Stainless Steel Selection for Ammonium Chloride Liquorice Paste Processing

The selection of stainless steel for ammonium chloride liquorice paste processing is governed by a three-variable corrosive load that is frequently underestimated in confectionery plant design: dissolved chloride ion concentration, pH, and sustained contact temperature. Ammonium chloride dissociates in the aqueous phase of liquorice paste to yield approximately 66.3 wt% chloride ion relative to the salt molecular weight; a 10 wt% ammonium chloride solution therefore exposes wetted surfaces to roughly 66,000 mg/L chloride. Saturated ammonium chloride solutions at dissolution temperatures exhibit pH values between 4.3 and 4.6 as a result of ammonium ion hydrolysis, which is sufficiently acidic to delay passive film repair on 300-series stainless steels. Process temperatures for dissolution, cooking, holding, and depositing typically span 20 °C to 120 °C, and liquorice paste total solids commonly range from 70 wt% to 85 wt%, producing adhesive wall layers that retain chloride ions at the metal surface. Under these conditions, S30400 and S30403 grades with pitting resistance equivalent numbers below 20 cannot provide reliable service in continuous contact with concentrated ammonium chloride paste, and the selection of S31603, enhanced molybdenum austenitic grades, duplex grades, or superaustenitic grades must be linked to specific process zones. Material compliance for food contact is not established solely by alloy name; it requires demonstration of corrosion resistance under the intended process chemistry, cleanability under the governing hygienic design criteria, and traceability to recognised material standards such as ASTM A240/A240M, ASTM A270/A270M, EN 10088-2, and ISO 2037. Additional conformity is typically assessed against NSF/ANSI 51, EC 1935/2004, and the general provisions for indirect food additives described in 21 CFR 174.5. The design approach must also incorporate surface finish requirements of Ra ≤ 0.8 μm for product contact areas as described in hygienic design guidance such as EHEDG Doc 8 and the relevant 3-A Sanitary Standards for food processing equipment. Where hot concentrated ammonium chloride solutions are present, the controlling failure modes are pitting corrosion, crevice corrosion, and chloride-induced stress corrosion cracking; uniform corrosion is secondary except at low pH and high temperature where active dissolution can occur.

Ammonium chloride is not a passive neutral salt in hot liquorice paste. The chloride anion competes with hydroxide and water for adsorption on the passive chromium oxide layer, and the slightly acidic ammonium ion buffering reduces the local pH at the metal surface during stagnation. This combination lowers the critical pitting temperature, defined by electrochemical testing, and makes the crevice environment in gasketed joints, threaded fittings, dead legs, and product deposits more aggressive than the bulk process fluid. The practical consequence is that material selection based only on bulk composition of the paste is insufficient; the local chloride concentration under a stagnant layer of liquorice solids can exceed the bulk concentration by evaporative concentration and by selective adsorption of water into the hydrophilic paste components. The corrosion risk is therefore highest in equipment zones where ammonium chloride crystals are first dissolved, where hot paste remains in static manifolds during production stops, where mechanical seals and O-rings create tight crevices, and where cleaning solutions containing oxidising chloride compounds are applied at elevated temperature. Welded joints that are not fully penetrated, lack back-purge protection, or retain heat tint further reduce the available pitting resistance by creating chromium-depleted zones and surface oxides with local electrochemical potential differences. The selection of a stainless steel grade for ammonium chloride liquorice paste processing therefore cannot be separated from the fabrication specification, the passivation method, the mechanical polish, the gasket geometry, and the validated cleaning procedure.

Why Does Molybdenum Addition Raise the Critical Pitting Temperature in Ammonium Chloride Pastes?

Molybdenum is the most influential alloying element for chloride pitting resistance in stainless steel as measured by the pitting resistance equivalent number, calculated as PREN = %Cr + 3.3(%Mo) + 16(%N). S30400 and S30403 with molybdenum contents below 0.5 wt% exhibit PREN values of approximately 18.0–19.0, whereas S31603 with 2.0–2.5 wt% molybdenum exhibits PREN values of approximately 23.0–25.0. The effect is disproportionate in chloride environments because molybdenum is incorporated into the passive film and promotes repassivation after chloride attack, shifting the critical pitting temperature upward. Published screening tests using ASTM G48 Method A ferric chloride solutions typically report critical pitting temperatures below 10 °C for S30400 and S30403, approximately 15–20 °C for S31603, and 30–35 °C for S32205 duplex and N08904 superaustenitic grades; published data for concentrated ammonium chloride-liquorice slurries is limited, but ferric chloride testing remains a conservative screening method because of its strongly oxidising chloride-rich chemistry. The critical crevice temperature is generally lower than the critical pitting temperature by 10–20 °C for the same alloy, which is the primary reason that liquorice paste equipment with gasketed connections, pump clearances, and stagnant manifolds fails by crevice corrosion before open pitting is observed on polished tank walls. Manganese sulfide inclusions act as initiation sites for pitting in 300-series stainless steels, and free-machining grades such as S30300 are wholly unsuitable for ammonium chloride paste service because their higher sulfur content produces a dense inclusion population that is readily attacked by chloride ions. For S31603, specifying only the minimum molybdenum content of 2.0 wt% can leave a production line at the bottom of the alloy’s resistance envelope; enhanced 316L-type grades with molybdenum between 2.5 wt% and 3.0 wt% and nickel between 12.5 wt% and 15.0 wt%, such as EN 1.4435, provide a more reproducible margin in hot ammonium chloride service.

The electrochemical response of stainless steel to ammonium chloride paste is further influenced by local chloride concentration. An ammonium chloride addition of 2.5 wt% in the finished liquorice paste corresponds to a dissolved chloride load of approximately 16,600 mg/L, while an addition of 7.0 wt% corresponds to approximately 46,400 mg/L if the salt is fully dissolved. In the dissolution zone, however, local concentrations can approach saturation, which at 100 °C is approximately 77.3 g/100 mL water, yielding chloride concentrations far above those in the finished product. This explains why S30400 dissolution vessels operating with saturated ammonium chloride at 80–90 °C exhibit localised attack after relatively short exposure, whereas the same alloy may appear serviceable in a cold finished-paste hopper at low salt content. For continuous contact with hot concentrated ammonium chloride paste, the minimum practical alloy is S31603 with enhanced molybdenum specification, and the preferred alloys for the most aggressive zones are S32205 duplex stainless steel, N08904, S31254 6% molybdenum superaustenitic, or S32750 superduplex stainless steel. The use of S31603 should be restricted to process zones where the bulk chloride concentration is below approximately 10,000 mg/L and the contact temperature remains below 40 °C with no stagnant crevice; when these limits are exceeded, surface finish and gasket design alone cannot compensate for insufficient alloy PREN.

A batch dissolution vessel that charges crystalline ammonium chloride into heated liquorice paste at atmospheric pressure should be considered the highest-risk metallic contact zone in the process. The vessel typically operates at 80–90 °C during dissolution, handles ammonium chloride concentrations that approach saturation at the point of addition, and contains internal components such as agitator shafts, baffles, temperature probe pockets, and bottom outlet flanges that create crevices and stagnation points. S30400 and S30403 are not recommended for this zone because their critical pitting temperatures in acidic chloride media are below the operating temperature and because their low molybdenum content leaves insufficient repassivation capacity when product residues remain on the wall during transfer. S31603 may be used for a dissolution vessel only when the ammonium chloride addition rate is controlled to avoid long exposure to saturated solution, when the vessel is continuously swept by agitation, and when the wetted surface is polished to Ra ≤ 0.8 μm and subsequently passivated in accordance with ASTM A967. A more robust specification for the dissolution vessel is S32205 duplex stainless steel with electropolished product-contact surfaces, flush-mounted instruments, continuous butt-welded internal attachments, and a conical outlet section that avoids horizontal ledges. Where process temperature exceeds 90 °C and the ammonium chloride concentration repeatedly reaches 20 wt% or more, substitution of S32750 superduplex or S31254 6% molybdenum superaustenitic is justified despite higher initial cost. The economic risk of pitting failure in a dissolution vessel is not limited to replacement cost but includes production downtime, product contamination from dissolved metal ions, and the potential for chloride-induced stress corrosion cracking propagating from pits under weld residual stress. Production-scale failure patterns observed in confectionery and food processing equipment indicate that pitting attacks preferentially initiate at bottom outlet flanges where settled ammonium chloride crystals remain after incomplete draining, and at unpolished weld roots where heat tint was not removed. The operational boundary is that the vessel should not be left wet with undissolved salt for more than 4 hours during shutdown; if shutdown exceeds this period, a potable water rinse and complete drainage should be performed. Threaded connections, internal brackets, and hollow agitator shafts should be avoided because they create inaccessible crevices that cannot be reliably passivated or inspected.

Material selection for this zone is also influenced by the rheological behaviour of liquorice paste. At total solids above 80 wt%, liquorice paste exhibits non-Newtonian pseudoplastic behaviour with a yield stress that requires positive-displacement pumping and high-torque agitation; the associated wall shear is not uniform, and slow-moving product layers near the wall can become chloride-rich through moisture transfer. This creates an under-deposit corrosion condition that is not well described by bulk hydrodynamic calculations. Agitator designs with low-speed anchor or scraping elements should be specified with a continuously swept wall clearance below 5 mm to minimise product build-up, and the scraper material should be selected to avoid galvanic coupling with the vessel wall. If the vessel is fabricated from S32205 duplex, the scraper should also be S32205 or a compatible polymer with no metallic reinforcement; use of carbon-steel or brass scraper blades is incompatible because galvanic coupling accelerates localised attack in chloride paste. The vessel jacket, if filled with hot water or steam and not subjected to ammonium chloride product, may be fabricated from carbon steel, but the outer vessel shell and insulation supports should be protected from ammonium chloride dust because the salt is hygroscopic and can form concentrated chloride films on external surfaces. For product-contact surfaces, the passivation specification should be referenced to ASTM A967 using either citric acid at 4–10 wt% and 20–50 °C for 30–60 minutes or nitric acid at 20–25 vol% and 40–50 °C for 30–60 minutes, followed by thorough rinsing and verification of neutral pH in the rinse water. A ferroxyl test may be applied in accordance with ASTM A380 to detect free iron contamination on product-contact surfaces, but it should not be used as a substitute for a full passivation procedure or for electrochemical validation of pitting resistance.

Surface Finish, Welding Heat Tints, and Passivation Determine Pitting Resistance

The as-welded surface condition of stainless steel is the weakest point in any ammonium chloride paste contact zone. Gas tungsten arc welding produces a heat tint oxide layer that contains chromium-depleted metal immediately beneath the oxide, and this condition can reduce the local critical pitting temperature by 10 °C or more compared with the parent material. Welding specification for liquorice paste equipment must require full-penetration orbital welds on hygienic tubing, back-purging with welding-grade argon until the oxygen content in the purge gas is below 0.5% by volume, and restricted use of filler wire to match or overmatch the base alloy. For S31603 product-contact piping and manifolds, filler metal should be ER316L with molybdenum in the upper half of the specification range, and for S32205 duplex, filler metal should be ER2209 with overmatching nickel content to maintain austenite-ferrite phase balance in the weld metal. Autogenous welds should be avoided in hot concentrated ammonium chloride service because the weld metal and heat-affected zone may solidify with a locally lower molybdenum content and a coarse dendritic structure that is more susceptible to pitting than the rolled plate or drawn tube. After welding, the entire weld zone must be mechanically cleaned, brushed with stainless steel wire that has not been used on carbon steel, and then pickled to remove heat tint. Pickling pastes containing nitric and hydrofluoric acids are effective but must be handled under strict chemical safety controls and must not be allowed to remain on the surface beyond the manufacturer’s specified time. After pickling and rinsing, the entire fabricated assembly should be passivated according to ASTM A967 and inspected for residual heat tint. A clean, silver-white weld surface with no blue or brown tint is the minimum acceptance criterion for hot chloride service; any residual tint indicates a chromium-depleted zone that will fail preferentially in ammonium chloride paste.

Surface roughness interacts directly with pitting resistance because chloride ions adsorb more readily in deep grooves and pits than on smooth electropolished surfaces. Mechanical finishing to Ra ≤ 0.8 μm is the baseline for food contact under hygienic design guidance, but for ammonium chloride paste processing the specification should also include electropolishing of high-risk zones such as dissolution vessel walls, depositor manifolds, pump casings, and gasket faces. Electropolishing removes embedded iron, manganese sulfide inclusions, and microcrevices created by mechanical grinding, and it produces a chromium-enriched passive layer that improves the localised corrosion resistance of S31603 and duplex stainless steels. The degree of improvement depends on the base alloy, the prior surface condition, and the test environment; published ferric chloride data for electropolished S31603 show moderate increases in critical pitting temperature compared with 220-grit mechanically finished surfaces, but the improvement cannot compensate for using an alloy with insufficient molybdenum in a hot concentrated ammonium chloride dissolver. For zones where the product contact surface is not accessible for electropolishing after assembly, such as small-diameter tube bends and valve bodies, the specification should require bright-annealed or internally polished tube with Ra ≤ 0.4 μm and pre-passivated fittings. The use of abrasive pads, grinding discs, or wire brushes contaminated with carbon steel is a common fabrication error that embeds free iron in the stainless surface and creates initiation sites for chloride pitting. A strict fabrication cleanliness protocol is required: stainless steel components must be stored separately from carbon steel, handled with clean gloves, and cut with tools reserved for stainless only. The presence of free iron on a product-contact surface can be detected by the ferroxyl test described in ASTM A380, but the test is qualitative and should be supplemented by visual inspection under adequate lighting at the weld root and gasket face.

When 2205 Duplex Replaces 316L in Hot Chloride Contact Equipment

Replacement of S31603 with S32205 duplex stainless steel in hot ammonium chloride paste equipment provides a significant increase in pitting resistance, mechanical strength, and resistance to chloride-induced stress corrosion cracking. S32205 has a PREN of approximately 34–36, a critical pitting temperature in ASTM G48 Method A ferric chloride of approximately 30–35 °C, and a minimum yield strength of 450 MPa, which is roughly double the 220 MPa minimum yield strength of S31603. The higher strength allows designers to use thinner wall sections for pressure-containing equipment such as jacketed cookers, heat exchanger shells, and pump casings, which reduces product hold-up and improves heat transfer. The thermal properties of S32205 are also favourable in hot chloride service: its thermal conductivity is approximately 19 W/m·K at 20 °C compared with approximately 15 W/m·K for S31603, and its coefficient of linear thermal expansion is approximately 13.0 × 10⁻⁶ K⁻¹ compared with approximately 16.0 × 10⁻⁶ K⁻¹ for S31603. In scraped-surface heat exchangers and jacketed cookers, this combination reduces thermal stress at welded attachments and allows more uniform heat transfer into the high-viscosity liquorice paste. Duplex stainless steel also has better resistance to erosion-corrosion at high-shear pump clearances and rotor-stator gaps because of its higher hardness and strength, which is important when liquorice paste contains undissolved salt crystals or concentrated caramelised residues that abrade the passive layer. However, S32205 is not immune to pitting in concentrated ammonium chloride paste. The local chloride concentration in the dissolver or in a stagnant depositor manifold can still exceed the repassivation capacity of the alloy, particularly when the pH falls below 4.0 or when the temperature exceeds 80 °C under sustained crevice conditions. Where these conditions are expected, S32750 superduplex with PREN above 42 or S31254 superaustenitic with 6 wt% molybdenum should be specified for the critical wetted surfaces.

Welding of S32205 duplex requires stricter procedural controls than welding of S31603. The heat input should be maintained within approximately 0.5–1.5 kJ/mm for tube and sheet thicknesses used in hygienic processing equipment, and the interpass temperature should not exceed 150 °C to avoid excessive ferrite content and the formation of chromium-rich sigma phase. Filler metal ER2209 with a nickel content higher than the base metal is used to maintain the ferrite-austenite balance in the weld metal; autogenous orbital welding of duplex tube is possible only when the weld is fully solution annealed and quenched, which is not practical for most food processing fabrications. The heat-affected zone of a duplex weld can contain a high fraction of ferrite with reduced local PREN, and this region may be preferentially attacked in ammonium chloride paste if the weld root is not pickled and passivated. The specification should therefore require post-weld solution annealing for large duplex vessels or heat exchangers if permitted by dimensional stability, or at minimum a controlled heat treatment cycle followed by full pickling and passivation of the weld root and cap. Surface polishing of duplex stainless steel is more difficult than S31603 because of its higher hardness and two-phase microstructure; electropolishing is the preferred finishing method for product-contact duplex surfaces because mechanical polishing with incorrect abrasive pressure can create crevices at the austenite-ferrite grain boundaries. For gasketed joints in duplex equipment, the gasket material must be selected to retain its compliance at the service temperature without extruding into the product flow path, and the gasket bore should match the flange bore within 0.5 mm to avoid a product ledge. EPDM gaskets are commonly used up to 100 °C; for higher temperatures, PTFE or silicone gaskets with FDA-compliant formulations may be applied, but the gasket hardness and compression set must be verified under the actual cleaning temperature. A crevice formed by a gasket with excessive compression or an undersized bore will negate the pitting resistance advantage of the duplex alloy.

Cleaning and sanitisation cycles impose a second corrosive regime on stainless steel in ammonium chloride liquorice paste processing. Caustic-based cleaners are generally compatible with stainless steel at the concentrations and temperatures used in clean-in-place systems, but chlorinated alkaline detergents that contain sodium hypochlorite can introduce oxidising chloride species that initiate pitting on S31603 and even on S32205 if used above recommended limits. Manufacturers of chlorinated alkaline cleaners typically recommend limiting free chlorine to 100 mg/L for 300-series stainless steel at temperatures below 40 °C and avoiding prolonged contact or drying on the surface; for hot ammonium chloride paste equipment, the safer practice is to eliminate chlorinated cleaners entirely and use non-chlorinated alkaline detergents followed by a mild acid wash and thorough potable water rinse. Acid washes used to remove mineral scale should be selected to avoid hydrochloric acid; phosphoric, citric, or nitric acid-based descaling agents are preferred because they do not increase chloride load and can contribute to re-passivation. The use of sodium hypochlorite as a terminal sanitiser should be prohibited in S31603 dissolver and depositor zones because the combination of hypochlorite oxidiser, low pH residual product film, and elevated temperature creates an aggressive pitting environment. If a chlorine-based sanitiser is required for regulatory reasons, the concentration should be limited to 50 mg/L free chlorine, the contact time should not exceed 15 minutes at 20–30 °C, and the surface should be rinsed with potable water until no chlorine residual is detected. Quaternary ammonium or peracetic acid-based sanitisers are generally less aggressive to stainless steel but must be validated for compatibility with the gasket material and for microbiological efficacy under the specific liquorice paste residues. After any acid or caustic cleaning cycle, the passivity of the stainless surface should be restored by a validated passivation step using citric or nitric acid in accordance with ASTM A967; repeated exposure to hot ammonium chloride paste gradually weakens the passive film, and an annual passivation programme is a practical operational boundary for production equipment.

Inspection of product-contact surfaces after cleaning should include borescopic examination of weld roots, gasket faces, and dead legs for early pitting or discoloration. A pit depth of less than 0.1 mm may be invisible to the unaided eye but can act as a stress raiser and a chloride trap in subsequent service. Surface profilometry should be used to verify that mechanical polishing has not degraded the roughness below the specified Ra ≤ 0.8 μm in accessible zones, and replica tape or portable profilometry can be applied to tube bends and pump casings. The operational boundary for continued use of S31603 equipment in ammonium chloride paste service is that no visible pitting, crevice attack, or heat tint is present after post-cleaning inspection. If pitting is detected, the affected component should be removed from service, the pit depth measured, and the alloy upgraded to S32205 or higher for the replacement. Equipment logs should record ammonium chloride concentration, product temperature, cleaning chemical concentrations, and inspection results so that any batch-to-batch increase in corrosion load can be identified before a failure occurs.

Depositor Manifolds, Dead-Leg Piping, and Gasket Crevices in Liquorice Paste Service

Depositor manifolds and dead-leg piping are the least accessible, most crevice-prone metallic zones in an ammonium chloride liquorice paste line. The manifold receives hot paste at 60–80 °C and distributes it to individual depositor nozzles; during production stops, paste remains in the manifold and cools, increasing viscosity and preventing complete draining. Ammonium chloride dissolved in the retained paste can concentrate through evaporation or moisture migration into surrounding product layers, and the resulting chloride-rich gel forms an under-deposit corrosion cell on the manifold floor and at the nozzle seat. S31603 manifolds with threaded nozzle connections or poorly matched gaskets are vulnerable to crevice corrosion at these points, and the attack often initiates at the root of a machined thread or at the interface between a PTFE gasket and a polished metal seat. For this zone, the manifold should be fabricated from S32205 or S32750 if the ammonium chloride content in the finished paste exceeds 4 wt% and the paste temperature exceeds 60 °C during extended holding. The internal volume should be designed to drain freely under gravity, with a slope of at least 3° toward the depositor outlet, and dead legs should be eliminated or maintained at a length-to-diameter ratio of ≤ 1.5 in accordance with hygienic design guidelines. Clamp fittings are preferred over threaded connections because they allow dismantling for inspection and because the gasket compression can be controlled to avoid excessive crevice volume. The gasket material must be specified with a hardness that allows 20–30% compression without extrusion, and the gasket bore should not protrude into the product flow stream. EPDM gaskets are generally acceptable up to 100 °C but may require replacement at intervals of 6–12 months because compression set in hot liquorice paste reduces sealing force and enlarges the crevice. PTFE gaskets offer better chemical compatibility but require higher bolt loads and may relax after thermal cycling; if PTFE is used, re-torquing after the first thermal cycle and periodic re-inspection are necessary.

Positive displacement lobe pumps, rotor-stator mixers, and scraped-surface heat exchangers create additional clearances that act as crevice sites in ammonium chloride paste. Rotor-to-case clearances in lobe pumps are typically below 0.3 mm, and the high-shear zone produces local heating that accelerates chloride attack if the material is S30300 free-machining stainless or inadequately passivated S31603. Pump casings and rotors should be fabricated from S31603 as a minimum, with enhanced molybdenum and electropolished surfaces for salt levels above 3 wt%, and from S32205 duplex or S32750 for continuous hot service above 70 °C. Mechanical seals and shaft seals represent unavoidable crevices; the seal faces should be silicon carbide or tungsten carbide with EPDM or perfluoroelastomer secondary seals, and the seal housing should be flushed with a small flow of water or paste-compatible liquid to prevent stagnation of ammonium chloride salts at the seal interface. Scraped-surface heat exchangers used to cook and cool liquorice paste contain a rotating shaft with blades or scrapers that contact the heat exchange surface under load; the clearance between scraper and wall should be maintained within 0.1–0.5 mm to prevent product burn-on and local chloride concentration. The heat transfer wall should be fabricated from S32205 duplex or S32750 for ammonium chloride paste above 4 wt% and wall temperatures above 120 °C; S31603 may be used only for short-term cooling of low-salt formulations with frequent inspection. Rotor and scraper materials must be galvanically compatible with the wall; use of carbon steel, brass, or aluminium scrapers is incompatible and will cause rapid galvanic pitting in the presence of ammonium chloride. The entire heat exchanger should be dismantled at the scheduled maintenance interval, and the product-contact surfaces should be inspected for pitting, crevice corrosion, and mechanical wear using borescopic and profilometric methods. Any loss of wall thickness greater than 0.2 mm in a scraped-surface heat exchanger tube should trigger replacement of the tube or the entire exchanger, because remaining wall thickness below design minimum reduces pressure integrity and increases the risk of stress corrosion cracking.

Inspection records should include borescopic imaging of weld roots, profilometry traverses at each product-contact gasket face, and repeat ferroxyl testing after any mechanical rework. The most severe operational boundary is not the bulk paste temperature or the bulk salt content, but the local stagnation condition at the end of a shift when hot liquorice paste remains in the depositor manifold and cools to ambient temperature over several hours. Equipment should be designed so that this retained mass is either minimal, removable by displacement, or held in a zone that has been upgraded to S32750 or S31254 with electropolished surfaces and crevice-free construction.