Crevice and Galvanic Corrosion in Packing Support Grid Connections
Packing support grids operate in one of the most continuously wetted regions of a packed tower. Liquid drains from the bed, contacts beams and fasteners, and may remain trapped on support ledges after shutdown.
The broad surfaces of a support grid may appear sound while serious corrosion develops inside clamps, beneath washers or between overlapping panels.
These concealed locations combine stagnant liquid, narrow crevices and sometimes dissimilar metals. Material selection based only on the main grid plate can therefore miss the most vulnerable parts of the assembly.
Why Crevices Are More Aggressive
A narrow gap between two surfaces restricts liquid exchange with the surrounding tower.
Inside the crevice, the chemistry can become different from the bulk process liquid. Depending on the service, local conditions may include:
Reduced oxygen
Changed pH
Concentrated chlorides
Accumulated solids
Higher contaminant concentration
Persistent moisture after shutdown
Passive corrosion-resistant alloys may lose their protective condition inside this stagnant environment.
Crevice corrosion can progress beneath a washer or clamp while the visible surface remains relatively clean.
Common Crevice Locations
High-risk areas include:
Grid panels overlapping each other
Beam splice plates
Bolted joints
Washers and bolt heads
Ledge clamps
Beam ends resting on support rings
Screen attachments
Intermittent welds
Shims
Deposits trapped against horizontal surfaces
A joint that is convenient to fabricate may unintentionally create a permanent liquid trap.
The connection design should be reviewed from the viewpoint of drainage and inspection, not only structural assembly.
Galvanic Corrosion Requires More Than Two Metals
Galvanic corrosion can occur when dissimilar conductive materials are electrically connected in the presence of an electrolyte.
A common example is a carbon-steel support attachment connected to a stainless-steel grid in a wet process environment.
The severity depends on:
Relative electrochemical behavior of the materials
Area ratio between the materials
Conductivity of the liquid
Temperature
Duration of wetting
Coating condition
Connection geometry
A small, less-resistant component connected to a much larger corrosion-resistant area can experience rapid attack.
Bolts, washers, clamps and weld filler should therefore be included in the material review.
“Stainless Steel” Is Not a Complete Specification
Different stainless-steel grades have different resistance to chlorides, acids and reducing environments.
Even when the correct grade is selected, fabrication can reduce corrosion resistance through:
Carbon-steel contamination
Heat tint
Poor weld cleaning
Embedded grinding particles
Rough cut edges
Incomplete passivation
Incorrect filler metal
The entire connection should receive the specified cleaning and surface treatment. A high-grade grid connected with unsuitable fasteners remains a weak system.
Corrosion Allowance Does Not Solve Every Problem
Adding thickness can extend service life under relatively uniform corrosion. It may not protect a clamp or bolt from localized attack.
A heavily corroded fastener can lose its function before the main beam shows significant thickness loss. Similarly, a crack-like crevice can penetrate a component without producing large visible metal loss.
Corrosion allowance should therefore be combined with:
Compatible material selection
Drainable geometry
Accessible inspection
Suitable surface treatment
Replacement planning
Control of dissimilar-metal contact
Localized corrosion requires prevention at the detail level.
Design Connections That Drain
Horizontal pockets should be minimized. Where surfaces must overlap, the orientation should allow liquid to leave during operation and shutdown.
Drainage review should consider the installed position inside the tower. A connection that drains on the shop floor may trap liquid when mounted vertically or against a curved vessel wall.
Deposits can also block nominal drain gaps. Openings should remain functional under the expected fouling conditions.
Seal welding may eliminate some crevices, but it must not be specified automatically. Incomplete sealing can create an even less accessible crevice, while welding can distort thin components or damage corrosion-resistant surfaces.
Use Electrical Isolation Carefully
Nonconductive washers, sleeves or pads can sometimes interrupt galvanic contact between dissimilar metals.
Isolation components must be evaluated for:
Chemical compatibility
Temperature resistance
Compressive strength
Creep
Installation damage
Required electrical continuity
Fire behavior
Long-term wetting
Partial isolation can be worse than a properly engineered metallic connection if liquid enters the interface and remains trapped.
Electrical isolation should therefore be a documented system, not an improvised plastic washer added during installation.
Consider the Vessel Attachment Material
The grid material may differ from the support ring, wall clips or vessel shell.
In a clad or lined tower, the connection can bridge the corrosion-resistant barrier and expose a base-metal attachment. The lining termination around the support must be sealed and inspectable.
Questions to resolve include:
Is the support ring solid alloy, clad or coated?
What material are the clips and welds?
Can liquid reach the base metal?
Does the clamp damage the lining?
Is the attachment continuously wet?
Can the area be repaired during shutdown?
The vessel and internals material specifications should be reviewed together.
Prioritize Connections During Inspection
Broad, easily visible surfaces often receive attention while hidden joints are overlooked.
Shutdown inspection should focus on:
Bolt shanks beneath washers
Clamp contact surfaces
Panel overlaps
Beam-end bearing areas
Weld toes
Support-ring interfaces
Deposit-covered joints
Areas showing rust streaks or discoloration
Removing selected fasteners may reveal damage not visible externally. Replacement should use the specified material rather than a readily available substitute.
Wall-thickness measurement is useful for accessible beams, but small connection components may require direct examination or replacement.
Investigate the Source of Corrosion Products
Rust-colored deposits do not always prove that the visible stainless-steel grid is failing. The source may be:
Carbon-steel installation tools
An incompatible bolt
Grinding contamination
A corroding support ring
Upstream equipment
Temporary construction hardware
Cleaning the stain without identifying the source allows the problem to return.
The pattern of corrosion products often helps locate the origin. Streaking below a clamp or beam end deserves particular attention.
Procurement Checklist
The support-grid specification should define:
Materials for panels and beams
Bolt, nut and washer materials
Clamp material
Weld filler
Surface finish
Pickling or passivation requirements
Corrosion allowance
Lining-interface details
Isolation components
Inspection access
Replacement fasteners
“Grid material: SS316L” is incomplete if the remaining connection components are not specified.