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Nitric Acid Passivation Standards and Specs

2007

I am a quality control manager in a small shop that has been given the job of validating an in-house passivation process. We manufacture medical implants made from certain 300 series stainless as well as titanium alloys, so passivation is a necessity.

By way of a summary, we are using a process that consists of an alkaline bath cleaning followed by a nitric acid bath @ 20-40 percent nitric acid @ room temperature, followed by a rinsing in de-ionized water

In researching the process, I have come up with a few questions. I realize that many questions relating to the actual process are subjective but any guidance would be appreciated.

My questions concern the maintenance and frequency in which tank solutions should be changed. With regard to rinsing solution (de-ionized water) it is clearly spelled out that ph should be in the 5.5 to 8.0 range. If it drops below 5.5 the rinse needs to be changed.

What is not clear however, is how often the nitric acid and cleaning solutions should be changed. In particular, the acid. It occurs to me that dissolving free iron from machining will eventually lead to the acid being saturated. So at what point does an acid bath become an iron bath that can actually contaminate the product? Additionally, what part does evaporation play and can a build-up of salts in the acid solution lead to problems? These are just a few of the questions that I have been wondering about as I educate myself.

Again, I realize that it's entirely subjective, based upon the frequency of use, number of parts being processed and even environmental conditions. However, I was hoping that someone could direct me to some reading material that might give me a basis for setting some criteria. Are there any official standards or specifications for the acid solution that could be referenced?

It should also be noted, that the vendor we work for does supply information for standards, but the actual process and details of that process are left up to us to decide upon. Therefore, the details are being left up to me to decide but I need some point of reference for making those determinations.

Any answers would be appreciated.

Vern

Vernon Lilly
Quality control manager - Westbrook, Connecticut, USA

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The industry standard limit on dissolved iron is 2% by weight.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007

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2007

Thank you Robert for the reply. At least that gives me a figure to work with. When you say industry standard, is that a standard thats down in writing, if so where would I find it?

As I continue to educate myself questions keep popping up.

In the standards that we are using it is stated that a concentration of nitric acid from 25%-40% by volume is used.
In turn the specific gravity for the mixed solution should be between 1.1193 and 1.2500

My question is, how are these figures determined? By all accounts specific gravity is a relative measurement, so how do I determine if these numbers are correct? Are there industry specific tables or formula's used?

Specifications being used are:
ASTM A967 -05
ASTM B600 -91(97)
ASTM A380
AMSSTD753 (method 102)

Vernon Lilly
- Westbrook, Connecticut