You will have seen them everywhere. Those metal siren boxes attached to houses which have rusted well beyond their use by (and working) date. This picture shows one in my neighbourhood. It is fairly typical. They are an eyesore when in this condition and they are probably not working anyway.
Anything made of steel will rust when exposed to moisture. The question becomes whether it can be weather-proofed well enough to stop moisture attacking the metal. The unit we replaced here is an Altron Condor AC36 alarm unit. Many of these have been used on houses throughout the country. It is robustly built with a powder-coated outer box, plus an inner steel box which is passivated and houses the siren, battery and control circuit board.
One dark and cold night, the alarm went off. Owners were home and the alarm was not even set. There was no means of stopping it. One has to get the ladder, climb the roof, undo the outer box, then the inner box,and disconnect the wiring from the siren. Or if going outside at night in the cold weather is not to your liking, then you have to get into the indoor unit, turn off power, remove the indoor unit battery and wait for the outdoor unit battery to run down.
To my way of thinking, any electronics in an outdoor unit can falter eventually and the above scenario never appeals to me. I always remove the outdoor batteries and also have a secret power disconnector in the house. That way if the outdoor electronics does set off with a mind of it's own, then power can be removed quite quickly from inside the house. Pro alarm installers will say that's not secure, however I think that in a domestic case, it is good enough, especially if the outdoor unit is mounted in a high, inaccessible position.
This picture shows the condition of the inner box of the Altron outdoor unit. This clearly shows significant rusting meaning water did get in. No doubt in the environment here, being 5km from the sea and being prone to extreme winds, it is difficult to keep salt laden sea air and moisture out, but that is what a product 'fit for purpose' has to do!
I eventually got into the inner box to have a look at the circuit board, but that was just for professional interest, since the owner had already decided to replace this unit. Once I ground off the rusted bolt heads, the AC36 circuit board had a lot of dust and dirt all over it, some corrosion and verdigris build-up, but wasn't in too bad a condition. Some may say that 18 years of service is all that can be expected, but to my mind, these sorts of products should be designed for longer.
The replacement unit is a 'plastic fantastic' of no discernable parentage, actually made of polycarbonate, so I expect rusting will not be a problem. It was bought on 'TradeMe' for $35. It is smaller, not as loud as the original, neither will it withstand many blows with a baseball bat, however it is high and fairly hard to access, so I expect it will be enough of a deterrent. Wiring was simple enough. This one needs two wires for 12V power, and two wires for the alarm trigger. It can be wired either to trigger with a pull down or a pull up to 12V depending on the needs of the indoor alarm. It has an anti-tamper switch, but this this will have only token usefulness. There is provision for an internal 6 volt battery but we didn't install one.
Axino-tech Consulting & Services , June 2012.