Electro-Chemical Hole Cutting
James Trevelyan
Department of Mechanical and Materials Engineering
The University of Western Australia
Nedlands 6907, Western Australia
E-mail: jamest@mech.uwa.edu.au
Web: http://www.mech.uwa.edu.au/jpt/
Technical advisers working on UXO neutralisation in Laos asked us if there are ways to cut holes in bomb casings to access the explosive charge, avoiding the need to detonate the bomb to render it safe. One technique being explored is abrasive water jet cutting. David Summers and his team at the University of Missouri, Rolla, USA, have performed an extensive series of tests on this technique.
It occurred to me that electro-chemical machining (ECM) might offer an effective alternative. It seemed possible that ECM could be achieved without the need for a high pressure water pump, reducing the amount and cost of the equipment.
Electro-chemical machining is, in effect, a greatly accelerated process of corrosion. Photographs of small marine animals which eat through 25 - 50 mm thick steel piping supporting oil platforms in the Carribean Sea inspired this work. These animals generate small electric currents which accelerate and concentrate corrosion of the steel under the animal.
A copper cathode (negative) promotes the following chemical reaction in a salt water bath by supplying electrons:
At the steel anode (positive), electrons are withdrawn, allowing iron to form soluble ions and dissolve in the ferrous form, creating a green solution:
The copper electrode is a small diameter tube through which salt solution is driven by a pump onto the steel surface.
In normal corrosion, the iron combines with atmospheric oxygen to form oxides (rust). However, in this situation, the iron stays in the salt solution before slowly forming the more familiar red-brown iron oxides. The process can be used equally well to inhibit corrosion. By making the steel the cathode (negative) instead, the electric current inhibits the tendency of the iron to convert into ions. This is widely applied to protect marine structures and is called 'cathodic protection'.
The aim of the experiment was to assess the possibility of using cheap and simple equipment for cutting holes in steel. The diagram shows the equipment used:
Bucket of concentrated salt
solution (Sodium Chloride - common salt)
Windscreen washer pump
Car battery with appropriate connections, switch and wiring
3 mm diameter copper tube
flexible connecting tubes
1 mm thick steel test plate.
The top surface of the steel plate was prepared by rubbing off the paint with steel wool.
The copper tube was held in place by hand, applying a gentle force to keep the copper tube 'floating' against the water pressure from the pump. Occasionally the copper tube touched the steel causing a small spark, but there was no tendency to weld. This would not occur with a proper positioning mechanism, of course.
In spite of the primitive equipment and the relatively low current (not measured, but the wiring could not supply more than about 3 - 4 amps) the metal removal rate was much faster than I expected. The initial removal of metal (see diagram below) was apparent after just a few seconds, and the tube could be used as a 'wand' to etch a pattern in the steel surface. After 2 - 3 minutes of steady pressure, a substantial hole was apparent, and after 4 minutes the hole stopped getting deeper. A small dark circle was apparent at the bottom of the hole: the underside of the paint layer. The water pressure was not sufficient to break this, but a slight push with a pointed tool dislodged a neat circular flake of paint to reveal a hole through the steel.
Initially the water flowed across the steel surface. However, once the hole deepened, the water sprayed back, and I lessened the pressure to avoid being covered with salt spray. Keeping the copper tube close to the steel promoted rapid etching and a 'smoke' was visible: tiny water bubbles of (presumably) hydrogen gas forming at the copper electrode.
The process was surprisingly effective and quick. The resulting hole was about 4 mm in diameter, though the end of the hole was about 2 mm in diameter. The remaining steel at the end of the hole was very thin and easily removed by hand.
The windscreen washer pump became very warm and is clearly not designed for continuous operation. A better engineered pump will be needed for extended tests.
The speed of operation clearly depends on the current density which can be achieved. The design of the copper tube electrode may influence this. The choice of electrolyte (salt solution) may also be important.
The most important aspects requiring immediate improvement are:
Design and development of a positioning mechanism for the copper tube electrode to provide automatic feed motion (see diagram),
automatic short circuit protection, subsequent tube position adjustment, and automatic re-starting,
enclosing the work space to contain the salt spray, and
background research on electro-chemical machining techniques to determine the most appropriate operating parameters.
This work was financially supported by the Night vision and Electronic Sensors Directorate of the US Army, Fort Belvoir, Virginia, and partly by private financial donations. Thanks to colleagues who helped: Robert Heath who designed and built the magnet, Brian McLean and Dino Busuladzic who spent hours under a hot sun searching for remaining metal fragments, and members of 13th Field Squadron, Australian Defence Force who operated the equipment and supported the trial.