Galvanic Corrosion between Aluminum Extrusion and Steel Fastners 3

[brut3]

I've been given the task in looking into fastener alternatives that our company can use to attach 6063-T6 Aluminum Extrusions to various substrates with different grades of carbon steel fasteners.

The particular application I'm looking at applying this fasteners in are for Drained/Back Ventilated cladding systems where an electrolytic liquid will be present and galvanic corrosion becomes one of primary design issues. We've typically been using SAE Grade 5 Dril-Flex fasteners (typically a curtain wall industry standard, see link below for specifics), however I'm looking into exploring the use of SAE grade 1 Buildex TEK fasteners with a bonded neoprene washer to help minimize the dissimilar metal interaction (see link below).

Before I suggest performing a salt spray test to compare these two fasteners would anyone expect to see similar results due to the neoprene washer between the materials?


Elco Dril-Flex Fastener Informatoin
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http://www.elcoconstruction.com/drilflex.asp

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http://files.engineering.com/getfil...41-162b535f6293&file=Stalgard_Silver_Spec.doc

Buildex TEK Fastener Data
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http://www.itwbuildex.com/bondedtek.htm

Thank you in advance for your input!

 

[TVP]

tbruton3,

First, are you aware that Elco also provides fasteners with EPDM sealing washers? Their product is called Flo-Seal®:

http://www.elcoconstruction.com/floseal.asp

Second, I prefer the Stalgard® coating from Elco (zinc rich basecoat, Al-rich topcoat) to Climaseal® from ITW (mechanical Zn plating + dip-spin thermoset polyester topcoat). It is better for bimetallic corrosion where Al & steel are the mating pieces.

Regarding your original question, the EPDM washer will certainly improve the corrosion performance of the finished assembly, because it provides an isolator between the Al and steel pieces, and increases the distance that any electrolyte must bridge between the two disparate metals. Testing just the fastener by itself in a salt spray environment will not show any real difference, unless you are intending to compare the nominal corrosion resistance of the Stalgard® coating vs. the Climaseal® coating, in which case the results will be moot because it is the system performance that is important, not the fastener in isolation.

 

[CoryPad]

I agree with TVP that the Stalgard finish is preferred to the Climaseal finish. One additional point is that the TEKS washer is galvanized, which is not as good as either Stalgard or Climaseal in contact with aluminum alloy structures.

 

[brut3]

I understand and agree that the Stalgard coating is a superior finish compared to the climaseal coating, however, the driving motivation behind finding an acceptable alternative would be a substantial savings per job seeing as the Dril-Flex fastener is about 4 times as expensive as the Buildex.

The overall goal is to have a cladding system that will not only be structurally sound but also provide adequate resistance to galvanic corrosion.

My next question stemming from this discussion is would there be a marked difference in the effective lifespan if the Buildex with Bonded washers were used instead of Dril-Flex?

 

[TVP]

Wow, I would not expect that a Buildex TEK screw with an EPDM washer would be 1/4 the cost of a Dril-Flex® screw that does not include a washer. Without knowing more about your specific design, mating part geometry, etc., I can only hazard a guess, which would be that these two parts would perform similarly, meaning +/- 25% before the onset of corrosion. The addition of the EPDM washer would offset some of the disadvantage of the coating.

 

[davenc]

Comparing the Buildex Teks fastener with the Elco DrilFlex is not a fair comparison. the DrilFlex is designed to withstand hydrogen assisted stress corrosion cracking. The Buildex Teks fastener is a case hardened product that will fail in the presence if aluminum and moisture. If this is a critical application you might want to think twice. If not a critcal application Elco makes fasteners similar to the Buildex Teks w/ the Stalgard coating.

 

[metcoat]

Can you go into a little more detail on the electrolytic liquid? How long will the liquid be in contact with the fasteners?

Metal Coatings Corporation - Houston TX

http://www.metcoat.com

 

[brut3]

The main application we're concerned about is when our cladding system will be placed in coastal environments which would make the electrolytic liquid in question salt water. I have to clarify that this isn't a completely open system where the fasteners will be in direct contact with water however the fasteners encounter water vapor that is constantly building up and evaporating off.

I hope this answered your question.