Stainless Steel Selection for Mercerization Wetting and Tenter Chain Tension Control

In continuous mercerizing ranges equipped with clip tenter frames, the stainless steel components serving the wetting bath and chain tension system are exposed to NaOH at 180–280 g/L, wetting agent blends at 0.5–3.0 g/L, and fabric widths from 1.8 m to 3.6 m. The tenter chain must maintain transverse restraint and warp tension while operating in an alkaline mist environment where residual chlorides from water, fabric preparation, or dyehouse carry-over can concentrate in stagnant crevices. Stainless steel is selected under ASTM A240/A240M-23a for plate and sheet, ASTM A276/A276M-17 for bar stock, ASTM A564/A564M-19a for precipitation-hardening pin and clip alloys, and ISO 6507-1:2018 for hardness verification. The wetting agent reduces surface tension from 72 mN/m to 28–35 mN/m at 20 °C to permit rapid NaOH diffusion into cotton fibers; the same surfactant package can also form viscous deposits at the chain rail interface and elevate sulfate or chloride carry-over into the chain joint. Tension control is maintained with strain-gauge load cells having 0.25–0.5 % full-scale nonlinearity, mounted in the rail return path, while the chain link pins operate with radial clearances of 0.05–0.15 mm. The selected stainless steel must therefore resist alkaline corrosion, chloride pitting, crevice corrosion, galling, and wear simultaneously, because a failure in any one mode transfers directly to width variation, fabric shrinkage, or tension-control drift.

Why Does UNS S31603 Stress Corrosion Crack in 20–25 wt% NaOH When Tenter Chain Tension Exceeds Routine Setpoints?

Caustic stress corrosion cracking in austenitic stainless steel is controlled by temperature, NaOH concentration, and tensile stress in the component. In the wetting bath itself, the saturator is normally operated at 18–20 °C to maximize fiber swelling and limit NaOH attack; under these conditions, UNS S31603 is generally resistant to caustic SCC in continuously immersed service, but crevice environments at chain pins, bushing shoulders, and clamp bolts can elevate localized NaOH concentration by evaporation. In lye recovery evaporators operating at 50–90 °C, caustic SCC of UNS S31603 has been documented in welds and cold-formed corners, which is why the material is normally excluded from hot recovery sections even though its uniform corrosion rate in 20 wt% NaOH at ambient temperature can remain below 0.1 mm/year. The relevant screening method is ASTM G36-94(2018) using boiling magnesium chloride; the test is severe and not directly equivalent to caustic service, but it differentiates susceptible austenitic grades from duplex and ferritic alloys. Intergranular corrosion following welding is assessed by ISO 3651-2:1998 or ASTM A262-15 Practice E. For UNS S31603, the carbon limit of 0.030 wt% is not sufficient to prevent sensitization in heavy-section welds without solution annealing. Tenter chain tension components that are cold-formed or welded after machining should be stress-relieved or specified as low-carbon grades with controlled ferrite to avoid residual stress concentrations above 0.2 % yield strength in caustic-laden crevices.

Residual tensile stress in tenter chain pins is introduced by press fits, staking, and cyclic bending. When the applied fabric tension imposes a mean stress above 150 MPa on a UNS S31603 pin, caustic SCC incubation can shorten in hot caustic, but at mercerization saturator temperatures it is not the primary failure mode. The operating boundary for UNS S31603 in continuous mercerization wetting is therefore set less by caustic SCC than by chloride pitting and crevice corrosion; however, if the tenter line also processes fabric previously bleached or dyed with chloride-containing formulations, chloride can concentrate in the chain lubrication film. This condition produces a mixed caustic-chloride electrolyte that can pit UNS S31603 at temperatures as low as 15 °C under stagnant conditions. UNS S32205 is selected for replacement pins because its higher chromium, molybdenum, and nitrogen content shifts pitting resistance to a PREN of 34–36, and its duplex microstructure provides a minimum yield strength of 450 MPa under ASTM A240/A240M-23a. The hardness of UNS S32205 in the solution-annealed condition is 250–290 HV, which is still below precipitation-hardening alloys but offers a balance between corrosion resistance and wear.

Pitting and Crevice Corrosion Resistance as a Function of Molybdenum and Nitrogen in Wetting Bath Return Lines

The pitting resistance equivalent number is calculated as PREN = %Cr + 3.3 × %Mo + 16 × %N and is used as a preliminary ranking tool for stainless steel in chloride-containing alkaline textile streams. Wetting bath return lines collect NaOH, wetting agent, fabric waxes, cotton pectin residues, and hard-water salts; evaporative concentration at the liquid line can increase chloride from incoming water levels of 50–100 mg/L to 500–1,000 mg/L in stagnant corners. UNS S31603 with a PREN of 23–25 has limited pitting resistance under these deposits, and its critical pitting temperature determined by ASTM G150-18 in neutral aerated chloride is commonly reported at 15–20 °C. UNS S32205 raises that threshold to approximately 35–40 °C. UNS S32750 is specified for return lines and lye recovery preheaters because its PREN of 40–43 corresponds to a critical pitting temperature above 80 °C in ASTM G150-18, and its minimum yield strength of 550 MPa allows thinner wall piping with reduced dead legs. UNS N08904 is an alternative where formability and weldability are more important than strength, but its lower nitrogen content gives a PREN of 34–36, and crevice corrosion at rubber-lined joints remains a risk.

Testing for pitting and crevice corrosion must follow ASTM G48-11(2020) Method B in ferric chloride solution at 25 ± 1 °C for 72 h, with a maximum mass loss criterion of 4.0 g/m² and no visible pits at 20× magnification. For duplex grades, the test temperature is often raised to 50 °C to differentiate UNS S32205 from UNS S32750. Intergranular corrosion after welding is checked using ASTM A262-15 Practice E or ISO 3651-2:1998, with a bend test after exposure to acidified copper sulfate solution; the acceptance criterion is no intergranular fissures. In mercerization wetting service, crevice corrosion occurs at the interface between tenter chain link plates and the bushing shoulder, particularly where cotton lint and surfactant residues form an oxygen-depleted zone. The specification should therefore include ASTM G48-11(2020) Method B on a crevice assembly or ASTM G78-15 crevice corrosion testing in an alkaline chloride environment, because the flat coupon pitting test underestimates real chain crevice behavior.

On wide tenter chains, the limiting wear mechanism is not alkaline corrosion but adhesive galling at the pin-bushing interface under oscillatory motion. The chain pin rotates slightly as the clip enters the rail, creating a contact stress in the range 50–150 MPa depending on clip load and geometry; under stop-start conditions, the pin and bushing can microweld when both are made from UNS S31603 with hardness below 180 HV. Galling resistance is evaluated by ASTM G98-17 or ASTM G196-08 and is strongly influenced by surface finish; a ground surface with Ra 0.2–0.4 µm galls less than a mill finish. For mercerization lines that run at 60–120 m/min, chain links made from UNS S32205 or UNS S21800 with a nitrided or work-hardened surface are preferred. UNS S21800 is specified under ASTM A276/A276M-17 and provides galling resistance because of its silicon content of 3.5–4.5 wt% and manganese content of 7.0–9.0 wt%, which promote a protective oxide film. Tension control load cells should not be fabricated from UNS S31603 because chloride from the wetting bath can initiate pitting under the strain-gauge bond line, causing zero drift; UNS S17400 or UNS S15500 per ASTM A564/A564M-19a is more suitable at hardness 420–450 HV after H900 aging. The load cell diaphragm must be passivated per ASTM A967/A967M-17 in citric acid and protected with an elastomeric barrier that is compatible with 20–25 wt% NaOH and with wetting agent solvents such as butyl glycol.

Chain tension control in mercerization requires maintaining uniform width while the cellulose fiber shrinks longitudinally and swells laterally. The load cells in the rail return lines register force changes as the fabric relaxes; if the tenter chain pins seize, the measured tension includes frictional noise that confuses the controller. The control loop typically uses a proportional-integral algorithm with a sampling rate of 10–50 ms; when chain vibration exceeds ±5 % of full-scale load, the resulting fabric width deviation can exceed ±1.0 % at the exit. Therefore, the stainless steel selected for chain components must combine a corrosion allowance for caustic wetting with a wear allowance for chain articulation. A material with high PREN but low hardness, such as UNS N08904, is unacceptable for pins because its annealed hardness of 150–190 HV leads to rapid adhesive wear; its use is confined to static wetted parts such as troughs and piping. A material with high hardness but poor chloride pitting resistance, such as UNS S21800 or UNS S17400, is confined to dry or intermittently wetted chain components with a phosphate or PTFE-based dry film lubricant. The lubrication interval must be tied to ASTM G133-05(2020) sliding wear results, not to vendor recommendations alone.

When Tenter Chain Pin and Clip Alloys Require Surface Engineering Instead of Higher Molybdenum Content

Surface hardening is required when the chain pin must resist both caustic-chloride pitting and adhesive wear, because higher molybdenum austenitic grades such as UNS N08904 or UNS S31254 do not provide sufficient hardness in the solution-annealed condition. Conventional plasma nitriding of UNS S31603 at 500–550 °C produces a compound layer and diffusion zone with hardness 800–1,100 HV, but chromium nitrides form preferentially and deplete the matrix adjacent to grain boundaries. The resulting pitting resistance after ASTM G48-11(2020) Method B is significantly reduced, and the process is generally not acceptable for mercerization wetting components. Low-temperature carburizing at 470–500 °C creates a carbon-supersaturated austenite layer with hardness 800–1,200 HV and maintains pitting resistance, as verified by ASTM G48 Method B and ISO 3651-2:1998. This treatment is applied to UNS S32205 pins and clips to achieve a surface hardness exceeding 600 HV while retaining a core yield strength of 450 MPa. The treated components are then tested for galling by ASTM G98-17 with a threshold galling stress above 50 MPa; published data for this specific configuration is limited, so acceptance criteria are usually established on a prototype fixture that replicates pin-bushing contact geometry.

An alternative for tenter chain tension load cells is precipitation-hardening UNS S17400 or UNS S15500 in the H900 or H1025 condition, with hardness between 390 and 440 HV and tensile strength above 1,300 MPa per ASTM A564/A564M-19a. These alloys contain 15–17.5 wt% Cr and 3.0–5.0 wt% Ni but have limited molybdenum, so their pitting resistance is lower than UNS S32205; they are suitable only where the load cell body is sealed and not continuously wetted. When the same load cell is subjected to alkaline mist with chloride, electroless nickel plating with 25–35 µm thickness may be used to provide a barrier, but the plating must be free of microcracks, and the passivation procedure must follow ASTM A967/A967M-17. The operational boundary is that UNS S17400 should not be exposed to 20 wt% NaOH above 60 °C in stressed conditions, and it should not be combined with chloride-containing cleaning acids or hypochlorite sanitizers, because crevice attack under the strain-gauge adhesive layer can occur within 72 h.

Mercerizing Wetting Agent Chemistry and Steel Surface Passivation in Saturated Lye Recovery Systems

Wetting agents used in mercerization are commonly sulfated or sulfonated hydrotropes, fatty alcohol ethoxylates, and alkyl polyglycosides. The sulfonated compounds such as sodium cumenesulfonate and sodium alkyl diphenyl oxide disulfonate are added at 0.5–3.0 g/L to lower the NaOH contact angle on greige cotton and to suppress the formation of caustic mists. The ethoxylated nonionics can form inverse solubility phases above their cloud point, leading to gummy deposits on chain rails; the selection of the stainless steel surface finish and passivation treatment therefore controls whether these deposits adhere and create oxygen concentration cells. A surface roughness of Ra 0.4–0.8 µm after electrochemical polishing is specified for wetted troughs to reduce deposit buildup, and the passivation treatment is performed per ASTM A967/A967M-17 using citric acid. In return lines, the oxidizing potential of the alkaline wetting bath is usually insufficient to maintain a fully passive film on UNS S31603 if the chloride concentration rises above 200 mg/L. Some continuous ranges therefore specify UNS S32205 or UNS S32750 for the return piping, while UNS S31603 is restricted to ambient-temperature troughs and unheated alkali storage.

Concentration of the lye from saturator expression and wash water is carried out in multiple-effect evaporators operating at 130–150 °C and NaOH concentrations from 20 wt% to 50 wt%. In this hot, concentrated caustic environment, UNS S31603 is at risk of both caustic SCC and general corrosion, and UNS S32205 is often at its upper temperature limit. The evaporation tubes are therefore fabricated from UNS S32750 or a nickel-based alloy such as UNS N06625 in severe service, with welding procedure qualification requiring intergranular corrosion testing by ISO 3651-2:1998 and positive material identification under API 578 or equivalent. The tension control components in the tenter frame are not exposed to these recovery temperatures, but if the tenter chain is cleaned with recovered caustic that contains wetting agent degradation products and traces of chelated iron, crevice corrosion may accelerate. The specification for replacement chain links should therefore require a mock-up test in the actual recovered lye at the maximum operating temperature of the chain, not only the standard ASTM G48-11(2020) ferric chloride test.