Use of Zincs

[msquared48]

Anyone heard of anyone sucessfully using sacrificial zincs at critical connections in bridges to control the corrosive effects of road salts?

Works on watercraft in salt environs. Why not bridges?

Mike McCann
McCann Engineering

 

[Zambo]

Galvanic corrosion protection systems are indeed used for reinforced concrete, prestressed concrete and steel bridges.

The systems vary from attaching galvanic anodes or impressed-current cathodic protection for reinforced concrete and prestressed bridges to zinc thermal spray coatings, hot-dip galvanizing and zinc rich paints for steel structures

 

[shin25]

I agree with Zambo,hot dipped galvanized steel, cold spray zinc coating are very common in the bridge bearing system.

 

[europipe]

Mike, I know galvanizing takes place through water and wet ground (conduction), but I never heard it works through air.
Maybe in very wet5 places?

 

[Zambo]

Europipe,
you are misunderstanding the process.

 

[msquared48]

europipe:

Not only in water environs, but it can happen with salts in small amounts of water.

It also happens with dissimilar metals, even the same alloy, but of different batches. I ran into one case, not associated with structural engineering, that brought out the latter point clearly to me. We had a dog that had a hysterectomy, and the surgery wounds would not close. Turns out that the metal staples they used to close the wounds were not of the same lot number. This caused galvanic action between the metals keeping the wound open. When the staples were removed and replaced with ones of the samelot number, the wound closed with no problem.

Going on with my original question, has this protection using zinc coatings only been going on recently? When did it start?

I constantly notice a big built-up steel girder highway bridge (link-span arrangement, 2 main girders) in Everett, Washington, with the bron discoloration of rusting at the pin connections, nowhere else, and is not being attended to. It its the span that begins the Boeing Access Road over I-5. It appears to me that this could be galvanic action due to dissimilar metals at the pin to plate connection. This bridge is painted which could be galvanized, is only about 25 years old, and the connection is highly visible.

Mike McCann
McCann Engineering

 

[msquared48]

Thinking further here, where the pin goes thru the plate, any galvanic coating will wear off with time causing the metals to contact directly. Any difference in the metallurgy of the two pieces could provide a venue for galvanic action.

Mike McCann
McCann Engineering

 

[shin25]

msquared,
Hot dipped galvanizing chemically bonds to the surface. If the bolts are also hot dipped galvanized then, the situation that you are explaining will take quite sometime to take place.

In the concrete bridge business, hot dipped galvanized bearing system is very common.

 

[msquared48]

shin25:

I trust that you are correct. I still have my professional doubts though in this situation considering the constant rotational movements due live load impact and temperature changes. And, galvanic action itself will remove the zinc, regardless of any rotational movement.

Mike McCann
McCann Engineering

 

[apsix]

Any difference in the metallurgy of the two pieces could provide a venue for galvanic action.

If they are both of carbon steel, even if they have differing metallurgies, I suspect that the galvanic corrosion potential difference would still be very small.

 

[civilperson]

Use sacks of magnesium bonded to the steel with copper braided cables for cathodic protection. Less noble than zinc, (I think).