Are you stripping the anodic layer before re-anodising?

Brian

The anodic layer is porous and the pores tend to close up during rinsing, especially in hot water. The dye is trapped in the pores in a heated dye tank.

You may have to run a series of test pieces to find the best method of rinsing before the dye tank, also the optimum time in the dye.

 Maybe your voltage is too low, anodising is a non conductor, and the voltage is normally increased as the coating gets thicker.

Edited By old mart on 27/07/2019 19:45:52

You do need to ensure good electrical contact throughout both anodising and plating, and anodising is trying to insulate the aluminium! Industrially, the work-holding jigs are commonly springy, which has production-handling advantages too, but keeps the contact with the work-piece.

Agitation: either user a simple stirrer, or, again borrowed from trade practice, bubble air through the fluid. I believe some people use an aquarium aerator for this.

Mark Smith advises on pH. It may be worth buying a pH meter and the better ones of those sold for testing home swimming-pool water might be accurate enough. They are colorimeters that examine a solution of a reagent in the water under test.

30 seconds in the caustic soda before anodising doesn't seem long enough to me for anything other than a freshly turned surface, especially if you are trying to strip a previous attempt at anodising. I leave it long enough to see the surface start to turn matt.

Neil

P.S. I use a Ti rod as a connector and ALWAYS clean up the contact points with wet and dry before each run.

Edited By Neil Wyatt on 28/07/2019 10:16:08

If the dye isn't taking then the anodise pores aren't forming properly, assuming you are using proper anodise dye & not ink or clothes dye ???

Like I said, I have never experienced black dye coming out grey, I have only ever seen dark shades of purple down to deep blue depending on the size of the pores. Is it possible that the part or the bath is contaminated???

I have only ever had repeatable results using Ti connections. My favourite method is to utilise a Ti fastener into a screw hole. Ti fasteners & welding wire can be had quite cheaply.

It is vitally important that you agitate the anodise bath & keep the temp ambient & the bigger the bath the better. You aren't using a jam jar are you???

If you're using a 720 calculator then you need to follow the 720 methods rules, it only works if you follow the rules & stick to the rules.

If you don't agitate the tank then you can/will get air bubbles & localised hot spots on the part, it is highly likely that this is what's causing the blotches. I no longer home anodise but I used to have a semi-pro setup using 25l tanks and there's lots of info & build diaries on t'interwebs of how to go about it.

I found that the Caswell Plating forum was the place, some very knowledgeable folk on there.

Regardless of what hes doing hes still using a final rinse of de-ionised water which is as far as i was taught is wrong. It has to be below pH of 2. I worked for a while at an aluminium extrusion plant which eventually was took over by Alco and we used to do alsorts of anodising for the MOD etc.. like missile and aircraft parts.

But this was 30 years ago so maybe things have changed.

We used to seal the coloured layer with lithium salts or sodium tetrofluoroborate solution if i remember right.

Edited By mark smith 20 on 28/07/2019 18:59:27