Corrosion at Aluminum/Steel Junction

Normal mild steel or alloy steel is very close to alumnium on the galvanic
table (-0.7 cf -0.75 V) so galvanic corrosion is unlikely ... however you
need to make sure the steel is protected from surface corrosion which is a
separate issue.

To use stainless steel bolts in aluminium is risky. Stainless steel will
have a potential of approx -0.2 V and so you have a -0.5 V potential which
is on the limits of acceptability. Is the interface was protected with a
product such as the Duralac recommended by Stealth Pilot then you might get
away with it. Otherwise pitting of the the aluminium around the strainless
fasteners might occur if moisture is present.

Stainless bolts in aluminium structure are not as bad as putting aluminium
bolts in stainless but still risky business if outdoors. An absolute no no
if in a salt atmosphere.




"george" wrote in message
oups.com...

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

Use only stainless steel.

"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

It is difficult to argue with someone who simply says '********' with no
explanation. However:

1. There are really several issues here. First we need to make the
assumption that both the aluminium and steel are adequately protected
against general surface corrosion. Lets assume that this is the case. As I
understood the original question, the issue was really "is steel and
aluminium going to cause any problems from the point of view of dissimilar
metal corrosion" (i.e. galvanic corrosion).

2. The answer, as per my previous post is, no. Following is some further
explanation:

3. In a case of galvanic corrosion you have two metals with different
voltage potentials. In this case say carbon steel bolts at -0.76 and
aluminium at -0.8 V (the actual numbers vary a little depending on where you
get them from). Stainless steel would be approx -0.2 V. Now in the presence
of a conductive media (i.e. moisture) a current flows from the more anodic
(aluminium in this case) to the more cathodic (steel) and the anode
(aluminium) will be eaten away.

4. Normally most corrosion control standards would say that a difference in
potential of 0.25V or less is acceptable. Clearly this is the case with
carbon steel and aluminium (difference is only 0.04 V). Thus the interface
is acceptable.

5. It is essential that the anodic material be the larger mass of metal
(which is the case awning under discussion). The smaller fasteners should
always be more cathodic. If it was the otherway around and we had aluminium
bolts in a steel frame then the bolts would certainly be consumed. Whilst
stainless steel fasteners have a voltage difference of 0.5+ V you can get
away with small stainless fasteners in aluminium because of the relative
mass of the two metals (see below in 7)

6. In any dissimilar metal situation is is advisable to put some form of
surface protection between the metals. Paint, sealant etc. If you can
exclude moisture you can generally get away with all sorts of sins ...
however the problems arise when the paint cracks and you get moisture
ingression so best not to rely on paint.

7. I refer you to MIL-STD-889 "Dissimilar Metals" and I quote "Generally the
closer one metal is to another the more compatible they will be" and another
"A small anodic area relative to the cathode area should be avoided. The
same metal or more cathodic metal should be utilised for small fasteners,
and bolts. The larger the relative anode area, the lower the galvanic
current density on the anode, the lesser the attack"

8. I also refer you to USAF T.O. 1-1-2 "You can with proper protection use
steel rivets in aluminium but you can never use aluminium rivets in steel"

9. I also refer you to AvP 970 which shows carbon steel and aluminium in the
same group indicating that they are safe for use together. (in MIL-STD-889
they are in adjacent groups but would still be considered OK ...
particularly when considering the leatives mass of the two metals).

Finally a practical illustration of what I am saying. I have, on the windows
of my house, flyscreens with aluminium frames. These are in a salt
environment 1/2 mile from the beach. They have stainless steel clips riveted
to them that are a simple angle of sheet stainless. They also have normal
carbon steel bolts through the frames in a nearby area. There is zero
evidence of corrosion between the steel bolts and the anodised alumnium
frame. The aluminium frame is completely and totally eaten away underneath
the stainless steel clips in a 1/2 x 1/2" area. Outside this region there is
no corrosion.



"Jim Watt" wrote in message
...
On Sat, 24 Jun 2006 22:43:45 +1000, "......... :-\)\)"
wrote:

Normal mild steel or alloy steel is very close to alumnium on the
galvanic
table (-0.7 cf -0.75 V) so galvanic corrosion is unlikely ...

B O L L O C K S


--
Jim Watt
http://www.gibnet.com

"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.

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:

:
:"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

In message .com,
writes
Hard to get zinc chromate in the average consumer product
anymore. It's been linked to cancer.

Milspec products a decade ago used to use strontium chromate in a grease
base, but even then the search was on for a less unfriendly replacement.

--
Paul J. Adam

"JP" wrote in message
...
Here's some links giving more precise explanation. The process is called
explosion welding (EXW).

http://www.key-to-nonferrous.com/Articles/Article51.htm

The following link is an excellent one

http://www.highenergymetals.com/Engi...b%20Page.p df

Who says you cant learn anything on usenet? g

Thanks. I had never heard of explosive welding, until now.
--
Jim in NC