4.3 Flue Gas Dew Point Corrosion & Aqueous Organic Acid Corrosion

Flue Gas Dew Point Corrosion — API RP 571 Section 3.29

1. Mechanism and Combustion Acid Thermodynamics

Flue Gas Dew Point Corrosion is an aggressive form of uniform and pitting attack that affects fired heaters, utility boilers, heat recovery steam generators (HRSGs), economizers, and air preheater systems. The damage occurs when sulfur-containing combustion flue gases cool below the condensation temperature of acidic vapors, depositing a corrosive liquid acid film onto metallic surfaces.

Fossil fuels utilized across refining operations (refinery fuel gas, residual fuel oil, asphalt, and petroleum coke) contain varying concentrations of organic sulfur and hydrogen sulfide ($H_2S$). During combustion in the radiant firebox, sulfur is oxidized primarily to sulfur dioxide ($SO_2$): $S + O_2 \rightarrow SO_2$

As the hot flue gas traverses the convection section, a minor fraction—typically 1% to 5% of the total $SO_2$—is further oxidized to sulfur trioxide ($SO_3$). This oxidation reaction is catalyzed by excess combustion oxygen ($O_2$) and the catalytic effect of hot iron oxide ($Fe_2O_3$) scale and vanadium pentoxide ($V_2O_5$) present on furnace tube surfaces at temperatures between 800 °F and 1100 °F (427 °C to 593 °C): $2SO_2 + O_2 \xrightarrow{Fe_2O_3, V_2O_5} 2SO_3$

In the presence of moisture produced by hydrocarbon combustion ($H_2O$), sulfur trioxide instantly reacts with water vapor to form gaseous sulfuric acid ($H_2SO_4$): $SO_3(g) + H_2O(g) \rightleftharpoons H_2SO_4(g)$

Combustion of Sulfur-Bearing Fuel in Fired Heater / Boiler
       │
       ├─► S + O2 → SO2 (Primary combustion product: 95 - 99% of sulfur)
       │
       └─► 2SO2 + O2 → 2SO3 (Catalyzed by excess air & tube scale: 1 - 5% of sulfur)
              │
              ▼  Combines with combustion moisture (H2O)
           H2SO4 (Sulfuric Acid Vapor)
              │
              ▼  Flue gas cools through Economizer & Air Preheater
       ┌──────────────────────────────────────────────────────────────┐
       │ ACID DEW POINT (250 °F - 320 °F / 121 °C - 160 °C):          │
       │ Sulfuric acid condenses as a concentrated liquid film        │
       │ (70 to 85 wt% H2SO4) directly onto cold metal surfaces!      │
       └──────────────────────────────────────────────────────────────┘
              │
              ▼  Flue gas cools further below Water Dew Point (120 °F - 140 °F / 49 °C - 60 °C)
       ┌──────────────────────────────────────────────────────────────┐
       │ SECONDARY WATER CONDENSATION:                                │
       │ SO2 and CO2 dissolve into liquid water, forming sulfurous   │
       │ (H2SO3) and carbonic (H2CO3) acids, compounding metal loss!  │
       └──────────────────────────────────────────────────────────────┘

2. Acid Dew Point vs. Water Dew Point Dynamics

A critical concept for refinery inspection and operations is the profound difference between the water dew point and the acid dew point:

• Water Dew Point: Pure water vapor condenses in flue gas at temperatures typically between 110 °F and 135 °F (43 °C to 57 °C) depending on fuel hydrogen content and moisture levels.
• Sulfuric Acid Dew Point: Because of the strong affinity between $SO_3$ and water vapor, sulfuric acid condenses at much higher temperatures. The dew point depends on the $SO_3$ concentration; RP 571 gives a typical value of about 280 °F (138 °C), and practical values commonly fall in the 250 °F to 320 °F (121 °C to 160 °C) band.
• Hydrochloric Acid Dew Point: Fuels containing chlorides can form HCl in the flue gas. RP 571 gives a typical HCl dew point of about 130 °F (54 °C), well below the sulfuric acid dew point.
• Concentration of Condensate: The initial droplets of condensate that precipitate onto metal surfaces at the acid dew point are not dilute; they consist of concentrated sulfuric acid containing 70 wt% to 85 wt% $H_2SO_4$. This concentrated acid rapidly attacks carbon steel and low-alloy steels, dissolving metal and precipitating thick, crusty iron sulfate scale: $Fe + H_2SO_4 \rightarrow FeSO_4 + H_2$

3. Critical Factors Governing Severity

1. Fuel Sulfur Content: Higher fuel sulfur content directly translates to higher $SO_2$ and $SO_3$ partial pressures, raising the acid dew point temperature.
2. Excess Combustion Air (Excess O2): Operating heaters with excessive air provides abundant free oxygen that drives the catalytic oxidation of $SO_2$ into $SO_3$. Reducing firebox excess oxygen from 5% down to 1.5–2.0% dramatically suppresses $SO_3$ synthesis.
3. Metal Surface Temperature vs. Bulk Gas Temperature: Corrosion occurs strictly as a function of metal wall temperature, not bulk flue gas temperature. In an air preheater, cold atmospheric combustion air enters at ambient temperature (e.g., 30 °F to 80 °F / -1 °C to 27 °C) and flows across the opposite side of thin heat exchanger plates or tubes. Even if the bulk flue gas exiting the unit is 350 °F (177 °C), the cold incoming air chills the metal plate surface below 260 °F (127 °C), causing severe acid condensation on the flue gas side.

4. Susceptible Equipment, Materials and Mitigation

Susceptible Equipment:

• Air Preheaters (APH): Particularly the cold-end baskets of rotary regenerative preheaters (Ljungström type) and cold-end tubes/plates of recuperative preheaters.
• Economizer Bundles: The lowest-temperature tube rows in boiler and HRSG economizers where cold boiler feedwater enters.
• Flue Gas Ducting & Dampers: Cold corners, expansion joints, uninsulated structural stiffeners, and damper frames where thermal bridging creates local cold spots.
• Induced Draft (ID) Fans & Stacks: ID fan housings, impellers, and carbon steel exhaust stack shells.

Materials Performance:

• Carbon Steel and Low Alloys (Cr-Mo): Rapidly corroded by condensing sulfuric acid (rates exceeding 50 to 200 mpy / 1.3 to 5 mm/yr).
• Austenitic 300-Series Stainless Steels: Not a cure for dew point corrosion. Condensing acid can still pit and corrode them, and RP 571 warns that 300 series SS feedwater heaters and economizer sections in heat recovery steam generators can suffer chloride SCC where chlorides are present in the flue gas and condense.
• Weathering Steel (Corten - ASTM A606/A588): Contains small additions of Cu, Cr, and Ni that form a slightly more adherent sulfate scale, providing a marginal 1.5× to 2× life extension in intermittent dew point service, but insufficient for continuous acid condensation.
• Enamel-Coated Carbon Steel: Vitreous porcelain enamel coatings applied to cold-end air preheater baskets provide excellent barrier protection against sulfuric acid.
• Borosilicate Glass and Fluoropolymer Tubes: Non-metallic borosilicate glass or Teflon-lined heat exchanger tubes are immune to acid dew point corrosion and are specified in deep heat recovery systems operating down to 180 °F (82 °C).

Operational Mitigation Strategies:

1. Maintain Metal Temperatures Above Dew Point: Ensure that the minimum metal temperature of cold-end heat exchange elements is maintained at least 25 °F to 50 °F (14 °C to 28 °C) above the calculated sulfuric acid dew point under all operating conditions.
2. Steam-Air Preheaters (SAPH): Install steam-heated air preheaters upstream of the main air preheater to heat cold incoming ambient combustion air to 140 °F to 180 °F (60 °C to 82 °C) before it contacts the flue gas heat exchange surfaces.
3. Low-Excess-Air Combustion: Operate burners with low excess air (<2% to 3% excess $O_2$) to suppress the oxidation of $SO_2$ into $SO_3$.
4. Fuel Desulfurization: Treat refinery fuel gas via amine treating units to maintain $H_2S$ levels below 50 to 160 ppmv.

Aqueous Organic Acid Corrosion — API RP 571 Section 3.7

1. Mechanism and Environmental Chemistry

Aqueous Organic Acid Corrosion refers to the degradation of carbon steel and low-alloy steels caused by low-molecular-weight, water-soluble organic acids (aliphatic fatty acids) dissolved in condensed aqueous phases. The principal organic acids encountered in petroleum refining include:

• Formic Acid ($HCOOH$, $pK_a = 3.75$) — The strongest and most corrosive of the series.
• Acetic Acid ($CH_3COOH$, $pK_a = 4.76$) — The most prevalent organic acid in refining overheads.
• Propionic Acid ($CH_3CH_2COOH$, $pK_a = 4.87$)
• Butyric Acid ($CH_3(CH_2)_2COOH$, $pK_a = 4.82$)

These organic acids are generated during reservoir thermal maturation or thermal/catalytic cracking reactions in distillation columns, delayed cokers, and FCC units. When process vapors cool below the water dew point, these volatile carboxylic acids selectively partition into the condensing water phase.

Once dissolved in water, the acid partially dissociates, releasing corrosive hydronium ions ($H^+$): $R\text{-}COOH \rightleftharpoons R\text{-}COO^- + H^+$

The reduction of hydrogen ions drives the anodic dissolution of carbon steel: $Fe + 2R\text{-}COOH \rightarrow Fe(R\text{-}COO)_2 + H_2$

Because ferrous acetate and ferrous formate salts are highly soluble in water, no protective passive film can establish on the metal surface. In oxygen-free systems, corrosion manifests as smooth, uniform wall loss. However, if dissolved oxygen enters the system, cathodic depolarization by $O_2$ accelerates the corrosion rate by a factor of 5 to 10×, producing severe localized pitting and grooving.

2. Crucial Distinction: Aqueous Organic Acid vs. Naphthenic Acid Corrosion

A frequent source of confusion on technical certification exams is differentiating Aqueous Organic Acid Corrosion from Naphthenic Acid Corrosion (API RP 571 Section 3.46):

3. Affected Systems and Metallurgy

• Affected Refinery Units: Wastewater strippers, sour water strippers, overhead condensing systems of crude distillation towers, delayed coker overhead accumulators, gas recovery units, and steam condensate systems receiving boiler feedwater contaminated with organic matter.
• Metallurgy Upgrades: Carbon steel and low-alloy steels exhibit poor resistance in warm acidic water (pH < 4.5). Upgrading to austenitic stainless steels—such as Type 304L or Type 316L SS—provides virtually complete immunity to dilute aqueous organic acids at typical process temperatures (<200 °F / 93 °C). For severe, concentrated hot organic acids, Alloy 825 or Titanium is selected.
• Mitigation: Upstream crude desalting, pH neutralization via caustic soda ($NaOH$) or neutralizer amine injection to maintain boot water pH between 6.0 and 7.5, and strict deaeration to eliminate dissolved oxygen.

4. Non-Destructive Examination (NDE) Methodologies

• Flue Gas Dew Point Inspection:
  • Visual Testing (VT) of air preheater cold-end baskets, ductwork corners, and stack bases for voluminous, reddish-white iron sulfate scale mounds and perforations.
  • Ultrasonic Thickness Testing (UT) on casing walls, ducting panels, and economizer U-bends.
  • Portable Acid Dew Point Meters (flue gas dew point probes) to experimentally measure the exact acid dew point in the flue gas duct during operation.
• Aqueous Organic Acid Inspection:
  • High-density grid ultrasonic thickness scanning (UT) on overhead accumulator boots, stripper piping elbows, and water draw-off lines.
  • Routine water sampling for pH, total acid number (TAN), and dissolved iron counts.