CF8M (316) surface rust on side of parts facing road.

[snellen]

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

 

[unclesyd]

We have always had trouble with CF8M castings showing varying degrees of rust in our atmosphere on the here on the Gulf Cost. If you install them in the winter they didn't rust until spring when the temperature and humidity get high.

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)

 

[EdStainless]

Some localities are even using MgCl these days. It isn't as bad on the concrete, but it is hell on stainless.

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

 

[snellen]

Thanks for the inputs guys. I have to find out if the city of Boston used MgCl last winter.

Gerard

 

[snellen]

Would you agree on J92205 (CD3MN, 4A)? Is the PRE high enough for this? The next step up, 5A (J93404) or CE3MN gets tricky to cast (crack prone).

Gerard

 

[EdStainless]

The J92205 should be a lot better than what you have.
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

 

[stanier]

We sometimes forget that the name of this material is "stainless steel" NOT "Doesnt stain at all steel"