Stainless Steel fasteners are used for aluminum and steel connections - also with the neoprene or rubber gaskets to separate the two materials.

Thanks,

Is it also true that if the steel studs are hot-dip galvanized that you don't need to separate the two materials with the neoprene or rubber gaskets?

That I do not know - I would probably feel safer with the galvanized studs.  Even painted studs have a layer of separation.

But just a small flaw in the galvanizing or the paint would initiate the reaction.

Hi openam

I have connected steel and aluminium together using threaded fasteners however I isolated them by using nylon spacers and nylon sleeves in the bolt holes to stop the bolts touching the aluminium.

regards desertfox

If you use rubber you could lose strength in you rivet joint.
You can use tape as a seperator. Fiberglass works well, even packing tape works it you have nothing else. Just make sure steel is clean and then primed, then apply tape over studs. Just make that it does not store water. If you want to install Al rivets you can prime the steel then apply primer to holes then install rivets wet (dipped in primer). Then prime the bucked end. Use zinc-chromate primer.

Cad or zinc plated hardware will also work. The following site gives more detail.

http://assist.daps.dla.mil/docimages/0000/32/77/36032.PD5

Your big issue is galvanic corrosion.  Aluminum and steel are far enough apart on the scale that you're going to have a battery in the presence on any electolyte (even the humidity in the air).  The only way is to insure that there is no electrolyte present at the joint.

The "Big SUV" of the private aircraft set - also known as the Helio Courier, has a cromoly tube frame aft and an aluminum semi-monocot frame up front.  Molten aluminum is sprayed on the cromoly frame to allow for connection of the aluminum portion of the airframe while still isolating out any electolyte that could cause galvanic corrosion.

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

Would anodizing the aluminum solve the issue of galvanic corrosion when it is in contact with stainless steel?

kjf1,

It doesn't solve the problem, but it may help.  There still exists the problem of areas of the aluminium part that aren't covered by aluminium oxide, either from lack of original coating (sharp edges, etc.), or due to coating damage.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

See this for a dwg. Search site for more?
http://www.alu-info.dk/Html/alulib/modul/A00175.htm

Hi all,

Interesting subject. We have an overhead conveyor system with an aluminum rail. We also used to use an aluminum splice plate at the rail splices but were plagued with the plates cracking.  We started using steel splice plates which are painted. There are 4 stainles steel fasteners on each side if the joint but these are not separated from the aluminum in the holes.  Does anyone see a potential problem with this?

Thanks for any help in advance.

It is possible there could be a problem if electrolyte is present and does not drain.  Luckily, you have a favorable anode:cathode area ratio, so the problem shouldn't be too bad.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Maybe nobody else has seen it done, but there is a company in Western Washington here that does explosion welding of Aluminum to steel. I've held a finished piece in my hands before.

In this case it would probably involve bonding an aluminum and steel plate, then attaching the fasteners to the steel portion. The aluminum still has to be protected from corrosion.

Tony Holt, P.E.
Machine Development Co.

See http://www.alu-info.dk/Html/alulib/modul/A00185.htm

the rate of corrosion is also dependent on the relative surface area's of the two parts connected. Aluminum is the material which corrodes when the two materials are connected. if you have a large surface area of aluminum and a small surface area of steel then (such as siding and nails) then the corrosion of the aluminum, even though accelerated, will be spread out over a large surface area and not tend to be as severe. you can reference "Chemistry: The Central Science" by brown Lemay and Bursten for an example problem on this exact subject

In the marine industry an aluminum pilot house is often fabricated then mounted onto a steel deck. The steel bolts used are isolated from the aluminum flange using plastic tube over the bolts and ss washers over plastic washers. A 1/4" gasket is used to seperate the flange from the steel deck. This works for many years before the gasket or plastic fails.
Some years ago I read an article about a special weldable alloy that solved this mounting problem. The alloy, in a rectangular shape 1/2" x 3" was tig welded to the house base flange. After the house was set in position the exposed alloy was welded to the deck. The problem is I cannot remember the periodical name containing this article. It was a Marine Industry periodical. Any answers?

The current issue of "Welding Journal" has an article about Resistance spot welding aluminum to steel with a transition material.  

In shipbuilding and aeronautics it is put a layer of MASTINOX paste, which isolate electically, consequently reducing corrosion

The original question was to join aluminum to steel and what to do about the fasteners.  Fasteners can be anything more noble than the materials you are joining.  Stainless steel would work.  Squirt sealant in the holes immediately before you insert the fasteners.  Chromated sealant works best.  It inhibits aluminum corrosion.  But it's also a carcinogen.  Non-chromated sealant is better than nothing.

Separate the steel and the aluminum.  Painting each side of joint before joining will work.  Sealant is better.  Sealants or paints with a corrosion inhibitor in it work better than those without. Gasketing material would also work for separating the steel and aluminum.  

Anodizing the aluminum first will help, but the anodize is too thin to work by itself completely.  It gets scratches.

 

Would galvanic corrosion occur between zinc-chromate coated steel and aluminum alloys of 5000 or 6000 series? The environment is harsh – sea humidity.  

I read, on the Internet, that if difference between Anodic Index numbers is more than 0.15 V, than the galvanic corrosion does occur. According to the same source:
Anodic Index for chromium plated surfaces is 0.60, for Aluminum other than 2000 Series – 0.90, for plain carbon and low alloy steel – 0.85.

So, (my second question) it appears that contact between plain carbon steel and aluminum will not cause galvanic corrosion. Is it correct?

Thank you.

It is not correct to state that galvanic corrosion will not occur between carbon steel and aluminium alloys.  This pairing definitely produces galvanic corrosion.

There is a possibility of corrosion (general and galvanic) for steel plated with zinc + chromate conversion coating in contact with aluminium alloys.  The zinc coating is practically worthless in harsh environments.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

CoryPad,

Thank you for the assistance.

However, how to relate your first answer with the Anodic Index rule of 0.15V or less for the galvanic corrosion to do not occur? Again, low allow steel - 0.85, aluminum - 0.90.

What coating is suitable for the marine environment?

Thank you, again.