Corrosion protection of steel hulls

Good day to everyone.

I am new on this forum. and was attracted by a series of comments about the use of zinc coatings on steel hulls, which for many folks globally seems to have been a real point of contention.
As a corrosion consultant for a few decades, I have worked on over 80 steel-hulled boats, ranging from elegant 60-foot steel yachts, a nuclear submarine (Astute Class), work-boats, tug-boats, survey vessels and fishing trawlers through to house-boats.

As always, all of these vessels share the same problems because they are all immersed in seawater, and there have been some instances where a houseboat or other craft has cruised from an inland river and into an estuary , where their magnesium anodes fizzled away like Alkaseltzer headache tablets!
So just to address certain ongoing discussions, it is a fact that the five major paint manufacturers in the world do discourage the use of zinc coatings underwater. Basically, if the coating system suffers a physical impact through to the zinc layer, the exposed area of zinc will become extremely anodic, because the rest of the zinc coating remains passivated by the mid-coats and antifoulings. This tends to make the zinc deplete at a much faster rate.

Just to be clear here, I am speaking about zinc silicate coatings, which have been in use for many decades now, and usually with great results. Zinc silicates are akin to having the boat hull galvanised, but the use of anodes is still very much in vogue because the zinc layer has been passivated by the sealer/mid-coats and antifouling, so they cannot 'throw' onto the propshaft or other immersed metalwork.
Another important point is the use of aluminium anodes on zinc-coated hulls, because the anodes remain electronegative to the zinc coating, and in the event of heavy abrasion or impact damage that leaves the zinc exposed, the aluminium anodes will protect the zinc layer.

On large steel boats with impressed-current systems on board, the reference electrode will be scanning the hull 24/7 and measuring the electrical potential of the hull, which must be kept below -850 mV at all times. Zinc silicates generate an electrical potential of 1040 mV and so offer cathodic protection on areas of questionable paint coatings. However, the reference electrode will be measuring this potential and automatically reduces the rectifier output to a reduced amperage/voltage because the zinc is offering good protection, albeit underneath the paint system.

In the event that the reference electrode gets damaged or overgrown with barnacles etc the zinc layer is still protecting the hull anyway.
Where a steel boat is built with an integral potable-water tank, then it is no unusual to blast-clean and coat the hull and water-tanks (and decks) all at the same time, giving a virtual 'galvanised' boat.

A zinc layer sealed with a good epoxy MIO coating can then have a standard alkyd deck paint applied on the decks, and instead of getting a durability of 6 - 8 years, you would get a durability of 12 - 15 years because the topcoat does not break down as fast. Why? Aged paints become porous, allowing ingress of water and chlorides. Once the sealer layer starts to break down, it allows moisture and salts to penetrate though to the steel, and this initiates a corrosion cell.
The salt solution underneath the coating system then forms an osmotic cell, drawing more and more moisture underneath the paint system to equalise the pressures above and below the paint system.
This leads to the formation of blisters, and corrosion under-creep.

With a zinc layer on the hull, once the moisture is able to penetrate through the sealer layer, it reaches the zinc layer which reacts and produces zinc oxides and other salt complexes, blocking any porosities within the coating system. So there cannot be any blistering or corrosion under-creep damaging the coating system from underneath, and the topcoat continues to give service while the UV slowly degrades it back to the sealer layer. At this point, the decks can be given a power wash at 300 bars using fresh water and this will remove all loose and flaking paint and any embedded chlorides (a critical point here). This is allowed to dry and a new sealer layer and topcoat can be applied. This is an energy-saving method to maintain the decks and hulls of many boats, with the requirement of re-blasting back to bare steel.

 

Thank you Theblacksmith for such an enlightening post. Corrosion and electrolysis is the bane of almost all modern water craft, (and is a bit of a mystery to the un-trained), and paint / coating degradation an issue for all boats for centuries. I have a series of question: is hardware store bought cold galv paint effective on a steel hull ? And then after that, and having read your info, would a hardware store bought aluminium containing paint over a cold galv paint give more / better protection to the galv paint; with a sealing / finish marine paint on top, followed by anti fouling paint over the submerged areas ? Or is hardware store bought paint greatly inferior to industrial rated coatings ?
Also, would affixing an aluminiun anode near a zinc anode be of any benefit ?

 

Dear Seasquirt.

Any paints or coatings purchased from a hardware store are designed for use on garden gates and domestic ironwork like wrought-iron railings, cast-iron grilles etc.
They are definitely not designed for marine use.
I should mention here that there is actually quite a difference between 'coatings' and 'paints':
Paints are normally cosmetic, whereas coatings are normally functional.
So paints can be high gloss, low gloss, textured, metallic and so on.
Coatings, on the other hand, are designed to be anti-corrosive, antifouling, high temperature (exhaust systems etc), hard-wearing (chain lockers, sea chests etc) potable water, non-skid (decks) and so on.

So paints are usually designed for atmospheric exposure to protect against UV, frost, ice, rain and general weathering, or else for indoor use for pure cosmetic appearance, or in the case of kitchens and bathrooms you need protection against steam damage and dirty hands from kids.
Waterbased emulsion paints contain anti-mould and anti-fungal additives and so do a good job around the house and especially where there is damp or potential for mould to develop.

On a boat, the first aspect to consider is that the boat and everything on board is going to be exposed to constant humidity from the air during it's entire life.
Immersed parts of the boat will be constantly exposed to either fresh water or salt water.

For that reason, coatings are designed to totally block the ingress of water and salts.
This can be a complex topic, but basically, on a coastal boat you will have ongoing exposure to airborne chlorides. Secondly, the air we breathe contains 20% nitrogen, and this gas dissolved in overnight atmospheric condensation or the condensed steam from cooking or showering on board a steel boat, will form a weak nitrous acid.

So between chlorides from the sea and the gas from the atmosphere, you will always have salts forming. This is why it is important when insulating a boat is to always use 'closed cell' foam, as it is totally waterproof.

The other types are not, and they can 'hold' moisture, and on the internal faces of hulls they can hold moisture against the steel surfaces, initiating corrosion cells.

Sorry to digress (a senior moment!! ) but to answer your questions, cold galv paint from a hardware store is totally useless on a steel boat. The binder used to manufacture it is normally only for the usual atmospheric exposure that a house undergoes.

Likewise, the aluminium paint you mentioned is designed to give an appealing cosmetic finish, and for that finish the manufacturer would choose leafing aluminium as opposed to aluminium powder, which is a lot denser.
This gives a very bright aluminium aspect upon drying, whereas a coating with aluminium powder is often much duller in appearance.

The aluminium particles are totally encapsulated within the paint binder, and so cannot react with the atmosphere, and under immersion conditions it cannot react with seawater. So it cannot offer any protective characteristics whatsoever.
Same as the cold galv paint, its density would have to be a magnitude of several times higher.

Zinc silicate coatings normally have a density above 3.0, and this is an indication of the zinc loadings in these coatings.
(Zinc anodes have a density of 7.1).
I hope this has answered some of your questions, but please feel free to ask away if you have more.

 

store are designed for use on garden gates anddomestic ironwork like wrought-iron railings, cast-iron grilles etc.
They are definitely not designed for marine use.
I should mention here that there is actually quite a difference between 'coatings' and 'paints':
Paints are normally cosmetic, whereas coatings arenormally functional.
So paints can be high gloss, low gloss, textured, metallic and so on.
Coatings, on the other hand, are designed to be anti-corrosive, antifouling, high temperature (exhaustsystems etc), hard-wearing (chain lockers, sea chestsetc) potable water, non-skid (decks) and so on.

So paints are usually designed for atmosphericexposure to protect against UV, frost, ice, rain andgeneral weathering, or else for indoor use for purecosmetic appearance, or in the case of kitchens andbathrooms you need protection against steamdamage and dirty hands from kids.
Waterbased emulsion paints contain anti-mould andanti-fungal additives and so do a good job around the house and especially where there is damp orpotential for mould to develop.

On a boat, the first aspect to consider is that the boatand everything on board is going to be exposed to constant humidity from the air during it's entire life.
Immersed parts of the boat will be constantlyexposed to either fresh water or salt water.

For that reason, coatings are designed to totally block the ingress of water and salts.
This can be a complex topic, but basically, on a coastal boat you will have ongoing exposure to airborne chlorides. Secondly, the air we breathe contains 20% nitrogen, and this gas dissolved in overnight atmospheric condensation or thecondensed steam from cooking or showering on board a steel boat, will form a weak nitrous acid.

So between chlorides from the sea and the gas from the atmosphere, you will always have salts forming. This is why it is important when insulating a boat is to always use 'closed cell' foam, as it is totally waterproof.

The other types are not, and they can 'hold' moisture, and on the internal faces of hulls they canhold moisture against the steel surfaces, initiatingcorrosion cells.

Sorry to digress (a senior moment!! ) but to answer your questions, cold galv paint from a hardwarestore is totally useless on a steel boat. The binderused to manufacture it is normally only for the usualatmospheric exposure that a house undergoes.

Likewise, the aluminium paint you mentioned is designed to give an appealing cosmetic finish, andfor that finish the manufacturer would chooseleafing aluminium as opposed to aluminium powder, which is a lot denser.
This gives a very bright aluminium aspect upon drying, whereas a coating with aluminium powder is often much duller in appearance.

The aluminium particles are totally encapsulated within the paint binder, and so cannot react with the atmosphere, and under immersion conditions it cannot react with seawater. So it cannot offer any protective characteristics whatsoever.
Same as the cold galv paint, its density would have to be a magnitude of several times higher.

Zinc silicate coatings normally have a density above 3.0, and this is an indication of the zinc loadings in these coatings.
(Zinc anodes have a density of 7.1).
I hope this has answered some of your questions, but please feel free to ask away if you have more.

 

What about polyurethane coatings or polyurea?
How about 3M 5200? or Durabak?
The claim for Durabak is the US navy is using it, maybe on decks.
Boat Paint & Marine Paint - Slip Resistant | Durabak

I talked to them and said it will protect metal underwater, wood, fiberglass.
Do you know any who have experience with that?

I figure 3M 5200 speaks for itself as to being durable and protecting underwater.
Only negative for Durabak it seems is bleach dissolves it.
I don't think bleach dissolves 5200.

 

Hot zinc spray is a good alternative to coatings.
@Theblacksmith: I am glad you made the distinction between paints and coatings. They are often used intercheangably. I would also add self etching zinc chromate as a coating.

 

All I see here at marinas is people using epoxy paints on steel hulls.