A different thread (thread391-518870) got sidetracked onto the topic of bonding to aluminum and stainless.
I started this thread to give the subject it's own track.
From that other thread:
TugBoatEngineer said:
TugboatEng (Marine/Ocean)4 May 24 21:05
MJ, are you interested in making longer lasting bonds to aluminum? I hear a lot of interest in using siloxanes to tie up aluminum oxide preventing the formation of aluminum hydrates when the structure is exposed to water.
MintJulep (Mechanical)8 May 24 11:54
@TugBoatEngineer: Yes, my industry (Passenger trains) has a long and continuing history of failed bonding of aluminum and stainless steel.
The vehicle structure is typically AL or SS. Manufacturers constantly want to bond floor panels to the structure - because it's faster and cheaper for them - and then in some years the bond fails. Door panels are SS or AL skin, bonded over a honeycomb core; they last maybe 10 years before the skins debond. Various internal panels are AL skin over something core, floor panels are SS bonded over plywood or balsa. Equipment boxes are riveted aluminum, with the joint faying surfaces "sealed so they don't leak" - until the seal bond fails.
But since most of the bonds are good through the end of warrantee the manufacturers don't care, and don't want to hear about proper surface preparation and extra, expensive things like siloxane to make the bonds last longer.
TugboatEng (Marine/Ocean)10 May 24 03:35
I wish I had a single specific source explaining the problems. Aluminum and stainless are particularly difficult to bond because they both form surface oxides very rapidly. They should be abrasive blasted immediately prior to assembly which usually isn't an option.
Buy explosively welded plates and cut strips from it.
The weld like-to-like on both sides.
With large flat surfaces it is very risky situation.
You need a fairly thick bond layer, and it must have some flex to it because of thermal expansion issues.
You blast each surface and apply the alloy specific primer.
And then the compatible adhesive between them to join.
This whole process is a pain and the materials are expensive.
In order to control the thickness of the bond layer, some adhesive manufacturers are using glass or rubber microspheres, the rubber microspheres add resilience as well.
One thought I was having that may allow the use of more conventional adhesives as opposed to the $60 per oz 3M epoxies is to use a strip of rubber in the joint. It's the same idea as double sided tape but with a more forgiving surface condition tolerance. Double sided tape does not work well on wavy surfaces. Many types of rubber can be bonded at 100% of their strength. Neoprene rubbers can exceed the tensile strength of epoxies.
