Copper antifoul on steel

[Halcytwo]

Hi
Any Metallurgists out there who can explain what would happen if a copper based antifoul was put directly onto a steel hulled boat.
I understand its common practice to epoxy first as a layer in between, does the epoxy serve any other purpose ?
Why epoxy and not something like Duralac
Thanks in advance.

 

[KellysEye]

I've never known copper based antifouling to be used on a steel boat we had one and met many others on our travels. I assume the epoxy is to electrically isolate the copper from the steel but I wouldn't try it or paint. Why take the risk.

 

[NormanS]

Most (all?) RN ships are (were?) painted with copper antifouling. Goes on red, and turns green after a while in the water. Keeps the weeds away, and doesn't seem to destroy the steel ships. I don't know what they used as an undercoat.

 

[vyv_cox]

No particular metallurgical reason I can think of. The main season that antifouling paint is not applied directly to a steel hull is that it has poor or non-existent protective qualities, so the steel would rust quickly. But also, most antifoulings need a primer to ensure good adhesion to the substrate. Antifouling like Coppercoat that comprises copper metal in an epoxy matrix needs an epoxy primer but a conventional type only needs a normal alkyd-type primer, such as Primocon.

 

[KellysEye]

> and doesn't seem to destroy the steel ships.

My other half's father was a Rear Admiral in charge of the RN Bath ship design base, warships are made of Aluminium, hence the fire problems seen in the Falklands when hit by a missile. Copper was used on the wooden navy ships but I doubt it's used on Aluminium for the same reason as steel.

>The main season that antifouling paint is not applied directly to a steel hull is that it has poor or non-existent protective qualities, so the steel would rust quickly.

Agree, antifoul is applied after two coats of zinc rich epoxy primer (not Primocon) on the bare steel. Putting copper antifouling on zinc rich epoxy on steel would make an interesting chemical experiment for someone.

 

[vyv_cox]

> and doesn't seem to destroy the steel ships.

My other half's father was a Rear Admiral in charge of the RN Bath ship design base, warships are made of Aluminium, hence the fire problems seen in the Falklands when hit by a missile. Copper was used on the wooden navy ships but I doubt it's used on Aluminium for the same reason as steel.

>The main season that antifouling paint is not applied directly to a steel hull is that it has poor or non-existent protective qualities, so the steel would rust quickly.

Agree, antifoul is applied after two coats of zinc rich epoxy primer (not Primocon) on the bare steel.

Very many people do not use epoxy primers. Whilst accepting that it is the best treatment, there are plenty of cases where it is too expensive to be justified. My rudder, the only steel part of the boat, is a case in point. It is primed with Primocon and antifouled using the cheapest conventional paint available.

 

[NormanS]

> and doesn't seem to destroy the steel ships.

My other half's father was a Rear Admiral in charge of the RN Bath ship design base, warships are made of Aluminium, hence the fire problems seen in the Falklands when hit by a missile. Copper was used on the wooden navy ships but I doubt it's used on Aluminium for the same reason as steel.

Much of the upperworks of RN ships is constructed in aluminium, but they have seldom found a need to use antifouling of any description on the upperworks. Check with the Rear Admiral.

 

[rogerthebodger]

On my steel hull boat I use Hampel Olympic 86950 which has a high load of cuprous oxide in it.

The hull was grit blasted and primed with epoxy tar before the application of 3 coats of the hampel and recoated every 2 years with one or two coats there after and to date corrosion of the steel hull and little wasting of the anodes.

The only thing I do between anti fouling is to give the hull an in water scrub with a stiff nylon brush every 3 to 4 months down as far as I can reach with my extending pool brush.

Data sheet

http://www.hempel.com/~/media/Sites/hempel/Files/Product Data Sheets/85000-89999/8695051110.pdf

 

[Pasarell]

Virtually all RN ships are built from steel. Some have aluminium or composite superstructures but as NormanS said that rarely needs antifouling. Steel corrodes quickly when exposed to salt water or air. For that reason it is usually blasted first and then coated with an anticorrosive primer. Sometimes that is zinc rich. For RN vessels usually not and there are various reasons why not. The primer can be a conventional single pack type similar to Primocon but when preparing to a high standard by blasting then it is normal to use a high performance primer such as an epoxy. Primocon is usually used where hand tool preparation is all that is possible. When applying an anticorrosive you aim for a specified film thickness, not a number of coats. Having said that 2 coats would be a minimum acceptable specification, whatever product is used, to give a good chance of avoiding holidays.
Copper based antifouing would adhere to bare steel reasonably well, in the same way as it would to GRP, but only until the steel starts to corrode. Antifoulings are normally porous to aid their fouling control properties and this would allow the steel to quickly start corroding. As the rust flaked off then the antifouling would go with it, probably still adhering to that rust! Putting a copper based antifouling directly onto steel would increase the rate of corrosion slightly compared to putting uncoated steel into the water.
On steel the primer and antifouling perform completely separate functions and sometimes a tie coat is needed between them to aid adhesion.