Galvanic Corrosion

by Hugo Baillargeon

In the context of stained glass restoration — specifically regarding the brown spots on lead and solder joints — the degradation is often a combination of chemical corrosion (acid attack) and galvanic corrosion.

Here is the technical breakdown:

1. The Mechanism

Galvanic corrosion (or bimetallic corrosion) occurs when two dissimilar metals are in physical contact in the presence of an electrolyte (moisture).

* The Cell: The lead came (the "matrix") and the solder (usually a tin/lead alloy) act as the two electrodes. While they are similar in composition, their microstructures and local potentials can vary slightly.

* The Electrolyte: This is the critical factor. Leftover flux residue (often acidic, such as zinc chloride) is hygroscopic, meaning it absorbs moisture from the air. This moisture creates the electrolyte required to complete the circuit.

* The Reaction: The electrolyte facilitates the movement of ions. If the alloy is not homogeneous—or if impurities from the flux are present—one area acts as the anode (where metal oxidizes and corrodes) and another as the cathode. The brown, crusty deposits are the resulting metallic oxides, chlorides, and salts forming at the anode site.

2. Why Flux is the Catalyst

In stained glass, the lead/solder joint is not just a mechanical bond; it is a chemical interface. When you use an active flux:

* Chemical Attack: The acid in the flux aggressively etches the lead/solder to remove oxidation and promote bonding.

* Persistent Electrolyte: If the flux is not thoroughly neutralized and washed away, it remains trapped in the tiny crevices between the glass and the came (the "lead-glass-solder" junction). Because the flux is hygroscopic, it pulls water into these crevices indefinitely.

* The Galvanic Bridge: The trapped, moisture-laden flux acts as a permanent electrolyte bridge, constantly "feeding" the corrosion process. This is why you see the dark, localized brown spots—these are the "hotspots" where the electrolyte concentration and chemical activity were highest.

3. The Engineering Perspective

From a materials science standpoint, this is a localized pitting corrosion. Because the lead is relatively soft and reactive, the galvanic potential difference doesn't need to be massive to cause significant damage over 10–15 years. The "lead net" might look fine overall, but the joints—the most critical structural points—have been compromised by the steady, years-long electrochemical leaching caused by those trapped contaminants.

Essentially, by failing to neutralize the flux, the previous restorer inadvertently built a tiny, slow-acting battery directly onto the window's frame.

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Thursday, 20 August 2026