How Mercury Can Damage Aluminum Tower Internals by Liquid-Metal Embrittlement

Aluminum tower internals provide low weight, useful cryogenic properties, and good performance in selected gas-processing and low-temperature services. They have one unusual vulnerability: elemental mercury can interact with aluminum and cause severe loss of integrity through amalgamation and liquid-metal embrittlement-related damage. The mercury concentration in the feed may be extremely small, yet accumulation or guard-system breakthrough can expose internal surfaces over time.

This risk cannot be managed through an ordinary uniform-corrosion allowance. Damage may be localized, crack-like, and assisted by stress. Once mercury breaches the protective aluminum oxide film, deterioration can progress without the widespread thinning expected from aqueous corrosion.

Why Aluminum Is Normally Protected

Aluminum rapidly forms a thin, adherent oxide layer that separates the underlying metal from many environments. Elemental mercury does not readily attack an intact oxide everywhere. The danger increases when fabrication defects, scratches, fretting, chemical cleaning, erosion, or stressed regions disrupt the film and permit mercury to contact bare aluminum.

Mercury can form an amalgam with aluminum and interfere with reformation of the protective oxide. In the presence of moisture, continuing oxidation may produce voluminous corrosion products. Under tensile stress, liquid-metal interaction can reduce ductility and promote crack initiation or propagation. Weld heat-affected zones, highly stressed joints, cold-worked features, and rubbing contacts deserve particular attention.

The exact damage behavior depends on mercury form, temperature, phase distribution, surface condition, alloy, stress, water presence, and exposure history. Total mercury concentration in a gas sample does not by itself define the local condition on an internal.

How Mercury Reaches Tower Internals

Mercury may enter with natural gas, condensate, or another process feed. Upstream mercury-removal beds are intended to protect cryogenic exchangers and downstream equipment, but protection depends on bed sizing, flow distribution, adsorption capacity, temperature, contaminant competition, and timely replacement.

Breakthrough can occur gradually or through channeling. Mercury can also be released during an upset, feed-source change, bypass operation, or disturbance of contaminated deposits. Sampling at a convenient upstream point may miss a transient or fail to represent the stream reaching the tower.

Inside the tower, mercury may condense, adsorb onto deposits, dissolve in liquid hydrocarbon, or accumulate at cold and stagnant locations. Distributor trough ends, tray crevices, support contacts, low points, and areas below feed entry can receive different exposure. Shutdown warming may redistribute mercury that was immobilized during cold operation.

Failure Consequences for Internals

Thin aluminum panels, packing sheets, support grids, fasteners, and distributor components have limited reserve if cracking begins. A local fracture can change liquid or vapor distribution long before the complete component collapses. Cracked packing elements may settle; failed support members can release packing; distributor leakage can create severe maldistribution.

Fragments moving through the tower may damage lower internals or downstream machinery. In severe cases, the pressure boundary or attached aluminum equipment may also be at risk, so an internal finding should not be treated as an isolated maintenance issue.

Because damage may be local, stable tower differential pressure does not prove absence of mercury attack. The first evidence may be guard-bed breakthrough data, mercury detected during maintenance, unexplained aluminum corrosion product, or cracking at a stressed feature.

Design and Material Decisions

The primary defense is preventing mercury from reaching susceptible aluminum equipment. Define the maximum mercury entering the protected system, the analytical method, detection limit, sampling frequency, alarm response, and mercury-removal-system performance. Include startup, regeneration, bypass, low-flow, and feed-change cases.

Material selection should evaluate whether aluminum is necessary and whether another compatible material can be used at the most exposed or highly stressed locations. Changing alloy alone should not be presented as complete protection unless supported by service-specific evidence.

Reduce high tensile stress, forced fit-up, sharp notches, and rubbing contacts. Protect surfaces during fabrication and installation. Scratches should be assessed under an approved procedure rather than polished aggressively without knowing the remaining thickness. Dissimilar-metal contacts, cleaners, and coatings must also be compatible with cryogenic service and process purity.

Avoid geometry that retains liquid mercury or contaminated condensate. Drainability is valuable, but mercury transport is phase-dependent, so drainage alone cannot replace feed purification. Instrument and sampling connections should allow the owner to verify the barrier before the stream enters the vulnerable tower section.

Inspection and Monitoring Strategy

Monitoring begins upstream. Trend inlet and outlet mercury measurements around the removal system, bed age, pressure drop, temperature, flow distribution indicators, and feed composition. A single non-detect result is only as reliable as the sampling system, analytical detection limit, and timing.

If breakthrough or contamination occurs, use a multidisciplinary assessment involving process, materials, inspection, operations, and occupational-hygiene specialists. Opening equipment containing mercury introduces serious personnel and waste-handling hazards. Decontamination, ventilation, sampling, protective equipment, and disposal must follow the site’s mercury procedure.

After safe access, inspect high-stress and retention locations for unusual deposits, cracking, surface disruption, and distortion. Appropriate nondestructive examination depends on component geometry and alloy. Thin corrugated packing and mesh may be difficult to examine reliably; replacement may be more defensible than limited sampling after significant exposure.

Do not grind, heat, weld, or handle contaminated aluminum casually. These actions may spread mercury, create vapor exposure, remove evidence, or worsen damage. Preserve samples and document orientation for laboratory analysis where failure mechanism confirmation is needed.

Procurement and Response Checkpoints

The tower-internal specification should identify aluminum alloy and condition, fabrication controls, surface-damage limits, cleaning restrictions, joint stresses, inspection requirements, and the process owner’s mercury design basis. The internal vendor should disclose aluminum components that may not be obvious in assemblies or proprietary packing.

The operating plan should define actions for mercury-removal-bed alarms or analytical breakthrough: rate reduction, isolation, diversion, shutdown, or continued operation under an approved assessment. Waiting for tower performance to deteriorate is not an adequate protective strategy.

Mercury attack on aluminum is a low-concentration, high-consequence mechanism. Reliable control depends on keeping mercury away, detecting barrier failure early, and treating any confirmed exposure as a system integrity event.