In the last few weeks, I have been clearing and organizing my garage workshop, and finding all sorts of interesting bits and bobs I had long forgotten, I came across old tins of paint and varnish that had dried out and also some plumbing tubs one of which was my plumbing flux.
I checked the container and found an expiry date on it which was long overdue and this made me wonder if plumbing flux expires or goes off?
I did some investigating and asked some questions here’s what I found out:
The performance of plumbing flux is not guaranteed beyond the shelf life of the product, it will remain usable beyond that date if stored in a sealed container and is protected from temperature extremes. The flux can degrade due to evaporation and may thicken and not work correctly as the product ages.
I also found out more about how flux works and why plumbing flux goes bad, I talked to a well-known flux manufacturer and got a direct answer which you can read at the end of this post, let’s have a look in more detail to understand:
What is Plumbing Flux?
Plumbing Flux is a mixture of chemicals that have two main ingredients, a wetting agent and an activator.
Flux usually supplied as a greasy paste similar in texture to vaseline and can be purchased online or in your local hardware store.
Plumbing flux is applied to a soldered joint and is used to create a ‘wet’ joint between two surfaces.
A Good flux will:
- Spread easily over the surface of the components to be soldered
- Clean the surfaces to be joined
- Keep the surfaces oxide-free
- Help the solder flow over the joint
Some forms of Flux are included in the core of the solder wire and these can be used for small intricate joints on electrical components like circuit boards.
How to use Plumbing Flux
For plumbing joints like those found when joining copper pipe, flux is applied separately to each part that will form the joint.
For copper pipework cleaning the areas to be joined with wire wool, this will remove any old dirt and expose the bare metal making for a good clean ‘wet’ solder joint.
The parts are coated with flux before assembly, in some brands like La-Co there is a brush supplied inside the lid of the container for easy application.
If not then using an old 1/2” paintbrush works well and can apply the flux evenly over the areas to be joined.
When the soldering has been completed its a good idea to wipe the joined pipework down with an old damp cloth to remove the residue and boiled flux,
By removing any old flux you can stop any discoloration or staining of the copper metal as the flux will degrade and go black or green with time.
How does flux work?
A solder joint bonds two compatible metals with a soft metal bonding agent called solder, this requires compatible materials and clean metal surfaces that are free of dirt and oxides, that can prevent a good bond.
Flux is applied with a brush to the parts to be joined and the areas that will form the bond.
When it is melted, the applied flux spreads out over the metal surfaces to be joined and cleans off particles of dirt and creates a barrier to the atmosphere which prevents any oxide layers from forming.
Surface oxidization takes place constantly when the surface is exposed to any oxygen in the atmosphere but the process increases when soldering, due to the higher temperatures.
The acid within the flux cleans metal surfaces and reacts with the oxide layer, this leaves the surface primed and enables a good joint.
This and allows the solder to flow between the surfaces and creates a good, clean joint also known as “wet joint”. If the flux does not flow it may form a bad joint often called a ‘Dry Joint’
The ingredients that make up the wetting agent of the flux are called the surfactant and allows the flux and solder to flow over the surfaces and create a good bond that will not only be strong it will also be conductive.
The acidic compound within the Flux and is also known as the activator, this part of the compound converts the oxides into soluble metal salts and cleans the surfaces of the joint and removes the oxides.
The effect of the flux is very similar to the way washing up liquid reacts in the washing up bowl as it dissolves greasy food when washing up dirty pots and pans.
Basically the wetting agent mixes with the dirt and any oils on the metal surface, dissolving them making them easier to remove
Does Plumbing flux go bad?
So to the question in hand about the shelf life of plumbing flux, my tub was over 2 years beyond the expiry date.
I had used the flux several times over the past few years without any issues that I could see, the joints I had soldered were working fine and I had no leaks, which didn’t help answer the question, so I contacted the manufacturer La-Co to ask if they could help.
They were really helpful and came back to me within a couple of hours explaining the reason that their plumbing flux products carry an expiry date, here’s their response:
‘The performance of LA-CO flux is not guaranteed beyond the shelf life of the product, but it will typically remain usable for some time beyond that date if it has been kept stored in a sealed container and protected from temperature extremes. The shelf life is placed on the product due to changes that can happen over time, such as evaporation or degradation of certain ingredients. The purpose of the flux is to help the solder flow into the joint and bond with the metal. These properties can become compromised as the product ages. The product may also thicken over time, making it difficult to apply the flux to the solder area.’
How long does flux last?
Following up on the response from LA-CO, I checked the typical shelf life of plumbing flux.
The shelf life of LA-CO flux is 3 years from the date of shipment from their warehouse, anything after that and the ingredients can be compromised.
Reasons for plumbing flux to go bad include:
- Leaving the lid off the tub exposing it to air
- Contamination from any previous soldering tasks
- Exposure to high or low temperature
- Evaporation of ingredients
You can tell if plumbing flux has gone off if it is has become thick and hard to spread using a brush. There is any discoloration, Flux is usually opaque white similar to vaseline, and typically becomes brown or stained when contaminated.
So if you have a plumbing or soldering task to complete but only have an old tub of Plumbing flux, its best to check the expiry date and the condition of the contents.
You can use old plumbing flux but it won’t give you the best performance and may compromise your soldering and the joints
So our recommendation is to follow the manufacturer’s guidelines and replace any out of date flux with a shiny new tub, that way you can be confident your plumbing won’t be affected by degraded products
For more information about how to use plumbing fluxes check out Episode 9 of The DIY Help Desk podcast using the link below
This article was written by: Richard Quinton – The DIY Help Desk Owner, Engineer & technical specialist.
Richard is one of the key partners in The DIY Help Desk team. He is a qualified Engineer, writer, and publisher, educated to Master’s level. He is a keen advocate of DIY and home improvements.
Richard enjoys helping others to learn new skills and reach their goals and believes that passing his knowledge and experience on through his writing is an effective way to positively impact the lifestyles and well-being of others on a larger scale.