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topic 5907

Phos Acid Anodizing Min.Coating Thickness


I am trying to find the minimum effective oxide coating created by phosphoric acid anodizing to support durable adhesive bonding for aluminum alloys. I know current is directly related to the thickness of the oxide coating and a current density of 8 amps per square foot should produce a sufficient surface for bonding. However, I would like to know what effect lower current and reduced coating thickness have on the durability of the bond, i.e. what is the minimum current density and resulting thickness to provide a suitable bonding surface. Thanks!

Ray Kaiser
- St. Paul, MN, USA


"Adhesive Bonding"
by L.H. Lee

from Abe Books
info on Amazon


As a relative novice to Phosphoric anodising all I can let you know is that in 3 years we have never really measured current density and have never found a reliable specification for coating thickness. The current draw seems to vary depending on the type of alloy being anodised and usually falls to virtually nil well before the process is complete.

The only two methods we have found are for verification of coating presence not for measurement of coating thickness. If you follow the Boeing model in BAC 5555 you can't get it that wrong. Verification of the coating visually is usually the most people ask for.

The only thing we do ensure is that there is electrical contact in at least 2 points on each component and there is good part to solution voltage.

Ciaron Murphy
- Great Britain



As Ciaron says, follow BAC 5555 and you will get good results. But as far as current density goes, you should be capable of supplying 8 amps per square foot, but you won't get that at 15 volts. The current density depends on the alloy, and we have found that we get about 1.5-2.0 amps per square foot for 2024 clad, at 15 volts.

Chad Wren
- Fort Worth, Texas


Thank you for your responses.


Do you use or know of any inspection other than visual to check the coating. We can verify visually that the parts have an anodic coating, however we are not currently capable of providing 8 amps/sq ft for large sheets and are wondering how this affects the coating thickness and subsequent bond strength and durability.

Ray kaiser
- St. Paul, Mn, USA

"Adhesive Bonding of Aluminum Alloys"
by Edward Thrall

from Abe Books
info on Amazon


A good and cheap way to check phosphoric anodized coatings is to use a tape-adhesion test. Simply place a piece of ordinary tape on the part and pull it off. If the coating is present in almost any thickness, the adhesive will remain on the part.

Because phosphoric acid is much more aggressive on aluminum than sulfuric acid, the surface area of the subsequent coating is much higher. The coating thickness itself is not that critical for adhesive applications, not to mention that the thickness is very limited in this electrolyte. The fact that you can not maintain 8 amps/sq ft could lead to nonuniform coating thickness, which is a bigger problem. If your current distribution is good, you shouldn't have anything to worry about.

Gerald Janssen
- Chicago, Illinois


Someone called me awhile back & was looking for someone who could run more voltage on a Phos line. Our current process is the same as ASTM D 3933, BAC 5555, Cessna standards, etc. (15 volts). The best way for you to test different amps to your product & find the results is to DO some R & D & start bonding the details & run pull test.

Donny Farris
- Ada, OK USA


We are trying to measure the current density and make a decision what will we do. How can we increase the current density and will we calculate the total surface by consideration of the two surfaces of the plates or only one surface. We are using the titanium clamps for holding the plates which have very little contact surfaces. Do these clamps effect the current density.?


Best Regards

kudret akØn
- ankara/turkey

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