Corrosion of box sections

[Rich_P]

I've been pondering over something small, but has big consequences for many Land Rovers.

Most Land Rovers built utilise the box section chassis that we all love and hate, love for its strengths and hate for how it also rots out like it does.

We know that the box section corrodes at a far greater rate internally than it does externally. We also know that applying paint and/or underseal/waxoil can help reduce the rate of corrosion in places.

What I don't know is that, if you took a box section chassis and did not paint it or give it any form of protection internally nor externally, would it still rot out from the inside far far faster than it could corrode inwards from the outside?

To add to the debate, would a chassis that has had paint or waxoil injected into the chassis corrode from the outside inwards at a faster rate than it would still likely corrode from the inside out? Or would corrosion rates be similar for both surfaces?

To finish off, would a chassis that has been lightly coated externally and heavily coated internally corrode from the outside in or not?

[bishbosh]

Corrosion needs three things to happen - a source of steel, a source of oxygen and a source of water.

Generally, the inside of the box retains moisture better so rusts faster as it isn't airtight either so oxygen gets there too.

Anything that isolates the steel from either moisture or oxygen will completely stop any corrosion.

The best thing for a box section is to weld it shut, but this isn't practical for chassis rails that have so many penetrations for bolts etc.

The risk with welding it shut is that your welds are not completely air / water tight and it still rusts!

The different permutations you talk of are all too variable to decide one way or the other which will rust faster.

[Rich_P]

Oh okay. You say that for corrosion to take play, the three "ingredients" are required. Would corrosion stop taking place if a barrier was created even if the surface had already begun corroding?

[Les Henson]

You can stop continued corrosion even if it's already started by denying it those elements that caused it in the first place.

Les.

[bobtail4x4]

Waits for fish oil man to make a comment

[missingsid]

Anything that isolates the steel from either moisture or oxygen will completely stop any corrosion.

The best thing for a box section is to weld it shut, but this isn't practical for chassis rails that have so many penetrations for bolts etc.

The risk with welding it shut is that your welds are not completely air / water tight and it still rusts!

So assuming perfect welding and isolation of holes (tubed holes), injecting Argon under preasure to fill the voids during the last weld would work??

Marc.

[Les Henson]

Any inert gas would do it, providing it removed the oxygen - CO2 would be cheaper, Helium (gotta be a good thing ), etc. Fill the chassis to a few PSi above atmospheric and then fit a pressure guage somewhere handy in order to keep an eye on things. I suppose it's doable, but would take some time detecting leaks and plugging them all, but a chassis loaded with CO2 or similar would be Corrosion free from the inside, which is where most of it starts.

Les.

[bishbosh]

No need for an inert gas - if it is welded shut the trapped oxygen will be consumed forming some initial surface rust. Once it has gone the corrosion will stop. There will not be enough trapped oxygen to fuel much corrosion.

[will_warne]

Or, possibly more practical and effective, insert a few bags of silica gel into the chassis just before welding the last face shut. It therefore doesn't matter if a small amount of water enters the chassis through a pin prick hole.

[FridgeFreezer]

Or, more practical than trying to hermetically seal a LR chassis - galvanise it or pump it full of waxoyl. Even in their unprotected form from the factory chassis can last ~20 years without major problems.

[nicks90]

absolutely!

my 90 is almost 15 years old now and has no patches or flakey nasty stuff on it - and thats because it has been waxoyled 3 times in its life. However because the rear crossmember gets a thorough wash after every event, the waxoyl gets blasted off so that gets an extra waxing once a year just to be safe.

[clbarclay]

On a techincal note, what we are talking about is 'rust' which is a specific form of corrosion. Other forms of corrosion like the oxidation can be beneficial, such as the oxidised of aluminium where the layer of oxidation forms a protective barrier preventing more oxidation. Aluminium though is not the most parctical material for a chassis.

Another way of reducing the effects is not to go near salt which is a catalyst for rust. Unfortunately for steel chassis, in england we spread vast amounts of salt on the roads and we have relativerly a lot of costal areas where sea salt in the wind gets blow around the chassis. A coulpe of years ago I saw a late 300 discovery (1997 IIRC) that had spend all its life near the coast and not even the axles were worth slavaging.