Brownells says galvanic corrosion isn't a real problem

[SWIGIN]

I disagree. The gasket material can be any of several different compounds depending on usage.
Aluminum alloys have a linear thermal expansion (10-6 in/(in oF)) of anywhere between roughly 9 & 13 depending on the alloy.
Iron & steel, depending on the alloy, has an expansion rate of around 6.
It will take several heat cycles between 70 & 500 degrees to get these two to play well together.

I'm referring to extremes that an AR may not ever experience.

Bolt an aluminum head down and heat it up with no gasket once and see if you lost torque.

The gasket is what gets smashed when the head expands and when cooled you have torque loss. Unless your stretching the attaching hardware.

Once the gasket is smashed all it can be smashed the hardware will hold torque.

[P89]

Sorry Zach, for this going so far off topic.

The fact of the matter is the cause of torque loss was a result of brinelling at the surface of the aluminum.
There's papers written on it by people smarter than both of us.

https://commons.emich.edu/cgi/viewco...context=mcnair
AN EXPERIMENTAL INVESTIGATION OF TORQUE LOSS IN A DIE CAST ALUMINUM THREADED FASTENER JOINT
Robert A.Green-Warren
Dr. MacArthur Stewart, Mentor

[Noah_Zark]

They are correct, even if they don't know why! If one recalls the 'galvanic series' chart from chemistry, the two metals are fairly close in potential, so the chances of galvanic corrosion are minimal.

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Steel and aluminum may seem close on the metals electromotive series, but steel, aluminum, and an electrolyte will set up a dandy little "battery" resulting in white corrosion of the aluminum. It's for this reason that steel ball joint tapers that mate into aluminum steering knuckles are coated with zinc, or else the female taper in the aluminum knuckle will corroded in a matter of a few years in the Salt Belt.

Noah


The anodizing on the AR upper acts as a non-conductive barrier.

[abner13]

Bolt an aluminum head down and heat it up with no gasket once and see if you lost torque.

The gasket is what gets smashed when the head expands and when cooled you have torque loss. Unless your stretching the attaching hardware.

Once the gasket is smashed all it can be smashed the hardware will hold torque.

Yes , aluminum and cast iron have different coefficients of thermal expansion. They get expand at different rates at the same temp. Gaskets also make up for imperfect sealing surfaces. The XR-750 factory dirt-track racer and limited production XR-1000 twin-carb Sportster used no head gasket. The Branch-ported heads were individually lapped to the cylinders.

The aluminum used for Harley crankcase and cylinder heads was garbage. But it was what it was in it's day. Modern aluminum alloys are light years ahead of that crap.

We'll explore green sand mould vs permanent die castings later.

[oldengineer]

The Navy built a few classes of DD/DDG/DE/DEG in the 1950s that had steel hulls and aluminum superstructures. There was a galvanic corrosion problem in spite of using a thick mica interface around the main deck where the structures were joined and a lot of sacrificial anodes. Ford has been using aluminum bodies on their trucks since 2015 and I haven't seen anything derogatory about them yet so hopefully they did their homework before going into production.

[P89]

Ford has been using aluminum bodies on their trucks since 2015 and I haven't seen anything derogatory about them yet so hopefully they did their homework before going into production.

Ford just copied what Land Rover was doing when they bought them out.

[Jhaydeno]

Steel and aluminum may seem close on the metals electromotive series, but steel, aluminum, and an electrolyte will set up a dandy little "battery" resulting in white corrosion of the aluminum. It's for this reason that steel ball joint tapers that mate into aluminum steering knuckles are coated with zinc, or else the female taper in the aluminum knuckle will corroded in a matter of a few years in the Salt Belt.

Noah


The anodizing on the AR upper acts as a non-conductive barrier.

I don’t disagree about the barrier, it acts no differently than a stainless or passivated galvanizing with a solid oxide layer to extremely low potential. The difference between those and iron oxide layers is that iron expands and contracts at different rates than the base metal, always exposing fresh metal at the boundary (I know you know this, but others may not).

Keep in mind the zinc in your example is nothing but a sacrificial anode, and if it’s oxidized and worn off, the aluminum is next to go. While electrolytes help, a galvanic cell in aluminum can be set up with something as simple as a particle of aluminum oxide, resulting in pit corrosion. About 25 years ago I had to do a failure analysis where the aluminum pitted 1/4” due to a simple oxide started in an acidic environment.

[DavidH]

Oh no! Bad news for everyone who uses a stainless barrel! Stainless & aluminum are the WORST together! Rapidly causing GC! LOL!

Noah said it first. The coatings on materials prevents Galvanic corrosion & electrolysis for the life of typical firearm. So, Brownells is technically correct. But, it’s still a good idea to research different metals characteristics and both GC & electrolysis.