Metal finishing Q&As since 1989

Passivation Pre-Cleaning and Multiple Materials in Same Bath

Does anyone know of any problems of running multiple stainless steels in the same bath? For instance, my company (a machine shop) runs a lot of 17-4, 15-5, 420, 440, 303 stainless steels in our ASTM A967 nitric 1 bath. This is the bath recommended for these materials. We have recently had some corrosion problems (in 17-4 & 303 sst), that appeared to be related to insufficient pre-cleaning of machining oils rather than passivation. However, in the process of solving this problem we inspected our baths for iron, and found high concentrations of it (up to 800 ppm). My thought is that this may be due to running quite a bit of 400 series stainless through the bath (and not monitoring iron content often enough).

My questions would be as follows:
1) Any problems with multiple materials?
2) Does anyone have good information as to when to change out a passivation bath due to iron content (I have been told 250-350 ppm, though cannot find it in any spec ( ASTM A967, ASTM A380 , AMSQQP35 [canceled] ))?
3) What are good "objective" ways to monitor precleaning baths and soap effectiveness? PH level, refractometer, water break test?

Thank you.

John:

The questions you ask are not all easy.

1) There is DEFINITELY a problem running various grades of ss in the same bath, especially if any of them are touching each other! This is a NO-NO. If you HAVE to run more than one grade, rack them.
2) I am not too familiar with the problems with iron concentrations in nitric baths, since we work with citric acid systems where this is not a problem. However, the 300ppm range is what we have been told, also. It can be a real problem if the iron content gets too high with nitric. I do not know of a specification on this, but I can check.
3) pH will not give you what you want to measure cleaning. Water break is the most common test used in the field, but a refractometer test could be useful in measuring the usability of the bath. If you use commercially available citric formulations this is not a problem, since they do a final cleaning as well as passivation, yielding better chrome oxide layers.

Let us know if we can help.

I would like to answer by your question order:

a.You can passivate and treat different types of Stainless steel but please avoid touching each other and follow up the metallic impurity contents of all the stages because you cannot expect degree of impurities in the bathes , especially watch Copper content and of course Iron content.

b. 300 mg per liter is a little bit high , try not to pass 100 mg per litter Iron and not more then 20 mg per liter copper.

c. The best way to control alkaline pre cleaning bath is by Free or Total Alkaline titration and replenishing the bath following the manufacturer's order . Discarding the bath is when the Total to free ration will be at least twice the beginning ratio. I'm not recommending water break test as controlling parameter for the bath you have to be more analytical and systematic and there are some kinds of stainless steel which have different comment to water break test that will mislead you to wrong conclusions concerning to bath life.

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