As soon as EO is produced it begins to age and degrade. Certain components, like the stabilisers in propellants, degrade quite quickly and others, like thick-cased projectiles designed to undergo extreme firing forces, can be quite robust over time. The exact speed and nature of the ageing process will depend on the construction and precise environmental ageing factors over time.
The differential rate of decay of different components can have a significant impact on the EOD operator. For instance, a munition may be fuzed with a cocked striker and the holding device may degrade more rapidly than the coiled spring, leading to a munition that sensitises over time. This means a munition can be in a very uncertain state depending on the nature and extent of corrosion.
This can also have an impact on detection and identification. The corrosion of ferrous materials generally makes them harder to detect with metal detectors and the corroded appearance may make the munition harder to identify.
As a rule of thumb, munitions designed to be projected or generally undergo significant forces will be more resilient with thicker casings, and fragmentation munitions tend to be more robust than thin-skinned blast munitions. Still, thin-skinned plastic munitions, with the right UV and environmental treatment, can be remarkably robust.
Primary Explosives
Different primary explosives deteriorate in different ways. Mercury fulminate produces quite stable compounds as it decomposes leading to a munition that is less likely to function. Tetracene on the other hand decomposes in the presence of moisture and > 60\(^{\circ}\) Celsius to form more sensitive compounds (eg 5-azidotetrazole).
Another common primary explosive degrades in a way that is more sensitive to the precise environmental conditions. A decay product of lead azide (hydrogen azide gas, HN\(_{3}\)) can react with metals such as copper, zinc, cadmium, or their alloys to form highly sensitive explosive compounds. This is why lead azide detonators are pressed into an aluminium liner when encased in such metals. On the other hand, whilst some moisture egress leads to the formation of hydrogen azide, if it is saturated it can lead to reduced functionality. Mines found in the Falkland Islands are suspected to have suffered from this defect.
Secondary Explosives
The main fill and booster explosives that have been found in ordnance from as far back as World War I has proven to be very stable. TNT and Comp B found from the world wars is still recovered in good condition with minimal deterioration that usually has no significant impact on performance. An exception is picric acid (or 2,4,6-trinitrophenol, TNP) which was used as a booster and HE fill but also reacted with certain metals to form sensitive compounds called picrates.
Propellants
Compounds such as nitrocellulose (NC) and nitro-glycerine (NG), often combined to produce double-based propellants begin reasonably stable but decompose over years, specially when subjected to high temperatures. There are particular bonds (the O-NO\(_{2}\) bond of the aliphatic nitrate esters) that decompose to form nitrogen dioxide and an alkoxyl radical. Stabilisers are used for these NG/NC compositions but themselves have a limited effectiveness period of around 20 years. Over time, propellants become dry and brittle, making them even more prone to ignition when exposed to heat or friction. Propellants are typically far more sensitive to ignition than HE and are the more likely cause of ammo depot fires.
In sealed containers (eg most rocket motors), the nitrogen dioxide gas remainds trapped and increases the pressure, which in turn accelerates the break-down of propellant. Eventually this can lead to spontaneous ignition or explosion in a process known as autocatalytic initiation. Several major stockpile explosions have been attributed to autocatalytic initiation.
Materials
The material choice used for the manufacture of munitions is the result of balancing performance, planned lifetime, and cost factors. Especially when exposed to the elements for an extended periods rather than stored in a controlled environment, the temperature cycling and generation of decomposition gases can lead to pressure changes that can crack certain types of casings, specially plastic ones.
The most important factor in how the munition ages in terms of the casing is the thickness because most degradation is a surface effect. The material used also plays a large role; aluminium and stainless steel produce oxides that form a protective film, but iron oxide is porous. Copper-based alloys like bronze and brass have some level of corrosion resistance. Bronze (alloy of copper and tin) is well suited to seawater applications and can be enhanced further by adding some silicon to the alloy. Brass’ (copper/zinc alloy) corrosion resistance can be improved with tin but is made worse by higher zinc.
Even given all this, if the rust is held in place by another material (such as clay), it may retain some degree of protection and so the thickness still plays an important role and the rate of corrosion will decrease if the outer oxide layer remains intact.
Plastics are resistant to corrosion in the ways that metals corrode, but can be degraded in other ways. Plastics are vulnerable to UV damage and in certain cases heat. As a result, thickness is still significant for the casing to protect the contents. Of all the casings, wood is the least resilient to weathering. Even treated it will tend to rot within a few years, which affects wooden mines like the Russian PMD-6 or bounding mines with wooden stakes. Despite all of that, if the munition ends up encased in something that itself protects it from the elements, it can last a remarkably long time intact.
Design and Production
The design can drastically impact the longevity of the munition, sometimes by design. Little gravel mines used by the US in Indochina were encased in cloth and only designed to function for a few days. Self-neutralisation as a result of moisture ingress is a design feature. On the other hand, thick fibreglass casings can be robust almost indefinitely in some cases.
The overall case material is not the only important design characteristic. Welds, seals, or even poor assembly can reduce how resistant the munition is to weathering.
Damage on Employment and Differential Weathering
Quite stark differences can exist between identical munitions landing short distances apart based on how it lands or is placed. A munition landing in a well sheltered position, above the water line, that did not sustain damage on arrival, can last much longer than one that breaks, abrades, or is subjected to harsher micro-climactic conditions.
The original publication this is based on is from the Geneva International Centre for Humanitarian Demining (GICHD) and can be found at gichd.org under Publications.
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