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Aluminum vs Stainless...why one versus the other?

Old 05-22-2015, 07:39 PM
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Default Aluminum vs Stainless...why one versus the other?

When is it better to use aluminum versus stainless steel for marine applications? Is aluminum favored for T-Tops because of it's relative cheaper cost? What about for small parts such as a bracket or clamp to attach to the T-Top frame? Might stainless steel be stronger for such an application? Any downside to combining the metals, such as building the frame with aluminum but using stainless attachment clamps? I'm looking to construct a frame for a storage box that I will attach to a T-Top.

Also, is it OK to drill into the undersurface of a hard top T-Top cover? I assume they are usually made of fiberglass and will hold a lag screw. Please correct me if I am wrong.

Thanks!
Old 05-22-2015, 09:43 PM
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Alum is lighter, don't want boat top heavy. You can attach SS fittings to alum. Top should be cored but typically have reinforced "pads" in mounting areas.
Old 05-22-2015, 09:53 PM
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any dissimilar metals will create corrosion when they touch. SS and aluminum seem to be one of the worst combinations. plastic bushings and teflon washers help.
Old 05-22-2015, 09:58 PM
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Aluminum bends and flexes ...stainless ...not so much .
Old 05-23-2015, 06:54 AM
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Awesome! Thanks so much for the replies...certainly helps for the planning stages
Old 05-23-2015, 08:39 AM
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I was told lately that there is no electrolyses between aluminum and stainless.? However, I would use a dialectic grease between them if I were doing the project. I use Plumber's silicon grease on all SS screws whether going into SS or aluminum and never (rarely) have a problem. On the weight of SS versus aluminum, one has to increase the diameter of aluminum stock to get a similar strength, thus the weight advantage of aluminum soon is diminished. Stainless steel is more expensive, harder to bend, and more difficult to weld, thus the use of aluminum, from my experience. Aluminum flexes and fatigues, thus cracks and bends occur, but you rarely see the same with SS. BTW, never say never!
Old 05-23-2015, 09:23 AM
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I disagree with stainless being more difficult to weld than aluminum.
Old 05-23-2015, 10:41 AM
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Originally Posted by Bayfly View Post
I was told lately that there is no electrolyses between aluminum and stainless.? However, I would use a dialectic grease between them if I were doing the project. I use Plumber's silicon grease on all SS screws whether going into SS or aluminum and never (rarely) have a problem. On the weight of SS versus aluminum, one has to increase the diameter of aluminum stock to get a similar strength, thus the weight advantage of aluminum soon is diminished. Stainless steel is more expensive, harder to bend, and more difficult to weld, thus the use of aluminum, from my experience. Aluminum flexes and fatigues, thus cracks and bends occur, but you rarely see the same with SS. BTW, never say never!
According to Wikipedia, the anode index of 'stainless' steel and aluminum is around .45. You want it much lower in a marine environment. Now, the specifics of that may vary quite a bit between the particular stainless alloy and the particular aluminum alloy, but in general the two metals should not be in close proximity in salt water. Your comments on stainless steel are spot on. It's a real PITA to work with.
Old 05-23-2015, 01:32 PM
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I built a Stainless top for my last boat and I think the extra cost and labor is worth it if your'e going to keep the boat a long time otherwise aluminum is fine. Polished S.S. is very corrosion resistant and will look great ten years later with minimal maintenance, after ten years in salt water anodized / painted aluminum is usually starting to pit and corrode.

Depending on which alloy's are used mixing aluminum and stainless will have some degree of electrolysis, not the best match. Buliding a stainless top requires more labor to build due to the increased work to bend and fit the pipe, and of course the added cost of the stainless steel.
Old 05-24-2015, 10:13 PM
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I'm a certified welder, and stainless isn't hard to weld. If you're not a welder, or haven't welded stainless on occasion then yes. Also depends if heli-arc, or stick welded. And any ferrous metal against non-ferrous IN a salt water environment is going to create problems. A simple test to see if something is ferrous, use a magnet--non ferrous it wont stick/attract-----ferrous it will. If weight is a consideration use aluminum, if not----- stainless. However be prepared to pay for SS.
Old 05-24-2015, 11:34 PM
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II have had aluminium boats for over 30 years. Always operated in salt water environment.

You don't want stainless steel in direct contact with aluminium and if you do, you should use something to isolate them from each other.

We use nylon washers under the heads of stainless screws or bolts etc, and also we coat them with either duralac or tef-gel before we put them in the hole. Do this, and use at least 316 grade marine stainless, and you will be OK.

I have seen what happens when these simple precautions are not taken. Electrolysis starts very quickly and soon corrodes in the area around where the two metals contact.

We are always very careful not to let sinkers, hooks and swivels fall onto the decks or make their way into the bilge. The old style brass swivels are the worst, they can eat a hole thru 1/16" alloy in a salt water environment in only a few weeks.

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