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topic 50885

Thickness of anodize coatings

A discussion started in 2009 but continuing through 2019

January 8, 2009

Q. We have a requirement to anodize parts that are pins per Mil-A-8625 [link is to free spec at Defense Logistics Agency, dla.mil] Type II Class I. The required pin diameter prior to anodize is .2459"/.2465". The required diameter after anodize and sealing is .2479"/.2485". Is this even possible to achieve? This is not a Hard Anodize process. Our specification also says "Anodic coatings, after sealing, shall weigh a minimum of 600 milligrams per square foot". How thick is this? Is this MIL spec, pin diameter prior to anodize, and coating thickness all consistent?

Bob Lunnon
MFG. ENGR. - Gainesville, Texas


simultaneous January 9, 2009

A. The difference is 0.0006 on the diameter, or 0.0003 on the side, which is very low for Type II and more like the newer IIB. If you will contact me I will read some of the ratios between coating weight and thickness from Sheasby/Pinner, which depends on alloy, time, current density, temperature, and acid concentration. The coating weight you call out does seem low for the thickness. 2024 at 0.00027 thickness and 0.00018 dimensional change shows 1104 mg, 6061 at 0.00028 which is 0.00019 dimensional change, shows 1758 mg, and 7075 at 0.00029 which is 0.00019 dimensional change shows 1594 mg. The difference between the thickness and the dimensional change is because the anodic film is approx. 1/2 under the original surface and approx. 1/2 above the original surface., again, depending on the parameters listed above.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina


January 10, 2009

A. Bob

Refer to Table IV in section 6.10.5 of MIL-A-8625 for typical thickness ranges. Asking for a .0020" thickness from a Type II coating is asking a bit much unless you have developed a special process.

Table 1 in section 3.7.1.1 lists the minimum coating weight (unsealed) for Type II coatings to be 1000 mg/ft2. Sealing will increase the coating weight. We see 1800 - 2000 mg/ft2 on 2024 T3 @ .0006" thickness (Type II). A .0020" coating will have a significantly higher weight even if the coating is exceptionally porous.

The coating weight, per unit of measure thickness, will vary depending upon operating conditions. Usually, coating weight or thickness is specified for a part. Occasionally, both will need to be controlled.

Is this a well designed part? Depends. Most 2-mil coatings are for wear resistance, corrosion resistance, or electrical resistance. Allowing such a low coating weight for this coating allows inferior coatings to be presented as acceptable. Determine what the coating is supposed to do, and revise the requirement accordingly. Why pay for a 2-mil thick coating if half that will suffice.

Willie Alexander
- Colorado Springs, Colorado


January 12, 2009

A. That's only a .002" coating, and while that's a bit thick for a type 2 coating, it certainly is achievable. I really can't comment intelligently about the weight of a type 2 coating at that thickness.

Marc Green
Marc Green
anodizer - Boise, Idaho


January 12, 2009

A. Bob: Type II anodize (Sulfuric) requires a coating thickness of .0003-.0007 inches. Because the coating converts the aluminum surface to aluminum oxide, the coating penetrates the surface as well as expands, therefore the net change in a diameter is usually the same as the coating thickness. Take your finish dimensions and adjust accordingly.

Kent Backus
Fort Worth, Texas, USA



Hard Coat Anodizing Coat Weight Measurement

October 14, 2019 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We have a plating line in house and since I'm still relatively new to the process, I am reviewing the MIL-A-8625 to clean up our paperwork. The spec for Type III is minimum coating weight of 4320 mg/ft^2 for every 0.001". According to a lab, the coating weight is measured from the before and after stripping of the panels. They take the thickness measurement with an eddy current tester for 1 side.The resulting number is to divide the coating weight by the one-sided measurement. Is this correct? I would think per the MIL-A-8625 that the coating weight, that was stripped from both sides, be divided by the total thickness? How is this MIL-A-8625 spec for type 3 interpreted?

Trung Lively
- Black Mountain, North Carolina USA


October 15, 2019

A. Hi Trung,

Coating weight for Type III coatings is optional upon request from the procuring authority. So, if you can measure the thickness by eddy current, you don't need to do coating weight. Unless your customer requires the test to be done, I wouldn't bother, personally.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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