Summary for hardcoat anodizing surface area Discussion
hardcoat anodizing surface area accuracy is discussed as a practical, not ultra-precise, requirement. Dori Kim asked whether 3-D solid models need to calculate surface area to the fourth decimal place for hardcoat anodizing current-density control and whether the engineering effort is worthwhile. Robert H. Probert replied that 4th-decimal accuracy is unnecessary; hard coat is typically run at about 24 amps per square foot, and ±10% or even ±20% in surface area can still work if other variables such as temperature, concentration, and dissolved aluminum are also controlled. He also pointed to a current-density method from Harold Weisner that can reduce tedious measurements and account for holes and recesses. Willie Alexander added that if an amp-hour or amp-minute counter is used, surface-area errors translate directly into thickness and current-density errors, but a 10% SA error is still within Mil-Spec for hard anodize. John Hu agreed that 100% confidence does not require 100% accuracy and said about 90% surface-area accuracy is enough.
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 3-D Solid Models. In the past we did not use current density therefore there are only a couple hundred parts with 3-D 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 Also we are not sure how much surface area tolerance we have before the standard calculation for time to anodize is no longer valid.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
Aluminum How-To
“Chromating – Anodizing – Hardcoating”
by Robert Probert
You’ll love this book. Finishing.com has sold a thousand copies without a return request 🙂
 | 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 relieve you of tedious measurements and also compensate for holes and recesses.
RECOMMENDED ANODIZING & CHROMATING BOOKS  “The Surface Treatment & Finishing of Aluminium and Its Alloys” by Wernick, Pinner & Sheasby (1959 – 2001)  “The Metallurgy of Anodizing Aluminum” by Jude Mary Runge (2018)  “Surface Treatment and Finishing of Aluminum” by Robert King (1988)  “Anodic Oxidation of Aluminum and Its Alloys” by V. F. Henley (1982)  “Aluminum Finishes Process Manual” by Reynolds Aluminum (1973) (as an Amazon Associate & eBay Partner, we earn from qualifying purchases) 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 100% confidence does not mean that surface area needs to be 100% accurate practically. John hu |
