How to Prevent Polythionic Acid Stress Corrosion Cracking of Stainless Tower Internals

Austenitic stainless-steel tower internals may operate for years in hot sulfur-containing service and then crack during a shutdown. The damaging environment is created not necessarily during normal processing, but when sulfide scale is exposed to oxygen and moisture. The resulting sulfur acids can include polythionic acids, which may cause intergranular stress corrosion cracking in sensitized stainless steel under tensile stress.

This shutdown mechanism is especially important in refinery and sulfur-processing equipment. Thin tray panels, mesh, distributor components, welds, clips, and cold-formed parts may contain high residual stress and extensive surface scale. They can crack with little general metal loss, so a clean-looking thickness reading does not establish fitness for service.

The Three Conditions Required for Cracking

Polythionic acid stress corrosion cracking requires a susceptible material condition, a specific acidic environment, and tensile stress. Removing any one of these conditions can prevent damage.

Susceptibility is commonly associated with sensitized austenitic stainless steel. During exposure to an unfavorable temperature range, chromium carbides can precipitate at grain boundaries, leaving adjacent regions depleted in chromium. Welding, long high-temperature operation, and certain heat treatments can create this condition. Low-carbon or stabilized grades reduce susceptibility, but grade designation alone does not prove the final fabricated and service-aged condition.

The environment forms when metal sulfide scale reacts with oxygen and liquid water during shutdown. Opening the tower admits air; water can come from washing, condensation, rain, steam-out, or humid air on a cold surface. Chlorides or other contaminants may add separate cracking or pitting risks.

Tensile stress includes operating stress, fabrication residual stress, weld shrinkage, cold forming, forced fit-up, and local bending at fasteners or supports. Internals can therefore be vulnerable even when their calculated operating stress is modest.

Why Tower Internals Deserve Separate Attention

Shutdown preservation plans often focus on the pressure boundary while assuming removable internals share the same risk. Internals experience different thermal histories, cold work, geometry, drainage, and scale retention. Mesh pads and structured surfaces trap deposits and wash water. Overlapping tray joints and support clips create crevices. Thin sections cool quickly and condense moisture before the shell instruments show a problem.

Cracking of one panel may lead to vapor bypass or loss of distribution. Multiple cracked clips can release a tray. Fragments from mesh or packing supports can travel into pumps or lower internals. Because cracks may be narrow and branched, damage can remain invisible until restart loads are applied.

Select a Shutdown Protection Strategy

The owner should use a written procedure based on the metallurgy, scale, expected outage duration, cleaning plan, and applicable industry guidance. Two broad strategies are commonly considered: keep oxygen away from the sulfide scale, or remove and neutralize the scale under controlled conditions.

An inert-gas strategy maintains an oxygen-deficient environment while the equipment contains scale and moisture. It requires verified isolation, continuous or monitored inerting, control of air ingress during draining, and safe provisions for personnel entry. A nominal nitrogen connection does not protect dead spaces if circulation is poor or openings are uncontrolled.

An alkaline washing and neutralization strategy removes or chemically controls reactive deposits before unrestricted air exposure. The procedure must define solution chemistry, concentration, temperature, contact time, coverage, sampling, drainage, rinsing, and disposal. Distributor laterals, tray undersides, mesh, crevices, and shadowed surfaces may not receive the same wash as open shell plate.

Water quality is critical. A wash intended to prevent one form of cracking should not introduce damaging chlorides or remain trapped in crevices. The shutdown sequence must avoid partially wetting sulfide scale and then leaving it exposed to air.

Material and Fabrication Decisions

For new internals, material selection should consider the full service thermal history, not only corrosion rate during operation. Low-carbon or stabilized stainless grades may be appropriate, subject to process-specific verification. Solution-annealed material can lose its favorable condition if later fabrication or long-term service creates sensitization.

Welding procedures should limit harmful thermal exposure and use appropriate filler metals. Heavy weld repair on thin parts can create residual stress and sensitized zones. Cold straightening, severe forming, and forced installation add stress at exactly the locations where deposits collect.

Material certificates and PMI do not reveal sensitization or residual stress. Where risk is significant, supplementary testing, fabrication controls, or material-condition verification may be required. The owner’s corrosion/materials specialist should define acceptance rather than leaving it to a generic stainless-steel specification.

Inspection Before Restart

If protection was lost, air entered unexpectedly, or improper wash water contacted scaled equipment, treat the event as a potential damage excursion. Document oxygen exposure, moisture, temperature, duration, and affected tower regions. Do not assume that drying later reverses cracks already formed.

Clean surfaces sufficiently for inspection without erasing evidence. Visual examination may find branched cracks near welds, bends, clips, and highly stressed joints, but surface crack testing is usually needed for credible assessment. Examination technique and coverage should reflect geometry; complex mesh may be impractical to inspect reliably and may require replacement if exposure was severe.

Inspect drainage pockets, tray joints, fastener holes, distributor corners, support welds, and heat-affected zones. Any crack should trigger evaluation of equivalent locations rather than isolated grinding. Repair welding requires an approved procedure and renewed shutdown protection because repaired areas may retain scale or residual stress.

Before restart, verify removal of wash liquid, reinstatement of internals, closure controls, and completion of the preservation record. The first heat-up should follow the operating procedure; it is not a substitute for inspection.

Polythionic acid cracking demonstrates why shutdown chemistry belongs in tower-internal design. The most dangerous exposure may begin only after the process has stopped.