SS Rust 3

[mjh368]

Hello Stainless Guru's

I am an injection molder in the medical industry. We are having a rust problem on some of our 420&440 SS molds. I am trying to identify the source of the rust. the molds are cooled with 70F water and the plastic melt temps are around 400F-460F.The material is Polypropylene. The other day we ran a dry cycle with no plastic and the mold components still rusted. Is the rust comping from wear?? how is our stainless rusting??
Sources of rust on stainless?

 

[StoneCold]

Have you ever cleaned the molds with steel wool or a wire brush that was not stainless? Are you coating them with a release agent that contains chlorides? Are they near the ocean?

Regards
StoneCold

 

[EdStainless]

It could even be that the molds were not properly cleaned and passivated to remove free iron from the surface.
SC has listed the most common causes. Do some research.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
eblessman@plymouth.com
or edstainless@earthlink.net

 

[pierdesign]

is there any copper present in the system?

 

[MikeHalloran]

Stainless steel is still > 60 pct iron. Pickling and passivation remove the iron near the surface. The remaining iron sort of hides behind the chromium and/or nickel that remains, but it's still there.

It's also possible that the material in the mold you bought is not actually what you think it is. Metal does get, ahem, accidentally misidentified, from time to time.

Did you by chance have the molds made offshore?





Mike Halloran
Pembroke Pines, FL, USA

 

[mcguire]

420 and 440, depending on chromium and carbon level and tempering temperature, can have very low levels of chromium in solution. You should assume all carbon is present as Cr23C6. So take the cabon level and multiply by 17 and subtract that from the reported chromium level. That is your working chromium level. If it below 10.5%, you're not really stainless. E.g., 420 with 0.20% carbon and 12.5% total chromium, has only ~9% chromium really working if it has been tempered at a high temperature.
That's the problem with martensitic stainless. Any slight difficulty in corrosivity of the environment or material preparation can leave you exposed to corrosion. There isn't any room for error.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[strider6]

Is you mold new or already in services? did you cleaned with some kind of chemical solutions?

S.

http://www.corrosionist.com