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Double Anodizing or Sequential Anodizing


An ongoing discussion beginning back in 2008 ...


Q. Is there an anodizing process called "Sequential Anodize" and "Double Anodize"? Could you explain the difference between the two anodizing processes? Could you highlight the advantage of them, if any?

Neo Tiang Hua
- Singapore


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.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

February 15, 2010

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.

John Hu
- Singapore

Double coat anodizing of aluminum

February 6, 2010

Q. I have a 150 °C steam application contained by aluminum. I've anodized it from shiny to a light grey color, if wet the anodized surface becomes conductive, so I think it can be anodized more without having to remove the previous anodized surface. And I found some parts advertised as "double anodized" so is it possible to double anodize? Also will I need to hard anodize to protect against the steam?

Brendan Murphy
product designer - London, Ontario, Canada

February 9, 2010

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".

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

February 9, 2010

Q. Are you implying that anodizing does not hold up over 85 °C? If so then please explain what the anodized aluminum cooking pots are coated with.

Brendan Murphy [returning]
- London, Ontario, Canada

February 11, 2010

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".

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

February 12, 2010

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.

Willie Alexander
- Colorado Springs, Colorado

Double anodizing for brilliant detail on matte parts

March 10, 2014

Q. Does anyone know the process of double anodizing ?

I urgently need to know the steps and how best to apply the double anodizing process. I need a matte piece, but with brilliant details, I'm currently doing the following process, but am having many problems:

Material: Aluminum 1100 - thickness: 0.5

1 - Brightening
2 - Neutralization
3 - Anodizing surface (fast)
4 - Hot sealing (2 minutes)
5 - Application of ink silk screen resistant to caustic soda
6 - matting in caustic soda
7 - Normal Anodizing
8 - Normal hot sealing

Part of the silk-screen ink may not have raised.

leonardo villalobos
professional - Sao Paulo - Brazil

March 17, 2014

A. Leonardo

In general, your process looks sound.

What problems are you having?

Willie Alexander
- Colorado Springs, Colorado

Can you clear anodize over color anodizing to make it darker?

August 25, 2018

Q. I'm looking to get a brick red on a part. Most of the companies I talk to only carry a bright red. I was wondering if you can first red anodize the part and then clear/hard anodize over that, adding to the thickness of the hard anodizing to darken the appearance of the red.

David Mucci
Motorcycle Builder - Portland, Oregon US

August 27, 2018

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.

Willie Alexander
- Green Mountain Falls, Colorado

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