Need Advice On Aluminum Alloy

A question on aluminum alloy as I am clueless when it comes to this. I tried doing a little research on it but didn't really get a definite answer. I'm fixing to pull the trigger on my casting deck extension. I am wanting it to be welded instead of riveted. My choices from the place I am getting the aluminum sheets are 5052 or 6061 aluminum. There is a $20 difference in the price. The deck will be 25" x 61" x 1/8". The 61" will taper down to 57", the shape of the bow. I will be putting a 1 1/2" to 2" 90 degree bend on both 61" and 57" ends of the sheet. I decided to use four 2" legs for support and they only come in 6063 alloy. So my two questions are:

1. 5052 or 6061 alloy for deck?
2. If I go with the 5052 alloy will there be any problems when welding it to the 6063 legs?

My gut is telling me I will be fine with the 5052 because most likely that is what my boat is made of.

Thank you.

5052 is far more corrosion resistant.

5052 is the better choice of the two, but the corrosion resistance isn't the greatest deciding factor. Strength losses in the HAZ (Heat Affected Zone) also play a role, among other considerations.

If you're concerned about corrosion resistance, you will need to remove all the mill finish from both sides of the sheet. The nice shiny, pretty stuff on brand new sheet aluminum is actually a product of manufacturing, and is not the same as the alloyed metal under it. As such, it has a slightly different galvanic potential to the metal underneath, and in an environment where water can become trapped, it will foster a corrosion cell quicker than the parent metal alone. This is why on nearly all of my work, you can see that the sheets have been sanded. A red surface conditioning pad (ScotchBrite), on a 4.5" or 7" grinder is the most cost effective method of removing this, though an acid wash will do it too. Furthermore, this needs to come off anyway, to better adhere whatever surface coating you have (carpet, paint, etc.).

Frankly, if I absolutely had to choose, I would choose a 'lesser' alloy that had properly been prepped, rather than a 'greater' alloy that had been left as delivered.

Just for the sake of discussion, 6061 extrusions are acceptable for underwater use, whereas 6063 isn't. And yes, 2" square is readily available in 6061. The interesting thing to point out here, is how even though the two alloys act differently when submerged, they weather very similarly while in air, and some of the other properties make it more desirable for above deck uses. So, if you were 'real' concerned, go with 6061. But, you'll save a little money with 6063, and in your application, will be absolutely fine.

No problem welding the two. 5356 would be the appropriate filler. In a case where both 4043 and 5356 are listed as acceptable, the properties of 5356 make it more suited for the boat use than 4043. Partially for corrosion resistance, and also for ductility.

Thank you very much for the great info guys.

dont sand the shiney off. No boat manufacturer does so. If your going to paint finish sanding is one thing. If this was the case imagine what tread plate would look like. If you guys want factual data I will provide it. I work for one of the largest steel suppliers in the country.

Bugpac said:

If this was the case imagine what tread plate would look like. .

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Mill finish is what I'm referring to. Tread Plate is available in mill finish and 'bright,' also known as 'tread brite,' though I think that may be one companies trade name (ie; Kleenex vs. facial tissue). Bright finish tread plate does not have the same surface coating as the mill finish sheet/plate, and is a completely different animal.

Here is some good reading. There's 135 pages, so I doubt anyone will read the full amount, but sections 3-5 are very informative on the rolling of aluminum sheet goods. Narrowing down further, section 5 explains baking on the coolant/lubricant that facilitates running the sheets through the rolls without sticking. This baked on lubricant is what the 5000 alloy sheets are shipped with.

https://www.aluminum.org/Content/Na...g_Aluminum_From_The_Mine_Through_The_Mill.pdf



To make it to where anyone can experiment and see with their own eyes, if one were to take a brand new piece of mill finish 5052 aluminum, and sand off the finish, no amount of sanding, finishing, polishing, etc, will take the aluminum back to the same finish the new piece was. You could polish it down to a mirror, but at no point along the way will it have the same color or reflective properties as the new sheet. It will, contrarily, have the same properties as polished 'tread brite.'

What that tells you, is that if there is no possible way to re-achieve that finish, that there is something different creating that finish - something that isn't down in the rest of the metal.

Bug, I'm sure in your job, you've done a fair amount of palletizing and depalletizing stacks of sheet aluminum. If so, you'll know that when stacked sheet to sheet, new mill finish aluminum is pretty slippery, especially when freshly stacked (possibly a slight amount of air between them). It is very easy to slide one sheet on top of the other, without much effort, and with minimal scratching to the sheet below. But, if you take a piece of sanded sheet, and even polish it down into the many thousand grits - certainly much smoother than the factory sheet, were you to analyze it with scientific tooling, it won't be as slick. In fact, it will often try and grab the sheet below and drag it as well. This is due to that lubricant.

The statement that no boat manufacture removes the mill finish isn't accurate either. It might be partially true if we are talking cheap press formed hulls (the Trackers, Lowes, G3s, etc. of the world). But, these are the same companies that still use pressure treated plywood for structural components, and don't bother with the expense of primer, instead stick paint right to bare aluminum. Or, pour flotation foam in a way that it doesn't allow for water to flow through the channels on the bottom of the hull. Not exactly the epitome of building things right.
Now, remember I said partially true, the hulls that get painted (and this includes the cheap production boats), get acid washed prior to painting. This acid bath does remove the mill finish, and gets the surface down to actual aluminum.

BUT, if we ignore the cheapest of the cheap boats, and start looking at workboat hulls designed for saltwater, you'll find nearly all of them are acid washed, including those sold bare. If you want to look at one, a customer of mine not too far up the road from you just purchased a LifeTyme hull - bare aluminum of course, and it was very clearly acid washed - not a spec of mill finish left on the hull.

And, don't listen to me. Read the link above. Or at least skim it - for those that have an interest in this sort of thing, it can be a good read. The next thing to read about would be galvanic differential, and poultice corrosion, and begin to see the correlation.

I just plan on roughing it up, primer and paint.

Acid wash and sandpaper aernt in the same zip code.

So to add to that. Imo it is a complete waste of time to sand sheet aluminum to glue carpet to. Millions of people everyday fabricate aluminum without doing this. If there was a need, we would have machines to remove this stuff. I have yet to see any manufacture sell aluminum sanded. We mirror finish it all the time.

Bugpac said:

Acid wash and sandpaper aernt in the same zip code.

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You're exactly right, the methods aren't in the same zip code, but they both are very effective means of getting to the end result - removing the lubricating finish from the metal. I personally like the scotchbrite method better, as exposure to hydrofluoric acid does have greater health risks than aluminum dust, and the handling/disposal procedures more expensive, but I do use acid when the situation warrants.

In larger industries, the acid wash is used more often than mechanical abrasion, as the higher cost of the proper and safe acid wash equipment is much more justifiable than in a small shop. The nice thing about the acid wash is that it can be done post assembly, so it can be done as the final step. In smaller shops, using wide belt/drum sanders isn't uncommon, and surface conditioning abrasives on angle grinders, handheld belt sanders, and burnishers are very commonly used. With these methods, the surface conditioning generally has to be done while the material is still in flat sheet form - working around bent shapes and weldments isn't practical.

Bugpac said:

Imo it is a complete waste of time to sand sheet aluminum to glue carpet to.

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I'd advise anyone installing carpet to read the directions on the particular glue they are using. Most (though, not all, to your credit), of the glues that I use do specifically advise roughing the surface prior to gluing. Some even list a minimum grit required to provide an adequate anchor pattern.

There is a difference here. Your saying you need to remove it all. Roughing up and what your suggesting doesnt coincide. Very few shops sand aluminum to build products. Your also confused on the process. Lubricants are only used in the hot rolling process. I will get some factual data. Not passed on data from opinions of others. I will share it here in the future.

I think we're going round and round in circles, but I'll bite.

Lubricants are only used in the hot rolling process.

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Nope, used in cold rolling to.

https://www.innovaltec.com/downloads/tech paper57.pdf

Your saying you need to remove it all.

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Yep, nailed it. Also, what I'm saying is that it doesn't matter what method you use to remove it. Whether chemically (hydroflouric acid wash), or mechanically (Scotchbrite surface conditioning) doesn't matter, because they both remove it.

Sure, there are reasons to choose one over the other - the mechanical means of removing it also provides an anchor pattern in the metal. Possibly undesirable in applications left bare, but on an application requiring an anchor pattern for a later surface coating application, it kills 2 birds with one stone.

Likewise, the acid wash won't leave an anchor pattern, instead leaving a more uniform finish. But, the point of this discussion is not to argue the merits of one over the other, but instead to point out that no matter which method is chosen, one of them needs to be done.

On a completely different note, for those that like lots of pictures, here is a link to a pretty impressive 25' Alaskan hull. Take note of the surface preparation....
https://aluminumalloyboats.com/viewtopic.php?f=49&t=4143

I know this is an old topic but bassboy1 gave some great information and answered my question. I found this while searching for some information on aluminum, I have to give it to you bassboy you are an aluminum expert, you certainly know aluminum better than anyone I've ever seen.

Don't overwhelm your self with differences in aluminum grades, we are only talking about Jon boats here, I use anything I can get cheap, be it sheet , aluminum conduit or good aluminum pipe, just sand n paint it, n touch it up when necessary. Trust me it will all weld if you want it to.