Aluminum and Steel Corrosion 1

[RDM]

Does anyone have any general information on ways separating aluminum steel when connecting them together? I am working on a remodel and the architect would like to use polished aluminum frames and they would have to be supported by existing steel framing.

Are there any special coatings, plastic washers, etc. that someone is aware of. Any help is appreciated. Thanks.

 

[JAE]

We have specified neoprene washers or pads to separate the two. Also specify stainless steel bolts.

 

[dik]

In this locale, it depends on the profile of the aluminum extrusions involved, but often:

The aluminum is coated with an asphaltic emulsion (one of the products is provided by Tremco; I can get a spec if you require) applied to the aluminum extrusion. They also use wood or neoprene spacer shims (approximately 1/4" thick)to separate the aluminum and steel and the fastening from the aluminum is done using stainless steel screws.

Once the frame is secured, I've also seen voids in the extrusion (or pairs) filled with sprayed foam (using duct tape to contain it). Add an ethafoam backer rod and caulk.

 

[haynewp]

Consider using cad or zinc plated steel bolts. I believe the cad is also superior for corrosive environments.

 

[Watermelon]

Don't know if you have posted on the corrosion engineering forum. If you haven't you may want to; there are probably some corrosion engineers there who do not check into the structural forum.

 

[dik]

I think there may be a concern about the coating being damaged during the installation with a reduction in corrosion resistance.

If you check with many aluminum window/wall suppliers you will find a large number of them use SS for this reason. Also many architects spec SS (I hope it's not because they don't know any better <G>).

Cad plating, however, allows you to use stronger fasteners.

 

[Ron]

Using an asphaltic emulsion as dik proposed is good. It isolates and lasts a long time. Additional isolation by use of spacers (rubber, neoprene, butyl) will add to this, but make sure the spacer material is compatible with the asphaltic emulsion or other coating.

Use stainless steel fasteners. Anything else will ultimately corrode.

Keep in mind that steel connected to aluminum theoretically results in the aluminum being corroded; however, as a practical matter, the initial corrosion by the aluminum passivates the surface very quickly and the predominance of corrosion occurs in the steel.

 

[GGedge]

Well it depends on whether the connection is going to get wet. If it isn't there is no problem just bolt them together. If they are going to get wet then things get more interesting.

However, if the existing steel is going to get wet then it will want to corrode anyway and should be provided with a protective coating anyway. This will prevent any bimetallic corrosion. Of course if the steel isn't corroded then the aluminium may try and act as a sacrificial anode and might corrode. I use the &quot;may&quot; and &quot;might&quot; deliberately because there is no guarantee especially if the aluminium is anodised as the oxide film may well prevent the passage of current. Safest thing to do though is to ensure that the steel is painted.

As to fixings the most common used type in Europe would be the equivalent of grade 316 and there is no reason why this shouldn't have good mechanical properties. An alternative would be to use a galvanized fixing.

 

[haynewp]

This site has a lot of good discussions on this for example:

http://www.finishing.com/160/63.html

Also, go here

http://assist2.daps.dla.mil/quicksearch/

and type in MIL-STD-889 in the document ID search.

 

[kenvlach]

Lots of good suggestions.

I strongly agree that the aluminum should be anodized.

Also, regardless of whatever bolts, washers, etc., are used, I recommend using some lubricant/galvanic isolator compound on the threads. See thread338-47695

 

[dik]

Thread 338-47695 has this link. It's a very good 'blurb' on corrosion and I though it should be replayed.

kenvlach:

Thanks for the lead

http://www.mcnallyinstitute.com/04-html/4-1.html

 

[MFDJ9]

RDM,
The company I work for manufactures window frames and curtainwall and we generally use PVC shims or plastic separators to isolate aluminum and steel. As for bolts,sometimes we use galvanized Grade 5 Steel bolts or if you are planning on using fasteners, you can try dril flexs. They're Grade 5 Steel fasteners with stalgard coating for corrosion resistance. It really depends where the bolts and fasteners are located..,if it is in the interior of the building,you don't really have to worry about corrosion because its a non exposed area..(not exposed to weathering).

 

[RDM]

Thank you all for the input. It was very helpful.

 

[kenvlach]

MFDJ9,
Re &quot;Grade 5 Steel fasteners with stalgard coating for corrosion resistance.&quot;

Please give info or a link for 'stalgard coating'
Maybe Al-coated steel?

 

[GGedge]

In reply to dik.

Be careful with the link you posted, especially the stuff on bimetallic corrosion.

The series on seawater is only really applicable to that environment and the series is based on potentials measured in sea water. As such it only really defines which metal will be the anode and which the cathode, not whether anything will happen (ie corrosion)or indeed the rate at which it might happen.

Also note that the idea that the further apart in the table the metals are (in other words the magnitude of potential difference) does not determine the rate of corrosion nor the severity of damage.

 

[MFDJ9]

kenvlach,
Here's the link to the Grade 5 Drilflex fasteners with stalgard coating.

http://www.elcoconstruction.com/construction/elcoproducts.html

You can also look up the capacities of the drilflex fasteners from ICBO report # ER-4780 which is on the internet.

 

[dik]

GGedge:
Thanks... correct about the seawater... posted the link from the thread because it was interesting. It was, in a way, off the topic.

 

[kenvlach]

GGedge, dik et al.,
The galvanic series for metals and alloys in neutral soils and water, ASM Handbook 13, Corrosion, p. 1288, is similar to that for seawater. Galvanic corrosion can occur as long as there is an electrolyte, and even rainwater and condensate have some conductivity due to dissolved CO₂, air pollution, dissolution of the contacted metals, residual cleaner, etc.

Agree with your earlier comments to anodize the aluminum and paint the steel, but still believe some thread sealant should be used, along with insulating washers as others have suggested.

MFDJ9,
thanks for the link. Couldn't find the exact definition of stalgard on the site, so sent them an email. I did get the impression that it is an aluminum coating with supplemental chromate.

 

[oijammu]

We normally use heavy duty masking tape between the two surfaces. It works especially between Aluminium roof sheeting and MS Z-purlins. I used it in abuilding in 1984, and no corrosion has not been detected todate

 

[dik]

kenvlach:

I'm trying to chase down some information about painting the steel... there's a school of thought that this may contribute to corrosion of the fastener. Any info on this from your end?