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

Solder Touch-ups

Can you go back and solder again after cleaning if you realize a part needs touched up or to add jump rings?

Yes, it is possible.

There are some conditions, of course.

• Clean area back to the bare solder

• Use a small amount of paste flux to avoid lots of clean up of flux. Paste flux will stay pretty much where it is put and does not spread so much as liquid flux does.

• Apply the solder as usual for the area.

• Clean again as for the original soldering, but without metals.

• Rinse in cold water and dry.

• Add the patina immediately

• Polish as usual

Finished!

More complete discussion and information is here.

https://verrier-processes.blogspot.com/2025/11/patina.html

Is Zinc poisonous?

 

Compared to several other metal ions with similar chemical properties, zinc is relatively harmless. Only exposure to high doses has toxic effects, making acute zinc intoxication a rare event. 

Rather than being a toxic metal ion, zinc is an essential trace element. Whereas intoxication by excessive exposure is rare, zinc deficiency is widespread and has a detrimental impact on growth, neuronal development, and immunity, and in severe cases its consequences are lethal. Zinc deficiency caused by malnutrition and foods with low bioavailability, aging, certain diseases, or deregulated homeostasis [equilibrium] is a far more common risk to human health than intoxication.

The Essential Toxin: Impact of Zinc on Human Health, by Laura M. Plum, Lothar Rink, and Hajo Haase

Hazards of Flux Fumes

 

Note:  These health risks are those associated with industrial exposure – frequent and for extended periods.  They do not apply directly to occasional and shorter periods of exposure.

Risks are assessed as acute and chronic.  Acute means immediate reaction.  Chronic means the effects are cumulative and may take years to appear.
 

Composition of Flux

The major components of commercial flux are varying combinations and proportions of zinc chloride (or ammonium chloride), hydrochloric acid, phosphoric acid, citric acid, and hydrobromic acid.  It comes in many forms and many brand names.  It is important to use water soluble flux in stained glass work to enable thorough cleaning.
 
 

 

Zinc Chloride Risks

Zinc chloride inhalation from smoke screen generators or smoke bombs may cause transient cough, sore throat, hoarseness, a metallic taste, and chest pain.  Exposure to high zinc chloride concentrations produces a chemical pneumonitis with marked dyspnoea, a productive cough, fever, chest pain and cyanosis. Pneumothorax and the adult respiratory distress syndrome (ARDS) have been reported. Fatalities have occurred….
http://www.inchem.org/documents/ukpids/ukpids/ukpid86.htm#:~:text=Toxicity%20Zinc%20chloride%20is%20corrosive,anorexia%2C%20fatigue%20and%20weight%20loss.
 

Ammonium Chloride Risks

Exposure to Ammonium Chloride is moderately hazardous, causing irritation, shortness of breath, cough, nausea, and headache. Most exposure is a result of contact with the fume form of this chemical (Ammonium Muriate Fume and Sal Ammoniac Fume), which is a finely divided particulate dispersed in the air. The fumes are capable of causing severe eye irritation. Consistent exposure can cause an asthma-like allergy or affect kidney function.
 
In the event of accidental contact, get immediate medical attention and follow these first aid measures:
·        Skin Contact: Immediately flush skin with water and disinfectant soap and use an emollient on irritated area.
·        Eye Contact: Rinse eye(s) with water for at least 15-20 minutes. Protect unexposed eye.
·        Ingestion: Rinse mouth thoroughly with water. Do NOT induce vomiting.
·        Inhalation: Move to fresh air and administer artificial respiration if needed.
https://www.msdsonline.com/2017/05/05/chemical-spotlight-ammonium-chloride/#:~:text=Exposure%20to%20Ammonium%20Chloride%20is,particulate%20dispersed%20in%20the%20air.
 
 

Hydrochloric Acid Risks

Hydrochloric acid is corrosive to the eyes, skin, and mucous membranes.  Acute (short-term) inhalation exposure may cause eye, nose, and respiratory tract irritation and inflammation and pulmonary edema in humans.  Acute oral exposure may cause corrosion of the mucous membranes, oesophagus, and stomach and dermal contact may produce severe burns, ulceration, and scarring in humans.
 

Acute Effects: 

Hydrochloric acid is corrosive to the eyes, skin, and mucous membranes.  Acute inhalation exposure may cause coughing, hoarseness, inflammation and ulceration of the respiratory tract, chest pain, and pulmonary edema in humans.  Acute oral exposure may cause corrosion of the mucous membranes, oesophagus, and stomach, with nausea, vomiting, and diarrhoea reported in humans.  [Skin] contact may produce severe burns, ulceration, and scarring…. Acute animal tests in rats, mice, and rabbits, have demonstrated hydrochloric acid to have moderate to high acute toxicity from inhalation and moderate acute toxicity from oral exposure.

 

Chronic Effects: 

(Non cancer): Chronic occupational exposure to hydrochloric acid has been reported to cause gastritis, chronic bronchitis, dermatitis, and photosensitization in workers.  Prolonged exposure to low concentrations may also cause dental discoloration and erosion.  Chronic inhalation exposure caused hyperplasia of the nasal mucosa, larynx, and trachea and lesions in the nasal cavity in rats.  The Reference Concentration (RfC) for hydrochloric acid is 0.02 milligrams per cubic meter (mg/m 3) … The RfC is an estimate … of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risk of deleterious noncancer effects during a lifetime.  It is not a direct estimator of risk but rather a reference point to gauge the potential effects.  At exposures increasingly greater than the RfC, the potential for adverse health effects increases.  Lifetime exposure above the RfC does not imply that an adverse health effect would necessarily occur.
https://www.epa.gov/sites/production/files/2016-09/documents/hydrochloric-acid.pdf
 

 
Phosphoric Acid Risks

Phosphoric acid can be very hazardous in the case of skin contact, eye contact, and ingestion. It can also cause irritation if vapours are inhaled. This chemical can cause damage to the skin, eyes, mouth, and respiratory tract. Because of the potential hazards posed by this chemical, it is important to use care when handling it.
 
Repeated or prolonged exposure to phosphoric acid mist can lead to chronic eye irritation, severe skin irritation, or prolonged respiratory tract issues.  In case of accidental exposure to phosphoric acid, follow these first aid guidelines:

Inhalation — Seek fresh air and immediate medical attention.

Eye Contact — Remove contact lenses if present. Immediately flush eyes with plenty of water for at least 15 minutes and get medical attention.

Skin Contact — Wash skin with soap and water. Cover any irritated skin with an emollient. Seek medical attention. 

Ingestion — Do NOT induce vomiting. Never give anything by mouth to an unconscious person. Seek medical attention if any adverse health symptoms occur.
https://www.msdsonline.com/2015/06/17/phosphoric-acid-safety-tips/
 
  

Citric Acid

Citric acid can be a minor skin irritant, causing itchy skin and even minor burns to those that are sensitive to it. Hands should be washed immediately if citric acid comes into contact with bare skin. Protective gloves should be worn during handling to avoid any accidental contact. The acid can also irritate the walls of the throat if ingested or burn the lining of your stomach if ingested in large quantities.
 
Eye Irritation - Citric acid is a severe eye irritant. Accidental contact with the eyes can occur … by touching the eyes after the acid has contacted the fingertips. …  Protective eyewear should be worn when working with citric acid under laboratory conditions. Eyes should be flushed with water immediately if they happen to come in contact with the acid.
https://sciencing.com/hazards-citric-acid-8165149.html

Remember that this irritation is equivalent to squirting lemon juice into your eye.  It is not a chronic risk.
 

Hydrobromic Acid (HBr)

Hydrobromic acid and hydrogen bromide gas are highly corrosive substances that can cause severe burns upon contact with all body tissues. The aqueous acid and gas are strong eye irritants and [tear producers]. Contact of concentrated hydrobromic acid or concentrated HBr vapor with the eyes may cause severe injury, resulting in permanent impairment of vision and possible blindness. Skin contact with the acid or HBr gas can produce severe burns. Ingestion can lead to severe burns of the mouth, throat, and gastrointestinal system and can be fatal. Inhalation of HBr gas can cause extreme irritation and injury to the upper respiratory tract and lungs, and exposure to high concentrations may cause death. … Hydrogen bromide has not been found to be carcinogenic or to show reproductive or developmental toxicity in humans.
https://web.stanford.edu/dept/EHS/cgi-bin/lcst/lcss/lcss47.html#:~:text=The%20aqueous%20acid%20and%20gas,gas%20can%20produce%20severe%20burns.
 
 
 

Precautions to be taken by glass workers

The risks outlined above are related to dealing with concentrated amounts of the materials in industrial settings.  Risk levels are much reduced in the craft setting.  The risks are mainly centred on breathing and eye exposure. 
 
It is important to wear masks of the quality that will deal with inorganic fumes.  In Europe these are designated as FFP2.  In general masks rated at N95, P95, or R95 are the level required for filtering out 95% of particles that are larger than 3microns.  Dust masks are not sufficient protection. 
 

Usually overlooked is eye protection.  The risks outlined here show that risks to eyes are equal to - or in some cases greater than – respiratory ones.  Eye protection is as important as breathing filters.  To fully protect the eyes, goggles of some sort are the minimum requirement.  Glasses will not be sufficient to prevent fumes reaching eyes.

 
For a “one stop solution” a full-face mask may be the simplest solution.  The filters on these are long lasting and replaceable.  They can be put on as one unit and are available in various face sizes.
 

At soldering temperatures, there are no lead or tin fumes created.  It is the fumes from the flux that are the risks in soldering.  These risks are small and can be dealt with by using adequate ventilation, masks, and goggles.

Solder Touch-ups

 

Can you go back and solder again after cleaning if you realize a part needs touched up or to add jump rings?

Yes, it is possible.

There are some conditions, of course.

• Clean the area back to the bare solder.
• Use a small amount of paste flux to avoid lots of clean up of flux. Paste flux will stay pretty much where it is put and does not spread so much as liquid flux does.
• Apply the solder as usual for the area.
• Clean again as for the original soldering, but without metals.
• Rinse in cold water and pat dry.
• Add the patina immediately.
• Polish as usual.

Finished!

More complete discussion and information is here.

Copper foil – to grind or not?

 

This is a question which always has two entrenched sides.  One for always doing it; one for grinding only when necessary for shape.

Some facts:

• The glue on foil is an impact adhesive.

• Impact adhesives stick most securely to smooth surfaces, and resins stick well to roughened surfaces.

• The adhesive is severely weakened by the heat of soldering.

It is a misconception that the adhesive on the back of copper foil tape is a structural element of panels. The adhesive is a temporary means of fixing the foil in place. The heat of soldering seriously degrades the adhesive. 

Therefore, the adhesive can only be a temporary measure to hold the foil in place while the came-like solder structure is created.

The structural element of copper foil panels is in the fin of solder connecting the beads on each side. This forms a came made of solder instead of lead. To be sound, there needs to be a domed bead of solder on each side with solder connecting them.

It doesn't matter in the long term whether the foil sticks well to the glass or not. Yes, it is easier to work if it does stick, but the strength remains in the came formed of solder.

Not every score and break is perfect, and often grinding is required to fit the pieces together. But when the score is exactly what is needed, there is no reason to grind it. The adhesive will stick better to the smooth than the roughened surface.

My conclusion is that it does not matter which side of the grinding issue you support. But why go to extra effort of grinding when there is no noticeable effect in the end? 

It is the came that is created by the fin of solder joining the beads on each side that provides the strength in copper foil panels.

When to Apply Patina?

 Is patina able to be applied days later or does it need to be applied immediately?

The best method is to clean the solder with non-metallic mild abrasives. If the bare solder is left for a while, the solder needs to be cleaned again, as oxidisation begins immediately. Any oxidisation inhibits the patina reaction and provides an uneven finish.

Once the solder has been cleaned with non-metallic scrubbing, wash in cold water, dry at once, and immediately apply the patina.

Can you leave the solder silver with polish/wax until you decide which color and go back later? 

It may be possible, but it is not a complete protection against oxidisation. It will, of course, reduce the amount of oxidisation and make the clean up a little easier. But, any polish needs to be thoroughly cleaned off and the solder scrubbed clean, washed, and dried before applying patina. So it may be most simple to cover the soldered panel with plastic cling film (Saran wrap?), or in a sealed plastic bag until it is time to clean the solder and patina.

In both cases the important elements are to

• clean back to bare solder,

• clean with water,

• dry immediately and

• apply the patina acid right away.