SS Anchor Bolts and Galvanized Base Plates

[rothrock]

A piping system design in a near marine tropical environment uses galvanized steel supports bolted to concrete foundations with stainless steel anchor bolts (ASTM A192 gr. B8).

The engineer is now concerned about dissimilar metal corrosion and wants the contact areas between the two metals coated with paint.

Is this really necessary? The galvanizing and SS bolts were upgrades to improve corrosion resistance from painted carbon steel.

Paint is a very generic term and the engineer wants us to select the coating and application process. Does anyone have a good product to recommend?

Thanks

 

[Metalguy]

The engineer is correct and wants to protect the support areas close to the bolts. Galvanizing has a limited lifetime-protects well as long as it lasts, and can steel even when scratched thru. But the voltage developed when coupled to *passive* SS is too much.

Whatever you do, make sure the SS bolts are well coated (removes the cathode).

 

[Metalguy]

Forgot to add--I'd use a good epoxy coating. Don't have any specific recos.

 

[DNJ]

You can hot dip galvanize the steel supports and use the SS anchor bolts directly with no other coating as long as you isolate the two metals from one another. I suggest you check out shoulder washers available from a number of US manufacturers. Choose a shoulder washer made from black polyethylene if the structure is in the direct sunlight as they will be protected from UV damage. The shoulder washer has a sleeve that fits inside the hole and a flat washer on top that should bear directly against the galv. steel structure. Then the SS bolt goes through the sleeve and should have a flat washers that sits on top of the flat surface of the shoulder washer. This is a lot cheaper and faster than coating the whole structure and a coating can still get nicked causing the galvanic couple to form.

Incidently, be sure to use Type 316 SS anchor bolts (ASTM A193 Gr B8M). 304 or 302 SS (ASTM A193 Gr B8)will not last long if this structure is in a demanding marine (splash area, windward area) environment.

Good Luck. DNJ

 

[mcguire]

Yes, as DNJ says, use 316. There is a tendency to make bolts from 303 because of its machinability, but its high sulfur content makes it very marginal and not up to the standards of 304. SCC is a consideration here with static load and chlorides, so alloy up to 316.