Double Anodizing or Sequential Anodizing

A. Hello Neo, now I'm curious too. Never heard of "sequential anodize". But I've heard of "double anodize" as meaning getting two different colors and/or textures on one piece through a masking process whereby any given area of the piece receives only one layer of anodizing, but the piece as a whole has seen two anodizing processes.

It would probably help if you gave us some context like where you heard about it, why it was mentioned, why it interested you, etc. People toss slang around all the time so we have no assurance that some other reader's idea of "sequential anodize" or "double anodize" will be the same as what you heard about if you heard nothing about it but the name.

A. I think you are referring to the process: aluminium first anodized in H3PO4 or H2SO4 bath then while it is wet, anodized in a Oxalic bath.

A. The aluminum oxide coating starts to crack above 85 °C. This is because the thermal expansion coefficient of the oxide is 1/5th that of the underlying aluminum metal. Aluminum Oxide does not conduct, if you are measuring through the coating, then the water in the pores is conducting thru to the aluminum underneath. Once sealed the inside of the pore is converted to "hydrated aluminum oxide" and you cannot unseal it and anodize over. Kept unsealed and wet, then you can re-start.

Hardcoat would be thicker and have a smaller pore (unless done at higher temperature) and that would protect your aluminum for a longer time than would lower thickness and larger pores. Pores are made smaller by low acid concentration, high current density, and low anodizing temperature, hence the very definition of "hardcoat".

A. I am quoting Sheasby/Pinner in their latest book, The Surface Treatment and Finishing of Aluminum and its Alloys, Sixth Edition, Volume 2, Page 1092, and personal experience with the microscope on anodized aluminum including cook ware that has not been Teflon coated. Think about what I said, "The underlying aluminum with heat, expands 5 times the oxide coating on top".

A. Brendan

Picture a rubber band (the aluminum) that has been painted (the anodize). If you stretch the band, the paint will fracture as is does not have elasticity the same as the rubber. When you relax the band, the coating is restored to the original appearance, provided the painting had adequate adhesion. The fracturing does not interfere with the performance of the coating (anodize), yet still exists.

A. Leonardo

In general, your process looks sound.

What problems are you having?

A. David

You can't anodize over anodize to alter the color. I would try samples of hard anodize (Type III) first at .0005", .001", and .0015" thicknesses and then dye. Different shops have different operating parameters so the natural color of hard anodize at a given thickness will vary, from light grey to a medium bronze; the alloy of aluminum being anodized will also influence the natural color. These natural colors are darker than the normal anodize used for everyday red, and will yield a darker red when dyed. If your anodize is too dark you will end up with a brown color.

Your end color will be a combination of the natural hard anodize + the red dye. Also note there are a number of different red dyes and each can behave differently. Might take a few tries to get what your after.