Why New SS316L Tower Internals Can Show Rust After Fabrication
A stainless-steel liquid distributor or support grid may be manufactured from verified SS316L and still show brown staining or early corrosion after fabrication.
Correct alloy grade is important, but material certificates and PMI do not confirm that the finished surface is clean and properly treated.
Common Causes of Early Rust Staining
Possible causes include:
- Carbon-steel grinding dust
- Carbon-steel wire brushes
- Shared fabrication tables
- Iron-contaminated lifting tools
- Weld spatter
- Heat tint
- Incomplete cleaning
- Chloride-containing shop contamination
- Damaged protective film
- Contaminated packaging
Free iron transferred onto stainless steel can rust even when the base alloy is correct.
Why Weld Heat Tint Matters
Welding produces colored oxide around the heat-affected zone. Beneath severe heat tint, the surface may have reduced corrosion resistance compared with properly cleaned stainless steel.
The required post-weld treatment may include:
- Mechanical cleaning
- Descaling
- Pickling
- Passivation
- Thorough rinsing and drying
The exact procedure depends on the project specification and fabrication condition.
Pickling and Passivation Are Not Identical
Pickling is used to remove scale, heat tint and affected surface material.
Passivation supports formation of a clean passive surface after contaminants are removed.
Passivation should not be assumed to remove heavy weld scale or embedded contamination by itself.
Why This Matters More for Distributors
Tower internals contain:
- Small calibrated holes
- Crevices
- Bolted joints
- Thin sheet
- Intermittently wetted surfaces
- Deposit-prone troughs
Rust products or surface attack can change outlet flow before major structural damage becomes visible.
Fabrication Controls
A suitable stainless fabrication area should control:
- Dedicated tools
- Grinding media
- Wire brushes
- Work tables
- Lifting contact
- Identification and traceability
- Weld-cleaning procedure
- Chemical-treatment records
- Final washing
- Packaging material
Inspection Before Shipment
Check:
- Material certificates
- PMI where specified
- Weld condition
- Heat-tint removal
- Surface contamination
- Outlet cleanliness
- Pickling/passivation records
- Rinse-water condition where required
- Dry packaging
- Separation from carbon-steel parts