Passive vs active 304/316 SS 1

[strokersix]

I'm attempting to evaluate galvanic couples for a piece of machinery we manufacture and do not understand when to use the passive versus active condition for austenitic stainless. I think the active condition should be used for intimate contact with dissimilar metals in a moist environment. If that's true then when would the passive condition be used? I've Googled a bit and not been able to reach a conclusion. Can someone please enlighten me?

Thanks.

 

[NickE]

All uses of stainless steel will see some benefit of passivation on corrosion.

Passivation is the process by which free iron and other contaminates are removed from the surface and the passive CrO layer is thickened.

 

[strokersix]

OK, but I don't understand how to use this information to predict field performance of the equipment in question. Specifically, I'm looking at cast 316 against painted aluminum extrusion (don't have the alloy in front of me at the moment) with screws through the stainless into the aluminum. Screws are currently carbon but may be stainless. Environment is agricultural chemicals, particularly fertilizers that cause corrosion failures. I would like to predict field performance and if there are extreme galvanic couples, what to do to seal moisture out.

 

[EdStainless]

The passive and active states relate to electrochemical activity. If the passive film on a metal surface is broken (mechanically or chemically) then the metal will shift from the passive potential to the active potential.
When you are evaluating galvanic corrosion potentials you should assume the worst case. Assume that the most anodic (least corrosion resistant) metal will be at its active potential, and corroding.
Yours is not an uncommon situation. Often the story is very complex. In direct contact the Al starts to corrode. The corrosion both loosens the joint and the corrosion by products trap moisture and chemicals. The result is that the steel bolts fail. A visual exam shows only light corrosion on the Al so the bolts are replaced and the process starts over.

If you are looking at stainless bolts you either need to make sure that the are way over sized (very low stresses) or that you use an alloy that is resistant to chloride stress cracking. You don't want to trade one problem for another.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Lars50]

My exprince is to never use stainless together with Aluminium. The Al will just corrode and leave a pocket of white powder around any bolt treaded into the Al. If corrosive environment similar to wet winter roads with salt, I would say that you will fail with this material mix.