Anodising Aluminium - Question for the Boffins

[mafioso]

I have been anodising aluminium for a while but there's a phenomenon which has been puzzling me for a long time. A mysterious 'stripe' appearing on the mounting plates I make for needles. Look at the pic and you can see right round the section.

The process involves the following steps

                                        1) First milled, drilled and tapped
                                        2) Hand finished by sanding on 3M waterpaper from 360 to 2000grit
                                        3) Degreasing by immersion a special rinse
                                        4) Chemical polishing by boiling in phosphoric acid
                                        5) De-smutting by immersion in nitric acid
                                        6) Anodising in sulphuric acid by passing current through the object

I never observed exactly when this 'stripe' appears until a few days ago as I rather like it.

On Friday night, when I anodised the plate in the pic, I decided to watch closely and was amazed to see it was already clearly visible BEFORE anodising, just after removal from the nitric acid.

I always assumed the 'decoration' was as a result of some electrical peculiarity occurring in the anodising bath and by the titanium jigs I use during anodising but this then must clearly be some chemical phenomena as I don't use the titanium jig in the nitric bath.

Can you think of an explanation for this riddle?

mafioso

 

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