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