Why is my prestige aluminium autoclave corroding adjacent to the external heating element?

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I have been using two prestige 2100 aluminium body autoclaves - these are really just like a pressure cooker used at home, but with additional control of the heating process. These are heated by an 240 Volt electric element, embedded into the base of the pressure vessel. The manufacture recommends using de-ionised or distilled water. I use water directly from our home water softener which uses an ion exchange system of operation, e.g. passes water across a resin bed, I understand this provides me with de-ionised (soft) water.

Over a period of 3 years regular use of the autoclaves these have both developed severe pitting to a depth of 3mm on the inside of the pressure vessels - but only within 20mm of the heating element. I read that De-ionised and distilled water will attract ions from the metal. However, could it be electolytic corrosion, caused by localised electrical currents in the metal adjacent to the magnetic field of the heating element? Or a combination of both? Comments appreciated on how to resolve this problem.

Tony Thom
User - Hampshire, England

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Your domestic water softener gives you soft water, but not de-ionised water. It is water whose "hard" cations (calcium, magnesium etc) have been exchanged for "soft" cations (probably sodium) so that the resulting water will not form insoluble crud with soap. It still contains cations. Your corrosion is probably linked directly to the use of this water. If you were to use distilled or de-ionised water, then you would not be exposing the aluminium to the alkali salts in your "soft" water.

Bill Reynolds
consultant metallurgist
Ballarat, Victoria, Australia