Demister Corrosion Causes and Material Upgrades

A demister can be dimensionally perfect and still fail in weeks if the alloy does not match the chemistry. Corrosion diagnosis should precede any “same as before” reorder.

Why thin wire changes the risk

A demister contains a large area of fine wire, so a small absolute loss of metal can remove a significant share of load-bearing section. The support grid may still look sound after the mesh has become brittle or fragmented. Corrosion allowance concepts used for thick vessel plate therefore do not transfer directly to knitted media.

The liquid film on the wire can also be more concentrated than the bulk process stream. Evaporation, condensation, deposits and intermittent washing may create local chloride, acid or salt conditions that are not represented by one normal-operation sample. Record start-up, shutdown, cleaning and credible upset chemistry as well as steady operation.

Common modes

• Uniform thinning — aggressive general corrosion; material or temperature mismatch
• Pitting — chlorides on stainless; desalination and coastal cooling water are frequent contexts
• Stress cracking — chloride stress corrosion cracking (SCC) risk on austenitic grades under tensile residual stress
• Galvanic attack — dissimilar conductive materials in electrical contact through a corrosive liquid film
• Under-deposit corrosion — fouling creates local chemistry cells

Diagnose before choosing an alloy

1. Map the location. Separate damage on mesh, grids, tie wires, welds and the vessel support. Edge-only damage, contact-point attack and uniform loss suggest different mechanisms.
2. Preserve evidence. Photograph before cleaning, retain representative wire and deposit samples, and record which face was upstream.
3. Check time and operating history. A failure after a wash-chemistry change or temperature excursion should not be treated as ordinary aging.
4. Review the complete environment. Include water content, pH where meaningful, halides, sulfur species, oxidants, solids and contaminants, plus normal and upset temperature.
5. Confirm the old material. A purchase description such as “stainless” is not proof of the installed grade. Use available traceability records and, where justified, agreed material verification.

Visual appearance alone cannot reliably distinguish all corrosion mechanisms. Red rust can come from an attached carbon-steel component, and a clean-looking polymer can still have lost strength through temperature or chemical exposure.

Upgrade logic (examples, not rules)

• Mild hydrocarbon / steam with unexpected chlorides → review 304 → 316L
• Chloride-rich aqueous service → consider duplex or appropriate plastic
• HF or specific reducing chemistries → Monel 400 may be a candidate, but only after reviewing concentration, water content, temperature, oxidants and upset cases; other nickel alloys or non-metallic options may be safer
• Oxidizing acid mist where metals struggle → review the exact concentration, temperature, water content and oxidants before considering PTFE, a compatible polymer or a special alloy; PP is not a general oxidizer-resistant default
• Grid, tie wires, hold-downs and mesh should be specified as a complete material set and checked for galvanic compatibility in the actual liquid film; belonging to the same broad material family is not enough by itself

Material families are not a corrosion chart

Moving from 304 to 316L, duplex, nickel alloy, titanium or polymer is not a universal ladder. Resistance changes by medium, concentration, temperature, aeration, contaminants, stress and fabrication condition. A material that is better in one chloride service may be worse in a reducing acid or oxidizing mixture.

For sour oil-and-gas service, the project may invoke requirements for resistance to H2S-related cracking. That review is separate from general and localized corrosion. Follow the project’s identified sour-service specification and environmental limits; never treat its material controls as a blanket statement that an alloy is corrosion-proof.

Documents that protect both sides

Specify the upgrade decision in the RFQ so it enters the PI as the manufacturing basis: the alloy grade, or, for monofilament products, the polymer type together with the raw material producer, its commercial designation and the applicable temperature limits. Each shipment then carries the ordinary material or batch traceability record listed in the PI to verify that stated basis. If EN 10204 Type 3.1 or Type 3.2 documentation, positive material identification (PMI) or additional testing is required, request it before material procurement and production so traceability, cost and lead time are confirmed upfront. A dimensional report does not identify material chemistry; a traceability record can verify the specified grade or batch, but neither document alone proves corrosion performance in service.

Freeze the grid, tie-wire, hold-down and attachment materials with the media. Also state whether welding, heat treatment, pickling, passivation, surface condition or contamination controls are project requirements. These are not implied by naming an alloy.

For a replacement investigation, the purchase file should contain the old pad identity, failure map, service data used for selection, approved material set and agreed evidence. This creates a reviewable decision trail without promising service life that the process data cannot support.

Send failure photos plus the fluid composition and temperature conditions during normal operation and credible upsets. A one-grade “upgrade for safety” without chemistry review often wastes money.

Use the materials-selection guide to build the RFQ and the inspection checklist to collect location-specific evidence before cleaning.

Authoritative references and limits

AMPP’s galvanic corrosion guidance confirms that dissimilar conductive materials, electrical contact and a corrosive electrolyte are all required for a galvanic couple; material names alone do not prove galvanic attack. AMPP’s materials-selection guidance also emphasizes environment, drainage, crevices, component compatibility and maintenance. For sour-service projects, use the applicable official project specification rather than an old “NACE compliant” label with no environmental limits.