Metal Marine Tanks (mainly welded aluminum)

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Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Wed Feb 17, 2016 11:06 am

slug, about welding aluminum; do you have the one pound gun or pistol grip style wire gun to feed aluminum ? Or do you have the push only steel type of gun that feeds wire from the cabinet but has no motor in the torch handle? Three types of MIG; #1 push only, which is very poor for aluminum due to wire flexibility making feed rate erratic; #2 "one pound gun" with the 3" wire roll in the hand torch housing works fine for aluminum due to short length of wire feed to the contact tip; #3 push pull guns like the Cobra's, Pythons with two wire feed motors, one in the cabinet and one in the handle/body of the torch.

#1 is totally frustrating in almost all cases and I would not recommend that torch in aluminum. Some welders do make it work but the welds I've seen are not very acceptable to me. #2,& #3 are the better choices of MIG gun for aluminum due to wire feeding consistency. If you have either of these gun types, and practice a bit on aluminum you can answer the question; do your welds look and break well enough to suit you?

If you're already geared up and familiar with SS, I'd consider that the best option, personally. Aluminum also needs Constant Current AC for TIG but then Constant Voltage DC for MIG, I'm not aware of any power supplies that provide both CV DC for MIG and CC AC for TIG. That implies two separate power supplies ; but SS both TIG and MIG can be done with DC so a CC/CV power supply will work for both depending on the features of any given model.

Most people would prefer the SS tank but the cost of all stages is a bit more than aluminum (which is considered higher than plastics) so few will decide to pursue that tank material. Everything I've shown here on tanks, could be applied to SS except the cutting methods you know have to be different- no carpenters' tools.

However, if you can get all the parts cut, you may also be able to have some of the tank pressed to shape (SS has little or no bend radius tooling issues) and combining those two solutions it seems like SS would be the effective material in your circumstance.

If you build in SS, then I'd make some suggestions on the design which are somewhat different than aluminum, mostly having to do with the baffles and mounting hardware welding joints. Let us know the design and I'll try to sketch a SS version that will apply.

thanks for the generous comments on the thread.

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Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Bill Edmundson » Thu Feb 18, 2016 6:27 pm

Kevin

Thanks for taking metal on. I can't. I can sure tell the winter in Alaska! The post get a lot longer :!: Mine would, too.

Bill
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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Thu Feb 18, 2016 8:07 pm

Bill, thanks for the online space to collect these War and Peace type articles of all sorts about the Miracle Metal! I'd give steel or SS a shot if anyone asked but then I'd probably try to turn the back from the "Dark Side" of metal ! Before we're done I may bump some of the past welding threads as I've found some old models that I may update, photo and post to try and focus a little there too?

[You know, most often, I get associated with metal even though I'm a wood enthusiast too! Not for boats so much, but I do like the stuff quite a bit.]

I decided to collect years of my posts in one place, Miss Gayle agreed to host the space****, so, tons of this was already written, about 80%, but I'd like these where there are some plans (near by) and builders (Glen-L Forum readers) who might use the info? Yep, it's winter and I'm still semi-retired so a good time to put out my long list of mistakes where others can read and avoid duplicating them! Winter yes; however, we've given up on the Old Time Alaskan winters for this new model of " five months of fall " which I quite like. It's like living in Juneau or Vancouver almost.

I'd thought there might be a few more questions with these posts in these few Metal threads but evidently I'm making sense? Can the "cat get the tongue" of a whole Forum?

**** Gayle may not have agreed to host all this if she'd been aware of how long I'd ramble on (!) ?

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Re: Metal Marine Tanks (mainly welded aluminum)

Post by gap998 » Fri Feb 19, 2016 12:20 pm

It's not rambling - it's comprehensive! :)
Gary

Planning a whole fleet, but starting with a Zip...I think.

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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Fri Feb 19, 2016 1:27 pm

gap998,
I'll rephrase that:

**** Gayle may not have agreed to host all this if she'd been aware of how long I'd ramble on (!) ?

**** Gayle may not have agreed to host all this if she'd been aware of how comprehensive I'd be (!) ?

Anyway, thanks for reading, I'll get after some more posts shortly.

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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Bill Edmundson » Fri Feb 19, 2016 3:04 pm

Kevin

You know I was just pocking a little fun. Thanks.

Bill
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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Sun Feb 28, 2016 4:48 pm

Bill, I knew you weren't giving me a real life hard time, just using the chance to add more to our topic here.

I think its time to try to explain the tanks' mounts? This will use a few photos, but it may be necessary to read the test a few times- gray-grey-on-gray-one-gray is not the most obviously explained imagery!

The tanks are mounted line engines, on the outboard and inboard sides- like a pair of engines - but most engines have their own engine "beds" or hull longitudinals two for each engine. I had only one centerline longitudinal and two outboard, but the tanks would span that 3' distance so the mounts shown will be two types or versions. One is the centerline version with holds up the inside edge of each tank; on mount carries both tanks' sides. On the outboard sides of each tank is another set of similar but not identical mounts.

Image
First image above, of the centerline mounts shows a U shape mount, with two side blocks; the two side blocks are:
#1 1/2" plate and will be welded roughly along the black marker line added to show the future weld;
#2 will be welded to the tank wall sides where a set of interior 'gussets have been welded to support the tank outside wall mounted plates;
#3 are flat edged on their lower surfaces to butt to the flat surfaces on the mount. (both mating/supporting surfaces will be acid etched and painted and are grooved to allow water to run out)

The main bracket or U is made from 1/2" 6"x6" angle extrusion. There is a 6" x1/2" plate welded to the angle to make the U, then two 2" x 6" 'blocks' are welded to the sides of the U's lower edges to make the support surfaces. The welds and surfaces of these mounts are grooved to allow standing water to drain, they were etched and painted before use.

At the top edge of the U, is a 1/4" x 6" x 4" flat bar as a collar tie, so the tanks can't move to widen or narrow the U, once the tanks are bolted to this support.

Under the U will be another inverted U to bolt through the centerline 4x12 glassed into the keel area of the boat.

Image
A mock up of the tanks' inner edges butted to the now unwelded tapped blocks that have a black marker edge line on them. This is how the mount will work. The wt will be carried by both the socket recess SS, cap screws AND the shoulder blocks on which the side welded tapped blocks rest.

Image
The same view in this photo as the previous image, but this time the tank's side wall tapped blocks are welded to the tank and the tank mount is installed on the centerline long. Everything had to be taken out and painted ! pain in the stern!

Image
Moving our point of view aft, and to starboard, the same mount is in this photo, above, as the previous one, showing how close the tanks fit, where they locate under the helm sole, and the cut out bulkhead used as access.

Image
We've seen this photograph before, this time its here to show the forward centerline mount from above, now that these mounts are shown in detail perhaps you can see this image was taken looking down the cabin sole hatch into the tank bilge area, and between the two tanks- photo taken facing aft.

Image
And the last images of the centerline double sided mounts- looking forward, standing on the keel plane of view, looking between the two tanks showing both mounts- with paint drips!

The collar ties are on, the forward collar tie is just a shine under the ribbed controls conduit hanging above the tanks' tops.

Image
Collar ties of 1/2" x 2" 6061-T6 bar were installed with welded tapped blocks to give threaded purchase to the two 1/2" x 7/8" SS socket recess cap screws used to pin the two tanks to one another at their tops. Being near the tank top, side wall doublers were not used to mount the tapped blocks for welding. There were collar ties between the tanks and from the tank tops' outer four corners to the hull's framing structure. Here truck load straps are used to keep the tanks aligned in space while the collar tie blocks are tacked to the tanks' fore and after vertical sides.

No movement is tolerated with almost a ton of fuel and tanks.

Image
This photograph shows the outboard tank mounts to a hull stringer and in this case (port side looking aft) the retainer angles added to hold the water tank into is void. These mount are like the centerline mounts except the only have one side and mount as an 'angle' to the stringers' inboard edges and do not straddle glassed in hull longitudinals.

Bolts had plastic tubing bushings (hole liners) and nylon washers under any SS washers to try to reduce galvanic corrosion sites, lots of thread goop, and were installed on mainly painted faces of, but some corrosion will still happen in time unless they're periodically removed then re-coated and replaced. Due to the space, and tank design some specialized socket extensions and circus performer reaches were used to get to all these fasteners- major inconvenience but it all works just fine.

That is an example of how large tanks (100gal.) need to be (could be) mounted - in this case in a fiberglass boat with hull longs of wood that were glassed into the hull while in the mold. I do not have any testing by coring these encased wooden hull longs. Therefore relying on them to be point loaded in compression is one thing... the collar ties, design of them mounts and the tanks own structural contributions to rigidity would help reduce or eliminate any torsion or side load on the wood.

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Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Mon May 22, 2017 10:51 am

There are lots of details that I've tried to list for planning a marine tank but one seems to have escaped my list! I was talking with another builder, recently, and he reminded me of the need to pay attention to tank fuel/gas draw risers (down comers if you prefer?).

His reasoning is valid and I've observed it most times but failed to mention it in this thread so I want to include these notes about draws/pipe size as related to both engine types, horse power and fuel flows.

Outboard engine fuel pumps are most often fed by 1/4" ID hose/tube/fittings; well, I'm not sure about 300hp engines or larger? Inboard engines are similarly plumbed with the occasional 3/8" lines but almost no engines except diesels use larger dia fuel lines. Diesels use some of their fuel flow volume to cool the fuel injectors in many cases- so they're not sucking in double the volume of fuel to burn- but those engines may have greater fuel line requirements over gas engines.

Large pipe (1/2" ) inside the tank as a draw will require more 'work' on the fuel pumps' suction/vacuum side to lift a greater volume of liquid up when the tanks are low. To reduce the pump load in suction smaller diameter pipes should be fitted to reduce the volume being lifted- that will not flow through the fuel lines if they're 1/4" or 3/8" ID.

What I have done regularly on many tanks is to fit a plastic hose with a cylindrical screen at the bottom- this keeps the 'junk' in the tank and also reduces the size of the draw- but.... I failed to note that issue as important to consider. I only mentioned the type of screen fitting that can be used- like those in chain saws, trimmers, other small engines- but I didn't relate the 1/2" riser pipe /fuel draw to the fact that these larger dia. draw pipes; very often get necked down with the 3/8" or 1/4" tubes fitted into that draw.

Then, we discussed the two main ideas of internal tank maintenance and covered the idea of a sump versus pulling all the junk up- into the filter to eliminate tank bottoms. Here again we touch this aspect of tank design; if there is a fuel draw bottom screen- inserted into the draw pipe- that implies a sump and water bottoms maintenance with a separate effort from just keeping an eye on the filters.

What also happens is due to fuel handling- this may be uncommon except in rural areas?- the fuel may become contaminated with water or other 'junk'. This material can settle out down to the tank bottom and if a 'glob' (no better word in my vocabulary?) of water and foreign material get to very small, fine mesh screen at the bottom of the tanks' fuel draw riser?? that contamination can restrict OR stop fuel flow.

If that does happen, the top fittings have to be removed and the liner with its bottom screen have to be lifted out and cleaned - I suppose you could blow some air down the riser/draw liner to the screen and either clear that blockage or flow enough to 'get home'?

So this update post focuses on the fuel riser/fuel draw pipe inside the tanks we've discussed and brings up the size of pipe that should be used- in both cases. If you're running an outboard then 1/4" or 3/8" pipe is more than adequate, unless you're planning the liner and screen pre-filter arrangement? In the case of a large (>V-6?) inboard, the fuel pumps are pretty strong compared to the needed flow- so this note is not as critical as it might be with outboards and tanks located well forward in the boat. Lifting more fuel than can flow in the lines aft- can be avoided by sizing these pipes smaller than I've shown in the posts above.

Probably avoid pump cavitation in the worst case, increase fuel pump life and reduce overall plumbing flow issues.

Cheers,
Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Post by regal » Sat Feb 24, 2018 4:24 pm

Kevin:

Stumbled on your post looking for information on brass/aluminum reactions. All I can say is ... wow. This is an incredible amount of information. I'm hoping you're still monitoring the thread, because I have a question about brass fittings in aluminum tanks that I would love your input on.

I have a 1990 cabin cruiser with a 5052 .9" aluminum tank. The boat only sees fresh water. A few years ago, a lake mechanic replaced the non-metal anti-siphon valve with a brass one. Now I'm leaking gas where the AS valve threads into the tank fitting.

So questions:
1. You mention that an electrolyte such as salt water is needed to allow galvanic corrosion. Is gasoline an electrolyte? I assume gas is conductive, but I don't know for sure.

2. You mention that it is common to weld into the aluminum tank NPT threaded fittings. On another site I saw a mention of these fittings being stainless steel, to allow brass components, but you imply that these welded fittings are aluminum also. The other site mentioned a USCG requirement of SS fittings in AL tanks, but I couldn't find anything saying as much. How can I tell what this threaded fitting is made of? I tried a magnet test (my welded fitting is non-magnetic) and a scratch test (my welded fitting seemed quite a bit harder to scratch than the tank itself, though that's hardly scientific).

3. Assuming that my welded fitting IS aluminum, how do I tell if I have corrosion? Or would it just be assumed given the brass/aluminum contact along with an actual leak?

4. Assuming that I DO have corrosion, what can I do about it? Is there anything I can do aside from replacing the tank? I suppose I could get a new welded fitting put on, though I don't know how practical that would be to do in place. Or do I just goop it up with thread paste (and either get a new AL valve, or a SS fitting to put between the tank and the valve)?

It's the elbow piece below that I'm referring to as "my welded fitting".
20180224_153743_resized.jpg
20180224_153702_resized.jpg
20180224_153718_resized.jpg
I appreciate any input you could offer.

Bill S
SLC, Utah

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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Sat Feb 24, 2018 11:11 pm

regal,
I'll try to reply to the questions as written first, then make some remarks or ask you more questions?
regal wrote:1. You mention that an electrolyte such as salt water is needed to allow galvanic corrosion. Is gasoline an electrolyte? I assume gas is conductive, but I don't know for sure.
Regal, I don't think hydrocarbons are electrolytic and in fact they provide so much free oxygen that grease smeared on aluminum bilges will protect from corrosion of an acidic bilge water! I do not think gasoline is in any way contributing to the leak's cause.
regal wrote:2. You mention that it is common to weld into the aluminum tank NPT threaded fittings. On another site I saw a mention of these fittings being stainless steel, to allow brass components, but you imply that these welded fittings are aluminum also. The other site mentioned a USCG requirement of SS fittings in AL tanks, but I couldn't find anything saying as much. How can I tell what this threaded fitting is made of? I tried a magnet test (my welded fitting is non-magnetic) and a scratch test (my welded fitting seemed quite a bit harder to scratch than the tank itself, though that's hardly scientific).
If the aluminum tank material is going to have fittings welded into the top- they have to be aluminum as no other metal will weld to aluminum. Many tank builders, installers and boat builders will protect the NOtoRIously soft and fragile aluminum threads with SS bushings- so a welded female thread aluminum fitting has its threads 'protected' by inserting a SS bushing- these reduce the thread size but are left installed in aluminum fitting even when other fittings are added or removed.

You show a brass King Nipple ( short nipple of pipe with M-NPT threads on one end and a set of conic ridges for insertion into the ID of elastomeric tubing/hose products) in the horizontal in your photos- some codes/practices/authorities argue that brass and aluminum are too reactive to have the King Nipple shown threaded directly into the aluminum fittings- regardless of the welded tank fitting, the street El you show or any other aluminum thread fitting. One reason is the two metals do form a galvanic cell- but then so does SS if not passivated and selected for the correct alloy. (remark about brass being in use at all?)

The street El Fitting threaded into the tank top welded in fitting- looks aluminum BUT.... it may be zinc plated, cadmium plated (usually yellowish?) or could be aluminum. I'm not sure and it does not appear marked? Many fittings come with makers' marks, or ANSI code labels to understand what they're made from and their application.

Few fluid connection fittings made in aluminum are ever made from marine metals. You mention you tank is 5052- almost without doubt the fitting is NOT 5052- but since there are 8 major alloy series of aluminum and plural dozens of alloys in some of those series that means there are hundreds of alloys in the aluminum market place- so using that term is like calling all sugary soft drinks that use brown coloring as "cokes" A bit generic and not conducive to your leak cure.

If the fitting is leaking then it's composition is not relevant - removing and replacing is the cure.
regal wrote:3. Assuming that my welded fitting IS aluminum, how do I tell if I have corrosion? Or would it just be assumed given the brass/aluminum contact along with an actual leak?
The fitting in the tank at the bottom is welded and it is aluminum - the male to female (Street El) fitting that is leaking may or may not be aluminum- don't matter -it's got to come out and be re-installed. (more on that below) If you have corrosion I'd expect to see some gray or off-white paste where the two threads meet- and I don't see that so I suspect you have a galled connection not corrosion?

I try to avoid assuming because I always make and 'a**' out of 'you' and 'me'- I don't see any interaction showing between the brass and the St El fitting, or any dew dripping off the brass showing tank top galvanic reaction- but then the latter could be present?
regal wrote:4. Assuming that I DO have corrosion, what can I do about it? Is there anything I can do aside from replacing the tank? I suppose I could get a new welded fitting put on, though I don't know how practical that would be to do in place. Or do I just goop it up with thread paste (and either get a new AL valve, or a SS fitting to put between the tank and the valve)?
Regardless of the cause of the leak you need to remove the male (St El) fitting and clean the threads inside the tank top female fitting, then re-install A male fitting to seal those threads and hold a King Nipple to connect the hose.

Here I am not clear what valve is involved? Is the St El and valve? I"m not familiar with that form of a valve? Is the brass King Nipple a valve- I'm not familiar with the form factor as a valve? So I'm not able to follow the wording in the question very well?

To attempt to answer your question: in almost all cases of Nation Pipe Thread's leaking the two fittings need to be tightened together- as NPT is a set of helical 'wedge's or tapered threads. To engage tighter the two threaded pieces are torqued tighter and that usually seals them- IF THE THREADS ARE INTACT? To learn this critical bit of info- you'll have to removed the upper fitting (aluminum colored St El) than means disconnecting the hose and removing the King Nipple and then removing the St El.

Inspect the threads and see if they are intact?

now for a possibility that could be involved?

if the St El (which I think you're referring to as a valve?) and is NOT a welded fitting, will not turn smoothly with a wrench while be removed- then the fitting was galled into the welded fitting (female threaded opening in the tank top) while it was installed.

Aluminum to aluminum NPT threads will gall at the blink of an eye and usually both fittings are destroyed in the process - in a second's time.

So, if this fitting is removed and the threads are galled- you'll have to #1 attempt to tap or 'chase' the threads in the female tank top welded fitting so it can attempt to be sealed again with another fitting- OR #2 the tank will have to be removed, purged/cleaned and the fitting reinstalled by welding.

hope to have helped you begin to explore the potential repair to your tank top leak? I am, as noted, a little unclear on some terms so my reply might be off course due to terminology use? Also I tried to reply to your questions - but have only shared my suspicions here at the end of the post.

Please feel free to add a little more explanation of where the valve is? and ask about any facets of what I understood and used in my reply?

Cheers,
Kevin Morin
Kenai, AK
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Re: Metal Marine Tanks (mainly welded aluminum)

Post by regal » Sun Feb 25, 2018 10:38 am

Kevin:

Thank you so much for the detailed response!

Let me address your confusion - which is caused in part by my assumptions (with your comment on assumptions fully in mind...) but mostly I think due to the angle I was looking at the tank top from. As you point out, there is a female threaded fitting welded onto the tank top, into which the "street-el" is threaded. Threaded into the St El is the anti-siphon valve - what you're calling the King Nipple. When I look down on the tank in the boat, the angle I have is directly over the St El - I don't see the threaded connection between the St El and the welded tank fitting. You can kind of see this in the middle picture I posted. From my vantage, it looked like the St El was welded directly into the tank. So when I was referring to my "welded fitting" I thought I was talking about everything between the tank and the King Nipple. When taking pictures (which I'm so glad I did now!), I of course shoved my phone into spots where my head can't go, and got better angles. Too bad I didn't notice the St El threaded connection when I saw the pictures - thanks for making me see the obvious!!

It's the King Nipple/St El connection that is leaking - now that I know I can remove both components, I can investigate and replace. Though your comments on galled threading make me nervous to try and remove the St El. As you say there is no visual evidence of corrosion, so maybe that was always a red herring. I'll pull the King Nipple-anti-siphon valve, check it out, try to get an angle into the female side of the St El to check out those threads, and go from there.

You say:
You show a brass King Nipple ( short nipple of pipe with M-NPT threads on one end and a set of conic ridges for insertion into the ID of elastomeric tubing/hose products) in the horizontal in your photos- some codes/practices/authorities argue that brass and aluminum are too reactive to have the King Nipple shown threaded directly into the aluminum fittings- regardless of the welded tank fitting, the street El you show or any other aluminum thread fitting. One reason is the two metals do form a galvanic cell- but then so does SS if not passivated and selected for the correct alloy. (remark about brass being in use at all?)
I read this as "I really shouldn't have a brass fitting in the St El, unless I can determine that it is NOT aluminum", correct?

I torqued the King Nipple into the St El as tight as I was comfortable, after I first noticed the leak. I was afraid of damaging the St El - which at the time I thought was welded in. You mentioned a couple of different thread tapes/pastes - but it doesn't sound like you're really a fan? Although, if it's the only thing that solves my gas leak, chances are I'll become a fan...

You say:
Few fluid connection fittings made in aluminum are ever made from marine metals.
I'm not sure what you mean here - what are "marine metals"?

I agree with what you've said a few times - the solution is to remove and replace the leaking components. Now that I've realized one of the components are welded in, I can follow that course. In the event that I DO end up pulling the St El, do you have any recommendations to keep the threads from galling on reinstallation?

Thank you so much for you time!

Bill

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Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Sun Feb 25, 2018 3:08 pm

Bill,
First, you're welcome to my few cents worth of experience (read; boy have I made countless mistakes with aluminum thread fittings!!) as it may apply to your tank top fittings.

Regarding the St El removal; if the fitting turns easily and smoothly- in other words the threads 'work'/function/perform as they were designed to do? Then you don't have galled threaded- however- IF the fitting takes lots of force to rotate, and , OR the fitting moves very hard tighter or looser- then there was inadequate 'goop' to lubricate the threads between the two fittings when the male St El end was installed.

Aluminum to aluminum is not a very good sliding/gliding/slipping surface material choice so dry, bare, threads in aluminum (NPT threads are tapered) will almost always gall. Galling is the process of microscopic 'hairs' or 'threads' of the cut metal surfaces becoming entwined and in essence- fused or cold welded to one another and this almost always results in the threads tearing off, and NPT fittings almost always fail to seal after they've galled.

So, if you can remove the fittings with easy smooth turning motion of the wrench- THEN- you're likely in very good shape- You can clean (304 SS wire brush the male thread and rinse in acetone) but the female thread may need a little more care and inspection- all cleaning residue drops in the tank! Pain in the Stern. (PitS)

If you have galled threads in the inside of the tank top welded fitting- THEN, you'll want to pull the tank and have it repaired- no all shops will weld on tanks that have held gasoline but some will. Steam clean the tank and its safe to repair.

Let me make a blanket statement to summarize my many opinions of aluminum metal installations- skip all copper based alloy metals (copper, brass, bronze, all related alloys) in favor of all SS and in SS: look for 316L alloy first and accept 304 if you can't find 316. THEN.... KEY idea- take time and effort to passivate all SS in contact/proximity/threaded/bolted/inserted into aluminum. Passivating SS is just standard practice but commonly skipped now days in too many marine applications.

Aluminum alloys of the 5000 and some 6000 series are considered marine grade aluminum alloys because of their chemical content they resist corrosion from salt water environments better than other alloys and are weldable and 6061 can be machined easily and produces good parts. There are many other alloys used for fittings and parts- their corrosion resistance varies with alloy as does their hardness, brittleness, toughness, and tensile strengths all vary with alloys.

To stop a new fitting (aluminum male NPT) from galling into another aluminum female fitting- best practice is don't do it. Use SS or even hot dipped galvanized (thread polishing and dressing is critical there) are both better choices and much less delicate threaded materials. If you have to use aluminum into aluminum then look for thread dope/goop/sealant that is fuel proof and do not over tighten.

One reason NPT is so poor an engineering choice of threads is because you have to tighten to seal but the orientation of the fitting is not possible to FULLY control with a tapered thread fitting. Best of all worlds (in threaded fluid connectors) is SAE/hydraulic/Straight Threads/O-Ring sealed fittings where the threads only pull the O-Ring into an expanded seal - the threads don't seal they are mechanical only- so they can work without sealing or 'jamming a fitting' into a wedged/tapered/torque sealed, conic, mechanical intersection volume.

The hydraulic type thread fitting could have been put in the tank top and the seal would be from an O'RIng (easily replaceable) as opposed to the NPT design fittings.

LockTite and other sealant vendors have a host of products that will lubricated but set up into a seal too- and there are plenty of other lube/sealants that are fuel proof as well. I have used many products and they seem to be improving over time.

HOpe this clears up some of my remarks,

cheers,
Kevin Morin
Kenai, AK
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Re: Metal Marine Tanks (mainly welded aluminum)

Post by regal » Mon Feb 26, 2018 9:04 pm

Kevin:

I think I know what needs to be done. I will post my results for posterity, though it may be a while. Aside from the 30 feet of foot-deep snow between me and the boat now, I don't relish sitting in the engine compartment in 40 degrees. Though, you're in Alaska, so I'm guessing I don't much sympathy from you!

Thanks again for all your input.

Bill

Kevin Morin
Posts: 699
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Tue Feb 27, 2018 9:30 am

Bill, I do understand that boat work out side the shop in the weather is not a high priority! I've got a skiff in the shop now to do some mod.s and repairs of old equipment mounting holes and all that would Not be happening outdoors.

It will be interesting to hear from you when the passes open, the weather warms and the boat is closer to hand- what you discover about these threads' problems?

I think that once the brass fitting is removed from the St El that in the next 1/4 twist of that remaining fitting - you'll know where you stand in regard the threads' state.

Please let us all know, as you say, for prosperity's sake and a good follow up on the tank thread.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
Posts: 699
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: Metal Marine Tanks (mainly welded aluminum)

Post by Kevin Morin » Mon May 07, 2018 11:19 am

Just some notes on the tank topic, an update from recent experience and confirmation of galvanic interaction of different metals in tanks.

A friend's friend bought a 13 year old welded aluminum boat, and found some corrosion in the bilge- and that lead to an exploration of the rest of the boat's bilges so any further possible corrosion was found. That lead to pulling the potable water tank and more corrosion- that leads to this post on the Tank Thread.

First, the water tank was made of aluminum and that is often considered poor practice due to the taste- but.... now more and more often a polypropylene tank is skipped and the aluminum tank used for dishes, showers, rinsing but bottled water is the sole source for drinking. So the metallic taste of the welded aluminum water tank is ignored by not drinking from the faucets.

Anyway- the builder in this case fitted the welded aluminum water tank (and I think the fuel tank too-but that's not been exposed yet) with brass/bronze fittings and the corrosion that resulted around the fittings; at the 1/2 pipe couples welded to the tanks is very visible.

The other matter that showed up is the tank bedding material- a raw rubber or boiler gasket type rubber sheet was used to put next to the tank- and that material evidently had some form of free carbon in the mix? Around the areas where the tank had been mounted to or strapped down with the rubber pads- there is corrosion where there was water/dew/moisture/bilge water?

I'm not sure how much water was present- not much in the main cabin areas I suspect- but... just the condensate of atmospheric humidity was sufficient to allow a metal to metal galvanic interaction. The base of each brass/bronze fitting was greenish and the near 1" or 2" circle around the fittings were powdery- and had begun to pit the aluminum.

I sill suspect the gas tank- not yet inspected- will be similar in plumbing and perhaps degradation. The boat's owner and his friends are going to open the bilge to clean the hull and see if there is anymore corrosion and perhaps remount some of the tankage, replumb, and replace fittings of copper alloyed metals with galvanized, SS or high strength nylon to try to avoid future deterioration by galvanic corrosion.

This post is a reminder to those planning a tank install- regardless of the hull's material; welded aluminum tanks will last a long time, give very reliable service and for their size and durability are a great deal-BUT..... if not installed correction, surface treated correctly or are in contact with dissimilar metals- they can have a very short life span!

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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