What corrosion process is going on in this steel deck?

[S/V Alchemy]

Originally Posted by stillbuilding

For dessert, I surmise that there is a snowball's chance in Hades of completely excising the divots of iron oxide in this steel deck and there must remain the potential for future corrosive "pox" and this may be one of the rare situations where painting with a "rust converter" after sand blasting may be appropriate.

Absolutely. POR-15 or Ospho paint (or something similar as long as the phosphoric conversion "locks" the rust) is annoying in terms of time and money, but is necessary. Also necessary is the cutting of limber holes and the metal prep needed to avoid or to mitigate standing water.

Steel can be made to last decades by putting in hours at the front end, but it's a case where both a through understanding of the science and a nice dollop of OCD are your friends.

[svseachange]

What's the solution for stillbuilding? Sand blast hull, use hammer to remove rust bubbles, apply six layers of two pot epoxy primer?

[UNCIVILIZED]

Originally Posted by svseachange

What's the solution for stillbuilding? Sand blast hull, use hammer to remove rust bubbles, apply six layers of two pot epoxy primer?

As long as the primer's put on within minutes of blasting. And the boat's located in Death Valley at the time, then that should do it.

[jakkum]

Yes, your pics are a piece of art.
Yes, nearly all ironoxides or all, have a bigger volume than the original iron. Perhaps some ten times.
Yes, all the rust, oxide has to bevremoved, esp the black, because it is so difficult to remove.
If you go over it with a needle hammer, it will look as if it is gone, but after a few hours you can again see that the black stuff still is in fact a layer. Often on the waterline.

In rhese cases I use an air chiselhammer with the chisel a bit blunted.
And ear and eye protection. Of course the noise but it can come off very violently.

If you have this under water and you do not mind doing a new paintjob, an overdose of magnesium anodes NEAR, NOT ON!! the ship, but connected with the ship with thin steel wire, can do a good job.
In seawater in a few days, in fresh water in a longer period, but it depends on the amount of anodes.
But find an expert to advise you.

With your deck it is different. I wonder if the black stuff is not soluble in foforic acid.......

[svseachange]

Originally Posted by UNCIVILIZED

As long as the primer's put on within minutes of blasting. And the boat's located in Death Valley at the time, then that should do it.

Are you saying to just be pragmatic and attack rust spots as they come? Kind of accept the boat is always going to be rusting away and then do what you can to delay the process?

We are newish to steel boat ownerhip (last boat was fibreglass) so I am still trying to work out best way to deal with issues like the OP raised. We want our steel boat to last about ten years.

[boden36]

The only real solution is abrasive blasting to near white metal. (class 2.5 at least).
To remove any salts you should then wash with a product like Chlor Rid, lightly reblast and then quickly paint. Slavishly follow the paint manufacturer's application instructions.
Regards,
Richard.

[ur2slo]

Originally Posted by UNCIVILIZED

As long as the primer's put on within minutes of blasting. And the boat's located in Death Valley at the time, then that should do it.

I'm sitting here, extolling the virtues of another useless thing I need to know.( Not your comment, your right on)

The molecular make up of rust......? ok

Still doesn't beat the thermal dynamics & cost basis to boil water thread.

But it might!



Let the flaming begin..........

[Oceanride007]

My boat is supposed to be made of Coreten, built 30 years ago. Had a bad issue under a leaky watertank, probably leaking for years before I bought it. Very surprised to find layers at least 3 perhaps more, like a onion, hard black layers about a 1mm, of course the best thing to do was replace with metal, but its over a diesel tank, in the keel.
Was swayed by local Paint Guru, to use a 2 pack binder, theory being to lock up the remaining corrosion, then normal primers etc over the top.

Its about 4 months now, next time on board I'll be looking for any growth before deciding to replace water tk.

But I did speak to others on the hardstand over the next few months, all (about 4) experienced people are using a similar product on hull external where its been disk back exposing bare steel, from International and although not perfect its as good as any other propriety method used in there experience.

[stillbuilding]

Well, I have gleaned useful info from the posts thank you.

Of course it is obviously rust but in a different form and I must confess I have not seen these sub-paint divots before.

There is too much of the topsides involved to be excavating all divots so I am grinding them off, sand blasting and painting with rust converter and then epoxies.

Will be interesting to see how it holds up.

SVSeachange, there are well described coatings to prevent rust and on a new build this should never be a significant issue for you so rest easy.

This boat is a rescued project boat which was never painted well and neglected for 20 years so is surely a one off.

[Aloha_float]

People at Hempel or International would probably provide a better answer. They likely have a total coating system to recommend including material prep for your steel boat. Cheap paint is not a good idea in any case as it is far more expensive to replace a corroded boat than it is to use the proper coating system.

[ur2slo]

Originally Posted by stillbuilding

Well, I have gleaned useful info from the posts thank you.

Of course it is obviously rust but in a different form and I must confess I have not seen these sub-paint divots before.

Not meaning to come off smart A** wise. It is the internet.

Your project is amazing.

I've delt with metals over my life time and yes, your correct, when you get the proper info, you become more knowledgeable, and its seems your smart enough to make sense of the saying........

Listen to everyone, but follow no one.



Intergranular corrosion........the delamination issues with plate I've delt with also.

I'm out..........

Keep asking, wondering, & and always........

Question Stupidity

[captlloyd]

Originally Posted by stillbuilding

Well, I have gleaned useful info from the posts thank you.

Of course it is obviously rust but in a different form and I must confess I have not seen these sub-paint divots before.

There is too much of the topsides involved to be excavating all divots so I am grinding them off, sand blasting and painting with rust converter and then epoxies.

Will be interesting to see how it holds up.

SVSeachange, there are well described coatings to prevent rust and on a new build this should never be a significant issue for you so rest easy.

This boat is a rescued project boat which was never painted well and neglected for 20 years so is surely a one off.

The rust you pictured is known as ''carbuckle''. And yes it is just an extreme form of the rusting process. A thickness of steel can rust and expand to an amazing thickness of this stuff, a 1/4 inch piece of steel can start this process and expand to 3/4 or more thickness while leaving some of the original thickness intact. If it has any thickness at all, it can be hard to sandblast away, it takes on a silicone-like hardness. Then a scaler or hammer is necessary. As stated before, remove this carbuckle and then sandblast and paint. Epoxy is your best bet for the deck. And your boat can last for many years.

[mbhouston]

I see this in my line of work in steel pipe exposed to atmosphere near salt water. Most likely chlorides were on the surface before painting. Sand blasting alone won't remove chlorides and in fact, makes the problem worse. Chlorides cause the coating to disbond from the steel and allows the corrosion process to continue. Steel surfaces need proper preparation before applying any type of coating. Near seawater, the surface needs to be washed to remove chlorides and coating applied as soon as it is dry. In an industrial setting, we use DI water. There are also chemical additives to dissolve the chlorides. I'm not sure what is available for a DIY'er, but I'm sure there are products available off the shelf somewhere.

[stillbuilding]

Originally Posted by Aloha_float

People at Hempel or International would probably provide a better answer. They likely have a total coating system to recommend including material prep for your steel boat. Cheap paint is not a good idea in any case as it is far more expensive to replace a corroded boat than it is to use the proper coating system.

Hi Aloha,
I have used Hempel paints for another boat - very good products and support. The simple enamels we banged on to the already rusty deck was a purely stop gap measure to get the boat to a boatyard for repair. Long story, but just did not happen as planned and here we are looking at carbuncles of corrosion.

Only reason I posted is that I had not seen this particular version of rust and was a bit surprise to see how aggressive it was under a paint barrier. Good answers received I must say.

Sections of adjacent steen have more usual delamination rust. See pic.

[stillbuilding]

Originally Posted by boden36

The only real solution is abrasive blasting to near white metal. (class 2.5 at least).
To remove any salts you should then wash with a product like Chlor Rid, lightly reblast and then quickly paint. Slavishly follow the paint manufacturer's application instructions.
Regards,
Richard.

Thanks for the advice Richard. I value your opinion, understanding your past achievements and abilities.

I think fine sand blasting is unlikely to remove the oxides so will follow with a rust converter and two part Epoxy primer, Undercoat And urethane.
Only a bit more effort than epoxies alone.

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