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Accuracy of surface area needed to hard coat

Q. I am currently working at an aerospace fuel systems manufacturer and stumbled upon a topic that no one at our company can answer with 100% confidence. We run current density using a standard rectifier and calculate surface area using 3D Solid Models. In the past we did not use current density therefore there are only a couple hundred parts with 3D Solid Models out of the thousands of parts that go through our anodize process. Does anyone know how accurate the surface area measurements need to be. Using the Solid Model allows for surface area calculations to the 4th decimal point (.000X). The reason I ask is because many wonder if it is wise to devote so much time to engineering these Solid Models for each and every part. Also we are not sure how much surface area tolerance we have before the standard calculation for time to anodize is no longer valid. Thank you so much!

Dori Kim
Metallurgical Engineer - Rockford
January 10, 2008

A. You do not need 4th decimal place accuracy for surface area. You want 24 amps per square foot for hard coat, but ± 10% or even 20% will work. How tight are your other parameters: temp, concentration, dissolved aluminum. ± 10% on any of those will also work.

That said, there was a current density gimmick published in the 1958 AES Proceedings by Dr Harold Weisner, and this gimmick is repeated in Aluminum How-To. It is a very simple method that will releive you of tedious measuremnets and also compensate for holes and receses.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

A. Do the math, & if you are not using an amp-hour or amp-minute counter, consider acquiring one.

We know that 1440 amp-minutes will produce a 2-mil thick coating on 144 sq. in of exposed aluminum. If you are off 10% in your SA calculations, a 2-mil thick target thickness will be off by 10%, well with in the Mil-Spec for hard anodize. Your calculated amperage (CD) will be off the same %. The Mil-Spec does not have a requirement for CD, and most shops will typically run 24-36 ASF, or higher (12-15 ASF for Type II).

If you are off 1% (-)on each dimension of a rectangular part, your SA will be less than 2% off actual.

If you are using an amp=minute counter, you can calculate your surface area using those values and your thickness values. You will need to compensate for racks if using aluminum tooling.

Willie Alexander
- Colorado Springs, Colorado

100% confidence does not mean that surface area needs to be 100% accurate practically.
Provided all other factors (such as temperature, bath composition, agitation, current density etc) are well controlled, in my opinion, 90% surface area calculation accuracy is enough.
4th decimal point calculation is totally unnecessary.

John hu
- Singapore

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