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Wet-End Corrosion and Washdown Design for Handling Equipment

Handling equipment at the wet end of a paper machine works in warm, acidic, chloride- and thiosulfate-bearing white water, is splashed and dried every shift, and is hosed down during cleaning, so corrosion and washdown are design inputs from the first drawing rather than coating choices at the end. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. This article works through the problem from the mill's side: what white water contains, where it attacks stainless steel, how 304L, 316L, and duplex grades compare, how welding and post-fabrication cleaning change the result, and how enclosures, sensors, controls, and lockout have to be built for washdown. Corrosion performance is set along one chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, and a grade or enclosure chosen wrongly at the start cannot be fixed by the controls that run the finished machine.

What makes the paper machine wet end corrosive to handling equipment?​

The wet end is corrosive because of what the process water carries and the temperature at which it carries it. White water, the process water drained from the stock at the forming section and reused, takes its chemistry from the pulp and the chemicals added to it. Laitinen's study of stainless steels in the paper machine environment gives a typical white water, for an acid, hydrosulfite-brightened mechanical pulp, as:

Constituent or conditionTypical range
Chloride, Cl⁻50 to 300 mg/l
Sulfate, SO₄²⁻200 to 1,600 mg/l
Thiosulfate, S₂O₃²⁻0 to 50 mg/l
Temperature45 to 55 °C
pH4 to 6

Chloride and thiosulfate are the two constituents that matter most to stainless steel handling equipment. Laitinen reports that pitting and crevice corrosion caused by chloride and thiosulfate are common at the paper machine wet end, especially in the splash zones.

The ASM Handbook treats the subject at mill scale. Dykstra's chapter on corrosion in the pulp and paper industry reviews the materials used to build paper machine components and the specific corrosion problems that affect them, and discusses the composition and corrosive nature of white water. A roll cart, felt-change lifter, doctor-blade handling device, or conveyor frame near the machine sits in the same water as the machine itself and needs the same material logic (Laitinen, VTT Publications 399, 1999, Introduction and Table 1; ASM Handbook Vol. 13C, 2006, Dykstra, Corrosion in the Pulp and Paper Industry, pp. 762–802).

Why do splash zones and wet-dry surfaces corrode fastest?​

A surface that stays fully wet is not the worst case at the wet end. Laitinen found attack most aggressive where surfaces are intermittently wet and dry, because the water evaporates and the chloride and thiosulfate ions concentrate in the deposits left behind. The chemistry under a dried deposit is therefore not the bulk white water chemistry in the table above.

The coupon exposures in that study put numbers on how quickly it happens:

  • In the wire-section splash zone, 304L coupons corroded within 1 month.
  • In the same splash zone, 316L coupons corroded within 2 months.
  • In the pick-up section splash zone, the 22Cr duplex grade S31803 showed crevice corrosion after 13 months.

For handling equipment, the implication is that location on the machine matters more than a general wet-end label. A felt-change lifter that is parked near the forming section, a cart that sits under a splash line, or a frame member that collects spray and dries between washdowns is in the splash-zone condition, while a component that is hosed and drained is closer to the fully wet condition. The design review for wet-end equipment therefore maps each part of the machine to its exposure, identifies every ledge, pocket, and horizontal surface where splash can dry to a deposit, and either drains it, closes it, or moves it (Laitinen, VTT Publications 399, 1999, Sect. 3 results).

How does thiosulfate attack stainless steel in white water?​

Thiosulfate is the wet end's less obvious hazard, because it causes pitting at parts-per-million concentrations and, under some conditions, without chloride. Garner's work on paper-machine white water found that:

  • Thiosulfate contamination of white water pits AISI 304 stainless steel and CA-15 cast stainless.
  • For sensitized 304, thiosulfate concentrations of 3 to 75 ppm caused pitting, and 5 to 20 ppm was particularly aggressive.
  • Pitting occurred without chloride at sulfate-to-thiosulfate molar ratios of 1.6 to 58.
  • Types 316 and 317L, Ferralium 255, and cast duplex grades were markedly more resistant.

A typical white water can carry up to 50 mg/l of thiosulfate, per Laitinen's Table 1. For dilute process water, 1 mg/l is approximately 1 ppm, so that range, on this conversion, overlaps the 5 to 20 ppm band Garner found particularly aggressive to sensitized 304. The mechanism has also been studied for newer alloys. He, Wang, and Singh tested the lean duplex grades UNS S32304, S32003, and S82441 in chloride-only and thiosulfate-containing simulated white water, and found that thiosulfate raised pitting susceptibility mainly by lowering the repassivation potential, so pits repassivate less readily. The higher-molybdenum grades S32003 and S82441 were more resistant than S32304, which showed preferential dissolution of its ferrite phase.

For a specifying engineer, the practical point is that chloride alone does not describe the wet-end environment. The mill's own white water analysis, including thiosulfate, is the input to grade selection for handling equipment that works in or near it (Garner 1985, Corrosion 41 no. 10; He, Wang, and Singh 2017, CORROSION 2017; Laitinen, VTT Publications 399, 1999, Table 1).

How do 304L, 316L, and duplex grades compare at the wet end?​

Laitinen compares the common grades by pitting resistance equivalent number (PRE) and by the chloride level at which localized corrosion began in that study:

GradeUNSPRECritical chloride for localized corrosion
304LS3040319 to 21about 100 mg/l
316LS3160324 to 25about 200 mg/l
22Cr duplexS3180335500 mg/l or more

Laitinen also reports that S31603, 316L, is the most common stainless steel in new paper machines. Setting the typical white water chloride range of 50 to 300 mg/l against the table, a comparison derived here by simple division rather than stated in the study, shows why grade choice cannot be made from habit:

  • At the upper end of the range, 300 mg/l is three times the roughly 100 mg/l critical value for 304L.
  • The same 300 mg/l is 1.5 times the roughly 200 mg/l value for 316L.
  • Only the duplex grade's critical value of 500 mg/l or more sits above the whole typical range.

Those critical values are from the study's test conditions, and thiosulfate lowers the margin further, so they are a ranking, not a design allowable. The selection logic for handling equipment follows from them. Structural members that stay dry, or that are hosed and drained, may justify 316L. Members in a splash zone, where deposits concentrate the ions, move toward duplex, although Laitinen's duplex coupons still showed crevice corrosion after 13 months in a splash zone, so drainage matters as much as grade. Among lean duplex grades, He, Wang, and Singh found the higher-molybdenum grades more resistant in thiosulfate-containing white water. Dykstra's ASM chapter provides the wider review of paper machine component materials (Laitinen, VTT Publications 399, 1999, Introduction and Sect. 3 results; He, Wang, and Singh 2017, CORROSION 2017; ASM Handbook Vol. 13C, 2006, Dykstra, pp. 762–802).

Where does corrosion concentrate on wet-end handling equipment?​

On a handling machine, the corrosion review goes feature by feature, starting where water can be trapped or can dry to a deposit:

  • Crevices. Lap joints, stitch-welded seams, bolted splices, and gasketed covers create crevices. Laitinen's duplex coupons corroded mainly in crevices, with only minor pitting, so a grade that resists pitting on open surfaces can still corrode in a crevice.
  • Horizontal ledges and pockets. Channels facing up, gusset pockets, and flat tops collect splash and dry it to a deposit, which is the condition Laitinen found most aggressive.
  • Fasteners and pins. A bolt or pin of a lower grade than the parent structure, or a fastener seated in a crevice, is exposed to the same attack with less corrosion resistance or a worse geometry than the members it joins.
  • Shaft ends and lubrication points. Paragraph 1910.261(k)(2) requires all ends of rotating shafts, including dryer drum shafts, to be completely guarded, and requires oil cups and grease fittings to be placed in a safe area remote from nip and heat hazards. On wet-end equipment, those guards and fittings are themselves exposed parts and need the same grade and drainage review.
  • Overhead structure. For cranes over the wet end, OSHA's periodic crane inspection under 1910.179(j)(3)(i) includes deformed, cracked, or corroded members, and 1910.179(j)(3)(x) includes electrical apparatus for pitting or deterioration of controller contactors, limit switches, and pushbutton stations.

The design responses are consistent across these features: continuous seal welds instead of stitch welds where a crevice would otherwise form, open or sloped sections that drain, fasteners at least equal in grade to the structure, and guards and fittings that can be hosed and inspected (Laitinen, VTT Publications 399, 1999, Sect. 3 results; OSHA 29 CFR 1910.261-2016, paragraph k.2; OSHA 29 CFR 1910.179-2016, paragraph j.3).

How do welding and post-fabrication cleaning affect corrosion resistance?​

A correct grade can still corrode early if fabrication leaves it in the wrong condition. Garner's results show the sensitivity: sensitized 304 pitted at thiosulfate levels as low as 3 ppm. Sensitization is a metallurgical change in austenitic stainless steel caused by heating in a particular temperature range, which is one reason thermal stress relief of welded austenitic stainless is handled differently from carbon steel, as the heat-treating library's article on stress relief of welded austenitic stainless explains.

Surface condition after fabrication is the second factor. Two documents address it directly:

  • TAPPI TIP 0402-35, 2021 revision, covers the post-fabrication cleaning of stainless steel equipment in pulp and paper mills, with the aim of maximizing its technical and corrosion performance.
  • ASTM A380/A380M-25 gives recommendations and precautions for cleaning, descaling, and passivating stainless steel parts, assemblies, equipment, and installed systems, to remove surface contaminants that may impair normal corrosion resistance.

For a wet-end lifter, cart, or conveyor frame, the practical sequence is weld, descale and clean the weld and heat-affected zone, passivate, then inspect, with the cleaning method named in the purchase order rather than left to the fabricator's default. Machined surfaces, such as bearing seats and pin bores, are included in the same cleaning review. UTEC Industrial performs automated vibratory stress relief (VSR) in-house and machines the welded frames of the handling equipment it builds (Garner 1985, Corrosion 41 no. 10; TAPPI TIP 0402-35, 2021; ASTM A380/A380M-25).

What does washdown mean for enclosures: NEMA 4X versus IP ratings?​

Electrical enclosures on wet-end handling equipment are rated under one of two systems, and they do not say the same thing. ANSI/NEMA 250-2020 classifies enclosures for electrical equipment rated 1,000 V maximum by Type. Its scope lists indoor Types 1, 2, 5, 12, 12K, and 13, indoor or outdoor Types 3, 3X, 3R, 3RX, 3S, 3SX, 4, 4X, 6, and 6P, and hazardous-location Types 7 and 9. The standard includes:

  • Corrosion-protection requirements for Types 3X, 3RX, 3SX, 4X, and 6P, in section 3.5.7. Type 4 is not in that list, which is the practical difference between a Type 4 and a Type 4X enclosure.
  • A hosedown water-ingress test, in section 5.7.
  • A corrosion-protection test, in section 5.10.
  • A conversion of NEMA Type ratings to IP code designations, in Annex A, Table A-1.

IEC 60529:1989+AMD1:1999+AMD2:2013 classifies the degrees of protection provided by enclosures for electrical equipment rated at not more than 72.5 kV, using the IP code. The code's first characteristic numeral rates protection against solid foreign objects and access to hazardous parts, and its second rates protection against water. For washdown, the second numeral needs care: an enclosure rated only for temporary or continuous immersion, 7 or 8, is not by that rating shown to be suitable for water jets, numerals 5 or 6, unless it is dual-coded, for example IP66/IP67.

The specification for a wet-end handling machine should therefore state the enclosure rating in the system the component is sold under, require a corrosion-protected Type such as 4X where a NEMA enclosure is used in a hosed, chemically wet area, and require jet and immersion ratings together where an IP-rated sensor or connector is hosed as well as splashed (ANSI/NEMA 250-2020, sections 1.1, 3.5.7, 5.7 and 5.10, Annex A; IEC 60529:1989+AMD1:1999+AMD2:2013 CSV, Clause 6).

How should controls and sensing be designed for wet-end handling equipment?​

The intelligence layer of a wet-end machine has to survive the same water as its structure, and it has to keep the machine safe while people hose it down. The design points are:

  • Electrical equipment scope. The machine's electrical equipment as a whole falls under IEC 60204-1:2016, which applies to the electrical, electronic, and programmable electronic equipment of machines, starting where the supply connects to the machine.
  • Panel location. Control panels and drives are located out of the splash and hose zone wherever the layout allows. Field devices that have to be in it, such as proximity sensors, encoders, limit switches, pushbuttons, and junction boxes, carry an enclosure rating chosen as described above.
  • Sensors and cabling. Every sensor and connector in the hose zone needs a jet rating as well as an immersion rating. Cable entries are sealed and oriented downward so water drains away from the entry.
  • Emergency stops. Paragraph 1910.261(k)(1) requires paper machine emergency stops, push buttons, pull cords, or other devices, interlocked with adequate braking action and tested periodically by using them to stop the machine. Wet-end handling equipment tied into the machine carries stop devices rated for the washdown exposure.
  • Safe torque-off. Allen-Bradley Kinetix 5700 servo drives have safe torque-off built into the drive.
  • Condition monitoring. Trending insulation faults, drive fault history, and limit-switch operation shows water ingress before it becomes a failed device, and gives the mill measured evidence for the crane inspection of electrical apparatus under 1910.179(j)(3)(x).

UTEC Industrial, a Rockwell Automation Recognized System Integrator, builds UL 508A control panels for the handling systems it builds, so the panel construction and enclosure rating are set at the same time as the machine (IEC 60204-1:2016; ANSI/NEMA 250-2020; IEC 60529:1989+AMD1:1999+AMD2:2013 CSV; OSHA 29 CFR 1910.261-2016, paragraph k.1; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700; OSHA 29 CFR 1910.179-2016).

How should washdown be sequenced with lockout?​

Washdown is cleaning, and the pulp and paper standard treats cleaning as a lockout task. Paragraph 1910.261(b)(1) requires the main power disconnect to be locked out before cleaning that requires close contact with the machinery, and (k)(2)(i) adds lockout devices at the power switch of every drive; the isolation rules are set out in the roll-handling article.

Hosing down a handling machine at close range meets that definition, and the design has to make the lockout practical:

  • Lockable disconnects at the machine. Each energy source, electrical, pneumatic, and hydraulic, needs a lockable isolation point outside the hose zone.
  • Control devices are not isolation. OSHA 29 CFR 1910.147 defines push buttons, selector switches, and other control-circuit-type devices as not being energy-isolating devices, so a "washdown mode" on the HMI does not replace lockout.
  • Stored energy. Under 1910.147(d)(5)(i), all potentially hazardous stored or residual energy must be relieved, disconnected, restrained, and otherwise rendered safe after lockout devices are applied. Raised lifters, pressurized cylinders, and loaded carts need bleed-down valves and mechanical blocking points that can be used with wet hands and gloves.
  • Lubrication access. Paragraph 1910.261(k)(2)(v) requires oil cups and grease fittings to be placed in a safe area remote from nip and heat hazards, which is also where relubrication after washdown should happen.

The result is a machine that can be isolated, bled down, washed, and relubricated without anyone reaching into a nip or relying on a control-circuit stop (OSHA 29 CFR 1910.261-2016, paragraphs b.1 and k.2; OSHA 29 CFR 1910.147-1989).

How are cranes, ropes, and lifting gear inspected in a wet-end environment?​

Corrosion on lifting equipment is a removal criterion, not a cosmetic finding. For overhead cranes, 1910.179(j)(3)(i) includes deformed, cracked, or corroded members in the periodic inspection. That inspection is performed at the 1 to 12-month intervals defined in 1910.179(j)(1)(ii), depending on the crane's activity, severity of service, and environment, and a crane over the wet end is a case for the shorter end of that range.

Ropes are covered by 1910.179(m)(1), which requires a thorough inspection of all running ropes at least once a month, with a certification record. The conditions it lists as causing an appreciable loss of strength include:

  • Reduction of rope diameter below nominal because of loss of core support, internal or external corrosion, or wear of outside wires.
  • Corroded or broken wires at end connections.
  • Corroded, cracked, bent, worn, or improperly applied end connections.

Paragraph 1910.261(k)(24)(ii) adds a pulp and paper requirement that all lifting equipment used for reels, meaning clamps, cables, and slings, shall be maintained in a safe condition and inspected regularly. For a felt-change lifter, reel lifter, or roll grab used near the wet end, the inspection plan therefore needs access points designed in, including removable covers over pins, inspection openings in box sections, and drain holes that can be checked, so the corroded condition these rules name can actually be seen (OSHA 29 CFR 1910.179-2016, paragraphs j.1, j.3 and m.1; OSHA 29 CFR 1910.261-2016, paragraph k.24).

What should a mill specify for wet-end handling equipment?​

A request for quotation that says only "stainless, washdown duty" leaves the fabricator to guess at every decision above. A complete specification states:

  • Exposure map: which parts of the machine are fully wet, splash-zone, or dry, and whether each is hosed.
  • White water chemistry: the mill's measured chloride, sulfate, and thiosulfate ranges, temperature, and pH, compared with the typical ranges and critical chloride values in Laitinen's study.
  • Grade by exposure: 316L, duplex, or lean duplex for each exposure zone, with the reasoning, and fastener and pin grades at least equal to the structure they join.
  • Fabrication: seal-welded joints where crevices would otherwise form, drainage of every ledge and pocket, post-weld treatment, and cleaning and passivation to ASTM A380/A380M-25, with TAPPI TIP 0402-35 (2021) as the pulp and paper reference.
  • Enclosures and devices: ANSI/NEMA 250-2020 Type, such as 4X, or IP code with jet and immersion ratings, for each panel, sensor, and connector in the hose zone.
  • Isolation for washdown: lockable disconnects, bleed-down valves, and blocking points consistent with 1910.261(b)(1) and 1910.147.
  • Inspection access: covers, openings, and drain holes placed so the inspections under 1910.179 and 1910.261(k)(24)(ii) can be performed.

UTEC Industrial performs factory acceptance testing and on-site commissioning, so the enclosure ratings, isolation points, and inspection access can be checked against this specification before the equipment is installed at the wet end (Laitinen, VTT Publications 399, 1999; ASTM A380/A380M-25; TAPPI TIP 0402-35, 2021; ANSI/NEMA 250-2020; OSHA 29 CFR 1910.261-2016; OSHA 29 CFR 1910.147-1989).

Related Articles

References​

  • Laitinen, T. Thiosulfate Pitting Corrosion of Stainless Steels in Paper Machine Environment. VTT Publications 399. Technical Research Centre of Finland, 1999.
  • Garner, A. (1985). "Thiosulfate Corrosion in Paper-Machine White Water." Corrosion, 41(10), 587–591. DOI 10.5006/1.3582988
  • He, L., Wang, Y., Singh, P.M. (2017). "Pitting Behavior of Lean Duplex Stainless Steels in Thiosulfate-containing Paper Machine Environments." CORROSION 2017, NACE International, 1–8. DOI 10.5006/C2017-09163
  • Cramer, S.D., Covino, B.S., Jr. (eds.). ASM Handbook, Volume 13C: Corrosion: Environments and Industries. ASM International, 2006. ISBN 978-0-87170-709-3.
  • TAPPI TIP 0402-35: Post-Fabrication Cleaning of Stainless Steel in the Pulp and Paper Industry. TAPPI Press, 2021.
  • ASTM A380/A380M-25: Standard Practice for Cleaning, Descaling, Pickling, and Passivation of Stainless Steel Parts, Equipment, and Systems. ASTM International, 2025.
  • ANSI/NEMA 250-2020: Enclosures for Electrical Equipment (1000 Volts Maximum). National Electrical Manufacturers Association, 2020.
  • IEC 60529:1989+AMD1:1999+AMD2:2013 CSV: Degrees of Protection Provided by Enclosures (IP Code). International Electrotechnical Commission, 2013.
  • IEC 60204-1:2016 (Ed. 6.0): Safety of Machinery -- Electrical Equipment of Machines -- Part 1: General Requirements. International Electrotechnical Commission, 2016.
  • OSHA 29 CFR 1910.261-2016: Pulp, Paper, and Paperboard Mills. U.S. Department of Labor, 2016.
  • OSHA 29 CFR 1910.179-2016: Overhead and Gantry Cranes. U.S. Department of Labor, 2016.
  • OSHA 29 CFR 1910.147-1989: The Control of Hazardous Energy (Lockout/Tagout). Occupational Safety and Health Administration, 1989.
  • Rockwell Automation 2198-UM002E-EN-P (2018): Kinetix 5700 Servo Drives User Manual. Rockwell Automation, 2018.

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