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Problems black anodising after vapour blasting

We have an aluminium component AL6082-T6 (equivalent to AL6061-T6) which we pre-machine, vapour blast the outer surfaces and then send for black anodising. After anodising the component undergoes final machining and then these surfaces are Alodined. The results has been horrific on these vapour blasted surfaces where it looks like the alodining process has "eaten" into the anodising as the whole surface which was vapour blasted is now smeared with a yellow/Alodine smudge and if looked at under the microscope one can see alodining in the pores of the anodising. The interesting thing is that the area that was not vapour blasted is pristine and the alodining has left this surface unaffected. Has anyone seen this effect? My feeling is that it obviously has to do with the vapour blasting. Could it be contamination during the vapour blasting? Could it be that the vapour blasting somehow changes the surface structure of the aluminium causing the anodising sealing process not to be effective on this surface? Any comments, ideas or experience would be appreciated.

Louis Kaltenrieder
Engineer / designer - South Africa
October 6, 2010

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Dear Sir,

Anodising process needs good pretreatment process with microetching. Your main problem which I feel is that the Alodine treatment after anodising. This Alodine chemicals is reacting with the anodising coating. Think to avoid after treatment.

DR.DIPANKAR SEN
- KOLKATA, India
October 7, 2010