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Electroless nickel technology for the protection of mild steel has moved on a lot since Dr House was first involved in a study of the subject.

However the basic facts of the technology remain. "We can't make a silk purse from a sow's ear and the substrate must be prepared correctly.

Today we would take steps we didn't consider 25 years ago, and at that point the British Electroless Nickel Society hadn't issued its Code of Practice, available from the Metal Finishing Association (www.sea. org. uk)", he says.

Dr House continues: "Twenty five years ago doing a study with UMIST, in which most of the chemical suppliers were present, we never seemed to get consistent results. So much seemed to depend on the substrate and on the mode of surface preparation. One factor, which became very clear, was that the use of hydrochloric acid pickles was to be avoided at all costs if the best corrosion protection was going to be achieved."

"Another 12 years or so later I was in Hamburg, looking at some work done by Professor Kreyer and his students on the effects of heat treatment of electroless nickel deposits. The cracks in the deposit were clearly visible, but if one increased the temperature those same cracks closed, but as I recall at the expense of the hardness."

Suitable heat treatment of the electroless nickel deposit will increase its hardness, and such treatment may well enhance the erosion and abrasion resistance of the deposit.

Unfortunately, the heat treatment (around 400ºC for 1 hour) also causes re-crystallisation between the nickel and the phosphorus.

The deposit shrinks and cracks, and thus the corrosion protection becomes severely impaired.

Far more recently, Nitec (UK) Ltd, has been investigating the same kind of approach to create deposits with good corrosion protection for use on items immersed in the North Sea.

Salt spray testing has been used to test for the point of corrosion to red rust spots. The results to date are very encouraging. Some eighteen months through the testing there is no sign of rusting. Normally 1000 hours would be considered a good result. The testing continues, and if this works for other applications the product lifetime is going to increase significantly and so we will not require as many replacement parts.

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