6.2 Sulfuric Acid Corrosion & Phosphoric Acid Corrosion

6.2 Sulfuric Acid Corrosion & Phosphoric Acid Corrosion

Mineral acids represent some of the most aggressive and chemically dynamic process environments encountered in petroleum refining and petrochemical manufacturing. Sulfuric Acid Corrosion (API RP 571 Section 3.62) and Phosphoric Acid Corrosion (API RP 571 Section 3.51) occur primarily in alkylation and polymerization units designed to convert light olefin gases into high-octane gasoline blendstocks. Both mechanisms demand strict operational discipline, comprehensive hydrodynamic control, and specialized metallurgical selection to avoid loss of containment.

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3.62 Sulfuric Acid Corrosion: Fundamental Mechanisms and Concentration Dynamics

Sulfuric acid ($\text{H}_2\text{SO}_4$) is a dense, highly reactive diprotic mineral acid. In petroleum refineries, it is utilized as a liquid acid catalyst in sulfuric acid alkylation units, as a chemical scrubbing agent in acid treating units, in spent acid regeneration (SAR) plants, and in the tail-gas condensing stages of sulfur recovery units (SRU).

Concentration, Temperature, and Velocity: What RP 571 Emphasizes

Sulfuric acid corrosivity depends on acid concentration, temperature, alloy content, velocity, contamination, and the presence of oxidizers. For carbon steel, API RP 571 highlights two thresholds that exam questions use directly:

• Concentration: carbon steel corrosion rates increase significantly at acid concentrations below about 65%. Alkylation units therefore circulate concentrated acid (typically 85% to 98%) in carbon steel, while dilute acid requires alloys or linings.
• Velocity: carbon steel corrosion rates increase significantly when velocity exceeds about 2 to 3 ft/s (0.6 to 0.9 m/s).

API RP 571 lists materials in order of increasing resistance to sulfuric acid: carbon steel, 316L SS, Alloy 20, high-silicon cast iron, high-nickel cast iron, Alloy B-2, and Alloy C-276.

The Ferrous Sulfate ($\text{FeSO}_4$) Passivating Mechanism on Carbon Steel

In concentrated acid (typically 93 wt% to 98 wt% $\text{H}_2\text{SO}_4$), carbon steel undergoes an initial oxidation reaction:

$\text{Fe} + \text{H}_2\text{SO}_4 \longrightarrow \text{FeSO}_4\text{ (solid)} + \text{H}_2\text{ (gas)}$

Because ferrous sulfate has low solubility in concentrated sulfuric acid, it precipitates onto the steel surface as an adherent, microscopically thin salt barrier layer. This salt film stifles further anodic metal dissolution, effectively passivating the carbon steel.

The Critical Vulnerability of the $\text{FeSO}_4$ Film: This protective scale is not a true, tough metallurgical oxide (like $\text{Cr}_2\text{O}_3$ on stainless steel). Instead, it is a mechanically soft, friable, non-adherent crystalline layer. Any increase in fluid velocity, shear stress, turbulence, or temperature, or any decrease in acid concentration, rapidly dissolves or strips the film, triggering immediate, violent corrosion.

Fluid Hydrodynamics, Critical Velocity Limits, and Water-Dilution Hazards

Two critical operational hazards dominate sulfuric acid service: velocity-induced scale stripping and exothermic water dilution.

1. The Carbon Steel Fluid Velocity Limit: 2 to 3 ft/s (0.6 to 0.9 m/s)

Because the ferrous sulfate film possesses virtually no mechanical bond strength, fluid flow velocity must be strictly managed:

• Maximum Velocity Ceiling: In concentrated (93% to 98%) acid piping, fluid velocity for carbon steel must be restricted to < 2 to 3 ft/s (0.6 to 0.9 m/s).
• Turbulent Stripping Threshold: At velocities exceeding 3 to 4 ft/s (0.9 to 1.2 m/s), or in regions of localized turbulence (downstream of pump discharges, control valves, orifice plates, 90° elbows, and branch tees), the shear stress of the flowing acid completely sweeps away the $\text{FeSO}_4$ scale. The bare carbon steel corrodes at rates exceeding 100 to 300 mpy, resulting in localized flow-accelerated erosion-corrosion.
• Piping Design Requirements: Sulfuric acid lines are deliberately designed with over-sized diameters to ensure low-velocity laminar flow, long-radius sweep elbows, and sweeping branch connections.

2. Operating Temperature Limits for Carbon Steel

Temperature dramatically increases the solubility of the ferrous sulfate film:

• Keep it cool: In alkylation service, carbon steel handling concentrated acid is typically kept at low temperature (commonly below about 100 °F / 38 °C), because corrosion rates climb quickly as temperature rises and the ferrous sulfate film becomes more soluble.

3. The Water-Dilution Exotherm: The Catastrophic Mixing Hazard

Sulfuric acid has an immense affinity for water. When concentrated acid is mixed with water, the hydration of sulfuric acid molecules is intensely exothermic:

$\text{H}_2\text{SO}_4 + n\text{H}_2\text{O} \longrightarrow \text{H}_2\text{SO}_4 \cdot n\text{H}_2\text{O} + \Delta H\text{ (Enormous Heat Liberation)}$

If water inadvertently enters a concentrated acid system (e.g., via leaking cooler tubes, moist compressed air, atmospheric breathing on storage tanks, or improper water-washing procedures):

1. Local temperature instantly spikes past 200 °F to 300 °F (93 °C to 149 °C), causing localized flash boiling.
2. The local acid concentration drops below the level carbon steel can tolerate (RP 571's roughly 65% threshold).
3. The combination of boiling temperatures and intermediate acid concentration causes carbon steel piping to perforate in hours.

Damage Morphology: Uniform Loss, Turbulent Gouging, and Hydrogen Grooving

Sulfuric acid corrosion generates three distinct physical damage morphologies:

1. Broad Uniform Thinning

In quiescent, low-velocity environments (such as acid storage tanks, settler drums, and oversized pipe runs), damage manifests as smooth, uniform metal loss across the entire wetted surface, accompanied by a powdery gray-white or greenish ferrous sulfate deposit.

2. Severe Turbulent Erosion-Corrosion and Grooving

In piping systems subject to high velocity or directional changes, attack manifests as aggressive, localized, flow-oriented grooving, horse-shoe scallops, and sharp knife-line thinning downstream of flow disruptions:

• Downstream of pump impellers and discharge flanges
• Immediately downstream of control valves, orifice plates, and thermowell quills
• Along the outer radius of short-radius elbows and tee junctions

3. "Hydrogen Grooving" (A Unique Acid Morphology)

Hydrogen grooving is an unmistakable macroscopic signature of sulfuric acid attack in horizontal piping and vessels containing slow-moving or stagnant concentrated acid:

                                  PIPE CROSS-SECTION
                              +------------------------+
                              |    HYDROGEN POCKET     |
                              |   /                  \ |
                              |  |  (Deep Gouge at )  ||
                              |  |  (Top of Pipe   )  ||
                              +--+--------------------+--+
                              |  ^                    ^  |
                              |  | Rising H2 Gas      |  |
                              |  | Bubble Tracks      |  |
                              |  | Strip FeSO4 Film   |  |
                              |                          |
                              |   CONCENTRATED H2SO4   |
                              +------------------------+

• Mechanism: As iron dissolves in concentrated acid, hydrogen gas is generated: $\text{Fe} + \text{H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2\uparrow$.
• In low-velocity horizontal lines, the buoyant hydrogen gas bubbles disengage and migrate vertically upward along the pipe walls toward the 12 o'clock crown.
• The upward physical path of these ascending gas bubbles creates localized convective micro-currents that mechanically scour away the fragile $\text{FeSO}_4$ film along narrow vertical tracks.
• Result: Parallel vertical grooves on the pipe sidewalls, feeding into a deep, continuous longitudinal trench along the top inside crown of the pipe.

Materials Selection and Metallurgical Upgrades for Sulfuric Acid

Selecting appropriate alloys for sulfuric acid requires matching material passivity to acid concentration, temperature, and fluid dynamics.

1. Carbon Steel (With Rigorous Constraints)

• Used for concentrated acid (RP 571: rates rise significantly below about 65% acid or above about 2 to 3 ft/s / 0.6 to 0.9 m/s), at low temperature.
• Must include a substantial corrosion allowance (typically 1/4 in. or 6.35 mm).
• Weld metal and heat-affected zones can corrode preferentially, so welds and turbulent locations receive close inspection.

2. Alloy 20 (UNS N08020 / Carpenter 20Cb-3)

Alloy 20 is an austenitic iron-nickel-chromium super-alloy developed specifically to overcome the vulnerabilities of stainless steels in sulfuric acid across intermediate concentrations:

• Nominal Chemistry: 32% to 38% Nickel, 19% to 21% Chromium, 2% to 3% Molybdenum, 3% to 4% Copper, and stabilized with Niobium (Columbium).
• Role of Copper: The addition of 3% to 4% copper provides unmatched resistance to sulfuric acid by stabilizing the protective passive oxide film in intermediate, reducing concentrations.
• Application: Widely specified for alkylation acid pump casings, valve bodies, mixing quills, orifice plates, and heat exchanger bundles operating up to 150 °F to 200 °F (66 °C to 93 °C) across all concentrations.

3. Nickel-Base Alloys (Alloy C-276, Alloy B-2)

• Alloy C-276 (UNS N10276): Contains 57% Ni, 16% Mo, 16% Cr, and 4% W. Provides outstanding resistance in boiling dilute acid, high-temperature spent acid regeneration gas streams, and mixed acid environments containing chlorides or sulfur dioxide.
• Alloy B-2 / B-3 (UNS N10665): High-molybdenum (28% Mo), low-chromium nickel alloy designed specifically for pure, non-oxidizing reducing acids. Highly resistant to intermediate sulfuric acid, but rapidly fails if oxidizing contaminants (ferric ions, cupric ions, or dissolved oxygen) are introduced.

4. Non-Metallic Materials and Linings

• Polytetrafluoroethylene (PTFE) and Perfluoroalkoxy (PFA): Completely chemically inert to sulfuric acid at all concentrations up to 400 °F (204 °C). Extensively used for lined piping spools, dip tubes, valve linings, and expansion joints.
• Acid-Brick Linings: Used in heavy spent-acid regeneration furnaces and large storage vessels.

3.51 Phosphoric Acid Corrosion: Catalytic Polymerization & Alkylation

Phosphoric Acid Corrosion (API RP 571 Section 3.51) is a specialized form of mineral acid degradation occurring in Catalytic Polymerization (Cat Poly) units, phosphoric acid alkylation, and chemical fertilizer processing.

Process Application and Acid Behavior

In Cat Poly units, light olefinic hydrocarbons (propylene and butylene from the FCC unit) are polymerized into high-octane motor gasoline oligomers or petrochemical feedstocks (dodecene, nonene). The catalyst used is either liquid phosphoric acid ($\text{H}_3\text{PO}_4$) or solid phosphoric acid (SPA), consisting of diatomaceous earth / kieselguhr pellets impregnated with concentrated polyphosphoric acid.

• Non-Oxidizing Behavior: Pure phosphoric acid is a moderately weak, non-oxidizing polyprotic mineral acid.
• Corrosion Mechanism on Carbon Steel: Carbon steel has poor resistance to concentrated phosphoric acid at elevated temperatures. Uninhibited hot phosphoric acid (> 120 °F / 49 °C) vigorously attacks carbon steel, driving active anodic dissolution and hydrogen evolution: $2\text{H}_3\text{PO}_4 + 3\text{Fe} \rightarrow \text{Fe}_3(\text{PO}_4)_2 + 3\text{H}_2$.

The Catalytic Destruction of Passivity by Halide Contaminants

Austenitic stainless steels (Types 304L and 316L) naturally passivate in pure phosphoric acid by establishing a protective chromium oxide / phosphate conversion film. However, this passivity is acutely vulnerable to halide contamination:

• Chlorides ($\text{Cl}^-$) and Fluorides ($\text{F}^-$): Chloride contamination (and fluorides where present) in the olefin feed or water can break down the passive film on stainless steels and sharply increase corrosion.
• Severe Localized Pitting: In the presence of halides, stainless steels suffer severe localized pitting, crevice corrosion, and stress corrosion cracking at temperatures above 140 °F (60 °C).

Affected Equipment in Cat Poly Units

• Cat Poly Reactors: Packed catalyst beds, internal screen baskets, and support grids.
• Reactor Effluent Piping and Fractionation Columns: Lines carrying unreacted hydrocarbons, acid carryover, and oligomer products.
• Reboilers and Exchangers: High metal skin temperatures accelerate phosphoric acid attack on reboiler tube bundles.

Materials Selection and Mitigation for Phosphoric Acid

• RP 571 temperature guidance for 100% phosphoric acid: Type 304L is satisfactory below about 120 °F (49 °C); Type 316L is required from about 120 °F to 225 °F (49 °C to 107 °C).
• Hotter or more dilute service: RP 571 notes that 316L and Alloy 20 are effective at concentrations up to about 85% at boiling temperatures; higher alloys (for example, Alloy 825) are used where chlorides cannot be excluded.
• Where attack concentrates: most corrosion occurs where free water mixes with the acid, such as water-wash points, low points, and reboilers.
• Water Washing Control: In SPA units, precise water hydration of the catalyst bed is critical. Excess water leaches concentrated liquid acid out of the catalyst pellets, flooding downstream piping with corrosive green acid water.

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Inspection, Non-Destructive Examination (NDE), and Monitoring

Because sulfuric and phosphoric acid degradation can rapidly transition from low uniform loss to aggressive localized grooving, inspection plans must focus on turbulent, high-risk locations.

1. Ultrasonic Thickness (UT) Scanning and Grid Mapping

• Grid UT Mapping: Performed systematically across piping elbows, tees, reducers, and pump suction/discharge spools in sulfuric acid circuits to detect localized wall thinning and flow gouging.
• Continuous High-Temperature UT Probes: Mounted at critical points downstream of acid injection quills and control valves.

2. Profile Radiography (PRT)

• The Gold Standard for Piping: Profile radiography is essential for small-bore piping (< 2 in. NPS), thermowells, orifice runs, and valve bodies. PRT clearly reveals localized erosion-corrosion gouging, wall loss beneath scale, and internal hydrogen grooving at the top of the pipe crown without requiring insulation removal.

3. Visual Internal Turnaround Inspection

• Inspect vessels and piping for displaced $\text{FeSO}_4$ salt cake, localized washboarding, and hydrogen grooving.
• Inspect storage tank bottom plates for accelerated thinning beneath settled water-acid sludge layers.