Silicone sealer.... man, it would have to be "painted" on thick. How about that tool grip dip stuff? Or paint on bedliner? As I said, the 5th wheel hitch is off currently, so eventually, I'd like to return her back into service.... so means I'll be putting it back on.

I was thinking of using grade 5 bolts, fine thread, lock nut with a flat washer on the inside and out. But, that would mean a steel washer sitting on the inside of the aluminum frame..... must think of way to isolate w/out it working loose....

 

I work for a large tanker operation, our trucks always gross out so weight is money. Early 80's the bean counters were specing aluminum frame rails, justified on weight savings. That was until the 5th wheel let go in a turn on one rig operating in the northeast causing a rollover. Root cause was the galvanic corrosion between the carbon steel brackets and the alum. caused by salt water getting between them. The bracket pulled right thru the frame at the area weakened by the corrosion. Alum is more reactive than steel causing it to corrode severely in the presence of steel and an electrolyte like salt and water. Bottom line is we replaced lots of frame rails, did it one side at a time, held everything up with jacks and after everything was detached from the rail pulled it out with a pickup, stuffed a new one back in. Contrary to what the bean counters said, it cost a pile of money, no more alum rails. One way around your problem might be to install an aluminum angle between the steel 5th wheel brackets and the frame. The alum angle will corrode where it is in contact with the steel bracket but the frame will be ok where it contacts the aluminum angle. You still have to isolate the steel bolts from the alum rails though, they make plastic sleeves for that. You will still have to replace the alum angle periodically because it will corrode also but that's better than the rail.

 

wrenchforfun,

What would you define as "periodically". Eventually, this old truck will end up as a farm truck (again). It prob will not get used during the winter months, only fall....

I'll have to take a closer look at eveything..... truck has yet to be brought home. Plus it's covered w/ snow....

 

It would depend on how much salt it sees, I guess if I was running it full time in the winter months in salt country I might pull pull the 5th wheel after a couple years just to see what the aluminum angle looks like. I'd still use the RTV between the alum angle and the steel. If it won't be used in the winter you can probably get away with just a good coating of RTV between the steel and alum. like the previous poster said. If the frame is severely corroded you might want to consider fishplating it with another piece of aluminum over the corroded area but then you are talking about drilling more holes in the frame in the area that is good to attach the fishplate so you again have to isolate the steel bolts/washers from the alum. I don't think you're supposed to weld on those frame rails. The secret is to keep the salt water from getting in between the steel and alum. If the frame is usable as is if you clean both the frame and 5th wheel brackets with a wire wheel real good then put a good layer of RTV between them so it seals the joint off good you should be ok esp. if it won't see salt. For the bolts you can gunk them with the RTV and also put it both under the steel washers and under the bolt heads and nuts so everything is sealed so water can't get in to an area that has steel to alum contact.

 

There's nothing wrong with aluminum truck frames. I've been working with them for years. Most problems start with loose fasteners that allow for increased movement and cracking to begin. Then someone that doesn't know anything about aluminum will try and weld it.I've stretched many short alum frame tractors into straight trucks for dump and farm use. I've seen these guys way overload them with wet silage or haylage. The alum frames hold up much better in the fields.The pitting you see in the photos' is electrolysis from direct contact with the 5th wheel frame. You can buy some etching primer that will prevent this from reoccurring. Surface prep will be the most important thing. Grade 5 bolts are NOT strong enough! Use grade 8 minimum or better yet use frame (HUCK) bolts. Make sure your bolts are cad plated to prevent them being frozen in the frame later. Kevin

 

Thanks for the tips!

As of right now, the frame is painted black. Last owner did that.

I'll have to take a closer look at the 5th wheel hitch and mounting location. That'll be when it comes home and is warmer.

As I said, it will prob. live out it's days here on the farm. Just pulling a small trailer during harvest. If used during winter.... there isn't much snow nor ice on the roads. And if there is any, they don't use "salt" here any more. Suppose to use something "friendlier".

 

Where you are you do not see the effect salt/water has on aluminum. I've seen it, many times. An etching primer will do nothing to isolate the steel from the aluminum. The aluminum has to be physically separated from the steel by a non-conductive barrier or the aluminum WILL corrode where it contacts the steel. It is simple chemistry, nothing will stop it except isolation of the two metals at the contact points. Ask someone who has (or had) an aluminum hulled boat running in salt water that did not have the proper protection taken to isolate the hull from any carbon steel, or stainless steel, components. If they did not catch it in time they have a hull full of pinholes in the vicinity of steel components.

 

This is known as a galvanic circuit.

To form a galvanic circuit, you need:

1) Two dissimilar metals
2) Physical or electrically-conductive contact between them
3) The presence of an electrolytic fluid, such as salt air or salt water.

Salt may not eat away at aluminum, but it will cause the aluminum to develop a white spotty oxide over time.

The greater the conductivity of and exposure to the electrolyte, the faster the galvanic corrosion occurs.

 

They should just make the whole 5th wheel assy out of aluminum! /ubbthreads/images/graemlins/wink.gif It would be nice and lightweight then....

Birken