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All About Guns What's your weapon of choice, and why? Discuss the beloved speargun here!

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Old 09-09-2020, 08:00 PM   #1
popgun pete
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Corrosion Article with respect to Underwater Equipment

Well worth reading when mixing metals, which is unavoidable in making spearguns.

http://www.marinasecuritygates.com/info/corrosion.pdf
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Old 09-11-2020, 02:58 AM   #2
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Re: Corrosion Article with respect to Underwater Equipment

Very instructive! I have witnessed this crevice corrosion in stainless shafts found at the bottom. The outside seems fine, but they crumble if you put any pressure on them (vise, pliers, etc). They are rusting from the inside out.

I have also seen this galvanic corrosion when you use stainless fasteners to attach handles and muzzles to aluminum tubes (frankenguns usually). I have two guns I need to keep a close eye on. If they corrode any further, I will have to make shorter guns or swap the tubes for carbon.

Now, how do we prevent this from happening in pipe guns? Avoid aluminum tubes? Because I wouldn't trust a nylon/delrin fastener to attach the muzzle/handle to the tube!

I recall the Sporasub Viper series avoided this problem with the two screws that held the handle to the tube thanks to the oversized holes in the aluminum and plastic inserts (part of the crocodile pins for the mono) on both sides. The muzzle was actually attached with a plastic screw (more for positioning the muzzle in the correct position).

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Old 09-11-2020, 06:56 AM   #3
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Re: Corrosion Article with respect to Underwater Equipment

Great article, thanks for posting it.
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Old 09-11-2020, 10:46 AM   #4
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Re: Corrosion Article with respect to Underwater Equipment

You could easily avoid any corrosion on aluminium pipe guns by having a sacrificial anode washer grounded with the aluminium. I am actually surprised this is not something that is used as a magnesium washer would cost almost nothing and you can just replace it after a few years when it start wearing out. A similar system of cathodic protection is very well tested with outboard engines and works very well.
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Old 09-11-2020, 11:11 AM   #5
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Re: Corrosion Article with respect to Underwater Equipment

If you break the galvanic cell connections then you can limit corrosion. A Riffe “Metaltech” is an example as wherever possible there are plastic washers under stainless steel items like bolts and nuts and the trigger mechanism sits in a plastic tub that also forms the gun's butt. When you take one apart you need to make sure you replace all the parts, particularly the thin plastic washers, otherwise corrosion will attack the alloy barrel.

Painting the gun can help, the Russian Seabear pneumatic has painted alloy body parts as the stainless steel barrel would gobble up the alloy sections if it was not electrically isolated. Where the barrel disappears into the mid-handle it is surrounded by some black stuff like silastic and the barrel is locked in by circlips buried in epoxy inside the gun's interior.

Zinc can also be used as a sacrificial washer. If guns are thoroughly soaked in freshwater you can stop corrosion as it is the saltwater’s conductivity that allows the electric circuit.

Guns can be completely built of titanium, but are heavy and you need to bring plenty of money if you want to buy one.
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Old 09-11-2020, 03:55 PM   #6
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Re: Corrosion Article with respect to Underwater Equipment

Interesting article for those that never heard of galvanic corrosion. It is amazing hom many people like to combine different metals without any protection.
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Old 09-11-2020, 07:38 PM   #7
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Re: Corrosion Article with respect to Underwater Equipment

Someone can tell me where can I find sacrificial washer in Zinc, i will need a bunch of 3/16" ID washers, thanks

Last edited by memosub; 09-11-2020 at 07:47 PM.
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Old 09-11-2020, 08:18 PM   #8
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Re: Corrosion Article with respect to Underwater Equipment

Quote:
Originally Posted by Marco View Post
Interesting article for those that never heard of galvanic corrosion. It is amazing hom many people like to combine different metals without any protection.
It's also amazing how many people are afraid of "dissimilar metals" They are so Paranoid that ANY Different metal next to another is TERRIBLE! They don't understand free electrons and cathode/anode reactions.
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Old 09-11-2020, 11:28 PM   #9
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Re: Corrosion Article with respect to Underwater Equipment

For stainless steel what can also dramatically help to prevent corrosion is to highly polish the metal and then passivate it or electropolish it. I remember testing SS trigger parts in 304 and 316 SS ... the 304 would always corrode much faster than 316, but if you polished the parts and then electropolish them ... the 304 would not corrode even with prolonged salt water bath corrosion testing.

Last edited by spearq8; 09-11-2020 at 11:44 PM.
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