Passivation of 440C Material

Question:

Can you explain what would cause 440C material to etch in a passivation bath of sodium dichromate/nitric operated at 120F? We have had the material analyzed, and everything seems correct. After glass beading the same parts that had etched, they do not react. We suspect a galvanic reaction of some type. The parts are usually processed in stainless steel baskets. N.S.

Answer:

Your problem sounds unusual since a 440C material should not etch in the sodium dichromate/nitric acid bath. The 440C is on the border of needing an additional oxidizing element like sodium dichromate to keep it from etching (based on chrome content of about 17% nominal). Therefore, probably the first thing is to make sure the bath is made up properly regarding concentration of nitric acid and sodium dichromate. There should be approximately 20% by volume nitric acid and 5% by weight sodium dichromate, and it should be used between room temperature to about 120F.

It would also be useful to investigate the 440C to make sure that it is the correct material and that it has received a proper heat treatment. Martensitic stainless steels are usually heated to a high temperature (about 1,900F), and then air quenched to increase hardness. The 440C has the highest carbon content of this class of stainless steels and should be the hardest of them all at this point. In order to make them more usable, they are usually stress relieved to achieve the correct balance between hardness and toughness for a given application. If the material is stress relieved at too high a temperature (in the range of approximately 650-1,000F), it will experience very poor corrosion resistance due to the formation of iron carbides and iron-chromium carbides. The material in the vicinity of the iron-chromium carbide is depleted of chromium and is technically no longer a stainless steel (in those localized areas that have experienced chrome depletion). The etching you see may be an effect of this processing and heat treatment prior to your receiving the material. I am not sure why the glass bead blasting seems to help it. Unless there is more to the glass bead blasting than you revealed, it will not effect a metallurgical or chemical change to the material. Possibly the material is on the verge of being passive, and the bead blasting may help to make a more uniform surface, but this is purely conjecture.

I would recommend you stay with austenitic stainless steel baskets when you process these parts. You did not indicate what material you were using in the interim, but I would hope it was not mild steel or a low chromium stainless steel. Both of these (especially the mild steel) would react quickly with the passivating solution, and I am not sure how you could even tell what was happening on the inside of the basket since the reaction with the basket itself would obscure almost everything else.