Corrosion of aluminum hull

Did the aluminum have a zinc-chromate undercoat, or was it anodized before painting?

The hull was constructed using a marine grade alloy and the paint system was supplied by International Paint company and applied in accordance with their procedures for vessels and inspected by their representative. To the best of my knowledge the primer was one of the epoxy coatings followed by the anti fouling coatings. The hull was not anodized.

The photos look like the corrosion started as blisters, but not in a pattern I usually see associated with stray current. Unfortunately the hull was hauled for other work before the problem became evident. The damage is confined to the lower underwater areas not easily visible from above.

My first thoughts were incompatable paints too. particularly copper based anti fouls. by your description however sounds similar to the paint blistering seen on motor vehicles. When burst the blisters realease a liquid (probably water but i've never checked) suggesting osmotic process. Painters claim its due to poor preperation or inadequate drying between coats. I could agree with the second reason though I think the speed of drying is important, too fast and the top seals and traps the liquid in the bottom which then slowly migrates to the metal (as the dryer surface side solidifies inward) creating the blister condition.

Could you share the photo's?

Yes if i can figure out how to upload it.

View of blister and the zinc close up. This looks more like osmosis not stray current corrosion. I am an electrical guy not an FRP expert but I have been around boat yards since the first case of blisters showed up in Canada back in 1968.

Anychance this is caused by cavitation? I'm not seeing any corrosion on the aluminum, but maybe evidence of pitting on the prop itself.

I am inclined to think it may be cavitation or at the very least high velocity flow induced problems. The prop itself does not exhibit the common signs of cavitation with severe erosion focused on the trailing edge. However, much of the problem area does get concentrated in the skeg/prop area. This suggests hydrodynamic flow /pressure issue.

One of rhe naval architects at work has a program he uses to predict hydrostatic pressure from wave formation. Unfortunately it relies heavily on data from tank testing with existing hullforms We do not have towing tank data for this particular hull shape. Its a Michael Tennant power cat.

By end of week we should have some additional data from the paint people. Meantime it falls on my shoulders to determine if anything electrical caused or contributed to the problem.

Just thought I would tap into the collective wisdom of the forum subscribers. Thanks!

Elnav.

Have you checked to see if the sacrificial anodes were painted over? Aluminum vesssels have zinc saccrificial anodes just like steel ones. Paint over those and the aluminum will oxidize. It may be as simple as that.

No the zincs were not painted. In this photo the zinc has ben removed. It sits in the recessed pocket below the prop.

I was wondering if a chemical analysis of the liquid or gel found inside the blisters would prove anything such as containing the leachate from the paint or paint solvent. Aluminum is ampothetic and creates a curious jelly substance when the aluminum passes current into salt water. I have not had any samples of fresh water corrosion residue to test. But I have seen osmosis problems in fresh water. However osmosis is normally associated with FRG hulls not aluminum hulls.

Are not FRG hulls epoxy over E-glass etc. and would epoxy paints have some of the same problems?

Just a thought

Brad

I notice you have a skegless propellor drive which doesnt appear to have a Zinc anode on it. Also the rudder area seems to have the worst of the problem. The wash around the propellor will cause electrolisis even in like metals. the SSshaft will encourage it also. you need a Zinc anode on the shaft as well, preferably between the hull and the propellor. I also suggest you repaint the affected components with a two part paint AFTER using an etch primer to remove the oxide layer. coating propellor and rudder in a urethane rubber paint will also help by sealing the aluminium from the water.

REPLY to Guest:

Below the waterline the hull was painted with 2 coats of International paint "Interprotect", then some smoothing with International Interfill 830, two more coats of Interprotect. Last, two coats of Trilux 33 antifouling. All the paints are from International (yachtpaint.com) and applied under their supervision.

Concerning the rudder, it does not contain any aluminum. It is FRP over a foam core. The visible damage was due to impact and the reason why the vessel was hauled.

I would be interested in hearing your reason for why the shaft zinc should be located btween hull and prop, instead of on the shaft prop nut end as shown. (I have my own theory but would like an independent opinion). Incidentally, neither the prop zinc or the hull mounted zincs show any serious degradation. Typical for fresh water use.

The propeller is in fact protected by a substantial skeg. The clearance between the prop hub and the cutless bearing is 14mm, not really enough for the typical split ball shaft zincs commonly used. Even the collar type would not fit properly. The prop aperture is of sufficient size according to the naval architect. Sisterships have not reported any problem with the configuration.

The recommendation for a rubber paint on the prop surprises me. This runs counter to conventional wisdom which suggest a very slick surface is required for propellors. Since the rudder is non metallic, corrosion protection is a moot point in this discusion.

However I would like to hear your reasoning for recommending a rubber coat instead of some other dielectric material having a stronger surface bond and smoother surface.

There is some paint removal evident on the FRP rudder blade; suggesting something other than simple galvanic or stray current corrosion as being the cause. I'm not surprised to see the vertical skeg that supports the rudder top and bottom having taken the brunt of the paint ablation. As you say velocity does increase corrosion. It also increases normal abrasion.

On the aluminum ferries in San Francisco Bay the corrosion protection was spec'd as having to be effective up to 25 knots. Well above the protection needed at rest. The contractor had some difficulty achieving that degree of protection. This hull does not travel that fast.

The question remains, did the blisters form as a result of corrosion or was the corrosion a result of the blisters forming from other causes. I'm 99% certain the cause was not electrical. Now we wait for the paint company inspector and their chemist's analysis.