rub rail corrosion/pitting

General boating discussion
Dingo44
Posts: 4
Joined: Fri Apr 02, 2021 2:32 pm

rub rail corrosion/pitting

#1

Post by Dingo44 »

Greetings all,

I have a 2yr old 26ft Pacific with a rub rail running the entire length of the boat. The boat is on a mooring 6mts of the year in salt water. I pulled the rub rail back last weekend and noticed there's pitting developing under the rail where salt deposits are trapped between the rail and the gunwale. Pacific also ran a thick piece of tape behind the rub rail which worked well along much of it's length but there are spots where salt has come between the boat and the tape, and hence water/pitting.

so how to proceed??

I've pulled the rail off on both sides. And was considering the following:
-clean up the pits and fill them with epoxy
-paint the entire area behind and just below the rub rail with epoxy so there's no direct metal/rub rail contact
-install non-metallic spacers between the rail and the boat to allow water to pass thru

What do the experts out there think? And what is the best way to clean those pits out before epoxy-ing (this will by DIY, and I live on an island with no access to a sandblaster)

If a coating of epoxy is the go, is West Systems G-Flex epoxy a good one to use in this application??

Thanks in advance to any and all insights-this is the best last great splurge in my life-I really need her to last

Dingo

ps I will try to post pics when my tech savvy wife gets home
dingahling
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Re: rub rail corrosion/pitting

#2

Post by dingahling »

Dingo,

I expect Kevin (KMorin) will be along shortly to advise, he is the resident expert on aluminum corrosion. In the meantime you can use the search box above to check out the numerous corrosion posts.
There is viewtopic.php?f=3&t=5692 this post from Tfitz whose project boat had the same issue under the rub rail, though if I remember correctly, the pitting was severe and yours being only 2 years old, should be much easier to deal with.
Unfortunately, all the post photos are unavailable, but there is info there to be had.

If I've learned any lessons from the corrosion posts here, I would say that wire brushing the pitted areas with baking soda or ammonia to neutralize the crevice corrosion (acid in the pitting) is first and foremost. I think KMorin or Chaps (another AAB stalwart) will better advise on the epoxy coating.

But for sure if possible, isolate the rub rail from the aluminum. I expect some here will say pitch the rub rail altogether.

Post pics, please. That will help responders a bunch.

Good luck
kmorin
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Re: rub rail corrosion/pitting

#3

Post by kmorin »

Dingo44, Welcome to the AAB.com Forum and sorry to hear of your new boat's corrosion.

I'm not that familiar with Pacifics in detail and most of them I've seen have an aluminum extrusion at the gunwale as rub rail. When you describe your boat you're referring to a rubber or vinyl shape that was installed along the sheer at the gunwale?

Was this bolted or did the 'tape' you describe hold it on? It would help if we knew the materials involved, and would take time to post some images?

I've seen (many, Many) skiffs over the years with a D cross section rubber extrusion that was bolted to the boat with an interior flat bar (inside the Flat of the D in the rubber ) and that was either drilled or self tapping screws were used from inside the boat to pull the flat bar against the hull?

Most of the pitting I've seen in these installations, including two I installed, were galvanic- where the SS fasteners were in close contact or threaded into aluminum and then allowed to be wetted over and over. I've concluded the corrosion was metal to metal (galvanic) and not just crevice corrosion enhanced by 'salt' in the close fit surfaces. The pitting I've seen was all close/near/adjacent to the bolt holes where the two metals' formed a "battery"/galvanic cell.

Image

Above is an image of the long term corrosion around the bolt holes holding on a rubber D shaped rub rail. The chalky, corrosion 'dust' or reduced aluminum crumbled out of the pitted area around the bolt hole.

Image

A little closer view as I was digging out some of the 1/4" angle (6061-T6 alloy) that was wrapped around this hull in order to give a nice flat, vertical surface to hold the D rubber.

Image

What I did was wire wheel out the pits, then cut our areas that were completely corroded and replaced with fitted patches! lots of work. In most areas I could simple TIG float the area and build up material and then sand flat - and that is what it took to get back to fully recovered rub rail (base) for this far advanced corrosion and extreme pitting.

I did not replace the rubber extrusion - I welded on a 1/2 pipe around the entire hull and if the skipper wants to use his rub rail against a softer surface- like a plastic boat?- then he'll have to use buoys or portable bumpers!

It would help others to understand more about your particular boat's exact rub rail installation to advise on re-installation.

That said; yes, getting the pitting cleaned out to the bottom is the first step in arresting a corrosion cell. A SS wire wheel, with fine wire brushes in the twisted (not the knotted style) on a 4" grinder motor is a good way to clean out the pits. And conveniently, your problem areas are at working ht. making the access much easier than if it were under the hull.

https://s.yimg.com/aah/cyberweld/weiler ... 3113-5.jpg this is an example of a wire wheel (SS wires) with 0.014" dia. wires that are soft so they do not gouge or excavate new parent metal- which will happen if you use the very stiff wired wheels also offered. This fine wire wheels are expensive and don't last very long compared to the thicker wire but the results are much better and worth the money (IMO).

Chaps, our resident corrosion recovery & coatings expert, has a very clean and detailed reply on a thread near this one. In which he details cleaning, and the series of coatings and fillers and re-coating that he provides in his bottom coating business. (Thread for corrosion recovery details. "Hull corrosion, Lifetimer. Advice sought." in this Category.

If time allows, images would help give useful advice; by the way- what did Pacific recommend when you contacted them about your pitting?

Cheers,
Kevin Morin
Kenai, AK
kmorin
Dingo44
Posts: 4
Joined: Fri Apr 02, 2021 2:32 pm

Re: rub rail corrosion/pitting

#4

Post by Dingo44 »

Hi, and thanks to you both for getting back to me. I hope these photos upload ok. Yes it is a heavy-duty D style rubber rail. There doesn't appear to be any corrosion around the bolt holes. As I hope the photos show there's a very distinct line of pitting where the bottom of the rail touches the boat. It is held in place by hex bolts which enter from the outside and lock washers with ss washers seated ontop of nylon washers from inside the boat. The head of the bolts do rest against a metal insert in the rail itself, not against the boat. But again the only corrosion appears to be at the underside of the rub rail.

I just bought the wire brush wheels you suggested Kevin, and will research the other post you mentioned to sort out what kind of filler to use. If I do not re-install the rub rail for this season, do I need to worry about the bolt holes being open and a source of future corrosion if left open? Should I put a sleeved bold thru that rests on nylon washers both sides to fill the holes?

To be fair, I could and should have done a better job of washing this boat down between runs to reduce the salt caking up. My mooring is remote and there's no electricity nor running water. Bringing it to a dock with fresh water adds an extra hour to a day on the water. That said I will certainly be more attentive in the future.

Again many thanks, grateful for the help/insights in sorting this out
Cheers,
Dingo
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kmorin
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Re: rub rail corrosion/pitting

#5

Post by kmorin »

Dingo,
not knowing better it appears Pacific has installed a flat bar to bolt to above their topsides and then only welded the extruded/shaped gunwale round inside to allow for a nice tight outside fit of the top toe of the rubber to the underside of their rounded oblong extrusion?

I agree that I don't see any galvanic corrosion signs around the bots- are they SS or glavanized? Hot dipped Galvanized would be a good choice since they have little reaction, if any, with aluminum and they're out of sight inside the rub rail therefore not unsightly as some consider hot dipped finish.

So the probable cause of the corrosion is crevice corrosion where water gets into a thin film and becomes acidic (shifts ph from neutral to acid) when the metal gets the oxygen out of the water (initially). These cells are discussed extensively else where on the site so I won't go into them.

I think this is a good place to follow Chaps' methods and techniques of cleaning out the pitting sites mechanically (wire brush) and perhaps with acid if you can find some where you live? Worst case you could 'boil down' some vinegar (concentrate the acidic solution) and using a hand held spray bottle when the skiff is warm; spray the areas around the pitting and let the acid help clean into the corroded areas. Won't hurt to tape a plastic drip sheet on the hull to avoid streaking up your topsides.

By wire brushing and acid washing you'd get the pit bottoms as cleaned out as possible- then using an epoxy primer seal the area and let dry. Followed by epoxy putty filler and sanding that flat- then re-primering the area and finally painting a 'bedding stripe' on the flat bar where the rub rail will remount.

The idea here is to clean out the metal to allow a good primer bond, then to fill and encapsulate the filler with a top primer- and that all allows a good epoxy/topcoat bond to the strip of hull where the rubber will be mounted.

While I agree with the idea of trying to make the rubber stand off the hull- to facilitate drying out this joint? I'm not sure I have any method what would do that effectively? Perhaps if you took a jig saw to the back of the rubber- making small vertical slices? you might create some form of drainage? Not sure?

I think a good paint film with good mechanical and chemical bond to the hull sides would give a better seal of the metal from crevice corrosion than 'standing off' ?

One idea that might be worth considering is to use a good quality marine sealant and calk that top seam once the area was cleaned and painted and the rubber reattached? If the corrosion is forming top down -as seems the case? then a bead of black 5200 on top of the rub rail and below the rounded overhanging extrusion would go along way to sealing that joint from water.

You could install the 5200 in two ways- one is before the rubber goes on and the other is after it's on. IN the former -you'd have more sealant down in the gap but have to work faster to get a rail on! In the latter you'd have more time to get the rail reattached but an outside seam fillet of calk isn't as likely to fully seal the joint.

The only thing I can see as a problem with the holes being left open is shipping some water into an internally enclosed space? I don't know the inside geometry or design of the hull so can't say? But they won't corrode as they're shown in the photographs- nothing to react there.

Hopefully you'll find more ideas and learn about other possible solutions? Pacific has to have some ideas?

cheers,
Kevin Morin
Kenai, AK
kmorin
Chaps
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Re: rub rail corrosion/pitting

#6

Post by Chaps »

Good info above. Its not a rare problem. One thing that I have repeatedly heard over the years is that black D rubber has chemical components that will galvanically react with aluminum particularly if the interface between the two is kept wet with sea water, likely culprit is the carbon black in the rubber which is conductive and electrochemically noble. My old Edwing had white D rubber around the gunwale and did not develop the issue and other than rain it saw little intentional fresh water flushing . . . in fact I can't remember ever washing it!
1987 24' LaConner pilothouse workboat, 225 Suzuki
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Dingo44
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Joined: Fri Apr 02, 2021 2:32 pm

Re: rub rail corrosion/pitting

#7

Post by Dingo44 »

Hi again,

Yes, it appears that it is caused by salt being trapped along the edges of the rail. The majority of the metal under the tape is shiny and new. Moreover, the backside of the rail itself only has salt deposits along its edges, none in the middle

I have ordered the wire brushes actually have a bottle of HCL here that I could use to clean out the pits. Can also get muriatic acid easily. Should I use a Scotch brite pad to do this AFTER hitting it with a wire brush?

I will research further as to the epoxy primer and putty, but are there brands out there that any of you would recommend? I know the West Systems GFlex epoxy can be mixed to whatever consistency is desirable. could I use this to fill the pits and then use a thinner coat to 'paint' over? Kevin, with regards to the 'bedding' stripe should this be an epoxy paint or could I again use the GFlex epoxy mixed to a desired consistency?

Interesting about the black rub rails and carbon causing the galvanic corrosion. Makes me glad I pulled the whole thing off when I did

Again many thanks all, I apologise if I'm asking overly simplistic questions and I appreciate your taking the time. I'm finally feeling like I have a fairly clear path to follow to get this worked out.

Dingo
kmorin
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Re: rub rail corrosion/pitting

#8

Post by kmorin »

Dingo44,
Dingo44 wrote: Sun Apr 04, 2021 9:41 am Yes, it appears that it is caused by salt being trapped along the edges of the rail.
I'd agree that may be what appears the cause or your pitting; but not actually the real cause shown of the pitting in your photos.

I think you may be seeing a white chalky,powdery substance and labeling that as "salt accumulation"? That is not the case (IMO). The entire boat has been bathed in salt- spray, wave action and floating in it! So, I'd ask; if this was the result of exposure to salt in water?? why isn't 100% of the boat reacting as this section has reacted?

My answer, offered many times on the site, is that some condition inside a closely spaced, unsealed interface, allowed a specific electro-chemical condition to overcome the naturally occurring aluminum oxide that the entire rest of the hull has intact?

Stated differently: What's different in the edge of the D rail to hull joint and the countless other locations on the hull that have not pitted/corroded/"failed"? I submit that the difference is the thin film of water created by the two material touching but not being fused/welded/fully sealed.

I believe what you're seeing as a white powder and labeling salt isn't salt at all. I think that white powder is the normal, well recognized and common by product of both electrolytic (galvanic) and crevice corrosion cells in aluminum? Of course not being there, and not seeing the 'powder' I can't say that this is fact... but its surely a symptom I've been working with since the late 1970's as I've dealt with welded aluminum boat corrosion issues on hundreds of boats.

So, while you are welcome to think this is "salt corrosion"- the evidence shown photographically doesn't confirm that idea. The white powder is most likely the PRODuct of the corrosion: NOT the cause- it's not salt.

You could also ask: "If salt is present over the entire hull; then why is there only corrosion where the water was trapped in a thin film and not allowed to drain?"

OR, you could also ask, as I'm doing here, "Why is the pitting confined to a 'covered', undrained, area where two different materials are touching and that tiny thin void space could be filled by salt water due to capillary action ?"

Hope you can form another view point about why your 'salt conclusion' is under question? This can happen from the galvanic differences of the free carbon in the D rubber as Chaps has mentioned, or it could begin from a well known crevice cell ph shift and subsequent corrosion.

Both resulting in pitting of the type shown, and the salt water is not a cause OR it would happen over the entire surface of the hull?

I have no experience with the West System in any use but wood bonding and encapsulation. So can't reply about using it as a bedding material, filler or it's grip on aluminum? My suggestion of a paint strip as bedding was based on marine epoxy paints' tenacious adherence to the primers which will (themselves) bond well to aluminum. West products may do this well but I have no experience.

A filler that does bond well to epoxy and bare aluminum is https://www.amazon.com/Fibreglass-Everc ... B000P72HG8 and I have seen its performance in regard filling pits. I ground the pits and filled them then bent the parent metal and unless the aluminum was bent to 90 the filler stayed 'stuck'. That is personal experience in my shop conditions so I'd advise testing the many products in your own shop to confirm your filler brand decision?

Perhaps Chaps can advise more (other brands, products, mfg's) on what's worked for him filling pits in welded aluminum boats?

Cheers,
Kevin Morin
Kenai, AK
kmorin
Dingo44
Posts: 4
Joined: Fri Apr 02, 2021 2:32 pm

Re: rub rail corrosion/pitting

#9

Post by Dingo44 »

Hi Kevin, my apologies. I was using 'salt' too liberally. I fully agree that the pitting has been caused by standing/stale water trapped between the rail and the boat that has become acidic over time. Again thank you for your input on all of this and taking the time to respond as thoroughly as you have. I will seek out the filler that you suggested.
Will update this post as the 'mend' gets underway, fingers crossed it's soon soon
Cheers,
Dingo
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