Chlor-Alkali Plant Pipe Fittings: Wet Chlorine, Caustic, and Hydrogen Circuit Material Selection

Wet Chlorine Circuit: The Most Aggressive Stream

Wet chlorine gas — chlorine at or above its dew point, containing dissolved moisture — is among the most aggressively corrosive process streams in the chemical industry. Dry chlorine (below the dew point) can be handled in carbon steel, but wet chlorine is a different matter entirely: moisture reacts with Cl₂ to form hypochlorous acid (HOCl) and hydrochloric acid (HCl), creating a strongly oxidising and acidic environment simultaneously. The combination attacks virtually all common construction materials. The primary material choices for wet chlorine fittings are:

• Hastelloy C-276 (UNS N10276): the industry standard for wet chlorine headers and transfer lines. Resistant to both the oxidising HClO and reducing HCl components of wet Cl₂. Used from ambient to approximately 150°C.
• Titanium Grade 2: excellent in wet chlorine at temperatures below approximately 80°C where the passive TiO₂ film is stable. Above 80°C or in the presence of reducing HCl from impurities, Grade 7 (Ti-0.15Pd) is preferred. Titanium is lighter and less expensive than C-276 but has stricter temperature limits.
• PVDF-lined fittings: for lower-pressure, ambient-temperature wet chlorine where metallic fittings are cost-prohibitive. Not a structural piping material — only for lined systems.

Stainless steel (316L or higher) is not acceptable in wet chlorine service — the combination of oxidising conditions and chloride concentration causes rapid pitting and crevice corrosion even at ambient temperature.

Caustic (NaOH) Circuit: Concentration and Temperature Drive Selection

Sodium hydroxide concentration and temperature together determine material selection for the caustic circuit. The membrane cell process produces 30–35% NaOH which is subsequently evaporated to 50% or higher. The failure mode in caustic is caustic stress corrosion cracking (caustic SCC or caustic embrittlement) — not general corrosion. Key material rules:

• Dilute NaOH (<25%) at <80°C: carbon steel (WPB) is acceptable. Corrosion rate is low and caustic SCC risk is negligible at this combination of concentration and temperature.
• 25–50% NaOH at 80–120°C: carbon steel requires stress relief (PWHT) of all welds per NACE SP0403 to eliminate residual stress that drives caustic SCC. Duplex and austenitic stainless are more susceptible to caustic SCC than carbon steel and must not be used.
• 50% NaOH at >120°C (evaporator service): Nickel 200 (UNS N02200) or Nickel 201 (low-carbon, for >315°C service) is the standard material. Nickel has outstanding caustic resistance at all concentrations up to 98% and temperatures to 300°C+.
• Hot concentrated NaOH (>50%, >150°C): Inconel 600 (UNS N06600) where additional strength over pure nickel is required. Inconel 600 is specifically immune to caustic SCC in the most aggressive conditions.

Hydrogen Circuit: Simpler but Still Specific

Hydrogen leaving the catholyte compartment is saturated with water vapour and contains traces of NaOH mist. The wet hydrogen stream is handled in carbon steel (WPB) for the bulk of the circuit, with: all welds stress-relieved (PWHT) because wet hydrogen at elevated pressure creates hydrogen embrittlement risk in hard weld metal; hardness limits ≤22 HRC per NACE MR0175 where H₂S from brine impurities might be present; and stainless steel (316L) in the caustic mist scrubber section where NaOH contamination is highest. Dry hydrogen after the drying column reverts to standard carbon steel without special requirements.

Brine Circuit

Saturated brine (approximately 25% NaCl) is handled in rubber-lined carbon steel, titanium, or FRP — the chloride concentration is too high for any stainless steel. Titanium Grade 2 is the standard metallic fitting material for brine circuits where metallic fittings are preferred over lined construction.

Summary Selection Table