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topic 10819

Anodized Coating Thickness and Sealing


Q. I am reading a specification and have some questions regarding the specified criteria for the clear anodized coating for a curtain wall on a project in Hong Kong.

1. Finish thickness specified for exterior aluminum surfaces is 40 microns (0.040 mm). First, can this thickness of clear anodizing be applied? Second, are there any negative results from applying this thickness (other than increased cost)? Third, Is there any benefit to having a this thickness of anodizing over a normal 18 to 30 micron thickness?

2. How can I determine the density of the coating thickness? Specifications say "density of coating shall not be less than 2.72 gm/cc". This obviously varies with the coating thickness so how can it be calculated?

3. What is a Nitric Acid wash to seal the finish? I have never heard of this. It sounds like a pretreatment to me rather than a seal. Specifications say "Nitric acid wash to seal is mandatory." I thought all seals on anodized coatings were done with deionized boiling water. Thank you for whatever help anyone can offer.

Ronald K [last name deleted for privacy by Editor]
- Hong Kong

simultaneous 2001

A. Ronald,

I can answer a couple of your questions rather easily. In reference to determining the coating density, the first step is to determine the average coating thickness of a test panel. The next step is to determine the coating weight of the panel by stripping the coating with a Chromic/Phosphoric acid mixture. The stripping process should be performed on a test panel that has not been sealed. Using the beginning and ending weights of the panel and the surface area, the coating weight can be determined. Use the thickness of the coating to determine the average density of the coating.

I have not heard of a nitric acid seal. It seems to me that sealing with this type of solution would leave a corrosive residue in the pores of the coating.

As far as the clear coat, could you please provide the specification you are conforming to so I can provide an adequate response.

Best of Luck.

Ira Donovan, M.S.F.
Kansas City, Missouri


A. 1) The thickness you are mentioning looks like a tad high for a normal sulfuric acid anodizing, as per MIL Spec Mil-A-8625 [link is to free spec at Defense Logistics Agency,]F Type II. In fact 40 microns corresponds to about 1.57 mil, which is almost about the thickness for a hard coat instead of an architectural anodizing! You need first to establish whether is it a hard coating or a regular sulfuric acid anodizing!

2) Density of the coating

The density of the coating thickness can be determined in the same way you determine the coating weight!

You anodize at least 2 test panels (same alloy as your load), with a known geometric area (let's say 10 by 10 cm) Measure your coating thickness. This would give you the volume of your coating:

Volume [cc] = 2 x Thickness (cm) x 10 (cm) x 10 (cm), accounting for both sides

Weigh you panels with the coating on (Winitial)

Strip, dry and reweigh stripped panels (Wfinal)

Coating density = (Winitial ñ Wfinal) [gr] / Volume coating [cc].

3) Seal Test and ' Nitric acid wash to seal ªÖ

Here, it looks like they are talking about the Acid Dissolution Test (ADT), ASTM B680 [link by ed. to spec at TechStreet] . Normally, the ADT is carried out without any pretreatment. However, new European standards require a pre-dip in a 65% (v/v) nitric acid for 10 minutes and then followed by the ADT!

Hope this would help or someone else would comment

Hocine Djellab
anodizing shop - Montreal, Quebec, CANADA


Q. In answer to Ira Donovan's question of Burns and McDonnell - Kansas City, MO The clear anodizing should conform to AAMA 611-92 an AAMA 607.1 and meet test requirements of ASTM B136-84 [link by ed. to spec at TechStreet] , ASTM B137 [link by ed. to spec at TechStreet], and ASTM B244 [link by ed. to spec at TechStreet].

Thanks for your help so far.

Ronald K [returning]
- Hong Kong

February 25, 2010

Q. I am very new to all of this and I am trying to do the coating weight based off ASTM B137 [link by ed. to spec at TechStreet]. My calculations do not match that of my laboratories. I am not sure how to do this calculation. Please Help.

Amber Spellman
Plating Shop Employee - Denver, Colorado

September 24, 2010

Q. I have a question about the procedure for stripping coating weight on ASTM B137. What is the purpose of swabbing surface with 10% NaOH in the section of Test Specimen (Sec 5)?

Steve Chien
- Auburn, Washington

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