Sealed up - preventing military equipment corrosion in tropical climates

Materials World magazine
,
24 Sep 2013

With government spending on defence a contentious subject for the general public, equipment developed with tax-payers’ money needs to be well preserved and maintained. Harry Lovell reports on a new material developed in Australia that could help combat corrosion in the tropics.

In a country that experiences a wide range of weather conditions virtually every day, it is hardly surprising that the prevention of corrosion is a primary consideration. Along Australia’s coastline, the effects of sea air and moisture are reflected by building design and materials. Humidity accompanied by high temperatures in particular is a significant factor in the country’s climate. Aside from the considerable range of climatic conditions within the country, the tropical environments of Australia’s nearest neighbours in the north serves as a reminder of the stringent demands on the equipment and materials used not only by defence forces, but also by industrial operations such as mining. Therefore, safeguarding against corrosion is an essential requirement for many sectors of Australian industry, whether they take place in the inner regions of the country or close to the sea. This has resulted in the ongoing development of protective materials capable of withstanding severe atmospheric conditions and providing an environment that incorporates protection against corrosion.

Australian defence forces are at the forefront of the demand for corrosion protection. In addition, the nature of military operations requires that when equipment reaches its ultimate destination, it should be in a condition requiring minimum attention prior to bringing into service. Medium- and long-term storage may still demand minimal preparation for use, and the working surfaces must be free of any corrosion, however minute it may be.

The occurrence of corrosion is well known. Around 2,500 years ago, Greek historian Herodotus mentioned the use of tin in conjunction with iron, and later Roman philosopher Pliny the Elder wrote of ferrum corrumpitar. In 1800, the first papers alluding to the electrochemical nature of iron appeared, while Faraday (1791–1867) produced his first and second laws, which were used for the calculation of corrosion rates in metals. Although corrosion was well known, combating and preventing its occurrence took many years. In the 1900s it was found that when dicyclohexylamineammonium nitrite (ICHAN) was vapourised and deposited on metals, it could provide protection against corrosion. However, this only applied to iron and aluminium, and they were deleterious to copper and bronze.

The search for corrosion protection for military equipment goes back to World War II, which brought the problem into sharp focus. Serious attention was first raised in respect to military stores held at Port Moresby in New Guinea. Corrosion was first apparent in radio sets but came to the forefront when hand grenades failed to explode. It was thought the high humidity had affected the potassium nitrate in the gunpowder component. This problem was exacerbated when the dry batteries in mine detectors failed, resulting in tanks advancing in areas the drivers thought to be clear then being confronted with exploding mines. This led to a joint effort by US and Australian forces to focus on the problem of corrosion and deterioration brought about by high humidity.

Their work revealed that tropic proofing was essential for a wide range of equipment, including not only metals but also organic materials, due to the action of fungi and effects of high levels of humidity. In particular, optical equipment was found to be prone to fungal attacks. While action against fungi could be improved to some extent by fungicides or fungistatics, overcoming the effects of moisture was just as challenging due to the high humidity profile of the region. The emphasis was understandably on instruments both electrical and optical, but the treatment of metal surfaces was not neglected. Ferrous metals and aluminium were initially addressed with varnishes and wax-based materials, however, these proved to be ineffectual over time. Clearly, high humidity accompanied by high temperatures accelerated corrosion. The army had a partial solution – Oil A, which had been available for some time but was only effective in drier climates. In addition, some suitable paints had been identified and rust preventatives, including zinc and aluminium napthenates, were introduced. The range of items needing to be treated included canvas, leather and food, and as in previous wars, R&D on corrosion accelerated the production of protective materials.

More recently, research into efficient and effective storage of large military equipment has become a focus for one Australian firm – Australian Inhibitors. Evidently, to achieve maximum protection it is essential to create a controlled environment. And so the research began.

More than a decade of work has resulted in a new product the company calls Ferro Foil, which is a complex laminate that combats the problems of corrosion and its effects on vulnerable materials. The material’s layers are comprised of oriented polypropylene, triple layer polypropylene, foil and finally triple-layer, co-extruded, volatile corrosion inhibitor (VCI) PE film. It is waterproof, greaseproof, flexible and heat-sealable.

All in the use
The material is formed into bags from sheet material, the equipment sealed inside and a vacuum drawn. The bags can cover a wide range of equipment sizes, from small arms to field guns and even larger. By vacuumising the bag prior to final sealing, the vapour phase inhibitor is free to fulfil its role. A 12-year storage trial carried out on a field gun demonstrated the capability of the system to prevent corrosion and deterioration of equipment, which were not only complex but, also incorporated different materials.

Ongoing development has resulted in the bags being fitted with valves, enabling insertion of gases such as nitrogen as well as facilitating topping up. This ability to modify the atmosphere and incorporate monitoring instruments has many applications. A major hurdle faced by the Australian army and other services is the movement of equipment in and out of the country. The armed services have to comply with the requirements laid down by the Department for Agriculture and Food, which also specifies appropriate cleaning procedures. Unused equipment placed in a sealed bag facilitates its passage through ports.

While the prevention of corrosion is a major target, the design and protective characteristics of the multilaminate material are of equal importance. Therefore, in the case of large or heavy pieces of equipment, containers or pallets are used to facilitate loading, unloading and storage.

Although initially developed for military use, the system has other applications, for example in the mining industry, where the environment is dusty and penetrative. Medical use is another possibility, such as in field hospitals where sterile equipment can be made available quickly and with some guarantee that sterility has been maintained in a pre-packaged state. Even valuable art works have been transported using nitrogen, together with environmental monitoring.

Overall, the packaging material and its protection against corrosion provides a robust system for military and other uses. It is capable of providing protection to very large pieces of equipment as well as small individual components and, with the incorporation of monitoring instruments, is able to serve a wide range of applications.  

Testing for success
To ensure the final film meets the requirements made of it, a number of testing procedures are essential.

Tensile strength ASTM D2103 (Standard Specification for Polyethylene Film and Sheeting) Flexible plastic materials, in this case a laminate, must be able to withstand tension during the manufacturing operations. In its functional role, the laminate is required to meet exacting demands. Tensional strength is a directional property in single films. However, cross lamination of materials imparts an optimal balance of non-directional tensile strength and elongation in materials, as well as increasing the resistance to tearing and elongation. The assessment of tensile strength is ideally determined by stretching the material and measuring the force required in MPa, then producing a stress strain curve. Alternatively, for routine purposes the use of a known weight over a predetermined time provides a simple monitoring procedure.

Break strength (TAPPI T470) This test is derived from the TAPPI test to measure the edge tearing resistance of paper. It is also used for plastic film to assess resistance to tearing during flexible packaging manufacture and to assess plastic films from the consumer aspect in respect to opening packs. Essentially, a strip of the laminate (with a v notch) is grasped between jaws and force applied until the tearing takes place.

Puncture resistance (ASTM 1709) Designed to evaluate the impact strength or toughness of plastic films, in this case used to assess the plastic/ foil laminate. The test uses a dart (available in two sizes) that falls from two predetermined heights. The drop weights can be varied according to requirements.

Water vapour transmission rate (ASDTM e96-2005) Determines the time taken for water vapour to pass through a material. A specimen is sealed to the open mouth of a test dish containing desiccant or water, then transferred to a controlled atmosphere. Weight is measured periodically and plotted as a function of time, with transmission rate determined by dividing the slope of the curve with the area of the dish opening.