Every fan of crime fiction knows that a good way to figure out whodunit is to check for fingerprints on bullets. A couple of years ago, Dr John Bond from our Forensic Research Centre – who is also the Scientific Support Manager with Northamptonshire Police – developed a way of visualising fingerprints on brass bullet casings even if they had been wiped clean.

This generated enormous interest. A panel of experts brought together by BBC Focus magazine cited it as one of the technologies ‘most likely to change the world’. Time magazine included it in a list of ’50 best inventions of the year’. It was even featured on US TV show America’s Most Wanted.

Now Dr Bond has applied the same principle to an industrial application by inventing a handheld device for quickly, cheaply and easily measuring corrosion on copper and brass machine parts.

The potential, as can be plainly seen, is enormous. Corroded parts need replacing before they break and cause damage or injury, but replacing them too soon creates needless expense. Accurately measuring corrosion, however, is often difficult, time-consuming and expensive. There are currently three common methods of corrosion analysis:

• Corrosion Coupons
  A corrosion coupon is simply an extra bit of metal attached to a machine which can be easily detached and weighed on a regular basis (typically every 90 days). The change in weight of the coupon as the metal oxidises indicates its level of corrosion and this is taken to be indicative of the overall machine.
• Electrical Resistance (ER)
  Like corrosion coupons, this involves attaching an extra, non-functioning bit of metal to a machine. But unlike the previous technique, there is no need to remove the piece to test it, Instead of weight difference, a small electrical charge is passed through the metal; as corrosion increases, so does resistance.
• Linear Polarisation Resistance (LPR)
  A complex electro-chemical technique which we won’t go into here. Suffice to say that it only works properly in clean, aqueous environments so its use is pretty limited. There are also a number of highly specialised corrosion analysis techniques.

And now there’s John Bond’s device (which doesn’t yet have a name) – a really simple, handheld doodad which could revolutionise the way that corrosion is monitored. It relies, like the fingerprint detection technique, on something called a ‘Schottky barrier diode’.

The name honours German physicist Walter H Schottky who, in 1938, came up with an explanation for what happens to an electrical current passing between a metal and a semi-conductor. The junction of the two substances is a Schottky barrier and can function as a diode (a device which allows a current to flow one way). This type of diode can be switched on and off much more rapidly than one formed from two semiconductors. The copper oxide cat’s whisker used to tune early radios is a very basic Schottky barrier diode – although it obviously would not have been called one at the time.

Brass is, of course, an alloy of copper and zinc (not to be confused with bronze, which is an alloy of copper and tin) and the new device works by measuring dezincification*; in other words, a change in the ratio of copper to zinc on the surface of the brass. Corrosion creates a layer of zinc oxide or copper oxide which acts as a semi-conductor and, with the remaining brass, forms a Schottky barrier. The ingenious bit is that, electrically, copper oxide behaves exactly the opposite to zinc oxide (what are termed 'p' type and 'n' type semiconductors). This makes it easy for Bond's device to be able to tell them apart.

By using two probes – one of zinc, one of platinum – Bond’s device is able to measure the drop in potential across the Schottky barrier diode in both directions, or 'forward and reverse bias', from which the level of dezincification can be calculated – and hence the level of corrosion. The tip of each probe has a diameter of about one millimetre which is small enough to give a good, clean contact without being so sharp that it damages the metal it’s testing.

If this all sounds terribly complicated, don't worry. The nub of it is that a cheap-to-manufacture, handheld device – basically a zinc rod and a platinum rod that you touch onto a brass machine part – can give an accurate, instant, direct readout of the level of corrosion. Which has got to be a lot, lot easier than measuring the weight or resistance of extra bits of metal.

A paper describing John Bond’s invention has been published online by the Review of Scientific Instruments. All that is needed now is a savvy business partner to manufacture and market the device. And a name.

• University press release
• Measuring dezincification of brass by Schottky barrier diodes formed between semiconductor corrosion products and brass (doi:10.1063/1.3484285)

*This may be the most glorious new word you’ll learn this week. Say it out loud. Isn’t it great?