aluminum and steel corrosion

huangyhg

aluminum and steel corrosion
does anyone have any general information on ways separating aluminum steel when connecting them together?  i am working on a remodel and the architect would like to use polished aluminum frames and they would have to be supported by existing steel framing.
are there any special coatings, plastic washers, etc. that someone is aware of.  any help is appreciated.  thanks.
we have specified neoprene washers or pads to separate the two.  also specify stainless steel bolts.
in this locale, it depends on the profile of the aluminum extrusions involved, but often:
the aluminum is coated with an asphaltic emulsion (one of the products is provided by tremco; i can get a spec if you require) applied to the aluminum extrusion.  they also use wood or neoprene spacer shims (approximately 1/4" thick)to separate the aluminum and steel and the fastening from the aluminum is done using stainless steel screws.
once the frame is secured, i've also seen voids in the extrusion (or pairs) filled with sprayed foam (using duct tape to contain it).  add an ethafoam backer rod and caulk.
consider using cad or zinc plated steel bolts. i believe the cad is also superior for corrosive environments.
don't know if you have posted on the corrosion engineering forum.  if you haven't you may want to; there are probably some corrosion engineers there who do not check into the structural forum.
i think there may be a concern about the coating being damaged during the installation with a reduction in corrosion resistance.
if you check with many aluminum window/wall suppliers you will find a large number of them use ss for this reason.  also many architects spec ss (i hope it's not because they don't know any better <g>).
cad plating, however, allows you to use stronger fasteners.
using an asphaltic emulsion as dik proposed is good.  it isolates and lasts a long time.  additional isolation by use of spacers (rubber, neoprene, butyl) will add to this, but make sure the spacer material is compatible with the asphaltic emulsion or other coating.
use stainless steel fasteners.  anything else will ultimately corrode.
keep in mind that steel connected to aluminum theoretically results in the aluminum being corroded; however, as a practical matter, the initial corrosion by the aluminum passivates the surface very quickly and the predominance of corrosion occurs in the steel.
well it depends on whether the connection is going to get wet.  if it isn't there is no problem just bolt them together.  if they are going to get wet then things get more interesting.
however, if the existing steel is going to get wet then it will want to corrode anyway and should be provided with a protective coating anyway.  this will prevent any bimetallic corrosion.  of course if the steel isn't corroded then the aluminium may try and act as a sacrificial anode and might corrode.  i use the "may" and "might" deliberately because there is no guarantee especially if the aluminium is anodised as the oxide film may well prevent the passage of current.  safest thing to do though is to ensure that the steel is painted.
as to fixings the most common used type in europe would be the equivalent of grade 316 and there is no reason why this shouldn't have good mechanical properties.  an alternative would be to use a galvanized fixing.
this site has a lot of good discussions on this for example:
lots of good suggestions.
i strongly agree that the aluminum should be anodized.
also, regardless of whatever bolts, washers, etc., are used, i recommend using some lubricant/galvanic isolator compound on the threads.  see
thread 338-47695 has this link.  it's a very good 'blurb' on corrosion and i though it should be replayed.
kenvlach:
thanks for the lead