SubscribeBecause it combines strength with light weight, aluminium is a popular choice for the manufacture of electrical interconnect components used in defence applications.
Adequately protecting aluminium against corrosion is, however, no easy matter, as Xavier Ducasse of Icore International explains. Aluminium is an intrinsically reactive metal and, as such, might be expected to be susceptible to rapid corrosion.
In practice, it protects itself by forming a corrosion-resistant surface oxide layer when exposed to the air. This layer is, however, comparatively fragile, and it certainly provides insufficient protection for aluminium components used in demanding defence applications.
Anodising, which provides a thicker oxide layer than that which forms naturally, provides much more effective corrosion protection. Unfortunately, the oxide layer has poor electrical conductivity, which means that anodising is not an appropriate solution for electrical interconnect components.
In the past, these considerations have led to the widespread use of cadmium plating. To be strictly accurate, this involves plating the component first with nickel and then with a layer of cadmium. This plating has excellent electrical conductivity, and is widely held to provide effective protection against corrosion. As we shall see, however, recent tests cast considerable doubt on the true effectiveness of this protection.
There is another issue which weighs against cadmium - its toxicity. For this reason, legislation has been enacted in many countries to limit its use. In Europe, for example, cadmium is one of the substances covered by the Restriction of Hazardous Substances (RoHS) Directive. While this does not apply to equipment for defence applications, it does underline the importance of finding alternatives for cadmium.
To achieve this objective, some vendors have proposed the use of electroless nickel plating on its own, without the cadmium top layer. Unfortunately, the nickel plating process gives rise to discontinuities in the form of micro-cracks in the plating. These cracks allow contaminants to penetrate easily to the aluminium surface, greatly reducing the value of the plating.
Let us now consider how the effectiveness of corrosion protection is evaluated. The most common approach is the elevated salt-spray test, which is typically carried out under conditions of near neutral pH. Even under these test conditions, electroless nickel plating performs poorly but, in contrast, cadmium plating gives a good account of itself, even when there is local penetration of the plating.
In the field, however, the story is very different, with local penetration of cadmium plating invariably being rapidly followed by corrosion of the aluminium subsurface. Icore has found that these practical results are closely replicated by the US Navy's combined cycle salt-spray and acid gas test, most probably because this test seeks to match more closely the pH levels encountered by in-service equipment. The in-service environment is often acidic as a result of engine exhaust gases combining with environmental moisture.
Put plainly, these tests conclusively demonstrate that neither cadmium nor electroless nickel plating provides adequate protection for aluminium interconnect components used in demanding defence applications.
Fortunately, alternatives have now become available. Arguably the most effective is to eliminate aluminium altogether and, instead, to manufacture connector backshells and similar components from modern composite materials. Like aluminium, these materials are lightweight and strong but, unlike aluminium, they are totally resistant to corrosion.
Composites do, however, have one major limitation - they are non-conductive. To provide the conductivity needed in electrical applications, they must be plated with a metal layer which is, of course, potentially open to corrosion.
Icore's solution is to plate the shell with a layer of copper, and to overlay this with a protective electroless nickel layer. In spite of the limitations of electroless nickel which were discussed earlier, extensive testing has revealed that connectors treated in this way show no noticeable corrosion after a full 2,000 hours of exposure in a salt-spray cabinet.
The reason is not hard to understand; both copper and nickel are much less chemically reactive than either aluminium or cadmium and are, therefore, much less susceptible to corrosion. Further, the electrochemical EMF generated by contact between copper and nickel is small, so the galvanic corrosion seen with many other combinations of metals does not take place.
This work confirms that composite connectors are an excellent solution to the problems of corrosion. It has to be acknowledged, however, that there are applications - such as those involving existing equipment or approved designs which cannot be easily modified - where the use of aluminium connectors is unavoidable.
In tests carried out by Icore where composite and metal components are used in combination, the composite products remain corrosion free, while the metal parts corrode to exactly the same extent as would be expected in all-metal systems.
To address this issue, a second approach to achieving enhanced corrosion resistance has been developed. This is the TTH system, Icore's proprietary co-deposited nickel-PTFE plating system for aluminium components, which has been proven to provide protection which is substantially better than the best cadmium plating.
Nickel-PTFE plating also offers other important benefits. For example, it is highly resistant to abrasion, making it a good choice for components used in applications such as bomb doors and aircraft landing gear, where stainless steel would, in the past, have been the only suitable option.
Further, the plating has a very low surface co-efficient of friction, which means that it resists the adhesion of contaminants. In marine applications, this is particularly useful since it eliminates the problem of barnacle growth.
The protection provided by TTH plating has been validated by extensive testing. In conventional neutral pH salt-spray tests, the ratings usually considered as acceptable for the various types of protective plating are 48 hours for nickel used alone, 250 hours for conventional nickel-PTFE, and 500 hours for cadmium over nickel. In contrast, independent tests have shown that TTH easily achieves ratings well in excess of 2,000 hours.
Equally impressive results were obtained in the US Navy's combined cycle salt-spray and acid gas test which was mentioned earlier as better reflecting real-life operating conditions.
The performance of the plating has also been confirmed by practical operating experience. In one example, the cadmium-plated electrical interconnects used in a marine helicopter tail rotor assembly had a typical service life of only a few months. The TTH replacements, which mate with the same stainless-steel connector, are still in operational use after six years, and show virtually no signs of corrosion.
Similar results have been reported for above-deck interconnects in US Navy AEGIS-class destroyers.
The experience of many users of electrical interconnects in the defence arena is that corrosion resistance is often inadequate, sometimes leading to problems and failures in mission-critical installations. The pressure to move away from the use of cadmium on environmental grounds has the potential to make this situation worse.
The most complete and effective solution is the adoption of composite components. These deliver all of the virtues of aluminium, while eliminating its high reactivity and corrosion susceptibility. If, however, metal components must be used, modern nickel-PTFE plating systems, such at the TTH system, provide results which are broadly comparable.
Finally, it is worth stressing that, whichever of these approaches is adopted, the corrosion resistance achieved will be many times better than that offered by traditionally manufactured components, and that this difference will be most marked when it really matters - that is, under the most demanding of operating conditions.
