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Old 03-09-2008, 00:21   #1
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materials, corrosion, and application

Okay Gord- this post is your baby! While in the case of my "how fast can my charger charge my batteries" post- where I still have no idea as to the answer- this time I want to learn to fish- not just have a sandwich!

I am completely not getting which metal to use where/when/why.

I see aluminum boats all the time.
The oars for my dinghy are aluminum.
My mast is aluminum. (I think?)
My mast steps are aluminum.
I was warned against using aluminum rivets for the steps because they would corrode.
I was advised to use aluminum propane tanks rather than steel because the steel would corrode.
Stainless steel doesn't corrode? But galvanized does?

Okay- lay on the calculus. I am ready for it.
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Old 03-09-2008, 01:45   #2
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Hi Jack - Not Gord but...

Materials selection is a complete engineering discipline unto itself. In my industry we have teams of engineers, many with Phds in metallurgy deciding which materials to use.

A nice primer on materials selection can be read here in about 20 minutes. I am sure Gord will post some nice reference material as well.

http://media.wiley.com/product_data/...0471359246.pdf



The reality is that there is a broad range of materials that would suit different applications to varying degrees.
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Old 03-09-2008, 03:09   #3
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AS a piping engineer I can tell you there is no easy answer.
Stainless steel we can use for most things except chlorine and sea water.
It's really good because if it doesn't corrode you don't have to allow for 3mm of material loss inside and outside for it to do it's job for at least twenty years.
Copper has the same properties but costs too much but is easily jointed so it's good for small bore stuff like house heating. Also kills sea life so protects boat bottoms from marine life.
Aluminium is a real hassle. Prone to attack from too many things. Not very strong, and if you make it stronger and harder with additives it's prone to cracking.
Wood is great. Surprisingly. Not much good for pipes because the strength is in the wrong direction, but take a length of timber and wrap it in GRP. STRONG. Resillient. Easy to repair and to make. And corrosion resistant. But a bit heavy.
There isn't a right choice, just more important parameters, weight or strength or prettiness.
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Old 03-09-2008, 04:44   #4
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Jack:
As others have indicated, you've asked a very broad question, which I would be unqualified to answer.
Notwithstanding, your query presents an opprtunity for a very interesting discussion; in which I will join, when I return home (2.5 weeks).
My current internet/computer access is painfully clumsy.
Regards.
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Old 03-09-2008, 04:52   #5
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Aluminum and and steel come on a very large number of alloys. Many of which are very appropriate to the marine environment. Bronze is another alloy that is also common. The alloy process yields drastically different properties and it seems almost amazing how different they can be. Nearly every marine application has a slightly different alloy used.

Any attempt at understanding all the possible materials is better handled in a specific context or application. Aluminum alloyed with magnesium can be cast to produce very strong parts yet that exact same material can not be extruded. The working of various metals is beyond anything you can expect to fully understand here.
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Old 04-09-2008, 00:02   #6
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Quote:
Originally Posted by Jack Long View Post
Okay Gord- this post is your baby! While in the case of my "how fast can my charger charge my batteries" post- where I still have no idea as to the answer- this time I want to learn to fish- not just have a sandwich!

I am completely not getting which metal to use where/when/why.

I see aluminum boats all the time.
The oars for my dinghy are aluminum.
My mast is aluminum. (I think?)
My mast steps are aluminum.
I was warned against using aluminum rivets for the steps because they would corrode.
I was advised to use aluminum propane tanks rather than steel because the steel would corrode.
Stainless steel doesn't corrode? But galvanized does?

Okay- lay on the calculus. I am ready for it.
Just to directly answer some of your questions;

Alum. rivets are made of a softer material, probably a 60 series, so they will mushroom out when applied. Whereas, alum hulls are made of a anti-corrosion material (5052) and aircraft stuff is a 75 series for it's strength.

Alum can be heat treated so when it's welded it's prone to cracking next to welds if it cools too fast. Larger structures are better due to their slower cooling rate. Smaller alum. boats are usually riveted where the larger ones are welded. Welding is a real trick. One has to pre-heat to start the weld and try not to let it get too hot as one runs the bead or it'll blow a hole thru it.

And SS will rust if not treated properly. There are many different series of SS too, a lot of which has iron.

But galvanize will not rust, it's the metal underneath that rusts. Galvanize is a zinc mixture plating that has no iron.
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Old 04-09-2008, 00:25   #7
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Compromise Example

One example of a compromise are aircraft wheels. They need to be light and strong and corrosion resistant due to the severe brake dust, runway environment. Magnesium alloy is used predominantly.

The problem is that magnesium wheel burn pretty good once they get started. On a high speed abort the brakes can throw a lot of heat into the wheels.

All in all - it's the best material.

The photo attached is of course a training picture.
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