Metal Marine Tanks (mainly welded aluminum)

Steel and aluminum boatbuilding. See: "Boatbuilding Methods", in left-hand column of the Home page, for information about alloys.

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Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Wed Jan 06, 2016 6:00 pm

On another thread that I'd missed reading (while the decisions about tanks were active) there were questions about metal tanks for boats. I'm going to work on an exploration of that topic here, under the Metal area of Building Materials because this seems like the logical place to talk about METAL tanks?

If not? Mod's please move to a more appropriate location?

Marine tanks are one topic that gets little (less than it might?) Forum space because tanks are not glamorous like a new boat build, or outboard profile images of a new skiff running in nice looking surroundings on a sunny day! Tanks aren't that appealing, you've got to have them- yeah- but... they're just buried somewhere in your boat and they're hardly the stuff of bikini freighters streaming a 100' wake in the sun! I mean the pretty girl is not sitting on the foredeck holding a welded tank! So tanks aren't that glamorous but they're important so they deserve a thread to explore them.

I'll focus on welded aluminum tanks in this series, similar to the companion thread about work techniques involved in welded aluminum boats. I'd like to prove to most builders of wood, fiberglass and aluminum boats that you can design "build" your own tanks. Yes, I do realize that most wooden and glass boat builders don't own welding equipment and don't necessarily know how to weld- but that will turn out to be the least amount of work in a welded marine tank. I'm using the term build to mean -be in charge of the final results- not to do every single step- we're not expecting you to mill or smelt aluminum sheet and maybe not to do the welding either?

In this thread I'll try to explore what you need to build your own tank from design concepts to finished work, and in the process I hope to convince you that #1 this is not rocket science reserved to a few professionals that you have to pay thousands of dollars to do for you.... #2 that the quality of the tank depends on your understanding the fundamentals of the metal work and following those basic rules of trade craft... & #3 that with common wood working power tools and some boatbuilding ingenuity you will end up with the 'perfect' tank for your boat regardless of what material your boat is made from.

Let's introduce some facts of life relating to Aluminum that we can learn- then ignore or just assume they're true.

#1 Aluminum cuts perfectly with AC power tools, no Plasma, no Laser and no Acetylene or Propane torch cutting is needed to make a tank of marine aluminum alloy. SO!!!! if you can cut wood with a power tool you can cut aluminum as well as anyone else! Skill saws, routers, jig saws, band saws and other wood cutting tools will work fine in aluminum all that is needed is the correct teeth per inch and some spray-on pan cooking grease!!

#2 Aluminum edges, once cut can be sanded on belt sanders, hand files and other edge dressing tools to produce extremely accurate, clean and ready-to-weld edges and that fact allow welded marine tanks of all shapes to be cut with power tools then prepared for welding very reasonable effort.

#3 Aluminum can be formed by press bending (braking); by shearing (cutting into rectangles and squares with a machine (power shear) that chops the metal with a huge powered knife) however, you can rely on #1 & #2, above, to do all the work required in 'making' your tank parts.

#4 Aluminum tanks can last many decades if A) they are designed well; B) they are built correctly and C) these tanks are maintained by the boat owner as needed.

So, I'll try to create a reference thread for those who might not build their own welded aluminum boat, but would consider taking on their own tanks as part of their overall build? In the process I hope that I'd give you some guidance on selecting a manufactured tank or, some ideas to consider if you had a tank made to order for your boat.

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

Postby Kevin Morin » Wed Jan 06, 2016 6:51 pm

Before we even begin to talk tanks, we need to talk aluminum; alloys, surface film materials and corrosion.

To become TRULY informed, really knowledgeable, you'd have to surf the web and read about the different series of aluminum alloys (many series of alloys ) but I'm not addressing that overview here. I'll confine my remarks to what I've worked with, and focus on that narrow bit that I can report to you with any accuracy.

Aluminum alloys are available in two types (alloy series of identification numbers) we care about in boats: 50 series and 60 series, all other alloy series are not as suited to boating environments so... ignore them! And in tanks made of plates or sheets, (I divide plate and sheet at 1/4"-0.250" thickness where the metal is plate above and sheet below and the 1/4" is EITHER!! you call it.) the alloys we'll consider are 50 series.

5052, 5086 and 5083 are the most prominently available alloys for marine tanks, with 5052 being the least expensive because it has the lowest mechanical properties. However, since almost all tanks we're going to explore are small compared to an entire hull, for example, the least mechanical strengths are more than enough for the average marine tank. That means the least expensive, most commonly available, and the lower (relative) strength, 5052 alloy, is fine for our tank work.

5052 used to be the only alloy available in the early 60's and before.. so lots and lots of boats were built entirely of that alloy, its not as strong as 5086 or '83 but then fir isn't quite as strong as oak and there are plenty of boats of both? Also, 'biaxial-poly-something-exotic-cloth' is probably stronger than fiberglass roving too! So!! there are plenty of boats built and floating today made of less than the most optimal materials, so we're going to work with 5052.

If you elect to purchase 5086 or 5083 for your tank work, that is fine..... the tanks will be structurally more rigid, but corrosion wise- all the explanations here will still apply. Structurally stronger tanks are no more corrosion resistant than slightly more malleable and less rigid alloys. These are two separate subjects, and care should be given in the study of this series of posts not to confuse surface corrosion with structural strength.

Now we need a couple of statements about aluminum that you'd need to "engrave on your brain" in order to give these statements their importance in learning to work with this metal.

#1 Aluminum's oxide film forms within a few seconds of bare aluminum being exposed to the atmosphere.

#2 Aluminum's self healing 'skin' or aluminum oxide film is the property that is the primary reason aluminum has a commercial value in mankind's use of materials.

Let's explore those two 'broad brush' statements so you can 'own' them. To do that, why not talk about steel? Steel's OXIDE is ferrous oxide also known as rust. Rust is a fully consuming deterioration of iron and steel that fully consumes the underlying metal until the parent metal is reduced to 'oxide' not metal.

However, aluminum is different, the oxide film (couple mills or 0.001 or 0.003 film) forms with oxygen THEN STOPS. This last statement is the key to aluminum's entire commercial value.

That translates to: Steel will rust till its gone & Aluminum will seal itself inside a few mills thick film that protects it forever!

OK, not true. There are three main ways to make that generalization false. They are the three main types of corrosion that continue to 'eat into' aluminum - despite the protective film. (doggone physics and chemistry anyway!)

These three types of surface corrosion of marine aluminum alloys, actually, all relate to one another at the molecular level so let's look at them in general terms. All three eventually come down to electrons flowing and molecules interacting with one another, but we group them separately because of the three different actions that induce these similar chemical reactions.

That may sound kind of hard to accept (?) but next post I'll try to make as much sense of that as my limited understanding provides.

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

Postby mrintense » Wed Jan 06, 2016 7:26 pm

Kevin,

I want to personally thank you for taking a considerable amount of your personal time to write these articles. Coming from an aircraft maintenance background in a previous career, I can appreciate much of what you have written so far, and I have certainly learned some new ideas from them. I was pleased when I saw you were going to address this particular subject because I have been wrestling with which choice I will ultimately have to make as regards fuel tank selection. I remember 5052 as being the one metal we could not use for aircraft work not because it was inferior, but because it didn't meet the specifications set by the manufacturers and the FAA.

We mostly worked with 2024 - T3 and 7075 grades of aluminum with occasional use of 6061. 5052 was reserved for tool making. It was certainly strong enough for that and we made some pretty big tools.

Anyway I diverge. I look forward to getting a marine perspective on fuel tanks because my personal boating experience is nil. Thank you again.
Carl
a.k.a. Clipper

Crafting a classically styled Vera Cruise named "Some Other Time"

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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Wed Jan 06, 2016 7:59 pm

mrintense,
Thank you for the note of appreciation with your kind words, always good to hear that something you say helps someone else to expand their own understanding.

IT always amazes me the level of research, testing and engineering work that has gone into the 8 or 9 series of aluminum alloys. I call it the Miracle Metal because that was the term used to describe aluminum alloys when they first appeared, I think in the 1890's or so? Probably one of the most researched and tested metals in use in the world?

Most of my posts will be based on my experience building welded boats, countless tanks, brackets and different gear for commercial fishing employing a 'learning as you go' or school of hard knocks method of education! It would be great to hear from the other Glen-L'ers who have done other projects so we'd be able to provide the widest scope of knowledge possible.

Thanks again, Carl, please post with questions or requests for clarifications if my series here is not clear? If your goal is to contribute to others' understanding and then readers won't ask questions.. well- there' s a gap in the feedback loop if I don't get to try and make clear what I've left vague or confusing.

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Re: Metal Marine Tanks (mainly welded aluminum)

Postby jamundsen » Wed Jan 06, 2016 8:04 pm

great info
Thanks Kevin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Thu Jan 07, 2016 1:06 pm

jamundsen, you're welcome, feel free to raise any questions about my posts that need correction of seem too vague?

Looking at one source of aluminum corrosion from several points of view.

Metals corrode in general areas or in small sites called pits by the same basic chemical action. I’m not going to get too techie, too far into chemical engineering here- but there are three main ways aluminum corrodes and they are: by different metals contact when both are wet; by acids or bases that are too strong; and by electrical energy that results in the same basic chemical activity at the molecular level. All three methods result in the same basic metal chemistry, the original alloy of aluminum is reduced to non-alloy and the molecules of metal are removed from the parent material.

All three are different paths to corrosion- but they result in the deterioration of aluminum alloys. Metal to Metal corrosion is called Galvanic corrosion; Acids and bases can corrode due to strong Chemical bonds robbing aluminum of its protective oxide (aluminum and oxygen combined) film and if aluminum is allowed to have a charge from a battery, for example, and get wet… then aluminum molecules can go into the water or whatever is wetting the metal from Stray (electrical) Currents.

So Galvanic, Chemical and Stray Current corrosion are all types of aluminum corrosion but when you research and explore you’ll find they all amount to the (molecular level) same event: aluminum molecules are removed from the parent metal into the wetting agent (salt water) or electrolyte. An electrolyte is just a conductive liquid that allows metal molecules to move from one metal structure to another, or to decompose as metal into non-metallic compounds.

What does this have to do with aluminum tanks? All three variations of metal corrosion can happen to an aluminum tank and all three can be avoided or mitigated to such a degree as not to affect a tank.

Let’s take the chemical or acid attack first, because 99.999% of people don’t seem to get this aspect of aluminum maintenance. Very strong acids or very strong bases (https://en.wikipedia.org/wiki/PH) are chemicals that have what we usually call ‘reactivity’; these chemicals will react with metals and aluminum is no exception. Strong acid or base will react with aluminum and ‘seize’ the molecules of the metal and convert them to other molecules hence the description reactivity – this could be called ‘UN-refining metal’ since the strong chemicals’ actions on the aluminum do not leave the original alloy when the reaction is done.

Now, this acid to aluminum reaction may be very fast due to “strong” (read article on Ph) or very mild and slow acting due to ‘mild’ or less active acids: BUTTTTTT the acid or base beyond certain ph limits will remove aluminum’s healing oxide layer and then ‘react’ down into the metal in a pit. This action is called pitting but the term is not limited to acid or chemical corrosion. Further; this reaction, especially the slow version can happen as a ‘wetting cycle’ over and over again, once started.

While we’re exploring acids let’s look at water, yep plain ole everyday tap water. Tap water is more or less neutral ph, like sea water is in terms of acidic or basic ph. However, if water loses its naturally occurring (entrained or mixed in) oxygen – then it becomes deaerated- where deaerated is “oxygen starved” water and ends up acting as acidic. Where does this happen? When water is trapped between two pieces of metal, like two pieces of aluminum- the water initially gives up its oxygen to the aluminum to form more oxide!

This is called thin film stagnation which essentially means the thickness of the layer of water is thinner than a hair, and once in that thin film, the stagnation or chemical changes happen much easier, faster and results in a stronger metal reaction than in a bucket full of the same water.

Wait! Hey! aluminum oxide is good stuff, it heals aluminum so that’s all good right? Well- it’s fine for a while- but the acidic action of that water then takes back the oxygen and the aluminum oxide is dismantled. There goes the protective film. As a result, pure aluminum is exposed to water that is slightly acidic-acting and with the oxide is stripped off the parent metal begins to corrode! This event – called crevice corrosion or poultice corrosion begins a cycle that will corrode aluminum which can’t get "Free Oxygen" to form aluminum oxide to ‘heal’ the surface and protect the base metal from corrosion!!!

Stagnate water will continue this corrosion cycle but fresh water rinses, with the free oxygen will often stop this cycle!

To fully grasp this series of events chemically, you’ll have to spend some serious reading time, a little head scratching time and some review time… but the result is this; unless aluminum can get oxygen to renew its oxide film- the metal can corrode from chemical agents.

A Solution? Give aluminum oxygen and it will heal itself- starve aluminum of oxygen and it can lose its oxide film and begin to corrode.

Next will be my rant portion of this post.

The fact is that aluminum mill scale, if not removed from the material will promote this type of corrosion. I realize that hundreds of tanks, just as many whole boats and countless other welded aluminum products do NOT HAVE the mill scale removed(!) but that doesn’t change the fact that this film will retain water, the water will become acidic and the metal will corrode if the mill scale is not removed. Mill scale, that shiny chrome looking film is porous, retains water VAPOR and is not galvanically the same as the underlying aluminum!!! How’s that for a built-in problem?

But then anyone who’s worked in steel knows the same is true of steel mill scale coatings. Steel mill scale will promote rust by holding water film and vapor, aluminum isn't much different except that if the mill scale is removed the metal will 'heal' but steel will continue to oxidize, but not a fast.

Image
Here is an example of corrosion beginning to form UNDER a clear coating of lacquer of some type on a, almost new, name brand production boat!!! The mill scale appears to have been left intact by this widely recognized name brand builder!!! (?) The owner used paint stripper, then acid to and finally some mechanical scrubbing with the 2nd coating of acid etch (!) to get the metal cleaned and corrosion site free.

First step to protecting a tank, clean the mill scale, inside an out with an acid wash (that is diluted and neutralized when the etch is done) or mechanical abrasion (more this later) so the aluminum has the best chance to form its aluminum oxide, self-healing film to protect from at least a couple of the corrosion's sources that may deteriorate your tank. Even a dry tank with mill scale has a layer of ambient water vapor retained in the porous film- therefore what may appear as a dry tank installation can begin to corrode due to the water held in the mill scale. That is what the tiny white ‘flowers’ are in the inside of boats, on tanks, and even on exposed topsides of some aluminum boats are. The beginning of mill scale supported crevice corrosion pit sites, from a vapor film of de-aerated water's action working on the oxide film under the mill scale.

Image
This 2nd photo, of this post, above, shows two tank top sheets, the left one is cleaned of mill scale, the right is not cleaned -yet.

Image
This photo of the same to sheets of 5086 x 0.187" (3/16") material shows another image of the surface comparison, the sheet on the right is mill scale as supplied by the vendor.

Image
This photo of the bottom baffles of a box-like tank show the surfaces of all materials in the construction have been brushed to remove the mill scale prior to tack up and assembly.

Once this tank was finished welded, inside and out, the tank(s) were rinsed/acid etched inside and out. The acid was introduced into the tank and the tank rolled around to expose all interior surfaces to the etching solution. The the acid was diluted until the rinse water was 7ph (neutralized) and the rinse was continued for 30 more minutes. Then the tank was air dried over nite and acetone was introduced to rinse the welding smut dust out and any remaining water.

The tanks were then left to dry for a few days, and finally etched outside and primer coated then painted to mount in the bilge of a glass fiber boat.

More details on this tank build as we progress. Next, let's go back to look at the other two main corrosion causes.

I hope this shows that cleaning the mill scale, and leaving bare is a good tank building step. However, anodizing, powder coating, and painting will all produce a mechanical or physical film barrier to crevice or poultice corrosion too. Not that these added layers are impervious to attack by corroding agents- they all can be overcome- but they do add another layer of defense from corrosion on the outside of the tanks. Most often hydrocarbons (gas or diesel) will provide a source oxygen and help to keep the oxide inside the tanks intact.

However when we discuss water bottoms, residual water at the bottom of the tank, we'll explore the fact this water corrodes tanks from the inside worse than most exterior surface corrosion so getting that condensate or impurity out of marine tanks is important and should be part of regularly maintenance.

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

Postby Kevin Morin » Fri Jan 08, 2016 12:05 am

Turning off the mill scale rant mode and going back to less contested areas of welded aluminum tanks- lets take a look a the other two main types of corrosion (decay/deterioration/damage) of 5052 or 5086 marine alloys that will commonly be used in plate built fuel tanks in our boats.

Marine alloy aluminum does not corrode in salt water; by itself. If bare marine alloy aluminum is left to sit in salt water without other metals near, the aluminum will last indefinitely, it is only the presence of different metals combined with salt water that begin to break down aluminum’s oxide and promote further corrosion. Galvanic corrosion is the term for this action because of the 'spread' on the Galvanic Scale of the two metals.

Both fresh and salt water are the main agents of aluminum corrosion, not the sole causes, but the agents that enhance the process as both stagnate water (mild acid) and as an electrolyte (liquid "wire"). Water and many other liquids can help break down aluminum oxide’s film or armor and expose the metal beneath to further corrosion reactions. (dead horse)

Metals all have different arrangements of atoms so they all have different amounts of electrical potential (Galvanic Potential) Lead and lead dioxide have enough ‘potential' that when the two metals are ‘wetted’ with a liquid (acid) that allows the electrons to flow; they begin reacting, which is a corrosion of one metal by the other. So a battery is just Galvanic corrosion inside a plastic box! The result is energy or voltage at the terminals but... inside one metal is getting corroded by the other with the liquid wire of an electrolyte to allow the electrons to flow- current.

If you build a welded aluminum tank and install brass fuel hose and plumbing fittings you have made a battery- it may hold fuel, and even last for a while but essentially you've created a copper alloy to aluminum alloy several volt battery at the thread fittings.

Don't do that. Do not put copper fittings in your aluminum tanks. SS is a steel alloy that has Chrome added and it does have huge Galvanic difference from aluminum BUTTTTT the surface of Stainless Steel (always try for 316L if possible) can be oxidized, like aluminum to minimize the battery formed by touching these two metals- most other metals do not have this surface oxidizing property like 316L alloy Stainless Steel.

Most tanks are connected to a; Fill, Vent, Draw, Bottom Sump, Tank Level and diesel tanks may have a fuel return fitting as well. That means there are several fittings implied. It is common to weld into the aluminum alloy tanks threaded female National Pipe Thread (thread pattern) fittings in order to buy from the widest available selection of hose and tube connection fittings. These aluminum threaded fittings are VERY, VERY (did I mention VERY ) fragile in the threaded areas- so a brass fitting will begin to corrode the female threads in a tank fitting in a very short time- even if thread paste is used.

VERY IMPORTANT POINT!!! Please don't miss this point. Aluminum threaded pipe couplings can be purchased, cut into halves and used to weld to the tank walls and pipes inside the tanks to accept SS fittings. HOWEVER... aluminum threads are very fragile and they may gall or fuse to the fittings installed in some cases.

What you need to do... buy the couplers, fit steel or SS pipe into the couplers- slice them on the band saw (technique will come later) and make sure you follow these steps.
#1 use a steel pipe of the same size, put VALVE grinding compound on the pipe's treads and
#2 hand tighten the coupler or half couple onto the steel pipe with the valve grinding compound on the male pipe threads
#3 tighten ONLY enough to get 'handy' not bound up
#4 release and repeat, 10x

What you're doing is 'lapping' the interior of the aluminum with the valve grinding compound and 'dressing the threads' of any cutting burrs- that are VERY common in threaded aluminum. When this operation is done and before the aluminum threaded pieces are welded to the tank or pipes, put all the couples in a bowl/bath/bucket of solvent and rinse the grinding compound out!

IF you'd like to skip this step, fine, buy four times the number of fittings you plan to need and you'll be fine.

Back to galvanic or dissimilar metal corrosion in marine alloy tanks. If you keep the fittings to aluminum and SS, and try your utmost to afford the 316L (expensive) fittings... then there are only a few more steps to insure your tanks won't corrode from dissimilar metals.

First is to "passivate" all the SS fittings once they are cleaned and deburred. All this means it to make the surface of the SS fittings passive. To do this buy a passivating gel and soak your parts. http://www.delstar.com/passivating.html http://www.mmsonline.com/articles/how-t ... teel-parts When the parts are cleaned and deburred, and passivated the SS will be as stable from crevice corrosion and galvanic damage of the aluminum as you can make them. (Well I guess you could have them platinum plated, but that's a bit more costly.)

Now the last is goop, thread paste, sealants, or some thread lubricant and isolation compound. There are many good products, and fuel will wash most of them out of threads so I'm not going to go into this subject too far. Stainless to Aluminum threads are a HUGE, MAJOR, Big, and inconvenient pain in the stern. They're fragile, create a battery of the two metals, but are super critical to safety and boating reliability. Lots of products out there- I've used my share but there are few really miracle products.

I've used http://tefgel.com/ and tape like this http://cleanfit.com/stainless_steel_pip ... 0891.shtml and the Glue Type sealants https://www.dultmeier.com/products/0.1158.1923/9336 and there are quite a few products!!! so I'd suggest testing, researching and asking others what they've used too http://www.rectorseal.com/index.php/pip ... ant-chart/

What about Galvanic Corrosion of aluminum tanks? Keep different metals apart and if they have to be in contact- keep them dry of water and gooped-up with some plastic film - but NEVER use a conductive paste like graphite thread paste! We'll look more at the different metals when it comes time to mount the tanks using non-aluminum thread fasteners.

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

Postby Kevin Morin » Fri Jan 08, 2016 5:50 pm

This surely is one talkative Forum!!! Please let me know if I'm providing useful rules of thumb or if the terms or discussions aren't making any sense? I'm used to living with all this information as the basis for building welded boats and not sure if my explanations are clearing fog, providing clarity to these issues? Hope so!

We've discussed aluminum alloy corrosion as regards chemical attack in crevice or poultice corrosion action removing molecules of metal once the oxide film is breached, and we explored a similar action started by different metals' 'battery action' eating at the aluminum. When oxygen is allowed to from a very thin film of a form of corrosion with the surface of aluminum alloys; a protective film, called aluminum oxide forms in a few seconds and then a balance of outside oxygen and film/oxide occurs that STOPS the corrosion from continuing. However, there are several ways to penetrate this oxide film and then set of continuing reactions with the aluminum under that film- and our job is to understand how to protect the oxide film to keep it intact and to stop and ongoing corrosion action that would happen if 'cells' of reaction were to continue without the protective oxide film.

Dry it off, give the metal oxygen, and keep that film that forms from being breached and aluminum will last a long time- failure to do these steps will result in the ongoing corrosion and metal failure.

Now. we need to review batteries themselves a bit- to recognize they're just contained corrosion inside a box and they produce electron flow (current or capital I in the symbols library) which can be a different (potentials/pressure in electrical terms and V for voltage in electrical symbols terms).

Batteries have two different metals (galvanically different potentials) which are covered inside a non-conductive plastic box by electrolyte or 'liquid wire' in this case - an acid.

We could say that if the battery's potential is not returned, via low resistance copper wire conductors, to the Batt. Neg from the Batt Pos- the hull could be used as a conductor- but that would alloy the metal of the hull to flow electrons out of the hull into the water! The hull would become corroded by the current flowing from the battery into the surrounding electrolyte (water) and metal would transfer from the hull to the water.

This is like electroplating! except there is no separate piece of metal to collect all the aluminum alloy molecules onto the surface to be plated. All stray current corrosion amounts to is one side of the electroplating cell. The dissolving side!

What does this have to do with welded aluminum tanks? Most built in tanks have a level sensor that connects to the dash or helm instruments to show the amount of fuel in the tank- these devices are typically DC powered so they could use the tank as the DC return or Neg leg of a circuit with the battery powered DC system.

Let's agree not to use the term GROUND when discussing welding or marine DC systems. This term is not true on a boat- boats float or they sit on trailers so... they don't have grounds- we should avoid the term as confusing with shore side grounds in the earth. The DC system (ignoring AC for this discussion) have a DC Plus and a DC Neg side to the circuit. The DC Plus is potential from the battery to the load, and the DC Negative is the same circuit from the Load to the Battery's Neg potential.

Let's not include any confusing terms- OK? DC Neg remains charged (potential) to 'earth-ground' it can destroy a metal boat, (all metal all boats) if the DC power system uses the hull metal as the DC Neg or DC return to Battery leg of a circuit because; that DC Neg will flow from the load through the hull to the water- easier than it will to the DC Neg of the battery. So you've turned the entire hull into 1/2 of an electroplating operation if you don't provide full, two wire, low resistance, zero Ohm termination conductors from the battery buss back that same buss.

A single wire, DC Plus, sending unit powered by on board DC may use the hull as DC Neg and 'read' at the level indicator on the helm instrument cluster..... but that will result in pitting any bare metal of the hull in water, it will destroy bottom paint protective films, it will lift and destroy any other defensive films that are submerged and is generally not acceptable boat construction.

How can tanks be corroded by stray current? If the wiring to the tank is not a low resistance, two wire, circuit then you risk using the tank as a sacrificial element in a quasi plating operation and that will result in tank material pitting, perhaps even fuel leaks.

https://en.wikipedia.org/wiki/Electroplating
http://www.finishing.com/faqs/howworks.html
http://science.howstuffworks.com/electr ... g-info.htm

This was the easiest of the three corrosion methods to cure/mitigate/eliminate/control BUT... we didn't discuss bonding of the tanks or the topic of bonding in general. This issue regarding electrically correct design and installation will come as we get into correct installation in future posts.

Next, we'll list the various elements of a marine tank for design purposes.

tap, tap, tap... is this thing on? Hellllll-low..... testing, one! two! three.... tap, tap .... (mike check)

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Re: Metal Marine Tanks (mainly welded aluminum)

Postby jamundsen » Sat Jan 09, 2016 7:04 am

Kevin
Keep this info coming.
So... Now I need to acid etch the insides of my new tank as well as the outside. How about alodine on the outside as well prior to epoxy primer?
John Amundsen
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Sat Jan 09, 2016 12:28 pm

jamundson,

Yes, etch the entire tank, inside and out, and allodyne should be used prior to primer, but there is a type of primer that is supposed to etch the metal as it is applied(?) MY experience with that self-etching primer is very limited and I prefer to;

#1 acid etch, ( I use Zep-a-Lume from the Zep Chemical Co. which I understand is not as easy to buy in some states? as it is here?)
#2 allow to foam and etch (when foaming stops or 5 minutes passes)
#3 rinse with fresh water hose for 10 minutes or more keeping the entire tank wet (outside)
#4 while the metal is wet from the etch no oxide will form
#5 apply allodyne (zinc chromate solution) to the wet metal and let dry.
#6 the test for a Chromium Oxide film is color- the entire tank should be yellowish/greenish/golden hues.
#7 if the tank has blue or blueish areas, then that should be redone from the acid step on- to get off the failed bond of chrome oxide.

#8 I'd etch inside the tank as a separate operation because you have to roll the tank around so much its not very easy to do both inside and outside at one time.

Incidentally, etching the tank's panels or plates/parts could be done before welding, after cutting and fitting all pieces. This could allow all the sloppy acid and rinse work processes of the strong, commercial grade and potentially dangerous acid; to be done on more compact pieces instead of an entire tank assembled. Then, when it's time to add the allodyne only a very weak solution of acid, hardly stronger than vinegar could be used to lift the new/thin/recent aluminum oxide film to give the allodyne's chromium 'salts' a clean surface to bond too.

Even if you choose not to etch the inside of the tank, at least the VERY least rinse (flush) it with acetone/gasoline/diesel!

How I learned that welding soot is very fine powder. I built a set of tanks for a boat that was being completed by her owner/builder in wood and glass and they were some of my first tanks about 75 gallon each. This small commercial fishing boat 28' (most of fleet is 36 to 40') has the tanks installed under the after deck and plumbed by the owner. The tanks were not etched or rinsed with acetone when complete. They were simply cut, fit, weld seams cleaned for MIG inside and TIG outside and welded completely inside and out.

The tops were single sided welds, but the bottoms, ends, and all interior welds were MIG inside and TIG outside but the MIG soot was not fully removed more than using an air hose to dust out the tanks.

The owner offered me a ride on his launch day, and I accepted and we left the dock after idling the engine for a bit, all systems seemed to be running fine so we headed out of the Kenai River into a small swell and followed the channel markers out a mile or so when the engine just quit. The tide was flooding so we have an immediate 3knot stream pushing up the mile back to the beach, with no engine.

I lifted the after deck hatch to make sure there was no fuel in the bilge, none; then I went to the gasoline engine where the skipper had the flame arrestor off and we cranked the engine. Nothing. The carb was dry! So we went to the Racor filter and the bowl was black! We dropped the bowl (back then you could use a fuel proof plastic bowl for gas and not all metal filters) and put the filter in a bucket, slipped on the new filter element, and wiped the bowl.

By now the surfs just a few hundred feet from the boat as we're broach and rolling onto the beach in shallow water... sealing up the filter the skipper fired up the engine and we headed back into the Kenai to the dock. I learned that cleaning tanks of welding soot is important by seeing the entirely coated filter element. When it dried out the soot would dissipate with a blown breath on the paper cartridge. The layer was so fine the dust was just a wisp but.... it was plenty to completely seal off the fuel flow !

So that is why I have rinsed every tank I've built with MIG- after that embarrassing experience. An acid etch of the tank's inside surfaces will do both jobs; remove MIG soot and take off mil scale and that seems to be less work than doing each job separately. I follow the water rinse with acetone just to make sure I've got the an foreign oil traces out too, even though I realize that hydrocarbons would be dissolved by the fuel. You could also use an alcohol rinse of the type of fuel/water dehydration or gas line antifreeze as this would form a solution with the water to remove it as well.

Note the obvious... ALL WELDING, ALL TESTING, all work on the tanks should be done before you begin the chemical work of any kind. No work should be done on a tank that involves ignitions sources when hyrdocarbons have been inside the tank.

Image
This scan of an old photograph shows your author as a young welder who survived (thank the Lord) the explosion of the tank in the foreground. The tank was emptied, rinsed, (I thought thoroughly) washed with soap and water, rinsed out, dried out and flooded with argon for 1/2 hour at a good flow rate, and when I lit up on the repair/remodel portion of the tank- it exploded while i was laying on it.

For those who are curious the tank top with me laying partly on it torn loose from the top TIG weld along the HAZ edge just inside the top panel's edges all around. Most of the keyhole welds on the top torn out the entire 1" to 2" round area of the top and most of the baffles and ends stayed welded, the shape of the tank was a long rectangle which is shown here (above) as folded up at the ends. I'm posing as the great white hunter type of picture with my trophy as a reminder to myself and others about being careful in tank repair/remodel prep.

My advice: just don't do it.

Please adopt this rule of thumb "Do NOT weld on tanks that have been filled with fuel, especially gasoline." I recommend that you leave that work for others, who, like me- are less intelligent than you! someone could get hurt.

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

Postby Kevin Morin » Sun Jan 10, 2016 10:35 pm

I'm going to use a box of flat plates for this example to introduce the first fundamentals of marine tanks, IMO. As we move further in this exploration I'll try to review bending which is very profitable in reducing weld seams (labor) and I'll look at some other shapes- it's pretty rare we get to build a cube for a marine tank.

The colors in these next few illustrations, as usually is my method, are to separate pieces since grey rectangles get a little confusing with we zoom in for detailed views. If this method causes confusion instead of reduces the same? PLEase let me know?? I've not labeled each item in some of the basic sketches but I could if that's necessary- again, please let me know?

My purpose in this post is to introduce just the fuel "plumbing parts"; we'll discuss mounting and level sensors separately, we don't need to show them here.

Image
First sketch above is a tank of plate, with some cutaways to view inside and see the items we're introducing.
The larger vertical pipe is the fill or downcomer if you're a pipe tradesman, the closest small diameter pipe is the draw, and the hot pink pipe is the water bottoms sump draw or pump out line. I used hot pink because this is an optional design element as we'll discuss below. There is a small fitting (gold) to the right that has no internal piping and will be connected to the tank vent system.

Image
Above, this image just zooms closer to the tank bottom so we can see the pipes/tubes as the near the bottom of the tank. The fill and the draw do not touch completely and the sump draw is inside the recessed pipe cup used to collect water bottoms.

Image
Last image in this intro shows that the fill and draw are not completely at the tank's bottom and the sump draw is close or at the sump bottom, NOTE some 'water' is shown in the sump for clarity of its purpose.

Discussing these items; Fill, Draw, Water bottoms Draw and Vent.

All the items shown above are just positioned to make them visible, there is no relationship to function shown- yet. These items are purely for discussion of tanks in theory, not a given tank plan.

The reason marine tanks have down comers is to introduce the in rushing fuel liquid to the low point in the tank, this allows the fuel to fill in a 1-1/2" or 2" pipe and not to froth or foam as much, this makes filling a deep tank much easier and allows the vent system, once the level covers the bottom of the fill/downcomer to handle all the gases coming off the top if the rising fuel from inside the tank AND keeps the fuel from venting and spewing up the fill pipe. Once the fill pipe's lower edge is covered- there is no back venting up the fill hose or pipe to spray fuel.

I show a pipe with a 1/2 pipe coupler welded at the top of the tank, this allows thread adapters to hose (king nipples) or other thread fittings to adapt to the fill hose or piping. This method will be typical of almost all welded aluminum tanks some female threaded fitting of aluminum welded to the tank to allow a SS or other fitting to be threaded to the tank and convert to machined fittings from the welded tank.

Fillers of 1-1/2" for gasoline are usually fine UNLESS your boating is in an area where you fuel up at a commercial fuel dock! If you fuel at commercial docks - you may find the hoses are larger and have a rate of fill that is much higher than a service station or gas station in your neighborhood. BEWARE... if you pull up to a commercial dock with small fill pipes and hose? you may have gasoline all over the decks and you WILL hear sailor-like language from the commercial users, things might be said about your mother that are not true!

Draws
Pipe or tube can be used with a thread fitting on top, just a small diameter version of the fill downcomer. Pipe and tube are measured differently one is a nomimal ID the other is a measured OD, and the alloys most commonly available of both product series differs quite a bit. I'll recommend you use pipe and make sure the alloy is 6061-T6 and that would be the same for all couplings as well.

The draw can be a diameter off or above the tank bottom but there are exceptions and we'll try to explore them as cases so you can choose how to design your tank. If you've taken a chain saw, gas powered weed whacker, or other small gasoline engine apart and noticed - the tank end of the gasline often has a small filter on a plastic tube as the draw? This arrangement can be used in marine fuel tanks to keep debris from being pulled up into the filter or fuel lines- the idea is that a plastic liner is installed in what I show as the green draw pipe to the left.

Now we'll need to take a detour on a branch of the tree, to discuss this topic of filtering and fuel handling. The reason to put this sub topic here is that we're between the two draws- one a water bottom draw the other the fuel draw and this sub-topic fits right between them.

As I view the fisherman, boat owner, builder and marine maintenance community there are two schools of thought regarding tank water bottoms. #1 leave the water in the tank and handle it separately, regularly, and with dedicated fittings and tank furniture. #2 , pull all the junk out with the fuel and separate everything in the fuel filters- that's what they're for.

So #1 leave contaminants in the tanks #2 pull them out with the fuel mixed together.

This is a decision you need to make for yourself. If you talk with an old time Navy hand you'll get some version of #1 and if you talk with a Racor salesman you'll hear that #2 is the 'only way to go'. Are modern filters able to handle a mixture of fuel and water- they say so, an there are lots of times we see water in the fuel bowl... so??? Is it best to get the water in a tank bottom into a sump that is specially built to hold that acidic water and pump that sump as part of boat maintenance? Well it does work to keep tanks 'dry' and all sorts of harbor grade fuel debris seems to be pulled out separately instead of sitting in the fuel filter or perhaps some part getting through to the engine?

I show a highly pink down comer with an identical fitting at the tank top and a sump that consists of a thick walled pipe section very shallow- less than an 1" deep- this is where you'd pump the sump or pull out water bottoms from a marine fuel tank. Water will collect in this little cup/sump/low point because it will be rolling around as droplets beneath the fuel and eventually find this point to sink down.

Water comes into the tank with the vents breathing in and out daily, in fact ever time the boat is in or out of the sun the tanks will vent out or drawn in due to the ambient temp changes so the air drawn in to a cool tank will condensate on the side walls- same in all tank materials. Water also comes in slugs from fuel docks, where the fuel storage and quality isn't always as good as you may find at the gas station. When the fuel stands in the tanks for a bit, water will settle out, with other debris and contaminants, and all this will collect in the sump- if you chose to use method #1 above.

When you fill a tank the fuel pours into the hose or deck fitting, usually, and down the fill pipe/downcomer to the bottom of the tank and initially some tank gases will flow back up around the fill nozzle/hose fitting but very quickly the bottom of the fill is covered with fuel sealing this from any gas escaping up the fill. From this point forward until the fill is complete all gas/fumes/vapors/air in the tank has to go up the vent fitting.

Current tank regulations (Coast Guard regs) require some separate study and we'll try to get that done clearly - but not here. The only aspect we need to consider now is the vent fitting(s) are on the top surface and have all their connections external and above the tank and none inside so the liquid could, technically come up to the top and continue to fill up the vent hose!

These are the main items of marine tanks' internal plumbing or piping and the main surface fittings needed to use the tank in routine operation, Fill, Draw and Vent- with the option to pull off water bottoms and clean the bottom of the tank with out opening an inspection hatches, none of which are shown or discussed at this level.

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

Postby mrintense » Mon Jan 11, 2016 3:31 am

Kevin,

You mention an either / or option for managing water / debris in the tank. In other words, using a sump or using a filter. It seems to me that there is no reason you could not use both as added insurance.

Coming from someone will no boating experience, these comments may seem stupid, but I genuinely do not know. I must assume then if one has the sump system, that as part of regular maintenance, some effort must be made to periodically pump out the excess water / debris.

How often might this be? Is there a special tool for this or is it something I should let the marina / mechanic handle?

For the filtering part of the process, the filters would obviously be somewhere where I could get at them, so changing any filters / emptying out a water bowl would certainly be within my realm of capabilities.

My particular boat is the Vera Cruise design, a 21 foot cabin cruiser using an outboard. I haven't decided yet if I want to go permanent aluminum tanks, although I am inclined to do so. But it also seems to me that portable polypropylene tanks might be okay as well since they could be periodically replaced if they become problematic, a much easier, and I suspect less expensive proposition than for aluminum tanks.

Thanks again for these wonderful articles that do much to explain the inner workings and considerations of fuel tanks.
Carl
a.k.a. Clipper

Crafting a classically styled Vera Cruise named "Some Other Time"

Clipper's Vera Cruise Build

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Re: Metal Marine Tanks (mainly welded aluminum)

Postby chugalug » Mon Jan 11, 2016 7:07 am

:D I like the idea of a water sump in your tank.
Last edited by chugalug on Mon Jan 11, 2016 9:58 pm, edited 1 time in total.
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Mon Jan 11, 2016 12:17 pm

mrintense, chugalug, sumps need to be tended or not bothered to build. The amount of time the tanks spend less than full combined with the local weather and humidity would show how often they needed pumped.

I don't find it stupid not to be exposed to facts related to a trade you've not pursued, (yet) and the saying goes that the only stupid question is one not asked. Online resources being what they are today, its possible some questions might be answered by wider searching but in a Forum setting, like we are here at Glen-L.com, I think all questions are valid during the topic's course because not only will they help me to fill in what I've failed to state, but they help others with the same question. The only place Forum questioning seems to presume on the poster/Mod.s/members/readers is if the thread has been done for a couple or years and a new poster joins and their first question is "I've heard this term 'sump' and I wondered if anyone here can tell me what that means" In cases like that it seems reasonable the new poster would consider reading the site more thoroughly incorporating the search function in choosing topics to explore...? So I don't find it stupid at all, the questions you're posing are completely logical and helpful- thanks.

http://www.walmart.com/ip/Oil-Change-Pump-Kit/11071257 this type of fuel proof hand pump will do the job of cleaning a water bottoms sump, just pull on the sump until a stroke of the pump brings up clean gasoline and wait (do the other tank) then go back for one more stroke to confirm the sump is 'dry' of contaminants and has only gas is the usual routine.

mrintense, using the single draw down in the sump into a filter is essentially the same as #2 method of relying on the filter since you're electing to let the filtration handle the separation. It would work fine, then the regular maintenance would focus more on the filter than the tank sump. As chugalug mentions, sometimes there are conditions where what seems like and unusually large volume of 'junk' comes out of the tank- so the only caution is to make sure you have two filters with swap valves so you can deal with any 'slugs' while you're running. Your idea would create the 'self tending' sump, in a manner of speaking, where the tending is transferred up to the fuel filter.

If a boat's cruising involves long runs, like many boats here in South Central Alaska do, it's common for these boats to carry volumes of fuel that vastly exceed the capacity that came with the boat, unless built in the PNW. For example, on particular river moose hunt on the Yukon & Koyukuk takes more than a thousand miles of motoring. In that case even 20-22' river type boats try to carry more than 200 gallons- and one 22' I worked on had two 150 gal. tanks - took up the entire cockpit!! In saltwater boats that cruise Prince Williams Sound, a 22' full cabin model looks to have 200 or more gallons when they leave the dock. Lots of these tanks are bunks, stowage lockers, seating and other living surfaces and built to do more than just hold fuel.

With that kind of contrast to stateside cruising where there are gas docks fairly close together, the comparison of tanks can be related to the cruise and course. If you're in a lake or river system with docks every few miles, or in coastal waters staying fairly close in? small volume removable tanks may work fine for your boat. So the case for one material or the other, and the case for size, portability and even location in the hull; all hinge to some degree on the volume of fuel required in planning.

I'm not putting a size on tanks for portability and as long as plastic tanks are kept out of the sun they seem to do fine, and as long as the stowed location can accommodate the stock sizes then metal tanks' main beneficial reasons may not apply to your boat? Building your own tank in welded aluminum provides a freedom of design, flexibility of location, and exact detailing of the tank's connections that result in the 'perfect' tank for your use.

In regard costs; please recall this entire series of posts in this thread are focused on explaining how anyone that can build a wood/glass boat can have a huge savings in cost (to their tanks) if you do all the planning, cutting, fitting, prepping and simply have the tank welded for you. The percentage of effort will be roughly 95%-5% and depending on the aluminum welder's skills and tools that could be 98%-2% where "all" the work is in cut fit and prep- tack and weld is nearly free by percent of work. Needless to say, this presumes the local presence of someone equipped and experienced welding aluminum - tanks included.

This topic would have to be read in combination with the 'Methods' thread running near it in order to have all the metal working details. Here, we're focusing narrowly on tanks but the metal working details remain as they are presented in the other thread's posts.

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

Postby Kevin Morin » Mon Jan 11, 2016 9:40 pm

Please recall that I'm writing for those who will not do their welding but that doesn't mean you can do these types of projects without knowing who and how the tank will be welded. The design of the tank will be closely associated with the who and how of the welding and that is what this post will explore.

First let's do the easy part, I've welded along time, and can still manage a decent bead so I will use my own welds as your visual standards to point out some points you can use for comparison. My purpose here is not to create ill will between you and your welder, if he can't weld as well as I can- that's not a deal breaker, if he's better than me- GREAT! What is important is to find a series of inspection features for you regarding aluminum welding which allow you to judge acceptable welds but more importantly to reject any welder who will perform poor welds and then accept them for sale to customers. That IS a deal breaker, these guys should be avoided, as they're not going to do the work you need to build a safe, long lasting marine tank.

Most all shops that are in business have already had a volume of work go out the door, so, it stands to reason their welds have held on, and the welders there are proud of their work so they won't accept poor welds or try to sell that level of weld to a client. What you need to avoid is someone who can't weld or is so uniformed they say "That guy is full of malarkey, he doesn't know what he's talking about" (I'm not full and I do know) and then try to substitute the standards I'm going to suggest for something less meaning your tank work could be in question?

If you don't know the difference between MIG and TIG welding, you should spend time researching the web for clear explanations. I'll just give some short phrases (for me, these are short!) for both definitions; TIG, or tungsten inert gas, uses an electrode of unconsumed tungsten to conduct the weld arc to melt (weld) the aluminum- and a separate filler rod of a specific alloy of aluminum is added to the wetted/molten/weld puddle independent of the electrode. (this is similar to gas welding with an electric torch instead of a gas torch if that helps paint a picture?) This method of welding requires the highest skill level of all electric arc welding methods and provides the purest and most controllable weld for aluminum, however the rate of welding is far slower than MIG.

MIG stands for metal inert gas welding, where a continuously fed wire is both the electrode and the filler in the same aluminum wire. This method is much easier to perform, much faster to put down but it is much less pure and in fact is naturally porous to some extent. This method is like stick welding with a stick that is a mile long and fed off a rolled up coil so it hardly runs out.

TIG is slower, higher purity and most controllable; MIG is faster, less pure, and is most useful and suited to long seams, as opposed to welding small diameter couples to tank tops, where constant direction change and the fast speed combine to make MIG mode VERY hard to get round small fittings but TIG performs those welds with ease.

However, a very qualifying test point when shopping for welding services; is- a shop, or individual welder, who uses both methods and will agree to provide the long seams inside MIG welded and all outside seams TIG welded. ON the other hand, as we'll see there are methods to build tanks using MIG inside and outside that work just fine.

At this point, if the two welding methods are not clear, go to Utube and look for Jodie's videos at Welding Tips and Tricks. com and watch him TIG and MIG aluminum and you'll see the tools, torches and power supplies required. I'm not suggesting tank building is worth buying these tools, I'm advocating you consider learning the differences so my remarks and drawings will have more meaning when you read on.

Image
Above is an image of three plates coming together in a fit called "inside edge to inside edge" so the Vixen filed or belt sanded edges of these three tanks sides/plates are held their adjacent part along the sharp edge of the outer edge of each part. This in ONLY ONE.... method of tank weld seam preparation but is introduced to discuss TIG welding of tanks- first.

This type of joint makes a great TIG joint because the weld fillet V of the plate/sheet edges is just about perfect for putting in uniform and clean, high purity TIG welds. IN fact this kind of welding is quite recreational, I've done it since the 1970's and it still makes me smile- its fun.

Almost all the inside seams of a tank with seams like this would be MIG welded inside, sometimes call back welded, and then the groove would be restored prior to putting the outside TIG weld on, with the exception of the top panel's seams.

Image
This photo a sump cup type of assembly shows a uniform TIG weld that has most of the attributes you're looking for. #1 the welds are uniform not irregular, #2 the welds tie in where the start and stop are uniform too, there is no crater or recessed dimple at the beginning and end of these welds, and #3 the welds are closely proportional to the two adjacent surfaces, the welds don't bulge over those references too much, and last #4 the edges are smoothly fused into the parent material.

Image
Tank tops can't be welded inside, unless that tank is big enough to crawl inside and we're not discussing that size! Here a tank top TIG weld is shown and the center of the weld has a keyhole which is correct. The weld is back welding itself all along the seam and we know this because the keyhole is the condition of the molten puddle flowing into the inner tank (back weld area) and producing a small even inside bead. This indicates there is full penetration and is the correct method for this weld joint.

Image
This photo above shows a close up of the previous photo but is in poor focus however the keyhole feature does show up. This is the correct tank top TIG weld and is achieved by tanking the top on with a very small gap (welded makes this call) and then welding to carrying the keyhole.

Image
In the photo above two fill downcomers (pipes that will be inside the tanks) are being welded to half couplers prior to installation. The weld on the left is in scale to the fillet, is reasonably uniform, starts and stops are not cratered and the edges conform to the two parent metal adjacent surfaces.

Next are MIG welds and again the key elements you're looking for as a non-welder are the same points in TIG but there can be an exception in MIG to appearance. MIG welds can be stepped or patterned but they could also be smoother - drag beads. (we're not going into the term drag bead in aluminum !!!) We could say patterned versus machine bead if that will help separate the two, once you begin to discus weld beads work methods there can be terms or description confusion I'd like to avoid.

Image
The photo above shows a drag or machine type weld bead where the MIG torch was just moved along the weld zone for the molten wire droplets filled the weld zone. The left end of the weld shows the weld was doubled back so the crater was on top of the main weld, and the right side shows the start, a common flaw area of MIG was doubled back as well, reduce cold start or cold lapping also a potential problem with aluminum MIG welds.

Image
Above is a photo of a typical patterned type of MIG weld in aluminum. The right side start area shows a wrap around start to avoid cold lapped beginning puddles and the same is true of the left side end of weld. The crater is relatively shallow and on top of the last puddle in the run of the weld. The top and toe of each puddle has fusion lines that flow smoothly into the surrounding metal and do not curl under- there is not bulging crown, most of the weld is slightly hollow or just about flat top to bottom, not too bulged.

MIG welds work great inside a tank if the sides are fit as the edges of the "inside edge to inside edge fit" shown above... BUT those types of outside weld grooves are harder to weld attractively with MIG. Another joint structure is commonly used but only if the forming equipment that is implied by these following images are available or afforded by the builder.

Lap joints are forms by the edge of one sheet or plate lapping the other plate, the difference in welding these joints is; the weld puddle almost cannot 'fall through' due to thin joint metal. INside edge to INside edge joints, if welded form the outside first, with MIG, or a top or bottom panel with no back weld, are much MUCH more difficult to achieve the needed quality compared to lap joints in the same location.

Image
Here, above, is a sketch of an tank panel with edges bent or press formed around the perimeter as a flange. If this is lapped inside OR outside a tank end, top, bottom touching the adjacent sides but not in an edge to edge fit; a MIG weld can be installed at very high wattage, resulting in a weld 2' long in seconds. The added amperage or power of the this type of joint being welded at very high speeds is a reduction in weld irregularities but it requires higher skill too.

The joint design also keeps weld heat and HAZ away from tank corners and is required by some classifications of tanks. The distance from the corner or bent area around the tank panel's perimeter of the weld; is specified in various Naval standards but for our discussion
#1 is there forming equipment in your working area?
#2 does you welder have adequate TIG skills to weld the tank or does he simply use a TIG torch to 'float out MIG leaks'?
#3 you're able to do the needed offset and thickness compensation needed to use this type of seam.

Notice the little crack lines at the joint of the folded tank end? That is a follow on discussion that comes in when we explore press braking preparation and planning.

Image
Just above is another lapped joint based tank construction weld joint detail. This time the body of the tank is made of one or two bend pieces with lapped closure, lower right corner and the end or top is a bent pan or flanged panel and the other set of joints are lapped as well.

IN these welds, we're presuming MIG, but this could all be done by TIG if needed, the shape deformity of welding this with TIG would probably be much higher, the tank may need extra internal furniture and structural pieces to remain uniform is this design joint is welded with TIG. HOWEVER, even MIG joints on this type would very commonly have all the starts or weld overlap "floated" with TIG to insure the faster process was not cold lapped where two welds begin and end together.

Image
The idea of outside fillets (first edge ot edge fit shown above) and the lapped fit are not the only weld joints that will work fine for tanks. This sketch shows a recessed end/side/top/bottom panel fit inside a bend tank side rectangle, with the top edge beveled so the weld will have a mechanical removed groove all around the edges so a in inside fillet, like those inside the tanks can be welded, but on the outside of the tank. The bevel is not shown all the way around the corner but that would be the method used to prepare this tank panel for welding.

ON the left is a fillet placed on top of the end panel, and that will seal but the better grade preparation is to provide the weld more root face to fuse into the the two adjacent parent metal edges.

Image
Last image in this series about tank joint design considerations; the recessed end/top/side panel (blue in the foreground) is beveled on the right side as the most preferred joint preparation- (this will need to be confirmed by testing with your welder for bevel angle and depth, as well as the depth of the panel's recess) but the weld to the left "Inside Fillet" on the unbeveled edge will work.

This post brought up reasons that you'd need to research and find a welding service, then working with them plan the joints to your tank project for the various mutually agreed techniques and methods of work, assembly and weld type and sequence. The primary considerations are - can 'he' weld? and two is the company willing to work with you- allowing you to plan, cut and bring them the tank for assembly at your design control? IF not, then you'll have to find another service to get this project done!

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin


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