Examining Galvanic and Deposition Corrosion

GALVANIC CORROSION · ALUMINUM · COPPER · ARCHITECTURAL MATERIALS

Copper and aluminum can create a significant corrosion problem when they are placed in electrical contact and exposed to moisture—but direct metal-to-metal contact is not the only way copper can accelerate attack on aluminum.

This architectural case study examines two related mechanisms: galvanic corrosion caused by direct contact between copper and aluminum, and deposition corrosion that can occur when copper-containing runoff reaches an aluminum surface.

Cross-section through a dress cap. The aluminum angle is clad with copper sheet, with white corrosion products visible on the aluminum.

Galvanic Corrosion Between Copper and Aluminum

Corrosion professionals are familiar with the galvanic series, which ranks metals and alloys according to their electrochemical behavior in a given environment. When sufficiently dissimilar metals are electrically connected in the presence of an electrolyte, the more active member of the pair can experience accelerated corrosion.

In this case, the problem developed on a multi-story public building in Canada where copper architectural elements were combined with aluminum glazing components.

How the glazing system was constructed

Large panes of glass were secured to the building’s metal framework with aluminum pressure plates approximately 6 cm wide. Elastomer strips between the plates and the glass provided the sealing surface, while screws through the pressure plates held each pane in place.

Decorative “dress caps” concealed the pressure plates and screws. These caps consisted of aluminum angle extrusions approximately 3.5 × 3.5 cm, with copper sheet mechanically wrapped around their longitudinal edges.

That construction placed copper and aluminum in direct contact. With moisture and rainwater providing an electrolyte, the combination created conditions favorable to galvanic attack of the aluminum.

The Corrosion Eventually Affected Building Performance

Significant corrosion developed on the aluminum components where the copper and aluminum were exposed to moisture. White aluminum corrosion products became visible around the affected areas.

According to the case study, the deterioration eventually contributed to leakage through the curtain-wall system and resulted in replacement of the glazing, pressure plates, and dress caps.

Separating the metals in the replacement design

The replacement design retained the desired copper appearance while changing the material beneath it. Instead of copper-clad aluminum angles, the new dress caps used glass fiber-reinforced plastic angles clad with copper sheet.

Using a nonmetallic substrate removed the direct copper-to-aluminum galvanic couple at the dress cap. However, that alone did not eliminate every possible pathway for copper-related attack.

Close-up of the dress cap showing copper cladding and corrosion products formed on the underlying aluminum.

Deposition Corrosion: When the Metals Are Not Directly Touching

Copper can also contribute to corrosion of aluminum without the two structural components being in direct metal-to-metal contact.

Copper roofs, gutters, flashing, and architectural trim undergo some atmospheric corrosion. Water flowing across those surfaces can carry dissolved copper species. If that runoff reaches an aluminum component, copper may deposit onto the aluminum surface while aluminum dissolves.

This process is commonly referred to as deposition corrosion.

The electrochemical process

In generalized form, deposition corrosion can be represented by a cathodic metal-deposition reaction coupled with anodic dissolution of another metal:

For the aluminum-copper system discussed in the article, the combined reaction was represented as:

In practical terms, copper deposits onto the aluminum while aluminum is oxidized and dissolves. The local copper deposits can then create additional cathodic sites on the aluminum surface.

A Simple Experiment Demonstrates the Mechanism

To demonstrate deposition corrosion, aluminum strips approximately 1.25 mm thick were partially immersed in a copper sulfate solution at ambient temperature.

After several weeks, metallic copper had deposited onto the exposed aluminum. The strips also exhibited substantial thinning and perforation.

Two aluminum strips after several weeks of copper sulfate exposure show dark copper deposits, severe thinning, and perforation. The center strip was retained for comparison.

Why Drainage Design Matters in Mixed-Metal Systems

The building example highlights an important distinction. Preventing direct contact between dissimilar metals can address one galvanic-corrosion pathway, but it does not necessarily prevent contamination from one metal from reaching another.

Where copper roofs, trim, gutters, or flashing are installed above aluminum windows, pressure plates, door frames, or other components, the path taken by rainwater and runoff deserves careful consideration.

Practical Considerations for Copper and Aluminum on Buildings

The Same Chemistry Can Be Useful in Another Context

The underlying electrochemical reaction is not always undesirable. A related process has historically been used in extractive metallurgy to recover copper from copper-bearing solutions.

In one application, acidic solutions percolate through crushed copper-containing ore and dissolve copper. The resulting copper-bearing solution is then brought into contact with scrap iron.

Copper is deposited while iron enters solution:

This process is often referred to as cementation metallurgy. It illustrates that the same electrochemical tendencies responsible for damaging corrosion can also be deliberately used in a controlled industrial process.

Think Beyond Direct Metal Contact

The most obvious lesson from this case is that direct coupling of copper and aluminum on a wet building exterior can create a substantial galvanic-corrosion risk.

The less obvious lesson may be equally important: separating the structural metals does not necessarily solve the problem if runoff from the more noble metal can still carry dissolved species onto the more active one. Material selection, electrical isolation, drainage, surface protection, and environmental exposure must be considered together.

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