• Topic 55760 "Aluminum Anodizing flakes off when heated to 125 °C"

"The Properties of Electrodeposited Metals and Alloys"
by William Safranek

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A. Safranek's "The Properties of Electrodeposited Metals and Alloys" .
covers the topic a bit. Anodic coatings have both very little elasticity and a lower coefficient of thermal expansion than the aluminum article they are formed on.

Result: either mechanical working or elevated temperatures can crack (craze) the coating.

A. Crazing of the anodize coating usually occurs when too much anodize coating thickness is deposited on part or when the temperature of the anodize tank is too hot. Hot temperature in the anodize tank will cause a soft anodize coating. Soft coating will craze.

A. I have had experience with the very some problem. One of the most common causes of crazing is thermal shock to the part and coating. This usually occurs when you are moving the part from a cold anodizing bath to a hot rinse or seal. The coating and substrate expand at different rate causing the crazing. One solution to this problem is to ramp your seal temperature after the part has entered the solution. Another solution would be to slowly increase the temperatures of your rinse as your parts progress to the hot tanks.

Thank you very much for your suggestions sir. For your information, under optical microscope we have seen straight cracks (Cross sectional view - as polished condition) in the hard anodic coating & are perpendicular to the thickness of coating.

A. 300 °F for 30 minutes can cause the anodize to craze or crack.

The anodize coating is a ceramic, thus has the high temperature resistance of most ceramics. The problem is the large differences in thermal expansion between the aluminum base metal (expands a lot) and the thermal expansion of the ceramic anodic layer (expands a little). This can put substantial mechanical stress on the anodize. The problem is worse when the anodize layer is thicker.

Ed. note: it is our sad duty to advise of the passing of Paul Yursis in August 2005.
Here is a brief obituary by Mike Caswell.

A. Dear Richard

Crazing is due to the thermal expansion and elasticity of aluminium is much greater that that of the anodic coating.

You could do the following thing to minimize the problem:
1. use higher anodising temperature
2. lower current density
3. a final short rinse in warm water (approx. 40 °C)

Anne

A. Another option would be to silk screen it with dyes made for anodizing prior to sealing the part. You can find a method or two describing how to do this in "The Surface Treatment of Aluminum and Its Alloys" by Wernick, Pinner and Sheasby, or you can contact anodizers that specialize in this practice and have them put on whatever design it is you are silk screening.

A. The best practice for high temperature painting is insure the anodic layer is around 5 µm to 8 µm - this allows painting up to 200 °C with no issues. Standard practice at most exterior trim anodize aluminum suppliers.

A. Crazing or cracking of anodic coatings is commonly due to mechanical stress, either applied load or differential thermal expansion. Bending causes linear, parallel cracks transverse to the bending, with cracks sometimes visible to the naked eye on the convex side of the bending. Point loading, as from a hardness indenter, creates radial cracking. Non-anodizing inclusions can also create anodize blisters and radial cracking. Crazing from thermal stresses can be 'dried lakebed' (except flat) or wavy lines (like wrinkled paint, but flat).

I have never seen nor read of cracks in anodize due solely to fluid attack. Fluids may possibly wick into crazing and, via etching or corrosion, create visible cracks. What is the fluid? 2024-T6? Cold worked and/or machined surface? Are inside or outside corners involved? Conventional, sealed sulfuric anodize (MIL-A-8625 / MIL-PRF-8625 [on DLA]F, Type II? Dyed? Thermal history, beginning with anodize bath? Please post a photo, maybe at 200x magnification.

See topics 27462, 27990, et al.

A. Nigel

In a past life, we hard anodized 3003 aluminum IC carriers that had comparable wall thickness. Our crazing was much more pronounced, and caused by the difference in co-efficient of expansion (search this site). The crazing occurred when we placed the parts in the rinse tank from the hard anodize tank. The crazing was audible as the part was introduced to the rinse. Adjustments in rinsing technique reduced the amount of crazing, but entirely.

Visually, the crazing might be objectionable, but you might test to see if the coating meets performance specifications.

Willie, Thanks for your response. We did feel that rinsing or sealing was a contributing factor in the 'crazing' on the anodised surface. We did in fact have a coating thickness of 40 microns on the component which was more than double the maximum specification requirement for this part. We have since stripped the part and the 'crazing' has gone. We are going to re-anodise to the required film thickness of 7.62 - 17.78 microns and seal in boiling water. It has also been suggested that the excessive anodise coating and the subsequent hot water sealing may have contributed to the 'crazing'. Any more comments would certainly help.Thanks again.

A. I see "scratches" but do not see "crazing". Come back with the alloy, coating thickness, and temp of the hot water. Be aware that cracking starts at 80 °C and gets worse with higher temp.

A. Alaatin

I think if you only change the coating thickness, the crazing will still occur, but due to the thinner coating, it will not be as noticeable. I suspect the alloy choice is a factor, although I can't guide you as to which would be better.

One way to determine if an alloy would craze under similar circumstances would be to seal the anodize in a boiling solution. Hard to believe the parts are crazing in the field if they were hot sealed in your shop.