Discussion in 'Earth Science' started by Success_Machine, Sep 10, 2002.
Do ocean going ships rust?
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One of the most annoying bits of being in the navy was getting out the rust-chipper and paint and all and taking care of rusty bits. Yuck!
Ok. Suppose I wanted to hang some metal frame structure just under the surface of the water, made out of , say rebar, how long would it last? Would a ceramic coating make it last longer?
how do you get rid of the rust adam?
The rust appears mostly above the waterline, as rust is oxidation and so you get more of it happening in the open air. If you need to drydock the ship to get rid of rust below the waterline, you're in serious trouble.
Yes, a good way to prevent rust is to coat the metal entirely with anything that will keep out the salt and air. Ceramic, fibreglass, plastic, whatever you have.
what about using a sacrfical anode?
Anodes and recitifiers have long been used to prevent electrolsis of metal in salt water. Another method is coatings of cement, expoxy, and some plastics.
Special paints have been developed to help prevent the formation of barnacles which usually grow in the top layer of the ocean where the water is the warmest. Say within the top 40' or so of water. This is normally painted on the hull of a ship when it is drydocked.
Wonder where the term "rust bucket" came from?
I think there are certain things you can do about the construction itself. Like using Stainless Steel 600 I think, mixing certain alloy's. Of course it would be lordly expensive to make the entire ship's bottom (is it called the hull?) of something like that. There are certain materials that do not rust at all, but you couldn't break them down to apply to the ship like a coating of paint.
I think they paint them over with an anti-rust paint every few days, maybe every week..something like that.
Now a days there are epoxy paints that are mixed and stand well in the enviorment. Only they are brittle. If any flexing is involved, such as a heavy object dropped on it, then they will crack and allow seepage to get in. There are also marine paints available that reduce uv damage to go with everything else.
Just about every time we docked in an Australian port, we got out the pneumatic needles guns and chipped off paint, then sanded it down, then put on a primer and undercoat, then a coat of "warship grey". We did this in patches, all over the ship, a bit at a time. While at sea we mostly polished when not on watch in the ops room or whatever. ALL over the ship, grey polish and grotty rags.
Which reminds me of a funny incident...
One time, at band camp...
But seriously. The captain was out on deck, wandering about checking on things. He happened to look up, and saw right up some chap's shorts (no, not me). He rushed into the bridge and then his voice came over the speakers all over the ship: "Now here this, this is the captain speaking. From now on, all personel are to wear their underwear! Consider this now part of the ship's standing orders. That is all."
What about the parts that are always under water - do they rust?
Eventually they rust too, yes. It just takes ages.
HAHAHAHHHAHA! That shorts/underwear anecdote is pretty funnY! Please Register or Log in to view the hidden image!
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the epoxy paint that you paint the hull with is called 'anti-fouling'.
Well as an ex-submariner it doesn't take ages. The slapped so many coats of paint on that it did take awhile. They were using a 6 coat system on surface ships if I recall right. 2 coats of primer, 2 of some blue epoxy and 2 of haze grey. The boatswain mates big complaint was missile launches which burned all the paint off under the launcher. They didn't have to use the needle guns tho.
Subs got special top coats that were so toxic that they had to build a tunnel to the boat in the dry dock when they were sand blasting and recoating.
Now boats have an anechoic polymer top coat. Rubbery stuff.
If you are looking for alloys that are resistant to seawater then Inconel is the gold standard, I think Alloy 20 aka Carpenter 20 are also highly resistant but very very costly. Stainless does not always do well in high chloride concentrations which seawater is. Titanium is good too.
>the epoxy paint that you paint the hull with is called 'anti-fouling'.
Well sometimes it is, there are epoxy primers too. What makes a paint anti-fouling is having a toxic component that leaches out to kill critters and algae that grow on the hull. They used to use mercury, copper, arsenic and/or organotin which is why the bottoms of a lot of marinas are now superfund sites.
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