Steel CB protection

[johngsandusky]

I've had two boats with iron keels. Both were primed and painted originally and held up well. On one I scraped scaled and sanded it by hand, primed with zinc paint, and copper bottom painted. That was over 20 years ago, it still looks good. I have no doubt that galvanizing is best, but many thousands of steel ships travel the world with no more than paint and primer on the hull. Many many classic boats have iron keels that are just painted too.

[Peerie Maa]

I have no doubt that galvanizing is best, but many thousands of steel ships travel the world with no more than paint and primer on the hull. Many many classic boats have iron keels that are just painted too.

Cast iron ballast keels and mild steel corrode at vastly different rates.

[MN Dave]

Don't forget the zinc anodes or impressed current systems on those thousands of ships:

I have no doubt that galvanizing is best, but many thousands of steel ships travel the world with no more than paint and primer on the hull. Many many classic boats have iron keels that are just painted too.

Cast iron ballast keels and mild steel corrode at vastly different rates.

This links to a pdf that has the results of testing done on zinc rich primers, followed by tests on rust converters.

Zinc rich paints:
1. Zinga Organic epoxy primer 96% Zn
2. Zinc Clad IV Solvent-based 85% Zn
3. Zinc Clad XI Water-based inorganic Zn silicate with no VOC and 90% Zn
4. N-5751M2 Solvent-based moisture cure 90% Zn
5. Intershield 300V Aluminum-rich epoxy
6. Epoxzen Organic epoxy with 90% Zn

Rust converters:
1. Gempler’s (water-based, tannic acid)
2. Loctite rust treatment (polymeric-based, barium sulfate)
3. Total Solutions (water-based, tannic acid)
4. Phoscote (phosphoric acid – current USMC product) <>>
5. VpCI CorrVerter (combined rust converter and primer)
6. Corroseal (water-based, tannic acid with primer)
7. Gem Rust Killer (under test)
The rust converters were coated with a non-corrosion inhibiting epoxy primer and a polyurethane topcoat. While they are good paints, there was no chance that the rust converters would look good in these tests. It is unfortunate that they didn't compare to a sample without a converter. All I can say is that none of the rust converters looks bad, since all they do is help the paint stick to the metal.

[alkorn]

Idle curiosity. US submarines are black.. What do they use, or are they telling?

Secret submarine stuff aside, hot-dip galvanizing an entire steel ship is not possible. Instead, they are protected with zincs that work on the same principle.

I wonder if you could design a steel centerboard with a protective zinc?

[Peerie Maa]

Secret submarine stuff aside, hot-dip galvanizing an entire steel ship is not possible. Instead, they are protected with zincs that work on the same principle.

I wonder if you could design a steel centerboard with a protective zinc?

The bilges of warships used to be flame sprayed with zinc, nearly as good as hot dip.
As to you question about designing a cb with a protective zinc, That is what we have been recommending - hot dipped.

[Phil Y]

Don't forget the zinc anodes or impressed current systems on those thousands of ships:



This links to a pdf that has the results of testing done on zinc rich primers, followed by tests on rust converters.

Zinc rich paints:
1. Zinga Organic epoxy primer 96% Zn
2. Zinc Clad IV Solvent-based 85% Zn
3. Zinc Clad XI Water-based inorganic Zn silicate with no VOC and 90% Zn
4. N-5751M2 Solvent-based moisture cure 90% Zn
5. Intershield 300V Aluminum-rich epoxy
6. Epoxzen Organic epoxy with 90% Zn

Rust converters:
1. Gempler’s (water-based, tannic acid)
2. Loctite rust treatment (polymeric-based, barium sulfate)
3. Total Solutions (water-based, tannic acid)
4. Phoscote (phosphoric acid – current USMC product) <>>
5. VpCI CorrVerter (combined rust converter and primer)
6. Corroseal (water-based, tannic acid with primer)
7. Gem Rust Killer (under test)
The rust converters were coated with a non-corrosion inhibiting epoxy primer and a polyurethane topcoat. While they are good paints, there was no chance that the rust converters would look good in these tests. It is unfortunate that they didn't compare to a sample without a converter. All I can say is that none of the rust converters looks bad, since all they do is help the paint stick to the metal.

Unfortunately I don't understand most of that! My boat is strip planked and glassed on the outside. The has 2 skegs and rudders. The skegs are steel. The rudders are stainless. In between them of course is a bronze prop. My own little power generation plant right there. At about 50 years old there is considerable corrosion in the skegs, and having had the boat for about 7 years now I've not yet found a paint system for the skegs and rudders that doesn't come out looking very much like a failure at the next haul out. Might be time to remove the skegs, get them thoroughly sand blasted and hot dipped by the sounds of it. But next best is probably sand blast in place and a zinc rich primer. I don't know what sticks to stainless though.

[ron ll]

I notice the chandleries around here are phasing out zincs in favor of aluminum anodes. Evidently all the zinc we are putting into the water is not very healthy.

[Phil Y]

Oh great, now we can't have decent antifouling or corrosion protection. And heaven forbid you might want to create any sawdust in your berth.

[MN Dave]

Unfortunately I don't understand most of that!

Wow! somebody tried to read one of my links! Sorry, it was a bit obtuse. Unfortunately, that wasn't as clearly written as it could have been, but wasn't much worse than the better articles. Welcome to my world. One thing about corrosion testing to remember is that corrosion tests are like an old army mule, they don't reproduce very well.

Might be time to remove the skegs, get them thoroughly sand blasted and hot dipped by the sounds of it. But next best is probably sand blast in place and a zinc rich primer. I don't know what sticks to stainless though.

Wait a minute, you did understand the link. It also said that painting the galvanized or zinc rich coating is a good idea.
As for what sticks to stainless steel, based on my observations of my stainless frying pan, eggs stick really well, followed by just about everything else, except paint.
Stainless needs to be really clean. it will help to lightly grit blast with a fine clean aluminum oxide grit that has never been used on steel. It also helps to clean with something like Alumaprep 33, which is pretty much Ospho with some degreaser. If your water is hard or high solids, rinse with RO or distilled water.

Do not galvanize or use cold galvanizing on stainless. The corrosion prof at MIT explained the effect on corrosion potentials to me and the part that I understood was that it it's bad to galvanize stainless. The details -- arrgh.

There are primers recommended for stainless that are also good for galvanized surfaces. Etching primers are good. I hate to recommend a chromate primer, but Pettit 6455 is good (apply thin, if you can't see through it, the paint will peel.) It is a version of the old DOD-P-15328 polyvinyl butyrate. I would also recommend an easier to use, less hazardous primer like Pettit 6980 Rustlok. The reason to paint stainless not to protect the stainless, it is to protect the other metals nearby. Painting stainless will reduce the large exposed area of cathode that causes galvanic corrosion of everything else.

Oh great, now we can't have decent antifouling or corrosion protection. And heaven forbid you might want to create any sawdust in your berth.

Aluminum anodes are better than zinc.
https://www.cruisingworld.com/how/zi...-anodes#page-2

Surprisingly, aluminum anodes are often no more costly than zinc ones. Also, because most are cadmium-free, aluminum anodes are less of a hazard to the marine environment. So, regardless of what a diver, boatyard manager or other industry pro tells you, there are several advantages, and no drawbacks, to switching to aluminum anodes.

[Phil Y]

Thanks Dave, etching primer it is then. For the stainless. Of course the other reason to paint stainless, is to stop stuff growing on it.

[BrianM]

If you chip the paint by grounding the plate rust will start and run under the paint coating.
On the other hand the steel cb made for a boat that I built in the '70s and had hot dipped is lying in my yard with no corrosion at all in evidence.

What he said... always galvanize if you can. No coating can approach the durability. It is self healing if scratched. Generally, hot dip galvanizing is charged by the weight of the material being dipped and spun. Our local high school Metalworking program build's park benches and has them galvanized, then powder coated. Some of my volunteer work with the kids as rewarded with an offer to allow me to throw some parts into the basket I had fabricated for a quick release bowsprit mechanism. It otherwise would have cost me about $50.

[BrianM]

Steel Centerboards.... a comment was made on how to fair the board to give it a lifting profile. I had a piece of 5/8" boiler plate my dad pulled from the scrap bin at work and gave me when I was building a sailing dory. I machined the edges square, then made some hardwood leading and trailing edges. I drilled and tapped the front and read edges of the boiler plate and after painting (I was in high school, so did not know about galvanizing) and simply attached the fairings to get a little better efficiency. NACA lifting foils are the ultimate solution, but the shape is impractical for a centerboard in terms of width and they have not flat surfaces to bear against a centerboard trunk.

[Jay Greer]

Oh great, now we can't have decent antifouling or corrosion protection. And heaven forbid you might want to create any sawdust in your berth.

Yes, Newport Harbor CA has that rule! Madding if you are, merely, sanding varnish without draping the area! Sanding a mast on the beach is a no no as well!
Jay

[willin woodworks]

And all those steel ships, at least in the US Navy, have men that do nothing all day but scale, prime and paint....you could always tell the boatswains mates on the mess deck because their boondockers were always red leaded or gray painted...

[MN Dave]

What he said... always galvanize if you can. No coating can approach the durability.

If you look at the hot dipped corrosion test panel on the lower right in the first picture, you may want to look a little harder at the zinc rich paints. I can guarantee that if you buy the same paints and run the exact same test again, the results will be a bit different. How different is hard to say. This time hot dip looked bad and next time it might do better.
zinc only.jpg
In the same test with the galvanized panel (probably from the same lot) painted and scratched down to the steel and hot dip was the best coating. Note that the Epoxyzen and galvanized swapped places from top to bottom once painted:
painted.jpg
EDIT: (not worth a bump)

Cast iron ballast keels and mild steel corrode at vastly different rates.

Not exactly vastly in the old NCEL testing on bare metal, but from the picture I'd have to say the paint held up a lot better on the iron. For the sake of... well nothing really, there is a lot of corrosion data here. The testing was at various depths down to 6000 ft and lots of alloys, so as you might expect, it has been too expensive to repeat, so these results are rare. At a depth of 5 feet, A36 (common mild construction steel) corroded at 6.2 mil/yr, wrought iron 4.8 mil/yr and grey iron 2.6 mil per yr. Other sources say mild steel corrodes 10 times as fast as cast iron. Corrosion test results are not all that consistent in detail, but the trend supports Nick's observation.

[RowAndSail]

Cast iron ballast keels and mild steel corrode at vastly different rates.

'tis but a scratch