4.1 Acidic Sour Water & Alkaline Sour Water (Ammonium Bisulfide) Corrosion

Acidic Sour Water Corrosion — API RP 571 Section 3.58

1. Mechanism Description and Aqueous Sulfide Chemistry

Acidic Sour Water Corrosion refers to the degradation of carbon steel and low-alloy steels in aqueous environments containing dissolved hydrogen sulfide ($H_2S$) at mildly acidic to near-neutral pH levels (typically pH 4.5 to 7.0). Sour water streams originate across refining operations wherever water vapor condenses in the presence of sulfur-containing hydrocarbon fractions.

When gaseous $H_2S$ dissolves in water, it establishes a two-step weak acid dissociation equilibrium: $H_2S(aq) \rightleftharpoons H^+ + HS^- \quad (pK_{a1} \approx 7.0)$ $HS^- \rightleftharpoons H^+ + S^{2-} \quad (pK_{a2} \approx 13.0)$

In acidic sour water, the primary cathodic reaction driving corrosion on carbon steel is the reduction of hydrogen ions ($H^+$) or direct molecular $H_2S$ reduction: $2H^+ + 2e^- \rightarrow 2H^\bullet \rightarrow H_2$ $2H_2S + 2e^- \rightarrow 2HS^- + H_2$

The complementary anodic oxidation of iron generates soluble ferrous ions: $Fe \rightarrow Fe^{2+} + 2e^-$

As ferrous ions react with sulfide species in solution, an insoluble corrosion product film—predominantly iron sulfide ($FeS$), initially precipitating as metastable mackinawite—forms directly upon the steel substrate: $Fe^{2+} + HS^- \rightarrow FeS\downarrow + H^+$

Bulk Sour Water (pH 4.5 - 7.0, Dissolved H2S, CO2)
       │
       ▼  Aqueous Boundary Layer
       │  - Mackinawite (FeS) scale precipitates
       │  - Quiescent, cyanide-free conditions: FeS passivates steel (<5 mpy)
       ▼
Carbon Steel Substrate (Anodic reaction: Fe → Fe2+ + 2e-)
       │
       └─► Atomic Hydrogen (H•) Generated: ─► Recombines to H2 gas (or enters steel!)

Under stable, quiescent, and neutral conditions, mackinawite forms a semi-protective barrier that impedes further ionic diffusion, lowering corrosion rates to acceptable levels (<3 to 5 mpy / 0.08 to 0.13 mm/yr). However, this protective film is mechanically delicate, chemically reactive, and readily destabilized by fluid turbulence, acidic pH depression, dissolved carbon dioxide ($CO_2$), or cyanide ions ($CN^-$).

2. Synergistic Factors: Carbon Dioxide and the Cyanide Menace

The CO2 Synergy (Carbonic Acid Attack)

In many refinery overhead systems, sour water streams contain significant concentrations of dissolved carbon dioxide ($CO_2$). Dissolved $CO_2$ hydrates to form carbonic acid ($H_2CO_3$): $CO_2(g) + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

Carbonic acid depresses the aqueous pH, shifting the equilibrium toward the dissolution of the protective $FeS$ scale. Furthermore, the combined presence of $H_2S$ and $CO_2$ produces a competitive precipitation between iron sulfide ($FeS$) and iron carbonate ($FeCO_3$). Because $FeS$ has a lower solubility product ($K_{sp}$) than $FeCO_3$, iron sulfide dominates, but the resulting scale lattice contains structural defects that diminish its protective passivity, accelerating galvanic coupling between the conductive $FeS$ scale and the underlying steel matrix.

The Cyanide Menace in FCC and Coker Gas Plants

Thermal and catalytic cracking of heavy feedstocks containing organic nitrogen compounds (e.g., pyridines, quinolines, carbazoles) generates significant quantities of hydrogen cyanide ($HCN$). In the presence of aqueous sour water in Fluid Catalytic Cracking (FCC) fractionator overheads, unsaturated gas plants, and delayed coker overheads, cyanide dissolves as free cyanide ions ($CN^-$).

Free cyanide exerts a catastrophic, two-fold impact on carbon steel:

1. Destruction of Protective Iron Sulfide Scale: Cyanide is a potent ligand for iron. It chemically strips the protective mackinawite scale off the metal surface by complexing ferrous ions into highly soluble ferrocyanide complexes: $FeS + 6CN^- \rightarrow [Fe(CN)_6]^{4-} + S^{2-}$ With the protective $FeS$ film stripped away, the bare carbon steel is continuously exposed to fresh sour water, driving general corrosion rates up to 50 to 100+ mpy (1.3 to 2.5+ mm/yr).
2. Hydrogen Recombination Poisoning: Cyanide acts as an aggressive recombination poison for atomic hydrogen. Under normal sour corrosion, atomic hydrogen adsorbed onto the metal surface recombines into molecular hydrogen gas ($H^\bullet + H^\bullet \rightarrow H_2$) that bubbles harmlessly into the process stream. Cyanide suppresses this recombination reaction, forcing atomic hydrogen to diffuse directly into the steel lattice. This causes severe environmental cracking: hydrogen blistering, hydrogen-induced cracking (HIC), and stress-oriented hydrogen-induced cracking (SOHIC) in pressure vessels and piping.

3. Mitigation of Cyanide: Polysulfide Injection Chemistry

To counteract cyanide-accelerated corrosion and hydrogen charging, refiners inject ammonium polysulfide (APS), chemically represented as $(NH_4)_2S_x$, into the wash water or sour gas stream upstream of overhead condensers.

Polysulfide chemically oxidizes reactive free cyanide ($CN^-$) into benign thiocyanate ($SCN^-$): $CN^- + S_x^{2-} \rightarrow SCN^- + S_{x-1}^{2-}$

Thiocyanate possesses negligible affinity for iron, cannot form soluble ferrocyanide complexes, and does not poison hydrogen recombination. Once free cyanide is neutralized to thiocyanate, a stable, protective iron sulfide ($FeS$) film re-establishes on the steel surface, hydrogen charging drops by orders of magnitude, and corrosion rates return to baseline. Refiners monitor polysulfide injection by sampling accumulator boot water, maintaining a slight excess of polysulfide (indicated by a distinct orange/red tint in the sour water, with the target residual set from boot-water analysis and vendor guidance).

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Alkaline Sour Water / Ammonium Bisulfide Corrosion — API RP 571 Section 3.5

1. Mechanism and Process Conditions

Ammonium Bisulfide ($NH_4HS$) Corrosion—often termed Alkaline Sour Water Corrosion—is an aggressive form of localized and velocity-accelerated attack occurring predominantly in hydroprocessing units (hydrotreaters, hydrodesulfurization [HDS], and hydrocrackers) and sour water stripper (SWS) overhead systems.

In hydroprocessing reactors, high-temperature, high-pressure catalytic hydrogenation converts organic sulfur and nitrogen present in the hydrocarbon feed into hydrogen sulfide ($H_2S$) and ammonia ($NH_3$): $\text{Organic Sulfur} + H_2 \xrightarrow{\text{CoMo/NiMo}} H_2S$ $\text{Organic Nitrogen} + H_2 \xrightarrow{\text{CoMo/NiMo}} NH_3$

As the hot reactor effluent cools through the heat exchanger train toward the Reactor Effluent Air Cooler (REAC), gaseous ammonia and hydrogen sulfide react reversibly to form solid ammonium bisulfide salt: $NH_3(g) + H_2S(g) \rightleftharpoons NH_4HS(s)$

To prevent dry salt deposition, refiners inject continuous wash water upstream of the REAC. The salt rapidly dissolves into the water phase, creating a concentrated alkaline sour water solution (pH typically 8.0 to 9.5): $NH_4HS(s) \xrightarrow{H_2O} NH_4^+(aq) + HS^-(aq)$

Although the solution is alkaline, concentrated $NH_4HS$ is extraordinarily destructive to carbon steel, low-alloy steels, and even 300-series stainless steels. The high concentration of bisulfide ions ($HS^-$) forms soluble iron complex species, actively dissolving protective iron sulfide films and causing severe thinning and localized gouging: $FeS + HS^- \rightleftharpoons FeS_2^{2-} + H^+ \quad \text{(Complex dissolution)}$

2. Critical Variables Governing NH4HS Corrosion Severity

1. $NH_4HS$ Concentration: API RP 571 states that solutions below about 2 wt% $NH_4HS$ are generally not corrosive to carbon steel and that concentrations above 2 wt% are increasingly corrosive. Industry REAC design guidance (such as API RP 932-B) commonly uses about 8 wt% as an upper design concentration for carbon steel with controlled velocity; above that, localized attack can become severe and alloy upgrades are usually required.
2. Fluid Velocity and Hydrodynamic Shear Stress: The iron sulfide film that forms in alkaline sour water is weakly adherent. Above a critical velocity threshold—typically 15 to 20 ft/s (4.6 to 6.1 m/s) for carbon steel—fluid shear stress physically strips the $FeS$ scale off the metal surface. This triggers a relentless erosion-corrosion cycle. For high-nickel alloys such as Alloy 825 (UNS N08825) and Alloy 625 (UNS N06625), the protective film tolerates fluid velocities exceeding 30 ft/s (9.1 m/s).
3. Piping Symmetry and Hydraulics: In multi-bay REAC units, unbalanced header piping creates maldistribution of water and gas phases. If one cooler bay receives excess vapor and inadequate wash water, localized flashing occurs, causing the $NH_4HS$ concentration to skyrocket above 10–15 wt%, resulting in rapid localized failure while adjacent cooler tubes show minimal wall loss.

3. Damage Morphology and Susceptible Equipment

• Morphology in Carbon Steel: Smooth, wavy metal loss with directional scallops, deep localized gouging, and "horseshoe" shaped impingement divots. Attack is heavily concentrated where flow direction changes or cross-sectional area decreases: inlet tube ends of air coolers (the first 6 to 12 inches / 150 to 300 mm from the tubesheet), return bends, header boxes, piping elbows, reducer fittings, and control valve trim.
• Material Performance: API RP 571 notes that carbon steel becomes less resistant as $NH_4HS$ concentration and velocity increase, while 300 series SS, duplex SS, aluminum alloys, titanium, and nickel-base alloys are more resistant, depending on concentration and velocity. Admiralty brass and other copper alloys are not suitable. Stainless steels can still pit under deposits or where chlorides and oxygen are present.
• Susceptible Systems: Hydrocracker and hydrotreater REAC circuits, separator drum overhead piping, sour water stripper overhead vapor lines, condenser tubes, and reflux circuits.

4. Prevention, Mitigation and Inspection Methodologies

Engineering Mitigation Strategies:

1. Continuous Wash Water Injection: Inject continuous, high-volume wash water upstream of the REAC to ensure that the condensed aqueous effluent maintains an $NH_4HS$ concentration below 8 wt% (industry best practice is to target <5 wt%, with many hydrocracker designs targeting <2 wt%).
2. Wash Water Quality: The injection water must be stripped sour water or steam condensate containing <10 to 20 ppb dissolved oxygen. Oxygen ingress into alkaline sour water generates elemental sulfur and thiosulfates, accelerating pitting and stress corrosion cracking.
3. Symmetrical Header Design: Implement balanced, symmetrical piping manifolds with equal length runs and identical pressure drop geometries to guarantee uniform distribution of two-phase vapor/liquid flow across all REAC bundles.
4. Metallurgy Upgrades: Upgrade carbon steel in high-velocity, high-concentration zones to:
   • Duplex Stainless Steel (2205 - UNS S31803 / S32205): Excellent resistance up to 8–10 wt% $NH_4HS$ and velocities up to 25 ft/s.
   • Nickel-Base Alloy 825 (UNS N08825): Industry benchmark for severe REAC service; resistant to $NH_4HS$ concentrations exceeding 12–15 wt% and velocities exceeding 30 ft/s.
   • Titanium (Grade 2 / Grade 12): Exceptional resistance in sour water stripper condensers, though susceptible to hydriding if galvanically coupled to steel under specific conditions.

Non-Destructive Examination (NDE):

• Ultrasonic Thickness Testing (UT Grid Scanning): Continuous automated or dense grid scanning on REAC inlet piping, elbows, reducer tees, and outlet manifolds.
• Internal Rotary Inspection System (IRIS): High-precision ultrasonic internal rotary inspection of air cooler tubes to detect inner diameter gouging, scalloping, and inlet tube erosion.
• Profile Radiography (PRT): Profile RT on small-bore piping, tube-to-tubesheet joints, and U-bends to detect localized underdeposit attack and thinning without insulation removal.
• Analytical Monitoring: Continuous monitoring of sour water boot pH, specific gravity (for $NH_4HS$ wt% estimation), dissolved iron counts, and cyanide concentrations.