The authoritative public forum
  for Metal Finishing since 1989
-----

Anodizing thin layer

2005

Hello all,

Could anyone help me with the following question?
On an insulating plate I have deposited chromium strips of 2 mm wide, 30 cm long, and only 0.3 micron thick. On top of each strip, 0.5 micron aluminum is deposited. I would like to anodize the aluminum part and keep the chromium.
The resistance of the strip is relatively high, as compared to a bulk metal piece. If I contact the strip on one end, and start anodizing, the voltage on the other end will be somewhat lower than on the contacted end, perhaps about 0.5 to 1 V.
My question is: how much influence can I expect on the thickness and quality of the anodized layer? In general: how sensitive does the anodization process depend on the applied voltage?

Many thanks in advance,

Karel Knechten
Research - Eindhoven, The Netherlands

---

2005

The thickness is a function of the voltage, electrolyte, alloy, current density, temperature, anodizing time... The thickness consists of a thin barrier layer (mostly dependent upon voltage) adjacent to the metal and a bulk cellular layer, which depends upon time & other variables). Anodic thickness is generally quite uniform, as thinner regions of lower electrical resistance to the flow of ions grow faster and hence, 'catch up' with slower growing thicker regions.
The quality and the uniformity of thickness depend upon maintaining constant parameters, including temperature. There must be sufficient cooling to remove both the heat of formation of alumina, 31 kJ per gram of Al anodized, and any ohmic heating in the metal strip. Near completion, the poorer conducting Cr must carry all the current. If it begins to get hot, resistance will rise further and you will experience 'burning.'

It is also important that the electrolyte not contact the Cr (short-circuit), so the initial Al deposit should be of uniform thickness and any bare Cr sides sealed with masking.

A few values:
Electrical resistance: Al strip: 0.26548 ohm/cm of length. Cr strip: 2.15 ohm/cm. (Al + Cr): 0.2363 ohm/cm.
Anodizing current (@ 12.5 Amps/ft2): 0.00269 Amps/cm length or 0.08073 Amps (total).
Estimated voltage drop over 30 cm: Initially 0.28 V, increasing to 2.6 V when the Al is ~gone (neglecting temperature increase effects).
Heat of alumina formation ( for 0.000027 gram Al/cm length): 0.837 J/cm.

What type of anodizing/electrolyte are you considering and why? Is this solely to produce an electrically insulating film, to study anodic films, or to perhaps dye the anodize? What is the substrate (ideally, an electrical insulator but good thermal conductor, plus acid-resistant)?
I recommend reading about various processes and anodic films in Chapter 6 "Fundamentals of Anodizing" in The Surface Treatment And Finishing of Aluminum And Its Alloys, 6th Edn.(or 5th) by P. G. Sheasby & R. Pinner.

See also letters 2369 www.finishing.com/23/69.shtml and 37497 www.finishing.com/374/97.shtml re barrier layer anodizing, used for thin insulating films as on Al foil in electrolytic capacitors.

You might also consider the brush anodizing process, which gives considerable manual control (reduces the current flow & heat generation by anodizing small sections at a time).

An alternative is simple oxidation of Al in boiling DI water or steam to boehmite, then heating (e.g., 300 C) to convert/dehydrate to alumina.

You have a rather tricky problem. Let us know your procedures and results. Good luck.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.