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.

Other posts on lead corrosion:

Varieties of lead corrosion

https://glasstips.blogspot.com/2017/10/lead-corrosion.html

Health Risks from Lead/Tin Solder

"Lead solder – is an open window or an open door in my greenhouse studio required, and in either case do I need some sort of extractor device."

The fumes are not from the lead or solder, they are from the flux.  Eye protection is important.  Gentle ventilation is sufficient.  A small fan to create a current of air to the outside will do of there is no other ventilation.

“If you give items away/sell them. What safety advice do you give to people.”

Lead is only dangerous if consumed.  It is not absorbed through the skin, only by transfer from the skin to the mouth.   Once the piece is handled, wash hands before eating, drinking or touching eyes.

“Is lead free solder a better option for small sculptures, which are not jewellery.”

The risks of lead poisoning are greatly exaggerated. If the pieces are not worn, the risks are minimal. If the pieces are not handled, there should be no transfer to the digestive system. Lead free solder is more difficult to work than a lead/tin alloy.

“What gloves are good to prevent cuts. I’m presuming disposable gloves are best for soldering.”

Skin tight cut resistant gloves are best, but are uncomfortable for long use periods. They can be used for soldering too. Honestly, I don’t use gloves for anything, and my blood lead levels have been below the minimum for the 30 years I have been working with lead and solder.

These are my opinons, of course.

The best guidance on health and safety for stained glass workers, is Greg Rawls’ website.  His career was in industrial health and safety until retirement a couple of years ago. He gives sensible, scientifically researched advice, which is not alarmist.