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Instantaneous stainless steel oxidation/corrosion in acid bath

Hi, I'm a university student doing a research on instantaneous oxidation of stainless steel in acid bath. I've been investigating potential problems of hypodermic needles of 304 SS grade oxidising/corroding rapidly in acid bath (targeted at improving passivation techniques and etching techniques used). I have currently 2 acid baths in question: The first being the nitric acid used in passivating the needles and the second being sufuric acid + phosphoric acid used in etching process to promote shiny stainless steel surface and remove burrs from the tip of the needles.

To my horror,Ii've obtained a few batches of oxidised needles (black needles, surface rough to touch >> looks like steel burn under candle flame) after the processes. The needles underwent full process of forming, annealing, drawing, cutting and grinding before alkaline cleaning and then followed with passivation in nitric acid and later etching (water rinsing in between). The mind boggling part about this phenomenon is that it is random and when it happens, a few batches of needles will be totally black while some batches will have black color appearing only at the heel of the needle, but at other times the needles appeared to be perfect.

All acid concentrations are according to ASTM standards.

Really appreciate your suggestions!

Sebastian Wijaya
University Student - Singapore
2005

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2005

Hi Sebastian,

Um, there's one thing perhaps missing in your research.

Once, back in 70's, I came across these largish tanks of
nitric that were full of hypo needles. Hundreds of thousands of them down at Englehardt Industries, I believe. This was in Toronto.

They, someone said, were in those tanks for quite a few months in order to get rid of the copper.

As you should know, the final 'calendering' process infers that hollow 'tubes' will definitely deflect and get squashed unless there is a 'fill' medium, in this case copper ... which has to be leached out, a long drawn out process. Incidentally, a similar process of 'fill' is often
used by people speciallizing in bending/forming metal pipe which is removed by just heating up ... otherwise the pipe
will not form beautifully.

Hence, I'd strongly assume that the final cutting, trimming and sharpening of the needles was done BEFORE
any final 'polishing' etc.

Could it be that some of the needles still had some copper
residue in them? Also, why 304 stainless? Why not 3l6?

Pardon my ignorance but my field was in the corrosion resistant plastics, hence my interest in the tankage.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).