Cleaning Stainless Steel

The following article has been authored by John Heaney.

Dissolution components are often made of 316 Stainless Steel (316 SS) or equivalent as it is a material specified in USP <711> for the construction of both Apparatus 1 (Baskets) and Apparatus 2 (Paddles).  316 SS has a reputation for being impervious to corrosion but that is not the case as it is merely resistant to corrosion.  It requires care and maintenance just like any other item in the lab.

316 SS’s corrosion resistance is due to the inclusion of additional Molybdenum to the mixture for the steel.  This means compared to other steels, 316 SS will be more likely to maintain its finish, and in some cases, dimensions if the item is used in common dissolution media such as buffers or hydrochloric acid.  As mentioned before, this does not mean it is impervious to corrosion or damage.  Acids of a sufficiently high concentration can still damage 316 SS, though the concentrations to cause that are greatly in excess of typical dissolution media which is 1N (1M) Hydrochloric acid (HCl). 

If someone notices corrosion on their 316 SS parts, what could be causing this if the most corrosive dissolution media is only 1N HCl?  The typical cause of corrosion in these cases is related to post-test clean-up.

Dissolution media is aqueous and typically either a buffer (primarily composed of salts) or HCl.  The concentrations of this media are kept near physiological levels, which generally 316 SS is resistant to.  If that media is left to evaporate over time, the concentration of the salts or the HCl will increase.  For example, if you left a vessel of dissolution media out long enough, eventually the salts that comprise it would precipitate out because there is not enough sink capacity for them to stay dissolved.  However, the remaining liquid at this point would essentially be at a glacial or maximum concentration.  For HCl, that is well in excess of 10N!  And while 316 SS is resistant to HCl, at those concentrations the resistance isn’t enough and damage will occur.

What isn’t thought of in many cases is this situation scales down into the drop or droplet range as well.  As drops of dissolution media are allowed to evaporate, the concentration of that drop gradually increases, changing from a drop of 1N HCl to a microscopic drop of glacial HCl. Glacial HCl is capable of corroding the 316 SS.  Once the corrosion or damage has a foothold, subsequent uses may cause the damage to spread and become easily visible.

So what are the best practices to avoid corrosion issues when using 316 SS after a dissolution test?

• Paddles and Basket Shafts
  • Worst: Raise the items out of the media and allow them to air dry.
    • This allows the salts or acid to concentrate to glacial concentrations causing damage to the 316 SS. If the tester cannot be cleaned immediately after the test, then leave everything in place and submerged in the media with covers on the vessels.  This will help keep evaporation to a minimum and will protect the 316 SS.
  • Good: Raise the items out of the media and immediately rinse them with deionized (DI) water.
    • This at least removes the bulk of the acid or buffer from the apparatus. It is not perfect as it is possible to miss spots but is far better than leaving the apparatus up and allowed to air dry with dissolution media on them.
  • Best: Rinse components in DI water and then clean them using Q-Sonic cleaner.
    • Rinsing the apparatus then soaking them in Q-Sonic cleaner while ultrasonicating is the best way to ensure they will remain corrosion free. Submersion of the apparatus ensures that no part is missed and the Q-Sonic cleaner will remove any deposits leftover from the test without damaging the 316 SS.
  • Sinkers and Baskets
    • Worst: Remove from media and let air dry.
      • For the same reasons as the apparatus, allowing the dissolution media to dry on sinkers and baskets will expose them to very high concentrations of acid or salt solution. Damage to the baskets or sinkers can be exacerbated due to the small wire size. This small wire size can hide damage that may result in larger openings than the specified mesh size. If the baskets or sinkers cannot be cleaned immediately after the test, they should be left in the vessels to prevent damage.
    • Good: Removing and immediately rinsing with DI water.
      • Rinsing the sinkers and baskets with DI water immediately after testing is much better than letting them dry, but unlike the larger apparatus, there’s a complication here. A basic rinse may not be sufficient to remove all the dissolution media from the basket or sinker mesh due to the numerous crevices located between the wires.
    • Best: Rinse sinkers and baskets in DI water, then place them separated into Q-Sonic cleaner and ultrasonicate.
      • Submerging the items will ensure that no spots are missed unlike rinsing. The Q-Sonic cleaner will help remove any deposits still present on the sinker or basket. There is another added benefit as the ultrasonication will also help remove any air bubbles which may prevent the cleaning solution from reaching everywhere.  There may be some concern about sonicating baskets as this may affect the welds; however, for QLA dissolution baskets this isn’t a problem as baskets are tested in production to have welds strong enough to hold 10 lbs of force.
    • Cannula
      • Worst: Pull the sample and allow to dry before cleaning.
        • As with other 316 SS items, allowing the sample to dry in or on the cannula will raise the concentration of the droplets of media and start inducing corrosion. Furthermore, as the cannula are hollow, this corrosion may start where it cannot be seen.  If proper cleaning immediately after the test is not possible, it may be best to consider QLA PEEK cannula.
      • Good: Rinse cannula immediately after sampling of all vessels is complete. Rinse both the outside and inside of the cannula with DI water.
        • When cleaning cannula, the outside needs to be cleaned but 5-10 mL of DI water (depending on the cannula) should be pushed through it using a syringe. The force applied by rinsing with a syringe will help ensure the inside of the cannula is scrubbed free of any deposits.
      • Best: Rinse cannula with Q-Sonic cleaner using a syringe, lay flat and submerge in Q-Sonic cleaner while ultrasonicating.
        • Cleaning the inside of the cannula is key not just for preventing corrosion but also ensuring no carry over occurs. While the force of DI water pushed through a syringe should be enough to dislodge any solid particles, the remains of gel-caps may be a bit more challenging due to how sticky they are.  Q-Sonic cleaner is a superior choice as it will help dissolve the remains of any material within the cannula.  However, it’s important to be sure that the cannula is filled with the cleaner as pockets of air would prevent any cleaning.

Proper cleaning of components is key to preventing corrosion of 316 SS.  At the very least, the items should be rinsed as soon as they’re brought out of the dissolution media to prevent the droplets from drying.  Ideally, they should be given a thorough cleaning using a mild detergent like Q-Sonic cleaner.