Protecting Space Frames against internal corrosion. Lotus Super Seven

A. Hello George,

Are you sure that corrosion is the reason that the frames fail? Stress can encourage corrosion, and corrosion can encourage cracking. But if the stress were less then the corrosion may not be a problem. With all the rivet holes you speak of it is likely that damp air can get inside the frame, but then when the frames hot or warm the moisture should be driven out too. You could brush plate zinc on the outside of the frame, as a sacrificial protection, and but aluminium is sacrificial to steel also, so the riveted on aluminium plates should, already, be helping there. The little spaces between the riveted on plates and the frame will be good places for corrosion to take place, try and avoid them or seal them at least. I'm sure you can plate the inside of the frame, using some ingenuity, maybe carbon rods (with spacers) placed inside the steel tubes before joining the tubes (the rods wired together too), then when the frames made pure in electrolyte and power it up. It doesn't sound too practical though, and if your then gonna drill lots of holes in it it does make much sense either.

Really the only thing to do is design it as well as you can, build it as best as you can. Remember not all steel or mild steel is the same, there are variations within grades (from manufacturer to manufacturer). Then if you want it plated (zinc or tin/zinc) take it to a professional. They'll do a better job, and know about the safety issues and deal with some chemicals you don't want to play with (i.e. chromates - plating just aunt the same without 'em). Then paint it as you like, or get a professional to do that too. Then take it home and park it in a nice dry garage.

I hope I've been some help, in a vague and general way.

Hi Freeman,

When I look at an outfit like NASCAR, I try to evaluate their focus. Do they care whether a frame lives longer than it's short life on the track? Probably not. Furthermore, and based on my limited exposure to their cars, they seem to use thicker, heavier, materials, with far fewer elements.

I don't think there's too many people or entities that care how long a structure like a space will live. Even with no protection, they hold up long enough to sell, and then it becomes the owners problem.

I have received a number of good tips, and I appreciate them all.

Thanks,

A. Hello Everyone.

Let me get this disclaimer out in front. I am not a plating guru. However I work for a company that has a large plating and wastewater operation. So I Have gotten a bit of a crash course.

As for your space frame or any other steel structure in which you want to prevent internal corrosion. My personal favorite is expanding foam. But you must use a good dense closed cell foam. The trick is to drill or find a hole in the particular tube you are filling near the end which is a snug fit for your applicator tube. There must be a way to vent the pressure at the other end. Considering your frame has already been poked full of holes and pop rivets. All you would have to do would be drill one out at each end of a tube fill from one end and then plug that hole with a pop rivet. Clean up the mess that comes out of the other end with the appropriate solvent. (do this before it hardens or you will be all day trying to clean it off) Then plug the second hole. Assuming that the metal is in decent shape and the rivets are holding on tight, the foam will expand to the full volume of the tube and force its way out any cracks or into the next tube it if wasn't a but weld.

If you have already checked and the hole space frame is constructed of open welds. Then someone could conceivably start filling from the lowest point on the frame with several rivets drilled out and have an assistant run around with a can of solvent and a rivet gun to reseal the holes as the foam climbs past them. But you would have to do it in a fairly short period of time. In other words be sure you have all the foam you will need before you start filling. My personal favorite is the stuff you find in aerosol cans at Wal-mart or hardware stores. It is more expensive by volume but you don't have to worry with mixing it properly and having it harden before you can get it where you need it.

I use it for sound deadening and rust prevention (and repair) in lower fenders of automobiles. (anyplace that will not need to drain)

Don't ever use in a enclosed area composed of thin material (i.e. sheet metal) that doesn't have enough space or vents for the pressurized foam can escape or else you will end up with a deformed panel.

I personally have a Plymouth Fury that was starting to rust in the rear quarters because of a sloppy weld. About ten years ago when a friend who worked at a body shop told me to try the foam. It worked perfectly. More recently I have filled just about any void in the body and frame that can be safely filled with it. (except the doors which of course need to drain) You can only hear the wind noise that comes through the glass. And there is no way for moisture to get in where it can cause corrosion. So far it has only added about 25 lbs (around 11.3 kg) to the total weight and has been a major improvement to the vehicle.

How does this relate to plating? I'm not sure but if you are having problems with internal corrosion in your plating shop it would be worth a try.

A. Years ago, Porsche built the space frames of at least some of their 917s of tubular aluminum. The state of the aluminum fabrication art was not so advanced - so cracking was a very real worry.

In order to allow for easy crack detection they would pressurize the inside of the tubular structure via a simple tire valve fitting - as long as pressure "held" all was well. Any loss of pressure indicated a crack - and finding the crack required only soap bubbles! Clever, yes?

Of course, upon initial construction of the frame, one would provide for the interconnection of all the tubes using a drilled connection at welded tube intersections as well as a high level of welding precision. Any structure that reliably holds pressure can be viewed in good confidence.

Internal corrosion of such a structure would be easily prevented by eliminating oxygen & moisture inside the tubes by simply evacuating the structure using a vacuum pump, (to 500 microns or so, holding for 30 minutes), and providing a mild positive pressure,(5 - 10 psi - no more), using DRY nitrogen.

No oxygen & no moisture equals no corrosion, EVER!

If one added a pressure gauge permanently piped into the tubular structure, you would have an clever & reliable corrosion prevention & crack detection system. One which weighs nearly nothing - costs almost nothing - with maximal reliability.

Just a thought....

Regarding attaching aluminum (or other) panels to tubular frames:

Wherever practical, one should avoid drilling rivet holes into the closed tube frame members - no matter how carefully installed, over time, the rivets will leak and allow ingress of atmosphere & moisture.

Filling the tubes with structural foam has much to recommend it, the foam can provide considerable buckling resistance - future frame repairs requiring welding could be more complicated, however.

Why not avoid the holes for rivets all together? During the fabrication phase weld on tabs and riveting strips at appropriate locations. A bit more weight,(and work), in return for "no worries".

When installing the chassis skins generally, consider using one of the good STRUCTURAL adhesives ⇨
to bond the panels to appropriate chassis tubes.

These adhesives are easily strong enough to absorb considerable structural loads over wide areas with few point loads.

This practice greatly reduces,(or eliminates), rattles & drafts - not to mention the stress risers that often lead to cracks between rivet holes in the skin panels.

Over a surprisingly short time, nearly all riveted structures "loosen up" and become much less stiff. Structural adhesives could go a long way towards eliminating this fault; a MUCH improved assembly.

Be forewarned: Aluminum adhesive bonding, (as well as welding), is much less reliable than with steel for the amateur and professional alike.

With steel panels, the constructor could forego many additional rivet tabs & etc. Structural adhesives could STILL be used with well designed spot welds REPLACING all those silly rivets.

Good rivets, after all, are EXPENSIVE and they do weigh something..more than you may think!

A full weight analysis would most likely show weight REDUCTION with this steel panel alternative! Any weight GAIN can be addressed with detailed design.

In any case, a steel panel alternative will almost certainly result in a stiffer, stronger & cheaper structure. It will likely have a much longer service life as well.

STEEL - may not be as "sexy" as aluminum - but very often it is a better choice for the "home based" craftsman.

With steel instead of aluminum skins, you will lose the ability to polish it. To get the "polished aluminum" look -check out the various films used for vehicle wrapping.