Video Extras 5: The Reef
The reef diaries
This corrosion
You never know when you’re going to need an underwater burglar alarm, but how to stop it from rusting? Discover how you can rely on sacrificial metals.
Follow Mike in this exclusive video extra as he grapples with the corrosive effects of seawater, as part of the fifth BBC/OU TV series Rough Science, based in Zanzibar
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Kate: Now then Michael B.
Mike: Yeah.
Kate: Can I make a little observation.
Mike: Yeah.
Kate: You're looking a little bit smug
[LAUGHTER]
Kate: Have you solved your corrosion problem?
Mike: I have. You realise - yeah, no - I was worried about the corrosion of the trip wire. So I did a little experiment last night. I look a nail like that.
Kate: Mmm. Mmm-huh.
Mike: And I just left it bobbing up and down in seawater.
Kate: Yeah.
Mike: Like our tripwire will be.
Kate: Yeah.
Mike: And look how it's rusted! Just overnight.
Kate: That's amazing, how much -
Mike: It's rusted.
Kate: Really has, hasn't it?
Mike: So we did need to worry about corrosion.
Kate: Yeah.
Mike: Now there are several ways you can stop that corrosion happening.
Kate: Yeah.
Mike: You could coat the wire in zinc, and that's - this screw is coated in zinc. It's galvanised.
Kate: Right, yeah.
Mike: You see.
Kate: It hasn't rusted at all no.
Mike: No, that's been in salt water overnight. Nothing.
Kate: So are you proposing to cover the trip wire in zinc?
Mike: [LAUGHS] No we don't have the resources to do that, no.
Kate: No.
Mike: But there's an even smarter way of dealing with corrosion. What we use is a bit of magnesium. It is more reactive than the iron in the nail. And it will react with the oxygen in the air.
Kate: Yeah.
Mike: It will corrode and it sacrifices itself for the iron. And you can see, you compare these two, they've been in salt water for the same amount of time. But this one has been connected up to magnesium and you can see, no corrosion.
Kate: That's extraordinary. So basically, the magnesium takes all the flack, if you like, instead of the iron.
Mike: That's right. It sacrifices itself for the sake of the iron.
Kate: So how are you going to build magnesium into the system? Does that mean you're going to now have to find metres and metres of the stuff?
Mike: We only need to put a little bit in and not many people know this, but a lot of pencil sharpeners are made of magnesium. The body of that is magnesium, so we just put that in, the appropriate point in…
Kate: What you just hang a pencil sharpener?
Mike: Yeah.
Kate: Within the system?
Mike: Yeah …
Kate: And that will do it?
Mike: It will stop our, our tripwire corroding. Er, strange but true. It's electro-chemistry.
Kate: Phew!
Content last updated: 01/03/2005
