In refinery sour water stripping, a few degrees, a trace of air, or the wrong alloy can take corrosion from manageable to catastrophic. Data from lab and field tests show how materials, pH control, chloride limits, and inhibitors cut rates from tens of mils per year to a few.
Sour water is the refinery problem child: a hot mix of dissolved hydrogen sulfide (H₂S), ammonia (NH₃), carbon dioxide (CO₂), and salts that eats equipment unless chemistry and materials are dialed in. In sour water stripping (SWS), moving from ~110°F to 250°F boosted carbon steel corrosion by about 640% in one study (www.corrosionsource.com), and even traces of air can quadruple it via polythionate formation (www.corrosionsource.com).
The chemistry is ruthless but predictable. At modest “acidic” pH (≈4.5–7), a protective iron sulfide (FeS) film forms on steel and curbs general attack (amcosite.wordpress.com). Push pH below ~4.5 with strong acids (e.g., free HCl from chloride hydrolysis) and the attack accelerates.
Corrosive chemistry and operating severity
Sour water typically sits around pH 4–7 and carries H₂S, NH₃, CO₂, and salts. Carbon steel in this brew can corrode at 10–80 mils per year (mpy; a mil is one‑thousandth of an inch) in hot service, with 66–87 mpy reported at 121°C at neutral-to-acid pH (www.corrosionsource.com). Temperature and oxidants dominate: in tests, carbon steel climbed from ~13.4 mpy at 43°C (pH~7) to ~87 mpy at 121°C (pH≈4), and air ingress raised rates ~4× (www.corrosionsource.com).
By contrast, 300-series austenitic stainless steels (304/316) show very low general corrosion under similar conditions (www.corrosionsource.com), provided chlorides and crevices are under control. In autoclave tests of refinery sour water, 304 stainless had no measurable general corrosion where carbon steel lost tens of mpy (www.corrosionsource.com).
Materials of construction and comparatives
Carbon and low‑alloy steel: Cheap, common, and vulnerable. Uninhibited carbon steel in hot sour water can run 10–80 mpy, with 66–87 mpy at 121°C under neutral-to-acid pH (www.corrosionsource.com). Adding 3–9% Cr (per NACE MR0175/MR0103) helps modestly, but H₂S can still drive sulfide stress cracking and hydrogen damage. Designs often assume a 3–5 mm corrosion allowance.
Austenitic stainless (300‑series): 304/316 resist general corrosion in sour water (www.corrosionsource.com), but chlorides and crevices can trigger pitting or stress corrosion cracking (SCC). Industry experience limits 304/316 typically to below ~60–80°C to avoid chloride SCC (amcosite.wordpress.com; amcosite.wordpress.com). In one case, crevice attack on 304 U-bends reached 12–71 mpy at pH ~3 (www.corrosionsource.com).
Duplex and super duplex stainless: Duplex (e.g., UNS S32205) offers roughly 2× the yield strength of 304 with better chloride and sulfide resistance; super-duplex (UNS S32750, ~25% Cr, 3.5% Mo, ≈5% Ni) handles higher Cl⁻ and H₂S. These alloys resist chloride SCC up to ~200°C and higher H₂S partial pressures (see ISO 15156/NACE MR0175), are common in sour-water ejectors, exchangers, and trays, and typically see a few mpy under moderate sour conditions.
High‑nickel alloys: Incoloy 825 (≈Ni–21Cr–30Fe–2Mo) and Inconel 625 (≈Ni–22Cr–9Mo) keep corrosion to <1–2 mpy in severe sour water and resist sulfide and chloride attack, including localized attack and steam‑out SCC (amcosite.wordpress.com). A field retrofit applied a high‑NiCrMo cladding to a 93 m² stripper shell; after 3 years, the cladding was “as‑sprayed” with no defects or corrosion (integratedglobal.com).
Illustrative comparison: 304SS yields ~0–10 mpy at pH≥5 and T<60°C (www.corrosionsource.com); 316L (with Mo) adds pitting resistance; duplex often <2 mpy; Incoloy 825/925 or 6‑Mo SS can be <1 mpy under the same conditions. In ammoniacal junctions (NH₄HS), higher Ni content improves safety against sulfide SCC where carbon steel can crack catastrophically.
pH control and acid selection
Because sour corrosion is pH‑sensitive, operations target pH above ~4.5 to sustain the FeS film (amcosite.wordpress.com). SWS feed may enter acidic but often alkalizes down the column because NH₃ strips less readily (www.digitalrefining.com). Raising a sour water from pH 4.0 to 7.0 at 43°C cut carbon‑steel corrosion from ~15.5 to 11.9 mpy (~25% reduction) (www.corrosionsource.com).
Acid choice matters. Hydrochloric acid dosing to pH 6–6.5 is sometimes used to prevent CaCO₃ scaling, but it creates free HCl via chloride hydrolysis, which aggressively attacks steel; sulfuric acid is worse (can form sulfuric side‑products with H₂S) (www.digitalrefining.com). Recommendation: target neutral/slightly alkaline pH (6–9) and use only mild acid/base washes (e.g., acetic acid or ammonia) during maintenance (www.digitalrefining.com).
Neutralizing programs often use amines; facilities dose such chemistries with dedicated metering to maintain stability. Where this is practiced, operators typically deploy a neutralizing amine regimen and meter it via an accurate dosing pump.
Chloride management and pretreatment
Chlorides undermine stainless passive films and promote pitting/SCC. Guidance keeps Cl⁻ to within a few hundred mg/L for 300‑series stainless; for higher chlorides, duplex or Ni‑base alloys are favored. NACE/MR0175 charts indicate duplex in H₂S up to ~70–80°C can tolerate chlorides, while 304/316L can crack above ~60°C when chlorides are present (amcosite.wordpress.com; amcosite.wordpress.com). Online monitoring of chloride (or conductivity) is advised.
In Indonesian inland refineries, feed water may carry moderate Ca/Mg; softening pretreatment helps avoid downstream Cl⁻ build‑ups in stripper bottoms. Plants often specify a softener upstream to stabilize SWS chemistry.
Chemical inhibitors and verification
Chemical inhibitors provide an added defense on carbon steel. Sour‑water programs tend to use filming/amphoteric compounds (fatty acids, imidazolines, thiols) and amine‑based neutralizers for H₂S; tertiary amine “sweetening” (thionoethylamines) and quaternary ammonium salts are common filmers. Anodic inhibitors (phosphate, nitrate) are less used.
Effectiveness varies and requires validation. In one set of tests, adding 250 ppm of a commercial filming inhibitor (“IPC 2625”) did not reduce carbon‑steel corrosion and slightly increased it (www.corrosionsource.com). By contrast, well‑chosen blends can reduce rates by 50–90% (vendor claims), though actual benefit is environment‑dependent. Typical treatment maintains 10–200 ppm inhibitor, monitored via corrosion probes, often injected at the stripper overhead or feed. Many refineries formalize this by adopting a targeted corrosion inhibitor program.
Oxygen and sour gas scavenging
Oxygen scavengers are essential because dissolved O₂, even at tens of parts per billion, can spur pitting. Common agents include sulfite and hydrazine. Programs that integrate an oxygen/H₂S scavenger at critical loops minimize oxidation in SWS circuits.
Engineering controls and coatings
Beyond chemistry and alloys, plant design matters. Temperature control via external reflux or a pump‑around at the column top lowers gas dew points and reduces overhead acid concentration; one study noted a top pump‑around “reduces corrosion potential” by condensing HCl/H₂S out of the vapor (www.mdpi.com). Flow design should minimize high‑velocity impingement (nozzles, elbows). Internals should resist fouling and allow wash‑downs.
For existing assets, corrosion‑resistant alloy (CRA) linings, weld‑overlays, or high‑velocity thermal spray (HVTS) can reset life. A documented case applied a Ni‑Cr‑Mo HVTS coating to a sour water stripper tower shell (93 m²); after 3+ years, inspections found the surface “as‑sprayed” with no defects or corrosion (integratedglobal.com). Hydrogen flux monitoring inspection has, indicate any signs of corrosion (integratedglobal.com). Application time in that case was about 2.5 days (integratedglobal.com).
Data-backed outcomes and rates
Temperature effect: raising sour‑water temperature from ~110°F to 250°F increased steel corrosion by ~640% (www.corrosionsource.com). pH effect: raising pH from 4.0→7.0 at 43°C cut carbon‑steel mpy by ~25% (from ~15.5 to 11.9 mpy) (www.corrosionsource.com).
Air/oxidant effect: air ingress raised steel corrosion ~4× (polythionates) (www.corrosionsource.com). Materials comparison: 304SS had essentially zero uniform corrosion where carbon steel showed tens of mpy (www.corrosionsource.com). Crevice pitting did appear on 304 at low pH, underscoring why 316L/6‑Mo or Ni‑alloys are often specified for worst‑case zones.
In practical units, carbon steel typically runs 10–50 mpy (0.25–1.25 mm/yr). Inhibitors must be screened: the 250 ppm IPC‑2625 case underperformed (www.corrosionsource.com), but operators report the right package can enable ~10–20 mpy instead of 50+ mpy. Maintaining a hot, dry overhead (≈80–85°C minimum) helps prevent ammonia‑salt condensation and corrosion spots (www.digitalrefining.com).
Regulatory context and cost drivers
Regulations require treated sour water to meet reuse/discharge limits such as NH₃ <10–20 mg/L and neutral pH (typically 6–9) before discharge or reuse (www.mdpi.com). Efficient H₂S/NH₃ removal via stripping is mandatory. In Indonesia, Ministry of Environment and Forestry (MoEF) wastewater rules align with these values, and a failing stripper can force costly reprocessing. Business drivers are clear: while Ni‑base alloys cost several times more than steel, upgrades or coatings can halve maintenance spend and avert shutdowns.
Summary of control strategy
Minimize sour‑water corrosion by: (1) selecting alloys with sufficient Cr/Mo/Ni for severity (e.g., 316L → duplex → Alloy 825/625) (amcosite.wordpress.com; integratedglobal.com); (2) strictly controlling pH (avoid pH<4.5) and chloride (keep it low) (amcosite.wordpress.com; www.corrosionsource.com); and (3) using tailored inhibitors with verified effectiveness. These steps, backed by lab and field data above, reduce corrosion rates from the tens of mpy to a few mpy, improving reliability and lowering lifetime cost.
Sources: Peer‑reviewed studies and industrial case reports (www.corrosionsource.com; integratedglobal.com; amcosite.wordpress.com; www.mdpi.com). All figures quote measured corrosion rates or operational outcomes under sour‑water conditions. Please refer to these for detailed technical guidance.
