steel protection on inside of small enclosed spaces

[bigbear69]

Hi, any ideas on protecting the steel on the inside of small enclosed spaces where there is no access to blast and paint etc.
I am attaching a scoop with boarding platform /steps etc to a steel hull yacht.the boarding platform will be inbuilt how do I protect the steel on the inside of this once is all welded up.
also planning on adding a overhead structure on the stern put solar panels and to hang the davits off. thinking of fabricating from steel and they will be hollow so need to protect the steel on the inside also.
i've heard of filling the enclosed space with diesel,The yacht has a diesel filled keel, is this the only way to protect the steel. or can I coat the inside before i install ?
Any help would be much appreciated.
cheers

[tazmann]

Hello
Probably a few different ways to go about it, I think I would build the platform out of galvanized material or pre blast and paint with a good weld through zinc primer, then if possable put access hatch big enough to reach inside with paint brush to touch up. Filling with diesel or oil would deffenabty help with rust but would allso add cosiderable weight.
The overhead structure, stainless tube or pipe would be the first choice, galvanized second and seal up the ends air tight .
Tom

[bigbear69]

cheers Tazman,
I was checking back through old threads and found one useful. All you need to do is fill void with inert gas ie Argon from your mig/tig bottle. Weld it all up then drill a hole and pressure test it to 2-3 psi to make sure there are no leaks. then displace all the air inside with argon and seal up the hole. no rust on the inside.
Any thoughts ideas on this process?
cheers

[Brent Swain]

If it's totally airtight , any oxygen will be used up quickly and it will not rust.

[tazmann]

bigbear69
I have heard of the argon trick but never tryed it, could do all 3 , zinc primer or galvanized. weld it up air tight with a couple small fittings then purge with argon, sure couldnt hurt and give a better piece of mind knowing you did the best you could.
Tom

[zero88]

Hi All,
another idea is to weld it up fully, then tap a small bolt or set screw into it and drop some waste oil into the void. The Oil will prevent rust and will climb all over the insde on its own.

I am doing similar on my build

Dirk

http://deboet.blogspot.com

[Eric Sponberg]

Here is the simplest, cheapest solution that is as old as the hills:

You can precoat the steel for a little extra protection, but as Brent points out, if you don't have oxygen, you don't have corrosion. So, before you weld up the final closing plate, place a lighted candle in the space that will burn long after the space is welded shut. With a long enough candle, it will consume all of the oxygen in the space. No oxygen, no corrosion. And you don't have to deal with extra gases or oils. Old ships that have been broken up have been found to have candles in the hidden spaces and the inside surfaces of the steel looking as fresh as the day they were made.

Eric

[Brent Swain]

A piece of dry ice would do the same . Make sure it has evaporated completely before sealing , or it could bulge and explode. CO2 from a fire extinguisher would also work, but I think the tiny amount of steel used in using up the oxygen would be so small as to be undetectable.
Brent

[marshmat]

I think it'd be hard to find anything cheaper, as effective, or easier than Eric's candle, if the compartment is completely sealed (ie, flotation space).

Filling with diesel seems to make sense only if you're going to use it as a fuel tank, in which case you'd have to treat the inside just as you would for a fuel tank.