The corrosion design of reactors for thiobisphenol formation from sulfur monochloride is controlled by the simultaneous presence of a chlorinating agent, dissolved hydrogen chloride, elemental sulfur, and trace moisture. In the electrophilic sulfidation of 2,6-di-tert-butylphenol, sulfur monochloride (S2Cl2) is charged below the liquid surface of a solvent-diluted phenol stream, and each mole of sulfide product releases 2 mol of HCl and 1 mol of elemental sulfur. The addition is exothermic and is controlled between 15 °C and 40 °C in batch campaigns; subsequent solvent recovery and product finishing raise metal temperatures to 90–120 °C. The reaction mass is maintained in a processing window of ±5 °C around the setpoint because elevated temperature accelerates polysulfide byproduct formation, while lower temperature can leave unreacted sulfur monochloride in the batch. Moisture ingress from undried feedstock or ambient air hydrolyzes S2Cl2 to SO2, HCl, and additional sulfur, forming the acidic chloride environment that drives localized attack. Alloy selection therefore cannot be based on neat sulfur monochloride alone; the water-wetted condensate formed during shutdown and steam-out is more aggressive than the dry reaction mass. Because the stream contains both sulfide and chloride species, material qualification must include chloride stress corrosion cracking and pitting tests in addition to uniform immersion tests. Recognized methods include ASTM G31-72(2004) for immersion corrosion, ASTM G48-11(2020) Method C for critical pitting temperature in ferric chloride, ASTM G35-98(2010) for polythionic acid stress corrosion cracking, and ISO 15156-3:2020 for sour-service limits where elemental sulfur and chloride can generate polythionic acids during shutdown. A corrosion allowance approach used for carbon steel is inappropriate in this service because attack is localized rather than uniform, and equipment failures have occurred at pitting depths far below calculated uniform metal loss. Amine-based corrosion inhibitors are not applied in the reaction mass because they react with HCl and sulfur chlorides to form solid amine hydrochlorides; corrosion control is achieved by alloy selection and moisture exclusion rather than by chemical inhibition.
Austenitic 316L stainless steel is frequently proposed for HCl-generating chlorinations because of its molybdenum content, but its calculated pitting resistance equivalent number, PREN = Cr + 3.3Mo + 16N, is only 24 to 26, whereas the Ni-Cr-Mo alloys used in this process exceed 65. The failure mode that governs exclusion is chloride stress corrosion cracking, which can occur in 316L in chloride-containing aqueous condensates at metal temperatures above approximately 60 °C; the boiling magnesium chloride test of ASTM G36-94(2018) is the standard screening method, but it is deliberately aggressive and is used to rank alloys rather than to predict field life. Sulfur species generated during thiobisphenol formation aggravate the risk because elemental sulfur reacts with moisture and oxygen to form polythionic acids, which attack sensitized grain boundaries in 300-series stainless steels; ASTM G35-98(2010) reproduces this intergranular stress corrosion cracking mechanism. Duplex grades such as 2205 (UNS S32205) offer higher PREN near 34 and better chloride stress corrosion cracking resistance than 316L, but they remain vulnerable in the hot acidic chloride vapour space, especially at crevices under sulfur deposits, and their dual-phase microstructure can suffer selective phase attack in polythionic acid. Consequently, wetted reactor components in the heated zones are not fabricated from 316L, 317L, or 22 Cr duplex stainless steel; these grades are confined to external lugs, jacket chambers, and non-wetted structural supports.
For the glass-lined jacketed reaction vessel, the primary corrosion boundary is not the base metal but the enamel layer, which is specified under ISO 28721-1:2019 and is suitable for HCl-generating chlorinations when the process stream contains no fluoride and when thermal shock remains within the manufacturer's allowable differential, typically 120 K for standard borosilicate glass linings. Glass-lined carbon steel is the first-cost option for batch sulfidation because the glass layer isolates the steel from HCl and sulfur chlorides; however, the presence of precipitated elemental sulfur and the need for high-shear dispersion can cause enamel erosion at baffles and nozzle entries. When the process design includes internal coils for exotherm control, the heat transfer resistance of the glass layer becomes limiting, and a solid Ni-Cr-Mo alloy vessel is selected instead. Alloy C-276 (UNS N10276) with nominal 16 wt% Mo and 4 wt% W is the most widely specified wrought alloy for this environment because it resists pitting and crevice attack in acid chloride brines. Alloy 22 (UNS N06022) contains higher chromium at 21 wt% and is preferred when the process stream contains residual oxidizing agents or when the reactor is cleaned with nitric acid between campaigns. Alloy 625 (UNS N06625) is less costly and is used for valves and fasteners, but its lower molybdenum content gives a PREN near 50, and it is not recommended for the highest-temperature condensate zones unless confirmed by field coupon testing.
| Alloy | UNS designation | Nominal Cr wt% | Nominal Mo wt% | Nominal W wt% | Approximate PREN |
|---|---|---|---|---|---|
| Alloy C-276 | N10276 | 16 | 16 | 4 | 65–69 |
| Alloy 22 | N06022 | 21 | 13 | 3 | 60–65 |
| Alloy 625 | N06625 | 21 | 9 | — | 50–52 |
| 316L stainless steel | S31603 | 17 | 2 | — | 24–26 |
| 2205 duplex stainless steel | S32205 | 22 | 3 | — | 34–36 |
Condensate return lines from vacuum distillation of crude thiobisphenol operate in a regime where HCl and water condense together on the upper tube surfaces, creating a continuously wetted acid film that is more corrosive than the bulk vapour. The dominant damage mechanisms are crevice corrosion at flanged joints, under-deposit pitting beneath sulfur fines, and chloride stress corrosion cracking at weld roots. Alloy C-276 and Alloy 22 are both specified for these lines, but the geometry must be designed to avoid tight crevices: butt-welded flanges are preferred over threaded fittings, and gasket faces are machined to a controlled surface finish of Ra 3.2 µm or smoother. Spiral-wound gaskets with PTFE filler are used for HCl service, and the gasket seating area is outside the crevice corrosion envelope only if the inner ring is flush with the pipe bore. Alloy 400 (UNS N04400) is sometimes available in plant inventories for hydrochloric acid, but it is not selected for S2Cl2-containing streams because sulfur chlorides can act as oxidizing agents and destabilize the nickel-copper passive film; published data for this specific configuration is limited, and the risk of sulfidation attack outweighs the lower material cost. Feed drying is specified when ambient relative humidity exceeds 60%, and the condensate system is designed to assume acid film pH below 1 during normal operation.
Material selection for thiobisphenol reactors is not completed by code calculations alone; a corrosion coupon program is required when the process stream contains sulfur monochloride because published laboratory data for this exact mixture is limited. Coupons of Alloy C-276, Alloy 22, and the proposed cast pump alloy are installed in the recirculation loop and in the vapour space with electrical isolation from the vessel wall. The exposure period is normally 90 to 180 days, after which the coupons are examined by optical microscopy and scanning electron microscopy for pitting, crevice, and intergranular attack. Uniform corrosion rates are calculated from mass loss per ASTM G31-72(2004), and critical pitting temperatures are determined on stressed and unstressed specimens per ASTM G48-11(2020) Method C. If polythionic acid stress corrosion cracking is a concern during shutdown, U-bend specimens are tested per ASTM G35-98(2010). Acceptance criteria used by many plants are a pitting depth below 25 µm and a uniform corrosion rate below 0.05 mm/yr for the planned inspection interval; these values are engineering targets rather than code-mandated limits. Field experience on agitated-batch and continuous plant lines indicates that coupon locations in dead legs and flange crevices provide more useful information than coupons placed in fully flowing liquid, because service failures in S2Cl2/HCl systems are localized rather than uniform.
| Process zone | Typical metal temperature | Exposed stream | Alloy or lining recommendation | Primary verification method |
|---|---|---|---|---|
| S2Cl2 feed piping | 15–40 °C | Neat sulfur monochloride | PTFE-lined carbon steel or Alloy C-276 for small-bore | ASTM G31 |
| Reactor liquid phase | 15–40 °C | Phenol, S2Cl2, HCl, elemental sulfur | Glass-lined steel or Alloy C-276 | ASTM G48 |
| Vacuum distillation column | 90–120 °C | Crude thiobisphenol, HCl vapour, solvent | Alloy C-276 or Alloy 22 | ASTM G28 |
| Condensate return line | 40–80 °C | HCl, water, traces of S2Cl2 | Alloy C-276 or Alloy 22, butt-welded flanges | ASTM G35 |
| Slurry filter | 20–60 °C | Sulfur cake, residual solvent | Alloy C-276 or Alloy 22 | ASTM G48 |
Fabrication of Alloy C-276 and Alloy 22 pressure-retaining components for thiobisphenol service requires separate welding procedure qualification under ASME BPVC Section IX and impact testing of the heat-affected zone when the minimum design metal temperature is below -29 °C. Alloy C-276 is welded with ERNiCrMo-4 filler metal and ENiCrMo-4 electrodes per AWS A5.14/A5.14M; interpass temperature is limited to 93 °C to minimize segregation and carbide precipitation in the weld metal. Wrought Alloy C-276 is normally solution annealed at 1121 °C followed by rapid quenching, but welded fabrications in pressure vessel service are often placed in service without post-weld heat treatment because the alloy is solid-solution strengthened and the weld corrosion resistance remains adequate if the interpass temperature is controlled. Stress corrosion cracking resistance of the weld is verified by bend tests and corrosion tests on welded coupon samples; ASTM G28-02(2015) Method A is used to detect sensitization in Ni-Cr-Mo alloys. When a glass-lined vessel is selected for the main reactor, the nozzle linings and agitator shaft are still fabricated from solid Alloy C-276 or Alloy 22, and the glass-to-metal transition at the nozzle is a critical inspection point because a pinhole in the enamel allows acid chloride to attack the carbon steel shell. Dissimilar joints between carbon steel and Ni-Cr-Mo alloys are avoided in wetted service unless a solid alloy pad or weld overlay of sufficient thickness is applied; iron contamination from carbon steel brushes or forming rolls is removed by pickling before service.
Mechanical seals and pump internals for sulfur monochloride feed and crude thiobisphenol slurry require a different alloy strategy than static pressure vessels because the sliding surfaces generate local temperature excursions and the seal environment is a thin film of acid chloride. Silicon carbide against silicon carbide seal faces with PTFE or perfluoroelastomer secondary seals are specified for process pumps in HCl service; however, perfluoroelastomer exposure to S2Cl2 must be verified by manufacturer immersion data because sulfur chlorides can attack some fluorinated elastomers. Pump casings in Alloy C-276 or cast Alloy CW-6MC (UNS N26625) are used, and impellers are solid Alloy C-276. Magnetic-drive pumps with PTFE-lined casings are used for small transfer lines to eliminate mechanical seal leakage, but the containment shell must be Alloy C-276 to resist permeation and acid attack. In slurry service with elemental sulfur solids, a recessed impeller or vortex pump is selected to reduce tip speed and erosion-corrosion at the impeller leading edge. Positive material identification is completed before installation per API RP 578 to verify alloy grades, and the inspection interval for seal and impeller components is established from the coupon data generated in the recirculation loop.