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Re: Stainless steel fasteners joining aluminum clip onto galvanized steel

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My suggestion is to separate the two steels, even in Los Angeles.  You can specify stainless steel shims , and as I understand it, there is now some special tapes that can be used to isolate the aluminum from the steel.  I don't know how well the tape works or lasts as a separation barrier between the steel and aluminum.
 
Michael Cochran S.E.
 
In a message dated 2/16/2005 9:10:34 A.M. Pacific Standard Time, Thomas.Honles(--nospam--at)ladwp.com writes:
Hello,

I was wondering what the members of the list think of the use of the various materials in this connection detail. We are currently reviewing an installation that was done on some roof overhang exposed steel framing. Some light architectural glass panels were added. The panels are aluminum framed and attached to support through a prefabricated system of aluminum rails and extruded angle clips. The aluminum is 6105-T5. The aluminum angle clips rest directly on hot dip galvanized structural steel framing, ASTM A501 or A53 Grade B, and are secured to the steel by Type 410 stainless steel self-tapping screws.

Because the system is well-drained, and not exposed to any particularly harsh environment here in Los Angeles, it seems that the corrosion issues should be negligible, and any galvanic corrosion potential in the dissimilar metals should result in the hot-dip zinc galvanizing acting as the sacrificial element. My check of a galvanic series table shows zinc being more anodic than any aluminum alloy. Also, zinc and aluminum appear closely together on the galvanic series, which leads me to believe that the corrosion potential is low and that the galvanized coating on the steel in the area of the connection would be durable for the life of the structure.

The panels are designed for wind loads and minimum gravity loads as would be applicable to a low-slope roof. Thanks in advance for your opinions.

Best regards,

Thomas Honles, SE, PE
Los Angeles, CA