Stains after Stainless Steel Passivation, Rinsing Requirements

A. Lucy,
"Stains" tend to be one of two different things: residue, or etching. Residue is the kind that will come off, even if you have to physically scratch at it. Etching is when the surface has been eaten into, so it can only be "removed" by redoing the surface finish (which of course also negates any passivation treatment that has been done).

Residue tends to be a factor when there is poor rinsing or dirty rinse water. Etching tends to be a factor when a chemical treatment attacks the surface, such as an acid bath that is a bit too strong for what the material can tolerate.

Do you know the concentration and pH of your citric detergent solution? 416 is a lower-end grade of stainless as far as resistance to corrosion and chemical attack goes. It's entirely possible for a low-pH citric acid bath to etch it.

A. adv. It is possible to just briefly electropolish parts in our Universal Electrolyte to remove stains without impacting dimensions and simultaneously having them passivated. Please contact me directly.

A. Hi Lucy,

I'm away from my desk but under ASTM A380 I think is a code N cleaning solution of 1% w/v each citric and saltpeter that is run at room temperature and grows bioslime like crazy, but is extremely gentle for cleaning many if not most stainless alloys including some of the really touchy ones.

I also think you should check out the carpenter steel method specific to 416. It includes a caustic clean bath prior to final passivation. I keep a pair of tanks that are a slightly modified version of theirs but the caustic clean step is an essential part of the process. It's posted somewhere deep in the technical part of their website and worth the dive.

Good luck!

A. Lucy,
2.5 may be too low a pH for 416 parts. If the passivation is already done and you just need cleaning/degreasing, I would look at a neutral detergent cleaner or alkaline cleaner.

A. Dom,
Assuming you are talking about stainless steel passivation here, since pipelines are generally stainless. Yes, you can rinse, in fact, you NEED to rinse. The passivation treatment with acid removes free iron from the surface, but most or all of the formation of the passive chromium oxide layer takes place when the iron-depleted surface comes into contact with air. Any stainless surface being passivated must be rinsed, dried or allow to dry, and left in contact with air for at least a few hours if possible before entering service.

The passivation will not strip off via rinsing because stainless passivation is a treatment, not a coating. The passive layer is formed from the metal itself.

A. Is rinsing after acid treatment standard practice in industry for stainless steel passivation? It's a required step, if you look at the passivation standards (ASTM A967, AMS2700, even the now defunct QQ-P-35C (canceled) [link is to free spec at Defense Logistics Agency, dla.mil]).

Carbon steel cannot be passivated by acid treatment. If you use citric acid on carbon steel you had better rinse it off or you may not have much steel left.

A. Hassan,
With citric acid , a water rinse is all you need. With nitric acid, a water rinse should be sufficient in most cases, but many people go with an alkaline neutralizing bath first just to make sure, especially if the parts have blind holes or other types of nooks and crannies where acid residue may become trapped.

A. We use a Ion exchange system for our final rinse. a two tank counterflow.
If you check your pH of the last tank often you should have no issue with a 3 to 5 minute rinse.

Rinse first, dry after.