44097

Type II vs Type III anodizing of 2024-T3  

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Can someone tell me the difference in wear properties between these two:

1. Material: 2024-T351, anodize per Mil-A-8625 [link is to spec at TechStreet], Type II, Class 2, Color Light Gold.
2. Material: 2024-T351, hard anodize per Mil-A-8625, Type III, Class 2, Color Black.

Also, why is 2024-T351 considered more difficult to hard anodize and what similar strength alloy would hard anodize more easily?

Stan Thomas
engr - Portland, OR, USA

1. Bad.
2. Poor. MIL-A-8625 allows hard anodized 2024 to have a Taber abrasive wear loss 7/3 x that of 6061 or 7075, and dyeing + sealing further softens the anodize.

Some effects of alloying are described in the Aluminum Anodizers Council 'Anodizing Reference Guide' http://www.anodizing.org/reference_guide.html

Simplified technical explanation: The Cu in 2000-series alloys forms intermetallics such as CuAl2 which dissolve rather than anodize. About 7 wt% CuAl2 in 2024, so the anodize is ~7% more porous than on a low-Cu alloy. The intermetallics also increase ohmic heating within the alloy, which in combination with thin spots in the anodize, can lead to a nasty, electrochemically-accelerated dissolution phenomenon known as 'burning.'

Many newer alloys have high strength and anodize better than the 1920's vintage 2024. Some with better corrosion resistance in higher strength tempers than the 1943 vintage 7075. The Metallic Materials Properties Development and Standardization (MMPDS) Handbook lists a number of 7000-series alloys: 7010, 7040, 7049/7149, 7050, 7055, 7075, 7150, 7175,7249, 7475; also 6013.

Yield strengths (ksi) for thicknesses up to 2 inches.
2024-T351 42-50
6013-T6 46-49
6013-T651 52
7010-T7451 61-63
7049/7149-T7351 60-64
7050-T73511 56-60
7055-T7751 85-88
7075-T7351 52-59
7150-T7751 74-78
7175-T73511 52-59
7475-T7351 53-61

All values from MMPDS-01 except 6013-T651 (MatWeb). 7040 (heavy plate only, > 3") and 7249 (Hand forgings) were omitted.

Ken Vlach - Goleta, California

Type III anodize of 2024 is a risky venture due to the higher concentration of copper within the alloy. Not to say it cant be done, we do it often, but the risk of material degredation is increased.

To answer your other question, 7075 would be more appropriate for a type III coating.

-Bill

Bill Grayson
metal finishing - San Jose, Ca, USA