Connecting a stainless steel plate to a carbon steel bushing to avoid galvanic corrosion

[EngTipsAccount1234]

I have a customer that wants to connect a T304 electropolished stainless steel plate to an embedded carbon steel bushing. This will be near a pool so will regularly be exposed to pool water. I'm wondering how to design the connection to avoid galvanic corrosion.

 

[EdStainless]

Don't connect them.
They would need to be electrically isolated from each other.
Such as by a plastic bushing/spacer.
And even then you are likely to get crevice corrosion.
And 304 will not stand up very well to pool water either.
Why is the bushing CS?
Is there any chance to change that material?
Maybe a high strength SS, or an engineered polymer?

 

[mfgenggear]

Or coat the bushing with copper. Or zinc

 

[TugboatEng]

On another note, any ferrite from the CS part is going to undo any benefits from electro polishing.

 

[EngTipsAccount1234]

The carbon steel bushing can't be changed unfortunately short of demolishing the concrete, which would be a last resort.

In the past we've used a carbon steel plate but painted, so that all components are carbon steel. And it has been sufficient.
In this application the painted carbon steel plate is rusting, which is diving the customer to want to switch to stainless steel.

I'm wondering if we stuck with the carbon steel plate but had it galvanized and powder coated if that would be a solution, or would the zinc plating still be introducing galvanic corrosion on the imbedded bushing.

 

[EdStainless]

If you can't 100% stop corrosion of the steel bushing and electrically isolate it from the SS then going to SS will gain you nothing.
Staying all CS sounds better.
When you say 'painted' do you mean simply a single coat?
The part should be blasted white, and then have a three layer coating system applied.
The primer is Zn rich, then there is a build or protection layer, and then a topcoat to seal.
This should be done to the bushing and the plate.

 

[EngTipsAccount1234]

Previous jobs would have only had a primer and single coat, the qualities of which I'm not sure but will investigate.

The three coat system makes since to me, however I'm unsure whether this would be effective on the bushing threads. Wouldn't these layers just get shredded once you install a bolt?

If hot-dip galvanizing is an option for the plate would this be better than a Zn rich primer?

 

[EngTipsAccount1234]

The painting on previous jobs consisted of a an alkyd based primer followed by a urethane alkyd based top coat, so nothing with specifically designed anti-corrosion attributes.

 

[EdStainless]

Galv would be better, but you should apply a good (and application specific) sealant over it.
In a wet environment like this the Zn could be all consumed in a mater of months leaving rusting steel.

 

[TugboatEng]

Alkyd? That's the lowest level of protection. Why are you not using an epoxy primer?