The oxidation process is a chemical union of iron, water and oxygen, which produces iron oxides. This is the rust build-up we see on many metal items that have been allowed to get wet. Many investigators believe that when an item has been submerged in water, the item oxidizes rapidly, when in fact this is not always the case. Because of the limited amount of oxygen in water (remember it takes oxygen as well as iron), oxidation occurs slowly. It's when the item is removed from the water and exposed to the surrounding air, which has roughly 20% oxygen that we often see the most damage from oxidation.
So, items like weapons etc., left on land exposed to ground and air moisture oxidize faster than those iron items placed in fresh water due to the amount of oxygen they are exposed to. However, add salt to the formula, and everything changes. Salt acts as a catalysis, it speeds the oxidation process up. In fact, it's believed that iron corrodes five times faster in sea water than in soil and 10 times faster in sea water than in air.
The next time you recover an item made with iron (one not needed for evidence), record the time it takes for rust to appear. You'll be surprised how quickly this process takes place on some items. Now imagine that the suspect's fingerprints were still resting on the metal surface when the item was removed from the water. Oxidation can lift the prints away from the surface, losing them forever.
Dissolved gases in water
You may ask how little oxygen is in fresh water to allow the oxidation process to slow underwater compared to on land. That's a hard question to answer because as public safety divers we deal with many different water environments, each having its own ecosystem which produces its own oxygen level. Oxygen, carbon dioxide and nitrogen are gases found dissolved in water. The amount of each depends on temperature (warm water holds less oxygen than cold water), the metabolic activity of underwater plants and animals (plants produce oxygen while animals use oxygen), and other environmental and biological factors. What does all this mean to you? It means that just about every body of water you deal with has a different percentage of oxygen. But there is one thing you can be sure of wherever you dive: a lot less oxygen is coming in contact with that metal item underwater than on land. Any doubts, stick your head underwater and try to breath. The photo to the right shows me after recovering a weapon that has been underwater for over 20 years. Note that I have my own oxygen supply.
Oxidation and Guns
Of all the metal items we will be asked to recover, guns no doubt carry the greatest potential for evidence to be destroyed by oxidation. However, just because you recover a weapon or other item covered with oxidation, this does not mean you will not be able to get any information or conduct any examination on it. With proper cleaning and handling, you can sometimes get the item to look almost functional.
When recovering a gun, you need to be watchful mainly for the following three conditions insofar as oxidation is concerned:
1. Rust forming on external surfaces of the gun and destroying fingerprints
2. Rust forming inside the barrel and preventing firearms comparative ballistics testing
3. Rust forming inside and destroying the internal workings such as the springs and trigger mechanisms
Package in water
Because of oxidation, most weapons and items that you know will oxidize and need to be taken to the lab or office for processing should be packaged in the same water they were found in. When selecting a container, choose one that can handle the item being recovered as well as the water needed to preserve it. Like always, before I recover a weapon of importance that will be going to the lab for processing, I always try to call the lab and ask them how they would like me to package and preserve it. They appreciate the call and everyone is happy.