Rust Suffocation?

[hpeer]

A wee story with no deep point.

Bought our new to us boat last spring. 275 gallon water tanks integral to the keel. Drained and cleaned. The 1987 epoxy had many, many blisters which were filled with water. Under the blisters the steel was new and shiny.

Why?

The only reason I can imagine is that, for some reason, the water had become depleted in oxygen leaving nothing to react with the iron.

In any case I sanded out (grinder) as much of the old epoxy as possible, two coats of 2 part zinc epoxy, two coats of 2 part epoxy primer, two coats of 2 part potable water certified top coat.

In normal matters I use the 2 part zinc epoxy with the 2 part top coat. Pain in the rump. I don't like coal tar epoxy in the bilge because it is black and you can't see rust stains.

[Delfin]

Originally Posted by psneeld

I've been using corroseal for over 30 years..it works sometimes and most of the time not...the prep is crucial and seems to be hit or miss because if you miss even a microscopic bit of rust...it's starts all over..and I'm not alone because the company I work for uses it by the 55 gallon container.

I say there are other just as effective methods..which a lot of others use/try...

What I'm saying is that you can't "smother" rust...you either eliminate it or convert it...but if there's even a tiny bit of "rust" left....it returns with vengence.

I'd be interested in hearing your experiences. I have found that you have to follow the instructions - 2 - 3 coats, wait 24 hours, then prime. When I have done that it seems to work really well. What variables are in play that results in failure, in your experience?

[Delfin]

Originally Posted by GordMay

As noted, you want to convert rust to black oxide, and protect that coating.

Rust is really Fe2O3, a reddish form of iron oxide. Iron has another oxide, Fe3O4, which is sometimes called “black oxide”, “black rust” or “hammerscale”.

Black oxide is a good protection for steel. Like aluminum oxide, black oxide molecules are the same size as iron molecules, so black oxide doesn’t grow or flake. Black oxide is true gun bluing, and the oxide found on some drill bits. Black oxide is also seen on iron and steel that has been hot-worked.

You can coat steel with black oxide by a careful regimen of rusting the right amount and boiling the rusted metal in water to convert it. This is how non-caustic gun bluing is done, and although it is tedious, it produces very attractive and durable results after several treatments.

When iron or steel starts to rust, it will puff up and expose clean metal to the open air, allowing rust to continue to the depths of the metal. There are a few products on the market which fall into the category of organic rust converters. These products contain phosphoric acid to convert rust to black oxide and polymers which bond to rust.

A unique advantage of phosphoric acid is that it leaves a fine coating of iron phosphate behind. Iron phosphate prevents rust. However, the iron phosphate coating is not very thick and not durable. Some additional protection is still required.

Auto body shops treat metal with acid metal wash, a solution of phosphoric acid, and alcohol before painting. Acid etching using phosphoric acid involves:
- Getting rid of waxes and oils on the metal surface
- Removing slight amounts of rust that form between sand blasting and painting
- Leaving a thin protective coat of iron phosphate

Gord, are you familiar with Corroseal? It is water soluble and benign, but turns rust black. I assume it is converting the rust to black oxide?

[captain58sailin]

Definitely want to prep the steel with chem prime or corroseal, I have had mixed results with corroseal. chem prime usually works very well in most applications, it is nasty so you need to wear your PPE.

[SteveA]

Am in the process of a similar refit, with all the interior out of my steel hull boat. Have applied a product called Rust Bullet and so far so good although pricey. Google it. The sales bumph deals at length with steel that cannot be properly prepared by blasting. Imressive product so far, and can be overcoated with what ever takes your fancy, although they argue that is not necessary.

[conachair]

Hi, a few thoughts after 5 years onboard a steel boat.

• Best practical book by far on the subject I've found is here - Metal boat maintenance-A do it yourself guide by Scott Fratcher in Engineering
  Cheap download. The author is particularly unkind about advice offered on internet forums It's worth clicking the name of the poster and viewing other posts by that poster. Some people do write unsubstantiated rubbish , others are close to genius.. you decide...
• Vital to keeping the inside rust free is having spiders down in the bilges Keep the water out and the inside dusty, achievable if the boat is all steel, apart from a few drips from the stuffing box.
• Phosphoric acid seems to work well, everyone has their own favorite, I use ospho. On a hot day you can put some on a seeming shiney clean piece of steel and watch it bubbling away as it reacts with the rust in the pits. I think epoxy on iron phosphate is better than on iron oxide. Difficult to wash the steel afterwards in awkward areas afterwards though.
• Best initial holdiong coat epoxy I've found so far is amerlok sealer, messy, takes ages to kick and seems to soak into everything. Mask well.
• After that I'm not sure the brand of the build coats make that much difference, I us ameron again, amerlok high build something or other. No reaction with the sealer and on a warm day you can get maybe 3 coats on.
• For touchups I have a few bottles with epoxy in, then use digital kitchen scales to measure the small amounts you need to mix for small jobs, the 2 parts might weigh a little different and are usual mixed by volume so a quick calc might be needed.
  
  
  Happy sanding
  

[stickybeak]

I'll second conachair's thoughts. Scott fratcher's ebook is money well spent.
He gets treated as a bit of a forum pariah as he's seen as flogging his books but I have taken his advice (particularly with the amerlock sealer) and it's worked. (a year and a half later, still working).

And conachair, nice trick with the kitchen scales!

b

[conachair]

Originally Posted by stickybeak

And conachair, nice trick with the kitchen scales!

Cheers, some say you can use them for cooking as well, wierd. Like using the fridge to store lettuce instead of beer and sealant.

In general steel boat paint is a tricky one as you need to wait 5 years to find out if it really works or not. But keeping the water outside is key.
Anywhere outside with wood sitting on steel seems to be asking for trouble as well, i'm slowly either getting rid of it all or stooling it off a little with plastic washers to leave an air gap. Not many long term cruisers in steel boats in the tropics have wood stuck to steel.

[GordMay]

I’ve not personally used Corroseal.

Evidently, Corroseal works best when applied to clean, dry, adhered rust. If the steel's not rusted, there's no "conversion".

From the Corroseal website < Corroseal Rust Converter | Metal Primer | Rust Paint >
“... Through an innovative chemical conversion process, Corroseal Rust Converter converts rust (iron oxide) into a stable substance, magnetite*. It also primes the surface with a high quality latex metal primer at the same time ...”

*Magnetite = Fe3O4

From their MSDS, “REPORTABLE COMPONENTS”
DIETHYLENE GLYCOL ETHYL ETHER
GALLIC ACID
ETHYLENE GLYCOL

[captain58sailin]

I have used corroseal with mixed results, conditions have to be near perfect. Phosphoric acid, will interact with the steel whether there is visible rust or not, have had much more consistent results. We use a 2 part international primer and paint after treating the steel which produces acceptable results on the exterior surfaces of the vessel. If used inside the vessel the fumes are quite strong and toxic. If you were able to coat in sections at the end of the day and leave the vessel to allow the paint to kick then you could get satisfactory results.

[Honey Ryder]

it looks like i may have had a stroke of luck...i got the cabin sole up and was expecting to find a ton of rust given that the old steel black tank had totally disintegrated...and found that the steel hull was in near perfect condition...no rust at all...i am guessing that the bilge was coated with some coal tar epoxy when she was built back in '79...there is a black colored surfacing...wow...that stuff is good...

makes a great advertisement for coal tar epoxy. the inside of the water tank in my keel is also in perfect condition, not a spec of rust. amazing after 30 years.

i'm now gonna try some localized spot blasting to see if i can get rid of the rust up near the portholes and on the horizontal sills, and then coat with zinc primer, epoxy primer and then some coal tar epoxy.

thank you to everybody who kindly volunteered information of this topic. really useful and greatly appreciated.