Corrosion potential in vacuum barrel guns

It has occurred to me that vacuum barrel guns could over time experience corrosion at the front end of the internal surface of the inner barrel. The muzzle always has some water cupped inside it before the spear tail is inserted into the gun. On loading the gun this water is trapped and drawn into the inner barrel and I expect that it vaporizes under the near vacuum conditions or forms droplets on the inner barrel wall. After a number of shots saltwater could gradually accumulate and be trapped in the inner barrel around the outside of the piston between the front piston seal and the shock absorber anvil after the gun is discharged. In a standard pneumatic speargun this will not happen as washing of the gun rinses this area out as it is easily accessed through the muzzle relief ports and thus any saltwater surrounding the front end of the piston is removed and also it can completely dry out.

A precaution may be to periodically push the spear part way down the barrel and tip some freshwater in the inner barrel at the same time with the gun held vertically and the sealed slider removed. This will not allow the area to dry, but it will remove the salt which is what causes the corrosion. Another possibility would be to drill some tiny breather holes transversely through the piston body, however you would want to avoid weakening the piston. The old metal pistons used to have a tiny side hole to avoid a hydraulic lock on inserting the tapered spear tail in the piston nose cavity, but the more compliant plastic pistons do not seem to need it.

Barrel corrosion at the front end will not affect the barrel surface area swept by the piston seals, but it will affect the surface that has to be crossed when removing and inserting the piston during maintenance. Something to watch out for if you dive in very salty conditions and just rinse your gun off without working the gun's piston after cleaning and before storing it.

Just to expand on my comment about "working the gun's piston". Nemrod in their earlier pneumatic spearguns for some reason placed a ring of ports right at the front end of the muzzle, drilled inwards from a sloping face that formed a truncated, cone shaped nose on the muzzle. This cone was surmounted by a rubber buffer, a doughnut shaped moulding that protected the front end of the gun when stood on its nose to drain after a dive, or after a post-dive wash. There were no other ports in the muzzle body which was knurled in a series of bands to afford a better manual grip when unscrewing it from the gun, however in my experience that was always a job for some multi-grip pliers.

The big problem was these ports were in front of the muzzle shock absorber, not immediately behind it, so they were not truly muzzle relief ports and could not provide an alternative flow path for the water that surrounded the shaft in the inner barrel. Another consequence was that some water was trapped around the front end of the piston as I have described above, although it could dry out eventually via the small hole in the side of the piston. Nemrod recognized that this water entrapment could lead to corrosion, so each Nemrod gun was supplied with a short plastic plunger with a larger diameter head that could be used to push the piston back while you introduced oil from an oil can into the muzzle with the gun held upright. The head on the plunger provided a larger surface area to push against with your hand. You then worked the piston up and down using the short stroke provided by the plunger and this was meant to preserve the gun against corrosion at the muzzle by spreading the oil around inside it. This maintenance action certainly stopped seizure of the steel piston nose through corrosion onto the face of the steel shock absorber anvil, but it never really stopped them rusting. The muzzle relief ports were eventually relocated to the rear of the muzzle on later models and Nemrod consequently stopped supplying the plunger as they must have realized their error in placing the ports where they would not open up the interior of the muzzle to flushing. This short plastic plunger was a handy device as you could put it into the muzzle, it was dome-shaped at the rear end to stop it jamming in the piston nose cavity, and you could then invert the gun and press it down onto the floor. This action pushed the piston back and let any water run out under the action of gravity. After that you could up end the gun, add the oil and carry out the maintenance procedure advocated by Nemrod.

It seems to me that a modern equivalent of this short plastic plunger is all that you would need to help periodically drain the muzzle area of a vacuum barrel gun. Being short it is better than a spear for this purpose as it will not jam in the piston and allows the muzzle to be held nearer to the ground. A bucket of water could assist in cleaning by placing both plunger and the front end of the gun in the bucket and pumping the gun a few times.

You can see the plastic plunger on the lower left of the "Comando" speargun and cardboard shipping box photo; it is the green plastic, rod shaped object at the end of the blue cord. The plunger is not intended to be carried during a dive, the cord attaches it to a rubber plug which blocks the muzzle bore during long term storage and which is hidden from view in this photo by the black plastic combined muzzle and speartip cover. Refer to the Deeper Blue thread on "Nemrod Comando Speargun Parts", second page, to see the photo.

The increased risk of water contamination with pneumovacuum spearguns suggests that they should be periodically opened up to change the oil and thereby get rid of any accumulation of water that has succeeded in penetrating past the piston seals. Too much water and the oil turns to an emulsion with a distinctive "milky" appearance to the lubricating fluid. While an oil film coating would be expected to protect internal metal parts you need to remember that oil floats on water, so oil may not adhere to the parts being protected and they may corrode when the water, especially saltwater, contacts these parts. Cadmium plated steel parts will get a spotty appearance as rust replaces the plating in those patches where the oil has been displaced by saltwater and the gun has been sitting in storage.

I note that the "Taimen" pneumovacuum gun now has a special tool or wrench to help dismantle the gun and I suspect that the reason for this tool being developed was to encourage changing the oil, there is even a video showing how to use the tool which is a multi-function item equipped with various jaws and locating lugs. Whereas with a pneumatic speargun the general view was that the oil change maintenance should be carried out every two years, my thoughts are that pneumovacuum guns should be having oil changes every year and possibly even sooner for guns subjected to heavy use. If you have the skills to fit a vacuum barrel muzzle kit then changing the oil more often should not be beyond most users of these kits. Having struggled with corroded gun parts in dismantling long neglected pneumatic guns I can tell you that prevention is a lot better than the cure.

That SAE 10 viscosity grade motorcycle fork oil can be used on most pneumatic "spring" spearguns, which includes both pneumatic and hydropneumatic models. Only small amounts are needed for external applications, just a few drops which will float off once the gun is submerged, then the water takes over the lubrication of seals. Excessive lubrication is to be avoided where sand is likely to be involved, so grease is not recommended unless used in the tiniest of amounts which is virtually wiped off again leaving not much more than a smear that puts a gleam onto the surface of the item. Don't use grease on the Aquatech guns. Spear shafts can benefit with a rub from an oily rag, even if they are made from stainless steel, or a squirt of WD40 which is then pretty much wiped away again, the stuff getting into the microgrooves on the metal. Surfaces that look smooth actually have a texture that will hold the lubricant. WD40 seems to affect some plastics, I use it on the muzzles of my metal piston pneumatic guns, but it can discolor some white plastic items making them more brittle and maybe susceptible to cracking, so I don't use it on plastic piston guns as there is no way of determining what type of plastic has been used. I have used WD40 for decades on the black plastic external parts with no problems, but that plastic has to withstand sun and sea, unlike the parts buried inside the gun, so is probably a more resistant material.

The "Taimen" is yet to arrive, I know that it departed Russia in mid-November. I would have expected it to be here by now, but all I can do is wait.

This thread is in a sense continued here, I never put the link in before and just found this thread again:- http://forums.deeperblue.com/threads/cleaning-vacuum-muzzle-guns.97710/

An aspect of corrosion in spearguns that is not often considered is the location, or locations, where the designer had envisaged that the gun was to be used. Saltwater is not the same everywhere as salinity levels can vary significantly, thus a gun that can survive in the "Black Sea" will not necessarily do so well in the "Pacific Ocean" if the operator thinks that the gun, being "built for saltwater use", can get by with zero maintenance no matter where it is used. If you look at this chart of salinity levels in the World's Oceans then you will see that saltwater definitely ain't the same everywhere.

https://en.wikipedia.org/wiki/List_of_bodies_of_water_by_salinity

Some guns that only ever saw service in freshwater lakes can get away with a mixture of brass, aluminium alloy, stainless steel and plain steel (either raw, cadmium or zinc plated) without instantly turning into a piece of junk. However get these weapons into something more briny and they begin to corrode away as the galvanic series exerts itself in the high electro-chemical conduction world of the marine environment. Even 316 stainless steel will corrode away with "crevice corrosion" if the chrome protection mechanism fails, resulting in this the most rust resistant of stainless steels just crumbling away. Titanium offers a high level of corrosion immunity, but creates a gun that will barely float unless it has an enormous tank capacity and thus a stratospheric price even with a stainless steel inner barrel fitted for wear resistance. As mass production guns have to be made to an affordable price the designer can only do so much with materials and must then look to a cleaning regime without involving the wholesale dismantling of the gun after every dive. Unfortunately selling guns is much more attractive than having guns last forever (as otherwise where would their business be?), hence just enough emphasis is put on cleaning to avoid owner complaints without really going into all the potential problems in any depth. You need to be aware of this "low key", don't scare the horses approach unless you expect to buy new guns on a regular basis as your older guns move along the slow conveyor belt towards the trash can. The rewards of understanding the vagaries of your equipment will be increased service life despite the increasing use of flimsy plastics in the high impact areas of pneumatic guns which I have long suspected was a plot to shorten the service life of pneumatic guns, but it also lowered their cost.

The remedy is to wash the muzzle out by pumping the spear back and forth in the muzzle after letting any trapped saltwater out before you begin and putting some freshwater in. Described here in a number of threads the procedure is quick and simple to do. Salty conditions here have ruined many guns as corrosion sets in when saltwater droplets evaporate and concentrate their salinity at a certain spot and burn into the metal. One manifestation is when in trying to remove the muzzle the entire inner barrel turns breaking the locating pegs in the rear of the handle or the rear bulkhead. Sometimes the muzzle will not turn until the clamping jaws on the chuck tear up the outer surface of the barrel and the thought occurs that maybe a replacement barrel is in order. I have seen the pitted entrance to alloy barrels beyond where the seals travel, so a vacuum barrel which has no muzzle relief ports to flush the area out will incur damage much sooner. However a lot depends on where you dive and the intervals between using the gun as stored guns have time to deteriorate. After forty years of spearfishing and repairing spearguns I have seen many things!

You need to read this thread as well as it is in a sense a continuation of this one:
https://forums.deeperblue.com/threads/cleaning-vacuum-muzzle-guns.97710/