Medical Device Cleaning: Martensitic Stainless Steel

Q: We use Alconox on the medical device parts we manufacture that need cleaning before shipment. Alconox is working in an excellent way with austenitic stainless steel parts, but is not working that well with martensitic stainless steel. Actually, at the end of the process cleaning martensitic steel, we notice some marks on the material. Any recommendations?

A: Fear not.  The detergent is not the problem.  Martensitic steel tends to rust in rinse water. The chelating agents in Alconox® Powdered Precision Cleaner will inhibit forming iron oxide during the cleaning process.  Once the detergent with the chelating agents is rinsed away, now you have bare martensitic steel with dissolved oxygen in the rinse water.

The best thing to do with martensitic steel is to rinse quickly with ambient temperature rinse water followed by rapid drying via a water removing process rather than a water evaporating process.  Dry by blowing, wiping, dipping in isopropanol, centrifuging, or other rinse water removing process will help to avoid formation of iron oxide with martensitic steel.

Simply put: Do not use hot water to rinse with.  Rinse quickly.  Do not allow the rinse water to evaporate.  Dry quickly by a water removing process.

That said, Citranox® Liquid Acid Cleaner and Detergent as opposed to Alconox powder, is a little bit better on martensitic steel because it does a more thorough job of removing surface iron oxide than Alconox detergent generally does.  To the extent you remove surface iron oxide, you slow down the rate of formation of iron oxide (rust) during rinsing.  The existence of iron oxide on the surface acts to catalyze the formation of more iron oxide.  So the more iron oxide that is removed from the surface during cleaning, it will oxidize more slowly during rinsing.   This does not mean that you still do not have to rinse quickly at ambient temperature, and then dry by a water removing method, it just means you have less risk when using Citranox than Alconox detergents.

Note that nitinol, when cleaned in a martensitic state, will have these recommendations apply there as well.

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