This article by Stan Gajda who lives in Pohnpei, Micronesia, gives still more expert advice on underwater recovery.
Conservation of Metal Objects Recovered from the Sea
If metal objects recovered from the sea after many years in contact with salt water are not treated properly, they will soon deteriorate and become totally useless. The exception is brass or stainless steel items.
Steel objects after many years of contact with salt water form on the steel surface iron chlorides which are unstable and if these objects are allowed to dry out, the iron chlorides are hygroscopic and continue to absorb atmospheric moisture. This in turn causes the boundary layer between the iron chlorides and the steel to blister off and damage the object and also causes the steel itself to continue rusting quite rapidly.
The initial treatment comprises of submerging the objects in fresh water. This helps to leach out the salts that are present in the artifact and also keeps the air away thus preventing deterioration.
In order to affect a permanent state of preservation and render the artifact suitable for permanent display it is necessary to treat the steel object(s) with an electrolysis process.
This process comprises of a suitable electrolyte that will conduct electric ions from sacrificial steel anodes to the steel artifact. The electrolyte used successfully is Caustic Soda (sodium hydroxide). This chemical is dissolved in water with enough dissolved to enable the current to pass without causing a complete electrical dead short.
A plastic tub is prepared that allow the artifact to be treated to be completely submerged. Mild steel anode plates of perhaps 16 gage are cut to allow coverage on both sides of the artifact. These plates are placed against the sides of the plastic tub and the artifact is placed in the center of the tub. A DC power supply is connected to the plates and the artifact. The positive cable is connected to the steel plates which are joined together with a wire loop so that both plates are connected positive and the negative cable is attached to the artifact, which becomes the cathode. The power supply is switched on and the current is adjusted so that just a few amps are flowing through the circuit. Very soon small bubbles will be observed streaming to the surface of the electrolyte from the artifact.
It is very important to keep the current down to just the minimum required to produce the gas stream from the artifact. Too much current will not hasten the process. It will cause the iron chloride layer to blister off the artifact and ruin the item being treated. This process will take a very long time. The current has to be applied continuously. As time passes the anode plates will gradually disappear due to the electrolysis process. Also the bubbles may slow right down and perhaps cease.
The remedy is to dismantle the electrolytic bath and renew the solution. When the plates are worn away to the extent of insufficient effective area remaining, new plates are made and fitted, and then the process continues.
What gradually happens to the artifact is that the unstable iron chlorides are converted into a stable compound and the unstable boundary layer is eliminated. Also the iron ions that are constantly migrating from the steel anodes to the steel artifact cathode combine with the artifact in the form of real iron and a lot of the unstable salt compounds formed in reaction of the steel with the sea water are converted to iron metal further enhancing and stabilizing the artifact.
Also the outer layer of the artifact that was the original steel surface when the object became submerged in the sea allows the coral and marine growth to release easily from the surface. This allows easy cleaning of the surface of the artifact without damage. This will then reveal the original surface features of the artifact when the treatment is fully completed. Machine marks and numbers and letters stamped on the steel will be readily visible.
The surface cleaning can be done from time to time while the artifact is undergoing the electrolysis treatment. Perhaps during electrode or solution changes. If the outside concretions and conglomerates do not come off easily, these should be left until later during the process. This will reduce the likelihood of surface damage and result in a better looking display item.
The time to process the large steel 50 caliber machine gun could run for up to a year of continuous electrolysis treatment. For example, the 30 caliber M1 Garand rifle on display in the Ace office took six months before the process was complete, to the stage where the bubbles stopped. That rifle was a difficult project because of the wood stock sections still attached which could not be removed and also the presence of stainless steel parts and brass cartridges in the magazine. The coral concretions were extremely difficult to remove from the wood and had to be done very slowly and laboriously with small tools. The wood was also very soft and fragile so some damage was unavoidable. But the end result was good. No deterioration of the metal parts of this rifle has occurred since the completion of treatment. At the time of this writing that rifle has been kept dry for more than ten years.
Also requiring treatment is an 30 caliber M1 carbine also recovered from the sea at the same time as the 50 caliber gun. This carbine has some wood still remaining and it would be very preferable to have this wood remain on this item in the interests of display appeal. The carbine is complete with a 15-round magazine attached. It is not possible to disassemble any part of this carbine without serious and permanent damage so the treatment will have to be the same as for the 50 caliber gun.
Also requiring treatment is a 20-round 45 caliber stick magazine for a Thompson machine gun which was also recovered from the sea at the same time as the other two weapons. All the artifacts being discussed here are from the lagoon off the northern beaches of Betio island in Tarawa, Kiribati, including the Garand rifle.
When making the electrical connection with the artifact to be treated, it is necessary to connect right to the steel of the object for the best possible contact. A small clamp with a sharp tip screwed into the artifact should be sufficient but the connection should be made where the small resulting damage will not be visible when the object is completed and on display.
When the electrolysis process is completed the artifact(s) are washed thoroughly in fresh water and then the plastic tub solution is replaced with a solution of Potassium Dichromate of a strength that shows a good yellow color. This chemical allows a final stabilization of the newly formed surfaces. The artifacts are left in this solution for perhaps two weeks and then removed and allowed to dry. Final surface cleaning is then completed on the items that are all metal. The next step is to apply linseed oil all over the artifact and to ensure it penetrates right through the interior. The items should be lightly warmed to allow better infusion of the linseed oil. After this application the oil is allowed to run off and dry. The outer surfaces can be wiped off to prevent a resultant shiny finish. That completes the preservation process of all-steel items.
In the case of the M1 Carbine, with the wood remaining on it, the wood will be still saturated with the finishing solution. If the wood is allowed to dry out it will shrink and warp and cause an unsightly mess. To prevent the wood from shrinking it is necessary to replace the water in the wood pores with a material that will keep the wood intact. To displace and remove the water, acetone is used because water dissolves in acetone. Sufficient acetone must be used to cover the carbine completely and soon the acetone will become milky in appearance. The acetone is replaced every couple of days until it does not show any further signs of water from the wood. The wood pores are now filled with acetone and if allowed to dry will quickly evaporate and cause the wood to collapse. To replace the acetone, the carbine is immediately placed in a bath of linseed oil completely covering it and left for several days. After removal and allowing the excess oil to drain off and further surface wiping, the carbine is allowed to dry out completely. It should then be stable with both the metal surfaces and the wood.
Since the linseed oil treatment used ten years ago, it is possible that better and improved compounds are available for the final treatment and preservation of the all-steel artifacts. Products available at Ace Hardware in aerosol form will probably be just as effective, for example LPS-3 and other similar products which leave a non-hardening greasy film on the items treated.
Remember – the process is slow and it can not be hurried up. It is very important to work carefully and handle the surface cleaning processes gently, a little at a time, particularly the wood on the M1 carbine. The final result will ensure a nice finish and will preserve these rare and genuine World War 2 Pacific combat relics indefinitely and allow them to be appreciated by others well into the future.
Other items that have been damaged by sea water corrosion can also have the continuing corrosion arrested or halted by using the same electrolysis process. The anodes must be made up though of material similar to the item being treated. For example magnesium or aluminum casings which may continue to decompose even after thorough cleaning can be stabilized by electrolysis.
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July 25th 2008