How to Prevent Galvanic Corrosion at Tower-Internal Connections

Tower internals frequently combine different metals: a stainless-steel distributor may rest on carbon-steel clips, alloy panels may use stainless fasteners, or a nickel-alloy component may connect to a lower-alloy support. Dissimilar metals do not automatically fail, but when they are electrically connected and exposed to a conductive liquid, galvanic current can accelerate attack of the less noble material. The most severe damage often occurs at small fasteners, support edges and hidden wet interfaces rather than on the main visible panel.

Understand the Complete Galvanic Cell

Galvanic corrosion requires three conditions: materials with different electrochemical behavior, electrical contact between them and an electrolyte bridging the surfaces. Removing any one condition can interrupt the cell.

The relative behavior of metals depends on the actual process environment. Temperature, acidity, chloride content, oxygen, deposits and passivation influence potential. A generic galvanic-series chart is useful for screening but cannot replace service-specific corrosion review.

The less noble metal generally becomes the anode and corrodes faster, while the more noble surface acts as the cathode. The effect is strongly influenced by exposed area. A small anodic carbon-steel clip connected to a large stainless-steel tray can experience intense local attack. Reversing the area ratio may reduce the galvanic current density on the anode, although independent corrosion of either material must still be considered.

Map Every Material Interface

Create an interface schedule covering the main internal, support rings, beams, clips, bolts, washers, weld overlays, gaskets and vessel lining. Include temporary and field-installed items; a stainless assembly can be compromised by one carbon-steel shim left beneath it.

Pay special attention to:

perimeter seals touching the shell;

beam seats and support-ring contacts;

bolted joints with wet crevices;

drain points where electrolyte concentrates;

welds between different alloy systems;

coated carbon steel connected to exposed alloy;

replacement hardware installed during turnaround.

The review should show whether each interface stays dry, is intermittently wetted, remains submerged or accumulates deposits. Intermittent wetting with oxygen access can be especially aggressive.

Select Compatible Materials as a System

The simplest control is to avoid an unfavorable couple. Match fasteners and small attachments to the corrosion resistance of the larger component, or select a combination demonstrated to be acceptable in the process liquid. Do not focus only on the distributor or tray material while leaving its clips and bolts unspecified.

Welded dissimilar-metal joints require review of base metals, filler, dilution, heat-affected zones and postweld condition. A sound weld does not eliminate galvanic or localized-corrosion risk.

An alloy upgrade should also be evaluated for side effects. Replacing only one small component with a more noble alloy can shift corrosion to the adjacent support. Material selection must follow the current path, not a “higher alloy is always safer” rule.

Use Electrical Isolation Carefully

Nonconductive gaskets, sleeves, washers or bearing pads can break metal-to-metal contact. For isolation to work, the detail must prevent alternate electrical paths through bolts, sharp edges, conductive deposits or damaged coatings.

The isolator must withstand temperature, compression, creep, chemical exposure and installation torque. A thin plastic washer may extrude, allowing the bolt to contact the plate. An insulating gasket can also create a crevice that retains concentrated liquid. Drawings should show the complete isolation kit and the assembly sequence, not merely name a gasket material.

Electrical isolation may conflict with static-control, grounding, lightning or electrical-safety requirements. Do not isolate a component until the process-safety and electrical teams confirm that bonding is unnecessary or provide another approved bonding path. Corrosion and electrical requirements must be resolved together.

Apply Coatings with the Correct Area Logic

Coating can separate a metal from the electrolyte, but partial damage changes the exposed-area ratio. A small holiday in a coated anodic component connected to a large bare cathode can concentrate attack at the defect.

Where coating is used, surface preparation, edge coverage, holiday testing and repair access are essential. Coating both members around the joint may be more robust than coating only the less noble metal, depending on service. Bolted interfaces and sharp support edges need particular attention because assembly can damage the film.

Coatings should not be expected to compensate for inaccessible water traps or uncontrolled material substitutions.

Design Out Wet Crevices and Retained Electrolyte

Promote drainage from beam seats, clamps, overlaps and low points. Avoid shims or backing strips that form narrow stagnant pockets. Seal welding may remove a crevice in some services, but it also changes fabrication, inspection and thermal movement and must be engineered accordingly.

Deposits can hold electrolyte after the bulk tower drains and can create locally different chemistry. Fouling service therefore needs accessible joints and a realistic cleaning method. If a connection cannot be inspected or washed, use a material system with sufficient tolerance for the hidden environment.

Inspection and Failure Diagnosis

Before installation, verify material grades, isolation components, coating condition and joint cleanliness. During assembly, control bolt torque so insulating sleeves and gaskets are not crushed. Confirm that metal shims, lifting wires and temporary braces do not bypass the isolation.

At turnaround, inspect the less noble side near the electrical contact. Rust streaks, grooving around washers, loss at beam seats, undercut beside welds and coating damage concentrated at connections are important clues. Measure remaining thickness at the connection rather than only on broad plate surfaces.

Galvanic corrosion should not be diagnosed from appearance alone. Crevice corrosion, contamination, erosion and general chemical attack can produce similar local damage. Review the material pair, wetting path, area ratio and location before selecting the repair.

Procurement and Drawing Checklist

The RFQ should state process chemistry, concentration range, temperature, contaminants, wet/dry behavior and cleaning conditions. Drawings and bills of material should identify every contacting material, fastener and isolation part. Require details for coating termination, sleeves, washers, drainage and field replacements.

Vendor review should answer four questions: Which member is expected to be anodic? What are the exposed cathode-to-anode areas? Can electrical contact or electrolyte continuity be interrupted reliably? How will the joint be inspected after installation? If these questions are unanswered, a material list alone is not a corrosion design.

Engineering Takeaway

Galvanic corrosion control requires management of the whole connection: material potential, area ratio, electrical continuity, electrolyte access, coating condition and drainage. Small attachments deserve as much attention as the major internal.