Calling all Metalurgists

[dguy]

I posted a similar question on PFF a few minutes ago, but I'm really not in the mood to do work today, so I'll burn up some time by typing here too.


Most backyard mechanics know that aluminum + steel + water = bad.  If you don't, try using aluminum rivets on a sheet steel patch, say...  on the floor of a vehicle, and check it out after the first winter.

So why the hell is replacing the rubber cradle bushings in the pre-88's with solid aluminum so popular?    

The benefits are obvious--solid-mount cradle, aluminum is relatively inexpensive and easy to work with.  But I would imagine that in the long run the bushings and/or surrounding steel are going to rot themselves to bits, and you'll one day hit a bump and find the entire drivetrain far behind you...

Anyone care to set me straight?  Please?  I feel like I'm missing something...  

[A.K.A]

---taken from another board---

aluminum doesn't RUST, but it DOES corrode ("rust" is generally used on iron-based metals, like steel or cast iron)...  however, when you have two metals in contact and add moisture, there is a galvanic reaction and one metal ends up being the sacrificial metal...  there are different alloying elements for as well as grades / types of aluminum, so it could be a specific combination (steel w/ alloying elements + aluminum w/ alloying elements) that makes the two metals galvanically equal...  pure metals are rarely used anyways...

prolly not the answer you are seeking, but a good start I hope!!

[aaron88]

When two dissimilar metals come together with some sort of electrolyte (ie. salt water) you get an electrical reaction (this is exactly how a battery works).  Due to the movement of electrons from one metal to the other, what you get is a corresponding reaction with the oxygen.  A build up a residue on one metal forms while the other slowly “corrodes”.  In this case between steel and aluminum, it is the aluminum that acts as the “anode” (the corroding metal), hence actually protecting the steel from corroding.  When aluminum corrodes a build up of aluminum oxide forms on the surface of the metal due to the corresponding electrical and chemical reaction.  Aluminum oxide is actually a ceramic material (one of the reasons it can be used as a much better wearing surface than hardened steel), ceramics as you might know also act as a good insulator.

The result:  The aluminum starts to corrode, then builds up a good residue of aluminum oxide and slowly stops corroding.  At this point the actual amount of corrosion is negligible.  But what does happens as a result is that the aluminum “insert” essentially fuses to its surroundings making it very difficult in some cases to extract.

The solution:  use some sort of anti-sizing compound, 3M tape, or some other sort of readily available anti-size material.  This will greatly slow down the corrosion process, essentially neutralizing the problem, but only if done properly.  Or if you don’t ever plan on removing the engine cradle in the future forget about it because it’s not going to fall apart anytime soon.

Aaron

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[dguy]

The result:  The aluminum starts to corrode, then builds up a good residue of aluminum oxide and slowly stops corroding.  At this point the actual amount of corrosion is negligible.  But what does happens as a result is that the aluminum “insert” essentially fuses to its surroundings making it very difficult in some cases to extract.

Ahh, so the reaction does actually stop at some point, instead of simply corroding away into nothingness.  Good to know...  thanks!

[aaron88]

Well…it doesn’t actually stop, but it does slow down to the point that you could say that it effectively stops.

Aaron

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