14.6 Galvanic Compatibility, Thermal Movement & Masonry Durability

Material Compatibility & Galvanic Corrosion Mechanics

The Electrochemical Mechanism of Galvanic Corrosion

Galvanic corrosion is an electrochemical reaction that occurs when three mandatory conditions are simultaneously satisfied:

1. Two electrochemically dissimilar metals with different electrode potentials are present.
2. An electrical conductive path directly connects the two metals (physical metallic contact).
3. An electrolyte (water, dew, rainfall, or salt-laden coastal moisture) bridges between the metals, enabling ionic transfer.

                         THE GALVANIC SERIES OF METALS

     ANODIC / ACTIVE (Corrodes Sacrificially to Protect Noble Metal)
       ▲   Magnesium & Magnesium Alloys
       │   Zinc (Galvanizing coatings, zinc cladding)
       │   Aluminum (Extruded 6063-T5, sheet cladding)
       │   Cadmium
       │   Carbon Steel, Cast Iron, Structural Steel
       │   Lead / Tin solders
       │   Architectural Bronze / Brass
       │   Copper (16-oz cold-rolled sheet)
       │   Stainless Steel 304 (Active)
       │   Stainless Steel 304 (Passivated)
       │   Stainless Steel 316 (Passivated)
       ▼   Titanium, Gold, Platinum
     CATHODIC / NOBLE (Protected from Electrochemical Attack)

When a galvanic circuit forms, the more active (anodic) metal corrodes at an accelerated rate, sacrificing its electrons to preserve the more noble (cathodic) metal. The farther apart two metals sit on the galvanic series, the greater the electrochemical voltage differential, and the more severe the galvanic corrosion.

The Critical Anode-to-Cathode Surface Area Ratio

The rate of galvanic degradation is governed by the ratio of the anodic surface area to the cathodic surface area:

• Small Anode + Large Cathode = Rapid, Catastrophic Failure: Fastening large copper facade panels (massive noble cathode) with zinc-coated carbon steel screws (small active anode) concentrates galvanic electron draw onto tiny fastener cross-sections. The steel screws will dissolve and shear off within months.
• Large Anode + Small Cathode = Diffuse, Negligible Attack: Fastening large structural steel plates (large active anode) with stainless steel or copper rivets (small noble cathode) spreads the galvanic reaction over a massive surface area, resulting in virtually undetectable, harmless corrosion.

The Copper Wash Runoff Catastrophe

Rainwater cascading over copper roofs, copper through-wall flashings, or bronze decorative trim dissolves microscopic concentrations of cupric ions ($Cu^{2+}$) and acidic copper salts. When this copper-laden runoff water washes downstream over aluminum siding, galvanized steel lintels, or zinc-coated gutters, the copper ions electrochemically plate out onto the active metal. This establishes millions of microscopic localized galvanic couples across the aluminum or zinc surface, causing aggressive pitting, localized perforation, and structural failure within 1 to 3 years.

Mandatory Architectural Rule: Water draining from copper or noble metals must never discharge onto downstream galvanized steel, aluminum, or zinc components. Copper flashings must drain exclusively into copper, stainless steel, or non-metallic PVC/cast-iron drainage systems.

Dielectric Separation Detailing

To integrate dissimilar architectural metals without galvanic corrosion, details must incorporate continuous dielectric breaks:

• Non-Conductive Isolation Gaskets: Continuous 1/8-inch neoprene, EPDM, Teflon, or high-density polyethylene (HDPE) sheets separating dissimilar metal brackets.
• Sleeved Fasteners: Stainless steel bolts connecting aluminum curtain wall clips to structural steel embeds must be fitted with non-conductive neoprene/nylon washers and dielectric sleeved bushings to eliminate metallic thread-to-hole contact.
• Protective Barrier Coatings: Heavy bitumastic coal-tar paints, epoxy barrier primers, or polyisobutylene tapes applied directly to aluminum surfaces before embedding them in contact with wet alkaline concrete or mortar.

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Thermal Movement & Expansion Joint Engineering

Physics of Thermal Expansion: The Expansion Formula

All construction materials expand when heated and contract when cooled. The linear thermal movement of an unconstrained building component is calculated using:

$\Delta L = \alpha \cdot L \cdot \Delta T$

Where:

• $\Delta L$ = Change in member length (inches)
• $\alpha$ = Linear coefficient of thermal expansion ($\text{in}/(\text{in}\cdot^\circ\text{F})$)
• $L$ = Total length of the member (inches)
• $\Delta T$ = Design temperature differential between extreme winter freeze and peak direct summer solar exposure ($^\circ\text{F}$)

Critical Engineering Takeaway: Extruded aluminum expands approximately twice as fast as steel and nearly four times faster than clay brick. In long continuous assemblies (such as aluminum curtain wall mullions or metal parapet copings), thermal expansion will buckle members, crush glazing, and shear anchor fasteners unless engineered movement joints are integrated.

Brick Masonry Expansion Joints vs. Concrete Control Joints

Architects must distinguish between movement joint typologies based on fundamental material physics:

• Clay Brick Masonry — Expansion Joints: Fired clay brick expands reversibly with temperature increases and expands irreversibly over decades as it slowly absorbs moisture from atmospheric humidity. Brick never shrinks back to its initial kiln dimensions. Therefore, brick facades require Expansion Joints—completely open, vertical joints extending through the entire brick wythe (free of mortar, wire ties, and structural rebar). The joint is backed by a compressible closed-cell polyethylene foam backer rod and sealed with an elastomeric silicone sealant. Per Brick Industry Association (BIA) standards:
  • Spaced at maximum 20 to 25 feet on center along straight walls.
  • Positioned within 4 to 8 feet of external building corners (where orthogonal wall expansion causes destructive corner rotation and diagonal cracking).
  • Located at changes in wall height, thickness, or foundation steps.
• Concrete & CMU — Control Joints: Concrete and concrete masonry units (CMU) undergo initial drying shrinkage as chemical hydration cures excess water out of the mix. CMU walls require Control Joints—continuous vertical weakened planes (formed with sash blocks and preformed rubber gaskets) spaced every 20 to 25 feet that induce drying shrinkage cracks to occur along clean, sealed lines rather than cracking through block faces.

Curtain Wall Slip Joints & Coping Details

Aluminum curtain wall vertical mullions must absorb both thermal expansion and structural building movement (interstory drift, concrete slab creep, and live load floor deflection). Vertical mullions incorporate an internal telescoping slip joint (expansion spigot) at each floor level. The mullion is anchored rigidly at one floor slab (fixed dead-load anchor) and secured with slotted holes at the adjacent floor slab (expansion anchor), allowing 1/2-inch to 3/4-inch free vertical slip without transferring structural floor loads into the glass vision panels. Parapet metal copings incorporate continuous 24-gauge concealed splice plates with a 1/4-inch thermal expansion gap every 10 to 12 feet, sealed with non-hardening butyl or silicone.

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Masonry Envelope Durability & Moisture Defense

Moisture Transport & Freeze-Thaw Spalling

Bulk moisture moves through exterior wall assemblies via gravity, wind-driven kinetic momentum, capillary action through microscopic pores, and air pressure differentials across facade fissures. When moisture penetrates porous brick, it creates serious freeze-thaw vulnerabilities:

• Freeze-Thaw Spalling Mechanics: Liquid water expands by approximately 9% in volume upon freezing at $32^\circ\text{F}$ ($0^\circ\text{C}$). If clay brick or stone absorbs moisture past its critical saturation threshold (typically exceeding 80% to 85% of its internal pore volume) and experiences subfreezing temperatures, the hydraulic expansion pressure generated within the pores exceeds the tensile strength of the fired clay matrix. The outer 1/8-inch to 1/2-inch fired face of the brick fractures, flakes off, and disintegrates (spalling), exposing the soft internal brick core to rapid erosion. Freeze-thaw resistance is evaluated under ASTM C67.

Efflorescence & Subflorescence

• Efflorescence: The deposition of a white, powdery crystalline crust of water-soluble inorganic mineral salts (calcium sulfate, sodium sulfate, potassium carbonate) on the exterior face of masonry. Efflorescence requires three concurrent conditions: (1) soluble salts present within the brick, mortar sand, or cement grout; (2) liquid moisture penetrating the wall to dissolve the salts; and (3) an evaporation mechanism that draws the salt solution to the exterior face, where the water evaporates and deposits the crystallized salts. While cosmetically unsightly, efflorescence is harmless and can be scrubbed off with water and mild acids.
• Subflorescence: A destructive failure occurring when moisture evaporates beneath the surface within the microscopic pore network of the brick rather than on the face. As water evaporates internally, growing salt crystals generate massive hydraulic crystallization pressures (exceeding 4,000 psi) that rupture the brick from the inside out, causing severe surface spalling.

The Cavity Wall Rainscreen Assembly & Flashing Detailing

The drained cavity wall (rainscreen assembly) is the most resilient exterior wall typology in modern architecture. It recognizes that no exterior cladding is 100% waterproof and relies on a secondary internal drainage defense:

Exterior                   Clear Cavity            Backup Structure
Brick Veneer               (Drainage & Capillary   (Continuous Insulation,
(Primary Cladding)          Break: Min 2 inches)    WRB, Sheathing, Studs)
   │                              │                      │
   ▼                              ▼                      ▼
  ┌──┐                          ┌───┐                  ┌───┐
  │  │                          │   │                  │   │ ◄── Weather-Resistive Barrier
  │  │                          │   │                  │   │     (WRB laps over flashing)
  │  │                          │   │                  │   │
  │  │                          │   │  8" Min Upturn   │   │
  │  │                          │   │   ┌──────────────┼───┤
  │  │                          │   │   │              │   │
  │  │                          │   │   │ Flashing     └───┘
  │  │                          │   │   │ Membrane
  │  │                          │   └───┘
  │  │                          │       
  │  │                          │       
  │  ├──────────────────────────┴─────────────────────────┐ Shelf Angle
  │  │ Weep Vent (24" o.c.)     ▲ Continuous Flashing     │
  └──┴──────────────────────────┴─────────────────────────┘

1. 2-Inch Clear Cavity: Provides an uninterrupted capillary break preventing water clinging to the back of the brick from bridging across to the backup wall. Mortar netting or high-loft drainage mats placed at the bottom of the cavity catch mortar droppings to ensure drainage pathways remain open.
2. Through-Wall Flashing: High-performance, puncture-resistant sheet membranes—such as 26-gauge Type 304 stainless steel, composite copper-fabric sheets, or 40-mil self-adhering rubberized asphalt—installed continuously above all shelf angles, window heads, foundation sills, and parapets.
3. 8-Inch Vertical Upturn: Flashing must extend up the backup wall a minimum of 8 inches, terminating beneath the self-adhered weather-resistive barrier in a shingle-lap fashion, mechanically sealed with termination bars and continuous sealant.
4. End Dams: At the ends of window lintels, shelf angles, or horizontal wall discontinuities, the flashing must be turned up 2 to 4 inches into a three-sided, watertight end dam. This prevents water travelling along the flashing from discharging off the ends into the interior wall cavity.
5. Weep Holes / Weep Vents: Installed in the vertical head joints of the brick course sitting directly on the flashing at a maximum spacing of 24 inches on center for open head joints or plastic cellular weep inserts (and 16 inches o.c. for cotton wicks). Weeps drain gravity runoff to the outdoors while equalizing cavity pressure with exterior gusts.