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topic 14182

Frosted Appearance on Passivated Parts

A discussion started in 2002 but continuing through 2019


Q. What would cause a frosted appearance after passivating 303 stainless steel? Could it be from pre cleaning or the passivate process?

Mario Rodriguez
- Phoenix, Arizona


A. Frosted appearance comes from etching away microscopic stuff, leaving a rougher surface which reflects light at many angles. Since you're working with 303, which has lots of sulfides to aid machinability, I'd suppose that the sulfides on the surface are being etched away by the either the pre-cleaning or the passivation. Either one could do it.

If you can stand the frosted look, put up with it because removal of those sulfides is really important for corrosion resistance.

Mike Mcguire
- Pittsburgh, Pennsylvania, USA


? You do not give enough information for me to give you a definitive answer but based on the information you have provided it seems you are getting etching of the part. This could be caused by passivating with oil left on the surface after inadequate cleaning. If you send more information about the process you are using I can possibly make more suggestions.

Good luck.

John Holroyd
- Elkhorn, Wisconsin


Q. The process I'm using is AMS-QQ-P-35 [affil. link to spec at Techstreet] type II. According to that spec, the frosted condition is cause for rejection. Parts had some oiled before process, I cleaned them as usual and passivated type II. Can a weak solution cause that condition?

Mario Rodriguez [returning]
- Phoenix, Arizona


A. Sorry about the delay in responding but I have been traveling.

It seems probable that your problem is caused by residual oil left on the part after cleaning. The cleaning process has to be extremely thorough if you are to properly passivate the parts. Etching is often caused by traces of oil reacting with the acid and forming an organic acid which will etch the parts very effectively. In fact this is a technique used by metallurgists to etch difficult metals.

First check to make sure that you have not got your passivation bath contaminated with oil and that it is at the proper strength for the procedure. If it is contaminated then change it.

Next I would suggest that you remove all the oil from the surface of your parts by wiping with paper towels, then wipe the parts with lacquer thinner [affiliate link to product info on Amazon] and paper towels using frequent changes of paper towels to be sure that you are not recontaminating the surface.

After the lacquer thinning the part should be soaked in a heated and agitated bath of caustic soda for at least half an hour. Finally rinse the part without touching the surface in any way and transfer the part to the passivating solution. If the part still etches I will be very surprised but I do not expect you will have any further problems. Please let me know how you go on with this.

Good Luck,

John Holroyd
- Elkhorn, Wisconsin


A. Mario:

Yes, a weak solution can cause a definite problem. The nitric acid used in AMS-QQ-P-35 [affil. link to spec at Techstreet] is a reducing acid in dilute solutions. I agree with the others that it is critical to clean the surface of residual sulfur and oils to get a good passive layer on 303. This can be done well with a 5% NaOH solution or a high pH alkaline cleaner. Citric acid proprietary formulations may be better for you if it is allowed by your customer. It is not allowed by AMS-QQ-P-35 [affil. link to spec at Techstreet], although many aerospace companies have tested and approved it. It is allowed by AMS2700 [affil. link to spec at Techstreet]. This can give you an improvement in performance without the danger of frosting the surface.

lee kremer
Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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February 4, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. ISSUES PASSIVATING 303 SST. PROBLEMS WITH ETCHING OF PARTS USING THE AMS-QQ-P-35 [affil. link to spec at Techstreet] procedure.


September 18, 2019

Q. Hi, we are having issues with frosting on 321 stainless after passivating to ASTM A380 [link is to the practice at TechStreet] . Could this be similar to the problems with passivation of 303 stainless?

Grant Hartman
Nortech Systems - Blue Earth, Minnesota

September 20, 2019

A. Grant,
321 isn't a high sulfur grade like 303. The notable alloy addition for 321 is titanium, which I wouldn't expect to cause issues. If your parts are getting etched, you may be using incorrect nitric acid bath parameters. I would suggest testing out a citric acid passivation method to see if that solves your problem.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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September 23, 2019

thumbs up sign  Thanks, I will see if our customer is open to Citric acid passivation. The main reason why I was asking is the inconsistency in the finish. Using the same lot of stainless and within the parts passivated at the same time we are seeing 50% with no issues and 50% with differing levels of discoloration to frosted.

Grant Hartman
- Blue Earth, Minnesota

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