Dissimilar metals 2

[archprecast]

I know that in general dissimilar metals are problematic and to be avoided. My specific question is about the pair: zamac and stainless steel. The specific use is a zamac female nut body (Star threaded insert, similar to a closed barrel nut) embedded in concrete and receiving stainless steel threaded rod. Is this inadvisable?

http://www.caryconcrete.com

 

[kenvlach]

No, but it may be tolerable indoors if the exposed portion can be coated/sealed to keep out water from washing the stairs, wet feet or boots, etc.

 

[archprecast]

The application is exterior and is used to connect a small pier or precast curbing to a foundation or coping to a parapet. The bed joint is mortar filled. So in reality, I'd say there is potential for moisture to get there, but not water since the contact takes place above the bed joint within the concrete.

This is a useful connection detail for us, but perhaps it is too risky. What extent of corrosion would you expect with a 3/8" diameter rod?

http://www.caryconcrete.com

 

[kenvlach]

Is it like this device: Cast-In Place Zamac Anchor?

http://www.ucanfast.com/cast_in_place_anchor.html

 

[archprecast]

Precisely.

http://www.caryconcrete.com

 

[EdStainless]

What would happen over time is that the Zn would corrode. With no standing water and the high pH of the grout this will be a slow process. I am thinking decades.
One thing to be aware of is that as the Zn corrodes the corrosion products take up more space, so the anchor will swell. Could this cause cracking and lead to failure?

If this were at sidewalk level and there was a chance of salt water from deicing getting in then it might fail in a season or two.

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Rust never sleeps
Neither should your protection

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

 

[swall]

I would have to see a long history of satisfactory use before I would ever approve something like this. In addition to the obvious corrosion issues raised by the other posters, Zamak can creep at moderately elevated temperatures. Zamak is high zink content and when zinc corrodes, a large volume of corrosion product is generated. I could see the possibility of these anchors corroding and the corrosion product forcing the anchor out or cracking the concrete. These products are die cast. With die castings, the best metal is on the surface. This metal will be the first to corrode, leaving the porous inner structure an your "anchor".

 

[archprecast]

I posted a sketch at

http://www.flickr.com/photos/60807902@N00/429657627/

to be more clear about the detail. In terms of the potential for water to get to the insert, it is more extreme than typical because one side is below grade. I am concerned about the "decades" issue. I've never seen a visible problem, and I'd have to break one open to assess the extent or if corrosion was actually occurring. But it looks to me like we should avoid this in the future and use only a stainless insert with stainless rod. Is it true that if I used galvanized (hot dipped, not electroplated) threaded rod with the Zamak insert I should anticipate no problems? Thanks to everyone for their comments.

http://www.caryconcrete.com

 

[EdStainless]

Well, sort of. If you use galv and Zamak then they will corrode at the same rate. You will not get selective attack of the anchor. The Zn will still corrode, but slower.

If you have never seen one of these corrode enough to crack then maybe it isn't a big issue. These are usually not under much load, are they?

Yes, you right to be concerned about a below grade application. Being constantly wet will lead to early failures.

What comes to my mind is the use of galv parts in general construction. In areas that are near the coast even simple parts inside exterior walls will corrode an fail.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

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