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Add additional thickness onto anodizing?




2006

Q. I'm an Malaysian newbie anodizer, recently I'm facing a problem which is, some of my anodized parts were undersize.

How to increase the hard anodized layer from 15 (0.015 mm) micron to 25 micron directly without stripping the anodized layer? Does it make sense?
is it possible by applying water lacquer or seal to buildup the thickness, but how about the functioning?

2nd,
if to strip-off the anodized layer..
what is the best ratio of caustic soda [affil links] solution to minimize the etch on aluminum?
or by using either HNO3 or H2SO4 solution dip?

Derric Lee
MF Tech - Malaysia



A. First answer is no (generally). Anodize thickness can sometimes be increased on 'fresh' parts -- not sealed or dried with heat. But 'old' anodize doesn't thicken, and turns to powder when the voltage exceeds the original anodize voltage.

Top coats are lower in abrasion resistance. Why was hard anodize specified?

Metal finishing suppliers have stripping solutions for anodizing racks which minimize attack on aluminum. Some are zinc-containing caustics, similar to zincate. The zinc is removed with a nitric desmut before re-anodizing.
The Surface Treatment and Finishing of Aluminum and Its Alloys (p. 955 in the 6th Edn.) lists several acidic stripping solutions. The 2 wt% chromic acid-3.5 vol% phosphoric acid solution (used hot) has negligible attack on aluminum and is used for determining anodize coating weight per ASTM B137.

Before re-anodizing, check whether overall dimensions or anodize thickness are more critical. If necessary, measure the aluminum thickness lost (perhaps 5 or 6 microns per surface) via micrometer and anodize thicker to reach specified dimensions.

Ken Vlach [deceased]
- Goleta, California

contributor of the year 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.

2006



Q. Thanks Ken Vlach,

I don't know why they require such thickness, it was raised from 15 micron/side, and now is 25 micron/side.
These anodized parts are used to carry gases and oils -- maybe for gas abrasion resistivity or maybe insulate the conductivity of metal.

By the way, what is happening for some of my hard anodized parts turn into light green color during sealing (ambient temp sealant)? It seems like dye.
It happened due to 2 different grades of material were mix (material mixed because the after hard anodized colour were different-yellowish and greyish).

Derric Lee [returning]
MF Tech - Malaysia
2006


A. A hard anodize thickness of 0.002 inch (51 microns) per MIL-A-8625 / MIL-PRF-8625 [on DLA]F Type III is usual in the USA, unless otherwise specified or for certain alloys. What specification are you using?

Nickel-containing seals are green but should not produce noticeable color. Check the product bulletin or ask your chemist for info. Maybe a tinted lacquer?

Different anodize colors are due to alloying elements. See the 'Anodizing Reference Guide' provided by the Aluminum Anodizers Council, http://www.anodizing.org/reference_guide.html

Also, oxalic acid in the anodizing solution causes yellowish anodize coloration. What anodize solution and aluminum alloys are you using?

Ken Vlach [deceased]
- Goleta, California

contributor of the year 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.

2006


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