }10 \times 10^{-6} /\text{°C}$

During process shutdowns, unit trips, or burner firing adjustments, the metal substrate contracts substantially faster than the outer ceramic oxide scale during cooling. This generates massive compressive interfacial shear stresses within the oxide layer. Because ceramic oxides possess high compressive strength but negligible shear and tensile ductility, the scale relieves stress by buckling, fracturing, and explosively delaminating from the metal surface—a process known as thermal spallation.

                                  COOLING CYCLE (Thermal Contraction)
                                     Metal Contraction >> Oxide Contraction
                                                       |
                                                       v
           Scale Under Massive Compressive Stress        Buckling & Through-Thickness Cracks
           vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv           \  /      \  /      \  /
          +-----------------------------------+          +--+      +--+      +--+
          |         Brittle Oxide Scale       |  ===>    |  |      |  |      |  | Spalls Off!
          +-----------------------------------+          +--+      +--+      +--+
          |                                   |          ========================
          |      Base Metal (Contracts Fast)  |          Bare Nascent Metal Exposed
          +-----------------------------------+          (Parabolic Clock Resets to Zero)

When the furnace is re-fired to operating temperatures, the exposed nascent metal surface is directly subjected to oxidizing gases. A new oxide scale begins growing from scratch at the maximum initial parabolic rate. Frequent thermal cycling repeatedly strips the protective barrier, converting what would have been a self-limiting parabolic process into a catastrophic, pseudo-linear rate of continuous metal loss.

Affected Equipment, Damage Morphology, and Inspection Strategies

High-temperature oxidation affects equipment exposed to direct combustion flames, high-temperature flue gases, or superheated steam.

1. Affected Refinery and Petrochemical Equipment

• Fired Heater Radiant Tubes: Radiant tubes in crude, vacuum, coker, platformer, and hydroprocessing heaters. Internal hydrocarbon coking acts as an insulating blanket, forcing operators to fire harder to maintain process outlet temperatures, driving external tube skin temperatures far beyond the oxidation limits of the tube metallurgy.
• Uncooled Tube Hangers, Supports, and Guides: Structural support castings (typically cast 25Cr-12Ni or 25Cr-20Ni HK-40) welded to furnace roofs, sidewalls, or convection tube sheets. Because they are uncooled by internal process fluid, their metal temperature reaches flue gas equilibrium (often 1600 °F to 2000 °F / 871 °C to 1093 °C).
• Steam-Methane Reformer Tubes & Steam Superheaters: Reformer catalyst tubes operating at 1600 °F to 1850 °F (871 °C to 1010 °C) external skin temperature; internal steam coils subject to high-temperature steam oxidation.
• Flare Tips, Burner Diffusers, and Incinerator Liners: Direct flame impingement zones, air/fuel premix diffusers, pilot tips, and flue gas dampers.

2. Macroscopic and Microscopic Damage Morphology

• Macroscopic Appearance: Affected surfaces display thick, encrusted, dark gray, black, or reddish-brown multi-layered scales. Severe scaling manifests as bloated, blistered, or exfoliated "tree bark" bark-like topography. Underneath exfoliated scale, the metallic wall displays uniform thinning, smooth planar metal loss, or irregular shallow cratering.
• Microscopic Examination: Cross-sectional metallography reveals multi-phase oxide scales adhering to or separating from the base metal, with oxide penetrating along surface grain boundaries (internal oxidation). In Cr-Mo and stainless steels, a chromium-depleted sub-surface zone is visible immediately adjacent to the metal-scale interface where chromium has diffused outward to feed chromia formation.

3. Inspection and Non-Destructive Examination (NDE)

• Infrared Thermography (IR): The primary non-intrusive monitoring technique for fired heaters during operation. Calibrated optical pyrometry and radiometric infrared cameras measure tube skin temperatures, identifying localized hot spots caused by internal coking or flame impingement before design oxidation thresholds are breached.
• Visual Inspection (VT): Conducted internally during turnaround outages. Visual inspection identifies heavy scale shedding, blistered tube surfaces, distorted or sagging uncooled hangers, and oxidized tube sheets.
• High-Temperature Ultrasonic Thickness Gauging (UT): Direct measurement of remaining sound metal thickness. In scaled components, loose surface oxide must be mechanically wire-brushed or grit-blasted to sound metal to prevent false acoustic echo readings from the oxide-metal interface.
• Laser Profilometry and Diameter Gauging: Automated crawling inspection tools deployed inside or outside furnace tubes measure external diameter expansion (creep bulging) coupled with external scale loss.