Removing Limescale Buildup on Aluminum Boat

So you want us to give you a marketable product so you can get rich.....is that it??

I would venture a guess that the secret is in treating the aluminum before commissioning the vessel...a layer that can be sacrificial, perhaps several layers, like epoxy paint maybe....

https://www.bottompaintstore.com/boat-bottom-paint-aluminum-safe-bottom-paint-c-13523_13531.html

https://www.streakmaster.com/aluminum_cleaner_cleaning.htm

Have you tried lemon juice, or vinegar? It'll take a lot of hand scrubbing or some type of an acid.

Yes, tried both and even though there is a small reaction it doesn't really help. Used various marine acids in test spots but still didn't work all that well and I'm concerned about damaging the aluminum.

The only think that has worked is a stainless wire brush on an angle grinder. However it leaves undesired swirl pattern in the aluminum so I'm not using that method as of now. The swirl pattern is like prepping for a future deposit of lime scale, similar to sanding a surface in preparation for gluing

Phosphoric acid -- as in Coke-Cola?

Didn't work. I used commercially marketed acids for cleaning this from pontoon, but only on a test spot. Very concerned about the acids potentially damaging the aluminum.

Dilute nitric acid? It won't attack the aluminiun as it renders it passive.

I'll try the nitric acid. Thanks wasn't aware that aluminum neutralized it without compromising the aluminum.

The concentration of the nitric acid is critical: try on a piece of scrap aluminium that you have first roughened with sandpaper. Find the strongest concentration that doesn't produce bubbles in the first 30 seconds, and use that first on a small section of the limescale to see if it does anything immediately. If it does, then it will be neutralizing the acid at the same time, so you're pretty safe on the affected areas. However, if you are going to leave it in contact with bare aluminium for any length of time before scrubbing, 2/3 concentration might be safer.

Actually I haven't had the scale analyzed, but definitively not a salt water issue as the boats have been Ohio manmade lakes. Highland county has a lot of hard water, with the large amounts of limestone covered with clay soil and a fair amounts of nitrogen runoff from local farms.

Thanks Captain Dan I asked because in the callowness of youth we used sodium hydroxide to react with Al and the resulting white scale was impervious to added NaOh.

Oh how lucky to have lakes to boat on. The sacrificial anode of Zinc may ease the problem as suggested elsewhere. Of course CaOH reacts with aluminium giving off Hydrogen as well so that could be a similar reaction.

Etch priming the Al before painting is a necessity, for the paint will peel off and expose another area for the scale to form.

CLR (calcium, Lime, Rust) cleaner:

https://www.amazon.com/s?k=clr+cleaner&hvadid=7005462241&hvbmt=be&hvdev=c&hvqmt=e&tag=mh0b-20&ref=pd_sl_6i75t31kkg_e

You are creating CaCO2, Mg(OH)3, Aluminum Oxide, ect. due to the corrosion and electrolysis of Aluminum metal in the salt (ionized solution) water.

best way to prevent this as others have stated, coat the aluminum metal with a non-conducting coating.

I would also put a Magnesium or Zinc sacrificial anode (to create connected to the pontoons to resist electrolysis of the aluminum metal and force the electrolysis at the sacrificial anode to create catholic protection of the metal.

These pontoon boats are solely used in fresh water man made lake of highland county Ohio, so I not sure electrolysis is the cause here. I do see some pitting on outboard lower units if they don't have the anodes installed (which we do monitor and correct), and l can l remove the scale with a angle grinder equipped with a wire brush, but the swirl pattern left in the pontoon is undesirable.

I missed the part that it was not in salt water, but you still will have the issues that I stated in my answer. your byproduct is most likely aluminum oxide which nothing will "dissolve". you will have to mechanically remove using a blast technique or belt sander or wire brush. how thick is this buildup?

is it necessary to completely remove? why not put a coating over top? possibly POR15

...."DO not use CLR on natural stone or marble, terrazzo, colored grout, painted or metallic glazed surfaces, plastic laminates, Formica, aluminum, steam irons, leaded crystal, refinished tubs or any damaged or cracked surface. ... CLR is corrosive."...

https://www.thecarycompany.com/facility-supplies/cleaners/clr/faq

yes corrosive. you can go about this 4 ways:

1) leave it as is

2) coat over top with something

3) use a corrosive agent (either acidic, basic or ionic) to attack the precipitant or the metal underneath (so it spalls off)

4) find a wetting agent that can wet the substrate and the precipitant so that it is easy to remove.

He wants something that attacks the lime deposit, not the aluminum.....

years back (more than 30), when I first started working in a lab, my mentor had some dilute HF sitting on the sink counter. on glassware that was stained or coated with something that was hard to scrub off, we would add some HF to the water and soak the glassware. The HF would etch the glass underneath and the crap would fall off. couldn't really tell that the glass was etched, just was very easy to clean glassware that now gets thrown out. So etching the metal underneath the contaminate is a viable way to remove it.

yes I know... HF has been vilified to the younger crowd as well as Lead, organic solvents, Mercury thermometers, ect. ect. ect.

here is a good one: I flush the eyewash and shower stations on my floor once a week. I am not allowed to dump the water off the loading dock. Can you imagine the water is considered contaminated water because it came from the tap due to the chlorine that was added to purify. Remember I said "off the loading dock".. the same loading dock that has trucks idle and drip oil that goes down that same drain but that is ok. sorry got side tracked. HF is not as dangerous as they make it out to be.. don't get it in your eyes like many other chemicals.

I agree, I would try some HF...works well, I used it for years....You just need to dilute it adequately...or maybe some fine sandpaper and wet sand it, then polish with a buffer...

https://www.jmnspecialties.com/Products/ALUMINUM-BRIGHT-HF-Aluminum-Cleaner

https://pilotshq.com/aluma-brite-aluminum-cleaner-gallon-25205-p-15857.html?gclid=Cj0KCQjwyerpBRD9ARIsAH-ITn-Y9ePe5NEpJWqY4ZoNU5cjbDwaCGAvJLruMbRceLMO-929v-uSEZkaAsxuEALw_wcB

How about blasting with baking soda, fine sand, or glass beads? They use this method for cleaning the tile at the waterline of swimming pools.

I did try wet blasting it with various media and it just bounced off. Lol I haven't tried dry blasting yet.

I used to have copper heater elements that would get calcium buildup that was quite thick. We used a (dry) baking soda blasting set up and it cleaned it right up. Smooth matte finish. What I'm finding on line generally talks of thousands of PSI, but I know we only had ~115PSI tops for what we did.

Here is a link I found for blasting a boat with soda and with sponge media!

https://www.sailmagazine.com/diy/blasting-your-hull

Very interesting article from the sailboat magazine. Will investigate it further

Please explain why the <...aluminum pontoons...> were not protected with a coating of some sort before the installation was completed?

Please explain why an electrolytic protection technique, such as a sacrificial anode as used in boats, has not been considered?

Protection of the pontoon is a function of the customer and the manufacturer. My experience as a repair shop is that all of the pontoons boats regardless of the manufacture accumulate this problem with in two years of being launched. This applies to those who are leaving there boats moored at a rented dock for the summer.

I have to make an assumption on the why about the anode protection as most of the boats do have anodes on there outboard and or I/O lower units. With that said the same lime cake still builds up on those too and can't be remove without damaging the paint underneath. I guess that the manufactures decided its better to sell new stuff than to make the old last forever.

have you tried CO2 bead blasting?

Haven't tried this method yet.

This may be a little exotic, but have you considered lasers?

The one I'm familiar with blasts rock to smithereens and reflects off aluminum.

I've seen it's been applied to rust removal.

I have marveled at these lasers and wish I had one to try. Unfortunately their price point is to high for me. Of course there are the scam adds offering them up for $19.95 on FB lol, but looking at them for real run north of $50k

Even a chineseum fiber laser with a hand held shroud / blower sucker and a beam shaping lenses ain't cheap to play with..

You might try expanding and contracting the aluminum with a steam cleaner and distilled white vinegar after soaking the hull for about 30 min keeping the vinegar wet...the vinegar should soften it a bit and the expansion and contraction of the aluminum should/might break it loose....this is how you clean aluminum pots....might need some steel wool to finish it off....

Aluminum expands better than contracts.

It's more important that the expansion coefficient between the two materials is different...

https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

its totally important. you wont have any change if they are equal.

everything will expand at the same rate therefore nothing changes

"Aluminum expands better than contracts."

Please explain/elaborate...

Without going into too much detail because I don't have the computer in front of me.

An interesting article I remember from many years ago was dealing with the problems of maintaining the holes of pressurized aircraft. The article went on to explain the problems associated with the fact that aluminum would expand when pressurized but wouldn't shrink back down the same way as if it Were made of steel which would obviously not be very good for an aircraft. I've also read about the thermal expansion coefficient or whatever of aluminum and how it will gain length but not return to its original size. We are talking minuscule amounts but when you have rivets holding a plane together etc it becomes important to know these things.

Holes? Hulls. Fuselage.I thought there was 30 minutes to edit Dang it. Have a good day.

I believe standard use of "expand" and "contract" refers to reversible changes such as thermal expansion/contraction, the changes in the size of wooden objects due to changes in water content, or elastic changes in the size of a container due to pressure differences.

If the fuselage of a plane does not return to its original size when the pressure is removed, then it didn't just expand, it stretched (surpassed the elastic expansion limit).

I'm sure some will call this nit-picking or just semantics, but I don't believe the phenomenon you refer to applies to removal of scale from an aluminum boat.

Nit picking is allowed. I'm pretty sure that I read this in Aviation magazine. Editors are not immune to nit picking either.

I can think of expanded steel mesh that you could also call stretched steel mesh.

Let's not forget stretch Armstrong. Who becomes expanded or stretched and then current tracks on his own.. See now I'm just getting confused.

I'm sure I could think of others, but thanks for The clarification.

I nit pick these little details in real life. Much to the bemusement of others so thank you very much.

.... Expansion or Stretch and Contraction Armstrong just doesn't have the same ring.

well first off, metals will expand and contract due to thermal expansion. they will always return to the same size unless you heated the metal beyond certain temperatures called yield point(depending on the metal/alloy).

metals that are under a tensile or compressive load will follow what is called a stress/strain curve. the metal will continue to follow the same curve as long as you have not passed the yielded strength. once you pass this point, you are now into plastic deformation once you have went into plastic deformation, you can never return to the same size that you started off with due to many different issues some are stress hardening, necking, twinning, phase transformation, first matrix cracking, among other issues.

You might have something here in that after a cold winter I can remove a lot more of the build up by just dry brushing, still a fare amount of the build up does remain though.

You could speed the process up considerably with rapid heating and cooling....They make a hot water steam unit that hooks inline with a pressure sprayer....The secret is to take advantage of the dissimilar expansion/contraction rates by rapid switching between cooling and heating....that is if in fact it is lime scale and not aluminum oxide, which would have similar characteristics....in that case it should be wet sanded off and coated immediately....

I think I would opt for the all-in-one unit with a diesel burner...

https://www.northerntool.com/shop/tools/product_200324565_200324565

I use Ducky on my boats. I think it's some variant of citric acid.

I would be very hesitant to blast it with any type of media. I saw an airplane that have been stripped with plastic beads and it left a very rough finish.

I've used nitric acid on aluminum and it didn't hurt it. Give it a try. Handle it carefully.

I agree - be wary of blasting. Surface finish is one concern, but I worry more about the induced surface stresses and the potential of crack propagation.

Is it a cosmetic issue? ....if not, why go to the trouble of removing something that stubbornly wants to stay there... use it as a secure base for a coat of environmentally friendly paint....albeit wash off the loose aquatic gunge.

Exfoliant Opportunity for Marine Cosmetologists

Indeed. Perhaps a thin but tough layer could be applied with a spray mechanism intentionally as a final manufacturing process just prior to drying and then painting. With careful formulation and perhaps electrical encouragement one might be able to apply such a layer much faster and more uniformly than natural accretion. With such a matrix one might be able to use a very tiny amount of antifoulant just prior to normal paint and thus optimize the localization of the environmentally troublesome copper(or whatever) and contain it under the final paint layers in much lower quantities than normally used. Precisely targeting antifoulant in careful dosage and into a firmly attached matrix might result in tolerable total quantity which gets released in such low levels that the effects remain mostly where they are needed at the metal surface of the structure(and beneath the paint.) Subsequent maintenance actions could be planned to remove paint thus leaving matrix and antifoulant in place (non-abrasive,perhaps thermal/chemical removal of paint) thus preserving the original antifoulant treatment beyond these maintenance actions and releasing only trace amounts of antifoulant as a result of each maintenance process. Voila, encapsulation and sequestration of the antifoulant thus surgically targeting it to biology which penetrates the paint. Perceive the normal paint as a kind of mucus which gets replaced frequently and easily.

When life gives you lemons, make lemonade. Imitate nature, she's had a very long time to perfect her techniques. Use paint which, like the glue of post-it notes is designed to be slightly thicker and far less tenacious so it can be removed taking the most recent fouling with it. Gently and frequently exfoliate the skin on the face of your boat.

_____________

thewildotter

except this isn't a biological issue. Chemical bi-product

Well, it is too late for his immediate problem but...

Biological is the harder issue. Chemical on top of easily removable paint is also handled by the "exfoliations" I described. The difficulty of removal of the chemical deposit is what I am addressing as a virtue as opposed to a curse but only if you plan ahead and make that deposit uniform and robust.

If, for example, he chose to sandblast off the existing stuff and was successful, then immediately applying an intentional layer of similarly tough stuff as an undercoat for easily removable paint could allow him to remove future unwanted deposits regardless of their source more easily in this process I have called exfoliation.

__________

thewildotter

Yes this is purely for cosmetic reasons(my opinion), although there is a performance increase, in speed associated with clean pontoons.

I think you mean smooth hull surfaces...is the surface irregular with deposits creating nooks and crannies? You could always just smooth it out, but I'm thinking the reason is that the scale forms much quicker when deposits smooth or otherwise are left behind...so we're back to just coating it with something...

Some have suggested bead blasting to remove the scale from the pontoon.

I would suggest the media used to blast could be nut shells or corn cob ground up. Nut shells are used to blast paint and buildup from furniture and the like without causing any damage to the base material. Wallnut which would be available to you or here crushed Macadamia shells over here may also prove effective.

As for corn cob well that is just what I use in my cartridge tumbler to remove discolouration from rifle shells and it works great. I have used it on heavily tarnished spoons and the like so using it as a blasting media may be beneficial.

Any matting of the surface will aid the adherence of etch primer. A two pack epoxy tar for the top coat should seal the surface. I used a product called Permatar on my Myers Manx when I built it years ago and it prevented all rust from forming.

Why not try converting the outer layer to corundum?

https://en.m.wikipedia.org/wiki/Plasma_electrolytic_oxidation

This is probably too expensive,but it depends on how many boats you have.

https://www.searobotics.com/products/hull-tank-cleaning/hullbug