Bottom Paint flaking/hull pitting

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allanb76
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Bottom Paint flaking/hull pitting

#1

Post by allanb76 »

First of all, I stumbled across this forum while trying to learn about my 2019 Silver streak issue....what a great site with a wealth of information! Sorry if this topic has been covered many times, but I still seem to find conflicting information everywhere.

I purchased the boat brand new in early 2020. Knowing I'd be keeping it in the water during the summers, I had it the bottom painted with Interprotect 2000E and Trilux II. I kept it in the water (salt) that summer (Jun to Oct) pulling it in August once to check things out. Aside from minor cleaning around the outboard bracket and trim tabs, everything looked good. I did notice my hull anode seemed to lose material fairly aggressively, but I thought that was a good thing as it was working well. Last summer I kept the boat in the same marina (different slip though). I didn't get around to pulling it half way through the season, but I had also noticed my anode wasn't working the same as the year before. In hind sight, that should've prompted me to pull the boat then. When I took it out in September I was shocked to see a fair amount of paint flaked on the pod. It was really hard to tell if there was minor pitting or what I was seeing is just oxidized bare aluminum. It did look a bit pitted to me though. There are a couple of areas in this pic that I sanded just to verify that it was in fact the epoxy barrier that flaked.
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My understanding is that the bottom paint should've protected against galvanic corrosion (although if the anode wasn't doing it's job, maybe not?). Perhaps it's a stray current issue? Could the stainless steel trim tabs cause the issue? I did change the anodes on them to aluminum as I had zinc on the first season and they didn't seem to be doing anything at all. I was also told to keep all anodes the same. My outboard and hull are aluminum anodes.

I've recently had the bottom paint fixed up and I really want to ensure this doesn't happen to me again. I've poked around with a multimeter a bit and as an electrician, one thing seems strange to me. The negative DC is not bonded to the hull. One could argue, the outboard is bolted to the hull and it is bonded to negative. However I discovered by removing the negatives from both outboards (one at a time) that the main motor is isolated from hull (thick paint on bracket?) and the kicker is bonded (small #12 or 10 wire). Could this be an issue? All electrical connections are in perfect shape. Every device has it's own negative wire running back to a common bus, so the hull is not intentionally being used as a path.

I will certainly be measuring the hull potential once it's back in the water. I've also been told it would be wise to paint (and isolate from aluminum?) the stainless steel trim tabs. I will likely try that as well. Any other suggestions would be greatly appreciated.
kmorin
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Re: Bottom Paint flaking/hull pitting

#2

Post by kmorin »

allan,
my comment or reply is about bonding the DC Neg Bus to the hull. This should be done from the battery Neg. or the Neg. buss if you have one ? It can be done with a battery sized cable, and a copper lug soldered to that cable (silver solder is good) and then shrink wrapped above the sleeve so the copper conductors don't get wetted and leach copper ions into the bilge if there is any bilge water?

A single point bond is correct (ABYC and long experience) and one where the lug or tab welded to the hull is in a dry location. The interface between the copper cable end and the welded aluminum tab is subject to both galvanic corrosion (increasing resistance over time) and crevice corrosion accelerated by the dissimilar metals if the junction is allowed to be wetted. Hot dipped zinc screw fasteners/bolts and nuts are good for this type of compression joint and unlike SS contribute very little to a galvanic cell at this bonding point.

This is not a "ground" and should not happen "inside" any circuit that carries a load- this is a bond of the DC Neg. Buss to the hull's welded structure, to keep the battery potential equal to the hull's- not a circuit path.

four years ago, I did inspect a boat in the same class as your hull by a different builder, as I recall, that had a similar problem, not identical; but similar. The tank level sender and the tank were not wired properly. Tanks' level sender should have a 'hot' wire coming as power from the gauge/indicator or directly from the fused DC Pos. Then another separate wire going back to the gauge/indicator and then to the DC Neg through the gauge. The tank's sender mount should be dry because if the tank's sender can be wetted to the tank? then it may conduct Pos Dc to the tank structure.

Tanks should be mounted so that there is a bond wire to the hull, which is intended to keep any possible charge off the tank- so it doesn't become a capacitor- this can be done by welding in the tank mounts, but if they're bolted? it's not bad idea to have this bond wire so the resistance at the mounts doesn't grow over time as the face of the two surfaces at the mounts gets a layer or higher resistance oxide or corrosion.

I the case I'm reporting; the sender's DC Neg (return) was bonded to the tank in one point and then returned to the gauge and then to DC Neg buss! this made the level circuit unreliable and a source of Pos. DC charging to the hull since the tank wasn't totally isolated. The bottom paint near the stern under that tank had begun to 'bubble' and flake off in a wider area.

Now, in that boat's history the instance of my inspection was a major rebuild, so the tank was rewired, the paint redone and the anodes replaced along with dozens of other changes. Therefore I can't report this was a cure for the bottom paint issues? That wiring looked good until the deck was pulled- it wasn't evident the DC Neg. Return leg was being used to (try to ) "bond" the tank to the DC Neg!

Hopefully Chaps, our resident bottom coating expert and most experienced bottom painter on the Forum will be along to remark about the SS trim tabs' galvanic potential to the hull, coating sequences, products and cures for your paint lifting on the engine extension?

cheers
Kevin Morin
Kenai, AK
kmorin
Chaps
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Re: Bottom Paint flaking/hull pitting

#3

Post by Chaps »

Good analysis on the potential wiring issues there Kevin, been learning from your posts for years.

On the paint problems it looks suspiciously like some low level positive DC is present in your hull, could be something hard to measure, less than a volt of +dc can raise hell. Anyway, what happens is that the +dc wants to go to earth ground like steel pilings or the mud below the boat so it exits the boat by pushing through the aluminum and stripping electrons off the surface of your aluminum plating that has the water exposure. When aluminum atoms start losing electrons they fall apart and dissipate into the water which leads to pitting and paint failure.

However, this issue and associated damage can usually at least be contained by a properly installed, sizeable, mil-spec aluminum hull anode that would become the pathway of least resistance for that stray current, The anode would get eaten up rapidly, yes, but thats what they are supposed to do when problems like this crop up. The OP's statement that the original hull anode wasted rapidly and that the replacement anode wasn't reacting very much tells me that it's likely the replacement anode is either zinc or poorly installed aluminum. In either case the dc+ will choose to exit the hull.

Stray current will cause corrosion issues far faster than galvanic corrosion. The paint failure around the tab hinges is galvanic and common when an aluminum boat is equipped with stainless tabs fastened onto the hull without any electrical isolation. That sets up a big galvanic cell (a closed circuit of electrons leaving the hull in that tab mount area and flowing waterborne to the tabs). So sort of similar to the stray current issue but less aggressive because the problem isn't being amplified by the presence of actual induced power. Anyway, as these electrons move in a circular fashion from the hull through open water to the tabs then flow back to the hull through the fasteners holding the tabs to the boat (its a local circuit) you get similar damage where the electrons are leaving behind deteriorating aluminum atoms on the hull causing the paint bubbling and pitting. You did the right thing by switching out the zinc tab anodes with aluminum anodes so now the tabs have an "easier meal" to focus their galvanic superiority on and they will start leaving your boat alone.

Another however, under the right conditions it could be the tabs causing the corrosion issues on the sides of the engine support. If the tabs are close enough to the engine support and the water salinity is high enough the electrons could be getting sucked across the gap to the side edges of the tabs.

Finally, I'm not too impressed by the paint job, not enough epoxy was applied. Epoxy is a good insulator but it needs to be at least 10 mil thick in order to keep the aluminum shielded.
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allanb76
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Re: Bottom Paint flaking/hull pitting

#4

Post by allanb76 »

Thanks for the great info guys! The one thing I don't understand is why any +DC would want to travel to "earth ground" like steel pilings, when a DC system has no reference to anything other than its original source. I read that cathodic over protection can lead to bottom paint flaking and I'm left wondering if changing those zinc trim tab anodes to aluminum may have created this scenario. I think my best bet is to buy a silver/silver chloride electrode and do some testing once it's in the water. Thanks Again.

Al
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Re: Bottom Paint flaking/hull pitting

#5

Post by Chaps »

Cathodic overprotection can occur if magnesium anodes are used in salt or brackish water. Is it possible that your boat was previously equipped with magnesium anodes? We have big lakes here close to our marine waters and it's not uncommon for a lake boat with magnesium anodes gets sold to someone that puts the boat into saltwater unknowingly and overprotection erupts causing paint failure.

The dc+ (if present and your boat is the source) will go to earth ground if the hull doesn't have a solid connection to the negative on the batteries. If that is not an issue on your boat then your paint failure on the sides is more likely being caused by a combination of the SS trim tabs and problematic hull anode protection along with inadequate epoxy under the bottom paint. The aluminum tab anodes you just added will slow that down considerably.

The other possibility is dc+ is coming aboard on the green wire from the marina if you have a shore power connection on your boat and don't have an isolator on that wire or (even better) an isolation transformer for your incoming AC. +DC gets aboard a marina AC safety ground circuit (green wire) typically from some other poorly wired boat in the marina, everyone is connected together on the green wire. That source of +dc will also seek out earth ground.
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IronwoodIsland
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Re: Bottom Paint flaking/hull pitting

#6

Post by IronwoodIsland »

Looks likely that the exposed dis-similar metal of the stainless trim tab is the culprit. I added a stainless cavitation plate to an outdrive to replace a broken off aluminum one. The entire outdrive disintegrated within the year, it was pretty old though.

I would consider trying to isolate the trim tab from the hull (hard to do) or probably better, clean and sand the trim tab, passivate with citric acid or the like and coat with some kind of underwater metal primer or epoxy. The more you can do to reduce the exposed area of the trim tab the more you will reduce galvanic currents that will affect your paint film on the boat hull.

I've been thinking of ways to create a trim tab purely in aluminum plate for our Ironwood boat to avoid this very problem.

By the way - are you in BC too? I used to get Flexdel Armor Coat antifouling shipped to BC but both Fedex and UPS declined to ship it this time. Now looking for something better than the copper laden but ineffective Trilux in Canada. About all I can find here is TotalBoat Aluminapaint AF which is a copper free dual biocide ablative. I really liked the durability of the Armor Coat but it looks like it's hard to get across the border now.
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Re: Bottom Paint flaking/hull pitting

#7

Post by Chaps »

There is an all aluminum trim tab that uses a poly hinge that we see on many new builds here in WA made by Lectrotab http://www.lectrotab.com/products/alumi ... m-tabs.php which won't affect alloy hulls though they should be equipped with R3 aluminum anodes (not zinc as they state in their ad!) as the tab gets eaten by the SS actuator bolts. Armor has been through a rough patch due to its chemistry falling out of favor with Canada. That company recently got bought out by Seahawk which has subsequently been bought out by International Paint (owners of Interlux). If Pettit Vivid is still available up there that would be a good choice. Same biocide as Trilux but seems to work better in the Pettit formulation. I'm not in BC (but was born there).
 
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Re: Bottom Paint flaking/hull pitting

#8

Post by IronwoodIsland »

Thanks Chaps - As usual an information packed comment! Plastic hinged, aluminum tabs - what a great idea.

Sounds like you have been following developments in aluminum friendly antifouling closely. Hope that the recent advances in antifouling paint continue as this is the main issue with keeping aluminum boats in the water full time.
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Re: Bottom Paint flaking/hull pitting

#9

Post by IronwoodIsland »

Interesting review of antifouling technologies by the Canadian Navy:

https://cradpdf.drdc-rddc.gc.ca/PDFS/un ... 60_A1b.pdf

Sadly according to a search I did on approved antifouling paints in Canada, the only approved paint (other than ablatives) registered for use in Canada is Trilux II (not Trilux 33 which has an additional biocide as well as the cuprous thiocyanate that Trilux II is based on)
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allanb76
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Re: Bottom Paint flaking/hull pitting

#10

Post by allanb76 »

Thanks again for all the info.   Here's what I did to try to resolve:

1.) Bonded the DC negative to the hull at the batteries ( was reliant on motors - found the main to be isolated, but kicker - was bonded)
2.) Had the bottom paint fixed up at a reputable shop.   My understanding is they sanded down the pod completely and applied new epoxy under Trilux II.  The rest of the hull only had minor flaking so they just applied fresh coat of Trilux II over top
3.)  Removed, primed IP200E and painted SS trim tabs.
4.) Tried to isolate trim tabs from hull - proved to be a very difficult task.  I did get a layer of 5200 between the tab and the mounting plates, but the difficult part for me was the screws.  I could not get a good fitting plastic washer setup to work well. 
5.) Replaced all anodes and checked connection to hull.  Nearly 0 ohms.
6.) Measured hull potential and fiddled with all electrical devices - pretty solid at -1100mv in all scenarios.  

Unfortunately, I'm still at a loss on this one (no pun intended).  I put the boat in the water May 15 and have been watching it fairly closely.   I had been peeking down at the pod mainly as that was the area most affected before.  All seemed to be good.  I was able to reach down and rub it with my hand.  I noticed no flaking and the anode seemed to be working - although not as aggressively as what I saw the first year with this boat.  Yesterday I removed the boat to check for growth and paint issues.   I was a bit disappointed to see a few chips off the repaired area (stern of pod) but it wasn't too bad so I thought I'd just touch it up.  I then crawled under the boat to see what it looked like....terrible.   Basically from mid hull back to the stern the paint is blistered and flaked quite badly.   

I'm now left wondering what my next steps are.   Is it possible this is just a result of not repainting the entire thing after last year.  The newly repaired area seems okay for the most part, but there are a few flakes which is concerning.  I understand the SS trim tab concern, but have talked to many that have stainless tabs on aluminum boats without issues.  Even the manufacturers use them.  I also had no issues the first year with just bare stainless.  The  only thing that was changed was the trim tab anodes.   I switched them from zinc to aluminum because I found they didn't do much the first year and many people recommended not mixing anode types.   My thought is that maybe the bit of extra aluminum anode is overprotecting the hull now, but -1100mv is okay according to everything I read.  Should I try a smaller hull anode?  

Is there risk with leaving the boat in the rest of the season?  I really can't tell if it's corrosion or just blistering paint.  I believe it's just blistering paint as almost all spots appear to just be dull grey.  If I flake off a piece of paint, it's fairly shiny underneath.  I've read that paint chips should be fixed right away, but if growth isn't a problem, what's the difference between it and a bare hull.  Plenty of guys keep their bare aluminum in the water for weeks at a time and just pressure wash them.  I'm starting to wish I chose that route :) 

Am I expecting too much of this bottom paint?  Is it normal wear and tear?  I see a lot of other boats (mainly fibreglass) and their bottom coats look like hell too.  
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Chaps
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Re: Bottom Paint flaking/hull pitting

#11

Post by Chaps »

As I mentioned previously and also based particularly on your new photos I think the primary issue is that the bottom job was poorly executed, most likely the shop waited too long after the final coat of epoxy primer was put on to then apply the first coat of bottom paint. Bottom paint needs to establish a chemical bond to epoxy which can only occur if the last coat of epoxy is slightly tacky as it is being overcoated with the antifoul. Once the epoxy gets fully cured and hard the bottom paint applied at that stage won't adhere well and will flake off. Another observation based on the new photos is that your round hull anode (and your tab anodes for that matter) look very much like they are zinc. The hull anode because I've only ever seen that style in zinc and I have removed many of them that were incorrectly attached to Canadian built aluminum boats. Also the tab anodes have no wear and show the characteristic color and finish of zinc after it oxidizes and scales over. The boat needs to be stripped and repainted properly IMO and correct mil-spec aluminum anodes installed.
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allanb76
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Re: Bottom Paint flaking/hull pitting

#12

Post by allanb76 »

Thanks.  They are most definitely aluminum anodes.  I quadruple checked and compared the weight to the Martyr spec sheets.  All Silver Streaks have used that hull anode for quite some time, I believe I agree the whole thing should be stripped.  In fact, I'm not even sure I'm going to bother with paint again.  It seems like more trouble than it's worth.  At least knowing the issue is just a poor paint job, I should be fine with keeping it in the water the rest of the season then.  
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Re: Bottom Paint flaking/hull pitting

#13

Post by allanb76 »

After some consult with the dealer I bought it from yesterday, I do agree that the anodes are not corroding like they should, which leads me to new questions.  Are Martyr a decent grade of anode?  I have a picture of the H2B4 (round heat exchanger anode on the hull) and the SKU on it, plus the actual mass match that of the aluminum model.   Is it possible it's just a really poor grade?  I'm stumped why I would measure -1100mv with a Ah/AgCl reference electrode if the anode was no good, or of another material.  

My plan of attack for the short term is to replace the anodes (again), do temporary touch ups with Primocon and Trilux II and continue to monitor for the rest of this season (6 weeks).  

Should the bilge area have it's own anode if there is standing water in it?  I've always assumed the hull anode would take care of that.

@ Ironwood Island, yes I am on Vancouver Island (Nanaimo).  I was unaware until recently that Trilux II still has copper in it.  With that in mind, It seems like a poor product to use around aluminum.  I guess it's rated okay to use due to the really small amounts?  I am really leaning toward soda blasting all the paint off and leaving it bare.  This bottom paint seems to be causing more headaches than it's worth.  
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Re: Bottom Paint flaking/hull pitting

#14

Post by Chaps »

Trilux contains copper thiocyanate, a galvanically less aggressive form of copper, its the same biocide as used in Vivid. Both can be safely used on aluminum boats with a decent coat or two of epoxy primer to isolate it from the hull. Is your boat done with Primocon under Trilux?  That's not a great combination actually.

Your voltage reading is in the right ballpark so anodes are likely ok. Submerged bilge areas are a separate water pool from the ocean so hull anodes on the outside don't do you any good under the deck. If you have much water down there and are seeing some corrosion in the bilge then bolt a small anode down low on a frame in the wet area and keep an eye on it.

A better paint job is needed IMO


 
 
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