Hull Corrosion

General boating discussion
Skiff of knowledge
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Hull Corrosion

#1

Post by Skiff of knowledge »

Hello everyone,
Thanks to those who contribute to the greater knowledge of all.I have found a wealth of information on how to deal with my issue and am very grateful.

I have a 17ft aluminum boat with a large area that has pits from corrosion.I have read many hrs of threads on this site dealing with methods of repair and the level of knowledge is impressive.
I have a few questions I am hoping to get answered.
In preparation for new paint and removal of contaminates Soda blasting was recommended as the best treatment,to create a mechanical bond for paint adhesion.
I have been unable to find a person in my area Nanaimo BC.

i see a service advertised as dustless blasting,is this suitable for cleaning, pit corrosion?I talked to a blaster who does boats all the time and uses 50/100 crushed glass
Would it give the anchor pattern need for mechanical bonding of the paint?
Is glass blasting to aggressive even if the operator has a lot of experience?
I have inquired about a silica sand attachment to a 2000plus pressure washer that is for rent,would this be a good solution?

I read articles on hull preparation and again I have a couple questions

after blasting it mentioned it is best to use an acid etch and I have a gallon of Zep-A-Lume E is this a good product?
Is a gallon enough?I was told to dilute it 50 50 with water?

I have been trying to get a strontium chromate primer but they are unavailable in canada.I have read this is the best product for a blasted hull.
Is there a good alternative that you would suggest?

I had the boat professionally painted with epoxy paint and anti-fouling a few years ago and it failed within six months ,with big patches just falling off.
Over the years more of the hull became bare and about six months ago it very suddenly was showing lots of blisters,with white powder forming like barnacles.Underneath the powder was pits some as deep as an1/16th or so.The hull thickness is 3/16ths
I read about repairs for the pits and while welding or sheet replacement is best I am leaning towards epoxy filling them.

Is there a epoxy filler that is recommended?
What percentage do you thin the first coat of epoxy primer before you fill the pits?
any idea of how many gallons of epoxy would be used on a 17ft boat?
Thank you very much for any help
Skiff of knowledge
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kmorin
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Re: Hull Corrosion

#2

Post by kmorin »

Skiff,
I'll try to reply? and to be helpful. However, its very hard to reply when you use open ended terms and references? We don't know what 'dustless blasting" is? so not sure how we'd reply to your question? If you provided a link, or a quote of a description I might try to reply but since I'm not familiar with the term? I can't reply intelligently. (I know that many posters think I don't reply intelligently anyway... so just go for it!)

Any service organization relies on the skill and experience of their tradesmen; welders, blasters, fitters and painters to do the work that comes in the door. We can't reflect on someone's skills! I'd like to, but its just not real world. "Is glass blasting...?" don't know. It might be (?) and then if someone's been doing this work for a while, and their customers are pleased; then that person's skills are meeting the demand of the market and solving boat owners' problems.

Sand in a pressure washer is a very good means to clean the pitting crater floors, as well as prepare an good anchor pattern on the metal for a primer. If you use the self etching primer (as some very reputable services do) then the etch and Allodyne step may be skipped. IF you want to go "old school" then etching and rinsing - applying Allodyne while wet is still a "go too" standard that works.

Zep-a-Lume is great acid mixture to treat aluminum in one of the two main paint prep sequences. However, dilution is a matter of temperature and area. If you have pure Zep and a cold 30' boat... might be a little more needed. If you have a little skiff and very warm temp's and mix the dilution with hot water? That sounds like it will remove the oxide and any remaining mill scale for Allodyne flooding.

Sorry your "professional" service wasn't up to those accepted levels of work. If the bottom coat simply pealed off, their work is how we shall know them. Not very much worth their costs.

Adding a Chrome Oxide film over bare aluminum is very reliable, and I don't personally know of an alternative. Perhaps (and very hopefully) Chaps our resident "Pro from Dover" in regards prepping and applying bottom paint to aluminum boats; will drop by and help your planning more than I can.

I have had some recent experience with an epoxy filler (Metal 2 Metal) but not in your exact application. Perhaps Chaps, will see your post and help with the remaining questions about his common practices?

a 17'er with a 3/16" (.187") bottom is pretty heavy duty! Hope you're able to plan a recovery from the pitting you've found.

Cheers,
Kevin Morin
Kenai,AK
kmorin
Skiff of knowledge
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Re: Hull Corrosion

#3

Post by Skiff of knowledge »

Thank you so much for the reply,

I read a rather in depth post you had written on prepping an aluminum boat with corrosion and I found it excellent in both clarity and information.
Chaps had also contributed with his expertise.It was excellent as well.I am following both your advice in my application as best I can. :beer:

I talked with the dustless blaster and found some more info.It is a medium of crushed glass 50/100 for my application blasted with water at the same time.Hence the term dustless...He quoted 200.an hr about 1200 to 1700 for his services.It seems the term dustless is used in other ways as I have seen a dry ice blaster using it as well.


I can rent a 5000 psi pressure washer with a silica sand attachment for 250 a day and if you think this would be a good choice it would save me a lot off money.I never blasted before but I have experience with NON ALLOY work and do a carpentry for a living including having a full woodworking shop,giving me understanding of finishing work.I feel I have the skill to be attentive to what is necessary if I have the right information.

I read in another post you mentioning about bad aluminum and I wondered if it happened around the time this boat was manufactured 2016.



I bought this boat in 2017, the boat was less than a year old.It had never been bottom painted and it must of been partially sitting on float as only the last third of the hull had signs of marine growth that had been scraped off.I looked at the hull from underneath and noticed the area but i did not look with my reading glasses, and it being only a little over six months old, I never imagined there being any corrosion.I bought the boat from a dealer in Washington and brought it home to Vancouver island ,and discovered on closer inspection pitting on the stern where it was left in the water.If I had bought this bought in Canada I would have returned it.

I got hold of the manufacturer and he told me to fill the large voids with a welder which I took to a shop and had done.

I then had the bottom painted and unfortunately(the painter told me later ,when he would not repair the boat, he had only painted new aluminum boats) he told me not to get it blasted which I had researched as the best solution.My mistake in assuming he being the professional, has the best knowledge.The back third of the hull still has the paint on it so no corrosion has reappeared.

I right away installed a battery switch which turns all power off except to the bilge pump.I turn it off whenever i am not using the boat.I read up on anodes and only use aluminum ones on the stern and outboard.I have read about your post of attaching them with hot dipped galvanized bolts and I will try this,I have been using stainless bolts.

This boat has been in the water full time as I live full time on a small island.I have made a platform to take it out of the water which I plan to do more of.

Cheers
Brent
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kmorin
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Re: Hull Corrosion

#4

Post by kmorin »

Brent, As to the difference in wet blasting with silica sand or glass (crushed, fused silica sand particles?) I can't really compare them unless I knew their relative pressures and particulate hardness? But I think the silica sand in water would be 'softer' and it sounds like you can do it yourself - and is cheaper. So I'd vote for the sand in water over the glass in water- but that is not informed by the hardness or sharpness of each set of particulate?

I'm not positive of the years (likely some online references left somewhere?) of the mill spec. failure of a large amount of aluminum that wasn't alloyed correctly in the Puget Sound. I think if was 2004-06 and was really showing up when some mis-alloyed sheet got into a couple hundred builds- some by the bigger builders like Nichols Bros. who builds ferry boats under CG/DHS supervision.

Anyway, its my understanding the alloy issue was/is solved by 2012? Not sure on those dates- so not for reference- just my vague recall. Others in the Puget Sound area will know more than I by far.

As to the corrosion of your boat, I'd ask you to consider a VERY close inspection of your wiring. This may well be stray current corrosion due to wiring? Reason I say that is you've mentioned your battery isolation switch is bypassed! So your bilge pump circuit is on when the switch is off! I realize the safety concerns but that raises the most likely concern of a fast onset of pitting to be a wiring issue? Depending on the type of on/off/level switch you have to monitor the bilge and to turn on the pump- there is a good possibility you have some DC-Neg to hull conduction? Remember that DC Neg is still positive in relation to the hull and water- until it is 'back' at the battery. Then the DC Neg bus should be bonded in one location to the hull.

I'm pretty sure there is no argument that particulate blasting of aluminum creates the best (high standard) bond or 'anchor pattern' to the surface of the metal? So, I think blasting is the best way to get a mechanical bond for the rest of the film coatings. OF course, if someone were using a super high pressure stream of very aggressive particles like garnet sand- and stayed too long in one place- they could sure do some thickness wear in short time. But with softer media, in a water stream and with movement over the surface blasting is the gold standard of surface profile prep.

re 'zincs' or anodes. If the current pitting is forward and somewhat uniformly from chine to keel, I'd want to look very closely at the wiring up there? As to bolting on w hot dipped fasteners- they're coated in zinc so not as galvanically different as SS to aluminum. But if you passivate SS (acid etchng for SS) and try to use 316L the bolts; they will be much less reactive to aluminum. However, if the SS does react and corrode inside a bolthole, or at the faces w washers, nuts and bolt heads- that will create a white chalk which is MUCh higher resistance than clean metal-to-metal conducting surfaces. So an anode is only able to do its job if the attachment is low (no) resistance allowing any stray current to dissipate into water without taking hull molecules along. If there are any external attachment points forward? not advocating a bolt through the hull, but a weld on plate; then using the tear drop anodes say 1/3 aft look like they'd be helpful with whatever is causing the pitting forward?

Do you have an through hull fittings, seacocks? transducers? anything that is not aluminum ? Like a valve screwed onto a though hull fitting? or a metal transducer housing through hull for depth sounder? Any of these could potentially be a source of both stray current and or galvanic corrosion.

Quite the lift you seem to have built! if you can pull the boat vertically out of the water.

Cheers,
Kevin Morin
Kenai, AK
Last edited by kmorin on Sat Jul 31, 2021 4:12 pm, edited 1 time in total.
Reason: correct date guestimates
kmorin
Skiff of knowledge
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Re: Hull Corrosion

#5

Post by Skiff of knowledge »

Thank you again, Kevin, for your informative reply.

I appreciate the vote of confidence for the do it myself blasting, and that my preference.
I was thinking of lying some boards down underneath boat and tipping it on its side blasting one side at a time?Holding over by tying to a pickup or tree.

The wiring was done, some by the manufacturer and ,others when the motor was installed.I installed some lights and a sounder/gps.I wired all the grounds to the same ground as the rest of the wiring.I do not know if it is grounded or Isolated from the hull.I will check it out.I have left the boat at a friends house in a town so all the work can be done there,so I can not check now.

As far as the outboard should I try to isolate it from the hull for the purpose of only having ground in one place on the hull?

There is on through hull hole for wash down pump and it has a stainless steel ball valve.

I have been wondering about wiring a float switch that is above the bilge so it is not in the water.Also making a cover for the boat so I can turn off all electricity when it is not in use,which is the majority of the time.I use it less 75 hrs a year as my main use is a 15min boat ride to where my pick-up is parked once every 10 days or so for resupplying.
I would like to test for stray currents and not sure how?

As for lifting the boat I have an overhead cable that has 2 2ton chain hoists that lift it vertically and a electric winch by my house that pulls it up the hillside.It is a bit primitive and as I am aging ,I am slowly improving it to simplify.In the future I would like to replace the chain hoists with electric winches.
I have an old boston whaler that is my back up right now and it is just hanging on the trolley.
I have tried to post a few more pictures of the winch system but I don't see the attachment button anymore.I may have used up my allocated allotment..
Thank you Cheers
Brent
kmorin
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Re: Hull Corrosion

#6

Post by kmorin »

Skiff of knowledge wrote: Sat Jul 31, 2021 7:31 pm The wiring was done, some by the manufacturer and ,others when the motor was installed.I installed some lights and a sounder/gps.I wired all the grounds to the same ground as the rest of the wiring.I do not know if it is grounded or Isolated from the hull.I will check it out
What is the saying about "many cooks in the soup"? Not saying I have any knowledge of the wiring being a problem but.... the hull looks like it is experiencing stray current corrosion. All the DC Neg (returns from loads) should be to one common bus, and that should be bonded to the hull in one location and the engine should be included. Sometimes it's convenient to do this bond from the batt. Neg near the engine but I don't believe the engine should be isolated from the hull- at all. If the DC neg is not bonded to the hull in one location then any given wetted connection can be a source of stray current using the hull as anode to the water.

Only way to check for stray current introduced by wiring is to begin eliminating circuit by circuit any chance of stray current. There's no test that is a handy and easy test you can do. There are several methods involving lifting the bonding wire so the system isn't bonded to hull- then with a milli-amp meter check the current from the DC neg to hull... But unless you'd sure of the entire wiring scheme's conductivity and routing? you could be seeing some or another appliance with a chassis stray current. There are lots of articles online for testing but most of them have to assume long list of conditions to be useful... and those conditions aren't something you can be confident are accurate without the detective work of tracing all the circuits in the first place!
Skiff of knowledge wrote: Sat Jul 31, 2021 7:31 pm There is on[e] through hull hole for wash down pump and it has a stainless steel ball valve.
Does the SS ball valve have a SS ball or a brass/bronze ball? Many lower cost SS body valves are supplied with brass/bronze balls to reduce costs. If the valve is closed then a brass surface is immersed in salt water near the aluminum hull. Is there any increased amount of corrosion pitting near the through hull opening? just the SS alone may be enough for localized pitting but I don't see one valve supplying the entire range of corrosion you show: not saying it won't contribute but it seems unlikely that much area was corroded by one valve body? Still, to insure the through hull's longer life- might pull the valve and passivate it before reinstalling using a Teflon pipe paste sealant.

I think you can post four or five pics per post? Not sure on that? but whatever you did on the first posts seemed to work?

Cheers,
Kevin Morin
Kenai, AK
kmorin
Skiff of knowledge
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Re: Hull Corrosion

#7

Post by Skiff of knowledge »

Thank you Kevin for sharing your expertise.

I am going to check and see if the ground is isolated or bounded to the boat.A simple continuity test should tell me,between the hull and the ground buss bar,I am guessing.

There is no noticeable pitting around through hull.I have the boat at my buddies place who is an electrical engineer so I am hopeful we will find out if there is any stray currents.
I am off to scrape the hull some more,the paint is pretty loose so I am scraping off what is easy.
Still no Idea of the best paint for the bare metal I have been phoning for tech support and also looking at airplane sites as they may have not been banned here yet...
Cheers Brent
kmorin
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Re: Hull Corrosion

#8

Post by kmorin »

Brent,
Sounds like your friend will have some insight into the various paths to the hull that stray current might take? Remember if the path is wet that there won't be any resistance to speak of? Getting a 'megger' (high resistance meter) might be a help if he's an EE he might know a contractor to borrow one?

Our AAB.com Forum pro is member name "Chaps" and he does business, as near as I know, as Agate Pass Marine and has a web page in that name (https://agatepassmarine.business.site/). Since his business is to do the exact type of work you've shown in your posts? I'd say he was the best, most informed and experienced Forum member that I can recommend for solid information.

I'm not saying Bob offers 'free phone' consultation-I don't know. I don't know how well he'd be disposed to give away his knowledge (in time consuming phone calls or emails) - except for his many (MANY) posts here giving countless exact details of what he's learned and how he does this type of work. On the Forum he's given all the information that I know about. I recall he's listed brands and types of products in his many posts- so searching for all his posts (1,000's) and maybe refining the Search to 'paint' or 'primer' (etc) of a more specific topic may help more with learning which brands have proven reliable?

I'm not spending your money for you, but depending on the level of transport effort to get to Agate Pass Marine; there is a proven, cost effective, totally reliable and honest service available for you to get your boat bottom taken care of: completely, professionally, with assured results and you'd know the in's-&- out's of the condition of your boat as you'd have good communications all through the project.

I'm not confident of your last bottom paint job vendor's knowledge, ethics or business practices? I've been involved in lots of businesses in the past- and when one detail is not up to par? It hasn't been rare for other details to be found less than "best practice". IF someone won't back up their work, or look into the causes of failure of their products as applied in their services- makes me cynically ask- what other short-cuts were taken or are common for that 'service'? Don't know them, don't want to. However, just saying if the paint is coming off easily- that really doesn't sound like the overall original adhesion was up to par?

Do remember that aircraft aluminum isn't usually the 50 and 60 series alloys we rely on to build welded aluminum boats. Not sure if their surface work on 'clad' sheeting is always applicable to 'pure' or non-clad 5052, 5086 or 6061 alloys?

Why would airplane sites be banned here? not sure I got that inference or detail?

Hope you'll keep us posted on your progress and what you find out about the causes of your corrosion problems?

Cheers,
Kevin Morin
Kenai, AK
kmorin
Skiff of knowledge
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Re: Hull Corrosion

#9

Post by Skiff of knowledge »

Thanks again Kevin,
I wish I could take the boat to Chaps.I looked at the mileage to boat there and it is about 110 miles by water.The trouble for me is the border.With covid it is very much more complex.
Sorry I wasn't very clear with the airplane sites.I was trying to source the strontium chromate primer epoxy paint and it seems that aircraft have some access that marine industry does not.I did get some adoline from the airplane site and plan to use it.
I have for the last couple of days been scraping the old paint off as I want to dispose of the old paint and make the blasting faster and it is near done.
My plan is to redo the hull myself and pay very close details to all the steps.
I really appreciate all the advice and thank you very much for your continued interest and expertise.
brent
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Chaps
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Re: Hull Corrosion

#10

Post by Chaps »

Thanks for the kind words Kevin, sorry Brent I have not checked in here for awhile.

Your hull issues could have one of several origins as Kevin has opined but to me at first blush it closely resembles what happens to an aluminum boat that has been coated with copper bottom paint. Whatever the cause of the problem it is repairable and preventable.

Pressure washer sand blasting is a suitable way to strip the coatings and prep the hull, that was my original method when I was just getting started. Dustless blasting is a much more refined and controllable method of blasting with a grit/water mix and often used in boatyards to prep bottoms because the method prevents the blast activity from getting airborne. I dry blast now because the operation is done indoors with dust and blast media contained and dealt with without the clean-up issues associated with wet, contaminated sand getting everywhere.

So assuming you find a deployable blasting method to scour the bottom and clean out all traces of affected metal and old coatings give it a good rinse and let it dry. Next saturate the entire bottom with any reputable marine epoxy barrier coat primer that has been thinned with its recommended reducer to a runny consistency so that it effectively soaks into the profiled aluminum and the now cleaned out corrosion pits. When that has dried you will then see the true extent of the damage and what needs to be filled with epoxy compound and faired out. Find a simple to use marine fairing putty like Quickfair or EZFair, mix small batches and apply to affected areas with a flexible steel putty knife and sand flat to surrounding unaffected hull surface when fully cured. Apply 3-4 more coats of epoxy barrier coat over the entire bottom then follow up with your non-copper bottom paint of choice before the last coat of epoxy is fully hard. You are done. No need for acids or conversion coatings at any time during this process. One good sized aluminum hull anode on the transom mounted properly and maintained will then take the brunt of any extraneous corrosive forces including self induced stray current issues. Rapid deterioration on the anode after the hull work is finished will be the tell-tale that informs you that you still may have a wiring problem that needs attention.
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Skiff of knowledge
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Re: Hull Corrosion

#11

Post by Skiff of knowledge »

Thank you so much for the helpful information,Chaps.
I have done some investigation on the wiring.I found the only place the wiring is grounded to the hull is through the outboard.The wiring grounding bus bar is isolated from the hull.
I am going to get a hold of the manufacturer to see how they wire their boats when they install outboards as this boat had the outboard installed elsewhere.
I found a site on the net and bought pettit alumaprotect for the primer coats and pettit protect for the barrier coats.I ordered pettit epoxy thinner to thin the first coat of epoxy primer.I also ordered pettit epoxy repair compound for the filling the pits after the first thinned coat is applied and dry.
Then Pettit HRT eco bottom paint for final coat,after 5 coats of the barrier coats
I really appreciate the advice/help
Thanks again
Brent
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Re: Hull Corrosion

#12

Post by allanb76 »

I'm curious if you've resolved your issue Skiff of Knowledge.  I have the same boat with a very similar issue.   I have not found anything wrong with the electrical/anodes, so I'm hoping it was just a poor paint job by the dealer when I bought new 2 years ago.  I would like the peace of mind of having it professionally repaired, but I'm not aware of anyone in the area (I live in Nanaimo) that is very reputable.  For that reason, I'm contemplating a road trip to Washington.  Hull corrosion scares the hell out of me and I want to stay ahead of it. 

How did the pressure washer work out?  Where did you rent it from?  
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