Stainless to aluminium corrosion

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jwilson

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I am long and sadly familiar with stainless bolts/studs/shafts getting stuck through corrosion when fitted into aluminium fittings on items such as my windlass, small outboards etc. I am about to do the Mk III modification to my quite thin-walled and slightly flexible Jeanneau-fitted black polyethylene diesel tank, and intend to have alloy plates inside and out of the top surface, the inner ones having alloy rivnuts set into them into which S/S machine screws will compress a cork nitrile gasket, with some Hylomar Blue to seal the threads.

Will SS/alloy interfaces still corrode when liberally and regularly sloshed with diesel? I am hoping not, as part of the Mk III modification is to have a decent and reasonably easily openable access panel for internal cleaning.

Before anyone suggests just getting a new tank made, the shape is a wildly complex plastic moulding to fit in an under-berth space. Any 'made up' tank would lose a lot of volume, and be quite difficult to fit securely.
 
Graham_Wright said:
Please explain.
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The diesel oil will stop the rusting of the mild steel through coating the plate with a protective coat of oil in the same way a coating steel tools with a thin film of oil stop the tool from rusting.

My diesel tanks are mild steel with no coating inside and will not rust. The mild steel tank on my old boat only had rusting on the top outside due to water pooling on the outside. The inside was perfect.
 
Rogershaw said:
The diesel oil will stop the rusting of the mild steel through coating the plate with a protective coat of oil in the same way a coating steel tools with a thin film of oil stop the tool from rusting.

My diesel tanks are mild steel with no coating inside and will not rust. The mild steel tank on my old boat only had rusting on the top outside due to water pooling on the outside. The inside was perfect.
Click to expand...

What about condensation? Will splashing ensure the underside of the plate is diesel coated?
 
Graham_Wright said:
Please explain.
Click to expand...

An interesting comment.

This is a photo I took a couple of years ago when I removed the fuel float sender from one of my diesel tanks:

IMG_1904.JPG


I was very surprised to see that the mild steel arm which connects the float to the resistance track was badly rusted. At the time I had a very mild diesel bug outbreak which I was dealing with so it is clearly the small amount of water in the fuel which causes mild steel to rust, although it's obviously not enough to cause a problem with the float. However, I would have preferred it if VDO had used stainless steel rod rather than mild steel.

Richard
 
vyv_cox said:
Why use aluminium inside the diesel tank? There will be no corrosion so just use mild steel with no risk of seizure with stainless fastenings.
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Equally there should be no problem will ali.
So if the OP prefers to use ali maybe because it's easier to work or it's 'in stock', it should not be any problem.
I might use stainless for the external plate, just to avoid superficial rust on the outside of the tank.
 
I'm using aluminium because without access to a proper metalwork shop it's far easier to cut, shape and drill than mild steel, even more so than stainless.
 
jwilson said:
I'm using aluminium because without access to a proper metalwork shop it's far easier to cut, shape and drill than mild steel, even more so than stainless.
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I hav not found thin stainless plate difficult to drill cleanly. I equally don't find it difficult to cut, in straight lines - making professional looking curved cuts much more difficult, with an angle grinder.

I'm interested to be educated - why is stainless of a similar thickness to mild steel difficult to cut and drill? On the assumption you are using good drill bits and lubrication.

Totally different question - what alloy of aluminium are you using?

Jonathan
 
Not sure what sort of aluminium alloy - a bit of 5mm sheet bought from local metals supplier.

Tested rivnuts into it yesterday, although I've used 4mm rivnuts before without a "setting tool" getting 6mm rivnuts to set just with two spanners was very difficult indeed, so gave up and tapped 6mm holes in the alloy plate. Hopefully finishing the job today.
 
Mild steel tanks rust from the outside, especially around the welds. My SS tank rusted along a welded seam. I had the tank steam cleaned, which exposed the pinholes. Heavy plastic tanks are probably best, although the rubber moulded ones are good if they are contained. I sailed on a Nic 32 with just such an arrangement and the tank was over 10 years old.
 
With modern bio fuels there is always the possibility of water in your fuel tank. This will rust the internal surfaces of your tank.
 
Did the job yesterday: My cunning plan of getting a two part tapped alloy backing plate inside the horrible polyethylene tank under a big alloy top plate was a bit of a struggle lining up multiple machine screws and holes but worked in the end. Of course as I was finishing the job I came up with an "extra cunning plan" to improve the access.

Pela-siphoned out 130 litres of diesel into six big drums. Despite a previous go at sucking gunge from the bottom of the tank with a pipe through a narrow hole once the tank was totally empty and I could get an arm and torch in there was a pocket of black jelly at the very lowest point, and the last couple of pints of diesel extracted were murky. I've been using Marine 16 anti-bug fairly regularly, bugs seem to have survived this, or there's been a very duff load of fuel put in.
 
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