Like almost all items, flux can go bad over time. This is the reason it has an expiration date. If used after its expiration date flux can become less active when interacting with solder metal. Flux needs to be stored in a cold but dry place for it to have an optimal shelf life.
Can Plumbing Flux go bad?

Is sweating pipes a concern for you? The process seems to depend on a lot of issues that need answering. Most frequently, we find questions regarding Plumbing flux. Perhaps you may be asking yourself if you can use old plumbing flux to bring your joints together. This question has been asked by many people already. The challenge many try answering it is because it’s not all that simple. Some people have been known to use plumbing flux 20 years after its shelf-life with no trouble at all while others have had problems with flux that is well less expired. As it turns out, a lot of factors go into flux and its shelf life. Let’s answer this question by exploring the factors important to this topic.

How does Plumbing Flux work?

To answer how plumbing flux goes bad, we need to dive into how it works. Plumbing flux comes with ingredients known as “activators”. They have the function to take away oxides from surfaces you may be soldering. The catch is that they also get rid of any oxides that are present in the plumbing flux itself. However, this activation only happens with soldering temperatures but lays dormant during colder or room temperatures. In other words, your flux activity depends on temperature.

Plumbing Flux and temperature

Flux activators can interact with themselves even while you leave the paste on the shelf. Not only does this mean you may end up not being able to use you your flux after its expiration date, but you also run the risk of having to throw it out before its expiration date is up. The activators may react with the powder with other particles, making it come out in a clumpy fashion rather than free-flowing. This means temperature control is a must. Clumps of flux can lead to clogging if activated by a heating process. Refrigerated storage of your flux is required if you want to realize the optimum shelf life. Not refrigerating your Plumbing flux, may lead it to go bad.
Source: https://www.indium.com/blog/solder-paste-expiration-shelf-life.php

Other considerations when using flux

Although using a flux within its expiration date is the best way to be safe, many have used flux long past its expiration date without it going bad whatsoever. There have been cases of people using paste from the 1960s without it going bad. Yet others have used the same flux they bought 8 months ago, and it already looks like it’ s losing consistency.
This has a lot to do with different products and their quality. It seems that good old fashion flux lasts a lot longer than water-soluble flux, so also keep that in mind. However, most codes require water-soluble flux to be used. If you are using water-soluble flux, it’s best not to use it after the expiration date.
Source: https://forum.heatinghelp.com/discussion/126984/whats-the-shelf-life-of-soildering-flux-does-it-go-bad

Conclusion

All in all, the best way to ensure your flux not going bad is to follow the expiration date while keeping it refrigerated and dry. Although many older cans of flux seem to last a lot older, you can always check the consistency. This can also have a lot to do with the varying qualities of products out there. Water-soluble flux seems to go bad much quicker than usual plumbing flux. If you have any questions regarding flux and its expiration, please let us know in the comments below. Remember to keep your flux cold, dry, and mind the expiration date.