Greetings all,
I have a UP aluminum sided car that has some nasty corrosion. I have made some attempt to repair, but apparently not good enough as some trouble spots have raised their ugly head one again. I have cut out cancerous areas and tried to weld patches into place ... some take, some don't. Not sure, but it seems the aluminum siding is not good quality ... maybe adulterated post war recycle? With the patcheing, if there was a frame member in the vicinity, I generally drilled and tapped to make the piece firm. Has the corrosion returned because we missed a wee bit? Thanks for any advice.

Old aluminum can be tough to work with. The alloys were just not corrosion resistant or weldable. I would recommend agains any efforts to weld it. Probably the thing to do is to cut out the cracked and corroded aluminum. Make replacement pieces, maybe out of a 5000 series aluminum. Install the patch with stainless or aluminum pop rivets. You could use stainless as a backing plate if needed for the patches. The stainless and aluminum should be electrically insulated from each other to stop future corrosion. Polyurethane paint is a suitable insulator. The repaired joints, including any overlapping pieces have to be water tight. Again polyurethane paint or calk or alumlastic, if it is still made, is the recommended sealants. There is/was a Mil Standard on dissimilar metal joint protection that goes into this in some detail. I do not recall the Mil Standard number.

By the way, I am no expert. I just was involved in maintenance of several fleets of aluminum cars in my years in rail and transit

Aluminum is a tricky metal to weld even when it is brand new. I welded a lot of aluminum when I worked in the shipyards of Seattle. It must be extremely clean. Even a tiny spec of paint or oil will cause problems. Trying to weld that crack is probably a lost cause. The aluminum in that crack is already corroded. The metal will be poor and crystallized. At the very minimum you would have to take a grinder and grind a nice clean groove where the crack is. Sand all the surface paint off around the crack, then hope you have enough metal to get a weld in. Not sure how thick that metal is but warping is a big problem even with 1/4 inch aluminum plate. If you have good metal you could try spot welding in different places, cleaning the weld, and then spot welding onto the good weld.

If this was my project I would section out the crack and patch it with new aluminum. I would cut at least 4" out (2" on either side of the crack) and then cut out 4 inches from where the crack ends. So a 12" crack would have a patch measuring 20" long by 4" wide. You should have solid aluminum under the paint and of course your patch will be brand new aluminum.

If welding is a problem section out the old aluminum from frame support to frame support. Replace with new aluminum and rivet the four corners (and anywhere else you can rivet it to). Fill the crack with silicon. If the patch needs more support buy a 1/16" (or larger) sheet of aluminum and silicon it over the back of the patch (patch will need to be flush with old metal). If I did a 6"X 6" patch my backing panel would probably be 12" X 12".

The cheap band aid approach is to sand the underside of the crack until it is shiny (if possible). Cut a strip of thin sheet metal. Place it over the length of the crack (interior). Then use industrial tin foil tape to secure it in place. Silicon the other side. Sand, bondo, and paint.

The cracking appears to be from compression. I would guess the cracks is right in the middle of two cross members or large empty spaces? Keep the cracks from spreading by drilling a 1/4" hole where they end.

You have a couple different problems going on here and you'll need to look into the cause as well as the repair. I can't say specifically how to fix it but here are some things to consider:

Cracking: The first thing that comes to mind is that aluminum has about twice the thermal expansion factor as steel. If the aluminum sheet has nowhere to go when it expands and contracts, it'll flex and work harden leading to cracks. An aluminum cladding has to be designed to move or you'll eventually have structural failures in the cladding.

Corrosion: That certainly looks like galvanic corrosion. In the presence of any moisture, even humidity, if aluminum is in direct contact with carbon steel, it becomes the sacrificial anode. The more moisture, the faster corrosion. The aluminum has to be electrically isolated from any carbon steel to eliminate any chance of galvanic corrosion. For mechanical fastening of aluminum to aluminum, use aluminum or stainless fasteners. Aluminum to steel requires an isolation gasket of some form and stainless fasteners that are not in contact with both metals at the same time. Cad plated fasteners work but it's impossible not to scratch off the plating.

Welding: The better choice for welding sheet aluminum is TIG. Good TIG welding of aluminum is almost an art form. And cleanliness is next to godliness.

If properly designed and installed, aluminum cladding can be a great way to go if cost is not a factor, but in a typical railroad environment.... I'll just leave that there.

[Edit] Forgot to mention.... Pure silicone is an excellent sealant for aluminum when applied and tooled properly. Just be aware that nothing will stick to it i.e. paint, filler, etc.

[Edit 2] That could be cracking of the heat affected zone if it's at the welds due to improper welding. Really good aluminum welders are hard to find.

Thank you very much, David, Tom and Eric ... will incorporate your suggestions into my repairs.

I use AlumPrep 33 to etch then alodine followed by a zinc chromate primer on my aluminum projects. I get everything from aircraftspruce.com.

Thanks Mr. Dewey! Will check out those products!

Just to add a bit more detail to SanDogDewey's post in case you want to know: This is a pretreatment for aluminum prior to applying coatings. The etchant is primarily a cleaner, Alodine is a chromate conversion coating which allows your paint or coating to stick. Coatings don't bond well directly to aluminum because of the nonporous natural oxide coating which forms instantaneously. It's the oxide that gives aluminum it's corrosion resistance.

SDD - noticed this is your first post. Welcome to the Interchange.

Thank you Eric ... storing all this great information in the gray matter until I finish with a generator install ... than I'll
try my hand, once again, with some body repair

years ago the shop I was in was working on a pre-war aluminum bodied Automobile and used an aluminum "brazing" rod to do some repair work. All seemed fine, but then the paint started blistering at the repairs. No amount of cleaning or even removal of metal would stop the issue. After finally consulting with the manufacturer of the rod we found out that it was not engineered for over-coating! Hmm, nothing in the literature mentioned that (small???) detail.