We are frequently asked about the shelf life of brazing flux. To best understand this topic it's helpful to differentiate between shelf life and service life.
Shelf life relates to the question of whether, after a defined storage period, the unopened product will deteriorate to the point it will not perform satisfactorily in service (chemical stability). This period is usually measured from the date of manufacturing, packing, inspection, or testing.
Brazed joint with flux protection
For brazing flux the stated unopened shelf life is typically 1 year. This is the recommendation in section 7.2 of American Welding Society (AWS) specification A5.31. The specification criteria are that during this time period flux will be able meet the test requirements of the specification.
From a practical standpoint the flux is useable beyond that time frame. AWS A5.31 notes that…"flux beyond the shelf life may be used if it continues to meet the user's requirements."
If flux suitability is in doubt you can test the brazed component(s) to ensure they meet application service requirements. You can also evaluate the flux by using the performance test regimen outlined in AWS specification AWS A5.31.
Regardless of the time involved water may evaporate leaving a harder flux consistency. To remedy this water may be added if the flux dries out. Water should be added incrementally and the flux stirred until it reaches a thick paste consistency.
Service life - sometimes called the expiration date - is the expected life of the product after it has been opened. Flux service life depends on several factors including storage conditions, dilution by operators during use, and contamination from brushes or atmosphere. Given these variables it is difficult to predict service life, but often flux usability can be extended by proper storage and confirmed by procedures and tests outlined.
In either case the best flux arbiter is how well it performs during brazing. Is it protecting the surface from oxidation? Does the braze filler metal wet and flow smoothly? Are you getting necessary braze alloy capillary penetration? If the answer to these questions is "yes", the flux is doing its job.