Environmental Degradation of Architectural Metals

Architectural metals have long served as both the functional and expressive components of America’s most enduring structures—from monumental civic buildings to remote coastal lighthouses. Yet in the absence of recurring maintenance, these same metals are highly vulnerable to deterioration driven by moisture, pollutants, chlorides, and thermal cycling.

1. Salt Aerosols and Chloride Deposition
Salt-laden air is the defining environmental challenge for coastal metal assemblies. Airborne chlorides:

• Deposit onto metal surfaces
• Absorb atmospheric moisture and form a thin electrolyte film
• Initiate localized pitting
• Undermine coatings from beneath
• Migrate into joints and bolt penetrations

Because salt remains hygroscopic, corrosion continues even during dry weather.
This condition matches what the NPS identifies as the primary cause of metal failure in coastal lighthouses.

At Yaquina, chloride-driven corrosion produced:

• section loss at cast iron interfaces
• accelerated pitting at horizontal surfaces
• widespread deterioration at mechanical attachment points
• deformation in thin ornamental elements

2. Thermal Cycling & Freeze–Thaw Expansion
Rapid temperature changes drive expansion and contraction of both metals and entrapped moisture. Freeze–thaw action exerts significant force, widening cracks and rupturing coatings.

At Yaquina, this manifested as:

• crack propagation radiating from bolt holes
• delamination of original coating systems
• deformation of panels under repetitive thermal stress

The NPS notes that lantern rooms, being both highly exposed and assembled from many attachment interfaces, are particularly vulnerable to this type of cyclical failure.

3. Pollutants, Acid Deposition, and Galvanic Activity
Sulfur dioxide, nitrogen oxides, and industrial particulates produce acidic moisture that destabilizes natural patinas and accelerates oxidation. Where ferrous and non-ferrous metals meet, galvanic corrosion becomes a secondary threat.

At Yaquina, mixed-metal assemblies (cast iron, steel, bronze) showed deterioration patterns consistent with both acidic deposition and galvanic activity, particularly where moisture was trapped against dissimilar metals.

• Blistering or lifting of paint
• Rust “bloom” at fasteners
• Crack lines radiating from bolt holes
• Powdering patina on bronze
• Deformation or misalignment of components
• Staining on adjacent surfaces

These early indicators typically suggest more significant deterioration concealed beneath coatings.

Oxidation — metal reacts with oxygen and moisture
Electrochemical corrosion — salts create an active electrolytic field
Crevice corrosion — moisture trapped in interfaces accelerates attack
Underfilm corrosion — coatings fail from the substrate outward

Initial Documentation and Controlled Disassembly
Upon mobilization, the lantern structure was comprehensively documented, photographed, and cataloged. Disassembly occurred methodically, with every component tagged and sorted according to material, function, and location. This controlled approach ensured traceability and preserved the ability to reconstruct assemblies accurately.

Blasting and Environmental Control
All parts were abrasive-blasted to remove coatings and corrosion products. Components were then stored in an environmentally controlled facility to prevent flash rusting and contamination, an essential step given the chloride content normally present on coastal metals.

Component Evaluation
Each piece was inspected for:

• cracks
• corrosion and pitting
• casting inclusions
• section loss
• dimensional irregularities

The evaluation confirmed advanced environmental deterioration consistent with long-term exposure.

Window Trim
All original window trim was deemed irreparable due to extensive section loss and deformation. Replacements were patterned from the historic profiles and recast to match original geometry and detailing.

Handrails
Existing handrails were both structurally compromised and inconsistent with original design intent. New stainless-steel handrails, fabricated by Allen Architectural Metals, replicated the historic configuration while offering improved durability.

Cracks & Structural Failures in Panels
Cracks were found at nearly every mechanical attachment point, excluding only the parapet walls and window mullions. Notably:

• Roof and deck panels exhibited stress cracks radiating from bolt penetrations.
• Two roof panels displayed full-length cracks discovered after blasting.
• One panel fully separated once surface coatings were removed.

These conditions matched the deterioration pathways documented in the NPS Lighthouse Preservation Handbook, particularly those associated with thermal stress and moisture intrusion.

Weld Repairs and Infill Strategy
Cracks were addressed using a consistent, conservation-aligned methodology:

• V-Groove Preparation
  Each crack was ground into a “V” profile to ensure adequate weld penetration.
• Weld Repair
  Certified personnel welded along the entire crack length using compatible filler materials.
• Surface Restoration
  Welds were ground flush to restore original profiles and maintain dimensional accuracy.
• Material Patching
  Areas of significant section loss were infilled with A36 steel, welded, and finished using the same method.

This approach aligns with preservation engineering best practices—retain original material wherever feasible, reinforce where possible, replace where necessary.

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