Does copper solder flux go bad?

Copper solder flux is an essential component for soldering copper pipes and fittings. It helps clean and prepare the copper surfaces for the solder to adhere properly. However, solder flux has a limited shelf life and can go bad over time. Knowing the shelf life of solder flux and signs of expired flux is important for maintaining effective solder connections.

Table of Contents

What is solder flux?

Solder flux is a chemical cleaning agent that is applied to the copper pipe and fittings prior to soldering. It serves several important functions:

Removes oxidation and surface contaminants from the copper that can prevent proper solder adhesion.

Applies a protective barrier to prevent re-oxidation during soldering.
Improves heat transfer between the copper and the solder.
Promotes even melting and flow of the solder.

The active ingredients in solder flux vary but often include some combination of rosin, organic acids, activators like zinc chloride, and solvents like isopropyl alcohol. These work together to clean and flux the copper surfaces being joined.

Does solder flux expire?

Yes, solder flux has a limited shelf life and can expire over time. Most solder flux will last 1-2 years when properly stored before opening. Once opened, the shelf life is reduced to about 6 months.

There are two main reasons solder flux expires:

The solvents and activators evaporate – As the flux sits, the solvents like alcohol start evaporating, thickening the flux. Activators also degrade over time. This makes the flux less effective at cleaning and fluxing.
Rosin solidifies – Rosin, a key ingredient in many fluxes, can oxidize and solidify over time. This can clog the flux and prevent proper flow and application.

Expired solder flux often becomes thick, gummy, and ineffective. It may clog the flux brush and fail to adequately clean and flux copper surfaces.

How to prolong shelf life

Proper storage is key to prolonging the shelf life of solder flux:

Store in a cool, dry place away from heat and sunlight which can degrade fluxes faster.
Keep the flux container tightly sealed when not in use.

Only open the container when ready to use the flux.
Close the lid tightly after each use.
Store the flux upright to prevent leaks.

Buying flux in smaller containers will also help use it up faster before it expires. Larger volumes take longer to use up.

Signs of expired solder flux

Here are some signs that indicate solder flux may be expired or going bad:

Thickened consistency – Fresh flux is usually a thin liquid that flows easily. Expired flux often becomes thickened or gummy.

Separation – The ingredients may separate with the rosin settling to the bottom.
Crystal formation – Crystals or globs of hardened rosin may form.
Excessive bubbling – When applied to the copper fitting, it bubbles and boils excessively.

Discolored – The flux may darken from its original color.
Weak, burnt odor – A strong, pungent odor indicates evaporation of the solvents.
Poor solder flow – The solder doesn’t adhere and flow onto the copper properly.

Dangers of using expired flux

While not necessarily dangerous in itself, using expired solder flux can lead to problems:

Poor solder bonds – Without proper fluxing action, the solder may not adhere correctly to the copper.
Leakage risks – Weak bonds can lead to leaks in the plumbing system.

Blockages – Outdated thickened flux may not wash away properly and clog plumbing.
Corrosion – Inadequate protection from oxidation may allow mineral buildup and corrosion.
Costly repairs – Leaks and blockages mean expensive repairs may be needed if old flux is used.

To avoid these risks, it’s recommended to always use fresh, quality solder flux within its expiration date. Discard and replace any expired or questionable flux.

How to test old solder flux

If you are unsure whether an existing flux is still good, there are ways to test it:

Check consistency – Does it still flow smoothly or is it thick and gummy?

Smell – Strong burnt odors likely mean solvents have evaporated.
Color – Darkening or separation may be signs of degradation.
Application test – Apply a small amount to scrap copper. Does it bubble excessively or impede solder flow?

Adhesion test – Try soldering scrap copper. Does the solder adhere smoothly and strongly or easily flake off?

Testing on scrap copper pieces first allows you to evaluate flux quality without compromising actual plumbing connections.

Removing expired solder flux

If you determine solder flux is expired, follow these steps:

Wear gloves and eye protection when handling.
Remove any existing flux residue from soldering tools with isopropyl alcohol.
Seal the container if still partially full. Discard in regular household trash.

Clean empty containers before disposal to remove any remaining residue.
Wash hands after handling expired flux.
Purchase new solder flux to replace the expired product.

Never pour flux down the drain. Follow local regulations for proper disposal of hazardous household chemicals.

Types of solder flux

There are a few main types of solder flux to be aware of:

Rosin flux

– Most common type of flux for copper plumbing.
– Made from rosin, a natural pine tree resin.
– Provides good flow and adhesion at low temperatures.
– Lower toxicity and irritant smoke compared to acid fluxes.
– Shelf life around 1-2 years when unopened.

Acid flux

– Contains zinc chloride or ammonium chloride.
– More effective for oxidized or contaminated copper.
– Corrosive to pipes so requires thorough washing.
– Short shelf life of 6 months or less.

No-clean flux

– Special flux that doesn’t require washing after soldering.
– Convenient to use.
– More expensive than traditional fluxes.
– Shelf life may be under 1 year.

Organic flux

– Made from organic acids like oleic, stearic, or other fatty acids.
– Low toxicity and smoke.
– Biodegradable and environmentally friendly.
– Very short shelf life of less than 6 months.

Buying new solder flux

When purchasing new solder flux, look for:

Reputable brand name known for quality.
Flux intended for copper plumbing, not electronics.

SAE spec FF-F-507 compliant formula.
Nice fluid consistency in the container.
No evidence of separation, crystal formation, or thickening.

Unexpired use-by or best-by date.
Sealed container without leaks.
Stored properly away from heat and sunlight.

A high-quality rosin or no-clean flux designed for copper plumbing will provide the best results. Only buy as much as you can use up within 6-12 months.

Proper use of solder flux

To ensure you get effective use of solder flux:

Shake or stir flux before each use to mix the ingredients.

Pour a small amount into a separate flux applicator cup.
Apply only as much as needed onto surfaces with a flux brush.
Apply flux just prior to soldering when it’s still wet and active.

Apply heat evenly to allow flux to vaporize and solder to flow.
Remove any residual flux after soldering is complete.
Keep flux sealed when not in use.

Proper flux application and soldering technique will allow even melted solder bonds less likely to fail or leak.

Solder flux alternatives

While solder flux is highly recommended, there are some potential alternatives:

Phosphoric acid – Can remove oxidation but provides no ongoing barrier during soldering.

Wire brush – Can mechanically remove oxides but is labor-intensive.
Sandpaper – Also removes oxidation but alters surface smoothness.
Baking soda paste – Mixed with water, it creates a mildly abrasive scrub.

Borax solution – Can dissolve some copper oxides.

However, these options fall short of providing the full benefits of a proper solder flux. They are best avoided for critical plumbing joints.

Storing solder flux long term

To store solder flux for extended periods:

Keep unused flux in its original sealed container.
Place containers in air-tight plastic bags or bins.
Store in a cool, dry place like a basement or pantry away from heat and moisture.

Avoid storage in hot locations like attics or garages.
Label containers with date of purchase and use-by date.
Write the expiration date on a calendar to prompt replacement.

Plan annual checking and rotation of stock.

With optimal storage conditions and careful stock rotation, solder flux may remain usable for up to 2 years when kept sealed. Opened flux should be used up within 6 months.

Disposing of solder flux

To properly dispose of solder flux:

Allow any residual flux left in containers to fully dry and harden.
Check local municipal household hazardous waste guidelines.
Take empty containers to approved disposal or recycling centers.

Clearly label containers “empty solder flux” if partially full.
Never pour flux or wash water down drains.
Wear gloves and wash hands after handling.

Consider donating unexpired flux to others who can use it up.

Proper flux disposal prevents contamination risks while allowing for safe reuse or recycling of containers.

Conclusion

Like any chemical product, solder flux has a limited useful life. Exposure to air and improper storage leads to evaporation, oxidation, and separation that render it ineffective. Watch for signs of aging like smell, consistency changes, poor performance, and crystal formation. Test questionable flux on scrap copper before use. Follow guidelines for safe storage, use, and disposal of flux. Replace expired flux to ensure reliable solder bonds on copper plumbing systems. With reasonable care and caution, quality solder flux can be an indispensable tool for the copper pipe fitter.