Front Office wrote:
I need to build a dozen outdoor canopies for an office building.
I want to use structural aluminum channel, bolted with steel bolts.

Where might expect corrosion to arise over the years, and would
it be mainly in the aluminum or in the steel bolts?

Thanks for any help.

Please post here or email me at grayscale[AT]verizon.net

Ralph

I would think stainless steel bolts or some form of fiber or plastic
(lucite) washer would provide the rigidity and separation. Not that
that view is universal:

http://www.eng-tips.com/viewthread.c...=154321&page=1

mattmlm (Structural)
9 May 06 10:21
I've got a project where Aluminum columns (and base plates) are being
anchored to a concrete pier/footing. The contractor used your everyday
common steel threaded anchor rods. In less than two weeks they are
completely rusted over. The contract documents called for hot-dipped
galvanized anchor bolts. (before judging, we did not provide the design
or specifications, simply consulting on a separate portion of the
larger project)

I'm thinking of having the contractor cut off and drill-in epoxy grout
anchors.

Will the galvanized anchor bolts react to the aluminum? Should I use
aluminum anchor bolts? If alum, what affect would the anchors have in
steel reinforced concrete?

Thank you,

MLM

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Goahead (Aerospace)
9 May 06 11:10
Galvanized steel anchor bolts should be OK with aluminum.
As an extra precaution the contact areas could be painted with
insulating paint.

http://www.welding-advisers.com/
TVP (Materials)
9 May 06 11:30
Al bolts are unlikely to be strong enough if they were not specifically
incorporated into the design. Zinc is the most compatible coating for
steel bolts that is also relatively cost effective.
Heckler (Mechanical)
9 May 06 12:09
Aluminum and Zinc are really close together on my Galvanic Corrosion
Potential Chart.
mattmlm (Structural)
9 May 06 12:24
Is that good thing or a bad thing?

MLM
theonlynamenottaken (Structural)
9 May 06 13:17
Its a good thing... the farther apart two metals are on the galvanic
series the greater potential between them, hence more aggressive
attack.

Aluminum embedded in and in direct contact with concrete is not
acceptable. A chemical reaction occurs and the concrete literally self
destructs from the expansion (product of the reaction, just like rust
on carbon steel that is allowed to corrode in concrete). I don't know
about epoxy grouted aluminum in concrete but I've seen slabs with
aluminum cast into it break themselves apart.
UcfSE (Structural)
9 May 06 13:23
I have a question to go along with this. I have noticed in the past
with aluminum panels that the manufacturer of the panels has
recommended stainless steel screws with some kind of special washer
between the steel and the aluminum. Does that line of thinking work
also in this case, say with using stainless steel anchor bolts or epoxy
bolts with an aluminum base plate?
mattmlm (Structural)
9 May 06 13:40
How good is stainless in direct contact with Aluminum? When this issue
first came up, the only thing that I thought of was stainless. Then
came the comments about the specs calling for hot-dipped galvanized.
I've always assumed that stainless was preferred, if not required. Is
that assumption correct?

MLM

Also, thanks for the input.
Heckler (Mechanical)
9 May 06 14:35
ASTM A-153 or ISO 1461 covers hot-dip galvanizing. I think it's more
of a cost factor when considering Stainless verses galvanized steel.
Goahead (Aerospace)
10 May 06 12:04
Stainless should not be in contact with aluminum. Stainless fasteners
should be Cadmium or Zinc plated before
being inserted in aluminum bodies. Usual anodizing or conversion
coating are not protective enough at the contact place.

http://www.welding-advisers.com/
IFRs (Petroleum)
26 May 06 16:21
In my experience (27 years in the petrochemical industry), aluminum and
stainless fasteners are acceptable in most environments. Althought
they are far apart on the galvanic scale, the typically much larger
area of the aluminum part compared to the fastener reduces to nearly
nil the tendency of the aluminum to corrode. However, in this case,
I'd definitly use galvanized anchors - they were specified, are
compatible, are probably more available not to mention less costly.
Note that the environment is a critical issue - If this is on the
seashore, or exposed to wet atmosphere, or under water, or vibrating
from machinery, etc then corrosion needs to be re-evaluated.