Galvanized Steel in Contact With Weathering Steel 3

[KootK]

I have specified some new, galvanized steel that is in direct contact with existing non-galvanized weathering steel. I did not specify any kind of isolation system (neoprene, epoxy paint etc) between the two materials. I felt that it was not necessary because:

1) The application is a dry, conditioned, indoor environment (the galvainzing was really aesthetic).

2) The surface areas of the galvanized and non-galvanized components are roughly the same. It is my understanding that galvanic corrosion occurs much more slowly when the anode and cathod are of similar surface areas.

3) The weathering steel has developed a patina which ought to provide some measure of electrical insulation between the contact surfaces.

Now that the structure is in place, questions have arisen about the lack of isolation between the galvanized and non-galvanized components, particularly at the structural elements near to an exterior opening. I have two related questions:

1) Do my arguments above hold water? The impact of items #2 and #3 are difficult to quantify.

2) Short of installing some form isolation between the galvanized and non-galvanized components (very tough here), is there any other way to limit the galvanic corrosion potential?

Thanks for you help.

KK

 

[csd72]

Is the weathering steel connected to steel that is exposed on the outside?

 

[KootK]

csd72,

No, the weathering steel is not connected to exposed steel. It's connected to a concrete platform, which is exposed beyond the enclosed structure.

If the weathering steel were connected to exposed steel, would that change your opinion of the situation? Would that then be the source of the moisture required for the galvanic corrosion?

KK

 

[Ron]

The worst likely outcome in this is that you will get preferential sacrificing of the galvanizing near the connection. Outside that, not really an issue.

 

[rowingengineer]

You could almost argue that you’re helping the existing structure. I agree with Ron.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[KootK]

Ron,

What exactly is "preferential sacrificing"? Is it something other than rusting?

Thanks for your help.

KK

 

[Ron]

In the area where the galvanized steel touches the weathering steel, it will serve as a sacrificial anode to both the steel it is covering and the steel to which it is connected. Since the weathering steel is otherwise unprotected, it will take advantage of the adjacent galvanizing, thus likely depleting it at a faster rate near the connection than farther away from the connection, though probably not to any great detriment to either.

As RE noted, the area of weathering steel in contact with the galvanizing will actually receive some extra protection.

 

[apsix]

Your point 1) trumps all other concerns; it's dry so corrosion won't be a problem.

Regarding point 2); I believe that the surface area of the more noble metal should be a lot less than that of the other to minimise corrosion. Therefore it's generally OK to use stainless steel bolts for galvanised members, but not to use galvanised bolts for a stainless structure.

 

[csd72]

looks like you need to do some reading:

http://www.galvanizeit.org/

The reason why I ask is because of the preferential sacrificing mentioned above. If any of the metal that it is electrically connected to has a tendency to start corroding then it will be the zinc in your galvanising that sacrifices itself instead.

This is used to advantage sometimes -look up sacrificial anode.

 

[Ron]

To further clarify my comments above, the "preferential sacrificing" to which I referred is the galvanizing being depleted, not the steel. Preferential refers to the localization of the depletion (nearer the connection) because of the larger area influence on the galvanizing.

 

[KootK]

@Ron:

Thanks for the clarification. Whether it was the steel or the galvanizing which what being preferentially depleted was where I was getting confused.

@Aspix:

The trouble is that the members of concern here are at the perimeter of the structure, about 5' from an exterior opening. Deicing salts are also going to be used on the concrete outside the opening as well. I was hoping to be able to derive some benefit from a mechanism other than dryness.

For the sake of what follows, let's say R = surface area of more noble metal / surface area of less noble metal. I agree, it is best for corrosion when R is small and worst when R is large. Since it's a continuum, however, there must be an in between condition where corrosion is moderate, right? This discussion has got me wondering about metal roof deck. We commonly use galvanized roof deck in direct contact with carbon steel. Why isn't galvanic corrosion an issue there? Is it just that you're generally dealing with a dry service environment?

@csd72:

I'm still a little unclear about the role of moisture / exposure at various locations. Here's my understanding based on your comments so far (and some reading):

1) No matter what is going on elsewhere, galvanic corrosion requires that there be an electrolyte present at the actual contact surface between the galvanized and non-galvanized components.

2) If a bimetallic cell does exist at the contact location, zinc from the galvanized component will be sacrificed to protect the non-galvanized component at locations that may be quite distant from the contact area. This protection / zinc sacrificing may even be extended to components that are connected to the member in contact with the galvanized piece. I would assume that there would need to be an electrolyte present at the connection of the two connected parts however.

Is that correct? I thought that the protection of the non-galvanized component would be limited spatially to an area pretty near to the contact area between the galvanized and non-galvanized pieces.

 

[csd72]

Moist air can be an electrolyte though usually a slow acting one.

Point 2 is exactly what I was getting at.

 

[KootK]

Also, no one has commented on the patina argument (point #3 in the original post). Any ideas there?

Is patina on carbon steel easily removed? I know that on some metals it is flaky and just falls off while, on others (bronze), it is quite stable.

Would one expect to lose the patina entirely when existing weathering steel is sand blasted and/or wire brushed?

Thanks for all the help so far.

KK

 

[KootK]

@csd72: good point regarding the moist air. Thanks.

I used to have a document that showed some sample calculations regarding the corrosion potential when anodes and cathodes of various surface area ratios were involved. Unfortunately, I've lost it. Does anyone know of any similar documents that might be available?

In particular, I'm curious about the definition of surface area for the anode and cathode. For example, would the surface area of the galvanized piece be:

1) Just the surface area of the galvanized member in contact or;

2) The surface area of the galvanized member in contact plus any other galvanized members that might be connected to the member in contact, either directly or indirectly?

 

[dik]

Just a caution... weathering steel will rust and stain adjacent surfaces...

Dik

 

[Ron]

KK...weathering steel has a chemistry and metallurgy that allows a fine, consistent corrosion to occur. As long that layer is present, very little deeper corrosion takes place. It becomes a passivating layer for the underlying steel. When the layer is removed by mechanical means, the patina will disappear, but reappear as the corrosion layer develops again.

As dik noted, the patina will stain adjacent surfaces.

The patina is an iron oxide, so it does not provide any electrical insulation that would prevent or preclude galvanic corrosion if connected to dissimilar metal (unless of course that dissimilar metal is the galvanized steel, which is OK).

 

[KootK]

Ron,

I'm a little confused by your last sentence, specifically the part in parentheses. Are there some situations in which a patina would provide some electrical insulation? Or are you simply saying that the patina is only good for protecting the element upon which it develops?

In my situation, I have a new galvanized column installed on top of an existing, weathering steel beam. The original patina would likely have been removed in the process of cleaning up the beam. Would the patina be expected to re-devlop between the beam and the column base plate as a result of galvanic corrosion? Would it provide any isulating benefit if it did? I suspect that the answer to both questions is no but I'd like to be certain.

Thanks

KK

 

[Ron]

KK..I would not expect the patina to develop between the galvanized base plate and the weathering steel beam for a very long time due to low oxygen exposure. I might happen but not so likely as with fully exposed weathering steel. Even if it develops, it will offer no "insulation" to the steel, but then, none is needed since you're in contact with galvanizing, not some significantly dissimilar material (such as aluminum or copper, etc.).

I would not expect any galvanic corrosion between the column and beam, in fact, the contact with the galvanized base plate will likely prevent corrosion of the weathering steel. I would expect no detrimental influence on either for this application.