Dear Jef and Sherwood
The German “Working Group Instrument Preparation” have investigated colourless anodized aluminium and coloured anodized aluminium of medical devices regarding the behaviour towards different process chemicals for reprocessing of medical devices and towards different water qualities.
With this answer I will explain the influence of different water qualities to anodized aluminium.
At first, my statement which is proved by al lot of experiences and investigations: Deionized water at high temperatures of 60° to 90 °C will protect and stabilize anodized aluminium surfaces. On the other side, softened water at 60° to 90 °C will destroy the anodized surface. The reason for that is as follows:
Softened water is produced by ion exchanger and Calcium and Magnesium salts are changed to Sodium Salts. Softened water has a high content of Sodium hydrogen carbonate. If this water is heated up Sodium carbonate will come into existence. So heated up softened water has an alkaline pH-value. The higher the carbonate hardness of water the higher the alkalinity of the produced softened water.
Enclosed you will see results of test washings:
Material:
Blue anodized aluminium specimen of company Aesculap, Tuttlingen which are the same as Aesculap containers.
Washer Disinfector:
Miele G 7735
Washing program:
Water was heated up to 93 °C for ten minutes without any process chemicals
Water Qualities:
Deionized water and softened water
Softened water:
1 time washed: 74 %
5 times washed: 27 %
10 times washed: 2 %
Deionized water:
1 time washed: 99 %
5 times washed: 86 %
10 times washed: 93 %
The thickness of the brand new aluminium oxide layer was 15 µm, expressed with “100%” in this chart. After one wash cycle with softened water the thickness was reduced and after some more washing cycles Aluminium oxide layer was completely removed with softened water.
On the other side, deionized water will protect the Aluminium oxide surface, the layer becomes more compact which was shown by additional measurement of the electricial conductivity.
More interesting results we found out with combinations of different detergents dissolved in different water qualities.
Dear Sherwood, the behaviour of anodized aluminium against different water qualities and against different detergents is more complicated as you describe in your statements. Examples: We can wash anodized aluminium in a washer disinfector with a washing solution of pH between 10 and 11 without any problems due to the inhibitors like silicates in cleaning agents, but using another washer disinfector which works with a pH neutral cleaning agent we can have problems with anodized aluminium due to the softened water. Some washer disinfectors have built in water softener, some not. Sometimes washer disinfectors are feeded with normal hard tap water, sometimes with softened water or deionized water. This is quite different and should be investigated in each case, when anodized aluminium is harmed.
Dear Sherwood,
even with 30 years of experience as a chemist you should never give up learning, same for me with 26 years of experience as a chemist.
Please have a look at http://www.a-k-i.org
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