8.2 Nitriding, Fuel Ash Corrosion & Refractory Degradation

8.2 Nitriding, Fuel Ash Corrosion & Refractory Degradation

Process integrity in high-temperature chemical converters, fired heaters, and fluid catalytic cracking units requires an in-depth understanding of how materials interact with nitrogen-bearing process gases, combustion ash byproducts, and refractory protective linings. API RP 571 categorizes these challenges under three specialized damage mechanisms: Nitriding (Section 3.47), Fuel Ash Corrosion (Section 3.30), and Refractory Degradation (Section 3.53). Each presents unique metallurgical vulnerabilities, temperature thresholds, and operational mitigation strategies.

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3.47 Nitriding

Mechanism Description

Nitriding is a high-temperature surface degradation mechanism caused by the diffusion of atomic nitrogen into the surface of metals and alloys. Under elevated process temperatures, nitrogen gas ($N_2$) or, more aggressively, dissociated ammonia ($NH_3$) reacts with the metallic surface. Nascent atomic nitrogen ($N$) dissolves into the crystalline lattice and diffuses inward, reacting with iron, chromium, aluminum, and molybdenum to form hard, brittle metal nitrides (such as iron nitrides $Fe_4N$ and $Fe_2N$, and chromium nitride $CrN$).

The chemical dissociation of ammonia on hot metallic surfaces serves as the primary catalytic driver in petrochemical operations:

$2\text{NH}_3 \stackrel{\Delta, \text{metal}}{\longrightarrow} 2\text{N}_{\text{adsorbed}} + 3\text{H}_2$

$\text{Fe} + \text{N}_{\text{adsorbed}} \longrightarrow \text{Fe}_x\text{N} \quad (\text{e.g., } \text{Fe}_4\text{N})$

$\text{Cr} + \text{N}_{\text{adsorbed}} \longrightarrow \text{CrN}$

Critical Factors and Temperature Thresholds

• Operating Temperature: API RP 571 states that nitriding begins above about 600 °F (316 °C) and becomes severe above 900 °F (482 °C). Ammonia and cyanides provide a much higher nitrogen activity than molecular nitrogen gas.
• Gas Composition: Ammonia partial pressure is the dominant operational variable. Even low concentrations of ammonia (such as in ammonia synthesis loops or sour water stripper overheads with excessive superheat) drive rapid surface nitridation.
• Alloy Composition: Chromium, iron, molybdenum, and aluminum readily form stable nitrides. In stark contrast, nickel does not form stable nitrides under refining conditions. Consequently, an alloy's resistance to nitriding increases with its nickel content; RP 571 notes that alloys containing about 30% to 80% nickel are more resistant.

Alloy Nickel Content vs. Nitriding Resistance:

Carbon Steel / Low-Alloy Steels  --->  Extreme susceptibility (forms deep, brittle Fe4N case)
300-Series Stainless Steels       --->  Moderate susceptibility (forms hard CrN; matrix Cr depleted)
Alloy 800H (30 - 35% Ni)          --->  Intermediate resistance
Alloy 600 (> 72% Ni)              --->  Outstanding resistance (Nickel does not form stable nitrides)

Morphology and Mechanical Impact

• Microstructure and Hardness: Nitriding produces a distinct, hardened surface case. Microhardness testing across the cross-section reveals extreme surface hardness, frequently exceeding 60 to 70 HRC (or > 800 to 1,000 HV), compared to a base metal hardness of 15 to 25 HRC.
• Case Depth: The nitrided case can range from a few mils in early stages to over 50 mils (1.27 mm) in long-term exposures.
• Thermal Spalling and Crazing: The nitrided case possesses a significantly lower coefficient of thermal expansion and substantially higher volume than the ductile substrate beneath it. During process temperature fluctuations or thermal cycling, immense shear stresses develop at the case-core boundary, resulting in extensive surface crazing, microcracking, and spalling (delamination) of the hardened layer.
• Loss of Toughness: While the surface is wear-resistant, the component loses all ductility; crack initiation occurs readily in the brittle nitrided layer and can propagate dynamically into the core via mechanical or thermal fatigue.

Affected Equipment and Units

• Ammonia synthesis converters, loops, and heat exchangers.
• Steam-methane reformer (SMR) burner tips and preheat coils when burning ammonia-rich off-gases.
• Nitric acid production plant piping and catalyst support grids.
• Heavy sour water stripper overhead condensers experiencing abnormal dry-out/superheat.

Prevention, Mitigation, and Inspection

• Alloy Upgrading: Upgrade vulnerable carbon steel and 300-series stainless steels to high-nickel alloys. Alloy 600 (UNS N06600, > 72% Ni) is the industry standard for severe nitriding environments. Alloys with intermediate nickel content, such as Alloy 800H (30–35% Ni), provide moderate protection.
• Inspection: Liquid Penetrant Testing (PT) detects shallow crazing and microcracking on accessible surfaces; field metallographic replication (FMR) identifies nitride needles and case depth; portable ultrasonic contact impedance (UCI) or microhardness testing identifies abnormally high surface hardness.

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3.30 Fuel Ash Corrosion

Mechanism Description

Fuel ash corrosion is an accelerated form of high-temperature liquid-phase attack that occurs on the fireside surfaces of fired heater tubes, superheaters, reheaters, tube hangers, and uncooled structural supports. It is caused by the deposition of combustion ash components that combine to form low-melting-point eutectic slag mixtures. Once these ash deposits melt and become liquid, they act as powerful chemical fluxing agents that dissolve the protective chromium oxide ($Cr_2O_3$) and alumina ($Al_2O_3$) scales on the tube surface, exposing bare metal to catastrophic oxidation and sulfidation.

Combustion of Ash-Bearing Heavy Fuels (Vanadium, Sodium, Sulfur, Potassium)
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                 Deposition of Ash Particulate on Hot Fireside Metal
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         Ash Components Form Eutectic Compounds (e.g., Na2O·6V2O5 / Alkali Sulfates)
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                                      v
      Metal Temperature Exceeds Slag Melting Point (T > 1000 °F for Oil Ash)
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                                      v
                MOLTEN LIQUID SLAG DISSOLVES PROTECTIVE OXIDE SCALE
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                                      v
           Catastrophic Liquid-Phase Fluxing, Deep Grooving & Rapid Wastage

Oil Ash vs. Coal Ash Corrosion Chemistry

Fuel ash corrosion manifests in two distinct industrial environments, differentiated by fuel chemistry, deposit constituents, and eutectic melting temperatures:

The Catalytic Role of Vanadium Pentoxide

Vanadium in heavy fuel oil oxidizes during combustion to form vanadium pentoxide ($V_2O_5$), which has an uncompounded melting point of 1,245 °F (674 °C). However, when sodium is present in the fuel, sodium oxide ($Na_2O$) and sulfur trioxide ($SO_3$) react with $V_2O_5$ to form complex sodium vanadates (such as sodium metavanadate $Na_2O \cdot V_2O_5$ and sodium vanadyl vanadate $Na_2O \cdot V_2O_4 \cdot 5V_2O_5$). These eutectic compounds melt at temperatures as low as 1,000 °F (538 °C). In the molten state, vanadates rapidly flux protective oxides and catalytically transport oxygen directly from the flue gas to the metal surface, generating metal loss rates exceeding 100 to 250 mpy (2.5 to 6.3 mm/yr).

Critical Factors

• Metal Skin Temperature: Molten ash attack cannot occur if the tube metal skin temperature remains below the eutectic melting point of the deposit. Corrosion rates escalate exponentially once metal skin temperatures exceed 1,000 °F to 1,100 °F (538 °C to 593 °C).
• Fuel Contaminant Concentrations: Vanadium concentrations above 50 ppm and sodium concentrations above 10 ppm in fuel oils represent high-risk operating conditions. An unfavorable $Na:V$ ratio near 1:3 produces the lowest-melting eutectics.
• Uncooled Structural Components: While internal fluid flow cools heater tubes to some degree, uncooled structural supports—such as tube hangers, guides, and roof brackets—operate at ambient flue gas temperatures (often 1,400 °F to 1,800 °F / 760 °C to 982 °C), making them the most frequently destroyed components in heavy fuel-fired heaters.

Morphology

• Hard, glassy, dark green-black or yellowish-brown crusty slag deposits adhered tightly to fireside tube surfaces.
• Underneath the slag, severe metal wastage appears as wide, deep grooves, irregular gouging, or extensive circumferential metal thinning.
• Extreme necking, burning, or complete disappearance of cast tube hangers and support beams.

Prevention and Mitigation

1. Fuel Quality and Additives: Switch to clean gaseous fuels (natural gas, refinery fuel gas) or low-vanadium fuel oils. When burning heavy fuels, inject magnesium-based fuel additives (magnesium oxide $MgO$ or magnesium hydroxide $Mg(OH)_2$) directly into the fuel at a recommended $Mg:V$ molar ratio of approximately 3:1. Magnesium reacts preferentially with vanadium to synthesize magnesium orthovanadate ($3MgO \cdot V_2O_5$), which has a melting point of 2,050 °F (1,121 °C)—far above tube operating temperatures. The ash remains a dry, non-corrosive solid powder that passes harmlessly through the heater.
2. Alloy Upgrades for Supports: Replace 300-series stainless steels or HK-40 (25Cr-20Ni) uncooled hangers with 50Cr-50Ni cast alloys with niobium (such as ASTM A560 Grade 50Cr-50Ni-Nb or IN-657). The massive 50% chromium content maintains a self-healing barrier that is remarkably resistant to molten slag fluxing.
3. Inspection: Visual inspection (VT) during outages after grit blasting; ultrasonic thickness (UT) measurements on tube fireside profiles.

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3.53 Refractory Degradation

Mechanism Description

Refractory linings—composed of castable refractories, dense bricks, or ceramic fiber blankets—provide essential thermal insulation and erosion protection for steel pressure vessels, fired heater casings, and transfer lines. Refractory degradation encompasses the gradual or sudden physical, mechanical, and chemical breakdown of these protective linings. When refractory degrades or falls away, the underlying structural carbon steel is exposed to extreme temperatures well beyond its design limits, leading to rapid bulging, oxidation, creep rupture, or loss of containment.

Primary Failure Modes

1. Thermal Shock and Thermal Spalling: Rapid temperature swings cause severe thermal expansion and contraction gradients between the hot face and cold face of the refractory. When thermal stresses exceed the tensile rupture strength of the ceramic material, spalling occurs, causing large chunks of refractory to crack and detach.
2. Metallic Anchor Failure: Refractory is anchored to the steel shell by metallic studs (V-anchors, Y-anchors, or hexmetal mesh). Anchors can fail by:
   • High-temperature oxidation and carburization from hot process gases bypassing the refractory.
   • Differential thermal expansion: Metallic anchors expand roughly three times faster than ceramic refractories. If anchor tips lack plastic expansion caps or wax coatings, the expanding metal shears the surrounding refractory, creating extensive spalling.
3. Catalyst Erosion: In Fluid Catalytic Cracking (FCC) units, high-velocity streams (60 to 100+ ft/s) of abrasive zeolite catalyst fines rapidly scour and erode castable linings in reactor cyclones, regenerator slide valves, and riser transfer lines.
4. Steam Explosions (Improper Dry-Out): Newly installed castable refractories contain substantial amounts of chemically bound and free hydraulic water. If the equipment is heated too rapidly during initial dry-out, internal water converts to high-pressure steam faster than it can migrate through the capillary pores. The resulting steam overpressure violently shatters and blows the refractory off the wall in an explosive failure.
5. Chemical Fluxing and Alkali Hydrolysis: In the presence of alkalis, water, or reducing gases, refractory binders (such as calcium aluminate cement) can decompose chemically, turning the dense refractory into a soft, crumbly chalk.

Critical Operating Factors and Curing Protocols

• Dry-Out Heating Rates: Curing schedules must strictly adhere to controlled heating ramps (typically 25 °F to 50 °F per hour / 14 °C to 28 °C per hour), with mandatory holding plateaus (soaks) at 220 °F to 250 °F (104 °C to 121 °C) to release free water and 550 °F to 650 °F (288 °C to 343 °C) to evacuate chemically bound water.
• Anchor Metallurgy: Carbon steel anchors are limited to low-temperature installations. High-temperature refractory systems require Type 304 or Type 310 stainless steel anchors. Above 1,800 °F (982 °C), ceramic anchors are utilized.
• Erosion Resistance: In high-velocity FCC environments, specialized dense, phosphate-bonded tabular alumina refractories installed inside hexmetal grids provide maximum abrasion resistance.

                         REFRACTORY DEGRADATION PATHWAYS
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         v                             v                             v
  Thermal Spalling /             Anchor System Failure           Steam Explosion
   Thermal Shock              (Oxidation / Expansion Shear)     (Rapid Initial Heat-Up)
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         +-----------------------------+-----------------------------+
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                     Refractory Loss / Void Formation
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                    HOT GAS BYPASS TO PRESSURE BOUNDARY
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             Shell Hot Spots (> 650 °F on Carbon Steel), Bulging, Rupture

Inspection and Monitoring

• Infrared (IR) Thermography: The single most critical in-service monitoring tool for refractory systems. Regular quantitative infrared scanning of external equipment shells detects localized refractory thinning, cracking, or bypass before shell failure occurs. Owners set maximum allowable shell skin temperatures from the shell's design temperature (often around 650 °F / 343 °C for carbon steel shells) and trend IR readings against those limits.
• Internal Visual Testing (VT): During outages, inspect for cracking (cracks $> 1/8\text{ in.}$ wide), pinch spalling, missing tiles, exposed anchor studs, and erosion gouges.
• Sounding (Hammer Testing): Tapping refractory with a ball-peen hammer assesses integrity; a clear ringing tone indicates sound bonding, whereas a dull, hollow thud indicates subsurface delamination.