Advice on making good the joint between GRP an Cast Steel.

[Ian_Edwards]

The joint in question is between the grounding plate, which is recessed into the hull and the GRP hull. The gap is filled with a non structural sealant, which has been poorly finished, it's very rough.
There's about 15m x 50mm to fair.
I'm intending to use West 205 105 with micro balloons, mixed to a thick paste.
The question is how do I treat the cast steel, which is rusting in places. My first thoughts are rotary wire brush, rust converter, special metal primer, then a epoxy barrier coat, followed by two coats of micro 350.
Can anyone see a miss match in the coatings, or think a better combination?
The boat is inside, but there is very little clearance between the hull and the concrete floor. Access is lying on my back on the concrete floor working over head.

 

[jwilson]

If this is a Southerly, bear in mind that sooner or later you may (will?) have to completely remove the grounding plate and keel mechanism from the GRP hull. Epoxy might make that eventual job rather hard.

 

[pandos]

The joint in question is between the grounding plate, which is recessed into the hull and the GRP hull. The gap is filled with a non structural sealant, which has been poorly finished, it's very rough.
There's about 15m x 50mm to fair.
I'm intending to use West 205 105 with micro balloons, mixed to a thick paste.
The question is how do I treat the cast steel, which is rusting in places. My first thoughts are rotary wire brush, rust converter, special metal primer, then a epoxy barrier coat, followed by two coats of micro 350.
Can anyone see a miss match in the coatings, or think a better combination?
The boat is inside, but there is very little clearance between the hull and the concrete floor. Access is lying on my back on the concrete floor working over head.

If it's a southerly, when I removed my keel it was sealed like how you describe and it was rusting.

I think you must have some movement between the hull and the edge of the keel. Or the joint will crack.

When I put it back I have been advised to fill the void with closed cell foam and caulk over that with sika flex or to use caulk saver strip and caulk over that..

I am probably going to use the expanding foam, and shave it back by about 5 mm from the surface and from the two edges and then paste on sikaflex.

My keel treatment is as yet unknown as I am waiting to see if I must grind it by hand or if I can have it blasted and primed by the yard...

 

[ducked]

The joint in question is between the grounding plate, which is recessed into the hull and the GRP hull. The gap is filled with a non structural sealant, which has been poorly finished, it's very rough.
There's about 15m x 50mm to fair.
I'm intending to use West 205 105 with micro balloons, mixed to a thick paste.
The question is how do I treat the cast steel, which is rusting in places. My first thoughts are rotary wire brush, rust converter, special metal primer, then a epoxy barrier coat, followed by two coats of micro 350.
Can anyone see a miss match in the coatings, or think a better combination?
The boat is inside, but there is very little clearance between the hull and the concrete floor. Access is lying on my back on the concrete floor working over head.

On triple keel Tridents (mild steel bilge, cast iron central keel) people seem to use International Metallic Primocon paint, along with Zinc anodes.

I'll probably be comparing that on at least one surface with variations on my own home made aluminium-based treatment on others, having 6 sides available in total.

It seems, from this account, that Zincs can be a mixed blessing, with "overprotection" and paint burn off possible. I dont really understand that but havnt read up about it much yet.

Cathodic protection

 

[vyv_cox]

On triple keel Tridents (mild steel bilge, cast iron central keel) people seem to use International Metallic Primocon paint, along with Zinc anodes.

I'll probably be comparing that on at least one surface with variations on my own home made aluminium-based treatment on others, having 6 sides available in total.

It seems, from this account, that Zincs can be a mixed blessing, with "overprotection" and paint burn off possible. I dont really understand that but havnt read up about it much yet.

Cathodic protection

There are a couple of paragraphs on this problem, known as cathodic disbondment, on this page Corrosion

 

[Tranona]

On triple keel Tridents (mild steel bilge, cast iron central keel) people seem to use International Metallic Primocon paint, along with Zinc anodes.

I'll probably be comparing that on at least one surface with variations on my own home made aluminium-based treatment on others, having 6 sides available in total.

It seems, from this account, that Zincs can be a mixed blessing, with "overprotection" and paint burn off possible. I dont really understand that but havnt read up about it much yet.

Cathodic protection

Using a primer like Primocon is reasonably effective on mild steel bilge plates, but the long term solution is to blast and epoxy coat as I did 2 years ago on my Golden Hind. anodes are a waste of time on mild steel as while they slowly waste they do not prevent rust once the coating has broken down. The photos in your link show this clearly. If you can't afford to blast and epoxy (not cheap - mine cost £700 (£250 less if you do the epoxy yourself) then just clean as best you can and use Jotun Vinygard smlmarinepaints.co.uk/products/vinyguard and antifoul of your choice. I use this smlmarinepaints.co.uk/products/CU-PRO

 

[ducked]

There are a couple of paragraphs on this problem, known as cathodic disbondment, on this page Corrosion

Thanks. No sokution to the problem suggested though.

I wondered if an alternative to zinc would help, but as far as I can tell both aluminium and magnesiom are more active than zinc so would be even worse.

Perhaps partly isolating the anode with paint or plastic gasket could moderate its activity but still allow it some protective effect, but I suppose quite a lot of trial and error might be involved. I wonder if voltage measurements might provide some guidance.

 

[ducked]

Using a primer like Primocon is reasonably effective on mild steel bilge plates, but the long term solution is to blast and epoxy coat as I did 2 years ago on my Golden Hind. anodes are a waste of time on mild steel as while they slowly waste they do not prevent rust once the coating has broken down. The photos in your link show this clearly. If you can't afford to blast and epoxy (not cheap - mine cost £700 (£250 less if you do the epoxy yourself) then just clean as best you can and use Jotun Vinygard smlmarinepaints.co.uk/products/vinyguard and antifoul of your choice. I use this smlmarinepaints.co.uk/products/CU-PRO

Thanks. Interesting.

I assume the "Vinygard" is a vinyl-based primer, as I believe the Primocon is, so thats consistent. Both links give a 403 error but a search finds some technical detail.

If anodes are contraindicated for the mild steel bilge keels, I wonder if the same applies to the cast iron central keel, where they would still be in line-of sight of the inside of the bilge keels.

There is also a prop shaft , and a rudder both of which usually have anodes, (though my rudder seems to be stainless and doesnt seem to have anodes fitted) so its a bit complicated

 

[rogerthebodger]

Having a steel boat Zinc anodes do protect mild steel where there is 2 different Matals that are electrically connected and imersed in an electrolyte (sea water)

i HAVE ANIDES ON MY MS hull and MS rudder but do little to protect from galvanic corrosion and does nothing to protect from rust.

My hull was grit blasted and painted with 2 coats of epoxy primer and 6 coats of epoxy tar before anti fouling.

I have NO yellol metal below the water line so very little in galvanic corrosion, I have 316 propeller, prop shaft and rudder shaft all insulated with plastic bearings / bushes

 

[vyv_cox]

Anodes are not very effective on keels - too small to do much. They only 'throw' a relatively short distance anyway, so it would take quite a few to have any effectiveness. A good paint system is the preferred option.

 

[Tranona]

Yes Vinygard is essentially the same as Primocon but half the price, although you have to buy a larger can. Iron keels are similar to mild steel with the exception that they often have traces of other metals of a lower potential in them, so a zinc node may show signs of wasting but won't stop corrosion (rust). There is no connection between the plates and the iron keel. The "line of sight" is only relevant where a remote anode is used, for example a hull mounted anode bonded to the shaft to protect the propeller. This should be ideally less than 1m distant as in the photos. The second one shows the bonding brushes inside the boat. my propeller also has, like many of its type a large anode bolted directly to the propeller which is the one that takes most of the load. You don't need either of these because you are able to fit the shaft anode bolted directly to the shaft.

Rudders are a bit more complicated, although from what I can see the Trident has a sheet metal rudder so the stock is likely to be the same metal as the blade. If that is the case, no anodes are needed

 

[ducked]

Having a steel boat Zinc anodes do protect mild steel where there is 2 different Matals that are electrically connected and imersed in an electrolyte (sea water)

i HAVE ANIDES ON MY MS hull and MS rudder but do little to protect from galvanic corrosion and does nothing to protect from rust.

My hull was grit blasted and painted with 2 coats of epoxy primer and 6 coats of epoxy tar before anti fouling.

I have NO yellol metal below the water line so very little in galvanic corrosion, I have 316 propeller, prop shaft and rudder shaft all insulated with plastic bearings / bushes

I've got at least 3 metals (SS of unknown grade(s), cast iron and mild steel) of unknown electrical connectivity, but of course immersed in electrolyte in service, so I dunno what'll happen, but I'll probably try both paint and anodes initially. I would be inclined to go for epoxy tar, but I understand its no longer available.

 

[Tranona]

I've got at least 3 metals (SS of unknown grade(s), cast iron and mild steel) of unknown electrical connectivity, but of course immersed in electrolyte in service, so I dunno what'll happen, but I'll probably try both paint and anodes initially. I would be inclined to go for epoxy tar, but I understand its no longer available.

There is nothing unusual about that mix, but you perhaps need to read up on galvanic action as it is only when the metals are in contact UNDERWATER (that is in the electrolyte) that there is a possibility of galvanic action. So your rudder is not connected , nor is the iron keel or the bilge plates. Only the propeller and shaft are connected so need an anode, an even then only if the propeller is made of one of the commonly used brass based alloys. My boat originally had an original proper bronze alloy propeller on a stainless steel shaft and managed without anodes since 1979. It is only when I renewed the engine, stern gear and propeller using a feathering propeller with a mix of bronze and stainless that anodes became necessary.

So KISS and just keep the shaft anode and coat all the rest of the metal underwater with the best coating you can afford. No other anodes needed.

 

[ducked]

https://www.sciencedirect.com/book/9780323524728/handbook-of-environmental-degradation-of-materials

"In very general terms, an exposed bare steel surface may require 110 mA/m2 (10.22 mA/ft2) for protection, while a steel surface coated with a good-quality coating system will use about 5 mA/m2 (0.46 mA/ft2). The current requirement will increase with time as the extent of coating breakdown (and thus exposed metal area) increases. "

I wonder if it might be practical to measure this?

I'd guess not practical for the private punter, since the current would have to go through the meter, which it wont with a directly bonded anode. I'd guess you could measure the relative potential of the two surfaces but I'm unclear how informative that would be.

The boundary condition reference standard for cathodic protection of steel underwater bits of glassfibre boats seem to be defined (at about -750mV) relative to a silver/silver chloride reference half-cell and it seems to be a fairly elaborate bit of kit.

An Overview of Cathodic Protection Potential Measurement

Reference Cell Testing; Know Thy Corrosion Protection Level – Editorial: Old vs. New | Steve D'Antonio Marine Consulting

I suppose sophisticated commercial impressed current systems both monitor and control the current and thus avoid "overprotection" damage