7000 series aluminum alloys

Dear Sir; I am a chemist at an aerospace industry in Israel.  Lately we have been experienced problems with corrosion when machining aluminum parts 7xxx.  We suspected the water to be the main reason as we use only tape water with 350ppm CaCO3 and 210ppm chlorides.  We noticed that the problem is more acute in winter.  We are going to move to deionized water.  Can you give your opinion on the phenomena and can you advice us about changing to deionized water. Sincerely Yours,  MY
Filed Under: 
Maintenance / Troubleshooting

Anonymous (not verified)

Fri, 2011-03-11 15:12

Dear MY. Certainly 350 ppm as CaCO3 is very hard water however, the 210ppm chlorides are far more troublesome.  Chlorides do cause corrosion and as such should be addressed.  I agree a deionizer well help resolve many hard water related problems, but remember the chloride issue must be handled separately.  The best approach is to work with your DI supplier and above all make them aware of your chloride situation.  They will have filters to take of both problems.  I Hope this helps thanks for reading metalworking Fluid Magazine. All Best, Dom 

Anonymous (not verified)

Fri, 2011-03-11 15:12

Yes I agree the water quality will have an influence and the level of chlorides quoted would not be acceptable even for ferrous materials. There are two other factors in my experience.  One is the length of machining time and the second is the alloying elements present in series 7000 alloys.  You need to work with a coolant that is specifically designed for this alloy.  It requires a high level of metal pasivator combined with a lower pH. I have seen several 'aluminum specific' coolant fail on this type of alloy. Rod Beaumont, Technical Applications Manager Polartech Ltd Rod Beaumont