In 2-naphthalenesulfonic acid manufacture, sulfonation of naphthalene with 96–100 wt% sulfuric acid at 160–165 °C favours the β-isomer, whereas low-temperature sulfonation near 80 °C yields predominantly the α-isomer. The selection of corrosion-resistant alloys for the sulfonation reactor, external circulation loop, heat exchanger, and transfer piping influences selectivity through three mechanisms: thermal resistance stability, metal ion release, and surface fouling. A corroded heat-transfer surface lowers the overall heat-transfer coefficient and forces the control system to increase hot oil or high-pressure steam temperature; local skin temperatures then exceed the 170 °C threshold where 1-isomer formation and polysulfonation accelerate. Iron, chromium, and nickel dissolution into the sulfonation mass also introduces redox-active centres that can promote colour body formation and alter downstream dye intermediate properties. The process is typically operated in batch or continuous stirred-tank mode with the sulfonating agent added in staged portions, and the acid inventory may contain up to 20% free SO₃ in oleum-based processes. Process pressure is usually close to atmospheric, but the vapour space contains sulfur dioxide, sulfur trioxide, water vapour, and traces of naphthalene; this vapour is more aggressive to partially wetted metal surfaces than the liquid bulk acid. Alloy qualification therefore requires separate evaluation for full immersion, vapour phase, splash zone, and weld heat-affected zone. Published data for the exact naphthalenesulfonic acid–sulfuric acid–water mixture is limited, so material selection relies on corrosion data from boiling sulfuric acid, oleum, and high-temperature immersion coupon testing rather than direct plant-derived alloy ranking.

What Limits Wrought 316L Use in the Hot Acid Transfer Loop?

Wrought UNS S31603 has been evaluated in sulfuric acid service mostly up to 40 wt% acid at intermediate temperatures; in the 96–100 wt% H₂SO₄ range at 165 °C, its passive Cr₂O₃ film dissolves and the alloy enters the transpassive or active corrosion domain, producing surface recession rates generally considered unacceptable for a chemical intermediate that must remain low in iron below 10 mg/kg. Published data for the exact 96 wt% H₂SO₄–naphthalene sulfonation mixture is limited, but plant experience indicates preferential attack at welded root passes and heat-affected zones unless the alloy is solution annealed after welding. The corrosion product is not a dense adherent film; it is a poorly attached chromium-nickel-iron sulfate layer that periodically sloughs, reducing the heat-transfer coefficient and generating suspended solids. ASTM A262-15 Practice C testing on as-welded 316L often reveals intergranular attack in boiling 65 wt% HNO₃, and although nitric acid is not the process environment, the test measures chromium-depleted zones that also underperform in hot concentrated sulfuric acid. For a 2-naphthalenesulfonic acid loop, even 0.10 mm/year uniform corrosion can release sufficient iron to cause off-specification colour after sulfonation; therefore UNS S31603 is usually confined to cold, dilute acid wash lines, flare header drains, or utility piping and excluded from the main sulfonation inventory. The alloy is also unsuitable for the vapour space of a naphthalene sulfonation vessel because acid condensation above the liquid level accelerates attack and produces acidic ferrous sulfate deposits that can drop into the batch and contaminate the final filter cake.

Where the acid inventory contains 20–25 wt% free sulfur trioxide and the return line experiences pressure pulses from the circulation pump, high-silicon cast iron provides one of the most reliable cast-material options. The alloy typically contains 14.5–16 wt% silicon and forms a continuous silica-rich surface film in hot concentrated sulfuric acid and in oleum. In cast cooler banks and outlet bends, this film remains adherent under turbulent flow conditions that would strip a less protective sulfate film from stainless steels or nickel alloys. The main process penalty is low mechanical toughness; high-silicon cast iron cannot be thermally shocked without cracking, and manufacturer installation guidance commonly requires preheating and controlled cool-down when the component is welded or clamped into a carbon steel shell. Equipment design therefore uses flanged cast sections, silicone-free PTFE gaskets, and free-floating tube bundles with expansion provisions rather than rigid welded connections. For a tube-and-shell acid cooler receiving 165 °C sulfonation liquor and rejecting heat to 140 °C hot oil, the high-silicon iron tubes maintain a clean heat-transfer surface longer than Alloy 20 or 904L, allowing the jacket temperature set point to remain within ±3 °C of the target. Because 2-naphthalenesulfonic acid selectivity is sensitive to local overheating, this temperature stability can reduce the formation of 1-isomer and oxidation byproducts. However, high-silicon cast iron should not be used for thin-walled piping subject to water hammer, for steam-traced lines where condensate can cause rapid local cooling, or for agitator shafts; its fracture toughness is insufficient for dynamic mechanical loads. In those positions, the process design must switch to a wrought nickel alloy or a lined component.

When Alloy 20 Is Retained in the Drying Preconcentrator

Alloy 20, designated UNS N08020, contains roughly 32–38 wt% nickel, 19–21 wt% chromium, 2–3 wt% molybdenum, and 3–4 wt% copper. The copper addition improves resistance to dilute sulfuric acid and to acid condensate streams, but the alloy has a defined upper temperature limit in concentrated sulfuric acid. In a drying preconcentrator operating under vacuum to remove water from crude sulfonation liquor at 80–100 °C, Alloy 20 can be acceptable if the acid strength does not exceed 93 wt% and chloride contamination remains below 50 mg/kg. Above 120 °C in 93–96 wt% H₂SO₄, the passive film loses stability and crevice attack initiates under flange facings, threaded couplings, and tube-to-tubesheet joints. ASTM G48-11 Method C can be used to determine the critical pitting temperature of Alloy 20 in ferric chloride solution, but it does not directly predict performance in hot sulfonated aromatic acid mixtures. For the preconcentrator, users commonly specify solution-annealed plate and forgings, restrict weld repairs to low heat input, and require dye penetrant examination per ASTM E1417 on all internal surfaces. If the preconcentrator must process acid above 93 wt% at temperatures above 120 °C, the selection should shift to Alloy 31, UNS N08031, or a nickel-molybdenum alloy such as UNS N10675, because Alloy 20 cannot maintain the low iron release needed for colour-sensitive 2-naphthalenesulfonic acid. Published data for this specific configuration is limited, but comparative boiling sulfuric acid testing indicates that Alloy 20 has a narrow operating window in the intermediate concentration range, while its performance in oleum is insufficient for process equipment exposed to free SO₃.

Tantalum Tubesheets in 96 wt% H₂SO₄ at 165 °C

Tantalum derives its corrosion resistance from a highly protective Ta₂O₅ film that remains stable in hot concentrated sulfuric acid under many conditions. In a tubular heat exchanger operating with 96 wt% H₂SO₄ at 165 °C on the tube side, tantalum can provide negligible metal release and a very smooth surface that minimises naphthalenesulfonic acid salt fouling. However, tantalum has two severe constraints in this service: hydrogen embrittlement and incompatibility with oleum. If tantalum is galvanically coupled to a less noble metal such as carbon steel or stainless steel without proper electrical isolation, atomic hydrogen generated during acid corrosion can enter the tantalum lattice and cause embrittlement. Tantalum tubesheets must therefore be isolated from steel shells, and nonmetallic gaskets must be used at all flanged connections. In free SO₃-containing environments above roughly 5 wt%, tantalum can suffer accelerated attack because the oxide film loses passivity in oleum. Consequently, tantalum is generally reserved for the hydration or water-diluted acid sections of the process, not for oleum feed lines. The cost and fabrication constraints of tantalum are extreme, and repair welding requires an inert-gas chamber or trailing shield with a qualified procedure under ASME BPVC Section IX. Because pinholes in a tantalum tube can be difficult to detect, helium leak testing and dye penetrant inspection are performed after fabrication. The high surface smoothness of tantalum also reduces the need for aggressive cleaning that would remove protective films from stainless steel alternatives.

Because field welding of nickel alloys in the sulfonation building is difficult under high humidity, pre-fabricated spool pieces with orbital gas tungsten arc welding are often specified. For UNS N10276 and UNS N10675, weld heat input must be controlled to avoid excessive grain-boundary precipitation; interpass temperature in the weld zone is often limited to 150 °C or 100 °C depending on the alloy. The internal surface finish of product-contact piping should be maintained at Ra ≤ 0.8 µm to reduce the accumulation of polysulfonated naphthalene residues and to limit crevice initiation at surface defects. Electropolishing after welding removes heat tint and restores the passive film; without electropolishing, the chromium-depleted heat tint becomes a preferential corrosion site in hot acid vapour. For lined pipe, PTFE or PFA liners are used in areas where acid temperature does not exceed the liner rating, typically 120–150 °C for continuous service depending on liner thickness and manufacturer data. Vacuum conditions require special liner locking because the liner can collapse or pull away from the steel wall. In the main sulfonation loop at 160–165 °C, lined piping is often replaced by solid nickel alloy or high-silicon cast iron sections because liner creep and permeation can introduce acid behind the liner. All welds in solid alloy piping are inspected by radiographic testing or phased array ultrasonic testing, and pressure testing follows ASME B31.3 for process piping. These fabrication controls are not optional; a single incomplete weld or surface defect can produce iron contamination that shifts the filter cake colour and creates a batch rejection.

Does Iron Leaching Alter the 2- to 1-Isomer Ratio During Isomerization?

Published data directly linking dissolved iron concentration to the 2-/1-isomer ratio in 2-naphthalenesulfonic acid production is limited. Dissolved iron does, however, promote redox side reactions involving residual naphthalene, sulfuric acid, and oxygen ingress. These side reactions consume sulfonating agent and generate naphthoquinone-type colour bodies that complicate downstream purification and reduce the apparent selectivity of the sulfonation stage. In high-temperature isomerization, the presence of iron oxides or iron sulfate fines can also act as heterogeneous nucleation sites for local polymer formation, creating fouling on heat-transfer surfaces. The resulting loss of heat-transfer efficiency forces the operator to raise jacket temperature, and the higher wall temperature increases the rate of α-isomer formation relative to β-isomer. Therefore, iron corrosion products do not necessarily alter the intrinsic isomerization kinetics; they degrade selectivity indirectly through fouling, acid consumption, and thermal control disturbance. Process monitoring commonly includes digestion of the sulfonation mass followed by inductively coupled plasma optical emission spectroscopy using ISO 11885 to quantify dissolved iron, chromium, nickel, and copper. If iron exceeds 25 mg/kg in the crude sulfonation liquor, the alloy is usually considered to be releasing unacceptable corrosion product for a high-purity intermediate. For equipment already installed, process engineers may reduce iron release by lowering acid temperature, reducing free SO₃ concentration, improving condensate drainage from traced lines, or replacing vulnerable weld joints with solution-annealed forged fittings. The iron contamination limit is not derived from a single standard but from downstream dye and pigment application specifications that reject visible colour development in the final coupling product.

Corrosion qualification for any of the candidate alloys should use immersion coupons placed in the actual sulfonation mixture rather than in reagent-grade sulfuric acid alone. The presence of naphthalenesulfonic acid, reaction byproducts, trace water, and free SO₃ changes the electrochemical behaviour of the passive film. ASTM G31-72(2017) provides the general laboratory immersion protocol, but it does not define acceptance criteria for a colour-sensitive aromatic sulfonic acid process. Weight-loss testing should be combined with surface examination, cross-sectional metallography, and analysis of the test solution for dissolved metal ions by ISO 11885. For welded qualification, ASTM G28 can be applied to nickel-chromium-molybdenum alloys to detect intergranular attack. For austenitic stainless steels, ASTM A262 Practice C remains the standard acid intergranular attack test. For pitting and crevice attack screening, ASTM G48 Method C may be used, although its ferric chloride chemistry is more aggressive than the sulfonation mass at room temperature. The test programme should include at least three replicates per condition because corrosion rates in hot concentrated sulfuric acid often show high scatter when passive films intermittently break. If a new alloy is to be introduced, plant trials should include a removable spool piece in the circulation line for 500–1000 h, with intermediate inspection at 250 h intervals. This duration is required because corrosion rates often increase after the initial passivation period when surface films begin to slough in high-velocity acid flow. A single 24-hour laboratory exposure is not reliable for selecting pressure-containing components in the main sulfonation loop.

In the hot acid circulation loop, pump and valve selection is as important as vessel and piping selection. Centrifugal pumps with high-silicon cast iron casings and tantalum or high-silicon iron impellers are used when the pump handles 150–165 °C acid containing suspended naphthalenesulfonic acid crystals. Vertical submerged pumps may be used for sump transfer, but the vapour space above the sump liquid must not contact unprotected 316L or carbon steel. Mechanical seals require care because the sulfonation liquor is both corrosive and fouling; a double seal with a clean flush or a magnetic-drive pump eliminates the shaft seal as a leak point. Magnetic-drive pumps with PTFE-lined casings and tantalum or C-276 internals are sometimes used at lower temperatures, but the liner and bearing materials must be rated for the specific acid strength. Valves in product contact are commonly lined plug valves with PFA liners and C-276 plugs, because the plug valve provides fewer crevices than a ball valve and can be serviced without removing the valve body. Metal-seated valves in 316L or Alloy 20 are generally unsuitable for the main sulfonation loop due to crevice attack and iron contamination. Instrument connections should be isolated with PTFE diaphragm seals, and flushing ports should be provided to clear solidified sulfonic acid from pressure taps. Batch-to-batch variation in valve performance is often caused by solids settling in dead legs; therefore the piping design should eliminate pockets and provide continuous circulation through all product-contact branches.

Corrosion Fatigue in High-Pressure Steam Jacketed Reactor Agitators

Agitator shafts and impellers in the sulfonation reactor are exposed to the vapour phase, hot acid spray, and cyclic bending stresses. In high-pressure steam jacketed glass-lined or alloy reactors, the agitator may experience thermal cycling from 80 °C at start-up to 165 °C at full reaction temperature, which superimposes thermal stress on the mechanical torsional and bending loads. Austenitic stainless steels such as 316L are not suitable for the shaft because corrosion pits initiate at surface defects and propagate under cyclic stress. UNS N10276 or UNS N08031 are preferred wrought alloys for shafts and impellers in this service because they combine corrosion resistance in hot sulfuric acid vapour with sufficient fatigue strength. The impeller design should avoid crevices where blades are bolted to the hub; welded hubs with smooth fillet radii reduce crevice initiation. If the reactor is glass-lined, the agitator can be coated with a fluoropolymer or glass layer, but any pinhole in the coating will allow acid to reach the substrate and create a fast-growing corrosion fatigue crack. Maintenance records from production-scale reactors show that most agitator failures occur not in the fully immersed blade but in the splash zone or at the mounting flange where acid can condense and concentrate during shutdown. During scheduled inspection, the shaft should be examined by liquid penetrant testing per ASTM E1417 and by wet fluorescent magnetic particle testing where applicable. The inspection interval should be shortened if the reactor is operated with more than 20% free SO₃ in the vapour space or if the agitator experiences more than 50 start-up/shutdown cycles per year.

Shutdown air ingress into a sulfuric acid sulfonation plant can create severe acidic condensate attack. During a turnaround, ambient air enters through open flanges and breathes into the vessel; the moisture in air combines with residual SO₃ and H₂SO₄ to form low-concentration acid films on cool metal surfaces. The diluted acid is more corrosive to stainless steels and nickel alloys than the original strong acid. The shutdown procedure should include nitrogen blanketing, drainage of low points, and drying of the vessel before opening. If a vessel must be opened for inspection, interior surfaces should be rinsed with a controlled amount of dilute acid in a defined sequence to avoid exothermic dilution. Personnel access must follow confined-space regulations, but the material selection is also affected: alloys that resist strong acid may corrode badly in humid, low-concentration acid films. This is why high-silicon cast iron components are sealed and dry-stored rather than left open to ambient air. After shutdown, all product-contact surfaces should be inspected for pitting, crevice attack, and delamination of PTFE or PFA liners. A single shutdown with inadequate moisture control can introduce chloride from plant water and initiate crevice corrosion that later fails during production. For this reason, the material selection for a sulfonation plant must consider not only the steady-state process chemistry but also the transient acid concentrations and temperatures that occur during start-up, shutdown, and steam-out.

Published data for this specific configuration is limited, and no single laboratory standard can fully predict field performance in 2-naphthalenesulfonic acid production. The final material selection is therefore based on a combination of vendor data for sulfuric acid and oleum, coupon testing in the actual process liquor, and careful review of failure histories from analogous sulfonation plants. The design should specify the alloy not as a single grade but as a complete package of chemical composition limits, solution annealing, weld procedure qualification, surface finish, and inspection acceptance criteria. For pressure-containing components, the design must also comply with ASME BPVC Section VIII Division 1 for vessels, ASME B31.3 for process piping, and the applicable material specifications under ASTM. In the European regulatory context, the Pressure Equipment Directive 2014/68/EU applies to the pressure boundary and imposes documentation requirements for material batches and welding procedures. Without these controls, a change in alloy vendor or welding procedure can introduce a batch-to-batch selectivity shift that is difficult to distinguish from changes in naphthalene feedstock or sulfuric acid quality.

The sulfonation selectivity of 2-naphthalenesulfonic acid cannot be separated from the mechanical reliability of the alloy surfaces that control heat transfer and metal contamination. A material that is marginally acceptable in a laboratory beaker may fail quickly in a production reactor where vapour condenses, solids settle, and thermal gradients are unavoidable. Therefore alloy selection must be treated as a process-control decision rather than a simple materials compatibility decision. The highest-risk zones are the hot acid circulation piping, the reactor jacket heat-transfer surface, the vapour space above the liquid level, and any dead leg where acid can concentrate. In these zones, the allowable alloy temperature margin should be at least 20 °C above the maximum normal operating temperature to account for process excursions. For example, if the sulfonation reactor operates at 165 °C, the selected alloy should have demonstrated resistance above 185 °C in the same acid strength under pilot-plant conditions or vendor test data. The margin is required because the sulfonation control loop will occasionally overshoot temperature during staged acid addition, and a single overshoot can initiate crevice corrosion that later propagates under normal conditions. Process operators should log the actual metal temperature of the reactor wall and circulation return line, not only the bulk liquid temperature, because the wall temperature at the steam or hot oil interface determines corrosion rate and isomer distribution near the heat-transfer surface.

In batch sulfonation reactors, the staged addition of sulfuric acid creates transient zones of low acid strength and high local temperature. These zones are most corrosive to stainless steels and nickel alloys because the dilution heat drives the acid temperature upward while the acid concentration is momentarily below the passivation range for the alloy. The use of a dip pipe made from Alloy 20 or 904L in this staged addition zone can produce rapid erosion-corrosion and iron release, even if the bulk reactor is lined or constructed from a more resistant material. The dip pipe should be high-silicon cast iron, tantalum, or PTFE-lined, and its outlet should be located near the agitator to promote rapid mixing. Similarly, instrumentation thermowells should be solid tantalum or fluoropolymer-coated in the splash zone, because a standard stainless steel thermowell creates a crevice at the flange and can fail by acid condensate attack during shutdown. The process design should avoid dissimilar metal joints that can form galvanic couples; if a tantalum thermowell is installed in a high-silicon cast iron boss, the joint must be electrically isolated and wetted surfaces must be free of sharp edges. In all cases, the alloy selection should be verified by a corrosion engineer familiar with sulfonation chemistry, not solely by mechanical design personnel, because the failure mode may be local contamination rather than visible wall loss.

For the external heat exchanger, a vertical arrangement with process acid on the tube side and hot oil on the shell side is commonly used to avoid gas blanketing and solids buildup. The tube-to-tubesheet joint is a critical crevice site; expanded tube joints are preferred over simple roller-expanded joints because the expansion must completely fill the tubesheet hole and leave no gap for acid ingress. For high-silicon cast iron tube sheets, the tubes are often joined by mechanical expansion into pre-formed grooves, and the tubesheet face is protected by a PTFE gasket at the channel cover. If a weld overlay is needed on a carbon steel tubesheet, the overlay should be UNS N10276 or UNS N08031 applied by a qualified overlay procedure under ASME BPVC Section IX, followed by ferrite measurement and ultrasonic examination for disbonding. The heat exchanger should be designed with removable channel covers so that the tube sheet and tube ends can be inspected without cutting the unit. During operation, the pressure differential between process and utility sides should be monitored to prevent hot oil from leaking into the acid or acid vapour from entering the oil system. Any leak contamination creates a hazardous exotherm on the oil side and must be detected immediately. The use of a double tubesheet with leak detection ports provides an additional barrier, but the added crevices must be carefully assessed for corrosion rather than automatically specified.