CF8M (316) surface rust on side of parts facing road.
CF8M (316) surface rust on side of parts facing road.
(OP)
Before winter we supplied some CF8M "posts" for building entrance protection (9/11....). We cast them and sold them here in Boston. I just got a call saying that the side facing the road is rusting. I have yet to take a ride over there but I was wondering if you guys have any input? The parts have been heat treated and passivated. Keep in mind that last winter was about the worst in history for us.
Thanks in advance.
Gerard Snellen
Wollaston Alloys
Thanks in advance.
Gerard Snellen
Wollaston Alloys





RE: CF8M (316) surface rust on side of parts facing road.
I personally never seen pickling(HNO3 + HF) and passivation (HNO3) prevent the surface rusting of CF8M. We have tried it all, mechanical, electropolish, shootpeening, and etc will nothing approaching a qualified success. The same thing happens on fixtures on a boat with 316 SS that is polished, if you don't rinse the boat down you will start to see microscopic pits/rust and if you check the hidden side it will be completely rust after a short exposure.
One thing with roadside exposure is that there is a mixture of CaCl2 and NaCl, in various proportions, and the worst offender is the CaCl2. It starts to attack Austenitic SS at around 0°C and at a low humidity (45%) while NACL becomes corrosive at 10°C and 76% humidity. (International Molybdenum Association Paper)
RE: CF8M (316) surface rust on side of parts facing road.
The chemistry just ins't corrosion resistant enough for the service. Road slush probably gets spashed on these, maybe the plows even pile it against them. If they don't get rinsed regularly they will pit and rust.
We recently supplied some 2205 for a project that is similar. The 316 posts that were in the open were fine. The ones that saw no rainfall rusted like mad.
Short term solution; blast to clean, citirc acid passivate, paint with a clear urethane.
Longterm; move to 2205.
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Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm
RE: CF8M (316) surface rust on side of parts facing road.
Gerard
RE: CF8M (316) surface rust on side of parts facing road.
Gerard
RE: CF8M (316) surface rust on side of parts facing road.
My only suggestion would be to set minimums for Cr=22%, Mo=3% and N=0.14%.
You also want the castings to pass a A923 test. If they are thick require impacts. They will need to be properly annealed to meet this.
= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm
RE: CF8M (316) surface rust on side of parts facing road.