Letter 14352

Grain structure visible through black anodising  

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I am currently having a problem with black anodising grade 6061 aluminium. The parts are cylindrical with and OD varying between 1 and 3cm. When the parts are black anodised the coating replicates the grain structure underneath and appears mottled, this is not passing quality inspection as the coating does not appear uniform (even though it most probably is). Can anything be done while anodising to prevent this or is there something that can be done to treat the aluminium to reduce the large sized grains evident on the surface? Or, would a different aluminium grade be better?

The parts also require turning, so that an easily machinable aluminium is required prior to the anodising.

Is the "mottled" effect due to the extrusion of the aluminium and the severe grain growth experience on the exterior of the rod because of this?

Any suggestions would be most welcome.

Katrina Parlevliet
- Ringwood, Victoria, Australia

Is your alkaline etch loaded with zinc from previously running 7075? Is the alkaline etch too high in dissolved aluminum. What are you deoxidizing in, how long, concentration, contamination level. Is the anodizing solution over 12 gm/L of dissolved aluminum? Can you see the mottled look after anodizing and before dying, if so try dippping in 5% Nitric Acid before dying. Is there high calcium in the dye? Is the dye tank stainless steel, then reverse the galvanic cell anywhichwayyou can. Try dying a piece in a beaker in the lab in a new make-up to see if the dye tank is contaminated.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

Your anodizer is over agressive in his etching, probably using a hot caustic or stong nitric / hydrofluoric acid etch. Suggest using NO caustic etch and only chromic/sulfuric deox prior to anodize. If indeed the grain is from the extrusion process, might try one of many acid "matte" etch products which tend to be elevated temperature fluoride baths.

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

Katrina,

I think that your evaluation is correct. I do not believe that it is anything to do with the anodising.

6061 is a 'hard' alloy and can be difficult to extrude AND achieve required mechanical properties.

I do not think that you can be confident to ALWAYS have a grain free appearance - the effect is merely cosmetic.

Try keeping the etch time to the absolute minimum.

Also, 6061 is one of the best free machinig 6000 series alloys, especially in T6 condition.

If you do not need high mechanical properties, you are probably aware that you could use 6063 or 6060 - this will be far less likely to have the grain effect, BUT, machinability would be compromised.

Best regards,

Martin Webb
- New Zealand

Are you sure your material is 6061? 7075 has a strong tendency to show grain structure when etched and anodized.

Guy Lester
- Ontario, CA, USA

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