Hull corrosion, Lifetimer. Advice sought.

[Laxputs]

Hi all. Would love your input on this please.

Details:
-Aluminum Lifetimer boat
-20 years + old
-Some minor pitting on hull in about 30 spots at a max depth of 1/3 of the hull. So corrosion eating into the hull. The pitting may have taken place before I owned it and it was hidden by the bottom paint.
-It was bottom painted for most of its life before I owned it. I'd rather not bottom paint anymore and have let it go.
-Only kept in ocean 4 months of year. Zincs. Pressure washed a couple times a year. I dive it regularly too.

Is there a product I can scrape into the pitting? Is there a JB Weld or aluminum filler out there? Any thoughts to fix the hull? Thanks.

[kmorin]

Laxputs, welcome to the AAB.com Forum.
Have you approached the builder and asked for help addressing the pitting?
Is the pitting outside the hull- seems like it from your post?
Do you know what type of bottom paint was used-is left on the hull? (might be wrong type?)
Is the hull on a trailer with bunks or rollers? Are the bunks covered with carpet? (bunk rot)
Was the hull left in a slip in a harbor during its previous owner's tenure?
Images of the pitting sites would help others to suggest causes and cures.

We have fairly extensive archives here, IMO they're worth the time (and it would be a fair bit of time) to research and read what's already been shown and discussed on this topic.

Cheers,
Kevin Morin
Kenai, AK

[Laxputs]

Thanks for the response.

1. I'll call Lifetimer and see what they suggest, good idea.
2. Pitting outside the hull, biggest is 1/2 a penny in size.
3. Not sure of the bottom paint.
4. Plastic rollers.
5. I bet the boat was left in a slip during previous ownership yes.

Wonder if I can apply a filler product and then perhaps should just bottom paint again even though I prefer not...

[kmorin]

laxputs,
the premium repair would be to TIG weld the corrosion sites after they are excavated to clean metal using a high speed carbide burr and then followed by sanding off the weld. This is the best because you end up with an intact all metal hull. Not the easiest repair in the world - agreed. And not the least expensive one either- but the best.

Next would be to excavate the pits, as above, and fill/spray etch, rinse, Allodyne, then primer and finally fill w an epoxy putty/filler then sand off- re-primer around the sanded area, and coat with bottom paint to avoid having the interface between the epoxy and parent metal become water soaked and begin corroding again- once the water can become a thin film the ph can shift pretty easily.

Excavating those pits with sand blasting, carbide cutter, or even a large diameter drill bit's tip is pretty important, as whatever is in the pits and not removed remains an active cell.

I did an article somewhere on the Forum about what I'd found in weld repairing pits under a hull. The pit's surface appearance is not always the extent of the pit's interior volume. I provide an illustration or two on that thread to show what I'd learned. Welding over a small (pencil dia.) appearing hole turned into a massive hidden cell (silver dollar sized!) which then formed a tiny "steam volcano" when heated with arc. Needless to say, that project was very time consuming to repair- the boat had a single wire to a fuel level sender and was using the hull as the negative or 'return' leg (DC Neg) for that fuel level indicator! (More than 1k pits of all sizes!)

Metal-2-Metal filler compound works well with aluminum but I've not had any experience with it below the waterline? But it does adhere to aluminum very well and sands to a nice feather edge and doesn't show under topsides painting.

Maybe Chaps will see this thread and give some advice? I think there's another, similar, thread close to this one discussing similar issues - have you had the chance to read that discussion?

In any case, guessing/estimating/discerning the cause of the corrosion seems like its important to avoid those conditions again? For example; is the wiring both AC & DC bonded to the hull in one single location? OR, as is very common in (and incorrect) some boats is the electrical system "isolated" from the hull? If there is no NEG. bond then stray current may be a real possibility even if you have some anodes intended to protect the hull?

Also, are the anodes actually zinc or the more effective aluminum alloy anodes currently in increasingly wide use?

Be interesting to learn what your builder's firm has to say?

Cheers,
Kevin Morin
Kenai, AK

[Laxputs]

Much appreciate the detailed thoughts Kevin. Thank you.

[Laxputs]

My plan as of now is to:
1. sandblast the bottom, get all the old paint off of it in case it's causing the corrosion.
2. have the electrical looked at to make sure proper grounds.
3. monitor it for the 4 moths season, dive it, pull, reassess at the end of the year.
4. keep the anodes in good shape.

The boat is worth about 25,000$, not sure it makes sense to spend 5,000$ on the hull...

Thanks, welcome any more thoughts.

[Chaps]

I see a lot of pitted boats at Agate Pass Marine, my bottom painting yard.

Typical causes: (not in any particular order and not limited to these)

A. Boat has been painted at some point with copper type bottom paint and/or the hull was not properly primed with epoxy before it was originally painted and/or it may have been used extensively in salt water without any protective coatings at all in combination with the other factors listed below.

B. Boat is equipped with zinc hull anodes instead of the more reactive (protective) aluminum anodes. If you already have aluminum hull anodes and are experiencing galvanic corrosion then the anodes are either worn out, not installed properly or too small for the size of boat. If your outboard motor main anode is deteriorating rapidly and your hull anodes are not wearing much or it all then your hull anodes are likely zinc and your outboard anode is trying to protect the entire boat which is why it gets used up rapidly. Avoid the temptation to fix this problem by changing your outboard anode to one made of zinc so they are all the same as this will lead to motor corrosion and hull corrosion. Zinc anodes are not reactive enough to protect aluminum boats, outboard motors and aluminum outdrives particularly since zinc gets coated with an oxide crust further reducing their effectiveness which is a problem aluminum anodes do not have. It's important to understand that aluminum anodes are manufactured along with other trace metals that make them more protective for aluminum boats than zinc anodes. This is true for all water uses - salt, brackish or typical fresh water. Aluminum boats that are moored and used exclusively in very clean and clear freshwater should use magnesium anodes.

C. Boat was manufactured with a variety of aluminum alloys used in submerged areas (not uncommon, actually) that react with each other (high grade 5086 hull plating chewing on lower grade alloy chine extrusions for instance) the cure being a proper paint job with epoxy primer and adequate aluminum anodes

D. Boat has stainless steel trim tabs chewing on everything made of aluminum they are attached to (like your boat!) The cure is to install aluminum trim tab anodes on the tabs. Doing so is not to protect the tabs but to give the stainless steel tabs something else to focus their galvanic aggressiveness on other than your boat hull.

E. Boat has stray DC currents coming aboard via the marina shore power green wire and exiting the hull via the water to earth ground (mud below the boat, closeby steel pile, etc) or for that matter traces of DC in the hull from your own boat's electrical system (bad wiring) doing the same thing, cure is to install a galvanic isolator on your incoming shore power circuit and/or fix the internal wiring issue that is allowing the hull to be DC active. It doesn't take much, 1/10th of a volt DC running around loose in your boat hull can cause pitting.

F. Boat has non-marine grade aluminum alloy used for construction (typically riveted skiffs, pontoons, pressed metal hulls, etc. Cure is to avoid leaving these types of boats in salt water for extended periods of time as they are typically hard to adequately protect using normal anti-corrosion techniques.

G. Boat has a combination of the above issues


Fixing a pitted boat with epoxy

The basic remedy that we (Agate Pass Marine) perform on boats with hull corrosion damage is a total blast strip of all marine growth and coatings and a light roughening of the hull metal using industrial sandblast equipment with a particular focus on cleaning out all traces of oxide crud and deteriorated metal present in the pitting.

Then we apply one coat of epoxy primer barrier coating that has been thinned to a consistency that allows it to soak deep into the blasted hull surface and deeply into the clean, rooted out corrosion pits. Then comes applying a quality epoxy filler paste into the pits, typically trowled in with a 1" wide thin steel putty knife. We give the filler a day or two to fully harden up then sand it off flush to the hull surface. The filler adheres better to the epoxy primer that we soaked into the pits than it would if we applied the filler to bare, unprimed pits.

Then we apply a second coat of epoxy primer over the areas that have been filled & sanded then follow up with two more coats of epoxy primer over the entire boat bottom. When the last coat of epoxy has dried to the point that it is very slightly tacky we start applying aluminum compatible bottom paint using a solvent type bottom paint (not water base) to ensure good adhesion to the epoxy primer.

We have not seen good results when attempts have been made to repair corrosion pitting with welding techniques

[Laxputs]

Wow. I'm humbled by that extensive reply. Thank you very much Chaps.

[Craigb]

Excellent response.

[Chaps]

Glad to help! Purpose of this site, thanks Les

[Laxputs]

Chaps can you recommend an appropriate epoxy primer?

[Chaps]

Any of the common marine barrier coat epoxies will work fine though I wouldn’t use a “high build” type at least on the first coat as you want it to soak in and penetrate.

[kmorin]

Chaps,
you just haven't been involved with a welding service with sufficient skill, equipment and interest to repair pits in hull plating!

The best you can do is to return metal plates to metal plates... eliminate the cause, reduce the influences that caused the conditions and return to the original hull specs: by welding.

That said.... not everyone wants to do Michelangelo TIG or even excavate the pits' to bare metal in order to get decent weld conditions. So, using goops and pastes, coatings and paints may be all that can be done unless you have access to someone with the skills and required methods needed to return a metal hull to an "All Metal Hull"?

I've surely seen some very poorly done attempts to recover pitted hulls so if the welders' skills and techniques aren't up to the task? Coatings and plastic fillers are better than having a botched welding job.

I recently saw an interior bilge pitting condition "repaired" by a welder who used MIG! The mess that welder made of that hull's interior bilge (bottom) was so astounding I was shocked (and I'm old and have seen some really poor work!!).

Cheers,
Kevin Morin
Kenai, AK