Corrosion at Aluminum/Steel Junction

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

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

Start your own Eng-Tips Group! Click Here!
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.

"Front Office" wrote in message
. ..
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

Ralph,
I'm 'going through' my 11 year old aluminum boat trailer right now which was
bolted together with galvanized bolts. I don't see the aluminum pitting I
would have expected around the bolt holes. I do have a few rusty bolts which
are easily replaced. I recommend you use this method. If you go with S/S
bolts you WILL have pitting around the aluminum holes (as the least noble
metal gets the damage) and it will not be easily repaired in the future.

Larry

My turbine powered boat project;
http://www.turbinefun.com

"Front Office" wrote in message
. ..
Where might expect corrosion to arise over the years, and would
it be mainly in the aluminum or in the steel bolts?

I suggest you take this question to rec.crafts.metalworking.

Vaughn

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


In a marine environment, the aluminum will go more quickly.

The ADAMS-class DDG and the FFs made during the 1960s had aluminum---or
should I say, al-loo-min-ee-um?-- bolted to a steel plate with a special
sort of bolt called a Huck bolt. They all experienced corrosion of the
aluminum. (black electrical tape separated the two metals.)

The TICONDROGA and SPRUANCE class ships had aluminum superstructures
which were welded to a "fusion bar" which consisted of a piece of very
clean and smoothly-machined piece of aluminum FLAT BAR joined to a very
clean piece and smoothly machined piece of steel flat bar via explosive
charges.


M.S.

"Mark Andrew Spence" wrote

The TICONDROGA and SPRUANCE class ships had aluminum superstructures which
were welded to a "fusion bar" which consisted of a piece of very clean and
smoothly-machined piece of aluminum FLAT BAR joined to a very clean piece
and smoothly machined piece of steel flat bar via explosive charges.

What was the purpose of the explosive charges? (besides, to go "bang" g)
--
Jim in NC

I am now more or less "trespassing" here but I would like to give an
explanation to this question.

The very smooth metal surfaces (bars), aluminum and steel are placed one on
top of an other and the explosive charge is placed, I guess above the
aluminum bar? Does this process take place in some kind of a purpose built
explosion chamber or are the materials simply buried underground (the weight
of e.g. a thick layer of sand would give enough resistance) to be fused, who
knows?

The idea is, that the detonation creates shock waves so intense, that the
metal atoms of the contact surface (layer) area are a sort of mixed
(knocked) together, hence the term fused. The process is probably called as
explosion welding? The created bond is very strong and there is no room for
any moisture to get in. That is why the corrosion can't take place between
the dissimilar materials. At least I figure this to be the case. The Mark
Andrew Spence may give more accurate explanation.

JP


"Morgans" wrote in
...

"Mark Andrew Spence" wrote

The TICONDROGA and SPRUANCE class ships had aluminum superstructures
which were welded to a "fusion bar" which consisted of a piece of very
clean and smoothly-machined piece of aluminum FLAT BAR joined to a very
clean piece and smoothly machined piece of steel flat bar via explosive
charges.

What was the purpose of the explosive charges? (besides, to go "bang"
g)
--
Jim in NC

"Morgans" wrote in message
...

"Mark Andrew Spence" wrote

The TICONDROGA and SPRUANCE class ships had aluminum superstructures
which were welded to a "fusion bar" which consisted of a piece of very
clean and smoothly-machined piece of aluminum FLAT BAR joined to a very
clean piece and smoothly machined piece of steel flat bar via explosive
charges.

What was the purpose of the explosive charges? (besides, to go "bang"
g)
--
Jim in NC

To make a compression weld.

Since steel and aluminum melt at different temps you can't heat weld them
like you would with normal steel-steel or heliarc(sp?) (heat weld in a
controlled atmosphere) like you would aluminum-aluminum.

"Morgans" wrote:

:
:"Mark Andrew Spence" wrote
:
: The TICONDROGA and SPRUANCE class ships had aluminum superstructures which
: were welded to a "fusion bar" which consisted of a piece of very clean and
: smoothly-machined piece of aluminum FLAT BAR joined to a very clean piece
: and smoothly machined piece of steel flat bar via explosive charges.
:
:What was the purpose of the explosive charges? (besides, to go "bang" g)

It's a method of driving two different metals together in such a way
that they sort of blend into each other rather than having to be
welded, bolted, or some such.

The same process came to be used for coins when we switched from
silver to copper-cored coins.

--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw

On Mon, 19 Jun 2006 18:34:46 +0000, 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

a product called Duralac is designed to prevent the electrolytic
corrosion between dissimilar metals.
it is an aviation product made in england. comes in a white and yellow
tube.
it is a toothpaste consistency yellow fluid containing zinc chromate.
it dries slowly to a rubber consistency.

the stuff is used for pop rivets with great benefit in corrosion
control. you just dip them in it prior to setting the pop in a wing
skin.

it is good stuff. two tubes in my aviation tool box.
Stealth Pilot