Galvanic corrosion in firearms

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Arkane
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I recently had a friend bring his AR by for me to look at, he thought his finish on his lower was coming off because it looked like it was "bubbling" in certain places. Turns out when he built his rifle a couple of years ago he used auto parts store moly grease to assemble it because the US Military Armorers manual calls for Molybdenum grease. The areas where steel and aluminum interact were in fact corroding. This is due to the graphite which is impregnated in that grease. The actual assembly grease that is supposed to be used is high/low temp Molybdenum Disulfide (GMD) mil spec MIL-G-21164 and it's variants. These contain no graphite and are referenced in the back of the armorers manual. This is the same grease we used on the 25mm Bushmaster guns so I know it'll handle the heat.

When you build or maintain your T91/A1 think about the areas where aluminum and steel interface and you need a assembly lubricant. Using a grease that conforms to that standard could save you a lot of headaches later on. Some of the greases I am aware of that meet this specification are Aeroshell 17, 33MS, and 64; Nyco GN-17, Shell ASG-17, Royco 64, and Honda Moly 60.
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e292644
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We using Loctite 38650 without any issues. This one has temperature range: -30 F to 1800 F rating.
Lightly coated on the barrel extension and medium light over the upper receiver thread.
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Arkane
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Looking at the MSDS for that Loctite it shows it has copper and graphite in it, so between two dissimilar metals (say aluminum and steel) you now have a combination of four anodic and cathodic metals which will exchange ions in an electrolytic path in the presence of air, grease, or water much like a battery. I've attached a chart showing the anodic and cathodic rankings of metals, notice that graphite is the most cathodic and aluminum alloys are up on the anodic scale.

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e292644
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You were right, switched to Aeroshell 33ms.
Thanks for the info.
Arkane wrote: Sat Jun 24, 2017 5:33 pm Looking at the MSDS for that Loctite it shows it has copper and graphite in it, so between two dissimilar metals (say aluminum and steel) you now have a combination of four anodic and cathodic metals which will exchange ions in an electrolytic path in the presence of air, grease, or water much like a battery. I've attached a chart showing the anodic and cathodic rankings of metals, notice that graphite is the most cathodic and aluminum alloys are up on the anodic scale.

Image
JPurp
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Aeroshell 33ms is the way to go. It is the original M16 spec.

It makes sense the engineers at Armalite that designed the AR15 had an aerospace background. Aeroshell was designed for aerospace use.

On a side note I have personally seen dozens of ARs that don't use anti-seize for barrel install and have yet to see a problem. Though technically galvanic reaction is a thing, my understanding is it is extremely rare in AR15s unless you are using very low quality parts.
e292644 wrote: Sun Jun 25, 2017 3:55 pm You were right, switched to Aeroshell 33ms.
Thanks for the info.
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