Galvanize High Strength Bolts and Structural Steel 2

[rfd23]

Hi,
I have concern on using galvanized bolts for structural steel connections due to bimetallic corrosion.
Have any one used galvanized bolt on painted structural steel. Main Purpose for galvanizing is to protect bolts
from corrosion, however i read that in this particular scenario (carbon steel painted plates-Galvanize bolt) will increase
corrosion rate to bolt.
insulator between bolt head/nut and steel surfaces do not seems to be a practical approach due to large number of connections and pre-tension requirement
of bolts.
For a particular project connections can be in range of 5000 to 10000 and
each connection will have min. 4 bolts.
Please share your input. Thanks in advance.

 

[KootK]

You won't get a galvanic cell operating between the bolt proper and the carbon steel. They're roughly the same material. Instead, what you'll get is a galvanic cell operating between the zinc on the bolt and both the carbon steel of the member and the carbon steel of the bolt. In essence, the zinc on the bolt will offer corrosive protection to both the bolts and the members. That said, because the zinc is being consumed by both the member and the bolts, it will be exhausted faster than if it were offering protection to the bolts alone. In that sense, the galvanic cell between main member and the zinc on the bolts may result in the bolts corroding sooner than they would if the main member were also galvanized. This is nowhere near the issue that it is when the entire fastener is the anode as it would be with a carbon steel main member and, say, zinc rivets.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rfd23]

Thanks kootk for detail description.
I can consider this situation better than CS bolt and CS painted plates.

Can you refer to any Refrence material for further insight into this subject.

 

[TehMightyEngineer]

Honestly this is a pretty good summary of the basics of galvanic corrosion:

https://en.wikipedia.org/wiki/Galvanic_corrosion

Definitely not everything though; slapping some zinc on something does not make it invincible. You need to get the electrons flowing from the right materials and often your "battery" can short-circuit away from your zinc.

For your purpose making the bolts HDG makes sense. Make sure your nuts are also HDG.

Also, for more fun reading, look at the clever engineered to failure design of the spider arms on front-load clothes washing machines. They get a predictable service life and a sacrificial anode all in one.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[KootK]

These guys are a pretty good resource: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SwinnyGG]

Also keep in mind that in order to create a galvanic couple, you need electrolyte.

If you keep connections dry they do not corrode.

 

[Huevo]

Hot dip galvanizing is not permitted for some higher strength bolts like A490 due to risk of hydrogen embrittlement.